Failure analysis methods and applications of irreversible capacity decay of lithium-ion batteries

Abstract

This invention provides a failure analysis method and application for irreversible capacity decay in lithium-ion batteries, relating to the field of battery analysis technology. The failure analysis method first converts the discharge capacity of fresh and failed cells into the discharge capacity of fresh and failed coin half-cells, then obtains the total capacity loss of the coin half-cells. Next, the fresh and failed cells are disassembled in a depleted state and fabricated into fresh positive electrode coin half-cells, failed positive electrode coin half-cells, fresh negative electrode coin half-cells, and failed negative electrode coin half-cells. Utilizing the relationship between different capacity losses, the irreversible capacity caused by the positive electrode structure, CEI growth, negative electrode structure, SEI and deactivated Li growth, negative electrode polarization, and lithium dendrite precipitation is calculated. By analyzing the proportion of each capacity loss in the total capacity loss, the capacity loss of a single lithium-ion cell is indirectly reflected, thereby achieving quantitative analysis of irreversible capacity decay.

Description

Technical Field

[0001] This invention belongs to the field of battery analysis technology, and relates to a failure analysis method and application for irreversible capacity decay of lithium-ion batteries. Background Technology

[0002] For lithium-ion batteries, capacity is determined by the combined capacity of the positive and negative electrodes and the amount of usable lithium ions. Capacity decay mechanisms can be broadly categorized into thermodynamic losses and kinetic losses, which are fundamentally different yet mutually influential. Thermodynamic losses are irreversible, such as the loss of active lithium and the active materials of the positive and negative electrodes; kinetic losses are reversible, mainly manifested as increases in impedance and polarization, such as increased ohmic impedance, SEI impedance, and charge transfer impedance caused by electrolyte drying, SEI film thickening, and a reduction in lithium insertion / extraction sites. From a thermodynamic perspective, the irreversible capacity decay of lithium-ion batteries can be attributed to the loss of usable lithium ions (LLI) and the loss of active material (LAM) of the positive and negative electrodes. In lithium-ion batteries with graphite as the negative electrode, LLI is mainly caused by the formation and growth of the SEI film on the graphite surface, and is the primary factor affecting capacity decay. Lithium-ion batteries with LTO negative electrodes typically do not experience LLI. Furthermore, incomplete re-intercalation of lithium after plating, resulting in "dead lithium," can also lead to LLI (Lithium-ionized lithium). LAM (Lithium-ionized aluminum oxide) mainly includes material loss in the positive and negative electrodes due to damage to the crystal structure, disruption of the conductive network, binder failure, and material peeling from the current collector. Generally, LMO positive electrodes and graphite negative electrodes are more susceptible to LAM, while LFP, NCM positive electrodes, and LTO negative electrodes typically experience less LAM.

[0003] Existing patents and technologies mostly study the effects of various impedances and polarizations on the reversible capacity decay of lithium-ion batteries from a kinetic perspective. However, from a thermodynamic perspective, there is a lack of systematic testing and evaluation methods for the irreversible capacity decay of lithium-ion batteries. The latter is not only closer to the essence of lithium-ion battery capacity decay, but also the foundation and prerequisite for the former.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] In view of the shortcomings and defects of the existing technology, the present invention aims to provide a failure analysis method and application for irreversible capacity decay of lithium-ion batteries.

[0006] To achieve the above objectives, the following technical solution is adopted:

[0007] The primary objective of this invention is to provide a failure analysis method for irreversible capacity decay of lithium-ion batteries, comprising the following steps:

[0008] S1. The discharge capacity of the fresh battery cell and the discharge capacity of the failed battery cell are converted into the discharge capacity of the fresh button half-cell and the discharge capacity of the failed button half-cell according to the area, and the total loss capacity of the button half-cell is determined; wherein, the fresh battery cell and the failed battery cell are lithium battery cells.

[0009] S2. Disassemble the fresh and failed cells from step S1, and assemble the positive electrode of the fresh cell and the positive electrode of the failed cell into a fresh positive electrode coin cell and a failed positive electrode coin cell, respectively.

[0010] The positive electrode structure loss capacity of the coin cell is determined based on the discharge capacity of the fresh positive electrode coin cell and the discharge capacity of the failed positive electrode coin cell.

[0011] Based on the discharge capacity of a fresh positive electrode coin cell and the discharge capacity of a fresh coin cell, determine the positive electrode deintercalation-intercalation loss capacity of the fresh coin cell; based on the discharge capacity of a failed positive electrode coin cell and the discharge capacity of a failed coin cell, determine the positive electrode deintercalation-intercalation loss capacity of the failed coin cell.

[0012] S3. Test the negative electrode sheets of the fresh cells and the negative electrode sheets of the failed cells obtained after disassembly to obtain the negative electrode residual capacity of the fresh cells and the negative electrode residual capacity of the failed cells, and convert them into the negative electrode residual capacity of the fresh button half-cell and the negative electrode residual capacity of the failed button half-cell according to the area.

[0013] Based on the positive electrode deintercalation-intercalation loss capacity and the negative electrode residual capacity of a fresh coin cell, determine the positive electrode CEI loss capacity of a fresh coin cell; based on the positive electrode deintercalation-intercalation loss capacity and the negative electrode residual capacity of a failed coin cell, determine the positive electrode CEI loss capacity of a failed coin cell; based on the positive electrode CEI loss capacity of a fresh coin cell and the positive electrode CEI loss capacity of a failed coin cell, determine the loss capacity caused by the increase in positive electrode CEI of a coin cell.

[0014] S4. Assemble the remaining negative electrode sheets of the fresh cell and the remaining negative electrode sheets of the failed cell into a fresh negative electrode button cell and a failed negative electrode button cell, respectively.

[0015] Based on the discharge capacity of fresh negative electrode coin cells and the discharge capacity of failed negative electrode coin cells, the capacity loss caused by negative electrode polarization and lithium dendrites and the capacity loss caused by negative electrode structure of coin cells are determined.

[0016] Based on the residual negative electrode capacity and discharge capacity of a fresh coin cell, determine the negative electrode SEI and deactivated Li loss capacity of the fresh coin cell; based on the residual negative electrode capacity and discharge capacity of a failed coin cell, determine the negative electrode SEI and deactivated Li loss capacity of the failed coin cell; based on the negative electrode SEI and deactivated Li loss capacity of both fresh and failed coin cells, determine the negative electrode SEI and Li loss capacity due to Li deactivation growth of the coin cell.

[0017] S5. By determining the proportions of the positive electrode structure loss capacity, the positive electrode CEI growth loss capacity, the negative electrode structure loss capacity, the negative electrode polarization and lithium dendrite loss capacity, and the negative electrode SEI and deactivated Li growth loss capacity in the total loss capacity of the coin cell, the irreversible capacity decay of lithium-ion batteries containing lithium battery cells can be quantitatively analyzed.

[0018] Furthermore, based on the above-described technical solution of the present invention, in step S1, the discharge capacity C of the fresh battery cell in the nth week of the charge-discharge cycle is... 新鲜 The discharge capacity C of the failed battery cell in the nth week of charge-discharge cycle 失效 The discharge capacity C of a fresh button half-cell in the nth week of charge-discharge cycle is calculated based on its area. 新鲜0 The discharge capacity C of a failed coin cell during the nth cycle of charge-discharge cycles. 失效0 And determine the total loss capacity C of the coin cell. total C total =C 新鲜0 -C 失效0 Wherein, the fresh cell and the failed cell are lithium-ion battery cells, and n is 2 or 3; and / or, in step S1, the failed cell is a cell whose capacity has decayed to 80% SOH.

[0019] Furthermore, based on the above technical solution of the present invention, in step S2, the fresh cell and the failed cell that have undergone charge-discharge cycle in step S1 are disassembled, and the positive electrode of the fresh cell and the positive electrode of the failed cell are assembled into a fresh positive electrode coin cell and a failed positive electrode coin cell, respectively.

