CN115684968A - Method for predicting capacity retention rate of lithium battery - Google Patents

Abstract

The invention belongs to the technical field of lithium ion battery performance evaluation, and particularly relates to a method for predicting capacity retention rate of a lithium battery. The method comprises the steps of (1) fitting to obtain a function of the loss capacity and the number of circulating circles of lithium in a negative pole piece of the lithium battery and a function of the loss capacity and the number of circulating circles of electrolyte lithium; (2) Fitting to obtain a function of the total loss capacity and the number of cycles of lithium; (3) And fitting to obtain a function of the discharge capacity and the number of circulation turns of the lithium battery in the circulation process through the total capacity, the total loss capacity, the first-turn calibration capacity and the second-turn calibration capacity of the lithium in the positive pole piece, and further obtaining a function of the capacity retention rate and the number of circulation turns of the lithium battery. The method predicts the capacity retention rate of the lithium battery based on the active lithium loss, can detect the cycle life and the health state of the lithium battery, shortens the development cycle of the battery, and saves the experiment cost; meanwhile, the method for predicting the capacity retention rate of the lithium battery has high accuracy and high goodness of fit between the predicted value and the experimental value.

Description

A method for predicting the capacity retention rate of lithium batteries

technical field

The invention belongs to the technical field of evaluating the performance of lithium-ion batteries, in particular to a method for predicting the capacity retention rate of lithium-ion batteries.

Background technique

Because of the advantages of light weight, high energy density, and long cycle life, lithium-ion batteries have made major breakthroughs in this field since the commercialization of lithium-ion batteries in 1991. However, there are still many constraints on the application of lithium-ion batteries in electric vehicles, such as slow charging rate, fast decay of cycle life, and thermal runaway. The goal that the field of lithium-ion batteries has been pursuing is to charge the battery to 80% SOC within 15 minutes, but the premise of realizing fast battery charging, improving battery life and safety is to ensure that the positive and negative electrode materials have high cycle stability and good performance. capacity retention. Generally speaking, the cycle stability and capacity retention rate of lithium-ion batteries have a great relationship with the structural changes before and after deintercalation of positive and negative electrode materials. The more stable the structure during lithium deintercalation, the better the cycle stability of lithium batteries. , the higher the capacity retention rate.

At present, there are relatively many methods for evaluating the battery capacity retention rate of lithium-ion in the industry. Generally, the long-cycle method is used to evaluate the cycle performance of positive/negative electrode materials and the capacity retention rate of lithium-ion batteries. This method is the most intuitive and accurate, but it is time-consuming and inefficient. Chinese patent document CN110658473A discloses a method for evaluating the storage performance of lithium-ion battery positive electrode materials, which evaluates capacity retention through the first discharge capacity ratio, and further introduces the stability of lithium battery positive and negative electrode materials. Chinese patent document CN113884930A discloses a method for predicting the cycle life and state of health of a power battery, using the equivalent Coulomb average value of the previous n weeks to predict the capacity retention rate of the cycle m weeks. These two methods are relatively fast and simple, but the accuracy rate is low, especially the result of evaluating the capacity retention rate by the first discharge capacity ratio has a large deviation from the actual result. At the same time, there are many data-driven and artificial intelligence-based methods in the existing technology to predict the remaining life and capacity retention rate of lithium-ion batteries, such as machine learning, neural networks, and Kalman filter methods. The prediction of these methods depends on the amount of data And the source of the data cannot reflect the attenuation mechanism of the lithium battery, and there are problems of low efficiency and large deviation.

Contents of the invention

Therefore, the technical problem to be solved in the present invention is to overcome defects such as large deviation between test results and actual results and low efficiency when predicting lithium battery capacity retention in the prior art, thereby providing a method for predicting lithium battery capacity retention .

For this reason, the present invention provides the following technical solutions.

