Lithium ion secondary battery

Abstract

The present invention provides a lithium ion battery which has excellent charge / discharge characteristics. A lithium ion secondary battery according to the present invention comprises: a positive electrode that has a porous positive electrode active material layer; a negative electrode; a separator; and a nonaqueous electrolyte solution. The lithium ion secondary battery is characterized in that the proportion of N-methylpyrrolidone to the weight of the positive electrode mixture in the positive electrode active material layer is 400 ppm or less.

Description

Lithium-ion secondary battery

[0001] The present invention relates to a lithium ion secondary battery.

[0002] The positive electrode active material layer of a lithium ion battery is typically produced by applying a positive electrode active material slurry containing positive electrode active material particles, a PVdF-based binder, N-methylpyrrolidone (boiling point: 204°C, abbreviated as NMP) as a dispersion medium, and a conductive additive onto a current collector and drying the coating layer (see, for example, Patent Document 1). The drying temperature for the coating layer is typically about 90°C to 130°C. Vacuum drying may also be used.

[0003] Japanese Patent Application Laid-Open No. 2019-212419

[0004] Typically, even when a coating layer of a positive electrode active material slurry containing N-methylpyrrolidone as a dispersion medium is dried, the N-methylpyrrolidone is not completely removed, and N-methylpyrrolidone remains in the positive electrode active material layer. The N-methylpyrrolidone remaining in the positive electrode active material layer may dissolve in the electrolyte solution and adversely affect the battery characteristics. The present invention has been made in view of these circumstances, and provides a lithium-ion battery with excellent charge-discharge characteristics.

[0005] The present invention provides a lithium ion secondary battery comprising a positive electrode having a porous positive electrode active material layer, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the proportion of N-methylpyrrolidone in the positive electrode active material layer relative to the weight of the positive electrode mixture is 400 ppm or less.

[0006] The lithium ion secondary battery of the present invention can have excellent charge-discharge characteristics by controlling the ratio of N-methylpyrrolidone to the weight of the positive electrode mixture in the positive electrode active material layer to 400 ppm or less, as demonstrated by experiments conducted by the present inventors.

[0007] 1 is a schematic cross-sectional view of a lithium-ion secondary battery according to one embodiment of the present invention. (a) is a schematic plan view of a positive electrode included in the lithium-ion secondary battery according to one embodiment of the present invention, and (b) is a schematic cross-sectional view of the positive electrode taken along dashed line A-A in (a). (a) is a schematic plan view of a negative electrode included in the lithium-ion secondary battery according to one embodiment of the present invention, and (b) is a schematic cross-sectional view of the negative electrode taken along dashed line B-B in (a). (b) is a schematic structural diagram of an electrode laminate included in the lithium-ion secondary battery according to one embodiment of the present invention. (a) to (e) are graphs showing the potential change of the positive electrode in the first charge-discharge cycle of coin cells in which the ratio of NMP to the weight of the positive electrode mixture is 4008 ppm, 1744 ppm, 400 ppm, 50 ppm, or 0 ppm. 1A to 1E are graphs showing the change in the potential of the positive electrode in the second charge-discharge cycle of coin cells in which the ratio of NMP to the weight of the positive electrode composite is 4008 ppm, 1744 ppm, 400 ppm, 50 ppm, or 0 ppm. 11 are graphs showing the relationship between the ratio of NMP to the weight of the positive electrode composite and the average discharge voltage in the first and second charge-discharge cycles. 12 are graphs showing the change in the discharge capacity retention rate in a charge-discharge cycle test.

[0008] An embodiment of the present invention will be described below with reference to the drawings. The configurations shown in the drawings and in the following description are merely examples, and the scope of the present invention is not limited to those shown in the drawings and the following description. A lithium-ion secondary battery 30 of this embodiment includes a positive electrode 5 having a porous positive electrode active material layer 2, a negative electrode 32, a separator 34, and a non-aqueous electrolyte solution 15, and is characterized in that the ratio of N-methylpyrrolidone to the weight of the positive electrode mixture in the positive electrode active material layer 2 is 400 ppm or less.

