Power storage device

Abstract

To provide a technique for better suppressing an increase in resistance in a low temperature environment in a power storage device using particles containing silicon as a negative electrode active material.SOLUTION: A power storage device disclosed herein includes a positive electrode, a negative electrode, and a nonaqueous electrolytic solution. The negative electrode contains, as a negative electrode active material, silicon-containing graphite particles which are graphite particles containing silicon. The nonaqueous electrolytic solution contains EC, DMC, EMC, a carboxylic acid ester having 4 or less carbon atoms, and FEC as nonaqueous solvents. When the total of EC, DMC, EMC, and carboxylic acid ester is assumed to be 100% by volume, the volume of the carboxylic acid ester is 1% by volume to 30% by volume. A content of FEC with respect to an entire nonaqueous solvent is 0.1% by mass to 5% by mass.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to an electricity storage device. [Background technology]

[0002] An example of an electricity storage device is a secondary battery such as a lithium-ion secondary battery. In recent years, this type of secondary battery has been suitably used as a portable power source for personal computers, mobile terminals, etc., and as a power source for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] Japanese Patent Application Laid-Open Publication No. 2017-50142 discloses an anode active material for lithium-ion secondary batteries. This anode active material includes a plurality of flake graphite particles and flake silicon particles whose surfaces are coated with carbon. The plurality of flake graphite particles are aggregated to form particles. Silicon particles are present between the plurality of flake graphite particles. The publication states that this configuration can provide an anode active material for lithium-ion secondary batteries that has high capacity and long life.

[0004] In the secondary battery disclosed in JP 2019-179724 A, the negative electrode includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector. The negative electrode active material layer contains a carbon material and a silicon compound containing silicon atoms. The nonaqueous solvent contains fluoroethylene carbonate at a content of 10% by volume or more. The surface of the negative electrode active material layer facing the negative electrode current collector is designated as the first surface, the surface opposite the first surface is designated as the second surface, the thickness of the negative electrode active material layer is designated as T, the region of the negative electrode active material layer extending from the first surface to a depth of 0.5T is designated as the lower layer, and the region of the negative electrode active material layer extending from the second surface to a depth of 0.5T is designated as the upper layer. The mass M1 of the silicon compound in the lower layer is greater than the mass M2 of the silicon compound in the upper layer. The publication also states that this configuration can suppress capacity loss even during rapid charging. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-50142 [Patent Document 2] Japanese Patent Application Publication No. 2019-179724 Summary of the Invention [Problem to be solved by the invention]

[0006] The present inventors wish to better suppress an increase in resistance in a low-temperature environment in an electricity storage device that uses silicon-containing particles as a negative electrode active material. [Means for solving the problem]

[0007] The technology disclosed herein provides an electricity storage device including a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode contains silicon-containing graphite particles as a negative electrode active material. The non-aqueous electrolyte contains EC, DMC, EMC, a carboxylic acid ester having 4 or less carbon atoms, and FEC as a non-aqueous solvent. When the total of EC, DMC, EMC, and the carboxylic acid ester is taken as 100% by volume, the volume ratio of the carboxylic acid ester is 1% by volume to 30% by volume. The content of FEC relative to the total non-aqueous solvent is 0.1% by mass to 5% by mass. This configuration makes it possible to better suppress an increase in resistance in a low-temperature environment in an electricity storage device using silicon-containing particles as a negative electrode active material. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic cross-sectional view of a lithium-ion secondary battery 100. As shown in FIG. [Figure 2] FIG. 2 is a schematic diagram of the electrode body 20. As shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0009] An embodiment of the energy storage device disclosed herein will be described below. The embodiment described herein does not particularly limit the technology disclosed herein. The technology disclosed herein is not limited to the embodiment described herein, unless otherwise specified. The drawings are schematic and do not necessarily reflect the actual product. Furthermore, members and parts that perform the same function are appropriately designated by the same reference numerals, and redundant explanations may be omitted. Furthermore, the notation "A to B" indicating a numerical range means "greater than or equal to A and less than or equal to B" unless otherwise specified, and also encompasses the meaning of "greater than A and less than B."

