Non-aqueous electrolyte solutions and lithium-ion secondary batteries

Abstract

The non-aqueous electrolyte solution involved in this embodiment includes lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium nitrate. In the non-aqueous electrolyte involved in this embodiment, the molar ratio of lithium nitrate to lithium hexafluorophosphate is 0.01 or more and 0.15 or less, and the molar ratio of lithium bis(fluorosulfonyl)imide to lithium hexafluorophosphate is 0.1 or more and 1.0 or less.

Description

Technical Field

[0001] This disclosure relates to a non-aqueous electrolyte solution and a lithium-ion secondary battery. This application claims priority based on Japanese Application No. 2024-150606, filed on September 2, 2024, and incorporates therein. Background Technology

[0002] Lithium-ion rechargeable batteries are widely used as power sources for mobile devices such as mobile phones and laptops, as well as hybrid vehicles.

[0003] Various studies have been conducted to improve the performance of lithium-ion rechargeable batteries. Cycling performance is one of the required characteristics of lithium-ion rechargeable batteries. Lithium-ion rechargeable batteries degrade after multiple charge-discharge cycles. Cycling performance refers to the capacity retention of a lithium-ion rechargeable battery after multiple charge-discharge cycles relative to its initial charge-discharge state. If the electrolyte decomposes on the surface of the negative electrode, the cycling performance of the lithium-ion rechargeable battery will decrease.

[0004] For example, Patent Document 1 discloses that the cycle characteristics of a lithium-ion secondary battery are improved by using a non-aqueous electrolyte containing a specified lithium salt and a specified glycol dimethyl ether.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2018-67501 Summary of the Invention

[0008] The technical problem the invention aims to solve

[0009] Even when using the non-aqueous electrolyte solution described in Patent Document 1, sufficient cycle characteristics are sometimes not observed with increasing charge-discharge cycles. Furthermore, lithium-ion secondary batteries are used in various environments and are required to operate appropriately at both low and high temperatures. Therefore, lithium-ion secondary batteries that exhibit sufficient cycle characteristics at both low and high temperatures are required.

[0010] This disclosure was made in view of the above-mentioned problems, and its object is to provide a lithium-ion secondary battery with excellent cycle characteristics at both low and high temperatures, and a non-aqueous electrolyte solution for the lithium-ion secondary battery.

[0011] Means for solving technical problems

[0012] To address the aforementioned technical issues, the following methods are provided.

[0013] (1) The non-aqueous electrolyte solution involved in the first method comprises lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium nitrate. In the non-aqueous electrolyte solution involved in the first method, the molar ratio of lithium nitrate to lithium hexafluorophosphate is 0.01 or more and 0.15 or less, and the molar ratio of lithium bis(fluorosulfonyl)imide to lithium hexafluorophosphate is 0.1 or more and 1.0 or less.

[0014] (2) The lithium-ion secondary battery involved in the second method has a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and the non-aqueous electrolyte solution involved in the above method.

[0015] (3) Alternatively, in the lithium-ion secondary battery described above, the S2p spectrum obtained by measuring the surface of the negative electrode using X-ray photoelectron spectroscopy (XPS) has: a first peak observed in the range of binding energy above 165 eV and below 175 eV, and a second peak observed in the range of binding energy above 155 eV and below 165 eV. Alternatively, the intensity ratio of the second peak to the first peak is 0.5 or more and less than 4.0.

[0016] (4) Alternatively, in the lithium-ion secondary battery described above, the F1s spectrum obtained by measuring the surface of the negative electrode using X-ray photoelectron spectroscopy (XPS) has: a third peak observed in the range of binding energy above 686 eV and below 690 eV, and a fourth peak observed in the range of binding energy above 682 eV and below 686 eV. Alternatively, the intensity ratio of the fourth peak to the third peak is 1.5 or more and less than 4.0.

[0017] (5) Alternatively, in the lithium-ion secondary battery described above, the C1s spectrum obtained by measuring the surface of the negative electrode using X-ray photoelectron spectroscopy (XPS) has: a fifth peak observed in the range of binding energy above 288 eV and below 292 eV, and a sixth peak observed in the range of binding energy above 280 eV and below 288 eV. Alternatively, the intensity ratio of the sixth peak to the fifth peak is above 2.0 and less than 5.0.

[0018] (6) Alternatively, in the lithium-ion secondary battery described above, the negative electrode has a negative electrode active material layer, which contains a negative electrode active material. Alternatively, the negative electrode active material may contain at least one of silicon-containing compound particles and graphite particles. Alternatively, the volume percentage of silicon-containing compound particles in the negative electrode active material may be 10% by volume or more and 100% by volume or less.

[0019] (7) Alternatively, in the lithium-ion secondary battery described above, the negative electrode has a negative electrode active material layer containing silicon compound particles. Alternatively, the weight percentage of silicon in the negative electrode active material layer is 1% by weight or more and 50% by weight or less.

[0020] Invention Effects

[0021] Lithium-ion secondary batteries using the non-aqueous electrolyte solution described above exhibit excellent cycle characteristics at both low and high temperatures. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the lithium-ion secondary battery according to the first embodiment.

[0023] Figure 2 The S2p spectrum is obtained by measuring the negative electrode surface of the lithium-ion secondary battery according to the first embodiment using X-ray photoelectron spectroscopy (XPS).

[0024] Figure 3 The F1s spectrum is obtained by measuring the negative electrode surface of the lithium-ion secondary battery according to the first embodiment using X-ray photoelectron spectroscopy (XPS).

[0025] Figure 4 The C1s spectrum is obtained by measuring the negative electrode surface of the lithium-ion secondary battery according to the first embodiment using X-ray photoelectron spectroscopy (XPS).

