Lithium-ion secondary batteries

Abstract

A lithium-ion secondary battery has a negative electrode, a positive electrode, a non-aqueous electrolyte containing lithium salt, and an aqueous electrolyte containing lithium salt. The aqueous electrolyte is in contact only with the positive electrode of the negative electrode and the positive electrode. The non-aqueous electrolyte is in contact with at least the negative electrode of the negative electrode and the positive electrode. The positive electrode contains a positive electrode active material, which contains a lithium transition metal composite oxide and a surface modification layer formed on the surface of the primary particles of the lithium transition metal composite oxide. The surface modification layer contains an alkaline earth metal element.

Description

Technical Field

[0001] This disclosure relates to lithium-ion secondary batteries. Background Technology

[0002] As a high-output, high-energy-density secondary battery, lithium-ion secondary batteries are widely used. They consist of a positive electrode, a negative electrode, and an electrolyte, allowing lithium ions to move between the positive and negative electrodes for charging and discharging. In previous secondary batteries, organic solvent-based electrolytes were used to achieve high energy density.

[0003] However, organic solvents are generally flammable, making safety a critical concern. Furthermore, the lower ionic conductivity of organic solvents compared to aqueous solutions, and their inadequate rapid charge-discharge characteristics, also pose a problem.

[0004] In view of these problems, secondary batteries using aqueous electrolytes containing water have been studied. For example, Patent Document 1 proposes a lithium-ion secondary battery that uses an aqueous solution containing a high concentration of alkaline salt as an aqueous liquid electrolyte. In addition, Patent Document 2 proposes a lithium-ion secondary battery comprising: a negative electrode filled with a non-aqueous solid electrolyte, a positive electrode, a separator disposed between the negative electrode and the positive electrode and filled with a non-aqueous solid electrolyte, and an aqueous liquid electrolyte.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent No. 6423453

[0008] Patent Document 2: Japanese Patent Application Publication No. 2018-198131 Summary of the Invention

[0009] In conventional lithium-ion secondary batteries with aqueous electrolytes, capacity reduction during charge-discharge cycles has become a problem.

[0010] One aspect of this disclosure is a lithium-ion secondary battery having a negative electrode, a positive electrode, a non-aqueous electrolyte containing a lithium salt, and an aqueous electrolyte containing a lithium salt, wherein the aqueous electrolyte is in contact only with the positive electrode, and the non-aqueous electrolyte is in contact with at least the negative electrode, wherein the positive electrode contains a positive electrode active material, the positive electrode active material comprising a lithium transition metal composite oxide and a surface modification layer formed on the surface of primary particles of the lithium transition metal composite oxide, the surface modification layer comprising at least one element selected from the group consisting of alkaline earth metals, rare earth elements, and elements from Group IIIb, Group IVb, and Group Vb of the periodic table.

[0011] According to this disclosure, a lithium-ion secondary battery capable of suppressing capacity reduction accompanying charge-discharge cycles can be provided. Attached Figure Description

[0012] Figure 1 This is a schematic cross-sectional view of an example of a lithium-ion secondary battery according to this embodiment. Detailed Implementation

[0013] One aspect of this disclosure is a lithium-ion secondary battery comprising a negative electrode, a positive electrode, a non-aqueous electrolyte containing a lithium salt, and an aqueous electrolyte containing a lithium salt. The aqueous electrolyte is in contact only with the positive electrode, and the non-aqueous electrolyte is in contact with at least the negative electrode. The positive electrode comprises a positive electrode active material, which includes a lithium transition metal composite oxide and a surface modification layer formed on the surface of primary particles of the lithium transition metal composite oxide. The surface modification layer comprises at least one element selected from the group consisting of alkaline earth metals, rare earth elements, and elements from Group IIIb, Group IVb, and Group Vb of the periodic table. By using the lithium-ion secondary battery of this disclosure, capacity reduction accompanying charge-discharge cycles can be suppressed. Although the mechanism for this effect is not entirely clear, it can be speculated as follows.

[0014] By allowing the aqueous electrolyte to contact only the positive electrode, the side reactions of water in the negative electrode are suppressed, thus enabling the charge-discharge reaction to proceed. However, on the other hand, this causes side reactions between the positive electrode active material and protons, leading to a decrease in capacity with charge-discharge cycles. However, as described in this disclosure, it is conceivable that by using a positive electrode active material with a surface modification layer comprising at least one element selected from the group consisting of alkaline earth metals, rare earth elements, and elements from Group IIIb, IVb, and Vb of the periodic table, on the surface of the primary particles of the lithium transition metal composite oxide, reactions with protons at the surface of the positive electrode active material can be suppressed, thereby suppressing the capacity decrease that accompanies charge-discharge cycles.

[0015] The following is a detailed description of an example of an embodiment of the lithium-ion secondary battery disclosed herein.

[0016] Figure 1 This is a schematic cross-sectional view of an example of a lithium-ion secondary battery according to this embodiment. Figure 1 The lithium-ion secondary battery 1 shown includes a positive electrode 10, a negative electrode 12, a separator 14, an aqueous electrolyte 16, a non-aqueous electrolyte 18, a positive electrode lead 20, a negative electrode lead 22, and a battery casing 24 that houses them.

[0017] The positive electrode 10 has a positive current collector 26 and a positive composite material layer 28 disposed on the positive current collector 26. A positive lead 20 is connected to the positive current collector 26. The positive lead 20 is housed in the battery housing 24 such that the front end of the positive lead 20 protrudes to the outside of the battery housing 24.

[0018] The negative electrode 12 has a negative electrode current collector 30 and a negative electrode composite material layer 32 disposed on the negative electrode current collector 30. A negative electrode lead 22 is connected to the negative electrode current collector 30. Moreover, the negative electrode lead 22 is housed in the battery housing 24 such that the front end of the negative electrode lead 22 protrudes to the outside of the battery housing 24.