[0009] The positive electrode 5 includes a positive electrode current collector 3 and a porous positive electrode active material layer 2 provided on the positive electrode current collector 3. The positive electrode current collector 3 is a sheet that serves as a base material for providing the positive electrode active material layer 2, and is a conductor that electrically connects the positive electrode connecting member 13 and the positive electrode active material layer 2. The positive electrode current collector 3 is, for example, aluminum foil. The positive electrode connecting member 13 is electrically connected to an external connection terminal 18a. The positive electrode active material layer 2 may be provided on one side of the positive electrode current collector 3, or may be provided on both sides of the positive electrode current collector 3.

[0010] The positive electrode active material layer 2 is a layer containing a positive electrode active material. The thickness of the positive electrode active material layer 2 (after pressing) is not particularly limited, but is preferably 150 μm or more. This increases the amount of positive electrode active material contained in the positive electrode 5, thereby increasing the battery capacity of the lithium-ion secondary battery 30. However, there is also the problem that the amount of N-methylpyrrolidone remaining in the positive electrode active material layer tends to increase. Furthermore, the positive electrode active material layer 2 may contain a binder and may also contain a conductive additive. Furthermore, the positive electrode active material layer 2 may contain a thickener.

[0011] The positive electrode active material contained in the positive electrode active material layer 2 is a material that is directly involved in the transfer of electrons accompanying charge transfer in the positive electrode 5. The positive electrode active material is, for example, LiCoO2, LiNiO2, LiNi x Co 1-x O2 (x=0.01-0.99), LiMnO2, LiMn2O4, LiCo x Mn y Ni z O2 (x + y + z = 1) or olivine-type LiFePO4 or Li x Fe 1-y M y PO4 (where 0.05≦x≦1.2, 0≦y≦0.8, and M is at least one of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, and Nb). The positive electrode active material layer 2 can contain these positive electrode active materials singly or in combination. Furthermore, the positive electrode active material particles (e.g., lithium iron phosphate (LiFePO4) particles) may have a conductive coating on their surfaces. This can improve the conductivity of the positive electrode active material surface where the intercalation reaction proceeds, thereby reducing the internal resistance of the positive electrode 5. The conductive coating is, for example, a carbon coating. The positive electrode active material particles may be contained in the positive electrode active material layer 2 as secondary particles. The average particle diameter of the secondary particles of the positive electrode active material can be 10 μm or more and 60 μm or less. The average particle size can be calculated by measuring the particle sizes of 100 secondary particles randomly selected from a cross-sectional photograph of the positive electrode active material layer 2 and averaging these particle sizes.

[0012] Examples of the conductive additive include acetylene black, fine particles of coke-based soft carbon, etc. When the positive electrode active material layer 2 contains the conductive additive, the conductivity of the positive electrode active material layer 2 can be improved, and the internal resistance of the positive electrode 5 can be reduced.

[0013] Examples of the binder include PVdF (polyvinylidene fluoride), an acrylic polymer binder, styrene butadiene rubber (SBR), and a polyimide resin. The positive electrode active material layer 2 contains a binder, which allows the positive electrode active material layer 2 to be bonded to the positive electrode current collector 3. The thickener is a substance that increases the viscosity of the positive electrode active material slurry used to form the positive electrode active material layer 2. Examples of the thickener include CMC (sodium carboxymethylcellulose or calcium carboxymethylcellulose).

[0014] For example, the cathode active material layer 2 can be formed by mixing and kneading cathode active material particles, a conductive additive, a binder, and a dispersion medium (such as water, N-methylpyrrolidone, an organic solvent, or a mixture thereof) to prepare a cathode active material slurry, applying this slurry to the cathode current collector 3, and drying the applied layer. Furthermore, when the dispersion medium contains N-methylpyrrolidone, a treatment for removing residual N-methylpyrrolidone from the cathode active material layer 2 may be performed. For example, the cathode active material layer 2 can be heated to a temperature near or higher than the boiling point of N-methylpyrrolidone (204°C), dried in a vacuum drying oven for several hours, or washed with a solvent (such as water). This treatment can remove residual N-methylpyrrolidone from the cathode active material layer 2.