[0026] Symbol Explanation

[0027] 10: Diaphragm

[0028] 20: Positive electrode

[0029] 22: Positive current collector

[0030] 24: Positive electrode active material layer

[0031] 30: Negative electrode

[0032] 32: Negative current collector

[0033] 34: Negative electrode active material layer

[0034] 40: Power generation components

[0035] 50: Exterior body

[0036] 52: Metal foil

[0037] 54: Resin layer

[0038] 60, 62: Terminal

[0039] 100: Lithium-ion secondary battery Detailed Implementation

[0040] Hereinafter, embodiments will be described in detail with appropriate reference to the accompanying drawings. The drawings used in the following description may differ from actual dimensions in some cases, for ease of understanding of features and to facilitate the depiction of key features. The materials, dimensions, etc., illustrated in the following description are examples only; the present invention is not limited to these examples and can be appropriately modified and implemented without altering its spirit.

[0041] Lithium-ion secondary batteries

[0042] Figure 1 This is a schematic diagram of the lithium-ion secondary battery according to the first embodiment. Figure 1 The lithium-ion secondary battery 100 shown includes a power generation element 40, an outer casing 50, and a non-aqueous electrolyte solution (not shown). The outer casing 50 covers the area surrounding the power generation element 40. The power generation element 40 is connected to the outside via a pair of terminals 60, 62 connected to it. The non-aqueous electrolyte solution is contained within the outer casing 50. Figure 1 The example shown illustrates a single power generation element 40 within the outer casing 50, but multiple power generation elements 40 can also be stacked. Furthermore, the lithium-ion secondary battery 100 can be any of the following types: cylindrical, square, laminated, or button-shaped.

[0043] (Power generation components)

[0044] The power generation element 40 includes a diaphragm 10, a positive electrode 20, and a negative electrode 30.

[0045] Positive electrode

[0046] The positive electrode 20, for example, has a positive current collector 22 and a positive active material layer 24. The positive active material layer 24 is in contact with at least one side of the positive current collector 22.

[0047] [Positive current collector]

[0048] The positive current collector 22 is, for example, a conductive plate. The positive current collector 22 is, for example, a thin metal sheet made of aluminum, copper, nickel, titanium, stainless steel, etc. Lightweight aluminum is suitable for use as the positive current collector 22. The average thickness of the positive current collector 22 is, for example, 10 μm or more and 30 μm or less. The positive current collector 22 can be a stretched film or a stamped film.

[0049] [Positive electrode active material layer]

[0050] The positive electrode active material layer 24 may contain, for example, a positive electrode active material. The positive electrode active material layer 24 may also contain conductive additives or binders as needed.

[0051] The positive electrode active material contains an electrode active material that enables the absorption and release of lithium ions, the removal and insertion (intercalation) of lithium ions, or the reversible doping and dedoping of lithium ions and anti-anions.

[0052] Positive electrode active materials are, for example, composite metal oxides. Examples of composite metal oxides include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), lithium manganese spinel (LiMn2O4), and those with the general formula: LiNi x Co y Mn z M a O2 compounds (in the general formula, x+y+z+a=1, 0≤x<1, 0≤y<1, 0≤z<1, 0≤a<1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr), lithium vanadium compounds (LiV2O5), olivine-type LiMPO4 (where M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO), lithium titanate (Li4Ti5O) 12 LiNi x Co y Al z O2 (0.9 < x + y + z < 1.1). As a general formula: LiNi x Co y Mn z M a The compound represented by O2, for example, can be selected from LiNi 0.92 Co 0.04 Mn 0.04 O2, LiNi 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.6 Co 0.2 Mn 0.1 Al 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.15 Al 0.05 O2, LiNi 0.7 Co 0.1 Mn 0.1 Al0.1 O2, LiNi 0.7 Mn 1.3 O4, LiNi 0.5 Mn 1.5 O4 and LiNi 0.3 Mn 1.7 One or more of the following: O4, etc. The positive electrode active material can also be an organic compound. For example, the positive electrode active material can also be polyacetylene, polyaniline, polypyrrole, polythiophene, polybenzoxene, etc.

[0053] The positive electrode active material can be a lithium-free material. Examples of lithium-free materials include FeF3, conjugated polymers containing organic conductive materials, Chevrel phase compounds, transition metal chalcogenides, vanadium oxides, niobium oxides, etc. A single lithium-free material can be used, or a combination of multiple materials can be used. When the positive electrode active material is lithium-free, for example, initial discharge is performed. Lithium is intercalated into the positive electrode active material through discharge. Alternatively, lithium can be pre-doped into the lithium-free positive electrode active material chemically or electrochemically.

[0054] Conductive additives improve the electronic conductivity between positive electrode active materials. Examples of conductive additives include carbon powder, carbon nanotubes, carbon materials, metal powders, mixtures of carbon materials and metal powders, and conductive oxides. Examples of carbon powders include carbon black, acetylene black, and Ketjen black. Examples of metal powders include powders of copper, nickel, stainless steel, and iron.

[0055] The content of the conductive additive in the positive electrode active material layer 24 is not particularly limited. For example, relative to the total mass of the positive electrode active material, conductive additive, and binder, the content of the conductive additive is 0.5% by mass or more and 20% by mass or less, preferably 1% by mass or more and 5% by mass or less.