[0019] Aqueous electrolyte 16, for example, is impregnated in the positive electrode composite layer 28 and contacts only the positive electrode 10 of the positive electrode 10 and negative electrode 12. Non-aqueous electrolyte 18, for example, is impregnated in the negative electrode composite layer 32 and contacts the negative electrode 12. Non-aqueous electrolyte 18 may also contact only the negative electrode 12 of the positive electrode 10 and negative electrode 12, or it may contact both the positive electrode 10 and negative electrode 12. A separator 14 is disposed between the positive electrode 11 and the negative electrode 12. The separator 14 may be wound around the negative electrode 12.

[0020] As the positive current collector 26 constituting the positive electrode 10, a foil of a metal that is electrochemically and chemically stable within the potential range of the positive electrode 10, or a thin film of the metal disposed on its surface, can be used. The shape of the positive current collector 26 is not particularly limited; for example, a mesh of the metal, a perforated sheet, an expanded metal, or other porous body can be used. Examples of materials for the positive current collector 26 include stainless steel, Al, aluminum alloy, and Ti. From the viewpoint of current collection performance and mechanical strength, the thickness of the positive current collector 26 is preferably, for example, 3 μm or more and 50 μm or less.

[0021] The positive electrode composite material layer 28 constituting the positive electrode 10 includes the positive electrode active material 34. Additionally, the positive electrode composite material layer 28 may also include a binder, a conductive material, etc. The positive electrode 10 can be manufactured by coating a positive electrode current collector 26 with a positive electrode composite material slurry, such as one containing the positive electrode active material 34, a binder, a conductive material, etc., and then drying and calendering the coating to form the positive electrode composite material layer 28 on the positive electrode current collector 26.

[0022] The positive electrode active material 34 comprises a lithium transition metal composite oxide and a surface modification layer. The surface modification layer is formed on the surface of the primary particles of the lithium transition metal composite oxide and contains at least one element selected from the group consisting of alkaline earth metals, rare earth elements, and elements from Group IIIb, Group IVb, and Group Vb of the periodic table. In addition, the positive electrode active material 34 may also contain: transition metal sulfides, metal oxides, lithium iron phosphate (LiFePO4), lithium iron pyrophosphate (Li2FeP2O7), or one or more lithium-containing polyanionic compounds containing transition metals; sulfide compounds (Li2S); oxygen-containing metal salts such as oxygen and lithium oxide.

[0023] From the viewpoint of charge-discharge efficiency, lithium transition metal composite oxides preferably contain at least one element selected from Ni, Co, Mn, and aluminum (Al). Among these elements, it is preferred that at least Ni, at least Co, at least both Ni and Mn, at least all three elements (Ni, Co, and Mn), or at least Ni, Co, and Al are included. Lithium transition metal composite oxides may also contain other additive elements besides these, such as zirconium (Zr), boron (B), magnesium (Mg), scandium (Sc), yttrium (Y), titanium (Ti), iron (Fe), copper (Cu), zinc (Zn), chromium (Cr), lead (Pb), tin (Sn), sodium (Na), potassium (K), barium (Ba), strontium (Sr), calcium (Ca), tungsten (W), molybdenum (Mo), niobium (Nb), and silicon (Si).

[0024] As a specific example of lithium transition metal composite oxides, Li can be cited as an example. x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1-y O z Li x Ni 1-y M y O z Li x Mn2O4, Li x Mn 2-y M yO4, LiMPO4, Li2MPO4F (in each chemical formula, M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3). Lithium transition metal composite oxides can be used alone or in combination. From the viewpoint of high capacity, lithium transition metal composite oxides preferably contain Ni at least 80 mol% relative to the total amount of transition metals other than lithium. Furthermore, from the viewpoint of crystal structure stability, lithium transition metal composite oxides are more preferably Li... a Ni b Co c Al d O2 (0<a≤1.2, 0.8≤b<1, 0<c<0.2, 0<d≤0.1, b+c+d=1).

[0025] In addition, lithium transition metal oxides can also be Li excess transition metal oxides, lithium transition metal halide oxides, etc. Li excess transition metal oxides, for example, are those with the general formula Li... 1+x Me 1-x O2 (0 < x) is represented. In addition, there are no particular limitations on lithium transition metal halide oxides as long as they contain halogen atoms. For example, from the perspective of structural stability of lithium transition metal oxides, lithium transition metal oxides containing fluorine atoms are preferred.

[0026] Lithium transition metal composite oxides are, for example, secondary particles formed by the aggregation of multiple primary particles. The particle size of the primary particles constituting the secondary particles is, for example, 0.05 μm to 1 μm. The particle size of the primary particles is measured in terms of the diameter of the circumcircle in a particle image observed using a scanning electron microscope (SEM). A surface modification layer exists on the surface of the primary particles. In other words, the surface modification layer exists on the surface of the secondary particles of the lithium transition metal composite oxide, or at the interface where the primary particles are in contact with each other.

[0027] The median particle size (D50) of lithium transition metal composite oxides on a volume basis is, for example, 3 μm to 30 μm, preferably 5 μm to 25 μm, and particularly preferably 7 μm to 15 μm. D50 refers to the particle size at which the cumulative frequency in the volume-based particle size distribution reaches 50% from the smallest particle size; it is also known as the median diameter. The particle size distribution of lithium transition metal composite oxides can be measured using a laser diffraction-based particle size distribution measuring device (e.g., Microtrack Bell Co., Ltd., MT3000II) with water as the dispersion medium.

[0028] The thickness of the surface modification layer formed on the surface of the primary particles of the lithium transition metal composite oxide is preferably in the range of 0.1 nm to 5 nm, for example, in terms of effectively suppressing the reaction with protons at the surface of the lithium transition metal composite oxide.

