Lithium ion secondary battery

Abstract

To provide a lithium ion secondary battery capable of improving coulombic efficiency.SOLUTION: A lithium ion secondary battery includes a negative electrode active material layer, a separator layer, and a positive electrode active material layer in this order. The negative electrode active material layer includes a silicon-based negative electrode active material. The negative electrode active material layer, the separator layer, and the positive electrode active material layer are impregnated with an electrolytic solution. The electrolytic solution contains an ionic liquid and lithium ions. The ionic liquid includes phosphonium ions and bis(fluorosulfonyl)imide ions.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to lithium-ion secondary batteries. [Background technology]

[0002] It is known that in lithium ion secondary batteries, a solid electrolyte interface (SEI) is formed at the interface between the electrode active material and the electrolyte. These SEIs are often a combination of the electrode active material and the electrolyte, and in some cases contain decomposition products of the electrolyte. The SEI functions as a protective layer on the surface of the electrode active material to protect the electrode active material and the electrolyte from corrosion and other undesirable side reactions. The following batteries have been disclosed as batteries having a stable SEI using an ionic liquid as the electrolyte.

[0003] For example, Patent Document 1 discloses a method for manufacturing a lithium ion battery having a protected anode with a preformed solid electrolyte interface (SEI), the method comprising: forming an SEI on a lithium electrode by performing a plurality of charge / discharge cycles on the lithium electrode in a first cell having an SEI-forming electrolyte to form a protected anode; the SEI-forming electrolyte includes lithium bis(fluorosulfonyl)imide (LiFSI) and an ionic liquid, the ionic liquid being selected from the group consisting of 1-methyl-1-propylpyrrolidinium, N-methyl-N,N-diethyl-N-propylammonium, and N,N-diethyl-N-methyl-N-(2-methoxyethyl)-ammonium. and an organic cation selected from the group consisting of fluorosulfonyl piperidinium, 1,1-methylpropyl piperidinium, N-methyl-N-(2-methoxyethyl)-pyrrolidinium, trimethylisopropylphosphonium, methyltriethylphosphonium, methyltributylphosphonium, and mixtures thereof, and (fluorosulfonyl)imide anion (FSI), and the method further comprises assembling a wet Li-ion voltaic cell, the wet Li-ion voltaic cell including the protected anode and a wet electrolyte in contact with the protected anode, the wet electrolyte including at least 50 ppm of water. According to the method for producing a lithium ion battery in Patent Document 1, it is said that an improved method for protecting a lithium anode against water in an electrolyte can be provided.

[0004] Patent Document 2 discloses an electrode system including an active material containing nanosilicon, a polyacrylonitrile polymer that binds the active material to conduct electricity and lithium ions, and an electrolyte in contact with the active material, the active material including a LiFSI salt and an ionic liquid having an anion of bis(fluorosulfonyl)imide (FSI).

[0005] Patent Document 3 discloses a system for generating electrical energy from chemicals in a compartmented cell, the system comprising at least two electrodes having at least one anode and at least one intercalation cathode, at least one separator separating the anode and the cathode, an ionic liquid electrolyte system having an ionic liquid solvent mixed with a minor amount of an ether co-solvent, and a lithium salt solute. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2018-200873 A [Patent Document 2] Special Publication No. 2017-539051 [Patent Document 3] Special Publication No. 2022-516205 Summary of the Invention [Problem to be solved by the invention]

[0007] As described above, the SEI functions as a protective layer on the surface of the electrode active material to protect the electrode active material and the electrolyte from corrosion and other undesirable side reactions. On the other hand, when a silicon-based negative electrode active material is used as the electrode active material in a liquid battery, the SEI may crack due to the expansion and contraction of the silicon-based negative electrode active material, forming a new surface of the SEI, which may cause the electrolyte to decompose and the SEI to continue to grow. Therefore, electrical energy is used in addition to charging and discharging, which may result in a low Coulombic efficiency.

[0008] Therefore, an object of the present disclosure is to provide a lithium ion secondary battery with improved coulombic efficiency. [Means for solving the problem]

[0009] The present disclosure achieves the above object by the following means.

