Secondary battery

Abstract

The present invention improves charge / discharge characteristics. This secondary battery comprises a positive electrode, a negative electrode, and an electrolyte solution. The positive electrode is provided with a positive electrode current collector containing aluminum. The electrolyte solution contains an electrolyte and a solvent. The electrolyte contains a bis(fluorosulfonyl) imide salt. The surface of the positive electrode current collector contains sulfur. A photoelectron spectrum obtained by X-ray photoelectron spectroscopy of the surface of the positive electrode current collector has a first signal having a peak in a range of 164.1-164.5 eV, and a second signal having a peak in a range of 168.9-169.3 eV. The ratio of the sum of the signal intensities of the first signal and the second signal to the total signal intensity of the S2p spectrum is 0.97 or less.

Description

secondary battery

[0001] The present invention relates to a secondary battery.

[0002] Patent Document 1 discloses a secondary battery in which the positive electrode has a positive electrode current collector made of aluminum or an aluminum alloy, and the electrolyte contains lithium bis(fluorosulfonyl)imide.

[0003] Japanese Patent Application Laid-Open No. 2015-133315

[0004] However, in the secondary battery disclosed in Patent Document 1, there is a possibility that the charge / discharge characteristics may be deteriorated due to the electrolyte.

[0005] The present invention has been made in view of the above problems, and an object of the present invention is to improve charge-discharge characteristics.

[0006] A secondary battery according to one aspect of the present invention is a secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode includes a positive electrode current collector containing aluminum, the electrolyte includes an electrolyte and a solvent, the electrolyte includes a bis(fluorosulfonyl)imide salt, and the surface of the positive electrode current collector contains sulfur, and a photoelectron spectrum obtained by X-ray photoelectron spectroscopy of the surface of the positive electrode current collector has a first signal having a peak in the range of 164.1 eV or more and 164.5 eV or less and a second signal having a peak in the range of 168.9 eV or more and 169.3 eV or less, and a ratio of the sum of the signal intensity of the first signal and the signal intensity of the second signal to the total signal intensity of an S2p spectrum is 0.97 or less.

[0007] According to the present invention, the charge-discharge characteristics can be improved.

[0008] Fig. 1 is a perspective view showing the configuration of a secondary battery according to one embodiment. Fig. 2 is an enlarged cross-sectional view showing the configuration of the battery element shown in Fig. 1. Fig. 3 is a diagram showing an XPS S2p spectrum of a positive electrode current collector surface according to an embodiment. Fig. 4 is a diagram showing an XPS F1s spectrum of a positive electrode current collector surface according to an embodiment. Fig. 5 is a diagram showing an XPS Al2p spectrum of a positive electrode current collector surface according to an embodiment.

[0009] Hereinafter, an embodiment of the present invention will be described, but the present invention is not limited to this embodiment.

[0010] (Secondary Battery) The secondary battery according to this embodiment is a secondary battery that obtains battery capacity by utilizing the occlusion and release of an electrode reactant, and includes a positive electrode, a negative electrode, and an electrolyte solution.

[0011] The type of electrode reactant is not particularly limited, and specifically includes light metals such as alkali metals and alkaline earth metals. Specific examples of alkali metals include lithium, sodium, and potassium. Specific examples of alkaline earth metals include beryllium, magnesium, and calcium. The electrode reactant may also be other light metals such as aluminum.

[0012] In the following description, an example will be given in which the electrode reactant is lithium. A secondary battery that obtains battery capacity by utilizing the absorption and desorption of lithium is, for example, a lithium ion secondary battery. In a lithium ion secondary battery, lithium is absorbed and desorbed in the ionic state.

[0013] Fig. 1 is a perspective view showing the configuration of a secondary battery according to one embodiment. Fig. 2 is an enlarged cross-sectional view showing the configuration of the battery element shown in Fig. 1. Fig. 1 shows a state in which an exterior film 10 and a battery element 20 are separated from each other, and the cross section of the battery element 20 is shown by a dashed line. Fig. 2 shows the cross section of only a portion of the battery element 20.

[0014] As shown in FIGS. 1 and 2, the secondary battery 1 includes an exterior film 10, a battery element 20, a positive electrode lead 31, a negative electrode lead 32, and sealing films 41 and .

[0015] As described above, the secondary battery 1 in Fig. 1 uses the exterior film 10 as an exterior member for housing the battery element 20. Therefore, the secondary battery 1 shown in Fig. 1 is a so-called laminate film type secondary battery.

[0016] 1, the exterior film 10 is an exterior member that houses the battery element 20, and has a bag-like structure that is sealed when the battery element 20 is housed inside. As a result, the exterior film 10 houses a positive electrode 210, a negative electrode 220, a separator 230, and an electrolyte solution (not shown), which will be described later.

[0017] In the example of Fig. 1, the exterior film 10 is a single film-like member that is folded in a folding direction F. The exterior film 10 has a recessed portion 10U for accommodating the battery element 20. The recessed portion 10U is a so-called deep-drawn portion.

[0018] Specifically, the exterior film 10 is a three-layer laminate film in which a fusion layer, a metal layer, and a surface protection layer are laminated in this order from the inside. When the exterior film 10 is folded, the outer peripheral edges of the opposing fusion layers are fused to each other. The fusion layer contains a polymer compound such as polypropylene. The metal layer contains a metal material such as aluminum. The surface protection layer contains a polymer compound such as nylon. The configuration (number of layers) of the exterior film 10 is not particularly limited and may be one layer, two layers, or four or more layers.

[0019] 1 and 2 , the positive electrode lead 31 is a positive electrode wiring connected to the positive electrode current collector 211 of the positive electrode 210, and is drawn out to the exterior of the exterior film 10. The positive electrode lead 31 includes at least one type of conductive material such as a metal material, and a specific example of the conductive material is aluminum. The shape of the positive electrode lead 31 is not particularly limited, and may be, for example, a thin plate shape or a mesh shape.

[0020] (Negative Electrode Lead) As shown in FIGS. 1 and 2 , the negative electrode lead 32 is a negative electrode wiring connected to the negative electrode current collector 221 of the negative electrode 220, and is drawn out to the exterior of the exterior film 10. The negative electrode lead 32 includes at least one type of conductive material such as a metal material. A specific example of the conductive material is copper. The shape of the negative electrode lead 32 is not particularly limited, and may be, for example, a thin plate shape or a mesh shape.

[0021] (Sealing Film) As shown in Fig. 1, the sealing film 41 is inserted between the exterior film 10 and the positive electrode lead 31. As shown in Fig. 1, the sealing film 42 is inserted between the exterior film 10 and the negative electrode lead 32. However, one or both of the sealing films 41 and 42 may be omitted.

[0022] The sealing film 41 is a sealing member that prevents outside air and the like from entering the inside of the exterior film 10. The sealing film 41 contains a polymer compound such as polyolefin that has adhesiveness to the positive electrode lead 31. A specific example of the polymer compound is polypropylene.

[0023] The sealing film 42 is a sealing member that prevents outside air and the like from entering the inside of the exterior film 10. The sealing film 41 contains a polymer compound such as polyolefin that has adhesiveness to the negative electrode lead 32. A specific example of the polymer compound is polypropylene.

[0024] (Battery Element) The battery element 20 is housed in the space of the recess 10U of the exterior film 10. The battery element 20 is a so-called power generating element. As shown in Figures 1 and 2, the battery element 20 includes a positive electrode 210, a negative electrode 220, a separator 230, and an electrolyte solution (not shown).

[0025] 1 , battery element 20 is a so-called wound electrode body. Therefore, positive electrode 210 and negative electrode 220 are wound around winding axis P while facing each other via separator 230. In the following description, the direction along winding axis P may be referred to as the Y direction, the longitudinal direction of battery element 20 perpendicular to winding axis P as the X direction, and the lateral direction of battery element 20 perpendicular to winding axis P as the Z direction.

[0026] In the example of FIG. 1 , the battery element 20 has a flat, three-dimensional shape. That is, the shape of a cross section (cross section along the XZ plane) of the battery element 20 intersecting the winding axis P of the battery element 20 is a flat shape defined by a major axis J1 and a minor axis J2. The major axis J1 is an imaginary axis extending in the X-axis direction and has a length greater than the length of the minor axis J2. The minor axis J2 is an imaginary axis extending in the Z-axis direction and has a length smaller than the length of the major axis J1. As a result, the cross-sectional shape of the battery element 20 is a flat, approximately elliptical shape. Note that the three-dimensional shape of the battery element 20 is merely an example and is not limited to the above.

