CN109037811B - A kind of charging method of lithium ion battery of graphite negative electrode system - Google Patents

Abstract

The invention relates to a charging method of a graphite cathode system lithium ion battery, which comprises a multi-stage constant current charging initial charging process with gradually increased charging multiplying power, a multi-stage constant current charging intermediate charging process with gradually decreased charging multiplying power and a constant voltage charging process. The charge rate at each stage in the initial charging process increases as the SOC (state of charge) increases, and the charge rate at each stage in the intermediate charging process decreases as the SOC increases. The charging method fully considers the lithium intercalation process and the lithium separation process of the graphite particles of the negative electrode from the electrochemical aspect, avoids the damage of the crystal structure of the graphite negative electrode and the battery capacity loss caused by lithium separation, can improve the cycle life of the lithium ion battery of the graphite negative electrode system, and can shorten the charging time within the SOC range of 20-80%.

Description

A kind of charging method of lithium ion battery of graphite negative electrode system

technical field

The invention belongs to the technical field of battery charging, and in particular relates to a charging method for a lithium-ion battery of a graphite negative electrode system.

Background technique

In the prior art, there are three lithium-ion battery charging methods, namely, a charging method based on suppressing temperature rise, a charging method based on SOC (battery state of charge or remaining capacity), and a charging method based on suppressing polarization voltage.

Among them, the charging method based on suppressing the temperature rise estimates the temperature rise in each stage of lithium-ion battery charging in real time, and takes the shortest charging time with the minimum temperature rise as the goal to design the current at each stage of charging. The SOC-based charging method believes that the lower the SOC, the greater the maximum charging current that the lithium-ion battery can withstand, that is, the charging current of this charging method increases with the SOC during the entire charging process from SOC=0% to SOC=100% and gradually decreased. The charging method based on the suppression of polarization voltage considers that the polarization voltage is the main reason for limiting the high current charging of lithium-ion batteries. This method aims to reduce the polarization voltage during charging, and divides the constant current stage in the traditional constant current and constant voltage charging into two parts For multi-stage constant current charging, a charging stop stage with zero current or a short discharge stage with negative current is added in the middle of each constant current charging stage.

Most of the above-mentioned charging methods for lithium-ion batteries only consider the external parameters of the battery such as temperature rise, SOC, polarization voltage during the charging process, and rarely consider the internal electrochemical characteristics of lithium-ion batteries, especially for graphite. For a lithium-ion battery with a negative electrode system, if the above method is used to charge the battery, the damage to the crystal structure of the graphite negative electrode and the loss of battery capacity caused by lithium precipitation cannot be avoided, resulting in a reduction in battery life.

SUMMARY OF THE INVENTION

For the charging method based on suppressing the temperature rise, the oxidizing property of the cathode material of the lithium-ion battery that is close to the fully charged state is greatly enhanced when the temperature is too high (above 60°C), and it is indeed possible to oxidize the electrolyte to release gas and heat, resulting in the deterioration of the electrolyte and the bulging of the battery. Even thermal runaway. However, below 55 °C, the temperature rise of the battery is beneficial to the deintercalation of lithium ions from the positive electrode, the diffusion in the electrolyte, and the insertion into the negative electrode material, which is beneficial to the charging process. This is also why the internal resistance of the battery decreases as the temperature increases. Therefore, the temperature rise should not be used as the basis for formulating the charging method. The correct approach is to set a higher temperature upper limit as the safety threshold. Only when the battery temperature exceeds the safety threshold, the effect of temperature on the charging process should be considered.

