Nonaqueous electrolyte secondary battery

Abstract

To provide a nonaqueous electrolyte secondary battery that can improve Coulomb efficiency while omitting a conventional negative electrode active material layer.SOLUTION: A nonaqueous electrolyte secondary battery according to an 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 disposed on the current collector, the layer not containing an organic binder.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] 2. Description of the Related 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 nonaqueous electrolyte secondary battery is usually 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). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2015-153714 A [Patent Document 2] JP 2007-123096 A [Patent Document 3] JP 2007-194204 A Summary of the Invention [Problem to be solved by the invention]

[0005] The above-mentioned conventional negative electrode active material layer contains a negative electrode active material, a carbon-based conductive material, and an organic binder, and is usually thick, at several tens of μm or more. Therefore, if such a negative electrode active material layer can be omitted from the negative electrode, the negative electrode can be made thinner and lighter. However, if the current collector is used as it is, the coulomb efficiency will be low.

[0006] Incidentally, Patent Documents 2 and 3 disclose 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. The thin film thus formed is different from a conventional negative electrode active material layer and does not contain a carbon-based conductive material or an organic binder. Therefore, although it has been conventionally known to form a layer not containing a carbon-based conductive material or an organic binder on a current collector, Patent Documents 2 and 3 do not disclose that the electrode is combined with a non-aqueous electrolyte solution containing an ionic liquid to form a battery, and that the coulombic efficiency is improved thereby.

[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 common negative electrode active material layer. [Means for solving the problem]

[0008] The present invention includes the embodiments set forth below. [1] A nonaqueous electrolyte secondary battery comprising a negative electrode, a positive electrode, and a nonaqueous electrolyte, the nonaqueous electrolyte being a nonaqueous electrolyte in which ionic crystals are dissolved in an ionic liquid, the negative electrode having 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 has a silicon content of 90 mass % or more. [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 and less than 1 μm. Effect of the Invention

[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 common negative electrode active material layer. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram of a nonaqueous electrolyte secondary battery according to one embodiment. [Diagram 2] FIG. 2 is an exploded perspective view of an evaluation cell used in the examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0012] The nonaqueous electrolyte secondary battery is a secondary battery that uses a nonaqueous electrolyte as an electrolyte, and examples of the nonaqueous electrolyte secondary battery include, in addition to 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 the negative electrode including the above-mentioned Si layer and uses an ionic liquid as the nonaqueous electrolyte, and the configuration of the battery 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 provided on the current collector 20. The positive electrode 14 includes a positive electrode current collector 24 and a positive electrode active material layer 26 including a positive electrode active material provided on the current collector 24.

[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 of, for example, a metal material such as copper, stainless steel, aluminum, nickel, titanium, conductive polymer, conductive glass, etc. In one embodiment, the current collector may be a metal layer formed of a metal material having electronic conductivity. The metal layer may be, for example, a metal layer such as copper whose surface is treated with tin, nickel, titanium, silver, etc., or a metal layer whose surface is 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 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, it is preferable to use a metal foil such as a copper foil.

[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) arranged on a current collector, and is a layer formed on the current collector (i.e., on the surface of the current collector) instead 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) that is blended to bind negative electrode active material particles together, and examples of the organic binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC).

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

[0022] Such a Si layer that does not include an organic binder or a carbon-based conductive material can be formed by, for example, a physical vapor deposition (PVD) method such as sputtering. If a physical vapor deposition method is used, an organic binder is not necessary, and a Si layer of a certain thickness can be formed without an organic binder, and the Coulomb efficiency can be increased. In addition, when formed by a physical vapor deposition method, the Si layer does not include a carbon-based conductive material, but the Coulomb efficiency can be increased even without the carbon-based conductive material.

[0023] The physical vapor deposition method, preferably the sputtering method, is not particularly limited, and a 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 together with silicon. Examples of such metal elements 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 made of silicon only.

[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. In addition, if the thickness is too large, the effect of thinning and reducing the weight of the negative electrode by providing it instead of the 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 usually used in non-aqueous electrolyte secondary batteries can be used, and examples of the positive electrode active material include composite oxides of lithium and transition metals, such as lithium cobalt oxide, lithium manganate, lithium iron phosphate, and lithium nickel oxide.

[0029] Examples of the organic binder 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 the conductive material 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 that is commonly used in non-aqueous electrolyte secondary batteries can be used, and examples thereof include porous resins such as polyethylene, polypropylene, polyolefin, polyimide, and polytetrafluoroethylene, ceramics, and nonwoven fabrics.

[0032] [Nonaqueous electrolyte] As the non-aqueous electrolyte, a non-aqueous electrolyte in which an ionic crystal is dissolved in an ionic liquid is used as described above. As the ionic crystal, a lithium salt is preferable. As the lithium salt, for example, LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC(CF3SO2)3, LiN(CF3SO2)2 (i.e., LiTFSI), LiN(FSO2)2 (i.e., LiFSI), LiBC4O8, etc. may be mentioned. These lithium salts may be used alone or in combination of two or more. As the lithium salt, LiFSI and / or LiTFSI are preferable.

