Pre-lithiated separator, preparation method therefor, and use thereof

Abstract

A pre-lithiated separator, a preparation method therefor, and a use thereof, relating to the technical field of lithium-ion batteries. The pre-lithiated separator comprises a substrate and a sustained-release pre-lithiation agent coating applied to the substrate on a positive electrode side thereof. The sustained-release pre-lithiation agent coating comprises a sustained-release lithium supplementing agent. The sustained-release lithium supplementing agent comprises a lithium supplementing agent and a sustained-release agent coated on an outer layer of the lithium supplementing agent. The preparation method for the pre-lithiated separator comprises: coating a sustained-release lithium supplementing agent on a surface of a separator substrate. Due to the continuous-release characteristics of a buffer, a protective SEI layer and a lithium metal alloy are always formed on a negative electrode, allowing a battery system to maintain a high lithium ion concentration, thereby improving the cycle performance of a battery. In addition, the surface of the separator is coated with the sustained-release lithium supplementing agent. The process is mature, the proportion of an active substance on the surface of the electrode is not reduced, the energy density of the battery is not affected, and the cycle performance can be improved while ensuring that the performance of the battery cell is not reduced.

Description

A pre-lithiation separator and a preparation method and application thereof TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a pre-lithiation separator and a preparation method and application thereof. BACKGROUND

[0002] Lithium ion batteries have greatly changed people's lives since their invention. The rapid development of electric vehicles, grid energy storage, and portable electronic devices has made the research and development of higher energy density lithium batteries more urgent. The negative electrode material of lithium battery is currently dominated by graphite, which has advantages such as high electrical conductivity and high stability, but has reached the theoretical specific capacity. The improvement of battery energy density drives the iteration of high-performance negative electrodes. Silicon-based negative electrodes have high theoretical specific capacity, but have problems such as volume expansion, poor conductivity, and unstable SEI film, and are often doped with graphite for application.

[0003] In the first cycle of lithium ion batteries, the formation of SEI film on the surface of the graphite negative electrode has a first irreversible capacity loss of 5-15%, and high-capacity silicon-based materials have a loss of 15-35%. Through pre-lithiation treatment of electrode materials, the irreversible lithium loss is compensated, which is one of the important means to solve the problem of large irreversible lithium loss of silicon-based materials. The current commonly used pre-lithiation technology is mainly divided into negative electrode pre-lithiation and positive electrode pre-lithiation. Negative electrode pre-lithiation, which introduces active lithium into the negative electrode, is used to compensate for the capacity loss caused by SEI growth. The main technical routes include chemical and electrochemical methods, but negative electrode pre-lithiation directly contacts with metal lithium, which is not compatible with existing battery production lines and has high modification cost. Positive electrode pre-lithiation, which pre-adds active lithium to the positive electrode, can release lithium ions to the negative electrode to compensate for the irreversible capacity in the first charge-discharge process. The main methods include additive, over-lithiated positive electrode material, etc. The process is simple, the cost is relatively low, and it can be directly added in the homogenization process of positive electrode slurry without additional process improvement, but the positive electrode lithium supplement additive generally has poor stability, is easy to react with carbon dioxide and water in the air, and has a high decomposition potential, which needs to be charged to a high potential in the first cycle to release its capacity and play the role of lithium supplement. In view of the problems of poor stability and conductivity of lithium supplement additives, CN115000536A patent coats a layer of semi-finished carbon layer containing hydroxyl on the surface of the additive, which can play the role of separating water and carbon dioxide in the air, and the semi-finished carbon layer containing hydroxyl has partial conductivity function, which can improve the conductivity of the positive electrode lithium supplement material.

[0004] Although pre-lithiation has been proven to effectively compensate for the irreversible lithium loss in the SEI film formation stage by introducing additional active lithium into the battery, it ignores the continuous lithium consumption that occurs throughout the cycle process. Controllable long-term lithium supplementation has a more profound effect on improving the cycle life of lithium batteries.