[0010] In this specification, the term "electricity storage device" refers to a device in which charging and discharging occur by the movement of charge carriers between a pair of electrodes (positive and negative electrodes) via an electrolyte. Electricity storage devices include secondary batteries such as lithium ion secondary batteries; and capacitors such as lithium ion capacitors and electric double layer capacitors. Below, an embodiment in which the electricity storage device is a lithium ion secondary battery will be described.

[0011] Fig. 1 is a schematic cross-sectional view of a lithium-ion secondary battery 100. Fig. 2 is a schematic diagram of an electrode assembly 20. As shown in Fig. 1, the lithium-ion secondary battery 100 includes the electrode assembly 20, a case 30, and a non-aqueous electrolyte solution 80.

[0012] 1 and 2, the electrode assembly 20 is a wound electrode assembly in which a long, sheet-like positive electrode 50 and a long, sheet-like negative electrode 60 are stacked together with a long, sheet-like separator 70 interposed therebetween and wound in the sheet longitudinal direction (hereinafter simply referred to as the "longitudinal direction"). In the electrode assembly 20, the exposed region 52a of the positive electrode 50 and the exposed region 62a of the negative electrode 60 protrude outward from both ends in the lateral direction perpendicular to the longitudinal direction.

[0013] As shown in FIGS. 1 and 2, the positive electrode 50 includes a long sheet-like positive electrode collector 52 and a positive electrode active material layer 54. The positive electrode collector 52 is, for example, aluminum foil. In this embodiment, the positive electrode collector 52 has a region where the positive electrode active material layer 54 is provided and an exposed region 52a where the positive electrode active material layer 54 is not provided and the surface of the positive electrode collector 52 is exposed. The positive electrode active material layer 54 is provided, for example, in a strip shape along the longitudinal direction on one or both sides (both sides here) of the positive electrode collector 52. The positive electrode active material layer 54 is not provided at an end (the left end in the drawing) in the short-side direction of the sheet (hereinafter simply referred to as the "short-side direction"). Here, the exposed region 52a is a strip-shaped region at the end (the left end in the drawing) in the short-side direction. As shown in FIG. 1, the current collector plate 42a is attached to the exposed region 52a.

[0014] The positive electrode active material layer 54 contains, for example, a positive electrode active material. The positive electrode active material is not particularly limited as long as the effects of the technology disclosed herein are realized, and any positive electrode active material having a conventionally known composition used for this type of application can be used. The positive electrode active material may be, for example, a lithium composite oxide, a lithium transition metal phosphate compound, or the like. The crystal structure of the positive electrode active material is not particularly limited, and may be a layered structure, a spinel structure, an olivine structure, or the like.

[0015] The lithium composite oxide is preferably a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element. Examples of the lithium transition metal composite oxide include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide. These positive electrode active materials may be used alone or in combination of two or more.

[0016] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of the additional elements include transition metal elements and typical metal elements, such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional elements may also be metalloid elements, such as B, C, Si, and P; or nonmetallic elements, such as S, F, Cl, Br, and I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0017] Examples of the lithium transition metal phosphate compound include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate. Examples of the positive electrode active material include LiNi 0.33 Co 0.33 Mn 0.33 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNiO2, LiCoO2, LiFeO2, LiMn2O4, LiNi 0.5 Mn 1.5 O4 and the like can be preferably used.

[0018] The positive electrode active material layer 54 may contain, in addition to the positive electrode active material, a conductive material, a binder, and the like. Examples of conductive materials include carbon black such as acetylene black (AB) and other carbon materials such as graphite. Examples of binders include polyvinylidene fluoride (PVDF). The content of the positive electrode active material relative to the entire positive electrode active material layer 54 is, for example, preferably 70 mass % or more, more preferably 80 mass % to 98 mass %, and even more preferably 85 mass % to 95 mass %. The content of the conductive material relative to the entire positive electrode active material layer 54 is, for example, 0.1 mass % to 20 mass %. The content of the binder relative to the entire positive electrode active material layer 54 is, for example, 0.5 mass % to 15 mass %.