For the SOC-based charging method, with the progress of charging, when the SOC of the lithium-ion battery exceeds a certain value (about 20%), since the position where lithium is easily inserted in the graphite particles of the negative electrode gradually decreases with the progress of the charging process, the lithium-ion battery The maximum charge current that can be tolerated does gradually decrease. However, when the lithium-ion battery power is relatively low (the SOC is within about 20%), the maximum charging current that the lithium-ion battery of the graphite anode system can withstand increases with the increase of the SOC. This needs to be explained in terms of the electrochemical properties of lithium-ion batteries with graphite anodes. The graphite particles of the negative electrode have a layered structure. When SOC=0, the layered structure of the graphite particles contains very little lithium, and the graphite interlayer spacing is at a minimum value. During the charging process from SOC=0 to SOC equal to a certain value (about 20%), as the SOC gradually increases, the amount of lithium intercalated in the graphite layer gradually increases, and the spacing between the graphite layers gradually increases; and as the SOC increases , the speed at which the graphite interlayer spacing increases gradually decreases. When the SOC exceeds a certain value (about 20%), the lithium content between the graphite layers is sufficient, and the graphite layer spacing basically no longer increases with the SOC, and is at a maximum value. Therefore, if the high-current fast charging is started directly at a very low SOC, the graphite layer on the surface of the graphite particles rapidly intercalates lithium, resulting in a rapid increase in the interlayer spacing; while the graphite layer inside the graphite particles does not have enough time to intercalate lithium, and the graphite layer spacing is still very large. Small. In this way, the stress between the surface layer and the interior of the graphite particles is very large, which may cause the C-C chemical bond to be broken and the graphite crystal structure to be destroyed, resulting in the loss of the available graphite anode material. The destruction of the graphite crystal structure caused by fast charging at low SOC and high current can be observed from the Raman spectrum of the graphite anode material.

In addition, when the SOC is lower than 10%, with the decrease of the SOC, the surface charge transfer resistance of the graphite particles of the negative electrode increases rapidly; under the same charging current, the potential of the negative electrode will also decrease rapidly with the increase of the charge transfer resistance. , when the anode potential is lower than 0 volts (relative to Li/Li ⁺ ), there will be a risk of lithium precipitation on the surface of the anode material. Therefore, as the SOC decreases, the charging current should also decrease to counteract the tendency of the negative electrode potential to decrease and prevent lithium precipitation in the negative electrode. In order to prevent the damage of the graphite crystal structure and the occurrence of lithium precipitation at low SOC, the maximum charging current that the graphite anode lithium-ion battery can withstand in the low SOC stage should be similar to an exponential relationship that gradually increases with the increase of SOC. a curve.

For the charging method based on the suppression of polarization voltage, the charging method of charge-stop-charge or charge-discharge-charge can indeed suppress the polarization of lithium concentration in the negative electrode graphite particles; Part of the lithium is deintercalated to reduce the concentration of lithium in the surface layer of the graphite particles, so that the concentration of lithium tends to be uniform, and the surface layer of the graphite particles is prevented from being overcharged to cause lithium precipitation or formation of lithium dendrites. However, this suppression of lithium concentration polarization only works during and around the period of charging stop or discharge. In other constant current charging stages, lithium concentration polarization will be rapidly re-established in the negative electrode graphite particles. In this way, in the charging cycle of charge-stop-charge-stop (or charge-discharge-charge-discharge), if the charging time/stop charging time (or charging time/discharging time) is set too large, it will not prevent lithium precipitation. Function; if the setting is too small, the charging time and discharge heat will be greatly increased. In order to shorten the charging time, the charging method of charge-stop-charge-stop (or charge-discharge-charge-discharge) often sets the current in the charging stage to be larger, so that the re-established lithium concentration polarization in the graphite particles will also be reduced. larger, more likely to cause lithium precipitation. In addition, the charging method of positive and negative pulses will make the charging device very complicated.

In fact, proper polarization during charging not only does not lead to negative electrode separation, but is beneficial to improve the charging speed of lithium-ion batteries; the difference in polarized lithium concentration inside and outside the graphite particles is beneficial to improve the diffusion rate of lithium into the graphite particles. In fact, as long as the lithium-ion battery is continuously charged at the maximum charging current that can be endured, the charging can be completed in the shortest time on the premise that no lithium precipitation does not damage the battery; the polarization generated during this period is conducive to speeding up the charging speed.

Based on the above considerations, the purpose of the present invention is to provide a charging method for a lithium ion battery of a graphite negative electrode system, which is used to solve the problem that the existing charging method of a lithium ion battery of a graphite negative electrode system easily leads to negative electrode separation and damages the crystal structure of the graphite negative electrode, resulting in reduced battery life. The problem.

