Analysis method for irreversible capacity loss of total battery

Abstract

The invention relates to a method for analyzing irreversible capacity loss of a total battery. According to the invention, the dead lithium amount after the battery cell circulation is obtained by adding the positive electrode active material loss, the negative electrode active material loss, the positive electrode active lithium loss and the negative electrode active lithium loss. The analysis method is simple and accurate in calculation, simple and convenient to test and low in cost, and an effective new analysis method is provided for total battery failure analysis.

Description

Technical Field

[0001] The present invention relates to the field of lithium batteries, and particularly to an analysis method for irreversible capacity loss of a full battery. Background Art

[0002] Cycle life is one of the core indicators of lithium-ion batteries. A longer cycle life means that the battery can satisfy more repeated charge and discharge cycles. However, the number of cycles is ultimately limited, and the capacity attenuation of lithium batteries can only be reduced but cannot be avoided. Therefore, finding out the reasons for cycle failure and improving the cycle life of lithium batteries has great value.

[0003] The factors affecting the cycle performance of the battery mainly include materials, manufacturing processes, and testing methods. Ultimately, it is the failure of materials (such as the positive electrode, negative electrode, separator, electrolyte, etc.) and the occurrence of lithium plating. The battery cycle failure mode can be decomposed into capacity loss caused by increased impedance and capacity loss caused by the loss of active lithium and active materials. Among them, the capacity loss caused by energy is reversible, also known as polarization loss. When discharging with a small current, the capacity loss caused by energy can be restored, while the capacity loss caused by the loss of active lithium and active materials is irreversible, and the capacity cannot be restored even when discharging with a small current.

[0004] Currently, due to the uncertainty of the content of dendritic lithium (dead lithium) generated during the battery cycle, it is impossible to intuitively know the accurate relationship between battery performance attenuation and the content of dendritic lithium, nor can we know the irreversible capacity loss of the battery and the proportion of each part of the loss in the irreversible capacity loss.

[0005] In the prior art, a method for detecting the lithium plating content of a lithium-ion battery is proposed, which includes the following steps: discharging the lithium-ion battery to be tested until the SOC is 0%; fully reacting the negative electrode plate obtained by disassembling the lithium-ion battery to be tested with water to obtain lithium hydroxide; determining the lithium plating content in the negative electrode plate according to the content of lithium hydroxide.

[0006] In addition, a control method for quantitative lithium plating of a lithium-ion battery is also disclosed in the prior art, which includes the following steps: (1) calibrating the initial capacity C0 of the battery, and then discharging the battery to the battery capacity C1 corresponding to the lithium plating amount to be studied; (2) disassembling the discharged battery and taking out the negative electrode plate inside the battery; (3) performing an impurity removal operation on the disassembled negative electrode plate; (4) assembling and combining the negative electrode plate after removing impurities with a newly prepared positive electrode plate to obtain a reassembled lithium-ion battery, and calculating the capacity C2 of the reassembled lithium-ion battery according to the positive electrode plate; (5) calculating the lithium plating amount m of the negative electrode plate according to the battery capacities C0, C1, and C2.

[0007] In the above two methods, only the amount of lithium plating on the negative electrode sheet is disclosed, and how to determine the amount of lithium plating in the full cell and how to obtain the irreversible capacity loss are not disclosed. Summary of the Invention

[0008] Problems to be Solved by the Invention

[0009] In order to comprehensively analyze the reasons for cell failure, it is necessary to know the accurate irreversible capacity loss of the full cell. Therefore, the object of the present invention is to provide a method capable of simply and accurately determining the irreversible capacity loss of the full cell.

[0010] Solutions for Solving the Problems

[0011] The present invention can accurately and ingeniously calculate the irreversible capacity loss of the lithium-ion full cell through simple tests, thus providing an effective new analysis method for the analysis of full cell failure.

[0012] Specifically, the present invention provides an analysis method for the irreversible capacity loss of a full cell, which includes the following steps:

[0013] Step S1: Take the first cell among multiple parallel cells and perform full charge and full discharge. After full discharge, use the positive electrode of the first cell as the positive electrode of BOL cell A, and use the negative electrode of the first cell as the positive electrode of BOL cell B. Among them, the negative electrodes of the BOL cell A and the BOL cell B are both lithium.

[0014] Step S2: Take the second cell among multiple parallel cells and perform charge and discharge cycles. The last cycle of the charge and discharge cycles is full charge and full discharge. After full discharge, use the positive electrode of the second cell as the positive electrode of EOL cell C, and use the negative electrode of the second cell as the positive electrode of EOL cell D. Among them, the negative electrodes of the EOL cell C and the EOL cell D are both lithium.