The invention provides a method for predicting the capacity retention rate of a lithium battery, comprising the following steps,

(1) The lithium battery to be tested is subjected to a cycle test, and the function M ^loss = m(x) of the loss capacity M ^loss and the number of cycles x of the lithium in the negative electrode sheet of the lithium battery is obtained by fitting, and the M ^loss is passed through x cycles Finally, the loss capacity of lithium caused by the negative pole piece;

The function N ^loss =n(x) of the loss capacity N ^loss of the electrolyte lithium in the lithium battery and the number of cycles x is obtained by fitting, and the N ^loss is the loss capacity of lithium caused by the electrolyte after x cycles;

(2) Through the loss capacity of lithium in the negative pole piece and the loss capacity of lithium in the electrolyte, the function of the total loss capacity Q ^loss of lithium and the number of cycles x is obtained by fitting, Q ^loss = q ₁ (x), wherein, Q ^loss is the total loss capacity of lithium caused by the negative electrode sheet and electrolyte after x cycles, recorded as the total loss capacity;

(3) According to the total capacity Q ^total of lithium in the positive pole piece, the total loss capacity Q ^loss , the first lap calibration capacity Q ₁ and the second lap calibration capacity Q ₂ , the discharge capacity Q of the lithium battery during the cycle is obtained by fitting The function of ^rete and the number of cycles x, Q ^rete = q ₂ (x), and then the function of the lithium battery capacity retention rate δ and the number of cycles x;

in,

Q ₁ is the calibrated first cycle capacity of the lithium battery at 0.33C, and Q ^rete is the actual discharge capacity of the lithium battery per cycle.

In the step (3), the total capacity Q ^total of lithium in the positive electrode sheet = aQ ^calc ;

Among them, a is the delithiation coefficient, which can be obtained from experience or the ratio of the discharge capacity obtained by discharging with a small current of 0.01-0.1C to the theoretical capacity Q ^calc ;

Q ^calc is the theoretical capacity of the positive electrode material obtained through first-principle calculations. Specifically, density functional (DFT) and generalized gradient approximation (GGA-PBE) theories are used for structural optimization, and the calculation methods can be calculated using common methods in the field.

In the step (2), the function of the total loss capacity Q ^loss and the number of cycles x is,

Q ^loss =M ^loss +N ^loss .

In the step (3), when Q ^loss ≤ Q ^total -Q ₁ , the discharge capacity of the lithium battery Q ^rete =Q ₁ +(x-1) ×(Q ₂ -Q ₁ );

When Q ^loss >Q ^total -Q ₁ , the discharge capacity of the lithium battery is Q ^rete =Q ^total -Q ^loss .

In the step (1), the function of the loss capacity M ^loss of lithium in the negative electrode sheet in the lithium battery and the number of cycles x is,

M ^loss = be ^cx ;

Among them, b and c are constants.

In the step (1), the function of the loss capacity N ^loss of lithium in the electrolyte solution of the battery and the number of cycles x is,

N ^loss = mx ³ +nx ² +kx+p;

Among them, m, n, k and p are all constants.

Further, after the cycle test of the lithium battery to be tested is completed, the residual lithium content in the negative pole piece is tested by inductively coupled plasma method, laser-induced breakdown spectroscopy or X-ray energy spectrum analysis method, and it is converted into the lithium content in the negative pole piece. Lithium loss capacity.

Further, after the cycle test of the lithium battery to be tested is completed, the residual lithium content in the electrolyte is tested by inductively coupled plasma method, laser-induced breakdown spectroscopy or X-ray energy spectrum analysis, and it is converted into the content of lithium in the electrolyte. loss of capacity.

The temperature of the cycle test is -10-60°C.

Further, when fitting the functions M ^loss and N ^loss , the number of cycles x is not less than 200.

The lithium battery system applicable to the present invention includes many systems such as iron-lithium system, ternary system, and cobalt-free system, and has high universality.

Capacity rise phenomenon: During the cycle of the battery, the discharge capacity of the next cycle is greater than the discharge capacity of the previous cycle, that is, Q ₁ <Q ₂ <Q ₃ <Q ₄ <...<Q _n >Q _n+1 >Q _{n +2} >Q _n+3 >..., the capacity retention rate is ≥100%.

Further, the discharge capacity of the battery with capacity increase phenomenon, when Q ^loss ≤ Q ^total -Q ₁ , the lithium battery discharge capacity Q ^rete =Q ₁ +(x-1)×(Q ₂ -Q ₁ );

When Q ^loss >Q ^total -Q ₁ , the discharge capacity of the lithium battery is Q ^rete =Q ^total -Q ^loss .