[0010] A lithium ion secondary battery comprising: The lithium ion secondary battery has a negative electrode active material layer, a separator layer, and a positive electrode active material layer in this order, The negative electrode active material layer contains a silicon-based negative electrode active material, and the negative electrode active material layer, the separator layer, and the positive electrode active material layer are impregnated with an electrolyte; The electrolyte contains an ionic liquid and lithium ions, The ionic liquid contains a phosphonium ion and a bis(fluorosulfonyl)imide ion. Lithium-ion secondary battery. Aspect 2: The lithium ion secondary battery according to aspect 1, wherein the phosphonium ion is a triethylpropylphosphonium ion. <Aspect 3> The specific surface area of ​​the silicon-based negative electrode active material is 25 m 2 / g or less. Aspect 4: The lithium ion secondary battery according to any one of aspects 1 to 3, wherein a molar ratio of the total number of moles of the phosphonium ion and the bis(fluorosulfonyl)imide ion to the total number of moles of the lithium ion and the bis(fluorosulfonyl)imide ion is 1.5 or less. Effect of the Invention

[0011] According to the lithium ion secondary battery of the present disclosure, the coulombic efficiency can be improved. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a graph comparing the average coulombic efficiencies of the lithium ion secondary batteries in Examples 1 and 2 and Comparative Examples 1 to 3. [Diagram 2] FIG. 2 is a graph comparing the average coulombic efficiency of the lithium ion secondary batteries in Examples 2 and 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, embodiments of the present disclosure will be described in detail. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the present disclosure.

[0014] In the present disclosure, a "composite" refers to a composition that can constitute a positive electrode (negative electrode) active material layer as it is or by further containing other components. Also, in the present disclosure, a "composite slurry" refers to a slurry that contains a dispersion medium in addition to a "composite" and can be applied and dried to form a positive electrode (negative electrode) active material layer.

[0015] The lithium ion secondary battery of the present disclosure may be a liquid battery in which the negative electrode active material layer, the separator layer, and the positive electrode active material layer are impregnated with an electrolyte solution, or may be a solid battery having a solid electrolyte layer as the separator layer. Note that, in the present disclosure, a "solid-state battery" means a battery that uses at least a solid electrolyte as the electrolyte, and therefore, in the present disclosure, a solid-state battery uses a combination of a solid electrolyte and an electrolyte solution as the electrolyte.

[0016] Lithium-ion secondary battery The lithium ion secondary battery of the present disclosure is A negative electrode active material layer, a separator layer, and a positive electrode active material layer in this order, The negative electrode active material layer contains a silicon-based negative electrode active material, and the negative electrode active material layer, the separator layer, and the positive electrode active material layer are impregnated with an electrolyte; The electrolyte contains an ionic liquid and lithium ions, The ionic liquid contains a phosphonium ion and a bis(fluorosulfonyl)imide ion.

[0017] According to the lithium ion secondary battery of the present disclosure, the coulombic efficiency can be improved.

[0018] The lithium ion secondary battery of the present disclosure includes a silicon-based negative electrode active material in the negative electrode active material layer and a phosphonium ion in the electrolyte. The silicon-based negative electrode active material is known to have a large expansion and contraction compared to other negative electrode materials, and the SEI formed in the silicon-based negative electrode active material cracks during charging and discharging, forming a new surface of the SEI, which causes the electrolyte to decompose and the SEI to continue to grow.

[0019] Although the details are unclear, it is speculated that the inclusion of phosphonium ions in the electrolyte creates an SEI that is effective in preventing the expansion and contraction of silicon-based negative electrode active materials, that is, an SEI that can suppress the problem of the SEI cracking and the formation of new SEI surfaces when using silicon-based negative electrode active materials, thereby causing the SEI to continue to grow, thereby improving the Coulombic efficiency. In addition, it is speculated that the phosphonium ions, having high electrochemical stability, suppress the excessive formation of the SEI, thereby improving the Coulombic efficiency.

[0020] (Structure of Lithium-Ion Secondary Battery) The lithium ion secondary battery of the present disclosure is The lithium ion secondary battery has a negative electrode active material layer, a separator layer, and a positive electrode active material layer in this order.

[0021] The lithium ion secondary battery of the present disclosure may further include a negative electrode current collector layer and a positive electrode current collector layer, or may have a negative electrode current collector layer, a negative electrode active material layer, a separator layer, a positive electrode active material layer, and a positive electrode current collector layer in this order.

