Nonaqueous electrolyte secondary battery

Abstract

This nonaqueous electrolyte secondary battery is provided with a positive electrode, a negative electrode, and a nonaqueous electrolyte, the negative electrode containing a silicon-containing carbon material, the silicon-containing carbon material containing an amorphous carbon phase and a silicon phase dispersed in the amorphous carbon phase, and the nonaqueous electrolyte containing a nonaqueous solvent and a salt dissolved in the nonaqueous solvent. The nonaqueous solvent contains fluoroethylene carbonate and a fluorine-containing carboxylic acid ester.

Description

Technical Field

[0001] The present invention relates to a non-aqueous electrolyte secondary battery. Background Art

[0002] Patent Document 1 proposes "a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte including a solvent and a solute, the non-aqueous electrolyte including a fluorinated compound represented by R1-C=O-OR2 (wherein R1 is a fluorinated alkyl group or alkoxy group, and R2 is a methyl group) and a chain carboxylic acid ester, and the content ratio of the fluorinated compound in the solvent being in the range of 5 to 30% by volume".

[0003] Patent Document

[0004] Patent Document 1: Japanese Patent Application Laid-Open No. 2014-67490 Summary of the Invention

[0005] Problems to be Solved by the Invention

[0006] By using a silicon-containing material as a negative electrode active material, a battery with a high capacity density can be achieved. On the other hand, the silicon-containing material expands and contracts significantly during charge and discharge, and the film (Solid Electrolyte Interface: SEI) derived from the non-aqueous electrolyte formed on the surface of the silicon-containing material is continuously damaged as the silicon-containing material expands and contracts. In the region where the SEI has been damaged, the non-aqueous electrolyte decomposes again to form an SEI. By repeating the formation and destruction of this film, the internal resistance of the battery gradually increases. On the other hand, the higher the content ratio of the silicon phase contained in the silicon-containing material, the more beneficial it is for increasing the capacity.

[0007] Means for Solving the Problems

[0008] One aspect of the present invention relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, the negative electrode including a silicon-containing carbon material, the silicon-containing carbon material including an amorphous carbon phase and a silicon phase dispersed in the amorphous carbon phase, the non-aqueous electrolyte including a non-aqueous solvent and a salt dissolved in the non-aqueous solvent, and the non-aqueous solvent including fluoroethylene carbonate and a fluorinated carboxylic acid ester.

[0009] Advantages of the Invention

[0010] According to the present invention, even if the negative electrode includes a silicon-containing carbon material as the silicon-containing material, an increase in the internal resistance can be suppressed.

[0011] The novel features of the present invention are set forth in the appended claims, and both the constitution and the content of the present invention, together with other objects and features of the present invention, will be further fully understood from the following detailed description with reference to the accompanying drawings. Brief Description of the Drawings

[0012] Figure 1 It is a longitudinal sectional view of a secondary battery according to an embodiment of the present invention. Detailed Embodiment

[0013] Hereinafter, embodiments of the present invention will be described by way of example, but the present invention is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may sometimes be exemplified, but other numerical values, materials, etc. may also be applied as long as the effects of the present invention can be obtained. In addition, in the present invention, constituent elements other than the characteristic parts may be applied to the constituent elements of known secondary batteries. When "the range of numerical value A to numerical value B" is mentioned in this specification, this range includes numerical value A and numerical value B. For example, when "A to B mol%" is mentioned, it is synonymous with "not less than A mol% and not more than B mol". In the following description, if the lower limit and the upper limit of the relevant numerical values of specific physical properties or conditions are exemplified, any one of the exemplified lower limits and any one of the exemplified upper limits can be arbitrarily combined as long as the lower limit is not above the upper limit. When multiple materials are exemplified, one of them can be selected and used alone, or two or more of them can be used in combination.

[0014] In addition, the present invention includes a combination of matters described in two or more claims arbitrarily selected from a plurality of claims described in the appended claims. That is, as long as no technical contradiction occurs, matters described in two or more claims arbitrarily selected from a plurality of claims described in the appended claims can be combined.

[0015] Non-aqueous electrolyte secondary batteries include lithium-ion secondary batteries that use at least a material capable of reversibly occluding and releasing lithium ions as a negative electrode active material, solid batteries including gel electrolytes, and the like.

[0016] In addition, in this specification, the so-called "internal resistance" means the direct current resistance value (DCIR). According to the present invention, the DCIR increase rate during charge and discharge cycles of a non-aqueous electrolyte secondary battery can be suppressed to be small, for example, in an environment of 25°C.

[0017] The non-aqueous electrolyte secondary battery of the present invention includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. A separator is usually disposed between the positive electrode and the negative electrode. The non-aqueous electrolyte usually has lithium ion conductivity.

[0018] The negative electrode contains a silicon-containing carbon material as the negative electrode active material. The silicon-containing carbon material is a material comprising an amorphous carbon phase and a silicon phase dispersed within the amorphous carbon phase. Hereinafter, such a silicon-containing carbon material is also referred to as "Si / AmoC". Since the negative electrode active material contains a silicon phase, a non-aqueous electrolyte secondary battery can achieve a high capacity. The more silicon phase is contained, the more beneficial it is for high-capacity. In addition, the non-aqueous electrolyte contains fluoroethylene carbonate (hereinafter also referred to as "FEC") and a fluorinated carboxylic acid ester (hereinafter also referred to as "carboxylic acid ester (F)").

[0019] Although Si / AmoC is a silicon-containing material, when used in combination with FEC and carboxylic acid ester (F), the increase rate of DCIR during charge and discharge cycling of a rechargeable non-aqueous electrolyte secondary battery can be suppressed to a small value. It is considered that this effect is brought about by the formation of a mixed film (SEI) on the surface of Si / AmoC by FEC and carboxylic acid ester (F) that is not easily destroyed. It is considered that a large amount of LiF is contained in such an SEI. If the non-aqueous electrolyte further contains a cyclic anhydride, the effect of suppressing the increase in DCIR during repeated charge and discharge cycling will be further enhanced.

[0020] On the other hand, when using a material comprising a SiO2 phase and a silicon phase dispersed within the SiO2 phase (hereinafter also referred to as "SiOx"), metallic Si, Si alloy, etc. as the negative electrode active material, if carboxylic acid ester (F) is used, the increase rate of DCIR will instead become larger. It is considered that the reason is that hydrofluoric acid (HF) is generated due to the decomposition of carboxylic acid ester (F), thereby deteriorating SiOx, metallic Si, Si alloy, etc.

[0021] Amorphous carbon does not react with HF, and the silicon phase dispersed within the amorphous carbon phase is coated with amorphous carbon. Therefore, it is considered that when using Si / AmoC among the silicon-containing materials, the effect of suppressing the increase in DCIR brought about by carboxylic acid ester (F) will become particularly obvious.

