CN114720885A - Method for accelerating evaluation of cycle performance of lithium ion battery - Google Patents

Abstract

The invention discloses an accelerated evaluation method for cycle performance of a lithium ion battery, which comprises the following steps: step S1, selecting a fresh battery and a reference battery which have the same battery system as the battery to be tested in advance, analyzing and obtaining a characteristic SOC interval of the battery system to be tested, and determining an accelerated test SOC interval; step S2, respectively carrying out accelerated cycle test on the battery to be tested and the reference battery in an accelerated test SOC interval to obtain an accelerated cycle capacity retention rate curve of the battery; and step S3, comparing the accelerated circulation capacity retention rate curves of the battery to be tested and the reference battery, and judging whether the circulation performance of the battery to be tested is good or bad relative to the circulation performance of the reference battery. The invention is suitable for the development of lithium ion battery products, is used for rapid comparative analysis of the cycle performance of the battery when screening materials and optimizing a system, can effectively shorten the development period of the battery, improves the research and development efficiency, and indirectly reduces the development cost of the battery by reducing the energy consumption of cycle testing.

Description

An accelerated evaluation method for the cycle performance of lithium-ion batteries

technical field

The invention relates to the technical field of performance testing of lithium ion batteries, in particular to an accelerated evaluation method for cycle performance of lithium ion batteries.

Background technique

At present, lithium-ion batteries have the advantages of high specific energy, many cycles, and long storage time. They are not only widely used in portable electronic devices (such as mobile phones, digital cameras and laptop computers), but also widely used in electric vehicles, In terms of large and medium-sized electric equipment such as electric bicycles and electric tools, the performance requirements of lithium-ion batteries are getting higher and higher.

Cycle life is the core indicator to characterize the performance of lithium-ion batteries. With the improvement of battery performance requirements in the field of energy storage systems and electric vehicles, at present, the cycle life of lithium-ion batteries has reached thousands or even tens of thousands of times. The cycle testing process has greatly delayed the development of battery products. How to speed up the evaluation of battery cycle performance and find a reasonable and effective cycle evaluation method has become a key technical problem to be solved in the lithium-ion battery industry.

At present, in the field of accelerated cycle evaluation of lithium-ion batteries, accelerated tests are mainly carried out by changing stress conditions such as battery temperature, pressure, voltage, and current. Quantitative or even qualitative changes in the reaction occur, resulting in inconsistency between the accelerated test results and the attenuation factors occurring in the actual measurement cycle, resulting in large deviations in the accelerated test results.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide an accelerated evaluation method for the cycle performance of a lithium ion battery in view of the technical defects existing in the prior art.

To this end, the present invention provides an accelerated evaluation method for the cycle performance of a lithium ion battery, comprising the following steps:

Step S1, for the battery to be tested whose cycle performance needs to be evaluated, pre-select a fresh battery and a reference battery of the same battery system as the battery to be tested, analyze and obtain the characteristic SOC range of the battery system to be tested with cycle decay, and determine the SOC range for the accelerated test ;

In step S2, the battery to be tested and the reference battery are respectively subjected to an accelerated cycle test within the SOC range of the accelerated test obtained in the first step, and the accelerated cycle capacity retention rate curves of the battery to be tested and the reference battery are obtained correspondingly;

Step S3, by comparing the accelerated cycle capacity retention rate curve of the battery to be tested and the accelerated cycle capacity retention rate curve of the reference battery to determine the cycle performance of the battery to be tested relative to the cycle performance of the reference battery.

It can be seen from the above technical solutions provided by the present invention that, compared with the prior art, the present invention provides an accelerated evaluation method for the cycle performance of lithium ion batteries, which is scientifically designed and suitable for the development of lithium ion battery products and for material screening. And system optimization, rapid comparative analysis of battery cycle performance can effectively shorten the battery development cycle, improve research and development efficiency, and indirectly reduce battery development costs by reducing cycle test energy consumption, which has good application prospects and promotion value.

For the present invention, firstly, according to the cycle decay analysis of the battery system to be tested, the accelerated test interval is determined, the accelerated cycle test is carried out on the battery in the actual cycle mode, and the charge and discharge capacities of the battery are measured in the actual cycle mode in different stages of the accelerated cycle, It is used to calculate the capacity retention rate, and further judge the cycle performance of the test battery relative to the reference battery by comparing the capacity retention rate of the test battery and the reference battery with the cycle times curve.

For the present invention, for the battery to be tested, since the charge and discharge capacities are kept the same during the accelerated cycle, if no side reaction occurs in the battery, the state of the battery after the cycle remains the same as before the cycle, that is, the process change rate is 0, Similarly, the higher the degree of side reactions in the battery, the greater the rate of change during accelerated cycling. Based on this, the analysis of DCIR (direct current resistance) and polarization voltage can be added during each acceleration cycle to assist in judging the cause of poor battery cycle performance and provide a reference for battery performance improvement.

For the method provided by the present invention, since the accelerated cycle analysis is limited in the characteristic decay interval, compared with the cycle test of the full SOC, the evaluation period of the battery cycle performance can be greatly shortened, and the research and development efficiency can be improved. At the same time, the analysis of the battery DCIR (direct current resistance) and the change rate of the polarization voltage process can be added in the acceleration cycle, which provides a reference for the battery cycle failure analysis.

Description of drawings

Fig. 1 is the flow chart of the accelerated evaluation method of a kind of lithium ion battery cycle performance provided by the present invention;

FIG. 2 is an accelerated evaluation method for the cycle performance of a lithium ion battery provided by the present invention. In Example 1, a fresh battery in the same battery system as the battery to be tested (that is, a fresh battery with a capacity retention rate of 100%) and a Schematic diagram of the characteristic SOC interval analysis curve of the cycle decay of the battery (that is, the reference battery with a capacity retention rate of 95%);

Fig. 3 is a kind of accelerated evaluation method of lithium ion battery cycle performance provided by the present invention, and the accelerated cycle capacity retention rate curve schematic diagram of the battery to be tested and the reference battery in Example 1;

Fig. 4 is a kind of accelerated evaluation method of the cycle performance of a lithium ion battery provided by the present invention, the DCIR (direct current resistance) comparison schematic diagram of the first 500 accelerated cycles of the battery to be tested and the reference battery in Example 1;

5 is a schematic diagram showing the comparison of the actual cycle results (ie, the actual cycle performance curve) of the battery to be tested and the reference battery in Example 1 for an accelerated evaluation method of the cycle performance of a lithium ion battery provided by the present invention.

Detailed ways

In order to make those skilled in the art better understand the solution of the present invention, the present invention is further described in detail below with reference to the accompanying drawings and embodiments.

Referring to FIG. 1 to FIG. 5 , the present invention provides an accelerated evaluation method for the cycle performance of a lithium ion battery, including the following steps:

Step S1, for the battery to be tested whose cycle performance needs to be evaluated, pre-select a fresh battery and a reference battery of the same battery system as the battery to be tested, and analyze and obtain the cycle decay of the battery system to be tested (that is, the battery system to which the battery to be tested belongs). The characteristic SOC (that is, the capacity retention rate, also called the state of charge) interval, and determine the accelerated test SOC interval;

In step S2, the battery to be tested and the reference battery are respectively subjected to an accelerated cycle test within the SOC range of the accelerated test obtained in the first step, and the accelerated cycle capacity retention rate curves of the battery to be tested and the reference battery are obtained correspondingly;

Step S3, by comparing the accelerated cycle capacity retention rate curve of the battery to be tested and the accelerated cycle capacity retention rate curve of the reference battery, to determine whether the cycle performance of the battery to be tested is relative to the cycle performance of the reference battery.

In steps S2 and S3, the fresh battery is a battery with no capacity fading (ie, the capacity retention rate is 100%).

In the present invention, in terms of specific implementation, the step S1 specifically includes the following operations:

Step S11, select a fresh battery and a reference battery, respectively perform a preset charge-discharge cycle operation, and collect the battery voltage V and the charging capacity Q1 and the discharging capacity Q2 of the fresh battery and the reference battery in real time;

Among them, the fresh battery is the battery without capacity decay;

A reference battery, which is a battery whose capacity decay is greater than or equal to a preset ratio (for example, 5%);

In the present invention, the fresh battery, the reference battery and the battery to be tested belong to the same battery system;

It should be noted that, for the present invention, two batteries of the same battery system as the battery to be tested (such as a developed battery of the same chemical system or the same type) are taken as the reference battery (that is, the reference battery in product development, and this is used as the evaluation). Benchmark, the performance of the battery produced in the subsequent development of different materials and processes is compared with this battery). Two reference batteries, one of which is a fresh battery and the other a battery that has undergone cycle testing, define the battery that has undergone cycle testing as the reference battery, and the capacity attenuation of the reference battery is ≥5%.

In the present invention, batteries of the same battery system refer to: cells of the same specification and model, that is, cells with the same specification and size and the same chemical system. For example, both are 18650 size lithium iron phosphate system batteries (which are cylindrical lithium iron phosphate cells). Of course, according to the needs of users, it can also be the same battery with the same size and a comparable chemical system (lithium cobalt oxide system or NCM ternary system).

