Lithium ion battery self-discharge evaluation method

Abstract

The invention discloses a lithium ion battery self-discharge evaluation method. The method comprises the following steps: S1, manufacturing a plurality of metal impurity single batteries and a plurality of control group single batteries; s2, executing a multi-step aging process on each metal impurity single cell and a control group single cell, and checking the timeliness of metal precipitation in each aging process; obtaining the voltage drop rate of each metal impurity single cell and the control group single cell; s3, comparing the voltage drop rate of each metal impurity single cell with a reference standard of a control group; and if yes, judging the battery as a bad battery. On the basis of the characteristics of simple structure, low cost and easy operation of a single battery, in combination with accurate regulation and control of high-SOC interval charge-discharge cycle, high-temperature aging and normal-temperature aging processes and in combination with disassembly of the single battery to detect the timeliness of metal precipitation in each stage, metal impurities introduced in the production process of the lithium ion battery are fully dissolved, and the production efficiency of the lithium ion battery is improved. The problem that metal impurities are difficult to screen out is solved.

Description

Technical Field

[0001] The invention relates to the technical field of lithium ion batteries, and in particular to a method for evaluating the self-discharge of lithium ion batteries. Background Art

[0002] Lithium-ion batteries have the advantages of high energy density, long life, and fast energy replenishment, and have been widely used in the field of electric vehicles in recent years. The consistency of lithium-ion battery voltage is not only related to the energy performance of the entire vehicle, but also affects the safety of the vehicle, that is, leading to overcharging and internal short circuits.

[0003] The inconsistency of lithium-ion battery voltage is generally caused by metal impurity contamination during the manufacturing process. As charging and discharging proceed, metal impurities are deposited on the negative electrode and the diaphragm and produce metal dendrites, causing a micro short circuit between the positive and negative electrodes, and finally causing the battery voltage to be lower than the normal voltage. Therefore, how to quickly and effectively identify self-discharge caused by metal impurities through the self-discharge screening process is a common problem faced by lithium-ion battery companies.

[0004] Regarding the screening process for self-discharge of lithium-ion batteries, the commonly used screening method in the industry is to place them for aging at a specific temperature and charge state, and judge the self-discharge defective products by the size of the voltage drop value or the voltage drop rate during the aging process. This screening method is not very targeted, the process time is long, and it does not fully identify the oxidation (precipitation) window for the introduction of metal impurities, which often results in energy waste, too long production cycle, and self-discharge leakage, resulting in the self-discharge batteries flowing into the application terminal.

[0005] Therefore, there is an urgent need to develop a technology that can solve the above technical problems. Summary of the invention

[0006] The purpose of the present invention is to provide a lithium ion battery self-discharge evaluation method in view of the technical defects in the prior art.

[0007] To this end, the present invention provides a lithium ion battery self-discharge evaluation method, which comprises the following steps:

[0008] Step S1, preparing a plurality of metal impurity single-chip batteries in advance, and preparing a plurality of control group single-chip batteries in advance;

[0009] Among them, the multiple metal impurity monolithic battery is a monolithic battery in which different kinds of metal impurity particles are pre-placed on the positive electrode sheet;

[0010] A plurality of control group single-chip batteries are a plurality of single-chip batteries without placing metal impurity particles on the positive electrode sheet;

[0011] Step S2, for each metal impurity single-chip battery and each control group single-chip battery, respectively, a pre-set multi-step aging process is performed, and after each step of the aging process is completed, the timeliness of metal precipitation in each step of the aging process is checked by disassembling the battery;

[0012] Among them, the multi-step aging process includes the first aging stage, the second aging stage and the third aging stage;

[0013] And, according to the voltage drop and aging time in the aging process, the voltage drop rate of each metal impurity single-chip battery and each control group single-chip battery is obtained;

[0014] Step S3, comparing the voltage drop rate of each metal impurity single-chip battery with a preset control group reference standard;

[0015] When the voltage drop rate of a metal impurity single-chip battery is greater than the reference standard of the control group, the metal impurity single-chip battery is judged as a bad battery, otherwise, the metal impurity single-chip battery is judged as a qualified battery.

