Method for improving purple area of circulating negative electrode interface of lithium ion battery and application

Abstract

The invention provides a method for improving a purple area of a circulating negative electrode interface of a lithium ion battery and application, and relates to the technical field of batteries. According to the method for improving the purple area of the circulating negative electrode interface of the lithium ion battery, a test fixture is mounted on a battery cell, a compressible buffer pad is arranged between the test fixture and the battery cell, and then a charge-discharge cycle test is performed on the battery cell; wherein the compressible buffer pad is arranged between the battery cell and the test clamp, the buffer characteristic of the compressible buffer pad is utilized, the contact mode of the clamp and the battery cell is changed, a buffer space is provided for charge-discharge cycle expansion of the battery cell, polarization accumulation in the cycle process can be improved, and generation and growth of a purple area at a negative electrode interface in the cycle process can be improved. The invention also provides application of the method for improving the purple area of the cycle negative electrode interface of the lithium ion battery in the field of batteries and module PACK.

Description

Technical Field

[0001] The invention belongs to the technical field of batteries and relates to a method and application for improving a purple zone at the negative electrode interface of a lithium ion battery cycle. Background Art

[0002] For lithium-ion batteries, especially automotive lithium-ion batteries, safety performance and warranty mileage are the most important measurement factors. Among them, the problem of the negative electrode interface when the battery is fully charged and disassembled at the EOL (End of Life) state is particularly important, especially whether there is a purple area on the negative electrode interface after cycling.

[0003] In lithium-ion batteries, because the negative electrode overhang is greater than the positive electrode width, three areas will be generated during the charge and discharge process. One is the normal golden lithium-embedded area directly corresponding to the positive and negative electrodes, one is the black (gray) area at the edge of the electrode without lithium embedding, and the other is the purple area between the normal golden lithium-embedded area and the black area without lithium embedding. Figure 1 and Figure 2 . As can be seen from the figure, the purple area is continuously distributed on the ear side and the non-ear side of the wound negative electrode sheet, and the width of the purple area is relatively wide. The principle of the purple area is that the increase in polarization during the cycle causes uneven lithium insertion. In the later stage of the cycle, the purple area will evolve into a lithium precipitation area, resulting in deterioration of the electrochemical performance and safety performance of the battery. User-side car companies are particularly concerned about whether the negative electrode interface is good after the battery cycle, especially how to avoid and improve the purple zone problem, which is particularly critical to the safety and electrochemical performance of the battery. At present, there is no report on the description of the purple zone problem at the negative electrode interface after lithium-ion battery cycling and related improvement ideas.

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

[0005] The present invention aims to provide a method and application for improving the purple zone at the negative electrode interface of a lithium ion battery cycle, so as to improve the above-mentioned problems existing in the prior art.

[0006] In order to achieve the above purpose, the following technical solutions are adopted:

[0007] The first object of the present invention is to provide a method for improving the purple zone of the negative electrode interface of a lithium ion battery cycle, comprising the following steps:

[0008] (a) Installing a test fixture on the battery cell and placing a compressible cushion between the test fixture and the battery cell;

[0009] (b) performing a charge-discharge cycle test on the battery cell obtained in step (a).

[0010] Further, based on the above technical solution of the present invention, in step (a), the pre-tightening force of the test fixture on the battery cell is 50-500Kgf.

[0011] Further, based on the above technical solution of the present invention, in step (a), the Shore hardness of the compressible cushion is 5-90HA;

[0012] and / or, the compression ratio of the compressible cushion is 10-90%;

[0013] And / or, the elastic modulus of the compressible cushion is 1-100 KPa;

[0014] and / or, the compressible cushion has a thickness of 0.5-3 mm;

[0015] And / or, the compressible buffer pad is made of at least one of polyethylene, polypropylene, polyamide, polyurethane, polyvinyl chloride, polyacrylonitrile, silicone, rubber, aerosol or foam.

[0016] Further, based on the above technical solution of the present invention, in step (a), the compressible buffer pad is arranged on one surface or two surfaces between the test fixture and the battery cell;

[0017] And / or, the compressible buffer pad covers 5-100% of the surface of the battery cell.

[0018] Furthermore, based on the above technical solution of the present invention, in step (a), the battery cell is a single-pole battery cell, a multi-pole battery cell or a full-pole battery cell.

[0019] Further, based on the above technical solution of the present invention, in step (b), the state of charge of the battery cell at the beginning of charging is 2-10% SOC, and the state of charge at the end of charging is 90-99% SOC.

[0020] Further, based on the above technical solution of the present invention, in step (b), the battery cell charge and discharge cycle system includes:

[0021] (b1) Step charging: above 80% SOC, the charging rate is ≤1C, and above 95% SOC, the charging rate is ≤1 / 3C. Constant current charging to the corresponding cut-off voltage, and the cut-off current is 0.05C;

[0022] (b2) leaving for 5-120 min;

[0023] (b3) Discharge at a constant current of ≤1C to the cut-off voltage;

[0024] (b4) leaving for 5-120 min;

[0025] (b5) Repeat (b1)-(b4) 1000-10000 times.

[0026] Further, based on the above technical solution of the present invention, in step (b1), constant current charging is performed until the SOC reaches above 95% and before the charging cut-off voltage, and the charging rate is 0.1C-1 / 3C.

[0027] Further, based on the above technical solution of the present invention, in step (b3), constant current discharge is performed to above 5% SOC, and the discharge rate is 0.2C-1 / 3C.

