Lithium supplement layer for lithium ion battery, separator and preparation method thereof and lithium ion battery

Abstract

This invention provides a lithium replenishment layer, a separator, a method for preparing the same, and a lithium-ion battery. The lithium in the replenishment layer is distributed in a gradient along its thickness, with the lithium concentration decreasing sequentially from the center to the two outer surfaces, allowing lithium to be slowly released from the outer surfaces to the outside. The lithium replenishment layer structure provided by this invention achieves effective slow-release lithium replenishment in lithium-ion batteries while reducing environmental requirements and lowering costs.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and relates to a lithium replenishment layer, a separator, and a method for preparing the same for lithium-ion batteries, as well as a lithium-ion battery. In particular, it relates to a lithium replenishment layer for use in a separator, a method for preparing the separator, and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries are widely used in various power markets, including passenger vehicles, energy storage, and consumer electronics, due to their high energy density and cycle performance. However, the energy density of lithium batteries has reached its technological limit due to the limitation of the specific capacity of cathode materials.

[0003] Currently, commonly used lithium replenishment methods include: one is negative electrode lithium replenishment, which involves coating the surface of the negative electrode sheet with a slurry of lithium metal powder, or directly combining the lithium sheet with the negative electrode sheet; the other is positive electrode lithium replenishment, which generally involves coating the positive electrode sheet with lithium oxide, lithium-rich compounds, etc., along with the positive electrode material. Regardless of whether it is positive or negative electrode lithium replenishment, it requires a more stringent workshop environment, and both lithium powder and lithium replenishment materials (lithium-rich compounds, lithium oxide) are much more expensive than the positive and negative electrode materials.

[0004] CN109004304A discloses a method for replenishing lithium in a soft-pack lithium-ion battery, a method for preparing a lithium-ion battery, and an intermediate lithium-replenishing battery. This method places the lithium-replenishing electrode in a gas bag containing electrolyte after the battery cell is packaged; that is, the lithium-replenishing electrode is also in the gas bag, but it also contains electrolyte, rather than directly using lithium sheets for lithium replenishment. CN109103419A discloses a lithium-ion battery negative electrode for lithium replenishment and its preparation method, which involves coating the surface of a pre-lithiated electrode with an organic thin film layer, the organic thin film layer being composed of electrolyte lithium salt dissolved in an organic solvent. The above methods suffer from problems such as high cost, significant safety risks, performance degradation, and unstable coulombic efficiency improvement.

[0005] Therefore, how to achieve effective lithium replenishment of lithium-ion batteries while reducing environmental requirements and lowering costs is a technical problem that urgently needs to be solved. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a lithium replenishment layer, a separator, a method for preparing the same, and a lithium-ion battery. The lithium replenishment layer structure provided by this invention achieves effective lithium replenishment and slow-release lithium replenishment in lithium-ion batteries while reducing environmental requirements, thereby improving battery cycle performance and lowering costs.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a lithium replenishment layer for a lithium-ion battery, wherein the lithium in the lithium replenishment layer is distributed in a gradient along the thickness direction of the lithium replenishment layer, and the concentration of the lithium decreases sequentially from the middle of the lithium replenishment layer to both sides of the lithium replenishment layer, so that the lithium is slowly released to the outside from both sides of the lithium replenishment layer.

[0009] The lithium replenishment layer structure provided by this invention has a lithium concentration that decreases from the middle to both sides of the lithium replenishment layer. As a result, the lithium in the lithium replenishment layer can be slowly released to the outside from both sides of the lithium replenishment layer, realizing slow-release lithium replenishment. This compensates for the active lithium lost during the use of lithium batteries, achieves effective lithium replenishment of lithium-ion batteries while reducing environmental requirements, and also reduces costs.

[0010] Preferably, the lithium replenishment layer is composed of multiple lithium replenishment monolayers stacked together.

[0011] Preferably, the number of lithium replenishment monolayers is 3 to 18, such as 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 or 18 layers.

[0012] In this invention, too many lithium replenishment layers are not conducive to improving battery energy density, while too few lithium replenishment layers will affect the effect of slow-release lithium replenishment, thereby further reducing the improvement of battery long-term performance.

