Patent Application
20240250267
This nonprovisional application is based on Japanese Patent Application No. 2023-007024 filed on Jan. 20, 2023 with the Japan Patent Office, the entire contents of which are hereby incorporated by reference.
The present invention relates to a positive electrode and a non-aqueous electrolyte secondary battery (hereinafter also referred to as a battery) including the positive electrode.
Each of Japanese Patent Laying-Open No. 2018-139213, Japanese Patent Laying-Open No. 2022-114665, and Japanese Patent Laying-Open No. 2022-100812 proposes that a carbon nanotube (hereinafter also referred to as a CNT) is used as a conductive material in a positive electrode of a battery.
When the CNT is used as the conductive material in a positive electrode composite material layer of the battery, an output property and a high-rate property can be improved due to high conductivity of the CNT. Further, when the content of the CNT is reduced in the positive electrode composite material layer and the content of a positive electrode active material is increased, a high energy density (high capacity) can be attained. On the other hand, when the CNT is used for the positive electrode composite material layer, an electrode fusing/disconnection property (hereinafter also referred to as a fusing/disconnection property) is decreased in response to a decrease in composite material layer resistance, disadvantageously.
An object of the present invention is to provide: a positive electrode that is excellent in charging/discharging efficiency and that can exhibit a high initial capacity and a high fusing/disconnection property; and a battery including the positive electrode.
The present invention provides the following positive electrode and battery.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.
Hereinafter, embodiments of the present invention will be described with reference to figures, but the present invention is not limited to the below-described embodiments. In each of all the figures described below, a scale is appropriately adjusted to facilitate understanding of each component, and the scale of each component shown in the figures does not necessarily coincide with the actual scale of the component.
A positive electrode includes a positive electrode composite material layer. The positive electrode composite material layer includes an active material particle and a CNT. A content of the CNT in the positive electrode composite material layer is 0.45 wt % or less based on 100 wt % of a total solid content of the positive electrode composite material layer as a reference. Since the content of the CNT in the positive electrode composite material layer is within the above range, charging/discharging efficiency tends to be excellent and high initial capacity and high fusing/disconnection property tend to be likely exhibited. The content of the CNT in the positive electrode composite material layer is preferably 0.40 wt % or less based on 100 wt % of the total solid content of the positive electrode composite material layer as a reference from the viewpoint of the charging/discharging efficiency, initial capacity, and fusing/disconnection property. The content of the CNT in the positive electrode composite material layer is preferably 0.15 wt % or more based on 100 wt % of the total solid content of the positive electrode composite material layer as a reference from the viewpoint of the charging/discharging efficiency. The CNT will be described in detail later.
A composite material layer area resistivity of the positive electrode composite material layer is 0.10 Ω·cm2 or more. When the composite material layer area resistivity of the positive electrode composite material layer is in the above-described range, the charging/discharging efficiency tends to be excellent and high initial capacity and high fusing/disconnection property tend to be likely exhibited. The composite material layer area resistivity of the positive electrode composite material layer is preferably 0.11 Ω·cm2 or more, and more preferably 0.12 Ω·cm2 or more from the viewpoint of the fusing/disconnection property. The composite material layer area resistivity of the positive electrode composite material layer is preferably less than 0.45 Ω·cm2, and more preferably 0.40 Ω·cm2 or less from the viewpoint of the charging/discharging efficiency. The composite material layer area resistivity of the positive electrode composite material layer can be controlled, for example, by adjusting the type of the CNT and the content of the CNT or by adjusting a condition of kneading process for a composite material layer slurry. The composite material layer area resistivity of the positive electrode composite material layer is measured in accordance with a method described in the below-described section “Examples”.
The active material particle may be a particle containing a lithium transition metal composite oxide. A crystal structure of the lithium transition metal composite oxide is not particularly limited, and may be a lamellar structure, a spinel structure, an olivine structure, or the like. As the lithium transition metal composite oxide, a lithium transition metal composite oxide including at least one of Ni, Co, and Mn as a transition metal element is preferable, and specific examples thereof include a lithium-nickel-based composite oxide, a lithium-cobalt-based composite oxide, a lithium-manganese-based composite oxide, a lithium-nickel-manganese-based composite oxide, a lithium-nickel-cobalt-manganese-based composite oxide, a lithium-nickel-cobalt-aluminum-based composite oxide, a lithium-iron-nickel-manganese-based composite oxide, and the like. When the active material particle includes nickel, a ratio of the content of nickel to the metal elements other than lithium in the active material particle may be, for example, 60 mol % or less. The active material particle may be surface-coated.
The active material particle may include, for example, a lamellar metal oxide.
The lamellar metal oxide is represented by the following formula (1):
In the formula (1), “a1” satisfies the relation “−0.3≤a1≤0.3”. “x1” satisfies the relation “0.1≤x1≤0.6”. “Mel” represents at least one selected from a group consisting of Co, Mn, Al, Zr, Ti, V, Cr, Fe, Cu, Zn, B, Mo, Sn, Ge, Nb, and W.