[0029] The surface modification layer contains at least one element selected from the group consisting of alkaline earth metals, rare earth elements, and elements from Groups IIIb, IVb, and Vb of the periodic table (hereinafter sometimes referred to as surface modification elements). Alkaline earth metals include Be, Mg, Ca, Sr, Ba, and Ra. Rare earth elements include Sc, Y, La, and Ce. Elements from Group IIIb of the periodic table include B, Al, Ga, In, and Tl; elements from Group IVb include C, Si, Ge, Sn, and Pb; and elements from Group Vb include N, P, As, Sb, and Bi.

[0030] The surface modification layer may, for example, contain a compound with a surface modification element. The compound containing the surface modification element may be in the form of oxides, hydroxides, carbonates, etc. Among alkaline earth metals, rare earth elements, and elements from Group IIIb, IVb, and Vb of the periodic table, Sr and Ca are preferred, for example, from the perspective of effectively suppressing the reaction with protons at the surface of lithium transition metal complex oxides. That is, the surface modification layer preferably contains at least one element from Sr and Ca.

[0031] The content of surface modification elements in the positive electrode active material 34 relative to the total molar number of metal elements other than Li is preferably in the range of 0.05 mol% to 20 mol%, for example, in terms of suppressing capacity reduction associated with charge-discharge cycles. Here, the composition of the surface modification layer and the composition of the lithium transition metal composite oxide in the positive electrode active material 34 can be determined by analyzing various parts of the cross-section of the primary particles of the positive electrode active material 34 using TEM-EDX.

[0032] The surface modification layer may also contain transition metal elements. For example, transition metal elements other than Ni and Co are preferred. Compared to Ni and Co, transition metal elements other than Ni and Co tend to have a higher effect on suppressing reactions with protons.

[0033] The method for manufacturing the positive electrode active material 34 includes, for example, the following steps: a first step, obtaining a transition metal oxide containing transition metal elements such as Ni and Co; a second step, mixing the transition metal oxide obtained in the first step, a lithium compound, and a compound containing surface modification elements to obtain a mixture; and a third step, calcining the mixture.

[0034] In the first step, for example, while stirring a solution containing metal salts such as Ni and Co, an alkaline solution such as sodium hydroxide is added dropwise to adjust the pH to the alkaline side (e.g., 8.5–12.5), thereby causing transition metal hydroxides containing transition metal elements such as Ni and Co to precipitate (co-precipitate). Next, the transition metal hydroxide is calcined to obtain transition metal oxides. The calcination temperature is not particularly limited, for example, 300°C–600°C.

[0035] In the second step, a mixture is obtained by combining the transition metal oxide, lithium compound, and compound containing surface-modifying elements obtained in the first step. Examples of lithium compounds include Li₂CO₃, LiOH, Li₂O₂, Li₂O, LiNO₃, LiNO₂, Li₂SO₄, LiOH·H₂O, LiH, and LiF. Examples of compounds containing surface-modifying elements include oxides, hydroxides, carbonates, sulfates, and nitrates of the surface-modifying elements. Preferably, the mixing ratio of the transition metal oxide and lithium compound obtained in the first step is adjusted such that the molar ratio of the metal element other than Li to Li is in the range of 1:0.98 to 1:1.1.

[0036] In the third step, the mixture obtained in the second step is calcined at a specified temperature and time to obtain a positive electrode active material 34 having a lithium transition metal composite oxide and a surface modification layer. The third step preferably employs a multi-stage calcination process, which includes, for example, a first calcination step calcining at a first heating rate under an oxygen stream to a first set temperature of 450°C to 680°C; and a second calcination step calcining the calcined material obtained in the first calcination step at a second heating rate under an oxygen stream to a second set temperature exceeding 680°C but below 800°C. The calcination is carried out, for example, in an oxygen stream with an oxygen concentration of 60% or higher, and the oxygen flow rate is set to 10 cm³ / s. 3 The roasting furnace has a flow rate of 0.2 mL / min to 4 mL / min, and a flow rate of 0.3 L / min or more per 1 kg of mixture.

[0037] Here, the first heating rate is set to a mode of 1.5℃ / min to 5.5℃ / min or higher, and the second heating rate is slower than the first heating rate and is set to a mode of 0.1℃ / min to 3.5℃ / min or higher.

[0038] The holding time of the first set temperature in the first roasting process is preferably 5 hours or less, more preferably 3 hours or less. The holding time of the first set temperature refers to the time during which the first set temperature is maintained after it has been reached. The holding time of the second set temperature in the second roasting process is preferably 1 to 10 hours, more preferably 1 to 5 hours. The holding time of the second set temperature refers to the time during which the second set temperature is maintained after it has been reached.

[0039] As the conductive material contained in the positive electrode composite layer 28, known conductive materials that improve the conductivity of the positive electrode composite layer 28 can be used, such as carbon black, acetylene black, Ketjen black, graphite, carbon nanofibers, carbon nanotubes, graphene and other carbon materials. As the binder material contained in the positive electrode composite layer 28, known binders that maintain good contact between the positive electrode active material 34 and the conductive material and improve the adhesion of the positive electrode active material 34 to the surface of the positive electrode current collector 26 can be used, such as fluoropolymers such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resins, polyolefins, carboxymethyl cellulose (CMC) or its salts, styrene-butadiene rubber (SBR), polyethylene oxide (PEO), polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP) and the like.

[0040] The negative electrode current collector 30 constituting the negative electrode 12 can be a foil of a metal that is electrochemically and chemically stable within the potential range of the negative electrode 12, or a thin film of the metal disposed on its surface. The shape of the negative electrode current collector 30 is not particularly limited; for example, a mesh of the metal, a perforated sheet, or an expanded metal porous body can be used. Examples of materials for the negative electrode current collector 30 include Al, Ti, Mg, Zn, Pb, Sn, Zr, and In. One or more of these materials can be used individually, or in alloys of two or more, as long as the material is composed of at least one main component. Furthermore, when two or more elements are included, alloying is not necessarily required. From the viewpoint of current collection performance and mechanical strength, the thickness of the negative electrode current collector 30 is preferably, for example, 3 μm or more and 50 μm or less.