[0022] The specificity of the above Si / AmoC can increase the amount of silicon phase dispersed within the amorphous carbon phase and also increase the mass ratio of Si / AmoC in the negative electrode active material. In other words, since the deterioration of Si / AmoC is suppressed, even if the usage amount is large, there is little concern about an increase in the decomposition reaction of the non-aqueous electrolyte due to the expansion and contraction of the silicon phase.

[0023] The average size of the silicon phase dispersed within the amorphous carbon phase is preferably controlled to be a small size. The larger the size of the silicon phase, the more likely it is to generate cracks in the amorphous carbon phase as the silicon phase expands and contracts. At this time, HF can invade from the cracks, so that HF reacts with the silicon phase. In other words, the smaller the size of the silicon phase, the less likely it is to generate cracks in the amorphous carbon phase, the higher the shielding effect of HF, and it is easy to exert the defensive property against HF brought about by the amorphous carbon phase.

[0024] Hereinafter, the components of the non-aqueous electrolyte secondary battery of the present invention will be further specifically described.

[0025] [Negative electrode]

[0026] The negative electrode contains a negative electrode active material. The negative electrode generally includes a negative electrode current collector and a layered negative electrode mixture (hereinafter referred to as the negative electrode mixture layer) held by the negative electrode current collector. The negative electrode mixture layer can be formed by coating a negative electrode slurry on the surface of the negative electrode current collector and drying it. The negative electrode slurry is obtained by dispersing the constituent components of the negative electrode mixture in a dispersion medium. The dried coating film can also be calendered as needed. The dispersion medium used for the negative electrode slurry is not particularly limited, and examples thereof include water, alcohol, N-methyl-2-pyrrolidone (NMP), and a mixed solvent thereof.

[0027] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain a binder, a thickener, a conductive agent, etc. as optional components.

[0028] (Negative electrode active material)

[0029] The negative electrode active material contains at least a Si-containing carbon material (Si / AmoC). The Si-containing carbon material may contain a fine lithium alloy in a state after occluding lithium ions. The negative electrode active material may further contain other materials capable of electrochemically occluding and releasing lithium ions. A carbonaceous material can be cited as an example of such a material.

[0030] In order to obtain good cycle characteristics and high capacity in a balanced manner, it is desirable to use Si / AmoC and a carbonaceous material in combination. For example, compared with SiO x In the negative electrode active material, the mass ratio of Si / AmoC can be set relatively large. The ratio of Si / AmoC in the total of Si / AmoC and the carbonaceous material is, for example, 5% by mass or more, can be 7% by mass or more, can also be 10% by mass or more, or 15% by mass or more. When more importance is attached to improving the cycle characteristics, the ratio of Si / AmoC in the total of Si / AmoC and the carbonaceous material is, for example, 50% by mass or less, can be 40% by mass or less, can be 30% by mass or less, or can also be 20% by mass or less. The ratio of Si / AmoC in the total of Si / AmoC and the carbonaceous material is, for example, 5% by mass or more and 50% by mass or less, can be 7% by mass or more and 40% by mass or less, can be 10% by mass or more and 30% by mass or less, or can also be 15% by mass or more and 20% by mass or less.

[0031] 《Carbonaceous material》

[0032] Examples of the carbonaceous material include graphite, graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). The carbonaceous material may be used alone or in combination of two or more. Among them, from the viewpoint of excellent charge-discharge stability and less irreversible capacity, the carbonaceous material is preferably crystalline carbon. Examples of the crystalline carbon include graphite materials such as natural graphite, artificial graphite, and graphitized mesophase carbon particles. The so-called crystalline carbon generally refers to the average plane spacing d of the (002) plane measured by X-ray diffraction method 002 of a carbonaceous material of 0.340 nm or less (for example, 0.3354 nm or more and 0.340 nm or less).

[0033] 《Si / AmoC》

[0034] Si / AmoC contains an amorphous carbon phase and a silicon phase or silicon particles dispersed in the amorphous carbon phase. The so-called amorphous carbon generally refers to the average plane spacing d of the (002) plane measured by X-ray diffraction method 002 of a carbon material greater than 0.34 nm. The amorphous carbon phase has lithium ion conductivity, so lithium ions can move between the silicon phase and the non-aqueous electrolyte. The amorphous carbon constituting the amorphous carbon phase can be, for example, hard carbon, soft carbon, or carbon other than them.

[0035] The amorphous carbon can be obtained, for example, by sintering a carbon source in an inert gas atmosphere and pulverizing the obtained sintered body. In addition, Si / AmoC can be obtained, for example, by mixing a carbon source and Si particles, crushing and stirring the mixture with a stirrer such as a ball mill, and then firing the mixture in an inert gas atmosphere. As the carbon source, for example, commercially available graphitizable carbon (soft carbon), carboxymethyl cellulose (CMC), polyvinylpyrrolidone, cellulose, saccharides such as sucrose, or water-soluble resins can be used. When mixing the carbon source and Si particles, for example, the carbon source and Si particles can also be dispersed in a dispersion medium such as alcohol. Alternatively, Si / AmoC can also be generated by a gas-phase reaction of a silicon source and a carbon source using the CVD method.

[0036] For example, compared with SiO xIn contrast, the content rate of the silicon phase contained in Si / AmoC can be set higher. The content rate of the silicon phase contained in Si / AmoC is, for example, 40% by mass or more, can be 50% by mass or more, and can also be 55% by mass or more. In addition, from the viewpoint of suppressing the influence of the expansion and contraction of the silicon phase as much as possible, the content rate of the silicon phase contained in Si / AmoC is, for example, 80% by mass or less, can be 70% by mass or less, and can also be 65% by mass or less. If it is within the above range, it is easy to achieve sufficient high capacity of the negative electrode, and the cycle characteristics are also easy to improve. The content rate of the silicon phase contained in Si / AmoC is, for example, 40% by mass or more and 80% by mass or less, can be 50% by mass or more and 70% by mass or less, and can also be 55% by mass or more and 65% by mass or less.

[0037] The content rate of the silicon phase contained in Si / AmoC can be measured by ICP (Inductively Coupled Plasma) optical emission spectrometry. Si, C, and O are included as main elements in Si / AmoC. It is considered that Si is included as metallic Si or SiO2, C is included as C, and O is included as SiO2. Therefore, when the amounts of Si, C, and O detected by ICP are a (mol), b (mol), and c (mol) respectively, the amount of substance x of metallic Si can be set as x = a - c / 2. Based on the above values of x, a, b, and c, the content rate of the silicon phase contained in Si / AmoC can be calculated.

[0038] In terms of easily ensuring sufficient reactivity between the silicon phase and lithium ions, the average particle size of Si / AmoC is, for example, desired to be 1 μm or more, and can also be 2 μm or more. In addition, in terms of easily alleviating the influence of the expansion and contraction of the silicon phase, the average particle size of Si / AmoC is desired to be 18 μm or less, and can also be 15 μm or less. The average particle size of Si / AmoC can be 1 μm or more and 18 μm or less, and can also be 2 μm or more and 15 μm or less.

