Non-aqueous electrolyte solution and lithium-ion secondary battery

Abstract

A non-aqueous electrolyte solution according to the present embodiment comprises lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium nitrate. In the non-aqueous electrolyte according to the present embodiment, the molar ratio of the lithium nitrate to the molar ratio of the lithium hexafluorophosphate is 0.01-0.15, and the molar ratio of the lithium bis(fluorosulfonyl)imide to the molar ratio of the lithium hexafluorophosphate is 0.1-1.0.

Description

Nonaqueous electrolyte solution and lithium ion secondary battery

[0001] This disclosure relates to a non-aqueous electrolyte solution and a lithium-ion secondary battery. This application claims priority to Japanese Patent Application No. 2024-150606, filed on September 2, 2024, the contents of which are incorporated herein by reference.

[0002] Lithium-ion secondary batteries are also widely used as a power source for mobile devices such as mobile phones and laptops, as well as hybrid cars.

[0003] Various studies are being conducted to improve the performance of lithium-ion secondary batteries. One of the characteristics required for lithium-ion secondary batteries is cycle performance. Lithium-ion secondary batteries deteriorate after multiple charge / discharge cycles. Cycle performance is the capacity retention rate of a lithium-ion secondary battery after multiple charge / discharge cycles compared to the lithium-ion secondary battery after the initial charge / discharge cycle. If the electrolyte decomposes on the surface of the negative electrode, the cycle performance of the lithium-ion secondary battery will deteriorate.

[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 predetermined lithium salt and a predetermined glyme.

[0005] JP 2018-67501 A

[0006] Even when the nonaqueous electrolyte solution described in Patent Document 1 is used, sufficient cycle characteristics may not be exhibited as the number of charge / discharge cycles increases. Furthermore, lithium ion secondary batteries are often used in a variety of environments and are therefore required to operate appropriately at both low and high temperatures. Therefore, there is a demand for lithium ion secondary batteries that exhibit sufficient cycle characteristics at both low and high temperatures.

[0007] The present disclosure has been made in view of the above problems, and aims to provide a lithium ion secondary battery that has excellent cycle characteristics at both low and high temperatures, and a nonaqueous electrolyte solution used in the lithium ion secondary battery.

[0008] In order to solve the above problems, the following means are provided.

[0009] (1) A nonaqueous electrolyte solution according to a first aspect includes lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium nitrate, wherein the molar ratio of the lithium nitrate to the lithium hexafluorophosphate is 0.01 or more and 0.15 or less, and the molar ratio of the lithium bis(fluorosulfonyl)imide to the lithium hexafluorophosphate is 0.1 or more and 1.0 or less.

[0010] (2) A lithium ion secondary battery according to a second aspect includes a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and the nonaqueous electrolyte solution according to the above aspect.

[0011] (3) In the lithium ion secondary battery according to the above aspect, an S2p spectrum obtained by measuring the surface of the negative electrode using X-ray photoelectron spectroscopy (XPS) may have a first peak observed in a binding energy range of 165 eV to 175 eV and a second peak observed in a binding energy range of 155 eV to 165 eV, and an intensity ratio of the second peak to the first peak may be 0.5 or greater and less than 4.0.

[0012] (4) In the lithium ion secondary battery according to the above aspect, an F1s spectrum obtained by measuring the surface of the negative electrode using X-ray photoelectron spectroscopy (XPS) may have a third peak observed in a binding energy range of 686 eV to 690 eV and a fourth peak observed in a binding energy range of 682 eV to 686 eV, and an intensity ratio of the fourth peak to the third peak may be 1.5 or greater and less than 4.0.

[0013] (5) In the lithium ion secondary battery according to the above aspect, a C1s spectrum obtained by measuring the surface of the negative electrode using X-ray photoelectron spectroscopy (XPS) may have a fifth peak observed in a binding energy range of 288 eV to 292 eV and a sixth peak observed in a binding energy range of 280 eV to 288 eV, and an intensity ratio of the sixth peak to the fifth peak may be 2.0 or greater and less than 5.0.

[0014] (6) In the lithium-ion secondary battery according to the above aspect, the negative electrode may have a negative electrode active material layer containing a negative electrode active material. The negative electrode active material may include at least one of silicon-containing compound particles and graphite particles. A volume ratio of the silicon-containing compound particles to the negative electrode active material may be 10% by volume or more and 100% by volume or less.

[0015] (7) In the lithium ion secondary battery according to the above aspect, the negative electrode may have a negative electrode active material layer containing silicon-containing compound particles, and a weight ratio of silicon in the negative electrode active material layer may be 1 wt % or more and 50 wt % or less.

[0016] The lithium ion secondary battery using the nonaqueous electrolyte solution according to the above embodiment has excellent cycle characteristics at both low and high temperatures.

[0017] 1 is a schematic diagram of a lithium ion secondary battery according to the first embodiment; FIG. 2 is an S2p spectrum obtained when the negative electrode surface of the lithium ion secondary battery according to the first embodiment is measured using X-ray photoelectron spectroscopy (XPS); FIG. 3 is an F1s spectrum obtained when the negative electrode surface of the lithium ion secondary battery according to the first embodiment is measured using X-ray photoelectron spectroscopy (XPS); and FIG. 4 is a C1s spectrum obtained when the negative electrode surface of the lithium ion secondary battery according to the first embodiment is measured using X-ray photoelectron spectroscopy (XPS).

