Nonaqueous electrolyte secondary battery

Abstract

Provided is a nonaqueous electrolyte secondary battery capable of improving coulombic efficiency while omitting a conventional negative electrode active material layer. A nonaqueous electrolyte secondary battery according to an embodiment includes a negative electrode, a positive electrode, and a nonaqueous electrolyte. The nonaqueous electrolyte is a nonaqueous electrolyte in which an ionic crystal is dissolved in an ionic liquid. The negative electrode has a current collector and a silicon-containing layer disposed on the current collector, and the layer does not contain an organic binder.

Description

Non-aqueous electrolyte secondary battery

[0001] An embodiment of the present invention relates to a non-aqueous electrolyte secondary battery.

[0002] BACKGROUND ART Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are used as high-voltage, high-energy density electricity storage devices, for example, as power sources for driving electronic devices.

[0003] The negative electrode of a non-aqueous electrolyte secondary battery is typically produced by applying a mixture containing a negative electrode active material such as graphite or silicon oxide, a carbon-based conductive material such as carbon nanotubes or acetylene black, and an organic binder such as polyvinylidene fluoride (PVDF) or styrene butadiene rubber (SBR) onto a current collector and drying the mixture to form a negative electrode active material layer (see Patent Document 1).

[0004] JP 2015-153714 A JP 2007-123096 A JP 2007-194204 A

[0005] The above-described conventional negative electrode active material layer contains a negative electrode active material, a carbon-based conductive material, and an organic binder, and is typically thick, at several tens of micrometers or more. Therefore, if such a negative electrode active material layer could be eliminated from the negative electrode, the negative electrode could be made thinner and lighter. However, if the current collector is used as is, the coulomb efficiency would be low.

[0006] Incidentally, Patent Documents 2 and 3 describe a technique for forming an electrode for a lithium-ion secondary battery, in which a thin film containing silicon is formed on a current collector by vacuum deposition or high-frequency sputtering. Unlike conventional negative electrode active material layers, the thin film formed in this manner does not contain a carbon-based conductive material or an organic binder. Therefore, while it is known to form a layer that does not contain a carbon-based conductive material or an organic binder on a current collector, Patent Documents 2 and 3 do not describe the construction of a battery by combining the electrode with a nonaqueous electrolyte containing an ionic liquid, nor the improvement in Coulombic efficiency that results from this.

[0007] An object of an embodiment of the present invention is to provide a non-aqueous electrolyte secondary battery that can improve coulomb efficiency while omitting a conventional, general negative electrode active material layer.

[0008] The present invention includes the following embodiments. [1] A nonaqueous electrolyte secondary battery including a negative electrode, a positive electrode, and a nonaqueous electrolyte, wherein the nonaqueous electrolyte is a nonaqueous electrolyte in which ionic crystals are dissolved in an ionic liquid, and the negative electrode has a current collector and a layer containing silicon disposed on the current collector, the layer not containing an organic binder. [2] The nonaqueous electrolyte secondary battery according to [1], wherein the layer does not contain a carbon-based conductive material. [3] The nonaqueous electrolyte secondary battery according to [1] or [2], wherein the layer contains 90 mass % or more of silicon. [4] The nonaqueous electrolyte secondary battery according to any one of [1] to [3], wherein the layer has a thickness of 1 nm or more but less than 1 μm.

[0009] According to an embodiment of the present invention, it is possible to provide a nonaqueous electrolyte secondary battery that can improve Coulomb efficiency (charge / discharge efficiency) while omitting a conventional, general negative electrode active material layer.

[0010] 1 is a schematic diagram of a non-aqueous electrolyte secondary battery according to one embodiment of the present invention; 2 is an exploded perspective view of an evaluation cell used in the examples;

[0011] The nonaqueous electrolyte secondary battery according to this embodiment includes a negative electrode, a positive electrode, and a nonaqueous electrolyte solution in which ionic crystals are dissolved in an ionic liquid. The negative electrode includes a current collector and a layer containing silicon (Si) (hereinafter, sometimes referred to as the "Si layer") disposed on the current collector, and the Si layer does not contain an organic binder. In this way, by using a negative electrode in which an organic binder-free Si layer is formed on a current collector instead of a conventional negative electrode active material layer, and combining this with a nonaqueous electrolyte solution containing an ionic liquid, the coulombic efficiency can be improved.

