Secondary formation method suitable for high-capacity battery

Abstract

The embodiment of the invention provides a secondary formation method suitable for a high-capacity battery, which comprises the following steps of: performing primary activation on a liquid-injected lithium battery core, and performing primary formation to obtain a primary formed lithium battery core; carrying out secondary activation on the primary formed lithium battery cell and then carrying out secondary formation to obtain a formed lithium battery cell; the vacuum degree of the primary formation ranges from-60 Kpa to-80 Kpa; the vacuum degree of the secondary formation is-70 Kpa to 85 Kpa; and the capacity of the formed lithium battery core is greater than or equal to 280Ah. Through the method provided by the invention, after the positive electrode of the lithium iron phosphate battery cell is supplemented with lithium, the negative electrode does not separate lithium when formation is performed to a high voltage, so that the wettability of an electrolyte on a pole piece is better, the lithium supplementing agent can be fully activated, the effect of effectively improving battery circulation is achieved, and the problem that the subsequent lithium supplementing agent reacts again to produce gas can be avoided; and the safety performance of the battery cell is greatly improved.

Description

Technical Field

[0001] The invention belongs to the technical field of batteries, and in particular relates to a secondary formation method suitable for large-capacity batteries. Background Art

[0002] Compared with battery structures with a capacity of 50Ah and 100Ah, large-capacity batteries (capacity ≥ 280Ah) have a higher volume energy density, use fewer components at the PACK end, and have more obvious cost advantages. They are the mainstream development direction of the future energy storage market.

[0003] In order to increase the volume energy density of the battery and reduce the cost of the battery cell, the surface density and compaction density of the positive and negative electrodes of large-capacity batteries are often higher, which also increases the difficulty of the electrode itself being wetted; once the electrode wettability is poor and the fit between the electrodes is low during formation, it is very likely to cause uneven lithium embedding at the negative electrode interface during formation, or even lithium precipitation. In addition, the existing market has high requirements for the energy density and cycle of large-capacity batteries, generally requiring a capacity decay of ≤20% after 4,000 or even 10,000 cycles, and conventional lithium iron phosphate is currently far from meeting these requirements.

[0004] At present, positive electrode lithium replenishment technology is an effective means to improve the cycle of large-capacity batteries. Lithium replenishment technology, also known as pre-lithiation or lithium insertion technology, is to add lithium to the battery before the lithium battery works to achieve the purpose of replenishing active lithium in the cycle process. It can effectively improve the battery cycle performance and energy density, thereby meeting the current and future needs for long cycle life of lithium-ion batteries. Common additives for positive electrode lithium replenishment are lithium ferrite (LFO) and lithium nickel oxide (LNO). Just adding a small amount during the positive electrode slurry can significantly improve the energy density and cycle performance of lithium batteries.

[0005] However, the release of lithium ions by lithium supplements is accompanied by the problem of gas production. Therefore, for batteries with added lithium supplements, it is necessary to fully activate the lithium supplement in the formation stage and discharge the gas produced by it in the formation stage. However, for LNO and LFO, they need to be charged to a high voltage of 4.0-4.2V to complete activation, which is far beyond the normal working voltage range of 2.5-3.65V for lithium iron phosphate batteries; if such a high voltage is reached in the formation stage, the side reactions in the battery will intensify and the internal environment will be quite unstable; plus the SE I film has just been formed and is not stable, and the traditional activation method leads to poor wettability of the electrode. Therefore, lithium iron phosphate batteries use traditional activation and formation processes to activate the lithium supplement after lithium supplementation at the positive electrode, which is easy to cause lithium precipitation at the interface of the negative electrode during formation. Summary of the invention

[0006] The purpose of the present invention is to provide a secondary formation method suitable for large-capacity batteries to solve the technical problems that the conventional activation and formation process is used to activate the lithium supplement agent after the positive electrode is supplemented with lithium, which easily leads to lithium precipitation at the interface of the negative electrode during formation and affects the cycle performance of the battery.