In the present invention, the battery to be tested and the reference battery may be of the same type or chemical system, but one of the positive electrode, negative electrode, electrolyte, and separator material (such as different positive electrodes or negative electrodes) or two (such as The positive and negative electrodes are different) or more (such as the positive electrode, the negative electrode, and the electrolyte are all different) materials are different.

In terms of specific implementation, the battery to be tested and the reference battery can be two batteries with the same components except for one electrode material (different negative electrode material or different positive electrode material), that is to say, they can be only two batteries with the same components. Two batteries with different negative electrode materials (negative electrode material or positive electrode material). That is, it can be a same system battery using two different negative electrode materials, or a same system battery using two different positive electrode materials. For example, it can be two 21700 type cylindrical lithium ion batteries with different negative electrode materials, or two 21700 type cylindrical lithium ion batteries with different positive electrode materials.

In terms of specific implementation, the battery to be tested and the reference battery may also be two batteries with the same components except for the different electrolytes. For example, it can be two 21700-type cylindrical lithium-ion batteries with different electrolytes.

In terms of specific implementation, the battery to be tested and the reference battery can also be two batteries with the same components except for the diaphragm. For example, two 21700-type cylindrical lithium-ion batteries with different separators may be used.

In the present invention, the battery to be tested and the reference battery may also have the same material components, but different manufacturing processes (such as different forming processes, etc.), that is, any materials and process conditions that may cause different battery cycle performance. Battery.

In step S1, the battery to be tested is a battery whose cycle performance needs to be evaluated, that is, a newly developed battery to be tested whose cycle performance is unknown and needs to be compared and analyzed with the reference battery to evaluate whether its cycle performance is better or worse than Reference cells of known performance.

In step S1, the difference between the battery capacity retention rates of the fresh battery and the reference battery is greater than a preset value (eg, greater than 5%). That is, the reference cells with a capacity fade of ≥5% have experienced significant performance degradation compared with fresh cells, which may include a decrease in electrode material activity, loss of active lithium, and an increase in cell polarization.

In step S11 , the preset charge-discharge cycle operation includes a discharge operation and a charge operation, specifically: firstly charge a preset charge current (0.05C～0.5C) with a constant current to a preset charge upper limit voltage, Then, discharge to the preset lower limit voltage by constant current discharge with a preset discharge current (0.05C～0.5C);

It should be noted that, for each specification and model of battery products, during development, combined with customer needs and based on a fixed chemical system design, the upper limit voltage of charging and the lower limit voltage of preset discharge can be determined, that is, the use of the product. The voltage range of the battery, and the upper limit voltage of the battery and the lower limit of the discharge voltage of the battery will be clearly given in the specification of the battery product.

Step S12, according to the preset first acquisition method or the preset second acquisition method, obtain a characteristic SOC interval in which the battery system to be tested undergoes cyclic decay;

In step S12, according to the preset first acquisition method, the characteristic SOC interval of the cyclic decay of the battery system to be tested is obtained, which specifically includes the following steps:

Step S121A, for the fresh battery and the reference battery, differentiate the battery voltage V (specifically, the charging voltage of the battery) by the charging capacity Q1 of the battery, respectively, to obtain the dQ/dV of the fresh battery and the reference battery;

Step S122A, for the fresh battery and the reference battery, take dQ/dV as the ordinate and the state of charge SOC corresponding to the charging capacity Q1 of the battery as the abscissa, and draw the capacity increments (IC) of the two in a graph. ) curve (dQ1/dV-SOC);

Step S123A, taking the capacity increment (IC) curve of the fresh battery as the reference curve, comparing the capacity increment (IC) curve of the reference battery with the reference curve of the fresh battery, and determining the operation according to the preset characteristic SOC interval, In the two incremental capacity (IC) curves (dQ1/dV-SOC), the cycle decay characteristics of the reference battery system (that is, equal to the test battery system because the reference battery system is the same as the test battery system) are determined SOC range; the reference battery system, which is equivalent to the battery system to be tested (also equivalent to the fresh battery system); the characteristic SOC range of the cycle decay of the reference battery system, including the lower limit value SOC _L and the upper limit value SOC _U ;

In the present invention, the characteristic SOC interval, that is, on the curves of the fresh battery and the reference battery (such as the capacity increment IC curve, that is, the dQ/dV-SOC curve), by comparing the peaks of the two curves one by one, Determine the SOC interval corresponding to the start position and end position of the peak in which the peak value is significantly reduced, or the peak value in which the peak value is significantly reduced and the peak position is significantly shifted; that is, the characteristic SOC interval must satisfy the phenomenon that the peak value is significantly reduced. Depending on the conditions, the peak shift may or may not occur. If the peak shift occurs, it is preferably the SOC interval corresponding to the peak where both the peak value is significantly reduced and the peak shift occurs.

In the present invention, in step S123A, the preset characteristic SOC interval determination operation includes the following steps:

First, on the incremental capacity (IC) curves (dQ/dV-SOC curves) of the fresh battery and the reference battery, by comparing the peaks of the two curves one by one, determine the peak where the peak of the peak is significantly reduced, Or the peak value of the wave peak is significantly reduced, and the peak position of the wave peak is significantly shifted, and then it is used as a characteristic peak of cyclic attenuation;

The peak value of the peak is significantly reduced, which means that the peak reduction ratio of the peak is greater than or equal to the preset peak reduction value;

The peak position of the wave crest is significantly shifted, which means that the corresponding SOC shift amplitude of the peak position of the wave crest is greater than or equal to the preset peak position shift value;

Then, the SOC interval corresponding to the start position and the end position of the characteristic peak of the cyclic decay is taken as the characteristic SOC interval.

In terms of specific implementation, the determination operation of the preset characteristic SOC interval specifically includes the following steps:

When the battery capacity decays by 5%, on the capacity increment (IC) curve (dQ/dV-SOC curve) of the fresh battery and the reference battery, the peaks of the two curves are compared one by one (that is, the fresh battery curve The 1st and 2nd peaks of the reference battery...the Nth peak, corresponding to the 1st peak, the 2nd peak...the Nth peak of the reference battery curve are compared, and N is a natural number greater than 2), determine A peak with a peak reduction ratio of ≥10%, or a peak with a peak reduction ratio of ≥10% and the peak position of the peak corresponding to SOC offset ≥3% is used as the characteristic peak of cyclic attenuation, preferably: the peak reduction ratio of the peak ≥10%, and the peak position of the peak corresponds to the peak of the SOC offset ≥3% as the characteristic peak of cyclic attenuation;

The abscissa corresponding to the starting position of the wave peak (ie, the characteristic peak of cyclic decay) is the lower limit value SOC _L of the characteristic SOC interval, and the abscissa corresponding to the end position of the wave crest is the upper limit value SOC _U of the characteristic SOC interval. That is to say, in the electrochemical reaction corresponding to this peak, the activity of the active material is attenuated (the peak value is reduced), and the polarization may be increased (the peak position corresponds to the voltage or SOC shift).

As shown in Figure 2, the peak height of the first peak of the dQ/dV-SOC curve (ie, the capacity increment curve) of the battery with 100% capacity retention rate (ie, fresh battery) is 11.3 Ah/V, and 95% capacity retention rate The peak height of the first peak of the battery with high rate is 8.5Ah/V, then the reduction ratio of the first peak peak reaches (11.3-8.5)/11.3=24.8%. Then the first wave peak is the characteristic peak of battery cycle decay, the abscissa SOC corresponding to the peak start position is 7%, which is the lower limit SOC _L , and the abscissa SOC corresponding to the peak end position is 18 %, which is the upper limit value SOC _U .

It should be noted that, in the present invention, in the testing process of the characteristic SOC interval, the selected battery is a reference battery and a fresh battery of the same system as the battery to be tested, that is, a developed product. During the test, select a fresh battery and a reference battery with at least 5% capacity decay for comparative testing. The test results of the two batteries are placed in one graph for comparison. With the capacity decay, select the peak or peak reduction and peak position with a peak reduction. The shifted peak is used as the characteristic peak of cyclic decay, so that the characteristic SOC interval is determined according to the starting and ending positions of the peak.

As shown in Figure 2, the battery with 100% capacity retention rate (fresh battery) and the battery with 95% capacity retention rate (reference battery) are batteries of the same system, but the battery with 95% capacity retention rate is cycle tested Batteries with capacity decay. As long as the chemical system and type of the battery remain unchanged, the characteristic SOC ranges of the two batteries, the fresh battery and the reference battery, are common.

In step S123A, in terms of specific implementation, the operation is determined according to the preset characteristic SOC interval, and in the two capacity increment (IC) curves, the characteristic SOC interval in which the cyclic decay of the reference battery system occurs is determined, specifically: in the fresh battery and the In the capacity increment (IC) curves of the two batteries of the reference battery, the state of charge (SOC) interval corresponding to the peak value of the peak of the curve (that is, the value of the highest point) is significantly reduced and the peak position of the peak is significantly shifted. , which is determined as the characteristic SOC range for the cycle decay of the reference battery system;

It should be noted that, in the present invention, the peak value of the peak on the capacity increment (IC) curve is the highest value of the peak presented by each peak, and the abscissa corresponding to the starting position and ending position of the peak is Its position, that is, the peak position, is indicated.