[0020] Based on the discharge capacity C of a fresh positive electrode coin cell in week n of a charge-discharge cycle. 新鲜c The discharge capacity C of a failed positive electrode coin cell during the nth cycle of charge-discharge cycles. 失效c Determine the positive electrode structure loss capacity C of the coin cell. cs C cs =C 新鲜c -C 失效c ;

[0021] Based on the discharge capacity C of a fresh positive electrode coin cell in week n of a charge-discharge cycle. 新鲜c The discharge capacity C of a fresh coin cell half-cell during the nth week of charge-discharge cycles. 新鲜0 Determine the positive electrode deintercalation-intercalation loss capacity C of a fresh coin half-cell. 新鲜cr C 新鲜cr =C 新鲜c -C 新鲜0 ;

[0022] Based on the discharge capacity C of the failed positive electrode coin cell in the nth week of charge-discharge cycle. 失效c The discharge capacity C of a failed coin cell during the nth cycle of charge-discharge cycles. 失效0 Determine the positive electrode deintercalation / intercalation loss capacity C of the failed coin cell. 失效cr C 失效cr =C 失效c -C 失效0 .

[0023] Furthermore, based on the above-mentioned technical solution of the present invention, in step S2, the fresh and failed battery cells that have undergone n charge-discharge cycles in step S1 are discharged to 0% SOC and then disassembled to obtain the positive electrode plates of the fresh and failed battery cells, respectively. These are then soaked in a solvent for 1-2 hours, dried in a vacuum oven at 60-90°C for 3-6 hours, and the active material on one side of the positive electrode plates is wiped off with anhydrous ethanol using lint-free paper. Single-sided circular positive electrode plates are punched using a punching machine, and the fresh and failed positive electrode coin cells are assembled in a glove box filled with a protective atmosphere; and / or,

[0024] In step S2, the fresh positive electrode coin cell and the failed positive electrode coin cell use the same charging and discharging current of 0.05-0.2C during charge and discharge cycles; the same upper limit of charging voltage of 3.5-3.8V; and the same lower limit of discharging voltage of 1.0-2.5V.

[0025] Furthermore, based on the above-described technical solution of this invention, in step S3, the negative electrode portions of the fresh battery cell and the failed battery cell obtained after disassembly are respectively tested to obtain the residual negative electrode capacity C of the fresh battery cell. 新鲜ar0 and the residual negative capacity C of the failed battery cell 失效ar0 The residual negative electrode capacity C of a fresh coin cell, converted by area. 新鲜ar The residual negative electrode capacity C of the failed coin cell 失效ar ;

[0026] Based on the positive electrode deintercalation-intercalation loss capacity C of a fresh coin half-cell 新鲜cr The residual capacity C of the negative electrode of a fresh coin cell 新鲜arDetermine the positive electrode CEI loss capacity C of a fresh coin half-cell. 新鲜CEI C 新鲜CEI =C 新鲜cr -C 新鲜ar ;

[0027] Based on the positive electrode deintercalation-intercalation loss capacity C of the failed coin half-cell 失效cr The residual negative electrode capacity C of the failed coin cell 失效ar Determine the positive electrode CEI loss capacity C of the failed coin cell. 失效CEI C 失效CEI =C 失效cr -C 失效ar ;

[0028] Based on the positive electrode CEI loss capacity C of a fresh coin half-cell 新鲜CEI The positive electrode CEI capacity loss C of the failed coin cell 失效CEI Determine the capacity loss C caused by the increase of CEI at the positive electrode of the coin half-cell. CEI C CEI =C 失效CEI -C 新鲜CEI .

[0029] Furthermore, based on the above technical solution of the present invention, in step S4, the remaining negative electrode sheets of the fresh cell and the remaining negative electrode sheets of the failed cell are assembled into a fresh negative electrode button cell and a failed negative electrode button cell, respectively.

[0030] Based on the charging capacity C of the first charge-discharge cycle of a fresh negative electrode coin cell. 新鲜a The charging capacity C in the nth cycle 新鲜ad And the charging capacity C of the first charge-discharge cycle of the failed negative electrode coin cell. 失效a The charging capacity C in the nth cycle 失效ad Determine the capacity loss C caused by negative electrode polarization and lithium dendrite formation in a coin cell. pl C pl =C 失效a -C 新鲜a And the capacity loss C of the negative electrode structure as C as =C 新鲜ad -C 失效ad .

[0031] Furthermore, based on the above-described technical solution of the present invention, in step S4, the residual negative electrode capacity C of the fresh coin cell is determined... 新鲜ar The first charge capacity C of a fresh negative-terminal coin cell 新鲜a Determine the negative electrode SEI and the capacity loss C due to Li deactivation in a fresh coin half-cell. 新鲜SEI C新鲜SEI =C 新鲜ar -C 新鲜a ;

[0032] Based on the residual negative electrode capacity C of the failed coin cell 失效ar The first charge capacity C of a failed negative terminal coin cell 失效a Determine the negative electrode SEI and the capacity loss C due to Li deactivation in the failed coin cell. 失效SEI C 失效SEI =C 失效ar -C 失效a This led to the determination of the negative electrode SEI and the capacity loss C caused by the growth of deactivated Li in the coin cell. SEI C SEI =C 失效SEI -C 新鲜SEI .

[0033] Furthermore, based on the above-mentioned technical solution of the present invention, in step S4, the fresh and failed battery cells that have undergone n charge-discharge cycles in step S1 are discharged to 0% SOC and then disassembled to obtain the negative electrode plates of the fresh and failed battery cells, respectively. These are then soaked in a solvent for 1-2 hours, dried in a vacuum oven at 60-90°C for 3-6 hours, and the active material on one side of the negative electrode plates is wiped off with deionized water using lint-free paper. Single-sided circular negative electrode plates are punched using a punching machine, and the fresh and failed negative electrode coin cells are assembled in a glove box filled with a protective atmosphere; and / or,

[0034] In step S4, the fresh negative electrode coin cell and the failed negative electrode coin cell use the same charging and discharging current of 0.05-0.2C during charge and discharge cycles; the same upper limit of charging voltage of 1.0-2.5V; and the same lower limit of discharging voltage of 0.001-0.1V.

[0035] Furthermore, based on the above-described technical solution of this invention, the lithium-ion battery is any one of a pouch battery, a prismatic battery, or a cylindrical battery; and / or,

[0036] The positive electrode active material of the lithium-ion battery includes one or more of nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, lithium iron phosphate, or lithium manganese oxide; and / or,

[0037] The negative electrode active material of the lithium-ion battery includes one or more of graphite, mesophase carbon microspheres, lithium titanate, hard carbon, silicon oxide, or silicon carbon; and / or,

[0038] The electrolyte of the lithium-ion battery includes one or more of propylene carbonate, ethylene carbonate, methyl ethyl carbonate, or ethyl carbonate as the solvent, one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, or lithium difluorosulfonylimide as the solute, and one or more of vinylene carbonate, fluoroethylene carbonate, 1,3-propanesulfonate lactone, or vinylene sulfate as the additive.

[0039] The second objective of this invention is to provide the application of the failure analysis method for irreversible capacity decay of lithium-ion batteries, which is the subject of the first objective of this invention, in the fields of battery analysis and selection.

[0040] Compared with the prior art, the technical solution of the present invention has at least the following technical effects:

[0041] (1) This invention provides a failure analysis method for irreversible capacity decay of lithium-ion batteries. First, the discharge capacity of fresh cells and failed cells (lithium-ion battery cells) is measured and converted into the discharge capacity of fresh coin half-cells and failed coin half-cells. Then, the total loss capacity of the failed coin half-cells is obtained. Then, the fresh cells and failed cells are disassembled in a depleted state and made into corresponding fresh positive electrode coin half-cells, failed positive electrode coin half-cells, fresh negative electrode coin half-cells, and failed negative electrode coin half-cells. Using the correlation between different loss capacities, the irreversible capacity caused by the positive electrode structure, CEI growth (thickening) at the positive end and the negative electrode structure, SEI and deactivated Li growth (SEI thickening and dead lithium dissolution), negative electrode polarization and lithium dendrite precipitation at the negative end of the coin half-cell is quantitatively calculated. By analyzing the proportion of various loss capacities in the total loss capacity of the coin half-cell, the capacity loss of the lithium-ion battery cells is indirectly reflected, thereby realizing the quantitative analysis of the irreversible capacity decay of lithium-ion batteries containing lithium-ion battery cells. This method addresses the testing and evaluation of irreversible capacity decay in lithium-ion batteries from a thermodynamic perspective, offering the advantage of quantitative analysis.