In order to solve the above-mentioned technical problems, the present invention proposes a charging method for a lithium-ion battery with a graphite negative electrode system, which includes the following steps:

Carry out constant current charging. The process of constant current charging includes an initial charging stage and an intermediate charging stage, and the remaining battery power is detected. When the remaining battery power is less than or equal to the first set value, it is the initial charging stage, and when the remaining battery power is greater than the first set value. When the value is fixed, it is the intermediate charging stage;

Divide the initial charging stage into the first group of constant current charging stages with more than two small stages, that is, the first group of constant current charging stages includes more than two sub-stages (small stages), and each The charging rate set corresponding to each sub-stage increases as the remaining battery power increases;

Divide the intermediate charging stage into a second group of constant current charging stages with more than two small stages, that is, the second group of constant current charging stages includes more than two sub-stages (small stages), and each The charging rate set corresponding to each sub-stage decreases as the remaining battery power increases;

The range of the remaining battery power of the first set value is 15% to 25%.

The present invention is based on the consideration of the electrochemical properties inside the lithium ion battery of the graphite negative electrode system. By setting the first group of constant current charging stages and the second group of constant current charging stages, in the first group of constant current charging stages, the corresponding set charging rate It increases with the increase of the remaining battery power, and the corresponding charging rate set in the second group of constant current charging stage decreases with the increase of the remaining battery power, which avoids the damage of the crystal structure of the graphite negative electrode and the battery caused by the precipitation of lithium. The capacity loss can not only improve the cycle life of the lithium-ion battery of the graphite anode system, but also shorten the charging time in the intermediate charging stage.

Further, during the constant current charging process, the terminal voltage of the battery is detected, and when the terminal voltage of the battery reaches the set charging cut-off voltage, the constant current charging is stopped and constant voltage charging is performed.

As a further limitation to the correspondingly set charging ratios in the first group of constant current charging stages, the correspondingly set charging ratios in the first set of constant current charging stages and the remaining battery power are in an increasing first exponential function relationship.

As a further limitation to the correspondingly set charging rates in the second group of constant current charging stages, the correspondingly set charging rates in the second group of constant current charging stages have a second exponential function relationship in which the remaining battery power is decreasing.

In order to ensure that the charging rate of the battery under low SOC charging is sufficiently small, further, the curvature of the first exponential function relationship is greater than the curvature of the second exponential function relationship.

In order to prevent the impact of high battery temperature on battery life, further, in the process of constant current charging, when the battery temperature reaches the set first temperature upper limit, reduce the currently set charging rate to continue charging; when the battery temperature reaches When the set second temperature upper limit is set, the charging is stopped; the set second temperature upper limit is greater than the first temperature upper limit.

Specifically, when the battery temperature is greater than or equal to the first temperature upper limit and less than the second temperature upper limit, the reduction range of the charging rate is determined according to the detected ambient temperature, and the higher the ambient temperature is, the greater the reduction rate of the charging rate is.

In order to prevent lithium precipitation when the battery is charged at a low temperature, further, when the battery temperature is lower than the set temperature lower limit, the currently set charging rate is reduced to continue charging. And when the battery temperature is lower than the set temperature lower limit, the reduction range of the charging rate is determined according to the difference between the battery temperature and the temperature lower limit, and the larger the difference is, the greater the reduction rate of the charging rate is.

The present invention also provides a charging method for a lithium ion battery of a graphite negative electrode system, comprising the following steps:

Carry out constant current charging. The process of constant current charging includes an initial charging stage and an intermediate charging stage, and the remaining battery power is detected. When the remaining battery power is less than or equal to the first set value, it is the initial charging stage, and when the remaining battery power is greater than the first set value. When the value is fixed, it is the intermediate charging stage;

Divide the initial charging stage into a first group of constant current charging stages with more than two small stages, and the charging rate set corresponding to the first group of constant current charging stages increases as the remaining battery power increases;

Divide the intermediate charging stage into a second group of constant current charging stages with more than two small stages, and the set charging rate corresponding to the second group of constant current charging stages decreases as the remaining battery power increases;

The first exponential function relationship between the charging rate set corresponding to the first group of constant current charging stages and the remaining power of the battery is an increasing first exponential function.

Further, the charging rate set corresponding to the second group of constant current charging stages has a second exponential function relationship in which the remaining battery power is decreasing.

