JP2017033839A - Positive electrode for lithium secondary battery, lithium secondary battery, and method for manufacturing positive electrode for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a positive electrode for a lithium secondary battery, capable of reducing a deterioration with time in discharge capacity due to storage while suppressing internal resistance; a lithium secondary battery; a method for manufacturing a positive electrode for a lithium ion secondary battery.SOLUTION: A positive electrode 10 for a lithium secondary battery contains a composite oxide represented by formula (I): LiMnCoNiM1O[where, M1 represents at least one element selected from among Fe, Cu, Al, Mg and Mo, and 0≤x≤0.33, 0≤a≤1.0, 0≤b≤1.0, 0≤c≤1.0, 0≤y≤1.0 and a+b+c+y=1 are satisfied.]. In transition metals that are present in a surface layer of the composite oxide, average oxidation numbers in a non-charged state are higher than tetravalent for Mn, higher than trivalent for Co, and higher than divalent for Ni. A boron-containing compound is present on a surface of the composite oxide.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a positive electrode for a lithium secondary battery, a lithium secondary battery, and a method for producing a positive electrode for a lithium ion secondary battery.

  Lithium secondary batteries are power supplies for portable electronic devices such as mobile phones and portable PCs, power supplies for household electrical devices, power storage devices, stationary power supplies such as uninterruptible power supplies, and driving power supplies for ships, railways, automobiles, etc. Practical use is being promoted widely in such fields. With respect to lithium secondary batteries, there have been high demands for battery size reduction, higher output, longer life, and the like. Therefore, for the purpose of developing a lithium secondary battery having a high energy density and a long service life, improvement of battery materials such as electrodes and electrolytes is being promoted.

  With respect to the non-aqueous electrolyte solution enclosed in the lithium secondary battery, decomposition of the non-aqueous solvent or the like due to the oxidation-reduction reaction with the electrode is particularly problematic. If the non-aqueous solvent that is a component of the non-aqueous electrolyte decomposes and the composition of the electrolyte changes, or if the decomposition product of the non-aqueous solvent accumulates on the electrode surface, the battery performance increases due to an increase in internal resistance. This is because the battery life is shortened and the life of the battery is shortened. Therefore, as a technique for suppressing the decomposition of the non-aqueous electrolyte, a number of techniques for modifying the surface of the active material and techniques for adding various additives to the non-aqueous electrolyte have been proposed. As a method for modifying the surface of the active material, there are a method of substituting different elements in the active material, a method of forming a film on the surface of the active material, and the like.

For example, Patent Document 1, the general formula _{Li z Ni 1-x-y} Co x M y O 2 ( however, M is, Mn, V, Mg, Mo, Nb, at least one selected from Ti and Al Element, 0.10 ≦ x ≦ 0.21, 0 ≦ y ≦ 0.08, 0.97 ≦ z ≦ 1.10) on the surface of primary particles composed of a lithium metal composite oxide. Lithium cobalt that adheres the fine particles composed of the cobalt compound in a uniformly distributed and dense state on the surface of the primary particles by performing a heat treatment at 300 to 700 ° C. And a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, which includes a step of forming fine particles having a diameter of 100 nm or less made of a composite oxide. Non-Patent Document 1 discloses a technique for preventing a decrease in discharge capacity by forming a ZrO ₂ film on the surface of LiCoO ₂ .

Japanese Patent No. 5181455

Yong Jeong Kima et al., Journal of The Electrochemical Society, 150 (12), A1723 (2003)

  As disclosed in Patent Document 1 and Non-Patent Document 1, it is possible to suppress elution of the active material by forming a film on the surface of the active material. In addition, when a film is formed, oxidative degradation of the active material due to oxygen, moisture, and the like, and decomposition of the non-aqueous solvent are also suppressed. Therefore, according to such a technique, the composition change of the non-aqueous electrolyte promoted under high temperature storage conditions and the deposition of the non-aqueous solvent decomposition product on the active material can be suppressed. It is possible to reduce the decrease in discharge capacity accompanying storage under high temperature conditions.

  However, in Patent Document 1 and Non-Patent Document 1, a film on the surface of the active material is formed of a metal oxide. Metal oxides generally tend to have low electronic conductivity. Also, it can be a great resistance to lithium ion conduction. And when the film by a metal oxide is formed with excess thickness, the internal resistance of a lithium secondary battery will increase and discharge capacity will fall. Therefore, it is possible to suppress the decomposition of the non-aqueous electrolyte without greatly increasing the internal resistance, and the composition change of the non-aqueous electrolyte and the deposition of the non-aqueous solvent decomposition product accelerated under high temperature storage conditions. There is a need for a technique that can more appropriately prevent a decrease in discharge capacity due to elution of active materials.

  Therefore, the present invention provides a positive electrode for a lithium secondary battery, a lithium secondary battery, and a positive electrode for a lithium ion secondary battery that can reduce a decrease in discharge capacity over time with storage while suppressing internal resistance. An object is to provide a manufacturing method.

The positive electrode for a lithium secondary battery according to the present invention in order to solve the above problems, the following general formula _{(I) Li 1 + x Mn} a Co b Ni c M1 y O 2 [ In the formula, M1 is Fe, Cu , Al, Mg, Mo, Zr, at least one element selected from the group consisting of 0 ≦ x ≦ 0.33, 0 ≦ a ≦ 1.0, 0 ≦ b ≦ 1.0, 0 ≦ c ≦ 1.0, 0 ≦ y ≦ 1.0, and a + b + c + y = 1 are satisfied. The transition metal present in the surface layer of the composite oxide has an average oxidation number in an uncharged state, tetravalent for Mn, trivalent for Co, and for Ni It is higher than divalent, and a boron-containing compound is present in the surface layer of the composite oxide.

Further, the lithium secondary battery according to the present invention comprises a positive electrode, a negative electrode, and a nonaqueous electrolyte, wherein the positive electrode, the following general formula _{(I) Li 1 + x Mn} a Co b Ni c M1 y O 2 [ In the formula, M1 is at least one element selected from the group consisting of Fe, Cu, Al, Mg, Mo, and Zr, and 0 ≦ x ≦ 0.33, 0 ≦ a ≦ 1.0, 0 ≦ b ≦ 1.0, 0 ≦ c ≦ 1.0, 0 ≦ y ≦ 1.0, and a + b + c + y = 1 are satisfied. The transition metal present in the surface layer of the composite oxide has an average oxidation number in an uncharged state, tetravalent for Mn, trivalent for Co, and for Ni It is higher than divalent, and a boron-containing compound is present on the surface of the composite oxide.

  The method for producing a positive electrode for a lithium secondary battery according to the present invention includes a composite oxide particle containing Li and at least one transition metal selected from the group consisting of Mn, Co and Ni, and oxofluoro A step of mixing a phosphorus compound and a solvent to bring the transition metal present in a surface layer of the composite oxide into a highly oxidized state, a step of washing and drying the particles of the composite oxide, and the composite oxidation And a step of forming a positive electrode mixture containing particles of the product on the positive electrode current collector and then forming the positive electrode current collector.

  According to the present invention, there is provided a positive electrode for a lithium secondary battery, a lithium secondary battery, and a positive electrode for a lithium ion secondary battery capable of reducing a temporal decrease in discharge capacity accompanying storage while suppressing internal resistance. A manufacturing method can be provided.

It is sectional drawing which shows typically the structure of the lithium secondary battery which concerns on one Embodiment of this invention. It is a figure which shows the oxidation state of manganese of the surface layer of lithium metal complex oxide. It is a figure which shows the oxidation state of cobalt of the surface layer of lithium metal complex oxide. It is a figure which shows the oxidation state of nickel of the surface layer of lithium metal complex oxide. It is a figure which shows the mass spectrometry result of the surface vicinity of a lithium metal complex oxide.

  As a result of intensive studies, the present inventors have found that a reaction product produced by a reaction between a boroxine compound and lithium hexafluorophosphate can improve the high-temperature storage characteristics of a lithium secondary battery. Specifically, the fluorinated boroxine compound produced by the reaction between the boroxine compound and lithium hexafluorophosphate is formed as a by-product while forming a film-like inclusion (boron-containing compound) on the surface of the positive electrode active material. It has been found that the oxofluorophosphorus compound represented by the general formula: POxFy exhibits an action of modifying the surface layer of the positive electrode active material. And by maintaining the surface layer of the boron-containing compound and the positive electrode active material, the capacity retention ratio after high-temperature storage can be reduced by suppressing the decomposition of the nonaqueous solvent and the elution of the positive electrode active material without greatly increasing the internal resistance. As a result, a long-life lithium ion secondary battery having high discharge capacity storage stability has been realized.

  Hereinafter, the manufacturing method of the positive electrode for lithium secondary batteries, the lithium secondary battery, and the positive electrode for lithium ion secondary batteries which concerns on one Embodiment of this invention is demonstrated in detail. In addition, the following description shows the specific example of the content of this invention, and this invention is not limited to these description. The present invention can be variously changed and modified by those skilled in the art within the scope of the technical idea disclosed in the present specification.