[0019] As shown in FIGS. 1 and 2, the negative electrode 60 includes a long, sheet-like negative electrode current collector 62 and a negative electrode active material layer 64. The negative electrode current collector 62 is, for example, copper foil. In this embodiment, the negative electrode current collector 62 has a region where the negative electrode active material layer 64 is provided and an exposed region 62a where the negative electrode active material layer 64 is not provided and the surface of the negative electrode active material layer 64 is exposed. The negative electrode active material layer 64 is provided, for example, in a strip shape along the longitudinal direction on one or both sides (both sides in this case) of the negative electrode current collector 62. The negative electrode active material layer 64 is not provided at the end in the short side direction (the end on the right side in the figure). Here, the exposed region 62a is a strip-shaped region at the end in the short side direction (the end on the right side in the figure). As shown in FIG. 1, a current collector 44a is attached to the exposed region 62a.

[0020] The negative electrode active material layer 64 includes, for example, a negative electrode active material. In this embodiment, the negative electrode active material includes silicon-containing graphite particles. The silicon-containing graphite particles may be, for example, graphite particles containing silicon-containing particles therein. In this case, the silicon-containing graphite particles may include, for example, graphite particles having voids and silicon-containing particles disposed within the voids. The voided graphite particles may function as substrate particles for the silicon-containing particles. Although not particularly limited, from the viewpoint of including a larger number of silicon-containing particles and realizing a higher capacity, a higher energy density, etc. of the lithium-ion secondary battery 100, the voided graphite particles are preferably porous graphite particles. Alternatively, in another embodiment, the voided graphite particles may be aggregates of flake graphite.

[0021] The average particle diameter of the silicon-containing graphite particles is generally 0.1 μm to 20 μm. From the viewpoint of realizing the effects of the technology disclosed herein, the average particle diameter is, for example, 0.3 μm or more, preferably 0.5 μm or more, more preferably 1 μm or more, and even more preferably 2 μm or more. From the viewpoint of increasing the degree of filling in the negative electrode active material layer 64, the average particle diameter is, for example, 15 μm or less, preferably 10 μm or less, and more preferably 5 μm or less. In this specification, the "average particle diameter" of particles refers to the particle diameter (D 50 The average particle size of the silicon-containing graphite particles may be, for example, the same as the average particle size of the porous graphite particles described above.

[0022] The silicon-containing particles contained in the silicon-containing graphite particles may be, for example, particles containing silicon and capable of functioning as a negative electrode active material. The silicon-containing particles may be, for example, silicon particles or silicon oxide particles. In this embodiment, the particle diameter of the silicon-containing particles is 1 μm or less. Although not particularly limited, from the viewpoint of containing more silicon-containing particles inside the graphite particles (for example, within the voids of the graphite particles), the particle diameter of the silicon-containing particles is generally 500 nm or less, for example, 250 nm or less, preferably 150 nm or less, more preferably 100 nm or less, and even more preferably 50 nm or less. From the same viewpoint, the particle diameter of the silicon-containing particles is, for example, 1 nm or more, preferably 3 nm or more, more preferably 5 nm or more, and even more preferably 10 nm or more. The particle diameter of the silicon-containing particles can be determined, for example, by obtaining a planar SEM image of the silicon-containing particles, randomly selecting a plurality of (e.g., 10 to 100) silicon-containing particles from the SEM image, calculating the particle diameter (circle-equivalent diameter) of each, and then calculating the arithmetic mean value thereof. Alternatively, the nominal value provided by the manufacturer or the like may be used.

[0023] Silicon-containing graphite particles can be obtained by known methods. For example, they can be obtained by mixing silicon-containing particles with a carbon precursor (e.g., petroleum pitch, coal pitch, phenolic resin, etc.), followed by carbonization and spheroidization. Alternatively, they can be obtained by mixing spherically granulated graphite substrate and silicon-containing particles in a dispersion medium, drying, and disposing the silicon-containing particles in the pores of the graphite particles as the substrate.

[0024] In this embodiment, the negative electrode active material further contains other graphite particles (hereinafter simply referred to as "other graphite particles"). The other graphite particles are, for example, graphite particles that are substantially free of silicon (preferably, graphite particles that are free of silicon). With respect to the other graphite particles, "substantially free of silicon" means that the silicon content in the other graphite particles is 1% by mass or less, preferably 0.5% by mass or less, and more preferably 0.3% by mass or less. The silicon content can be calculated by a conventional method such as ICP analysis. The other graphite particles may be, for example, artificial graphite or natural graphite. The other graphite particles may have an amorphous carbon coating layer on their surfaces. Although not particularly limited, the other graphite particles have, for example, a substantially spherical shape. In this specification, with respect to the other graphite particles, the term "substantially spherical shape" means that the other graphite particles have an average aspect ratio of 1 to 2 (preferably 1 to 1.5) based on observation with a scanning electron microscope (SEM). The average aspect ratio can be determined, for example, by obtaining a planar SEM image of the other graphite particles, randomly selecting a plurality of graphite particles (e.g., 10 to 100) from the SEM image, calculating the aspect ratio of each, and then calculating the arithmetic mean value thereof. The average particle diameter of the other graphite particles may be, for example, 5 μm to 30 μm, or may be 15 μm to 25 μm.

