Preparation process of lithium ion battery and lithium ion battery

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a preparation process of a lithium ion battery and the lithium ion battery. Before the lithium ion battery is used, circulation is carried out for 2-10 weeks at a minimum rate, and the lithium ion battery is used for carrying out secondary load training on a positive electrode current collector and a negative electrode current collector and carrying out pre-lithiation on a negative electrode; by controlling the charge-discharge multiplying power of the battery and the lithium ion de-intercalation speed of the battery, the stress borne by the positive and negative current collectors during expansion and contraction of the battery is controlled, so that the positive and negative current collectors are repeatedly acted by the stress less than the fatigue limit, low-load strengthening is realized, the fatigue strength of the current collectors is further improved, and the service life of the battery is prolonged. And the fatigue damage of the current collector is reduced, so that the current collector has longer service life in the expansion and contraction process of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a preparation process of a lithium ion battery and a lithium ion battery. Background Art

[0002] The widespread use of lithium-ion batteries at this stage has led people to place higher demands on the energy density and cycle performance of lithium-ion batteries. In order to improve the energy density and capacity of batteries, high-energy-density batteries such as silicon-doped lithium-ion batteries that can embed more lithium ions have begun to be used.

[0003] During the cycling process of lithium-ion batteries, the intercalation and deintercalation of lithium ions causes the battery to repeatedly expand / contract in width and thickness. This is especially true for high-energy-density batteries, where the greater the number of intercalated and deintercalated lithium ions, the greater the expansion / contraction of the battery, which in turn generates stress. For batteries with a wound structure, the structural discontinuity of the corners can easily lead to stress concentration. Under the action of cyclic stress, the current collector suffers fatigue damage, with the outermost corner of the aluminum foil breaking first and then gradually intensifying and spreading to the inner ring, seriously affecting the performance of the battery cell.

[0004] Based on this, it is urgent to invent a preparation process of a lithium ion battery and a lithium ion battery to solve the above technical problems. Summary of the Invention

[0005] One of the purposes of the present invention is to provide a preparation process for a lithium ion battery to improve the fatigue strength of the current collector in response to the deficiencies of the prior art.

[0006] To solve the above technical problems, this application adopts the following technical solutions:

[0007] A process for preparing a lithium-ion battery is provided, comprising the following steps:

[0008] S1. Coating a positive electrode active material on at least one surface of a positive electrode current collector to form a positive electrode sheet, and coating a negative electrode active material on at least one surface of a negative electrode current collector to form a negative electrode sheet;

[0009] S2, stacking the positive electrode sheet, the separator, and the negative electrode sheet in sequence and winding them to form a core, and encapsulating the core with an aluminum-plastic film to prepare a dry battery cell;

[0010] S3, baking and drying the dry battery cell;

[0011] S4, injecting electrolyte into the dry cell and then performing formation, volume separation, and secondary sealing processes to produce a finished lithium-ion battery;

[0012] S5. Before using the lithium-ion battery, cycle it at a very low rate for 2-10 times to perform sub-load training on the positive and negative current collectors and pre-lithiate the negative electrode;

[0013] Among them, the minimum magnification is less than the fatigue limit magnification, and the fatigue limit magnification is the smaller value of the magnification when the fatigue limit of the positive current collector is reached and the magnification when the fatigue limit of the negative current collector is reached.

[0014] Furthermore, the magnitude of the minimum magnification is 40% - 100% of the fatigue limit magnification.

[0015] Furthermore, the ratio of the minimum magnification to the standard usage magnification of the battery is between 1 / 12 and 1 / 4.

[0016] Furthermore, it further includes step S6: cycling the lithium-ion battery at different magnifications and measuring the stresses received by the positive current collector and the negative current collector.

[0017] Furthermore, in step S1, it also includes that at each corner position of the positive electrode sheet, part of the positive electrode active material is removed by laser cleaning to form one or more grooves arranged at intervals along the winding direction of the positive electrode sheet.