FIG. 1 is a cross-sectional view schematically showing the structure of a lithium secondary battery according to an embodiment of the present invention.
As shown in FIG. 1, the lithium secondary battery 1 according to this embodiment includes a positive electrode 10, a separator 11, a negative electrode 12, a battery can 13, a positive electrode current collecting tab 14, a negative electrode current collecting tab 15, an inner lid 16, and an internal pressure release. A valve 17, a gasket 18, a positive temperature coefficient (PTC) resistance element 19, a battery lid 20, and an axis 21 are provided. The battery lid 20 is an integrated part composed of the inner lid 16, the internal pressure release valve 17, the gasket 18 and the resistance element 19.

  The positive electrode 10 and the negative electrode 12 are provided in a sheet shape, and are overlapped with each other with the separator 11 interposed therebetween. The positive electrode 10, the separator 11, and the negative electrode 12 are wound around the axis 21 to form a cylindrical electrode group. The form of the electrode group is not the cylindrical form shown in FIG. 1, but the form wound in a flat circular form, the form in which strip-shaped electrodes are laminated, and the bag-like separator in which the electrodes are stored is laminated. Thus, various forms exemplified by a multi-layered structure and the like are possible.

  The axis 21 can be provided in any cross-sectional shape suitable for supporting the positive electrode 10, the separator 11, and the negative electrode 12. Examples of the cross-sectional shape include a cylindrical shape, a columnar shape, a rectangular tube shape, and a square shape. Further, the shaft center 21 can be made of any material having good insulation. Examples of the material include polypropylene and polyphenylene sulfide.

  The battery can 13 is preferably formed of a material having good corrosion resistance with respect to the non-aqueous electrolyte. Moreover, it is preferable that the battery can 13 is formed of a material in which the region in contact with the non-aqueous electrolyte is difficult to be alloyed with lithium. Specifically, the material of the battery can 13 is preferably aluminum, an aluminum alloy, stainless steel, nickel-plated steel, or the like. Stainless steel is advantageous in that it has good corrosion resistance because a passive film is formed on the surface and has a strength that can withstand an increase in internal pressure. Aluminum and aluminum alloys are advantageous in that the energy density per weight can be improved because they are lightweight.

  The battery can 13 can have an appropriate shape such as a cylindrical shape, a flat oval shape, a flat oval shape, a square shape, or a coin shape, depending on the form of the electrode group. The inner surface of the battery can 13 may be subjected to surface processing for improving corrosion resistance and adhesion.

  The positive electrode current collecting tab 14 and the negative electrode current collecting tab 15 for drawing current are connected to the positive electrode 10 and the negative electrode 12 by spot welding, ultrasonic welding, or the like. The electrode group provided with the positive electrode current collecting tab 14 and the negative electrode current collecting tab 15 is housed in the battery can 13. At this time, the positive electrode current collecting tab 14 is electrically connected to the bottom surface of the battery lid 20, and the negative electrode current collecting tab 15 is electrically connected to the inner wall of the battery can 13. A plurality of the positive electrode current collecting tab 14 and the negative electrode current collecting tab 15 may be provided in the electrode group as shown in FIG. By providing a plurality, for example, when the lithium secondary battery 1 is applied as a drive power source for an automobile or the like, it is possible to cope with a large current.

  A non-aqueous electrolyte solution containing a non-aqueous solvent and a supporting salt is injected into the battery can 13. The method for injecting the non-aqueous electrolyte may be a method of injecting directly with the battery lid 20 opened, or a method of injecting from the inlet provided in the battery lid 20 with the battery lid 20 closed. There may be. The opening of the battery can 13 is sealed by joining the battery lid 20 by welding, caulking, or the like. The battery lid 20 is provided with a relief valve so that it is opened when the internal pressure of the battery can 13 rises excessively.

  As the non-aqueous solvent, for example, a chain carbonate, a cyclic carbonate, a chain carboxylic acid ester, a cyclic carboxylic acid ester, a chain ether, a cyclic ether, an organic phosphorus compound, an organic sulfur compound, or the like can be used. These compounds may be used alone as a nonaqueous solvent, or two or more kinds may be mixed and used.

  As the chain carbonate, a compound having a chain alkyl group having 1 to 5 carbon atoms is preferable. Specific examples of such a chain carbonate include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, and the like. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, vinylene carbonate, 1,2-butylene carbonate, and 2,3-butylene carbonate.

  Examples of the chain carboxylic acid ester include methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, and propyl propionate. Examples of the cyclic carboxylic acid ester include γ-butyrolactone, γ-valerolactone, and δ-valerolactone.

  Examples of the chain ether include dimethoxymethane, diethoxymethane, 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 1,3-dimethoxypropane, and the like. Examples of the cyclic ether include tetrahydrofuran, 2-methyltetrahydrofuran, 3-methyltetrahydrofuran and the like.

  Examples of the organic phosphorus compound include phosphoric acid esters such as trimethyl phosphate, triethyl phosphate, and triphenyl phosphate, phosphorous acid esters such as trimethyl phosphite, triethyl phosphite, and triphenyl phosphite, And trimethylphosphine oxide. Examples of the organic sulfur compound include 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, sulfolane, sulfolene, dimethyl sulfone, ethyl methyl sulfone, methyl phenyl sulfone, and ethyl phenyl sulfone. .

  These compounds that can be used as the non-aqueous solvent may have a substituent, or may be a compound in which an oxygen atom is substituted with a sulfur atom. As a substituent, halogen atoms, such as a fluorine atom, a chlorine atom, a bromine atom, are mentioned, for example. When two or more compounds are used in combination as a non-aqueous solvent, in particular, a compound having a high relative dielectric constant and a relatively high viscosity such as a cyclic carbonate or a cyclic lactone and a viscosity such as a chain carbonate are relatively It is preferable to combine with a low compound. For example, when the cyclic carbonate and the chain carbonate are used in combination, the ratio of the cyclic carbonate is preferably 40% by volume or less, more preferably 30% by volume or less, and further preferably 20% by volume or less. preferable.

Examples of the supporting salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₂ , Li (CF ₃ SO ₂ ) ₂ N, Li (C ₂ F ₅ SO ₂ ) ₂ N, Li (F _{2 SO 2) 2 N, LiF,} Li 2 CO 3, LiPF 4 (CF 3) 2, LiPF 4 (CF 3 SO 2) 2, LiBF 3 (CF 3), LiBF 2 (CF 3 SO 2) 2, lithium bis Lithium salts such as oxalate borate and lithium difluorooxalate borate can be used. As the supporting salt, one of these may be used alone, or two or more thereof may be mixed and used.

As an electrolytic solution, an electrolytic solution containing ethylene carbonate or propylene carbonate and dimethyl carbonate or ethyl methyl carbonate as a non-aqueous solvent and lithium hexafluorophosphate (LiPF ₆ ) as a supporting salt, LiPF ₆ and LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₂ , Li (CF ₃ SO ₂ ) ₂ N, Li (C ₂ F ₅ SO ₂ ) ₂ N, and Li (F ₂ SO ₂ ) ₂ N An electrolyte containing at least one of the above as a supporting salt is particularly suitable. Ethylene carbonate and propylene carbonate have a high dielectric constant, and ethylene carbonate has an advantage that the graphite electrode is less likely to peel off than propylene carbonate or the like. Dimethyl carbonate and ethyl methyl carbonate have low viscosity. On the other hand, lithium hexafluorophosphate is particularly suitable as a supporting salt because of its particularly good solubility and ionic conductivity. When LiBF ₄ or the like that is hardly hydrolyzed is used in combination with such lithium hexafluorophosphate, the high-temperature storage characteristics of the battery may be improved.

  For example, for the lithium hexafluorophosphate, the concentration of the supporting salt is preferably in the range of 0.6 mol / L to 1.8 mol / L per electrolytic solution. This is because good ionic conductivity is easily realized when the concentration of the supporting salt is 0.6 mol / L or more. Further, when the concentration of the supporting salt is 1.8 mol / L or less, the ratio of the non-aqueous solvent is ensured to a certain level or more, so that the ion conduction resistance is rarely excessive.

  Non-aqueous electrolyte is a film-forming agent that forms a coating on the surface of the negative electrode active material, an overcharge inhibitor that suppresses overcharging of the battery, and a flame retardant that improves the flame retardancy (self-extinguishing) of the non-aqueous electrolyte. Various additives including a wettability improving agent for improving the wettability of the agent, the electrode and the separator may be contained. Note that these additives can be used together with other nonaqueous solvents and supporting salts in an amount added as an additive even for compounds that can be used as nonaqueous solvents and supporting salts.

  Examples of the film forming agent include carboxylic acid anhydrides such as vinylene carbonate used as a solvent, sulfur compounds such as 1,3-propane sultone, lithium bisoxalate borate (LiBOB), and trimethyl borate (TMB). A boron compound etc. are mentioned. It is known that a SEI (Solid Electrolyte Interphase) film is formed on the surface of the negative electrode active material by a decomposition product of a nonaqueous electrolytic solution. Although the SEI film has an effect of suppressing the decomposition of the non-aqueous electrolyte, if it is formed excessively, it may increase the internal resistance or consume a large amount of charge when formed. When a film-forming agent such as vinylene carbonate is added to the non-aqueous electrolyte, such a SEI film can be reformed into a film that can be stably charged and discharged, thereby extending the battery life. is there.