[0022] <Negative electrode current collector layer> The material used for the negative electrode current collector layer is not particularly limited, but may be any material generally used as a negative electrode current collector for lithium ion secondary batteries. Examples of materials used for the negative electrode current collector layer include, but are not limited to, Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, stainless steel, or a carbon sheet. In particular, from the viewpoint of ensuring reduction resistance and being difficult to alloy with lithium, the material used for the negative electrode current collector layer may contain at least one metal selected from Cu, Ni, and stainless steel, or may be made of a carbon sheet. The negative electrode current collector layer may have some kind of coating layer on its surface for the purpose of adjusting the resistance, etc.

[0023] The shape of the negative electrode current collector layer is not particularly limited, but examples thereof include a foil shape, a plate shape, a mesh shape, etc. Among these, a foil shape is preferred.

[0024] The thickness of the negative electrode current collector layer is not particularly limited, but may be 0.1 μm or more, or 1 μm or more, and may be 1 mm or less, or 100 μm or less.

[0025] <Negative electrode active material layer> The negative electrode active material layer contains at least a silicon-based negative electrode active material, and may further contain, optionally, a conductive assistant, a binder, a solid electrolyte, and the like. The negative electrode active material layer may also contain various other additives. The contents of the negative electrode active material, the conductive assistant, the binder, and the like in the negative electrode active material layer may be appropriately determined according to the intended battery performance. For example, the content of the negative electrode active material may be 40% by mass or more, 50% by mass or more, or 60% by mass or more, or may be 100% by mass or less, or 90% by mass or less, with the entire negative electrode active material layer (total solid content) being 100% by mass.

[0026] (Negative electrode active material) The negative electrode active material layer may contain a negative electrode active material other than a silicon-based negative electrode active material.

[0027] Examples of silicon-based negative electrode active materials include, but are not limited to, silicon, silicon oxide, silicon carbide, silicon nitride, and solid solutions thereof. Silicon-based negative electrode active materials can also include metal elements other than silicon, such as Fe, Co, Sb, Bi, Pb, Ni, Cu, Zn, Ge, In, Sn, Ti, etc.

[0028] The shape of the silicon-based negative electrode active material is not particularly limited, and may be any shape that is common to silicon-based negative electrode active materials for lithium ion secondary batteries. The shape of the silicon-based negative electrode active material may be particulate. The silicon-based negative electrode active material may be primary particles, or secondary particles formed by agglomeration of a plurality of primary particles. The specific surface area of ​​the silicon-based negative electrode active material is not particularly limited, and may be 25 m 2 The specific surface area of ​​the silicon-based negative electrode active material may be, for example, 0.1 m 2 / g or more, 0.2m 2 / g or more, 0.4m 2 / g or more, 0.6m 2 / g or more, 0.8m 2 / g or more, or 1m 2 / g or more, 2 / g or less, 40m 2 / g or less, 30m 2 / g or less, or 25m 2 The specific surface area can be determined by the BET method using nitrogen as an adsorbate.

[0029] The negative electrode active material other than the silicon-based negative electrode active material is not particularly limited, but may be a material capable of absorbing and releasing metal ions such as lithium ions, or may be metallic lithium. Examples of materials capable of absorbing and releasing metal ions such as lithium ions include, but are not limited to, Sn alloy-based negative electrode active materials and carbon materials. Examples of Sn alloy-based negative electrode active materials include tin, tin oxide, tin nitride, and solid solutions thereof. In addition, the Sn alloy-based negative electrode active material may include metal elements other than tin, such as Fe, Co, Sb, Bi, Pb, Ni, Cu, Zn, Ge, In, Ti, Si, etc. The carbon material is not particularly limited, and examples thereof include hard carbon, soft carbon, graphite, etc.

[0030] The shape of the negative electrode active material other than the silicon-based negative electrode active material is not particularly limited, but may be a general shape as a negative electrode active material for a lithium ion secondary battery. The shape of the negative electrode active material other than the silicon-based negative electrode active material may be particulate. The negative electrode active material other than the silicon-based negative electrode active material may be primary particles or secondary particles formed by agglomeration of a plurality of primary particles.

[0031] The proportion of the silicon-based negative electrode active material contained in the negative electrode active material layer is not particularly limited, and may be 50% by mass to 100% by mass, 60% by mass to 100% by mass, 70% by mass to 100% by mass, 80% by mass to 100% by mass, or 90% by mass to 100% by mass, relative to the negative electrode active material layer.

