Non-aqueous electrolyte secondary battery

Abstract

This non-aqueous electrolyte secondary battery comprises: an electrode body including a positive electrode and a negative electrode; and a non-aqueous electrolyte. The non-aqueous electrolyte secondary battery has a positive electrode current collector and a positive electrode mixture layer formed on a surface of the positive electrode current collector. The negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on a surface of the negative electrode current collector. The positive electrode mixture layer contains a positive electrode active material. A carbonaceous coating containing one or more elements selected from the group consisting of alkali metals other than Li and alkaline earth metals is formed on a surface of the positive electrode active material, and the negative electrode mixture layer contains a negative electrode active material containing a silicon-containing material and a silicate compound.

Description

Nonaqueous electrolyte secondary battery

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery, and more particularly to a low-resistance non-aqueous electrolyte secondary battery.

[0002] Conventionally, non-aqueous electrolyte secondary batteries have been widely used as high-power, high-capacity secondary batteries, which include a positive electrode, a negative electrode, and a non-aqueous electrolyte, and are charged and discharged by transferring Li ions or the like between the positive electrode and the negative electrode. Patent Document 1 discloses a technique for reducing the resistance value of non-aqueous electrolyte secondary batteries by forming a carbonaceous coating on the surface of a lithium transition metal composite oxide, which is a positive electrode active material.

[0003] International Publication No. 2023 / 203952

[0004] In recent years, non-aqueous electrolyte secondary batteries have been required to have ever higher output and capacity. The present inventors have conducted extensive research and found that by forming a carbonaceous coating on the surface of a positive electrode active material and incorporating a negative electrode active material containing a silicon-containing material and a silicate compound in a negative electrode mixture layer, it is possible to specifically reduce the resistance value during charging compared to existing technologies such as the technology described in Patent Document 1.

[0005] An object of the present disclosure is to provide a high-capacity, low-resistance non-aqueous electrolyte secondary battery.

[0006] A non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes an electrode assembly including a positive electrode and a negative electrode, and a non-aqueous electrolyte. The battery has a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. The negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The positive electrode mixture layer includes a positive electrode active material. A carbonaceous coating containing one or more elements selected from the group consisting of alkali metals excluding Li and alkaline earth metals is formed on the surface of the positive electrode active material. The negative electrode mixture layer includes a negative electrode active material containing a silicon-containing material and a silicate compound.

[0007] According to a nonaqueous electrolyte secondary battery according to one aspect of the present disclosure, it is possible to reduce the battery resistance while achieving a high capacity.

[0008] 1 is a schematic diagram of montmorillonite, an example of a silicate compound, in an axial cross-sectional view of a non-aqueous electrolyte secondary battery according to an embodiment of the present invention;

[0009] In recent years, non-aqueous electrolyte secondary batteries have been required to have increasingly higher output and capacity. Patent Document 1 discloses a technology for reducing resistance by improving the positive electrode active material contained in the positive electrode. However, the characteristics required of non-aqueous electrolyte secondary batteries are becoming increasingly more demanding, necessitating further technological innovation. The present inventors conducted extensive research and discovered that improving both the positive electrode and the negative electrode can achieve both higher capacity and higher output. Specifically, the present inventors discovered that by forming a carbonaceous coating containing one or more elements selected from the group consisting of alkali metals (excluding Li) and alkaline earth metals on the surface of the positive electrode active material, and incorporating a negative electrode active material containing a silicon-containing material and a silicate compound into the negative electrode mixture layer, it is possible to specifically reduce resistance during charging compared to existing technologies. According to the inventors' research, the reduction in resistance when both the positive electrode and the negative electrode are improved is greater than the simple sum of the reductions in resistance when only the positive electrode is improved and the reductions in resistance when only the negative electrode is improved.

[0010] An example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail below. Hereinafter, a cylindrical battery in which a wound electrode assembly is housed in a cylindrical exterior body will be exemplified. However, the electrode assembly is not limited to the wound type and may be a laminated type in which multiple positive electrodes and multiple negative electrodes are alternately stacked one by one with separators interposed therebetween. Furthermore, the exterior body is not limited to a cylindrical shape and may be, for example, prismatic or coin-shaped, or may be a battery case made of a laminate sheet including a metal layer and a resin layer.

[0011] FIG. 1 is an axial cross-sectional view of a cylindrical secondary battery 10 according to an embodiment. As shown in FIG. 1 , the secondary battery 10 includes a wound electrode assembly 14, an electrolyte, and an exterior body 16 that accommodates the electrode assembly 14 and the electrolyte. The electrode assembly 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The exterior body 16 is a cylindrical metal container with a bottom and an opening on one axial side, and the opening of the exterior body 16 is closed by a sealing body 17. Hereinafter, for convenience of explanation, the sealing body 17 side of the battery will be referred to as the top, and the bottom side of the exterior body 16 will be referred to as the bottom.