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

[0034] The anion component contained in the ionic liquid is, for example, BF4 - , PF6 - , SbF6 - , NO3 -, CF3SO3 - , (FSO2)2N - (i.e., FSI anion), (CF3SO2)2N - (i.e., TFSI anion), (C2F5SO2)2N - , (CF3SO2)3C - , CF3CO2 - , C3F7CO2 - , CH3CO2 - , (CN)2N - These may be used alone or in combination of two or more. Among these, the FSI anion and / or the 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 5-membered ring or a 6-membered ring as a skeleton. Examples of cyclic structures such as a 5-membered ring or a 6-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, particularly preferred are chain or cyclic compounds containing a nitrogen element, because they are chemically and electrochemically stable. Examples of the cation containing a nitrogen element include alkyl ammonium 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 a cylindrical type, a coin type, a square type, or any other shape. The basic configuration of the battery is the same regardless of the shape, and can be modified in design depending on the purpose. For example, in the case of a cylindrical type, a wound body in which a negative electrode and a positive electrode are wound with a separator interposed therebetween is housed in a battery can, and a nonaqueous electrolyte is injected and sealed with insulating plates placed above and below. 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 laminated and housed in a coin type battery can, and a nonaqueous electrolyte is injected and sealed. EXAMPLES

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

[0040] The electrolytes 1 to 3 used in the examples and comparative examples are as follows. ·Electrolyte 1:1.0mol / kgLiFSI / IL-120 The lithium salt is lithium bis(fluorosulfonyl)imide at a concentration of 1.0 mol / kg. The ionic liquid IL-120 is 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide (MPPy-FSI).

[0041] ·Electrolyte 2: 1.0mol / kgLiFSI / IL-110 The lithium salt is lithium bis(fluorosulfonyl)imide at a concentration of 1.0 mol / kg. The ionic liquid IL-110 is 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIm-FSI).

[0042] ·Electrolyte 3:1MLiFSI / EC:DMC=1:1 +5 mass%FEC The lithium salt is lithium bis(fluorosulfonyl)imide, and its concentration is 1 mol / L. The lithium salt is dissolved in an organic solvent to produce a non-aqueous electrolyte, and the organic solvent is ethylene carbonate / dimethyl carbonate=1 / 1 (volume ratio), to which 5 mass% of fluoroethylene carbonate (FEC) is 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 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 a sputtering time of 1 minute, 30 minutes, and 60 minutes, the surface of the Si layer after sputtering was gradually scraped off using argon etching, and the depth of the scrape at which the amount of Cu on the surface exceeded 50 atomic % in XPS measurement was determined as the thickness of the Si layer. In this way, the thickness of the Si layer is determined by SEM observation when it is 500 nm or more, and by XPS measurement when it is less than 500 nm.

[0044] [Coulombic efficiency] (Preparation of evaluation cells) 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 Figure 2, a 13 mm diameter metallic Li foil was attached to the bottom surface of the first body of the two-electrode cell for electrochemical measurement. A 20 mm diameter polyimide separator that had been 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 holder and a spring to prepare the evaluation cell.

[0045] (Coulombic efficiency measurement conditions) A constant current charge / discharge device (Nagano Corporation, "BTS2004W") was used as the measurement device, and the evaluation cell was charged and 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. Then, the cut-off voltage was 1.0 V vs. Li / Li + The battery was discharged at a constant current up to 1000 V. This was repeated 50 times, and the average Coulomb efficiency in the Li dissolution and precipitation reaction was calculated. The average Coulomb efficiency was calculated as the average value for each cycle (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 Si film was formed on the surface of the Cu foil using a sputtering device SVC-700TM SG (Sanyu Electronics Co., Ltd.). A Si target (diameter 49 mm, thickness 0.5 mm, purity 5N) was used as the target.

[0047] Inside the device 10 -4 After evacuation to a pressure of less than 1 Pa, argon (Ar) was supplied up to 1 Pa. Then, a Si film was formed on the surface of the Cu foil by sputtering for 1 minute with a current value of 20 mA and a voltage of 250 V applied, 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 a negative electrode and the above-mentioned electrolyte 1, and the coulombic efficiency was measured. The results are shown in Table 1 below.

[0049] [Example 2] The sputtering time was 30 minutes, and the other conditions were the same as in Example 1 to obtain a Si film-treated Cu foil. The thickness of the Si layer in the obtained Si film-treated Cu foil was 150 nm. The Si film-treated Cu foil was used as a negative electrode, and an evaluation cell was produced using the above-mentioned electrolyte 1, and the coulombic efficiency was measured.

[0050] [Example 3] The sputtering time was set to 60 minutes, and otherwise the same procedure as in Example 1 was followed to obtain a Si-film-treated Cu foil. The thickness of the Si layer in the obtained Si-film-treated Cu foil was 380 nm. The Si-film-treated Cu foil was used as a negative electrode, and an evaluation cell was produced using the above-mentioned electrolyte 1, and the Coulomb efficiency was measured.

[0051] [Example 4] The sputtering time was 90 minutes, and the other conditions were the same as in Example 1 to obtain a Si-film-treated Cu foil. The thickness of the Si layer in the obtained Si-film-treated Cu foil was 630 nm. The Si-film-treated Cu foil was used as a negative electrode, and an evaluation cell was produced using the above-mentioned electrolyte 1, and the coulombic 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 coulombic 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 being subjected to sputtering treatment and the above-mentioned electrolyte 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 coulombic efficiency was measured.

[0055] [Table 1]

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

[0057] In addition, the various numerical ranges described in the specification can be arbitrarily combined with their upper and lower limits, and all such combinations are considered to be preferred numerical ranges described in the specification. In addition, 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 gist of the invention. These embodiments and their omissions, substitutions, modifications, etc. are included in the scope and gist of the invention as well as the invention and its equivalents described in the claims. [Explanation of symbols]

[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... Positive electrode active material layer

Claims

1. A non-aqueous electrolyte secondary battery comprising a negative electrode, a positive electrode, and a non-aqueous electrolyte, The non-aqueous electrolyte is a non-aqueous electrolyte in which ionic crystals are dissolved in an ionic liquid, 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. 10. The non-aqueous electrolyte secondary battery of claim 1, wherein the layer does not include a carbon-based conductive material.

3. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the layer has a silicon content of 90 mass % or more.

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

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