It should be noted that the decrease in the peak value of the wave peak indicates that the reactivity of the active material (positive electrode or negative electrode) in which the electrochemical reaction occurs in the battery decreases. When the battery capacity decreases by 5%, on the corresponding dQ/dV-SOC curve (capacity increment IC curve), if the peak reduction ratio is ≥10%, the influence caused by the test error can be excluded, and it can be determined that the peak-to-peak value of the curve is significant reduce.

It should be noted that for the incremental capacity (IC) curves of the fresh battery and the reference battery, the peak positions of the peaks on the two curves are shifted, usually due to the increased polarization of the battery. When the battery capacity is attenuated by 5%, on the corresponding dQ/dV-SOC curve, the SOC corresponding to the starting position or ending position of the characteristic peak is offset by ≥3%, which can exclude the influence caused by the test error and can be determined as the peak of the curve. The peak position is significantly shifted.

In step S12, according to the preset second acquisition method, the characteristic SOC interval of the cyclic decay of the battery under test is obtained, which specifically includes the following steps:

Step S121B, for the fresh battery and the reference battery, use the battery voltage V during the charging period to _charge or the battery voltage V during the _discharge period, respectively, perform differential processing on the charging capacity Q1 or the discharging capacity Q2 of the battery to obtain the fresh battery and the reference battery. than the dV _charge /dQ1 or dV _discharge /dQ2 of the battery;

Step S122B, for the fresh battery and the reference battery, respectively take dV _charge /dQ1 or dV _discharge /dQ2 as the ordinate, and take the state of charge SOC corresponding to the battery's charging capacity Q1 or discharge capacity Q2 as the abscissa, in a graph. The differential voltage (DV) curve (dV/dQ-SOC) of the two is obtained by drawing in

Step S123 B, taking the differential voltage curve of the fresh battery as the reference curve, comparing the differential voltage of the reference battery with the reference curve of the fresh battery, and determining the operation according to the preset characteristic SOC interval, in the two differential voltage curves, Determine the characteristic SOC range of cycle decay of the reference battery system (that is, the battery system to be tested, because the reference battery system is the same as the battery system to be tested); the reference battery system is equivalent to the battery system to be tested (also equivalent to the fresh battery) system);

The characteristic SOC interval of the reference battery system with cyclic decay, including the lower limit value SOC _L and the upper limit value SOC _U ;

In the present invention, the characteristic SOC interval, that is, on the curve of the fresh battery and the reference battery (such as the differential voltage curve, that is, the dV/dQ-SOC curve), by comparing the peaks of the two curves one by one, which occurs The SOC interval corresponding to the start position and end position of the peak with a significantly reduced peak value, or the peak value with a significantly reduced peak value and a significantly shifted peak position; that is, the characteristic SOC interval must satisfy the condition that the peak value is significantly reduced, while The peak position shift may or may not occur, and if the peak position shift occurs, it is preferably an SOC interval corresponding to the peak in which both the peak value is significantly reduced and the peak position shift occurs.

In the present invention, in step S123B, the preset characteristic SOC interval determination operation includes the following steps:

First, on the differential voltage curve (dV/dQ-SOC curve) of the fresh battery and the reference battery, by comparing the peaks of the two curves one by one, determine the peak in which the peak of the peak is significantly reduced, or both the peak The peak value is significantly reduced, and the peak position of the peak is significantly shifted, and then it is used as the characteristic peak of cyclic attenuation;

The peak value of the peak is significantly reduced, which means that the peak reduction ratio of the peak is greater than or equal to the preset peak reduction value;

The peak position of the wave crest is significantly shifted, which means that the corresponding SOC shift amplitude of the peak position of the wave crest is greater than or equal to the preset peak position shift value;

Then, the SOC interval corresponding to the start position and the end position of the characteristic peak of the cyclic decay is taken as the characteristic SOC interval.

In terms of specific implementation, the determination operation of the preset characteristic SOC interval specifically includes the following steps:

When the battery capacity decays by 5%, on the differential voltage curve (dV/dQ-SOC curve) of the fresh battery and the reference battery, compare the peaks of the two curves one by one (that is, the first curve of the fresh battery curve). , the second peak...the Nth peak, which corresponds to the comparison with the first peak, the second peak...the Nth peak of the reference battery curve, N is a natural number greater than 2), determine the peak value of the peak decreases The peak of the ratio ≥ 10%, or the peak reduction ratio of the peak is ≥ 10% and the peak position of the peak corresponds to the SOC offset amplitude ≥ 3% as the characteristic peak of the cyclic attenuation, preferably: the peak reduction ratio of the peak is ≥ 10%, And the peak position of the wave peak corresponds to the peak of the SOC offset amplitude ≥ 3% as the characteristic peak of cyclic attenuation;

The abscissa corresponding to the starting position of the wave peak (ie, the characteristic peak of cyclic decay) is the lower limit value SOC _L of the characteristic SOC interval, and the abscissa corresponding to the end position of the wave crest is the upper limit value SOC _U of the characteristic SOC interval. That is to say, in the electrochemical reaction corresponding to this peak, the activity of the active material is attenuated (the peak value is reduced), and the polarization may be increased (the peak position corresponds to the voltage or SOC shift).

It should be noted that, in the present invention, in the testing process of the characteristic SOC interval, the selected battery is a reference battery and a fresh battery of the same system as the battery to be tested, that is, a developed product. During the test, select a fresh battery and a reference battery with at least 5% capacity decay for comparative testing. The test results of the two batteries are placed in one graph for comparison. With the capacity decay, select the peak or peak reduction and peak position with a peak reduction. The shifted peak is used as the characteristic peak of cyclic decay, so that the characteristic SOC interval is determined according to the starting and ending positions of the peak.

It should be noted that the battery with 100% capacity retention rate (fresh battery) and the battery with 95% capacity retention rate (reference battery) are batteries of the same system, except that the battery with 95% capacity retention rate is cycle tested Batteries with capacity decay. As long as the chemical system and type of the battery remain unchanged, the characteristic SOC ranges of the two batteries, the fresh battery and the reference battery, are common.

In the present invention, the characteristic SOC interval is obtained by comparing and analyzing the curves of two batteries with different capacity retention ratios (ie, a fresh battery and a reference battery with a cycle capacity decay ≥5%).

In step S123 B, in terms of specific implementation, the operation is determined according to the preset characteristic SOC interval, and in the two differential voltage curves, the characteristic SOC interval in which the cycle decay occurs in the reference battery system is determined, specifically: in the fresh battery and the reference battery In the differential voltage curves of these two batteries, the state of charge (SOC) interval corresponding to the peak value of the peak of the curve (that is, the value of the highest point) is significantly reduced and the peak position of the peak is significantly shifted, and is determined as the reference battery system. The characteristic SOC interval where cyclic decay occurs;

It should be noted that, in the present invention, the peak value of the peak on the differential voltage curve is the highest value of the peak presented by each peak, and the abscissa corresponding to the starting position and ending position of the peak indicates its position. , the peak position.

It should be noted that the decrease in the peak value of the wave peak indicates that the reactivity of the active material (positive electrode or negative electrode) in which the electrochemical reaction occurs in the battery decreases. When the battery capacity decays by 5%, on the corresponding differential voltage curve, if the peak value reduction ratio is greater than or equal to 10%, the influence caused by the test error can be excluded, and the peak-to-peak value of the curve can be determined to be significantly reduced.

It should be noted that for the differential voltage curves of the fresh battery and the reference battery, the peak positions of the peaks on the two curves are shifted, which is usually caused by the increase of the battery polarization. When the battery capacity is attenuated by 5%, on the corresponding dQ/dV-SOC curve, the SOC corresponding to the starting position or ending position of the characteristic peak is offset by ≥3%, which can exclude the influence caused by the test error and can be determined as the peak of the curve. The peak position is significantly shifted.

Step S13, according to the characteristic SOC interval in which the battery system to be tested undergoes cyclic decay, determine an accelerated test SOC interval; the accelerated test SOC interval includes a lower limit value SOC _CL and an upper limit value SOC _CU ;

The accelerated test SOC interval includes all the characteristic SOC intervals in which the battery system to be tested undergoes cyclic decay, or includes part of the characteristic SOC interval in which the battery system to be tested undergoes cyclic decay.

In step S13, in terms of specific implementation, in order to fully shorten the test period, the lower limit value SOC _CL =SOC _L ±10% of the accelerated test SOC interval, and the upper limit value SOC _CU =SOC _U ±10% of the accelerated test SOC interval; In terms of implementation, it is preferable that SOC _CL =SOC _L ±5%, and SOC _CU =SOC _U ±5%.