[0039] The average particle size of Si / AmoC means the particle size (volume average particle size) at which the volume cumulative value is 50% in the particle size distribution measured by the laser diffraction scattering method. As the measuring device, for example, "LA-750" manufactured by HORIBA, Ltd. can be used.

[0040] The average particle size of Si / Amo-C can also be measured by disassembling the non-aqueous electrolyte secondary battery and observing the cross-section of the negative electrode mixture layer of the taken-out negative electrode using SEM or TEM. At this time, the average particle size is obtained by arithmetically averaging the maximum particle sizes of any 100 particles.

[0041] Si / AmoC can be removed from the battery by the following method. First, disassemble the battery in a fully discharged state to remove the negative electrode, then wash the negative electrode with ethyl methyl carbonate or dimethyl carbonate anhydrous to remove the non-aqueous electrolyte components. The negative electrode has a negative electrode current collector and a negative electrode mixture layer supported on its surface. Therefore, the negative electrode mixture layer is peeled off from the negative electrode current collector and pulverized in a mortar to obtain a sample powder. Next, dry the sample powder in a dry atmosphere for 1 hour, and then immerse it in slightly boiling 6M hydrochloric acid for 10 minutes to remove components other than Si / AmoC such as binders. Then, wash the sample powder with ion-exchanged water, filter it, and dry it at 200°C for 1 hour, whereby the carbonaceous material and Si / AmoC can be isolated. In addition, the so-called fully discharged state is a state where the depth of discharge (DOD) is 90% or more (the state of charge (SOC) is 10% or less). The separation of the carbonaceous material and Si / AmoC can be carried out by a sieve or centrifugation.

[0042] 《Silicon Phase》

[0043] The silicon phase is the phase of silicon (Si) monomer, and lithium ions are occluded and released repeatedly with the charge and discharge of the battery. Through the Faraday reaction participated by the silicon phase, capacity can be exhibited.

[0044] The silicon phase is usually dispersed in the amorphous carbon phase in a granular form. The capacity of the silicon phase is large, and the degree of expansion and contraction during charge and discharge is also large. Therefore, it is desirable that the average size of the granular silicon phase is small. For example, the average particle diameter of the silicon phase is preferably 20 nm or less, can be less than 20 nm, or can be 15 nm or less. By refining the silicon phase in this way, the volume change of Si / AmoC during charge and discharge becomes smaller, the generation of cracks in the amorphous carbon phase can be reduced, and the defense against HF brought by the amorphous carbon can be improved.

[0045] The average particle diameter of the silicon phase can be measured by using the cross-sectional image of Si / AmoC obtained by a transmission electron microscope (TEM). Specifically, the average particle diameter of the silicon phase is obtained by arithmetically averaging the maximum particle diameters of any 100 silicon phases.

[0046] The silicon phase can be composed of multiple microcrystals. The microcrystal size of the silicon phase is very small, preferably 50 nm or less. If the microcrystal size of the silicon phase is as small as this size, the volume change caused by the expansion and contraction of the silicon phase during charge and discharge can be further reduced. The lower limit value of the microcrystal size of the silicon phase is not particularly limited, for example, it is 1 nm or more. The microcrystal size of the silicon phase is calculated from the half-width of the diffraction peak attributed to the (111) plane in the X-ray diffraction pattern of the silicon phase (monomer Si) according to the Scherrer equation.

[0047] (Negative Electrode Binder)

[0048] As the negative electrode binder, a resin material can be used, for example. Examples of the binder include fluororesin, polyolefin resin, polyamide resin, polyimide resin, acrylic resin, vinyl resin, rubber-like materials (such as styrene-butadiene copolymer (SBR)), etc. The binder can be used alone as one kind, or two or more kinds can be used in combination.

[0049] (Thickener)

[0050] Examples of the thickener include cellulose derivatives such as cellulose ether. Examples of the cellulose derivatives include carboxymethyl cellulose (CMC) and its modified products, methyl cellulose, etc. The thickener can be used alone as one kind, or two or more kinds can be used in combination.

[0051] (Negative electrode conductive material)

[0052] Examples of the negative electrode conductive material include carbon nanotubes (CNT), carbon fibers other than CNT, and conductive particles (such as carbon black, graphite), etc.

[0053] (Negative electrode current collector)

[0054] As the negative electrode current collector, a metal foil can be used, for example. The negative electrode current collector can also be porous. Examples of the material of the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy, etc. The thickness of the negative electrode current collector is not particularly limited. For example, it is 1 to 50 μm, and it can also be 5 to 30 μm.

[0055] [Positive electrode]

[0056] The positive electrode contains a positive electrode active material. The positive electrode usually includes a positive electrode current collector and a layered positive electrode mixture (hereinafter referred to as "positive electrode mixture layer") held by the positive electrode current collector. The positive electrode mixture layer can be formed by coating a positive electrode slurry on the surface of the positive electrode current collector and drying it. The positive electrode slurry is obtained by dispersing the constituent components of the positive electrode mixture in a dispersion medium. If necessary, the dried coating film can be calendered. The positive electrode mixture contains a positive electrode active material as an essential component, and can contain a binder, a thickener, etc. as optional components. The dispersion medium used for the positive electrode slurry is not particularly limited, and examples include water, alcohol, NMP, and their mixed solvents, etc.

[0057] (Positive electrode active material)

[0058] The positive electrode active material only needs to be a material that can be used as the positive electrode active material of a non-aqueous electrolyte secondary battery (such as a lithium ion secondary battery). However, from the viewpoint of high capacity, it is desirable to contain a lithium transition metal composite oxide (hereinafter also referred to as "composite oxide N"), and this lithium transition metal composite oxide contains at least nickel as a transition metal. The ratio of the composite oxide N in the positive electrode active material is, for example, 70% by mass or more, and can be 90% by mass or more, or can also be 95% by mass or more.

[0059] From the viewpoint of ensuring high capacity, the content rate of Ni in the composite oxide N relative to metals other than lithium may be 80 atomic % or more, may be 90 atomic % or more, or may be 95 atomic % or more. From the viewpoint of structural stability, the content rate of Ni in the composite oxide N relative to metals other than lithium may be 99 atomic % or less, may be 98 atomic % or less, or may be 97 atomic % or less.

[0060] For example, the composite oxide N may be: a lithium transition metal composite oxide having a layered rock salt type structure and containing Ni and at least one selected from the group consisting of Co, Mn, and Al. Hereinafter, the following lithium transition metal composite oxide will also be referred to as "composite oxide HN": a lithium transition metal composite oxide having a layered rock salt type structure, containing Ni and at least one selected from the group consisting of Co, Mn, and Al, and having a ratio of Ni to metal elements other than Li of 80 atomic % or more. The ratio of the composite oxide HN in the composite oxide N used as a positive electrode active material is, for example, 90 mass % or more, may be 95 mass % or more, or may be 100%.