[0005] There are also some patents for long-acting lithium supplementation and slow-release lithium supplementation, such as Chinese patent CN114284463A, which sets a diaphragm on both sides of the lithium-containing metal body, the lithium-containing metal body is in a whole piece structure, the diaphragm on both sides is provided with an opening, and the part of the lithium-containing metal body not covered by the diaphragm forms a first-effect lithium supplementing area, and the part covered by the diaphragm forms a slow-release lithium supplementing area, but this method has a complex production process, which is not conducive to mass production and application. For example, Chinese patent CN117117357A coats a silazane, siloxane, sulfonic silane or carbodiimide compound on the surface of the lithium supplementing agent, mixes it with the positive electrode slurry, and coats it on the current collector, consumes water and acidic substances in the battery during the charge and discharge cycle of the battery, and slowly releases lithium ions to achieve a long-acting lithium supplementing effect, but the positive electrode lithium supplementing also has a problem of reducing the mass fraction of active materials, which is not conducive to the improvement of the energy density of the battery. SUMMARY

[0006] The present application aims to provide a pre-lithiated separator for supplementing irreversible lithium loss in the first cycle of a lithium ion battery; the present application also aims to provide a preparation method of a pre-lithiated separator; and the present application further aims to provide an application of a pre-lithiated separator.

[0007] The present application provides a pre-lithiated separator, which comprises a substrate and a slow-release pre-lithiated agent coating layer coated on the positive electrode side of the substrate, wherein the slow-release pre-lithiated agent coating layer comprises a slow-release lithium supplementing agent, and the slow-release lithium supplementing agent comprises a lithium supplementing agent and a slow-release agent coated on the outer layer of the lithium supplementing agent.

[0008] Further, the thickness of the slow-release pre-lithiated agent coating layer is 1-5 μm; and the lithium supplementing agent is selected from at least one of lithium oxide, lithium sulfide, lithium zirconate, lithium titanate, lithium ferrite, lithium nickelate, lithium oxalate, lithium silicate, Li2S / Co composite and Li2O / Co composite.

[0009] Further, the slow-release agent is selected from one of magnesium nitrate, copper nitrate and sodium nitrate.

[0010] The surface of the lithium supplementing agent is coated with a nitrate salt to improve the air stability of the lithium supplementing agent, and during the operation of the battery, the nitrate salt will dissociate into metal ions and nitrate ions in the electrolyte, thereby releasing the coated lithium supplementing agent. The free metal ions will be reduced to form lithium metal alloy by the metal lithium, and the nitrate ions can form a stable protective SEI film on the negative electrode surface, and the released lithium supplementing agent can supplement the continuous lithium loss occurring during the cycle process. Due to the sustained release characteristics of the nitrate salt, the negative electrode will always form a protective SEI layer and lithium metal alloy, and the battery system can maintain a high lithium ion concentration, thereby improving the cycle performance of the battery.

[0011] Further, the preparation method of the slow-release lithium supplement agent is: dissolving the slow-release agent in an organic solvent to configure a solution, uniformly spraying the solution on the surface of the lithium supplement agent, and depositing a layer of slow-release agent on the surface of the lithium supplement agent after the solvent volatilizes.

[0012] The slow-release agent is coated on the surface of the lithium supplement agent.

[0013] Further, the mass ratio of the lithium supplement agent to the slow-release agent is 2:1-1:30; and the organic solvent is at least one selected from N-methyl pyrrolidone (NMP), N,N-dimethylacetamide (DMAC), N,N-dimethylformamide (DMF), acetone, methanol, and ethanol.

[0014] Further, the substrate is selected from one of polyethylene (PE), polypropylene (PP), polyethylene-polypropylene (PE / PP) composite film, polyethylene terephthalate (PET), polyimide (PI), aramid, and cellulose diaphragm.

[0015] Further, the substrate comprises a single-sided or double-sided ceramic coating.

[0016] The application also provides a preparation method of a pre-lithiated diaphragm, comprising the following steps:

[0017] S1: preparing a slow-release pre-lithium agent coating slurry: dispersing the slow-release lithium supplement agent, the binder, and the dispersant in an organic solvent and uniformly mixing;

[0018] S2: coating the slow-release pre-lithium agent coating slurry: coating the slow-release pre-lithium agent coating slurry prepared in step S1 on the positive electrode side of the substrate to obtain the pre-lithiated diaphragm as described above.