[0027] The positive electrode 210 includes a positive electrode current collector 211 and a positive electrode active material layer 212. In the positive electrode 210, the positive electrode current collector 211 is laminated between the positive electrode active material layers 212. However, the positive electrode active material layer 212 may be provided only on one surface of the positive electrode current collector 211, on the side where the positive electrode 210 faces the negative electrode 220.

[0028] The positive electrode current collector 211 contains aluminum and may be, for example, aluminum foil. The surface condition of the positive electrode current collector 211 will be described later.

[0029] The positive electrode active material layer 212 is a layer containing a positive electrode active material capable of absorbing and releasing lithium. The positive electrode active material layer 212 contains a positive electrode active material. The positive electrode active material layer 212 preferably further contains a positive electrode binder and a positive electrode conductive agent. The positive electrode active material layer 212 is not limited to the materials listed above, and may further contain, for example, a dispersant.

[0030] The positive electrode active material is preferably a lithium-containing compound such as a lithium-containing composite oxide or a lithium-containing phosphate compound. The lithium-containing composite oxide is an oxide containing lithium and one or more elements other than lithium as constituent elements. The lithium-containing composite oxide has, for example, a layered rock salt type or a spinel type crystal structure. The lithium-containing phosphate compound is a phosphate compound containing lithium and one or more elements other than lithium as constituent elements. The lithium-containing phosphate compound has, for example, an olivine type crystal structure. A specific example of the lithium-containing composite oxide is LiNiO ２ , LiCoO ２ , LiCo０．９８ Al ０．０１ Mg ０．０１ O ２ , LiNi ０．５ Co ０．２ Mn ０．３ O ２ , LiNi ０．８ Co ０．１５ Al ０．０５ O ２ , LiNi ０．３３ Co ０．３３ Mn ０．３３ O ２ , Li １．２ Mn ０．５２ Co ０．１７５ Ni ０．１ O ２ , Li １．１５ (Mn ０．６５ Ni ０．２２ Co ０．１３ ) O ２ , LiMn ２ O ４ Specific examples of lithium-containing phosphate compounds include LiFePO ４ , LiMnPO ４ , LiFe ０．５ Mn ０．５ P.O. ４ , LiFe ０．３ Mn ０．７ P.O. ４ And so on.

[0031] The positive electrode binder contained in the positive electrode active material layer 212 is a binder for the resin contained in the positive electrode 210, and improves the binding between the positive electrode active material particles. The positive electrode binder may be any material, and may include, for example, one or more of synthetic rubber and polymer compounds. Examples of synthetic rubber used as the positive electrode binder include styrene butadiene rubber, fluorine-based rubber, and ethylene propylene diene. Examples of polymer compounds used as the positive electrode binder include polyvinylidene fluoride (PVdF) and polyimide.

[0032] The content of the positive electrode binder relative to the total of the positive electrode active material, positive electrode conductive agent, and positive electrode binder is preferably 0.3 mass% to 2.0 mass%, more preferably 0.3 mass% to 1.5 mass%, even more preferably 0.3 mass% to 1.0 mass%, and even more preferably 0.3 mass% to less than 1.0 mass%. In the following description, the content of the positive electrode binder relative to the total of the positive electrode active material, positive electrode conductive agent, and positive electrode binder may be simply referred to as the positive electrode binder content. This improves the binding between the positive electrode active material particles, improves charge / discharge characteristics, and reduces the interfacial resistance between the positive electrode current collector 211 and the positive electrode active material layer 212. Here, the positive electrode binder content can be measured by thermogravimetry.

[0033] The positive electrode conductive agent contained in the positive electrode active material layer 212 is a conductive agent other than the positive electrode active material contained in the positive electrode 210, and improves the conductivity between the positive electrode active material particles. The positive electrode conductive agent may be any material, including, for example, a carbon material. Examples of carbon materials used as the positive electrode conductive agent include graphite, carbon black, acetylene black, and ketjen black. However, the positive electrode conductive agent is not limited to these materials, and may also be a metal material, a conductive polymer, or the like, as long as it is a conductive material.

[0034] The positive electrode active material layer 212 preferably contains carbon black and carbon nanotubes (CNTs) as positive electrode conductive agents. When the positive electrode active material layer 212 contains carbon black as a positive electrode conductive agent, the conductivity between the positive electrode active material particles can be improved, and the internal resistance of the positive electrode 210 can be reduced. When the positive electrode active material layer 212 contains CNTs as a positive electrode conductive agent, the adhesion between the positive electrode active material particles can be improved, and the conductivity between the positive electrode active material particles can be improved. Here, whether the positive electrode active material layer 212 contains carbon black and CNTs can be determined by observing a cross section of the positive electrode active material layer 212 with a scanning electron microscope (SEM).

[0035] The content of the positive electrode conductive agent relative to the total of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder is preferably 1.0 mass % or more and 2.5 mass % or less. This makes it possible to reduce the interfacial resistance between the positive electrode current collector 211 and the positive electrode active material layer 212 while improving the charge / discharge characteristics. Here, the content of the positive electrode conductive agent can be measured by thermogravimetry. In the following description, the content of the positive electrode conductive agent relative to the total of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder may be simply referred to as the content of the positive electrode conductive agent.

[0036] The content of carbon black relative to the total of the positive electrode active material, positive electrode conductive agent, and positive electrode binder is preferably 0.3 mass % or more and 2.5 mass % or less, and more preferably 0.3 mass % or more and 1.5 mass % or less. This makes it possible to reduce the interfacial resistance between the positive electrode current collector 211 and the positive electrode active material layer 212 while improving the charge / discharge characteristics. Here, the carbon black content is determined by the ratio of the specific surface area attributable to carbon black to the specific surface area of ​​the electrode and the specific surface area (100 m) of the carbon black. ２ / g-300m ２ In the following description, the content of carbon black relative to the total of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder may be simply referred to as the carbon black content.

[0037] The CNTs used as the positive electrode conductive agent are preferably at least one of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). The average fiber diameter of the CNTs is preferably 1 nm or more and 20 nm or less. The average fiber length of the CNTs is preferably 5 μm or more and 200 μm or less. This can further reduce the interfacial resistance between the positive electrode current collector 211 and the positive electrode active material layer 212 while improving the charge / discharge characteristics.

[0038] The negative electrode 220 includes a negative electrode current collector 221 and a negative electrode active material layer 222. In the negative electrode 220, the negative electrode current collector 221 is laminated between the negative electrode active material layers 222. However, the negative electrode active material layer 222 may be provided only on one surface of the negative electrode current collector 221, on the side where the negative electrode 220 faces the positive electrode 210.

[0039] The negative electrode current collector 221 is a conductor, and for example, copper foil or the like can be used.

[0040] The negative electrode active material layer 222 is a layer containing a negative electrode active material capable of absorbing and releasing lithium. The negative electrode active material layer 222 is not limited to being made of only a negative electrode active material, and may contain, for example, a conductive agent and a binder.

[0041] The negative electrode active material preferably contains at least one of a carbon material and a metal-based material. This allows for a high energy density to be obtained. Specific examples of carbon materials used as the negative electrode active material include graphitizable carbon, non-graphitizable carbon, natural graphite, and artificial graphite. The metal-based material used as the negative electrode active material is a material containing, as a constituent element, one or more of metal elements and metalloid elements that can form an alloy with lithium. Specific examples of metal elements and metalloid elements used as the negative electrode active material include silicon and tin. The metal-based material used as the negative electrode active material may be a simple substance, an alloy, a compound, a mixture of two or more types, or a material containing two or more types of phases. Specific examples of metal-based materials used as the negative electrode active material include TiSi ２ , SiO ｘ (0<x≦2), etc.

[0042] The negative electrode active material layer 222 is not limited to containing only the negative electrode active material.

[0043] For example, the negative electrode active material layer 222 may further contain a negative electrode binder. The negative electrode binder includes at least one of synthetic rubber, polymer compound, etc. Specific examples of synthetic rubber used as the negative electrode binder include styrene butadiene rubber, fluorine-based rubber, and ethylene propylene diene. Specific examples of polymer compounds used as the negative electrode binder include polyvinylidene fluoride, polyimide, and carboxymethyl cellulose.

[0044] For example, the anode active material layer 222 may further contain an anode conductive agent. The anode conductive agent includes at least one of a carbon material, a metal material, and a conductive polymer compound. Specific examples of carbon materials used as the anode conductive agent include particulate carbon materials such as carbon black, acetylene black, and ketjen black, and fibrous carbon materials such as carbon nanotubes. The carbon nanotubes are, for example, single-wall carbon nanotubes (SWCNTs). This can improve the electronic conductivity of the particle surfaces of the first anode active material. The mass ratio of the anode conductive agent to the anode active material layer 222 is preferably 5% or less, more preferably 2% or less. This can improve the paintability of the anode slurry.

