Manufacturing method for lithium-ion secondary batteries

Abstract

To provide a method for manufacturing lithium-ion secondary batteries that can suppress the oxidative decomposition of ester-based solvents. [Solution] The present invention provides a method for manufacturing a lithium-ion secondary battery, comprising: an electrode body manufacturing step of manufacturing an electrode body comprising a positive electrode and a negative electrode, wherein the positive electrode has a positive electrode composite material containing a positive electrode active material and lithium phosphate; a battery cell manufacturing step of manufacturing a battery cell by sealing the electrode body and a non-aqueous electrolyte containing an ester-based solvent in an outer casing; and an activation step of activating the battery cell by applying a predetermined voltage at an ambient temperature of 0°C or lower.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a lithium ion secondary battery. [Background Art]

[0002] Patent Document 1 discloses a lithium ion secondary battery comprising: a positive electrode having a positive electrode mixture containing a positive electrode active material and lithium phosphate; and a non-aqueous electrolyte containing a carbonate-based solvent. [Prior Art Literature] [Patent Literature]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2019-50155 [Summary of the Invention] [Problem to be Solved by the Invention]

[0004] In lithium ion secondary batteries, a non-aqueous electrolyte containing an ester-based solvent may sometimes be used instead of a non-aqueous electrolyte containing a carbonate-based solvent. However, since the redox potential of the ester-based solvent is lower than that of the carbonate-based solvent, oxidative decomposition of the ester-based solvent is likely to occur when the battery is activated. While lithium phosphate contained in the positive electrode mixture of the positive electrode has the function of suppressing oxidative decomposition of the ester-based solvent, in lithium ion secondary batteries using a non-aqueous electrolyte containing an ester-based solvent, it is necessary to further suppress oxidative decomposition of the ester-based solvent.

[0005] The present invention has been made in view of the above problem, and an object of the present invention is to provide a method for manufacturing a lithium ion secondary battery capable of suppressing oxidative decomposition of an ester-based solvent. [Means for Solving the Problem]

[0006] To solve the above-mentioned problems and achieve the objective, the present invention provides a method for manufacturing a lithium-ion secondary battery, comprising: an electrode body manufacturing step of manufacturing an electrode body comprising a positive electrode and a negative electrode, wherein the positive electrode has a positive electrode composite material containing a positive electrode active material and lithium phosphate; a battery cell manufacturing step of manufacturing a battery cell by sealing the electrode body and a non-aqueous electrolyte containing an ester-based solvent in an outer casing; and an activation step of activating the battery cell at 0°C or below by applying a predetermined voltage.

[0007] As a result, the lithium-ion secondary battery manufacturing method according to the present invention can produce a lithium-ion secondary battery that can suppress the oxidative decomposition of ester-based solvents.

[0008] Furthermore, in the above, it is preferable that the ambient temperature is between -30°C and 0°C.

[0009] Furthermore, in the above, it is preferable that the non-aqueous electrolyte contains 10% to 70% of the ester solvent. [Effects of the Invention]

[0010] The method for manufacturing a lithium-ion secondary battery according to the present invention has the effect of being able to manufacture a lithium-ion secondary battery that can suppress the oxidative decomposition of ester-based solvents. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 shows the relationship between the ester solvent ratio and the residual volume retention rate. [Figure 2] Figure 2 shows the relationship between the ester solvent ratio and the amount of gas generated. [Figure 3] Figure 3 shows the relationship between the activation temperature and the residual capacity retention rate. [Figure 4] Figure 4 shows the relationship between the activation temperature and the amount of gas generated. [Modes for carrying out the invention]

[0012] The following describes an embodiment of the method for manufacturing a lithium-ion secondary battery according to the present invention. However, the present invention is not limited to this embodiment.

[0013] The method for manufacturing a lithium-ion secondary battery according to this embodiment includes an electrode body manufacturing step, a battery cell manufacturing step, and an activation step.