[0019] Further, in step S1, the raw materials of the slow-release pre-lithium agent slurry include, by mass percentage:

[0020] organic solvent 45-80%;

[0021] slow-release lithium supplement agent 20-45%;

[0022] binder 0.5-7%;

[0023] stabilizer 0.1-1.5%;

[0024] dispersant 0.1-1.5%.

[0025] Further, the binder is selected from at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP); the stabilizer is sodium hydroxymethyl cellulose or polyamide wax; and the dispersant is selected from at least one of polyacrylamide and sodium fatty alcohol ether sulfate.

[0026] The application also discloses a lithium ion battery comprising a positive electrode, a negative electrode, an electrolyte and the pre-lithiated separator obtained by the preparation method, and the lithium ion battery has a capacity retention rate of more than 93% after 1200 cycles at room temperature.

[0027] The pre-lithiated separator disclosed by the application coats the slow-release lithium supplement agent on the surface of the separator. Due to the sustained-release characteristics of the buffer, the negative electrode can always form a protective SEI layer and lithium metal alloy, and the battery system can maintain a high lithium ion concentration, thereby improving the cycle performance of the battery. Moreover, the slow-release lithium supplement agent is coated on the surface of the separator, and the process is mature and does not reduce the proportion of active substances on the surface of the electrode, so the energy density of the battery is not affected, and the cycle performance can be improved while the performance of the battery cell is not reduced. DETAILED DESCRIPTION

[0028] In order to make the technical scheme of the application clearer, the application will be further described in detail below in combination with specific embodiments. EMBODIMENT

[0029] Preparation of the slow-release lithium supplement agent: the mass ratio of lithium oxide to magnesium nitrate is 1:5, the magnesium nitrate is dissolved in a methanol solution, and is uniformly sprayed on the surface of the lithium oxide; the methanol is completely volatilized by standing at room temperature or drying in an oven; at least 5 rounds of spraying and drying are performed until the magnesium nitrate solution is completely sprayed; and the slow-release lithium supplement agent is obtained after drying.

[0030] Preparation of the pre-lithiated separator:

[0031] S1: preparation of a slow-release pre-lithium agent coating slurry: 30 g of the slow-release lithium supplement agent, 1.5 g of PVDF, 0.35 g of sodium hydroxymethyl cellulose and 0.15 g of polyacrylamide are dispersed in 68 g of NMP, and are uniformly stirred.

[0032] S2: coating of the slow-release pre-lithium agent coating slurry: the slow-release pre-lithium agent slurry prepared in step S1 is coated on the surface of the polyethylene-based film by spraying, and after the organic solvent is volatilized, the thickness of the coating is 2 microns. EMBODIMENT

[0033] The difference between this embodiment and embodiment 1 is that the lithium oxide is replaced by lithium ferrite.

[0034] Preparation of the slow-release lithium supplement: lithium ferrite and magnesium nitrate were mixed in a mass ratio of 1:5. The magnesium nitrate was dissolved in a methanol solution and uniformly sprayed on the surface of the lithium ferrite. The mixture was left to stand at room temperature or was dried in an oven to completely volatilize the methanol. The spraying and drying were repeated for at least 5 cycles until the magnesium nitrate solution was completely sprayed. After drying, the slow-release lithium supplement was obtained.

[0035] Preparation of the pre-lithiated separator:

[0036] S1 Preparation of the slow-release pre-lithium agent coating slurry: 30 g of the slow-release lithium supplement, 1.5 g of PVDF, 0.35 g of sodium hydroxymethyl cellulose, and 0.15 g of polyacrylamide were dispersed in 68 g of NMP and stirred uniformly.

[0037] S2 Coating of the slow-release pre-lithium agent coating slurry: the slow-release pre-lithium agent slurry prepared in step S1 was coated on the surface of the polyethylene-based film by spraying. After the organic solvent was volatilized, the coating thickness was 2 μm. Example

[0038] The difference between this example and Example 1 is that the "magnesium nitrate" is replaced with "copper nitrate".