[0028] The second object of the present invention is to provide the application of the above method for improving the purple zone of the cycle negative electrode interface in the battery field.

[0029] Compared with the prior art, the technical solution of the present invention has at least the following technical effects:

[0030] (1) The present invention provides a method for improving the purple zone at the negative electrode interface of a lithium-ion battery during cycling, by arranging a compressible buffer pad between a battery cell and a fixture, utilizing the buffering properties of the compressible buffer pad and changing the contact mode between the fixture and the battery cell, providing a buffer space for the battery cell to expand during charge and discharge cycles, thereby improving the polarization accumulation during the cycle and improving the generation and growth of the purple zone at the negative electrode interface during the cycle.

[0031] (2) The present invention also provides an application of the above-mentioned method for improving the purple zone of the negative electrode interface of a lithium-ion battery cycle. In view of the advantages of the above-mentioned method for improving the purple zone of the negative electrode interface of a lithium-ion battery cycle, it has good application in the field of batteries and module PACKs. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a negative electrode interface diagram of a single cell of Comparative Example 1 of the present invention after being disassembled after 500 cycles;

[0033] Figure 2 for Figure 1 A partial enlarged view of

[0034] Figure 3 This is a negative electrode interface diagram of a single cell of Example 1 of the present invention after being disassembled after 500 cycles;

[0035] Figure 4 for Figure 3 A partial enlarged view of

[0036] Figure 5 This is a negative electrode interface diagram of a disassembled single cell of Example 10 of the present invention after 500 cycles;

[0037] Figure 6 for Figure 5 A partial enlarged view of . DETAILED DESCRIPTION

[0038] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme in the embodiment of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention. The process parameters of the following embodiments that do not specify specific conditions are usually based on conventional conditions.

[0039] The endpoints and any values ​​of the ranges disclosed in the present 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 each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in the present invention.

[0040] In the existing single cell performance test, the fixture is usually directly installed on the cell, that is, the fixture and the cell are directly in contact with each other for cycle testing. However, the inventors found that the negative electrode interface of the lithium-ion battery after charge and discharge cycles has a purple area problem, that is, there is a purple area between the golden normal lithium-embedded area and the black area without lithium-embedded area. Figure 1 If the purple zone problem is not solved or improved in time, the purple zone may evolve into a lithium precipitation zone in the later stage of the cycle, resulting in deterioration of battery performance and increased safety risks.

[0041] In order to improve the problem of purple area at the negative electrode interface after cycling, the inventor has conducted a lot of research and analyzed the reasons for the existence of purple area at the negative electrode interface after cycling. It is believed that it may be caused by the following reasons: during the cycle, the positive and negative electrode plates inside the battery cell show expansion and contraction effects as the charging and discharging process, especially in the later stage of the cycle, the lithium insertion state at different positions is different, the degree of bulging is also different, the expansion force is getting bigger and bigger, the battery cell bulging is getting more and more serious, the gap between the battery cell and the clamp is getting smaller and smaller, the extrusion between the battery cell and the clamp is getting bigger and bigger, the contact expansion force at different positions is getting more and more inconsistent, and the internal pole pieces are squeezed tighter and tighter between the clamp and the battery cell, and the electrolyte inside the pole piece is squeezed out. At the same time, the internal pole piece is subjected to more and more force, and there may be damage inside, which affects the lithium ion transmission between the positive and negative electrodes to form a purple area. The more serious purple area may evolve into a lithium precipitation area, which has an adverse effect on the electrochemical performance and safety performance of the battery. Based on this, the inventor has conducted a lot of practice and found that there is a certain relationship between the contact mode between the clamp and the battery cell and the purple area at the negative electrode interface. The problem of the purple area at the negative electrode interface can be improved by changing the contact mode between the clamp and the battery cell, and the present invention is specially proposed.

[0042] According to a first aspect of the present invention, there is provided a method for improving the purple zone of the negative electrode interface of a lithium ion battery cycle, comprising the following steps:

[0043] (a) Installing a test fixture on the battery cell and placing a compressible cushion between the test fixture and the battery cell;

[0044] (b) performing a charge-discharge cycle test on the battery cell obtained in step (a).

[0045] Specifically, in step (a), the compressible cushion can be compressed when subjected to external pressure (such as compression from the clamp and the battery cell) so as to have certain cushioning properties.

[0046] A compressible buffer pad is set between the battery cell and the clamp. The buffering properties of the compressible buffer pad are utilized and the contact mode between the clamp and the battery cell is changed to provide a buffer space for the expansion of the battery cell during charge and discharge cycles. This can improve the polarization accumulation during the cycle and improve the generation and growth of the purple zone at the negative electrode interface during the cycle.

[0047] As an optional embodiment of the present invention, in step (b), the state of charge of the battery cell at the beginning of charging is 2-10% SOC, for example, 2% SOC, 3% SOC, 4% SOC, 5% SOC, 6% SOC, 7% SOC, 8% SOC, 9% SOC or 10% SOC, and the state of charge at the end of charging is 90-99% SOC, for example, 90% SOC, 92% SOC, 93% SOC, 94% SOC, 95% SOC, 96% SOC, 97% SOC, 98% SOC or 99% SOC, etc.