[0015] Step S3: Charge / discharge the BOL cell A and the EOL cell C under the same conditions to obtain the gram capacity C1 of the positive electrode active material of the BOL cell A and the gram capacity C2 of the positive electrode active material of the EOL cell C. Then, the capacity loss caused by the loss of the positive electrode material is (C1 - C2) × m1, where m1 is the initial total amount of the positive electrode active material.

[0016] Step S4: Charge / discharge the BOL cell B and the EOL cell D respectively under the same charge / discharge conditions as in Step S3 to obtain the gram capacity C3 of the negative electrode active material of the BOL cell B and the gram capacity C4 of the negative electrode active material of the EOL cell D. Then, the capacity loss caused by the loss of the negative electrode material is (C3 - C4) × m2, where m1 is the initial total amount of the negative electrode active material.

[0017] Step S5: Determine the capacity loss C5 caused by the loss of active lithium in the positive electrode;

[0018] Step S6: Determine the capacity loss C6 caused by the loss of active lithium in the negative electrode;

[0019] Step S7: The irreversible capacity loss C of the full cell is: C = (C1 - C2) × m1 + (C3 - C4) × m2 + C5 + C6.

[0020] According to the above analysis method, among them, the capacity loss C5 caused by the loss of active lithium in the positive electrode is obtained by any one of the following methods:

[0021] The capacity loss C5 caused by the loss of active lithium in the positive electrode is C0' - C0, where C0 is the capacity of the first battery cell when fully charged, and C0' is the capacity of the second battery cell when fully charged;

[0022] Respectively determine the positive electrode active lithium capacities C7 and C8 of the EOL battery C and the BOL battery A after charge / discharge in the step S3. Then, the capacity loss caused by the loss of active lithium in the EOL battery C is C 10 - C7, the capacity loss caused by the loss of active lithium in the BOL battery A is C9 - C8, and the capacity loss C5 caused by the loss of active lithium in the positive electrode is (C 10 - C7) - (C9 - C8); where C9 is the capacity of the first battery cell when fully discharged, and C 10 is the capacity of the second battery cell when fully discharged;

[0023] Respectively take the positive electrode of the first battery cell after full charge and the positive electrode of the second battery cell after full charge, and perform ICP testing to obtain the positive electrode lithium capacity of the first battery cell and the positive electrode lithium capacity of the second battery cell. The capacity loss C5 caused by the loss of active lithium in the positive electrode is the positive electrode lithium capacity of the second battery cell minus the positive electrode lithium capacity of the first battery cell.

[0024] According to the above analysis method, among them, the capacity loss C6 caused by the loss of active lithium in the negative electrode is obtained by any one of the following methods:

[0025] The capacity loss C6 caused by the loss of active lithium in the negative electrode is C 10 - C9, where C9 is the capacity of the first battery cell when fully discharged, and C 10 is the capacity of the second battery cell when fully discharged;

[0026] Respectively determine the negative electrode active lithium capacities C 11 and C 12 of the EOL battery D and the BOL battery B after charge / discharge in the step S4. Then, the capacity loss caused by the dead lithium in the negative electrode of the EOL battery D is C0' - C 11 , and the capacity loss caused by the dead lithium in the negative electrode of the BOL battery B is C0 - C 12, the capacity loss C6 caused by the loss of active lithium in the negative electrode is (C0'-C 11 )-(C0-C 12 );

[0027] Take the negative electrode of the first battery cell after full discharge and the negative electrode of the second battery cell after full discharge respectively, and perform ICP testing to obtain the negative electrode lithium capacity of the first battery cell and the negative electrode lithium capacity of the second battery cell. The capacity loss C6 caused by the loss of active lithium in the negative electrode is the negative electrode lithium capacity of the second battery cell minus the negative electrode lithium capacity of the first battery cell.

[0028] According to the above analysis method, among them, the positive electrode active lithium capacities C7 and C8 are obtained by calculating according to the molecular mass ratio of the specific capacity of the positive electrode material.

[0029] According to the above analysis method, among them, the negative electrode active lithium capacities C 11 and C 12 are obtained by calculating according to the molecular mass ratio of the specific capacity of the negative electrode material.

