Lithium ion battery diaphragm, preparation method thereof and lithium ion battery

Abstract

The invention provides a lithium ion battery diaphragm, which comprises a base membrane and a lithium supplement coating coated on the surface of the base membrane, the lithium supplement coating comprises a lithium supplement agent, a conductive agent and a binder, the mass ratio of the lithium supplement agent to the lithium supplement coating is W1, the mass ratio of the conductive agent to the lithium supplement coating is W2, and the mass ratio of the binder to the lithium supplement coating is W3; the thickness of the lithium supplementing coating is T, W1, W2, W3 and T meet the following relations: 0.1 * W1 < = T < = 5 * (W2 + W3), and W1 + W2 + W3 = 100%. Compared with the prior art, the lithium ion battery diaphragm provided by the invention has the advantages that the lithium supplementing agent is coated on the base membrane and is separated from the positive electrode slurry, so that the working procedure that the lithium supplementing agent is in contact with air is reduced, and deterioration caused by moisture absorption of the lithium supplementing agent is reduced; according to the lithium supplementing coating in the formula proportion, the lithium supplementing effect of the lithium supplementing agent is guaranteed, charging and discharging of the positive electrode material are not affected, and the discharging capacity and the energy density of the positive electrode material are improved.

Description

Technical Field

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

[0002] As the negative electrode material, silicon carbon forms a thicker SEI layer on the surface of the silicon negative electrode during the battery formation process, which consumes a lot of Li + , which reduces the first coulombic efficiency of the battery, reduces the battery capacity, and thus reduces the battery energy density. Based on this, a lithium supplement is needed to make up for the loss of Li. Li5FeO4 as a cathode lithium supplement has the advantages of high capacity and is one of the most competitive cathode lithium supplements.

[0003] Normally, Li5FeO4 is directly used as a cathode additive and mixed with cathode materials and coated to improve the first efficiency and discharge capacity of the battery. However, Li5FeO4 is easy to absorb water. In the processes of coating, laser cleaning, rolling, slitting, film making, winding, and top and side sealing, it is easy to react with water and deteriorate, causing the charging capacity to drop sharply, which greatly affects the lithium replenishment effect and fails to achieve the purpose of improving the first efficiency and ED. Secondly, Li5FeO4 has poor electronic conductivity and limited charging capacity. When it is added to the cathode sheet, its gram capacity is limited by the low content of conductive agent in the cathode formula, so the capacity is limited; and it is non-conductive itself, which is equivalent to adding a new non-conductive substance on the basis of the original cathode formula, which will affect the gram capacity of LiCoO2, so that its effect as a lithium replenisher will not be exerted. On the contrary, it will deteriorate the discharge capacity, reduce ED, and deteriorate the rate and cycle performance.

[0004] In view of this, it is indeed necessary to provide a technical solution to the above problems. Summary of the invention

[0005] One of the purposes of the present invention is to provide a lithium-ion battery separator to address the deficiencies of the prior art, thereby improving the water absorption and deterioration of the lithium supplement additive Li5FeO4 in the positive electrode material and reducing the gram capacity problem of the positive electrode material.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A lithium-ion battery separator comprises a base film and a lithium-replenishing coating coated on the surface of the base film, wherein the lithium-replenishing coating comprises a lithium-replenishing agent, a conductive agent and a binder, wherein the mass ratio of the lithium-replenishing agent to the lithium-replenishing coating is W1, the mass ratio of the conductive agent to the lithium-replenishing coating is W2, and the mass ratio of the binder to the lithium-replenishing coating is W3; the thickness of the lithium-replenishing coating is T, and W1, W2, W3, and T satisfy the relationship: 0.1*W1≤T≤5*(W2+W3), W1+W2+W3=100%.

[0008] Preferably, W1 is 20% to 98%, W2 is 1% to 40%, and W3 is 1% to 40%. Preferably, the thickness T of the lithium supplement coating is 0.1 μm to 3 μm.