[0016] It can be seen from the technical solution provided by the present invention that, compared with the prior art, the present invention provides a lithium-ion battery self-discharge evaluation method, which is scientifically designed and solves the problem that metal impurities in lithium-ion batteries are difficult to screen out based on the characteristics of simple structure, low cost and easy operation of single-chip batteries. The present invention targets metal impurities that are easily introduced during the production process of lithium-ion batteries, establishes a screening process for metal dissolution based on single-chip batteries and metal redox potential, and tests the timeliness of metal precipitation by disassembling single-chip batteries, thereby formulating a lithium-ion battery screening process, which has great practical significance.

[0017] A lithium-ion battery self-discharge evaluation method provided by the present invention is based on the characteristics of a simple single-chip battery structure, low cost, and easy operation, combined with accurate regulation of high SOC range charge-discharge cycles, high-temperature aging, and room-temperature aging processes, to achieve full dissolution of metal impurities introduced in the lithium-ion battery production process, solve the problem that metal impurities are difficult to screen out, and improve the timeliness and economy of the self-discharge screening process. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A basic flow chart of a lithium-ion battery self-discharge evaluation method provided by the present invention;

[0019] Figure 2 A schematic diagram of the structure of a single-chip battery in an embodiment of a lithium-ion battery self-discharge evaluation method provided by the present invention;

[0020] Figure 3aA schematic diagram (photo) of a negative electrode sheet on a single-chip battery as a control group (i.e., a blank control group) when no metal particles are placed in the single-chip battery based on a lithium-ion battery self-discharge evaluation method provided by the present invention;

[0021] Figure 3b to Figure 3f They are schematic diagrams (photographs) of a lithium-ion battery self-discharge evaluation method provided by the present invention, when metal impurity particles including copper (Cu), iron (Fe), stainless steel (SUS304), cold-rolled carbon steel (SPCC), nickel (Ni) and the like are introduced into a single-chip battery, and after 10 charge-discharge cycles at 80%-90% SOC (aging stage 1), and 2 days of high-temperature aging at 45°C and 70% SOC (aging stage 2), metal impurity particles of copper (Cu), iron (Fe), stainless steel (SUS304), cold-rolled carbon steel (SPCC), and nickel (Ni) are obviously precipitated at the negative electrode respectively;

[0022] Figure 4a A schematic diagram (photo) of a separator on a single-chip battery as a control group (i.e., a blank control group) when no metal particles are placed in the single-chip battery based on a lithium-ion battery self-discharge evaluation method provided by the present invention;

[0023] Figure 4b to Figure 4f They are schematic diagrams of a lithium-ion battery self-discharge evaluation method provided by the present invention, when metal impurity particles including copper (Cu), iron (Fe), stainless steel (SUS304), cold-rolled carbon steel (SPCC), nickel (Ni) and the like are introduced into a single-chip battery, and after 10 charge-discharge cycles at 80%-90% SOC (aging stage 1), and 45°C 70% SOC high-temperature aging for 2 days (aging stage 2), copper (Cu), iron (Fe), stainless steel (SUS304), cold-rolled carbon steel (SPCC), and nickel (Ni) metal impurity particles are obviously precipitated at the diaphragm;

[0024] Figure 5 A lithium ion battery self-discharge evaluation method provided by the present invention is provided. For a single-chip battery, after 10 80%-90% SOC charge-discharge cycles at 25°C (first aging stage), 45°C 70% SOC aging for 2 days (second aging stage), and 25°C 30% SOC aging for 5 days (third aging stage), a schematic diagram comparing the voltage drop rate caused by different metals and the voltage drop rate when no metal particles are placed (i.e., a control group battery without adding metal impurity particles);

[0025] In the figure, 1. diaphragm, 2. negative electrode, 3. positive electrode, 4. metal impurity particles. DETAILED DESCRIPTION

[0026] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

[0028] In the description of this patent, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", and "set" should be understood in a broad sense, for example, it can be fixedly connected or set, or it can be detachably connected or set, or connected or set in one piece. For ordinary technicians in this field, the specific meanings of the above terms in this patent can be understood according to specific circumstances.