[0030] According to the above analysis method, among them, the charging / discharging in the step S2, the step S3, and the step S4 is carried out under the following conditions: the temperature is 20°C to 30°C, the rate is 0.05C - 0.1C, and the voltage range is 2.7V - 4.2V;

[0031] The number of charging / discharging cycles in the step S2 is 300 to 800 cycles.

[0032] According to the above analysis method, among them, the BOL battery A, the BOL battery B, the EOL battery C, and the EOL battery C are all coin cells.

[0033] Effects of the Invention

[0034] The above technical solutions of the present invention have the following beneficial effects:

[0035] (1) The analysis method for the irreversible capacity loss of the full battery of the lithium battery of the present invention is simple and accurate in calculation.

[0036] (2) The test method used in the analysis method of the present invention is simple and the cost is relatively low.

[0037] (3) The present invention provides an effective new analysis method for the failure analysis of full batteries. Detailed Embodiments

[0038] Hereinafter, the content of the present invention will be described in detail. The description of the technical features recorded below is based on representative embodiments and specific examples of the present invention, but the present invention is not limited to these embodiments and specific examples. It should be noted that:

[0039] In this specification, the numerical range expressed as "numerical value A to numerical value B" means the range including the end point numerical values A and B.

[0040] In this specification, the numerical range expressed as "above" or "below" means the numerical range including this number.

[0041] In this specification, the meaning expressed by "can" includes both the meaning of performing a certain process and not performing a certain process.

[0042] In this specification, the use of "optional" or "optional" means that certain substances, components, execution steps, applied conditions and other factors are used or not used.

[0043] In this specification, the "normal temperature" or "room temperature" used means the indoor environmental temperature of "23 ± 2°C".

[0044] In this specification, the unit names used are all international standard unit names, and if not otherwise stated, the "%" used represents the weight or mass percentage content.

[0045] In this specification, the use of "substantially" and "essentially" means that the standard deviation from the theoretical model, theoretical data or target data is within the numerical range of 3%, preferably 2%, more preferably 1%.

[0046] In this specification, when using the terms "comprise" and / or "include", it indicates the existence of features, steps, operations, devices, components and / or their combinations.

[0047] In this specification, the "some specific / preferred embodiments", "other specific / preferred embodiments", "embodiments", etc. mentioned refer to the specific elements (e.g., features, structures, properties and / or characteristics) related to the embodiment, which are included in at least one of the embodiments described here, and may or may not exist in other embodiments. In addition, it should be understood that the elements can be combined in various embodiments in any suitable manner.

[0048] The present invention mainly calculates the amount of dead lithium after the cycle of the battery cell and the lithium loss caused by each part of the positive and negative electrodes through the inductively coupled plasma (ICP) test and the coulombic efficiency test of the button cell of the battery cell after formation (Beginning of Life, BOL) and after cycling (End of Life, EOL). The test method in the analysis method of the irreversible capacity loss of the full battery of the present invention is simple, low in cost, simple in calculation and accurate in result, providing an effective new analysis method for the failure analysis of the full battery.

[0049] Specifically, the amount of dead lithium after battery cycling is divided into four parts: loss of positive electrode active material, loss of negative electrode active material, positive electrode dead lithium, and negative electrode dead lithium. Adding these four parts together can obtain the amount of dead lithium after cell cycling.

[0050] The analysis method for the irreversible capacity loss of the full battery of the lithium battery of the present invention includes the following steps:

[0051] Step S1: Take the first cell among multiple parallel cells and perform full charge and full discharge. After full discharge, use the positive electrode of the first cell as the positive electrode of BOL battery A, and use the negative electrode of the first cell as the positive electrode of BOL battery B. Among them, the negative electrodes of both BOL battery A and BOL battery B are lithium;

[0052] Step S2: Take the second cell among multiple parallel cells and perform charge-discharge cycling. The last cycle of the charge-discharge cycling is full charge and full discharge. After full discharge, use the positive electrode of the second cell as the positive electrode of EOL battery C, and use the negative electrode of the second cell as the positive electrode of EOL battery D. Among them, the negative electrodes of both EOL battery C and EOL battery D are lithium;

[0053] Step S3: Charge / discharge BOL battery A and EOL battery C under the same conditions to obtain the gram capacity C1 of the positive electrode active material of BOL battery A and the gram capacity C2 of the positive electrode active material of EOL battery C. Then, the capacity loss caused by the loss of the positive electrode material is (C1 - C2) × m1, where m1 is the initial total amount of the positive electrode active material;

[0054] Step S4: Charge / discharge BOL battery B and EOL battery D respectively under the same charge / discharge conditions as in Step S3 to obtain the gram capacity C3 of the negative electrode active material of BOL battery B and the gram capacity C4 of the negative electrode active material of EOL battery D. Then, the capacity loss caused by the loss of the negative electrode material is (C3 - C4) × m2, where m1 is the initial total amount of the negative electrode active material;

[0055] Step S5: Determine the capacity loss C5 caused by the loss of positive electrode active lithium;

[0056] Step S6: Determine the capacity loss C6 caused by the loss of negative electrode active lithium;

[0057] Step S7: The irreversible capacity loss C of the full battery is: C = (C1 - C2) × m1 + (C3 - C4) × m2 + C5 + C6.