[0041] The negative electrode composite material layer 32 constituting the negative electrode 12 includes the negative electrode active material 36. Additionally, the negative electrode composite material layer 32 may include a binder material, a conductive material, etc. The conductive material and binder material can be the same materials used on the positive electrode 10 side. The negative electrode 12 can be manufactured by coating a negative electrode composite material slurry, such as containing the negative electrode active material 36, a binder material, and a conductive material, onto the negative electrode current collector 30, drying and calendering the coating film, and forming the negative electrode composite material layer 32 on the negative electrode current collector 30.

[0042] The negative electrode active material 36 is not particularly limited to any material that can be used as a negative electrode active material in conventional lithium-ion secondary batteries. Examples include carbon materials such as artificial graphite, natural graphite, hard carbon, soft carbon, carbon nanotubes, and activated carbon; metals or alloys such as Li, Si, and Sn; and metal compounds such as metal oxides, metal sulfides, and metal nitrides. As alloys, examples include Li-containing alloys such as lithium-aluminum alloys, lithium-tin alloys, lithium-lead alloys, and lithium-silicon alloys. Additionally, as metal oxides, examples include lithium titanate (Li₄Ti₅O₂). 12 Examples of suitable metal nitrides include cobalt oxides, iron oxides, etc. Other examples include lithium-containing nitrides such as lithium cobalt nitrides, lithium iron nitrides, and lithium manganese nitrides. Furthermore, sulfide compounds can also be included. Specifically, for example, from the perspective of improving battery energy density, the negative electrode active material 36 preferably contains carbon materials; for example, from the perspective of improving battery capacity, the negative electrode active material 36 preferably contains Li-containing alloys or metal oxides. That is, the negative electrode active material 36 preferably contains at least one of carbon materials, Li-containing alloys, and metal oxides.

[0043] The aqueous electrolyte 16 containing lithium salt is, for example, an aqueous liquid electrolyte containing lithium salt and an aqueous solvent, or an aqueous solid electrolyte composed of lithium salt, an aqueous solvent, and a matrix polymer. An aqueous solid electrolyte is obtained, for example, by drying a precursor solution in which lithium salt is dissolved in an aqueous solvent, and then mixed or dissolved with a matrix polymer. From the perspective of improving battery characteristics, the aqueous electrolyte 16 is preferably, for example, an aqueous liquid electrolyte.

[0044] The aqueous solvent is a solvent containing water, which can be water alone or a mixture of water and other solvents. The water content, relative to the total amount of the aqueous solvent, is preferably 50% or more by volume, for example, from the perspective of improving the safety of the lithium-ion secondary battery 1.

[0045] Furthermore, the amount of water relative to the lithium salt contained in the aqueous electrolyte 16, in terms of the lithium salt:water molar ratio, is preferably 1:4 or less, more preferably in the range of 1:0.5 to 1:4, and even more preferably in the range of 1:0.5 to 1:3 molar ratio. If the amount of water relative to the lithium salt contained in the aqueous electrolyte 16 is within the above range, compared to cases outside the above range, for example, the potential window of the aqueous electrolyte 16 may be widened, and the applied voltage to the lithium-ion secondary battery 1 can be further increased.

[0046] Solvents other than water included in aqueous solutions can include, for example, organic solvents such as esters, ethers, nitriles, alcohols, ketones, amines, amides, sulfur compounds, and hydrocarbons. Alternatively, halogen-substituted solvents, such as those formed by replacing at least a portion of the hydrogen atoms with halogen atoms like fluorine, can also be used. Specifically, from the perspective of improving the battery characteristics of the lithium-ion secondary battery 1, cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, and butyl carbonate are preferred; chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; and fluorinated carbonates containing fluorine as a constituent element, such as fluoroethylene carbonate, dimethyl fluorocarbonate, and methyl fluoropropionate. Particularly in the above examples, for example, from the perspective of suppressing battery self-discharge, cyclic carbonates or fluorinated carbonates containing fluorine as a constituent element are preferred. Furthermore, among the fluorinated carbonates listed above, fluoroethylene carbonate is preferred. These organic solvents can be used individually or in combination of two or more.

[0047] The amount of organic solvent relative to the lithium salt contained in the aqueous electrolyte 16 is preferably in the range of 1:0 to 1:2.5, more preferably in the range of 1:0 to 1:2, based on the molar ratio of lithium salt to organic solvent. If the amount of organic solvent relative to lithium salt is within the above range, the battery characteristics of the lithium-ion secondary battery can sometimes be improved compared to cases outside the above range.

[0048] Lithium salts can be any compounds that can dissolve in aqueous solvents and dissociate to allow lithium ions to be present in the aqueous electrolyte 16. Examples of such lithium salts include salts with inorganic acids such as perchloric acid, sulfuric acid, and nitric acid; salts with halide ions such as chloride ions and bromide ions; and salts with organic anions containing carbon atoms in their structure.

[0049] Organic anions that constitute lithium salts include, for example, the anions represented by the following general formulas (i) to (vi).

[0050] (R 1 SO2)(R 2 SO2)N - (i)

[0051] (R 1 R 2 Each is independently selected from alkyl or halogen-substituted alkyl groups. R 1 and R 2 (The elements can be chosen to bond together to form a ring.)

[0052] R 3 SO3 - (ii)

[0053] (R 3 Selected from alkyl or halogen-substituted alkyl groups.

[0054] R 4 CO2 - (iii)

[0055] (R 4 Selected from alkyl or halogen-substituted alkyl groups.

[0056] (R 5 SO2)3C - (iv)

[0057] (R 5 Selected from alkyl or halogen-substituted alkyl groups.

[0058] [(R 6 SO2)N(SO2)N(R 7 SO2)] 2- (v)

[0059] (R 6 R 7 Selected from alkyl or halogen-substituted alkyl groups.