[0025] Although not particularly limited, from the viewpoint of achieving a high capacity and a high energy density of the lithium-ion secondary battery 100, the negative electrode active material layer 64 may contain, for example, 0.3% by mass or more of the silicon-containing graphite particles, preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 1.5% by mass or more, when the total of the silicon-containing graphite particles and the graphite particles is taken as 100% by mass. On the other hand, from the viewpoint of suppressing expansion and contraction of the negative electrode 60 during charging and discharging, the negative electrode active material layer 64 may contain, for example, 10% by mass or less of the silicon-containing graphite particles, preferably 7.5% by mass or less, and more preferably 5% by mass or less, when the total of the silicon-containing graphite particles and the graphite particles is taken as 100% by mass.

[0026] The negative electrode active material layer 64 may contain a conductive material in addition to the negative electrode active material. Examples of the conductive material that can be used include carbon black such as acetylene black (AB), carbon nanotubes such as single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT), and carbon fibers.

[0027] The ratio of the negative electrode active material to the entire negative electrode active material layer 64 is, for example, 70 mass % or more, preferably 80 mass % or more, more preferably 85 mass % to 99 mass %, and may be 90 mass % to 95 mass %. The ratio of the conductive material to the entire negative electrode active material layer 64 may be, for example, 0.01 mass % to 3 mass %.

[0028] The negative electrode active material layer 64 may contain a binder in addition to the negative electrode active material. Examples of binders include carboxymethyl cellulose (CMC), polyacrylic acid (PAA), styrene butadiene rubber (SBR), and polyvinylidene fluoride (PVDF). Among these, carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) are preferably used. The proportion of the binder when the entire negative electrode active material layer 64 is taken as 100% by mass may be, for example, 0.5% by mass to 10% by mass.

[0029] The separator 70 may be a porous sheet (film) made of a resin material such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide. The porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0030] The case 30 is, for example, an outer container that houses the electrode assembly 20 and the nonaqueous electrolyte 80. Here, the case 30 is a flat, rectangular case. As shown in FIG. 1 , the case 30 has a positive electrode terminal 42, a negative electrode terminal 44, a safety valve 36, and an inlet (not shown). The positive electrode terminal 42 is, for example, a terminal for external connection on the positive electrode side. Here, the positive electrode terminal 42 is electrically connected to the positive electrode 50 of the electrode assembly 20 via a current collector 42a. The negative electrode terminal 44 is, for example, a terminal for external connection on the negative electrode side. Here, the negative electrode terminal 44 is electrically connected to the negative electrode 60 of the electrode assembly 20 via a current collector 44a. The safety valve 36 is, for example, a thin-walled portion that is configured to release internal pressure in the case 30 when the internal pressure rises above a predetermined level. The inlet is, for example, a portion through which the nonaqueous electrolyte 80 is injected into the case 30.

[0031] By incorporating silicon into the negative electrode, it is possible to achieve even higher capacity and higher energy density of the electricity storage device. On the other hand, negative electrodes containing silicon undergo large expansion and contraction during charging and discharging of the electricity storage device. To reduce this expansion and contraction, for example, small particle size silicon is used. However, when small particle size silicon is used, silicon particles may aggregate together within the negative electrode. When such aggregation occurs, for example, the gaps between the silicon particles become smaller, making contact between the silicon particles and the electrolyte more difficult. This may result in, for example, in insufficient formation of an SEI coating on the silicon particles, which may result in a decrease in output characteristics in low-temperature environments and the durability of the electricity storage device. Therefore, the present inventors have investigated the composition of non-aqueous electrolytes.

[0032] The nonaqueous electrolyte 80 contains, for example, a nonaqueous solvent and a supporting salt. In this example, the nonaqueous solvent contains ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), a carboxylic acid ester having four or less carbon atoms, and fluoroethylene carbonate (FEC).