[0058] It can be understood that "full charge" refers to the process of charging the cell from 0% SOC to 100% SOC, and "full discharge" refers to the process of discharging the cell from 100% SOC to 0% SOC.

[0059] The following elaborates on each step in detail.

[0060] In steps S1 and S2 of the present invention, for the disassembled positive and negative electrodes, an impurity removal step can be carried out. For example, the disassembled positive and negative electrodes can be immersed in an organic solvent such as dimethyl carbonate for a period of time, such as 0.5 to 2 hours, and then washed and dried. The positive and negative electrodes after impurity removal are used to assemble new batteries A - D.

[0061] The "multiple parallel battery cells" mentioned in steps S1 and S2 can be at least 2 respectively, for example, can be 2, 3, 4, etc. In step S2, the charge - discharge cycle is carried out under the following conditions:

[0062] The temperature is 20°C - 30°C, for example, 25°C;

[0063] The rate is 0.05C - 0.1C, for example, 0.05C, 0.08C, 0.1C, etc.;

[0064] The voltage range is 2.7V - 4.2V, for example, 2.7V, 3.0V, 3.2V, 3.5V, 3.8V, 4.0V, 4.2V, etc.;

[0065] The number of charge - discharge cycles can be 300 - 800 cycles, for example, can be 300 cycles, 400 cycles, 500 cycles, 600 cycles, 700 cycles, 800 cycles, etc.

[0066] The charging / discharging in steps S3 and S4 is carried out under the same conditions. The specific conditions for charging / discharging are as follows:

[0067] The temperature is 20°C - 30°C, for example, 25°C;

[0068] The rate is 0.05C - 0.1C, for example, 0.05C, 0.08C, 0.1C, etc.;

[0069] The voltage range is 2.7V - 4.2V, for example, 2.7V, 3.0V, 3.2V, 3.5V, 3.8V, 4.0V, 4.2V, etc.

[0070] Step S3 is the step of calculating the loss of the positive electrode active material. In step S3, the gram capacity of the active material of the EOL battery C and the BOL battery A are respectively measured. By calculating the difference between the gram capacities before and after, that is, the difference between the two subtracted and then multiplied by the total amount of the initial positive electrode active material, the total loss of the positive electrode active material of the full battery can be obtained.

[0071] Step S4 is the calculation step for the loss of negative electrode active material. In step S4, the gram capacity of the active material of the EOL battery D and the BOL battery B are respectively measured. By calculating the difference in gram capacity before and after, that is, subtracting the two and then multiplying by the total amount of the initial active material of the negative electrode, the total loss of the negative electrode active material of the full battery can be obtained.

[0072] Step S5 is the calculation step for the dead lithium in the positive electrode. It is obtained by the difference between the capacity of the fully charged EOL cell (the capacity of the second fully charged cell) and the capacity of the fully charged BOL cell (the capacity of the first fully charged cell). That is, the capacity loss C5 caused by the loss of active lithium in the positive electrode is C0 ’ -C0, where C0 is the capacity of the first fully charged cell, C0 ’ is the capacity of the second fully charged cell;

[0073] Among them, the capacity C0 of the first fully charged cell and the capacity C0 of the second fully charged cell ’ can obtain the capacity value by fully charging the battery with an electrochemical workstation. However, this method of directly calculating the dead lithium in the positive electrode by the difference in the fully charged cell capacity has a large error. Therefore, optimization scheme 1 and optimization scheme 2 are further proposed:

[0074] Optimization scheme 1: Subtracting the active lithium in the positive electrode from the capacity during full charge can obtain the capacity loss caused by dead lithium in the positive electrode, that is, the dead Li in the positive electrode generated after cycling. Taking the difference between the capacity loss caused by dead lithium in the positive electrode of the EOL and the dead lithium in the positive electrode of the BOL can obtain the capacity loss caused by the loss of active lithium in the positive electrode. That is, respectively determine the positive electrode active lithium capacities C7 and C8 of the EOL battery C and the BOL battery A after charging / discharging in the step S3. Then the capacity loss caused by dead lithium in the positive electrode of the EOL battery C is C 10 -C7, the capacity loss caused by dead lithium in the positive electrode of the BOL battery A is C9 - C8, and the capacity loss C5 caused by the loss of active lithium in the positive electrode is (C 10 -C7)-(C9 - C8); where C0 is the capacity of the first fully charged cell, C0 ’ is the capacity of the second cell, C9 is the capacity of the first fully discharged cell, C 10 is the capacity of the second fully discharged cell.