The active material particle can include a plurality of large particles and a plurality of small particles. The large particles have an average particle size larger than that of the small particles. The average particle size of the large particles may be, for example, 12 μm or more and 20 μm or less, and is preferably 14 μm or more and 18 μm or less. The average particle size of the small particles may be, for example, 2 μm or more and 6 μm or less, and is preferably 3 μm or more and 6 μm or less. In the present specification, the average particle size represents a particle size (hereinafter also referred to as D50) corresponding to a cumulative particle volume of 50% from the small particle size side with respect to the total particle volume in the volume-based particle size distribution. The average particle size can be measured by a laser diffraction/scattering method. A ratio of the average particle size of the small particles to the average particle size of the large particles can be, for example, 1/10 to ½.
The content of the active material particle in the positive electrode composite material layer may be, for example, 70 wt % or more, is preferably 80 to 99 wt %, and is more preferably 90 to 99 wt % or less, based on 100 wt % of the total solid content of the positive electrode composite material layer as a reference.
The CNT is fibrous carbon having such a structure that graphene that forms a carbon hexagonal network is rounded into a tubular shape. The CNT has a high aspect ratio and a property with an excellent electron conductivity. Examples of the CNT include: a single-walled CNT composed of one layer of graphene; a multi-walled CNT composed of two or more layers of graphene; and the like. The multi-walled CNT is preferable from the viewpoint of thermal and chemical stabilities. The CNT preferably include a CNT surface-modified by acid treatment, heat treatment, or the like.
As the CNT, a commercially available CNT may be purchased and used, or a CNT produced by a conventionally known CNT manufacturing method may be used. Examples of such a method include a chemical vapor deposition (CVD) method, an arc discharging method, a laser evaporation method, and the like.
The positive electrode composite material layer may include a conductive material other than the CNT. As the conductive material other than the CNT, for example, a carbon black such as acetylene black (AB) or another carbon material (for example, graphite) can be suitably used.
The positive electrode composite material layer can include a binder. Examples of the binder include polyvinylidene difluoride (PVDF), polytetrafluoroethylene (PTFE), polyimide (PI), polyamide (PA), polyamideimide (PAI), butadiene rubber (BR), styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), styrene-ethylene-butylene-styrene block copolymer (SEBS), carboxymethyl cellulose (CMC), a combination thereof, and the like. The content of the binder in the positive electrode composite material layer may be, for example, 0.5 to 10 wt %, and is preferably 1.0 to 5 wt %, based on 100 wt % of the total solid content of the positive electrode composite material layer as a reference. The positive electrode composite material layer may include a known additive agent that can be used for the positive electrode.
In a positive electrode 100 shown in
Positive electrode composite material layer 20 may have a thickness of, for example, 10 μm or more and 200 μm or less. Positive electrode composite material layer 20 can have a high density. The density of positive electrode composite material layer 20 may be, for example, 3.5 g/cm3 or more, or may be 3.7 g/cm3 or more, 3.8 g/cm3 or more, or 3.9 g/cm3 or more. The density of positive electrode composite material layer 20 may be, for example, 4.0 g/cm3 or less.
As shown in
In the positive electrode slurry preparation (A1), a positive electrode slurry including the active material particles and the CNTs is prepared. The positive electrode slurry is prepared by dispersing the active material particles in a dispersion medium. An organic solvent can include, for example, at least one selected from a group consisting of N-methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF), dimethylformamide (DMF), methyl ethyl ketone (MEK) and dimethyl sulfoxide (DMSO). Any amount of the organic solvent is usable. The positive electrode slurry can have any solid content concentration (mass fraction of the solid content). The slurry can have a solid content concentration of, for example, 40% to 80%. Any stirring device, mixing device, or dispersing device can be used for the mixing.
The application (B1) can include applying the positive electrode slurry onto a surface of the substrate so as to form a coating film. In the present embodiment, the positive electrode slurry can be applied to the surface of the substrate by any application device. For example, a slot die coater, a roll coater, or the like may be used. The application device may be capable of performing multi-layer application.
The drying (C1) can include heating and drying the coating film. In the present embodiment, any drying device can be used as long as the coating film can be heated. For example, the coating film may be heated by a hot-air dryer or the like. The organic solvent can be evaporated by heating the coating film. With this, the organic solvent can be substantially removed.
The compression (D1) can include compressing the dried coating film to form positive electrode composite material layer 20. In the present embodiment, any compression device can be used. For example, a rolling machine or the like may be used. The dried coating film is compressed to form positive electrode composite material layer 20, thereby completing positive electrode 100. Positive electrode 100 can be cut into a predetermined planar size in accordance with a specification of the battery. Positive electrode 100 may be cut to have a strip-shaped planar shape, for example. Positive electrode 100 may be cut to have a quadrangular planar shape, for example.
The battery can be a lithium ion battery.
The non-aqueous electrolyte includes a solvent and a lithium salt. The solvent is aprotic. The solvent can include any component. The solvent may include, for example, at least one selected from a group consisting of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), 1,2-dimethoxyethane (DME), methyl formate (MF), methyl acetate (MA), methyl propionate (MP), and γ-butyrolactone (GBL).