  Examples of the overcharge inhibitor include biphenyl, biphenyl ether, terphenyl, methyl terphenyl, dimethyl terphenyl, cyclohexylbenzene, dicyclohexylbenzene, triphenylbenzene, hexaphenylbenzene, and the like. As the flame retardant, for example, organic phosphorus compounds such as trimethyl phosphate and triethyl phosphate, fluorides of the above non-aqueous solvents including borate esters, and the like can be used. As the wettability improver, for example, chain ethers such as 1,2-dimethoxyethane can be used.

  The positive electrode 10 (positive electrode for a lithium secondary battery) includes a positive electrode active material capable of inserting and extracting lithium ions. Specifically, the positive electrode 10 includes, for example, a positive electrode mixture layer formed of a positive electrode mixture in which a positive electrode active material, a binder, and a conductive agent are mixed, and a positive electrode mixture layer coated on one side or both sides. It is set as the structure provided with the processed positive electrode electrical power collector.

  As the positive electrode active material, lithium metal containing lithium (Li), at least one transition metal selected from the group consisting of manganese (Mn), cobalt (Co), and nickel (Ni), and oxygen (O) A composite oxide (composite oxide) is included. The positive electrode active material may be an aggregate composed only of primary particles and secondary particles of a lithium metal composite oxide, or can absorb and release lithium ions from the primary particles and secondary particles of the lithium metal composite oxide. It may be an assembly of particles of other compounds.

  The positive electrode 10 is characterized in that the transition metal present in the surface layer of the lithium metal composite oxide is in a highly oxidized state. Specifically, the transition metal present in the surface layer of each particle of the lithium metal composite oxide is compared with the transition metal present in the particle during both the occlusion and release of lithium ions, and the average oxidation. The number exists in a state where it is shifted to a slightly expensive number side. This highly oxidized state is formed, for example, by treating a lithium metal composite oxide with a compound (oxofluorophosphorus compound) represented by the general formula: POxFy, as will be described later. In the present specification, the term “compound” includes an atomic group existing as an ion, for example, a fluorophosphate anion, an atomic group constituting a part of one compound molecule, or the like. The surface layer is defined as a region from the surface of the crystal grain to a depth of 30 nm. In this embodiment, the lithium metal composite oxide, which is the positive electrode active material, is brought into a highly oxidized state using an oxofluoroline compound. It is not limited to the process by this, You may use the lithium metal complex oxide made into the highly oxidized state by the other method.

Specifically, as the lithium metal composite oxide, the following general formula (I)
_{Li 1 + x Mn a Co b} Ni c M1 y O 2 ··· (I)
[In the formula, M1 is at least one element selected from the group consisting of Fe, Cu, Al, Mg, Mo, Zr, and 0 ≦ x ≦ 0.33, 0 ≦ a ≦ 1.0, 0 ≦ b ≦ 1.0, 0 ≦ c ≦ 1.0, 0 ≦ y ≦ 1.0, and a + b + c + y = 1 are satisfied. And a layered oxide having a layered rock salt type crystal structure can be used. As the layered oxide, a manganese-based layered oxide containing Mn and having a> 0 is particularly preferable. According to the manganese-based layered oxide, it is possible to increase the theoretical capacity and safety while the material cost is low. By adopting a multi-component composition containing Co and Ni together with Mn, a stable crystal structure can be obtained. It can also be obtained.

Further, as the lithium metal composite oxide, specifically, the following general formula (II)
Li _{1 + x} Mn _2-xy M2 _y O ₄ (II)
[Wherein, M2 is at least one element selected from the group consisting of Ni, Co, Fe, Cu, Al, Mg, Mo, Zr, and 0 ≦ x ≦ 0.33, 0 ≦ y ≦ 1.0 and 2-xy> 0 are satisfied. And a spinel type oxide having a spinel type crystal structure can be used. According to the spinel type oxide, although the material cost is low, the safety is high and a stable crystal structure is realized even in a high voltage range.

  The average oxidation number of the transition metal contained in the lithium metal composite oxide usually varies with charge / discharge of the lithium secondary battery. For example, in the case of the whole layered oxide, Mn is in a minute range near about 4 valence, Co is in a range from about 3 to about 4 valence, Ni is in a range from about 2 to about 4 valence. The charge can change. Further, in the spinel oxide as a whole, the formal charge of Mn can change in the range from about 3 to about 4.

  On the other hand, in the transition metal existing in the surface layer of the layered oxide, the average oxidation number in an uncharged state is tetravalent for Mn, trivalent for Co, and 2 for Ni by treatment with an oxofluorophosphorus compound. Higher than the price. Moreover, in the transition metal which exists in the surface layer of a spinel type oxide, the average oxidation number in a non-charging state becomes higher than trivalence about Mn. Alternatively, by the treatment with the oxofluoroline compound, the transition metal constituting the lithium metal composite oxide is combined with fluorine (F) having a high electronegativity, and the effect caused by the decrease in charge density on the transition metal But it can be explained. In addition, in the non-charged state, after synthesizing the lithium metal composite oxide, in addition to the uncharged state in which the lithium secondary battery is not initially charged or the state in which the lithium secondary battery is completely discharged, the state is charged to less than about 1% SOC. The discharged state is included. Such a shift in the average oxidation number in the non-charged state can be confirmed by performing X-ray photoelectron spectroscopy (XPS) on the positive electrode recovered by disassembling the lithium secondary battery. It is.

  Specifically, the shift in the average oxidation number means that other atoms derived from the treatment with the oxofluorophosphorus compound are bonded to the transition metal present in the surface layer of the lithium metal composite oxide. In the lithium metal composite oxide, the surface layer is modified in this way, so that the electrochemical activity on the surface side is suppressed. And the situation where the transition metal contained in lithium metal complex oxide elutes in a non-aqueous electrolyte, or the non-aqueous solvent which contacted the transition metal is oxidatively decomposed is prevented. For this reason, even in a lithium secondary battery in a highly charged state or the like, it is possible to reduce a decrease in discharge capacity with time due to storage.

  Specifically, the transition metal present in the surface layer of the lithium metal composite oxide is modified to a state in which fluorine atoms and the like are bonded by treatment with an oxofluorophosphorus compound. In a state in which fluorine atoms or the like are bonded, lithium ion conduction is hardly hindered compared to the case where a film of a metal oxide or the like is formed on the surface of the positive electrode active material. Therefore, by modifying the surface layer in this way, the elution of transition metals and the decomposition of the non-aqueous solvent are suppressed without greatly increasing the internal resistance of the lithium secondary battery.

Specific examples of the oxofluoroline compound include fluorophosphate anions such as POF ₂ ⁻ , PO ₂ F ₂ ⁻ , PO ₃ F ²⁻ , salts thereof, POF ₂ , PO ₂ F ₂ , PO _3. An organophosphorus compound having an atomic group represented by F or the like is used. These oxofluorophosphorus compounds have a phosphorus atom having a relatively high electron withdrawing property. For example, it is represented by an intermediate product produced by nucleophilic reaction of anions such as POF ₂ ⁻ , PO ₂ F ₂ ⁻ , PO ₃ F ²⁻ , POF ₂ , PO ₂ F ₂ , PO ₃ F and the like. The organophosphorus compound having an atomic group that is formed exhibits an acidic action due to the presence of such a phosphorus atom. In addition, the oxofluoroline compound is hydrolyzed with water that may be present in a minute amount as represented by the following reaction formulas to generate hydrogen fluoride (HF).
^{_{PO 2 F 2 - + H 2}} O ⇔ PO 3 F 2- + HF + H + ··· (1)
PO ₃ F ² + H ₂ O ＰＯ PO ₄ ³ + HF + H ⁺ (2)
Therefore, the surface layer of the lithium metal composite oxide can be modified by these components.

  The positive electrode 10 also has a feature that a boron-containing compound having a B—O bond is present on the surface of the lithium metal composite oxide in a state provided in the lithium secondary battery. In the lithium metal composite oxide, the surface layer is modified by the action of the oxofluorophosphorus compound, so that a SEI (Solid Electrolyte Interphase) film-like inclusion by a boron-containing compound is present at the interface with the non-aqueous electrolyte. It is designed to be well formed. Inclusions of this boron-containing compound are unlikely to become resistance to lithium ion conduction compared to coatings such as metal oxides, so that the non-aqueous solvent can be decomposed without greatly increasing the internal resistance of the lithium secondary battery. Suppression is achieved.

  The boron-containing compound can be generated on the surface of the lithium metal composite oxide by adding a boron compound to the non-aqueous electrolyte of the lithium secondary battery. Examples of the boron compound include lithium borate salts such as lithium bisoxalate borate and lithium difluorooxalate borate, boric acid esters such as trimethyl borate, and boroxine compounds.