[0075] Optimization scheme 2 (ICP test method): When the cell is charged, lithium is transferred from the positive electrode to the negative electrode. Therefore, the capacity of the lithium remaining on the positive electrode is the capacity loss caused by the loss of active lithium in the positive electrode; that is, respectively take the positive electrode of the first fully charged cell and the positive electrode of the second fully charged cell after full charge, and perform ICP testing to measure the lithium content on the positive electrode sheet, so as to obtain the positive electrode lithium capacity of the first cell and the positive electrode lithium capacity of the second cell. The capacity loss C5 caused by the loss of active lithium in the positive electrode is the positive electrode lithium capacity of the second cell minus the positive electrode lithium capacity of the first cell.

[0076] In addition, from the capacities obtained above, the BOL positive dead Li / EOL positive dead Li can be obtained.

[0077] Among them, the positive active lithium capacities C7 and C8 can be calculated from the gram capacity of the positive electrode material according to the molecular mass ratio.

[0078] Step S6 is the calculation method for negative dead lithium. It is obtained from the difference between the EOL fully discharged cell capacity (the capacity of the second cell fully discharged) and the BOL fully discharged cell capacity (the capacity of the first cell fully discharged). That is, the capacity loss C6 caused by the loss of negative active lithium is C 10 -C9, where C9 is the capacity of the first cell fully discharged, and C 10 is the capacity of the second cell fully discharged.

[0079] As described in the calculation method of positive dead lithium, the capacity C9 of the first cell fully discharged and the capacity C 10 of the second cell fully discharged can obtain the capacity values by fully charging the battery with an electrochemical workstation. However, this method of directly calculating the positive dead lithium by the difference in the fully charged capacities of the cells has a large error. Therefore, optimization scheme 1 and optimization scheme 2 are further proposed:

[0080] Optimization scheme 1: Subtracting the negative active lithium from the capacity during full discharge can obtain the capacity loss caused by negative dead lithium, that is, the negative dead Li generated after cycling. Taking the difference between the capacity loss caused by EOL negative dead lithium and the BOL negative dead lithium can obtain the capacity loss caused by the loss of negative active lithium. That is, respectively determine the negative active lithium capacities C 11 and C 12 of the EOL cell D and the BOL cell B after charge / discharge in the step S4. Then, the capacity loss caused by negative dead lithium of the EOL cell D is C0'-C 11 , and the capacity loss caused by negative dead lithium of the BOL cell B is C0-C 12 . The capacity loss C6 caused by the loss of negative active lithium is (C0'-C 11 )-(C0-C 12 ).

[0081] Optimization scheme 2: (ICP test method): When the cell discharges, lithium is transferred from the negative electrode to the positive electrode. Therefore, the capacity of lithium remaining on the negative electrode is the capacity loss caused by the loss of negative active lithium. That is, respectively take the negative electrode of the first cell after full discharge and the negative electrode of the second cell after full discharge, and perform ICP tests to measure the lithium content on the negative electrode sheet, so as to obtain the negative lithium capacity of the first cell and the negative lithium capacity of the second cell. The capacity loss C6 caused by the loss of negative active lithium is the negative lithium capacity of the second cell minus the negative lithium capacity of the first cell.

[0082] In addition, from the capacities obtained above, the BOL negative dead Li / EOL negative dead Li can be obtained.

[0083] The negative electrode active lithium capacity C mentioned above 11 and C 12 can be calculated by the molecular mass ratio of the specific capacity of the negative electrode material.

[0084] In step S7, the loss of the positive electrode active material, the loss of the negative electrode active material, the dead lithium of the positive electrode, and the dead lithium of the negative electrode are added together to obtain the amount of lithium deposition in the full cell, that is, the irreversible capacity loss C of the full cell is: C = (C1 - C2) × m1 + (C3 - C4) × m2 + C5 + C6.