[0048] In step (b), SOC is the state of charge, which is used to reflect the remaining capacity of the battery. Its value is defined as the ratio of the remaining capacity to the battery capacity, usually expressed as a percentage. For example, when SOC = 0.5, it means that the remaining capacity of the battery is 50% of the total capacity. During the cycle, the battery cell should not be fully charged and discharged as much as possible. Try to use a cycle test that is more in line with the user's operating conditions, which is also conducive to improving the purple area of ​​the negative electrode interface.

[0049] The present invention provides a method for improving the purple zone at the negative electrode interface of a lithium-ion battery during cycling. By changing the contact mode between a clamp and a battery cell during the charge and discharge cycle and cooperating with a specific charge and discharge cycle system, the purple zone at the negative electrode interface can be effectively reduced, thereby reducing the adverse effect of the purple zone on the performance of the battery cell.

[0050] As an optional embodiment of the present invention, in step (a), the preload force of the test fixture on the battery cell is 50-500Kgf. If the preload force is too small (lower than 50Kgf), the test fixture and the battery cell cannot achieve a good fit, resulting in a too long path during the lithium ion charging and discharging process, increased polarization, and is not conducive to high-rate charging and discharging. If the preload force is too small, it may even cause the fixture and the battery cell to be unable to be installed. If the preload force is too large (higher than 500Kgf), the test fixture has too much pressure on the pole piece of the battery cell, which is particularly easy to damage the pole piece inside the battery cell during the cycle, and is more likely to aggravate the deterioration of the cycle and the purple zone, and the worst causes safety problems. Therefore, typical but non-restrictive preload forces are 50Kgf, 100Kgf, 150Kgf, 200Kgf, 250Kgf, 300Kgf, 350Kgf, 400Kgf, 450Kgf or 500Kgf, etc. It has been found in practice that the preload range of 150-300kgf has the best improvement effect on the purple zone, and the corresponding cycle and voltage change during the cycle are the smallest.

[0051] The compressible cushion can be made of a polymer material, so that the compressible cushion has elastic deformation capability. As an optional embodiment of the present invention, in step (a), the material of the compressible cushion includes at least one of polyethylene, polypropylene, polyamide, polyurethane, polyvinyl chloride, polyacrylonitrile, silica gel, rubber, aerosol or foam. The compressible cushion can be a single-layer structure made of the above single material, or a multi-layer structure, or a single-layer structure made of at least two of the above materials, or a multi-layer structure, or a composite structure layer.

[0052] There are also certain optimizations for the properties of the compressible cushion.

[0053] As an optional embodiment of the present invention, in step (a), the compression ratio of the compressible cushion is 10-90%, and a typical but non-limiting compression ratio is 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90%, etc.

[0054] As an optional embodiment of the present invention, in step (a), the elastic modulus of the compressible buffer pad is 1-100KPa; a typical but non-limiting elastic modulus is 1KPa, 10KPa, 20KPa, 30KPa, 40KPa, 50KPa, 60KPa, 70KPa, 80KPa, 90KPa or 100KPa, etc.

[0055] As an optional embodiment of the present invention, in step (a), the Shore hardness of the compressible cushion is 5-90HA; typical but non-limiting Shore hardness is 5HA, 10HA, 20HA, 30HA, 40HA, 50HA, 60HA, 70HA, 80HA or 90HA, etc. If the Shore hardness of the compressible cushion is too large (higher than 90HA), the compressible cushion basically cannot play a buffering role, and the degree of improvement in the purple area effect is significantly reduced. If the Shore hardness is too large, the compressible cushion is not easy to compress. When it is applied to the application scenario of the PACK package, the battery cell cannot be loaded into the PACK package.

[0056] By further limiting the compression ratio, elastic modulus and Shore hardness of the compressible buffer, the application scenario of the PACK package can be better simulated. Generally, the total thickness of the battery cell plus the total thickness of the compressible buffer pad is greater than the thickness between the two end plates of the PACK package. Usually, it needs to be compressed to a certain ratio. Different compression ratios can be monitored according to the size of the preload force. The size of the preload force is related to the safety and performance of the PACK package. Due to the shortage of PACK package testing resources and in order to save R&D costs, most companies in the industry refer to the evaluation data of single battery cells, which can basically provide a good guiding significance.

[0057] There are also certain restrictions on the thickness of the compressible buffer pad. As an optional embodiment of the present invention, in step (a), the thickness of the compressible buffer pad is 0.5-3mm; if the thickness is too thin (less than 0.5mm), the battery cell and the fixture are basically in full contact after a certain preload, and the buffering effect during the cycle cannot be achieved. There is a certain damage to the pole piece, and the purple zone after the cycle cannot be improved, which affects the performance of the battery cell; if the thickness of the compressible buffer pad is too thick (greater than 3mm), it is easy to cause poor heat dissipation of the battery cell, which is not conducive to faster charging and discharging; and the thicker the thickness, the greater the amount used, and the cost will also increase. In addition, since the length of the two end plates of the chassis of the PACK package is fixed, the thicker the thickness of the compressible buffer pad will lead to the following problems: first, it cannot be installed, and second, it will reduce the energy density, and the cruising range of the car will be greatly reduced, which is not conducive to the application of actual application scenarios. Therefore, the typical but non-restrictive thickness is 0.5mm, 1.0mm, 1.5mm, 2.0mm, 2.5mm or 3.0mm, etc. After experimental verification, the optimal thickness of the compressible cushion is 1-2 mm.