[0012] The positive electrode 11, negative electrode 12, and separator 13 constituting the electrode assembly 14 are all rectangular, elongated bodies that are spirally wound in the longitudinal direction and alternately stacked in the radial direction of the electrode assembly 14. The separator 13 isolates the positive electrode 11 and the negative electrode 12 from each other. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 to prevent lithium precipitation. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the longitudinal and lateral directions. The two separators 13 are formed to be at least slightly larger than the positive electrode 11 and are arranged, for example, to sandwich the positive electrode 11. The electrode assembly 14 includes a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like. In the electrode assembly 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the lateral direction of the positive electrode 11 and the negative electrode 12 is the axial direction. That is, the end faces in the lateral direction of the positive electrode 11 and the negative electrode 12 form the end faces in the axial direction of the electrode body 14 .

[0013] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the exterior body 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the exterior body 16 by welding or the like, and the exterior body 16 serves as the negative electrode terminal.

[0014] A gasket 28 is provided between the exterior body 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior body 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior body 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior body 16 by the grooved portion 22 and the open end of the exterior body 16 that is crimped to the sealing body 17.

[0015] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked, and functions as a safety valve. Each component constituting the sealing body 17 has, for example, a disk or ring shape, and each component except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0016] The positive electrode 11, the negative electrode 12, the separator 13, and the non-aqueous electrolyte that constitute the secondary battery 10 will be described in detail below, with the positive electrode 11 and the negative electrode 12 being particularly described below.

[0017] [Positive Electrode] The positive electrode 11 has a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. The positive electrode mixture layer is preferably formed on both sides of the positive electrode current collector. The positive electrode current collector can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 11, or a film having such a metal disposed on the surface layer. The thickness of the positive electrode current collector is, for example, 10 μm or more and 30 μm or less.

[0018] The positive electrode mixture layer contains, for example, a positive electrode active material, a conductive agent, and a binder. The thickness of the positive electrode mixture layer is, for example, 10 μm to 150 μm on one side of the positive electrode current collector. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing the positive electrode active material, the conductive agent, etc. to the surface of the positive electrode current collector, drying the coating, and then rolling the coating to form positive electrode mixture layers on both sides of the positive electrode current collector.

[0019] The basis weight of the positive electrode mixture layer is 280 g / m ２ It is preferable that the weight is 300 g / m or more. ２It is more preferable that the above ratio is satisfied. This allows for a high capacity battery. However, if the basis weight of the positive electrode mixture layer is increased, the positive electrode mixture layer becomes thicker, which tends to cause uneven charge / discharge in the thickness direction and deteriorate the charge / discharge cycle characteristics. In particular, when the negative electrode mixture layer contains a large amount of a negative electrode active material containing a silicon-containing material, the deterioration of the charge / discharge cycle characteristics is significant.

[0020] In this embodiment, as described above, a carbonaceous coating containing one or more elements selected from the group consisting of alkali metals excluding Li and alkaline earth metals is formed on the surface of a positive electrode active material, and a negative electrode mixture layer is made to contain a silicon-containing material and a silicate compound. In other words, when a carbonaceous coating containing one or more elements selected from the group consisting of alkali metals excluding Li and alkaline earth metals is formed on the surface of a positive electrode active material and a silicate compound is contained in the negative electrode mixture layer, the diffusibility of the nonaqueous electrolyte within the positive electrode mixture layer is ensured even if the basis weight of the positive electrode mixture layer is increased. As a result, a high capacity battery can be achieved while maintaining charge / discharge cycle characteristics. The upper limit of the basis weight of the positive electrode mixture layer is, for example, 380 g / m ２ is.

[0021] The basis weight of the positive electrode mixture layer is the unit area (unit: m) of the positive electrode current collector at the portion where the positive electrode mixture layer is formed. ２ ) is calculated by dividing the mass of the positive electrode mixture layer formed on one surface of the positive electrode current collector by the area of ​​the portion of the positive electrode mixture layer formed on that surface of the positive electrode current collector. In addition, in cases where the positive electrode mixture layer is formed unevenly on both surfaces of the positive electrode current collector, the basis weight of the positive electrode mixture layer is calculated for each surface in the same manner as above. In this case, the basis weight of the positive electrode mixture layer on both surfaces is 280 g / m ２ It is preferable that the weight is 300 g / m or more. ２It is more preferable that the weight per unit area of ​​the positive electrode mixture layer is equal to or greater than 1000. Furthermore, when a positive electrode mixture layer is formed on only one surface of the positive electrode current collector, the weight per unit area of ​​the positive electrode mixture layer is calculated by dividing the mass of the positive electrode mixture layer formed on one surface of the positive electrode current collector by the area of ​​the portion of that surface of the positive electrode current collector on which the positive electrode mixture layer is formed. Note that, when the positive electrode current collector has a partially exposed portion of the current collector on which the positive electrode mixture layer is not formed, the area of ​​the exposed portion is not included in the "portion on which the positive electrode mixture layer is formed" in calculating the weight per unit area of ​​the positive electrode mixture layer.