[0007] To achieve the above-mentioned purpose, an embodiment of the present invention provides a secondary formation method suitable for large-capacity batteries, comprising the following steps: activating a lithium battery cell after injection once and then performing a primary formation to obtain a primary formed lithium battery cell; activating the primary formed lithium battery cell for a second time and then performing a secondary formation to obtain a formed lithium battery cell; the vacuum degree of the primary formation is -60Kpa to -80Kpa; the vacuum degree of the secondary formation is -70Kpa to 85Kpa; the capacity of the formed lithium battery cell is ≥280Ah.

[0008] As a preferred embodiment, the temperature of the primary activation is 40° C. to 50° C., the humidity of the primary activation is ≤5% RH; and the time of the primary activation is 28 h to 64 h.

[0009] As a preferred embodiment, the primary formation is achieved by the following method: charging at 20° C. to 30° C. with a first constant current until the battery capacity of the primary formation lithium battery cell reaches a SOC state of 20% to 40%.

[0010] As a preferred implementation, the first current is a constant current of 0C to 0.3C.

[0011] As a preferred embodiment, the constant current charging is performed once to three times.

[0012] As a preferred embodiment, the voltage of the primary-formed lithium battery cell is less than 3.5V.

[0013] As a preferred embodiment, the temperature of the secondary activation is 40° C. to 50° C., the humidity of the secondary activation is ≤5% RH; and the time of the secondary activation is 12 h to 24 h.

[0014] As a preferred embodiment, the secondary formation is achieved by the following method: at 20°C to 30°C, first charging the lithium battery with a second constant current until the voltage of the lithium battery is 3.65V; then charging the lithium battery with a third constant current until the voltage of the lithium battery is 4.0V to 4.2V.

[0015] As a preferred implementation, the second current is greater than the first current; the second current is greater than the third current.

[0016] As a preferred implementation, the second current is a constant current of 0.15C to 0.5C.

[0017] As a preferred implementation, the third current is a constant current of 0.01-0.1C.

[0018] As a preferred embodiment, the lithium battery cell after injection is a lithium iron phosphate battery cell; the voltage of the formed lithium battery cell is 4.0V to 4.2V.

[0019] The technical solution proposed in the present invention has the following beneficial effects: the present application forms a stable SE I film at the negative electrode of the battery cell by performing a first activation followed by a first formation, and then a second activation followed by a second formation, and controlling the conditions of each operation, thereby ensuring that the negative electrode will not produce lithium dendrites and lithium precipitation when it is formed to a high voltage, so that the electrolyte has better wettability to the electrode sheet. Through the method of the present application, after the positive electrode of the lithium iron phosphate battery cell is replenished with lithium, it can be ensured that the negative electrode will not precipitate lithium when it is formed to a high voltage, and the lithium replenisher can be fully activated, which effectively improves the battery cycle, and can also avoid the problem of subsequent lithium replenisher reacting again to produce gas, greatly improving the safety performance of the battery cell. Compared with the traditional process of first activating and then forming, the secondary formation process provided by the present application has a lower negative pressure throughout the process, which also effectively reduces the production cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a photograph of the negative electrode interface of the disassembled lithium battery cell prepared by the secondary formation method of Example 1 of the present invention;

[0021] Figure 2 This is a photograph of the negative electrode interface of the lithium battery cell after disassembly, which was prepared by the conventional activation-first-then-formation method of Comparative Example 1;

[0022] Figure 3 The graph is a cycle trend diagram of the formed lithium battery cell of Example 1 and the lithium battery cell of Comparative Example 1 at room temperature. DETAILED DESCRIPTION

[0023] 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.

[0024] It should be noted that if the embodiments of the present invention involve directional indications (such as up, down, left, right, front, back, top, bottom...), the directional indications are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0025] In this application, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0026] It should be noted that when an element is referred to as being "fixed to" or "disposed on" another element, it may be directly on the other element or there may be an intermediate element. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be an intermediate element at the same time.