[0061] The higher the ratio of Ni, the more lithium ions can be extracted from the composite oxide HN during charging, and the capacity can be increased. However, in the composite oxide HN with increased capacity, Ni has a tendency to have a higher valence. In addition, when the Ni ratio increases, the ratio of other elements relatively decreases. At this time, the crystal structure is likely to become unstable, and side reactions are likely to occur with repeated charge and discharge. On the surface of the particles of the composite oxide HN with a high Ni content, Ni is likely to change into a crystal structure that is difficult to reversibly occlude and release lithium ions.

[0062] In the non-aqueous secondary battery of the present invention, although the composite oxide HN with a high Ni content is used for the positive electrode in this way, and a silicon-containing material (Si / AmoC) is used for the negative electrode and used in combination with FEC and carboxylic acid ester (F), an increase in DCIR can be suppressed.

[0063] Co, Mn, and Al contribute to the stabilization of the crystal structure of the composite oxide HN with a high Ni content. However, from the viewpoint of reducing manufacturing costs, it is desirable that the Co content rate be as small as possible. The composite oxide HN with a small Co content (or no Co) may also contain Mn and Al.

[0064] The composite oxide HN can be represented, for example, by the formula: Li α Ni (1-x1-x2-y-z) Co x1 Mn x2 Al y M z O 2+βIt is represented by. Element M is an element other than Li, Ni, Co, Mn, Al, and oxygen.

[0065] In the above formula, α representing the atomic ratio of lithium is, for example, 0.95 ≤ α ≤ 1.05. α increases or decreases due to charge and discharge. In (2 + β) representing the atomic ratio of oxygen, β satisfies -0.05 ≤ β ≤ 0.05.

[0066] 1 - x1 - x2 - y - z (= v) representing the atomic ratio of Ni is, for example, 0.8 or more, can be 0.85 or more, can also be 0.90 or more, or 0.95 or more. In addition, v representing the atomic ratio of Ni can be 0.98 or less, can also be 0.95 or less. When limiting the range, these upper and lower limits can be arbitrarily combined.

[0067] x1 representing the atomic ratio of Co is, for example, 0.1 or less (0 ≤ x1 ≤ 0.1), can be 0.08 or less, can be 0.05 or less, or can also be 0.01 or less. If x1 is 0, it includes the case where Co is below the detection limit.

[0068] x2 representing the atomic ratio of Mn is, for example, 0.1 or less (0 ≤ x2 ≤ 0.1), can be 0.08 or less, can be 0.05 or less, or can also be 0.03 or less. x2 can be 0.01 or more, or can also be 0.03 or more. Mn contributes to the stabilization of the crystal structure of the composite oxide HN, and since the composite oxide HN contains inexpensive Mn, it is beneficial for cost reduction. When limiting the range, these upper and lower limits can be arbitrarily combined.

[0069] y representing the atomic ratio of Al is, for example, 0.1 or less (0 ≤ y ≤ 0.1), can be 0.08 or less, can be 0.05 or less, or can also be 0.03 or less. y can be 0.01 or more, or can also be 0.03 or more. Al contributes to the stabilization of the crystal structure of the composite oxide HN. When limiting the range, these upper and lower limits can be arbitrarily combined.

[0070] z representing the atomic ratio of element M is, for example, 0 ≤ z ≤ 0.10, can be 0 ≤ z ≤ 0.05, or can also be 0.001 ≤ z ≤ 0.01.

[0071] Element M can be at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, Sc, and Y. Among them, it is considered that if the composite oxide HN contains at least one selected from the group consisting of Nb, Sr, and Ca, the surface structure of the composite oxide HN will be stabilized, the resistance will be reduced, and the dissolution of the metal can be further suppressed. Element M will be more effective if it tends to exist near the surface of the particles of the composite oxide HN.

[0072] The content of the elements constituting the composite oxide N can be measured using an inductively coupled plasma atomic emission spectroscopy (ICP-AES), an electron probe micro analyzer (EPMA), an energy dispersive X-ray spectroscopy (EDX), or the like.

[0073] For example, the composite oxide N is secondary particles formed by aggregation of a plurality of primary particles. The particle size of the primary particles is, for example, 0.05 μm or more and 1 μm or less. The average particle size of the secondary particles of the composite oxide N is, for example, 3 μm or more and 30 μm or less, and may also be 5 μm or more and 25 μm or less.

[0074] In this specification, the average particle size of the secondary particles means the particle size (volume average particle size) at which the volume accumulation value is 50% in the particle size distribution measured by the laser diffraction scattering method. This particle size is sometimes referred to as D50. As the measuring device, for example, "LA-750" manufactured by HORIBA, Ltd. can be used.

[0075] (Positive electrode binder)

[0076] As the positive electrode binder, for example, a resin material can be used. Examples of the binder include a fluororesin, a polyolefin resin, a polyamide resin, a polyimide resin, an acrylic resin, and a vinyl resin. The binder can be used alone or in combination of two or more.

[0077] (Positive electrode conductive material)

[0078] Examples of the positive electrode conductive material include carbon nanotubes (CNT), carbon fibers other than CNT, and conductive particles (for example, carbon black and graphite).

[0079] (Positive electrode current collector)

[0080] As the positive electrode current collector, for example, a metal foil can be used. The positive electrode current collector can also be porous. Examples of the porous current collector include a mesh, a punched sheet, and an expand metal. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium. The thickness of the positive electrode current collector is not particularly limited, and is, for example, 1 to 50 μm, and may also be 5 to 30 μm.

[0081] [Non-aqueous electrolyte]

[0082] The non-aqueous electrolyte contains a non-aqueous solvent and a salt (electrolyte salt). The non-aqueous solvent contains at least fluoroethylene carbonate (FEC) and a fluorinated carboxylic acid ester (carboxylic acid ester (F)). FEC and the carboxylic acid ester (F) form an excellent mixed film (SEI) on the surface of the silicon-containing material. The carbonate ester and the carboxylic acid ester into which a fluorine atom is introduced have a reduced electron density by introducing a fluorine atom with strong electron-withdrawing property as a substituent, and thus are less likely to be oxidized at the positive electrode. Thereby, side reactions can be suppressed on both sides of the positive electrode and the negative electrode.

[0083] The non-aqueous electrolyte containing a non-aqueous solvent is usually a liquid electrolyte, but may also be in a state where its fluidity is restricted by a gelling agent or the like. In the case of a lithium-ion secondary battery, a lithium salt can be used as the salt.

[0084] (FEC)

[0085] FEC is excellent as an SEI forming material. Among them, when the carboxylic acid ester (F) and FEC are used in combination, a more stable SEI can be formed. On the other hand, when the non-aqueous electrolyte does not contain FEC and contains only the carboxylic acid ester (F) as an additive, the strength of the SEI is insufficient.

