A composite film for electrochemical prelithiation and a prelithiation method

Abstract

This invention discloses a composite film and a method for electrochemical pre-lithiation. The composite film includes an insulating base film; copper foil and lithium metal are laminated on the insulating base film; the composite film can be bent along the indentation on it to wrap the battery cell. At this time, regions A and C of the composite film correspond to the side surface of the battery cell, region B corresponds to the bottom surface of the battery cell, and region D corresponds to the front surface of the battery cell; regions A, B, and C are the insulating base film, copper foil, and lithium metal laminated sequentially; region D is the insulating base film and the copper foil laminated sequentially, with the copper foil extending outward to form a tab. This invention utilizes the composite film to wrap the battery cell, realizing the circuit connection between the lithium source and the negative electrode. After liquid injection, lithium ions will migrate from the edge of the negative electrode to the center under the action of potential difference, achieving the purpose of pre-lithiation of the battery cell.

Description

Technical Field

[0001] This invention relates to a composite membrane and a pre-lithiation method for electrochemical pre-lithiation, belonging to the field of cell pre-lithiation. Background Technology

[0002] During the first charge of a lithium-ion battery, the surface of the negative electrode reacts with the electrolyte to form a solid electrolyte film (SEI). While this electrolyte film is beneficial to the cycle stability of the lithium-ion battery, it consumes lithium ions from the positive electrode. Moreover, this reaction is irreversible, which leads to a decrease in the battery's initial coulombic efficiency (ICE) and reduces the battery's capacity.

[0003] Currently, in order to solve the problem of low initial coulombic efficiency of anode materials, chemical reduction method, artificial SEI film method and electrochemical pre-lithiation method have been developed. Among them, electrochemical pre-lithiation method is the most direct way to solve the low ICE of lithium-ion anode materials.

[0004] Currently, most electrochemical pre-lithiation processes are still in the experimental exploration stage, making it difficult to effectively achieve mass production of pre-lithiated cells. Pre-lithiation operations are complex and have low consistency, and the uniformity and degree of pre-lithiation after pre-lithiation are difficult to control, which can easily lead to excessive lithium replenishment and lithium plating.

[0005] Currently, some research teams directly embed the lithium source inside the cell during electrochemical pre-lithiation, connecting it to the cell's negative electrode. After electrolyte injection, lithium ions slowly migrate to the negative electrode under the influence of potential difference. Most of these teams place the lithium source on the top and bottom of the cell. While this method has some effect, the migration path is long because the lithium ion migration channel is on the side of the cell, resulting in incomplete lithium consumption, especially at farther distances, posing a safety risk. Furthermore, as lithium ions are consumed, the cell surface becomes uneven, which can affect the electrode interface under stress and thus impact cell stability. Summary of the Invention

[0006] The purpose of this invention is to provide a composite film and a pre-lithiation method for electrochemical pre-lithiation. This invention utilizes a composite film of composite copper foil and lithium metal to wrap the battery cell. After liquid injection, lithium ions will migrate from the edge of the negative electrode to the center under the action of potential difference, thereby achieving the purpose of pre-lithiation of the battery cell.

[0007] The composite membrane for electrochemical pre-lithiation provided by the present invention includes an insulating substrate; wherein, copper foil and lithium metal are laminated on the insulating substrate;

[0008] The composite film can be wrapped around the battery cell after being bent along the indentation on it. At this time, the A and C areas of the composite film correspond to the side of the battery cell, the B area of ​​the composite film corresponds to the bottom of the battery cell, the D area of ​​the composite film corresponds to the front of the battery cell, and the E area of ​​the composite film corresponds to the back of the battery cell.

[0009] Region A, Region B, and Region C are sequentially composited with the insulating base film, the copper foil, and the lithium metal;

[0010] The D region consists of the insulating base film and the copper foil that are sequentially laminated. The copper foil extends outward to form a tab, which is welded to the negative electrode of the battery cell during pre-lithiation. The copper foil acts as a conductive medium to achieve the effect of short-circuiting the negative electrode of the battery cell with the lithium source.

[0011] The E region is the insulating base film.

[0012] In the above-mentioned composite film, the insulating base film can be a polyester film or a polyethylene film;

[0013] The thickness of the insulating substrate is 1–500 μm, preferably 20–100 μm.