In step S2, an accelerated cycle test is performed on the battery to be tested in the accelerated test SOC interval obtained in the first step, and the accelerated cycle capacity retention rate curve of the battery to be tested is obtained correspondingly, which specifically includes the following steps:

Step S21A, with the actual cycle format of the reference battery, perform a preset multiple (for example, 3) charge-discharge cycle operations on the battery to be tested (each charge-discharge cycle operation includes a discharge operation and a charge operation), and The battery charge capacity and discharge capacity obtained during the last charge-discharge cycle operation are taken as the initial charge capacity C ₀ and initial discharge capacity D ₀ of the battery to be tested;

It should be noted that the actual cycle system refers to the charge and discharge cycle system formulated for life evaluation based on customer needs during battery development, including the upper limit voltage of charging, the lower limit voltage of discharge, charging current and discharging current, and resting time. and so on. As in the embodiment, for a 52Ah iron-lithium battery, the cycle system can be: constant current charging at 1C until its upper limit voltage is 3.65V, then changing to constant voltage charging, cutting off when the current drops to 0.02C, and leaving it at rest 15-30min, constant current discharge at 1C, until the lower limit voltage of 2.0V is reached, let stand for 15-30min, and then continue the charge and discharge cycle.

For the actual cycle system, product developers formulate the cycle system used for life evaluation according to customer needs during battery development, and with the consent of the customer, the specific content of the system is clearly specified in the specification.

It should be noted that, in the present invention, because the battery to be tested is an experimental battery made by material diversification (replacement of the same type of material due to performance improvement or cost reduction demand) or process optimization on the basis of the reference battery ( Research and development experimental battery), the battery to be tested is successful (that is, the performance of the battery to be tested is better than or equivalent to the reference battery) or failed (that is, the performance of the battery to be tested is inferior to the reference battery), and the same type of battery to be replaced as the reference battery The actual cycle system of the battery, the test battery and the reference battery is the same. The actual cycle system of the battery to be tested can be obtained from the specification book of the reference battery.

Step S22A, taking the actual cycle system possessed by the reference battery as the accelerated cycle system, and sequentially performing multiple (for example, n, n is a natural number greater than 1) stages of accelerated cycle tests (that is, repeatedly executing multiple stages of acceleration. cycle test), and after each stage of accelerated cycle test, obtain the charge capacity and discharge capacity of the battery to be tested, and record the total number of accelerated cycles at the end of each stage of accelerated cycle test (that is, from the first stage of Calculated from the acceleration cycle test to the end of the acceleration cycle test at this stage, the total number of acceleration cycles executed);

In step S22A, the test contents of the accelerated cycle tests of multiple stages are the same;

Accelerated loop testing for each stage, including the following operations:

The first step is to charge the battery to the lower limit SOC _CL of the accelerated test SOC interval with a pre-sized charging current (such as a small current of 0.05-0.5C), and then leave it for a preset period of time (such as 10-30 minutes);

The second step is to perform the same accelerated cycle test operation on the battery several times (for example, N times, N is a natural number greater than 1, for example, N times is 50 times);

Each acceleration cycle test operation is specifically as follows: selecting the charging current Ic and the discharging current Id corresponding to the SOC interval of the acceleration test in the actual cycle system of the battery to be tested, and then successively using the charging current Ic to perform a preset charging time tc of the battery to be tested. The charging operation and the discharging operation for the preset discharge duration td are performed on the battery with the discharging current Id, so that the charging and discharging capacities of the battery in the acceleration test SOC interval (from the lower limit value SOC _CL to the upper limit value SOC _CU ) are the same; that is, Ic* tc=Id*td;

The actual cycle system of the battery to be tested is equivalent to the actual cycle system of the reference battery;

It should be noted that the charging current Ic and the discharging current Id corresponding to the SOC interval of the accelerated test in the actual cycle system of the battery to be tested can be obtained from the actual cycle system specified in the specification of the reference battery belonging to the same battery system, such as In the embodiment, the actual cycle system is 1C constant current charging to the upper limit voltage of 3.65V, then switching to constant voltage charging, stopping when the current drops to 0.02C, standing for 15-30min, and then performing constant current discharge at 1C until it reaches 0.02C. The lower limit voltage is 2.0V. Therefore, the charge current and discharge current of the battery to be tested in the accelerated test SOC range are both 1C.

The third step is to continue to perform the charging and discharging cycle operation of the battery to be tested for a preset number of times (for example, 2 to 5 times) with the actual cycle format of the reference battery.

In the third step, as mentioned above, the actual cycle system refers to the charge and discharge cycle system formulated for life evaluation according to customer needs during battery development, including upper limit voltage of charge, lower limit voltage of discharge, charge current and Discharge current and standing time etc. For example, it can be: charge the battery to be tested with 1C constant current to the upper limit voltage of 3.65V, and then switch to 3.65V constant voltage charging, stop when the current drops to 0.02C, let it stand for 15min-30min, and then conduct constant current discharge at 1C, until the lower limit voltage of 2.0V is reached.

The fourth step is to use the charging capacity and discharging capacity of the battery obtained during the last full-discharge charging and discharging cycle operation as the charging capacity and discharging capacity of the battery to be tested after the accelerated cycle test at each stage;

It should be noted that in the third step, full discharge means charging and discharging according to the actual cycle system, that is, charging to the upper limit voltage of the battery, and discharging to the lower limit voltage of the battery: for example: 1C constant The current is charged to the upper limit voltage of 3.65V, and then switched to 3.65V constant voltage charging. When the current drops to 0.02C, stop, let it stand for 15min ~ 30min, and then conduct constant current discharge at 1C until it reaches the lower limit voltage of 2.0V. Then let stand for 15min-30min, and then carry out the same charge and discharge cycle.

In the present invention, the full-charge discharge is relative to the acceleration cycle. The acceleration cycle is performed within a certain SOC interval, while the full-charge discharge is performed within the entire SOC interval for charging and discharging. Here, the purpose of full-charge discharge is performed. Yes: After a certain period of accelerated cycle, the test battery has the remaining charge and discharge capacity when tested according to the actual cycle format, so as to calculate and evaluate the capacity retention rate.

It should be noted that when the above-mentioned number of cycles of charge and discharge reaches N times, the battery is charged and discharged for 2-5 times in the actual cycle to be investigated, and the charge and discharge capacity of the last cycle is calculated. It is recorded as the charge capacity _CN and the discharge capacity DN after _N accelerated cycles of the battery. At this time, the corresponding battery charge capacity retention rate is _CN /C ₀ , and the discharge capacity retention rate is _DN /D ₀ .

Step S23A, according to the charge capacity and discharge capacity of the battery to be tested in the accelerated cycle test of each stage, and the initial charge capacity C ₀ and initial discharge capacity D ₀ obtained in step S21A, calculate and obtain the charge capacity of the battery to be tested in each stage. Battery charge capacity retention rate and battery discharge capacity retention rate in accelerated cycle test;

In step S23A, the battery charging capacity retention rate of the battery under test in the accelerated cycle test of each stage is equal to the charging capacity C to _{be tested} in the accelerated cycle test of each stage divided by the initial charging capacity C ₀ The quotient of ; that is, equal to C to _{be tested} /C ₀ ;

In step S23A, the battery discharge capacity retention rate of the battery to be tested in the accelerated cycle test of each stage is equal to the discharge capacity D of the battery to be _tested in the accelerated cycle test of each stage divided by the initial discharge capacity D ₀ The quotient of ; that is, equal to D to _{be measured} /D ₀ .

Step S24A, take the battery charge capacity retention rate and the battery discharge capacity retention rate of the battery to be tested in each stage of the accelerated cycle test as the ordinate, and the total number of acceleration cycles corresponding to the end of each stage of the accelerated cycle test as the horizontal axis. Coordinate, draw and obtain the accelerated cycle capacity retention curve of the battery to be tested.

In terms of specific implementation, between step S24A and step S23A, a step may also be included:

Step S25A, compare the battery charging capacity retention rate and the battery discharging capacity retention rate of the battery under test in each stage of the accelerated cycle test with the preset battery life cut-off capacity retention rate in real time, and determine whether the battery charging capacity retention rate or the battery discharge capacity retention rate is preset. When the battery discharge capacity retention rate is less than the preset battery life cut-off capacity retention rate, if yes, continue to execute step S24A, otherwise, return to step S22A and step S23A.

In terms of specific implementation, the preset battery life cut-off capacity retention rate is usually 80%, that is, the battery discharge capacity after cycling is reduced to 80% of the initial (fresh battery) discharge capacity.

It should be noted that, through steps S25A and S2, the conditions for the end of the battery under test acceleration cycle can be changed from nN times (that is, n stages, and N times of acceleration cycle test operations are performed in each stage) to the battery capacity retention rate. The charge capacity retention rate or the discharge capacity retention rate at the end of battery life is defined.