[0027] In addition, if there are descriptions involving "first", "second", etc. in the embodiments of the present invention, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or suggesting their 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 at least one of the features. In addition, the technical solutions between the various embodiments can be combined with each other, but they must be based on the ability of ordinary technicians in the field to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0028] The method of the present application solves the problems of poor electrode wettability caused by the traditional activation method due to the large surface density and compaction density of the electrode in large-capacity batteries, and lithium deposition caused by poor liquid seepage in the middle during formation; it also solves the problems of easy rupture of the SE I film and lithium deposition at the negative electrode when the lithium iron phosphate battery requires high-voltage formation due to positive electrode lithium replenishment, and gives full play to the role of the lithium replenisher, thereby improving the cycle performance of the battery cell.

[0029] Specifically, the embodiment of the present application provides a secondary formation method suitable for large-capacity batteries, comprising the following steps: activating the lithium battery cell after liquid injection once and then performing a primary formation to obtain a primary formed lithium battery cell; activating the primary formed lithium battery cell for a secondary time and then performing a secondary formation to obtain a formed lithium battery cell; the vacuum degree of the primary formation is -60Kpa~-80Kpa (depending on the needs of actual use, it can be -60Kpa, or -65Kpa, or -70Kpa, or -80Kpa, etc.); the vacuum degree of the secondary formation is -70Kpa~85Kpa (depending on the needs of actual use, it can be -70Kpa, or -75Kpa, or -80Kpa, or -85Kpa, etc.); the capacity of the formed lithium battery cell is ≥280Ah.

[0030] After the first formation, the voltage of the battery cell is less than 3.5V, and the internal environment of the battery cell is relatively stable. Under this state, the battery cell continues to be activated for a period of time, the SE I film state will gradually stabilize, and continue to be activated after low current charging, which can greatly improve the wettability of the electrolyte to the electrode.

[0031] As a preferred embodiment, the temperature of the primary activation is 40°C to 50°C (depending on the actual use needs, it can be 40°C, or 42°C, or 45°C, or 50°C, etc.), and the humidity of the primary activation is ≤5%RH; the time of the primary activation is 28h to 64h (depending on the actual use needs, it can be 28h, or 35h, or 55h, or 64h, etc.).

[0032] As a preferred embodiment, the primary formation is achieved by the following method: at 20°C to 30°C (depending on the needs of actual use, it can be 20°C, or 22°C, or 25°C, or 30°C, etc.), charging with a first current constant current to a battery capacity of the primary formation lithium battery core of 20% to 40% (depending on the needs of actual use, it can be 20%, or 25%, or 30%, or 40%, etc.) SOC state (i.e. State of Charge, representing the battery capacity when the battery is fully charged).

[0033] As a preferred implementation, the first current is a constant current of 0C to 0.3C (depending on actual use requirements, it can be 0.01C, or 0.1C, or 0.2C, or 0.3C, etc.).

[0034] As a preferred embodiment, the constant current charging is performed once to three times.

[0035] As a preferred embodiment, the voltage of the primary-formed lithium battery cell is less than 3.65V.

[0036] Preferably, the one-time formation operation includes: firstly letting the activated lithium battery cell stand for 1 minute, and then charging at a constant current of 0.05C for 30 minutes (the upper limit voltage of the battery cell is 3.5V); then letting it stand for 5 minutes, and then charging at a constant current of 0.1C to 0.15C for 90 minutes (the upper limit voltage of the battery cell is 3.5V);

[0037] Preferably, the one-time formation operation further comprises: standing for another 5 minutes, and then charging at a constant current of 0.15C to 0.2C for 50 minutes to 60 minutes (the upper limit voltage of the battery cell is 3.5V to 3.65V).