[0022] Examples of the conductive agent contained in the positive electrode mixture layer include carbon black (CB) such as acetylene black (AB) and ketjen black, carbon nanotubes (CNT), graphene, graphite, and other carbon-based particles. These may be used alone or in combination of two or more. The content of the conductive agent in the positive electrode mixture layer is, for example, 0.1% by mass or more and 10% by mass or less with respect to the total mass of the positive electrode mixture layer.

[0023] Examples of binders contained in the positive electrode mixture layer include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyimide-based resins, acrylic-based resins, polyolefin-based resins, and polyacrylonitrile (PAN). These may be used alone or in combination of two or more. The content of the binder in the positive electrode mixture layer is, for example, 0.1% by mass or more and 10% by mass or less with respect to the total mass of the positive electrode mixture layer.

[0024] The positive electrode active material includes, for example, secondary particles formed by aggregation of primary particles. The particle size of the primary particles constituting the secondary particles of the positive electrode active material is, for example, 0.02 μm to 2 μm. The particle size of the primary particles is measured as the diameter of the circumscribed circle in a particle image observed with a scanning electron microscope (SEM). The average particle size of the secondary particles of the positive electrode active material is, for example, 2 μm to 30 μm. Here, the average particle size refers to the volume-based median diameter (D50). D50 refers to the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size, and is also called the median diameter. The particle size distribution of the secondary particles of the positive electrode active material can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrac-Bell Corporation) using water as a dispersion medium.

[0025] The positive electrode active material contained in the positive electrode mixture layer includes, for example, a lithium transition metal composite oxide. The lithium transition metal composite oxide is, for example, a compound represented by the general formula Li ａ Ni ｘ M1 ｙ M2 ｚ O ２－ｂ (wherein 0.8≦a≦1.2, 0.50≦x≦0.95, 0≦y<0.50, 0≦z≦0.05, 0≦b≦0.05, x+y+z=1, M1 is at least one element selected from the group consisting of Co, Mn, and Al, and M2 is at least one element selected from the group consisting of W, Nb, Ti, Zr, B, Sb, Si, Fe, Mo, Sn, Ca, Sr, and Bi) The proportion of metal elements contained in the lithium transition metal composite oxide can be measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), or the like.

[0026] The lithium transition metal composite oxide has a layered rock salt structure belonging to, for example, space group R-3m, space group C2 / m, etc. From the viewpoints of high capacity and stability of the crystal structure, it is preferable that the lithium transition metal composite oxide has a layered rock salt structure belonging to space group R-3m. The layered rock salt structure of the lithium transition metal composite oxide may include a transition metal layer, a Li layer, and an oxygen layer. The reversible movement of Li ions present in the Li layer causes the charge / discharge reaction of the battery to proceed.

[0027] A carbonaceous coating containing one or more elements selected from the group consisting of alkali metals excluding Li and alkaline earth metals is formed on the surface of the positive electrode active material. By forming a carbonaceous coating on the surface of the positive electrode active material and by including a silicate compound and a negative electrode active material containing a silicon-containing material described below in the negative electrode mixture layer, the electrical resistance of the nonaqueous electrolyte secondary battery can be specifically reduced. Here, the surface of the positive electrode active material refers to the surface of the secondary particles of the positive electrode active material. That is, the carbonaceous coating is present on the surface of the secondary particles of the positive electrode active material. The carbonaceous coating may be present in a dotted form so as to cover at least a portion of the surface of the secondary particles of the positive electrode active material, or may be present so as to cover the entire surface of the secondary particles. The carbonaceous coating may also be present at the interface where primary particles of the positive electrode active material contact each other.

[0028] As described above, the carbonaceous coating contains one or more elements selected from the group consisting of alkali metals excluding Li and alkaline earth metals, and preferably contains at least one of Na and K. The carbonaceous coating may further contain S. The carbonaceous coating may also contain Li. The presence of S in the carbonaceous coating can be confirmed by energy dispersive X-ray spectroscopy (TEM-EDX). The presence of alkali metals and alkaline earth metals such as Na and K in the carbonaceous coating can be confirmed by X-ray photoelectron spectroscopy (XPS).

[0029] The thickness of the carbonaceous coating is, for example, 30 nm or less. The lower limit of the thickness of the carbonaceous coating is, for example, 1 nm. The thickness of the carbonaceous coating is measured, for example, from an image of a cross section of the positive electrode active material observed with a transmission electron microscope (TEM).

[0030] The positive electrode mixture layer may contain a positive electrode active material that does not have a carbonaceous coating on its surface. For example, the positive electrode mixture layer may contain a lithium transition metal composite oxide that does not have a carbonaceous coating on its surface. In the positive electrode mixture layer, the proportion of the positive electrode active material that has a carbonaceous coating on its surface is preferably 90 mass% or more, and more preferably 99 mass% or more, relative to the total mass of the positive electrode active material.