Further, the discharge capacity of the battery without capacity rise phenomenon, Q ^rete =Q ^total -Q ^loss .

The technical solution of the present invention has the following advantages:

1. The method for predicting the lithium battery capacity retention rate provided by the present invention, the method comprises, (1) the lithium battery to be tested is carried out cycle test, obtains the lithium loss capacity M ^loss and the cycle cycle of the lithium in the lithium battery negative pole piece by fitting The function M ^loss of the number x = m(x), M ^loss is the loss capacity of lithium caused by the negative pole piece after x cycles; the loss capacity N ^loss and cycle number of electrolyte lithium in the lithium battery are obtained by fitting The function N ^loss of x=n(x), N ^loss is the loss capacity of lithium caused by the electrolyte after x cycles; (2) through the loss capacity of lithium in the negative electrode sheet and the loss capacity of lithium in the electrolyte, The function of the total loss capacity Q ^loss of lithium and the number of cycles x is obtained by fitting, Q ^loss = q ₁ (x), where Q ^loss is the total loss of lithium caused by the negative electrode sheet and the electrolyte after x cycles The capacity is recorded as the total loss capacity; (3) The lithium battery is obtained by fitting the total capacity Q ^total , the total loss capacity Q ^loss , the first lap calibration capacity Q ₁ and the second lap calibration capacity Q ₂ of the lithium in the positive pole piece The function of the discharge capacity Q ^rete in the cycle process and the number of cycles x, Q ^rete =q ₂ (x), and then the function of the lithium battery capacity retention rate δ and the number of cycles x. The method predicts the lithium battery capacity retention rate based on active lithium loss, which can detect the cycle life and health status of the lithium battery, shortens the battery development cycle, and saves experimental costs; at the same time, the method for predicting the lithium battery capacity retention rate in the present invention The accuracy rate is high, and the predicted value is in good agreement with the experimental value.

The method for predicting the capacity retention rate of the lithium battery provided by the present invention can also judge whether the battery has a phenomenon of rising capacity, and predict the phenomenon of rising battery capacity.

2. The method for predicting the capacity retention rate of lithium batteries provided by the present invention, the present invention predicts the capacity retention rate of the full battery based on the active lithium loss capacity of the negative electrode plate and electrolyte, and accurately reflects the attenuation mechanism of the battery capacity. Simple and efficient, suitable for batteries of many systems such as iron-lithium system, ternary system, cobalt-free system, etc. The method provided by the invention is less restricted by temperature and rate conditions, has a wide range of applications, and has high universality .

The method provided by the invention can predict the capacity retention rate of the battery with the capacity rise phenomenon, and can also predict the capacity retention rate of the battery without the capacity rise phenomenon, and has a wide range of applications.

Description of drawings

In order to more clearly illustrate the specific implementation of the present invention or the technical solutions in the prior art, the following will briefly introduce the accompanying drawings that need to be used in the specific implementation or description of the prior art. Obviously, the accompanying drawings in the following description The drawings show some implementations of the present invention, and those skilled in the art can obtain other drawings based on these drawings without any creative work.

Fig. 1 is a schematic flow chart of assembling a monolithic battery in Example 1 of the present invention;

Fig. 2 is the fitting curve of the loss capacity and the number of cycles of lithium in the negative electrode material in Example 1 of the present invention;

Fig. 3 is the fitting curve of the loss capacity and cycle number of lithium in the electrolyte in the embodiment of the present invention 1;

Fig. 4 is the predicted value and actual measured value of the capacity retention rate of the lithium battery in Example 1 of the present invention under different cycle numbers.

Detailed ways

The following examples are provided in order to further understand the present invention better, are not limited to the best implementation mode, and do not limit the content and protection scope of the present invention, anyone under the inspiration of the present invention or use the present invention Any product identical or similar to the present invention obtained by combining features of other prior art falls within the protection scope of the present invention.

Those who do not indicate specific experimental steps or conditions in the embodiments can be carried out according to the operation or conditions of the conventional experimental steps described in the literature in this field. The reagents or instruments used were not indicated by the manufacturer, and they were all commercially available conventional reagent products.