[0045] The separator 230 is a film that insulates the positive electrode 210 from the negative electrode 220. The separator 230 is provided between the main surface of the positive electrode 210 and the main surface of the negative electrode 220 so that the positive electrode 210 and the negative electrode 220 do not come into direct contact with each other.

[0046] The separator 230 is preferably made of a material that is electrically stable, chemically stable with respect to the positive electrode active material, the negative electrode active material, and the electrolyte, and is insulating. The separator 230 can be made of, for example, a polymer nonwoven fabric, a porous film, or a layer of glass or ceramic fibers. The separator 230 is more preferably made of a porous polyolefin film. This improves battery safety by preventing short circuits and providing a shutdown effect.

[0047] The electrolyte solution is impregnated into each of the positive electrode 210, the negative electrode 220, and the separator 230. In the example of Fig. 1, the electrolyte solution fills the space inside the exterior member 30. The electrolyte solution is a non-aqueous electrolyte solution containing an electrolyte salt and a solvent that dissolves the electrolyte salt.

[0048] The electrolyte salt is lithium bis(fluorosulfonyl)imide (LiFSI, LiN(SO ２ F ２ ) ２The electrolyte salt may contain other electrolyte salts used as electrolyte salts for lithium ion batteries. The other electrolyte salts may be light metal salts such as lithium salts. Specific examples of lithium salts include lithium hexafluorophosphate (LiPF ６ ), lithium monofluorophosphate (Li ２ PFO ３ ), lithium difluorophosphate (LiPF ２ O ２ ) and lithium tetrafluoroborate (LiBF ４ ), lithium bis(oxalato)borate (LiB(C ２ O ４ ) ２ It is preferable that the electrolyte salt is a lithium salt containing boron (B), such as lithium trifluoromethanesulfonate (LiCF), etc. This allows P and B to be contained in the surface of the positive electrode current collector 211. Note that the other electrolyte salt is not limited to the above, and may be lithium trifluoromethanesulfonate (LiCF), etc. ３ SO ３ ), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF ３ SO ２ ) ２ ), lithium tris(trifluoromethanesulfonyl)methide (LiC(CF ３ SO ２ ) ３ ) etc.

[0049] The solvent comprises at least one member of a first group consisting of ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), and gamma butyrolactone (GBL).

[0050] The solvent may further contain at least one of a carbonate ester and a chain carboxylic acid ester other than the compounds included in Group 1. Examples of the carbonate ester other than the compounds included in Group 1 include diethyl carbonate (DEC) and ethyl methyl carbonate (EMC). Examples of the chain carboxylic acid ester include propyl propionate (PrPr), ethyl propionate (PrEt), methyl propionate, propyl acetate (AcPr), ethyl acetate, and methyl acetate.

[0051] The solvent may further include other nonaqueous solvents used as nonaqueous solvents in lithium-ion batteries. Examples of such nonaqueous solvents include ethers such as 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, and 1,4-dioxane. The ethers may be compounds in which some or all of the hydrogen atoms have been substituted with fluorine atoms, such as 1,1,2-tetrafluoroethyl 2,2,2,3,3-tetrafluoropropyl ether.

[0052] The electrolytic solution may contain substances other than the electrolyte salt and the solvent, such as additives.

[0053] The electrolyte may contain additives such as unsaturated cyclic carbonates such as vinylene carbonate, methylene ethylene carbonate, and vinyl ethylene carbonate; halogenated cyclic carbonates such as monofluoroethylene carbonate and difluoroethylene carbonate; hydrofluoroethers; sulfonates; phosphates; acid anhydrides; and isocyanates. Specific examples of sulfonates include propane sultone and propene sultone. Specific examples of phosphate esters include trimethyl phosphate and triethyl phosphate. Specific examples of acid anhydrides include succinic anhydride, 1,2-ethanedisulfonic anhydride, and 2-sulfobenzoic anhydride. Specific examples of isocyanates include hexamethylene diisocyanate.

[0054] The molar ratio of the original solvent to lithium ions calculated from the vibrational spectroscopy spectrum of the electrolyte solution is greater than 0 and equal to or less than 1.76. This allows for further improvement in charge / discharge characteristics. In the present disclosure, the original solvent refers to solvent molecules contained in the electrolyte solution that are not solvated with lithium ions. The original molar ratio of the original solvent to lithium ions according to the present disclosure is calculated by the original amount of substance M of the solvent calculated by the following formula (1): ０ Here, when the electrolyte solution contains multiple types of solvents, the original substance amount M of each solvent can be calculated by the following formula (1): ０ The original molar ratio of the solvent to the lithium ions can be calculated by calculating the weighted average weighted by the molar ratio of the multiple solvents and dividing it by the amount of lithium ions contained in the electrolyte solution. ０ = M ａｌｌ -M Ｎ = M ａｌｌ -c ｔ / {1+Γ(I o / I ｓ )} (1) Here, in formula (1), M ａｌｌ is the total amount of solvent contained in the solution, M Ｎ is the amount of solvent molecules solvated by lithium ions, c ｔ is the solvent concentration, Γ is the vibrational spectrum intensity ratio per unit concentration, I ｏ is the peak intensity of the solvent itself, I ｓ Here, the vibrational spectrum intensity ratio Γ per unit concentration is ｏ is the concentration of the solvent that is not solvated with lithium ions, c ｓ When the concentration of the solvent solvating the lithium ion is ｓ / c ｏ is the horizontal axis and I ｓ / I ｏ It can be calculated from the slope of a graph plotted with σ on the vertical axis.

[0055] Here, "peak of the solvent itself" means a peak observed at the peak position (wave number) when a solvent not solvated with lithium ions is subjected to vibrational spectroscopy, i.e., when only the solvent is subjected to vibrational spectroscopy. Also, "peak of the solvent of the solvate" refers to a peak position when a solvent solvated with lithium ions is subjected to vibrational spectroscopy. Here, the peak of the solvent of the solvate can be distinguished from the peak of the solvent itself because the vibrational spectroscopy spectrum is shifted relative to the peak of the solvent itself. Therefore, the height from the baseline to the peak top of each of the peak of the solvent itself and the peak of the solvent of the solvate is called the peak intensity I. ｏ , I ｓ In addition, when there are multiple peaks of the solvent itself and the solvent of the solvate in the vibrational spectroscopy spectrum of the electrolyte solution, the peak intensity I ｏ , I ｓ The ratio of peaks is easy to determine based on the peak intensity I ｏ , I ｓ In addition, when the shift of the peak of the solvent of the solvate relative to the peak of the original solvent is small and the peak of the original solvent and the peak of the solvent of the solvate overlap to form a gentle peak, the peak intensity I can be calculated by performing peak separation using a known means. ｏ , I ｓ The ratio may be calculated.

[0056] Table 1 shows examples of the wave numbers of the peaks of the solvent itself and the solvate solvent in the vibrational spectroscopy spectrum of the electrolyte solution of the present invention. The wave numbers of the peaks of the solvent itself and the solvate solvent shown in Table 1 are respectively I ｏ and I ｓcan be used to measure. Here, as shown in Table 1, the FEC peak is near the EC peak and the DMC peak, and therefore may overlap with each other. Since it can be assumed that FEC has the same vibrational spectroscopy spectrum intensity ratio Γ per unit concentration as EC and DMC, when a solvent contains at least one of EC and DMC and FEC, the original amount of substance of the solvent may be calculated using a composition in which FEC is replaced with the same amount of EC. Note that the values ​​shown in Table 1 are merely examples, and the wavenumber of the observed peak may differ from the wavenumber shown in Table 1 depending on the vibrational spectroscopy spectrum measurement device, measurement environment, and measurement conditions.

[0057]

[0058] In the present disclosure, the vibrational spectroscopy of an electrolyte solution is measured by Raman spectroscopy or Fourier transform infrared spectroscopy using an electrolyte solution extracted from a secondary battery to be measured using a centrifuge. Here, the measurement of the vibrational spectroscopy of an electrolyte solution is preferably performed in an environment where the influence of atmospheric moisture can be reduced or ignored. The measurement of the vibrational spectroscopy of an electrolyte solution can be performed under low-humidity or humidity-free conditions, such as a dry room or a glove box, or by using an electrolyte solution sealed in a transparent, sealed container as a sample. The electrolyte solution extracted from the secondary battery to be measured can be quantitatively analyzed using ICP (Inductively Coupled Plasma), NMR (Nuclear Magnetic Resonance), or GC-MS (Gas Chromatography Mass Spectrometry). Based on the obtained composition of the electrolyte solution, an electrolyte solution having the same composition as the electrolyte solution in the secondary battery to be measured can be prepared, and the prepared electrolyte solution can be used as the sample for measuring the vibrational spectroscopy spectrum.

