Lithium ion secondary battery
The lithium-ion secondary battery design with a silicon-based active material and specific electrolyte composition stabilizes the SEI film, enhancing initial coulombic efficiency by efficiently utilizing lithium ions.
Patent Information
- Application Number
- JP2024065795
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2024-04-15
- Publication Date
- 2025-10-27
AI Technical Summary
The SEI film in lithium-ion secondary batteries with silicon-based negative electrode active materials cracks due to expansion and contraction, leading to electrolyte decomposition and reduced initial coulombic efficiency due to a large specific surface area and contact area with the electrolyte.
A lithium-ion secondary battery design incorporating a silicon-based negative electrode active material with a specific surface area of 20 to 60 m²/g, impregnated with an electrolyte solution containing triethylpropylphosphonium bis(fluorosulfonyl)imide as the ionic liquid and lithium bis(fluorosulfonyl)imide as the lithium salt, with a molar ratio of 2.0 to 7.0, to stabilize the SEI film.
Improves the initial coulombic efficiency by forming a suitable SEI film that efficiently utilizes lithium ions, despite the large surface area of the silicon-based active material.
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Figure 2025162462000001_ABST
Abstract
Description
[Technical Field]
[0001] The present disclosure relates to lithium-ion secondary batteries. [Background technology]
[0002] In lithium-ion secondary batteries, a solid electrolyte interfacial (SEI) film is known to form at the interface between the electrode active material and the electrolyte. This SEI film is often a combination of the electrode active material and the electrolyte and may contain decomposition products of the electrolyte. The SEI film functions as a protective layer on the surface of the electrode active material, protecting the electrode active material and the electrolyte from corrosion and other undesirable side reactions. The following electrode systems are known that use ionic liquids as electrolytes and have stable SEI films:
[0003] Patent Document 1 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 containing LiFSI salt and an ionic liquid having a bis(fluorosulfonyl)imide (FSI) anion. The electrode system in Patent Document 1 is said to be able to provide high energy density and long cycle life at low cost. [Prior art documents] [Patent documents]
[0004] [Patent Document 1] International Publication No. 2016 / 070120 Summary of the Invention [Problem to be solved by the invention]
[0005] As described above, the SEI film functions as a protective layer on the surface of the electrode active material, protecting the electrode active material and the electrolyte from corrosion and other undesirable reactions. When a silicon-based negative electrode active material is used as the electrode active material in a liquid-phase battery, the SEI film may crack due to the expansion and contraction of the silicon-based negative electrode active material, forming a new SEI film surface, which may lead to continued decomposition of the electrolyte and growth of the SEI film.
[0006] To address the collapse of the SEI film due to the expansion and contraction of silicon-based negative electrode active materials, it is conceivable to use low-expansion silicon-based negative electrode active materials. However, low-expansion silicon-based negative electrode active materials have a large specific surface area and a large contact area with the electrolyte, which results in the formation of a relatively large SEI film, which in turn reduces the initial coulombic efficiency.
[0007] Therefore, an object of the present disclosure is to provide a lithium ion secondary battery that includes a silicon-based negative electrode active material with a large specific surface area and that can improve the initial coulombic efficiency. [Means for solving the problem]
[0008] The present disclosure achieves the above object by the following means.
[0009] <Aspect 1> 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, The specific surface area of the silicon-based negative electrode active material is 20 to 60 m 2 / g, the negative electrode active material layer, the separator layer, and the positive electrode active material layer are impregnated with an electrolyte solution, the electrolyte solution contains an ionic liquid and a lithium salt, the ionic liquid comprises triethylpropylphosphonium bis(fluorosulfonyl)imide; the lithium salt comprises lithium bis(fluorosulfonyl)imide; and the molar ratio of the number of moles of the ionic liquid to the number of moles of the lithium salt is 2.0 to 7.0; Lithium-ion secondary battery. <Aspect 2> 2. The lithium ion secondary battery according to aspect 1, wherein the molar ratio is 5.0 to 7.0. <Aspect 3> The specific surface area is 20 to 30 m 2 / g. [Effects of the Invention]
[0010] According to the lithium ion secondary battery of the present disclosure, the initial coulombic efficiency can be improved in a lithium ion secondary battery containing a silicon-based negative electrode active material with a large specific surface area. [Brief explanation of the drawings]
[0011] [Figure 1] FIG. 1 is a schematic cross-sectional view illustrating a lithium ion secondary battery according to the present disclosure. [Figure 2] FIG. 2 is a bar graph comparing the initial coulombic efficiency of each electrolyte solution for the Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION
[0012] 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 gist of the present disclosure.