[0018] Furthermore, the relationship between the depth h of the groove and the thickness H of the positive electrode active material layer is 0.5H < h < H.

[0019] Furthermore, the position of the groove area covers the center of the corner and does not exceed the corner, and the following relationship exists between the width s of the groove area and the core thickness n: d < s < πn - d, where d is the tolerance of the laser cleaning equipment.

[0020] Furthermore, in step S3, the baking temperature is 85 - 100°C and the baking time is 24 - 72H.

[0021] Furthermore, for lithium-ion batteries equipped with silicon-carbon materials, the baking temperature is not higher than 95°C; for lithium-ion batteries with pure graphite anodes, the baking temperature is not higher than 100°C; for lithium-ion batteries, the baking duration is not more than 72H.

[0022] The beneficial effect of the present invention is that the present invention realizes low-load strengthening (coaxing) of the current collector of the lithium-ion battery by cycling the lithium-ion battery for multiple cycles at an extremely low rate before use. Low-load strengthening (coaxing effects) means that the strength (including static strength and fatigue strength) of a metal material with strain aging can be strengthened and improved after a certain number of repeated actions and exercises of low-amplitude loads (lower than the fatigue limit). By controlling the charge and discharge rates of the battery and the speed of lithium ion insertion and extraction from the battery, the stress on the positive and negative current collectors during battery expansion and contraction is controlled, so that the positive and negative current collectors are repeatedly subjected to stress less than the fatigue limit, thereby improving the fatigue strength of the current collector, reducing fatigue damage caused by the current collector, and making the current collector have a longer service life during the expansion and contraction of the battery.

[0023] A second object of the present invention is to provide a lithium-ion battery prepared by any of the above-mentioned lithium-ion battery preparation processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0025] Figure 1 Schematic diagram of the structure of the positive electrode sheet 1 in the present invention;

[0026] Figure 2 Schematic diagram of the structure of the lithium-ion battery in the present invention.

[0027] Wherein: 1-groove; H-thickness of the positive electrode active material layer; h-depth of the groove; n-thickness of the core; s-width of the groove area. DETAILED DESCRIPTION

[0028] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by technicians in the technical field of the present application. The terms used herein in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application.

[0029] The present invention will be further described below in conjunction with the accompanying drawings, but this does not limit the present invention.

[0030] One aspect of the present application provides a process for preparing a lithium-ion battery, comprising the following steps:

[0031] S1. Coating a positive electrode active material on at least one surface of a positive electrode current collector to form a positive electrode sheet, and coating a negative electrode active material on at least one surface of a negative electrode current collector to form a negative electrode sheet;

[0032] S2, stacking the positive electrode sheet, the separator, and the negative electrode sheet in sequence and winding them to form a core, and encapsulating the core with an aluminum-plastic film to prepare a dry battery cell;

[0033] S3, baking and drying the dry battery cells;

[0034] S4, injecting electrolyte into the dry cell and then undergoing formation, volume separation, and secondary sealing processes to produce a finished lithium-ion battery;

[0035] S5. Before using the lithium-ion battery, cycle it at a very low rate for 2-10 times to perform sub-load training on the positive and negative current collectors and pre-lithiate the negative electrode;

[0036] Among them, the minimum rate is less than the fatigue limit rate, and the fatigue limit rate is the smaller value of the rate when the fatigue limit of the positive electrode collector is reached and the rate when the fatigue limit of the negative electrode collector is reached. The present application charges and discharges the lithium-ion battery multiple times, causing the battery to expand and contract multiple times, thereby generating stress on the positive electrode collector and the negative electrode collector to achieve sub-load training. Sub-load training refers to the metal material with strain aging being subjected to a certain number of low-amplitude loads (lower than the fatigue limit) and training repeatedly, and its strength (including static strength and fatigue strength) can be strengthened and improved. The present application improves the static strength and fatigue strength of the positive electrode collector and the negative electrode collector by sub-load training, so that the positive electrode collector and the negative electrode collector will produce less fatigue damage and are not prone to breakage after the expansion and contraction of the battery during multiple charge and discharge, thereby improving the service life of the positive electrode collector and the negative electrode collector, and the fatigue limit of the collector after sub-load training is also improved, which can adapt to the requirements of the charge and discharge of higher-rate batteries for the collector.