Further, the curvature of the first exponential function relationship is greater than the curvature of the second exponential function relationship.

Further, in the process of the constant current charging, when the battery temperature reaches the set first temperature upper limit, reduce the currently set charging rate to continue charging; when the battery temperature reaches the set second temperature upper limit, stop charging; The set second upper temperature limit is greater than the first upper temperature limit.

Further, when the battery temperature is greater than or equal to the first temperature upper limit and less than the second temperature upper limit, the reduction range of the charging rate is determined according to the detected ambient temperature, and the higher the ambient temperature is, the greater the reduction rate of the charging rate is.

Description of drawings

Fig. 1 is a kind of charging method schematic diagram provided by experiment one of the present invention;

Fig. 2 is the schematic diagram of the conventional charging method provided by experiment two;

FIG. 3 is a graph showing the test results of the discharge capacity retention rate obtained by cyclic charging and discharging using the charging methods of Experiment 1 and Experiment 2.

Detailed ways

The specific embodiments of the present invention will be further described below with reference to the accompanying drawings.

The present invention provides a charging method for a lithium ion battery of a graphite negative electrode system, comprising the following steps:

The charging process of the graphite anode lithium-ion battery is divided into three charging stages, namely the initial charging stage, the intermediate charging stage and the final charging stage. The initial charging stage includes several small constant current charging stages with gradually increasing charging rates, and the intermediate charging stage. It includes several small stages of constant current charging with gradually decreasing charging rates, and the final charging stage is the constant voltage charging stage.

The transition between the initial charging stage and the intermediate charging stage is based on SOC=15% to 25%, and the transition between the intermediate charging stage and the final charging stage is based on the terminal voltage of the battery reaching the set charging cut-off voltage.

In order to avoid the destruction of the crystal structure of lithium and graphite during low SOC charging, the maximum charging rate of each constant current charging stage during the initial charging process should be close to the increasing exponential function relationship f ₁ (x) relative to the gradually increasing trend of SOC. In order to avoid lithium precipitation due to local overcharge of graphite particles when charging with a large SOC, the trend of the gradual decrease of the maximum charging rate relative to the SOC in each constant current charging stage during the intermediate charging process should be close to a decreasing exponential function relationship f ₂ ( x), where the curvature of the f ₁ (x) function curve is larger than that of f ₂ (x), because the damage to the graphite crystal structure by charging mainly occurs in the lower SOC stage, and the curvature of the f ₁ (x) function curve A larger value can ensure that the charging rate during low SOC charging is sufficiently small. In practical applications, first determine the functional equations f ₁ (x), f ₂ (x) and the SOC division interval in the initial charging process and the intermediate charging process, and then determine the initial charging process and the intermediate charging process according to the functional equation and the SOC division interval. The charging rate in each constant current charging stage.

Set the first temperature upper limit T _max1 and the second temperature upper limit T _max2 , monitor the battery temperature during the charging process, reduce the charging rate when the battery temperature reaches the temperature upper limit T _max1 and continue charging, the higher the ambient temperature or the worse the battery heat dissipation condition, the decrease The larger the battery temperature; the charging stops when the battery temperature reaches the upper temperature limit T _max2 .

A lower temperature limit T _min is set, and the battery temperature is monitored during the charging process. When the battery temperature is less than T _min , the charging rate should be reduced, and the greater the difference between the battery temperature and T _min , the greater the decrease.

In order to prove the effectiveness of the charging method of the present invention, experiment 1 and experiment 2 are carried out for comparative analysis. Experiment 1 is the charging method of the present invention, and experiment 2 is the charging method of conventional constant current and constant voltage charging, and experiment 1 and experiment 2 use the same charging method. Lithium iron phosphate lithium ion battery with graphite negative electrode system, the positive electrode of the battery is mixed with 95.3% LiFePO ₄ +2% PVDF + 2.7% SP (conducting agent), and the negative electrode of the battery is 98% artificial graphite + 1% SBR + 1% CMC is mixed, the diaphragm is PP/PE/PP composite membrane, the electrolyte is composed of organic solvent (30%EC+30%PC+40%DEC), 1mol/L LiPF ₆ and additives (0.5%VC, 5%FEC, 4% VEC) composition.