[0029] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0030] In order to enable those skilled in the art to better understand the scheme of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and implementation modes.

[0031] See also Figure 1 The present invention provides a method for evaluating self-discharge of a lithium-ion battery, comprising the following steps:

[0032] Step S1, preparing a plurality of metal impurity single-chip batteries in advance, and preparing a plurality of control group single-chip batteries in advance;

[0033] Among them, the multiple metal impurity monolithic battery is a monolithic battery in which different kinds of metal impurity particles are pre-placed on the positive electrode sheet;

[0034] A plurality of control group single-chip batteries are a plurality of single-chip batteries without placing metal impurity particles on the positive electrode sheet;

[0035] Step S2, for each metal impurity single-chip battery and each control group single-chip battery, respectively, a pre-set multi-step aging process is performed, and after each step of the aging process is completed, the timeliness of metal precipitation in each step of the aging process is checked by disassembling the battery (that is, checking whether the metal impurity particles are precipitated on the negative electrode sheet and the diaphragm of the battery and the precipitation situation, and checking the influence of the aging process on the timeliness of metal precipitation);

[0036] Among them, the multi-step aging stage process includes: the first aging stage, the second aging stage and the third aging stage;

[0037] And, according to the voltage drop and aging time in the aging process (specifically the third aging stage), the voltage drop rate (i.e., the ratio of the voltage drop to the aging time) of each metal impurity monolithic battery and each control group monolithic battery is obtained;

[0038] Step S3, comparing the voltage drop rate of each metal impurity single-chip battery with a preset control group reference standard;

[0039] When the voltage drop rate of a metal impurity monolithic battery is greater than the reference standard of the control group, the metal impurity monolithic battery is judged to be a bad battery (i.e., a battery with poor self-discharge); otherwise, the metal impurity monolithic battery is judged to be a qualified battery (i.e., a battery with good self-discharge).

[0040] In step S3, in specific implementation, the reference standard of the control group is equal to the sum of the average value (Kave) of the voltage drop rate of all the single-chip batteries in the control group and three times the standard deviation (δ), that is, equal to Kave+3δ.

[0041] In specific implementation, the average voltage drop rate of all multiple control group single-chip batteries (Kave) is the average voltage drop rate of all multiple control group single-chip batteries in the third aging stage;

[0042] For each single-chip battery in the control group, the voltage drop rate k of the single-chip battery in the control group in the third aging stage can be calculated based on the initial voltage V1 corresponding to the initial time t1 of the third aging stage and the end voltage V2 corresponding to the end time t2 of the aging stage;

[0043] k=(V1-V2) / (t2-t1); the unit is mV / h.

[0044] Among them, δ represents the standard deviation, which is the arithmetic square root of the variance. It means that when the control group obeys the normal distribution, the standard deviation is defined as the square root of the arithmetic mean of the squares of the deviations of the population values ​​from their mean. It reflects the degree of compliance of the data within the group, and the mean ±3δ represents a 99.97% probability that the data obeys this interval. Since the points with large voltage drop rates are abnormal points, the mean + 3δ is set as the standard for poor self-discharge.

[0045] It should be noted that, for the present invention, based on the oxidation potential of metal impurities, an aging process is set (i.e., a corresponding aging process is formulated, and the timeliness of the aging process on metal precipitation is confirmed by disassembling the single chip) to meet the precipitation of metal impurities, and the precipitation of metal impurities is confirmed by disassembling the single chip battery during the aging process, and combined with the voltage drop during the aging process, a self-discharge screening process for lithium-ion batteries is formulated.