[0014] The electrode fabrication process is a process for fabricating an electrode body comprising a positive electrode and a negative electrode, wherein the positive electrode has a positive electrode active material and a positive electrode composite material containing lithium phosphate (Li3PO4). The positive electrode active material has a positive electrode potential of approximately 4.3 [V] (vs. metallic Li), for example, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 can be used.

[0015] The battery cell manufacturing process involves enclosing an electrode body and a non-aqueous electrolyte containing an ester-based solvent in an outer casing to produce a battery cell. The non-aqueous electrolyte contains, for example, methyl propionate (MP) or methyl acetate (MA) as the ester-based solvent. However, the ester-based solvent contained in the second non-aqueous electrolyte is not limited to methyl propionate or methyl acetate, and other ester-based solvents may also be used. Furthermore, it is preferable that the non-aqueous electrolyte contains, for example, 10% to 70% of the ester-based solvent.

[0016] The activation process is a process of activating the battery cell by applying a predetermined voltage at an ambient temperature of 0°C or lower. The ambient temperature (activation temperature) in the activation process is preferably between -30°C and 0°C, and more preferably between -20°C and -10°C.

[0017] In the activation step of the method for manufacturing a lithium ion secondary battery according to the embodiment, by setting the environmental temperature (temperature during activation) to 0°C or lower, oxidative decomposition of the ester-based solvent contained in the non-aqueous electrolyte of the battery cell can be suppressed, and the protective function of lithium phosphate (Li₃PO₄) can be fully exhibited. Further, in the activation step of the method for manufacturing a lithium ion secondary battery according to the embodiment, by setting the temperature during activation to 0°C or lower, Li₃PO₄ in the positive electrode mixture of the positive electrode forms a protective film, whereby the effect of suppressing oxidative decomposition on the surface of the positive electrode can be obtained. EXAMPLES

[0018] Hereinafter, the technology of the present disclosure will be described in further detail with reference to Examples A1 to A13 and Comparative Examples B1 to B30, but the technology of the present disclosure is not limited to the following examples.

[0019] [Preparation of Positive Electrode] As the positive electrode active material for each of Examples A1 to A13 and Comparative Examples B1 to B30, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O₂ was used. An electrode was produced by preparing a slurry at a ratio (wt%) of active material:conductive auxiliary:PVdF = 87:10:3, and applying the slurry onto an Al foil. For the levels in which lithium phosphate (Li₃PO₄) was mixed (Examples A1 to A13 and Comparative Examples B19 to B30), electrodes were produced by substituting 5 wt% of the active material ratio.

[0020] [Preparation of Negative Electrode] For each of the negative electrodes of Examples A1 to A13 and Comparative Examples B1 to B30, a natural graphite-based material having an average particle diameter of 20 µm was used. A carbon material, a styrene-butadiene copolymer (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed at a ratio (wt%) of carbon material:SBR:CMC = 98:1:1 with water as a dispersion solvent, and the mixture was applied onto a copper foil to obtain an electrode. The weight of the negative electrode mixture was adjusted so that the capacity ratio relative to the capacity of the positive electrode was about 1.1.

[0021] [Electrode weight, film thickness, and density verification] For each of the electrodes prepared for Examples A1 to A13 and Comparative Examples B1 to B30, punching was performed using a Φ12 punch. The weight and film thickness of the punched Φ12 electrodes were measured, and the basis weight and density were calculated by dividing these measurements by the weight and thickness of the conductive foil used.

[0022] [Electrode fabrication process] For each of Examples A1 to A13 and Comparative Examples B1 to B30, an electrode body was prepared by facing the positive and negative electrodes through a polypropylene / polyethylene / polypropylene three-layer porous membrane with an air permeability of 300 seconds, obtained by the Gurley test method.

[0023] [Nonaqueous electrolyte] For each of Examples A1 to A13 and Comparative Examples B1 to B30, a non-aqueous solvent was prepared to have the specified solvent type and composition. Then, using 1.0 M [mol / L] of LiPF6 as the electrolyte, non-aqueous electrolytes (electrolytes) were prepared for each of Examples A1 to A13 and Comparative Examples B1 to B30 by adjusting them to the specified volume ratio and molar concentration.