[0085] In the present invention, for the convenience of testing, the above-mentioned BOL cell A, BOL cell B, EOL cell C, and EOL cell C are all coin cells.

[0086] Example

[0087] The embodiments of the present invention will be described in detail below in conjunction with the examples. However, those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. Those not specified in the examples are carried out under conventional conditions or conditions recommended by the manufacturer. Those reagents or instruments not specified in the manufacturer are all conventional products that can be obtained through commercial purchase.

[0088] Example 1

[0089] Take 2 groups of parallel battery cells and calculate the amount of lithium deposition in the full cell of this type of battery cell. The calculation method is as follows:

[0090] Take 3 battery cells in the first group. One battery cell is fully charged, and the other 2 battery cells are fully charged and fully discharged respectively. Disassemble the battery cell that is only fully charged, and obtain all the Li capacity (i.e., the full discharge capacity C0) of the positive electrode as 25.41 Ah through ICP testing. Then disassemble one of the fully discharged battery cells, and take out the positive electrode and negative electrode of the battery cell to prepare coin cell A and coin cell B respectively. The positive electrode of coin cell A uses the positive electrode of this battery cell, and the negative electrode uses a lithium sheet. The positive electrode of coin cell B uses the negative electrode of this battery cell, and the negative electrode uses a lithium sheet;

[0091] The ICP instrument is Agilent5110, and the RF power is 1150.

[0092] Take 3 battery cells in the second group. Charge and discharge these 3 battery cells 500 times in a cycle. After the cycle, fully charge one of the battery cells, and fully charge and fully discharge the other 2 battery cells respectively. Disassemble the battery cell that is only fully charged, and obtain all the Li capacity C0 of the positive electrode through ICP testing ’, then disassemble one fully discharged battery cell, take out the positive and negative electrodes of the battery cell to prepare button cell C and button cell D respectively. The positive electrode of button cell C uses the positive electrode of this battery cell, and the negative electrode uses a lithium sheet. The positive electrode of button cell D uses the negative electrode of this battery cell, and the negative electrode uses a lithium sheet;

[0093] Battery cell cycling conditions: 25 °C, 0.05C, voltage range 2.7 - 4.2V.

[0094] Calculation method for loss of positive active material: Subtract the weight of the aluminum foil from the weight of the positive electrode tab of the battery cell, and then multiply by the proportion of the positive active material in the positive electrode slurry for preparing the battery cell. After calculation, the total positive active material of the battery cell is 150 g (i.e., m1). Charge / discharge button cell A (at a rate of 0.05C, voltage range 2.7 - 4.2V) to obtain the gram capacity C1 of the BOL positive active material as 152 mAh / g; similarly, charge / discharge button cell C (at a rate of 0.05C - 0.1C, voltage range 2.7 - 4.2V) to obtain the gram capacity C2 of the EOL positive active material as 140 mAh / g. Calculate the capacity caused by the loss of the positive material through the formula (C1 - C2) × m1, which is 1.8 Ah.

[0095] Calculation method for loss of negative active material: After calculation, the total negative active material of the battery cell is 71 g (i.e., m2). Charge / discharge button cell B (at a rate of 0.05C - 0.1C, voltage range 2.7 - 4.2V) to obtain the gram capacity C3 of the BOL negative active material as 270 mAh / g; similarly, charge / discharge button cell D (at a rate of 0.05C - 0.1C, voltage range 2.7 - 4.2V) to obtain the gram capacity C4 of the EOL negative active material as 210 mAh / g. Calculate the capacity caused by the loss of the negative material through the formula (C3 - C4) × m2, which is 4.26 Ah.

[0096] Calculation method for dead lithium in the positive electrode: When fully discharged, the capacity C0 of all Li in the positive electrode is calculated by ICP as 25.41 Ah; the active lithium capacity C7 of the EOL positive electrode material can be obtained by charging button cell C to obtain the gram capacity of the positive electrode material and calculating according to the percentage of the lithium molecular mass in the positive electrode material molecular mass, and C7 is 24.38 Ah. Through C 10 -C7, the capacity loss caused by dead lithium in the EOL positive electrode is obtained as 1.03 Ah. Similarly, the active lithium capacity C8 of the BOL positive electrode material can be obtained by calculating according to the molecular mass ratio of the positive electrode material gram capacity, which is 25.11 Ah. Through C9 - C8, the capacity loss caused by dead lithium in the BOL positive electrode can be obtained as 0.3 Ah. Taking the difference between the two, the capacity loss C5 caused by the loss of active lithium in the positive electrode is 0.73 Ah.