[0042] (2) This invention provides an application of a failure analysis method for irreversible capacity decay of lithium-ion batteries. Given the advantages of the above-mentioned failure analysis method for irreversible capacity decay of lithium-ion batteries, it has certain guiding significance in the fields of battery analysis and battery development (including the selection of positive and negative electrodes, electrolytes and auxiliary materials during cell development). Attached Figure Description

[0043] Figure 1 This is the electrochemical polarization impedance diagram of a fresh and a failed battery cell when analyzed using EIS testing in Comparative Example 1 of this invention.

[0044] Figure 2 This is a DC internal resistance diagram of a fresh and a failed battery cell when HPPC testing and analysis were performed in Comparative Example 1 of this invention.

[0045] Figure 3 This is a dV / dQ curve of a fresh battery cell and a failed battery cell in Comparative Example 1 of the present invention. Detailed Implementation

[0046] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Process parameters not specifically specified in the following embodiments are generally performed under conventional conditions.

[0047] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0048] Conventional analytical methods such as hybrid pulse DC internal resistance testing (HPPC), electrochemical impedance spectroscopy (EIS), and galvanostatic intermittent titration (GITT) are all qualitative analyses of capacity decay based on ohmic polarization, electrochemical polarization, and solid-state diffusion polarization. These methods can qualitatively analyze the causes of capacity decay from a kinetic perspective (including electrolyte drying, binder failure, SEI thickening, etc.), but they are difficult to quantitatively determine the specific proportion of active lithium ion loss and the percentage of failure of positive and negative electrode materials. Therefore, this invention is proposed.

[0049] According to a first aspect of the present invention, a failure analysis method for irreversible capacity decay of lithium-ion batteries is provided, comprising the following steps:

[0050] S1. The discharge capacity of the fresh cell and the discharge capacity of the failed cell are converted into the discharge capacity of the fresh button cell and the discharge capacity of the failed button cell respectively according to the area, and the total loss capacity of the button cell is determined; wherein, the fresh cell and the failed cell are lithium battery cells.

[0051] S2. Disassemble the fresh and failed cells from step S1, and assemble the positive electrode of the fresh cell and the positive electrode of the failed cell into a fresh positive electrode coin cell and a failed positive electrode coin cell, respectively.

[0052] The positive electrode structure loss capacity of the coin cell is determined based on the discharge capacity of the fresh positive electrode coin cell and the discharge capacity of the failed positive electrode coin cell.

[0053] Based on the discharge capacity of a fresh positive electrode coin cell and the discharge capacity of a fresh coin cell, determine the positive electrode deintercalation-intercalation loss capacity of the fresh coin cell; based on the discharge capacity of a failed positive electrode coin cell and the discharge capacity of a failed coin cell, determine the positive electrode deintercalation-intercalation loss capacity of the failed coin cell.

[0054] S3. Test the negative electrode sheets of the fresh cells and the negative electrode sheets of the failed cells obtained after disassembly to obtain the negative electrode residual capacity of the fresh cells and the negative electrode residual capacity of the failed cells, and convert them into the negative electrode residual capacity of the fresh button half-cell and the negative electrode residual capacity of the failed button half-cell according to the area.

[0055] Based on the positive electrode deintercalation-intercalation loss capacity and the negative electrode residual capacity of a fresh coin cell, determine the positive electrode CEI loss capacity of a fresh coin cell; based on the positive electrode deintercalation-intercalation loss capacity and the negative electrode residual capacity of a failed coin cell, determine the positive electrode CEI loss capacity of a failed coin cell; based on the positive electrode CEI loss capacity of a fresh coin cell and the positive electrode CEI loss capacity of a failed coin cell, determine the loss capacity caused by the increase in positive electrode CEI of a coin cell.

[0056] S4. Assemble the remaining negative electrode pieces of the fresh cell and the remaining negative electrode pieces of the failed cell into a fresh negative electrode coin cell and a failed negative electrode coin cell, respectively.

[0057] Based on the discharge capacity of fresh negative electrode coin cells and the discharge capacity of failed negative electrode coin cells, the capacity loss caused by negative electrode polarization and lithium dendrites and the capacity loss caused by negative electrode structure of coin cells are determined.

[0058] Based on the residual negative electrode capacity and discharge capacity of a fresh coin cell, determine the negative electrode SEI and deactivated Li loss capacity of the fresh coin cell; based on the residual negative electrode capacity and discharge capacity of a failed coin cell, determine the negative electrode SEI and deactivated Li loss capacity of the failed coin cell; based on the negative electrode SEI and deactivated Li loss capacity of both fresh and failed coin cells, determine the negative electrode SEI and Li loss capacity due to Li deactivation growth of the coin cell.

[0059] S5. By determining the proportion of the positive electrode structure loss capacity, the positive electrode CEI growth loss capacity, the negative electrode structure loss capacity, the negative electrode polarization and lithium dendrite loss capacity, and the negative electrode SEI and deactivated Li growth loss capacity in the total loss capacity of the coin cell, the irreversible capacity decay of lithium-ion batteries containing lithium battery cells can be quantitatively analyzed.

[0060] This invention first measures the discharge capacity of fresh and failed battery cells (lithium-ion battery cells) and converts this into the discharge capacity of fresh and failed coin cell half-cells. Then, it obtains the total capacity loss of the failed coin cell half-cell. Next, it disassembles the fresh and failed cells in a depleted state and fabricates corresponding fresh positive electrode coin cell half-cells, failed positive electrode coin cell half-cells, fresh negative electrode coin cell half-cells, and failed negative electrode coin cell half-cells. Using the relationship between different capacity losses, it quantitatively calculates the positive electrode structure and CEI growth (thickness) at the positive terminal of the failed coin cell half-cell. This study investigates the irreversible capacity loss caused by the negative electrode structure, SEI and deactivated Li growth (SEI thickening and dead lithium dissolution), negative electrode polarization, and lithium dendrite precipitation in coin cells. It also analyzes the proportion of various capacity losses in the total capacity loss of coin cells, indirectly reflecting the capacity loss of individual lithium-ion cells. This allows for quantitative analysis of the irreversible capacity decay of lithium-ion batteries containing individual lithium-ion cells. This failure analysis method is not only simple, feasible, and capable of quantitative calculation, but also exhibits good consistency of test data.

[0061] It should be noted that, since various capacity loss measurements require assembling the positive and negative electrodes into positive and negative half-cells respectively, and the negative electrode of the half-cell is usually a lithium metal sheet, which is very reactive and prone to reaction, the assembly of the half-cell needs to be carried out in a vacuum environment free of water and oxygen. Considering the actual assembly conditions, it is easier to assemble the half-cell into a button cell in a glove box. Therefore, when measuring the capacity loss of a single lithium battery cell (such as a pouch cell, a square cell, or a cylindrical cell), it can be disassembled and made into a button cell to indirectly analyze and measure the capacity loss of the single lithium battery cell.

[0062] As an optional embodiment of the technical solution of the present invention, in step S1, the fresh battery cell is the battery cell after being subjected to formation and capacity testing.

[0063] As an optional embodiment of the technical solution of the present invention, in step S1, the failed cell is a cell whose capacity has decayed to 80% SOH.

[0064] As an optional embodiment of the technical solution of the present invention, the failure analysis method for irreversible capacity decay of lithium-ion batteries includes the following steps:

[0065] S1. The discharge capacity C of a fresh battery cell in the nth charge-discharge cycle. 新鲜 The discharge capacity C of the failed battery cell in the nth week of charge-discharge cycle 失效 The discharge capacity C of a fresh button half-cell in the nth week of charge-discharge cycle is calculated based on its area. 新鲜0 The discharge capacity C of a failed coin cell during the nth cycle of charge-discharge cycles. 失效0 And determine the total loss capacity C of the coin cell.total C total =C 新鲜0 -C 失效0 Wherein, the fresh battery cell and the failed battery cell are lithium-ion battery cells, and n is 2 or 3;

[0066] S2. Disassemble the fresh and failed cells that have undergone charge-discharge cycles in step S1, and assemble the positive electrode plates of the fresh and failed cells into fresh positive electrode coin cells and failed positive electrode coin cells, respectively.