[0058] When a fixture and a battery cell are used for cycle testing in the prior art, the fixture is usually in direct contact with both sides of the battery cell. In the present invention, a compressible buffer pad is arranged between the test fixture and the battery cell, and can be specifically arranged on one surface where the test fixture contacts the battery cell (single-sided installation), or can be arranged on two surfaces where the test fixture contacts the battery cell (double-sided installation), that is, the compressible buffer pad has a certain coverage on the surface of the battery cell. If no coverage is done, there is no buffer at all between the battery cell and the fixture. During the cycle, due to the thickening of the SEI film, the thickness of the pole piece increases, but the distance between the fixture and the battery cell is fixed, and the clamp squeezes the internal pole piece of the battery cell more and more, and the purple area grows larger and larger, which is not conducive to the performance of the battery cell.

[0059] There is a certain optimization for the coverage area or coverage ratio of the compressible buffer pad on the surface of the battery cell, which can be determined in combination with performance data. As an optional embodiment of the present invention, in step (a), the coverage area of ​​the compressible buffer pad on the surface of the battery cell is 5-100%. Typical but non-limiting coverage areas are 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 100%, etc. When the coverage area of ​​the compressible buffer pad on the surface of the battery cell is 100%, it can be considered that the compressible buffer pad is fully covered on the surface of the battery cell. The fully covered compressible buffer pad has a worse heat dissipation capacity than the compressible buffer pad with incomplete coverage (less than 100%), and the amount of the fully covered compressible buffer pad is large, and the cost will also increase. At the same time, the inventor has verified through experiments that the compressible buffer pad does not need to be fully covered on the surface of the battery cell to achieve the same improvement effect. The best covering method is to hollow out the middle, and only a certain width of the pole pieces at the two ends of the pole piece or a certain width of the four sides of the edge pole piece is needed. Different shapes can also be made. The best effect can be achieved with a coverage ratio of 5-20%.

[0060] At present, the fast charging system battery cells are mainly based on the winding system and the same-side pole ear method. The present invention is not only applicable to winding battery cells, but also to stacked battery cells of different systems. Battery cells of different systems and different structural methods have the same purple zone problem. Among them, the purple zone phenomenon of the wound battery cell only appears on the upper and lower pole ear sides and non-pole ear sides of the pole piece. In addition to the upper and lower pole ear sides and non-pole ear sides of the pole piece, the purple zone of the stacked battery cell also appears on the non-pole ear sides of the left and right pole pieces in addition to the purple zone on the upper and lower pole ear sides and non-pole ear sides of the pole piece. The production principle is the same. The inventor found through experiments that the battery cell with the same-side pole ear method also has an obvious purple zone phenomenon at the negative electrode interface after cycling. This is mainly due to the uneven current field and temperature field during the charge and discharge cycle at a higher rate under the above-mentioned structural method, which leads to uneven lithium insertion at different positions of the charge and discharge overcurrent, and then forms a purple zone. Therefore, certain improvements are made to the pole ear structure design of the battery cell to improve the purple zone problem at the negative electrode interface.

[0061] As an optional embodiment of the present invention, in step (a), the battery cell is a single-pole ear battery cell, a multi-pole ear battery cell or a full-pole ear battery cell, preferably a full-pole ear battery cell. The full-pole ear design has better uniformity of temperature field and current field than the positive and negative pole ears on the same side, and has obvious improvement on the purple area during the cycle, and also has obvious improvement on polarization improvement and cycle improvement.

[0062] As an optional embodiment of the present invention, in step (a), the tabs of the battery cell are arranged on opposite sides. After simulation and experimental data verification, the temperature field and current field at each position inside the battery cell are more uniform when the tabs are arranged on opposite sides compared with the tabs on the same side, especially when the tab width is the same as the pole piece width.

[0063] By further defining the above-mentioned ear structure, the purple area problem at the negative electrode interface can be improved to a certain extent.

[0064] At the same time, there are also certain optimizations for the battery cell charge and discharge cycle system. The prior art generally adopts a 100% DOD discharge cycle, a 1C rate discharge is generally used in the discharge process, and a constant current and constant voltage charge is generally used in the charging process to a cut-off voltage of 0.05C current cut-off. The charge and discharge strategy of the single battery cell is more stringent than the actual vehicle PACK package BMS charging strategy, and this charging method deviates from the actual PACK package BMS control measurement, and is out of the current market application end car companies The application scenario cannot feedback the actual application scenario. Moreover, this charging strategy is prone to cumulative overcharging problems during the cycle, which is easy to cause polarization growth, resulting in an increase in the purple area of ​​the negative electrode interface during the cycle, and ultimately leading to safety problems. Therefore, the present invention, on the basis of changing the contact method between the clamp and the battery cell, and optimizing the charge and discharge cycle strategy, further improves the problem of the purple area of ​​the negative electrode interface through the combination of the two methods.

[0065] As an optional embodiment of the present invention, in step (b), the battery cell charge and discharge cycle system includes:

[0066] (b1) Step charging, then charging at a constant current of ≤1C (e.g., 1C, 2 / 3C, 1 / 3C, 0.1C, etc.) above 80% SOC (80% SOC-95% SOC), and ≤1 / 3C (e.g., 1 / 3C, 0.2C or 0.1C, etc.) above 95% SOC to the corresponding cut-off voltage, with a cut-off current of 0.05C;

[0067] The charging strategies are different, and the material systems have different corresponding SOC charging rates. Generally, the charging rate is from low to high to low from low SOC to high SOC. This is mainly because the impedance is large at low SOC and to prevent overcharging at high rates at high SOC.