Example 1

The present embodiment provides a method for predicting the capacity retention rate of a lithium battery, comprising the following steps,

(1) Taking NCM613 (specific composition: LiNi _0.6 Co _0.1 Mn _0.3 O ₂ ) as an example of a monolithic battery as the positive electrode material and graphite as the negative electrode material, the preparation method of the monolithic battery is as follows:

(1) Pretreatment of positive and negative electrodes: cutting the positive and negative electrodes, wiping the tabs, cleaning, and pasting glue;

(2) Assembly of monolithic battery: As shown in Figure 1, stack the sheets according to the method of negative electrode sheet-diaphragm-positive electrode sheet-diaphragm-negative electrode sheet, and then weld the tabs and seal them with liquid in order to get assembled For a good battery, the injection coefficient is higher than the normal coefficient; in the present embodiment, the diaphragm is a polypropylene diaphragm (PP diaphragm), and the electrolyte is A60. Specifically, the present embodiment is a monolithic battery, and the required electrolyte is softer than There are relatively many packs and square shell batteries, so the injection volume is 4.0ml.

(3) The assembled monolithic battery is left to stand at 40°C for 3-24 hours to ensure that the electrolyte completely infiltrates the positive and negative electrode materials, and further pre-charged and formed to obtain a monolithic battery. In this embodiment, the standing time is 3 hours.

At 25°C, 0.33C constant current and constant voltage (abbreviated as CCCV) was used to calibrate the first-cycle discharge capacity of the monolithic battery, denoted as Q ₁ , and the measured first-cycle discharge capacity Q ₁ of the monolithic battery in this embodiment was 147mAh. Further, the second cycle discharge capacity of the monolithic battery was calibrated, denoted as Q ₂ , and the measured second cycle discharge capacity Q ₂ of the monolithic battery in this embodiment was 147.1042mAh.

(2) Test of active lithium loss capacity

(1) Take 36 single-chip batteries with high consistency for cycle test, use 3 batteries for charge and discharge test in each cycle, and obtain the discharge capacity of lithium battery under different cycle numbers, that is, 12 sets of cycle numbers, each Set up 3 parallel samples under the number of cycles. The test conditions of the cycle test are 25°C, 0.8C constant current for charge and discharge test, respectively cycle 50cycles, 100cycles, 200cycles, 300cycles, 400cycles, 500cycles, 600cycles, 700cycles, 800cycles, 900cycles, 1000cycles, 1100 cycles; after the cycle test is over Perform post-processing on the negative pole piece, specifically, discharge at a low current of 0.1C to remove all the active lithium in the negative pole piece, disassemble the battery, and keep the negative pole piece and electrolyte separately for subsequent testing. Further, record the discharge capacity of the single-chip battery at each number of cycles, and take the average value among parallel samples to obtain the discharge capacity Q ^x of the lithium battery at different cycles, denoted as Q ¹ , Q ² ... …Q ¹² , and then the measured capacity retention rate δ' of the battery under different cycle numbers, the measured capacity retention rate δ' is the ratio of Q ^x to Q ₁ .

(2) Test of the loss capacity of lithium in the negative electrode sheet: use dimethyl carbonate (DMC) to clean the above-mentioned negative electrode sheet, and remove the overhang area through oxidation (the overhang area refers to the length and width of the negative electrode sheet. area) after treatment, use an appropriate amount of deionized water to soak the negative electrode sheet, remove the current collector part, and finally conduct an inductively coupled plasma (ICP) test to test the residual lithium content in the negative electrode sheet. The residual lithium in this part is the negative electrode. According to the lithium content of the test, the lithium lost by the sheet is converted into the capacity of the negative electrode sheet. The conversion formula is as follows, that is, the loss capacity of each negative electrode sheet,

Among them, m is the mass of the negative electrode material area after removing the overhang and current collector of the negative electrode sheet; F is the Faraday constant; wt% is the lithium mass percentage obtained by ICP test; M _Li is the relative molecular mass of Li.