[0067] Based on the discharge capacity C of a fresh positive electrode coin cell in week n of a charge-discharge cycle. 新鲜c The discharge capacity C of a failed positive electrode coin cell during the nth cycle of charge-discharge cycles. 失效c Determine the positive electrode structure loss capacity C of the coin cell. cs C cs =C 新鲜c -C 失效c ;

[0068] Based on the discharge capacity C of a fresh positive electrode coin cell in week n of a charge-discharge cycle. 新鲜c The discharge capacity C of a fresh coin cell half-cell during the nth week of charge-discharge cycles. 新鲜0 Determine the positive electrode deintercalation-intercalation loss capacity C of a fresh coin half-cell. 新鲜cr C 新鲜cr =C 新鲜c -C 新鲜0 ;

[0069] Based on the discharge capacity C of the failed positive electrode coin cell in the nth week of charge-discharge cycle. 失效c The discharge capacity C of a failed coin cell during the nth cycle of charge-discharge cycles. 失效0 Determine the positive electrode deintercalation / intercalation loss capacity C of the failed coin cell. 失效cr C 失效cr =C 失效c -C 失效0 ;

[0070] S3. Test the negative electrode portions of the fresh battery cells and the failed battery cells obtained after disassembly to obtain the residual negative electrode capacity C of the fresh battery cells. 新鲜ar0 and the residual negative capacity C of the failed battery cell 失效ar0 The residual negative electrode capacity C of a fresh coin cell, converted by area. 新鲜ar The residual negative electrode capacity C of the failed coin cell 失效ar ;

[0071] Based on the positive electrode deintercalation-intercalation loss capacity C of a fresh coin half-cell 新鲜cr The residual capacity C of the negative electrode of a fresh coin cell 新鲜arDetermine the positive electrode CEI loss capacity C of a fresh coin half-cell. 新鲜CEI C 新鲜CEI =C 新鲜cr -C 新鲜ar ;

[0072] Based on the positive electrode deintercalation-intercalation loss capacity C of the failed coin half-cell 失效cr The residual negative electrode capacity C of the failed coin cell 失效ar Determine the positive electrode CEI loss capacity C of the failed coin cell. 失效CEI C 失效CEI =C 失效cr -C 失效ar ;

[0073] Based on the positive electrode CEI loss capacity C of a fresh coin half-cell 新鲜CEI The positive electrode CEI capacity loss C of the failed coin cell 失效CEI Determine the capacity loss C caused by the increase of CEI at the positive electrode of the coin half-cell. CEI C CEI =C 失效CEI -C 新鲜CEI ;

[0074] S4. Assemble the remaining negative electrode sheets of the fresh cell and the remaining negative electrode sheets of the failed cell into a fresh negative electrode button cell and a failed negative electrode button cell, respectively.

[0075] Based on the charging capacity C of the first charge-discharge cycle of a fresh negative electrode coin cell. 新鲜a The charging capacity C in the nth cycle 新鲜ad And the charging capacity C of the first charge-discharge cycle of the failed negative electrode coin cell. 失效a The charging capacity C in the nth cycle 失效ad Determine the capacity loss C caused by negative electrode polarization and lithium dendrite formation in a coin cell. pl C pl =C 失效a -C 新鲜a And the capacity loss C of the negative electrode structure as C as =C 新鲜ad -C 失效ad ;

[0076] Based on the residual negative electrode capacity C of a fresh coin cell 新鲜ar The first charge capacity C of a fresh negative-terminal coin cell 新鲜a Determine the negative electrode SEI and the capacity loss C due to Li deactivation in a fresh coin half-cell. 新鲜SEI C 新鲜SEI =C 新鲜ar -C 新鲜a ;

[0077] Based on the residual negative electrode capacity C of the failed coin cell 失效ar The first charge capacity C of a failed negative terminal coin cell 失效a Determine the negative electrode SEI and the capacity loss C due to Li deactivation in the failed coin cell. 失效SEI C 失效SEI =C 失效ar -C 失效a This led to the determination of the negative electrode SEI and the capacity loss C caused by the growth of deactivated Li in the coin cell. SEI C SEI =C 失效SEI -C 新鲜SEI .

[0078] S5. Determining the capacity loss C of the positive electrode structure in a coin cell. cs Capacity loss due to positive electrode CEI growth C CEI negative electrode structure loss capacity C as Capacity loss C due to negative electrode polarization and lithium dendrites pl Capacity loss C due to the growth of negative electrode SEI and deactivated Li SEI In total loss capacity C total The proportion of [the component] is used to quantitatively analyze the irreversible capacity decay of lithium-ion batteries containing individual lithium-ion cells.

[0079] It should be noted that in step S1, n in the nth week of the charge-discharge cycle is usually 2 or 3. This is mainly because the ambient temperature of the storage environment after the fresh battery cell is produced may affect the battery cell capacity (the battery cell discharge capacity is higher at high temperatures and lower at low temperatures). Therefore, by cyclically charging and discharging the battery cell at a constant temperature (e.g., 25°C) for 2 or 3 weeks, the battery cell can be activated and its capacity can be kept stable.

[0080] Furthermore, for a freshly produced battery cell (after capacity testing), its capacity is generally considered to remain almost unchanged after the 2nd, 3rd, or 6th cycle. This is mainly because a fresh battery cell can cycle for more than 2500 cycles, and even after 2500 cycles, the cell's capacity retention rate is generally still above 80%. Therefore, the capacity of a fresh battery cell after the 2nd or 3rd charge-discharge cycle and its capacity after the 6th cycle can be considered unchanged. Based on this, the total loss capacity C measured in step S1 of this invention... total This corresponds to the capacity loss in subsequent steps S2-S5, which involve assembling a coin cell half-cell from fresh and failed cells that have undergone n charge-discharge cycles in step S1 and then performing another n charge-discharge cycles.

[0081] Compared to traditional qualitative analysis methods, this invention, by disassembling lithium-ion battery cells into empty cells and fabricating positive and negative electrode coin cells, can accurately analyze the fate of lost active lithium, including the proportion of capacity loss caused by SEI growth and dead lithium, the proportion of capacity loss caused by CEI growth, the proportion of capacity loss caused by negative electrode polarization and lithium dendrite precipitation, the proportion of capacity loss caused by positive electrode material loss, and the proportion of capacity loss caused by negative electrode material loss. This has certain guiding significance for the selection of positive and negative electrode materials and electrolytes in the development of lithium-ion battery cell platform systems.

[0082] As an optional embodiment of the technical solution of the present invention, in step S1, the lithium battery cell is selected from one of the following: pouch cell, square cell, or cylindrical cell.

[0083] As an optional embodiment of the technical solution of the present invention, in step S2, the fresh cell and the failed cell that have undergone n charge-discharge cycles in step S1 are discharged to 0% SOC and then disassembled to obtain the positive electrode sheet of the fresh cell and the positive electrode sheet of the failed cell. They are soaked in a solvent (e.g., DMC, ethanol or deionized water) for 1-2 hours, and then dried in a vacuum oven at 60-90℃ for 3-6 hours. The active material on one side of the positive electrode sheet (positive electrode sheet of the fresh cell and positive electrode sheet of the failed cell) is wiped off with anhydrous ethanol using lint-free paper. A single-sided circular positive electrode sheet is punched out using a punching machine. The fresh positive electrode coin cell and the failed positive electrode coin cell are assembled in a glove box filled with a protective atmosphere.

[0084] As an optional embodiment of the technical solution of the present invention, in step S2, the fresh positive electrode coin cell and the failed positive electrode coin cell (positive electrode coin cell) use the same charging and discharging current of 0.05-0.2C during charge and discharge cycles; the same upper limit of charging voltage of 3.5-3.8V; and the same lower limit of discharging voltage of 1.0-2.5V.

[0085] As an optional embodiment of the technical solution of the present invention, in step S3, some negative electrode sheets of the fresh battery cell obtained after disassembling the battery and some negative electrode sheets of the failed battery cell are scraped to remove powder, and ICP detection is performed to obtain the residual negative electrode capacity C of the fresh battery cell. 新鲜ar0 and the residual negative capacity C of the failed battery cell 失效ar0 .