[0060] [(R 8 SO2)N(CO)N(R 9 SO2)] 2- (vi)

[0061] (R 8 R 9 Selected from alkyl or halogen-substituted alkyl groups.

[0062] In the above general formulas (i) to (vi), the alkyl group or halogen-substituted alkyl group preferably has 1 to 6 carbon atoms, more preferably 1 to 3, and even more preferably 1 to 2. The halogen used as the halogen-substituted alkyl group is preferably fluorine. The number of halogen substitutions in the halogen-substituted alkyl group is less than or equal to the number of hydrogen atoms in the original alkyl group.

[0063] R 1 ~R 9 These are groups represented, for example, by the following general formula (vii).

[0064] C n H a F b Cl c Br d I e (vii)

[0065] (n is an integer greater than or equal to 1, and a, b, c, d, and e are integers greater than or equal to 0, satisfying 2n+1 = a + b + c + d + e.)

[0066] Specific examples of the organic anion represented by the above general formula (i) include bis(trifluoromethanesulfonyl)imide (TFSI; [N(CF3SO2)2)). - ), bis(perfluoroethanesulfonyl)imide (BETI; [N(C2F5SO2)2) - (perfluoroethanesulfonyl)(trifluoromethanesulfonyl)imide ([N(C2F5SO2)(CF3SO2)]) - ), etc. As specific examples of the organic anions represented by the above general formula (ii), CF3SO3 can be listed as an example. - C2F5SO3 - Etc. Specific examples of organic anions represented by the above general formula (iii) include, for example, CF3CO2. - C2F5CO2 - Etc. As a specific example of the organic anion represented by the above general formula (iv), tris(trifluoromethanesulfonyl)carbonate ([(CF3SO2)3C]) can be cited. - ), tri(perfluoroethanesulfonyl)carbonate ([(C2F5SO2)3C] - ), etc. As specific examples of the organic anions represented by the above general formula (V), sulfonyl bis(trifluoromethanesulfonyl)imide ([(CF3SO2)N(SO2)N(CF3SO2)] can be listed as an example. 2- ), sulfonyl bis(perfluoroethanesulfonyl)imide ([(C2F5SO2)N(SO2)N(C2F5SO2)] 2- ), sulfonyl (perfluoroethanesulfonyl)(trifluoromethanesulfonyl)imide ([(C2F5SO2)N(SO2)N(CF3SO2)] 2- ), etc. Specific examples of the organic anions represented by the above general formula (vi) include, for example, carbonyl bis(trifluoromethanesulfonyl)imide ([(CF3SO2)N(CO)N(CF3SO2)] 2- ), carbonyl bis(perfluoroethanesulfonyl)imide ([(C2F5SO2)N(CO)N(C2F5SO2)] 2- ), carbonyl (perfluoroethanesulfonyl)(trifluoromethanesulfonyl)imide ([(C2F5SO2)N(CO)N(CF3SO2)] 2- )wait.

[0067] Examples of organic anions other than those in the above general formulas (i) to (vi) include bis(1,2-phenylene glycol (2-)-O,O')borate, bis(2,3-naphthodiol (2-)-O,O')borate, bis(2,2'-biphenylene glycol (2-)-O,O')borate, and bis(5-fluoro-2-ol-1-benzenesulfonic acid-O,O')borate.

[0068] As the anion constituting the lithium salt, an imide anion is preferred. Preferred specific examples of imide anions include, in addition to the imide anions listed as organic anions represented by the above general formula (i), bis(fluorosulfonyl)imide (FSI; [N(FSO2)2]). - (Fluorosulfonyl)(trifluoromethanesulfonyl)imide (FTI; [N(FSO2)(CF3SO2)]) - )wait.

[0069] Lithium salts containing lithium ions and imide anions are preferred for effectively suppressing battery self-discharge, for example, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(perfluoroethanesulfonyl)imide (LiBETI), lithium (perfluoroethanesulfonyl)(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide (LiFSI), and lithium (fluorosulfonyl)(trifluoromethanesulfonyl)imide (LiFTI), with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) being more preferred. It should be noted that these can be used alone or in combination of two or more.

[0070] Other specific examples of lithium salts include CF3SO3Li, C2F5SO3Li, CF3CO2Li, C2F5CO2Li, (CF3SO2)3CLi, (C2F5SO2)3CLi, (C2F5SO2)2(CF3SO2)CLi, (C2F5SO2)(CF3SO2)2CLi, [(CF3SO2)N(SO2)N(CF3SO2)]Li2, [(C2F5SO2)N(SO2)N(C2F5SO2)]Li2, [(C2F5SO2)N(SO2)N(CF3SO2)]Li2, [(CF3SO2)N(CO)N(CF3SO2)]Li2, [(C [2F5SO2)N(CO)N(C2F5SO2)]Li2, [(C2F5SO2)N(CO)N(CF3SO2)]Li2, bis(1,2-phenylene glycol (2-)-O,O')lithium borate, bis(2,3-naphthalene glycol (2-)-O,O')lithium borate, bis(2,2'-biphenylene glycol (2-)-O,O')lithium borate, bis(5-fluoro-2-ol-1-benzenesulfonic acid-O,O')lithium borate, lithium perchlorate (LiClO4), lithium chloride (LiCl), lithium bromide (LiBr), lithium hydroxide (LiOH), lithium nitrate (LiNO3), lithium sulfate (Li2SO4), lithium sulfide (Li2S), lithium hydroxide (LiOH), etc. These can be used individually or in combination of two or more.

[0071] The lithium salt contained in the aqueous electrolyte 16 preferably contains lithium ions and imide anions, for example, from the perspective of improving the battery characteristics of lithium-ion secondary batteries, and the concentration of lithium salt in the aqueous electrolyte is preferably 4.5 mol / L to 6 mol / L.