In step S2, an accelerated cycle test is performed on the reference battery in the accelerated test SOC interval obtained in the first step, and the accelerated cycle capacity retention rate curve of the reference battery is obtained correspondingly, which specifically includes the following steps:

Step S21B, performing a preset multiple (for example, 3) charge-discharge cycle operations on the reference battery with the actual cycle format of the reference battery (each charge-discharge cycle operation includes a discharge operation and a charge operation), and Take the battery charge capacity and discharge capacity obtained during the last charge-discharge cycle operation as the initial charge capacity C ₁ and initial discharge capacity D ₁ of the reference battery;

Step S22B, taking the actual cycle system possessed by the reference battery as the accelerated cycle system, and sequentially performing multiple (for example, n, n is a natural number greater than 1) stages of accelerated cycle tests (that is, repeatedly executing multiple stages) on the reference battery. Accelerated cycle test), and in the accelerated cycle test of each stage, obtain the charging capacity and discharge capacity of the reference battery, and record the total number of acceleration cycles corresponding to the end of the accelerated cycle test of each stage (that is, from the first From the start of the acceleration cycle test of the stage to the end of the acceleration cycle test of the stage, the total number of acceleration cycles executed);

In step S22B, the test contents of the accelerated cycle tests of multiple stages are the same;

Accelerated loop testing for each stage, including the following operations:

The first step is to charge the battery to the lower limit SOC _CL of the accelerated test SOC interval with a pre-sized charging current (such as a small current of 0.05-0.5C), and then leave it for a preset period of time (such as 10-30 minutes);

The second step is to perform the same accelerated cycle test operation on the battery several times (for example, N times, N is a natural number greater than 1);

Each acceleration cycle test operation is specifically as follows: selecting the charging current Ic and the discharging current Id corresponding to the SOC interval of the acceleration test in the actual cycle system of the reference battery, and then successively using the charging current Ic to perform a preset charging time tc on the reference battery. The charging operation and the discharging operation of the preset discharge duration td are performed on the battery with the discharge current Id, so that the charging and discharging capacities of the battery in the acceleration test SOC interval (from the lower limit SOC _CL to the upper limit SOC _CU ) are the same; that is, Ic *tc=Id*td;

The actual cycle system of the battery to be tested is equivalent to the actual cycle system of the reference battery;

It should be noted that, in the actual cycle system of the reference battery, the corresponding charging current Ic and discharge current Id in the SOC interval of the accelerated test can be obtained from the actual cycle system specified in the specification of the reference battery. For example, in the embodiment, The actual cycle system is 1C constant current charging to the upper limit voltage of 3.65V, then switching to constant voltage charging, stopping when the current drops to 0.02C, standing for 15-30min, and then performing constant current discharge at 1C until it reaches the lower limit voltage of 2.0V . Therefore, the charging current and discharging current of the reference battery in the accelerated test SOC range are both 1C.

In the third step, in the actual cycle mode, continue to perform a preset multiple (for example, 2 to 5 times) charging and discharging cycle operations for the reference battery to be fully discharged;

The fourth step is to use the charging capacity and discharging capacity of the battery obtained during the last full-discharge charging and discharging cycle operation as the charging capacity and discharging capacity of the reference battery after the accelerated cycle test at each stage;

It should be noted that in the third step, full discharge means charging and discharging according to the actual cycle system, that is, charging to the upper limit voltage of the battery, and discharging to the lower limit voltage of the battery: for example: 1C constant The current is charged to the upper limit voltage of 3.65V, and then switched to 3.65V constant voltage charging. When the current drops to 0.02C, stop, let it stand for 15min ~ 30min, and then conduct constant current discharge at 1C until it reaches the lower limit voltage of 2.0V. Then let stand for 15min-30min, and then carry out the same charge and discharge cycle.

In the present invention, the full-charge discharge is relative to the acceleration cycle. The acceleration cycle is performed within a certain SOC interval, while the full-charge discharge is performed within the entire SOC interval for charging and discharging. Here, the purpose of full-charge discharge is performed. Yes: After a certain period of accelerated cycling, the reference battery has the remaining charge and discharge capacity when tested according to the actual cycle format, so as to calculate and evaluate the capacity retention rate.

Step S23B, according to the charge capacity and discharge capacity of the reference battery in the accelerated cycle test of each stage, and the initial charge capacity C ₁ and initial discharge capacity D ₁ obtained in step S21B, calculate and obtain the reference battery in each stage. The battery charge capacity retention rate and the battery discharge capacity retention rate in the accelerated cycle test;

In step S23B, the battery charging capacity retention rate of the reference battery in each stage of the accelerated cycle test is equal to the reference battery's charging capacity C _N in each stage of the accelerated cycle test divided by the initial charging capacity Quotient of C ₁ ; i.e. equal to C _reference /C ₁ ;

In step S23B, the battery discharge capacity retention rate of the reference battery in each stage of the accelerated cycle test is equal to the discharge capacity of the reference battery in each stage of the accelerated cycle test _{DN Reference} divided by the initial discharge capacity The quotient of D ₁ is equal to D _ref /D ₁ .

Step S24 B, taking the battery charge capacity retention rate and the battery discharge capacity retention rate of the reference battery in each stage of the accelerated cycle test as the ordinate, and the total number of acceleration cycles corresponding to the end of each stage of the accelerated cycle test as The abscissa is plotted to obtain the accelerated cycle capacity retention curve of the reference battery.

In terms of specific implementation, between step S24B and step S23B, a step may also be included:

Step S25B, compare the battery charging capacity retention rate and the battery discharge capacity retention rate of the reference battery in each stage of the accelerated cycle test with the preset battery life cut-off capacity retention rate in real time, and determine the battery charging capacity retention rate or When the battery discharge capacity retention rate is less than the preset battery life cut-off capacity retention rate, if yes, continue to execute step S24B, otherwise, return to execute step S22B and step S23B.

In terms of specific implementation, the preset battery life cut-off capacity retention rate is usually 80%, that is, the battery discharge capacity after cycling is reduced to 80% of the initial (fresh battery) discharge capacity.

It should be noted that, through steps S25B and S2, the conditions for the end of the acceleration cycle of the reference battery can be changed from nN times (that is, n stages, and N times of acceleration cycle test operations are performed in each stage) to the battery capacity retention rate. It is defined by the charge capacity retention rate or the discharge capacity retention rate when the battery life cut-off condition is reached.

In step S3, in terms of specific implementation, if the accelerated cycle capacity retention rate curve (that is, the charge and discharge capacity retention rate curve) of the battery to be tested is located in the accelerated cycle capacity retention rate curve of the reference battery (that is, the charge and discharge capacity retention rate curve) curve), it is judged that the cycle performance of the battery to be tested is better than that of the reference battery;

In step S3, in terms of specific implementation, if the accelerated cycle capacity retention rate curve (that is, the charge and discharge capacity retention rate curve) of the battery to be tested is located in the accelerated cycle capacity retention rate curve of the reference battery (that is, the charge and discharge capacity retention rate curve) curve), it is judged that the cycle performance of the battery to be tested is inferior to that of the reference battery.

In the present invention, in step S3, in terms of specific implementation, if the accelerated cycle capacity retention rate curve (that is, the charge and discharge capacity retention rate curve) of the accelerated cycle of the battery to be tested is different from the accelerated cycle capacity retention rate curve of the reference battery ( That is to say, the charge and discharge capacity retention rate curves) are basically coincident, then the accelerated cycle test in step S2 needs to be repeatedly performed, and the accelerated cycle capacity retention rate curves of the battery under test and the reference battery are repeatedly obtained until the battery under test and the reference battery are accelerated. The cycle capacity retention rate curve is completely separated, and then according to the relative position (ie above or below) of the accelerated cycle capacity retention rate curve of the test battery and the reference battery, to judge the cycle performance of the test battery compared to the reference battery The advantages and disadvantages of cycle performance;

Wherein, when the degree of overlap between the accelerated cycle capacity retention rate curve of the battery to be tested and the accelerated cycle capacity retention rate curve of the reference battery is greater than or equal to a preset first ratio (for example, 85%), it is determined that the two basically overlap;

Wherein, when the degree of overlap between the accelerated cycle capacity retention rate curve of the battery to be tested and the accelerated cycle capacity retention rate curve of the reference battery is less than or equal to a preset second ratio (for example, 5%), it is judged that the two are completely separated;

The preset first ratio is greater than the preset second ratio.

Among them, it should be noted that when the acceleration cycle test of step S2 is currently repeatedly performed, in the acceleration cycle test of multiple stages included in the acceleration cycle test of step S2, the acceleration cycle test corresponding to the end of the last stage of the acceleration cycle test The total number of cycles is greater than the total number of acceleration cycles corresponding to the end of the acceleration cycle test of the last stage in the acceleration cycle test of multiple stages included in the acceleration cycle test of step S2 when the acceleration cycle test of step S2 is performed before. For example, if the previous total number of times is 200 times, then the total number of acceleration cycles when step S2 is repeatedly performed is required to be greater than 200 times, for example, 300 times.

In the present invention, in step S3, the accelerated cycle capacity retention rate curve of the reference battery can be used as benchmark data to establish a database, and can be directly used as a reference curve for comparative analysis in the later screening and evaluation of batteries of the same system.

Based on the above technical solutions, for the present invention, the accelerated test interval is first determined according to the cycle decay analysis of the battery system to be tested, the accelerated cycle test is performed on the battery in the actual cycle mode, and the battery is measured in the actual cycle mode in different stages of the accelerated cycle. The charge and discharge capacity of the test battery is used for the calculation of the capacity retention rate, and the cycle performance of the test battery is judged relative to the reference battery by comparing the capacity retention rate of the test battery and the reference battery with the cycle times curve.

Compared with the prior art, the accelerated evaluation method for the cycle performance of the lithium ion battery provided by the present invention has the following beneficial technical effects:

1. For the method provided by the present invention, first determine the characteristic SOC range of the battery under test and the SOC range of the accelerated test, and then perform the accelerated cycle test in the SOC range of the accelerated test, and analyze the capacity retention rate after the accelerated cycle. Compared with the traditional full SOC cycle test, the evaluation time can be significantly shortened.