[0086] The content rate of FEC contained in the non-aqueous solvent is, for example, 5% by volume or more, may be 10% by volume or more, or may be 15% by volume or more. The content rate of fluoroethylene carbonate contained in the non-aqueous solvent is, for example, 30% by volume or less, or may be 25% by volume or less. The range of the content rate of FEC contained in the non-aqueous solvent is, for example, 5% by volume to 30% by volume, or may be 10% by volume to 25% by volume.

[0087] (carboxylic acid ester (F))

[0088] Examples of the fluorinated carboxylic acid ester include an alkyl ester of a carboxylic acid into which a fluorine atom is introduced, a fluorinated alkyl ester of a carboxylic acid into which no fluorine atom is introduced, and a fluorinated alkyl ester of a carboxylic acid into which a fluorine atom is introduced. Specific examples thereof include the trifluoropropionate shown in the formula (1) (hereinafter, also referred to as trifluoropropionate (1))

[0089] [Chemical formula 1]

[0090]

[0091] (wherein, R1 is C 1-3 alkyl group);

[0092] The fluorinated carboxylic acid ester shown in the formula (2) (hereinafter, also referred to as fluorinated carboxylic acid ester (2))

[0093] [Chemical formula 2]

[0094]

[0095] (In the formula, X1, X2, X3 and X4 are each a hydrogen atom or a fluorine atom, one or two of X1 to X4 are fluorine atoms, R2 is a hydrogen atom, C 1-3 alkyl or fluorinated C 1-3 alkyl, R3 is C 1-3 alkyl or fluorinated C 1-3 alkyl);

[0096] The fluoroalkyl carboxylate shown by the formula (3) (hereinafter, also referred to as fluoroalkyl carboxylate (3))

[0097] [Chemical formula 3]

[0098]

[0099] (In the formula, R4 is C 1-3 alkyl, R5 is fluorinated C 1-3 alkyl), etc.

[0100] In the formula (1), examples of the C 1-3 alkyl represented by R1 include methyl, ethyl, n-propyl, and isopropyl. Among them, methyl or ethyl is preferred. The non-aqueous electrolyte may contain one kind of trifluoropropionate (1), or may contain two or more kinds of trifluoropropionates (1). In particular, methyl 3,3,3-trifluoropropionate (FMP) with R1 being methyl can obtain high antioxidant properties at low viscosity. Therefore, it is preferable to use a trifluoropropionate (1) containing at least FMP. The ratio of FMP in the trifluoropropionate (1) is, for example, 50% by mass or more, preferably 80% by mass or more, and FMP may also be used alone.

[0101] In the formula (2), as the C 1-3 alkyl and fluorinated C 1-3 alkyl represented by R2 and R3, the C 1-3 alkyl moieties are respectively exemplified by those exemplified for R1. In the fluorinated C 1-3 alkyl, the number of fluorine atoms can be appropriately determined according to the carbon number of the alkyl, preferably 1 to 5, and may also be 1 to 3. Examples of the fluorinated C 1-3 alkyl include fluoromethyl, fluoroethyl, difluoromethyl, trifluoromethyl, 2,2,2-trifluoroethyl, etc. Among them, as R2, a hydrogen atom or C 1-3 alkyl is preferred, and a hydrogen atom is particularly preferred. As R3, C 1-3 alkyl is preferred.

[0102] In formula (2), one or two of X1 to X4 may be a fluorine atom. When one of X1 to X4 is a fluorine atom, the position of the fluorine atom may be either the α-position (e.g., X1) or the β-position (e.g., X3) of the carbonyl group in formula (2). When two of X1 to X4 are fluorine atoms, the positions of the fluorine atoms may be only the α-position (X1 and X2) of the carbonyl group in formula (2), may be only the β-position (X3 and X4), or may be the α-position and the β-position (e.g., X1 and X3). Among them, it is preferred that at least one of X1 and X2 is a fluorine atom (that is, the α-position of the carbonyl group is a fluorine atom).

[0103] Examples of the fluorinated carboxylic acid ester (2) include ethyl 2-fluoropropionate (αF-EP), ethyl 3-fluoropropionate, ethyl 2,2-difluoropropionate, ethyl 2,3-difluoropropionate, ethyl 3,3-difluoropropionate, etc. Among them, a fluorinated carboxylic acid ester having a fluorine atom at the α-position is preferred, and the fluorinated carboxylic acid ester (2) preferably contains at least αF-EP.

[0104] In formula (3), as the C shown by R4 1-3 alkyl group and R 5 shown fluorinated C 1-3 alkyl group's C 1-3 alkyl part, examples respectively include those exemplified by R1. The number of fluorine atoms in R5 can be selected according to the number of carbon atoms of the C 1-3 alkyl group, preferably 1 to 5, more preferably 1 to 3. As R4, methyl or ethyl is preferred, and from the viewpoint of reducing viscosity, methyl is preferred. As R5, trifluoromethyl, 2,2,2-trifluoroethyl, etc. are preferred, and particularly preferably 2,2,2-trifluoroethyl derived from easily available 2,2,2-trifluoroethanol.

[0105] In the carboxylic acid fluoroalkyl ester (3), 2,2,2-trifluoroethyl acetate (FEA) is preferred. Therefore, it is preferred to use a carboxylic acid fluoroalkyl ester (3) containing at least FEA.

[0106] It is considered that the trifluorocarboxylic acid ester (1) can bring about high durability performance of the SEI. The carboxylic acid fluoroalkyl ester (3) has the effect of enhancing the film-forming ability of the fluorinated carboxylic acid ester (2) and can further inhibit the decomposition of the trifluorocarboxylic acid ester (1). It is considered that the carboxylic acid fluoroalkyl ester (3) does not contain fluorine in R4 and does not generate HF detachment due to alkali, so the durability of the generated film is high.

[0107] The content rate of the carboxylic acid ester (F) contained in the non-aqueous solvent is, for example, 10% by volume or more, can be 20% by volume or more, can be 30% by volume or more, and can also be 35% by volume or more or 40% by volume or more. The content rate of the carboxylic acid ester (F) contained in the non-aqueous solvent is, for example, 80% by volume or less, can be 70% by volume or less, and can also be 60% by volume or less. The range of the content rate of the carboxylic acid ester (F) contained in the non-aqueous solvent can be, for example, 10% by volume to 80% by volume, can be 35% by volume to 80% by volume, and can also be 40% by volume to 70% by volume.

[0108] Among the carboxylic acid esters (F), trifluorocarboxylic acid esters (1) and fluoroalkyl carboxylates (3) are particularly preferred. The total content ratio of the trifluorocarboxylic acid ester (1) and the fluoroalkyl carboxylate (3) in the carboxylic acid ester (F) can be 50% by volume or more, can be 70% by volume or more, and can also be 90% by volume or more.

[0109] It is particularly desirable to use at least one selected from the group consisting of methyl 3,3,3-trifluoropropionate and 2,2,2-trifluoroethyl acetate as the carboxylic acid ester (F). 50% by volume or more, even 70% by volume or more or 90% by volume or more of the carboxylic acid ester (F) can also be composed of at least one selected from the group consisting of methyl 3,3,3-trifluoropropionate and 2,2,2-trifluoroethyl acetate.