[0024] The electrolytes in Examples A1 to A3 contain 30 vol% EC, 60, 50, and 30 vol% EMC, and 10, 20, and 40 vol% MP. The electrolyte in Example A4 contains 30 vol% EC and 70 vol% MP. The electrolytes in Examples A5 to A7 contain 30 vol% EC, 60, 50, and 30 vol% EMC, and 10, 20, and 40 vol% MA. The electrolytes in Examples A8 to A13 contain 30 vol% EC and 70 vol% MA.

[0025] The electrolyte composition of Comparative Example B1 contains 30 vol% EC and 70 vol% EMC. The electrolyte compositions of Comparative Examples B2 to B4 contain 30 vol% EC, 60, 50, and 30 vol% EMC, and 10, 20, and 40 vol% MP. The electrolyte composition of Comparative Example B5 contains 30 vol% EC and 70 vol% MP. The electrolyte compositions of Comparative Examples B6 to B8 contain 30 vol% EC, 60, 50, and 30 vol% EMC, and 10, 20, and 40 vol% MA. The electrolyte composition of Comparative Example B9 contains 30 vol% EC and 70 vol% MA.

[0026] The electrolyte composition of Comparative Example B10 contains 30 vol% EC and 70 vol% EMC. The electrolyte compositions of Comparative Examples B11 to B13 contain 30 vol% EC, 60, 50, and 30 vol% EMC, and 10, 20, and 40 vol% MP. The electrolyte composition of Comparative Example B14 contains 30 vol% EC and 70 vol% MP. The electrolyte compositions of Comparative Examples B15 to B17 contain 30 vol% EC, 60, 50, and 30 vol% EMC, and 10, 20, and 40 vol% MA. The electrolyte composition of Comparative Example B18 contains 30 vol% EC and 70 vol% MA.

[0027] The electrolytes for Comparative Examples B19 and B28 contain 30 vol% EC and 70 vol% EMC. The electrolytes for Comparative Examples B20 to B22 contain 30 vol% EC, 60, 50, and 30 vol% EMC, and 10, 20, and 40 vol% MP. The electrolyte for Comparative Example B23 contains 30 vol% EC and 70 vol% MP. The electrolytes for Comparative Examples B24 to B26 contain 30 vol% EC, 60, 50, and 30 vol% EMC, and 10, 20, and 40 vol% MA. The electrolytes for Comparative Examples B27, B29, and B30 contain 30 vol% EC and 70 vol% MA.

[0028] [Battery cell manufacturing process] For each of Examples A1 to A13 and Comparative Examples B1 to B30, a battery cell was fabricated by enclosing the electrode body and electrolyte in an outer casing (sealing with lamination).

[0029] [Activation process] For each battery cell of Examples A1 to A13 and Comparative Examples B1 to B30, the initial charge was performed using a constant current method in a constant temperature bath adjusted to the activation temperature, which is the ambient temperature for each example. The charge was performed at a current of 0.1[C] until the predetermined voltage was reached, and then discharged to 3.0[V] at a current of 0.3[C] using the constant current method. This process was repeated three times.

[0030] The activation temperature for Examples A1 to A8, Comparative Examples B10 to B18, and Comparative Example B28 is 0°C. The activation temperature for Example A9 is -10°C. The activation temperature for Example A10 is -15°C. The activation temperature for Example A11 is -20°C. The activation temperature for Example A12 is -25°C. The activation temperature for Example A13 is -30°C. The activation temperature for Comparative Examples B1 to B9 and Comparative Examples B19 to B27 is 25°C. The activation temperature for Comparative Example B29 is 10°C. The activation temperature for Comparative Example B30 is 5°C.

[0031] [Initial characteristic evaluation] For each of the battery cells in Examples A1 to A13 and Comparative Examples B1 to B30 after the activation process, charging was performed using a constant current-constant voltage method with a current value of 0.1[C] up to 4.3[V], and then constant voltage charging was performed until the current value during constant voltage charging became 1 / 50[C], resulting in a fully charged state. Subsequently, the capacity obtained when the battery was discharged using a constant current method with a current value of 0.2[C] up to 3.0[V] was defined as the initial capacity.