[0009] Preferably, the base film thickness is T2, and the ratio of the lithium-replenishing coating thickness T1 to the base film thickness T2 is 1:3-4.

[0010] Preferably, the lithium supplement is at least one of Li5FeO4, Li2NiO2, and Li2C4O4.

[0011] Preferably, the conductive agent includes at least one of CNT, carbon black, acetylene black, and Ketjen black, and the binder includes at least one of PVDF, SBR, CMC, PAA, PAN, and PTFE.

[0012] Preferably, the base film comprises any one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester and natural fiber.

[0013] The second object of the present invention is to provide a method for preparing a lithium ion battery separator, comprising the following steps:

[0014] S1: mixing a lithium replenishing agent, a conductive agent and a binder to prepare a lithium replenishing coating slurry;

[0015] S2: coating the lithium-replenishing coating slurry obtained in step S1 on the surface of the base film, drying, and rolling to obtain the lithium-ion battery separator.

[0016] The third object of the present invention is to provide a lithium-ion battery, comprising the lithium-ion battery separator described in any of the above paragraphs, a battery cell formed by winding a positive electrode sheet and a negative electrode sheet, and an electrolyte.

[0017] Preferably, the side of the lithium-ion battery separator coated with the lithium-replenishing coating is arranged opposite to the positive electrode sheet.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: the lithium-ion battery separator provided by the present invention comprises a base film and a lithium replenishing coating coated on the surface of the base film, and the lithium replenishing agent is coated on the base film and separated from the positive electrode slurry, thereby reducing the process of the lithium replenishing agent contacting the air and thus reducing the lithium replenishing agent absorbing moisture and causing deterioration; in addition, the lithium replenishing coating under the formula ratio of the present invention ensures the lithium replenishing effect of the lithium replenishing agent, does not affect the charging and discharging of the positive electrode material, and improves the discharge capacity and energy density of the positive electrode material; at the same time, the lithium de-lithiation product of the lithium replenishing agent has similar physical properties to the positive electrode material, which can enhance the adhesion between the separator and the positive electrode sheet and reduce the risk of lithium precipitation caused by poor adhesion. DETAILED DESCRIPTION

[0019] To make the technical solutions and advantages of the present invention more clear, the present invention and its beneficial effects will be further described in detail below in conjunction with specific implementation methods, but the implementation methods of the present invention are not limited thereto.

[0020] The terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0021] The first aspect of the present invention is to provide a lithium-ion battery separator, comprising a base film and a lithium-replenishing coating coated on the surface of the base film, wherein the lithium-replenishing coating comprises a lithium-replenishing agent, a conductive agent and a binder, wherein the mass ratio of the lithium-replenishing agent to the lithium-replenishing coating is W1, the mass ratio of the conductive agent to the lithium-replenishing coating is W2, and the mass ratio of the binder to the lithium-replenishing coating is W3; the thickness of the lithium-replenishing coating is T, and W1, W2, W3, and T satisfy the relationship: 0.1*W1≤T≤5*(W2+W3), W1+W2+W3=100%.

[0022] Among them, W1 is 20% to 98%, W2 is 1% to 40%, and W3 is 1% to 40%; when W1 is lower than this range, the lithium supplement effect is not significant enough, and when W1 is too high, the capacity of the lithium supplement will be reduced; the W2 parameter needs to be between 1% and 40%. At the same time, if the conductive agent content is lower than the above range, the capacity of the lithium supplement will be low. If the conductive agent content is too high, the coating thickness needs to be increased to achieve the lithium supplement effect, which will increase the thickness of the battery cell and reduce the energy density of the battery cell.

[0023] In some embodiments, the lithium supplement agent is Li5FeO4, Li2NiO2, Li2C4O4, preferably Li5FeO4. Li5FeO4 has the advantages of high discharge capacity and improved battery first efficiency. When the lithium supplement agent Li5FeO4 is used for lithium supplementation, the physical properties of the lithium desorption product LiFeO2 of Li5FeO4 are similar to those of the positive electrode material LiCoO2. When the diaphragm coating binder is the same, the lithium desorption product LiFeO2 has a stronger interaction with the positive electrode sheet LiCoO2 than the ordinary ceramic coated diaphragm, thereby enhancing the adhesion between the diaphragm and the positive electrode sheet and reducing the lithium precipitation problem caused by poor diaphragm adhesion.