[0059] The surface state of the positive electrode current collector 211 will be described in detail below. In the following description, X-ray photoelectron spectroscopy analysis will be described as XPS (X-ray Photoelectron Spectroscopy). In the following description, wide scans and various narrow scans on the surface of the positive electrode current collector 211 refer to XPS performed on the surface of the positive electrode current collector 211 after disassembling the secondary battery 1 and removing it, under the conditions shown in Table 1. In the wide scans and various narrow scans on the surface of the positive electrode current collector 211, it is preferable to perform multiple measurements using the number of measurements shown in Table 1 to obtain an averaged spectrum.

[0060]

[0061] 3 is a diagram showing an XPS S2p spectrum of the surface of the positive electrode current collector according to this embodiment. The surface of the positive electrode current collector 211 contains sulfur. Whether the surface of the positive electrode current collector 211 contains sulfur can be determined by wide scanning or narrow scanning of the surface of the positive electrode current collector 211. If a peak is detected in the S2p spectrum as shown in FIG. 3, it can be said that the surface of the positive electrode current collector 211 contains sulfur. In the present disclosure, the S2p spectrum refers to a spectral range of 160 eV to 180 eV.

[0062] 3, the photoelectron spectrum obtained by XPS of the surface of the positive electrode current collector 211 has a first signal S1 having a peak in the range of 164.1 eV to 164.5 eV and a second signal S2 having a peak in the range of 168.9 eV to 169.3 eV. The presence or absence of the first signal S1 and the second signal S2 can be determined by performing peak separation on the S2p spectrum obtained by S2p narrow scanning of the surface of the positive electrode current collector 211 using a Gaussian / Lorentzian mixed function.

[0063] Signal intensity I of the first signal Ｓ１ and the signal intensity of the second signal I Ｓ２ The sum of the signal intensities of the S2p spectrum, I Ｓｔ Ratio to (I Ｓ１ +I Ｓ２ ) / I Ｓｔis 0.97 or less. This allows for good charging and discharging. Ｓ１ and the signal intensity I of the second signal S2 Ｓ２ refers to the difference between the peak top and the baseline of each of the first signal S1 and the second signal S2. In addition, the sum of the signal intensities of the S2p spectrum, I Ｓｔ This refers to the difference between the peak top and the baseline of a composite signal St obtained by combining the signals S1 to S3 obtained by peak separation of the S2p spectrum.

[0064] 4 is a diagram showing an XPS F1s spectrum of the surface of the positive electrode current collector according to this embodiment. The surface of the positive electrode current collector 211 preferably further contains fluorine. Whether the surface of the positive electrode current collector 211 contains fluorine can be determined by wide scanning or narrow scanning the surface of the positive electrode current collector 211. If a peak is detected in the F1s spectrum as shown in FIG. 4, it can be said that the surface of the positive electrode current collector 211 contains fluorine. In the present disclosure, the F1s spectrum refers to a range of 679 eV or more and 697 eV or less.

[0065] 4, the photoelectron spectrum obtained by XPS of the surface of the positive electrode current collector 211 has a third signal F1 having a peak between 685.7 eV and 686.1 eV, and a fourth signal F2 having a peak between 687.4 eV and 687.8 eV. The presence or absence of the third signal F1 and the fourth signal F2 can be determined by performing peak separation on the F1s spectrum obtained by an F1s narrow scan of the surface of the positive electrode current collector 211 using a Gaussian / Lorentzian mixed function.

[0066] Signal intensity I of the third signal F1 Ｆ１ and the signal intensity I of the fourth signal F2 Ｆ２ The sum of the signal intensities of the F1s spectrum, I Ｆｔ Ratio to (I Ｆ１ +I Ｆ２ ) / I Ｆｔ is 0.80 or more. The signal intensity I of the third signal F1 Ｆ１ and the signal intensity I of the fourth signal F2 Ｆ２refers to the difference between the peak top and the baseline of each of the third signal F1 and the fourth signal F2. Also, the sum of the signal intensities of the F1s spectrum, I Ｆｔ This refers to the difference between the peak top and the baseline of a composite signal St obtained by combining the signals F1 to F4 obtained by peak separation of the F1s spectrum.

[0067] 5 is a diagram showing an XPS Al2p spectrum of the surface of the positive electrode current collector according to this embodiment. It is more preferable that the surface of the positive electrode current collector 211 further contains aluminum. Here, if a peak is detected in the Al2p spectrum as shown in FIG. 5 in a wide scan or an Al2p narrow scan of the surface of the positive electrode current collector 211, it can be said that the surface of the positive electrode current collector 211 contains aluminum. In this disclosure, the Al2p spectrum refers to a range of 61 eV to 84 eV.

[0068] 5 , the photoelectron spectrum obtained by XPS of the surface of the positive electrode current collector 211 has a fifth signal Al1 having a peak at 75.4 eV to 75.8 eV and a sixth signal Al2 having a peak at 76.4 eV to 76.8 eV. The presence or absence of the fifth signal Al1 and the sixth signal Al2 can be determined by performing peak separation using a Gaussian / Lorentzian mixed function on the Al2p spectrum obtained by an Al2p narrow scan of the surface of the positive electrode current collector 211.

[0069] Signal intensity I of the fifth signal A11 Ａｌ１ and the signal intensity I of the sixth signal Al2 Ａｌ２ The sum of the signal intensities of the Al2p spectrum, I Ａｌｔ Ratio to (I Ａｌ１ +I Ａｌ２ ) / I Ａｌｔ is 0.30 or more. This can improve the charge / discharge characteristics. Ａｌ１ and the signal intensity of the sixth signal I Ａｌ２ The difference between the peak top and the baseline of the fifth signal A11 and the sixth signal A12 is the sum of the signal intensities of the A12p spectrum, I ＡｌｔThis refers to the difference between the peak top and the baseline of a composite signal Alt obtained by combining the signals Al1 to Al5 obtained by peak separation of the Al2p spectrum.

[0070] It is more preferable that the surface of the positive electrode current collector 211 further contains at least one of boron and phosphorus. This can further improve charge / discharge characteristics. Here, if a peak is detected in the B1s spectrum during a wide scan or a B1s narrow scan of the surface of the positive electrode current collector 211, it can be said that the surface of the positive electrode current collector 211 contains boron. In the present disclosure, the B1s spectrum refers to a range of 183 eV to 203 eV. Furthermore, if a peak is detected in the P2p spectrum during a wide scan or a P2p narrow scan of the surface of the positive electrode current collector 211, it can be said that the surface of the positive electrode current collector 211 contains phosphorus. In the present disclosure, the P2p spectrum refers to a range of 127 eV to 147 eV.

[0071] As described above, the secondary battery 1 according to the first embodiment is a secondary battery including a positive electrode 210, a negative electrode 220, and an electrolyte. The positive electrode 210 includes a positive electrode current collector 211 containing aluminum. The electrolyte contains an electrolyte and a solvent. The electrolyte contains a bis(fluorosulfonyl)imide salt. The surface of the positive electrode current collector 211 contains sulfur. A photoelectron spectrum obtained by X-ray photoelectron spectroscopy of the surface of the positive electrode current collector 211 has a first signal having a peak in the range of 164.1 eV to 164.5 eV and a second signal having a peak in the range of 168.9 eV to 169.3 eV. The signal intensity I of the first signal Ｓ１ and the signal intensity of the second signal I Ｓ２ The sum of the signal intensities of the S2p spectrum, I Ｓｔ Ratio to (I Ｓ１ +I Ｓ２ ) / I Ｓｔ is 0.97 or less. This can improve the charge / discharge characteristics.

[0072] In a preferred embodiment, the surface of the positive electrode current collector 211 contains fluorine. A photoelectron spectrum obtained by X-ray photoelectron spectroscopy of the surface of the positive electrode current collector 211 has a third signal having a peak in the range of 685.7 eV to 686.1 eV, and a fourth signal having a peak in the range of 687.4 eV to 687.8 eV. The signal intensity I of the third signal Ｆ１ and the signal intensity of the fourth signal I Ｆ２ The sum of the signal intensities of the F1s spectrum, I Ｆｔ Ratio to (I Ｆ１ +I Ｆ２ ) / I Ｆｔ is 0.80 or more. This can further improve the charge / discharge characteristics.

