JP2013012387A - Electrolyte and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress deterioration due to oxidation reductive decomposition of solvent or the like in electrolyte, in a lithium ion secondary battery using a positive pole active substance formed from lithium manganese-based oxide which contains lithium (Li) element and tetravalent manganese (Mn) element and which has a crystal structure belonging to a layer-shaped rock-salt structure.SOLUTION: By adding thiophene into electrolyte, additive agent is oxidized earlier than when organic solvent is oxidized on a positive electrode surface during activation treatment or high-temperature storage and a coating is considered to be formed, thus increasing capacity recovery factor.

Description

  The present invention relates to an electrolytic solution used for a lithium ion secondary battery and a lithium ion secondary battery using the electrolytic solution.

In recent years, along with the development of portable electronic devices such as mobile phones and notebook computers, and the practical application of electric vehicles, secondary batteries with small and light weight and high capacity are required. At present, lithium ion secondary batteries using lithium cobaltate (LiCoO ₂ ) as a positive electrode material and a carbon-based material as a negative electrode material have been commercialized as high capacity secondary batteries that meet this requirement. Such a lithium ion secondary battery has a high energy density and can be reduced in size and weight, and thus has attracted attention as a power source in a wide range of fields. However, since LiCoO ₂ is manufactured using Co, which is a rare metal, as a raw material, it is expected that resource shortages will become serious in the future. Furthermore, since Co is expensive and has a large price fluctuation, it has been desired to develop a cathode material that is inexpensive and has a stable supply.

Therefore, the use of a lithium manganese oxide-based composite oxide containing manganese (Mn), whose constituent elements are inexpensive and whose supply is stable, is considered promising. Among them, a substance called Li ₂ MnO ₃ that contains only tetravalent manganese ions and does not contain trivalent manganese ions that cause manganese elution during charge and discharge has attracted attention. Li ₂ MnO ₃ has been thought to be impossible to charge and discharge until now, but recent research has found that it can be charged and discharged by charging to 4.8V. However, Li ₂ MnO ₃ needs further improvement with respect to charge / discharge characteristics.

Development of xLi ₂ MnO ₃・ (1-x) LiMeO ₂ (0 <x ≦ 1), which is a solid solution of Li ₂ MnO ₃ and LiMeO ₂ (Me is a transition metal element), has been actively developed to improve charge / discharge characteristics. It is. Note that Li ₂ MnO ₃ can also be expressed as a general formula Li (Li _0.33 Mn _0.67 ) O ₂ and belongs to the same crystal structure as LiMeO ₂ . Therefore, xLi ₂ MnO ₃ · (1-x) LiMeO ₂ may also be described as Li _1.33-y Mn _0.67-z Me _{y + z} O ₂ (0 ≦ y <0.33, 0 ≦ z <0.67) .

  However, a lithium ion secondary battery using a lithium manganese oxide composite oxide containing tetravalent manganese ions as a positive electrode active material needs to be activated prior to use to be activated. In this activation step, there was a phenomenon that lithium ions were released from the lithium manganese oxide-based positive electrode active material, oxygen was desorbed, and the electrolyte solution was oxidized and decomposed by the oxygen. Further, even when stored in a charged state in a high-temperature storage test, there was a phenomenon that the electrolyte solution and the like decomposed on the positive electrode surface. When the electrolytic solution or the like is oxidatively decomposed as described above, an insulating film is formed on the electrode surface, and the internal resistance is increased, resulting in a problem that the charge / discharge capacity after storage is reduced.

In addition, in a lithium ion secondary battery using a carbon material such as graphite as the negative electrode active material, the organic solvent in the electrolytic solution is reduced and decomposed on the negative electrode surface during charging, and an insulating film called SEI (Solid Electrolyte Interface) is formed in the negative electrode. Formed on the surface. This SEI is mainly composed of LiF, LiCO _{3 and the} like, and these are irreversible materials, and the amount of lithium available for charging / discharging decreases, resulting in an irreversible capacity.