[0024] The second aspect of the present invention is to provide a method for preparing a lithium ion battery separator, comprising the following steps:

[0025] S1: mixing a lithium replenishing agent, a conductive agent and a binder to prepare a lithium replenishing coating slurry;

[0026] S2: coating the lithium-replenishing coating slurry obtained in step S1 on the surface of the base film, drying, and rolling to obtain the lithium-ion battery separator.

[0027] The third aspect of the present invention is to provide a lithium ion battery, comprising a positive electrode sheet, a negative electrode sheet and a separator between the positive electrode sheet and the negative electrode sheet, wherein the separator is the lithium ion battery separator described above.

[0028] The positive electrode sheet includes a positive electrode collector and a positive electrode material layer coated on at least one surface of the positive electrode collector. The positive electrode active material layer may include but is not limited to layered oxide positive electrode materials, spinel structure positive electrode materials, olivine structure positive electrode materials, and polyanion positive electrode materials; the layered oxide positive electrode materials include lithium nickel oxide (such as lithium nickel oxide), lithium cobalt oxide (such as lithium cobalt oxide LiCoO2), nickel cobalt manganese oxide (such as NCM), nickel cobalt aluminum oxide (such as NCA), etc.; the spinel structure positive electrode material includes lithium manganese oxide (LiMn2O4); the olivine structure positive electrode material includes lithium iron phosphate (LiFePO4); the polyanion positive electrode material includes phosphates and sulfates.

[0029] The positive electrode current collector can be any material suitable for use as a positive electrode current collector for lithium-ion batteries in the art, such as aluminum foil, copper foil, nickel foil, stainless steel or carbon material. The negative electrode plate includes a negative electrode current collector and a negative electrode active material layer coated on at least one surface of the negative electrode current collector, and the negative electrode active material layer can include but is not limited to one or more of graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, silicon-based materials, tin-based materials, lithium titanate or other metals that can form alloys with lithium.

[0030] The graphite may be selected from one or more of artificial graphite, natural graphite and modified graphite; the silicon-based material may be selected from one or more of elemental silicon, silicon oxide compounds, silicon carbon composites and silicon alloys; the tin-based material may be selected from one or more of elemental tin, tin oxide compounds and tin alloys. The negative electrode current collector is usually a structure or part for collecting current. The negative electrode current collector may be any material suitable for use as a negative electrode current collector for lithium-ion batteries in the art. For example, the negative electrode current collector may include but is not limited to metal foil, and more specifically may include but is not limited to copper foil.

[0031] The secondary battery also includes an electrolyte, which includes an organic solvent, an electrolyte lithium salt and an additive. Among them, the electrolyte lithium salt can be LiPF6 and / or LiBOB used in high-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, and LiPF6 used in low-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, LiPF6, and LiTFSI used in anti-overcharge electrolytes; it can also be at least one of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC and EC; it can also be a chain carbonate, including DFC, DMC, or EMC; it can also be a carboxylic acid ester, including MF, MA, EA, MP, etc. The additives include but are not limited to at least one of film-forming additives, conductive additives, flame retardant additives, anti-overcharge additives, additives for controlling the content of H2O and HF in the electrolyte, additives for improving low-temperature performance, and multifunctional additives.

[0032] To make the technical solutions and advantages of the present invention more clear, the present invention and its beneficial effects will be further described in detail below in conjunction with specific implementation methods, but the implementation methods of the present invention are not limited thereto.