[0033] The carboxylic acid ester is not particularly limited, as long as it has, for example, four or less carbon atoms and can achieve the effects of the technology disclosed herein. For example, a carboxylic acid ester having a lower viscosity than EC, DMC, and EMC can be preferably used. Examples of the carboxylic acid ester include methyl acetate, ethyl acetate, and methyl propionate. The above-mentioned types of carboxylic acid esters may be used alone or in combination of two or more. In this embodiment, the volume of the carboxylic acid ester in the nonaqueous electrolyte solution 80 is 1% by volume to 30% by volume, where the total of EC, DMC, EMC, and carboxylic acid ester is 100% by volume.

[0034] In this embodiment, FEC refers to a compound in which some of the hydrogen (H) atoms in EC have been substituted with fluorine (F) atoms. The type of FEC is not limited as long as the effects of the technology disclosed herein can be achieved. Examples of FEC that can be preferably used include 4-fluoroethylene carbonate; difluoroethylene carbonates such as 4,4-difluoroethylene carbonate and 4,5-difluoroethylene carbonate; 4,4,5-trifluoroethylene carbonate; and 4,4,5,5-tetrafluoroethylene carbonate. Of these, 4-fluoroethylene carbonate is preferably used as FEC. The above-mentioned types of FEC may be used alone or in combination of two or more.

[0035] In this embodiment, the content of FEC in the nonaqueous electrolyte solution 80 is 0.1% by mass to 5% by mass or less relative to the total nonaqueous solvent. In the technology disclosed herein, FEC functions, for example, as a film-forming agent. From the viewpoint of better realizing this function, the content of FEC in the total nonaqueous solvent is, for example, 0.3% by mass or more, preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more. On the other hand, from the viewpoint of suppressing an increase in resistance due to the formation of a film and suppressing gas generation, the content of FEC in the total nonaqueous solvent is preferably 4.5% by mass or less, more preferably 4% by mass or less.

[0036] From the viewpoint of enhancing the stability of the nonaqueous electrolyte solution 80 and realizing more stable charge and discharge, when the total of EC, DMC, EMC, and the carboxylic acid ester is taken as 100 volume %, the total volume percentage of EC and DMC is preferably 50 volume % or more, and more preferably 55 volume % or more. From the viewpoint of including components sufficient to realize the effects of the technology disclosed herein, the total volume percentage of the above-mentioned EC and DMC is, for example, 90 volume % or less, preferably 80 volume % or less, and more preferably 70 volume % or less. From the viewpoint of imparting an appropriate viscosity to the nonaqueous electrolyte solution 80, when the total of EC, DMC, EMC, and the carboxylic acid ester is taken as 100 volume %, the volume percentage of EMC is, for example, 5 volume % to 45 volume %, and preferably 10 volume % to 40 volume %.

[0037] In addition to the above-mentioned components, the nonaqueous solvent may contain other organic solvents, such as various carbonates, ethers, esters, nitriles, sulfones, and lactones, that are used in this type of application, as long as the effects of the technology disclosed herein can be achieved. In this case, the volume ratio of the other organic solvents to the total nonaqueous solvent is approximately 20% by volume or less, for example, 10% by volume, preferably 5% by volume or less, more preferably 3% by volume or less, and even more preferably 1% by volume or less, and the closer to 0% by volume the better. The nonaqueous electrolyte 80 may further contain a supporting salt. Examples of supporting salts include lithium salts such as LiPF6, LiBF4, and LiClO4. The concentration of the supporting salt may be, for example, 0.7 mol / L to 1.4 mol / L. The nonaqueous electrolyte 80 may contain additives used in this type of application, as needed. The additives may include, for example, film-forming agents such as LiB(C2O4)2 (LiBOB) and LiBF2 (C2O4); gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB); thickeners; and the like.

[0038] The lithium ion secondary battery 100 can be used for a variety of purposes. Suitable applications include a driving power source mounted on vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). The lithium ion secondary battery 100 can be used, for example, as a storage battery for small-sized power storage devices. The lithium ion secondary battery 100 can also be used, for example, in the form of an assembled battery in which a plurality of batteries are connected in series and / or parallel.