[0031] Next, an example of a method for producing a positive electrode active material having a carbonaceous coating formed on the surface thereof will be described.

[0032] The carbonaceous coating is formed, for example, as follows: (1) An anionic surfactant is dissolved in water to prepare an aqueous solution; (2) A powder of a lithium transition metal composite oxide, which is a positive electrode active material, is added to the aqueous solution and stirred to prepare a suspension in which the lithium transition metal composite oxide is dispersed in the aqueous solution; (3) The suspension is filtered, and the resulting residue is calcined to obtain a positive electrode active material having a carbonaceous coating formed on its surface.

[0033] Anionic surfactants can be adsorbed onto the surface of lithium transition metal composite oxides in water to form a substantially uniform carbonaceous coating. The surfactant is not particularly limited as long as it contains C and one or more elements selected from the group consisting of alkali metals excluding Li and alkaline earth metals. The surfactant preferably contains at least one of Na and K. The surfactant may further contain S. Examples of surfactants include sodium lauryl sulfate and potassium lauryl sulfate. The concentration of the surfactant in the aqueous solution is, for example, 0.1% to 10% by mass.

[0034] [Negative Electrode] The negative electrode 12 has a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The negative electrode mixture layer is preferably formed on both sides of the negative electrode current collector. The negative electrode current collector can be a foil of a metal such as copper or a copper alloy that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface layer. The thickness of the negative electrode current collector is, for example, 5 μm or more and 30 μm or less.

[0035] The negative electrode mixture layer contains a negative electrode active material and a silicate compound, and has a thickness of, for example, 10 μm to 150 μm on one side of the negative electrode current collector.

[0036] The negative electrode active material includes, for example, a carbon material and a silicon-containing material. The carbon material is, for example, graphite. The graphite may be any of natural graphite such as flake graphite, lump graphite, and amorphous graphite, or artificial graphite such as lump artificial graphite and graphitized mesophase carbon microbeads. The silicon-containing material can occlude more lithium ions per unit mass than carbon materials such as graphite. Therefore, by using a silicon-containing material as the negative electrode active material, a high capacity battery can be achieved.

[0037] The silicon-containing material contained in the negative electrode mixture layer may be any material containing silicon element, and examples thereof include silicon alloys, silicon compounds, and composite materials containing silicon element, among which composite materials containing silicon element are preferred. The volume-based median diameter (D50) of the composite material is generally smaller than the volume-based median diameter (D50) of the carbon material. The volume-based median diameter (D50) of the composite material is, for example, 1 μm or more and 15 μm or less. Note that one type of silicon-containing material may be used alone, or two or more types may be used in combination.

[0038] A suitable silicon-containing material is a composite particle containing an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase. The silicon phase is formed by dispersing silicon element in the form of fine particles. The composite particle may also be formed with a conductive layer covering a portion of the surface of the ion-conducting phase. The conductive layer is made of a material with higher conductivity than the ion-conducting phase and forms a good conductive path in the negative electrode mixture layer. The conductive layer contains, for example, conductive carbon and covers 30% to 70% of the surface area of ​​the ion-conducting phase. The coverage of the conductive layer can be calculated, for example, using X-ray photoelectron spectroscopy (XPS).

[0039] The ion-conducting phase is a continuous phase composed of an aggregate of particles finer than the silicon phase. The ion-conducting phase is, for example, at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. The silicide phase is a phase of a compound composed of Si and an element more electropositive than Si, such as NiSi, Mg ２ Si, TiSi ２ Examples include:

[0040] The silicate phase preferably contains at least one element selected from lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, and radium, for example, from the viewpoint of high lithium ion conductivity, etc. Among these, a silicate phase containing lithium (hereinafter sometimes referred to as a lithium silicate phase) is preferred, for example, from the viewpoint of high lithium ion conductivity, etc.

[0041] The lithium silicate phase can be, for example, a compound of the formula: Li ２ｚ SiO ２＋ｚ (0<z<2). From the viewpoints of stability, ease of preparation, lithium ion conductivity, and the like, z preferably satisfies the relationship 0<z<1, and more preferably z=1 / 2.

[0042] Another example of a suitable silicon-containing material is a silicon dioxide material having a sea-island structure in which fine silicon phases are uniformly dispersed in an amorphous silicon oxide phase, and which is generally represented by the general formula SiO ｘ (0<x≦2). The main component of the silicon oxide may be silicon dioxide. The content ratio (x) of oxygen element to silicon element is, for example, 0.5≦x<2.0, preferably 0.8≦x≦1.5.

[0043] Another example of a suitable silicon-containing material is a composite particle having a sea-island structure in which fine silicon phases are substantially uniformly dispersed in a carbon phase. The carbon phase is preferably an amorphous carbon phase. The carbon phase may contain a crystalline phase component, but preferably contains a larger amount of amorphous phase components. The amorphous carbon phase is, for example, composed of a carbon material having an average interplanar spacing of (002) planes of more than 0.34 nm as measured by X-ray diffraction. The amorphous carbon phase is, for example, composed of amorphous carbon particles. Note that when the silicon-containing material is a composite particle having a sea-island structure in which fine silicon phases are substantially uniformly dispersed in a carbon phase, the total mass of silicon element contained in the silicon-containing material may be equal to the mass of the silicon phase.

