A method for evaluating the initial efficiency of a battery

Abstract

The present invention belongs to the technical field of battery performance, and particularly relates to a method for evaluating the initial efficiency of a battery, comprising the following steps: Step S1, take at least two negative electrode active materials of the same polarity, test the particle size of each active material, and obtain at least two particle size distributions D X ; Step S2, calculate the particle size ratio Z = D X1 / D X2 , and satisfy 0.1 ≤ Z = D X1 / D X2 ≤ 1, X1 < X2; Step S3, calculate the average value Z0 of each particle size ratio, compare the Z0 of each negative electrode active material to obtain the minimum value, and the battery assembled with the negative electrode active material corresponding to the minimum value has the highest initial efficiency. The method for evaluating the initial efficiency of a battery according to the present invention can effectively and accurately evaluate a battery with high initial efficiency, thereby contributing to the assembly and matching of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery performance, and particularly relates to a method for evaluating the first efficiency of a battery. Background Art

[0002] A lithium-ion battery is an energy storage device with a high energy density and capable of providing stable energy. It also has excellent life characteristics and is relatively friendly to the environment. With the setting of the national carbon neutrality goal, the development and improvement of lithium-ion battery technology have become particularly important. The first efficiency of a lithium-ion battery is an important indicator in the preparation process of lithium-ion batteries. In product design and material evaluation, whether the first efficiency can reach the set value is particularly important for subsequent product development.

[0003] After the full battery is manufactured, it first undergoes a charge-discharge cycle. During charging, lithium ions are removed from the positive electrode material and embedded in the negative electrode. During discharging, the lithium stored in the negative electrode loses electrons to form lithium ions, which pass through the electrolyte and then are embedded in the positive electrode again. Usually, the lithium ions removed from the positive electrode during charging cannot be 100% re-embedded in the positive electrode during discharging. For the positive electrode material, the capacity loss is mainly caused by the change in the material structure due to the first de-lithiation. The change in the structure causes a reduction in the lithium-insertion positions in the material. Usually, in half-cell tests, the first efficiency of ternary materials is the lowest, about 85%-88%, and the first efficiency of lithium iron phosphate series materials is relatively high, about 95%-99% or even 100%. For carbon-based negative electrodes, the first efficiency is mainly caused by the formation of SEI. Usually, in half-cell tests, the first efficiency is about 88-92%. Since the lithium titanate negative electrode hardly forms an SEI film, the first efficiency can reach about 97%. For the emerging silicon-carbon negative electrode, it is only about 50%. After the battery is filled with electrolyte, two processes of formation and grading are carried out. During formation, the capacity is charged, and during the first charging process of grading, the sum of the capacities of the two is the capacity of the first charge of the full battery. The second step of grading is generally to discharge to the cut-off voltage of the battery. The first efficiency of the full battery = the discharge capacity of the second step of grading / (the charging capacity of formation + the charging capacity of the first step of grading). Usually, during the battery preparation process, the capacity of the battery can be determined according to the first efficiency of the active material. When the first efficiency of the active material used in the battery cell is not accurately controlled, the design will not be very accurate. Therefore, there is no method to evaluate the first efficiency of the battery. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for evaluating the first efficiency of a battery, which can effectively and accurately evaluate a battery with a high first efficiency, thereby contributing to the assembly and matching of the battery.

[0005] To achieve the above purpose, the present invention adopts the following technical solutions:

[0006] A method for evaluating the first efficiency of a battery, comprising the following steps:

[0007] Step S1: Take at least two kinds of negative electrode active materials with the same polarity, test the particle size of each active material, and obtain at least two kinds of particle size distributions D X ;

[0008] Step S2: Calculate the particle size ratio Z = D X1 / D X2 , and satisfy 0.1 ≤ Z = D X1 / D X2 ≤ 1, X1 < X2;

[0009] Step S3: Calculate the average value Z0 of each particle size ratio, compare the Z0 of each negative electrode active material to obtain the minimum value. The battery assembled with the negative electrode active material corresponding to the minimum value and the positive electrode active material has the highest initial efficiency. Among them, the initial efficiency of the positive electrode active material is greater than that of the negative electrode active material.