[0039] As described above, the lithium ion secondary battery 100 is an electricity storage device including a positive electrode 50, a negative electrode 60, and a non-aqueous electrolyte solution 80. The negative electrode 60 contains silicon-containing graphite particles, which are graphite particles containing silicon, as a negative electrode active material. The non-aqueous electrolyte solution contains EC, DMC, EMC, a carboxylic acid ester having four or less carbon atoms, and FEC as a non-aqueous solvent. When the total of EC, DMC, EMC, and the carboxylic acid ester is taken as 100% by volume, the volume ratio of the carboxylic acid ester is 1% by volume to 30% by volume. The content of FEC relative to the entire non-aqueous solvent is 0.1% by mass to 5% by mass.

[0040] In the lithium-ion secondary battery 100, the negative electrode 60 contains silicon-containing graphite particles as the negative electrode active material, thereby achieving high capacity and high energy density. The nonaqueous electrolyte 80 contains EC, DMC, and EMC as nonaqueous solvents, thereby achieving stable charge and discharge. The nonaqueous electrolyte 80 contains a carboxylic acid ester having four or fewer carbon atoms as the nonaqueous solvent, thereby reducing the viscosity of the nonaqueous electrolyte 80. This allows the nonaqueous electrolyte 80 to better contact the silicon component and graphite component contained in the silicon-containing graphite particles. The nonaqueous electrolyte 80 further contains FEC as a nonaqueous solvent. FEC functions, for example, as a film-forming agent for forming an SEI film. Therefore, by including FEC in the nonaqueous electrolyte 80, an SEI film can be more efficiently formed on the negative electrode active material. In the lithium-ion secondary battery 100, when the total of EC, DMC, EMC, and carboxylic acid ester is taken as 100% by volume, the volume of the carboxylic acid ester is 1% to 30% by volume, and the content of FEC relative to the total non-aqueous solvent is 0.1% to 5% by mass. This appropriately reduces the viscosity of the non-aqueous electrolyte 80 even in a low-temperature environment, allowing the non-aqueous electrolyte 80 to sufficiently contact the silicon component and graphite component of the negative electrode active material. This makes it possible to suppress an increase in resistance in a low-temperature environment. Furthermore, it makes it possible to efficiently form an SEI coating. This makes it possible to suppress a decrease in capacity retention due to charge-discharge cycles.

[0041] The carboxylic acid ester may be at least one of methyl acetate and methyl propionate, which can better realize the effects of the technology disclosed herein.

[0042] When the total of EC, DMC, EMC, and carboxylic acid ester is taken as 100% by volume, the total volume percentage of EC and DMC may be 50% by volume or more. The volume percentage of EMC may be 10% by volume to 40% by volume. This makes it possible to stabilize the charge and discharge of the lithium ion secondary battery 100 and improve the durability of the lithium ion secondary battery 100.

[0043] The silicon-containing graphite particles may be composed of graphite particles having voids and silicon-containing particles disposed within the voids. The voids in the graphite particles can mitigate the expansion and contraction of the silicon-containing particles that accompany the charging and discharging of the lithium-ion secondary battery 100. This allows the effects of the technology disclosed herein to be more effectively realized.

[0044] The particle diameter of the silicon-containing particles may be 1 μm or less. This allows the silicon-containing particles to be contained in a larger proportion of the silicon-containing graphite particles. This improves the conductivity of the negative electrode active material while achieving high output. In addition, this configuration allows the contact area between the silicon-containing particles and the nonaqueous electrolyte solution 80 to be increased. This allows the effects of the technology disclosed herein to be more effectively achieved.

[0045] The lithium ion secondary battery 100 may further contain graphite particles that are substantially free of silicon as the negative electrode active material, thereby improving the conductivity of the negative electrode active material layer 64.

[0046] Test examples relating to the technology disclosed herein will be described below, but it is not intended that the technology disclosed herein be limited to those shown in the following test examples.

[0047] [Manufacturing test cells] <Examples 1 to 6, Comparative Examples 1 and 2> LiNi as a positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (LNCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were prepared. These were combined with N-methylpyrrolidone (NMP) as a solvent in a mass ratio of LNCM:AB:PVdF = 92:5:3 and kneaded using a stirring granulator to prepare a positive electrode slurry. The positive electrode slurry was applied to both sides of a 15 μm thick aluminum foil and dried. The foil was then pressed to the specified thickness and processed to the specified dimensions to obtain a positive electrode.