  A positive electrode 10 (a positive electrode for a lithium secondary battery) including a lithium metal composite oxide in which a surface layer is modified with an oxofluorophosphorus compound and a boron-containing compound is interposed on a surface thereof is, for example, a non-lithium secondary battery. A method of adding a boroxine compound and lithium hexafluorophosphate to an aqueous electrolyte, a method of adding an oxofluorophosphorus compound and a boron compound to a non-aqueous electrolyte of a lithium secondary battery, and a lithium metal composite oxide as a positive electrode It can be obtained by any of the methods of treating with an oxofluoroline compound in advance at the time of production.

In the method of adding a boroxine compound and lithium hexafluorophosphate to the non-aqueous electrolyte of a lithium secondary battery, specifically, the following general formula (III):
(RO) ₃ (BO) ₃ (III)
[Wherein, each R is independently an organic group having 1 to 6 carbon atoms. ]
The boroxine compound represented by these can be used. By reacting such a boroxine compound and lithium hexafluorophosphate (LiPF ₆ ), which is also added as a supporting salt, in a non-aqueous electrolyte, a boroxine compound having a boron atom having a valence higher than 3 The following general formula (IV):
POxFy (IV)
[Wherein, x is a number from 1 to 3, and y is a number from 1 to 5. ] Or a compound having an atomic group (oxofluoroline compound) can be produced. In addition, the oxidation number of the phosphorus atom in the oxofluoro phosphorus compound to be generated is 3 or 5.

  Examples of the organic group (R) of the boroxine compound include linear or branched alkyl groups having 1 to 6 carbon atoms, cycloalkyl groups, and the like. Specific examples of such an organic group (R) include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, an isobutyl group, and a cyclohexyl group. The organic group (R) may contain a halogen atom, nitrogen atom, sulfur atom and the like exemplified by fluorine atom, chlorine atom and bromine atom.

The organic group (R) of the boroxine compound is preferably a divalent organic group having 1 to 6 carbon atoms. When the organic group (R) is monovalent, the structure of the boroxine compound is not stable, so that the reaction with the lithium metal composite oxide tends to be difficult. In addition, when the organic group (R) is trivalent, the insolubility of the boroxine compound becomes too high, so that the reaction with the lithium metal composite oxide becomes difficult. On the other hand, when the organic group (R) is divalent, the boroxine compound is not easily dissociated into boronic acid in the nonaqueous electrolytic solution, and it is advantageous in that appropriate solubility can be obtained. Among these compounds, tri-iso-propoxy boroxine (TiPBx) ((CH ₃ ) ₂ CHO) ₃ (BO) ₃ ), which has relatively good stability and solubility, is preferable. .

The boroxine compound represented by the general formula (III) can be synthesized by, for example, a condensation reaction of B (OR) ₃ and boric anhydride (B ₂ O ₃ ). Further, a compound having all three different alkyl groups such as B (OR ₁ ) (OR ₂ ) (OR ₃ ) as R constituting B (OR) ₃ is used, or only two different compounds are used. Alternatively, it is possible to synthesize boroxine compounds having different organic groups in one molecule by reacting them by changing the number of moles thereof.

  The addition amount of the boroxine compound is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, based on the total amount of the nonaqueous solvent and the supporting salt. Moreover, it is preferable to set it as 2.0 mass% or less with respect to the total amount of a nonaqueous solvent and a support salt, and it is more preferable to set it as 1.5 mass% or less. When a boroxine compound is added, lithium hexafluorophosphate added as a supporting salt may be lost due to the reaction with the boroxine compound and the conductivity of lithium ions may be impaired. Therefore, it is preferable to limit the upper limit of the addition amount in this way within the range of the normal addition amount of lithium hexafluorophosphate.

The reaction between the boroxine compound represented by the general formula (III) and lithium hexafluorophosphate proceeds rapidly at room temperature and normal pressure when the boroxine compound and lithium hexafluorophosphate are mixed in a solvent. Specifically by this reaction, anions (corresponding to atomic groups) such as PO ₂ F ₂ ⁻ and PO ₃ F ²⁻ and (RnO) POF ₂ (wherein Rn represents an organic group). ) And the like are produced. Therefore, the surface layer of the lithium metal composite oxide can be modified with the oxofluorophosphorus compound by adding the boroxine compound and lithium hexafluorophosphate to the non-aqueous electrolyte of the lithium secondary battery.

  Further, in the reaction of the boroxine compound represented by the general formula (III) and lithium hexafluorophosphate, a boroxine compound having a boron atom having a valence higher than trivalent is generated together with the oxofluorophosphorus compound. Specifically, this boroxine compound forms a fluoride of a boroxine compound represented by the general formula (III) (fluorinated boroxine compound). Since the fluorinated boroxine compound has a negatively charged boron atom, the dissociation of the supporting salt is stabilized by the interaction between the boron atom and the lithium ion. Further, the boroboroxine fluoride compound can generate inclusions due to the boron-containing compound on the surface of the lithium metal composite oxide.

FIG. 2A is a diagram showing an oxidation state of manganese on the surface layer of the lithium metal composite oxide. Moreover, FIG. 2B is a figure which shows the oxidation state of cobalt of the surface layer of lithium metal complex oxide. Moreover, FIG. 2C is a figure which shows the oxidation state of nickel of the surface layer of lithium metal complex oxide.
In FIG. 2A, FIG. 2B, and FIG. 2C, the X-ray photoelectron spectroscopy measurement was performed about the lithium metal complex oxide contained in a lithium secondary battery, and the result of having analyzed the electronic state of 2p orbital of each transition metal of surface layer is shown. Yes.

Specifically, the lithium secondary battery includes a ternary layered oxide represented by LiMnCoNiO ₂ as a lithium metal composite oxide (positive electrode active material) and an amorphous carbon on the surface of natural graphite as a negative electrode active material. A carbon material coated with bismuth, a mixed solution in which ethylene carbonate (EC) and ethylmethyl carbonate (EMC) are mixed at a volume ratio of 1: 2 as a nonaqueous solvent, and lithium hexafluorophosphate (LiPF ₆ ) as a supporting salt In an amount of 1.0 mol / L. The rated capacity of this lithium secondary battery is about 1600 mAh.

  2A, FIG. 2B, and FIG. 2C are the results (No. 1) in the lithium secondary battery in an uncharged state after being manufactured without adding an additive to the non-aqueous electrolyte. Further, the thin broken line shows the result (No. 2) in the lithium secondary battery that was prepared by adding vinylene carbonate, which is a negative electrode-side film-forming agent, to 1 part by mass to the non-aqueous electrolyte, and then slightly charged. ). In addition, the thin solid line is a result of the lithium secondary battery slightly charged after being prepared by adding 1 part by weight of vinylene carbonate and 1 part by weight of triisopropoxyboroxine to the nonaqueous electrolytic solution ( No. 3). The thick broken line shows the result of a lithium secondary battery that was prepared by adding 1 part by weight of vinylene carbonate and 5 parts by weight of triisopropoxyboroxine to the non-aqueous electrolyte solution, and then charging it slightly ( No. 4).

  In any test, the X-ray photoelectron spectroscopy measurement is performed with a beam width of 1 mm square or less, and incident X-rays are measured as 1200 eV soft X-rays, which are scattered on the surface layer of lithium metal composite oxide (up to 30 nm depth) The X-ray is selectively detected. The background of each measurement result is subtracted by the Shirley method, and the measurement value of the binding energy is corrected with reference to 285 eV of carbon (C1s). The charging of the lithium secondary batteries (No. 2 to No. 4) is constant voltage charging up to 6 mAh at a charging voltage of 4.2 V, and the measurement is performed on the positive electrode that is disassembled after the charging and recovered. Is going.

  As shown in FIG. 2A, regarding the manganese of the surface layer of the lithium metal composite oxide, in the uncharged lithium secondary battery (No. 1), the binding energy is in each of the region near 642 eV and the region near 654 eV. A strong signal indicating that the average oxidation number is tetravalent manganese has been detected. Moreover, the same signal is detected in the lithium secondary battery (No. 2) produced by adding vinylene carbonate. On the other hand, in the lithium secondary batteries (No. 3 and No. 4) prepared by adding triisopropoxyboroxine, these signals have a binding energy in a region around 644 eV and a region around 656 eV. It has shifted beyond. This indicates that manganese was oxidized to an average oxidation number higher than tetravalent, or manganese and fluorine with high electronegativity were combined, and the charge density on manganese decreased and shifted to a higher energy side. .

  On the other hand, as shown in FIG. 2B, for the cobalt of the surface layer of the lithium metal composite oxide, in the uncharged lithium secondary battery (No. 1), the binding energy is in the region near 780 eV and in the region near 796 eV. In each case, a strong signal indicating that the average oxidation number is trivalent cobalt is detected. Moreover, the same signal is detected in the lithium secondary battery (No. 2) produced by adding vinylene carbonate. On the other hand, in the lithium secondary batteries (No. 3 and No. 4) prepared by adding triisopropoxyboroxine, these signals have a binding energy in a region around 784 eV and a region around 797 eV. It has shifted beyond. As in the case of manganese, cobalt was oxidized to an average oxidation number higher than trivalent, or cobalt and fluorine with high electronegativity were combined to reduce the charge density on cobalt and shifted to a higher energy side. Is shown.