2. The present invention first determines the characteristic SOC interval in which the battery under test undergoes cycle decay. In this SOC interval, the cycle decay characteristic of the battery system under test is significant, so it can be used as the SOC interval of the cycle acceleration test to shorten the cycle evaluation time.

3. For the present invention, in the selected accelerated test SOC interval, the battery is subjected to an accelerated cycle test according to the charging current Ic and discharge current Id in the actual cycle system, and the charging and discharging time is cut off, so that the battery is in the accelerated test SOC interval. The charge and discharge capacities within (from the lower limit value SOC _CL to the upper limit value SOC _CU ) are the same, that is, Ic*tc=Id*td. Since the accelerated cycling process keeps the charge and discharge capacities the same, if there is no side reaction in the battery, the state of the battery after the cycle remains the same as before the cycle, that is, the process change rate is 0. Similarly, the higher the degree of side reactions in the battery , the greater the rate of change during the acceleration cycle.

Based on this, the present invention can assist in judging the cause of poor battery cycle performance by adding analysis of DCIR (direct current resistance) and polarization voltage during each acceleration cycle, and provide a reference for battery performance improvement.

4. For the present invention, in the rapid evaluation of the cycle performance of the battery to be tested, the measured battery charge and discharge capacity retention rates are plotted on the ordinate and the corresponding total number of accelerated cycles as the abscissa, respectively, to obtain the battery to be tested. The accelerated cycle capacity retention rate curve of the reference battery; the cycle performance of the electrode material to be tested is evaluated by comparing the position of the curve. Instead of only analyzing by comparing the capacity retention ratios of one or several times under the same accelerated cycle times, the present invention ensures the accuracy and comprehensiveness of the analysis results, especially when the capacity retention ratios of the battery to be tested and the reference battery are When the curve crosses, the result of the curve analysis can maintain the accuracy, but if only the capacity retention rate points of the previous few times are analyzed, misjudgment may occur.

In order to understand the technical solutions of the present invention more clearly, the technical solutions of the present invention are described below through specific embodiments.

Example 1.

The present invention will be described in detail below with reference to the accompanying drawings by taking the test of a commercial square lithium ion battery as an example, so as to further illustrate the substantial features and significant progress of the present invention.

In this embodiment, the test sample is a square iron lithium lithium ion experimental battery with a 1C capacity of 52Ah. The test cell is the same type as the reference cell, but uses a different negative electrode.

The battery testing equipment is a conventional charge-discharge instrument, and the equipment used in this embodiment is the Arbin BT2000 charge-discharge test system.

In embodiment 1, the accelerated evaluation method of lithium ion battery cycle performance provided by the present invention includes the following steps:

Step 1: Determine the characteristic SOC range and the accelerated test SOC range in which the battery system to be tested undergoes cyclic decay.

The specific operation is as follows: First, select a fresh battery of the same system as the battery to be tested (that is, a fresh battery with a capacity retention rate of 100%) and a battery after cycling (that is, a reference battery with a capacity retention rate of 95%) for cycle decay characteristics. SOC interval analysis. Specifically, the battery is charged and discharged with a small current of 0.1C, and the battery voltage is differentiated by the charging capacity to obtain dQ/dV. Capacity increment IC curve (Fig. 2). It can be seen from Figure 2 that in the range of 7% SOC to 18% SOC, with the decrease of the battery capacity retention rate, the height of the first lithium intercalation peak on the IC curves of the two capacity increments decreases significantly, which is the attenuation of the battery system to be tested. The most significant SOC interval. Therefore, the characteristic SOC interval of the battery cycle decay in this system is 7% SOC to 18% SOC, that is, the lower limit value SO _L of the characteristic SOC interval is 7%, and the upper limit value SOC _U is 18%.

The acceleration test SOC interval is determined based on the characteristic SOC interval of the cyclic decay. The acceleration test SOC interval is a characteristic SOC interval that includes or partially includes the cyclic decay. The interval includes the lower limit value SOC _CL and the upper limit value SOC _CU of the SOC, which is sufficient To shorten the test period, generally SOC _CL =SOC _L ±10%, SOC _CU =SOC _U ±10%, more preferably SOC _CL =SOC _L ±5%, SOC _CU =SOC _U ±5%.

In Example 1, considering the influence of the test period and the large battery polarization in the low state of charge interval, the accelerated test interval is selected to be 10%-20% SOC, that is, the SOC _CL is 10%, and the SOC _CU is 20%.

Step 2: Accelerate the cycle test of the battery to be tested and the reference battery in the accelerated test interval (10% SOC-20% SOC), and obtain the accelerated cycle capacity retention rate curves of the battery to be tested and the reference battery. Specifically include the following operations: .

Step 1: Take the battery to be tested and the reference battery, carry out 3 charge-discharge cycles with the actual cycle system to be investigated (1C charge-discharge), and record the charge and discharge capacity of the third cycle as the initial charge capacity and initial discharge capacity.

The actual cycle system investigated (1C charge and discharge) is specifically: charge and discharge the battery at 1C=53A, the constant current charging cut-off voltage is 3.65V, the constant voltage charging until the current drops to 0.05C=2.65A cut-off, the discharge cut-off The voltage is 2.5V. In Table 1, record the initial charge capacity C ₀ and initial discharge capacity D ₀ of the battery to be tested in the third cycle, and the initial charge capacity C _0S and initial discharge capacity D _0S of the reference battery.

Step 2: Charge the battery to the lower limit of the accelerated test interval (10% SOC-20% SOC) with a small current of 0.2C, that is, 10% SOC, and let it stand for 10-30 minutes.

Step 3: Select the charging current Ic and the discharging current Id corresponding to the accelerated test SOC interval (10%-20% SOC) of the battery to be tested in the actual cycle system, and end the charging and discharging time so that the battery is in the characteristic SOC interval The charge and discharge capacities are the same inside (10%-20% SOC), ie Ic*tc=Id*td. Ic=53A, tc=360s, Id=53A, td=360s, the number of charging and discharging cycles of the battery in the accelerated test interval (10%-20% SOC) is set to 500 times.

Step 4: When the number of cycles of charging and discharging above reaches 500 (that is, N is 500) times, the battery under test and the reference battery are charged and discharged for 2 full-discharge cycles in the actual cycle format of 1C charge and discharge, and The charge and discharge capacity of the last cycle is recorded as the charge capacity _CN and discharge capacity D _N after 500 accelerated cycles of the battery. At this time, the corresponding battery charge capacity retention rate C _N /C ₀ and discharge capacity retention rate is D _N /D ₀ .

Step 5, repeat n times (n is equal to 3) the acceleration cycle process from steps 2 to 4 (the acceleration cycle process from steps 2 to 4 is the acceleration cycle test of one stage) The acceleration cycle of the battery was completed when the number of cycles was 1500. Then, the charge capacity retention rate and discharge capacity retention rate of the battery after 500, 1000, and 1500 accelerated cycles were obtained (that is, the battery charge capacity retention rate and the battery discharge capacity retention rate in these three stages were obtained), which were recorded in Table 1.

Table 1: Schematic representation of the charge-discharge capacity and retention rate of the reference battery and the test battery during accelerated cycling.

Step 6: Take the battery charge and discharge capacity retention rate measured in the above steps as the ordinate, and plot the corresponding total number of accelerated cycles as the abscissa to obtain the accelerated cycle capacity retention rate curves of the battery to be tested and the reference battery, respectively , as shown in Figure 3.

The third step is to quickly evaluate the cycle performance of the battery under test: by comparing the accelerated cycle capacity retention rate curve of the test battery with the accelerated cycle capacity retention rate curve of the reference battery, determine the cycle performance of the test battery relative to the reference battery. The advantages and disadvantages of cycle performance.

It can be seen from Figure 3 that during 1500 acceleration cycles, the capacity retention rate curve of the battery to be tested is below the reference battery, and the distance between the two curves is getting larger and larger, and it is impossible to cross again. Therefore, The cycle performance of the battery to be tested is judged to be inferior to that of the reference battery.

In order to further understand the reasons for the poor cycle performance of the battery to be tested, the DCIR (direct current resistance) during the first 500 accelerated cycles of the battery was analyzed, as shown in Figure 4.

It should be noted that the calculation of the DCIR value is carried out with reference to a well-known method in the industry, that is, when the battery starts to discharge, the voltage difference at the discharge time of 0 seconds and 3 seconds is divided by the discharge current. It is not repeated here.

It can be seen from Figure 4 that the DCIR of the tested battery and its growth rate with the cycle process are higher than those of the reference battery, which may be the reason for its poor cycle, and also provides a certain reference direction for battery performance improvement.

In this embodiment, by performing an accelerated cycle test on the battery under test and the reference battery, it can be known that the cycle performance of the battery under test is inferior to that of the reference battery, and is consistent with the actual cycle test result, as shown in Figure 5.

After inspection, the accelerated cycle evaluation method for the cycle performance of the lithium ion battery provided by the present invention only takes 36 days to evaluate the cycle performance between the battery to be tested and the reference battery.