[0038] As a preferred embodiment, the temperature of the secondary activation is 40°C to 50°C (depending on the actual needs, it can be 40°C, or 42°C, or 45°C, or 50°C, etc.), and the humidity of the secondary activation is ≤5%RH; the time of the secondary activation is 12h to 24h (depending on the actual needs, it can be 12h, or 15h, or 20h, or 24h, etc.).

[0039] As a preferred embodiment, the secondary formation is achieved by the following method: at 20°C to 30°C (depending on the actual use needs, it can be 20°C, or 22°C, or 25°C, or 30°C, etc.), first charge the lithium battery cell with a second constant current until the voltage of the lithium battery cell is 3.65V; then charge the lithium battery cell with a third constant current until the voltage of the lithium battery cell is 4.0V to 4.2V.

[0040] As a preferred embodiment, the second current is greater than the first current; the second current is greater than the third current. In this way, the SE I film is given a stable process, and it is not easy to generate lithium dendrites and lithium precipitation when the secondary formation is to a high voltage (4.0V to 4.2V). And after the primary formation, the embedding of a small amount of lithium ions into the negative electrode can open the graphite channel, thereby improving the infiltration of the electrolyte into the electrode sheet.

[0041] As a preferred implementation, the second current is a constant current of 0.15C to 0.5C (depending on actual use requirements, it can be 0.15C, or 0.2C, or 0.35C, or 0.5C, etc.).

[0042] As a preferred embodiment, the third current is a constant current of 0.01-0.1 C. In this way, the voltage plateau time can be effectively increased, thereby achieving the effect of fully activating the lithium supplement agent.

[0043] Preferably, the secondary formation operation is as follows: first, the primary formation lithium battery cell is allowed to stand for 1 minute, and then 0.15C-0.2C constant current is charged until the upper limit voltage of the battery cell is 3.65V; then, it is allowed to stand for 5 minutes, and then 0.1C constant current is charged until the upper limit voltage of the battery cell is 3.95V; then, it is allowed to stand for 10 minutes, and then 0.01C-0.02C constant current is charged until the upper limit voltage of the battery cell is 4.2V.

[0044] As a preferred embodiment, the lithium battery cell after injection is a lithium iron phosphate battery cell; the voltage of the formed lithium battery cell is 4.0V to 4.2V (depending on the actual use requirements, it can be 4.0V, or 4.1V, or 4.2V, etc.).

[0045] Through the method of the present application, the total duration of the entire activation and formation process remains unchanged, while the wettability of the electrode and the stability of the formed SEI film are greatly improved. In addition, the current is small during the primary formation, and the formation negative pressure can be appropriately reduced to -60Kpa~-80Kpa. During the secondary formation, the SE I film is relatively stable as a whole, and the negative pressure can also be controlled at -70Kpa~85Kpa. The negative pressure of the entire process is greatly reduced compared to the primary formation, which effectively reduces the production cost.

[0046] Example 1

[0047] A secondary formation method suitable for a large-capacity battery comprises the following steps: subjecting a lithium battery cell (a 314Ah battery cell is subjected to a single injection of 1150g electrolyte) to primary activation (activated for 52h at 45±5°C and humidity≤5%RH) and then primary formation (the vacuum degree of the primary formation is -65Mpa and the temperature is 25±5°C) to obtain a primary formed lithium battery cell; subjecting the primary formed lithium battery cell to secondary activation (activated for 24h at 45±5°C and humidity≤5%RH) and then secondary formation (the vacuum degree is -80Mpa and the temperature is 25±5°C) to obtain a formed lithium battery cell.

[0048] Among them, the operation of primary formation is as follows: first let the activated lithium battery cell stand for 1 minute, and then charge it at a constant current of 0.05C for 30 minutes (the upper limit voltage of the battery cell is 3.5V); then let it stand for 5 minutes, and then charge it at a constant current of 0.1C for 90 minutes (the upper limit voltage of the battery cell is 3.5V); then let it stand for 5 minutes, and then charge it at a constant current of 0.15C for 50 minutes (the upper limit voltage of the battery cell is 3.65V).