[0012] Non-aqueous electrolyte secondary batteries are secondary batteries that use a non-aqueous electrolyte as an electrolyte, and examples thereof include lithium ion secondary batteries, sodium ion secondary batteries, potassium ion secondary batteries, calcium ion secondary batteries, magnesium ion secondary batteries, etc. Among these, lithium ion secondary batteries are preferred.

[0013] The nonaqueous electrolyte secondary battery may include a separator disposed between the positive electrode and the negative electrode. The nonaqueous electrolyte secondary battery according to this embodiment uses a negative electrode including the above-described Si layer and an ionic liquid as the nonaqueous electrolyte, and the configuration thereof may be that of a known nonaqueous electrolyte secondary battery.

[0014] 1 is a schematic diagram showing the configuration of a nonaqueous electrolyte secondary battery 10 according to one embodiment. The nonaqueous electrolyte secondary battery 10 includes a negative electrode 12, a positive electrode 14, a separator 16 disposed between the negative electrode 12 and the positive electrode 14, and a nonaqueous electrolyte 18. The negative electrode 12 includes a negative electrode current collector 20 and a Si layer 22 disposed on the current collector 20. The positive electrode 14 includes a positive electrode current collector 24 and a positive electrode active material layer 26 disposed on the current collector 24 and containing a positive electrode active material.

[0015] [Negative Electrode] The negative electrode includes a current collector and a Si layer as described above.

[0016] The negative electrode current collector is not particularly limited as long as it has electronic conductivity and can extract current to the outside. The current collector can be formed, for example, from a metal material such as copper, stainless steel, aluminum, nickel, or titanium, a conductive polymer, or conductive glass. In one embodiment, the current collector may be a metal layer formed from a metal material having electronic conductivity. The metal layer may be, for example, a metal layer such as copper whose surface has been treated with tin, nickel, titanium, or silver, or a metal layer whose surface has been oxidized.

[0017] The shape of the current collector is not particularly limited. For example, the current collector may be a film such as a foil made of a metal material, or may be a sheet, a plate, or a molded body such as a porous body or a foam. The current collector may also be a metal layer formed on a resin film or sheet. As the current collector, a metal foil such as copper foil is preferably used.

[0018] The thickness of the current collector is not particularly limited, and may be, for example, 1 to 1000 μm, 5 to 100 μm, or 10 to 50 μm.

[0019] The Si layer is a layer containing silicon (Si) disposed on a current collector, and is formed on the current collector (i.e., on the surface of the current collector) in place of a conventional negative electrode active material layer. The Si layer formed on the current collector may be provided on one side or both sides of the current collector.

[0020] The Si layer does not contain an organic binder. Here, the organic binder is generally an organic compound (resin) blended to bind negative electrode active material particles together, and examples thereof include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC).

[0021] The Si layer preferably does not contain a carbon-based conductive material, which is a carbon-based conductive material generally blended into the negative electrode active material layer as a conductive additive or conductive filler, and examples thereof include carbon nanotubes, conductive carbon black (acetylene black, ketjen black, etc.), graphite, and carbon fiber.

[0022] Such a Si layer that does not contain an organic binder or a carbon-based conductive material can be formed by, for example, a physical vapor deposition (PVD) method such as sputtering. Physical vapor deposition does not require an organic binder, and a Si layer of a certain thickness can be formed without an organic binder, thereby improving the Coulomb efficiency. Furthermore, when formed by physical vapor deposition, the Si layer does not contain a carbon-based conductive material, but the Coulomb efficiency can be improved even without the carbon-based conductive material.

[0023] The physical vapor deposition method, preferably sputtering, is not particularly limited, and any known method can be applied. For example, a silicon film can be formed on the surface of the current collector by supplying an inert gas (e.g., Ar) at 0.2 to 2.0 Pa into a vacuum chamber in which a current collector and a silicon target are placed, and applying a high voltage.

[0024] The Si layer may contain a metal element in addition to silicon. Examples of such a metal element include tin (Sn), aluminum (Al), nickel, titanium, and silver. The Si layer is mainly composed of silicon, and the silicon content is preferably 90 mass % or more, more preferably 95 mass % or more, more preferably 98 mass % or more, and even more preferably 99 mass %. In one embodiment, the Si layer may be a layer consisting only of silicon.

[0025] The thickness of the Si layer is preferably 1 nm or more and less than 1 μm. The thicker the Si layer, the more the Coulomb efficiency can be improved, but if the thickness is too large, the effect tends to plateau and decrease. Furthermore, if the thickness is too large, the effect of thinning and reducing the weight of the negative electrode by providing it instead of a conventional negative electrode active material layer is reduced. The thickness of the Si layer is more preferably 5 to 900 nm, more preferably 10 to 800 nm, more preferably 100 to 700 nm, and even more preferably 200 to 500 nm.