[0072] In the case where the aqueous electrolyte 16 is an aqueous solid electrolyte, the matrix polymers contained may include, for example, polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyacrylonitrile (PAN), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyethylene oxide (PEO), and polymethyl methacrylate (PMMA). Alternatively, it may be a mixture of acrylonitrile and acrylic acid as monomers, which are thermally polymerized to form a high molecular weight substance.

[0073] The content of the matrix polymer relative to the total amount of the aqueous electrolyte 16 is preferably 1% by mass or more and 15.0% by mass or less, more preferably 3% by mass or more and 10% by mass or less. By setting it within the above range, for example, gelation or solidification of the aqueous electrolyte 16 becomes easier.

[0074] When the aqueous electrolyte 16 is an aqueous solid electrolyte, it can be coated onto the entire positive electrode 10, or at least onto the positive electrode composite material layer 28. An aqueous solid electrolyte is obtained, for example, by coating the positive electrode 10 with a precursor solution formed by dissolving a lithium salt in an aqueous solvent and then mixing or dissolving a matrix polymer, or by immersing the positive electrode 10 in the precursor solution, coating the positive electrode 10 with the precursor solution, and then drying it. When the aqueous electrolyte 16 is an aqueous liquid electrolyte, the entire positive electrode 10 can be immersed in the aqueous liquid electrolyte, or only the aqueous liquid electrolyte can be impregnated into the positive electrode composite material layer 28.

[0075] The non-aqueous electrolyte 18 containing lithium salt is either a non-aqueous liquid electrolyte containing lithium salt and an organic solvent, or a non-aqueous solid electrolyte composed of lithium salt, organic solvent, and a matrix polymer. The non-aqueous solid electrolyte is prepared, for example, by heating and drying a precursor solution in which lithium salt is dissolved in an organic solvent and then mixed or dissolved with a matrix polymer. From the perspective of improving battery characteristics, the non-aqueous electrolyte 18 is preferably a non-aqueous liquid electrolyte.

[0076] Organic solvents include those known to have been used in conventional non-aqueous secondary batteries, such as esters, ethers, nitriles, alcohols, ketones, amines, amides, sulfur compounds, and hydrocarbons. Among these, esters, ethers, nitriles, amides, and mixtures of two or more of these solvents are preferred for improving battery performance.

[0077] Examples of esters include cyclic carbonates such as ethylene carbonate, propylene carbonate, and butyl carbonate; chain carbonates such as dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, and isopropyl methyl carbonate; and carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and γ-butyrolactone.

[0078] Examples of ethers include cyclic ethers such as 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-epoxybutane, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-eucalyptol, and crown ethers; cyclic ethers such as 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, and dibutyl ether. Chain ethers such as dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, etc.

[0079] The organic solvent preferably contains a halogen substitute formed by substituting the hydrogen atoms of the aforementioned solvents with halogen atoms such as fluorine. Particularly preferred are at least one of fluorinated cyclic carbonates, fluorinated chain carbonates, and fluorinated ethers. Preferred examples of fluorinated cyclic carbonates include 4-fluoroethylene carbonate, 4,5-difluoroethylene carbonate, 4,4-difluoroethylene carbonate, 4,4,5-trifluoroethylene carbonate, and 4,4,5,5-tetrafluoroethylene carbonate. Preferred examples of fluorinated chain carbonates include ethyl 2,2,2-trifluoroacetate, methyl 3,3,3-trifluoropropionate, and methyl pentafluoropropionate. Preferred examples of fluorinated ethers include 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether.

[0080] For example, in order to suppress the decrease in lithium-ion conductivity of the non-aqueous electrolyte 18, the organic solvent preferably includes a cyclic organic solvent such as a cyclic carbonate, and more preferably includes a cyclic organic solvent comprising 10% or more of the total volume of the organic solvent. In the case of including a matrix polymer, it is more preferable to include a cyclic organic solvent comprising 80% or more of the total volume of the organic solvent.

[0081] Lithium salts include well-known lithium salts used in conventional non-aqueous secondary batteries, such as LiPF6, LiBF4, LiAsF6, LiClO4, LiCF3SO3, LiN(FSO2)2, and LiN(ClF). 2l+1 SO2)(C m F 2m+1 SO2)(l and m are integers greater than or equal to 1), LiC(C p F 2p+1 SO2)(C q F 2q+1 SO2)(Cr F 2r+1 The lithium salts can be, for example, the lithium salts listed above used in the aqueous electrolyte 16, such as SO2 (where p, q, and r are integers greater than or equal to 1), Li[B(C2O4)2] (lithium bis(oxalate)borate (LiBOB)), Li[B(C2O4)F2], Li[P(C2O4)F4], Li[P(C2O4)2F2], LiPO2F2, etc. Additionally, the lithium salts can be, for example, the lithium salts listed above used in the aqueous electrolyte 16.

[0082] The matrix polymer can be the same material as the aqueous electrolyte 16. The content of the matrix polymer, for example, relative to the total amount of the non-aqueous electrolyte 18, is preferably 1% by mass or more and 15.0% by mass or less, more preferably 3% by mass or more and 10% by mass or less. By setting it within the above range, for example, the solidification of the non-aqueous electrolyte 18 becomes easier.

[0083] For example, from the perspective of effectively suppressing contact between the negative electrode 12 and water, the non-aqueous electrolyte 18 preferably has water repellency with a solubility of 2g or less relative to 100g of water at 25°C. The water repellency of the non-aqueous electrolyte 18 can be improved, for example, by increasing the proportion of organic solvents with water-repellent substituents or fluorinated organic solvents.