[0014] In the aforementioned composite film, the thickness of the copper foil can be 5–50 μm, preferably 5–10 μm, serving as the conductive medium between lithium metal and the negative electrode.

[0015] In the composite film described above, the thickness of the lithium metal is 10–500 μm, preferably 100–200 μm.

[0016] In the composite film described above, the copper foil and the lithium metal are composited by hot pressing, rolling or bonding.

[0017] Based on the composite membrane, the present invention provides a method for chemical pre-lithiation, comprising the following steps:

[0018] The battery cell is wrapped with the composite film, and after being filled with electrolyte, it is left to stand for pre-lithiation. Because there is a potential difference between the negative electrode of the battery cell and the lithium metal, lithium ions detach from the lithium metal and migrate to the negative electrode of the battery cell, and diffuse from the edge of the negative electrode to the center. Therefore, standing achieves automatic pre-lithiation.

[0019] The battery cell shall be wrapped in the following manner:

[0020] The A and C regions of the composite film correspond to the side surface of the battery cell, and the B region of the composite film corresponds to the bottom surface of the battery cell. In this way, the migration distance of lithium ions when diffusing into the negative electrode can be greatly reduced, and the pre-lithiation efficiency is improved accordingly. The lithium on the composite film can be consumed more completely in a limited time, thereby increasing the safety of the pre-lithiated battery cell.

[0021] The D region of the composite film corresponds to the front side of the battery cell; the tab corresponds to the negative tab of the battery cell.

[0022] Before wrapping the battery cell with the composite film, a separator is wrapped around the battery cell to prevent the electrode from coming into direct contact with the lithium metal, thereby improving the safety of the pre-lithium battery cell.

[0023] The pre-lithiation temperature is between 25°C and 60°C, and the time is between 24h and 96h.

[0024] In the pre-lithiation process of this invention, the dew point of any process step in which lithium metal is exposed to air must be less than -40°C.

[0025] In conventional battery cell production processes, insulation film is typically installed after welding. In this invention, a composite film is installed first, followed by tab welding, where the copper tabs on the composite film are welded to the negative electrode of the battery cell; or insulation film is installed after welding, and then a secondary welding process is performed to weld the copper tabs on the composite film to the negative electrode of the battery cell.

[0026] When using the method of this invention for pre-lithiation, lithium ions diffuse inward along the edge of the negative electrode. Therefore, when the lithium source is placed on the side or bottom of the cell, the migration distance of lithium ions is shortest, resulting in a more significant pre-lithiation rate and effect compared to other locations. Furthermore, the lithium on the insulating bottom film is more easily and completely consumed, increasing the safety performance of the pre-lithiated cell. This invention mainly utilizes an insulating bottom film composited with metallic lithium to replace conventional polyester film. Mass production of pre-lithiated cells can be achieved with slight modifications to the current normal cell manufacturing process, and the manufacturing cost is relatively low. Attached Figure Description

[0027] Figure 1 This is a planar composition diagram of the composite membrane used for electrochemical pre-lithiation according to the present invention.

[0028] Figure 2 This is a structural diagram of the composite membrane used for electrochemical pre-lithiation according to the present invention.

[0029] Figure 3 This is a schematic diagram of lithium ion migration during pre-lithiation of the composite membrane used for electrochemical pre-lithiation according to the present invention.

[0030] Figure 4 The graph shows the change of total voltage over time during the pre-lithiation process in Embodiment 2 and Comparative Example 3 of the present invention. Detailed Implementation