[0046] In step S1, in specific implementation, it should be noted that, for the present invention, multiple metal impurity particles of different types are artificially introduced into the positive electrode sheet of each single-chip battery to simulate the types and sizes of metal impurities introduced during the battery production process, that is, metal impurities generated by the current collector, battery shell, positive and negative electrode main materials, slurry and welding processes are artificially introduced during the battery manufacturing process;

[0047] In step S1, in specific implementation, the single-chip battery is a soft-pack battery with a sandwich structure;

[0048] A monolithic battery is a laminated battery, which includes a plurality of structures distributed sequentially from top to bottom, specifically including: a layer of separator, a layer of negative electrode sheet, a layer of separator, a layer of positive electrode sheet, a layer of separator, a layer of negative electrode sheet and a layer of separator distributed sequentially from top to bottom;

[0049] The aluminum foil on the positive electrode sheet is ultrasonically welded to the aluminum external pole ear, and the aluminum external pole ear serves as the positive electrode lead-out pole ear;

[0050] The copper foil on the negative electrode sheet is ultrasonically welded to the copper-plated nickel tab, and the copper-plated nickel tab serves as the negative electrode lead-out tab;

[0051] The lead-out ends of the positive electrode lead-out tab and the negative electrode lead-out tab are on the same side.

[0052] In step S1, in a specific implementation, the structural adhesive on the positive electrode lead-out tab and the negative electrode lead-out tab is heat-sealed with the aluminum-plastic film.

[0053] In step S1, in a specific implementation, the particle size range of the metal impurity particles is: 15 microns to 150 microns.

[0054] It should be noted that before the single-chip battery is heat-sealed with aluminum-plastic film, multiple different types of metal impurity particles with a particle size distribution ranging from 15 microns to 150 microns are artificially introduced onto the positive electrode sheet. Subsequently, the single-chip battery is manufactured according to the following process, including drying, liquid injection, infiltration, pre-formation, main formation and aging.

[0055] In step S1, in specific implementation, different types of metal impurity particles are selected based on the impurity pollution sources identified by the lithium-ion battery in a specific process, but not limited to this. The metal impurities come from the current collector, battery shell, positive and negative electrode materials, slurry, metal welding slag, etc. The metal impurity particles include any of copper (Cu), iron (Fe), nickel (Ni), zinc (Zn), stainless steel (SUS304) and cold-rolled carbon steel (SPCC). The particle size range of the metal impurity particles is 15μm-150μm.

[0056] In step S2, in specific implementation, a pre-set multi-step aging process is executed, and the single-chip battery is disassembled after each step of the aging process to verify the timeliness of the aging process on metal precipitation, including the following three stages:

[0057] The first aging stage: the high SOC range charge and discharge cycle stage, at temperature T1, the battery is charged and discharged for a preset number of times (for example, 10 times) in a specific SOC range, with the charging current and discharging current being C1 and C2 respectively, and then the battery is adjusted to SOC2 at temperature T1;

[0058] The second aging stage: high temperature aging stage, the battery is stored at temperature T2 for a storage time of t2;

[0059] The third aging stage: the room temperature aging stage, at the temperature T3, the state of charge will be adjusted to SOC3, and the storage time is set to t3.

[0060] In terms of specific implementation, the first aging stage is an aging stage in which the timeliness is confirmed after disassembly.

[0061] Further, in the first aging stage, the aging temperature of the first aging stage is T1, 25°C≤T1≤35°C, the charge and discharge SOC cycle interval is 80%-90%, the pulse charging current is 0.5C-1C, the pulse discharging current is 0.5C-1.5C, the charge and discharge cycle number is 5-20 times (each cycle number includes one charging and one discharging operation), and then the battery is adjusted to SOC2 at the temperature T1;

[0062] In the present invention, in step S3, after the first aging stage is completed, the timeliness of metal precipitation in each aging process is checked by disassembling the battery, which specifically includes the following operations:

[0063] First, disassemble the metal impurity monolithic battery;

[0064] Then, it is detected whether the metal impurity particles are effectively precipitated on the negative electrode sheet and the separator of the metal impurity monolithic battery;

[0065] Effective precipitation has been achieved, that is, the proportion of the metal impurity particles to the surface area of ​​the negative electrode sheet and the separator exceeds a preset first proportion.