[0013] Preferably, the thickness of the lithium replenishment monolayer is 0.1 to 1.3 μm, such as 0.1 μm, 0.3 μm, 0.5 μm, 0.8 μm, 1 μm or 1.3 μm.

[0014] In this invention, if the thickness of the lithium replenishment monolayer is too thin, less than 0.1 μm, it will increase the difficulty of the coating process; while if the thickness of the lithium replenishment monolayer is too thick, it will reduce the lithium ion release effect.

[0015] Preferably, when the number of lithium replenishment monolayers is an odd number n, the middle layer of the lithium replenishment layer is the (n-1) / 2+1th lithium replenishment monolayer.

[0016] Preferably, when the number of lithium replenishment monolayers is an even number m, the middle of the lithium replenishment layer consists of the m / 2th lithium replenishment monolayer and the m / 2+1th lithium replenishment monolayer.

[0017] In this invention, the number of lithium-supplementing monolayers can be either odd or even. When it is an odd number of layers, the middle layer has the highest lithium concentration; when it is an even number of layers, the two middle layers have the highest lithium concentration.

[0018] Preferably, the lithium-replenishing monolayer comprises a lithium-containing compound and a binder.

[0019] Preferably, the mass percentage of lithium-containing compounds in the lithium-supplementing monolayer is 10% to 90%, such as 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90%.

[0020] Preferably, the difference in the mass percentage of lithium compounds between adjacent lithium-filled monolayers is 1% to 20%, such as 1%, 3%, 5%, 8%, 10%, 13%, 15%, 18%, or 20%.

[0021] In this invention, if the difference in the mass percentage of lithium compounds between adjacent lithium replenishment monolayers is less than 1%, it will affect the release effect of the lithium replenishment layer and be detrimental to the long-term performance improvement of the battery. If it is greater than 20%, it will cause the lithium replenishment layer to release lithium ions too quickly, resulting in poor long-term performance improvement.

[0022] Preferably, the lithium-containing compound includes Li4FeO5, Li5FeO4, Li2NiO2, Li6CoO4, Li2MnO3, Li6MnO4, Li2RuO3, Li5ReO6, and Li 0.65 Ni 1.35 Any one or a combination of at least two of the following: O2, Li2O, Li2O2, LiF, Li2S, Li3N, Li2DHBN, Li2C2O4, LiPF6, C4BLiO8, or C2BF2LiO4.

[0023] Preferably, the adhesive comprises any one or a combination of at least two of PVDF, PTFE, or PMMA.

[0024] In a second aspect, the present invention provides a separator comprising a lithium replenishment layer for lithium-ion batteries as described in the first aspect.

[0025] When the separator provided by this invention contains the aforementioned lithium replenishment layer, the lithium in the replenishment layer spontaneously replenishes the electrolyte, achieving slow-release gradient lithium replenishment and timely replenishment of lithium ions, thereby improving the lifespan of the lithium battery. Simultaneously, after the lithium in the replenishment layer is released, the positions of the lithium ions in the original coating form pores, thereby increasing the membrane porosity and preventing pore blockage caused by side reactions, which would reduce the ion transport channels and thus improve the lithium ion transport rate. This achieves effective lithium replenishment for lithium-ion batteries while reducing the requirements for the manufacturing environment and manufacturing costs.

[0026] If the outermost lithium concentration in the lithium replenishment layer of the separator is higher, the lithium salt concentration in the electrolyte will be higher when the outermost lithium is released into the electrolyte. This results in each layer of the lithium replenishment separator having a lower lithium salt content than the last. This structure is not conducive to the release of lithium salt, and the large difference in lithium salt concentration between the outermost lithium salt and the lithium salt in the electrolyte can easily lead to the direct release of a large amount of lithium salt, making it difficult for the lithium salt in the subsequent lithium replenishment layer to be released, thus failing to achieve the slow-release effect.

[0027] Preferably, the diaphragm further includes a first base film layer, which is disposed in the middle of the lithium replenishment layer and extends along the first base film layer to both sides of the lithium replenishment layer, wherein the lithium concentration in the lithium replenishment layer gradually decreases.