[0033] Example 1

[0034] The preparation method of the lithium ion battery separator in this embodiment is as follows:

[0035] S1: Mix the lithium replenishing agent Li5FeO4, the conductive agent CNT, and the binder PVDF according to the weight ratio of 9.8:0.1:0.1 to prepare a lithium replenishing coating slurry;

[0036] S2: coating the lithium-replenishing coating slurry obtained in step S1 on the surface of the polyethylene film, drying it, and rolling it to obtain the lithium-ion battery separator.

[0037] The formula and parameters of the lithium supplement coating of Example 1 are shown in Table 1 below.

[0038] Embodiment 2-7

[0039] The difference between Example 2-7 and Example 1 is that the proportions of the components of the lithium supplement coating are different, see Table 1 for details.

[0040] The rest is the same as in Example 1 and will not be described again here.

[0041] Example 8

[0042] The difference from Example 1 is that the thickness of the polyethylene film is 3 μm.

[0043] The rest is the same as in Example 1 and will not be described again here.

[0044] Comparative Example 1

[0045] The difference from Example 1 is that no lithium replenishing layer is provided.

[0046] Comparative Example 2

[0047] The difference between Comparative Example 2 and Example 1 is that the lithium replenishing agent in the lithium replenishing coating is Li6CoO4;

[0048] The rest is the same as in Example 1 and will not be described again here.

[0049] Comparative Example 3

[0050] The difference from Example 1 is that the proportions of the components of the lithium supplement coating are different, see Table 1 for details.

[0051] The rest is the same as in Example 1 and will not be described again here.

[0052] Comparative Examples 4 to 6

[0053] The difference between Comparative Examples 4 to 6 and Example 1 is that the thickness of the lithium supplement coating is different, see Table 1 for details.

[0054] The rest is the same as in Example 1 and will not be described again here.

[0055] Table 1

[0056]

[0057]

[0058] The separators prepared in Examples 1-8 and Comparative Examples 1-6 were assembled into soft-pack batteries with positive electrode sheets, negative electrode sheets and electrolytes, and charge and discharge tests were performed, and the data were recorded in Table 2 below:

[0059] At a rate of 0.5C, charge the cell voltage from the shipping state (3.95V) to 4.53V, and then discharge it to 3.0V at a rate of 0.2C; record the thickness, length, width, discharge process capacity and energy of the cell in the shipping state (3.95V). The platform voltage is calculated by the formula: voltage = discharge energy / capacity. The final energy density is calculated by the formula: energy density = fractional capacity * platform voltage / (cell thickness * cell length * cell width)

[0060] Table 2

[0061]

[0062]

[0063] Result analysis: By comparing the embodiment with comparative example 1, it can be seen that after the diaphragm is modified with the lithium supplement, the capacity of the battery cell will be significantly improved, that is, the lithium supplement Li5FeO4 on the surface of the diaphragm plays an additional role in providing Li + The role of the negative electrode can replenish the Li + .

[0064] From the comparison of Examples 1 to 5 and Comparative Examples 1 to 2, it can be seen that when the thickness of the lithium supplement coating is the same, as the content of the lithium supplement in the lithium supplement slurry formula decreases, the discharge capacity of the battery cell continues to decrease, which shows that a high lithium supplement content is the key to maximizing energy density. However, compared with Comparative Example 3, it can be found in Example 1 that the higher the lithium supplement content, the better. The 98% content is close to the limit of the lithium supplement content. Further increasing the lithium supplement content to 99.8%, the CNT and PVDF content is reduced to 0.1%, and the electronic conductivity is significantly reduced. The lithium supplement capacity cannot be fully utilized, and the battery capacity will be reduced. Therefore, it is necessary to select a suitable lithium supplement content to increase the energy density of the battery cell.

[0065] From Example 1 and Comparative Example 2, at the same ratio, the lithium supplement Li5FeO4 has higher capacity and energy density than the lithium supplement Li6CoO4, and a lower K value. The lower the self-discharge rate of the battery, the better the battery performance.