[0032] (Conductive assistant) The conductive assistant is not particularly limited. The conductive assistant may be, for example, vapor grown carbon fiber (VGCF), acetylene black (AB), Ketjen black (KB), carbon nanotube (CNT), carbon nanofiber (CNF), etc., but is not limited thereto. The conductive assistant may be, for example, particulate or fibrous, and its size is not particularly limited. The conductive assistant is not particularly limited, but may be used alone or in combination of two or more kinds.

[0033] (Binder) The binder is not particularly limited. The binder may be, for example, polyvinylidene fluoride (PVdF), butadiene rubber (BR), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), polyacrylic acid (PAA), or other materials, but is not limited thereto. The binder is not particularly limited, but may be used alone or in combination of two or more types.

[0034] (solid electrolyte) The material of the solid electrolyte is not particularly limited, and may be, for example, a sulfide solid electrolyte, an oxide solid electrolyte, or a polymer electrolyte.

[0035] Examples of sulfide solid electrolytes include, but are not limited to, sulfide-based amorphous solid electrolytes, sulfide-based crystalline solid electrolytes, and argyrodite-type solid electrolytes. Specific examples of sulfide solid electrolytes include Li2S-P2S5-based (Li7P3S 11 , Li3PS4, Li8P2S9, etc.), Li2S-SiS2, LiI-Li2S-SiS2, LiI-Li2S-P2S5, LiI-LiBr-Li2S-P2S5, Li2S-P2S5-GeS2(Li 13 GeP3S 16 , Li 10 GeP2S 12 ), LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li 7-x P.S. 6-x Cl x etc.; or combinations thereof, but are not limited to these.

[0036] An example of an oxide solid electrolyte is Li7La3Zr2O 12 , Li 7-x La3Zr 1-x Nb x O 12 , Li 7-3x La3Zr2Al x O 12 , Li 3x La 2 / 3-x TiO3, Li 1+x Al x Ti2-x (PO4)3, Li 1+x Al x Ge 2-x (PO4)3, Li3PO4, or Li 3+x PO 4-x N x (LiPON), but is not limited to these.

[0037] The sulfide solid electrolyte and the oxide solid electrolyte may be glass or crystallized glass (glass ceramics).

[0038] Examples of the polymer electrolyte include, but are not limited to, polyethylene oxide (PEO), polypropylene oxide (PPO), and copolymers thereof.

[0039] The shape of the negative electrode active material layer is not particularly limited, and may be, for example, a sheet-like negative electrode active material layer having a substantially flat surface. The thickness of the negative electrode active material layer is not particularly limited, and may be, for example, 0.1 μm or more, 1 μm or more, or 10 μm or more, and may be 2 mm or less, 1 mm or less, or 500 μm or less.

[0040] The negative electrode active material layer can be manufactured by applying a known method. For example, the negative electrode active material layer can be easily formed by dry or wet forming a negative electrode mixture containing the above-mentioned various components. The negative electrode active material layer may be formed together with the negative electrode current collector layer or may be formed separately from the negative electrode current collector layer.

[0041] <Separator Layer> The separator layer is not particularly limited, but for example, a polyolefin-based, polyamide-based, polyimide-based nonwoven fabric can be used.

[0042] <Cathode active material layer> The positive electrode active material layer contains at least a positive electrode active material, and may further contain an optional conductive assistant, a binder, a solid electrolyte, and the like. The positive electrode active material layer may also contain various other additives. The contents of the positive electrode active material, the conductive assistant, the binder, and the like in the positive electrode active material layer may be appropriately determined according to the intended battery performance. For example, the content of the positive electrode active material may be 40% by mass or more, 50% by mass or more, 60% by mass or more, or 100% by mass or less, or 90% by mass or less, with the entire positive electrode active material layer (total solid content) being 100% by mass.

[0043] (Cathode active material) The material of the positive electrode active material is not particularly limited. Examples of the positive electrode active material include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), lithium nickel-cobalt-manganese oxide (NCM), LiCO 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, lithium nickel-cobalt-aluminate (NCA; LiNi x Co y Al z O2), Li 1+x Mn 2-x-y M y The material may be, but is not limited to, a Li-Mn spinel substituted with a different element having a composition represented by O4 (M is one or more metal elements selected from Al, Mg, Co, Fe, Ni, and Zn).