[0044] The mass ratio of the silicon phase to the total mass of the silicon-containing material is preferably 30% by mass or more. In this case, the discharge capacity is increased, and the battery is likely to have high output. Furthermore, the mass ratio of the silicon phase to the total mass of the silicon-containing material is preferably 60% by mass or less. In this case, the volume change of the negative electrode mixture layer during charge and discharge can be reduced, the expansion and contraction of the electrode body 14 during charge and discharge can be suppressed, and the battery is likely to have excellent durability. Therefore, the mass ratio of the silicon phase to the total mass of the silicon-containing material is preferably 30% by mass or more and 60% by mass or less.

[0045] The crystallite size of the silicon phase constituting the composite particles is, for example, 10 nm or more and 30 nm or less, and is calculated by Scherrer's equation from the half-width of the analytical peak assigned to the Si(111) plane in the X-ray diffraction pattern of the silicon phase.

[0046] The ratio of the total mass of silicon contained in the silicon-containing material to the mass of the negative electrode mixture layer is preferably 20 mass% or more, which increases the discharge capacity of the negative electrode mixture layer and enables the realization of a high-capacity battery.

[0047] Furthermore, the ratio of the total mass of silicon contained in the silicon-containing material to the mass of the negative electrode mixture layer is preferably 50% by mass or less, more preferably 40% by mass or less. If the ratio of the total mass of silicon contained in the silicon-containing material to the mass of the negative electrode mixture layer exceeds 50% by mass, the volume change of the negative electrode mixture layer during charging and discharging may become excessively large, and the diffusibility of the non-aqueous electrolyte in the electrode body 14 may be reduced. Therefore, the ratio of the total mass of silicon contained in the silicon-containing material to the mass of the negative electrode mixture layer is preferably 20% by mass or more and 50% by mass or less, more preferably 20% by mass or more and 40% by mass or less. Note that the mass of silicon element may be approximately the same as the mass of the silicon phase. That is, the ratio of the mass of the silicon phase to the mass of the negative electrode mixture layer may be 20% by mass or more and 50% by mass or less.

[0048] The total mass of silicon contained in a silicon-containing material can be measured using ICP (Inductively Coupled Plasma). ICP is a type of optical emission spectroscopy. When plasma energy is applied to an analytical sample from the outside, the contained component elements (atoms) are excited.

[0049] The silicate compound contained in the negative electrode mixture layer has lithophilicity and high lithium diffusivity. The silicate compound includes, for example, a layered silicate compound such as a layered silicate mineral. The layered silicate mineral has a structure in which multiple silicate layers are stacked. The layered silicate mineral may be, for example, particulate. The layered silicate mineral may be, for example, plate-like particles, spherical particles, chunky particles, rod-like particles, etc. The layered silicate mineral is, for example, montmorillonite (MMT). Layered silicate minerals such as montmorillonite have water swelling properties, cation adsorption capacity, and cation diffusivity.

[0050] 2 is a schematic diagram of montmorillonite, a silicate compound contained in the negative electrode. Montmorillonite is a (Na, Ca) ０．３３ (Al, Mg) x (Si ４ O １０ ) (OH) ２ ・nH ２ It has a composition represented by O.

[0051] Specifically, montmorillonite has a layered structure in which a sheet-like tetrahedron layer 31 in which Si and O tetrahedra are two-dimensionally bonded and a sheet-like octahedron layer 32 in which Al and OH octahedra are two-dimensionally bonded share some O ions, and a basic layer 30 is formed in which one octahedron layer 32 is symmetrically sandwiched between two tetrahedron layers 31. In montmorillonite, the spacing between the two basic layers 30 expands as water molecules enter between them. In addition, Na ＋ , Ca ２＋ , K. ＋A layer of exchangeable cations such as montmorillonite is formed. This allows silicate compounds, including montmorillonite, to have a high adsorption capacity for the cation lithium ion. It is also known that silicate compounds reduce the energy consumption of lithium ion diffusion within the crystal. This is thought to be why silicate compounds have a high diffusivity for lithium ions.

[0052] The layered silicate compound is preferably at least one type of smectite. Suitable smectites include, in addition to the above-mentioned montmorillonite, halloysite, hectorite, beidellite, nontronite, saponite, and sauconite. Among the layered silicate compounds, smectite has a high affinity with lithium ions and more effectively improves the diffusibility of lithium ions in the negative electrode mixture layer. The negative electrode mixture layer contains, for example, at least one type of smectite selected from the group consisting of montmorillonite, halloysite, hectorite, beidellite, nontronite, saponite, and sauconite. Among these, montmorillonite is preferred.