[0018] Hereinafter, the embodiments will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may show characteristic portions enlarged for convenience in order to make the features easier to understand, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto. Appropriate changes can be made within the scope of the present invention.

[0019] "Lithium-ion secondary battery" FIG. 1 is a schematic diagram of a lithium-ion secondary battery according to a first embodiment. The lithium-ion secondary battery 100 shown in FIG. 1 includes a power generating element 40, an exterior body 50, and a non-aqueous electrolyte solution (not shown). The exterior body 50 covers the periphery of the power generating element 40. The power generating element 40 is connected to the outside via a pair of terminals 60, 62 connected to the power generating element 40. The non-aqueous electrolyte solution is accommodated within the exterior body 50. Although FIG. 1 illustrates a case where one power generating element 40 is contained within the exterior body 50, multiple power generating elements 40 may be stacked. The lithium-ion secondary battery 100 may be any of a cylindrical type, a prismatic type, a laminate type, a button type, and the like.

[0020] (Power Generation Element) The power generation element 40 includes a separator 10 , a positive electrode 20 , and a negative electrode 30 .

[0021] <Positive Electrode> The positive electrode 20 includes, for example, a positive electrode current collector 22 and a positive electrode active material layer 24. The positive electrode active material layer 24 is in contact with at least one surface of the positive electrode current collector 22.

[0022] [Positive Electrode Current Collector] The positive electrode current collector 22 is, for example, a conductive plate material. The positive electrode current collector 22 is, for example, a thin metal plate made of aluminum, copper, nickel, titanium, stainless steel, or the like. Aluminum, which is lightweight, is suitably used for the positive electrode current collector 22. The average thickness of the positive electrode current collector 22 is, for example, 10 μm or more and 30 μm or less. The positive electrode current collector 22 may be an expanded film or a punched film.

[0023] [Positive Electrode Active Material Layer] The positive electrode active material layer 24 contains, for example, a positive electrode active material. The positive electrode active material layer 24 may contain a conductive additive and a binder as necessary.

[0024] The positive electrode active material includes an electrode active material that can reversibly absorb and release lithium ions, desorb and insert (intercalate) lithium ions, or dope and dedope lithium ions with counter anions.

[0025] The positive electrode active material is, for example, a composite metal oxide. The composite metal oxide is, for example, lithium cobalt oxide (LiCoO ２ ), lithium nickel oxide (LiNiO ２), lithium manganese oxide (LiMnO ２ ), lithium manganese spinel (LiMn ２ O ４ ), and the general formula: LiNi ｘ Co ｙ Mn ｚ M ａ O ２ (wherein 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, and Cr), lithium vanadium compounds (LiV ２ O ５ ), olivine-type LiMPO ４ (wherein M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, or VO), lithium titanate (Li ４ Ti ５ O １２ ), LiNi ｘ Co ｙ Al ｚ O ２ (0.9<x+y+z<1.1). General formula: LiNi ｘ Co ｙ Mn ｚ M ａ O ２ Examples of the compound represented by the formula include LiNi ０．９２ Co ０．０４ Mn ０．０４ O ２ , LiNi ０．９ Co ０．０５ Mn ０．０５ O ２ , LiNi ０．８ Co ０．１ Mn ０．１ O ２ , LiNi ０．６ Co ０．２ Mn ０．２ O ２ , LiNi ０．９ Co ０．０５ Mn ０．０５ O ２ , LiNi ０．６ Co ０．２ Mn ０．１ Al ０．１ O ２ , LiNi０．６ Co ０．２ Mn ０．１５ Al ０．０５ O ２ , LiNi ０．７ Co ０．１ Mn ０．１ Al ０．１ O ２ , LiNi ０．７ Mn １．３ O ４ , LiNi ０．５ Mn １．５ O ４ and LiNi ０．３ Mn １．７ O ４ The positive electrode active material may be an organic material, such as polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

[0026] The positive electrode active material may be a lithium-free material, such as FeF ３ Examples of the non-lithium-containing material include conjugated polymers containing organic conductive materials, Chevrel phase compounds, transition metal chalcogenides, vanadium oxides, and niobium oxides. The non-lithium-containing material may be any one of these materials alone or in combination. When the positive electrode active material is a non-lithium-containing material, for example, discharge is first performed. Lithium is inserted into the positive electrode active material by discharging. Alternatively, a non-lithium-containing positive electrode active material may be pre-doped with lithium chemically or electrochemically.

[0027] The conductive additive enhances the electronic conductivity between the positive electrode active materials. Examples of the conductive additive include carbon powder, carbon nanotubes, carbon materials, metal powder, a mixture of carbon materials and metal powder, and conductive oxides. Examples of the carbon powder include carbon black, acetylene black, and ketjen black. Examples of the metal powder include powders of copper, nickel, stainless steel, and iron.

[0028] There are no particular limitations on the content of the conductive additive in the positive electrode active material layer 24. For example, the content of the conductive additive relative to the total mass of the positive electrode active material, the conductive additive, and the binder is 0.5 mass% or more and 20 mass% or less, and preferably 1 mass% or more and 5 mass% or less.