[0010] Preferably, in step S1, D X includes two or more of D10, D20, D30, D40, D50, D60, D70, D80, D90, D99.

[0011] Preferably, in step S1, there are three groups of negative electrode active materials.

[0012] Preferably, in step S3, the battery includes a positive electrode sheet, a separator, a negative electrode sheet, an electrolyte, and a housing. The separator is used to separate the positive electrode sheet and the negative electrode sheet, and the housing is used to encapsulate the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte. The positive electrode sheet and / or the negative electrode sheet includes the active material with the minimum average value Z0 of the above particle size ratio.

[0013] Preferably, the positive electrode sheet includes a positive electrode current collector and a positive electrode active coating provided on at least one surface of the positive electrode current collector. The positive electrode active coating includes a positive electrode active material, and the positive electrode active material includes one or more of a phosphate material and a nickel-cobalt-manganese ternary material.

[0014] Preferably, the chemical formula of the positive electrode active material is LiM x N y PO4, where M and N are one or more of Fe, Mn, and Ni, and x + y = 1.

[0015] Preferably, the negative electrode sheet includes a negative electrode current collector and a negative electrode active coating provided on at least one surface of the negative electrode current collector. The negative electrode active coating includes a negative electrode active material, and the negative electrode active material includes one or more of graphite, artificial graphite, carbon nanotubes, conductive carbon, and coke.

[0016] Preferably, the negative electrode active material is graphite, and the positive electrode active material is lithium iron phosphate.

[0017] Preferably, the separator includes a polyethylene-based film, a polypropylene-based film, or a composite film. The composite film includes a base film and a coating layer provided on at least one surface of the base film. The coating layer includes one or more of a ceramic layer and an adhesive layer.

[0018] Preferably, the electrolyte includes lithium hexafluorophosphate, an additive, and a solvent.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: The method for evaluating the initial efficiency of a battery according to the present invention can effectively and accurately evaluate a battery with a high initial efficiency, thereby contributing to the assembly and matching of batteries. Specific Embodiments

[0020] The following combines specific embodiments to further describe the present invention in detail, but the embodiments of the present invention are not limited thereto.

[0021] A method for evaluating the initial efficiency of a battery includes the following steps:

[0022] Step S1: Take at least two negative electrode active materials of the same pole, test the particle size of each active material, and obtain at least two particle size distributions D X ;

[0023] Step S2: Calculate the particle size ratio Z = D X1 / D X2 , and satisfy 0.1 ≤ Z = D X1 / D X2 ≤ 1, X1 < X2;

[0024] Step S3: Calculate the average value Z0 of each particle size ratio, compare the Z0 of each negative electrode active material to obtain the minimum value. The battery assembled with the negative electrode active material corresponding to the minimum value and the positive electrode active material has the highest initial efficiency. Among them, the initial efficiency of the positive electrode active material is greater than that of the negative electrode active material. The method for evaluating the initial efficiency of a battery according to the present invention can effectively and accurately evaluate a battery with a high initial efficiency, thereby contributing to the assembly and matching of batteries, improving the assembly time, and improving the performance of the assembled battery. The method for evaluating the initial efficiency of the battery according to the present invention can be evaluated according to the negative electrode active material or the positive electrode active material. The active materials for evaluation need to be the same positive electrode or the same negative electrode.

[0025] In some embodiments, D X in step S1 includes two or more of D10, D20, D30, D40, D50, D60, D70, D80, D90, and D99. The particle size distribution DX The value of X should be as evenly distributed as possible to accurately reflect the physical properties of the active material. In some embodiments, D

[0026] In some embodiments, the active material of three groups of negative electrodes is included in the step S1. By using two or more groups of active materials for comparison, better active materials with better performance can be obtained more, so as to improve the battery performance.

[0027] In some embodiments, the battery in the step S3 includes a positive electrode sheet, a separator, a negative electrode sheet, an electrolyte, and a housing. The separator is used to separate the positive electrode sheet and the negative electrode sheet, and the housing is used to encapsulate the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte. The positive electrode sheet and / or the negative electrode sheet include the active material with the average value Z0 and the minimum value of the above particle size ratio. Using better active materials for the electrode sheets is beneficial to improving the performance of the assembled battery. Both the positive and negative electrodes use the active materials obtained by screening, and better electrode sheets and batteries can be obtained through synergistic cooperation. Among them, the cell structure in the evaluated battery is wound or laminated.