[0013] In the present disclosure, a "composite" refers to a composition that can constitute an electrode active material layer, etc., either 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 an electrode active material layer, etc.
[0014] <Lithium-ion secondary battery> The lithium ion secondary battery of the present disclosure comprises: 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, The specific surface area of the silicon-based negative electrode active material is 20 to 60 m 2 / g, the negative electrode active material layer, the separator layer, and the positive electrode active material layer are impregnated with an electrolyte solution, the electrolyte solution contains an ionic liquid and a lithium salt, the ionic liquid comprises triethylpropylphosphonium bis(fluorosulfonyl)imide; the lithium salt comprises lithium bis(fluorosulfonyl)imide; and The molar ratio of the number of moles of the ionic liquid to the number of moles of the lithium salt is 2-7.
[0015] According to the lithium ion secondary battery of the present disclosure, the initial coulombic efficiency can be improved in a lithium ion secondary battery containing a silicon-based negative electrode active material with a large specific surface area.
[0016] The present inventors have discovered that in a lithium ion secondary battery containing a silicon-based negative electrode active material with a large specific surface area, triethylpropylphosphonium bis(fluorosulfonyl)imide (P2223-FSI) as an ionic liquid, and lithium bis(fluorosulfonyl)imide (Li-FSI) as a lithium salt, there is a relationship between the molar ratio of the number of moles of P2223-FSI to the number of moles of Li-FSI and the initial coulombic efficiency that differs from that observed with other ionic liquids.
[0017] This relationship is as follows: in lithium-ion secondary batteries containing P2223-FSI as the ionic liquid, the initial coulombic efficiency increases as the molar ratio of the number of moles of P2223-FSI (ionic liquid) to the number of moles of Li-FSI (lithium salt) increases, i.e., the lithium ion concentration in the electrolyte decreases; on the other hand, in lithium-ion secondary batteries containing other ionic liquids, the initial coulombic efficiency decreases as the molar ratio of the number of moles of ionic liquid to the number of moles of lithium salt increases, i.e., the lithium ion concentration in the electrolyte decreases.
[0018] Based on these findings, the present inventors have conceived of a lithium ion secondary battery that includes a silicon-based negative electrode active material with a large specific surface area, and that can improve the initial coulombic efficiency by including P2223-FSI as an ionic liquid and Li-FSI as a lithium salt, and by setting the molar ratio of the number of moles of ionic liquid to the number of moles of lithium salt within a predetermined range.
[0019] Without being limited by theory, it is speculated that when P223-FSI is used as the ionic liquid during the initial charge / discharge cycle, the high electrochemical stability of the P2223 ion leads to the formation of an SEI film suitable for silicon-based negative electrode active materials. Therefore, even if the molar ratio is high, i.e., even if the P2223 ion concentration is high and the lithium ion concentration is low, the high electrochemical stability of the P2223 ion leads to the formation of an SEI film suitable for silicon-based active materials with a large specific surface area, allowing the lithium ions present in the system to be efficiently utilized, thereby improving the initial coulombic efficiency.