[0039] Preparation of the slow-release lithium supplement: lithium ferrite and magnesium nitrate were mixed in a mass ratio of 1:5. The magnesium nitrate was dissolved in a methanol solution and uniformly sprayed on the surface of the lithium ferrite. The mixture was left to stand at room temperature or was dried in an oven to completely volatilize the methanol. The spraying and drying were repeated for at least 5 cycles until the magnesium nitrate solution was completely sprayed. After drying, the slow-release lithium supplement was obtained.

[0040] Preparation of the pre-lithiated separator:

[0041] S1 Preparation of the slow-release pre-lithium agent coating slurry: 30 g of the slow-release lithium supplement, 1.5 g of PVDF, 0.35 g of sodium hydroxymethyl cellulose, and 0.15 g of polyacrylamide were dispersed in 68 g of NMP and stirred uniformly.

[0042] S2 Coating of the slow-release pre-lithium agent coating slurry: the slow-release pre-lithium agent slurry prepared in step S1 was coated on the surface of the polyethylene-based film by spraying. After the organic solvent was volatilized, the coating thickness was 2 μm. Example

[0043] The difference between this example and Example 1 is that the "lithium oxide, magnesium nitrate mass ratio 1:5" is replaced with "lithium oxide, magnesium nitrate mass ratio 1:15".

[0044] Preparation of the slow-release lithium supplement: lithium ferrite and magnesium nitrate were mixed in a mass ratio of 1:5. The magnesium nitrate was dissolved in a methanol solution and uniformly sprayed on the surface of the lithium ferrite. The mixture was left to stand at room temperature or was dried in an oven to completely volatilize the methanol. The spraying and drying were repeated for at least 5 cycles until the magnesium nitrate solution was completely sprayed. After drying, the slow-release lithium supplement was obtained.

[0045] Preparation of the pre-lithiated separator:

[0046] S1 Preparation of slow-release pre-lithium agent coating slurry: 30 g of slow-release lithium supplement, 1.5 g of PVDF, 0.35 g of sodium hydroxymethyl cellulose, and 0.15 g of polyacrylamide were dispersed in 68 g of NMP and stirred uniformly.

[0047] S2 Coating of slow-release pre-lithium agent coating slurry: The slow-release pre-lithium agent slurry prepared in step S1 was coated on the surface of a polyethylene-based film by spraying, and after the organic solvent was volatilized, the coating thickness was 2 μm.

[0048] The difference between the present comparative example and Example 1 is that the lithium supplement is not treated with a slow-release agent.

[0049] (1) Preparation of pre-lithium agent slurry: 5 g of lithium oxide, 1.5 g of PVDF, 0.35 g of sodium hydroxymethyl cellulose, and 0.15 g of polyacrylamide were dispersed in 93 g of NMP and stirred uniformly.

[0050] (2) Preparation of lithium supplement separator: The pre-lithium agent slurry prepared in step 2 was coated on the surface of a polyethylene-based film by spraying, and after the organic solvent was volatilized, the coating thickness was 2 μm.

[0051] Conventional 2 μm ceramic-coated polyethylene separator.

[0052] The separators obtained in Examples 1-4 and Comparative Examples 1-2 were assembled into small soft-pack batteries with ternary positive electrodes, silicon-carbon negative electrodes, and electrolyte, and the assembled batteries were pre-charged, formed, and analyzed for data at room temperature (1C / -1C, 25°C), and the results are shown in Table 1 below.