[0028] In this invention, the first base film layer can be located in the middle of the lithium replenishment layer, that is, the lithium concentration is highest at the position in the lithium replenishment layer that is in direct contact with both sides of the first base film layer.

[0029] Preferably, the thickness of the first base film layer is 1 to 15 μm, such as 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm or 15 μm, and more preferably 3 to 5 μm.

[0030] Preferably, the porosity of the first base film layer is 20-60%, such as 20%, 30%, 35%, 38%, 39%, 40%, 41%, 42%, 43%, 45%, 50% or 60%, and more preferably 38-43%.

[0031] Preferably, a second base film layer is provided on at least one surface of the lithium replenishment layer, and more preferably, a second base film layer is provided on both surfaces of the lithium replenishment layer.

[0032] In this invention, when a second base film layer is provided on both sides of the lithium replenishment layer, during the use of the lithium-ion battery, lithium in the lithium replenishment layer is slowly released from the middle to the second base film layers on both sides, compensating for the active lithium lost during battery use and achieving long-term, efficient lithium replenishment. If the second base film layer is only provided on one side of the lithium replenishment layer, lithium ions will accumulate on one side to a thicker thickness and be unevenly distributed.

[0033] Both the first and second base films form an effective concentration gradient difference between lithium ions in the lithium replenishment layer and the electrolyte, which is conducive to the timely release of lithium ions in the lithium replenishment layer, thereby quickly making up for the lost lithium ions and improving the long-term performance of the lithium-ion battery.

[0034] Preferably, the thickness of the second base film layer is 1 to 15 μm, such as 1 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 8 μm, 10 μm, 13 μm or 15 μm, and more preferably 3 to 5 μm.

[0035] Preferably, the porosity of the second base film layer is 20-60%, such as 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55% or 60%, and more preferably 38-43%.

[0036] Preferably, a safety coating is also provided on the surface of the second base film layer away from the lithium replenishment layer.

[0037] Preferably, the lithium replenishment layer is composed of multiple lithium replenishment monolayers stacked together.

[0038] Preferably, the number of lithium replenishment monolayers is 3 to 18, such as 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 or 18 layers.

[0039] In the separator provided by this invention, an excessive number of lithium-replenishing monolayers is detrimental to improving battery energy density, while an insufficient number of lithium-replenishing monolayers affects the effect of slow-release lithium replenishment, thereby further reducing the improvement of long-term battery performance.

[0040] Preferably, the thickness of the lithium replenishment monolayer is 0.1 to 1.3 μm, such as 0.1 μm, 0.3 μm, 0.5 μm, 0.8 μm, 1 μm or 1.3 μm.

[0041] In the diaphragm provided by the present invention, if the thickness of the lithium replenishment monolayer is too thin, less than 0.1 μm, it will increase the difficulty of the coating process; while if the thickness of the lithium replenishment monolayer is too thick, it will reduce the lithium ion release effect.

[0042] Preferably, when the number of lithium replenishment monolayers is an odd number n, the middle layer of the lithium replenishment layer is the (n-1) / 2+1th lithium replenishment monolayer.

[0043] Preferably, when the number of lithium replenishment monolayers is an even number m, the middle of the lithium replenishment layer consists of the m / 2th lithium replenishment monolayer and the m / 2+1th lithium replenishment monolayer.

[0044] In this invention, the number of lithium-supplementing monolayers can be either odd or even. When it is an odd number of layers, the middle layer has the highest lithium concentration; when it is an even number of layers, the two middle layers have the highest lithium concentration.

[0045] Preferably, the lithium-replenishing monolayer comprises a lithium-containing compound and a binder.

[0046] Preferably, the mass percentage of lithium-containing compounds in the lithium-supplementing monolayer is 10% to 90%, such as 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90%.

[0047] Preferably, the difference in the mass percentage of lithium compounds between adjacent lithium-filled monolayers is 1% to 20%, such as 1%, 3%, 5%, 8%, 10%, 13%, 15%, 18%, or 20%.

[0048] In the separator provided by this invention, if the difference in the mass percentage of lithium compounds between adjacent lithium replenishment monolayers is less than 1%, it will affect the release effect of the lithium replenishment layer and be detrimental to the long-term performance improvement of the battery. If it is greater than 20%, it will cause the lithium replenishment layer to release lithium ions too quickly, resulting in poor long-term performance improvement.