[0056] The adhesive in the positive electrode active material layer 24 binds the positive electrode active materials together. Known adhesives can be used as the adhesive. The adhesive is preferably a material that is insoluble in the electrolyte, has oxidation resistance, and possesses adhesive properties. Examples of adhesives include fluoropolymers. Examples of adhesives include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamide-imide (PAI), polybenzimidazole (PBI), polyethersulfone (PES), polyacrylic acid and its copolymers, metal ion crosslinkers of polyacrylic acid and its copolymers, polypropylene (PP) or polyethylene (PE) grafted with maleic anhydride, or mixtures thereof. PVDF is particularly preferred as the adhesive used for the positive electrode active material layer.

[0057] The content of the binder in the positive electrode active material layer 24 is not particularly limited. For example, relative to the total mass of the positive electrode active material, conductive additive, and binder, the content of the binder is 1% by mass or more and 15% by mass or less, preferably 1.5% by mass or more and 5% by mass or less. If the content of the binder is low, the adhesive strength of the positive electrode 20 becomes weak. If the content of the binder is high, the binder is electrochemically inert and does not contribute to the discharge capacity, thus the energy density of the lithium-ion secondary battery 100 becomes low.

[0058] <Negative electrode>

[0059] The negative electrode 30, for example, has a negative current collector 32 and a negative active material layer 34. The negative active material layer 34 is formed on at least one side of the negative current collector 32. The negative electrode 30 is an example of a negative electrode for a lithium-ion secondary battery.

[0060] [Negative current collector]

[0061] The negative current collector 32 is, for example, a conductive plate. The negative current collector 32 can be made of the same material as the positive current collector 22. The negative current collector 32 can be an extended film or a stamped film.

[0062] [Negative electrode active material layer]

[0063] The negative electrode active material layer 34 contains negative electrode active material. The negative electrode active material layer 34 may also contain binders, conductive additives, etc., as needed.

[0064] The negative electrode active material can be a compound capable of absorbing and releasing ions, and can be a known negative electrode active material used in lithium-ion secondary batteries. Examples of negative electrode active materials include metallic lithium, lithium alloys, carbon materials, and materials capable of alloying with lithium. Examples of carbon materials include graphite (natural graphite, artificial graphite), carbon nanotubes, difficult-to-graphitize carbon, easily-graphitize carbon, and low-temperature calcined carbon, all capable of absorbing and releasing ions. Examples of materials capable of alloying with lithium include silicon, tin, zinc, lead, and antimony. Materials capable of alloying with lithium can be these elemental metals, or alloys or oxides containing these elements. Additionally, materials capable of alloying with lithium can also be composites whose surface is at least partially covered by a conductive material (e.g., carbon materials).

[0065] The negative electrode active material may also contain at least one of silicon-containing compound particles and graphite particles. Alternatively, the negative electrode active material may consist solely of silicon-containing compound particles. These silicon-containing compound particles may be elemental Si, SiC, or SiO. x (x, for example, satisfies 0.8≤x≤2.0), or it can be MSi (M is an alkaline earth metal or a transition metal).

[0066] In the negative electrode active material, the volume percentage of silicon-containing compound particles can be 10% or more and less than 100% by volume, or 10% or more and less than 100% by volume, or 10% or more and less than 50.5% by volume. For example, when the negative electrode active material is composed of silicon-containing compound particles and graphite particles, their combined volume percentage is 100% by volume.

[0067] The volume ratio of silicon-containing compound particles in the negative electrode active material was determined through the following steps. First, the negative electrode active material layer was divided into three parts along its thickness direction, namely, an upper layer, a middle layer, and a lower layer. Then, SEM (Scanning Electron Microscopy) images of three different locations within each of the upper, middle, and lower layers were taken. A total of nine cross-sectional SEM images were measured. The negative electrode active material was extracted from each of the cross-sectional SEM images. The extraction of the negative electrode active material can be performed using the difference in image contrast. Next, silicon-containing compound particles were extracted from the negative electrode active material. The extraction of silicon-containing compound particles can be performed using the difference in image contrast or compositional analysis using methods such as EDX (Energy Dispersive X-ray Spectroscopy). Then, the area ratio of silicon-containing compound particles relative to the negative electrode active material was determined in each image, and its average value was calculated. Since the cross-sectional SEM images were measured on any surface, the area ratio in the cross-sectional SEM images is approximately consistent with the overall volume ratio. Therefore, this average value was taken as the volume ratio of silicon-containing compound particles in the negative electrode active material.

[0068] Furthermore, in the negative electrode active material layer 34, the weight ratio of silicon can be 1% or more and 50% or less, or 1% or more and 10.2% or less. The weight ratio of silicon can be measured using X-ray fluorescence (XRF) analysis. XRF measurements are performed at three different points within the plane of the negative electrode active material layer 34, and the average of the calculated silicon ratios is obtained to determine the weight ratio of silicon in the negative electrode active material layer 34. The spacing between the measurement points during XRF measurements is wider than the diameter of the measuring sphere (the beam used in the measurement). For example, with a beam diameter of 1.2 mm, the distance between adjacent points of the three measurements is set to be greater than 1.2 mm.

[0069] The conductive additives and binders used in the negative electrode active material layer 34 can be the same substances as those used in the positive electrode active material layer 24.

[0070] The content of the binder in the negative electrode active material layer 34 is not particularly limited. For example, relative to the total mass of the negative electrode active material, conductive additive, and binder, the content of the binder is 0.5% by mass or more and 20% by mass or less, preferably 5% by mass or more and 15% by mass or less. If the content of the binder is low, the adhesive strength of the negative electrode 30 becomes weak. If the content of the binder is high, the binder 1 becomes electrochemically inert and does not contribute to the discharge capacity, thus the energy density of the lithium-ion secondary battery 100 becomes low.