Therefore, for example, Patent Document 1 describes that in a lithium ion secondary battery using LiMn ₂ O ₄ , LiMnO ₂ or the like as a positive electrode active material, thiophene is added to the electrolytic solution, and the negative electrode surface is protected by the addition of thiophene. It is described that the interface impedance of the negative electrode can be lowered by making the film dense.

  For example, Patent Document 2 describes that by adding thiophene to the electrolyte of a lithium ion secondary battery, a film is formed on the surface of the negative electrode and irreversible reaction occurring between the negative electrode and the electrolyte is prevented. ing.

Further, for example, in Patent Document 3, in a lithium ion secondary battery using LiMn ₂ O ₄ or the like as a positive electrode active material, it is described that thiophene is added to an electrolytic solution, and a film is formed on the positive electrode by the addition of thiophene, It is described that transition metals such as Mn can be prevented from eluting from the positive electrode into the electrolyte.

JP 2002-184462 A JP 2002-008720 A JP 2000-090970 A

  However, none of the above-mentioned Patent Documents 1-3 uses a positive electrode active material that generates oxygen by an activation treatment, and suppresses decomposition of an electrolyte solution or the like on the surface of the positive electrode using such a positive electrode active material. It was difficult to do. Moreover, since the negative electrode active material uses a carbon material such as graphite, deterioration due to generation of SEI is inevitable.

  The present invention has been made in view of such circumstances, and in a lithium ion secondary battery using a positive electrode active material that expresses a high capacity even though activation treatment is required, the charge / discharge capacity after storage is reduced. It is a problem to be solved.

A feature of the electrolytic solution of the present invention that solves the above problems is that it has a positive electrode active material comprising a lithium manganese-based oxide that contains a lithium (Li) element and a tetravalent manganese (Mn) element and whose crystal structure belongs to a layered rock salt structure. An electrolyte used in a lithium ion secondary battery having a positive electrode,
It contains a solvent or liquid dispersion medium, an electrolyte dissolved or dispersed in the solvent or liquid dispersion medium, and an additive selected from thiophene and thiophene derivatives dissolved in the solvent or liquid dispersion medium.

  In addition, the lithium ion secondary battery of the present invention using the electrolytic solution of the present invention is characterized by the electrolyte solution of the present invention and a layered rock salt structure including a lithium (Li) element and a tetravalent manganese (Mn) element. A positive electrode having a positive electrode active material composed of a lithium manganese oxide belonging to the above and a negative electrode.

  The electrolytic solution of the present invention contains a solvent or liquid dispersion medium, an electrolyte dissolved or dispersed in the solvent or liquid dispersion medium, and an additive selected from thiophene and thiophene derivatives dissolved in the solvent or liquid dispersion medium. .

  This additive has a smaller absolute value of HOMO than an organic solvent generally used as an electrolytic solution, and is easily oxidized. Therefore, during the activation process and during high temperature storage, the additive is preferentially oxidized over the organic solvent, suppresses the oxidation of the organic solvent, and is stable on the positive electrode active material composed of a lithium manganese oxide belonging to a layered rock salt structure. It is thought that the coated film is formed. Moreover, since the film formed from the additive chosen from thiophene and a thiophene derivative has electroconductivity, the influence on a battery reaction can be made small.

  Furthermore, this additive has a smaller absolute value of LUMO than an organic solvent generally used as an electrolytic solution, and is easily reduced. Therefore, it is reduced before the organic solvent on the negative electrode surface during charging. Therefore, it is considered that reduction of the organic solvent is suppressed, and as a result, the generation of excessive SEI is suppressed.

  That is, by using an electrolytic solution containing an additive selected from thiophene and thiophene derivatives, the storage characteristics of the lithium ion secondary battery are improved and the capacity recovery rate is increased.