[0020] On the other hand, when other ionic liquids are used as the ionic liquid during the initial charge / discharge cycle, an SEI film is also formed, but it is presumed that the SEI film grows more easily than when using an electrolyte containing the P2223 ion, which has high electrochemical stability. Therefore, when the molar ratio is large, the ions contained in the ionic liquid, especially the cations, have a large contact area with the electrolyte for the silicon-based active material with a large specific surface area, resulting in the formation of a relatively large SEI film, which is presumed to result in a low initial coulombic efficiency.
[0021] FIG. 1 is a cross-sectional schematic diagram showing one embodiment of the lithium ion secondary battery of the present disclosure, but the present disclosure is not limited to this case.
[0022] The lithium ion secondary battery 100 has a negative electrode active material layer 110, a separator layer 120, and a positive electrode active material layer 130 in this order. The negative electrode active material layer 110 has a specific surface area of 20 to 60 m 2 The negative electrode active material layer 110, the separator layer 120, and the positive electrode active material layer 130 are impregnated with an electrolyte. 2 In a lithium ion secondary battery containing a silicon-based negative electrode active material having a capacitance of 1.0 μm / g, the initial coulombic efficiency can be improved by using an electrolyte solution containing a specific ionic liquid and a predetermined amount of lithium salt.
[0023] The lithium ion secondary battery of the present disclosure has a negative electrode active material layer, a separator layer, and a positive electrode active material layer, and may further have an optional negative electrode current collector layer and a positive electrode current collector layer.
[0024] <Negative electrode current collector layer> The material used for the negative electrode current collector layer is not particularly limited, and any material commonly used for negative electrode current collectors in lithium-ion secondary batteries can be appropriately used. 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, and carbon sheet. The negative electrode current collector layer may have a coating layer on its surface for the purpose of adjusting resistance, etc.
[0025] The shape of the negative electrode current collector layer is not particularly limited, but examples thereof include foil, plate, mesh, etc. Among these, foil is preferred.
[0026] 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.
[0027] <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 additive, 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, conductive additive, binder, solid electrolyte, and the like in the negative electrode active material layer may be appropriately determined depending on the desired battery performance. For example, when the entire negative electrode active material layer (total solid content) is taken as 100 mass%, the content of the negative electrode active material may be 40 mass% or more, 50 mass% or more, or 60 mass% or more, or may be 100 mass% or less, or 90 mass% or less.
[0028] (Silicon-based negative electrode active material) 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 may also contain metal elements other than silicon, such as Fe, Co, Sb, Bi, Pb, Ni, Cu, Zn, Ge, In, Sn, and Ti.
[0029] The shape of the silicon-based negative electrode active material is not particularly limited, and may be any shape common to silicon-based negative electrode active materials for lithium-ion secondary batteries. The silicon-based negative electrode active material may be in the form of particles. The silicon-based negative electrode active material may be in the form of primary particles or secondary particles formed by agglomeration of multiple primary particles.
[0030] The specific surface area of silicon-based negative electrode active materials is 20 to 60 m 2 / g, 20-30m 2 The specific surface area is preferably 20 m / g, for example. 2 / g or more, 22m 2 / g or more, 24m 2 / g or more, 26m 2 / g or more, 28m 2 / g or more, 30m 2 / g or more, or 60m 2 / g or less, 50m 2 / g or less, 40m 2 / g or less, 35m 2 / g or less, 30m 2 The specific surface area can be determined by the BET method using nitrogen as an adsorbate.
[0031] (Conductive additive) The conductive additive is not particularly limited. The conductive additive 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 additive may be, for example, particulate or fibrous, and its size is not particularly limited. The conductive additive is not particularly limited, but one type may be used alone, or two or more types may be used in combination.
[0032] (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), or other materials, but is not limited to these. The binder is not particularly limited, and one type may be used alone, or two or more types may be used in combination.
[0033] (solid electrolyte) The material of the solid electrolyte is not particularly limited, but may be a sulfide solid electrolyte, an oxide solid electrolyte, a polymer electrolyte, or the like. Examples of sulfide solid electrolytes include Li2S-P2S5-based (Li7P3S 11 Examples of oxide solid electrolytes include, but are not limited to, Li7La3Zr2O 12 , Li 7-x La3Zr 1-x Nb x O 12 Examples of the polymer electrolyte include, but are not limited to, polyethylene oxide (PEO).