[0110] The non-aqueous electrolyte may further contain an additive. However, the non-aqueous electrolyte recovered from the non-aqueous electrolyte secondary battery may also contain almost no additive. In this case, the oxidation product or reduction product of the additive is included as a film component on the positive electrode surface or the negative electrode surface. Even in such a case, the non-aqueous electrolyte taken from the non-aqueous electrolyte secondary battery usually still has an additive above the detection limit, so it can be confirmed that the non-aqueous electrolyte contains an additive.

[0111] The non-aqueous electrolyte may also contain an unsaturated carbonate as an additive. As the unsaturated carbonate, vinylene carbonate (VC), ethylene vinylene carbonate (VEC), divinylene ethylene carbonate (DVEC), etc. can be used.

[0112] The content rate of the unsaturated carbonate contained in the non-aqueous electrolyte only needs to be a concentration above the detection limit. The content rate of the unsaturated carbonate in the non-aqueous electrolyte is, for example, 0.01% by mass or more, can be 0.1% by mass or more, and can also be 0.5% by mass or more. The content rate of the unsaturated carbonate in the non-aqueous electrolyte is, for example, 3% by mass or less, can be 2% by mass or less, and can also be 1% by mass or less.

[0113] The non-aqueous electrolyte may also contain an acid anhydride as an additive. The acid anhydride is considered to have the effect of forming a film on the negative electrode and improving the high-temperature cycle characteristics of the secondary battery. Although the acid anhydride sometimes increases the internal resistance, when a small amount of the acid anhydride is added in combination with FEC and carboxylic acid ester (F), the effect of suppressing the increase in the internal resistance caused by FEC and carboxylic acid ester (F) can be improved. It is speculated that the reason is that during the charge-discharge cycle, when cracks are generated in the negative electrode active material, the acid anhydride rapidly reacts on the newly generated surface of the cracks to form a low-resistance protective film.

[0114] As the acid anhydride, from the viewpoint of effectively using its constituent elements for forming the protective film, a cyclic acid anhydride having a simple structure is preferred. Examples of such acid anhydrides include diglycolic acid anhydride, maleic anhydride, succinic anhydride, acetic anhydride, phthalic anhydride, benzoic anhydride, etc. The acid anhydride may be used alone or in combination of two or more. Among them, glycolic anhydride, succinic anhydride, etc. are preferred.

[0115] The content rate of the acid anhydride contained in the non-aqueous electrolyte may be a concentration above the detection limit. For example, the content rate of the acid anhydride in the non-aqueous electrolyte may be 0.01% by mass or more, may be 0.1% by mass or more, or may be 0.5% by mass or more. The content rate of the acid anhydride in the non-aqueous electrolyte is, for example, 3% by mass or less, may be 2% by mass or less, or may be 1% by mass or less. The content rate of the acid anhydride contained in the non-aqueous electrolyte may be 0.01% by mass or more and 3% by mass or less, may be 0.1% by mass or more and 2% by mass or less, or may be 0.5% by mass or more and 1% by mass or less.

[0116] The content rate of each component in the non-aqueous electrolyte can be determined, for example, by gas chromatography under the following conditions.

[0117] Machine used: GC-2010 Plus manufactured by Shimadzu Corporation

[0118] Column: HP-1 manufactured by J&W Company (film thickness 1 μm, inner diameter 0.32 mm, length 60 m)

[0119] Column temperature: Heat from 50 °C to 90 °C at a heating rate of 5 °C / minute, maintain at 90 °C for 15 minutes, then heat from 90 °C to 250 °C at a heating rate of 10 °C / minute, and maintain at 250 °C for 15 minutes

[0120] Split ratio: 1 / 50

[0121] Linear velocity: 30.0 cm / second

[0122] Inlet temperature: 270 °C

[0123] Injection volume: 1 μL

[0124] Detector: FID 290 °C (sens. 10 1 )

[0125] (non-aqueous solvent)

[0126] The non-aqueous electrolyte may contain a non-aqueous solvent other than FEC and carboxylic acid ester (F). Examples of such non-aqueous solvents include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), etc. Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), etc. Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL), γ-valerolactone (GVL), etc. Examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate (MA), ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, etc. These non-aqueous solvents may be used alone or in combination of two or more.

[0127] (salt)

[0128] In the case of a lithium-ion battery, a lithium salt can be used as the salt (electrolyte salt). Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, LiB 10 Cl 10 , lithium salts of lower aliphatic carboxylic acids, LiCl, LiBr, LiI, borate salts, imide salts. Examples of borate salts include lithium difluorooxalate borate, lithium bis(oxalato)borate, etc. Examples of imide salts include lithium bis(fluorosulfonyl)imide (LiN(FSO2)2), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2), etc. The non-aqueous electrolyte may contain only one kind of electrolyte salt or may contain two or more kinds. The concentration of the electrolyte salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0129] [Separator]

[0130] It is desirable to sandwich a separator between the positive electrode and the negative electrode. The separator has a high ion permeability and has appropriate mechanical strength and insulation properties. As the separator, a microporous film, a woven fabric, a non-woven fabric, etc. can be used. As the material of the separator, polyolefins such as polypropylene and polyethylene are preferred.

[0131] As an example of the structure of a non-aqueous electrolyte secondary battery, there can be cited a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and an electrolytic solution are housed in an outer package. However, it is not limited thereto, and it can also be applied to other forms of electrode groups. For example, it can also be a laminated electrode group formed by laminating a positive electrode and a negative electrode with a separator interposed therebetween. The form of the non-aqueous electrolyte secondary battery is also not limited, and for example, a cylindrical shape, a square shape, a coin shape, a button shape, a laminated shape, etc. are acceptable.

[0132] Hereinafter, with reference to Figure 1 the structure of the non-aqueous electrolyte secondary battery will be described. Figure 1 Fig. is a longitudinal sectional view of a cylindrical secondary battery as an example of the present embodiment. However, the present invention is not limited to the following configuration.

[0133] The non-aqueous electrolyte secondary battery (hereinafter referred to as battery 10) includes an electrode group 18, a non-aqueous electrolyte (not shown), and a bottomed cylindrical battery can 22 that houses them. A sealing body 11 is caulked to the opening of the battery can 22 with a gasket 21 interposed therebetween. Thus, the inside of the battery is sealed. The sealing body 11 includes a valve body 12, a metal plate 13, and an annular insulating member 14, and the annular insulating member 14 is interposed between the valve body 12 and the metal plate 13. The valve body 12 and the metal plate 13 are connected to each other at their respective central portions. A positive electrode lead 15a led out from the positive electrode 15 is connected to the metal plate 13. Therefore, the valve body 12 functions as an external terminal of the positive electrode. A negative electrode lead 16a led out from the negative electrode 16 is connected to the inner surface of the bottom of the battery can 22. An annular groove portion 22a is formed near the open end of the battery can 22. A first insulating plate 23 is disposed between one end surface of the electrode group 18 and the annular groove portion 22a. A second insulating plate 24 is disposed between the other end surface of the electrode group 18 and the bottom of the battery can 22. The electrode group 18 is formed by winding the positive electrode 15 and the negative electrode 16 with a separator 17 interposed therebetween.