[0066] It should be noted that the purpose of the first aging stage is to cycle in the high SOC range to generate a high oxidation potential, which can realize the oxidation and dissolution of metal impurities. In order to accelerate the migration of metal ions, the metal precipitation aging results can be confirmed by disassembling the metal impurity monolithic battery (that is, confirming the number of cycles in the first stage. The number of cycles required in the first stage is the number of cycles that can allow the metal impurity particles to be oxidized and dissolved in the battery and migrate to the negative electrode). At this time, the metal impurity particles are not effectively precipitated on the negative electrode sheet and the diaphragm of the metal impurity monolithic battery (effectively precipitated, that is, the proportion of the metal impurity particles to the surface area of ​​the negative electrode sheet and the diaphragm exceeds the preset first proportion, and the preset first proportion can be, for example, 40%).

[0067] In terms of specific implementation, the second aging stage is an aging stage in which the timeliness is confirmed after disassembly.

[0068] Further, in the second aging stage, the aging temperature T2 of the second aging stage is 45°C ± 3°C, the SOC2 is 60% ≤ SOC2 ≤ 75% SOC, and the storage time t2 = 2-5 days;

[0069] In the present invention, in step S3, after the second aging stage is completed, the timeliness of metal precipitation in each aging process is checked by disassembling the battery, which specifically includes the following operations:

[0070] First, disassemble the metal impurity monolithic battery;

[0071] Then, the metal impurity particles are detected to see whether they are obviously precipitated on the negative electrode sheet and the separator of the metal impurity monolithic battery;

[0072] Obviously precipitated, that is, the proportion of the metal impurity particles to the surface area of ​​the negative electrode sheet and the separator exceeds the preset second proportion.

[0073] The preset second ratio of the second aging stage is greater than the preset first ratio of the first aging stage.

[0074] It should be noted that the high temperature of the second aging stage continues the oxidation precipitation of metal impurities, and the status is confirmed after the metal impurity monolithic battery is fully charged and disassembled (that is, the storage time of the second aging stage is confirmed, and the storage time required for the second aging stage is the storage time that allows the metal impurity particles to be obviously precipitated on the negative electrode sheet and the diaphragm of the battery). The metal impurity particles are obviously precipitated on the negative electrode sheet and the diaphragm of the metal impurity monolithic battery, that is, the proportion of the metal impurity particles to the surface area of ​​the negative electrode sheet and the diaphragm exceeds the preset second proportion, and the preset second proportion can be, for example, 70%. The preset second proportion of the second aging stage is greater than the preset first proportion of the first aging stage.

[0075] In specific implementation, in the third aging stage, the aging temperature of the third aging stage is T3, 25°C≤T3≤35°C;

[0076] 15%≤SOC3≤30%, storage time t3=3-7 days;

[0077] It should be noted that in the third aging stage, self-discharge is detected mainly through the voltage drop caused by metal precipitation, and the self-discharge rate is detected through the voltage drop effect caused by metal impurities.

[0078] It should be noted that, for the present invention, based on the aging results of the three aging stages, an aging process for self-discharge screening of lithium-ion batteries is specifically formulated, and the specific timeliness of the aging process (such as the number of charge and discharge cycles and storage time) is confirmed by disassembling a single battery.

[0079] In summary, for the present invention, the design principle of its overall technical solution is:

[0080] First, a metal impurity monolithic battery is manufactured, during which metal impurity particles that are easily introduced during the battery production process are artificially introduced, and a monolithic battery without the introduction of metal impurity particles is manufactured as a control group monolithic battery;

[0081] Then, a multi-step aging process is formulated based on the metal oxidation potential and the purpose of accelerating metal precipitation. Taking advantage of the simple structure and easy disassembly of the monolithic battery, the metal precipitation in each step of the process is checked for timeliness. The aging process includes high SOC pulse cycles performed in the first aging stage (used to accelerate gold dissolution, accelerate the migration of metal ions to the negative electrode and precipitation at the negative electrode), high SOC and high temperature aging performed in the second aging stage (used to meet the oxidation potential of metal dissolution and continue the dissolution and precipitation of metals), and normal temperature voltage drop screening performed in the third aging stage (used to detect the voltage drop caused by dendrites produced by metal precipitation at room temperature).