[0034] The shape of the negative electrode active material layer is not particularly limited, and may be, for example, a substantially flat sheet-like negative electrode active material layer. 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.
[0035] The negative electrode active material layer can be produced by applying a known method. For example, the negative electrode active material layer can be easily formed by dry or wet molding a negative electrode composite 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.
[0036] <Separator layer> The material of the separator layer is not particularly limited, and may be any material commonly used for separator layers in lithium ion secondary batteries, such as polyolefin, polyamide, or polyimide nonwoven fabric.
[0037] <Cathode active material layer> The positive electrode active material layer contains at least a positive electrode active material, and may further contain, optionally, a conductive additive, 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, conductive additive, binder, solid electrolyte, and the like in the positive electrode active material layer may be appropriately determined depending on the desired battery performance. For example, when the entire positive electrode active material layer (total solid content) is taken as 100 mass%, the content of the positive electrode active material may be 40 mass% or more, 50 mass% or more, 60 mass% or more, or 100 mass% or less, or 90 mass% or less.
[0038] (Cathode active material) Positive electrode active materials include, for example, lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), and nickel-cobalt-manganese lithium oxide (NCM:LiCO 1 / 3 Ni 1 / 3 Mn 1 / 3 O2), but is not limited to these cases.
[0039] 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 the form of particles. The positive electrode active material may be in the form of primary particles or secondary particles 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 50% cumulative value in the volume-based particle size distribution determined by laser diffraction / scattering method.
[0040] For the conductive additive, binder, and solid electrolyte that can be contained in the positive electrode active material layer, reference can be made to the description above in "<Negative electrode active material layer>".
[0041] The shape of the positive electrode active material layer is not particularly limited, and may be, for example, a substantially flat sheet-like positive electrode active material layer. 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.
[0042] The positive electrode active material layer can be produced by applying a known method. For example, the positive electrode active material layer can be easily formed by dry or wet molding a positive electrode composite 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.
[0043] <Positive electrode current collector layer> The material used for the positive electrode current collector layer is not particularly limited, and any material commonly used for positive electrode current collectors in lithium-ion secondary batteries can be appropriately used. Examples of materials used for the positive electrode current collector layer include, but are not limited to, Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. The positive electrode current collector layer may have a coating layer on its surface for purposes such as adjusting resistance. The positive electrode current collector layer may also be a metal foil or a substrate on which the above metals are plated or vapor-deposited.
[0044] The shape of the positive electrode current collector layer is not particularly limited, but examples thereof include foil, plate, mesh, etc. Among these, foil is preferred.
[0045] 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.
[0046] 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.
[0047] <Electrolyte> In the lithium-ion secondary battery of the present disclosure, the electrolyte solution contains an ionic liquid and a lithium salt. The contents of the ionic liquid and the lithium salt, relative to the total mass of the electrolyte solution (100 mass %), are not particularly limited, but may be 50 mass % or more, 60 mass %, 70 mass % or more, or 80 mass % or more, or 100 mass % or less, or 95 mass % or less.
[0048] (ionic liquid) The ionic liquid comprises triethylpropylphosphonium bis(fluorosulfonyl)imide.
[0049] The ionic liquid is not particularly limited, but may contain an ionic liquid other than triethylpropylphosphonium bis(fluorosulfonyl)imide. When the entire ionic liquid is taken as 100 mass%, the content of triethylpropylphosphonium bis(fluorosulfonyl)imide is not particularly limited, but may be 50 mass% or more, 60 mass%, 70 mass% or more, or 80 mass% or more, or 100 mass% or less, or 95 mass% or less.
[0050] (lithium salts) The lithium salt contains a lithium ion and a bis(fluorosulfonyl)imide ion.