[0015] Alternatively, the positive electrode active material layer 2 may be subjected to a press treatment. In the press treatment, pressure can be applied to the positive electrode active material layer 2 so that the porosity of the positive electrode active material layer 2 falls within the range of 20% to 40%, for example.

[0016] The negative electrode 32 is an electrode having a negative electrode active material layer 36. The negative electrode active material layer 36 is, for example, a porous layer containing a negative electrode active material provided on a sheet-shaped negative electrode current collector 38. The negative electrode current collector 38 is electrically connected to the negative electrode connection member 14. The negative electrode connection member 14 is electrically connected to the external connection terminal 18b. The negative electrode current collector 38 is, for example, copper foil. The negative electrode active material is a substance that directly participates in the transfer of electrons accompanying charge transfer in the negative electrode. Examples of negative electrode active materials include graphite, partially graphitized carbon, hard carbon, soft carbon, lithium titanate (LTO), Sn alloys, and metallic lithium. The negative electrode active material layer 36 can contain these negative electrode active materials alone or in combination.

[0017] The separator 34 is sheet-shaped and is disposed between the positive electrode 5 and the negative electrode 32. The separator 34, together with the positive electrode 5 and the negative electrode 32, can constitute an electrode stack 22 as shown in FIG. 4 . The provision of the separator 34 can prevent a short-circuit current from flowing between the positive electrode 5 and the negative electrode 32. The separator 34 is not particularly limited as long as it can prevent a short-circuit current from flowing and is permeable to ions that conduct between the positive and negative electrodes. For example, the separator 34 can be a microporous polyolefin film, a cellulose sheet, or an aramid sheet. The separator 34 may also be a nonwoven fabric containing at least one of cellulose fibers, polyester fibers, polypropylene fibers, polyacrylonitrile fibers, polyethylene terephthalate fibers, and glass fibers.

[0018] 4 , the electrode laminate 22 may have a structure in which a plurality of positive electrodes 5 and a plurality of negative electrodes 32 are stacked so that the positive electrodes 5 and the negative electrodes 32 are arranged alternately. The electrode laminate 22 may also have a structure in which a separator 34 is arranged between adjacent positive electrodes 5 and negative electrodes 32. The electrode laminate 22, together with the positive electrode connecting member 13 and the negative electrode connecting member 14, may be covered with a shrink film 25.

[0019] The nonaqueous electrolyte 15 can use carbonates, lactones, ethers, esters, ionic liquids, etc. as solvents, and two or more of these solvents can be mixed. Among these, a mixture of cyclic carbonates and chain carbonates is particularly preferred. The nonaqueous electrolyte 15 is a solution in which a lithium salt solute, such as LiCF3SO3, LiAsF6, LiClO4, LiBF4, LiPF6, LiBOB, LiN(CF3SO2)2, or LiN(CF2F5SO2), is dissolved in an organic solvent. If necessary, additives such as VC (vinylene carbonate), PS (propane sultone), VEC (vinyl ethyl carbonate), PRS (propene sultone), and flame retardants can be added alone or in combination. Solid electrolytes, etc., can also be used.

[0020] The proportion of N-methylpyrrolidone remaining in the positive electrode active material layer 2 relative to the weight of the positive electrode mixture in the positive electrode active material layer 2 is 400 ppm or less. From the viewpoint of the concentration of N-methylpyrrolidone in the nonaqueous electrolyte solution 15, the concentration of N-methylpyrrolidone in the nonaqueous electrolyte solution 15 in the positive electrode active material layer 2 is preferably 2.0 wt % or less, and more preferably 1.6 wt % or less. This can prevent the internal resistance of the lithium ion secondary battery 30 from increasing, thereby increasing the discharge voltage of the lithium ion secondary battery 30. Furthermore, the lithium ion secondary battery can have excellent charge-discharge cycle characteristics. The "weight of the positive electrode mixture in the positive electrode active material layer 2" is the weight of the solid content of the positive electrode active material layer 2. This weight can be calculated by subtracting the weight of the positive electrode current collector 3 from the weight of the dried positive electrode 5.