[0071] The conductive additive in the negative electrode active material 34 improves the electronic conductivity between the negative electrode active materials. The conductive additive can be made of the same material as the positive electrode active material layer 24.

[0072] The content of the conductive additive in the negative electrode active material layer 34 is not particularly limited. For example, relative to the total mass of the negative electrode active material, conductive additive, and binder, the content of the conductive additive is 5% by mass or more and 20% by mass or less, preferably 1% by mass or more and 12% by mass or less.

[0073] <Septum>

[0074] The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 isolates the positive electrode 20 from the negative electrode 30, preventing short circuits between them. The separator 10 extends in-plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

[0075] The separator 10 is, for example, a porous structure with electrical insulation properties. The separator 10 is, for example, a monolayer or laminate of a polyolefin film. The separator 10 can also be an extended membrane of a mixture of polyethylene or polypropylene. The separator 10 can be a nonwoven fabric made of fibers selected from at least one of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 can also be a solid electrolyte, for example. Solid electrolytes include, for example, polymeric solid electrolytes, oxide solid electrolytes, and sulfide solid electrolytes. The separator 10 can also be an inorganic coated separator. An inorganic coated separator is formed by coating the surface of the above-mentioned membrane with a mixture of resins such as PVDF or CMC and inorganic materials such as alumina or silica. Inorganic coated separators have excellent heat resistance and suppress the precipitation of transition metals dissolved from the positive electrode to the negative electrode surface.

[0076] <Non-aqueous electrolyte solution>

[0077] A non-aqueous electrolyte solution is encapsulated within an outer casing 50 and immersed in a power generation element 40. The non-aqueous electrolyte solution may contain, for example, a non-aqueous solvent and an electrolytic salt. The electrolytic salt is dissolved in the non-aqueous solvent.

[0078] Non-aqueous solvents include, for example, aprotic organic solvents. Organic solvents include, for example, cyclic carbonates, chain carbonates, ethers, and mixtures thereof. Additionally, the solvent can also be an ionic liquid.

[0079] Cyclic carbonates solubilize the electrolyte. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, butyl carbonate, and fluoroethylene carbonate. Preferably, the cyclic carbonate contains at least fluoroethylene carbonate. Fluoroethylene carbonate (FEC) has a high redox potential and is easily reduced and decomposed. Through the partial reduction and decomposition of FEC, the electrolyte or remaining solvent in the electrolyte is difficult to decompose. Furthermore, during the initial use of the lithium-ion secondary battery, FEC forms a stable SEI coating on the surface of the negative electrode active material. The SEI coating prevents direct contact between the negative electrode active material and the electrolyte, and also prevents the decomposition of the electrolyte.

[0080] Chain carbonates reduce the viscosity of cyclic carbonates. Examples of chain carbonates include diethyl carbonate, dimethyl carbonate, and methyl ethyl carbonate. In addition, non-aqueous solvents may include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, and 1,2-diethoxyethane.

[0081] In non-aqueous electrolyte solutions, the electrolytic salts include lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium nitrate (LiNO3). The molar ratio of lithium nitrate to lithium hexafluorophosphate is 0.01 or more and 0.15 or less. Furthermore, the molar ratio of lithium bis(fluorosulfonyl)imide to lithium hexafluorophosphate is 0.1 or more and 1.0 or less.

[0082] If the non-aqueous electrolyte solution contains three electrolyte salts in the above ratio, a coating (SEI coating) with sufficient thickness is formed on the surface of the negative electrode 30 (negative electrode active material layer 34) that does not significantly impede the movement of Li. The coating is formed on the surface of the negative electrode 30 (negative electrode active material layer 34) by the reduction and decomposition of a portion of the non-aqueous electrolyte solution. The coating hinders further decomposition of the non-aqueous electrolyte solution.

[0083] If the coating is not thick enough, the non-aqueous electrolyte solution will come into contact with the negative electrode 30, increasing the likelihood of decomposition of the non-aqueous electrolyte solution. Decomposition of the non-aqueous electrolyte solution is one of the reasons for reduced cycle performance. Additionally, the coating may decompose due to heat during high-temperature operation. If the coating has sufficient thickness on the negative electrode surface, the cycle performance of the lithium-ion secondary battery is less likely to deteriorate even during high-temperature operation.

[0084] Furthermore, if the coating significantly hinders the movement of Li, Li deposition will occur on the surface of the coating. If lithium dendrites form on the surface of the coating, the cycle characteristics of the lithium-ion secondary battery will decrease. Compared to high-temperature operation, lithium ions are less mobile at low temperatures, making Li deposition more likely. If the coating contains the three electrolytes in the above ratio, it is difficult for the coating to achieve high resistance, and the cycle characteristics of the lithium-ion secondary battery will not decrease even at low temperatures.

[0085] The chemical bonding state of the coating can be analyzed by measuring the surface of the negative electrode 30 using X-ray photoelectron spectroscopy (XPS).

[0086] Figure 2 The S2p spectrum was obtained by measuring the surface (coating) of the negative electrode 30 using X-ray photoelectron spectroscopy (XPS). Figure 2 The solid line represents the result when the non-aqueous electrolyte solution contains lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium nitrate (LiNO3) in the ratio specified above. Figure 2 The dashed line represents the result when the non-aqueous electrolyte solution contains lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) but not lithium nitrate (LiNO3).

[0087] The S2p spectrum exhibits a first peak p1 observed in the range of binding energies above 165 eV and below 175 eV, and a second peak p2 observed in the range of binding energies above 155 eV and below 165 eV. Hereinafter, the ordinal numbers of the peaks, such as the first peak p1, are merely distinguishing symbols and are independent of peak intensity. The first peak originates from SO4. 2- The first peak, the second peak originates from S. 2- The peak.