[0032] [Measurement of battery cell volume] For each battery cell in Examples A1 to A13 and Comparative Examples B1 to B30, the battery cell volume (total volume of the battery cell) was measured using the Archimedes method.

[0033] [Storage test (durability test)] For each battery cell of Examples A1 to A13 and Comparative Examples B1 to B30, charging was performed in a 25°C constant temperature bath using a constant current-constant voltage method with a current value of 0.1°C to 4.3V. Afterward, the constant temperature bath temperature was raised to 60°C and the cells were stored for 100 days. After storage, the constant temperature bath was returned to 25°C, and the temperature was controlled for 3 hours. Then, the cells were discharged using a constant current method with a current value of 0.2°C to 3.0V, and the remaining capacity and battery cell volume were measured. The remaining capacity was calculated as (discharge capacity after storage) / (discharge capacity before storage). The amount of gas generated during the storage test was calculated as (battery cell volume after storage test) - (battery cell volume before storage test).

[0034] Table 1 shows the cathode composite material, activation temperature, electrolyte composition, remaining capacity retention rate, and gas generation amount for Examples A1 to A13. Table 2 shows the cathode composite material, activation temperature, electrolyte composition, remaining capacity retention rate, and gas generation amount for Comparative Examples B1 to B9. Table 3 shows the cathode composite material, activation temperature, electrolyte composition, remaining capacity retention rate, and gas generation amount for Comparative Examples B10 to B18. Table 4 shows the cathode composite material, activation temperature, electrolyte composition, remaining capacity retention rate, and gas generation amount for Comparative Examples B19 to B30. In Tables 1 to 4, non-aqueous solvents are abbreviated as EC (ethylene carbonate), EMC (ethyl methyl carbonate), MP (methyl propionate), and MA (methyl acetate).

[0035] [Table 1]

[0036] [Table 2]

[0037] [Table 3]

[0038] [Table 4]

[0039] Figure 1 shows the relationship between the ester solvent ratio and the remaining volume retention rate. Figure 2 shows the relationship between the ester solvent ratio and the amount of gas generated. Note that the ester solvent ratio in Figures 1 and 2 refers to the ratio of ester solvents (MP, MA) contained in the electrolyte. Figure 3 shows the relationship between the activation temperature and the remaining volume retention rate. Figure 4 shows the relationship between the activation temperature and the amount of gas generated.

[0040] As is clear from Tables 1 to 4 and Figures 1 to 4, Examples A1 to A13 achieve a higher residual capacity retention rate and lower gas generation compared to Comparative Examples B1 to B30. Specifically, in Examples A1 to A13, by setting the activation temperature to 0°C or below, the oxidative decomposition of the ester-based solvent is suppressed, a protective film is formed on the lithium phosphate (Li3PO4) in the positive electrode composite material, and the protective function is fully realized, thereby reducing the amount of gas that may be generated when a lithium-ion secondary battery (battery cell) is discharged after long-term storage. Furthermore, in Examples A1 to A13, by suppressing the oxidative decomposition of the ester-based solvent through activation at a temperature of 0°C or below, a high residual capacity retention rate can be obtained.

Claims

1. An electrode body comprising a positive electrode and a negative electrode, wherein the positive electrode has a positive electrode composite material containing a positive electrode active material and lithium phosphate, and an electrode body manufacturing step for manufacturing the electrode body, A battery cell manufacturing step involves enclosing the electrode body and a non-aqueous electrolyte containing an ester-based solvent in an outer casing to produce a battery cell, An activation step in which the battery cell is activated by applying a predetermined voltage when the ambient temperature is 0°C or lower, Having, A method for manufacturing a lithium-ion secondary battery, characterized by the following features.

2. The ambient temperature is between -30°C and 0°C. A method for manufacturing a lithium-ion secondary battery according to claim 1.

3. The non-aqueous electrolyte contains the ester solvent in an amount of 10% to 70%. A method for manufacturing a lithium-ion secondary battery according to claim 1 or 2, characterized by the above.

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