In addition, as shown in Figure 4, according to the results of 1000 accelerated cycle tests (about 24 days), it has been determined that the cycle performance of the battery to be tested is worse than that of the reference battery. This result is consistent with the actual cycle test results, while the existing conventional cycle test takes 36 days and can only be cycled about 250 times, and the existing conventional cycle test still cannot fully and accurately determine the battery to be tested from the actual cycle curve. The cycle performance of the reference battery, therefore, the use of the accelerated evaluation method of the present invention can effectively improve the research and development efficiency, shorten the battery research and development cycle, and at the same time, the analysis of the DCIR can provide a reference for the improvement of the battery cycle performance.

To sum up, compared with the prior art, an accelerated evaluation method for the cycle performance of a lithium ion battery provided by the present invention has a scientific design, is suitable for the development of lithium ion battery products, and is used for material screening and system optimization. The rapid comparative analysis of the battery cycle performance can effectively shorten the battery development cycle and improve the research and development efficiency. At the same time, by reducing the energy consumption of the cycle test, the battery development cost can be indirectly reduced, which has good application prospects and promotion value.

For the present invention, firstly, according to the cycle decay analysis of the battery system to be tested, the accelerated test interval is determined, the accelerated cycle test is carried out on the battery in the actual cycle mode, and the charge and discharge capacities of the battery are measured in the actual cycle mode in different stages of the accelerated cycle, It is used to calculate the capacity retention rate, and further judge the cycle performance of the test battery relative to the reference battery by comparing the capacity retention rate of the test battery and the reference battery with the cycle times curve.

For the present invention, for the battery to be tested, since the charge and discharge capacities are kept the same during the accelerated cycle, if no side reaction occurs in the battery, the state of the battery after the cycle remains the same as before the cycle, that is, the process change rate is 0, Similarly, the higher the degree of side reactions in the battery, the greater the rate of change during accelerated cycling. Based on this, the analysis of DCIR (direct current resistance) and polarization voltage can be added during each acceleration cycle to assist in judging the cause of poor battery cycle performance and provide a reference for battery performance improvement.

For the method provided by the present invention, since the accelerated cycle analysis is limited in the characteristic decay interval, compared with the cycle test of the full SOC, the evaluation period of the battery cycle performance can be greatly shortened, and the research and development efficiency can be improved. At the same time, the analysis of the battery DCIR (direct current resistance) and the change rate of the polarization voltage process can be added in the acceleration cycle, which provides a reference for the battery cycle failure analysis.

The above are only the preferred embodiments of the present invention. It should be pointed out that for those skilled in the art, without departing from the principles of the present invention, several improvements and modifications can be made. It should be regarded as the protection scope of the present invention.

Claims (10)

1. An accelerated evaluation method for cycle performance of a lithium ion battery is characterized by comprising the following steps:

step S1, for a battery to be tested needing to evaluate the cycle performance, selecting a fresh battery and a reference battery which have the same battery system as the battery to be tested in advance, analyzing to obtain a characteristic SOC interval of the battery system to be tested, wherein the characteristic SOC interval is subjected to cycle attenuation, and determining an accelerated test SOC interval;

step S2, respectively carrying out accelerated cycle testing on the battery to be tested and the reference battery in the accelerated test SOC interval obtained in the first step, and correspondingly obtaining accelerated cycle capacity retention rate curves of the battery to be tested and the reference battery;

and step S3, comparing the accelerated cycle capacity retention rate curve of the battery to be tested with the accelerated cycle capacity retention rate curve of the reference battery, and judging whether the cycle performance of the battery to be tested is good or bad relative to the cycle performance of the reference battery.

2. The method for accelerated evaluation of cycle performance of lithium ion batteries according to claim 1, wherein said step S1 specifically comprises the following operations,

step S11, selecting a fresh battery and a reference battery, respectively performing preset charge-discharge cycle operation, and acquiring the battery voltage V, the charge capacity Q1 and the discharge capacity Q2 of the fresh battery and the reference battery in real time;

wherein, the fresh battery is a battery without capacity attenuation;

the reference battery is a battery with capacity attenuation larger than or equal to a preset proportion;

the fresh battery, the reference battery and the battery to be tested belong to the same battery system;

step S12, obtaining a characteristic SOC interval of the battery system to be tested, wherein the characteristic SOC interval is cyclically attenuated according to a preset first obtaining mode or a preset second obtaining mode;

step S13, determining an accelerated test SOC interval according to the characteristic SOC interval of the cyclic attenuation of the battery system to be tested; the accelerated test SOC interval comprises a lower limit value SOC_CLAnd upper limit value SOC_CU；

The SOC interval comprises all characteristic SOC intervals of the battery system to be tested, or comprises part of characteristic SOC intervals of the battery system to be tested, wherein the characteristic SOC intervals comprise cyclic attenuation of the battery system to be tested.

3. The method for accelerated evaluation of cycle performance of lithium ion batteries according to claim 2, wherein in step S12, a characteristic SOC interval of the battery system to be tested, in which cycle degradation occurs, is obtained according to a preset first obtaining manner, specifically comprising the following steps:

step S121A, for the fresh battery and the reference battery, respectively carrying out differential processing on the charging voltage V of the battery by the charging capacity Q1 of the battery to obtain the dQ/dV of the fresh battery and the reference battery;

step S122A, respectively taking dQ/dV as an ordinate and the state of charge SOC corresponding to the charging capacity Q1 of the battery as an abscissa for the fresh battery and the reference battery, and drawing and obtaining capacity increment curves of the fresh battery and the reference battery in a graph;

step S123A, taking the capacity increment curve of the fresh battery as a reference curve, comparing the capacity increment curve of the reference battery with the reference curve of the fresh battery, determining operation according to a preset characteristic SOC interval, and determining the characteristic SOC interval of the reference battery system in the two capacity increment curves, wherein the characteristic SOC interval is subjected to cyclic attenuation;

a reference battery system which is equal to the battery system to be tested;

characteristic SOC interval of reference battery system with cyclic degradation, including lower limit value SOC_LAnd upper limit value SOC_U；

In step S123A, a characteristic SOC interval determination operation is preset, including the steps of:

firstly, on capacity increment curves of a fresh battery and a reference battery, peaks of two curves are compared one by one to determine a peak in which the peak is obviously reduced or a peak in which the peak is obviously reduced and the peak position of the peak is obviously shifted and then the peak is used as a characteristic peak of cyclic attenuation;

the peak value of the peak is obviously reduced, namely the reduction ratio of the peak value of the peak is greater than or equal to a preset peak value reduction value;

the significant deviation of the peak position of the peak refers to that the SOC deviation amplitude corresponding to the peak position of the peak is larger than or equal to a preset peak position deviation value;

then, the SOC section corresponding to the start position and the end position of the characteristic peak of the cyclic decay is set as the characteristic SOC section.

4. The method for accelerated evaluation of cycle performance of lithium ion batteries according to claim 2, wherein in step S12, a characteristic SOC interval of the battery to be tested for cycle degradation is obtained according to a preset second obtaining manner, specifically comprising the following steps:

step S121B, for fresh battery and reference battery, respectively, with the battery voltage V of the battery during charging_{Charging (CN)}Or battery voltage V during discharge_PutDifferentiating the charging capacity Q1 or the discharging capacity Q2 of the battery to obtain the dV of the fresh battery and the reference battery_{Charging device}dQ1 or dV_Put/dQ2；

Step S122B, respectively applying dV to the fresh battery and the reference battery_{Charging device}dQ1 or dV_PutThe method comprises the following steps that (/ dQ 2) is used as an ordinate, the SOC corresponding to the charging capacity Q1 or the discharging capacity Q2 of a battery is used as an abscissa, and a differential voltage curve of the two is obtained in a drawing mode;

step S123B, taking the differential voltage curve of the fresh battery as a reference curve, comparing the differential voltage of the reference battery with the reference curve of the fresh battery, determining operation according to a preset characteristic SOC interval, and determining the characteristic SOC interval of the reference battery system in the two differential voltage curves, wherein the characteristic SOC interval is subjected to cyclic attenuation; a reference battery system which is equal to the battery system to be tested;

the characteristic SOC interval of the reference battery system subjected to cyclic degradation comprises a lower limit value SOC_LAnd upper limit value SOC_U；

In step S123B, a characteristic SOC interval determination operation is preset, including the steps of:

firstly, on a differential voltage curve of a fresh battery and a reference battery, peaks of two curves are compared one by one to determine a peak in which the peak of the peak is significantly reduced, or a peak in which the peak of the peak is significantly reduced and the peak position of the peak is significantly shifted, and then the peak is used as a characteristic peak of cyclic attenuation;

the peak value of the peak is obviously reduced, namely the reduction ratio of the peak value of the peak is greater than or equal to a preset peak value reduction value;

the significant deviation of the peak position of the peak refers to that the SOC deviation amplitude corresponding to the peak position of the peak is larger than or equal to a preset peak position deviation value;

then, the SOC sections corresponding to the start position and the end position of the characteristic peak of the cyclic decay are set as characteristic SOC sections.