[0029] The binder in the positive electrode active material layer 24 binds the positive electrode active material together. Known binders can be used. The binder is preferably one that is insoluble in the electrolyte, has oxidation resistance, and has adhesive properties. The binder is, for example, a fluororesin. Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamideimide (PAI), polybenzimidazole (PBI), polyethersulfone (PES), polyacrylic acid and its copolymers, metal ion crosslinked polyacrylic acid and its copolymers, maleic anhydride-grafted polypropylene (PP) or polyethylene (PE), and mixtures thereof. PVDF is particularly preferred as the binder used in the positive electrode active material layer.

[0030] The binder content in the positive electrode active material layer 24 is not particularly limited. For example, the binder content relative to the total mass of the positive electrode active material, conductive additive, and binder is 1% by mass or more and 15% by mass or less, and preferably 1.5% by mass or more and 5% by mass or less. If the binder content is low, the adhesive strength of the positive electrode 20 will be weakened. If the binder content is high, the binder will be electrochemically inactive and will not contribute to the discharge capacity, resulting in a low energy density of the lithium-ion secondary battery 100.

[0031] <Negative Electrode> The negative electrode 30 includes, for example, a negative electrode current collector 32 and a negative electrode active material layer 34. The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32. The negative electrode 30 is an example of a negative electrode for a lithium ion secondary battery.

[0032] [Negative Electrode Current Collector] The negative electrode current collector 32 is, for example, a conductive plate material. The negative electrode current collector 32 may be the same as the positive electrode current collector 22. The negative electrode current collector 32 may be an expanded film or a punched film.

[0033] [Negative Electrode Active Material Layer] The negative electrode active material layer 34 contains a negative electrode active material. The negative electrode active material layer 34 may contain a binder, a conductive additive, and the like, as necessary.

[0034] The negative electrode active material may be any compound capable of absorbing and releasing ions, and known negative electrode active materials used in lithium-ion secondary batteries can be used. 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, non-graphitizable carbon, easily graphitizable carbon, and low-temperature calcined carbon, which can absorb and release ions. Examples of materials capable of alloying with lithium include silicon, tin, zinc, lead, and antimony. Materials capable of alloying with lithium may be, for example, these elemental metals, or alloys or oxides containing these elements. Furthermore, materials capable of alloying with lithium may be composites in which at least a portion of the surface is coated with a conductive material (e.g., a carbon material).

[0035] The negative electrode active material may include, for example, at least one of silicon-containing compound particles and graphite particles. The negative electrode active material may also include silicon-containing compound particles alone. The silicon-containing compound particles may be simple silicon, SiC, or SiO ｘ (where x satisfies, for example, 0.8≦x≦2.0) or MSi (where M is an alkaline earth metal or a transition metal).

[0036] The volume ratio of the silicon-containing compound particles in the negative electrode active material may be 10% by volume or more and 100% by volume or less, 10% by volume or more and less than 100% by volume, or 10% by volume or more and 50.5% by volume or less. For example, when the negative electrode active material is composed of silicon-containing compound particles and graphite particles, the total volume ratio of these is 100% by volume.

[0037] The volume ratio of silicon-containing compound particles in the negative electrode active material is determined by the following procedure. First, the negative electrode active material layer is divided into three parts in the thickness direction, namely, an upper layer, a middle layer, and a lower layer. Then, cross-sectional SEM (scanning electron microscope) images are taken at three different locations within the plane of each of the upper layer, middle layer, and lower layer. A total of nine cross-sectional SEM images are measured. The negative electrode active material is extracted from each cross-sectional SEM image. The negative electrode active material can be extracted by utilizing differences in image contrast. Next, silicon-containing compound particles are extracted from the negative electrode active material. The silicon-containing compound particles may be extracted by utilizing differences in image contrast or by using composition analysis such as EDX (energy dispersive X-ray spectroscopy). Then, the area ratio of the silicon-containing compound particles to the negative electrode active material is determined in each image, and the average value is calculated. Since the cross-sectional SEM images are measured on an arbitrary plane, the area ratio in the cross-sectional SEM image approximately corresponds to the overall volume ratio. Therefore, this average value is regarded as the volume ratio of the silicon-containing compound particles in the negative electrode active material.

[0038] Furthermore, the weight ratio of silicon element in the negative electrode active material layer 34 may be 1 wt % or more and 50 wt % or less, or 1 wt % or more and 10.2 wt % or less. The weight ratio of silicon element can be measured using X-ray fluorescence analysis (XRF). The weight ratio of silicon element in the negative electrode active material layer 34 can be determined by performing XRF measurement at three different points on the surface of the negative electrode active material layer 34 and calculating the average of the silicon element ratios calculated at each point. The spacing between measurement points when performing XRF measurement is wider than the measurement sphere (beam diameter used for measurement). For example, when the beam diameter is 1.2 mm, the spacing between adjacent measurement points is set to be greater than 1.2 mm.

[0039] The conductive additive and binder used in the negative electrode active material layer 34 may be the same as those used in the positive electrode active material layer 24 .

[0040] The binder content in the negative electrode active material layer 34 is not particularly limited. For example, the binder content relative to the total mass of the negative electrode active material, conductive additive, and binder is 0.5 mass% or more and 20 mass% or less, and preferably 5 mass% or more and 15 mass% or less. If the binder content is low, the adhesive strength of the negative electrode 30 will be weakened. If the binder content is high, the binder 1 will be electrochemically inactive and will not contribute to the discharge capacity, resulting in a low energy density of the lithium-ion secondary battery 100.