[0044] The positive electrode active material may have a coating layer, but is not particularly limited to this. The coating layer is a layer containing a material that has lithium ion conductivity, is low in reactivity with the positive electrode active material and the solid electrolyte, and can maintain the shape of the coating layer without flowing even when it comes into contact with the active material and the solid electrolyte. Specific examples of materials constituting the coating layer include LiNbO3, Li4Ti5O 12 , Li3PO4, etc., but are not limited to these.

[0045] The shape of the positive electrode active material is not particularly limited as long as it is a general shape for a positive electrode active material of a lithium ion secondary battery. The positive electrode active material may be, for example, in a particulate form. The positive electrode active material may be a primary particle or a secondary particle formed by agglomeration of a plurality of primary particles. The average particle diameter D of the positive electrode active material 50 The average particle size D may be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, and may be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less. 50 is the particle size (median size) at an integrated value of 50% in the volume-based particle size distribution determined by a laser diffraction / scattering method.

[0046] For the conductive assistant, binder, and solid electrolyte, the above description of "Negative electrode active material layer" can be referred to.

[0047] The shape of the positive electrode active material layer is not particularly limited, and may be, for example, a sheet-like positive electrode active material layer having a substantially flat surface. The thickness of the positive electrode active material layer is not particularly limited, and may be, for example, 0.1 μm or more, 1 μm or more, or 10 μm or more, and may be 2 mm or less, 1 mm or less, or 500 μm or less.

[0048] <Positive electrode current collector layer> The material used for the positive electrode collector layer is not particularly limited, but may be any material generally used as a positive electrode collector for lithium ion secondary batteries. Examples of materials used for the positive electrode collector layer include, but are not limited to, Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, stainless steel, etc. The positive electrode collector layer may have a coating layer on its surface for the purpose of adjusting resistance, etc. The positive electrode collector layer may be a metal foil or a substrate on which the above metal is plated or vapor-deposited.

[0049] The shape of the positive electrode current collector layer is not particularly limited, but examples thereof include a foil shape, a plate shape, a mesh shape, etc. Among these, a foil shape is preferred.

[0050] The thickness of the positive electrode current collector layer is not particularly limited, but may be 0.1 μm or more, or 1 μm or more, and may be 1 mm or less, or 100 μm or less.

[0051] The positive electrode active material layer can be manufactured by applying a known method. For example, the positive electrode active material layer can be easily formed by dry or wet forming a positive electrode mixture containing the above-mentioned various components. The positive electrode active material layer may be formed together with the positive electrode current collector layer, or may be formed separately from the positive electrode current collector layer.

[0052] <Electrolyte> In the lithium ion secondary battery of the present disclosure, the negative electrode active material layer, the separator layer, and the positive electrode active material layer are impregnated with an electrolyte solution.

[0053] The electrolyte of the lithium ion secondary battery of the present disclosure contains an ionic liquid and lithium ions. The electrolyte may contain a supporting salt (lithium salt). The lithium ions contained in the electrolyte are not particularly limited, but may be those obtained by ionizing the supporting salt.

[0054] (Ionic Liquid) The ionic liquid contained in the electrolyte of the lithium ion secondary battery of the present disclosure contains a phosphonium ion and a bis(fluorosulfonyl)imide ion.

[0055] (phosphonium ion) The phosphonium ion can be represented by the following general formula (1). [PR 1 R 2 R 3 R 4 ] + …(1) In general formula (1), R 1 ~R 4 are the same or different and each represents an optionally substituted alkyl group (preferably an unsubstituted alkyl group), or a hydrogen atom (provided that R 1 ~R 4is not a hydrogen atom at the same time). The alkyl group is not particularly limited as long as the phosphonium ion can form an ionic liquid. The alkyl group may be either branched or linear, but is preferably linear. The alkyl group may be, for example, an alkyl group having 1 to 5 carbon atoms, preferably an alkyl group having 1 to 3 carbon atoms. The alkyl group is not particularly limited, but specific examples thereof include a methyl group, an ethyl group, a propyl group, a butyl group, and a pentyl group. Among them, from the viewpoint of the coulombic efficiency of the lithium ion secondary battery, the phosphonium ion is preferably, but is not particularly limited to, a triethylpropylphosphonium ion.

[0056] (Supporting salt) Examples of the supporting salt include, but are not limited to, inorganic lithium salts and organic lithium salts. Examples of the inorganic lithium salt include, but are not limited to, LiPF6, LiBF4, LiClO4, and LiAsF6. Examples of the organic lithium salt include, but are not limited to, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(FSO2)2, and LiC(CF3SO2)3. Among them, since an ionic liquid containing bis(fluorosulfonyl)imide ions is present in the electrolyte, LiN(FSO2)2 is preferred from the viewpoint of being the same anion, but is not limited to this.