Specifically, the process of experiment 1 is as follows:

At room temperature of 23°C, the battery is charged according to the lithium-ion battery charging method of the present invention. The charging process is shown in Figure 1, which specifically includes the following steps:

The basis for selecting the transition from the initial charging stage to the intermediate charging stage is SOC=20%, and the charging rate when SOC=20%-30% is selected is 2.5C.

In the "charge rate-SOC" coordinate system in Fig. 1, the exponential function curve f ₁ (x) in the initial charging process is determined with points (0, 0.05) and (0.2, 2.5), which can be regarded as the initial The maximum charge rate curve that the battery can withstand during charging. Since the damage of the graphite crystal structure mainly occurs in the very low SOC range, the curvature of the exponential function curve should be large, and the exponential function curve equation is set as:

f ₁ (x)=b ₁ +a ₁ e ^5x

得到

即f₁(x)＝-1.3758+1.4258e^5x。solve the equation by

get

That is, f ₁ (x)=-1.3758+1.4258e ^5x .

In the "charge rate-SOC" coordinate system in Fig. 1, the exponential function curve f ₂ (x) of the intermediate charging process is determined with points (0.3, 2.5) and (1, 0.05), which can be regarded as intermediate charging The maximum charge rate curve that the battery can withstand during the process. The curvature of the curve f ₂ (x) should be smaller than that of the f ₁ (x) curve, and the exponential function curve equation is set as:

f ₂ (x)=b ₂ +a ₂ e ^-x

得到

即f₂(x)＝-2.3668+6.5694e^-x。solve the equation by

get

That is, f ₂ (x)=-2.3668+6.5694e ^-x .

The division interval of SOC is selected to be 10%, and then the charging rate of each constant current charging stage in the initial charging process and the intermediate charging process can be determined according to the equations of f ₁ (x) and f ₂ (x) (the specific values are marked in Fig. 1), the charging rate value of each constant current charging stage should be below the maximum charging rate curve that the battery can withstand (see Figure 1). Since the damage to the graphite crystal structure by charging is mainly concentrated in the lower SOC stage, the present invention divides 0%-10% SOC into two constant-current charging stages, 0%-5% SOC and 5%-10% SOC. Since the charging rate of 0.05C is too small, the present invention sets the charging rate of the two constant-current charging stages of 0%-5% SOC and 90%-100% SOC to 0.1C.

Since the lithium iron phosphate battery is used in experiment 1, the charging cut-off voltage can be selected as 3.65v. During the charging process, when the battery terminal voltage reaches the set charging cut-off voltage of 3.65v, it will be switched to constant voltage charging. When the charging rate drops to 0.05C stop charging.

The experiment of the present invention uses a lithium iron phosphate battery, and the ambient temperature is 23°C, and the upper temperature limit T _max1 =45°C and T _max2 =50°C can be set accordingly. The battery temperature is detected in real time during the entire charging process. When the battery temperature reaches 45°C, the charging rate is reduced to 80% of the corresponding charging rate in Figure 1. When the battery temperature drops below 45°C for 10 minutes, the battery is recharged according to the charging rate in Figure 1 according to the current SOC.

Select 30 batteries for 500 charge-discharge cycle tests. The specific steps are:

The battery is charged by the charging method of Experiment 1 of the present invention, charged from 0% SOC to 100% SOC, put on hold for 20 minutes, discharged at a discharge rate of 1C to SOC=10%, and then discharged at a small rate of 0.2C until the terminal voltage of the battery is 2.5v, then set aside for 20min.

After repeating this for 499 times, the average discharge capacity retention rate of 30 batteries is shown in curve 1 in Fig. 3 .

The process of experiment 2 is as follows:

At room temperature of 23 °C, the same lithium-ion battery as in experiment 1 was used for conventional constant current and constant voltage charging, as shown in Figure 2. The specific steps are as follows:

Starting from 0% SOC, charge at a constant rate of 1C until the cut-off voltage of 3.65v. Charge at a constant voltage of 3.65v, and stop charging when the charge rate decreases to 0.05C.

Select 30 batteries for 500 charge-discharge cycle tests. The specific steps are:

Use the charging method of Experiment 2 to charge, from 0% SOC to 100% SOC, and then discharge according to the discharge method of cyclic discharge recorded in Experiment 1. The average discharge capacity retention rate of 30 batteries is shown in Figure 3. Curve 2 shown.