[0086] As an optional embodiment of the technical solution of the present invention, in step S4, the fresh cell and the failed cell that have undergone n charge-discharge cycles in step S1 are discharged to 0% SOC and then disassembled to obtain the negative electrode sheet of the fresh cell and the negative electrode sheet of the failed cell. They are soaked in a solvent (e.g., DMC, ethanol or deionized water) for 1-2 hours, and then dried in a vacuum oven at 60-90℃ for 3-6 hours. The active material on one side of the negative electrode sheet (the negative electrode sheet of the fresh cell and the negative electrode sheet of the failed cell) is wiped off with deionized water using lint-free paper. A single-sided circular negative electrode sheet is punched out using a punching machine. The fresh negative electrode button half-cell and the failed negative electrode button half-cell are assembled in a glove box filled with a protective atmosphere.

[0087] As an optional embodiment of the technical solution of the present invention, in step S4, the fresh negative electrode coin cell and the failed negative electrode coin cell (negative electrode coin cell) use the same charging and discharging current, which is 0.05-0.2C, during the charge and discharge cycle; the same upper limit of the charging voltage, which is 1.0-2.5V; and the same lower limit of the discharging voltage, which is 0.001-0.1V.

[0088] The failure analysis method for irreversible capacity decay of lithium-ion batteries provided by this invention can be used for failure analysis of various types of lithium-ion batteries. As an optional embodiment of the technical solution of this invention, the lithium-ion battery is selected from any one of pouch batteries, prismatic batteries, or cylindrical batteries.

[0089] As an optional embodiment of the technical solution of the present invention, the positive electrode active material of the lithium-ion battery includes one or more of nickel-cobalt-manganese ternary materials (NCM), nickel-cobalt-aluminum ternary materials (NCA), lithium iron phosphate (LFP), or lithium manganese oxide (LMO).

[0090] As an optional embodiment of the technical solution of the present invention, the negative electrode active material of the lithium-ion battery includes graphite, mesophase carbon microspheres (MCMB), lithium titanate (LTO), hard carbon, and silicon oxide (Si). x One or more of O or silicon-carbon (SiC).

[0091] As an optional embodiment of the technical solution of the present invention, the electrolyte of the lithium-ion battery includes one or more of propylene carbonate (PC), ethylene carbonate (EC), ethyl methyl carbonate (EMC), or ethyl carbonate (DMC) as the solvent, one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), or lithium bis(fluorosulfonyl)imide (LiFSI) as the solute, and one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propanesulfonate lactone (PS), or vinylene sulfate (DTD) as the additive.

[0092] As an optional embodiment of the technical solution of the present invention, the positive electrode active material of the above-mentioned lithium-ion battery is lithium iron phosphate, and the negative electrode active material is artificial graphite.

[0093] As an optional embodiment of the technical solution of the present invention, the above-mentioned lithium-ion battery positive electrode sheet contains the following components in the following mass fractions: 96%-98% lithium iron phosphate, 0.5%-2% conductive carbon black, and 1.5%-2.5% polyvinylidene fluoride (PVDF).

[0094] As an optional embodiment of the technical solution of the present invention, the above-mentioned lithium-ion battery negative electrode sheet contains the following components in the following mass fractions: 95%-97% artificial graphite, 0.5%-1.5% conductive carbon black, 1.0%-2.0% sodium carboxymethyl cellulose (CMC), and 1.5%-2.5% styrene-butadiene rubber (SBR).

[0095] As an optional embodiment of the technical solution of the present invention, the surface density of the coating of the above-mentioned lithium-ion battery positive electrode sheet is 340-400 g / m². 2 (e.g., 340g / m) 2 350g / m 2 360g / m 2 370g / m 2 380g / m 2 390g / m 2 400g / m 2 wait).

[0096] As an optional embodiment of the technical solution of the present invention, the surface density of the coating of the above-mentioned lithium-ion battery negative electrode sheet is 140-200 g / m². 2 (e.g., 140g / m 2 150g / m 2 160g / m 2 170g / m 2 180g / m 2 190g / m 2 200g / m 2 wait).

[0097] As an optional embodiment of the technical solution of the present invention, the compaction density of the above-mentioned lithium-ion battery positive electrode is 2.3-2.8 g / cm³. 3 (e.g., 2.3g / cm) 3 2.4g / cm 3 2.5g / cm 3 2.6g / cm 3 2.7g / cm 3 2.8g / cm 3 wait).

[0098] As an optional embodiment of the technical solution of the present invention, the compaction density of the above-mentioned lithium-ion battery negative electrode is 1.45-1.75 g / cm³. 3 (e.g., 1.45g / cm) 3 1.5g / cm 3 1.55g / cm 3 1.6g / cm 3 1.65g / cm 3 1.7g / cm 3 1.75g / cm 3 wait).

[0099] According to a second aspect of the present invention, the failure analysis method for irreversible capacity decay of lithium-ion batteries provided in the first aspect of the present invention is also provided for application in the fields of battery analysis and selection.

[0100] Given the advantages of the failure analysis method for irreversible capacity decay of lithium-ion batteries provided by this invention, it can be used in the material selection for the development of lithium-ion battery platform systems.

[0101] The present invention will now be described in further detail with reference to specific embodiments and comparative examples.

[0102] Example 1

[0103] This embodiment provides a failure analysis method for irreversible capacity decay of lithium-ion batteries (lithium iron phosphate-graphite pouch cells). The pouch cell uses lithium iron phosphate as the positive electrode and artificial graphite as the negative electrode. It is fabricated using a stacking process to produce a pouch cell with 13 negative electrode layers and 12 positive electrode layers.

[0104] The positive electrode ratio is LiFePO4:SP:PVDF = 97.0%:1.0%:2.0%;

[0105] The negative electrode ratio is C:SP:CMC:SBR = 96.2%:1.0%:1.2%:1.6%;

[0106] The coating surface density of the positive electrode sheet is 381 g / m². 2 ;

[0107] The coating surface density of the negative electrode sheet is 179 g / m². 2 ;

[0108] The compaction density of the positive electrode sheet is 2.54 g / cm³. 3 ;

[0109] The compaction density of the negative electrode sheet is 1.62 g / cm³. 3 .

[0110] The above-mentioned failure analysis method for irreversible capacity degradation of pouch cells specifically includes the following steps:

[0111] S1. Take one fresh (pouch cell) and one cell that has cycled to 80% SOH (cells with 80.0% SOH after 2311 cycles at 35℃ 1C / 1C, hereinafter referred to as failed (pouch cells)). Place them in a test cabinet (Xinwei test cabinet) for 8 hours, discharge them to 2.5V using a constant current of 0.1C, let them rest for 10 minutes, charge them to 3.65V using a constant current and constant voltage of 0.1C, cut off the current at 0.05C, let them rest for 10 minutes, and then discharge them to 2.5V using a constant current of 0.1C again. Cycle for 3 weeks. Record the 0.1C discharge capacity of the fresh cell and the failed cell in the third week of charge-discharge cycle. Based on the positive electrode area of ​​the pouch cell and the electrode area of ​​the coin cell, calculate the 0.1C discharge capacity C of a fresh coin cell with a diameter of 12mm in the third week of charge-discharge cycle. 新鲜0 The 0.1C discharge capacity C of the failed coin cell during the third week of charge-discharge cycles. 失效0 And calculate the total loss capacity C of the coin cell. total C total =C 新鲜0 -C 失效0 The charge-discharge cycle constant temperature (charge-discharge cycle test) is 25°C.