_The test results of 36 batteries are recorded as M _1-1 ^loss , M _1-2 loss , M 1-3 ^loss , ^M _2-1 ^loss , M _2-2 ^loss , M _2-3 ^loss , M _3-1 ^loss , M _3-2 ^loss , M _3-3 ^loss ... M _12-1 ^loss , M _12-2 ^loss , M _12-3 ^loss , the specific values are shown in Table 1, and the average of M ^loss under the number of cycles of each group is calculated The values are denoted as M ₁ ^loss , M ₂ ^loss , M ₃ ^loss ·······M ₁₂ ^loss , the specific values are shown in Table 1, and the function of the active lithium loss capacity of the negative electrode sheet and the number of cycles x is obtained by fitting M ^loss =m(x), specifically M ^loss =27.502e ^0.0007x . The test results and fitted curves of lithium loss capacity and cycle times in the negative electrode sheet are shown in Figure 2.

Table 1 The active lithium loss capacity M ^loss of the negative electrode sheet

Number of cycles Parallel sample 1(mAh) Parallel sample 2(mAh) Parallel sample 3 (mAh) Average (mAh) 50 27.52 28.93 28.95 28.47 100 30.23 30.54 30.26 30.34 200 32.18 31.33 31.87 31.79 300 35.24 34.88 35.49 35.20 400 36.19 36.12 36.92 36.41 500 38.77 38.39 38.24 38.47 600 44.43 43.28 44.16 43.96 700 46.12 45.45 44.93 45.50 800 46.78 46.98 46.97 46.91 900 55.11 55.59 54.74 55.15 1000 59.30 60.25 58.49 59.35 1100 61.97 62.66 63.72 62.78

Lithium loss capacity test in the electrolyte: Since the battery will consume a certain amount of electrolyte after a long cycle, the amount of electrolyte will decrease and cannot meet the test requirements. Therefore, it is necessary to dilute the electrolyte after the above charge and discharge test, and then use Inductively coupled plasma (ICP) tests the residual lithium content in the electrolyte; at the same time, it is also necessary to test the lithium content of the pure electrolyte. The lithium content increment of the electrolyte solution after removing the lithium content of the blank electrolyte solution is converted into the corresponding capacity of the electrolyte solution, the calculation formula is as follows, the lithium content increment of the electrolyte solution is the loss capacity of lithium in the electrolyte solution,

Wherein, n is the quality of electrolyte; F is Faraday's constant; wt% is the lithium content that ICP test obtains, and this lithium content has removed the lithium content in the blank electrolyte, and the unit is mass percent; M _Li is the relative molecular mass of Li .

_The test results of 36 batteries ^are recorded as N _1-1 ^loss , N _1-2 loss , N _1-3 ^loss , N 2-1 ^loss , N _2-2 ^loss , N _2-3 ^loss , N _3-1 ^loss , N _3-2 ^loss , N _3-3 ^loss ·······N _12-1 ^loss , N _12-2 ^loss , N _12-3 ^loss , the specific values are shown in Table 2. The average value of N ^loss is recorded as N ₁ ^loss , N ₂ ^loss , N ₃ ^loss ... N ₁₂ ^loss , the specific values are shown in Table 2, and the loss capacity and circulation cycle of lithium in the electrolyte are obtained by fitting The function N ^loss =n(x) of the number x, specifically, N ^loss =3×10 ⁹ x ³ −4×10 ⁶ x ² +0.0047x+0.8054. The test results and fitted curves of lithium loss capacity and cycle times in the electrolyte are shown in Figure 3.

Table 2 The loss capacity N ^loss of the electrolyte

(2) According to the loss capacity of lithium in the negative electrode sheet and the loss capacity of lithium in the electrolyte, the function of the total active lithium loss capacity Q ^loss and the number of cycles x is obtained by fitting, Q ^loss =q ₁ (x) = M ^loss +N ^loss =m(x)+n(x)=, 27.502e ^0.0007x +3×10 ⁹ x ³ −4×10 ⁶ x ² +0.0047x+0.8054. Among them, Q ^loss is the active lithium loss capacity caused by the negative electrode sheet and electrolyte after x cycles, which is recorded as the total active lithium loss capacity.