[0049] The operation of the secondary formation is as follows: first let the primary formation lithium battery cell stand for 1 minute, then charge it with a constant current of 0.15C until the upper limit voltage of the battery cell is 3.65V; then let it stand for 5 minutes, then charge it with a constant current of 0.1C until the upper limit voltage of the battery cell is 3.95V; then let it stand for 10 minutes, then charge it with a constant current of 0.02C until the upper limit voltage of the battery cell is 4.2V.

[0050] After formation, the formed lithium battery cell is disassembled, and the disassembled negative electrode sheet is shown in the following figure: Figure 1 shown; from Figure 1 It can be seen that the negative electrode interface has good uniformity and no lithium deposition. The battery cell cycle performance test results are as follows: Figure 3 As shown. Figure 3 It can be seen that the cycle attenuation trend of the battery cell of Example 1 of the present application is significantly better than the cycle attenuation trend of the battery cell of Comparative Example 1.

[0051] Example 2

[0052] A secondary formation method suitable for large-capacity batteries comprises the following steps: subjecting a lithium battery cell (a 280Ah battery cell is subjected to a single injection of 1050g electrolyte) to primary activation (activated for 48h at 45±5°C and humidity ≤5%RH) and then primary formation (the vacuum degree of the primary formation is -60Mpa and the temperature is 25±5°C) to obtain a primary formed lithium battery cell; subjecting the primary formed lithium battery cell to secondary activation (activated for 12h at 45±5°C and humidity ≤5%RH) and then secondary formation (the vacuum degree is -75Mpa and the temperature is 25±5°C) to obtain a formed lithium battery cell.

[0053] The operation of the primary formation is as follows: first, let the activated lithium battery cell stand for 1 minute, and then charge it at a constant current of 0.05C for 30 minutes (the upper limit voltage of the battery cell is 3.5V); then let it stand for 5 minutes, and then charge it at a constant current of 0.15C for 90 minutes (the upper limit voltage of the battery cell is 3.5V).

[0054] The operation of the secondary formation is as follows: first let the primary formation lithium battery cell stand for 1 minute, then charge it with a constant current of 0.2C until the upper limit voltage of the battery cell is 3.65V; then let it stand for 5 minutes, then charge it with a constant current of 0.1C until the upper limit voltage of the battery cell is 3.95V; then let it stand for 10 minutes, then charge it with a constant current of 0.01C until the upper limit voltage of the battery cell is 4.2V.

[0055] Example 3

[0056] A secondary formation method suitable for a large-capacity battery comprises the following steps: subjecting a lithium battery cell after injection (a 300Ah battery cell is injected with 1080g of electrolyte solution once) to primary activation (activated for 52h at 45±5°C and humidity≤5%RH) and then primary formation (the vacuum degree of the primary formation is -70Mpa and the temperature is 25±5°C) to obtain a primary formed lithium battery cell; subjecting the primary formed lithium battery cell to secondary activation (activated for 12h at 45±5°C and humidity≤5%RH) and then secondary formation (the vacuum degree is -75Mpa and the temperature is 25±5°C) to obtain a formed lithium battery cell.

[0057] Among them, the operation of primary formation is as follows: first let the activated lithium battery cell stand for 1 minute, and then charge it at 0.05C constant current for 30 minutes (the upper limit voltage of the battery cell is 3.5V); then let it stand for 5 minutes, and then charge it at 0.1C constant current for 90 minutes (the upper limit voltage of the battery cell is 3.5V); then let it stand for 5 minutes, and then charge it at 0.2C constant current for 60 minutes (the upper limit voltage of the battery cell is 3.5V).