[0053] The negative electrode mixture layer may further contain a binder. Examples of binders contained in the negative electrode mixture layer include styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof (PAA-Na, PAA-K, etc., or a partially neutralized salt), polyvinyl alcohol (PVA), etc. These may be used alone or in combination of two or more.

[0054] The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a silicate compound, a binder, and the like to the surface of a negative electrode current collector, drying the coating, and then rolling the coating to form a negative electrode mixture layer on both sides of the negative electrode current collector.

[0055] [Separator] The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Alternatively, the separator 13 may be a multilayer separator including a polyethylene layer and a polypropylene layer, and the surface of the separator 13 may be coated with a material such as an aramid-based resin or ceramic.

[0056] [Non-aqueous electrolyte] The non-aqueous electrolyte has, for example, lithium ion conductivity. The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0057] The liquid electrolyte (electrolytic solution) contains, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. Examples of the halogen-substituted compound include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0058] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0059] Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl phenyl ether. and chain ethers such as ethyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0060] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lower aliphatic lithium carboxylates, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of phosphates include lithium difluorophosphate (LiPO ２ F ２Examples of the borate salt include lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFFOB). Examples of the imide salt include lithium bisfluorosulfonylimide (LiN(FSO ２ ) ２ ), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonyl imide (LiN(C ２ F ５ SO ２ ) ２ Among these, LiPF is preferred from the viewpoint of ionic conductivity, electrochemical stability, etc. ６ The concentration of the lithium salt may be, for example, 4 mol or less, or 3 mol or less, preferably 1.8 mol or less, and more preferably 0.8 mol or more and 1.8 mol or less, per 1 L of the non-aqueous solvent.

[0061] The non-aqueous electrolyte may contain an additive such as an unsaturated carbonate ester, an acid anhydride, a phenol compound, a benzene compound, a nitrile compound, an isocyanate compound, a sultone compound, a sulfate compound, a borate ester compound, a phosphate ester compound, or a phosphite ester compound.

[0062] Examples of unsaturated cyclic carbonates include vinylene carbonate, 4-methylvinylene carbonate, 4,5-dimethylvinylene carbonate, 4-ethylvinylene carbonate, 4,5-diethylvinylene carbonate, 4-propylvinylene carbonate, 4,5-dipropylvinylene carbonate, 4-phenylvinylene carbonate, 4,5-diphenylvinylene carbonate, vinylethylene carbonate, and divinylethylene carbonate. One type of unsaturated cyclic carbonate may be used alone, or two or more types may be used in combination. Some of the hydrogen atoms in the unsaturated cyclic carbonate may be substituted with fluorine atoms. The acid anhydride may be an anhydride formed by intermolecular condensation of multiple carboxylic acid molecules, but is preferably an acid anhydride of a polycarboxylic acid. Examples of polycarboxylic acid anhydrides include succinic anhydride, maleic anhydride, and phthalic anhydride.

[0063] Examples of phenolic compounds include phenol, hydroxytoluene, etc. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, cyclohexylbenzene (CHB), etc.

[0064] Examples of nitrile compounds include adiponitrile, pimelonitrile, propionitrile, and succinonitrile. Examples of isocyanate compounds include methyl isocyanate (MIC), diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and bisisocyanatomethylcyclohexane (BIMCH). Examples of sultone compounds include propane sultone and propene sultone. Examples of sulfate compounds include ethylene sulfate, ethylene sulfite, dimethyl sulfate, and lithium fluorosulfate. Examples of borate ester compounds include trimethyl borate and tris(trimethylsilyl)borate. Examples of phosphate ester compounds include trimethyl phosphate and tris(trimethylsilyl)phosphate. Examples of phosphite ester compounds include trimethyl phosphite and tris(trimethylsilyl)phosphite.

[0065] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc. can be used. As the inorganic solid electrolyte, a material known in all-solid-state lithium ion secondary batteries, etc. (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halogen-based solid electrolyte, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs the non-aqueous solvent and gels is used. As the polymer material, for example, a fluororesin, an acrylic resin, a polyether resin, etc. can be used.

[0066] Hereinafter, the present disclosure will be further described with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

[0067] <Test Cell A> [Preparation of Positive Electrode Active Material] Sodium lauryl sulfate as a surfactant was dissolved in water to prepare a 10 mass % aqueous solution. ０.８ Mn ０．２ O２ A lithium transition metal composite oxide of 1,200 g / L was added and stirred for 5 minutes to prepare a suspension with a concentration of 1200 g / L. The residue obtained by filtering this suspension was vacuum-dried at 180°C for 2 hours to obtain the positive electrode active material of Example 1. Observation by transmission electron microscope (TEM) revealed that the thickness of the carbonaceous coating was 10 nm. Observation by energy dispersive X-ray spectroscopy (TEM-EDX) revealed a peak attributable to S near 2.2 keV, confirming the presence of S in the carbonaceous coating. Analysis by X-ray photoelectron spectroscopy (XPS) also confirmed the presence of Na in the carbonaceous coating.