[0057] (Ratio of ionic liquid to lithium ions) The molar ratio of the total number of moles of phosphonium ions and bis(fluorosulfonyl)imide ions to the total number of moles of lithium ions and bis(fluorosulfonyl)imide ions may be 6.0 or less, 4.0 or less, 2.0 or less, or 1.5 or less, or 0.2 or more, 0.4 or more, 0.6 or more, 0.8 or more, or 1.0 or more, from the viewpoint of Coulomb efficiency. The molar ratio of the total number of moles of phosphonium ions and bis(fluorosulfonyl)imide ions to the total number of moles of lithium ions and bis(fluorosulfonyl)imide ions is preferably 1.5 or less, from the viewpoint of improving Coulomb efficiency. When the molar ratio is small, the coordination state in the electrolyte changes, the reduction stability is improved, and the Coulomb efficiency is improved.

[0058] The electrolyte may contain a solvent other than the ionic liquid, but is not limited thereto. Examples of the solvent include, but are not limited to, cyclic carbonates and chain carbonates. Examples of the cyclic carbonates include, but are not limited to, ethylene carbonate (EC), propylene carbonate (PC), but are not limited to, ethylene carbonate (BC). Examples of the chain carbonates include, but are not limited to, dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Examples of the solvent include, but are not limited to, acetates such as methyl acetate and ethyl acetate, and ethers such as 2-methyltetrahydrofuran. Examples of the solvent include, but are not limited to, γ-butyl lactone, sulfolane, N-methylpyrrolidone (NMP), and 1,3-dimethyl-2-imidazolidinone (DMI). The solvent may be water.

[0059] The method for forming the lithium ion secondary battery is not particularly limited, and a known method can be adopted. For example, the method for forming the lithium ion secondary battery may include a step of arranging a negative electrode active material layer, a separator layer, and a positive electrode active material layer in this order, injecting an electrolyte solution, and impregnating the negative electrode active material layer, the separator layer, and the positive electrode active material layer with the electrolyte solution, but is not limited to this case. EXAMPLES

[0060] The present disclosure will be described in more detail with reference to the following examples, but the scope of the present disclosure is not limited to these examples. The half-cell used in the examples of the present disclosure is mainly composed of a silicon-based negative electrode active material layer and an ionic liquid, and a simple evaluation is performed using metallic lithium as the counter electrode, in order to clearly compare the coulombic efficiency of the half-cell.

[0061] Example 1 <Preparation of negative electrode active material layer A1> Silicon-based negative electrode active material with a specific surface area of ​​1 m 2 A negative electrode mixture slurry was prepared by mixing silicon having a molecular weight of 1000 to 1000 g / g, a polyimide binder as a binder, vapor grown carbon fiber (VGCF) and hollow carbon (KB) as conductive additives, and an appropriate amount of solvent. The obtained negative electrode mixture slurry was applied to a Cu foil as a negative electrode current collector and dried to prepare a negative electrode active material layer A1 on the negative electrode current collector.

[0062] <Adjustment of electrolyte B1> Lithium bis(fluorosulfonyl)imide (hereinafter referred to as "Li-FSI") as a supporting salt containing lithium ions was dissolved in triethylpropylphosphonium bis(fluorosulfonyl)imide (hereinafter referred to as "P2223-FSI") as an ionic liquid such that the molar ratio of P2223-FSI to Li-FSI (P2223-FSI / Li-FSI) was 6, to prepare electrolyte B1.

[0063] <Preparation of lithium-ion secondary battery C1> The negative electrode active material layer A1 and a lithium foil as a counter electrode were placed opposite each other with a separator interposed therebetween and housed in a container, and then the electrolyte solution B1 was poured into the container and the container was sealed to prepare a lithium ion secondary battery C1.

[0064] Electrochemical evaluation of lithium-ion secondary battery C1 The lithium ion secondary battery C1 was charged and discharged for 100 cycles under constant current-constant voltage conditions (current value: 0.1 C) with a charging capacity of 1200 mAh / g. The coulombic efficiency in each cycle was measured, and then the average value of the coulombic efficiency from the 4th cycle to the 100th cycle was calculated to obtain the average coulombic efficiency up to the 100th cycle. The average coulombic efficiency up to the 100th cycle of the lithium ion secondary battery C1 was 99.0%. Note that, since the coulombic efficiency decreases from the 1st to 3rd cycles due to the initial formation of SEI, the average coulombic efficiency was calculated from the 4th cycle.