[0026] [Positive Electrode] The positive electrode includes a current collector and a positive electrode active material layer as described above.

[0027] Examples of the current collector used in the positive electrode include those commonly used in non-aqueous electrolyte secondary batteries, such as foils made of metal materials such as aluminum, titanium, stainless steel, nickel, etc. Furthermore, the surfaces of these metals may be treated with carbon, nickel, titanium, silver, etc.

[0028] The positive electrode active material layer is a layer containing a positive electrode active material, and may contain a conductive material and an organic binder together with the positive electrode active material. As the positive electrode active material, the conductive material, and the organic binder, those typically used in non-aqueous electrolyte secondary batteries can be used. Examples of the positive electrode active material include composite oxides of lithium and transition metals, such as lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, and lithium nickel oxide.

[0029] Examples of organic binders used in the positive electrode include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC).

[0030] Examples of conductive materials used in the positive electrode include carbon-based conductive materials such as carbon nanotubes, conductive carbon black (acetylene black, ketjen black, etc.), graphite, and carbon fibers.

[0031] [Separator] As the separator, any separator commonly used in non-aqueous electrolyte secondary batteries can be used, including, for example, porous resins such as polyethylene, polypropylene, polyolefin, polyimide, and polytetrafluoroethylene, ceramics, and nonwoven fabrics.

[0032] [Non-aqueous electrolyte] As the non-aqueous electrolyte, a non-aqueous electrolyte in which ionic crystals are dissolved in an ionic liquid is used as described above. As the ionic crystals, a lithium salt is preferred. As the lithium salt, for example, LiPF ６ , LiBF ４ , LiClO ４ , LiAsF ６ , LiCF ３ SO ３ , LiC(CF３ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ (i.e., LiTFSI), LiN(FSO ２ ) ２ (i.e., LiFSI), LiBC ４ O ８ These lithium salts may be used alone or in combination of two or more. As the lithium salt, LiFSI and / or LiTFSI are preferred.

[0033] The ionic liquid that dissolves the lithium salt is composed of a cation component and an anion component, is liquid at room temperature (25° C.), is non-volatile, and has a relatively high decomposition temperature.

[0034] Examples of anion components contained in the ionic liquid include BF ４ － , P.F. ６ － , SbF ６ － , NO ３ － , C.F. ３ SO ３ － , (FSO ２ ) ２ N － (i.e., FSI anion), (CF ３ SO ２ ) ２ N － (i.e., TFSI anion), (C ２ F ５ SO ２ ) ２ N － , (CF ３ SO ２ ) ３ C － , C.F. ３ CO ２ － , C ３ F ７ CO ２ － , C.H. ３ CO ２ － , (CN) ２ N－ These may be used alone or in combination of two or more. Among these, FSI anion and / or TFSI anion are preferred.

[0035] Examples of cationic components constituting the ionic liquid include compounds containing elements such as N, P, S, O, C, and Si, and cations having a chain structure or a cyclic structure such as a five-membered ring or a six-membered ring as a skeleton. Examples of cyclic structures such as a five-membered ring or a six-membered ring include heterocyclic structures such as a furan ring, a thiophene ring, a pyrrole ring, a pyridine ring, an oxazole ring, an isoxazole ring, a thiazole ring, an isothiazole ring, a furazan ring, an imidazole ring, a pyrazole ring, a pyrazine ring, a pyrimidine ring, a pyridazine ring, a pyrrolidine ring, a piperidine ring, a benzofuran ring, an isobenzofuran ring, an indole ring, an isoindole ring, an indolizine ring, and a carbazole ring. Among these cationic components, chain or cyclic compounds containing a nitrogen element are particularly preferred because of their chemical and electrochemical stability. Examples of the cation containing a nitrogen element include alkylammonium such as triethylammonium, 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-methyl-1-propylpyrrolidinium, methylpropylpiperidinium, etc. These cations may be used alone or in combination of two or more.

[0036] An ionic liquid according to a preferred embodiment comprises an anion component comprising an FSI anion and / or a TFSI anion, and at least one cation component selected from the group consisting of 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-methyl-1-propylpyrrolidinium, and methylpropylpiperidinium.