[0041] The conductive additive in the negative electrode active material layer 34 enhances the electronic conductivity between the negative electrode active materials. The conductive additive may be the same as that in the positive electrode active material layer 24.

[0042] There are no particular limitations on the content of the conductive additive in the negative electrode active material layer 34. For example, the content of the conductive additive relative to the total mass of the negative electrode active material, the conductive additive, and the binder is 5% by mass or more and 20% by mass or less, and preferably 1% by mass or more and 12% by mass or less.

[0043] <Separator> The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 separates the positive electrode 20 from the negative electrode 30 and prevents short-circuiting between the positive electrode 20 and the negative electrode 30. The separator 10 extends in-plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

[0044] The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is, for example, a monolayer or laminate of a polyolefin film. The separator 10 may be a stretched membrane of a mixture of polyethylene, polypropylene, or the like. The separator 10 may be a fibrous nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 may be, for example, a solid electrolyte. The solid electrolyte may be, for example, a polymer solid electrolyte, an oxide-based solid electrolyte, or a sulfide-based solid electrolyte. The separator 10 may also be an inorganic-coated separator. The inorganic-coated separator is formed by coating the surface of the above-mentioned film with a mixture of a resin such as PVDF or CMC and an inorganic material such as alumina or silica. The inorganic-coated separator has excellent heat resistance and suppresses the deposition of transition metals eluted from the positive electrode onto the negative electrode surface.

[0045] <Non-aqueous Electrolyte Solution> The non-aqueous electrolyte solution is sealed in the exterior body 50 and impregnates the power generating element 40. The non-aqueous electrolyte solution includes, for example, a non-aqueous solvent and an electrolytic salt. The electrolytic salt is dissolved in the non-aqueous solvent.

[0046] The non-aqueous solvent is, for example, an aprotic organic solvent. The organic solvent is, for example, a cyclic carbonate, a chain carbonate, an ether, a mixture thereof, or an ionic liquid.

[0047] The cyclic carbonate solvates the electrolyte. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and fluoroethylene carbonate. The cyclic carbonate preferably contains at least fluoroethylene carbonate. Fluoroethylene carbonate (FEC) has a high oxidation-reduction potential and is easily reduced and decomposed. Reductive decomposition of a portion of the fluoroethylene carbonate (FEC) makes it difficult for the electrolyte and remaining solvent in the electrolyte to decompose. Furthermore, fluoroethylene carbonate (FEC) forms a stable coating (SEI coating) on ​​the entire surface of the negative electrode active material during the initial use of the lithium ion secondary battery. The SEI coating prevents direct contact between the negative electrode active material and the electrolyte, preventing decomposition of the electrolyte.

[0048] The chain carbonate reduces the viscosity of the cyclic carbonate. Examples of the chain carbonate include diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. The non-aqueous solvent may also contain methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, and 1,2-diethoxyethane.

[0049] The non-aqueous electrolyte solution contains lithium hexafluorophosphate (LiPF ６ ), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium nitrate (LiNO ３ The molar ratio of lithium nitrate to lithium hexafluorophosphate is 0.01 or more and 0.15 or less. The molar ratio of lithium bis(fluorosulfonyl)imide to lithium hexafluorophosphate is 0.1 or more and 1.0 or less.

[0050] When the non-aqueous electrolyte solution contains the three types of electrolytic salts in the above ratio, a coating (SEI coating) having a sufficient thickness and not significantly inhibiting the movement of Li is formed on the surface of the negative electrode 30 (negative electrode active material layer 34). The coating is formed on the surface of the negative electrode 30 (negative electrode active material layer 34) by the reductive decomposition of a portion of the non-aqueous electrolyte solution. The coating inhibits further decomposition of the non-aqueous electrolyte solution.

[0051] If the coating is not thick enough, the nonaqueous electrolyte solution and the negative electrode 30 come into contact, increasing the possibility of decomposition of the nonaqueous electrolyte solution. Decomposition of the nonaqueous electrolyte solution is one of the causes of deterioration in cycle characteristics. Furthermore, during high-temperature operation, the coating may decompose due to heat. If a coating of sufficient thickness is provided on the surface of the negative electrode, the cycle characteristics of the lithium-ion secondary battery are less likely to deteriorate even during high-temperature operation.

[0052] Furthermore, if the coating significantly inhibits the movement of Li, Li deposition occurs on the surface of the coating. If Li dendrites form on the surface of the coating, the cycle characteristics of the lithium-ion secondary battery will deteriorate. During low-temperature operation, lithium ions are less likely to move than during high-temperature operation, making Li deposition more likely to occur. When the three types of electrolytic salts are contained in the above ratio, the coating is less likely to become highly resistive, and the cycle characteristics of the lithium-ion secondary battery are less likely to deteriorate even during low-temperature operation.

[0053] 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).

[0054] 2 shows the S2p spectrum obtained when the surface (coating) of the negative electrode 30 is measured using X-ray photoelectron spectroscopy (XPS). The solid line in FIG. 2 indicates the S2p spectrum obtained when the nonaqueous electrolyte solution is lithium hexafluorophosphate (LiPF ６ ), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium nitrate (LiNO ３ ) in the above-mentioned predetermined ratio. The dotted line in FIG. 2 shows the results when the non-aqueous electrolyte solution contains lithium hexafluorophosphate (LiPF ６ ) and lithium bis(fluorosulfonyl)imide (LiFSI), but lithium nitrate (LiNO ３ ) is not present.