  On the other hand, as shown in FIG. 2C, regarding the nickel of the surface layer of the lithium metal composite oxide, in the uncharged lithium secondary battery (No. 1), the binding energy is in the region near 856 eV and in the region near 873 eV. In each case, a strong signal indicating that the average oxidation number is divalent nickel is detected. Moreover, the same signal is detected in the lithium secondary battery (No. 2) produced by adding vinylene carbonate. On the other hand, in the lithium secondary battery (No. 3, No. 4) produced by adding triisopropoxyboroxine, these signals have a binding energy in the region near 859 eV and the region near 878 eV. It has shifted beyond. As in the case of manganese and cobalt, nickel is oxidized to an average oxidation number higher than divalent, or nickel and fluorine with high electronegativity are combined to reduce the charge density on nickel and shift to a higher energy side. It shows that.

  As these results show, when a boroxine compound and lithium hexafluorophosphate are added to the non-aqueous electrolyte of a lithium secondary battery, the average oxidation number of transition metals present on the surface layer of the lithium metal composite oxide is expensive. It can be seen that it can be shifted to the several side. This action is due to the oxofluorophosphorus compound produced by the reaction between the boroxine compound and lithium hexafluorophosphate, and cannot be obtained with a film-forming agent such as vinylene carbonate. In addition, from the signal intensity, in the lithium secondary batteries (No. 3 and No. 4) prepared by adding triisopropoxyboroxine, the nonaqueous solvent decomposition deposited on the surface of the lithium metal composite oxide It is recognized that the thickness of the product is smaller than that of the lithium secondary battery (No. 2) produced by adding vinylene carbonate.

FIG. 3 is a diagram showing a mass analysis result in the vicinity of the surface of the lithium metal composite oxide. FIG. 3A shows the result of a lithium secondary battery produced without adding an additive to the non-aqueous electrolyte, and FIG. 3B shows the lithium secondary battery produced by adding vinylene carbonate to the non-aqueous electrolyte. As a result of the secondary battery, FIG. 3C shows a result of the lithium secondary battery prepared by adding triisopropoxyboroxine to the nonaqueous electrolytic solution. FIG. 3D shows a result of vinylene carbonate and nonaqueous electrolytic solution. The result in the lithium secondary battery produced by adding both triisopropoxyboroxine is shown.
3 (a) to 3 (d), time-of-flight secondary mass spectrometry (TOF-SIMS) is performed in a negative mode for a lithium metal composite oxide contained in a lithium secondary battery. The result of having conducted and analyzed the composition of the atomic group interposed in the surface vicinity of lithium metal complex oxide (positive electrode active material) is shown. The vertical axis in FIGS. 3A to 3D is a count per 0.0009 amu. Moreover, the lithium secondary battery contains lithium hexafluorophosphate (LiPF ₆ ) as a supporting salt, like the lithium secondary battery.

As shown in FIG. 3A, on the positive electrode surface of the lithium secondary battery produced without adding an additive to the non-aqueous electrolyte, C ₂ derived from the non-aqueous solvent is present in a region around a mass number of about 43. A strong signal of H ₃ O ions has been detected. Further, as shown in FIG. 3B, on the positive electrode surface of the lithium secondary battery prepared by adding vinylene carbonate to the non-aqueous electrolyte, in the same manner, in the region around the mass number of about 43, C ₂ H A strong signal of ₃ O ions has been detected.

On the other hand, as shown in FIG. 3C, on the positive electrode surface of the lithium secondary battery prepared by adding triisopropoxyboroxine to the nonaqueous electrolytic solution, C region has a mass of about 43. A signal of metaboric acid (BO ₂ ) ions is detected with a stronger intensity than ₂ H ₃ O ions. In addition, as shown in FIG. 3 (d), on the positive electrode surface of the lithium secondary battery prepared by adding both vinylene carbonate and triisopropoxyboroxine to the nonaqueous electrolytic solution, the mass number is about the same. A signal of metaboric acid (BO ₂ ) ions is detected in a region near 43 with a stronger intensity than that of C ₂ H ₃ O ions.

  As these results show, inclusion of a boron-containing compound derived from the boroxine compound is formed when a boroxine compound and lithium hexafluorophosphate are added to the non-aqueous electrolyte of the lithium secondary battery. . At this time, the non-aqueous solvent decomposition product deposited on the surface of the lithium metal composite oxide is relatively reduced, and it is recognized that the decomposition of the electrolytic solution is suppressed.

On the other hand, in the method of adding an oxofluorophosphorus compound and a boron compound to a non-aqueous electrolyte of a lithium secondary battery, the oxofluorophosphorus compound is a fluoro such as POF ₂ ⁻ , PO ₂ F ₂ ⁻ , PO ₃ F ^2−. An organophosphorus compound having an atomic group represented by phosphate anion, salts thereof, POF ₂ , PO ₂ F ₂ , PO ₃ F, or the like is added separately. As the oxofluorophosphorus compound to be added, a fluorophosphate anion or a salt thereof is particularly suitable. A single kind of oxofluoroline compound may be used, or a plurality of kinds may be used in combination.

  When using a fluorophosphate anion, the fluorophosphate anion may be dissolved in an aprotic non-aqueous solvent in advance. The non-aqueous solvent can be selected from the group of non-aqueous solvents used for the non-aqueous electrolyte. In particular, the fluorophosphate anion is preferably dissolved in the same non-aqueous solvent as that used for the non-aqueous electrolyte.

  Moreover, when using the salt of a fluorophosphate anion, it is preferable to use salts, such as an alkali metal, an alkaline-earth metal, and an earth metal. Examples of the alkali metal include lithium, sodium, potassium, cesium and the like. Examples of the alkaline earth metal include magnesium, calcium, strontium, barium and the like. In addition, examples of the earth metal include aluminum, gallium, indium, and thallium. As the salt of the fluorophosphate anion, transition metal salts such as iron, cobalt, and nickel may not be used from the viewpoint of avoiding unnecessary electrochemical reaction involving charge consumption, insertion into the active material, and the like. It is preferable to use a metal salt having a large valence.

  As boron compounds, lithium borate salts such as lithium bisoxalate borate and lithium difluorooxalate borate, boric acid esters such as trimethyl borate, boroxine compounds represented by the general formula (III), etc. may be added. Good. A single type of boron compound may be used, or a plurality of types may be used in combination.

  On the other hand, in the method in which the lithium metal composite oxide is previously treated with the oxofluorophosphorus compound at the time of manufacturing the positive electrode, the prepared lithium metal composite oxide particles are treated with the oxofluorophosphorus compound, and the positive electrode mixture containing the particles Is used to manufacture a positive electrode for a lithium secondary battery. This manufacturing method of a positive electrode for a lithium secondary battery is a method including a surface layer treatment step, a cleaning step, and an electrode formation step.

  In the surface layer treatment step, particles of lithium metal composite oxide (composite oxide) containing Li and at least one transition metal selected from the group consisting of Mn, Co, and Ni, an oxofluorophosphorus compound, and a solvent And the transition metal present in the surface layer of the lithium metal composite oxide (composite oxide) is brought into a highly oxidized state.

  The lithium metal composite oxide particles can be prepared according to a general method for synthesizing a positive electrode active material. For example, compounds containing Li, such as lithium carbonate, lithium hydroxide, lithium acetate, lithium nitrate, lithium chloride, lithium sulfate, and carbonates, hydroxides, acetates, nitrates, sulfates, oxides, etc. containing transition metals And a lithium metal composite oxide are synthesized using a solid phase method, a coprecipitation method, a sol-gel method, a hydrothermal method, or the like. Then, the synthesized product is appropriately crushed to form lithium metal composite oxide particles.

Examples of the oxofluorophosphorus compound used for the treatment of lithium metal composite oxide particles include the fluorophosphate anions such as POF ₂ ⁻ , PO ₂ F ₂ ⁻ , PO ₃ F ²⁻ , salts thereof, and POF _2. An organic phosphorus compound having an atomic group represented by PO ₂ F ₂ , PO ₃ F, or the like may be used. When using a fluorophosphate anion, salt thereof, etc., the lithium hexafluorophosphate is not consumed by the reaction between the boroxine compound and lithium hexafluorophosphate. There is an advantage that it is lost. As the fluorophosphate anion and its salt, lithium monofluorophosphate or lithium difluorophosphate is preferable from the viewpoint of not affecting the battery reaction.

  As an oxofluorophosphorus compound used for the treatment of lithium metal composite oxide particles, a reaction product of a boroxine compound represented by the above general formula (III) and lithium hexafluorophosphate may be used. When a reaction product of a boroxine compound and lithium hexafluorophosphate is used, a boron-containing compound derived from a fluorinated boroxine compound can be generated on the surface of the lithium metal composite oxide. It is possible to omit the addition of a boron compound performed after the production. As the boroxine compound, triisopropoxyboroxine having relatively good stability and solubility is preferable.