[0031] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0032] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0033] like Figure 1The diagram shown is a plan view of the composite film for electrochemical pre-lithiation provided by the present invention. It includes an insulating base film, which can be a polyester film or a polyethylene film with a thickness of 1–500 μm. Copper foil and lithium metal are laminated onto the insulating base film. The copper foil has a thickness of 5–50 μm and serves as the conductive medium between the lithium metal and the negative electrode. The lithium metal has a thickness of 10–500 μm. The copper foil and lithium metal can be laminated by hot pressing, calendering, or adhesive bonding. When the composite film is bent along the indentations, it can wrap the battery cell. At this time, regions ① and ② of the composite film correspond to the sides of the battery cell, region ③ corresponds to the bottom surface of the battery cell, and region ④ corresponds to the front surface of the battery cell, as shown below. Figure 2 As shown, regions ①, ②, and ③ are sequentially laminated polyester film, copper foil, and lithium metal; region ④ is sequentially laminated polyester film and copper foil. The copper foil extends outward to form a tab, which is welded to the negative electrode of the battery cell during pre-lithiation. The copper foil acts as a conductive medium to achieve the effect of short-circuiting the negative electrode of the battery cell with the lithium source. Figure 3 As shown, there is a potential difference between the negative electrode and lithium metal in the battery cell. Under the influence of this potential difference, lithium ions diffuse from the side of the battery cell from the edge of the negative electrode towards the center.

[0034] Comparative Example 1: Fabrication of ordinary unpre-prepared lithium battery cells

[0035] 1. The battery cell is a laminated cell, with a cell design of 48 positive and 47 negative, and the cell system is LFP / C;

[0036] 2. The electrolyte used is Tinci 021 model electrolyte;

[0037] 3. The battery cell is wrapped with a standard insulating protective film;

[0038] 4. Aging at 45℃, capacity testing at 25℃; cycling at 25℃, with a cycling current of 1C / 1C.

[0039] Example 1: Pre-lithiation of battery cells using composite film coating

[0040] 1. The battery cell is a laminated cell, with a cell design of 48 positive and 47 negative, and the cell system is LFP / C;

[0041] 2. The electrolyte used is Tinci 021 model electrolyte;

[0042] 3. The composite film of this invention is used to wrap the battery cell;

[0043] 4. The thickness of the composite lithium metal on the composite film is 100μm, the insulating substrate is a polyester film with a thickness of 50μm, and the copper foil has a thickness of 8μm.

[0044] 5. The pre-lithiation time is 60 hours;

[0045] 6. Aging at 45℃, capacity testing at 25℃; cycling at 25℃, with a cycling current of 1C / 1C.

[0046] Example 2: Pre-lithiation of battery cells using composite film coating

[0047] The specific process parameters are the same as in Example 1, except that the thickness of the composite lithium metal on the composite film is 200 μm.

[0048] Comparative Example 2: Pre-lithiation of lithium strips by placing them on the front of the battery cell.

[0049] 1. The battery cell is a laminated cell, with a cell design of 48 positive and 47 negative, and the cell system is LFP / C;

[0050] 2. The electrolyte used is Tinci 021 model electrolyte;

[0051] 3. Place the lithium copper composite strip on both the top and bottom sides of the battery cell, and weld the copper tabs of the lithium copper composite strip to the negative electrode of the battery cell.

[0052] 4. The thickness of lithium metal on the lithium-copper composite strip is 100μm;

[0053] 5. Use ordinary insulating protective film to wrap the battery cell;

[0054] 6. The pre-lithiation time is 60 hours;

[0055] 7. Perform chemical aging at 45℃, capacity testing at 25℃; cycle at 25℃ with a cycle current of 1C / 1C.

[0056] Comparative Example 3: Pre-lithiation of lithium strips by placing them on the front of the battery cell.

[0057] The specific process parameters are the same as those in Comparative Example 2, except that the thickness of the lithium metal on the lithium-copper composite strip is 200 μm.

[0058] The above embodiments and comparative examples were subjected to composition and room temperature cycling, and the parameters were recorded under the same conditions. The results are shown in Table 1.

[0059] The testing methods for each parameter are as follows:

[0060] 1. Voltage: After the cell reaches the pre-lithiation time, the voltage of the positive and negative electrodes of the cell is measured. At this time, the degree of pre-lithiation can be preliminarily judged based on the voltage. The higher the voltage, the more lithium ions migrate and the better the pre-lithiation effect.

[0061] 2. Capacity and capacity retention: Tested using a battery tester and through charge-discharge cycles. During capacity testing, a 0.33C charge-discharge cycle was used, and during cycling, a 0.5C charge-discharge cycle was used.