[0066] From Example 1 and Comparative Examples 4 to 6, it is found that when the content of lithium supplement agent in the formula is the same, increasing the thickness of the lithium supplement coating can further increase the capacity of the battery cell, but at the same time it will significantly increase the thickness of the battery cell, resulting in a decrease in the energy density of the battery cell; at the same time, if the lithium supplement coating is reduced to 0.1 μm, the energy density of the battery cell is difficult to be significantly improved due to the insignificant increase in capacity; therefore, the appropriate thickness of the lithium supplement coating is also the key to maximizing the energy density.

[0067] In summary, the lithium-ion battery separator provided by the present invention comprises a base film and a lithium replenishing coating coated on the surface of the base film. The lithium replenishing agent is coated on the base film and separated from the positive electrode slurry, thereby reducing the process of the lithium replenishing agent contacting the air and thus reducing the lithium replenishing agent absorbing moisture and causing deterioration; in addition, the lithium replenishing coating under the formula ratio of the present invention ensures the lithium replenishing effect of the lithium replenishing agent, does not affect the charging and discharging of the positive electrode material, and improves the discharge capacity and energy density of the positive electrode material; at the same time, the lithium de-lithiation product of the lithium replenishing agent has similar physical properties to the positive electrode material, which can enhance the adhesion between the separator and the positive electrode sheet and reduce the risk of lithium precipitation caused by poor adhesion.

[0068] According to the disclosure and teaching of the above description, those skilled in the art to which the present invention belongs can also change and modify the above embodiment. Therefore, the present invention is not limited to the above specific embodiment, and any obvious improvement, replacement or modification made by those skilled in the art on the basis of the present invention belongs to the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for the convenience of description and do not constitute any limitation to the present invention.

Claims

1. A lithium-ion battery separator, comprising a base film and a lithium-supplementing coating coated on the surface of the base film, characterized in that: The lithium replenishing coating includes a lithium replenishing agent, a conductive agent and a binder, wherein the mass ratio of the lithium replenishing agent to the lithium replenishing coating is W1, the mass ratio of the conductive agent to the lithium replenishing coating is W2, and the mass ratio of the binder to the lithium replenishing coating is W3; the thickness of the lithium replenishing coating is T1, and W1, W2, W3, and T1 satisfy the relationship: 0.1*W1≤T1≤5*(W2+W3), W1+W2+W3=100%.

2. The lithium-ion battery separator according to claim 1, characterized in that The W1 is 20% to 98%, the W2 is 1% to 40%, and the W3 is 1% to 40%.

3. The lithium ion battery separator according to claim 1, characterized in that: The thickness of the base film is T2, and the ratio of the thickness of the lithium-replenishing coating T1 to the thickness of the base film T2 is 1:3-4.

4. The lithium-ion battery separator according to claim 1, characterized in that The thickness T1 of the lithium supplement coating is 0.1 μm to 3 μm.

5. The lithium ion battery separator according to claim 1, characterized in that: The lithium supplement is at least one of Li5FeO4, Li2NiO2, and Li2C4O4.

6. The lithium-ion battery separator according to claim 1, characterized in that: The conductive agent includes at least one of CNT, carbon black, acetylene black, and Ketjen black, and the binder includes at least one of PVDF, SBR, CMC, PAA, PAN, and PTFE.

7. The lithium-ion battery separator according to claim 1, characterized in that: The base film includes any one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester and natural fiber.

8. The method for preparing a lithium ion battery separator according to any one of claims 1 to 7, characterized in that: The following steps are involved: S1: mixing a lithium replenishing agent, a conductive agent and a binder to prepare a lithium replenishing coating slurry; S2: coating the lithium-replenishing coating slurry obtained in step S1 on the surface of the base film, drying, and rolling to obtain the lithium-ion battery separator.

9. A lithium ion battery, characterized in that: The invention comprises a lithium-ion battery separator as claimed in any one of claims 1 to 7, a battery core formed by winding a positive electrode sheet and a negative electrode sheet, and an electrolyte.

10. The lithium ion battery according to claim 9, characterized in that: The side of the lithium-ion battery separator coated with the lithium supplement coating is arranged opposite to the positive electrode sheet.

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