(3) The total capacity Q ^total of lithium in the positive pole sheet = aQ ^calc ;

Wherein, a is the delithiation coefficient, which can be obtained based on empirical values, and can also be the ratio of the discharge capacity obtained by discharging with a small current of 0.01-0.1C to the theoretical capacity Q ^calc . According to experience, the delithiation coefficient a of positive electrode materials is generally between 0.25-1. For example, the delithiation coefficient of layered structure materials is generally between 0.4-0.7, and the delithiation coefficient of olivine structure is between 0.65-1. The detachment coefficient of the spinel structure is about 0.75. In this embodiment, the positive electrode sheet NCM613 is a layered structure. According to experience, the value of a is 0.5; Q ^calc is the theoretical capacity of the positive electrode material, which is calculated by first principles. Specifically, density functional (DFT) and generalized gradient approximation (GGA-PBE) theory were used for structural optimization. In this embodiment, the theoretical capacity Q ^calc is 362 mAh, and the total capacity Q ^total of lithium in the positive electrode sheet is 181 mAh.

The function q ₂ (x) of the actual capacity Q ^rete of the lithium battery during the cycle and the cycle number x:

When Q ^loss ≤ Q ^total -Q ₁ , the discharge capacity of lithium battery Q ^rete =Q ₁ +(x-1)×(Q ₂ -Q ₁ );

When Q ^loss > Q ^total - Q ₁ , the discharge capacity of lithium battery Q ^rete = Q ^total - Q ^loss ;

The function of the predicted capacity retention rate δ of lithium battery and the number of cycles x:

Furthermore, for some batteries, the phenomenon that the discharge capacity of the latter cycle is greater than the discharge capacity of the previous cycle will appear during the early cycle, that is, Q ₁ <Q ₂ <Q ₃ <Q ₄ <...<Q _n >Q _n+1 ＞Q _n+2 ＞Q _n+3 ＞……, commonly known as the capacity increase phenomenon, when these batteries have capacity increase phenomenon, satisfy Q ^loss ≤ Q ^total -Q ₁ , when the battery capacity increase phenomenon disappears, satisfy Q ^loss > Q ^total - Q ₁ ; for batteries with a capacity increase phenomenon, the battery discharge capacity Q ^rete is calculated by the following formula:

When Q ^loss ≤ Q ^total -Q ₁ , the discharge capacity of lithium battery Q ^rete =Q ₁ +(x-1)×(Q ₂ -Q ₁ );

When Q ^loss >Q ^total -Q ₁ , the discharge capacity of the lithium battery is Q ^rete =Q ^total -Q ^loss .

For other batteries, in the cycle process, the discharge capacity of the last cycle is smaller than the discharge capacity of the previous cycle from beginning to end. This kind of battery will not appear the phenomenon of capacity increase. During the whole cycle process, the discharge capacity of the lithium battery meets Q ^loss >Q ^total −Q ₁ , therefore, the discharge capacity Q ^rete =Q ^total −Q ^loss of the battery without the capacity increase phenomenon.

Specific to this embodiment, the battery capacity of this embodiment has an increase phenomenon, therefore, the battery discharge capacity Q ^rete satisfies the following formula,

When Q ^loss ≤ Q ^total - Q ₁ , the discharge capacity of lithium battery Q ^rete = Q ₁ + (x-1) × (Q ₂ - Q ₁ ) = 147+(x-1) × (147.1042-147) = 147+0.1042(x－1);

When Q ^loss > Q ^total - Q ₁ , the discharge capacity of lithium battery Q ^rete = Q ^total - Q ^loss = 181-27.502e ^0.0007x -3×10 ⁹ x ³ +4×10 ⁶ x ² -0.0047x-0.8054 ;

The function of the predicted capacity retention rate δ of lithium battery and the number of cycles x:

The capacity retention rate of the lithium battery of this embodiment under different cycle times can be predicted by formula 1, and the cycle life and health status of the lithium battery can be predicted. The measured capacity retention rate δ' and the predicted capacity retention rate δ of the lithium battery under different cycle numbers are shown in Figure 4, as can be seen from Figure 4, the lithium battery capacity retention rate predicted by the method provided by the present invention is close to the measured value, accurate high degree.

Apparently, the above-mentioned embodiments are only examples for clearly illustrating, rather than limiting the implementation. For those of ordinary skill in the art, on the basis of the above description, other changes or changes in different forms can also be made. It is not necessary and impossible to exhaustively list all the implementation manners here. And the obvious changes or variations derived therefrom are still within the scope of protection of the present invention.