[0112] S2. After discharging both the fresh and failed cells from step S1 to 0% SOC (empty discharge disassembly), disassemble them to obtain the positive electrode sheet. Soak it in DMC for 1 hour, then dry it in an 80℃ vacuum oven for 4 hours. Remove it and wipe off the active material on one side of the positive electrode sheet with lint-free paper dipped in anhydrous ethanol. Punch a single-sided circular positive electrode sheet using a punching machine. Assemble the positive electrode coin cell in a glove box filled with a protective atmosphere. The specific assembly method is as follows:

[0113] Positive electrode coin cells were prepared by assembling a single-sided circular positive electrode sheet (lithium iron phosphate electrode sheet), a separator, a lithium sheet, a gasket, and a spring sheet in that order, corresponding to fresh positive electrode coin cells and failed positive electrode coin cells, respectively. The lithium iron phosphate electrode sheet was cut to a diameter of 12 mm. The lithium iron phosphate electrode sheet in the fresh positive electrode coin cell was derived from the positive electrode sheet of a fresh cell, and the lithium iron phosphate electrode sheet in the failed positive electrode coin cell was derived from the positive electrode sheet of a failed cell. A single-sided ceramic polyethylene (PE) base membrane was used as the separator. The electrolyte was a 1 mol / L lithium hexafluorophosphate (LiPF6) solution, and the solvent was a volume mixture of ethylene carbonate (EC) and ethyl carbonate (DMC) in a 1:1 ratio.

[0114] Fresh positive electrode coin cell and degraded positive electrode coin cell were placed in a test chamber for 8 hours. They were then discharged to 2.0V using a constant current of 0.1C, allowed to rest for 10 minutes, and then charged to 3.75V using a constant current and constant voltage of 0.1C. The cutoff current was 0.05C, and the cells were allowed to rest for 10 minutes. This cycle was repeated for 3 weeks. The discharge capacity C of the fresh positive electrode coin cell in the third week was recorded. 新鲜c The discharge capacity C of the failed positive electrode coin cell in the third week 失效c The positive electrode structure loss capacity C of the coin cell can be calculated. cs C cs =C 新鲜c -C 失效c ; and based on the discharge capacity C of a fresh positive electrode coin cell in the third week. 新鲜c The discharge capacity C of a fresh coin cell in week 3 新鲜0 Calculate the positive electrode deintercalation loss capacity C of a fresh coin half-cell. 新鲜cr C 新鲜cr =C 新鲜c -C 新鲜0 Based on the discharge capacity C of the failed positive electrode coin cell in the third week... 失效c The discharge capacity C of the failed coin cell in week 3 失效0 Calculate the positive electrode deintercalation / intercalation loss capacity C of the failed coin cell. 失效cr C 失效cr =C 失效c -C 失效0 ;

[0115] S3. After disassembling the fresh and failed battery cells, scrape off the powder from the negative electrode sheets obtained separately and perform ICP testing to obtain the residual negative electrode capacity C of the fresh battery cell. 新鲜ar0 and the residual negative capacity C of the failed battery cell 失效ar0 The residual negative electrode capacity C of a fresh coin cell, converted by area. 新鲜ar The residual negative electrode capacity C of the failed coin cell 失效ar ;

[0116] Based on the positive electrode deintercalation-intercalation loss capacity C of a fresh coin half-cell 新鲜cr The residual capacity C of the negative electrode of a fresh coin cell 新鲜ar Determine the positive electrode CEI loss capacity C of a fresh coin half-cell. 新鲜CEI C 新鲜CEI =C 新鲜cr -C 新鲜ar ;

[0117] Based on the positive electrode deintercalation-intercalation loss capacity C of the failed coin half-cell 失效cr The residual negative electrode capacity C of the failed coin cell 失效ar Determine the positive electrode CEI loss capacity C of the failed coin cell.失效CEI C 失效CEI =C 失效cr -C 失效ar ;

[0118] Based on the positive electrode CEI loss capacity C of a fresh coin half-cell 新鲜CEI The positive electrode CEI capacity loss C of the failed coin cell 失效CEI Determine the capacity loss C caused by the increase of CEI at the positive electrode of the coin half-cell. CEI C CEI =C 失效CEI -C 新鲜CEI ;

[0119] S4. Assemble the remaining negative electrode sheets of the fresh battery cell and the remaining negative electrode sheets of the failed battery cell into a fresh negative electrode coin cell and a failed negative electrode coin cell, respectively. The specific assembly method is as follows:

[0120] A negative electrode coin cell was prepared by assembling graphite electrodes, separators, lithium sheets, gaskets, and spring contacts in the following order. The graphite electrodes were cut to a diameter of 12 mm. The graphite electrodes in the fresh negative electrode coin cell were derived from the negative electrode electrodes of fresh cells, while those in the failed negative electrode coin cell were derived from the negative electrode electrodes of failed cells. A single-sided ceramic polyethylene (PE) membrane was used as the separator. The electrolyte was a 1 mol / L lithium hexafluorophosphate (LiPF6) solution, with the solvent being a volume mixture of ethylene carbonate (EC) and ethyl carbonate (DMC) in a 1:1 ratio.

[0121] Fresh negative electrode coin cell and failed negative electrode coin cell were placed in a test chamber for 8 hours. They were then charged to 0.005V using a constant current of 0.1C, rested for 10 minutes, and then charged to 2.0V using a constant current and constant voltage of 0.1C, with a cutoff current of 0.05C. After resting for 10 minutes, the charge capacity C of the fresh negative electrode coin cell was recorded for the first charge-discharge cycle. 新鲜a The charging capacity C in the third cycle 新鲜ad The 0.1C charge capacity C of the failed negative electrode coin cell during the first week of charge-discharge cycles. 失效a The charging capacity C in the third cycle 失效ad ;

[0122] Based on the charge capacity C of the first week of charge-discharge cycle of a fresh negative electrode coin cell. 新鲜a The charging capacity C of the failed negative electrode coin cell during the first week of charge-discharge cycle 失效a Calculate the capacity loss C of a coin cell caused by negative electrode polarization and lithium dendrite formation. pl C pl, =C 失效a -C 新鲜a ;

[0123] Based on the charge capacity C of a fresh negative electrode coin cell during the third week of charge-discharge cycles. 新鲜ad The charging capacity C of a failed negative electrode coin cell during the third week of charge-discharge cycles. 失效ad Calculate the negative electrode structure capacity loss C of a coin cell. as C as =C 失效ad -C 新鲜ad ;

[0124] Based on the residual negative electrode capacity C of a fresh coin cell 新鲜ar The first charge capacity C of a fresh negative-terminal coin cell 新鲜a Determine the negative electrode SEI and the capacity loss C due to Li deactivation in a fresh coin half-cell. 新鲜SEI C 新鲜SEI =C 新鲜ar -C 新鲜a ;

[0125] Based on the residual negative electrode capacity C of the failed coin cell 失效ar The first charge capacity C of a failed negative terminal coin cell 失效a Determine the negative electrode SEI and the capacity loss C due to Li deactivation in the failed coin cell. 失效SEI C 失效SEI =C 失效ar -C 失效a This led to the determination of the negative electrode SEI and the capacity loss C caused by the growth of deactivated Li in the coin cell. SEI C SEI =C 失效SEI -C 新鲜SEI .

[0126] S5. Determine the capacity loss C of the positive electrode structure of the coin half-cell. cs Capacity loss due to positive electrode CEI growth C CEI negative electrode structure loss capacity C as Capacity loss C due to negative electrode polarization and lithium dendrites pl Capacity loss C due to the growth of negative electrode SEI and deactivated Li SEI In total loss capacity C total The proportion of [the component] is used to quantitatively analyze the irreversible capacity decay of lithium-ion batteries containing individual lithium-ion cells.

[0127] The data obtained from the test in this embodiment is as follows:

[0128] The 0.1C discharge capacity of fresh and degraded cells in the third week of charge-discharge cycles is used to calculate the corresponding coin cell capacity based on area conversion. This gives the discharge capacity C of a fresh coin cell in the third week of charge-discharge cycles. 新鲜0 The discharge capacity C of the failed coin cell half-cell in the third week of charge-discharge cycle is 2.280mAh.失效0 The capacity is 1.910mAh, and the total capacity loss C of the coin cell is calculated. total It has a capacity of 0.370mAh.

[0129] The discharge capacity C of a fresh positive coin cell in week 3 新鲜c The discharge capacity C of the 2.798mAh coin cell with the failed positive electrode in the third week is... 失效c The capacity loss C of the positive electrode structure of the coin cell is calculated to be 2.775 mAh. cs The capacity is 0.023 mAh, and the positive electrode structure loss accounts for 6.22%. The positive electrode deintercalation / intercalation loss capacity C of the fresh coin cell is calculated. 新鲜cr The positive electrode deintercalation / intercalation loss capacity C of the failed coin cell is 0.518mAh. 失效cr It has a capacity of 0.865mAh.