  The electrolytic solution of the present invention contains a solvent or liquid dispersion medium, an electrolyte dissolved or dispersed in the solvent or liquid dispersion medium, and an additive selected from thiophene and thiophene derivatives dissolved in the solvent or liquid dispersion medium. .

  As the solvent or liquid dispersion medium, an organic solvent is generally used. The organic solvent is not particularly limited, but preferably contains a chain ester from the viewpoint of load characteristics. Examples of such chain esters include chain carbonates typified by dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate, and organic solvents such as ethyl acetate and methyl propionate. These chain esters may be used alone or in admixture of two or more. Particularly, for improving low-temperature characteristics, the above-mentioned chain esters occupy 50% by volume or more in the total organic solvent. In particular, it is preferable that the chain ester occupies 65% by volume or more of the total organic solvent.

  However, as an organic solvent, in order to improve the discharge capacity rather than using only the above chain ester, an ester having a high dielectric constant (dielectric constant: 30 or more) should be mixed with the chain ester. Is preferred. Specific examples of such esters include, for example, cyclic carbonates represented by ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, γ-butyrolactone, ethylene glycol sulfite, and the like. A cyclic ester such as carbonate is preferred. Such an ester having a high dielectric constant is preferably contained in an amount of 10% by volume or more, particularly 20% by volume or more in the total organic solvent from the viewpoint of discharge capacity. Further, from the viewpoint of load characteristics, 40% by volume or less is preferable, and 30% by volume or less is more preferable.

  As the electrolyte, a non-aqueous electrolyte dissolved in an organic solvent is generally used, but a gel-like solid electrolyte in which a liquid dispersion medium is dispersed can also be used. In some cases, it is possible to include water as the liquid dispersion medium.

As the electrolyte to be dissolved in an organic solvent, for _{example, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiCF 3 CO 2, Li 2 C 2 F 4 ( SO ₃ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2) are used alone or in combination of two or more. Among these, LiPF ₆ and LiC ₄ F ₉ SO ₃ that can obtain good charge / discharge characteristics are preferably used.

The concentration of the electrolyte in the electrolytic solution is not particularly limited, but is preferably about 0.3 to 1.7 mol / dm ³ , particularly about 0.4 to 1.5 mol / dm ³ .

  Moreover, in order to improve the safety | security and storage characteristic of a battery, you may contain an aromatic compound in electrolyte solution. As the aromatic compound, benzenes having an alkyl group such as cyclohexylbenzene or t-butylbenzene, biphenyl, or fluorobenzenes are preferably used.

  The additive thiophene derivatives include 3-methylthiophene, 3-ethylthiophene, 3-propylthiophene, 3-butylthiophene, 3-pentylthiophene, 3-hexylthiophene, 3-heptylthiophene, 3-n-octylthiophene, 3-nonylthiophene, 3-decylthiophene, 3-undecylthiophene, 3-dodecylthiophene, 3-octadecylthiophene, 2-acetylthiophene, 2-acetyl-3-methylthiophene, 2-acetyl-5-methylthiophene, 2 , 3-dimethylthiophene, 2,5-dimethylthiophene, 2-bromo-3-methylthiophene, ethyl 3-thiopheneacetate, 3-methyl-2-thiophenecarboxylic acid, 2-amino-4,5-dimethyl-3- It can be selected from ethyl thiophenecarboxylate, 2-nitrothiophene, 2,3-dinitrothiophene, 3-aminothiophene and the like.

  This additive must be dissolved in an organic solvent, and addition beyond the solubility should be avoided. The addition amount of the additive in the electrolytic solution varies depending on the type of the additive and the organic solvent, but is desirably in the range of 0.01% by mass to less than 2.0% by mass. If the additive is less than 0.01% by mass, it is difficult to achieve the added effect, and if it is 2.0% by mass or more, the effect is lowered and the internal resistance of the lithium ion secondary battery is increased.

  Table 1 shows the calculated HOMO and LUMO energy of the additive and the organic solvent. The calculation method was based on AM1.