[0082] Finally, according to the initial aging voltage V1 and initial time t1 of the single-chip battery in the third aging stage, the final voltage V2 and final time t2 after room temperature aging in the third aging stage, the voltage drop rate is calculated: K = (V1-V2) / (t2-t1), in mV / h. Due to the different types of metal foreign matter, some metal impurity single-chip batteries will cause the K value to decay particularly quickly and have abnormal points. Another part of the metal impurity single-chip batteries will cause the K value to be greater than the sum of the overall self-discharge average value (Kave) of the control group single-chip batteries without metal impurity particles and 3 times the standard deviation (δ) (i.e. Kave+3δ), and are considered unqualified.

[0083] In order to more clearly understand the technical solution of the present invention, the technical solution of the present invention is described below through specific embodiments.

[0084] Example 1.

[0085] The first step is to prepare a soft-pack battery with a sandwich structure, including a separator 1, a negative electrode sheet 2, a separator 1, a positive electrode sheet 3, a separator 1, a negative electrode sheet 2 and a separator 1, see Figure 2 As shown; metal impurity particles 4 are placed on the positive electrode sheet 3;

[0086] Among them, the positive electrode sheet is a NCM ternary high nickel (Ni9) positive electrode, the size of the electrode powder area is 37.5mm*44mm, the positive electrode ceramic span is 3mm, the aluminum foil width is 5mm, and the positive electrode external lug size is 4mm*54mm, which is ultrasonically welded with the aluminum foil on the positive electrode sheet as the positive electrode lead-out lug;

[0087] The negative electrode sheet is made of artificial graphite, the size of the electrode powder area is 46*56mm, the copper foil width is 6mm, the diaphragm size is 4mm*66mm, the copper nickel-plated pole ear is ultrasonically welded to the copper foil on the negative electrode sheet, and the negative electrode lead-out ear is used as the negative electrode. The lead-out ends of the positive and negative pole ears are on the same side, and the structural glue on the pole ear is heat-sealed with the aluminum-plastic film.

[0088] Before the aluminum-plastic heat sealing, metal impurity particles 4 of different materials with a particle size distribution of 15μm to 150μm, 304 stainless steel (SUS304), iron (Fe), copper (Cu), nickel (Ni), cold carbon rolled steel (SPCC) were artificially introduced on the positive electrode 3. Subsequently, the single-chip battery was manufactured according to the following process: including drying at 95℃-12h, ensuring sufficient infiltration, excessive electrolyte, infiltration time of 12h, pre-charged at 0.05C-55min, 0.2C-70min; main charge at 0.33C to 4.25V, constant voltage to 0.05C, stand for 30min, and discharge at 0.33C to 2.5V.

[0089] In the second step, a three-stage aging process is performed;

[0090] The aging operation in the first aging stage is as follows: temperature T1 = 25°C ± 3°C, SOC cycle range 80% - 90%, pulse charging current 0.5C, pulse discharge current 1C, cycle number 10, then adjust the battery state of charge to SOC = 70%, and check the metal precipitation by disassembling the single-chip battery.

[0091] The aging operation of the second aging stage is at a temperature of T2 = 45°C ± 3°C and a storage time of t2 = 2 days. The metal precipitation is checked by disassembling the single cell. See Table 1 below.

[0092] Table 1: Schematic diagram of different types of metal impurity particles being obviously precipitated at the negative electrode and the separator

[0093]

[0094] See also Figures 3a to 3f , Figures 4a to 4f As shown, after 10 charge-discharge cycles at 80%-90% SOC (aging stage 1) and 2 days of high-temperature aging at 45°C and 70% SOC (aging stage 2), different metal impurity particles were obviously precipitated at the negative electrode and the diaphragm.

[0095] In the third aging stage, the temperature is T3 = 25 ± 3°C, the SOC3 is adjusted to 30%, and the storage time is t3 = 5 days.

[0096] The third step is to count the self-discharge failures through the voltage drop of the battery after the third aging stage is completed.

[0097] Based on the above situation, combined with the verification results of monolithic battery disassembly at different aging stages and the results of voltage drop screening during the aging process, the aging screening process was formulated as follows: 10 cycles at 25°C 80%-90% SOC, aging at 45°C 70% SOC for 2 days, and aging at 25°C 30% SOC for 5 days.