[0055] The S2p spectrum has a first peak p1 observed in the binding energy range of 165 eV to 175 eV, and a second peak p2 observed in the binding energy range of 155 eV to 165 eV. Hereinafter, the ordinal numbers indicating peaks such as the first peak p1 are merely symbols for distinguishing the peaks and are not related to the magnitude of the peak intensity. The first peak is SO ４ ２－ The second peak is due to S ２－ This is a peak derived from

[0056] 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 a result on the surface of the negative electrode 30 of the lithium ion secondary battery 100 after 100 cycles. When the intensity ratio of the second peak p2 to the first peak p1 is 0.5 or more, SO ４ When the intensity ratio of the second peak p2 to the first peak p1 is less than 4.0, the resistance of the coating to lithium ions is low, and Li deposition can be suppressed even in a low-temperature environment.

[0057] 3 shows an F1s spectrum obtained by measuring the surface (coating) of the negative electrode 30 using X-ray photoelectron spectroscopy (XPS). The solid line in FIG. 3 indicates the F1s spectrum obtained when the nonaqueous electrolyte solution is lithium hexafluorophosphate (LiPF ６ ), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium nitrate (LiNO ３ ) in the above-mentioned predetermined ratio. The dotted line in FIG. 3 shows the results when the non-aqueous electrolyte solution contains lithium hexafluorophosphate (LiPF ６ ) and lithium bis(fluorosulfonyl)imide (LiFSI), but lithium nitrate (LiNO ３ ) is not present.

[0058] The F1s spectrum has 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 is a peak derived from a C—F bond, and the fourth peak p4 is a peak derived from a Li—F bond.

[0059] The intensity ratio of the fourth peak p4 to the third peak p3 is preferably 1.5 or more 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. As shown in FIG. 3 , if the fourth peak p4 and the third peak p3 partially overlap, peak separation is performed from the spectrum. Peak separation can be performed using XPS software. Specifically, this is performed using the following procedure: A composite waveform is created based on a composite function using a Gaussian function and a Lorentzian function, and the composite waveform is compared with the measured waveform. The composite waveform is modified to minimize the difference between the two, and fitting is performed between the synthetic waveform and the measured waveform to determine the optimal waveform for combining the measured waveform.

[0060] When the intensity ratio of the fourth peak p4 to the third peak p3 is 1.5 or more, the thickness of the coating containing Li—F is sufficient, and thermal decomposition of the coating is unlikely to occur even in a high-temperature environment. When the intensity ratio of the fourth peak p4 to the third peak p3 is less than 4.0, the resistance of the coating to lithium ions is low, and Li deposition can be suppressed even in a low-temperature environment.

[0061] 4 shows a C1s spectrum obtained by measuring the surface (coating) of the negative electrode 30 using X-ray photoelectron spectroscopy (XPS). The solid line in FIG. 4 indicates the C1s spectrum obtained when the nonaqueous electrolyte solution is lithium hexafluorophosphate (LiPF ６ ), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium nitrate (LiNO ３ ) in the above-mentioned predetermined ratio. The dotted line in FIG. 4 shows the results when the non-aqueous electrolyte solution contains lithium hexafluorophosphate (LiPF ６ ) and lithium bis(fluorosulfonyl)imide (LiFSI), but lithium nitrate (LiNO ３ ) is not present.

[0062] The C1s spectrum has a fifth peak p5 observed in a binding energy range of 288 eV to 292 eV, and a sixth peak p6 observed in a binding energy range of 280 eV to 288 eV. The fifth peak p5 is a peak derived from a C—F bond, and the sixth peak p6 is a peak derived from a C—C bond or a C—H bond.

[0063] The intensity ratio of the sixth peak p6 to the fifth peak p5 is preferably 2.0 or greater and less than 5.0. Here, the intensity ratio of the sixth peak p6 to the fifth peak p5 is a result on the surface of the negative electrode 30 of the lithium ion secondary battery 100 after 100 cycles.

[0064] When the intensity ratio of the sixth peak p6 to the fifth peak p5 is 2.0 or more, a relatively high molecular weight coating containing alkyllithium is obtained, and this coating covers the surface of the negative electrode, thereby suppressing contact between the electrolyte and the negative electrode. When the intensity ratio of the sixth peak p6 to the fifth peak p5 is less than 5.0, a coating containing carbon and fluorine and having low resistance to lithium ions is formed, thereby suppressing Li deposition even in a low-temperature environment.

[0065] The non-aqueous electrolyte solution may also contain an SEI film-forming material, a surfactant, etc. Examples of additives include vinylene carbonate, vinylethylene carbonate, phenylethylene carbonate, succinic anhydride, lithium bisoxalate, lithium tetrafluoroborate, dinitrile compounds, propane sultone, butane sultone, propene sultone, 3-sulfolene, fluorinated allyl ether, and fluorinated acrylate.

[0066] <Exterior Body> The exterior body 50 seals the power generating element 40 and the non-aqueous electrolyte solution inside. The exterior body 50 prevents the non-aqueous electrolyte solution from leaking to the outside and prevents moisture and the like from entering the lithium-ion secondary battery 100 from the outside.

[0067] 1, the exterior body 50 has a metal foil 52 and a resin layer 54 laminated on each side of the metal foil 52. The exterior body 50 is a metal laminate film in which the metal foil 52 is coated on both sides with a polymer film (resin layer 54).