[0028] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material. The positive electrode active material can be one or more combinations of compounds represented by, but not limited to, the chemical formula such as Li a Ni x Co y M z O 2-b N b (where 0.95 ≤ a ≤ 1.2, x > 0, y ≥ 0, z ≥ 0, and x + y + z = 1, 0 ≤ b ≤ 1, M is selected from one or more combinations of Mn and Al, and N is selected from one or more combinations of F, P, and S), and the positive electrode active material can also be one or more combinations of, but not limited to, LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5One or a combination of more than one of O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, TiS2, etc. The positive electrode active material can also be subjected to a modification treatment, and the method for modifying the positive electrode active material should be known to those skilled in the art. For example, methods such as coating and doping can be used to modify the positive electrode active material, and the materials used for the modification treatment can be a combination of one or more of, including but not limited to, Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, W, etc. The positive electrode current collector is usually a structure or component that collects current, and the positive electrode current collector can be various materials suitable for use as the positive electrode current collector of a lithium-ion battery in the art. For example, the positive electrode current collector can be, including but not limited to, a metal foil, etc., and more specifically can be, including but not limited to, an aluminum foil, etc. Preferably, the positive electrode active material is LiM x N y PO4, where M and N are Fe, Mn, Ni, etc., and x + y = 1.

[0029] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on the surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material. The negative electrode active material can be one or several of, including but not limited to, 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. Among them, the graphite can be selected from one or several of artificial graphite, natural graphite, and modified graphite; the silicon-based materials can be selected from one or several of elemental silicon, silicon oxides, silicon-carbon composites, and silicon alloys; the tin-based materials can be selected from one or several of elemental tin, tin oxides, and tin alloys. The negative electrode current collector is usually a structure or component that collects current, and the negative electrode current collector can be various materials suitable for use as the negative electrode current collector of a lithium-ion battery in the art. For example, the negative electrode current collector can be, including but not limited to, a metal foil, etc., and more specifically can be, including but not limited to, a copper foil, etc. Preferably, the negative electrode active material is graphite, where D10 = 1 - 10 μm, D50 = 10 - 20 μm, D90 = 20 - 30 μm, D99 = 30 - 80 μm; where D10 represents the particle size when the cumulative volume ratio of the graphite reaches 10%, and similarly, D50 represents the particle size when the cumulative volume ratio of the graphite reaches 50%. The negative electrode above satisfies 0.1 ≤ Z = Da / Db ≤ 1, where Da and Db are the particle size dimensions at different volume ratios of the material, and at the same time satisfy a < b; for different graphites, with the same a and b values, the corresponding Z value for different graphites is a definite value. When the Z value is smaller, the initial efficiency of preparing the battery cell is higher.

[0030] The separator can be made of various materials suitable for lithium-ion battery separators in the art. For example, it can be a combination of one or more of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers, etc. Preferably, the separator is a polyethylene-based film or a polypropylene-based film, or a composite film, and the separator film can be coated with a ceramic layer or an adhesive layer.

[0031] The electrolyte 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 overcharge-preventing 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 linear carbonate, including DFC, DMC, or EMC; it can also be a carboxylic acid ester, including MF, MA, EA, MP, etc. The additive includes at least one of, but not limited to, a film-forming additive, a conductive additive, a flame-retardant additive, an overcharge-preventing additive, an additive for controlling the content of H2O and HF in the electrolyte, an additive for improving low-temperature performance, and a multifunctional additive. Preferably, the electrolyte lithium salt is a series of lithium hexafluorophosphate, and there are no restrictions on the additive and the solvent.

[0032] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode active coating disposed on at least one surface of the positive electrode current collector. The positive electrode active coating includes a positive electrode active material, and the positive electrode active material includes one or several of a phosphate material and a nickel-cobalt-manganese ternary material. Preferably, the positive electrode active material is a phosphate material.