[0051] The lithium salt is not particularly limited, but may contain a lithium salt other than lithium bis(fluorosulfonyl)imide. When the total amount of the lithium salt is 100 mass%, the content of lithium bis(fluorosulfonyl)imide is not particularly limited, but may be 50 mass% or more, 60 mass%, 70 mass% or more, or 80 mass% or more, and may be 100 mass% or less, or 95 mass% or less.
[0052] (Ratio of ionic liquid to lithium salt) In the lithium-ion secondary battery of the present disclosure, the molar ratio of the number of moles of ionic liquid to the number of moles of lithium salt is 2.0 to 7.0. This molar ratio is not particularly limited, but is preferably 5.0 to 7.0 from the viewpoint of initial coulombic efficiency. This molar ratio may be 2.0 or more, 3.0 or more, 4.0 or more, 5.0 or more, or 6.0 or more, and may be 7.0 or less, 6.5 or less, or 6.0 or less. An electrolyte solution having the above molar ratio can be prepared, for example, by dissolving a predetermined amount of lithium bis(fluorosulfonyl)imide (LiFSI) as a lithium salt in triethylpropylphosphonium bis(fluorosulfonyl)imide (P2223-FSI) as an ionic liquid.
[0053] <<Uses of lithium secondary batteries>> The lithium ion secondary battery in the present disclosure may be, for example, an in-vehicle battery, or may be used as a power source for moving objects other than vehicles (e.g., trains, ships, and aircraft), or may be used as a power source for electrical appliances such as information processing devices. [Example]
[0054] 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. Note that the half-cells used in the examples of the present disclosure are mainly composed of a silicon-based negative electrode active material and an ionic liquid, and a simple evaluation was performed using metallic lithium as the counter electrode, in order to clearly compare the initial coulombic efficiency of each cell.
[0055] <Production Example 1> <Formation of Negative Electrode Active Material Layer A1> Silicon (specific surface area 25m) as a silicon-based negative electrode active material 2 A negative electrode composite slurry was prepared by mixing the sintered body (A1) containing 10 ...
[0056] <Production Example 2> <Formation of Negative Electrode Active Material Layer A2> Silicon (specific surface area 25m 2 / g), silicon (specific surface area 33m 2 A negative electrode active material layer A2 was formed in the same manner as in Production Example 1, except that a 1000 vol. / g powder was used.
[0057] <<Production Example 3>> <Formation of Negative Electrode Active Material Layer A3> Silicon (specific surface area 25m 2 / g), silicon (specific surface area 50m 2 A negative electrode active material layer A3 was formed in the same manner as in Production Example 1, except that a 1000 vol. / g powder was used.
[0058] <<Production Example 4>> <Preparation of Electrolyte Solution B1> Electrolyte solution B1 was prepared by dissolving lithium bis(fluorosulfonyl)imide (hereinafter referred to as "Li-FSI") as a lithium salt in triethylpropylphosphonium bis(fluorosulfonyl)imide (hereinafter referred to as "P2223-FSI") as an ionic liquid so that the molar ratio of P2223-FSI to Li-FSI (P2223-FSI / Li-FSI) was 3.0.
[0059] Example 5 <Preparation of Electrolyte Solution B2> An electrolyte solution B2 was prepared in the same manner as in Production Example 4, except that P2223-FSI was dissolved so that the molar ratio of P2223-FSI to Li-FSI (P2223-FSI / Li-FSI) was 6.0.
[0060] Example 1 <Production of lithium-ion battery (LIB) 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 (hereinafter also referred to as "LIB-C1").
[0061] Electrochemical evaluation of LIB-C1 LIB-C1 was charged and discharged for one cycle under constant current-constant voltage conditions (current value: 0.1 C) with a charge capacity of 1200 mAh / g. The initial coulombic efficiency was calculated as the ratio of the initial discharge capacity to the initial charge capacity (initial discharge capacity / initial charge capacity). The initial coulombic efficiency of LIB-C1 was 74.2%.