[0037] The secondary load training needs to meet the requirement that the stress received is less than the fatigue limit. When the stress received is greater than the fatigue limit, the current collector will be damaged. Therefore, in order to ensure that the stress generated by the expansion and contraction of the battery on the current collector during the extremely small rate cycle is appropriate, it is necessary to measure the rate at which the fatigue limit of the positive and negative current collectors is reached. This can be achieved by embedding a PVDF piezoelectric film between the current collector and the active material layer, and measuring the stress received by the positive and negative current collectors by inferring the interface stress through the charge output. The charging can be adjusted at different rates to obtain the rate at which the fatigue limit of the positive and negative current collectors is reached. It can also be achieved through multiple experiments based on the fracture of the positive and negative current collectors. The fatigue limit rate of the positive and negative current collectors is estimated under certain circumstances. The smaller value of the two is used as the fatigue limit rate to ensure that when the charge and discharge rate does not reach the fatigue limit rate, the positive and negative current collectors will not reach the fatigue limit. Controlling the minimum rate to be less than the fatigue limit rate can prevent the stress on the positive and negative current collectors from exceeding the fatigue limit. The range of the number of cycles of the minimum rate cycle is 2-10 cycles to ensure that the effect of the sub-load training is appropriate and that too many cycles will not cause excessive reaction of the electrode material and unnecessary side reactions, which will have a negative impact on the battery performance, and can meet the needs of battery pre-lithiation.

[0038] The fatigue limit, also known as the endurance limit, refers to the maximum stress value that can be sustained after an infinite number of stress cycles without failure. A material's fatigue limit is an inherent property of the material and varies depending on the cycling characteristics, the form of specimen deformation, and the environment in which the material is located. Determination requires fatigue testing, using several smooth, small-sized specimens and conducting tests on a dedicated fatigue testing machine. For some common materials and shapes, calculations can be performed using empirical formulas by consulting reference materials or by computer simulation using finite element analysis (FEA). The fatigue limits of positive and negative current collectors can be determined using these methods. Since the calculation and measurement of fatigue limits are common knowledge, they will not be further elaborated here.

[0039] Preferably, the minimum rate is 40% to 100% of the fatigue limit rate. When the stress on the positive and negative electrode collectors is closer to the fatigue limit, the strengthening and improvement effect on the positive and negative electrode collectors is more obvious. Within the above range, the strengthening effect on the positive and negative electrode collectors is appropriate when cycling at a minimum rate, and the strengthening and improvement of the positive and negative electrode collectors can be completed within a fewer number of cycles.

[0040] Preferably, the ratio of the minimum magnification to the standard usage magnification of the battery is between 1 / 12 and 1 / 4. The standard usage magnification of the battery refers to the ratio between the maximum current at which the battery can stably discharge within a certain period of time and its nominal capacity. During the charge and discharge process of the battery, irreversible capacity attenuation and thickness expansion will occur. Therefore, in order to prevent the minimum magnification cycle of the secondary load exercise from having a greater impact on the capacity attenuation and thickness expansion of the battery, it is necessary to control the minimum magnification within the above range. When the minimum magnification is within the above range, it can adapt to the requirements of current collector strengthening of different battery systems and has a relatively small impact on battery capacity attenuation and thickness expansion.

[0041] Specifically, it further includes step S6: cycling the lithium-ion battery at different magnifications and measuring the stresses received by the positive current collector and the negative current collector. Through step S6, the relationship between the charge and discharge magnification of the battery and the stresses received by the positive current collector and the negative current collector can be obtained, and the magnification at the fatigue limit of the positive current collector and the negative current collector can be obtained.