[0084] When the non-aqueous electrolyte 18 is a non-aqueous solid electrolyte, it can be coated only on the surface of the negative electrode composite layer 32. However, since water side reactions also occur on the negative electrode current collector 30 and the negative electrode lead 22, it is preferable to coat it entirely on the negative electrode 12, and even more preferably, on the negative electrode lead 22 (excluding the portion protruding from the battery casing 24). The non-aqueous solid electrolyte is obtained, for example, by coating the negative electrode 12 with a precursor solution made by dissolving lithium salt in an organic solvent and then mixing or dissolving a matrix polymer, or by immersing the negative electrode 12 with the negative electrode lead 22 attached in the precursor solution, coating the precursor solution onto the negative electrode 12, and then drying it. When the non-aqueous electrolyte 18 is a non-aqueous liquid electrolyte, only the non-aqueous liquid electrolyte can be impregnated into the negative electrode composite layer 32, but it is preferable to immerse the entire negative electrode 12 in the non-aqueous liquid electrolyte.

[0085] The separator 14 is not particularly limited as long as it has the function of allowing lithium ions to pass through and electrically separating the positive electrode 10 from the negative electrode 12. It can be, for example, a solid electrolyte with lithium-ion conductivity, or a porous sheet made of resin, inorganic materials, etc. Specific examples of solid electrolytes include water-resistant solid electrolytes such as LATP. Specific examples of porous sheets include microporous films, woven fabrics, and nonwoven fabrics. Materials for the separator 14 include olefin resins such as polyethylene and polypropylene; polyamides, polyamide-imide, cellulose, etc. Inorganic materials constituting the separator 14 include glass and ceramics such as borosilicate glass, silica, alumina, and titanium dioxide. The separator 14 can be a laminate containing a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin resin. Alternatively, it can be a multilayer separator containing a polyethylene layer and a polypropylene layer, or a separator with an aromatic polyamide resin, ceramic, or other material coated on its surface.

[0086] From the perspective of effectively suppressing the backflow of water to the negative electrode 12 and the side reactions of water in the negative electrode 12, the separator 14 is preferably coated with a water-repellent non-aqueous solid electrolyte. The water-repellent non-aqueous solid electrolyte coated on the separator 14 can be the same as that described in the non-aqueous electrolyte 18 above.

[0087] Battery casing 24 can be made of metal, resin, or laminated film. Materials for metal casings include, for example, nickel, iron, and stainless steel. Materials for resin casings include, for example, polyethylene and polypropylene. Laminated films are, for example, multilayer films in which a resin film covers a stainless steel foil. Materials for the resin film include, for example, polypropylene, polyethylene, nylon, and polyethylene terephthalate.

[0088] The lithium-ion secondary battery of this embodiment is used in various forms such as square, cylindrical, flat, thin, coin-shaped, and laminated.

[0089] <Example>

[0090] The present disclosure will be further described below with reference to embodiments, but the present disclosure is not limited to these embodiments.

[0091] <Example 1>

[0092] [negative electrode]

[0093] A negative electrode composite slurry was prepared by mixing graphite (as the negative electrode active material) and PVDF (as the binder material) in N-methyl-2-pyrrolidone (NMP) at a solid component mass ratio of 96:4. This negative electrode composite slurry was then coated onto a negative electrode current collector made of copper foil. After drying the coating, it was calendered using calendering rollers. Finally, it was cut into specified electrode sizes to obtain the negative electrode. The coating weight of the negative electrode composite slurry and the filling density of the negative electrode composite layer were both 22.6 g / m³. -2 1.0gcm -3 .

[0094] [positive electrode]

[0095] The Ca content relative to the general formula Ni 0.90 Co 0.05 Al 0.05 The metal composite oxide shown in O2 was mixed with strontium hydroxide (Sr(OH)2) in a manner where the total amount of Ni, Co, and Al was 0.1 mol%. Then, lithium hydroxide monohydrate (LiOH·H2O) was mixed in such a manner that the total amount of Ni, Co, Al, and Sr was mixed with Li in a molar ratio of 1:1.02. This mixture was calcined from room temperature to 650°C at a heating rate of 2°C / min under an oxygen flow of 95% oxygen concentration (10 L / min per kg of mixture), followed by calcination from 650°C to 720°C at a heating rate of 1°C / min. Impurities were removed by washing the calcined material with water, yielding a positive electrode active material with a surface modification layer containing Sr formed on the primary particle surface of the lithium transition metal composite oxide. The composition of the positive electrode active material of Example 1 was analyzed by ICP-AES, and the results showed that Li... 0.99 Ni 0.899 Co 0.05 Al 0.05 Sr 0.001 O2.

[0096] A positive electrode composite slurry was prepared by mixing the above-mentioned positive electrode active material, carbon black as a conductive material, and PVDF as a binder in NMP at a mass ratio of 91:7:2. Next, the positive electrode composite slurry was coated onto a positive electrode current collector made of Ti foil. After the coating was dried, it was calendered using calendering rolls. Then, it was cut into specified electrode sizes to obtain the positive electrode. The coating amount of the positive electrode composite slurry and the filling density of the positive electrode composite layer were both 35.0 g / m³. -2 2.8gcm -3 .

[0097] [Non-aqueous liquid electrolyte]

[0098] A non-aqueous liquid electrolyte was prepared by dissolving 1 M LiTFSI in a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volume ratio of 20:80.

[0099] [Aqueous liquid electrolyte]

[0100] An aqueous liquid electrolyte containing dissolved LiTFSI and LiBETI was prepared by mixing LiTFSI, LiBETI, and water in a molar ratio of 0.7:0.3:2.0.

[0101] [Separator]

[0102] An electrolyte was prepared by dissolving 1 M LiTFSI in a mixed solution of ethylene fluorocarbonate (FEC) and methyl 3,3,3-trifluoropropionate (FMP) at a volume ratio of 9:1. Next, relative to the electrolyte, 4% by mass of polymethyl methacrylate (PMMA) and 8% by mass of polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) were dissolved in a solvent consisting of 10 times the volume of THF (tetrahydrofuran) and 10 times the volume of acetone. The electrolyte was mixed in this solution to prepare a precursor solution of a non-aqueous solid electrolyte. A porous sheet was then immersed in the precursor solution of the non-aqueous solid electrolyte and dried at 60°C for 1 hour to obtain a water-repellent non-aqueous solid electrolyte coated on the porous sheet. The porous sheet coated with this non-aqueous solid electrolyte was used as a separator.