Claims (10)

1. A method for predicting the capacity retention rate of a lithium battery is characterized by comprising the following steps,

(1) Carrying out cycle test on the lithium battery to be tested, and obtaining the loss capacity M of lithium in the negative pole piece of the lithium battery through fitting ^loss Function M of the number of cycles x ^loss ＝m(x)，M ^loss Is prepared byThe lost capacity of lithium caused by the negative pole piece after x cycles;

obtaining the loss capacity N of the electrolyte lithium in the lithium battery through fitting ^loss Function N of the number of loop turns x ^loss ＝n(x)，N ^loss Is the lost capacity of lithium caused by the electrolyte after x cycles;

(2) Fitting to obtain the total loss capacity Q of lithium through the loss capacity of lithium in the negative pole piece and the loss capacity of lithium in the electrolyte ^loss As a function of the number x of cycles, Q ^loss ＝q ₁ (x) Wherein, Q ^loss The total loss capacity of lithium caused by the negative pole piece and the electrolyte after x-circle circulation is recorded as the total loss capacity;

(3) By total capacity Q of lithium in positive pole piece ^total Total loss capacity Q ^loss First turn calibration capacity Q ₁ And second turn rating capacity Q ₂ And fitting to obtain the discharge capacity Q of the lithium battery in the circulation process ^rete As a function of the number of loop turns x, Q ^rete ＝q ₂ (x) Further obtaining a function of the capacity retention ratio delta of the lithium battery and the number x of cycle turns;

wherein,

Q ₁ is the first circle capacity, Q, of the lithium battery calibrated at 0.33C ^rete Is the actual discharge capacity per cycle of the lithium battery.

2. The method according to claim 1, wherein in the step (3), the total capacity Q of lithium in the positive pole piece ^total ＝aQ ^calc ；

Wherein a is a lithium removal coefficient, and can be obtained by empirical value or discharge capacity and theoretical capacity Q obtained by discharging with 0.01-0.1C low current ^calc The ratio of (A) to (B);

Q ^calc is the theoretical capacity of the positive electrode material calculated by the first principle.

3. According to claimThe method of claim 2, wherein the total lost capacity Q ^loss As a function of the number x of cycles,

Q ^loss ＝M ^loss +N ^loss 。

4. the method according to any one of claims 1 to 3, wherein in the step (3), when Q is ^loss ≤Q ^total －Q ₁ Discharge capacity Q of lithium battery ^rete ＝Q ₁ +(x-1)×(Q ₂ -Q ₁ )；

When Q is ^loss ＞Q ^total －Q ₁ Discharge capacity Q of lithium battery ^rete ＝Q ^total -Q ^loss 。

5. The method of any one of claims 1 to 4, wherein in step (1), the lost capacity M of lithium in the negative electrode sheet of the lithium battery ^loss As a function of the number x of cycles,

M ^loss ＝be ^cx ；

wherein b and c are constants.

6. The method according to any one of claims 1 to 5, wherein in step (1), the lost capacity N of lithium in the electrolyte of the battery ^loss As a function of the number x of cycles,

N ^loss ＝mx ³ +nx ² +kx+p；

wherein m, n, k and p are constants.

7. The method as claimed in claim 5, wherein after the cycle test of the lithium battery to be tested is finished, the content of the residual lithium in the negative electrode plate is tested by adopting an inductive coupling plasma method, a laser-induced breakdown spectroscopy method or an X-ray energy spectroscopy method, and the residual lithium is converted into the loss capacity of the lithium in the negative electrode plate.

8. The method as claimed in claim 6, wherein after the cycle test of the lithium battery to be tested is finished, the content of the residual lithium in the electrolyte is tested by an inductively coupled plasma method, a laser-induced breakdown spectroscopy method or an X-ray energy spectroscopy method, and is converted into the lost capacity of the lithium in the electrolyte.

9. The method according to any one of claims 1 to 8, wherein the temperature of the cycle test is-10 to 60 ℃.

10. Method according to any of claims 1-9, characterized in that the function M is fitted ^loss And N ^loss The number of cycles x is not less than 200.

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