[0062] Examples 2 to 6 <Fabrication and electrochemical evaluation of lithium-ion secondary batteries (LIBs) C2-6> Lithium ion secondary batteries C2 to C6 (hereinafter also referred to as "LIB-C2 to C6") were produced in the same manner as in Example 1, except that negative electrode active material layers and electrolyte solutions shown in Tables 1, 2, and 3 were used instead of negative electrode active material layer A1 and electrolyte solution B1. Electrochemical evaluation of LIB-C2 to C6 was carried out in the same manner as in Example 1, and the initial coulombic efficiencies of LIB-C2 to C6 were as shown in Tables 1, 2, and 3.
[0063] Example 6 <Preparation of electrolyte b1> An electrolyte solution b1 was prepared in the same manner as in Production Example 4, except that P2223-FSI was dissolved so that the molar ratio of P2223-FSI to Li-FSI (P2223-FSI / Li-FSI) was 1.5.
[0064] <<Production Example 7>> <Preparation of electrolyte b2> Electrolyte solution b2 was prepared by dissolving Li-FSI as a lithium salt in 1-ethyl-3-methylimidazolium bisbis(fluorosulfonyl)imide (hereinafter referred to as "EMIm-FSI") as an ionic liquid so that the molar ratio of EMIm-FSI to Li-FSI (EMIm-FSI / Li-FSI) was 1.5.
[0065] <<Production Example 8>> <Preparation of electrolyte b3> An electrolyte solution b3 was prepared in the same manner as in Production Example 7, except that EMIm-FSI was dissolved so that the molar ratio of EMIm-FSI to Li-FSI (EMIm-FSI / Li-FSI) was 3.0.
[0066] Example 9 <Preparation of electrolyte b4> An electrolyte solution b4 was prepared in the same manner as in Production Example 7, except that EMIm-FSI was dissolved so that the molar ratio of EMIm-FSI to Li-FSI (EMIm-FSI / Li-FSI) was 6.0.
[0067] Example 10 <Preparation of electrolyte b5> Electrolyte b5 was prepared by dissolving Li-FSI as a lithium salt in 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide (hereinafter referred to as "P13-FSI") as an ionic liquid so that the molar ratio of P13-FSI to Li-FSI (P13-FSI / Li-FSI) was 1.5.
[0068] Example 11 <Preparation of electrolyte b6> An electrolyte solution b6 was prepared in the same manner as in Production Example 10, except that P13-FSI was dissolved so that the molar ratio of P13-FSI to Li-FSI (P13-FSI / Li-FSI) was 6.0.
[0069] Comparative Examples 1 to 18 <Fabrication and electrochemical evaluation of lithium-ion secondary batteries (LIBs) c1 to c18> Lithium ion secondary batteries c1 to c18 (hereinafter also referred to as "LIB-c1 to c18") were produced in the same manner as in Example 1, except that negative electrode active material layers and electrolyte solutions shown in Tables 1, 2, and 3 were used instead of negative electrode active material layer A1 and electrolyte solution B1. Electrochemical evaluation of LIB-c1 to c18 was carried out in the same manner as in Example 1, and the initial coulombic efficiencies of LIB-c1 to c18 were as shown in Tables 1, 2, and 3.
[0070] Example 12 <Preparation of electrolyte b7> Lithium hexafluorophosphate (LiPF6) as a supporting salt containing lithium ions was dissolved in ethylene carbonate (EC)-dimethyl carbonate (DMC)-ethyl methyl carbonate (EMC) to adjust the LiPF6 concentration to 1.2 M. Furthermore, fluoroethylene carbonate (FEC) was added to this solution as a stabilizer.
[0071] 《Reference examples 1~3》 <Fabrication and electrochemical evaluation of lithium-ion secondary batteries (LIBs) r1 to r3> Lithium ion secondary batteries r1 to r3 (hereinafter also referred to as "LIB-r1 to r3") were produced in the same manner as in Example 1, except that negative electrode active material layers and electrolyte solutions shown in Tables 1, 2, and 3 were used instead of negative electrode active material layer A1 and electrolyte solution B1. Electrochemical evaluation of LIB-r1 to r3 was carried out in the same manner as in Example 1, and the initial coulombic efficiencies of LIB-r1 to r3 were as shown in Tables 1, 2, and 3.