[0068] The metal foil 52 can be, for example, aluminum foil. The resin layer 54 can be a polymer film such as polypropylene. The materials constituting the inner and outer resin layers 54 can be different. For example, the outer material can be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), and the inner polymer film can be made of polyethylene (PE), polypropylene (PP), or the like.

[0069] <Terminals> The terminals 60 and 62 are connected to the negative electrode 30 and the positive electrode 20, respectively. The terminal 62 connected to the positive electrode 20 is a positive electrode terminal, and the terminal 60 connected to the negative electrode 30 is a negative electrode terminal. The terminals 60 and 62 are responsible for electrical connection to the outside. The terminals 60 and 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screw fastening. It is preferable to protect the terminals 60 and 62 with insulating tape to prevent short circuits.

[0070] "Method for manufacturing lithium-ion secondary battery" The lithium-ion secondary battery 100 is fabricated by preparing and assembling the negative electrode 30, the positive electrode 20, the separator 10, the non-aqueous electrolyte solution, and the exterior body 50. An example of a method for manufacturing the lithium-ion secondary battery 100 will be described below.

[0071] The positive electrode 20 is produced by, for example, sequentially carrying out a slurry production step, an electrode application step, a drying step, and a rolling step.

[0072] The slurry preparation step is a step of preparing a slurry by mixing a positive electrode active material, a conductive additive, and a binder in a solvent, such as water or N-methyl-2-pyrrolidone.

[0073] The electrode coating step is a step of coating the surface of the positive electrode current collector 22 with a slurry. The method of coating the slurry is not particularly limited. For example, a slit die coating method or a doctor blade method can be used as the method of coating the slurry. The slurry is coated at room temperature, for example.

[0074] The drying step is a step of removing the solvent from the slurry. For example, the negative electrode current collector 32 coated with the slurry is dried in an atmosphere at 80°C to 350°C.

[0075] The rolling step is performed as necessary. The rolling step is a step of applying pressure to the positive electrode active material layer 24 to adjust the density of the positive electrode active material layer 24. The rolling step is performed using, for example, a roll press device.

[0076] The negative electrode 30 can be produced by the same procedure as that for the positive electrode 20. The separator 10 and the outer casing 50 can be commercially available products.

[0077] Next, the prepared positive electrode 20 and negative electrode 30 are stacked so that the separator 10 is located between them to prepare the power generating element 40. A terminal 62 is connected to the positive electrode 20 of the power generating element 40, and a terminal 60 is connected to the negative electrode 30. When the power generating element 40 is a wound body, the positive electrode 20, the negative electrode 30, and one end side of the separator 10 are wound around the axis.

[0078] Next, the power generation element 40 is sealed in the exterior body 50. The non-aqueous electrolyte solution is poured into the exterior body 50. After the non-aqueous electrolyte solution is poured, the power generation element 40 is impregnated with the non-aqueous electrolyte solution by reducing the pressure, heating, etc. The exterior body 50 is sealed by applying heat, etc. Instead of pouring the non-aqueous electrolyte solution into the exterior body 50, the power generation element 40 may be impregnated with the non-aqueous electrolyte solution. After the non-aqueous electrolyte solution is poured into the power generation element 40, it is preferable to leave it to stand for 24 hours.

[0079] Next, the power generating element 40 is subjected to an initial charge and discharge. During this initial charge, a coating is formed on the surface of the negative electrode 30.

[0080] When forming the coating, it is preferable to inject the nonaqueous electrolyte solution and then heat the coating at a temperature in the range of 50° C. to 70° C. for about 2 hours. By carrying out this treatment, the intensity ratio of the second peak p2 to the first peak p1 becomes 0.5 or more and less than 4.0.

[0081] Furthermore, when forming the coating, it is preferable to perform an initial charging process by performing two-stage charging at different charge rates and holding the battery at each charge rate for a certain period of time. For example, it is preferable to perform two hours of constant-current charging at a charge rate of 0.1 C (the current value at which charging is completed in one hour when constant-current charging is performed at 25°C), followed by charging at a charge rate of 0.2 C until the battery voltage reaches 4.3 V, and then holding the battery at a constant voltage of 4.3 V for 10 minutes or more. By performing this process, the intensity ratio of the fourth peak p4 to the third peak p3 becomes 1.5 or more and less than 4.0.

[0082] When forming the coating, it is preferable to perform charge and discharge in 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 becomes 2.0 or more and less than 5.0.

[0083] By performing the first charge and discharge, a coating is formed on the surface of the negative electrode 30, and the lithium ion secondary battery 100 according to this embodiment is obtained.

[0084] The lithium-ion secondary battery 100 according to this embodiment has excellent cycle characteristics both in low-temperature and high-temperature environments. This is because the nonaqueous electrolyte solution contains a predetermined electrolyte salt at a predetermined ratio, and thus a coating having low resistance to Li and a sufficient thickness is formed on the surface of the negative electrode 30.

[0085] The above describes the embodiments of the present invention in detail with reference to the drawings. However, each configuration and combination thereof in each embodiment is an example, and additions, omissions, substitutions, and other modifications of the configurations are possible within the scope that does not deviate from the spirit of the present invention.

[0086] Example 1 A positive electrode slurry was applied to one surface of an aluminum foil having a thickness of 15 μm. The positive electrode slurry was prepared by mixing a positive electrode active material, a conductive additive, a binder, and a solvent.