[0062] 3. Residual Lithium Metal: After 500 cycles, the cell is disassembled, the pre-lithiation film is removed and dried, and the weight after drying is recorded as W2. The weight before the pre-lithiation film is reassembled is recorded as W1. The amount of lithium metal consumed at this time is W3 = W2 - W1, and the initial amount of lithium metal is W0. W0 = S (lithium metal area) × H (lithium metal thickness) × ρ (lithium metal density); that is, residual lithium metal = (W0 - W3) / W0 × 100%.

[0063] Table 1 Comparison of verification effects of different pre-lithiation schemes

[0064]

[0065]

[0066] As can be seen from the data in Table 1, the pre-lithiation of the battery cell using the method of the present invention significantly improves the capacity and capacity retention rate compared with Comparative Example 1. The effect is better when using a lithium metal thickness of 200μm, but the residual amount of lithium was observed to be high after the battery cell was disassembled.

[0067] Using the method of this invention to pre-lithiate the battery cell, the capacity and capacity retention rate are significantly improved compared with conventional front-side pre-lithiated battery cells (Comparative Examples 2-3). After disassembling the battery cell, the residual amount of lithium was found to be greatly reduced.

[0068] The positive and negative electrode voltages were monitored during the wetting process of the side pre-lithiation (Example 2, lithium is placed on the side and bottom of the cell) and the front pre-lithiation (Comparative Example 3, lithium is placed on both the top and bottom of the cell) in the above embodiments and comparative examples. The voltage curves are shown below. Figure 4 As shown.

[0069] Depend on Figure 4 It can be seen that when the method of the present invention is used to pre-lithiate the battery cell, the starting voltage of the positive and negative electrodes and the voltage rise rate are significantly better than the conventional pre-lithiation scheme. The higher the voltage between the positive and negative electrodes, the more lithium ions are embedded in the negative electrode, that is, the deeper the pre-lithiation.

[0070] This application provides a method for electrochemical pre-lithiation of electrodes, which can be inserted into existing mature production lines with minimal impact. By pre-lithiating the electrodes in advance to offset the consumption of lithium ions by the formation of the negative electrode SEI film, the battery energy and energy density can be effectively improved. At the same time, the appearance of the pre-lithiated cells produced by this pre-lithiation process remains basically unchanged, and the safety performance and reliability are relatively stable.

Claims

1. A composite membrane for electrochemical pre-lithiation, comprising an insulating substrate; characterized in that: The insulating substrate is laminated with copper foil and lithium metal; The composite film can wrap the battery cell after being bent along the indentation on it. At this time, the A and C areas of the composite film correspond to the side of the battery cell, the B area of ​​the composite film corresponds to the bottom of the battery cell, and the D area of ​​the composite film corresponds to the front of the battery cell. Region A, Region B, and Region C are sequentially composited with the insulating base film, the copper foil, and the lithium metal; The D region consists of the insulating base film and the copper foil, which are sequentially laminated together, with the copper foil extending outward to form a tab.

2. The composite membrane according to claim 1, characterized in that: The insulating base film is a polyester film or a polyethylene film; The thickness of the insulating substrate is 1–500 μm.

3. The composite membrane according to claim 1 or 2, characterized in that: The thickness of the copper foil is 5–50 μm.

4. The composite membrane according to claim 1 or 2, characterized in that: The thickness of the lithium metal is 10–500 μm.

5. The composite membrane according to claim 1 or 2, characterized in that: The copper foil and the lithium metal are composited by hot pressing, rolling or bonding.

6. The application of the composite film according to any one of claims 1-5 in pre-lithiation of battery cells.

7. An electrochemical pre-lithiation method, comprising the following steps: The battery cell is wrapped with the composite film described in any one of claims 1-5, and after being inserted into the casing and injected with electrolyte, it is left to stand for pre-lithiation. The battery cell shall be wrapped in the following manner: The A and C regions of the composite film correspond to the side surface of the battery cell, the B region of the composite film corresponds to the bottom surface of the battery cell, and the D region of the composite film corresponds to the front surface of the battery cell; the tab corresponds to the negative tab of the battery cell.

8. The method according to claim 7, characterized in that: Before wrapping the battery cell with the composite film, a separator is wound around the battery cell.

9. The method according to claim 7 or 8, characterized in that: The pre-lithiation temperature is between 25°C and 60°C, and the time is between 24h and 96h.

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