[0103] [Test Battery]

[0104] A negative electrode with a negative lead is immersed in a non-aqueous liquid electrolyte to obtain a negative electrode impregnated with the non-aqueous liquid electrolyte. Similarly, a positive electrode with a positive lead is immersed in an aqueous liquid electrolyte to obtain a positive electrode impregnated with the aqueous liquid electrolyte. An electrode body with a separator between the negative and positive electrodes is then constructed. Figure 1 The battery is housed in a battery casing to create a test battery.

[0105] <Example 2>

[0106] Except that calcium hydroxide (Ca(OH)2) was used instead of strontium hydroxide (Sr(OH)2) in the preparation of the positive electrode active material, the test cell was prepared in the same manner as in Example 1.

[0107] <Comparative Example 1>

[0108] Except that strontium hydroxide (Sr(OH)2) was not used in the preparation of the positive electrode active material, the test cell was prepared in the same manner as in Example 1.

[0109] The test batteries of each embodiment and Comparative Example 1 were charged at a constant current of 0.2C to 4.2V, then charged at a constant voltage of 4.2V until reaching 0.02C, and stopped for 20 minutes. Then, they were discharged at a constant current of 0.2C to 2.9V, and stopped for 20 minutes. This charge-discharge cycle was repeated for 40 cycles, and the capacity retention rate was calculated using the following formula. The results are shown in Table 1. It should be noted that a higher capacity retention rate indicates that the capacity reduction accompanying the charge-discharge cycle is more suppressed.

[0110] Capacity retention (%) = (Discharge capacity in the 40th cycle ÷ Discharge capacity in the 1st cycle) × 100

[0111] [Table 1]

[0112]

[0113] Examples 1 and 2, compared to the comparative example, showed high capacity retention rates. Therefore, it can be said that by using a positive electrode active material with a surface modification layer containing Ca and Sr formed on the surface of primary particles of a lithium transition metal composite oxide, the capacity reduction during charge-discharge cycles of lithium-ion secondary batteries using aqueous electrolytes can be suppressed.

[0114] <Example 3>

[0115] A solution of an aqueous electrolyte precursor, containing 10% by mass of polyvinyl alcohol (PVA) relative to the aqueous liquid electrolyte, was coated onto the surface of the positive electrode and dried at 25°C for 12 hours, instead of immersing the positive electrode in the aqueous liquid electrolyte. Otherwise, the test cell was made in the same manner as in Example 2.

[0116] <Comparative Example 2>

[0117] In the preparation of the positive electrode active material, calcium hydroxide (Ca(OH)2) was not used. Otherwise, the test battery was prepared in the same manner as in Example 3.

[0118] [Table 2]

[0119]

[0120] Example 3, compared to Comparative Example 2, showed a higher capacity retention rate. Therefore, it can be said that even when using an electrolyte containing a matrix polymer, by using a positive electrode active material with a surface-modified layer formed on the surface of primary particles of a lithium transition metal composite oxide, the capacity reduction during charge-discharge cycles of a lithium-ion secondary battery using an aqueous electrolyte can be suppressed.

[0121] Explanation of reference numerals in the attached figures

[0122] 1. Lithium-ion secondary battery

[0123] 10 Positive electrode

[0124] 12 Negative electrode

[0125] 14. Separators

[0126] 16 Aqueous electrolytes

[0127] 18 Non-aqueous electrolytes

[0128] 20 Positive lead

[0129] 22 Negative lead

[0130] 24 Battery casing

[0131] 26 Positive current collector

[0132] 28 Positive electrode composite material layers

[0133] 30 Negative current collector

[0134] 32 Negative electrode composite material layer

[0135] 34 Positive electrode active material

[0136] 36 Negative Electrode Active Material

Claims

1. A lithium-ion secondary battery, comprising a negative electrode, a positive electrode, a non-aqueous electrolyte containing a lithium salt, and an aqueous electrolyte containing a lithium salt. The aqueous electrolyte is in contact only with the positive electrode of the negative electrode and the positive electrode. The non-aqueous electrolyte is in contact with at least the negative electrode of the negative electrode and the positive electrode. The positive electrode contains a positive electrode active material. The positive electrode active material comprises a lithium transition metal composite oxide and a surface modification layer formed on the surface of the primary particles of the lithium transition metal composite oxide. The surface modification layer comprises at least one element selected from the group consisting of alkaline earth metals, rare earth elements, and elements from Group IIIb, Group IVb, and Group Vb of the periodic table. The non-aqueous electrolyte is a liquid.

2. The lithium-ion secondary battery according to claim 1, wherein, The surface finishing layer contains a transition metal.

3. The lithium-ion secondary battery according to claim 1 or 2, wherein, The surface modification layer contains at least one element selected from Sr and Ca.

4. The lithium-ion secondary battery according to any one of claims 1 to 3, wherein, The negative electrode contains a negative electrode active material. The negative electrode active material comprises at least one of carbon materials, Li-containing alloys, and metal oxides.

5. The lithium-ion secondary battery according to any one of claims 1 to 4, wherein, The aqueous electrolyte is a liquid.

6. The lithium-ion secondary battery according to any one of claims 1 to 5, wherein, The lithium salt in the aqueous electrolyte comprises lithium ions and imide anions, and the concentration of the lithium salt in the aqueous electrolyte is 4.5 mol / L to 6 mol / L.

7. The lithium-ion secondary battery according to any one of claims 1 to 6, wherein, A porous separator coated with a water-repellent non-aqueous solid electrolyte is disposed between the positive electrode and the negative electrode.

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