  Since the additive has a smaller HOMO absolute value and a smaller LUMO absolute value than the organic solvent, it can be seen that the additive is easily oxidized on the positive electrode during charging and is easily reduced on the negative electrode.

  The electrolytic solution of the present invention can have the same structure as the conventional one except that it contains the above-mentioned organic solvent and an additive selected from thiophene and thiophene derivatives, in which a lithium metal salt as an electrolyte is dissolved It can be.

  The lithium ion secondary battery of the present invention mainly includes a positive electrode, a negative electrode, and an electrolytic solution of the present invention. Moreover, the separator pinched | interposed between a positive electrode and a negative electrode is provided similarly to a general lithium ion secondary battery.

The positive electrode includes a positive electrode active material including a lithium (Li) element and a tetravalent manganese (Mn) element, and a lithium manganese-based oxide whose crystal structure belongs to a layered rock salt structure. This positive electrode active material has a composition formula: xLi ₂ M ₁ O ₃ · (1-x) LiM ₂ O ₂ (0 ≦ x ≦ 1), and M ₁ is one or more kinds in which tetravalent Mn is essential. The basic composition is a lithium manganese-based oxide represented by a metal element, M ₂ being two or more metal elements in which tetravalent Mn is essential. Needless to say, composite oxides that slightly deviate from the above composition formula due to defects of Li, M ₁ , M _2, or O (oxygen) inevitably occurring are also included. Due to the presence of Mn that is less than tetravalent, the average oxidation number of Mn in the entire composite oxide obtained is allowed to be 3.8 to 4 valence. As the metal element other than tetravalent Mn in M ₁ and M _2, at least one selected from the group consisting of Cr, Fe, Co, Ni, Al, and Mg can be used.

  This positive electrode active material includes at least a metal compound raw material containing one or more metal elements essential for Mn and a theory of Li contained in the target composite oxide containing lithium hydroxide and substantially free of other compounds. Manufactured by performing a raw material mixture preparation step that mixes a molten salt raw material containing Li exceeding the composition to prepare a raw material mixture, and a melting reaction step in which the raw material mixture is melted and reacted at the melting salt raw material or higher. can do. By using a molten salt of lithium hydroxide, a lithium manganese oxide containing Li and tetravalent Mn and belonging to a layered rock salt structure is synthesized as a main product.

  Then, the raw material mixture is heated to a temperature higher than the melting point of lithium hydroxide, and the raw material mixture is reacted in a molten salt, whereby a particulate composite oxide is obtained. This is because the raw material mixture is alkali-melted and mixed uniformly in the molten salt. In addition, by reacting in a molten salt consisting essentially of lithium hydroxide, crystal growth is suppressed even when the reaction temperature is high, and a composite oxide having nano-order primary particles can be obtained.

As a metal compound raw material for supplying tetravalent Mn, one or more metal compounds selected from oxides, hydroxides, and metal salts containing one or more metal elements essential to Mn are used. This metal compound is essential for the metal compound raw material. Specifically, manganese dioxide (MnO ₂ ), manganese trioxide (Mn ₂ O ₃ ), manganese monoxide (MnO), manganese trioxide (Mn ₃ O ₄ ), manganese hydroxide (Mn (OH) ₂ ) , Manganese oxyhydroxide (MnOOH), metal compounds in which a part of Mn of these oxides, hydroxides or metal salts is substituted with Cr, Fe, Co, Ni, Al, Mg, or the like. One or two or more of these may be used as the essential metal compound. Among these, MnO ₂ is preferable because it is easy to obtain and is relatively easy to obtain.

  Here, Mn of the metal compound is not necessarily tetravalent, and may be Mn of tetravalent or less. This is because the reaction proceeds in a highly oxidized state, and even bivalent or trivalent Mn becomes tetravalent. The same applies to the transition element substituting Mn.