[0098] See also Figure 5 As shown, for a single-chip battery, after 10 80%-90% SOC charge and discharge cycles at 25°C (the first aging stage), 45°C 70% SOC aging for 2 days (the second aging stage), and 25°C 30% SOC aging for 5 days (the third aging stage), the control group reference standard is used as the standard, and the control group reference standard is equal to: the sum of the average voltage drop rate (Kave) and three times the standard deviation (δ) of all multiple control group single-chip batteries. The voltage drop rate generated by different metal impurities on different metal impurity monolithic batteries is compared with the standard value of the control group voltage drop (i.e., the control group reference standard). If the voltage drop is higher than the standard value, it is judged as a bad situation.

[0099] See also Figure 5For the present invention, the voltage drop rate k of the single-chip battery during the aging process can be calculated based on the initial voltage V1 corresponding to the initial time t1 of the third aging stage and the end voltage V2 corresponding to the end time t2 of the aging. k = (V1-V2) / (t2-t1); therefore, for multiple single-chip batteries with different metal impurity particles added, the corresponding voltage drop rates can be obtained respectively.

[0100] Figure 5 Schematic diagram of the comparison of the voltage drop rate of the metal impurity monolithic battery with different metal impurity particles and the voltage drop rate of the control monolithic battery. Figure 5 In the figure, it reflects the response of the voltage drop rate of the metal impurity single-chip battery after the metal precipitation of the single-chip battery with different metal impurity particles compared with the single-chip battery of the control group; if the voltage drop rate caused by the metal impurity particles is greater than the sum of the mean voltage drop rate of the single-chip battery in the control group and 3 times the standard deviation (Kave+3δ), the metal impurity single-chip battery is judged as a defective product, otherwise, it is judged as a qualified product.

[0101] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for evaluating self-discharge of a lithium-ion battery, characterized in that: The following steps are involved: Step S1, preparing a plurality of metal impurity single-chip batteries in advance, and preparing a plurality of control group single-chip batteries in advance; Among them, the multiple metal impurity monolithic battery is a monolithic battery in which different kinds of metal impurity particles are pre-placed on the positive electrode sheet; A plurality of control group single-chip batteries are a plurality of single-chip batteries without placing metal impurity particles on the positive electrode sheet; Step S2, for each metal impurity single-chip battery and each control group single-chip battery, respectively, a pre-set multi-step aging process is performed, and after each step of the aging process is completed, the timeliness of metal precipitation in each step of the aging process is checked by disassembling the battery; Among them, the multi-step aging process includes the first aging stage, the second aging stage and the third aging stage; And, according to the voltage drop and aging time in the aging process, the voltage drop rate of each metal impurity single-chip battery and each control group single-chip battery is obtained; Step S3, comparing the voltage drop rate of each metal impurity single-chip battery with a preset control group reference standard; When the voltage drop rate of a metal impurity single-chip battery is greater than the reference standard of the control group, the metal impurity single-chip battery is judged as a bad battery, otherwise, the metal impurity single-chip battery is judged as a qualified battery.

2. The lithium ion battery self-discharge evaluation method according to claim 1, characterized in that: Monolithic batteries are soft-pack batteries with a sandwich structure; A monolithic battery is a laminated battery, which includes a plurality of structures distributed sequentially from top to bottom, specifically including: a layer of separator, a layer of negative electrode sheet, a layer of separator, a layer of positive electrode sheet, a layer of separator, a layer of negative electrode sheet and a layer of separator distributed sequentially from top to bottom; The aluminum foil on the positive electrode sheet is ultrasonically welded to the aluminum external pole ear, and the aluminum external pole ear serves as the positive electrode lead-out pole ear; The copper foil on the negative electrode sheet is ultrasonically welded to the copper-plated nickel pole tab, and the copper-plated nickel pole tab serves as the negative electrode lead-out tab.

3. The lithium ion battery self-discharge evaluation method according to claim 2, characterized in that: The lead-out ends of the positive electrode lead-out tab and the negative electrode lead-out tab are on the same side.