[0033] In some embodiments, the chemical formula of the positive electrode active material is LiM x N y PO4, where M and N are one or several of Fe, Mn, and Ni, and x + y = 1.

[0034] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode active coating disposed on at least one surface of the negative electrode current collector. The negative electrode active coating includes a negative electrode active material, and the negative electrode active material includes one or several of graphite, artificial graphite, carbon nanotubes, conductive carbon, and coke. Preferably, the negative electrode active material is graphite. According to the degree of graphitization, the negative electrode active material can include soft carbon and hard carbon.

[0035] In some embodiments, the negative electrode active material is graphite, and the positive electrode active material is lithium iron phosphate. The first efficiency of the secondary battery prepared by using the above two active materials in combination is higher and the performance is better.

[0036] In some embodiments, the separator includes a polyethylene-based film, a polypropylene-based film, or a composite film. The composite film includes a base film and a coating layer provided on at least one surface of the base film. The coating layer includes one or more of a ceramic layer and an adhesive layer.

[0037] In some embodiments, the electrolyte includes lithium hexafluorophosphate, an additive, and a solvent. Lithium hexafluorophosphate, as the lithium salt, accounts for 10% to 20% by weight of the electrolyte, the additive accounts for 5% to 10% by weight of the electrolyte, and the solvent accounts for 70% to 85% by weight of the electrolyte. Preferably, the weight fraction of the lithium salt is 10%, 12%, 14%, 15%, 16%, 18%, 20%, the weight fraction of the additive is 5%, 6%, 7%, 8%, 9%, 10%, and the weight fraction of the solvent is 70%, 72%, 75%, 77%, 79%, 80%, 82%, 84%, 85%.

[0038] Example 1

[0039] 1. Prepare the positive electrode sheet: The positive electrode formula is LiFePO4: CNT: SP: PVDF mass ratio = 95%: 1%: 2%: 2%; it is mixed and stirred in a stirring tank and then coated on the positive electrode current collector. The positive electrode is compacted to 2.55 g / cm 3 , and the prepared positive electrode sheet is denoted as P.

[0040] 2. Prepare the negative electrode sheet: The negative electrode material is a carbon-based material. The negative electrode formula is C: SP: CMC: SBR = 97%: 0.6%: 1.4%: 1%; it is mixed and stirred in a stirring tank and then coated on the foil. The compaction density is 1.6 g / cm 3 , the sheet prepared from the negative electrode material 1 is marked as N1, the sheet prepared from the negative electrode material 2 is marked as N2, and the sheet prepared from the negative electrode material 3 is marked as N3.

[0041] 3. Prepare the battery cell: Use the positive electrode sheet P and the negative electrode sheets N1 / N2 / N3, and perform winding or laminating on the positive and negative electrode sheets and the separator. Through baking and liquid injection, formation and grading are carried out to prepare three groups of battery cells marked as A, B, and C respectively.

[0042] 4. The formation material temperature is 40 °C. The total charging capacity in the formation stage is denoted as C1. The first step of grading is constant voltage charging, and the charged capacity is denoted as C2. The second step is constant voltage charging, and the charged capacity is denoted as C3. The third step of grading is constant current discharging, and the discharged capacity is denoted as C4. The first efficiency of the whole battery cell = C4 / (C1 + C2 + C3).

[0043] The formation and grading data of three groups of battery cells were processed, and the particle sizes of three types of anode materials were statistically analyzed as follows:

[0044] Table 1 Particle size distribution of three types of anode materials

[0045] Particle size Anode material 1 / μm Anode material 2 / μm Anode material 3 / μm D10 8.03 7.8 5.5 D50 13.1 16.6 13.4 D90 20.6 28.9 25.6 D99 26.5 47.5 48.5

[0046] Table 2 Comparison of particle size relationships of three types of anode materials

[0047]

[0048]