[0048] Graphite particles (graphite particles substantially free of silicon) and silicon-containing graphite particles were prepared as negative electrode active materials. The graphite particles had an average particle diameter of 22 μm. The silicon-containing graphite particles were composed of void-containing graphite particles with an average particle diameter of 3 μm and silicon particles disposed within the voids. The silicon particles had a particle diameter of 30 nm. Acetylene black (AB) was prepared as a conductive material. Carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) were prepared as binders. The prepared materials and water as a dispersion medium were kneaded using a stirrer to prepare a negative electrode slurry. The negative electrode slurry was prepared so that the mass ratio of graphite particles / silicon-containing graphite particles / AB / CMC / SBR was 93:2:1:1:3. The negative electrode slurry was applied to both sides of a copper foil with a thickness of 10 μm, dried, and then pressed to a predetermined thickness and processed to a predetermined size to obtain a negative electrode.

[0049] A separator was prepared using a porous polyolefin sheet (20 μm thick) with a three-layer structure of PP / PE / PP, with a heat-resistant layer (4 μm thick). Leads were attached to each of the positive and negative electrodes, and the electrodes were stacked with the separator interposed between them to produce an electrode assembly. The electrode assembly was inserted into an aluminum laminate sheet exterior, a nonaqueous electrolyte was poured into it, and the opening of the exterior was sealed to produce a test cell for each example. The composition of the nonaqueous electrolyte was as shown in the corresponding column in Table 1. FEC was 4-fluoroethylene carbonate. LiPF6 was dissolved in the nonaqueous electrolyte as a supporting electrolyte at a concentration of 1 mol / L. Note that "-" in Table 1 indicates that the corresponding component was not contained. "MA" in Table 1 stands for methyl acetate. "MP" in Table 1 stands for methyl propionate.

[0050] [Table 1]

[0051] [Initial capacity measurement] The test cell was subjected to one charge-discharge cycle in an environment of 25°C, in which constant current charging (CC charging) was performed at a charge rate of 0.05 C until the voltage between the positive and negative electrodes reached 4.2 V, and then constant current discharging (CC discharging) was performed at a discharge rate of 0.05 C until the voltage between the positive and negative electrodes reached 2.5 V. The discharge capacity at this time was measured and used as the initial capacity.

[0052] [Low temperature resistance evaluation] The initial capacity obtained as described above was taken as SOC 100%, and the test cell was adjusted to SOC 50% in a thermostatic chamber at 25°C. The test cell in this state was placed in a thermostatic chamber at -10°C and discharged for 10 seconds at current values ​​of 1C, 1.5C, 2C, and 3C, and the battery voltage was measured in each case. The current values ​​and battery voltage were plotted to determine the IV characteristics during discharge. The IV resistance during discharge was calculated from the slope of the resulting line. The ratio of each example to Comparative Example 1, which was taken as 100%, was calculated. The results are shown in the corresponding columns in Table 2.

[0053] [Cycle characteristic evaluation] The test cell was placed in a thermostatic chamber at 25°C and subjected to CC charging at 0.33 C up to 4.14 V, followed by CV charging until the current reached 0.1 C, and then fully charged. It was then CC discharged at 0.33 C down to 3 V. This cycle constitutes one charge-discharge cycle, and 200 charge-discharge cycles were performed. The following formula (X): Capacity retention rate (%) = (discharge capacity after 200 cycles / initial capacity) x 100 (X) The capacity retention rate after 200 cycles was calculated for each test cell based on the above formula. The results are shown in the corresponding column in Table 2.

[0054] [Gas generation rate evaluation] The volume of each test cell was measured using Archimedes' method. Specifically, the test cell was immersed in water at 25°C, and the volume of the test cell was measured based on the change in mass. The volume was measured before and after the cycle test, and the volume was calculated using the following formula (Y): Gas generation rate (%) = [{(volume after 200 cycles) - (initial volume)} / (initial volume)] x 100 (Y) The gas generation rate after 200 charge / discharge cycles was calculated for each test cell based on the above formula. The results are shown in the corresponding columns in Table 2.