[0097] Calculation method of dead lithium in the negative electrode: The dead lithium in the negative electrode can be obtained by performing ICP tests on the negative electrodes of the first group of fully discharged battery cells and the second group of fully discharged battery cells, and calculating the difference in the Li amounts of the two before and after cycling. One fully discharged battery cell after 500 cycles of the second group was disassembled, and the lithium capacity of the negative electrode was measured to be 7.06 Ah by ICP test. One fully discharged battery cell of the first group was disassembled, and the lithium capacity of the negative electrode was measured to be 3.10 Ah by ICP test. The difference between the two gives the loss of active lithium C6 caused by the negative electrode as 3.96 Ah.

[0098] Adding the loss of positive electrode active material, the loss of negative electrode active material, the dead lithium in the positive electrode, and the dead lithium in the negative electrode, the irreversible capacity loss C of the full battery is 1.8 + 4.26 + 0.73 + 3.96 = 10.75 Ah.

[0099] Example 2

[0100] Battery cell cycling conditions: 25 °C, 0.05C, voltage range 2.7 - 4.2V.

[0101] Calculation method of loss of positive electrode active material: After calculation, the total positive electrode active material of the battery cell is 151 g (i.e., m1). The specific capacity C1 of the positive electrode active material at BOL of battery A is 140 mAh / g, and the specific capacity C2 of the positive electrode active material at EOL of battery C is 122 mAh / g. The capacity caused by the loss of positive electrode material is calculated by the formula (C1 - C2) × m1 to be 2.7 Ah.

[0102] Calculation method of loss of negative electrode active material: After calculation, the total negative electrode active material of the battery cell is 65 g (i.e., m2). The specific capacity C3 of the negative electrode active material at BOL of battery B is 240 mAh / g, and the specific capacity C4 of the negative electrode active material at EOL of battery D is 180 mAh / g. The capacity caused by the loss of negative electrode material is calculated by the formula (C3 - C4) × m2 to be 3.9 Ah.

[0103] Calculation method of dead lithium in the positive electrode: The dead lithium in the positive electrode can be obtained by calculating the difference in the Li amounts of the two before and after cycling through ICP test. The lithium capacity of the positive electrode of the fully charged battery cell after cycling is measured to be 1.1 Ah by ICP test, and the lithium capacity C 10 of the positive electrode of the fully charged battery cell before cycling is 0.5 Ah. The difference between the two gives the capacity loss C5 caused by the loss of active lithium at the positive electrode as 0.6 Ah.

[0104] Calculation method of dead lithium in the negative electrode: The dead lithium in the negative electrode can be obtained by calculating the difference in the Li amounts of the two before and after cycling through ICP test. The lithium capacity of the negative electrode of the fully discharged battery cell after cycling is measured to be 6.1 Ah by ICP test, and the lithium capacity of the negative electrode of the fully discharged battery cell before cycling is 4.2 Ah. The difference between the two gives the capacity loss C6 caused by the loss of active lithium at the negative electrode as 1.9 Ah.

[0105] Adding the loss of active lithium in the positive electrode, the loss of active lithium in the negative electrode, the dead lithium in the positive electrode, and the dead lithium in the negative electrode, the irreversible capacity loss C of the full cell is 2.7 + 3.9 + 0.6 + 1.9 = 9.1 Ah.

[0106] Industrial Applicability

[0107] The analysis method for the irreversible capacity loss of the full cell of the lithium battery of the present invention is simple to calculate, accurate, and has a relatively low cost, providing an effective new analysis method for the failure analysis of the full cell.