[0068] [Preparation of Positive Electrode] The positive electrode active material, acetylene black, and polyvinylidene fluoride (PVDF) were mixed in a solid content mass ratio of 98:1:1, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry. The positive electrode mixture slurry was applied to both sides of a positive electrode current collector made of aluminum foil, and after drying the coating, the coating was rolled using a roller and cut to a predetermined electrode size to obtain a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode current collector. The basis weight of the positive electrode mixture layer was 340 g / m ２ It was.

[0069] [Preparation of Negative Electrode] A mixture of graphite and SiC in a mass ratio of 65:35 was used as the negative electrode active material. The negative electrode active material, montmorillonite, styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC) were mixed in a solid mass ratio of 100:1:1:1, and an appropriate amount of water was added to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried. The coating was then rolled using a roller and cut to a predetermined electrode size to obtain a negative electrode having a negative electrode mixture layer formed on both sides of the negative electrode current collector. The ratio of the total mass of silicon contained in the silicon-containing material to the mass of the prepared negative electrode mixture layer was 21% by mass.

[0070] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 20:5:75. Lithium hexafluorophosphate (LiPF ６) was dissolved in the solution to a concentration of 1.3 mol / L to prepare a non-aqueous electrolyte (non-aqueous electrolytic solution).

[0071] [Preparation of Test Cell (Non-Aqueous Electrolyte Secondary Battery)] An aluminum lead was attached to a portion of the positive electrode, and a nickel lead was attached to a portion of the negative electrode. The positive and negative electrodes were spirally wound with a polyolefin separator between them to prepare a wound electrode assembly. Insulating plates were placed on the top and bottom of the electrode assembly, and the electrode assembly was housed in an outer can. The negative electrode lead was welded to the bottom of a cylindrical outer can with a bottom, and the positive electrode lead was welded to a sealing member. An electrolyte was poured into the outer can, and the opening of the outer can was sealed with a sealing member via a gasket to prepare a non-aqueous electrolyte secondary battery as a test cell.

[0072] <Test Cell B> Test Cell B was obtained in the same manner as Test Cell A, except that sodium lauryl sulfate was not added in the preparation of the positive electrode active material, and a carbonaceous coating was not formed on the surface of the positive electrode active material, and that montmorillonite was not used in the preparation of the negative electrode, and montmorillonite was not contained in the negative electrode mixture layer.

[0073] <Test Cell C> Test cell C was obtained in the same manner as test cell A, except that sodium lauryl sulfate was not added in the preparation of the positive electrode active material and a carbonaceous coating was not formed on the surface of the positive electrode active material.

[0074] <Test Cell D> Test cell D was obtained in the same manner as test cell A, except that montmorillonite was not used in the preparation of the negative electrode, and montmorillonite was not contained in the negative electrode mixture layer.

[0075] [Evaluation of Direct Current Resistance (DCIR)] Test cells A to D were charged at a constant current of 0.05 C in a temperature environment of 25° C. until a constant state of charge (SOC) was reached. After that, the cells were left to stand for 3 hours, and then charged at a current of 0.5 C for 10 seconds. The resistance value was calculated from the voltage difference before and after the charging, and this was taken as DCIR.

[0076] Table 1 shows the DCIR evaluation results for test cells A to D when the SOC was set to 30%. Table 2 shows the DCIR evaluation results for test cells A to D when the SOC was set to 70%. Table 3 shows the DCIR evaluation results for test cells A to D when the SOC was set to 90%. The DCIR evaluation results in Tables 1 to 3 show the difference in DCIR with test cell B (reference) as the standard.

[0077]

[0078]

[0079]

[0080] In all of Tables 1 to 3, Test Cell A had a lower DCIR than any of Test Cells B to D. In addition, the difference in DCIR between Test Cell A and the reference was greater than the sum of the difference in DCIR between Test Cell C and the reference and the difference in DCIR between Test Cell D and the reference.

[0081] The mechanism by which the DCIR reduction effect in Test Cell A was achieved is presumed to be as follows. Test Cell A has a carbonaceous coating containing Na formed on the surface of the positive electrode active material, thereby reducing the resistance at the surface of the positive electrode active material compared to Test Cells B and C, which do not have the carbonaceous coating formed on the surface of the positive electrode active material. This facilitates the release of lithium ions from the positive electrode active material during charging, resulting in reduced reaction resistance. Furthermore, in addition to the formation of a carbonaceous coating containing Na on the surface of the positive electrode active material, montmorillonite (a silicate compound) is contained in the negative electrode mixture layer. This reduces the reaction resistance at the surface of the negative electrode active material compared to Test Cells B and D, which do not have the silicate compound contained in the negative electrode mixture layer. Because the silicate compound can temporarily accept lithium ions released from the positive electrode active material during charging, the lithium ions accepted by the silicate compound do not remain on the surface of the negative electrode active material containing the silicon-containing material, but are instead occluded on the surface of the negative electrode active material. Therefore, it is presumed that test cell A had a lower DCIR than any of test cells B to D, and the difference in DCIR between test cell A and the reference was greater than the sum of the difference in DCIR between test cell C and the reference and the difference in DCIR between test cell D and the reference.