4. The lithium ion battery self-discharge evaluation method according to claim 1, characterized in that: In step S1, the particle size of the metal impurity particles ranges from 15 microns to 150 microns; The sources of metal impurity particles include current collector, battery shell, positive and negative electrode main materials, slurry and welding process; The metal impurity particles include any of copper Cu, iron Fe, nickel Ni, zinc Zn, stainless steel SUS304, and cold-rolled carbon steel SPCC.

5. The lithium ion battery self-discharge evaluation method according to claim 1, characterized in that: In step S2, a pre-set multi-step aging process is executed, and the single-chip battery is disassembled after each step of the aging process to verify the timeliness of the aging process on metal precipitation, including the following three stages: The first aging stage: the high SOC range charge and discharge cycle stage, at temperature T1, a preset number of charge and discharges are performed in a specific SOC range, the charge current and discharge current are C1 and C2 respectively, and then the battery is adjusted to SOC2 at temperature T1; The second aging stage: high temperature aging stage, the battery is stored at temperature T2 for a storage time of t2; The third aging stage: the room temperature aging stage, at the temperature T3, the state of charge will be adjusted to SOC3, and the storage time is set to t3.

6. The lithium ion battery self-discharge evaluation method according to claim 5, characterized in that: In the first aging stage, the aging temperature of the first aging stage is T1, 25℃≤T1≤35℃, the charge and discharge SOC cycle range is 80%-90%, the pulse charging current is 0.5C-1C, the pulse discharge current is 0.5C-1.5C, the charge and discharge cycle number is 5 times-20 times, and then the battery is adjusted to SOC2 at temperature T1.

7. The lithium ion battery self-discharge evaluation method according to claim 5, characterized in that: In the second aging stage, the aging temperature T2 of the second aging stage is 45°C ± 3°C, the SOC2 is 60% ≤ SOC2 ≤ 75% SOC, and the storage time t2 = 2-5 days; In the third aging stage, the aging temperature of the third aging stage is T3, 25°C≤T3≤35°C; 15%≤SOC3≤30%, and the storage time is t3=3-7 days.

8. The lithium ion battery self-discharge evaluation method according to claim 1, characterized in that: In step S3, after the first aging stage is completed, the timeliness of metal precipitation in each aging process is checked by disassembling the battery, which specifically includes the following operations: First, disassemble the metal impurity monolithic battery; Then, it is detected whether the metal impurity particles are effectively precipitated on the negative electrode sheet and the separator of the metal impurity monolithic battery; Effective precipitation has been achieved, that is, the proportion of metal impurity particles to the surface area of ​​the negative electrode sheet and the separator exceeds a preset first proportion; In step S3, after the second aging stage is completed, the timeliness of metal precipitation in each aging process is checked by disassembling the battery, which specifically includes the following operations: First, disassemble the metal impurity monolithic battery; Then, the metal impurity particles are detected to see whether they are obviously precipitated on the negative electrode sheet and the separator of the metal impurity monolithic battery; Obviously precipitated, that is, the proportion of the metal impurity particles to the surface area of ​​the negative electrode sheet and the separator exceeds the preset second proportion. The preset second ratio in the second aging stage is greater than the preset first ratio in the first aging stage.

9. The method for evaluating self-discharge of a lithium-ion battery according to any one of claims 1 to 8, characterized in that: In step S3, the reference standard of the control group is equal to the sum of the average voltage drop rate Kave of all the single-chip batteries in the control group and three times the standard deviation δ.

10. The lithium ion battery self-discharge evaluation method according to claim 9, characterized in that: The average voltage drop rate of all the multiple control group single-chip batteries, Kave, is the average voltage drop rate of all the multiple control group single-chip batteries in the third aging stage; For each single-chip battery in the control group, the voltage drop rate k of the single-chip battery in the control group in the third aging stage is calculated based on the initial voltage V1 corresponding to the initial time t1 of the third aging stage and the end voltage V2 corresponding to the end time t2 of the aging stage; k = (V1 - V2) / (t2 - t1).

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