[0037] The concentration of the salt such as the lithium salt in the non-aqueous electrolyte is not particularly limited, but is preferably 0.1 to 3.0 mol / kg, and more preferably 0.5 to 2.0 mol / kg.

[0038] The nonaqueous electrolyte secondary battery according to the embodiment can be formed into any shape, such as a cylindrical shape, a coin shape, a prismatic shape, or any other shape. The basic configuration of the battery is the same regardless of the shape, and the design can be modified depending on the purpose. For example, in the case of a cylindrical battery, a negative electrode and a positive electrode are wound with a separator interposed therebetween, and the resulting wound body is housed in a battery can, and a nonaqueous electrolyte is poured into the battery can, and insulating plates are placed on the top and bottom of the battery can, and the battery can is sealed. In addition, when applied to a coin-type battery, a disc-shaped negative electrode, a separator, a disc-shaped positive electrode, and a stainless steel plate are stacked and housed in a coin-type battery can, and a nonaqueous electrolyte is poured into the battery can, and the battery can is sealed.

[0039] The present invention will be explained in more detail below based on examples and comparative examples, but the present invention is not limited thereto.

[0040] The electrolyte solutions 1 to 3 used in the examples and comparative examples are as follows: Electrolyte solution 1: 1.0 mol / kg LiFSI / IL-120 The lithium salt was lithium bis(fluorosulfonyl)imide, with a concentration of 1.0 mol / kg. The ionic liquid IL-120 was 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide (MPPy-FSI).

[0041] Electrolyte 2: 1.0 mol / kg LiFSI / IL-110 The lithium salt was lithium bis(fluorosulfonyl)imide, and its concentration was 1.0 mol / kg. The ionic liquid IL-110 was 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIm-FSI).

[0042] Electrolyte 3:1M LiFSI / EC:DMC = 1:1 + 5 mass% FEC: The lithium salt was lithium bis(fluorosulfonyl)imide, with a concentration of 1 mol / L. This nonaqueous electrolyte was prepared by dissolving the lithium salt in an organic solvent, with a volume ratio of ethylene carbonate / dimethyl carbonate of 1 / 1, to which 5 mass% fluoroethylene carbonate (FEC) was added.

[0043] The measurement and evaluation methods in the examples and comparative examples are as follows. [Si Layer Thickness] For the samples with a sputtering treatment time of 90 minutes, the cross section of the Cu foil after sputtering was observed with an SEM to measure the thickness of the Si layer. For the samples with sputtering treatment times of 1 minute, 30 minutes, and 60 minutes, the surface of the Si layer after sputtering was gradually removed using argon etching, and the removal depth at which the amount of Cu on the surface exceeded 50 atomic % was determined as the thickness of the Si layer by XPS measurement. In this way, the thickness of the Si layer was determined by SEM observation when the thickness was 500 nm or more, and by XPS measurement when the thickness was less than 500 nm.

[0044] [Coulombic efficiency] (Preparation of evaluation cell) The evaluation cell was constructed in a glove box under an argon atmosphere with a dew point of −70° C. or less. As shown in FIG. 2 , a 13 mm diameter metal Li foil was attached to the bottom surface of the first body of the two-electrode cell for electrochemical measurements. A 20 mm diameter polyimide separator impregnated with an electrolyte under reduced pressure for 30 minutes was placed on top of the foil. A 14 mm diameter negative electrode was placed on top of the foil, and the cell was sealed with the second body via an electrode press and a spring to prepare an evaluation cell.

[0045] (Conditions for measuring coulombic efficiency) A constant current charge / discharge device ("BTS2004W" manufactured by Nagano Corporation) was used as the measurement device, and the evaluation cell was charged / discharged in a constant temperature room at 25° C. The current density was 1.00 mA / cm ２ Then, 1 mAh of Li was deposited on the negative electrode current collector at a constant current. After that, the cutoff voltage was 1.0 V vs. Li / Li. ＋ This cycle was repeated 50 times to calculate the average coulombic efficiency in the Li dissolution / precipitation reaction. The average coulombic efficiency was calculated as the average value for each cycle, calculated as 100×discharge capacity / charge capacity (%).

[0046] Example 1 A 15 μm thick Cu foil (NC-WS manufactured by Furukawa Electric Co., Ltd.) was used as a current collector. A SVC-700TM SG (Sanyu Electronics Co., Ltd.) was used as a sputtering device to form a Si film on the Cu foil surface. A Si target (diameter 49 mm, thickness 0.5 mm, purity 5N) was used as the target.