[0073] In a preferred embodiment, the surface of the positive electrode current collector 211 contains aluminum. A photoelectron spectrum obtained by X-ray photoelectron spectroscopy of the surface of the positive electrode current collector 211 has a fifth signal having a peak in the range of 75.4 eV to 75.8 eV, and a sixth signal having a peak in the range of 76.4 eV to 76.8 eV. The signal intensity I Ａｌ１ and the signal intensity of the sixth signal I Ａｌ２ The sum of the signal intensities of the Al2p spectrum, I Ａｌｔ Ratio to (I Ａｌ１ +I Ａｌ２ ) / I Ａｌｔ is 0.30 or more. This can further improve the charge / discharge characteristics.

[0074] In a more desirable embodiment, the surface of the positive electrode current collector 211 contains at least one of boron and phosphorus, which can further improve the charge / discharge characteristics.

[0075] In a more desirable embodiment, the positive electrode 210 further includes a positive electrode active material layer 212. The positive electrode active material layer 212 includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder. The positive electrode conductive agent includes carbon black and carbon nanotubes. The content of the positive electrode conductive agent relative to the total of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder is 1.0 mass% or more and 2.5 mass% or less. The content of carbon black relative to the total of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder is 0.3 mass% or more and 1.5 mass% or less. This makes it possible to reduce the interfacial resistance between the positive electrode current collector 211 and the positive electrode active material layer 212 while improving charge / discharge characteristics.

[0076] In a more preferable embodiment, the content of the positive electrode binder relative to the total of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder is 0.3 mass % or more and 2.0 mass % or less, thereby improving the charge / discharge characteristics and further reducing the interfacial resistance between the positive electrode current collector 211 and the positive electrode active material layer 212.

[0077] In a more preferred embodiment, the solvent contains at least one of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, and gamma-butyrolactone. The molar ratio of the solvent to lithium ions calculated from the vibrational spectroscopy of the electrolyte solution is greater than 0 and less than or equal to 1.76. This further improves charge / discharge characteristics.

[0078] (Method for manufacturing secondary battery) Here, an example of a method for manufacturing the secondary battery 1 according to the first embodiment will be described. The method for manufacturing the secondary battery 1 according to the first embodiment includes a step of fabricating the positive electrode 210, a step of fabricating the negative electrode 220, a step of preparing an electrolyte solution, a step of assembling the secondary battery 1, and a charge / discharge step.

[0079] In the process of producing the positive electrode 210, the positive electrode 210 is produced by the following method. A positive electrode active material, a positive electrode binder, and a positive electrode conductive agent are mixed to prepare a positive electrode mixture. The prepared positive electrode 210 mixture is then dispersed in a dispersion liquid such as N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture slurry, and the prepared positive electrode mixture slurry is then uniformly applied to both surfaces of the positive electrode current collector 211. The obtained coating is dried with hot air or the like, and then compression-molded using a roll press or the like to produce the positive electrode 210. A positive electrode 210 lead is attached to the portion of the produced positive electrode 210 where the positive electrode current collector 211 is exposed.

[0080] In the process of producing the negative electrode 220, the negative electrode 220 is produced by the following method. A negative electrode active material and a negative electrode binder are mixed to prepare a negative electrode mixture. The prepared negative electrode mixture is dispersed in a dispersion liquid such as NMP to prepare a negative electrode mixture slurry, and the negative electrode mixture is then uniformly applied to both sides of a negative electrode current collector. The resulting coating is dried using hot air or the like, and then compression-molded using a roll press or the like to produce the negative electrode 220. A negative electrode 220 lead is attached to the exposed portion of the negative electrode current collector 221 of the produced negative electrode 220.

[0081] In the step of preparing the electrolyte solution, an electrolyte salt is dissolved in a solvent to prepare the electrolyte solution.

[0082] In the process of assembling the secondary battery 1, the secondary battery 1 is assembled by the following method. The positive electrode 210, separator 230, and negative electrode 220 are stacked in this order and wound longitudinally to produce an electrode assembly. The produced electrode assembly is loaded into an exterior member, and three sides of the exterior member are heat-sealed, leaving one side unheat-sealed to form an opening. Thereafter, an electrolyte is poured into the opening of the exterior member, and the remaining side of the exterior member is heat-sealed in a reduced-pressure environment to seal the exterior member and form a laminate cell.

[0083] In the charge / discharge process, the fabricated laminate cell is subjected to one charge / discharge cycle. Here, one charge / discharge cycle can be performed under charge / discharge conditions A, B, or C, for example. Charge / discharge condition A refers to a condition in which a first charge, a first standing time, a second charge, a second standing time, and a discharge are performed in the following order at a temperature of 33°C: First charge method: CCCV, First charge rate: 0.2C, First charge control voltage: 3.0V, First cutoff time: 1 hour, First standing time: 12 hours, Second charge method: CCCV, Second charge rate: 0.2C, Second charge control voltage: 4.2V, Second cutoff time: 8 hours, Second standing time: 12 hours, Discharge method: CC, Discharge rate: 0.2C, Discharge end voltage: 2.5V. Charge / discharge condition B refers to a condition in which the second standing time of charge / discharge condition A is changed to 5 minutes and the temperature for charge / discharge is changed to 25°C. Charge-discharge condition C is a condition in which the second standing time of charge-discharge condition A is changed to 72 hours, and the temperature for charge-discharge is changed to 25° C. This allows a good coating to be formed on the positive electrode current collector 211, and makes the secondary battery 1 electrochemically stable.

[0084] Through the above steps, the secondary battery 1 according to the first embodiment can be fabricated. Note that the above-described method for manufacturing a secondary battery is an example, and the method is not limited to this.

[0085] EXAMPLES Examples will be described below, but the present invention is not limited to these examples.

[0086] Table 3 shows Examples 1-1 to 1-3 and Comparative Example 1-1. In the "Charge / Discharge Conditions" column of Table 3, "A," "B," and "C" indicate that the laminate cell was charged and discharged under the above-described charge / discharge conditions A, B, and C, respectively, in the charge / discharge process. In the "Charge / Discharge Conditions" column of Table 3, "D" indicates that the laminate cell was charged and discharged under charge / discharge condition D in the charge / discharge process. Here, charge / discharge condition D refers to a condition in which charging, standing, and discharging are performed in the following order at a temperature of 25°C. That is, unlike charge / discharge conditions A, B, and C, in which charging is performed twice, charge / discharge condition D involves a single charge. Charging method: CCCV; Charge rate: 0.2C; Charge control voltage: 4.2V; Cutoff time: 8 hours; Standing time: 5 minutes; Discharge method: CC; Discharge rate: 0.2C; Discharge end voltage: 2.5V

[0087]

[0088] Example 1-1 The positive electrode according to Example 1-1 was fabricated by the following method. ０．８２ Co ０．１４ Al ０．４ O ２ ), polyvinylidene fluoride (PVdF) as a positive electrode binder, and carbon black as a positive electrode conductive agent were mixed in a mass ratio of 91:3:6 to prepare a positive electrode mixture. The prepared positive electrode mixture was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture slurry, and then a strip-shaped aluminum foil with a thickness of 12 μm was prepared as a positive electrode current collector, and the prepared positive electrode mixture slurry was uniformly applied to both sides of the aluminum foil. The obtained coating was dried with hot air and then compression-molded using a roll press to prepare a positive electrode. A positive electrode lead was attached to the exposed portion of the positive electrode current collector of the prepared positive electrode.

[0089] The negative electrode according to Example 1-1 was prepared by the following method. A negative electrode mixture was prepared by mixing artificial graphite as the negative electrode active material and PVdF as the negative electrode binder in a mass ratio of 93:7. The prepared negative electrode mixture was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a negative electrode mixture slurry, and then a copper foil having a thickness of 15 μm was prepared as a negative electrode current collector, and the negative electrode mixture was uniformly applied to both sides of the copper foil. The obtained coating was dried with hot air and then compression-molded using a roll press to prepare a negative electrode. A negative electrode lead was attached to the exposed portion of the negative electrode current collector of the prepared negative electrode.

[0090] The separator in Example 1-1 was a microporous polyethylene film having a thickness of 15 μm.

[0091] The electrolyte solution according to Example 1-1 was prepared by dissolving LiFSI as an electrolyte salt in a solvent prepared by mixing EC and PrPr in a mass ratio of 53:47, so that the concentration of LiFSI was 2.00 mol / kg.