[0088] The intensity ratio of the second peak p2 to the first peak p1 is preferably 0.5 or more and less than 4.0. Here, the intensity ratio of the first peak p1 to the second peak p2 is the result on the surface of the negative electrode 30 of the lithium-ion secondary battery 100 after 100 cycles. If the intensity ratio of the second peak p2 to the first peak p1 is 0.5 or more, the thickness of the coating with SO4 bonds is sufficient, and thermal decomposition of the coating is difficult to occur even at high temperatures. In addition, if the intensity ratio of the second peak p2 to the first peak p1 is less than 4.0, the impedance of the coating to lithium ions is small, and Li precipitation can be suppressed even at low temperatures.

[0089] Figure 3 The F1s spectrum was obtained by measuring the surface (coating) of the negative electrode 30 using X-ray photoelectron spectroscopy (XPS). Figure 3The solid line represents the result when the non-aqueous electrolyte solution contains lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium nitrate (LiNO3) in the ratio specified above. Figure 3 The dashed line represents the result when the non-aqueous electrolyte solution contains lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) but not lithium nitrate (LiNO3).

[0090] The F1s spectrum exhibits a third peak, p3, observed in the binding energy range of 686 eV to 690 eV, and a fourth peak, p4, observed in the binding energy range of 682 eV to 686 eV. The third peak, p3, originates from the CF bond, and the fourth peak, p4, originates from the Li-F bond.

[0091] The intensity ratio of the fourth peak p4 to the third peak p3 is preferably 1.5 or higher and less than 4.0. Here, the intensity ratio of the fourth peak p4 to the third peak p3 is the result on the surface of the negative electrode 30 of the lithium-ion secondary battery 100 after 100 cycles. Figure 3 As shown, peak separation is performed from the spectrum when the fourth peak p4 partially overlaps with the third peak p3. Peak separation can be performed using XPS software. Specifically, the following steps are followed: Create a synthesized waveform based on a synthesis function using Gaussian and Lorentz functions, and compare this synthesized waveform with the measured waveform. Correct the synthesized waveform to minimize the difference between the two, and fit the synthesized waveform to the measured waveform to find the optimal waveform for synthesizing the measured waveform.

[0092] If the intensity ratio of the fourth peak p4 to the third peak p3 is greater than 1.5, the Li-F coating thickness is sufficient, and thermal decomposition of the coating is difficult to occur even at high temperatures. Conversely, if the intensity ratio of the fourth peak p4 to the third peak p3 is less than 4.0, the coating has low impedance to lithium ions, and Li precipitation can be suppressed even at low temperatures.

[0093] Figure 4 The C1s spectrum was obtained by measuring the surface (coating) of the negative electrode 30 using X-ray photoelectron spectroscopy (XPS). Figure 4 The solid line represents the result when the non-aqueous electrolyte solution contains lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium nitrate (LiNO3) in the ratio specified above. Figure 4 The dashed line represents the result when the non-aqueous electrolyte solution contains lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) but not lithium nitrate (LiNO3).

[0094] The C1s spectrum exhibits a fifth peak (p5) observed in the binding energy range of 288 eV to 292 eV, and a sixth peak (p6) observed in the binding energy range of 280 eV to 288 eV. The fifth peak (p5) originates from the CF bond, and the sixth peak (p6) originates from the CC or CH bond.

[0095] The intensity ratio of the sixth peak p6 to the fifth peak p5 is preferably 2.0 or higher and less than 5.0. Here, the intensity ratio of the sixth peak p6 to the fifth peak p5 is the result on the surface of the negative electrode 30 of the lithium-ion secondary battery 100 after 100 cycles.

[0096] If the intensity ratio of the sixth peak p6 to the fifth peak p5 is 2.0 or higher, a coating with a higher molecular weight containing alkyl lithium can be obtained. This coating covers the negative electrode surface, thereby suppressing the contact between the electrolyte and the negative electrode. Alternatively, if the intensity ratio of the sixth peak p6 to the fifth peak p5 is less than 5.0, it contains carbon and fluorine, forming a coating with low impedance relative to lithium ions, which can suppress Li deposition even at low temperatures.

[0097] In addition, non-aqueous electrolyte solutions may also contain SEI coating forming materials, surfactants, etc. Additives include, for example, vinylene carbonate, vinyl ethylene carbonate, phenylene carbonate, succinic anhydride, lithium bis(oxalate), lithium tetrafluoroborate, dinitrile compounds, propane sulpholactone, butane sulpholactone, propene sulpholactone, 3-cyclobutene sulfone, fluorinated allyl ethers, and fluorinated acrylates.

[0098] <Exterior body>

[0099] The outer casing 50 seals the power generation element 40 and the non-aqueous electrolyte solution inside. The outer casing 50 prevents the non-aqueous electrolyte solution from leaking to the outside and prevents moisture and other substances from entering the lithium-ion secondary battery 100 from the outside.

[0100] For example, Figure 1 As shown, the outer casing 50 has a metal foil 52 and resin layers 54 laminated on each side of the metal foil 52. The outer casing 50 is a metal laminate formed by coating a polymer film (resin layer 54) onto both sides of the metal foil 52.

[0101] For example, aluminum foil can be used as the metal foil 52. The resin layer 54 can be a polymer film such as polypropylene. The materials constituting the resin layer 54 can also be different on the inner and outer sides. For example, a high-melting-point polymer such as polyethylene terephthalate (PET) or polyamide (PA) can be used as the outer material, while polyethylene (PE) or polypropylene (PP) can be used as the material of the polymer film on the inner side.