[0049] Preferably, the lithium-containing compound includes Li4FeO5, Li5FeO4, Li2NiO2, Li6CoO4, Li2MnO3, Li6MnO4, Li2RuO3, Li5ReO6, and Li 0.65 Ni 1.35 Any one or a combination of at least two of the following: O2, Li2O, Li2O2, LiF, Li2S, Li3N, Li2DHBN, Li2C2O4, lithium hexafluorophosphate, lithium bis(oxalato)borate, or lithium difluorooxalato)borate.

[0050] Preferably, the adhesive comprises any one or a combination of at least two of PVDF, PTFE, or PMMA.

[0051] Thirdly, the present invention provides a method for preparing a diaphragm as described in the second aspect, the method comprising the following steps:

[0052] The separator is obtained by coating at least one side surface of the substrate with a lithium-filled layer.

[0053] The lithium concentration in the lithium replenishment layer decreases sequentially from the middle of the lithium replenishment layer towards the substrate, or the lithium concentration in the lithium replenishment layer decreases sequentially from the substrate to both sides of the lithium replenishment layer.

[0054] In the preparation method provided by the present invention, the lithium replenishment layer can be prepared adaptively according to the specific structure of the separator.

[0055] Preferably, the coating includes:

[0056] Multiple lithium replenishment monolayers are sequentially coated on the surface of the base film to obtain the lithium replenishment layer.

[0057] Preferably, the substrate includes a first base film layer, which is disposed in the middle of the lithium replenishment layer, and the method for preparing the separator includes:

[0058] Multiple lithium replenishment monolayers are sequentially coated on both sides of the first base film layer to obtain a lithium replenishment layer. The lithium concentration in the lithium replenishment layer gradually decreases along both sides of the first base film layer to the lithium replenishment layer.

[0059] Preferably, the substrate includes a second base film layer, the second base film layer being disposed on both sides of the lithium replenishment layer, and the method for preparing the separator includes:

[0060] Multiple lithium replenishment monolayers are sequentially coated on one side surface of the second base film layer to obtain a lithium replenishment layer. Then, another second base film layer is laminated on the surface of the lithium replenishment layer. The lithium concentration in the lithium replenishment layer gradually decreases from the middle of the lithium replenishment layer to both sides of the second base film layer.

[0061] When the substrate includes a first base film layer and a second base film layer, the first base film layer is disposed in the middle of the lithium replenishment layer, and the second base film layer is disposed on both sides of the lithium replenishment layer. The method for preparing the separator includes:

[0062] Multiple lithium replenishment monolayers are sequentially coated on both sides of the first base film layer to form lithium replenishment layers on both sides of the first base film layer. Second base film layers are respectively disposed on the surfaces of the lithium replenishment layers away from the first base film layer. The lithium concentration in the lithium replenishment layers gradually decreases from the first base film layer to the two side second base film layers.

[0063] Multiple lithium replenishment monolayers are sequentially coated on one side of the second base film layer to obtain a lithium replenishment layer. Then, a first base film layer is set on the surface of the lithium replenishment layer, and multiple lithium replenishment monolayers are sequentially coated. Then, a second base film layer is laminated. The lithium concentration in the lithium replenishment layer gradually decreases from the first base film layer to the second base film layers on both sides.

[0064] Fourthly, the present invention also provides a lithium-ion battery, the lithium-ion battery comprising a separator as described in the second aspect or a separator prepared by the preparation method described in the third aspect.

[0065] The lithium-ion battery provided by this invention, in addition to the separator provided above, also includes a positive electrode sheet, a negative electrode sheet, an electrolyte, a positive tab, a negative tab, and an aluminum-plastic film; the specific preparation process and all raw materials are conventional technologies.

[0066] Optionally, the positive electrode includes a positive current collector and a positive active material layer located on at least one side of the positive current collector. The positive active material layer includes a positive active material, a conductive agent, and a binder. The negative electrode includes a negative current collector and a negative active material layer located on at least one side of the negative current collector (which may also be a pure metal negative electrode). The negative active material includes a negative active material, a conductive agent, and a binder (which may also be a pure metal negative electrode).