[0042] Preferably, in step S1, at each corner position of the positive electrode sheet, part of the positive electrode active material is removed by laser cleaning to form one or more grooves 1 arranged at intervals along the winding direction of the positive electrode sheet. By these means, the expansion stress at the corner can be fully relieved, and the alternating bending moment received by the current collector can be reduced.

[0043] Preferably, the relationship between the depth h of the groove 1 and the thickness H of the positive electrode active material layer is 0.5H < h < H. The expansion gap formed by the depth of the groove 1 within the above range can meet the requirements for relieving the corner stress.

[0044] Preferably, the position of the groove 1 area covers the center of the corner and does not exceed the corner. The width s of the groove 1 area has the following relationship with the core thickness n: d < s < πn - d, where d is the tolerance of the laser cleaning equipment, that is, the position of the groove 1 cleaning area does not exceed the position of the entire corner, but can cover the center position of the corner.

[0045] Preferably, in step S3, the baking temperature is 85 - 100 °C and the baking time is 24 - 72H. For lithium-ion batteries equipped with silicon-carbon materials, the baking temperature is not higher than 95 °C. Without affecting the performance of the battery core, increasing the baking temperature and baking duration as much as possible is beneficial to relieving or eliminating the internal stress of the current collector and further improving the strength and elongation of the current collector.

[0046] Preferably, for lithium-ion batteries equipped with silicon-carbon materials, the baking temperature is not higher than 95 °C; for lithium-ion batteries with pure graphite anodes, the baking temperature is not higher than 100 °C; for lithium-ion batteries, the baking duration is not more than 72H. If this limit is exceeded, the electrical performance of the battery will be affected.

[0047] A second aspect of the present invention provides a lithium-ion battery prepared by the above-mentioned lithium-ion battery preparation process.

[0048] Example 1

[0049] The present invention provides a lithium ion battery and a preparation process thereof, comprising the following steps:

[0050] S1. Preparation of lithium-ion battery positive electrode.

[0051] Prepare the positive electrode slurry according to a reasonable ratio, stir it evenly and then apply it on the aluminum foil; the positive electrode slurry specifically includes lithium cobalt oxide as the main material, a conductive agent and a binder;

[0052] Confirm the position of each corner of the positive electrode by trial winding, and use laser cleaning to remove the powder at the corner, as shown in the attached Figure 1 The cleaning slot is the same length as the electrode and has a width of 5mm (the equipment tolerance is ±1mm, and the core thickness is designed to be 4.8mm). The cleaning depths are set to 25%, 50%, 75%, and 100%, i.e. 1.2mm, 2.4mm, 3.6mm, and 4.8mm, respectively, and are recorded as groups A / B / C / D. In addition, a control group 1 is set up, in which the corners are not cleaned.

[0053] S2. Use the conventional lithium-ion battery electrode preparation process to prepare the corresponding negative electrode, which is then wound together with the positive electrode and separator to assemble into a core, and then encapsulated with aluminum-plastic film to prepare a dry battery cell.

[0054] For the negative electrode slurry, it specifically includes 90% graphite + 10% silicon as the main ingredients, and the rest are conductive agent, binder and dispersant;

[0055] S3. Use the optimized process to bake the dry battery cells.

[0056] The prepared dry battery cell was placed in a vacuum oven and baked at 95 °C for 66 h with a vacuum degree of -90 kPa.