[0072] [Table 1]
[0073] [Table 2]
[0074] [Table 3]
[0075] In Table 1, the specific surface area is 25m 2The initial coulombic efficiency of lithium ion secondary batteries containing silicon-based negative electrode active materials with an ionic liquid content of 0.1g / g was compared. In lithium ion secondary batteries containing P2223-FSI as the electrolyte, the initial coulombic efficiency increased as the molar ratio to the lithium salt increased, and the lithium ion secondary batteries of Examples 1 and 2 exhibited high initial coulombic efficiencies. On the other hand, lithium ion secondary batteries containing EMIm-FSI or P13-FSI as the electrolyte had lower coulombic efficiencies than lithium ion secondary batteries containing P2223-FSI, and the initial coulombic efficiency tended to decrease as the molar ratio of the number of moles of ionic liquid to the number of moles of lithium salt increased.
[0076] In Table 2, the specific surface area is 33 m 2 In lithium ion secondary batteries containing silicon-based negative electrode active materials with a specific surface area of 50 m / g, the initial coulombic efficiency of the electrolyte was compared. 2 The initial coulombic efficiencies of the electrolytes were compared for lithium-ion secondary batteries containing silicon-based negative electrode active materials with a 0.05% ionic strength (0.05%) / g. Tables 2 and 3 showed similar trends to those in Table 1. The bar graph in Figure 2 summarizes the results in Tables 1, 2, and 3.
[0077] Although the details are unclear, it is speculated that when P223-FSI was used as the ionic liquid during the first charge / discharge, the high electrochemical stability of the P2223 ion led to the formation of an SEI film suitable for silicon-based anode active materials. Therefore, even when the molar ratio was high, i.e., the P2223 ion concentration was high and the lithium ion concentration was low, the high electrochemical stability of the P2223 ion led to the formation of an SEI film suitable for silicon-based active materials with a large specific surface area, allowing the lithium ions present in the system to be efficiently utilized, thereby improving the initial Coulombic efficiency.
[0078] On the other hand, when other ionic liquids were used as the ionic liquid during the initial charge / discharge cycle, an SEI film was also formed, but it is presumed that the SEI film grew more easily than when using an electrolyte containing the P2223 ion, which has high electrochemical stability. Therefore, when the molar ratio was increased, the ions, especially the cations, contained in the ionic liquid had a large contact area with the electrolyte for the silicon-based active material, which had a large specific surface area, and a relatively large SEI film was formed, which is presumed to be the reason for the low initial coulombic efficiency.
[0079] 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. [Explanation of symbols]
[0080] 100 Lithium-ion secondary battery 110 Negative electrode active material layer 120 separator layer 130 Cathode active material layer
Claims
1. 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, The specific surface area of the silicon-based negative electrode active material is 20 to 60 m ２ / g, the negative electrode active material layer, the separator layer, and the positive electrode active material layer are impregnated with an electrolyte solution; the electrolyte solution contains an ionic liquid and a lithium salt, the ionic liquid comprises triethylpropylphosphonium bis(fluorosulfonyl)imide; the lithium salt comprises lithium bis(fluorosulfonyl)imide; and the molar ratio of the number of moles of the ionic liquid to the number of moles of the lithium salt is 2.0 to 7.0; Lithium-ion secondary battery.
2. 2. The lithium ion secondary battery according to claim 1, wherein the molar ratio is 5.0 to 7.
0.
3. The specific surface area is 20 to 30 m ２ The lithium ion secondary battery according to claim 1 or 2, wherein the ionic strength is 0.15g / g.
Citation Information
Patent Citations
Stable silicon-ionic liquid interface lithium-ion batteries
WO2016070120A1