[0067] The types of raw materials mentioned above are not particularly limited. Any known substance can be used in this application without departing from the inventive concept of this application.

[0068] Optionally, the positive electrode active material includes, but is not limited to, lithium cobalt oxide (LiCoO2) and lithium nickel cobalt manganese oxide (LiNi). x Mn y Co 1-x- yO2 (NMC), lithium nickel cobalt aluminum oxide (LiNiCoAlO2, NCA), lithium manganese oxide (LiMn2O4), lithium manganese iron phosphate (LiMn) x Fe 1-x PO4 (abbreviated as LMFP), lithium vanadium phosphate (Li3V2(PO4)3), lithium vanadium oxide phosphate (LiVOPO4), lithium iron phosphate (LiFePO4), lithium titanate (Li2TiO3), and one or more lithium-rich manganese-based materials.

[0069] Optionally, the negative electrode active material includes graphite, non-graphite carbon, and non-carbon-based graphite materials. In other embodiments, the negative electrode active material is a silicon-based negative electrode active material containing silicon, such as silicon alloys, silicon oxide, or combinations thereof, and in some cases may be mixed with graphite. In other embodiments, the negative electrode may include a carbon-based negative electrode active material containing one or more of graphite, graphene, carbon nanotubes (CNTs), and combinations thereof. In yet another embodiment, the negative electrode active material includes one or more lithium-accepting negative electrode active materials, such as lithium titanium oxide (Li4Ti5O). 12 One or more transition metals (such as tin (Sn)), one or more metal oxides (such as vanadium oxide (V2O5), tin oxide (SnO), titanium dioxide (TiO2)), titanium niobium oxide (Ti) x Nb y O z , where 0≤x≤2, 0≤y≤24 and 0≤z≤64, metal alloys (such as copper-tin alloy (Cu6Sn5)) and one or more metal sulfides (such as iron sulfide (FeS)).

[0070] Optionally, there are no particular restrictions on the positive and negative current collectors, as long as they are conductive and do not cause chemical changes in the battery. Specifically, copper, stainless steel, aluminum, nickel, titanium, or metal current collectors with carbon or other surface treatments can be used. The positive electrode tab is typically made of aluminum, and the negative electrode tab is typically made of copper or a copper-nickel composite tab.

[0071] Optionally, the adhesive is a component used to assist in the bonding of active materials, conductive materials, etc., and to the current collector. Specifically, it may include at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, styrene-butadiene rubber, polyacrylic acid, polyacrylonitrile, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber.

[0072] Conductive agents can be used to assist and improve the conductivity in secondary batteries, and there are no particular limitations, as long as they are conductive without causing chemical changes. Specifically, they may include graphite, such as natural or artificial graphite; carbon materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking black; conductive fibers, such as carbon fibers and metal fibers; conductive tubes, such as carbon nanotubes; metal powders, such as fluorocarbon powders, aluminum powders, and nickel powders; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxides; and polyphenylene derivatives.

[0073] Electrolytes consist of electrolyte salts and solvents.

[0074] Optionally, the electrolyte salt includes, but is not limited to, one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0075] Optionally, the solvent includes, but is not limited to, one or more of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0076] Optionally, the electrolyte also includes additives, which may be negative electrode film-forming additives, positive electrode film-forming additives, or additives that can improve certain battery performance, such as additives that improve battery overcharge performance, or additives that improve battery high-temperature or low-temperature performance. Any known type of additive can be used in this application without departing from the inventive concept. There are no special requirements for the mixing method of the additives; for example, they can be directly mixed with conductive agents, active materials, and binders to form a mixture.