[0049] Table 3 Initial efficiency of three groups of battery cells

[0050] Group A B C First efficiency 85.58％ 87.34％ 89.23％

[0051] As shown in Table 1, the physical properties of the anode active materials of the three groups are as above, and the particle size distributions of different sizes of particles in each group are obtained, such as D10, D50, D90, and D99. According to the particle size distribution of the anode active substances in Table 1, various particle size ratios can be calculated, such as Z = D10 / D50, Z = D10 / D90, Z = D10 / D99, Z = D50 / D90, Z = D50 / D99, Z = D90 / D99. According to Table 2, the average value ZA0 of the particle size ratio of the first group is 53.85%, the average value ZB0 of the particle size ratio of the second group is 40.58%, and the average value ZC0 of the particle size ratio of the third group is 34.51%. Therefore, the average value ZC0 of the particle size ratio of the third group is the smallest, and the initial efficiency of the prepared battery cells will be correspondingly higher. As shown in Table 3, the first efficiency of the batteries in the third group is as high as 89.23%. Using this method, the initial efficiency performance of similar materials in full batteries can be quickly determined to achieve the purpose of quickly selecting materials.

[0052] According to the disclosure and teaching of the above specification, those skilled in the art to which the present invention pertains can also make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments, and any obvious improvements, substitutions, or variations made by those skilled in the art based on the present invention fall within the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A method for evaluating the initial efficiency of a battery, characterized in that, It includes the following steps: Step S1: Take at least two kinds of anode active materials with the same polarity, test the particle size of each active material, and obtain at least two particle size distributions D X ; Step S2, calculate the particle size ratio Z = D X1 / D X2 , and satisfy 0.1 ≤ Z = D X1 / D X2 ≤ 1, X1 < X2; Step S3: Calculate the average value Z0 of the proportion of each particle size, compare the Z0 of each negative electrode active material to obtain the minimum value. The battery assembled with the negative electrode active material corresponding to the minimum value and the positive electrode active material has the highest initial efficiency, where the initial efficiency of the positive electrode active material is greater than that of the negative electrode active material.

2. The method for evaluating the initial efficiency of a battery according to claim 1, characterized in that In step S1, D X includes two or more of D10, D20, D30, D40, D50, D60, D70, D80, D90, and D99.

3. The method for evaluating the initial efficiency of the battery according to claim 1, wherein, The step S1 includes active materials of three groups of negative electrodes.

4. The method for evaluating the initial efficiency of a battery according to claim 1, characterized in that In the step S3, the battery includes a positive electrode sheet, a separator, a negative electrode sheet, an electrolyte, and a housing. The separator is used to separate the positive electrode sheet and the negative electrode sheet, and the housing is used to encapsulate the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte. The positive electrode sheet and / or the negative electrode sheet includes the active material with the minimum average value Z0 of the above-mentioned particle size proportion.

5. The method for evaluating the initial efficiency of a battery according to claim 4, characterized in that, The positive electrode sheet includes a positive electrode current collector and a positive electrode active coating provided on at least one surface of the positive electrode current collector. The positive electrode active coating includes a positive electrode active material, and the positive electrode active material includes one or more of a phosphate material and a nickel cobalt manganese ternary material.

6. The method for evaluating the initial efficiency of the battery according to claim 5, wherein The chemical formula of the positive electrode active material is LiM x N y PO4, where M and N are one or more of Fe, Mn, and Ni, and x + y = 1.

7. The method for evaluating the initial efficiency of the battery according to claim 5, wherein, The negative electrode sheet includes a negative electrode current collector and a negative electrode active coating provided on at least one surface of the negative electrode current collector. The negative electrode active coating includes a negative electrode active material, and the negative electrode active material includes one or more of graphite, artificial graphite, carbon nanotubes, conductive carbon, and coke.

8. The method for evaluating the initial efficiency of a battery according to claim 7, wherein The negative electrode active material is graphite, and the positive electrode active material is lithium iron phosphate.

9. The method for evaluating the initial efficiency of a battery according to claim 4, wherein, The separator includes a polyethylene-based film, a polypropylene-based film, or a composite film. The composite film includes a base film and a coating layer provided on at least one surface of the base film. The coating layer includes one or more of a ceramic layer and an adhesive layer.

10. The method for evaluating the initial efficiency of the battery according to claim 4, wherein The electrolyte includes lithium hexafluorophosphate, an additive, and a solvent.

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