[0055] [Table 2]

[0056] As described above, in the test cells of Examples 1 to 6, the negative electrode contained silicon-containing graphite particles as the negative electrode active material. The nonaqueous electrolyte contained EC, DMC, EMC, a carboxylic acid ester having 4 or less carbon atoms, and FEC as the nonaqueous solvent. When the total of EC, DMC, EMC, and the carboxylic acid ester was taken as 100% by volume, the volume of the carboxylic acid ester was 1% to 30% by volume. The content of FEC relative to the total nonaqueous solvent was 0.1% to 5% by mass. The results shown in Table 2 demonstrate that this configuration can better suppress an increase in resistance in a low-temperature environment in an electricity storage device using silicon-containing particles as the negative electrode active material.

[0057] The techniques disclosed herein may include the techniques described in the following sections. Section 1: An electricity storage device comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte solution, the negative electrode contains silicon-containing graphite particles, which are graphite particles containing silicon, as a negative electrode active material; the non-aqueous electrolyte contains, as a non-aqueous solvent, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, a carboxylic acid ester having four or less carbon atoms, and fluoroethylene carbonate; a volume ratio of the carboxylic acid ester is 1% by volume or more and 30% by volume or less, when the total of the ethylene carbonate, the dimethyl carbonate, the ethyl methyl carbonate, and the carboxylic acid ester is 100% by volume; The content of the fluoroethylene carbonate relative to the total amount of the non-aqueous solvent is 0.1% by mass or more and 5% by mass or less. Section 2: Item 2. The electricity storage device according to item 1, wherein the carboxylic acid ester is at least one of methyl acetate and methyl propionate. Section 3: When the total of the ethylene carbonate, the dimethyl carbonate, the ethyl methyl carbonate, and the carboxylic acid ester is taken as 100% by volume, the total volume ratio of the ethylene carbonate and the dimethyl carbonate is 50% by volume or more, Item 3. The electricity storage device according to item 1 or 2, wherein the volume ratio of the ethyl methyl carbonate is 10% by volume or more and 40% by volume or less. Section 4: 4. The electricity storage device according to any one of items 1 to 3, wherein the silicon-containing graphite particles include graphite particles having voids and silicon-containing particles disposed in the voids. Section 5: 5. The electricity storage device according to any one of items 1 to 4, wherein the silicon-containing particles have a particle diameter of 1 μm or less. Item 6: Item 6. The electricity storage device according to any one of items 1 to 5, further comprising graphite particles that are substantially free of silicon as the negative electrode active material.

[0058] Although the embodiments of the technology disclosed herein have been described above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above. [Explanation of symbols]

[0059] 20 Electrode body 30 cases 42 Positive terminal 44 Negative terminal 50 positive electrode 60 negative electrode 70 Separator 80 Nonaqueous electrolyte 100 Lithium-ion secondary battery

Claims

1. An electricity storage device comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte solution, the negative electrode contains silicon-containing graphite particles, which are graphite particles containing silicon, as a negative electrode active material; the non-aqueous electrolyte contains, as a non-aqueous solvent, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, a carboxylic acid ester having four or less carbon atoms, and fluoroethylene carbonate; a volume ratio of the carboxylic acid ester is 1% by volume or more and 30% by volume or less, when the total amount of the ethylene carbonate, the dimethyl carbonate, the ethyl methyl carbonate, and the carboxylic acid ester is 100% by volume; The content of the fluoroethylene carbonate relative to the total amount of the non-aqueous solvent is 0.1% by mass or more and 5% by mass or less.

2. The electricity storage device according to claim 1 , wherein the carboxylic acid ester is at least one of methyl acetate and methyl propionate.

3. When the total of the ethylene carbonate, the dimethyl carbonate, the ethyl methyl carbonate, and the carboxylic acid ester is taken as 100% by volume, the total volume ratio of the ethylene carbonate and the dimethyl carbonate is 50% by volume or more, The electricity storage device according to claim 1 or 2, wherein a volume ratio of the ethyl methyl carbonate is 10% by volume or more and 40% by volume or less.

4. The electricity storage device according to claim 1 or 2, wherein the silicon-containing graphite particles include graphite particles having voids and silicon-containing particles disposed in the voids.

5. The electricity storage device according to claim 1 , wherein the silicon-containing particles have a particle diameter of 1 μm or less.

6. The electricity storage device according to claim 1 or 2, wherein the negative electrode active material further contains graphite particles that are substantially free of silicon.

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