[0102] <Terminal>

[0103] Terminals 60 and 62 are connected to the negative terminal 30 and the positive terminal 20, respectively. Terminal 62, connected to the positive terminal 20, is the positive terminal, and terminal 60, connected to the negative terminal 30, is the negative terminal. Terminals 60 and 62 are responsible for external electrical connections. Terminals 60 and 62 are formed of conductive materials such as aluminum, nickel, and copper. The connection method can be welding or threaded fixing. To prevent short circuits, it is preferable to protect terminals 60 and 62 with insulating tape.

[0104] [Manufacturing method of lithium-ion secondary batteries]

[0105] A lithium-ion secondary battery 100 is manufactured by preparing and assembling a negative electrode 30, a positive electrode 20, a separator 10, a non-aqueous electrolyte solution, and an outer casing 50. An example of a manufacturing method for the lithium-ion secondary battery 100 will be described below.

[0106] The positive electrode 20 is manufactured by sequentially performing a slurry preparation process, an electrode coating process, a drying process, and a rolling process.

[0107] The slurry preparation process involves mixing the positive electrode active material, conductive additives, and binder with a solvent. Solvents may include, for example, N-methyl-2-pyrrolidone.

[0108] The electrode coating process involves applying a slurry to the surface of the positive current collector 22. There are no particular limitations on the slurry application method. For example, slot die coating or blade coating can be used. The slurry is applied, for example, at room temperature.

[0109] The drying process is the process of removing solvent from the slurry. For example, the negative electrode current collector 32 coated with slurry is dried in an atmosphere of 80°C to 350°C.

[0110] The rolling process can be performed as needed. The rolling process involves applying pressure to the positive electrode active material layer 24 and is a process for adjusting the density of the positive electrode active material layer 24. The rolling process can be performed, for example, using a rolling mill device.

[0111] The negative electrode 30 can be manufactured using the same steps as the positive electrode 20. The separator 10 and the outer casing 50 can be commercially available products.

[0112] Next, the diaphragm 10 is stacked between the positive electrode 20 and the negative electrode 30 to form a power generation element 40. Terminal 62 is connected to the positive electrode 20 of the power generation element 40, and terminal 60 is connected to the negative electrode 30. In the case that the power generation element 40 is a wound body, the positive electrode 20, the negative electrode 30 and the diaphragm 10 are wound around one end of the diaphragm 10 as an axis.

[0113] Then, the power generation element 40 is sealed into the outer casing 50. A non-aqueous electrolyte solution is injected into the outer casing 50. By depressurizing or heating after injection, the non-aqueous electrolyte solution is immersed in the power generation element 40. The outer casing 50 is then sealed by heating or the like. Alternatively, the power generation element 40 may be immersed in the non-aqueous electrolyte solution without injecting it into the outer casing 50. After injecting the non-aqueous electrolyte solution into the power generation element 40, it is preferable to let it stand for 24 hours.

[0114] Next, the power generation element 40 is subjected to its first charge and discharge. During this first charge, a coating is formed on the surface of the negative electrode 30.

[0115] During the coating formation, it is preferable to heat the solution at a temperature range of 50°C to 70°C for approximately 2 hours after injecting the non-aqueous electrolyte solution. This treatment results in an intensity ratio of the second peak p2 to the first peak p1 of 0.5 or higher and less than 4.0.

[0116] Furthermore, during the coating formation process, as an initial charging treatment, it is preferable to perform a two-stage charging at different charging rates, maintaining each charging rate for a certain period of time. For example, it is preferable to charge for 2 hours at a constant current rate of 0.1C (the current value at which charging ends within 1 hour when constant current charging is performed at 25°C), then charge at a charging rate of 0.2C until the battery voltage reaches 4.3V, and further maintain a constant voltage state of 4.3V for at least 10 minutes thereafter. By performing this treatment, the intensity ratio of the fourth peak p4 to the third peak p3 becomes 1.5 or more and less than 4.0.

[0117] When forming the coating, it is preferable to perform charge-discharge within a temperature range of 20°C to 30°C. By performing this treatment, the intensity ratio of the sixth peak p6 to the fifth peak p5 is greater than 2.0 and less than 5.0.

[0118] By performing an initial charge and discharge cycle, a coating is formed on the surface of the negative electrode 30, thereby obtaining the lithium-ion secondary battery 100 according to this embodiment.

[0119] The lithium-ion secondary battery 100 described in this embodiment exhibits excellent cycle characteristics under both low-temperature and high-temperature conditions. This is because a non-aqueous electrolyte solution with a specified ratio of electrolyte salts is used to form a coating with low impedance and sufficient thickness relative to Li on the surface of the negative electrode 30.

[0120] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the structures and combinations thereof in each embodiment are merely examples. Without departing from the spirit of the present invention, additions, omissions, substitutions, and other changes to the structures are possible.

[0121] (Example)

[0122] [Example 1]

[0123] A positive electrode paste is coated on one side of an aluminum foil with a thickness of 15 μm. The positive electrode paste is made by mixing positive electrode active material, conductive additive, binder and solvent.

[0124] The positive electrode active material used is LiNi as a lithium oxide. 0.92 Co 0.04 Mn 0.04 Acetylene black was used as the conductive additive. Polyvinylidene fluoride (PVDF) was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. The mass ratio of the positive electrode active material, conductive additive, and binder was set at 90 wt%: 5 wt%: 5 wt%. They were mixed in the solvent to prepare the positive electrode slurry. The loading of the positive electrode active material in the dried positive electrode active material layer was set at 25 mg / cm³. 2 Solvent was removed from the positive electrode slurry in a drying oven to prepare the positive electrode active material layer. The positive electrode active material layer was then pressurized using a roller press to produce the positive electrode.