[0065] Example 2 <Adjustment of electrolyte B2> An electrolyte solution B2 was prepared by dissolving Li-FSI as a supporting salt containing lithium ions in P2223-FSI as an ionic liquid such that the molar ratio of P2223-FSI to Li-FSI (P2223-FSI / Li-FSI) was 1.5.

[0066] <Preparation of lithium-ion secondary battery C2 and electrochemical evaluation> The negative electrode active material layer A1 and a lithium foil as a counter electrode were placed opposite each other via a separator and housed in a container. Then, the electrolyte B2 was poured into the container, and the container was sealed to prepare a lithium ion secondary battery C2. The electrochemical evaluation of the lithium ion secondary battery C2 was carried out in the same manner as the lithium ion secondary battery C1. The average coulombic efficiency of the lithium ion secondary battery C2 up to the 100th cycle was 99.5%.

[0067] Example 3 <Preparation of negative electrode active material layer A2> The specific surface area of ​​the silicon-based negative electrode active material is 25m 2A negative electrode mixture slurry was prepared by mixing silicon having a molecular weight of 1000 g / g, a polyimide binder as a binder, vapor grown carbon fiber (VGCF) and hollow carbon (KB) as conductive additives, and an appropriate amount of solvent. The obtained negative electrode mixture slurry was applied to a Cu foil as a negative electrode current collector and dried to prepare a negative electrode active material layer A2 on the negative electrode current collector.

[0068] <Preparation of lithium-ion secondary battery C3 and electrochemical evaluation> The negative electrode active material layer A2 and a lithium foil as a counter electrode were placed opposite each other through a separator and housed in a container. Then, the electrolyte B2 was poured into the container, and the container was sealed to prepare a lithium ion secondary battery C3. The electrochemical evaluation of the lithium ion secondary battery C3 was carried out in the same manner as the lithium ion secondary battery C1. The average coulombic efficiency of the lithium ion secondary battery C3 after 100 cycles was 99.6%.

[0069] Example 4 <Preparation of negative electrode active material layer A3> The specific surface area of ​​the silicon-based negative electrode active material is 33 m 2 A negative electrode mixture slurry was prepared by mixing silicon having a molecular weight of 1000 g / g, a polyimide binder as a binder, vapor grown carbon fiber (VGCF) and hollow carbon (KB) as conductive additives, and an appropriate amount of solvent. The obtained negative electrode mixture slurry was applied to a Cu foil as a negative electrode current collector and dried to prepare a negative electrode active material layer A3 on the negative electrode current collector.

[0070] <Preparation of lithium-ion secondary battery C4 and electrochemical evaluation> The negative electrode active material layer A3 and a lithium foil as a counter electrode were placed opposite each other through a separator and housed in a container. Then, the electrolyte B2 was poured into the container, and the container was sealed to prepare a lithium ion secondary battery C4. The electrochemical evaluation of the lithium ion secondary battery C4 was carried out in the same manner as the lithium ion secondary battery C1. The average coulombic efficiency of the lithium ion secondary battery C4 up to the 100th cycle was 99.0%.

[0071] Comparative Example 1 <Preparation of lithium-ion secondary battery c1 and electrochemical evaluation> The lithium foil as the negative electrode active material layer and the lithium foil as the counter electrode were placed opposite each other through a separator and housed in a container. Next, the electrolyte B1 was poured into the container, and the container was sealed to prepare a lithium ion secondary battery c1. The electrochemical evaluation of the lithium ion secondary battery c1 was carried out in the same manner as the lithium ion secondary battery C1. The average coulombic efficiency of the lithium ion secondary battery c1 up to the 100th cycle was 83.4%.

[0072] Comparative Example 2 <Adjustment of electrolyte b1> Li-FSI as a supporting salt containing lithium ions was dissolved in 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (hereinafter referred to as "EMIm-FSI") as an ionic liquid such that the molar ratio of EMIm-FSI to Li-FSI (EMIm-FSI / Li-FSI) was 6, to prepare electrolyte b1.