[0068] Taking the average 2.5C (10-80% SOC, that is, the charging range under the average fast charge rate of 2.5C is 10-80% SOC) fast charging system as an example, 0-5% SOC 1 / 3C rate charging, 5-10% SOC 1C rate charging, 10-40% SOC 3C rate charging, 40-60% SOC 2.5C rate charging, 60-70% SOC 2.25C rate charging, 70-80% SOC 1.8C rate charging, 80-95% SOC 1C rate charging, 95-100% SOC 1 / 3C rate charging, the cycle number range is between 1500-6000 times;

[0069] (b2) leaving for 5-120 min, preferably 20-60 min (e.g., 20 min, 30 min, 40 min or 60 min, etc.);

[0070] (b3) discharging at a constant current of ≤1C (e.g., 1C, 2 / 3C, 1 / 3C, 0.1C, etc.) to the cut-off voltage;

[0071] (b4) leaving for 5-120 min, preferably 20-60 min (e.g., 20 min, 30 min, 40 min or 60 min, etc.);

[0072] (b5) Repeat (b1)-(b4) 1000-10000 times (for example, 1000 times, 2000 times, 2500 times, 3000 times, 4000 times, 5000 times, 6000 times, 8000 times or 10000 times, etc.).

[0073] As an optional embodiment of the present invention, in step (b1), constant current charging is performed to a SOC of 95% or more and before the charging cut-off voltage, and the charging rate is 0.1C-1 / 3C, such as 0.1C, 0.2C, 0.3C or 1 / 3C. A small charging rate at the terminal SOC has a small effect on the polarization accumulation during the cycle, which is more conducive to the improvement of the purple zone.

[0074] As an optional embodiment of the present invention, in step (b3), constant current discharge is performed to above 5% SOC, and the discharge rate is 0.2C-1 / 3C, for example, 0.2C, 0.3C or 1 / 3C.

[0075] The present invention optimizes the charging and discharging mechanism to avoid over-discharging during the cycle process under high SOC of the battery cell and the discharge process at a high rate, improves the terminal voltage polarization accumulation during the cycle, and at the same time, adds a compressible buffer material between the battery cell and the test fixture to give the battery cell a certain expansion space during the cycle. Different material combinations and material composition, material thickness and compression ratio optimize the purple area of ​​the negative electrode interface after the cycle, improve the safety and cycle performance of the battery cell, and the full-ear design structure optimizes the current field and temperature field of different positions of the battery cell pole piece, so that the temperature distribution at each position is more uniform, and the SOC distribution at each position of the pole piece during the charging and discharging process is more uniform, and the above-mentioned method is used in combination for the best effect. And the above-mentioned method of improving the purple area of ​​the negative electrode interface of the cycle of the present invention can be applied to various battery systems, including but not limited to iron-lithium systems, ternary systems, etc.

[0076] According to the second aspect of the present invention, there is also provided application of the method for improving the purple zone of the cycle negative electrode interface provided by the first aspect of the present invention in the field of batteries and module PACKs.

[0077] In view of the advantages of the above-mentioned method for improving the purple zone of the cycle negative electrode interface of the present invention, it has good application in the field of batteries and module PACK.

[0078] The present invention will be further described in detail below with reference to specific embodiments and comparative examples.

[0079] Example 1

[0080] This embodiment provides a method for improving the purple zone of the cycle negative electrode interface, comprising the following steps:

[0081] (a) A cell with a full-ear structure with the ear on the opposite side is used. The test fixture uses a 200kgf preload force on the single cell (5EA). The initial state of charge of the single cell on the upper fixture is 27% SOC. A compressible cushion (single-layer silicone cushion) is added on both sides between the single cell and the test fixture; wherein the compression ratio of the compressible cushion is 50%, the elastic modulus is 20Kpa, the Shore hardness is 30HA, the thickness is 1.5mm, and the compressible cushion covers the hollowing out in the middle of the upper and lower and left and right positions of the cell, with a coverage ratio of 10%;

[0082] (b) The battery cell obtained in step (a) is first charged to 3.65V at 1 / 3C constant current and constant voltage, with a cut-off current of 0.05C and a shelf time of 30min, and then discharged to 2.5V at 1 / 3C constant current. After the initial capacity of the battery cell is calibrated, a charge and discharge cycle test (cycle test) is performed, wherein the state of charge of the battery cell at the beginning of charging is 4% SOC and the state of charge at the end of charging is 99% SOC. The specific cycle charging strategy is as follows:

[0083] (b1) Step charging with constant current (charging rate of 0.2C for SOC above 95%) to the corresponding cut-off voltage, cut-off current of 0.05C;

[0084] Specifically, for the average 2.5C (10-80% SOC) iron-lithium fast charging system, the specific charging strategy is: 4-5% SOC 1 / 3C rate constant current charging, 5-10% SOC 1C rate constant current charging, 10-40% SOC 3C rate constant current charging, 40-60% SOC 2.5C rate constant current charging, 60-70% SOC 2.25C rate constant current charging, 70-80% SOC 1.8C rate constant current charging, 80-95% SOC 1C rate constant current charging, 95-99% SOC 0.33C rate constant current constant voltage charging to 3.65V, cut-off current 0.1C cut-off;

[0085] (b2) Leave for 30 min;

[0086] (b3) 1 / 3C constant current discharge to cut-off voltage 2.5V;

[0087] (b4) Leave for 30 min;

[0088] (b5) Repeat (b1)-(b4) 2500 times.