[0134] (Supplementary Note)

[0135] Based on the above description, the following technology is disclosed.

[0136] (Technology 1)

[0137] A non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, and a non-aqueous electrolyte,

[0138] the negative electrode includes a silicon-containing carbon material,

[0139] the silicon-containing carbon material includes an amorphous carbon phase and a silicon phase dispersed in the amorphous carbon phase,

[0140] the non-aqueous electrolyte includes a non-aqueous solvent and a salt dissolved in the non-aqueous solvent,

[0141] the non-aqueous solvent includes fluoroethylene carbonate and a fluorinated carboxylic acid ester.

[0142] (Technology 2)

[0143] The non-aqueous electrolyte secondary battery according to Technology 1, wherein the content of the silicon phase in the silicon-containing carbon material is 40% by mass or more or 50% by mass or more.

[0144] (Technology 3)

[0145] The non-aqueous electrolyte secondary battery according to Technology 1 or 2, wherein the average particle diameter of the silicon phase is less than 20 nm.

[0146] (Technology 4)

[0147] The non-aqueous electrolyte secondary battery according to any one of Technologies 1 to 3, wherein the content of fluoroethylene carbonate in the non-aqueous solvent is 5% by volume or more and 30% by volume or less.

[0148] (Technology 5)

[0149] The non-aqueous electrolyte secondary battery according to any one of Technologies 1 to 4, wherein the content of the fluorinated carboxylic acid ester in the non-aqueous solvent is 10% by volume or more and 80% by volume or less.

[0150] (Technology 6)

[0151] The non-aqueous electrolyte secondary battery according to any one of Technologies 1 to 5, wherein the content of the fluorinated carboxylic acid ester in the non-aqueous solvent is 35% by volume or more and 80% by volume or less.

[0152] (Technology 7)

[0153] The non-aqueous electrolyte secondary battery according to any one of Technologies 1 to 6, wherein the fluorinated carboxylic acid ester is at least one selected from the group consisting of methyl 3,3,3-trifluoropropionate and 2,2,2-trifluoroethyl acetate.

[0154] (Technology 8)

[0155] The non-aqueous electrolyte secondary battery according to any one of Technologies 1 to 7, wherein the non-aqueous electrolyte further contains a cyclic acid anhydride.

[0156] (Technology 9)

[0157] The non-aqueous electrolyte secondary battery according to any one of Technologies 1 to 8, wherein the positive electrode contains a lithium transition metal composite oxide, and the lithium transition metal composite oxide contains at least nickel as a transition metal.

[0158] (Technology 10)

[0159] The non-aqueous electrolyte secondary battery according to Technique 9, wherein the content ratio of Ni contained in the lithium transition metal composite oxide is 80 atomic % or more with respect to all metals other than lithium.

[0160] Hereinafter, the present invention will be specifically described based on Examples and Comparative Examples. However, the present invention is not limited to the following Examples.

[0161] <<Example 1>>

[0162] A non-aqueous electrolyte secondary battery was produced in the following order and evaluated.

[0163] (1) Production of positive electrode

[0164] LiNi as the composite oxide HN was used 0.91 Co 0.04 Al 0.05 O2 as the positive electrode active material. 100 parts by mass of the composite oxide HN (average particle diameter 12 μm), 1 part by mass of carbon nanotubes, 1 part by mass of polyvinylidene fluoride, and an appropriate amount of NMP were mixed to obtain a positive electrode slurry. Then, the positive electrode slurry was coated on both sides of an aluminum foil, and after drying the coating film, rolling was performed to form a positive electrode mixture layer on both sides of the aluminum foil, thereby obtaining a positive electrode.

[0165] (2) Production of negative electrode

[0166] A silicon-containing carbon material (average particle diameter 5 μm) and graphite (average particle diameter 20 μm) were mixed at a mass ratio of 10:90 to obtain a negative electrode active material. The silicon-containing carbon material (Si / AmoC) was prepared as follows.

[0167] [Preparation of Si / AmoC]

[0168] 〈First step〉

[0169] Readily graphitizable carbon (soft carbon) was prepared as a carbon source.

[0170] <Second step>

[0171] The carbon source and raw material silicon (3N, average particle diameter 10 μm) were mixed. In the mixture, the mass ratio of the carbon source to the raw material silicon was set to 40:60.

[0172] The mixture was filled into a jar (made of SUS, volume: 500 mL) of a planetary ball mill (manufactured by Fritsch, P-5). 24 SUS balls (diameter 20 mm) were placed in the jar and the lid was closed, and the mixture was pulverized for 50 hours at 200 rpm in an inert atmosphere.

[0173] <Third step>

[0174] Next, the powdery mixture was taken out in an inert atmosphere and sintered at 800 °C for 4 hours under the pressure applied by a hot press in the inert atmosphere to obtain a sintered body of the mixture.

[0175] <Step 4>

[0176] Next, the obtained sintered body was pulverized and passed through a 40-μm mesh to obtain Si / AmoC particles composed of an amorphous carbon phase and silicon particles dispersed in the amorphous carbon phase.

[0177] The silicon content rate in the Si / AmoC particles was 60% by mass, the average particle size was 6 μm, and the average particle size of the silicon phase was less than 20 nm.

[0178] 98 parts by mass of a negative electrode active material, 1 part by mass of sodium salt of CMC (CMC-Na), 1 part by mass of SBR, and an appropriate amount of water were mixed to prepare a negative electrode paste. Next, the negative electrode paste was coated on both sides of a copper foil serving as a negative electrode current collector, the coating film was dried, and then calendered to form a negative electrode mixture layer on both sides of the copper foil to obtain a negative electrode.

[0179] (3) Preparation of non-aqueous electrolyte

[0180] In a mixed solvent containing FEC and FMP (methyl 3,3,3-trifluoropropionate) in a volume ratio of 20:80, LiPF6 was dissolved at a concentration of 1.0 mol / L to prepare a non-aqueous electrolyte. 2% by mass of vinylene carbonate (VC) was added to the non-aqueous electrolyte.

[0181] (4) Fabrication of battery

[0182] One end of an aluminum positive electrode lead was attached to the positive electrode. One end of a nickel negative electrode lead was attached to the negative electrode. The positive electrode and the negative electrode were wound with a polyethylene spacer to fabricate an electrode group. After the electrode group was vacuum-dried at 105 °C for 2 hours, it was housed in a bottomed cylindrical battery case that also served as a negative terminal. The battery case was made of an iron case. Next, after injecting the non-aqueous electrolyte into the battery case, the opening of the battery case was sealed with a metal sealing body that also served as a positive terminal. At this time, a resin gasket was interposed between the sealing body and the opening end of the battery case. The other end of the positive electrode lead was connected to the sealing body, and the other end of the negative electrode lead was connected to the inner bottom surface of the battery case. Thus, a 2170-type cylindrical battery A1 with a design capacity of 5 Ah was fabricated.