[0073] <Preparation of lithium-ion secondary battery c2 and electrochemical evaluation> The negative electrode active material layer A1 and lithium foil as a counter electrode were placed opposite each other via a separator and housed in a container. Next, the electrolyte solution b1 was poured into the container, and the container was sealed to produce a lithium ion secondary battery c2. The electrochemical evaluation of the lithium ion secondary battery c2 was carried out in the same manner as the lithium ion secondary battery C1. The average coulombic efficiency of the lithium ion secondary battery c1 up to the 100th cycle was 96.8%.

[0074] Comparative Example 3 <Production of lithium-ion secondary battery c3 and electrical evaluation> The lithium foil as the negative electrode active material layer and the lithium foil as the counter electrode were placed opposite each other through a separator and housed in a container. Next, the electrolyte solution b1 was poured into the container, and the container was sealed to produce a lithium ion secondary battery c3. The electrochemical evaluation of the lithium ion secondary battery c3 was carried out in the same manner as the lithium ion secondary battery C1. The average coulombic efficiency of the lithium ion secondary battery c1 up to the 100th cycle was 97.2%.

[0075] Table 1 shows the results of Examples 1 to 4 and Comparative Examples 1 to 3. Fig. 1 shows a graph comparing the average coulombic efficiencies of the lithium ion secondary batteries in Examples 1 and 2 and Comparative Examples 1 to 3. Fig. 2 shows a graph comparing the average coulombic efficiencies of the lithium ion secondary batteries in Examples 2 and 3.

[0076] [Table 1]

[0077] The lithium ion secondary batteries C1 to C4, which used an electrolyte solution containing the ionic liquid P2223-LSI containing phosphonium ions for a silicon-based negative electrode active material, were able to achieve a high average Coulombic efficiency of 99.0% or more even after 100 cycles (Examples 1 to 4). 2 The lithium ion secondary batteries C2 and C3, in which the average Coulombic efficiency was 1.5 or less / g and the molar number of the ionic liquid relative to the lithium salt was 1.5 or less, exhibited high average Coulombic efficiency.

[0078] In the lithium-ion secondary batteries C1 to C4, the electrolyte contains phosphonium ions, which form an SEI that is effective in expanding and contracting the silicon-based negative electrode active material, that is, an SEI that can suppress the problem of the SEI cracking and forming a new surface when a silicon-based negative electrode active material is used, thereby causing the SEI to continue to grow. It is presumed that this is why the average Coulombic efficiency is improved. In addition, since the phosphonium ions have high electrochemical stability, excessive formation of the SEI is suppressed, which is why it is presumed that the average Coulombic efficiency is improved.

[0079] On the other hand, the lithium-ion secondary battery c1, which does not contain a silicon-based negative electrode active material, had a low average Coulombic efficiency of 83.4% after 100 cycles, suggesting that an electrolyte containing phosphonium ions is effective for silicon-based negative electrode active materials. In addition, the lithium-ion secondary battery c2, which uses an electrolyte containing the ionic liquid EMIm-FSI, which does not contain phosphonium ions, also had a low average Coulombic efficiency of 96.8% after 100 cycles, suggesting the usefulness of phosphonium ions for silicon-based negative electrode active materials.

[0080] Although preferred embodiments of the lithium ion secondary battery of the present disclosure have been described, those skilled in the art will recognize that modifications are possible without departing from the scope of the claims.

Claims

1. A lithium ion secondary battery, the lithium ion secondary battery has a negative electrode active material layer, a separator layer, and a positive electrode active material layer in this order; The negative electrode active material layer contains a silicon-based negative electrode active material, and the negative electrode active material layer, the separator layer, and the positive electrode active material layer are impregnated with an electrolyte; The electrolyte contains an ionic liquid and lithium ions, The ionic liquid contains a phosphonium ion and a bis(fluorosulfonyl)imide ion. Lithium-ion secondary battery.

2. 2. The lithium ion secondary battery according to claim 1, wherein the phosphonium ion is a triethylpropylphosphonium ion.

3. The specific surface area of ​​the silicon-based negative electrode active material is 25 m ２ The lithium ion secondary battery according to claim 1, wherein the capacitance is 0.1 μm or less.

4. The lithium ion secondary battery according to any one of claims 1 to 3, wherein the molar ratio of the total number of moles of the phosphonium ion and the bis(fluorosulfonyl)imide ion to the total number of moles of the lithium ion and the bis(fluorosulfonyl)imide ion is 1.5 or less.

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