[0089] Example 2

[0090] This embodiment provides a method for improving the purple zone of the cycle negative electrode interface, comprising the following steps:

[0091] (a) A non-full-ear structure design cell with the same-side ear is adopted, and the test fixture uses a 300kgf preload force on the single cell (5EA). The initial state of charge of the single cell in the upper fixture is 27% SOC. A compressible cushion (a layer of composite cushion, i.e., an outer layer of foamed silica gel, a middle layer of PP material, and an inner layer of aerosol) is added to only one side between the single cell and the test fixture; wherein the compression ratio of the compressible cushion is 40%, the elastic modulus is 35Kpa, the Shore hardness is 50HA, the thickness is 1.0mm, and the compressible cushion covers the hollowing out in the middle of the upper and lower and left and right positions of the cell, with a coverage ratio of 10%;

[0092] (b) The battery cell obtained in step (a) is first charged to 3.65V with a 1 / 3C constant current and constant voltage, with a cut-off current of 0.05C and a shelf time of 30min, and then discharged to 2.5V with a 1 / 3C constant current. After the initial capacity of the battery cell is calibrated, a charge and discharge cycle test (cycle cabinet test) is performed, wherein the state of charge of the battery cell at the beginning of charging is 3% SOC, and the state of charge at the end of charging is 98% SOC. The specific cycle charging strategy is as follows:

[0093] (b1) Step charging with constant current (charging rate of 0.2C for SOC above 95%) to the corresponding cut-off voltage, cut-off current of 0.05C;

[0094] Specifically, for the ternary fast charging system with an average of 2.5C (10-80% SOC), the specific charging strategy is: 3-5% SOC 1 / 3C constant current charging, 5-10% SOC 1C constant current charging, 10-40% SOC 3C constant current charging, 40-60% SOC 2.5C constant current charging, 60-70% SOC 2.25C constant current charging, 70-80% SOC 1.8C constant current charging, 80-95% SOC 1C constant current charging, 95-98% SOC 0.33C constant current charging to 3.65V, cut-off current 0.1C cut-off;

[0095] (b2) Leave for 30 min;

[0096] (b3) 1 / 3C constant current discharge to cut-off voltage 3.0V;

[0097] (b4) Leave for 30 min;

[0098] (b5) Repeat (b1)-(b4) 2500 times.

[0099] Example 3

[0100] This embodiment provides a method for improving the purple zone of the cycle negative electrode interface, comprising the following steps:

[0101] (a) A non-full-ear structure design cell with the same-side ear is adopted, the test fixture uses a 400kgf preload force on the single cell (5EA), the initial state of charge of the single cell on the upper fixture is 27% SOC, and a compressible cushion is added on both sides between the single cell and the test fixture (the compressible cushion includes a middle layer PP material plate, and a foamed silicone pad located on the front and back sides of the middle layer PP material plate, a total of three layers); wherein, the compression ratio of the compressible cushion is 30%, the elastic modulus is 20Kpa, the Shore hardness is 40HA, the thickness is 1.2mm, and the coverage ratio is 100%;

[0102] (b) The battery cell obtained in step (a) is first charged to 3.65V at 1 / 3C constant current and constant voltage, with a cut-off current of 0.05C and a shelf time of 30min, and then discharged to 2.5V at 1 / 3C constant current. After the initial capacity of the battery cell is calibrated, a charge and discharge cycle test (cycle test) is performed, wherein the state of charge at the beginning of charging of the battery cell is 3% SOC and the state of charge at the end of charging is 98% SOC. The specific cycle charging strategy is as follows:

[0103] (b1) Step charging with constant current (charging rate of 0.2C for SOC above 95%) to the corresponding cut-off voltage, cut-off current of 0.05C;

[0104] Specifically, for the average 2.5C (10-80% SOC) iron-lithium fast charging system, the specific charging strategy is: 3-5% SOC 1 / 3C constant current charging, 5-10% SOC 1C constant current charging, 10-40% SOC 3C constant current charging, 40-60% SOC 2.5C constant current charging, 60-70% SOC 2.25C constant current charging, 70-80% SOC 1.8C constant current charging, 80-95% SOC 1C constant current charging, 95-98% SOC 0.33C constant current charging to 3.65V, cut-off current 0.1C cut-off;

[0105] (b2) Leave for 30 min;

[0106] (b3) 1 / 3C constant current discharge to cut-off voltage 2.5V;

[0107] (b4) Leave for 30 min;

[0108] (b5) Repeat (b1)-(b4) 2500 times.

[0109] Example 4

[0110] This embodiment provides a method for improving the purple zone of the cycle negative electrode interface, except that the preload force in step (a) of embodiment 1 is adjusted from 200 kgf to 50 kgf, the rest is the same as embodiment 1.

[0111] Example 5

[0112] This embodiment provides a method for improving the purple zone of the cycle negative electrode interface, except that the preload force in step (a) of embodiment 1 is adjusted from 200 kgf to 500 kgf, the rest is the same as embodiment 1.

[0113] Example 6

[0114] This embodiment provides a method for improving the purple zone of the cycle negative electrode interface, except that the thickness of the compressible buffer pad in step (a) of Example 1 is adjusted from 1.5 mm to 3.0 mm, the rest is the same as Example 1.