[0077] Compared with the prior art, the present invention has the following beneficial effects:

[0078] The lithium replenishment layer structure provided by this invention features a lithium concentration that decreases from the center to both sides of the layer. This allows lithium in the replenishment layer to be slowly released to the outside from both sides, achieving slow-release lithium replenishment and compensating for the active lithium lost during lithium battery use. When the separator contains the aforementioned lithium replenishment layer, the lithium in the layer spontaneously replenishes the electrolyte, achieving slow-release gradient lithium replenishment and timely replenishing lithium ions, thereby improving the lifespan of the lithium battery. Simultaneously, after the lithium in the replenishment layer is released, the positions of the lithium ions in the original coating form pores, thereby increasing the porosity of the separator and preventing pore blockage caused by side reactions, which would reduce the ion transport channels and thus improve the lithium ion transport rate. This achieves effective lithium replenishment for lithium-ion batteries while reducing the requirements for the preparation environment and the preparation cost. Attached Figure Description

[0079] Figure 1 This is a schematic diagram of the diaphragm provided in Example 1.

[0080] Wherein, 1-base film, 2-lithium replenishment layer. Detailed Implementation

[0081] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0082] Examples 1-12

[0083] Examples 1-12 provide a separator, which includes a base film, a lithium replenishment layer and a base film stacked sequentially (i.e., the lithium replenishment layer is located between two base films); the lithium replenishment layer is composed of multiple lithium replenishment monolayers stacked together, and the lithium concentration in the lithium replenishment layer gradually decreases from the middle of the lithium replenishment layer to the base films on both sides; the specific number of lithium replenishment monolayers, the thickness of the lithium replenishment monolayers, the mass percentage of lithium compounds in the middle lithium replenishment monolayers, and the difference in the mass percentage of lithium compounds in adjacent lithium replenishment monolayers are all shown in Table 1.

[0084] The membrane is prepared as follows:

[0085] Based on the specific structure of the diaphragm, lithium-supplementing monolayer slurries with different lithium concentrations were sequentially coated on the surface of a 3μm thick polyethylene diaphragm (porosity 30%) (the concentration first increases and then decreases, consistent with the pattern in the structure). The coating thickness of each lithium-supplementing monolayer was 0.1μm. After coating, a 3μm thick polyethylene diaphragm was also laminated on the other side of the lithium-supplementing layer (the side without a base film). After drying, the diaphragm was obtained.

[0086] The lithium-supplemented single-layer slurry is composed of lithium hexafluorophosphate (LiPF6), polymethyl methacrylate and N-methylpyrrolidone (NMP).

[0087] Figure 1 A schematic diagram of the structure of the diaphragm provided in Example 1 is shown, wherein the base film 1 is located on both sides of the lithium replenishment layer 2, and the lithium replenishment layer 2 is composed of multiple lithium replenishment monolayers stacked together.

[0088] Table 1

[0089]

[0090] Example 13

[0091] The difference between this embodiment and Embodiment 1 is that the base film in this embodiment is only disposed on one side of the lithium replenishment layer.

[0092] In the preparation method, after obtaining the lithium replenishment layer, no further composite with another base film is performed.

[0093] The remaining structure and preparation method are consistent with those in Example 1.

[0094] Example 14

[0095] The difference between this embodiment and Embodiment 1 is that in this embodiment, the base film is located in the middle of the lithium replenishment layer, that is, the base film layer is between the ninth and tenth lithium replenishment monolayers (there are nine base film monolayers on each side of the base film layer); the lithium concentration in the lithium replenishment layer decreases sequentially from the base film to both sides of the lithium replenishment layer.

[0096] In the preparation method, the same number of lithium-supplemented monolayers are coated on both sides of the base film layer.

[0097] The remaining preparation methods are consistent with those in Example 1.

[0098] Comparative Example 1

[0099] The difference between this comparative example and Example 1 is that the diaphragm in this example is only a single-layer base membrane.

[0100] Comparative Example 2

[0101] The difference between this comparative example and Example 5 is that the lithium replenishment layer in this comparative example is a single-layer structure with a thickness of 0.9 μm, which is consistent with the ratio of the 9th and 10th intermediate layers in Example 5.

[0102] The remaining preparation methods and parameters are consistent with those in Example 5.

[0103] Comparative Example 3

[0104] The membrane comprises a base membrane, a lithium replenishment layer, and another base membrane stacked sequentially. The lithium replenishment layer consists of 18 lithium replenishment monolayers stacked together, and the lithium concentration in the lithium replenishment layer gradually increases from the middle of the lithium replenishment layer to the base membranes on both sides. In the lithium replenishment monolayer closest to the base membrane, the mass percentage of lithium-containing compounds in the lithium replenishment monolayer is 90%, the difference in the mass percentage of lithium between adjacent lithium replenishment monolayers is 10%, and the mass percentage of lithium-containing compounds in the middle 9th and 10th lithium replenishment monolayers is 10%.