[0125] A negative electrode paste was coated on one side of a 10 μm thick copper foil. The negative electrode paste was made by mixing a negative electrode active material, conductive additives, binders, and solvents.

[0126] The negative electrode active material consisted of graphite particles and silicon-containing compound particles with carbon powder supported on the surface of silicon powder. Acetylene black was used as the conductive additive. Polyvinylidene fluoride (PVDF) was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. The mass ratio of the negative electrode active material, conductive additive, and binder was set to 94 wt%: 2 wt%: 4 wt%. These were mixed in the solvent to prepare the negative electrode slurry. The loading of the negative electrode active material in the dried negative electrode active material layer was set to 6.1 mg / cm³. 2Solvent was removed from the negative electrode slurry in a drying oven to prepare the negative electrode active material layer. The negative electrode was then fabricated by pressing the negative electrode active material layer using a roller press. The negative electrodes fabricated under the same conditions were evaluated, and the results showed that the volume percentage of silicon-containing compound particles was 50.5% by volume, and the volume percentage of graphite particles was 49.5% by volume. Furthermore, the weight percentage of silicon in the negative electrode active material layer was 10.2% by weight.

[0127] Secondly, a non-aqueous electrolyte solution was prepared. The solvent for the non-aqueous electrolyte solution was set as ethylene carbonate (EC): ethyl methyl carbonate (EMC): diethyl carbonate (DEC) = 30 vol%: 50 vol%: 20 vol%. Additionally, LiPF6, LiFSI, and LiNO3 were added as electrolytic salts to the electrolyte. The molar ratio of LiNO3 to LiPF6 was set to 0.15, and the molar ratio of LiFSI to LiPF6 was set to 0.30.

[0128] (Evaluation of the fabrication of lithium-ion secondary batteries)

[0129] A power generation element was obtained by laminating the negative and positive electrodes with the positive and negative active material layers facing each other across a separator (porous polyethylene sheet). This power generation element was then inserted into an aluminum laminated film outer packaging, and heat-sealed except for one perimeter to form a closed section. Finally, a non-aqueous electrolyte solution was injected into the outer packaging, and the power generation element was heated to 60°C and maintained for approximately 2 hours. Afterward, the remaining section was sealed using a vacuum sealing machine while depressurizing the remaining area.

[0130] Next, the power generation element undergoes its initial charge and discharge. This initial charge and discharge is performed in two stages. First, it is charged for 2 hours at a constant current rate of 0.1C (the current value at the end of 5 hours of constant current charging at 25°C), followed by charging at a rate of 0.2C until the battery voltage reaches 4.3V. The battery voltage is then maintained at a constant 4.3V for 10 minutes. Then, it is discharged at a constant current rate of 0.2C until the battery voltage reaches 2.0V. After the initial charge and discharge, the power generation element is heated to 30°C and subjected to another charge and discharge cycle. The discharge capacity after the initial charge and discharge is measured, and the battery capacity Q1 at the time of the initial discharge is calculated.

[0131] (Measurement of volume retention after 100 cycles)

[0132] The cycle characteristics of lithium-ion secondary batteries were measured. The cycle characteristics were measured using a secondary battery charge-discharge test apparatus (manufactured by Hokuto Electric Co., Ltd.). Cycle characteristics were measured in both a low-temperature environment (25°C) and a high-temperature environment (45°C). The lithium-ion secondary batteries used for the low-temperature measurement and those used for the high-temperature measurement were different samples prepared under the same conditions.

[0133] During the cycle characteristic measurement, 100 charge-discharge cycles were performed. In each cycle, the battery was charged at a constant current of 2.0C until the battery voltage reached 4.3V, and discharged at a constant current of 4.0C until the battery voltage reached 2.5V. The discharge capacity after 100 charge-discharge cycles was measured, and the battery capacity Q2 after 100 cycles was calculated.

[0134] The capacity retention rate E after 100 cycles is calculated from the capacities Q1 and Q2 obtained above. The capacity retention rate E is calculated by E = Q2 / Q1 × 100. The capacity retention rate of the lithium-ion secondary battery of Example 1 under low temperature conditions is 95%, and the capacity retention rate of the lithium-ion secondary battery of Example 1 under high temperature conditions is 85%.

[0135] In addition, the lithium-ion secondary battery after 100 charge-discharge cycles (25°C) was decomposed, and the surface of the negative electrode was analyzed using XPS. Then, the intensity ratio of the second peak p2 to the first peak p1 in the S2p spectrum, the intensity ratio of the fourth peak to the third peak in the F1s spectrum, and the intensity ratio of the sixth peak to the fifth peak in the C1s spectrum were determined.

[0136] [Examples 2-10]

[0137] Examples 2-10 differ from Example 1 in that the molar ratios of LiPF6, LiFSI, and LiNO3 in the non-aqueous electrolyte solution were changed, as were the proportions of silicon-containing compound particles and graphite particles in the negative electrode active material. Cycling characteristics and XPS analysis were performed under the same conditions as in Example 1. The composition of the coating formed on the negative electrodes in Examples 2-10 differs from that in Example 1 due to the different molar ratios of LiPF6, LiFSI, and LiNO3. The intensity ratios of the peaks determined by XPS in Examples 2-10 differ from those in Example 1.