As the compound containing a metal element that substitutes a part of Mn, one or more second metal compounds selected from oxides, hydroxides, and metal salts may be used. Specific examples of the second metal compound include cobalt oxide (CoO, Co ₃ O ₄ ), cobalt nitrate (Co (NO ₃ ) ₂ · 6H ₂ O), cobalt hydroxide (Co (OH) ₂ ), nickel oxide (NiO), nickel nitrate (Ni (NO ₃ ) ₂ · 6H ₂ O), nickel sulfate (NiSO ₄ · 6H ₂ O), aluminum hydroxide (Al (OH) ₃ ), aluminum nitrate (Al (NO ₃ ) ₃・ 9H ₂ O), copper oxide (CuO), copper nitrate (Cu (NO ₃ ) ₂ .3H ₂ O), calcium hydroxide (Ca (OH) ₂ ) and the like. One or two or more of these may be used as the second metal compound.

  The melt reaction step is a step in which the raw material mixture is melted and reacted. The reaction temperature is the temperature of the raw material mixture in the melting reaction step, and it should be higher than the melting point of the molten salt raw material, but if it is less than 500 ° C, the reaction activity of the molten salt is insufficient and the desired complex oxidation containing tetravalent Mn It is difficult to produce a product with high selectivity. If the reaction temperature is 550 ° C. or higher, a complex oxide with high crystallinity can be obtained. The upper limit of the reaction temperature is lower than the decomposition temperature of lithium hydroxide, and is desirably 900 ° C. or lower, more preferably 850 ° C. or lower. If manganese dioxide is used as the metal compound for supplying Mn, the reaction temperature is preferably 500 to 700 ° C, more preferably 550 to 650 ° C. If the reaction temperature is too high, a molten salt decomposition reaction occurs, which is not desirable. If the reaction temperature is maintained for 30 minutes or more, more desirably 1 to 6 hours, the raw material mixture reacts sufficiently.

  Further, when the melting reaction step is performed in an oxygen-containing atmosphere, for example, in the air, a gas atmosphere containing oxygen gas and / or ozone gas, a complex oxide containing tetravalent Mn is easily obtained in a single phase. In an atmosphere containing oxygen gas, the oxygen gas concentration is preferably 20 to 100% by volume, more preferably 50 to 100% by volume. As the oxygen concentration is increased, the particle size of the composite oxide synthesized tends to be reduced.

The structure of the composite oxide obtained by the above production method is a layered rock salt structure. It can be confirmed by X-ray diffraction (XRD), electron diffraction, etc. that the layered rock salt structure is the main component. In addition, the layered structure can be observed with a high-resolution image using a high-resolution transmission electron microscope (TEM). If the resulting composite oxide is represented by a composition formula, xLi ₂ M ₁ O ₃ · (1-x) LM ₂ O ₂ (0 ≦ x ≦ 1, where M ₁ is essential for tetravalent Mn. metal elements, M ₂ is a metal element essentially containing tetravalent Mn. Incidentally, Li is 60% or less further 45% or less by atomic ratio may be replaced with a hydrogen element (H). the M ₁ is most preferably tetravalent Mn, but less than 50% or even less than 80% may be substituted with other metal elements.

The metal elements other than tetravalent Mn constituting M ₁ and M ₂ are selected from Ni, Al, Co, Fe, Mg, and Ti from the viewpoint of chargeable / dischargeable capacity when used as an electrode material. preferable. Needless to say, composite oxides that slightly deviate from the above composition formula due to defects of Li, M ₁ , M _2, or O (oxygen) inevitably occurring are also included. Therefore, the average oxidation number of Mn contained in the average oxidation number and M ₂ of M ₁ is allowed to 3.8 to 4 valence.

Specific examples include Li ₂ MnO ₃ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.5 Mn _0.5 O ₂ , or a solid solution containing two or more of these. A part of Mn, Ni, and Co may be substituted with other metal elements. The obtained composite oxide as a whole may have the basic composition as the exemplified oxide, and may slightly deviate from the above composition formula due to unavoidable metal element or oxygen deficiency.