[0021] The amount of N-methylpyrrolidone remaining in the positive electrode active material layer 2 can be measured by various methods. For example, the concentration of N-methylpyrrolidone in the nonaqueous electrolyte solution 15 in the pores of the positive electrode active material layer 2 can be measured by extracting the nonaqueous electrolyte solution 15 using centrifugal extraction from the positive electrode active material layer 2 of the positive electrode 5 removed from the lithium-ion secondary battery 30 and measuring the N-methylpyrrolidone concentration of the extracted nonaqueous electrolyte solution 15. Furthermore, based on the measured N-methylpyrrolidone concentration and the weight of the positive electrode mixture in the positive electrode active material layer 2, the proportion of N-methylpyrrolidone remaining in the positive electrode active material layer 2 relative to the weight of the positive electrode mixture in the positive electrode active material layer 2 can be calculated. The proportion of N-methylpyrrolidone remaining in the positive electrode active material layer 2 relative to the weight of the positive electrode mixture in the positive electrode active material layer 2 may be 0 ppm to 400 ppm, 0.01 ppm to 400 ppm, 0.1 ppm to 400 ppm, or 1 ppm to 400 ppm. The concentration of N-methylpyrrolidone in the nonaqueous electrolyte solution 15 in the positive electrode active material layer 2 may be 0 wt % to 2.0 wt %, or 0.01 wt % to 2.0 wt %.

[0022] The battery case 11 is a battery exterior housing that houses the electrode stack 22 (including the positive electrode 5, the negative electrode 32, and the separator 34) and the nonaqueous electrolyte 15. The battery case 11 may be formed into a bag shape by welding a laminate film at a welding portion. In this case, the lithium ion secondary battery 30 is a pouch battery. The battery case 11 may be a metal case or a hard resin case. The battery case 11 may also have a lid member 12.

[0023] Experimental method: A positive electrode was prepared in which N-methylpyrrolidone was not contained in the positive electrode active material layer, and N-methylpyrrolidone was added to the electrolyte to simulate a state in which N-methylpyrrolidone remained in the positive electrode active material, thereby experimentally investigating the effect of N-methylpyrrolidone concentration.

[0024] Preparation of Lithium-Ion Secondary Battery Positive electrode active material powder (lithium iron phosphate powder), conductive additive (carbon black powder), thickener (carboxymethylcellulose sodium (CMC) BSH6 manufactured by Daiichi Kogyo Seiyaku Co., Ltd. (degree of etherification: 0.65 to 0.75, viscosity of 1% CMC aqueous solution at 25 ° C: 3000 to 4000 mPa s)), and acrylic polymer binder (latex dispersion aqueous solution AXA391, active ingredient concentration: 40 wt %, glass transition temperature Tg: -35 ° C, pH: 7 to 9, B-type viscosity: 5 to 50 cP, average particle size: 180 μm, particle size distribution: peak at 170 μm (100 to 500 μm)) were mixed to a solids weight ratio of positive electrode active material powder: conductive additive: thickener: acrylic polymer binder = 95:2.5:1:1.5. Water was added to this mixed powder and kneaded to prepare a positive electrode active material slurry. This positive electrode active material slurry was applied to an aluminum foil (positive electrode current collector sheet) (application mass: 2.57 g / 100 cm 2 The coated film was dried to form a positive electrode active material layer on the positive electrode current collector sheet. The dried positive electrode current collector sheet with the positive electrode active material layer was punched out to a diameter of 15 mm to prepare a positive electrode.