The lithium salt is a supporting electrolyte. The lithium salt is dissolved in a solvent. The lithium salt may include, for example, at least one selected from a group consisting of LiPF6, LiBF4, and LiN(FSO2)2. The lithium salt may have a molar concentration of, for example, 0.5 mol/L or more and 2.0 mol/L or less, or may have a molar concentration of 0.8 mol/L or more and 1.2 mol/L or less.
The non-aqueous electrolyte may further include any additive agent in addition to the solvent and the lithium salt. For example, the electrolyte solution may include an additive agent at a mass fraction of 0.01% or more and 5% or less. The additive agent may include, for example, at least one selected from a group consisting of vinylene carbonate (VC), vinylethylene carbonate (VEC), and the like.
Hereinafter, the present invention will be described in more detail with reference to examples. “%” and “parts” in the examples are mass % and parts by mass unless otherwise stated particularly.
Active material particles 1 (lithium nickel composite oxide with a particle size of 14 to 18 μm) and active material particles 2 (lithium nickel composite oxide with a particle size of 3 to 6 μm) were mixed at a ratio of 1:1, mixing was performed such that the mixture had 98.55 parts by mass, CNTs each serving as a conductive material had 0.45 parts by mass, and PVdF serving as a binder had 1.0 parts by mass, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further added, thereby preparing a positive electrode composite material slurry. The slurry was applied to a current collector composed of aluminum foil, thereby forming a positive electrode composite material layer. Thereafter, drying was performed, rolling was performed to attain a predetermined thickness using a roller, cutting was performed to attain a predetermined size, and an aluminum tab was attached, thereby forming a positive electrode.
Each of positive electrodes was produced in the same manner as in Example 1 except that a conductive material and a blending ratio of the conductive material in the composite material layer as shown in Table 1 were employed.
A graphite negative electrode active material, carboxymethyl cellulose (CMC) serving as a thickener, and styrene-butadiene rubber (SBR) serving as a binder were measured in amount to attain a mass ratio of 98:1:1, and were dispersed in water, thereby preparing a negative electrode composite slurry. The negative electrode composite slurry was applied to a current collector composed of a copper foil, thereby forming a negative electrode composite material layer. Thereafter, drying was performed, rolling was performed to attain a predetermined thickness using a roller, cutting was performed to attain a predetermined size, and a nickel tab was attached, thereby forming a negative electrode.
Ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:30:40. Lithium hexafluorophosphate (LiPF6) was added to the mixed solvent to attain a concentration of 1.15 mol/liter. Further, vinylene carbonate (VC) was added to have an addition ratio of 1.0 mass % with respect to the total mass of this mixed solvent, thereby preparing a non-aqueous electrolyte.
The volume resistivity of the aluminum foil serving as the current collector was 2.70E-06 [Ω], and the composite material layer area resistivity (Ω·cm2) of the positive electrode composite material layer of the positive electrode plate was measured using an electrode resistance measurement system (RM2610 provided by HIOKI). Results are shown in Table 1.
The positive electrode and the negative electrode produced above were stacked with a separator composed of polyolefin being interposed therebetween, thereby producing a stack type electrode assembly. This electrode assembly was accommodated in an exterior package composed of an aluminum laminate sheet, the non-aqueous electrolyte prepared above was introduced thereinto, and the opening of the exterior package was then sealed, thereby obtaining a test cell.
For the test cell produced above, constant-current charging was performed under a temperature condition of 25° C. until 4.25 V was attained at a current density of 0.2 mA/cm2, and constant-voltage charging was further performed until a current density of 0.04 mA/cm2 was attained at 4.25 V, thereby finding a charging capacity (initial capacity). After 10 minutes of resting, constant-current discharging was performed until 3.0 V was attained at a current density of 0.2 mA/cm2, thereby finding a discharging capacity. The charging/discharging efficiency was calculated in accordance with the following formula using the discharging capacity and the charging capacity.
Charging/Discharging Efficiency=[Discharging Capacity]/[Charging Capacity]×100
A ratio was found when each of the initial capacity and charging/discharging efficiency of Comparative Example 1 was regarded as 100. Results are shown in Table 1.
The test cell for evaluation on the electrode fusing/disconnection property was connected in parallel to a constant-voltage power supply having a maximum current of 200 A, and a nail penetration test was performed to cause flow of a current so as to simulate a large cell. A heat generation rate (W) after electrode fusing (after nail removal) was measured, and a ratio of the heat generation rate when the heat generation rate of Comparative Example 1 was regarded as 100 was found as the fusing/disconnection property. Results are shown in Table 1.
In Table 1, when the initial capacity was 100 or more, the charging/discharging efficiency was 99.5 or more, and the electrode fusing/disconnection property was 80 or more, it was determined as “O”, and otherwise, it was determined as “X”. In each of Examples 1 to 5 according to the present invention, excellent charging/discharging efficiency, high initial capacity, and high fusing/disconnection property could be attained.
Although the embodiments of the present invention have been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation. The scope of the present invention is defined by the terms of the claims, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2023-007024 | Jan 2023 | JP | national |