[0105] The membrane is prepared as follows:

[0106] Based on the specific structure of the diaphragm, lithium-supplementing monolayer slurry with different lithium concentrations (concentration increases from the middle to both sides) is sequentially coated on the surface of a 3μm thick polyethylene diaphragm. The coating thickness of each lithium-supplementing monolayer is 0.1μm. After coating, a 3μm thick polyethylene diaphragm is also laminated on the other side of the lithium-supplementing layer (the side without a base film). After drying, the diaphragm is obtained.

[0107] The lithium-supplemented single-layer slurry is composed of lithium hexafluorophosphate (LiPF6), polymethyl methacrylate and N-methylpyrrolidone (NMP).

[0108] The separators provided in Examples 1-14 and Comparative Examples 1-3 were assembled with positive electrode sheets, negative electrode sheets, separators, and electrolytes into lithium batteries, and their formation capacity and cycle life were tested. The positive electrode material was a 9-series NCM (nickel-cobalt-manganese ternary material); the negative electrode material was a hybrid negative electrode material of graphite and silicon.

[0109] The specific test conditions were room temperature, 1C / 1C cycle, and voltage range of 2.7-4.2V; the test results are shown in Table 2.

[0110] Table 2

[0111]

[0112]

[0113] The data results from Examples 1-9 show that an excessively thick lithium-added monolayer is not conducive to improving long-term performance.

[0114] The data results from Examples 1 and 11 show that when the number of lithium-added monolayers is too high, it will increase the difficulty of lithium-ion release and lead to a poorer cycle retention rate.

[0115] The data results from Examples 1 and 12 show that if the difference in the mass ratio of lithium-containing compounds between adjacent lithium-added monolayers is too large, it will affect the lithium-ion release rate too quickly, resulting in no significant long-term performance improvement.

[0116] The data results from Examples 1 and 13 show that the base film is only located on one side of the lithium replenishment layer, resulting in a thick accumulation of lithium ions on one side and uneven distribution.

[0117] The data results from Examples 1 and 14 show that when the base film is located in the middle of the lithium replenishment layer, the performance can be significantly improved, but it is slightly inferior compared to the two sides.

[0118] The data results from Example 1 and Comparative Example 1 show that without any treatment of the base film, the problem of rapid long-term performance degradation cannot be solved.

[0119] The data results from Example 1 and Comparative Example 2 show that if the lithium concentration in the lithium replenishment layer is not distributed in a gradient (i.e., gradually decreases from the middle to both sides), orderly lithium replenishment cannot be obtained, thus failing to effectively improve long-term performance.

[0120] The data from Example 1 and Comparative Example 3 show that if the lithium concentration in the lithium replenishment layer is highest on one side of the lithium replenishment layer, the problem of extending the long-term lifespan cannot be solved.

[0121] In summary, the lithium replenishment layer structure provided by this invention achieves slow-release lithium replenishment by decreasing the lithium concentration from the center to both sides of the layer, thus compensating for the loss of active lithium during the use of the lithium battery. When the separator contains the aforementioned lithium replenishment layer, the lithium in the replenishment layer spontaneously replenishes the electrolyte, achieving slow-release gradient lithium replenishment and timely replenishing lithium ions, thereby improving the lifespan of the lithium battery. Simultaneously, after the lithium in the replenishment layer is released, the positions of the lithium ions in the original coating form pores, thereby increasing the porosity of the separator and avoiding the reduction of ion transport channels caused by pore blockage due to side reactions, thus also improving the lithium ion transport rate. This invention achieves effective lithium replenishment for lithium-ion batteries while reducing the requirements for the preparation environment and the preparation cost.