[0047] Inside the device 10 －４ After evacuating the chamber to a vacuum of 1 Pa or less, argon (Ar) was supplied until the pressure reached 1 Pa. Thereafter, a Si film was formed on the surface of the Cu foil by sputtering for 1 minute at a current of 20 mA and a voltage of 250 V, thereby obtaining a Si film-treated Cu foil (referred to as "Cu-Si" in Table 1 below).

[0048] The thickness of the Si layer on the obtained Si film-treated Cu foil was 10 nm. An evaluation cell was produced using the Si film-treated Cu foil as the negative electrode and the above-mentioned electrolyte solution 1, and the coulomb efficiency was measured. The results are shown in Table 1 below.

[0049] [Example 2] A Si film-treated Cu foil was obtained in the same manner as in Example 1, except that the sputtering time was 30 minutes. The thickness of the Si layer on the obtained Si film-treated Cu foil was 150 nm. An evaluation cell was produced using the Si film-treated Cu foil as the negative electrode and the above-mentioned electrolyte solution 1, and the coulomb efficiency was measured.

[0050] [Example 3] A Si film-treated Cu foil was obtained in the same manner as in Example 1, except that the sputtering time was 60 minutes. The thickness of the Si layer on the obtained Si film-treated Cu foil was 380 nm. An evaluation cell was produced using the Si film-treated Cu foil as the negative electrode and the above-mentioned electrolyte solution 1, and the coulomb efficiency was measured.

[0051] [Example 4] A Si film-treated Cu foil was obtained in the same manner as in Example 1, except that the sputtering time was 90 minutes. The thickness of the Si layer on the obtained Si film-treated Cu foil was 630 nm. An evaluation cell was produced using the Si film-treated Cu foil as the negative electrode and the above-mentioned electrolyte solution 1, and the coulomb efficiency was measured.

[0052] Example 5 An evaluation cell was produced using the Si-film-treated Cu foil (Si layer thickness: 380 nm) obtained in Example 3 as the negative electrode and the above-mentioned electrolyte solution 2, and the coulomb efficiency was measured.

[0053] Comparative Example 1 An evaluation cell was produced using a 15 μm thick Cu foil (NC-WS manufactured by Furukawa Electric Co., Ltd.) as the negative electrode without sputtering treatment and the above-mentioned electrolyte solution 1, and the coulombic efficiency was measured.

[0054] Comparative Example 2 An evaluation cell was produced using the Si-film-treated Cu foil (Si layer thickness: 380 nm) obtained in Example 3 as the negative electrode and the above-mentioned electrolyte solution 3, and the coulomb efficiency was measured.

[0055]

[0056] As shown in Table 1, in contrast to Comparative Example 1, which is a blank in which Cu foil was used as the negative electrode as it was as a current collector, Examples 1 to 4, in which a silicon layer formed on Cu foil by sputtering was used as the negative electrode and electrolyte solution 1 containing an ionic liquid was used, showed improved Coulombic efficiency. The Coulombic efficiency tended to improve more as the thickness of the Si layer increased. Furthermore, Example 5, in which the type of ionic liquid contained in the electrolyte was changed, also showed improved Coulombic efficiency compared to Comparative Example 1. On the other hand, in Comparative Example 2, in which organic solvent electrolyte solution 3 was used instead of an ionic liquid as the electrolyte, no improvement in Coulombic efficiency was observed compared to Comparative Example 1.

[0057] The various numerical ranges described in this specification can be arbitrarily combined with their respective upper and lower limit values, and all such combinations are considered to be preferred numerical ranges described in this specification. Furthermore, a numerical range described as "X to Y" means from X to Y.

[0058] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their omissions, substitutions, modifications, etc. are included within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents.

[0059] 10...Non-aqueous electrolyte secondary battery, 12...Negative electrode, 14...Positive electrode, 16...Separator, 18...Non-aqueous electrolyte, 20...Current collector, 22...Si layer, 24...Current collector, 26...Cathode active material layer

Claims

1. A nonaqueous electrolyte secondary battery comprising a negative electrode, a positive electrode, and a nonaqueous electrolyte, wherein the nonaqueous electrolyte is a nonaqueous electrolyte in which ionic crystals are dissolved in an ionic liquid, and the negative electrode has a current collector and a layer containing silicon disposed on the current collector, the layer not containing an organic binder.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein said layer does not contain a carbon-based conductive material.

3. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the silicon content in said layer is 90 mass % or more.

4. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the thickness of said layer is 1 nm or more and less than 1 μm.

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