[0092] The secondary battery according to Example 1-1 was assembled by the following method. The positive electrode and negative electrode prepared above were stacked with the separator interposed therebetween, tightly adhered, and wound longitudinally to produce an electrode assembly. The prepared electrode assembly was loaded into an exterior member, and three sides of the exterior member were heat-sealed, with one side left unheat-sealed to form an opening. The exterior member was a laminate film formed by laminating a 25 μm-thick nylon film as the outermost layer, a 40 μm-thick aluminum foil as the metal layer, and a 30 μm-thick polypropylene film as the insulating layer. The electrolyte prepared above was then injected through the opening in the exterior member, and the remaining side of the exterior member was heat-sealed in a reduced-pressure environment to seal the exterior member and form a laminate cell.

[0093] In Example 1-1, the fabricated laminate cell was charged and discharged under charge and discharge conditions A. In this way, a battery according to Example 1-1 was fabricated.

[0094] <XPS Measurement> In Example 1-1, XPS measurement was carried out on the surface of the positive electrode current collector by the following method. In the XPS measurement, the battery was disassembled in a glove box filled with argon, and the positive electrode current collector was removed and washed with dimethyl carbonate to prepare a sample. Thereafter, the sample was introduced into an X-ray photoelectron spectrometer (PHI5000 VersaProve, manufactured by ULVAC-PHI, Inc.) without exposing it to the atmosphere, and the X-ray beam used for measurement was set to a monochromated Al-Kα ray (1486.6 eV) with a diameter of approximately 100 μm, and wide scans and various narrow scans were performed under the conditions in Table 2 above. In the XPS measurement, charge neutralization was performed using an electron beam and an ion beam. Peak separation was performed using a Gaussian / Lorentzian mixed function for the S2p, F1s, Al2p, and P2p spectra obtained from each of the S2p narrow scan, F1s narrow scan, Al2p narrow scan, and wide scan, and the signal intensity ratio (I Ｓ１ +I Ｓ２ ) / I Ｓｔ , (I Ｆ１ +I Ｆ２ ) / I Ｆｔ , (I Ａｌ１ +I Ａｌ２ ) / I Ａｌｔ The presence or absence of P2p spectrum was measured. Ｓ１ +I Ｓ２ ) / I Ｓｔ , (I Ｆ１ +I Ｆ２ ) / I Ｆｔ , (I Ａｌ１ +I Ａｌ２ ) / I Ａｌｔ The values ​​of and the presence or absence of P2p spectrum are shown in Table 3. In the column "Presence or absence of P2p" in Table 3, "Y" indicates that a peak was detected in the P2p spectrum in the wide scan, and "N" indicates that a peak was not detected in the P2p spectrum in the wide scan.

[0095] <<Vibrational Spectroscopy Measurement>> In Example 1-1, vibrational spectroscopy of the electrolyte solution was measured by the following method. In the vibrational spectroscopy measurement, the prepared electrolyte solution was sealed in a sealed glass container to prepare a sample. The sample was then introduced into a Raman spectrometer (manufactured by Nanophoton Inc.), and the wavelength of the excitation laser used in the measurement was set to 758 nm. As a result, the peak intensity I of the original solvent obtained was ｏ and the peak intensity I of the solvent of the solvate ｓ Based on this, the original amount of substance M of the solvent can be calculated by the above-mentioned formula (1). ０ The original molar ratio of the solvent to lithium ions was calculated by calculating the original molar ratio M of the solvent for each type of solvent. ０ The amount of the solvent was calculated by calculating the weighted average weighted by the molar ratio of the multiple solvents. As a result, the molar ratio of the solvent to lithium ions was calculated as shown in Table 3.

[0096] <Cycle Characteristics Test> In Example 1-1, a cycle characteristics test was carried out in the following manner.

[0097] The secondary battery prepared above was charged and discharged 100 times in an environment of 60°C under the following conditions, and the discharge capacity at the first cycle and the discharge capacity at the 100th cycle were measured. The ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle was calculated as the cycle retention rate. That is, the cycle retention rate was calculated based on the following formula: Cycle retention rate (%) = (discharge capacity at the 100th cycle / discharge capacity at the 1st cycle) x 100. Charging method: CCCV, Charging rate: 0.1 C, Charging control voltage: 4.2 V, Charging cut-off current: 0.05 C, Discharging method: CC, Discharging rate: 0.1 C, Discharging cut-off voltage: 2.5 V

[0098] <Storage Property Test> In Example 1-1, a storage property test was carried out by the following method.

[0099] The secondary battery prepared above was subjected to a first charge / discharge cycle at 23°C under the following conditions, and the discharge capacity before storage was measured: Charging method: CCCV, Charging rate: 0.1C, Charging control voltage: 4.2V, Charging cut-off current: 0.05C, Discharging method: CC, Discharging rate: 0.1C, Discharging cut-off voltage: 2.5V.

[0100] The battery was then placed in a thermostatic chamber and stored in an 80°C environment for 10 days, and then discharged under the following conditions to measure the discharge capacity after storage. Charge and discharge were performed in an environment of 23°C. The ratio of the discharge capacity before storage to the discharge capacity after storage was calculated as the storage retention rate. That is, the storage retention rate was calculated based on the formula: storage retention rate (%) = (discharge capacity after storage / discharge capacity before storage) x 100. Discharge method: CC Discharge rate: 0.1C End-of-discharge voltage: 2.5V

[0101] <Low Temperature Load Characteristic Test> In Example 1-1, a low temperature load characteristic test was carried out in the following manner.

[0102] The secondary battery fabricated above was subjected to the first charge-discharge cycle in an environment of 23°C under the following conditions. Thereafter, the second to 100th charge-discharge cycles were performed in an environment of -10°C under the same conditions as the first charge-discharge cycle, under the following conditions. The ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle was calculated as the low-temperature load retention rate. That is, the low-temperature load retention rate was calculated based on the following formula: low-temperature load retention rate (%) = (discharge capacity at the 100th cycle / discharge capacity at the first cycle) x 100. Charging method: CCCV, charging rate: 0.1 C, charge control voltage: 4.2 V, charge cut-off current: 0.05 C, discharging method: CC, discharge rate: 0.1 C, discharge cut-off voltage: 2.5 V

[0103] (Example 1-2) As shown in Table 3, in Example 1-2, a secondary battery was produced in the same manner as the battery according to Example 1-1, except that the conditions for charging and discharging the produced laminated cell were changed to charge and discharge condition B, and measurements and tests were carried out.

[0104] (Example 1-3) As shown in Table 3, in Example 1-3, a secondary battery was produced in the same manner as the battery according to Example 1-1, except that the conditions for charging and discharging the produced laminated cell were changed to charge and discharge condition C, and measurements and tests were carried out.

[0105] (Comparative Example 1-1) As shown in Table 3, in Comparative Example 1-1, a secondary battery was fabricated in the same manner as the battery according to Example 1-1, except that the conditions for charging and discharging the fabricated laminated cell were changed to charging and discharging condition D, and measurements and tests were performed.

[0106] As shown in Table 3, (I Ｓ１ +I Ｓ２ ) / I Ｓｔ In Examples 1-1 to 1-3 where I Ｓ１ +I Ｓ２ ) / I Ｓｔ The charge-discharge characteristics were improved compared to Comparative Example 1-1, where I was >0.97. Ｓ１ +I Ｓ２ ) / I Ｓｔ It is understood that when the ratio is ≦0.97, the charge / discharge characteristics can be improved.

[0107] As shown in Table 3, (I Ｆ１ +I Ｆ２ ) / I Ｆｔ ≧0.80 and (I Ａｌ１ +I Ａｌ２ ) / I Ａｌｔ In Example 1-3, where I Ｆ１ +I Ｆ２ ) / I Ｆｔ <0.80 and (I Ａｌ１ +I Ａｌ２ ) / I Ａｌｔ The charge-discharge characteristics were improved compared to Example 1-2, where I Ｆ１ +I Ｆ２ ) / I Ｆｔ It can be seen that when the ratio is ≧0.80, the charge / discharge characteristics can be improved.

[0108] As shown in Table 3, (I Ａｌ１ +I Ａｌ２ ) / I Ａｌｔ ≧0.30 and (I Ｆ１ +I Ｆ２ ) / IＦｔ In Example 1-1, where I Ａｌ１ +I Ａｌ２ ) / I Ａｌｔ <0.30 and (I Ｆ１ +I Ｆ２ ) / I Ｆｔ <0.80, the charge-discharge characteristics were improved. Ａｌ１ +I Ａｌ２ ) / I Ａｌｔ It can be seen that when the ratio is ≧0.30, the charge / discharge characteristics can be improved.