[0058] The operation of the secondary formation is as follows: first let the primary formation lithium battery cell stand for 1 minute, then charge it with a constant current of 0.2C until the upper limit voltage of the battery cell is 3.65V; then let it stand for 5 minutes, then charge it with a constant current of 0.1C until the upper limit voltage of the battery cell is 3.95V; then let it stand for 10 minutes, then charge it with a constant current of 0.01C until the upper limit voltage of the battery cell is 4.2V.

[0059] Comparative Example 1 (conventional example)

[0060] After the battery cell with a capacity of 314Ah is injected with 1150g of electrolyte, it is activated for 76h in an environment with a temperature of 45±5℃ and a humidity of ≤5%RH. After the activation, it is formed to a high voltage. The specific formation process is shown in Table 1 below.

[0061] Table 1 Specific chemical process parameters of comparative example 1

[0062]

[0063]

[0064] After formation, the battery cell is disassembled, and the disassembled negative electrode sheet is shown in the following figure. Figure 2 shown; from Figure 2 It can be seen that lithium is deposited on the edge of the negative electrode. The battery cell cycle performance test results are as follows Figure 3 As shown. Figure 3 It can be seen that the cycle attenuation trend of the battery cell of comparative example 1 is much lower than the cycle attenuation trend of the battery cell of example 1 of the present application.

[0065] The above description is only a preferred embodiment of the present invention, and does not limit the patent scope of the present invention. All equivalent structural changes made by using the contents of the present invention specification and drawings under the inventive concept of the present invention, or directly / indirectly applied in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A secondary formation method suitable for large-capacity batteries, characterized in that: The method comprises the following steps: activating the injected lithium battery cell once and then performing a formation once to obtain a formed lithium battery cell; activating the formed lithium battery cell twice and then performing a secondary formation to obtain a formed lithium battery cell; the vacuum degree of the primary formation is -60Kpa to -80Kpa; the vacuum degree of the secondary formation is -70Kpa to 85Kpa; and the capacity of the formed lithium battery cell is ≥280Ah.

2. The secondary formation method for large-capacity batteries according to claim 1, characterized in that: The temperature of the primary activation is 40° C. to 50° C., the humidity of the primary activation is ≤5% RH; and the time of the primary activation is 28 h to 64 h.

3. The secondary formation method for large-capacity batteries according to claim 1, characterized in that: The primary formation is achieved by the following method: charging at 20° C. to 30° C. with a first constant current until the battery capacity of the primary formation lithium battery cell is in a SOC state of 20% to 40%.

4. The secondary formation method for large-capacity batteries according to claim 3, characterized in that: The first current is a constant current of 0C to 0.3C.

5. The secondary formation method for large-capacity batteries according to claim 1, characterized in that: The number of constant current charging is 1 to 3 times; the voltage of the lithium battery cell formed once is less than 3.5V.

6. The secondary formation method for large-capacity batteries according to claim 1, characterized in that: The temperature of the secondary activation is 40° C. to 50° C., the humidity of the secondary activation is ≤5% RH; and the time of the secondary activation is 12 h to 24 h.

7. The secondary formation method for large-capacity batteries according to claim 3, characterized in that: The secondary formation is achieved by the following method: at 20°C to 30°C, first charging the lithium battery with a second constant current until the voltage of the lithium battery is 3.65V; then charging the lithium battery with a third constant current until the voltage of the lithium battery is 4.0V to 4.2V.

8. The secondary formation method for large-capacity batteries according to claim 7, characterized in that: The second current is greater than the first current; the second current is greater than the third current.

9. The secondary formation method for large-capacity batteries according to claim 7, characterized in that: The second current is a constant current of 0.15C to 0.5C; The third current is a constant current of 0.01-0.1C.

10. The secondary formation method for large-capacity batteries according to claim 1, characterized in that: The lithium battery cell after injection is a lithium iron phosphate battery cell; the voltage of the formed lithium battery cell is 4.0V to 4.2V.

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