[0057] S4. After confirming that the moisture content is qualified, the dry cell is further processed, the electrolyte is injected, and then the formation, capacity division, and second sealing process are performed to make a finished lithium-ion battery. The battery is then cycled at a rate of 0.5C for 6 weeks to pre-lithiate the negative electrode, where the battery standard rate is used;

[0058] The pre-lithiation battery was cycled for 1200 cycles. The point where the aluminum foil began to break was monitored by CT. The results are shown in Table 1:

[0059] Table 1

[0060]

[0061] By comparing the experimental groups A / B / C / D and the control group 1 in Table 1, it can be seen that when the cleaning depth is too small, it is impossible to fully relieve the expansion stress at the corners. This is because when the powder removed is too little and the depth is too shallow, although the expansion stress of the active material on the surface along the winding direction of the pole piece is relieved due to the provision of expansion space, the expansion stress cannot be relieved for the active material close to the aluminum foil because the groove 1 cannot reach the deeper position and no expansion space is provided along the winding direction of the pole piece. Moreover, the expansion of the active material of the battery is mostly along the winding direction of the pole piece, resulting in excessive stress on the current collector and thus causing it to break. When cleaning is not performed at the corners, since no expansion space is provided at the deeper and shallower positions of the active material layer, the stress is greater and the aluminum foil is more likely to break.

[0062] Example 2

[0063] S1. Preparation of lithium-ion battery positive electrode.

[0064] Prepare the positive electrode sheet according to the method of Example 1, and use laser cleaning to remove the powder at the corners, as shown in the attached Figure 1 , the cleaning slot is the same length as the electrode, the width is 5mm, and the cleaning depth is 3.6mm (70%);

[0065] S2 prepared according to the method of Example 1 dry cells;

[0066] S3. Use the optimized process to bake the dry battery cells.

[0067] The prepared dry cells were placed in a vacuum oven and each group was baked according to the parameters in Table 2:

[0068] Table 2

[0069] 95℃ 90℃ 85℃ 80℃ 66H Group E Group F Group G Group H 48H Group I 36H Group J 24H Group K 12H Control group 2

[0070] S4. After all groups of water content are qualified, the electrolyte is injected and then undergoes formation, volume separation, and secondary sealing processes to produce finished lithium-ion batteries. The batteries are then cycled at a rate of 0.5C for 6 weeks to pre-lithiate the negative electrode.

[0071] The pre-lithiation battery was cycled and the point where the aluminum foil began to break was monitored by CT. The results are recorded in Table 3:

[0072] Table 3

[0073]

[0074] From the comparison between experimental groups E to K and control group 2, it can be seen that when the baking temperature is insufficient, the internal stress removal effect of the current collector is poor. After multiple cycles, the internal stress of the current collector can easily cause the current collector to break. When the baking time is insufficient, the internal stress of the current collector cannot be sufficiently released, which can also easily cause the current collector to break.

[0075] Example 3

[0076] S1. Preparation of lithium-ion battery positive electrode.

[0077] Prepare the positive electrode sheet according to the method of Example 1, and use laser cleaning to remove the powder at the corners, as shown in the attached Figure 1 , the cleaning slot is the same length as the electrode, the width is 5mm, and the cleaning depth is 3.6mm (70%);

[0078] S2 prepared according to the method of Example 1 dry cells;

[0079] S3. Use the optimized process to bake the dry battery cells.

[0080] The prepared dry battery cell was placed in a vacuum oven and baked at 95 °C for 66 h with a vacuum degree of -90 kPa.

[0081] S4. After confirming that the moisture content is qualified, the dry battery cell is further processed, the electrolyte is injected, and then the battery undergoes formation, capacity division, and secondary sealing processes to produce a finished lithium-ion battery. The battery is then cycled for 6 weeks at rates of 0.05 / 0.2 / 0.25 / 0.3 / 0.35 / 0.5, respectively, and recorded as the M / N / O / P / Q / R group. A control group 3 is set up, which does not undergo pre-lithiation treatment and directly starts the cycle test. The fatigue limit rate of the finished lithium-ion battery is 0.55c. The fatigue limit rate test method is to embed a PVDF piezoelectric film between the current collector and the active material layer, and charge and discharge the finished lithium-ion battery at different rates, and record the stress on the PVDF piezoelectric film at different rates. The fatigue limit of the current collector is measured using a dedicated fatigue testing machine, and the fatigue limit rate is queried according to the fatigue limit.