Claims

1. An analysis method for the irreversible capacity loss of a full cell, characterized in that, It includes the following steps: Step S1: Take the first battery cell among multiple parallel battery cells and perform full charge and full discharge. After full discharge, use the positive electrode of the first battery cell as the positive electrode of BOL battery A, and use the negative electrode of the first battery cell as the positive electrode of BOL battery B. Wherein, the negative electrodes of the BOL battery A and the BOL battery B are both lithium; Step S2: Take the second battery cell among multiple parallel battery cells and perform charge-discharge cycles. The last cycle of the charge-discharge cycles is full charge and full discharge. After full discharge, use the positive electrode of the second battery cell as the positive electrode of EOL battery C, and use the negative electrode of the second battery cell as the positive electrode of EOL battery D. Wherein, the negative electrodes of the EOL battery C and the EOL battery D are both lithium; Step S3: Charge / discharge the BOL battery A and the EOL battery C under the same conditions to obtain the gram capacity C1 of the positive electrode active material of the BOL battery A and the gram capacity C2 of the positive electrode active material of the EOL battery C. Then the capacity loss caused by the loss of the positive electrode material is (C1 - C2) × m1, where m1 is the initial total amount of the positive electrode active material; Step S4: Charge / discharge the BOL battery B and the EOL battery D respectively under the same charge / discharge conditions as in Step S3 to obtain the gram capacity C3 of the negative electrode active material of the BOL battery B and the gram capacity C4 of the negative electrode active material of the EOL battery D. Then the capacity loss caused by the loss of the negative electrode material is (C3 - C4) × m2, where m1 is the initial total amount of the negative electrode active material; Step S5: Determine the capacity loss C5 caused by the loss of positive electrode active lithium; Step S6: Determine the capacity loss C6 caused by the loss of negative electrode active lithium; Step S7: The irreversible capacity loss C of the full battery is: C = (C1 - C2) × m1 + (C3 - C4) × m2 + C5 + C6.

2. The analysis method according to claim 1, wherein The capacity loss C5 caused by the loss of positive electrode active lithium is obtained by any one of the following methods: The capacity loss C5 caused by the loss of positive electrode active lithium is C0' - C0, where C0 is the capacity of the first battery cell during full charge, and C0' is the capacity of the second battery cell during full charge; Determine the positive electrode active lithium capacities C7 and C8 of the EOL battery C and the BOL battery A respectively after charging / discharging in the step S3. Then, the capacity loss caused by the active lithium loss of the EOL battery C is C 10 -C7, and the capacity loss caused by the active lithium loss of the BOL battery A is C9 - C8. The capacity loss C5 caused by the positive electrode active lithium loss is (C 10 -C7)-(C9 - C8); where C9 is the full discharge capacity of the first battery cell, and C 10 is the full discharge capacity of the second battery cell; Respectively take the positive electrode of the first battery cell after full charge and the positive electrode of the second battery cell after full charge, and perform ICP tests to obtain the lithium capacity of the positive electrode of the first battery cell and the lithium capacity of the positive electrode of the second battery cell. The capacity loss C5 caused by the loss of positive electrode active lithium is the lithium capacity of the positive electrode of the second battery cell minus the lithium capacity of the positive electrode of the first battery cell.

3. The analysis method according to claim 1 or 2, wherein The capacity loss C6 caused by the loss of negative electrode active lithium is obtained by any one of the following methods: The capacity loss C6 caused by the loss of active lithium in the negative electrode is C 10 - C9, where C9 is the full discharge capacity of the first battery cell, and C 10 is the full discharge capacity of the second battery cell; Determine the negative active lithium capacities C 11 and C 12 of the EOL battery D and the BOL battery B respectively after charging / discharging in the step S4. Then, the capacity loss caused by the dead lithium at the negative electrode of the EOL battery D is C0’ - C 11 , and the capacity loss caused by the dead lithium at the negative electrode of the BOL battery B is C0 - C 12 . The capacity loss C6 caused by the loss of negative active lithium is (C0’ - C 11 ) - (C0 - C 12 ); Respectively take the negative electrode of the first battery cell after full discharge and the negative electrode of the second battery cell after full discharge, and perform ICP tests to obtain the lithium capacity of the negative electrode of the first battery cell and the lithium capacity of the negative electrode of the second battery cell. The capacity loss C6 caused by the loss of negative electrode active lithium is the lithium capacity of the negative electrode of the second battery cell minus the lithium capacity of the negative electrode of the first battery cell.

4. The analysis method according to claim 2, wherein, The positive electrode active lithium capacities C7 and C8 are calculated according to the molecular mass ratio based on the gram capacity of the positive electrode material.

5. The analysis method according to claim 3, wherein The negative electrode active lithium capacity C 11 and C 12 is obtained by calculating according to the molecular mass ratio of the specific capacity of the negative electrode material.

6. The analysis method according to any one of claims 1-5, wherein, The charging / discharging in the step S2, the step S3, and the step S4 is carried out under the following conditions: the temperature is 20°C to 30°C, the rate is 0.05C - 0.1C, and the voltage range is 2.7V - 4.2V; The number of charge-discharge cycles in the step S2 is 300 to 800 cycles.

7. The analysis method according to any one of claims 1-6, wherein, The BOL battery A, the BOL battery B, the EOL battery C, and the EOL battery C are all coin cells.