[0130] The negative electrode residual capacity of the detected fresh battery cell is converted into the negative electrode residual capacity C of a fresh coin cell based on its area. 新鲜ar The capacity is 0.3620mAh. The residual negative electrode capacity of the detected failed cell is converted into the residual negative electrode capacity C of the failed coin cell based on its area. 失效ar The capacity loss C at the positive electrode of the fresh coin cell is 0.6644 mAh; 新鲜CEI The capacity loss C at the positive electrode of the failed coin cell is 0.1560 mAh. 失效CEI The capacity loss due to the increase in positive electrode CEI is 0.2006mAh; C CEI The capacity is 0.0446mAh, and the loss due to the increase in positive electrode CEI accounts for 12.05%.

[0131] The capacity of a fresh negative-pole coin cell during its first week of 0.1C charging is C. 新鲜a 0.000mAh, the 0.1C charging capacity of a failed negative electrode coin cell half-cell in the first week. 失效a Given a capacity of 0.000mAh, calculate the capacity loss C of the coin cell caused by negative electrode polarization and lithium dendrite formation. pl The capacity is 0.000mAh; the 0.1C charging capacity of the fresh negative electrode coin cell in the 3rd week is C. 新鲜ad The 0.1C charging capacity of the failed negative electrode coin cell half-cell is 2.857mAh, which is the 0.1C charging capacity in the third week. 失效ad Given a capacity of 2.854 mAh, calculate the negative electrode structure capacity loss C of the coin cell. as The capacity is 0.003mAh, and the loss rate of the negative electrode structure is 0.81%.

[0132] The negative electrode SEI and capacity loss C of coin half-cell due to Li deactivation 新鲜SEIThe capacity loss C from the SEI of the failed cell's negative electrode and the deactivated Li is 0.3620mAh. 失效SEI The capacity C of the coin cell was calculated to be 0.6644 mAh, which is the negative electrode SEI and the capacity loss due to the growth of deactivated Li. SEI The capacity is 0.3024mAh, and the capacity loss due to the growth of the negative electrode SEI and deactivated Li accounts for 81.73%.

[0133] The details are shown in Table 1.

[0134] Table 1

[0135]

[0136] The data in the table shows that the thickening of the negative electrode SEI film and the formation of dead lithium are the main reasons for the irreversible capacity decay of lithium-ion batteries. Therefore, the proper combination of negative electrode graphite and electrolyte and the formation process to form a thin and dense SEI film are of great significance.

[0137] The above analytical methods are applicable not only to lithium iron phosphate lithium-ion battery systems, but also to other mainstream lithium-ion battery systems such as ternary systems, which will not be elaborated here.

[0138] Comparative Example 1

[0139] The difference from Example 1 is that the cause of capacity decay is qualitatively analyzed through impedance analysis (AC impedance test (EIS), hybrid pulse DC internal resistance test (HPPC) and ohmic internal resistance test (ACR)) and dV / dQ.

[0140] Specifically, electrochemical impedance spectroscopy was first performed on the fresh and failed battery cells of Example 1, and the test results were as follows: Figure 1 , Figure 2 As shown in Table 2.

[0141] Among them, EIS was used to analyze the electrochemical polarization impedance of fresh and failed battery cells, and the specific results are as follows: Figure 1 As shown in the figure. The test results in the figure show that, compared to fresh cells, the R of pouch cells... sei and R ct There has been a significant increase.

[0142] The DC internal resistance of fresh and failed battery cells was analyzed using HPPC testing (2C 10s). Specific results are as follows: Figure 2 As shown in the figure, the DCR of the battery cell increases dramatically after 2000 cycles.

[0143] Meanwhile, the AC internal resistance of fresh and failed cells was analyzed using ACR, and the specific results are shown in Table 2.

[0144] Table 2

[0145] battery cells Fresh battery cells Failed battery cells ACR(mΩ) 4.561 5.163

[0146] In addition, dV / dQ analysis of fresh and failed pouch cells yielded similar results. Figure 3 As shown in the figure, the shift in peak position indicates a certain loss of negative electrode material, with significant loss of active lithium.

[0147] The above analysis shows that as cycling continues, the ohmic internal resistance, electrochemical polarization, and solid-phase diffusion impedance of the battery cell all increase continuously. Furthermore, the negative electrode material suffers some loss due to continuous cycling, while active lithium is lost in large quantities during cycling. However, the above analysis cannot quantitatively analyze the specific destination of the lost active lithium or the specific loss ratio of the positive and negative electrode materials.

[0148] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention shall be within the scope of protection of the pending claims of the present invention.

Claims

1. A failure analysis method for irreversible capacity decay of lithium-ion batteries, characterized in that, Includes the following steps: S1. The discharge capacity of the fresh battery cell and the discharge capacity of the failed battery cell are converted into the discharge capacity of the fresh button half-cell and the discharge capacity of the failed button half-cell according to the area, and the total loss capacity of the button half-cell is determined; wherein, the fresh battery cell and the failed battery cell are lithium battery cells. S2. Disassemble the fresh and failed cells from step S1, and assemble the positive electrode of the fresh cell and the positive electrode of the failed cell into a fresh positive electrode coin cell and a failed positive electrode coin cell, respectively. The positive electrode structure loss capacity of the coin cell is determined based on the discharge capacity of the fresh positive electrode coin cell and the discharge capacity of the failed positive electrode coin cell. Based on the discharge capacity of a fresh positive electrode coin cell and the discharge capacity of a fresh coin cell, determine the positive electrode deintercalation-intercalation loss capacity of the fresh coin cell; based on the discharge capacity of a failed positive electrode coin cell and the discharge capacity of a failed coin cell, determine the positive electrode deintercalation-intercalation loss capacity of the failed coin cell. S3. Test the negative electrode sheets of the fresh cells and the negative electrode sheets of the failed cells obtained after disassembly to obtain the negative electrode residual capacity of the fresh cells and the negative electrode residual capacity of the failed cells, and convert them into the negative electrode residual capacity of the fresh button half-cell and the negative electrode residual capacity of the failed button half-cell according to the area. Based on the positive electrode deintercalation-intercalation loss capacity and the negative electrode residual capacity of a fresh coin cell, determine the positive electrode CEI loss capacity of a fresh coin cell; based on the positive electrode deintercalation-intercalation loss capacity and the negative electrode residual capacity of a failed coin cell, determine the positive electrode CEI loss capacity of a failed coin cell; based on the positive electrode CEI loss capacity of a fresh coin cell and the positive electrode CEI loss capacity of a failed coin cell, determine the loss capacity caused by the increase in positive electrode CEI of a coin cell. S4. Assemble the remaining negative electrode sheets of the fresh cell and the remaining negative electrode sheets of the failed cell into a fresh negative electrode button cell and a failed negative electrode button cell, respectively. Based on the discharge capacity of fresh negative electrode coin cells and the discharge capacity of failed negative electrode coin cells, the capacity loss caused by negative electrode polarization and lithium dendrites and the capacity loss caused by negative electrode structure of coin cells are determined. Based on the residual capacity of the negative electrode and the discharge capacity of the fresh negative electrode coin half-cell, determine the negative electrode SEI and the deactivated Li loss capacity of the fresh coin half-cell. Based on the residual capacity of the negative electrode and the discharge capacity of the failed coin cell, determine the negative electrode SEI and the capacity loss due to deactivated Li in the failed coin cell; based on the negative electrode SEI and the capacity loss due to deactivated Li in the fresh coin cell and the negative electrode SEI and the capacity loss due to deactivated Li in the failed coin cell, determine the negative electrode SEI and the capacity loss due to deactivated Li growth in the coin cell. S5. By determining the proportions of the positive electrode structure loss capacity, the positive electrode CEI growth loss capacity, the negative electrode structure loss capacity, the negative electrode polarization and lithium dendrite loss capacity, and the negative electrode SEI and deactivated Li growth loss capacity in the total loss capacity of the coin cell, the irreversible capacity decay of lithium-ion batteries containing lithium battery cells can be quantitatively analyzed.