[0087] The positive electrode active material is LiNi as a lithium oxide. ０．９２ Co ０．０４ Mn ０．０４The conductive additive was acetylene black. The binder was polyvinylidene fluoride (PVDF). The solvent was N-methyl-2-pyrrolidone. The mass ratio of the positive electrode active material, conductive additive, and binder was 90 wt %:5 wt %:5 wt %. These were mixed in a solvent to prepare a positive electrode slurry. The amount of positive electrode active material carried in the positive electrode active material layer after drying was 25 mg / cm. ２ The solvent was removed from the positive electrode slurry in a drying furnace to prepare a positive electrode active material layer, which was then pressed with a roll press to prepare a positive electrode.

[0088] A negative electrode slurry was applied to one surface of a copper foil having a thickness of 10 μm. The negative electrode slurry was prepared by mixing a negative electrode active material, a conductive additive, a binder, and a solvent.

[0089] The negative electrode active material used was graphite particles and silicon-containing compound particles in which carbon powder was supported on the surface of silicon powder. The conductive additive used was acetylene black. The binder used was polyvinylidene fluoride (PVDF). The solvent used was N-methyl-2-pyrrolidone. The mass ratio of the negative electrode active material, conductive additive, and binder was 94 wt %:2 wt %:4 wt %. These were mixed in a solvent to prepare a negative electrode slurry. The amount of negative electrode active material supported in the negative electrode active material layer after drying was 6.1 mg / cm. ２ The solvent was removed from the negative electrode slurry in a drying furnace to form a negative electrode active material layer. The negative electrode active material layer was pressed with a roll press to form a negative electrode. When the negative electrode fabricated under the same conditions was evaluated, the volume ratio of the silicon-containing compound particles was 50.5% by volume, and the volume ratio of the graphite particles was 49.5% by volume. The weight ratio of silicon element in the negative electrode active material layer was 10.2% by weight.

[0090] Next, a non-aqueous electrolyte solution was prepared. The solvent of the non-aqueous electrolyte solution was ethylene carbonate (EC): ethyl methyl carbonate (EMC): diethyl carbonate (DEC) = 30 vol %: 50 vol %: 20 vol %. The electrolyte solution contained LiPF as an electrolytic salt. ６ and LiFSI and LiNO ３ LiPF was added. ６ LiNO ３The molar ratio of LiPF was set to 0.15. ６ The molar ratio of LiFSI to HCl was 0.30.

[0091] (Preparation of Lithium-Ion Secondary Battery for Evaluation) A power generation element was obtained by stacking a negative electrode and a positive electrode with a separator (porous polyethylene sheet) interposed between them so that the positive electrode active material layer and the negative electrode active material layer faced each other. This power generation element was inserted into an exterior body made of aluminum laminate film and heat-sealed except for one peripheral location to form a closed opening. Finally, a nonaqueous electrolyte solution was injected into the exterior body, and the power generation element was heated to a temperature range of 60°C and held for about 2 hours. Thereafter, the remaining location was heat-sealed while reducing the pressure using a vacuum sealer.

[0092] Next, the power generation element was subjected to an initial charge and discharge. The initial charge and discharge were carried out in two stages. First, the power generation element was charged for 2 hours at a constant current charge rate of 0.1 C (a current value at which charging is completed in 5 hours when constant current charging is carried out at 25°C), and then charged at a charge rate of 0.2 C until the battery voltage reached 4.3 V. The battery voltage was maintained at a constant voltage of 4.3 V for 10 minutes. Thereafter, the power generation element was discharged at a constant current discharge rate of 0.2 C until the battery voltage reached 2.0 V. After the initial charge and discharge, the power generation element was heated to 30°C and charged and discharged once more in the same manner as above. The discharge capacity after the initial charge and discharge was detected, and the battery capacity Q at the time of initial discharge was calculated. １ asked for.

[0093] (Measurement of capacity retention rate after 100 cycles) The cycle characteristics of the lithium ion secondary battery were measured. The cycle characteristics were measured using a secondary battery charge / discharge tester (manufactured by Hokuto Denko Corporation). The cycle characteristics were measured in a low-temperature environment at an ambient temperature of 25°C and a high-temperature environment at an ambient temperature of 45°C. The lithium ion secondary battery used for the low-temperature environment measurement and the lithium ion secondary battery used for the high-temperature environment measurement were different samples fabricated under the same conditions.

[0094] When measuring the cycle characteristics, 100 charge / discharge cycles were performed. In each cycle, charging was performed at a constant current of 2.0 C until the battery voltage reached 4.3 V, and discharging was performed at a constant current of 4.0 C until the battery voltage reached 2.5 V. The discharge capacity after 100 charge / discharge cycles was measured, and the battery capacity Q after 100 cycles was calculated. ２ asked for.

[0095] The capacity Q calculated above １ , Q ２ The capacity retention rate E after 100 cycles was calculated from the above. The capacity retention rate E was calculated as follows: E = Q ２ / Q １ × 100. The capacity retention rate of the lithium ion secondary battery of Example 1 in a low temperature environment was 95%, and the capacity retention rate of the lithium ion secondary battery of Example 1 in a high temperature environment was 85%.

[0096] After 100 charge / discharge cycles (25°C), the lithium-ion secondary battery was disassembled and the surface of the negative electrode was analyzed by XPS to determine 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.