  The mixing amount of the oxofluorophosphorus compound with respect to the lithium metal composite oxide is preferably 4 parts by mass or less, and more preferably 3 parts by mass or less. When the mixing amount of the oxofluorophosphorus compound is 4 parts by mass or less, in the lithium secondary battery, a decrease in discharge capacity with time due to storage can be satisfactorily reduced. On the other hand, the mixing amount of the oxofluoroline compound is preferably 0.1 parts by mass or more, more preferably 0.25 parts by mass or more, and further preferably 1 part by mass or more. The mixing amount of the oxofluorophosphorus compound for modifying the surface layer of the lithium metal composite oxide may be excessive with respect to the particle surface in consideration of the amount of lithium metal composite oxide and the specific surface area. If it is 1 part by mass or more, a significant modification effect can be obtained.

  The solvent for mixing the lithium metal composite oxide particles and the oxofluorophosphorus compound may be either a protic non-aqueous solvent or an aprotic non-aqueous solvent. For example, an appropriate nonaqueous solvent such as methanol, ethanol, propanol, isopropanol, ethylene glycol, diethylene glycol, glycerin, dimethyl sulfoxide, N-methyl-2-pyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide or the like is used. be able to. As the mixing means, for example, an appropriate mixer such as a planetary mixer, a disper mixer, a rotation / revolution mixer, or the like can be used.

  The washing step is a step of washing and drying the lithium metal composite oxide (composite oxide) particles reacted with the oxofluorophosphorus compound. For example, the particles of the lithium metal composite oxide are washed with a solvent of the same type as the solvent used for mixing the lithium metal composite oxide and the oxofluorophosphorus compound, and unreacted substances and the like are removed. Thereafter, the solvent is removed by drying to obtain a lithium metal composite oxide having a modified surface layer.

  The electrode forming step is a step in which a positive electrode mixture containing particles of a lithium metal composite oxide (composite oxide) reacted with an oxofluorophosphorus compound is applied to the positive electrode current collector and then molded. The positive electrode mixture can be prepared, for example, by mixing lithium metal composite oxide particles, a binder, a conductive agent, and an appropriate solvent such as N-methyl-2-pyrrolidone (NMP). it can. Moreover, as a method for applying the positive electrode mixture, for example, a doctor blade method, a dipping method, a spray method, or the like can be used. A positive electrode mixture is applied to one or both sides of a positive electrode current collector, dried, and then compression molded to a predetermined electrode density to produce a positive electrode.

  As the binder, for example, an appropriate material such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polychlorotrifluoroethylene, polypropylene, polyethylene, acrylic polymer, or a copolymer thereof can be used. . As the conductive agent, for example, carbon particles such as graphite, carbon black, acetylene black, ketjen black, and channel black, carbon fibers, and the like can be used. The mixing ratio of the conductive agent is preferably 5% by mass or more and 20% by mass or less with respect to the positive electrode active material.

  As the positive electrode current collector, a metal foil, a metal plate, an expanded metal, a punching metal, or the like made of aluminum, stainless steel, or the like can be used. The thickness of the positive electrode current collector is preferably 15 μm or more and 25 μm or less. The metal foil may be produced by either a rolling method or an electrolytic method. In addition, the surface of the positive electrode current collector may be subjected to a surface treatment for improving oxidation resistance.

  The thickness of the positive electrode mixture layer formed by compression molding is preferably 50 μm or more and 250 μm or less, although it depends on the type and particle size of the lithium metal composite oxide, the performance required for the battery, and the like. Moreover, what is necessary is just to adjust the density of a positive mix layer according to the kind of material to be used, and the performance requested | required of a battery. In general, the lithium metal composite oxide is present in the positive electrode mixture layer in a state where secondary particles in which primary particles are aggregated are formed, but the particle size of the secondary particles is equal to the particle size of the primary particles. There is a tendency to depend. Therefore, the electrode density can be improved by optimizing the particle size and particle shape of the primary particles.

  The separator 11 is provided to prevent the positive electrode 10 and the negative electrode 12 from being in direct contact with each other and causing a short circuit. As the separator 11, a microporous film such as polyethylene, polypropylene, and aramid resin, a film in which the surface of such a microporous film is coated with a heat resistant material such as alumina particles, and the like can be used.

  The negative electrode 12 can be formed of a general negative electrode for a lithium ion secondary battery capable of inserting and extracting lithium ions. For example, the negative electrode 12 includes a negative electrode active material, a binder, and a negative electrode current collector. The negative electrode active material constituting the negative electrode may be any of a carbon material, a metal material, a composite compound material, and the like, for example. The negative electrode active material may be formed of one of these materials, or two or more of them may be used in combination.

  Examples of the carbon material constituting the negative electrode include natural graphite, coke derived from petroleum, coal or charcoal, or artificial crystalline carbon material obtained by high-temperature treatment of pitch at about 2500 ° C. or higher, and such coke. Alternatively, mesophase carbon obtained by low-temperature treatment of pitch, amorphous carbon material such as hard carbon, activated carbon, and the like can be given. Carbon materials include materials in which amorphous carbon is coated on the surface of crystalline carbon, materials in which the crystallinity of the surface of crystalline carbon is reduced by mechanical treatment, and organic polymers on the surface of crystalline carbon. Further, a material carrying boron, silicon or the like, carbon fiber, or the like may be used.

  Examples of the metal material constituting the negative electrode include metallic lithium and alloys of lithium and aluminum, tin, silicon, indium, gallium, magnesium, and the like. The metal material may be a material in which a metal such as lithium, aluminum, tin, silicon, indium, gallium, and magnesium or an alloy thereof is supported on the surface of the carbon material. In addition, examples of the composite compound material constituting the negative electrode include composite oxides of lithium and iron, zinc, copper, nickel, cobalt, manganese, titanium, silicon, and nitrides thereof.

  The negative electrode 12 can be obtained, for example, by mixing a negative electrode active material together with a binder with a binder to form a negative electrode mixture, applying the negative electrode mixture to a negative electrode current collector, and then drying and compression molding. it can. As a method for applying the negative electrode mixture, for example, a doctor blade method, a dipping method, a spray method, or the like can be used.

  The thickness of the negative electrode mixture layer formed by compression molding is preferably 50 μm or more and 200 μm or less, although it depends on the type and particle size of the negative electrode active material, the performance required for the battery, and the like. Moreover, what is necessary is just to adjust the density of a negative electrode compound-material layer according to the kind of material to be used, and the performance requested | required of a battery. For example, when a general graphite electrode is formed, it is preferably 1.3 g / cc or more and 1.8 g / cc or less. On the other hand, when the electrode is formed of a carbon material having low crystallinity, it is preferably 1.0 g / cc or more and 1.3 g / cc or less.

  As the binder, an appropriate material such as an aqueous binder such as carboxymethyl cellulose and a styrene-butadiene copolymer, or an organic binder such as polyvinylidene fluoride (PVDF) can be used. The amount of the aqueous binder is preferably 0.8% by mass or more and 1.5% by mass or less per solid content of the negative electrode mixture. On the other hand, the amount of the organic binder is preferably 3% by mass or more and 6% by mass or less per solid content of the negative electrode mixture.

  As the negative electrode current collector, a metal foil, a metal plate, an expanded metal, a punching metal, or the like made of copper, a copper alloy containing copper as a main component, or the like can be used. The thickness of the negative electrode current collector is preferably 7 μm or more and 20 μm or less. The metal foil may be produced by either a rolling method or an electrolytic method. Further, the surface of the negative electrode current collector may be subjected to a surface treatment for improving oxidation resistance.

  According to the lithium secondary battery 1 described above, since the transition metal present in the surface layer of the lithium metal composite oxide is in a highly oxidized state, the elution of the positive electrode active material (lithium metal composite oxide) and the positive electrode and non-aqueous electrolysis are performed. Occurrence of oxidative decomposition of the nonaqueous solvent occurring at the interface with the liquid is reduced. In addition, since a boron-containing compound is present on the surface of the positive electrode active material (lithium metal composite oxide), contact between the positive electrode and the non-aqueous electrolyte is prevented, and the occurrence of oxidative decomposition of the non-aqueous solvent is even better. To be reduced. In addition, the intervening boron-containing compound does not significantly inhibit the conduction of lithium ions compared to metal oxides and the like, and continues to accumulate excessively with charge and discharge, such as decomposition products of nonaqueous solvents. There are few things. Therefore, it is possible to reduce a decrease in discharge capacity over time associated with storage in a high charge depth state, a high temperature state, or a high temperature environment while suppressing internal resistance of the lithium secondary battery.

  EXAMPLES Hereinafter, although an Example is shown and this invention is demonstrated concretely, the technical scope of this invention is not limited to this.

  As an example of the present invention, a lithium secondary battery including a lithium metal composite oxide treated with an oxofluorophosphorus compound as a positive electrode active material was produced, and the high-temperature storage characteristics of the lithium secondary battery were evaluated.