[0183] <<Example 2>>

[0184] When preparing the non-aqueous electrolyte, FEA (2,2,2-trifluoroethyl acetate) was used to replace FMP, and the battery A2 was fabricated in the same manner as in Example 1 except for this.

[0185] Example 3

[0186] Battery A3 was fabricated in the same manner as in Example 1, except that diglycolic anhydride (DGA) was added to the non-aqueous electrolyte at a mass ratio of 0.5%.

[0187] Example 4

[0188] Battery A4 was fabricated in the same manner as in Example 1, except that succinic anhydride (SUCA) was added to the non-aqueous electrolyte at a mass ratio of 0.5%.

[0189] Example 5

[0190] When preparing Si / AmoC, the ball mill processing conditions were changed and the average particle size of the silicon phase was controlled to be about 100 nm. Otherwise, battery A5 was fabricated in the same manner as in Example 1.

[0191] Comparative Example 1

[0192] When preparing the non-aqueous electrolyte, a mixed solvent containing EC and EMC in a volume ratio of 20:80 was used to replace the mixed solvent containing FEC and FMP in a volume ratio of 20:80. Otherwise, battery B1 was fabricated in the same manner as in Example 1.

[0193] Comparative Example 2

[0194] Except for using SiO x (x = 0.9, average particle size 6 μm) to replace Si / AmoC, battery B2 was fabricated in the same manner as in Comparative Example 1.

[0195] Comparative Example 3

[0196] Except for using SiO x (x = 0.9, average particle size 6 μm) to replace Si / AmoC, battery B3 was fabricated in the same manner as in Example 1.

[0197] (5) Evaluation

[0198] <Initial DCIR>

[0199] At a temperature of 25 °C, the battery is charged at a constant current of 0.2It until the voltage reaches 4.2V. Then, it is charged at a constant voltage of 4.2V until the current reaches 0.02It. After that, it is paused for 20 minutes. Thus, a battery with SOC 100% is obtained. The obtained battery with SOC 100% is discharged at a constant current of 0.3It until the state of charge (SOC) is 10%. For the battery with SOC 10%, the voltage values are measured when discharging for 10 seconds at current values of 0A, 0.1A, 0.5A, and 1.0A respectively. When the relationship between the discharge current value and the voltage value after 10 seconds is approximated as a straight line by the least squares method, the initial DCIR is calculated from the absolute value of the slope at this time.

[0200] 〈Charge and Discharge Cycles〉

[0201] The charge and discharge cycles of the battery after measuring the initial DCIR are carried out for 100 cycles under the following conditions.

[0202] <Charge>

[0203] In an environment of 25 °C, it is charged at a constant current of 0.2It until the voltage reaches 4.2V. Then, it is charged at a constant voltage of 4.2V until the current reaches 0.02It. The battery after constant voltage charging is paused for 20 minutes.

[0204] <Discharge>

[0205] After pausing, in an environment of 25 °C, it is discharged at a constant current of 0.3It until the voltage reaches 2.5V.

[0206] <DCIR Rise Rate>

[0207] For the battery after 100 cycles, DCIR(100) is obtained by the same procedure as for finding the initial DCIR, and the DCIR rise rate is calculated by the following formula.

[0208] DCIR Rise Rate (%) = 100×(DCIR(100) - Initial DCIR) / Initial DCIR

[0209] The evaluation results of each example and each comparative example are shown in Table 1.

[0210]

[0211] As shown in Table 1, it can be understood that in terms of suppressing the rise of DCIR, FEC and carboxylic acid ester (F) when using SiO xIt does not function effectively at [time], but functions effectively for Si / AmoC. Additionally, it can be understood that by adding a trace amount of acid anhydride to the non-aqueous electrolyte, the effect of suppressing the increase in DCIR can be improved. Moreover, it can be understood that the smaller the average particle size of the silicon phase, the more obvious the effect of suppressing the increase in DCIR.

[0212] Industrial Applicability

[0213] The non-aqueous electrolyte secondary battery of the present invention is applicable to main power sources such as mobile communication devices and portable electronic devices, as well as vehicle-mounted power sources, etc., but its use is not limited to these.

[0214] The present invention has been described with reference to the currently preferred embodiments, but the above invention should not be construed in a limiting sense. After reading the above disclosure, those skilled in the art will clearly understand various modifications and changes without any doubt. Therefore, all modifications and changes without departing from the core spirit and scope of the present invention can be construed as being included in the scope of the appended claims.

[0215] 10: Secondary battery

[0216] 11: Sealing body

[0217] 12: Valve body

[0218] 13: Metal plate

[0219] 14: Insulating member

[0220] 15: Positive electrode

[0221] 15a: Positive electrode lead

[0222] 16: Negative electrode

[0223] 16a: Negative electrode lead

[0224] 17: Spacer

[0225] 18: Electrode group

[0226] 21: Gasket

[0227] 22: Battery can

[0228] 22a: Groove portion

[0229] 23: First insulating plate

[0230] 24: Second insulating plate

[0231] 16: Negative electrode

Claims

1. A non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode contains a silicon-containing carbon material. The silicon-containing carbon material contains an amorphous carbon phase and a silicon phase dispersed in the amorphous carbon phase. The non-aqueous electrolyte contains a non-aqueous solvent and a salt dissolved in the non-aqueous solvent. The non-aqueous solvent contains fluoroethylene carbonate and a fluorinated carboxylic acid ester.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the content ratio of the silicon phase contained in the silicon-containing carbon material is 40% by mass or more.

3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the content ratio of the silicon phase contained in the silicon-containing carbon material is 50% by mass or more.

4. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the average particle size of the silicon phase is less than 20 nm.

5. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the content ratio of the fluoroethylene carbonate contained in the non-aqueous solvent is 5% by volume or more and 30% by volume or less.

6. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the content ratio of the fluorinated carboxylic acid ester contained in the non-aqueous solvent is 10% by volume or more and 80% by volume or less.

7. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the content ratio of the fluorinated carboxylic acid ester contained in the non-aqueous solvent is 35% by volume or more and 80% by volume or less.

8. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the fluorinated carboxylic acid ester is at least one selected from the group consisting of methyl 3,3,3-trifluoropropionate and 2,2,2-trifluoroethyl acetate.

9. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the non-aqueous electrolyte further contains a cyclic acid anhydride.

10. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the positive electrode contains a lithium transition metal composite oxide, and the lithium transition metal composite oxide contains at least nickel as a transition metal.

11. The non-aqueous electrolyte secondary battery according to claim 10, wherein the content ratio of Ni contained in the lithium transition metal composite oxide is 80 atomic% or more with respect to all metals other than lithium.

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