[0025] Next, N-methylpyrrolidone (abbreviated as NMP) was added to a 1M LiPF electrolyte (carbonate-based solvent) so that the ratio of NMP to the positive electrode composite of the positive electrode active material layer was 50 ppm, 400 ppm, 1744 ppm, or 4008 ppm. A nonaqueous electrolyte solution was prepared. A nonaqueous electrolyte solution without NMP (NMP ratio: 0 ppm) was also prepared. Table 1 shows the conversion table between the ratio of NMP to the weight of the positive electrode composite (NMP amount (ppm) = (100 × NMP (g) / [NMP (g) + positive electrode composite (g)])) and the NMP concentration in the electrolyte (NMP amount (wt%) = (100 × NMP (g) / [NMP (g) + electrolyte (g)])). Next, for each of the prepared nonaqueous electrolyte solutions, the prepared positive electrode, a glass fiber separator (diameter: 19 mm), a negative electrode (metallic lithium, diameter: 16 mm), and the prepared nonaqueous electrolyte solution were placed in a coin cell case to prepare five lithium ion secondary batteries (coin cells).

[0026]

[0027] Charge-Discharge Cycle Test A charge-discharge cycle test was conducted using each of the fabricated lithium-ion secondary batteries. Specifically, 1C charge-discharge cycles were repeated with an upper limit voltage of 3.9 V and a lower limit voltage of 2.0 V. Figures 5(a) to 5(e) are graphs showing the change in the positive electrode potential during the first charge-discharge cycle of each coin cell (negative electrode potential: 0 V), and Figures 6(a) to 6(e) are graphs showing the change in the positive electrode potential during the second charge-discharge cycle of each coin cell (negative electrode potential: 0 V). Figure 7 is a graph showing the relationship between the ratio of NMP to the weight of the positive electrode composite and the average discharge voltage during the first charge-discharge cycle and the second charge-discharge cycle. FIG. 8 is a graph showing the relationship between the number of charge / discharge cycles and the discharge capacity retention rate of a coin cell in which the ratio of NMP to the weight of the positive electrode composite was 0 ppm, a coin cell in which the ratio of NMP to the weight of the positive electrode composite was 50 ppm, a coin cell in which the ratio of NMP to the weight of the positive electrode composite was 400 ppm, and a coin cell in which the ratio of NMP to the weight of the positive electrode composite was 4008 ppm.

[0028] 5 to 7, in the coin cells in which the ratio of NMP to the weight of the positive electrode composite was 0 ppm, 50 ppm, or 400 ppm, an average discharge voltage of 3.17 V or more was measured in the first charge-discharge cycle, but in the coin cells in which the ratio of NMP to the weight of the positive electrode composite was 1744 ppm or more, the average discharge voltage decreased as the ratio of NMP increased. This is thought to be because the internal resistance of the coin cell increased as the ratio of NMP increased.

[0029] 6, it can be seen that in the second charge-discharge cycle, the batteries in which the ratio of NMP to the weight of the positive electrode composite was between 0 ppm and 400 ppm showed a smaller plateau at the end of charge and a slower start of the voltage drop at the end of discharge than the battery in which the ratio of NMP to the weight of the positive electrode composite was 1744 ppm. This is thought to be because the internal resistance of the coin cell increased as the ratio of NMP increased.

[0030] 8, the rate at which the discharge capacity retention rate versus the number of cycles decreases increases as the proportion of NMP relative to the weight of the positive electrode composite increases, indicating that the charge-discharge cycle characteristics decrease as the proportion of NMP increases.

[0031] 2: Positive electrode active material layer 3: Positive electrode current collector 5: Positive electrode 11: Battery case 12: Lid member 13: Positive electrode connecting member 14: Negative electrode connecting member 15: Non-aqueous electrolyte 18a, 18b: External connection terminals 22: Electrode laminate 25: Shrink film 30: Lithium ion secondary battery 32: Negative electrode 34: Separator 36: Negative electrode active material layer 38: Negative electrode current collector

Claims

1. A lithium ion secondary battery comprising a positive electrode having a porous positive electrode active material layer, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the ratio of N-methylpyrrolidone to the weight of the positive electrode mixture in the positive electrode active material layer is 400 ppm or less.

2. The lithium ion secondary battery according to claim 1, wherein the concentration of N-methylpyrrolidone in the non-aqueous electrolyte is 2.0 wt % or less.

3. The lithium ion secondary battery according to claim 1 or 2, wherein the positive electrode active material layer contains lithium iron phosphate particles as a positive electrode active material.

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