[0082] On the other hand, since Test Cell C contains montmorillonite (silicate compound) in the negative electrode mixture layer, the inclusion of montmorillonite (silicate compound) in the negative electrode mixture layer has the effect of reducing the reaction resistance on the surface of the negative electrode active material, but since a carbonaceous coating containing Na is not formed on the surface of the positive electrode active material, it is thought that the effect of reducing the reaction resistance on the surface of the positive electrode active material was not obtained.For this reason, it is inferred that the DCIR of Test Cell C was reduced compared to that of the reference Test Cell B, but not compared to that of Test Cell A.

[0083] Furthermore, although Test Cell D has the effect of reducing the reaction resistance on the surface of the positive electrode active material by forming a carbonaceous coating containing Na on the surface of the positive electrode active material, it does not contain montmorillonite (a silicate compound) in the negative electrode mixture layer, and it is thought that lithium ions released from the positive electrode active material during charging end up remaining on the surface of the negative electrode active material containing a silicon-containing material. For this reason, it is inferred that the DCIR of Test Cell D was reduced compared to that of the reference Test Cell B, but not compared to that of Test Cell A.

[0084] This shows that by forming a carbonaceous coating containing one or more elements selected from the group consisting of alkali metals excluding Li and alkaline earth metals on the surface of the positive electrode active material, and by incorporating a negative electrode active material containing a silicon-containing material and a silicate compound into the negative electrode mixture layer, it is possible to specifically reduce the DCIR during charging.

[0085] The present disclosure is further described by the following embodiments. Aspect 1: A non-aqueous electrolyte secondary battery comprising: an electrode assembly including a positive electrode and a negative electrode; and a non-aqueous electrolyte; wherein the positive electrode has a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector; the negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector; the positive electrode mixture layer contains a positive electrode active material; and a carbonaceous coating containing one or more elements selected from the group consisting of alkali metals excluding Li and alkaline earth metals is formed on the surface of the positive electrode active material; and the negative electrode mixture layer contains a negative electrode active material containing a silicon-containing material and a silicate compound. Aspect 2: The non-aqueous electrolyte secondary battery according to Aspect 1, wherein a ratio of the total mass of silicon contained in the silicon-containing material to the mass of the negative electrode mixture layer is 20 mass% or more. Configuration 3: The nonaqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the ratio of the total mass of silicon contained in the silicon-containing material to the mass of the negative electrode mixture layer is 50 mass% or less. Configuration 4: The nonaqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the ratio of the total mass of silicon contained in the silicon-containing material to the mass of the negative electrode mixture layer is 40 mass% or less. Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the silicon-containing material includes an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase. Configuration 6: The nonaqueous electrolyte secondary battery according to Configuration 5, wherein the ion-conducting phase is at least one phase selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. Configuration 7: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 6, wherein the silicate compound includes montmorillonite. Configuration 8: The positive electrode mixture layer has a basis weight of 280 g / m ２ The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 7. Configuration 9: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 8, wherein the carbonaceous coating further contains S. Configuration 10: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 9, wherein the carbonaceous coating contains at least one of Na and K.

[0086] REFERENCE SIGNS LIST 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Exterior body, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved portion, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Basic layer, 31 Tetrahedron layer, 32 Octahedron layer

Claims

1. A non-aqueous electrolyte secondary battery comprising: an electrode assembly including a positive electrode and a negative electrode; and a non-aqueous electrolyte; wherein the positive electrode has a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector; the negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector; the positive electrode mixture layer contains a positive electrode active material; a carbonaceous coating containing one or more elements selected from the group consisting of alkali metals excluding Li and alkaline earth metals is formed on the surface of the positive electrode active material; and the negative electrode mixture layer contains a negative electrode active material containing a silicon-containing material and a silicate compound.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the ratio of the total mass of silicon contained in said silicon-containing material to the mass of said negative electrode mixture layer is 20 mass % or more.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the ratio of the total mass of silicon contained in said silicon-containing material to the mass of said negative electrode mixture layer is 50 mass % or less.

4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the ratio of the total mass of silicon contained in said silicon-containing material to the mass of said negative electrode mixture layer is 40 mass % or less.

5. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the silicon-containing material comprises an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase.

6. The nonaqueous electrolyte secondary battery according to claim 5, wherein the ion-conducting phase is at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase.

7. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the silicate compound includes montmorillonite.

8. The basis weight of the positive electrode mixture layer is 280 g / m ２ The nonaqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein 9. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the carbonaceous coating further contains S.

10. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the carbonaceous coating contains at least one of Na and K.

Images