[0115] Example 7

[0116] This embodiment provides a method for improving the purple zone of the cycle negative electrode interface, except that the thickness of the compressible buffer pad in step (a) of Example 1 is adjusted from 1.5 mm to 0.5 mm, the rest is the same as Example 1.

[0117] Example 8

[0118] This embodiment provides a method for improving the purple zone of the cycle negative electrode interface, except that the Shore hardness of the compressible buffer pad in step (a) of Example 1 is adjusted from 30HA to 100HA, and the rest is the same as Example 1.

[0119] Example 9

[0120] This embodiment provides a method for improving the purple zone of the negative electrode interface during cycling. Except for designing the battery cell with a single-pole lug structure with the lugs on the same side in step (a), the remaining steps are the same as those in embodiment 1.

[0121] Example 10

[0122] This embodiment provides a method for improving the purple zone of the cycle negative electrode interface, comprising the following steps:

[0123] (a) A single-pole ear structure with the same-side pole ear is used to design the battery cell. The test fixture uses a 200kgf preload force on the single battery cell. The initial state of charge of the single battery cell in the upper fixture is 27% SOC. A compressible buffer pad (single-layer silicone pad) is added on both sides between the single battery cell and the test fixture; wherein the compression ratio of the compressible buffer pad is 50%, the elastic modulus is 20Kpa, the Shore hardness is 30HA, the thickness is 1.5mm, and the compressible buffer pad covers the hollowing out in the middle of the upper and lower and left and right positions of the battery cell, with a coverage ratio of 10%;

[0124] (b) The battery cell obtained in step (a) is first charged to 3.65V at 1 / 3C constant current and constant voltage, with a cut-off current of 0.05C and a shelf time of 30min, and then discharged to 2.5V at 1 / 3C constant current. After the initial capacity of the battery cell is calibrated, a charge and discharge cycle test (cycle on-board test) is performed, wherein the state of charge at the beginning of charging of the battery cell is 0% SOC, and the state of charge at the end of charging is 100% SOC. The specific cycle charging strategy is as follows:

[0125] (b1) Step charging with constant current to the corresponding cut-off voltage, and the cut-off current is 0.05C;

[0126] The specific charging strategy for the iron-lithium system is: 0-10% SOC 1.5C constant current charging, 10-40% SOC 3C constant current charging, 40-60% SOC 2.5C constant current charging, 60-70% SOC 2.25C constant current charging, 70-80% SOC 1.8C constant current charging, 80-100% SOC 1C constant current constant voltage charging to 3.65V, cut-off current 0.05C cut-off;

[0127] (b2) Leave for 30 min;

[0128] (b3) 1C constant current discharge to cut-off voltage 2.5V;

[0129] (b4) Leave for 30 min;

[0130] (b5) Repeat (b1)-(b4) 2500 times.

[0131] Comparative Example 1

[0132] This comparative example provides a method for improving the purple zone of the cycle negative electrode interface, comprising the following steps:

[0133] (a) The battery cell is designed with a non-full-ear structure with the same-side ear. The test fixture uses a 300kgf preload force on the single battery cell (5EA). The initial state of charge of the single battery cell in the upper fixture is 27% SOC. There is no compressible buffer pad between the single battery cell and the test fixture.

[0134] (b) The battery cell obtained in step (a) is first charged to 3.65V at 1 / 3C constant current and constant voltage, with a cut-off current of 0.05C and a shelf time of 30min, and then discharged to 2.5V at 1 / 3C constant current. After the initial capacity of the battery cell is calibrated, a charge and discharge cycle test (cycle on-board test) is performed, wherein the state of charge at the beginning of charging of the battery cell is 0% SOC, and the state of charge at the end of charging is 100% SOC. The specific cycle charging strategy is as follows:

[0135] (b1) Step charging with constant current to the corresponding cut-off voltage, and the cut-off current is 0.05C;

[0136] The specific charging strategy for the iron-lithium system is: 0-10% SOC 1.5C constant current charging, 10-40% SOC 3C constant current charging, 40-60% SOC 2.5C constant current charging, 60-70% SOC 2.25C constant current charging, 70-80% SOC 1.8C constant current charging, 80-100% SOC 1C constant current constant voltage charging to 3.65V, cut-off current 0.05C cut-off;

[0137] (b2) Leave for 30 min;

[0138] (b3) 1C constant current discharge to cut-off voltage 2.5V;

[0139] (b4) Leave for 30 min;

[0140] (b5) Repeat (b1)-(b4) 2500 times.

[0141] In order to better illustrate the technical effects of each embodiment and comparative example, the following experimental examples are specially set up.

[0142] Each embodiment and comparative example uses 5 single cells as a group to perform cycle tests simultaneously. When the charge and discharge cycles reach 500 cycles, 2 single cells in each group are selected to measure the voltage increase after 500 cycles. Then they are disassembled and the width of the purple zone is measured. The remaining single cells in each group continue to perform cycle tests until the end of the test.

[0143] At the same time, the contents in Table 1 are explained as follows:

[0144] (1) The width of the purple area after 500 cycles, where the purple area refers to the non-complete lithium insertion area. The upper and lower parts refer to the orientation of the pole piece. Taking the wound battery as an example, the upper part represents the pole ear side of the pole piece, and the lower part represents the non-pole ear side. The same is true for stacked models, except that stacked batteries can also be divided into left and right. This example only takes the wound battery as an example;

[0145] (2) The voltage increase after 500 cycles refers to the increase in the cut-off voltage at the corresponding SOC during the cycle;

[0146] (3) EOL cycle number refers to the number of charge and discharge cycles experienced when the battery capacity drops to 80% of the initial capacity.