[0138] [Examples 11-18]

[0139] Examples 11-18 differ from Example 1 in that the molar ratio of LiPF6, LiFSI, and LiNO3 in the non-aqueous electrolyte solution was changed, and the charge / discharge conditions during the initial charge / discharge were also changed. In Examples 13 and 14, the heating and holding process of the power generation element was not performed after the injection of the non-aqueous electrolyte solution. In Examples 15 and 16, the charge / discharge rate of the second stage was changed to 0.7C during the initial charge / discharge, and constant current charging and discharging were performed. In Examples 17 and 18, the charge / discharge process with the power generation element heated was not performed after the initial charge / discharge. Cyclic characteristics and XPS analysis were performed under the same conditions as in Example 1.

[0140] [Examples 19-21]

[0141] In Examples 19-21, the molar ratio of LiPF6, LiFSI, and LiNO3 in the non-aqueous electrolyte solution was kept constant, while the ratio of silicon-containing compound particles to graphite particles in the negative electrode active material was varied. Cyclic characteristics and XPS analysis were performed in Examples 19-21 in the same manner as in Example 1.

[0142] [Comparative Examples 1-8]

[0143] The difference between Comparative Examples 1-8 and Example 1 is that the molar ratio of LiPF6, LiFSI and LiNO3 in the non-aqueous electrolyte solution was changed.

[0144] Comparative Example 1 did not add LiNO3 to the non-aqueous electrolyte solution.

[0145] Comparative Example 2 involved the addition of an excess of LiNO3 to a non-aqueous electrolyte solution.

[0146] Comparative Example 3 did not add LiFSI to the non-aqueous electrolyte solution.

[0147] Comparative Example 4 did not add LiNO3 or LiFSI to the non-aqueous electrolyte solution.

[0148] Comparative Example 5 involved the excessive addition of LiFSI to a non-aqueous electrolyte solution.

[0149] Comparative Example 6 did not add LiNO3 and LiPF6 to the non-aqueous electrolyte solution.

[0150] Comparative Example 7 did not add LiFSI to the non-aqueous electrolyte solution, and the amount of LiNO3 added was also relatively small.

[0151] Comparative Example 6 did not add LiNO3 to the non-aqueous electrolyte solution, and the amount of LiFSI added was also relatively small.

[0152] The conditions and measurement results of Examples 1 to 21 and Comparative Examples 1 to 8 are summarized in Table 1.

[0153] Table 1

[0154]

[0155] Examples 1-21 exhibited superior cycling characteristics under both low-temperature and high-temperature conditions compared to Comparative Examples 1-8. This is believed to be because the ratio of electrolytic salts in the non-aqueous electrolyte solution was within a specified range, thereby forming a desired coating on the surface of the negative electrode. The coating possessed sufficient thickness, thus suppressing the reaction between the electrolyte and the negative electrode caused by the thermal decomposition of the coating, even under high-temperature conditions. Furthermore, the impedance of the coating relative to Li ions did not become excessively high, thereby suppressing Li deposition.

[0156] Furthermore, Examples 1-4, 10, and 11 exhibit superior cycling characteristics under both low-temperature and high-temperature conditions compared to Examples 13-18. It is believed that by controlling not only the molar ratio of LiPF6, LiFSI, and LiNO3 in the non-aqueous electrolyte solution, but also the state of the coating as measured by XPS, the cycling characteristics of the lithium-ion secondary battery can be improved.

Claims

1. A non-aqueous electrolyte solution, wherein, It contains lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium nitrate. The molar ratio of lithium nitrate to lithium hexafluorophosphate is greater than 0.01 and less than 0.

15. The molar ratio of the lithium bis(fluorosulfonyl)imide to the lithium hexafluorophosphate is 0.1 or more and 1.0 or less.

2. A lithium-ion secondary battery, wherein, It comprises: a positive electrode, a negative electrode, a membrane located between the positive electrode and the negative electrode, and the non-aqueous electrolyte solution as described in claim 1.

3. The lithium-ion secondary battery according to claim 2, wherein, The S2p spectrum obtained by measuring the surface of the negative electrode using X-ray photoelectron spectroscopy (XPS) has the following characteristics: a first peak observed in the range of binding energy above 165 eV and below 175 eV, and a second peak observed in the range of binding energy above 155 eV and below 165 eV. The intensity ratio of the second peak to the first peak is greater than 0.5 and less than 4.

0.

4. The lithium-ion secondary battery according to claim 2, wherein, The F1s spectrum obtained by measuring the surface of the negative electrode using X-ray photoelectron spectroscopy (XPS) has the following characteristics: a third peak observed in the range of binding energies above 686 eV and below 690 eV, and a fourth peak observed in the range of binding energies above 682 eV and below 686 eV. The intensity ratio of the fourth peak to the third peak is greater than 1.5 and less than 4.

0.

5. The lithium-ion secondary battery according to claim 2, wherein, The C1s spectrum obtained by measuring the surface of the negative electrode using X-ray photoelectron spectroscopy (XPS) has the following characteristics: a fifth peak observed in the range of binding energies above 288 eV and below 292 eV, and a sixth peak observed in the range of binding energies above 280 eV and below 288 eV. The intensity ratio of the sixth peak to the fifth peak is greater than 2.0 and less than 5.

0.

6. The lithium-ion secondary battery according to claim 2, wherein, The negative electrode has a negative electrode active material layer, which contains negative electrode active material. The negative electrode active material has at least one of silicon-containing compound particles and graphite particles. The volume percentage of silicon-containing compound particles in the negative electrode active material is more than 10% by volume and less than 100% by volume.

7. The lithium-ion secondary battery according to claim 2, wherein, The negative electrode has a negative electrode active material layer, which contains silicon-containing compound particles. The weight percentage of silicon in the negative electrode active material layer is more than 1% by weight and less than 50% by weight.

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