2. The failure analysis method for irreversible capacity decay of lithium-ion batteries according to claim 1, characterized in that, In step S1, the discharge capacity C of the fresh battery cell in the nth week of charge-discharge cycle is measured. 新鲜 The discharge capacity C of the failed battery cell in the nth week of charge-discharge cycle 失效 The discharge capacity C of a fresh button half-cell in the nth week of charge-discharge cycle is calculated based on its area. 新鲜0 The discharge capacity C of a failed coin cell during the nth cycle of charge-discharge cycles. 失效0 And determine the total loss capacity C of the coin cell. total C total =C 新鲜0 -C 失效0 Wherein, the fresh battery cell and the failed battery cell are lithium-ion battery cells, and n is 2 or 3; and / or, In step S1, the failed cell is the cell whose capacity has decayed to 80% SOH.

3. The failure analysis method for irreversible capacity decay of lithium-ion batteries according to claim 2, characterized in that, In step S2, the fresh and failed cells that have undergone charge-discharge cycles in step S1 are disassembled, and the positive electrode plates of the fresh and failed cells are assembled into fresh positive electrode coin cells and failed positive electrode coin cells, respectively. Based on the discharge capacity C of a fresh positive electrode coin cell in week n of a charge-discharge cycle. 新鲜c The discharge capacity C of a failed positive electrode coin cell during the nth cycle of charge-discharge cycles. 失效c Determine the positive electrode structure loss capacity C of the coin cell. cs C cs =C 新鲜c -C 失效c ; Based on the discharge capacity C of a fresh positive electrode coin cell in week n of a charge-discharge cycle. 新鲜c The discharge capacity C of a fresh coin cell half-cell during the nth week of charge-discharge cycles. 新鲜0 Determine the positive electrode deintercalation-intercalation loss capacity C of a fresh coin half-cell. 新鲜cr C 新鲜cr =C 新鲜c -C 新鲜0 ; Based on the discharge capacity C of the failed positive electrode coin cell in the nth week of charge-discharge cycle. 失效c The discharge capacity C of a failed coin cell during the nth cycle of charge-discharge cycles. 失效0 Determine the positive electrode deintercalation / intercalation loss capacity C of the failed coin cell. 失效cr C 失效cr =C 失效c -C 失效0 .

4. The failure analysis method for irreversible capacity decay of lithium-ion batteries according to claim 3, characterized in that, In step S2, the fresh and failed battery cells, after undergoing n charge-discharge cycles in step S1, are discharged to 0% SOC and then disassembled to obtain the positive electrode plates of the fresh and failed cells. These are soaked in a solvent for 1-2 hours, then dried in a vacuum oven at 60-90℃ for 3-6 hours. The active material on one side of the positive electrode plates is wiped off with anhydrous ethanol using lint-free paper. Single-sided circular positive electrode plates are punched using a punching machine. The fresh and failed positive electrode coin cells are then assembled in a glove box filled with a protective atmosphere; and / or, In step S2, the fresh positive electrode coin cell and the failed positive electrode coin cell use the same charging and discharging current of 0.05-0.2C during charge and discharge cycles; the same upper limit of charging voltage of 3.5-3.8V; and the same lower limit of discharging voltage of 1.0-2.5V.

5. The failure analysis method for irreversible capacity decay of lithium-ion batteries according to claim 3, characterized in that, In step S3, the negative electrode portions of the fresh battery cell and the failed battery cell obtained after disassembly are tested separately to obtain the residual negative electrode capacity C of the fresh battery cell. 新鲜ar0 and the residual negative capacity C of the failed battery cell 失效ar0 The residual negative electrode capacity C of a fresh coin cell, converted by area. 新鲜ar The residual negative electrode capacity C of the failed coin cell 失效ar ; Based on the positive electrode deintercalation-intercalation loss capacity C of a fresh coin half-cell 新鲜cr The residual capacity C of the negative electrode of a fresh coin cell 新鲜ar Determine the positive electrode CEI loss capacity C of a fresh coin half-cell. 新鲜CEI C 新鲜CEI =C 新鲜cr -C 新鲜ar ; Based on the positive electrode deintercalation-intercalation loss capacity C of the failed coin half-cell 失效cr The residual negative electrode capacity C of the failed coin cell 失效ar Determine the positive electrode CEI loss capacity C of the failed coin cell. 失效CEI C 失效CEI =C 失效cr -C 失效ar ; Based on the positive electrode CEI loss capacity C of a fresh coin half-cell 新鲜CEI The positive electrode CEI capacity loss C of the failed coin cell 失效CEI Determine the capacity loss C caused by the increase of CEI at the positive electrode of the coin half-cell. CEI C CEI =C 失效CEI -C 新鲜CEI .

6. The failure analysis method for irreversible capacity decay of lithium-ion batteries according to claim 5, characterized in that, In step S4, the remaining negative electrode sheets of the fresh cell and the remaining negative electrode sheets of the failed cell are assembled into a fresh negative electrode coin cell and a failed negative electrode coin cell, respectively. Based on the charging capacity C of the first charge-discharge cycle of a fresh negative electrode coin cell. 新鲜a The charging capacity C in the nth cycle 新鲜ad And the charging capacity C of the first charge-discharge cycle of the failed negative electrode coin cell. 失效a The charging capacity C in the nth cycle 失效ad Determine the capacity loss C caused by negative electrode polarization and lithium dendrite formation in a coin cell. pl C pl =C 失效a -C 新鲜a And the capacity loss C of the negative electrode structure as C as =C 新鲜ad -C 失效ad .

7. The failure analysis method for irreversible capacity decay of lithium-ion batteries according to claim 6, characterized in that, In step S4, based on the residual negative electrode capacity C of the fresh coin cell... 新鲜ar The first charge capacity C of a fresh negative-terminal coin cell 新鲜a Determine the negative electrode SEI and the capacity loss C due to Li deactivation in a fresh coin half-cell. 新鲜SEI C 新鲜SEI =C 新鲜ar -C 新鲜a ; Based on the residual negative electrode capacity C of the failed coin cell 失效ar The first charge capacity C of a failed negative terminal coin cell 失效a Determine the negative electrode SEI and the capacity loss C due to Li deactivation in the failed coin cell. 失效SEI C 失效SEI =C 失效ar -C 失效a This led to the determination of the negative electrode SEI and the capacity loss C caused by the growth of deactivated Li in the coin cell. SEI C SEI =C 失效SEI -C 新鲜SEI .

8. The failure analysis method for irreversible capacity decay of lithium-ion batteries according to claim 6, characterized in that, In step S4, the fresh and failed battery cells, after being charged and discharged n times in step S1, are discharged to 0% SOC and then disassembled to obtain the negative electrode plates of the fresh and failed cells. These are then soaked in a solvent for 1-2 hours, dried in a vacuum oven at 60-90℃ for 3-6 hours, and the active material on one side of the negative electrode plates is wiped off with deionized water using lint-free paper. Single-sided circular negative electrode plates are punched using a punching machine, and the fresh and failed negative electrode coin cells are assembled in a glove box filled with a protective atmosphere; and / or, In step S4, the fresh negative electrode coin cell and the failed negative electrode coin cell use the same charging and discharging current of 0.05-0.2C during charge and discharge cycles; the same upper limit of charging voltage of 1.0-2.5V; and the same lower limit of discharging voltage of 0.001-0.1V.

9. The failure analysis method for irreversible capacity decay of lithium-ion batteries according to any one of claims 1-8, characterized in that, The lithium-ion battery is any one of a pouch battery, a prismatic battery, or a cylindrical battery; and / or, The positive electrode active material of the lithium-ion battery includes one or more of nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, lithium iron phosphate, or lithium manganese oxide; and / or, The negative electrode active material of the lithium-ion battery includes one or more of graphite, mesophase carbon microspheres, lithium titanate, hard carbon, silicon oxide, or silicon carbon; and / or, The electrolyte of the lithium-ion battery includes one or more of propylene carbonate, ethylene carbonate, methyl ethyl carbonate, or ethyl carbonate as the solvent, one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, or lithium difluorosulfonylimide as the solute, and one or more of vinylene carbonate, fluoroethylene carbonate, 1,3-propanesulfonate lactone, or vinylene sulfate as the additive.

10. The application of the failure analysis method for irreversible capacity decay of lithium-ion batteries according to any one of claims 1-9 in the field of battery analysis and selection.

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