[0147] Table 1

[0148]

[0149] At the same time, taking Example 1, Example 10 and Comparative Example 1 as examples, the negative electrode interface diagrams of the disassembled single cells after 500 cycles are compared. Figure 1 and Figure 2 As shown, the negative electrode interface diagram of the disassembled single cell of Example 1 after 500 cycles is as follows Figure 3 and Figure 4 As shown, the negative electrode interface diagram of the disassembled single cell of Example 10 after 500 cycles is as follows Figure 5 and Figure 6 shown.

[0150] Combining the data from Table 1 Figure 1-Figure 6 It can be seen that compared with comparative example 1, the purple area of ​​each embodiment of the present invention is reduced to varying degrees. Moreover, through comparison, it can be seen that the combination of the three improvement methods of compressible buffer pad + full-ear structure + charging strategy optimization (such as embodiment 1) is the best for improving the purple area. Figure 3 and Figure 4 Among the three improvement methods, namely, introducing a compressible cushion, adopting a full-ear structure, and optimizing the charging strategy, introducing a compressible cushion between the fixture and the battery cell (such as Example 10) has the most obvious improvement effect on the purple area. Figure 5 and Figure 6 .

[0151] At the same time, it can be seen from the data in Table 1 that too large or too small preload force has no significant effect on the improvement of the purple zone after the cycle. When a preload force of 150-300kgf is used, the improvement effect on the purple zone after the cycle is better.

[0152] The method provided by the present invention can also simultaneously reduce the difference in cycle tests between the PACK package and the single cell, making the discount coefficients of the single cell cycle and the PACK package cycle closer. At present, in the industry, the cycle discount ratio of single cells and PACK packages is generally 70-80%, and the present invention can increase the discount ratio coefficient to about 80-90%, improving the user experience. The main reason is that it improves the purple area problem of the negative electrode interface during the cycle process, and can also improve the voltage alarm and other related problems caused by the large dynamic pressure difference at the end of the charge and discharge cycle of the PACK package and the module, thereby avoiding overcharging and over-discharging of the module and the PACK package, and improving the safety of the battery cell and the module PACK.

[0153] The above description is only 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 are within the scope of protection of the pending claims of the present invention.

Claims

1. A method for improving the purple zone of the negative electrode interface of a lithium ion battery cycle, characterized in that: The following steps are involved: (a) Installing a test fixture on the battery cell and placing a compressible cushion between the test fixture and the battery cell; (b) performing a charge-discharge cycle test on the battery cell obtained in step (a).

2. The method for improving the purple zone of the negative electrode interface of a lithium ion battery according to claim 1, characterized in that: In step (a), the pre-tightening force of the test fixture on the battery cell is 50-500Kgf.

3. The method for improving the purple zone of the negative electrode interface of a lithium ion battery according to claim 1, characterized in that: In step (a), the Shore hardness of the compressible cushion is 5-90HA; and / or, the compression ratio of the compressible cushion is 10-90%; And / or, the elastic modulus of the compressible cushion is 1-100 KPa; and / or, the compressible cushion has a thickness of 0.5-3 mm; And / or, the compressible buffer pad is made of at least one of polyethylene, polypropylene, polyamide, polyurethane, polyvinyl chloride, polyacrylonitrile, silicone, rubber, aerosol or foam.

4. The method for improving the purple zone of the negative electrode interface of a lithium ion battery according to claim 1, characterized in that: In step (a), the compressible buffer pad is disposed on one surface or two surfaces between the test fixture and the battery cell; And / or, the compressible buffer pad covers 5-100% of the surface of the battery cell.

5. The method for improving the purple zone of the negative electrode interface of a lithium ion battery according to claim 1, characterized in that: In step (a), the battery cell is a single-pole battery cell, a multi-pole battery cell or a full-pole battery cell.

6. The method for improving the purple zone of the negative electrode interface of a lithium ion battery according to claim 1, characterized in that: In step (b), the state of charge of the battery cell at the beginning of charging is 2-10% SOC, and the state of charge at the end of charging is 90-99% SOC.

7. The method for improving the purple zone of the negative electrode interface of a lithium ion battery according to claim 6, characterized in that: In step (b), the battery cell charge and discharge cycle system includes: (b1) Step charging: above 80% SOC, the charging rate is ≤1C, and above 95% SOC, the charging rate is ≤1 / 3C. Constant current charging to the corresponding cut-off voltage, and the cut-off current is 0.05C; (b2) leaving for 5-120 min; (b3) Discharge at a constant current of ≤1C to the cut-off voltage; (b4) leaving for 5-120 min; (b5) Repeat (b1)-(b4) 1000-10000 times.

8. The method for improving the purple zone of the negative electrode interface of a lithium ion battery according to claim 7, characterized in that: In step (b1), constant current charging is performed until the SOC reaches 95% or above and before the charging cut-off voltage, and the charging rate is 0.1C-1 / 3C.

9. The method for improving the purple zone of the negative electrode interface of a lithium ion battery according to claim 7, characterized in that: In step (b3), the battery is discharged at a constant current to a SOC of 5% or above, and the discharge rate is 0.2C-1 / 3C.

10. Application of the method for improving the purple zone at the interface of a circulating negative electrode according to any one of claims 1 to 9 in the field of batteries and module PACKs.

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