[0109] Table 4 shows Examples 1-4 to 1-21. In the "Charge / Discharge Conditions" column of Table 4, "A" indicates that the laminate cell was charged and discharged under the above-described charge / discharge condition A in the charge / discharge process. In the "Presence or Absence of P2p" column of Table 4, "Y" indicates that a peak was detected in the P2p spectrum in the wide scan, and "N" indicates that a peak was not detected in the P2p spectrum in the wide scan.

[0110]

[0111] Example 1-4 In Example 1-4, a secondary battery was fabricated in the same manner as the battery of Example 1-1, except that an electrolyte solution was prepared by dissolving LiFSI as an electrolyte salt in a mixed solvent in the mass ratio shown in Table 4 to a concentration of 2.20 mol / kg. Measurements and tests were then carried out.

[0112] (Examples 1-5 to 1-21) In Examples 1-5 to 1-21, the solvents shown in Table 4 were mixed in the mass ratios shown in Table 4, and LiFSI and LiPF were added as electrolyte salts. ６ A secondary battery was fabricated in the same manner as the battery according to Example 1-1, except that an electrolyte solution was prepared by dissolving the above in an amount shown in Table 4, and measurements and tests were carried out.

[0113] As shown in Table 4, in Examples 1-5 to 1-21 in which the P2p spectrum was detected in the XPS measurement, the charge-discharge characteristics were improved compared to Example 1-4 in which the P2p spectrum was not detected. Therefore, it can be seen that the presence of phosphorus (P) on the surface of the positive electrode current collector can improve the charge-discharge characteristics.

[0114] Table 5 shows Examples 1-4 and 1-22 to 1-55. In the "Charge / Discharge Conditions" column of Table 5, "A" indicates that the laminate cell was charged and discharged under the above-described charge / discharge condition A in the charge / discharge process. In the "Presence or Absence of P2p" column of Table 5, "Y" indicates that a peak was detected in the P2p spectrum in the wide scan, and "N" indicates that a peak was not detected in the P2p spectrum in the wide scan.

[0115]

[0116] Examples 1-22 to 1-25 In Examples 1-22 to 1-55, secondary batteries were fabricated in the same manner as the battery of Example 1-1, and measurements and tests were performed, except that an electrolyte solution was prepared by dissolving LiFSI as an electrolyte salt in a solvent prepared by mixing the solvents shown in Table 5 in the mass ratios shown in Table 5, to a concentration shown in Table 5.

[0117] As shown in Table 5, Examples 1-1 to 1-4 and Examples 1-22 to 1-55, in which the molar ratio of the original solvent to lithium ions was greater than 0 and 1.76 or less, showed sufficient charge-discharge characteristics. Therefore, it can be seen that the charge-discharge characteristics can be improved by having the molar ratio of the original solvent to lithium ions be greater than 0 and 1.76 or less.

[0118] Table 6 shows Examples 2-1 to 2-3 and Comparative Example 2-1. In the "Charge / Discharge Conditions" column of Table 6, "A," "B," "C," and "D" indicate that the laminate cell was charged and discharged under the above-described charge / discharge conditions A, B, C, and D, respectively, in the charge / discharge process.

[0119]

[0120] (Example 2-1) As shown in Table 6, in Example 2-1, the positive electrode active material according to Example 1-1, carbon black (manufactured by Denka Co., Ltd.) as a positive electrode conductive agent, MWCNT as a positive electrode conductive agent, and polyvinylidene fluoride (PVdF) as a positive electrode binder were mixed in a mass ratio of 96:1:1:2 to prepare a positive electrode mixture, and a secondary battery was fabricated in the same manner as the battery according to Example 1-1. Here, the MWCNT used as the positive electrode conductive agent according to Example 2-1 had a fiber diameter in the range of 7 nm to 12 nm and a fiber length in the range of 100 μm to 200 μm. Furthermore, in Example 2-1, in addition to the XPS measurement, cycle characteristic test, and storage characteristic test according to Example 1-1, a positive electrode interface resistance measurement and a 5 C discharge test were also performed.

[0121] <<Measurement of Positive Electrode Interface Resistance>> In measuring the positive electrode interface resistance according to Example 2-1, the secondary battery prepared above was subjected to one cycle of charge and discharge in an environment of 23° C. under the following charge and discharge conditions: Charging method: CCCV, Charging rate: 0.1 C, Charging control voltage: 4.2 V, Charging cut-off current: 0.05 C, Discharging method: CC, Discharging rate: 0.1 C, Discharging cut-off voltage: 2.5 V.

[0122] Thereafter, the positive electrode was removed, and the interface resistance between the positive electrode current collector and the positive electrode active material layer was measured using an electrode resistance meter (electrode resistance meter for lithium ion secondary batteries) manufactured by Hioki E.E. Corporation. Specifically, first, the electrode probe of the meter was brought into contact with the surface of the positive electrode active material layer in an atmospheric pressure environment, and then a constant current was applied to the surface of the positive electrode active material layer with the electrode probe. Based on the obtained potential distribution of the positive electrode, the interface resistance between the positive electrode current collector and the positive electrode active material layer was calculated as the positive electrode interface resistance. In Table 6 and subsequent tables, the positive electrode interface resistance measurements are shown as a relative ratio, with the positive electrode interface resistance in Example 2-4 described below being set to 100.

[0123] <<5 C Discharge Test>> In the 5 C discharge test according to Example 2-1, the secondary battery prepared above was subjected to one cycle of charge and discharge in an environment of 23° C. under the following first charge and discharge conditions, and the discharge capacity at the first cycle was measured: Charging method: CCCV, Charging rate: 0.1 C, Charging control voltage: 4.2 V, Charging cut-off current: 0.05 C, Discharging method: CC, Discharging rate: 0.1 C, Discharging cut-off voltage: 2.5 V.

[0124] Thereafter, a second charge / discharge cycle was performed under the second charge / discharge condition described below, and the discharge capacity at the second cycle was measured. The charge / discharge cycle was performed in an environment of 23°C. The ratio of the discharge capacity at the second cycle to the discharge capacity at the first cycle was calculated as the 5C discharge retention rate. That is, the 5C discharge retention rate was calculated based on the following formula: 5C discharge retention rate (%) = (discharge capacity at the second cycle / discharge capacity at the first cycle) x 100. Charging method: CCCV, Charging rate: 0.1C, Charging control voltage: 4.2V, Charging cut-off current: 0.05C, Discharging method: CC, Discharging rate: 5C, Discharging cut-off voltage: 2.5V

[0125] (Comparative Example 2-1) As shown in Table 6, in Comparative Example 2-1, a secondary battery was produced in the same manner as the battery of Example 2-1, except that the conditions for charging and discharging the produced laminate cell were changed to charge and discharge condition D, and measurements and tests were performed.

[0126] (Example 2-2) As shown in Table 6, in Example 2-2, a secondary battery was produced in the same manner as the battery according to Example 2-1, except that the conditions for charging and discharging the produced laminated cell were changed to charge and discharge condition B, and measurements and tests were performed.

[0127] (Example 2-3) As shown in Table 6, in Example 2-3, a secondary battery was produced in the same manner as the battery according to Example 2-1, except that the conditions for charging and discharging the produced laminated cell were changed to charge and discharge condition C, and measurements and tests were performed.

[0128] (Example 2-4) As shown in Table 6, in Example 2-4, a secondary battery was produced in the same manner as the battery of Example 2-1, except that an electrolyte solution was produced using a solvent obtained by mixing the solvents shown in Table 6 in the mass ratio shown in Table 6, and measurements and tests were performed.

[0129] As shown in Table 6, (I Ｓ１ +I Ｓ２ ) / I Ｓｔ In Examples 2-1 to 2-4 where the ratio is ≦0.97, (I Ｓ１ +I Ｓ２ ) / I Ｓｔ The charge-discharge characteristics were improved compared to Comparative Example 2-1, where I was >0.97. Ｓ１ +I Ｓ２ ) / I Ｓｔ It is understood that when the ratio is ≦0.97, the charge / discharge characteristics can be improved.

[0130] As shown in Table 6, (I Ｆ１ +I Ｆ２ ) / I Ｆｔ ≧0.80 and (I Ａｌ１ +I Ａｌ２ ) / I Ａｌｔ In Example 2-3, where I Ｆ１ +I Ｆ２ ) / I Ｆｔ <0.80 and (I Ａｌ１ +I Ａｌ２ ) / I Ａｌｔ The cycle characteristics were improved compared to Example 2-2, where I Ｆ１ +I Ｆ２ ) / I Ｆｔ It can be seen that when the ratio is ≧0.80, the cycle characteristics can be improved.