It can be seen from the test results of the full-charge and discharge cycle test of the lithium-ion battery of the graphite negative electrode system in Fig. 3 that compared with the conventional constant current and constant voltage charging method, the charging method of the present invention can significantly improve the cycle of the lithium-ion battery of the graphite negative electrode system. life.

The above table is a comparison of the charging time between Experiment 1 and Experiment 2. It can be seen from the table that if the lithium-ion battery is charged from empty to fully charged state, that is, from 0% SOC to 100% SOC, the charging of Experiment 1 is used. The time required for charging by the method will be much longer than that in experiment two; however, if charging from 20% SOC to 80% SOC, the charging time required by the charging method in experiment one will be 5.3 minutes faster than in experiment two; in particular, if From 20% SOC to 60% SOC, the charging method of experiment one will be 10.1min faster than that of experiment two. Therefore, the charging method of the present invention is especially suitable for fast charging in the range of 20%-80% SOC and fast supplementary charging in the range of 20%-60% SOC. Although the charging time of experiment 1 of the present invention outside the range of 0% to 100% SOC is long, the damage of the crystal structure of the graphite negative electrode and the loss of battery capacity caused by lithium precipitation are avoided, and the life of the battery is effectively guaranteed.

To sum up, the charging method of the present invention fully considers the lithium intercalation process and the lithium evolution process of the negative electrode graphite particles from the electrochemical level in the formulation process, and improves the lithium ion battery cycle life under the premise of ensuring the graphite negative electrode system lithium ion battery. The charging speed when charging in the range of 20% to 80% SOC.

The above description is only a preferred experiment of the present invention, and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the scope of the claims of the present invention.

Claims (5)

1. A charging method of a graphite cathode system lithium ion battery is characterized by comprising the following steps:

performing constant current charging, wherein the constant current charging process comprises an initial charging stage and an intermediate charging stage, detecting the residual electric quantity of the battery, and the initial charging stage is performed when the residual electric quantity of the battery is less than or equal to a first set value, and the intermediate charging stage is performed when the residual electric quantity of the battery is greater than the first set value;

dividing the initial charging stage into a first group of constant current charging stages with more than two small stages, wherein the charging multiplying power correspondingly set in the first group of constant current charging stages is increased along with the increase of the residual electric quantity of the battery;

dividing the intermediate charging stage into a second group of constant current charging stages with more than two small stages, wherein the charging multiplying power correspondingly set in the second group of constant current charging stages is reduced along with the increase of the residual electric quantity of the battery;

the range of the residual battery capacity of the first set value is 15% -25%;

the charging multiplying power set correspondingly in the first group of constant current charging stages and the battery residual capacity are in a first exponential function relationship of increasing; the charging multiplying power set correspondingly in the second group of constant current charging stages and the residual electric quantity of the battery are in a descending second exponential function relationship; the curvature of the first exponential function is greater than the curvature of the second exponential function.

2. The method of claim 1, wherein during the constant current charging, the terminal voltage of the battery is detected, and when the terminal voltage of the battery reaches a predetermined charge cut-off voltage, the constant current charging is stopped and the constant voltage charging is performed.

3. The method for charging a graphite cathode system lithium ion battery according to claim 1 or 2, wherein in the constant-current charging process, when the battery temperature reaches a set first upper temperature limit, the charging is continued by reducing the currently set charging rate; stopping charging when the temperature of the battery reaches a set second upper temperature limit; the second upper temperature limit is set to be greater than the first upper temperature limit.

4. The method of charging a graphite cathode system lithium ion battery according to claim 3, wherein when the battery temperature is equal to or higher than the first upper temperature limit and lower than the second upper temperature limit, the decrease width of the charge rate is determined based on the detected ambient temperature, and the decrease width of the charge rate increases as the ambient temperature increases.

5. The method of claim 1, wherein when the temperature of the battery is lower than a set lower temperature limit, the charging is continued by reducing the currently set charging rate, and the reduction of the charging rate is determined according to the difference between the temperature of the battery and the lower temperature limit, and the larger the difference is, the larger the reduction of the charging rate is.

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