The positive electrode of the lithium secondary battery was produced using a lithium metal composite oxide (layered oxide) represented by LiMn _0.33 Co _0.33 Ni _0.33 O ₂ as a positive electrode active material. The positive electrode active material used has an average particle diameter of 10 μm and a specific surface area of 0.8 m ² / g. As the conductive agent, a mixture in which massive graphite and acetylene black were mixed at a mass ratio of 9: 2 was used. As the binder, polyvinylidene fluoride (PVDF) was used. PVDF was used by dissolving in advance in N-methyl-2-pyrrolidone (NMP) to a concentration of 5% by mass. As the positive electrode current collector, an aluminum foil having a thickness of 20 μm was used.

The positive electrode was produced according to the following procedure. First, the synthesized lithium metal composite oxide particles, an oxofluorophosphorus compound, and a solvent were mixed to bring the surface layer of the lithium metal composite oxide into a highly oxidized state. Methanol was used as the solvent. As the oxofluorophosphorus compound, lithium difluorophosphate (LiPO ₂ F ₂ ) was used. The lithium metal composite oxide and the oxofluorophosphorus compound were reacted by continuing to stir all day and night. Then, the solution after the reaction was filtered, and the recovered lithium metal composite oxide was washed with methanol and then dried to obtain a positive electrode active material.

  Subsequently, a positive electrode active material, a conductive agent, and PVDF were mixed at a mass ratio of 85: 10: 5 to obtain a slurry-like positive electrode mixture. Next, the obtained positive electrode mixture was uniformly applied to the positive electrode current collector and dried at 80 ° C. The positive electrode current collector was coated with a positive electrode mixture on both sides in the same procedure. Then, the positive electrode mixture coated on both surfaces of the positive electrode current collector was compression-molded by a roll press and cut so that the application width of the positive electrode mixture was 5.4 cm and the application length was 50 cm. Thereafter, a positive electrode current collector tab made of aluminum foil was welded to the cut positive electrode current collector to obtain a positive electrode for a lithium secondary battery.

The negative electrode of the lithium secondary battery was produced using natural graphite as the negative electrode active material. The natural graphite used had an average particle size of 20 μm, a specific surface area of 5.0 m ² / g, and a surface spacing of 0.368 nm. As the binder, carboxymethyl cellulose and a styrene-butadiene copolymer were used. The carboxymethyl cellulose and styrene-butadiene copolymer were used by being dispersed in water in advance. As the negative electrode current collector, a rolled copper foil having a thickness of 10 μm was used.

  The negative electrode was produced according to the following procedure. First, a negative electrode active material, carboxymethyl cellulose, and a styrene-butadiene copolymer were mixed at a mass ratio of 98: 1: 1 to obtain a slurry-like negative electrode mixture. Next, the obtained negative electrode mixture was uniformly applied to a negative electrode current collector and dried. The negative electrode current collector was coated with a negative electrode mixture on both sides in the same procedure. Then, the negative electrode mixture coated on both surfaces of the negative electrode current collector was compression-molded by a roll press, and cut so that the application width of the negative electrode mixture was 5.6 cm and the application length was 54 cm. Thereafter, a negative electrode current collector tab of copper foil was welded to the cut negative electrode current collector to obtain a negative electrode for a lithium secondary battery.

  The lithium secondary battery was in the form of a cylinder shown in FIG. Specifically, the produced positive electrode and negative electrode are overlapped with a polyethylene separator interposed between them and wound in a spiral shape, and stored in a cylindrical battery can having a diameter of 18 mm and a length of 650 mm. It was prepared by pouring a water electrolyte and caulking the battery lid. In addition, as the lithium secondary battery, a plurality of test batteries (test batteries 1 to 14) in which the mixed amount of the oxofluorophosphorus compound and the composition of the non-aqueous electrolyte were different were prepared.

The non-aqueous electrolyte was prepared by adding different amounts of oxofluorophosphorus compound to each of the plurality of test batteries (test batteries 1 to 14) with respect to the non-aqueous solvent and the supporting salt. As the non-aqueous solvent, a mixed solution in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1: 2 was used. As the supporting salt, lithium hexafluorophosphate (LiPF ₆ ) was used in an amount of 1.0 mol / L. To each non-aqueous electrolyte, trimethyl borate or vinylene carbonate was added as an additive in the following combinations.

[Test battery 1]
As the test battery 1, a lithium secondary battery in which the mixed amount of the oxofluorophosphorus compound was 1 part by mass with respect to the lithium metal composite oxide and no additive was added to the nonaqueous electrolytic solution was produced.

[Test battery 2]
As the test battery 2, a lithium secondary battery in which the mixed amount of the oxofluorophosphorus compound was 3 parts by mass with respect to the lithium metal composite oxide and no additive was added to the nonaqueous electrolytic solution was produced.

[Test battery 3]
As the test battery 3, a lithium secondary battery in which the mixed amount of the oxofluorophosphorus compound was 0.5 parts by mass with respect to the lithium metal composite oxide and no additive was added to the nonaqueous electrolytic solution was produced.

[Test battery 4]
As the test battery 4, a lithium secondary battery in which the amount of oxofluorophosphorus compound mixed was 0.25 parts by mass with respect to the lithium metal composite oxide and no additive was added to the non-aqueous electrolyte was produced.

[Test battery 5]
As the test battery 5, a lithium secondary battery in which the mixed amount of the oxofluorophosphorus compound is 1 part by mass with respect to the lithium metal composite oxide and vinylene carbonate is added to the non-aqueous electrolyte in an amount of 1 part by mass is produced. did.

[Test battery 6]
As the test battery 6, a lithium secondary battery in which the mixed amount of the oxofluorophosphorus compound is 1 part by mass with respect to the lithium metal composite oxide and vinylene carbonate is added in an amount of 2 parts by mass to the non-aqueous electrolyte is produced. did.

[Test battery 7]
As the test battery 7, a lithium secondary battery in which the mixed amount of the oxofluorophosphorus compound is 1 part by mass with respect to the lithium metal composite oxide and the boric acid ester is added in an amount of 1 part by mass to the nonaqueous electrolyte Produced.

[Test battery 8]
As the test battery 8, a lithium secondary battery in which the mixed amount of the oxofluorophosphorus compound is 1 part by mass with respect to the lithium metal composite oxide and the boric acid ester is added in an amount of 2 parts by mass to the non-aqueous electrolyte. Produced.

[Test battery 9]
As the test battery 9, the mixing amount of the oxofluorophosphorus compound is 1 part by mass with respect to the lithium metal composite oxide, the boric acid ester is 1 part by mass in the non-aqueous electrolyte, and the vinylene carbonate is 1 part by mass. Lithium secondary batteries added in respective amounts were prepared.

[Test battery 10]
In the test battery 10, the mixing amount of the oxofluorophosphorus compound is 1 part by mass with respect to the lithium metal composite oxide, the non-aqueous electrolyte is 1 part by mass of boric acid ester, and 2 parts by mass of vinylene carbonate. Lithium secondary batteries added in respective amounts were prepared.

[Test battery 11]
As the test battery 11, a lithium secondary battery in which no additive was added to the non-aqueous electrolyte was produced using a lithium metal composite oxide that was not treated with an oxofluorophosphorus compound.

[Test battery 12]
As the test battery 12, a lithium secondary battery in which the mixed amount of the oxofluorophosphorus compound was 5 parts by mass with respect to the lithium metal composite oxide and no additive was added to the nonaqueous electrolytic solution was produced.

[Test battery 13]
As the test battery 13, a lithium secondary battery in which vinylene carbonate was added in an amount of 1 part by mass to a non-aqueous electrolyte using a lithium metal composite oxide not treated with an oxofluorophosphorus compound was produced.

[Test battery 14]
As the test battery 14, a lithium secondary battery in which a boric acid ester was added in an amount of 1 part by mass to a non-aqueous electrolyte using a lithium metal composite oxide that was not treated with an oxofluorophosphorus compound was produced.

  Next, the high-temperature storage characteristics of each manufactured lithium secondary battery were evaluated. For high-temperature storage characteristics, measure the initial discharge capacity before high-temperature storage and the discharge capacity after high-temperature storage according to the following procedure, and calculate the ratio of the discharge capacity after storage to the initial discharge capacity (capacity maintenance ratio). It was evaluated by.

  Specifically, first, the lithium secondary battery was charged at a constant current and low voltage for 5 hours at a charging current of 1500 mA and a charging voltage of 4.2 V in a thermostatic chamber kept at 25 ° C. Next, a constant current was discharged to a final voltage of 3.0 V at a discharge current of 1500 mA. And charging / discharging of a total of 3 cycles was repeated on the conditions similar to this charging / discharging conditions. At this time, the discharge capacity at a discharge current of 1500 mA measured in the third cycle was defined as the initial discharge capacity.