[0097] Examples 2 to 10 are LiPF in a non-aqueous electrolyte solution. ６ and LiFSI and LiNO ３ The differences from Example 1 are that the molar ratio of the silicon-containing compound particles and the graphite particles in the negative electrode active material were changed, and that the ratio of the silicon-containing compound particles and the graphite particles in the negative electrode active material was changed. The other conditions were the same as in Example 1, and cycle characteristics and XPS analysis were performed. The composition of the coating formed on the negative electrode in Examples 2 to 10 was LiPF ６ and LiFSI and LiNO ３ The molar ratios of the compounds are different from those of Example 1. In Examples 2 to 10, the relationship between the peak intensity ratios determined by XPS is different from that of Example 1.

[0098] Examples 11 to 18 are LiPF in a non-aqueous electrolyte solution. ６ and LiFSI and LiNO ３The differences from Example 1 are that the molar ratio of and the charge / discharge conditions during the initial charge / discharge were changed. In Examples 13 and 14, the heating and holding step of the power generation element after the injection of the nonaqueous electrolyte solution was not performed. In Examples 15 and 16, the second-stage charge / discharge rate was changed to 0.7 C during the initial charge / discharge, and constant current charge and constant current discharge were performed. In Examples 17 and 18, the charge / discharge step with the power generation element in a heated state was not performed after the initial charge / discharge. The other conditions were the same as in Example 1, and cycle characteristics and XPS analysis were performed.

[0099] Examples 19 to 21 are examples of LiPF in a non-aqueous electrolyte solution. ６ and LiFSI and LiNO ３ The molar ratio of the silicon-containing compound particles to the graphite particles in the negative electrode active material was fixed, and the ratio of the silicon-containing compound particles to the graphite particles in the negative electrode active material was changed. In Examples 19 to 21, the cycle characteristics and XPS analysis were performed in the same manner as in Example 1.

[0100] "Comparative Examples 1 to 8" Comparative Examples 1 to 8 are LiPF 5 in a non-aqueous electrolyte solution. ６ and LiFSI and LiNO ３ The difference from Example 1 is that the molar ratio of LiNO was changed. ３ In Comparative Example 2, no LiNO was added to the non-aqueous electrolyte solution. ３ In Comparative Example 3, no LiFSI was added to the non-aqueous electrolyte solution. In Comparative Example 4, LiNO was added to the non-aqueous electrolyte solution. ３ In Comparative Example 5, LiFSI was added in excess to the non-aqueous electrolyte solution, and LiNO was not added. In Comparative Example 6, LiFSI was added to the non-aqueous electrolyte solution. ３ and LiPF ６ In Comparative Example 7, LiFSI was not added to the non-aqueous electrolyte solution, and LiNO ３ In Comparative Example 6, the amount of LiNO added to the non-aqueous electrolyte solution was also small. ３ No LiFSI was added, and the amount of LiFSI added was small.

[0101] The conditions and measurement results for Examples 1 to 21 and Comparative Examples 1 to 8 are summarized in Table 1.

[0102]

[0103] Compared with Comparative Examples 1 to 8, Examples 1 to 21 exhibited excellent cycle characteristics in both low-temperature and high-temperature environments. This is believed to be due to the formation of a desired coating on the surface of the negative electrode by ensuring that the ratio of electrolyte salt contained in the nonaqueous electrolyte solution was within a predetermined range. By providing a coating with sufficient thickness, it is possible to suppress the reaction between the electrolyte and the negative electrode due to thermal decomposition of the coating, even in high-temperature environments. Furthermore, by preventing the coating from becoming too resistant to Li ions, it is possible to suppress Li deposition.

[0104] Furthermore, Examples 1 to 4, 10, and 11 were superior in cycle characteristics in low-temperature and high-temperature environments compared to Examples 13 to 18. ６ and LiFSI and LiNO ３ It is believed that the cycle characteristics of the lithium ion secondary battery were improved by controlling not only the molar ratio but also the state of the coating measured by XPS.

[0105] REFERENCE SIGNS LIST 10 Separator 20 Positive electrode 22 Positive electrode current collector 24 Positive electrode active material layer 30 Negative electrode 32 Negative electrode current collector 34 Negative electrode active material layer 40 Power generating element 50 Exterior body 52 Metal foil 54 Resin layer 60, 62 Terminal 100 Lithium ion secondary battery

Claims

1. A non-aqueous electrolyte solution comprising lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium nitrate, wherein the molar ratio of the lithium nitrate to the lithium hexafluorophosphate is 0.01 or more and 0.15 or less, and 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 comprising a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and the nonaqueous electrolyte solution according to claim 1.

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

0.

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

0.

5. The lithium ion secondary battery according to claim 2, wherein a C1s spectrum obtained by measuring the surface of the negative electrode using X-ray photoelectron spectroscopy (XPS) has a fifth peak observed in a binding energy range of 288 eV or more and 292 eV or less and a sixth peak observed in a binding energy range of 280 eV or more and 288 eV or less, and the intensity ratio of the sixth peak to the fifth peak is 2.0 or more 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 containing a negative electrode active material, the negative electrode active material has at least one of silicon-containing compound particles and graphite particles, and the volume ratio of the silicon-containing compound particles to the negative electrode active material is 10% by volume or more and 100% by volume or less.

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

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