5. The method for accelerated evaluation of cycle performance of lithium ion batteries according to claim 3 or 4, wherein in step S1, the battery to be tested and the reference battery are two batteries having identical battery components except for different negative electrode materials, positive electrode materials, electrolytes or separators;

in step S1, the difference between the capacity retention rates of the fresh battery and the reference battery is greater than a predetermined value;

in step S13, the lower limit value SOC of the accelerated test SOC range_CL＝SOC_LPlus or minus 10 percent and upper limit value SOC of SOC interval accelerated test_CU＝SOC_U±10％；

In step S11, the preset charge-discharge cycle operation includes a discharge operation and a charge operation, specifically: the method comprises the steps of firstly charging the battery to a preset charging upper limit voltage by a charging current with a preset magnitude at a constant current, and then discharging the battery to a preset discharging lower limit voltage by a discharging current with a preset magnitude at a constant current.

6. The method for accelerated evaluation of cycle performance of lithium ion batteries according to claim 1, wherein in step S2, the accelerated cycle test is performed on the battery to be tested within the accelerated test SOC interval obtained in the first step, and an accelerated cycle capacity retention rate curve of the battery to be tested is correspondingly obtained, specifically comprising the following steps:

step S21A, the actual cycle standard of the reference battery is used for carrying out preset multiple times of charge-discharge cycle operation on the battery to be tested, and the charge capacity and the discharge capacity of the battery obtained in the last charge-discharge cycle operation are used as the initial charge capacity C of the battery to be tested₀And initial discharge capacity D₀；

Step S22A, taking the actual cycle standard of the reference battery as the accelerated cycle standard, sequentially carrying out accelerated cycle tests of multiple stages on the battery to be tested, obtaining the charging capacity and the discharging capacity of the battery to be tested after the accelerated cycle test of each stage, and recording the total number of accelerated cycles corresponding to the accelerated cycle test of each stage;

step S23A, according to the charging capacity and discharging capacity of the battery to be tested in the accelerated cycle test of each stage and the initial charging capacity C obtained in the step S21A₀And initial discharge capacity D₀Calculating and obtaining the battery charging capacity retention rate and the battery discharging capacity retention rate of the battery to be tested in the accelerated cycle test of each stage;

step S24A, drawing an accelerated cycle capacity retention rate curve of the battery to be tested by taking the battery charging capacity retention rate and the battery discharging capacity retention rate of the battery to be tested in the accelerated cycle test of each stage as vertical coordinates and taking the total number of accelerated cycles corresponding to the accelerated cycle test of each stage as horizontal coordinates;

in step S2, performing an accelerated cycling test on the reference battery within the accelerated testing SOC interval obtained in the first step, and correspondingly obtaining an accelerated cycling capacity retention rate curve of the reference battery, specifically including the following steps:

step S21B, the reference battery is subjected to preset multiple times of charge-discharge cycle operation according to the actual cycle standard of the reference battery, and the charge capacity and the discharge capacity of the battery obtained in the last charge-discharge cycle operation are used as the initial charge capacity C of the reference battery₁And initial discharge capacity D₁；

Step S22B, taking the actual circulation system of the reference battery as the accelerated circulation system, sequentially carrying out accelerated circulation tests of multiple stages on the reference battery, obtaining the charging capacity and the discharging capacity of the reference battery in the accelerated circulation test of each stage, and recording the total number of accelerated circulation corresponding to the accelerated circulation test of each stage;

step S23B, based on the charge capacity and discharge capacity of the reference cell in the accelerated cycle test at each stage, and the initial charge capacity C obtained in step S21B₁And initial discharge capacity D₁Calculating and obtaining the battery charging capacity retention rate and the battery discharging capacity retention rate of the reference battery in the accelerated cycle test of each stage;

and step S24B, drawing and obtaining an accelerated cycle capacity retention rate curve of the reference battery by taking the battery charging capacity retention rate and the battery discharging capacity retention rate of the reference battery in the accelerated cycle test of each stage as vertical coordinates and taking the total number of accelerated cycles corresponding to the accelerated cycle test of each stage as horizontal coordinates.

7. The method for accelerated evaluation of cycle performance of lithium ion batteries according to claim 6, wherein in step S22A, the accelerated cycle test of each stage specifically comprises the following operations:

firstly, charging the battery to a lower limit value SOC of an acceleration test SOC interval by using a charging current with a preset magnitude_CLThen standing for a preset time;

secondly, performing preset multiple times of same accelerated cycle test operation on the battery;

the test operation of each accelerated cycle is as follows: selecting a charging current Ic and a discharging current Id corresponding to an accelerated test SOC interval in an actual circulation system of a battery to be tested, and then sequentially carrying out charging operation with a preset charging time tc on the battery to be tested by using the charging current Ic and discharging operation with a preset discharging time td on the battery by using the discharging current Id, so that the charging and discharging capacities of the battery in the accelerated test SOC interval are the same; i.e., Ic tc ═ Id ═ td;

the actual circulation system of the battery to be tested is equal to the actual circulation system of the reference battery;

thirdly, continuously performing preset charging and discharging circulation operation of fully charging and discharging the battery to be tested for multiple times in an actual circulation mode;

fourthly, the charging capacity and the discharging capacity of the battery obtained in the last full charge and discharge cycle operation are used as the charging capacity and the discharging capacity of the battery to be tested after the accelerated cycle test of each stage;

in step S22B, the accelerated cycle test of each stage specifically includes the following operations:

firstly, charging the battery to a lower limit value SOC of an acceleration test SOC interval by using a charging current with a preset magnitude_CLThen standing for a preset time;

secondly, performing preset multiple times of same accelerated cycle test operation on the battery;

the test operation of each accelerated cycle is as follows: selecting a charging current Ic and a discharging current Id corresponding to an accelerated test SOC interval in an actual circulation system of a reference battery, and then sequentially carrying out charging operation with a preset charging time tc on the reference battery by using the charging current Ic and discharging operation with a preset discharging time td on the battery by using the discharging current Id, so that the charging and discharging capacities of the battery in the accelerated test SOC interval are the same; i.e., Ic tc ═ Id td;

the actual circulation system of the battery to be tested is equal to the actual circulation system of the reference battery;

thirdly, continuously performing preset charging and discharging circulation operation of full charge and discharge for multiple times on the reference battery according to the actual circulation system of the reference battery;

and fourthly, taking the charging capacity and the discharging capacity of the battery obtained in the last full-charge and full-discharge charge-discharge cycle operation as the charging capacity and the discharging capacity of the reference battery after the accelerated cycle test at each stage.

8. The method for accelerated evaluation of cycle performance of lithium ion batteries according to claim 6, wherein between step S24A and step S23A, further comprising a step of:

step S25A, comparing the battery charging capacity retention rate and the battery discharging capacity retention rate of the battery to be tested in the accelerated cycle test of each stage with a preset battery life ending capacity retention rate in real time, and judging whether the battery charging capacity retention rate or the battery discharging capacity retention rate is smaller than the preset battery life ending capacity retention rate, if so, continuing to execute step S24A, otherwise, returning to execute step S22A and step S23A;

and/or, between the step S24B and the step S23B, the method further comprises the step of:

step S25B, comparing the battery charge capacity retention rate and the battery discharge capacity retention rate of the reference battery in the accelerated cycle test of each stage with a preset battery life-cut capacity retention rate in real time, and when determining whether the battery charge capacity retention rate or the battery discharge capacity retention rate is smaller than the preset battery life-cut capacity retention rate, if so, continuing to execute step S24B, otherwise, returning to execute step S22B and step S23B.

9. The method for accelerated evaluation of cycle performance of lithium ion batteries according to claim 1, wherein in step S3, if the curve of the accelerated cycle capacity retention rate of the battery to be tested is located above the curve of the accelerated cycle capacity retention rate of the reference battery, it is determined that the cycle performance of the battery to be tested is better than the cycle performance of the reference battery;

in step S3, if the accelerated cycle capacity retention rate curve of the battery to be tested is located below the accelerated cycle capacity retention rate curve of the reference battery, it is determined that the cycle performance of the battery to be tested is inferior to the cycle performance of the reference battery.

10. The lithium ion battery cycle performance accelerated evaluation method of claim 1, wherein in step S3, if the accelerated cycle capacity retention rate curve of the battery to be tested in accelerated cycle substantially coincides with the accelerated cycle capacity retention rate curve of the reference battery, the accelerated cycle test of step S2 needs to be repeatedly performed to repeatedly obtain the accelerated cycle capacity retention rate curves of the battery to be tested and the reference battery until the accelerated cycle capacity retention rate curves of the battery to be tested and the reference battery are completely separated, and then the cycle performance of the battery to be tested is judged to be superior to the cycle performance of the reference battery according to the relative position of the accelerated cycle capacity retention rate curves of the battery to be tested and the reference battery;

when the coincidence degree of the accelerated cycle capacity retention rate curve of the battery to be tested and the accelerated cycle capacity retention rate curve of the electric reference battery is larger than or equal to a preset first proportion, judging that the accelerated cycle capacity retention rate curve and the accelerated cycle capacity retention rate curve are basically coincident;

when the coincidence degree of the accelerated cycle capacity retention rate curve of the battery to be tested and the accelerated cycle capacity retention rate curve of the reference battery is smaller than or equal to a preset second proportion, judging that the two are completely separated;

the first proportion is preset and is larger than the second proportion.

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