  Subsequently, the lithium secondary battery whose initial discharge capacity was measured was stored at a high temperature. All of the storage conditions were a battery voltage of 4.2 V, an environmental temperature of 50 ° C., and a storage time of 60 days. After 60 days, the lithium secondary battery was transferred to a thermostat kept at 25 ° C. and left to stand for 10 hours or more to remove heat, and then measured for discharge capacity after high-temperature storage under the same charge / discharge conditions. Went.

  Table 1 shows the results of calculating the capacity retention rate (%) of the discharge capacity after high-temperature storage with respect to the initial discharge capacity. In Table 1, POxFy is the amount (wt%) of the oxofluorophosphorus compound (POxFy) mixed in the lithium metal composite oxide, and “-” indicates that each component is not added.

  In the test cells 1 to 10 including the lithium metal composite oxide treated with the oxofluorophosphorus compound, the average oxidation number in the non-charged state of the transition metal contained in the lithium metal composite oxide is higher than tetravalent for Mn, Co was higher than trivalent, and Ni was higher than divalent. On the other hand, in the test battery 11 including the lithium metal composite oxide not treated with the oxofluorophosphorus compound, the average oxidation number of the transition metal contained in the lithium metal composite oxide in the non-charged state is Mn, Co, or Ni. All were low compared with the test batteries 1-10. As shown in Table 1, it can be confirmed that the capacity retention rate after high-temperature storage is improved in the test batteries 1 to 10 as compared with the test battery 11. It can be said that this result shows that the elution of transition metals and the decomposition of the nonaqueous solvent are suppressed by using a highly oxidized lithium metal composite oxide as the positive electrode active material.

  In each of the test batteries 1 to 10 including the lithium metal composite oxide treated with the oxofluorophosphorus compound, the AC internal resistance measured after high-temperature storage is particularly reduced in the frequency region near 1 Hz. It was recognized that This result indicates that the oxofluorophosphorus compound may contribute to the reduction of charge transfer resistance at the positive electrode side interface. Therefore, by treating the lithium metal composite oxide with the oxofluorophosphorus compound in advance during the production of the positive electrode, the surface layer of the lithium metal composite oxide can be reduced as in the case where the oxofluorophosphorus compound is added to the non-aqueous electrolyte. It can be judged that it has been modified.

  In addition, as shown in Test Battery 1 to Test Battery 4 and Test Battery 11, the capacity retention rate tends to increase as the mixing amount of the oxofluorophosphorus compound with respect to the lithium metal composite oxide increases. . On the other hand, as shown in the test battery 12, when the mixing amount of the oxofluorophosphorus compound is 5 parts by mass with respect to the lithium metal composite oxide, In comparison, the capacity maintenance rate is low. As a result, there is an appropriate range for the mixing amount of the oxofluorophosphorus compound. When the oxofluorophosphorus compound is excessive, the lithium ion reactivity in the electrode reaction is impaired, and the lithium metal composite having a high oxidation state is lost. This indicates that oxide may not be obtained. Therefore, the mixing amount of the oxofluoroline compound with respect to the lithium metal composite oxide is preferably about 4 parts by mass or less, and more preferably 3 parts by mass or less.

  Moreover, as the test battery 5 to the test battery 6 and the test battery 13 show, the capacity | capacitance maintenance factor tends to become high in the lithium secondary battery which added vinylene carbonate to the nonaqueous electrolyte. This result shows that the reductive decomposition of the non-aqueous solvent generated on the negative electrode side is suppressed by vinylene carbonate. The action on the positive electrode side caused by the treatment with the oxofluorophosphorus compound and the action on the negative electrode side by vinylene carbonate This shows that the decomposition of the electrolyte is satisfactorily suppressed. The degree of suppression in the test battery 5 and the test battery 6 is larger than that of the test battery 1 and the test battery 13, and it can be seen that a synergistic effect is achieved.

  Moreover, as the test battery 7 to the test battery 8 and the test battery 14 show, the capacity | capacitance maintenance factor tends to become high in the lithium secondary battery which added the trimethyl borate to the non-aqueous electrolyte. This result indicates the possibility that the lithium metal composite oxide treated with the oxofluorophosphorus compound reacted with trimethyl borate to form inclusions on the surface of the lithium metal composite oxide that suppress the decomposition of the non-aqueous solvent. ing. Therefore, it can be said that by using an oxofluorophosphorus compound in combination with a boron compound, various film forming agents, and the like, it is possible to further reduce a decrease in discharge capacity with time.

DESCRIPTION OF SYMBOLS 1 Lithium secondary battery 10 Positive electrode 11 Separator 12 Negative electrode 13 Battery can 14 Positive electrode current collection tab 15 Negative electrode current collection tab 16 Inner cover 17 Internal pressure release valve 18 Gasket 19 Positive temperature coefficient resistance element 20 Battery cover 21 Axis center

Claims (14)

The following general formula (I)
_{Li 1 + x Mn a Co b} Ni c M1 y O 2 ··· (I)
[In the formula, M1 is at least one element selected from the group consisting of Fe, Cu, Al, Mg, Mo, Zr, and 0 ≦ x ≦ 0.33, 0 ≦ a ≦ 1.0, 0 ≦ b ≦ 1.0, 0 ≦ c ≦ 1.0, 0 ≦ y ≦ 1.0, and a + b + c + y = 1 are satisfied. A composite oxide represented by
The transition metal present in the surface layer of the composite oxide has an average oxidation number in a non-charged state, tetravalent for Mn, trivalent for Co, and higher than divalent for Ni,
A positive electrode for a lithium secondary battery, wherein a boron-containing compound is present on the surface of the composite oxide. Comprising a composite oxide containing Li and at least one transition metal selected from the group consisting of Mn, Co and Ni;
The transition metal present in the surface layer of the composite oxide has bonded a fluorine atom,
A positive electrode for a lithium secondary battery, wherein a boron-containing compound is present on the surface of the composite oxide.   The positive electrode for a lithium secondary battery according to claim 1, wherein the composite oxide contains Mn. A positive electrode, a negative electrode, and a non-aqueous electrolyte,
The positive electrode has the following general formula (I)
_{Li 1 + x Mn a Co b} Ni c M1 y O 2 ··· (I)
[In the formula, M1 is at least one element selected from the group consisting of Fe, Cu, Al, Mg, Mo, Zr, and 0 ≦ x ≦ 0.33, 0 ≦ a ≦ 1.0, 0 ≦ b ≦ 1.0, 0 ≦ c ≦ 1.0, 0 ≦ y ≦ 1.0, and a + b + c + y = 1 are satisfied. A composite oxide represented by
The transition metal present in the surface layer of the composite oxide has an average oxidation number in a non-charged state, tetravalent for Mn, trivalent for Co, and higher than divalent for Ni,
A lithium secondary battery, wherein a boron-containing compound is present on the surface of the composite oxide. A positive electrode, a negative electrode, and a non-aqueous electrolyte,
The positive electrode comprises a composite oxide containing Li and at least one transition metal selected from the group consisting of Mn, Co and Ni;
The transition metal present in the surface layer of the composite oxide has bonded a fluorine atom,
A lithium secondary battery, wherein a boron-containing compound is present on the surface of the composite oxide.   The lithium secondary battery according to claim 4, wherein the composite oxide contains Mn. In the non-aqueous electrolyte, the following general formula (III):
(RO) ₃ (BO) ₃ (III)
[Wherein, each R is independently an organic group having 1 to 6 carbon atoms. ]
The lithium secondary battery according to claim 4, wherein a boroxine compound represented by the formula:   The lithium secondary battery according to claim 4, wherein a boric acid ester is added to the nonaqueous electrolytic solution.   The lithium secondary battery according to claim 4 or 5, wherein vinylene carbonate is added to the non-aqueous electrolyte. A composite oxide particle containing Li and at least one transition metal selected from the group consisting of Mn, Co, and Ni, an oxofluorophosphorus compound, and a solvent are mixed to form a surface layer of the composite oxide. Bringing the transition metal present in the substrate into a highly oxidized state;
Washing and drying the composite oxide particles;
A method for producing a positive electrode for a lithium ion secondary battery, comprising: applying a positive electrode mixture containing particles of the composite oxide to a positive electrode current collector and then forming the positive electrode current collector. 11. The lithium ion secondary according to claim 10, wherein the oxofluoroline compound is POF ₂ ⁻ or a salt thereof, and PO ₂ F ₂ ^— or a salt thereof, or PO ₃ F ^{2 —} or a salt thereof. A method for producing a positive electrode for a battery.   The method for producing a positive electrode for a lithium ion secondary battery according to claim 11, wherein the oxofluorophosphorus compound is lithium monofluorophosphate or lithium difluorophosphate. The oxofluoroline compound has the following general formula (III):
(RO) ₃ (BO) ₃ (III)
[Wherein, each R is independently an organic group having 1 to 6 carbon atoms. ]
The method for producing a positive electrode for a lithium ion secondary battery according to claim 10 or 11, wherein the reaction product is a reaction product of a boroxine compound represented by the formula (1) and lithium hexafluorophosphate.   The method for producing a positive electrode for a lithium ion secondary battery according to claim 13, wherein the oxofluorophosphorus compound is triisopropoxyboroxine.

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