[0122] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A diaphragm, characterized in that, The separator includes a lithium replenishment layer for lithium-ion batteries; A second base film layer is provided on both sides of the lithium replenishment layer; The lithium in the lithium replenishment layer is distributed in a gradient along the thickness direction of the lithium replenishment layer, and the lithium concentration decreases from the middle of the lithium replenishment layer to both sides of the lithium replenishment layer, so that the lithium is slowly released to the outside from both sides of the lithium replenishment layer. The lithium replenishment layer is composed of multiple lithium replenishment single-layer stacks; The number of layers in the lithium replenishment monolayer is 3 to 18; The thickness of the lithium replenishment monolayer is 0.1~1.3μm; When the number of lithium replenishment monolayers is an odd number n, the middle of the lithium replenishment layer is the (n-1) / 2+1th lithium replenishment monolayer. When the number of lithium replenishment monolayers is an even number of layers m, the middle of the lithium replenishment layer consists of the m / 2th lithium replenishment monolayer and the m / 2+1th lithium replenishment monolayer. The lithium-replenishing monolayer comprises a lithium-containing compound and a binder; The difference in the mass percentage of lithium compounds between adjacent lithium-filled monolayers is 1-20%; The method for preparing the diaphragm includes the following steps: The separator is obtained by coating at least one side surface of the substrate with a lithium-filled layer. Among them, the lithium concentration in the lithium replenishment layer decreases sequentially from the middle of the lithium replenishment layer towards the substrate; The substrate includes a second base film layer, which is disposed on both sides of the lithium replenishment layer. The method for preparing the separator includes: Multiple lithium replenishment monolayers are sequentially coated on one side of the second base film layer to obtain a lithium replenishment layer. Then, another second base film layer is laminated onto the surface of the lithium replenishment layer. The lithium concentration in the lithium replenishment layer gradually decreases from the middle of the lithium replenishment layer to both sides.

2. The diaphragm according to claim 1, characterized in that, The lithium-containing compound in the lithium-replenishing monolayer accounts for 10-90% of the total mass.

3. The diaphragm according to claim 1, characterized in that, The lithium-containing compounds include Li4FeO5, Li5FeO4, Li2NiO2, Li6CoO4, Li2MnO3, Li6MnO4, Li2RuO3, Li5ReO6, and Li 0.65 Ni 1.35 Any one or a combination of at least two of the following: O2, Li2O, Li2O2, LiF, Li2S, Li3N, Li2DHBN, Li2C2O4, LiPF6, C4BLiO8, or C2BF2LiO4.

4. The diaphragm according to claim 1, characterized in that, The adhesive includes any one or a combination of at least two of PVDF, PTFE, or PMMA.

5. The diaphragm according to claim 1, characterized in that, The thickness of the second base film layer is 1~15μm.

6. The diaphragm according to claim 5, characterized in that, The thickness of the second base film layer is 3~5μm.

7. The diaphragm according to claim 1, characterized in that, The porosity of the second base film layer is 20-60%.

8. The diaphragm according to claim 7, characterized in that, The porosity of the second base film layer is 38-43%.

9. The diaphragm according to claim 1, characterized in that, A safety coating is also provided on the surface of the second base film layer that is away from the lithium replenishment layer.

10. A method for preparing a diaphragm as described in any one of claims 1-9, characterized in that, The preparation method includes the following steps: The separator is obtained by coating at least one side surface of the substrate with a lithium-filled layer. Among them, the lithium concentration in the lithium replenishment layer decreases sequentially from the middle of the lithium replenishment layer towards the substrate; The substrate includes a second base film layer, which is disposed on both sides of the lithium replenishment layer. The method for preparing the separator includes: Multiple lithium replenishment monolayers are sequentially coated on one side of the second base film layer to obtain a lithium replenishment layer. Then, another second base film layer is laminated onto the surface of the lithium replenishment layer. The lithium concentration in the lithium replenishment layer gradually decreases from the middle of the lithium replenishment layer to both sides.

11. The method for preparing the diaphragm according to claim 10, characterized in that, The coating includes: Multiple lithium replenishment monolayers are sequentially coated on the surface of the base film to obtain the lithium replenishment layer.

12. A lithium-ion battery, characterized in that, The lithium-ion battery includes a separator as described in any one of claims 1-9 or a separator prepared by the preparation method described in claim 10 or 11.

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