[0131] As shown in Table 6, (I Ａｌ１ +I Ａｌ２ ) / I Ａｌｔ ≧0.30 and (I Ｆ１ +I Ｆ２ ) / I Ｆｔ In Example 2-1, where I Ａｌ１ +I Ａｌ２ ) / I Ａｌｔ <0.30 and (I Ｆ１ +I Ｆ２ ) / I Ｆｔ The charge-discharge characteristics were improved compared to Example 2-2, where I Ａｌ１ +I Ａｌ２ ) / I ＡｌｔIt can be seen that when the ratio is ≧0.30, the charge / discharge characteristics can be improved.

[0132] As shown in Table 6, Examples 2-1 to 2-4, which contained carbon black and carbon nanotubes as the positive electrode conductive agent, exhibited sufficient charge / discharge characteristics and low positive electrode interfacial resistance. Therefore, it is clear that the inclusion of carbon black and carbon nanotubes as the positive electrode conductive agent can improve charge / discharge characteristics and suppress the interfacial resistance between the positive electrode current collector and the positive electrode active material layer.

[0133] Table 7 shows Examples 2-5 to 2-18. In the "Charge / Discharge Conditions" column of Table 7, "A" indicates that the laminate cell was charged and discharged under the above-described charge / discharge condition A in the charge / discharge process.

[0134]

[0135] (Examples 2-5 to 2-18) In Examples 2-5 to 2-18, a positive electrode was prepared using the positive electrode materials (positive electrode active material, carbon black, MWCNT, SWCNT, and positive electrode binder) shown in Table 7 in the mass ratio shown in Table 7, and an electrolyte solution was prepared using a solvent obtained by mixing the solvents shown in Table 7 in the mass ratio shown in Table 7. Except for this, secondary batteries were prepared in the same manner as the battery of Example 2-1, and measurements and tests were performed. Here, in Examples 2-7, 2-17, and 2-18, SWCNT was used as the positive electrode conductive agent. The SWCNT used in Examples 2-7, 2-17, and 2-18 had an average fiber diameter of 1.6 nm and an average fiber length of 5 μm or more.

[0136] As shown in Table 7, in Examples 2-5 and 2-8 to 2-18, in which the content of the positive electrode conductive agent was 1.0 mass% or more and 2.5 mass% or less, the 5C discharge retention ratio and storage retention ratio were greater than those in Examples 2-6 and 2-7, in which the content of the positive electrode conductive agent was less than 1.0 mass%. Therefore, it is clear that the charge / discharge characteristics can be improved by setting the content of the positive electrode conductive agent to 1.0 mass% or more and 2.5 mass% or less.

[0137] As shown in Table 7, in Examples 2-8, 2-9, and 2-11 to 2-18, the positive electrode interface resistance was smaller than that of Examples 2-5 to 2-7 and 2-10, in which the carbon black content was less than 0.3 mass%. Therefore, it can be seen that by setting the carbon black content to 0.3 mass% or more and 1.5 mass% or less, the interface resistance between the positive electrode current collector and the positive electrode active material layer can be suppressed.

[0138] As shown in Table 7, in Examples 2-5 to 2-18, in which the content of the positive electrode binder was 0.3% by mass or more and 2.0% by mass or less, low positive electrode interface resistance and good charge / discharge characteristics were exhibited. In particular, in Examples 2-9 to 2-18, in which the content of the positive electrode binder was 0.3% by mass or more and 1.5% by mass or less, better charge / discharge characteristics were exhibited, and in Examples 2-11 to 2-18, in which the content of the positive electrode binder was 0.3% by mass or more and 1.0% by mass or less, lower positive electrode interface resistance and even better charge / discharge characteristics were exhibited, and in Examples 2-16 and 2-17, in which the content of the positive electrode binder was 0.3% by mass or more and less than 1.0% by mass, the best charge / discharge characteristics were exhibited. Therefore, it is clear that when the content of the positive electrode binder is 0.3 mass % or more and 2.0 mass % or less, preferably 0.3 mass % or more and 1.5 mass % or less, more preferably 0.3 mass % or more and 1.0 mass % or less, and even more preferably 0.3 mass % or more and less than 1.0 mass %, the positive electrode interface resistance can be reduced and good charge / discharge characteristics can be obtained.

[0139] Table 8 shows Examples 2-19 to 2-24. In the "Charge / Discharge Conditions" column of Table 8, "A" indicates that the laminate cell was charged and discharged under the above-described charge / discharge condition A in the charge / discharge process.

[0140]

[0141] (Examples 2-19 to 2-24) In Examples 2-19 to 2-24, a positive electrode was prepared using the positive electrode materials (positive electrode active material, carbon black, MWCNT, SWCNT, and positive electrode binder) shown in Table 8 in the mass ratio shown in Table 8, and an electrolyte solution was prepared by dissolving LiFSI as an electrolyte salt in a solvent prepared by mixing the solvents shown in Table 8 in the mass ratio shown in Table 8 to a concentration of 2.20 mol / kg. Except for this, secondary batteries were prepared in the same manner as the battery of Example 2-1, and measurements and tests were performed.

[0142] As shown in Table 8, (I Ｆ１ +I Ｆ２ ) / I Ｆｔ In Examples 2-20 to 2-24, where I Ｆ１ +I Ｆ２ ) / I Ｆｔ Compared with Example 2-19, where the positive electrode interface resistance was <0.80, the positive electrode interface resistance was reduced and the cycle characteristics were improved. Ｆ１ +I Ｆ２ ) / I Ｆｔ It can be seen that when the ratio is ≧0.80, the positive electrode interface resistance can be reduced and the cycle characteristics can be improved.

[0143] The above-described embodiments are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention may be modified or improved without departing from the spirit thereof, and equivalents thereof are also included in the present invention.

[0144] REFERENCE SIGNS LIST 1 secondary battery 10 exterior film 20 battery element 31 positive electrode lead 32 negative electrode lead 41, 42 sealing film 30 exterior member 210 positive electrode 211 positive electrode current collector 212 positive electrode active material layer 220 negative electrode 221 negative electrode current collector 222 negative electrode active material layer 230 separator

Claims

1. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode comprises a positive electrode current collector containing aluminum, the electrolyte contains an electrolyte and a solvent, the electrolyte contains a bis(fluorosulfonyl)imide salt, the surface of the positive electrode current collector contains sulfur, and a photoelectron spectrum obtained by X-ray photoelectron spectroscopy of the surface of the positive electrode current collector has a first signal having a peak in the range of 164.1 eV to 164.5 eV and a second signal having a peak in the range of 168.9 eV to 169.3 eV, and the ratio of the sum of the signal intensity of the first signal and the signal intensity of the second signal to the total signal intensity of an S2p spectrum is 0.97 or less.

2. The secondary battery according to claim 1, wherein the surface of the positive electrode current collector contains fluorine, a photoelectron spectrum obtained by X-ray photoelectron spectroscopy of the surface of the positive electrode current collector has a third signal having a peak in the range of 685.7 eV to 686.1 eV and a fourth signal having a peak in the range of 687.4 eV to 687.8 eV, and a ratio of the sum of the signal intensity of the third signal and the signal intensity of the fourth signal to the total signal intensity of the F1s spectrum is 0.80 or greater.

3. The secondary battery according to claim 1 or 2, wherein the surface of the positive electrode current collector contains aluminum, a photoelectron spectrum obtained by X-ray photoelectron spectroscopy of the surface of the positive electrode current collector has a fifth signal having a peak at 75.4 eV or more and 75.8 eV or less and a sixth signal having a peak at 76.4 eV or more and 76.8 eV or less, and a ratio of the sum of the signal intensity of the fifth signal and the signal intensity of the sixth signal to the total signal intensity of the Al2p spectrum is 0.30 or more.

4. The secondary battery according to any one of claims 1 to 3, wherein the surface of the positive electrode current collector contains at least one of boron and phosphorus.

5. The secondary battery according to any one of claims 1 to 4, wherein the positive electrode further comprises a positive electrode active material layer, the positive electrode active material layer including a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder, the positive electrode conductive agent including carbon black and carbon nanotubes, the content of the positive electrode conductive agent relative to the total of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder is 1.0 mass% or more and 2.5 mass% or less, and the content of the carbon black relative to the total of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder is 0.3 mass% or more and 1.5 mass% or less.

6. The secondary battery according to claim 5, wherein the content of the positive electrode binder is 0.3 mass % or more and 2.0 mass % or less relative to the total of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder.

7. The secondary battery according to any one of claims 1 to 6, wherein the solvent contains at least one of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, and gamma-butyrolactone, and the molar ratio of the solvent to lithium ions calculated from the vibrational spectroscopy spectrum of the electrolyte solution is greater than 0 and not greater than 1.76.

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