[0082] After pre-lithiation, each group was subjected to a cycle test. The node where the aluminum foil began to break was monitored by CT. The results are recorded in Table 4:

[0083] Table 4

[0084]

[0085]

[0086] From the comparison between the M / N / O / P / Q / R group and the control group 3, it can be seen that when the minimum rate is too small, the sub-load training effect is insufficient, and the improvement in the fatigue strength of the current collector is too small to meet the cycle requirements of the battery, resulting in easy breakage. When the rate is too large, it will exceed the fatigue limit, causing damage to the current collector in the pre-lithium stage, and the current collector is prone to breakage after multiple cycles.

[0087] The foregoing description shows and describes several preferred embodiments of the present invention. However, as previously mentioned, it should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. Rather, the present invention may be used in various other combinations, modifications, and environments and may be modified within the scope of the present invention through the teachings above or through techniques or knowledge in the relevant art. Modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention are intended to be within the scope of the appended claims.

Claims

1. A process for preparing a lithium-ion battery, characterized in that: It includes the following steps: S1. Coating a positive electrode active material on at least one surface of a positive electrode current collector to form a positive electrode sheet, and coating a negative electrode active material on at least one surface of a negative electrode current collector to form a negative electrode sheet; S2. Stacking the positive electrode sheet, the separator, and the negative electrode sheet in sequence and then winding them to form a wound core, and encapsulating the wound core with an aluminum-plastic film to prepare a dry battery cell; S3. Baking and drying the dry battery cell; S4. Injecting electrolyte into the dry battery cell and then going through processes of formation, grading, and secondary sealing to produce a finished lithium-ion battery; S5. Before using the lithium-ion battery, cycling it at an extremely small rate for 2 - 10 cycles to perform sub-load conditioning on the positive electrode current collector and the negative electrode current collector and pre-lithiation on the negative electrode; Wherein, the extremely small rate is less than the fatigue limit rate, and the fatigue limit rate is the smaller value between the rate when reaching the fatigue limit of the positive electrode current collector and the rate when reaching the fatigue limit of the negative electrode current collector.

2. The process for preparing a lithium-ion battery according to claim 1, wherein: The magnitude of the extremely small rate is 40% - 100% of the fatigue limit rate.

3. The process for preparing a lithium-ion battery according to claim 2, wherein: The ratio of the extremely small rate to the standard usage rate of the battery is between 1 / 12 and 1 / 4.

4. The process for preparing a lithium-ion battery according to claim 1, wherein: It further includes step s6. Cycling the lithium-ion battery at different rates and measuring the stresses received by the positive electrode current collector and the negative electrode current collector.

5. The process for preparing a lithium-ion battery according to claim 1, wherein: In step S1, it further includes that at each corner position of the positive electrode sheet, laser cleaning is used to remove part of the positive electrode active material to form one or more grooves (1) arranged at intervals along the winding direction of the positive electrode sheet.

6. The process for preparing a lithium-ion battery according to claim 5, wherein: The relationship between the depth h of the groove (1) and the thickness H of the positive electrode active material layer is 0.5H < h < H.

7. The process for preparing a lithium-ion battery according to claim 6, wherein: The position of the groove (1) area covers the center of the corner and does not exceed the corner, and the relationship between the width s of the groove (1) area and the thickness n of the wound core is as follows: d < s < πn - d, where d is the tolerance of the laser cleaning equipment.

8. The process for preparing a lithium-ion battery according to claim 1, wherein: In step S3, the baking temperature is 85 - 100 °C, and the baking time is 24 - 72H.

9. The process for preparing a lithium-ion battery according to claim 1, wherein: For a lithium-ion battery equipped with silicon-carbon material, the baking temperature is not higher than 95 °C; for a lithium-ion battery with a pure graphite negative electrode, the baking temperature is not higher than 100 °C; for a lithium-ion battery, the baking duration is not more than 72H.

10. A lithium-ion battery, characterized in that: Prepared by the preparation process of any lithium-ion battery according to claims 1 - 9.

Images