[0053] Table 1 Performance test results of lithium ion batteries assembled from Examples 1-4 and Comparative Examples 1-2

[0054]

[0055] From Table 1, compared with the conventional ceramic coated separator of Comparative Example 2, the first charge capacity of the separators of Comparative Example 1 and Examples 1-4 is obviously improved, which is due to the higher specific energy of the lithium supplement agent. Since the irreversible capacity of the lithium supplement agent is large, the first discharge capacity is not greatly improved, and the first coulombic efficiency slightly decreases. By comparing the room temperature cycle capacity retention rate data of Comparative Example 1 and Comparative Example 2, it is found that there is little difference between them, indicating that the lithium supplement agent without coating of the buffer agent only effectively compensates for the irreversible lithium loss in the SEI film forming stage, and has little effect on the continuous lithium consumption occurring in the entire cycle process. Compared with Comparative Example 2, the capacity of the battery of Examples 1-4 is greatly improved because the lithium supplement agent also has capacity to play. Compared with the ordinary lithium supplement layer separator, i.e. Comparative Example 1, the capacity retention rate in the entire cycle process is greatly improved. Not only is the capacity of the battery greatly improved, but also the entire cycle process is significantly improved, and the capacity retention rate is maintained at more than 93% after 1200 cycles at room temperature.

[0056] The above-described examples only express several embodiments of the present application, and the description is more specific and detailed, but it should not be understood as limiting the scope of the patent of the present application. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, several modifications and improvements can be made, which all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A prelithiated separator, characterized by, The cathode side of the substrate is coated with a slow-release pre-lithium coating, which comprises a slow-release lithium supplement agent, and the slow-release lithium supplement agent comprises a lithium supplement agent and a slow-release agent coated on the outer layer of the lithium supplement agent.

2. A prelithiated separator according to claim 1, wherein, The thickness of the slow-release pre-lithium coating is 1-5 μm; the lithium supplement agent is selected from at least one of lithium oxide, lithium sulfide, lithium zirconate, lithium titanate, lithium ferrite, lithium nickelate, lithium oxalate, lithium silicate, Li2S / Co composite, and Li2O / Co composite; and the slow-release agent is selected from one of magnesium nitrate, copper nitrate, and sodium nitrate.

3. The prelithiated separator of claim 1, wherein, The preparation method of the slow-release lithium supplement agent is to dissolve the slow-release agent in an organic solvent to form a solution, spray the solution on the surface of the lithium supplement agent, and deposit a layer of slow-release agent on the surface of the lithium supplement agent after the solvent is volatilized.

4. A prelithiated separator according to claim 3, wherein, The mass ratio of the lithium supplement agent to the slow-release agent is 2:1-1:30; and the organic solvent is selected from at least one of N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, acetone, methanol, and ethanol.

5. The prelithiated separator of claim 1, wherein, The substrate is selected from one of polyethylene, polypropylene, polyethylene-polypropylene composite film, polyethylene terephthalate, polyimide, aramid, and cellulose diaphragm.

6. A prelithiated separator according to claim 5, wherein, The substrate comprises a single-sided or double-sided ceramic coating.

7. A method for preparing a pre-lithiated separator, characterized in that, The method comprises the following steps: S1: preparing a slow-release pre-lithium coating slurry by dispersing a slow-release lithium supplement agent, a binder, and a dispersant in an organic solvent and mixing them uniformly; S2: coating the slow-release pre-lithium coating slurry obtained in step S1 on the cathode side of the substrate to obtain a pre-lithiated diaphragm as claimed in any one of claims 1-6.

8. The method of claim 7, wherein the pre-lithiated separator is prepared by the steps of: In step S1, the raw materials of the slow-release pre-lithium coating slurry include, by mass percentage: organic solvent 45-80%; slow-release lithium supplement agent 20-45%; binder 0.5-7%; stabilizer 0.1-1.5%; dispersant 0.1-1.5%.

9. The method of claim 8, wherein the prelithiated separator is prepared by the steps of: The binder is selected from at least one of polyvinylidene fluoride, polytetrafluoroethylene, and polyvinylidene fluoride-hexafluoropropylene copolymer; the stabilizer is sodium hydroxymethyl cellulose or polyamide wax; and the dispersant is selected from at least one of polyacrylamide and sodium fatty alcohol ether sulfate.

10. A lithium-ion battery, characterized in that The lithium ion battery comprises a cathode, an anode, an electrolyte, and a pre-lithiated diaphragm obtained by the preparation method of any one of claims 7-9, and has a capacity retention rate of greater than 93% after 1200 cycles at room temperature.

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