  The positive electrode of the lithium ion secondary battery of the present invention has a current collector and an active material layer bound on the current collector. The active material layer was made into a slurry by adding a positive electrode active material comprising a lithium manganese-based oxide whose crystal structure belongs to a layered rock salt structure, a conductive additive, a binder resin, and an appropriate amount of an organic solvent as necessary. The product can be produced by applying the product onto a current collector by a method such as a roll coating method, a dip coating method, a doctor blade method, a spray coating method, or a curtain coating method, and curing the binder resin.

  The negative electrode of the lithium ion secondary battery of the present invention can be formed by forming metallic lithium, which is a negative electrode active material, into a sheet shape, or by pressing a sheet into a current collector network such as nickel or stainless steel. . A lithium alloy or a lithium compound can also be used in place of metallic lithium. Moreover, you may use the negative electrode which consists of a negative electrode active material and binder resin which can occlude / release lithium ion like a positive electrode. Examples of the negative electrode active material that can be used include a fired organic compound such as natural graphite, artificial graphite, and phenol resin, and a powdery carbon material such as coke. As the binder resin, a fluorine-containing resin, a thermoplastic resin, or the like can be used as in the positive electrode.

It is also preferable to use a powder made of silicon oxide represented by SiO _x (0.3 ≦ x ≦ 1.6) as the negative electrode active material. It is known that SiO _x decomposes into Si and SiO ₂ when heat-treated. This is called a disproportionation reaction. If silicon monoxide SiO is a homogeneous solid with a Si to O ratio of approximately 1: 1, it separates into two phases, the Si phase and the SiO ₂ phase, due to the internal reaction of the solid. . The Si phase obtained by separation is very fine. In addition, the SiO ₂ phase covering the Si phase has a function of suppressing the decomposition of the non-aqueous electrolyte. However, when only silicon oxide is used as the negative electrode active material, cycle characteristics may be insufficient. In such a case, it is desirable to use silicon oxide in combination with a carbon material such as graphite. .

  Moreover, there is no limitation in particular also in binder resin and conductive support material which are used for a positive electrode and a negative electrode, What is necessary is just to be usable with a general lithium ion secondary battery. The conductive aid is for ensuring the electrical conductivity of the electrode, and for example, a mixture of one or more carbon material powders such as carbon black, acetylene black, and graphite may be used. it can. The binder resin plays a role of connecting the positive electrode active material and the conductive additive. For example, a fluorine-containing resin such as polyvinylidene fluoride, polytetrafluoroethylene, or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene is used. be able to.

  As the current collector used for the positive electrode and the negative electrode, a metal mesh or a metal foil can be used. For example, a porous or non-porous conductive substrate made of a metal material such as stainless steel, titanium, nickel, aluminum, copper, or a conductive resin can be used. Examples of the porous conductive substrate include a mesh body, a net body, a punching sheet, a lath body, a porous body, a foamed body, a fiber group molded body such as a nonwoven fabric, and the like. Examples of the non-porous conductive substrate include a foil, a sheet, and a film. As a method for applying the composition for forming an electrode mixture layer, a conventionally known method such as a doctor blade or a bar coater may be used.

  As the organic solvent for adjusting the viscosity used in the slurry, N-methyl-2-pyrrolidone (NMP), methanol, methyl isobutyl ketone (MIBK) and the like can be used.

  As the separator, a separator having sufficient strength and capable of holding a large amount of electrolyte is preferable. From such a viewpoint, the separator is made of polyolefin such as polypropylene, polyethylene, a copolymer of propylene and ethylene with a thickness of 5 to 50 μm. A microporous film or non-woven fabric is preferably used.

  The shape of the lithium ion secondary battery constituted by the above components can be various, such as a cylindrical type, a stacked type, and a coin type. In any case, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body. And connecting between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal leading to the outside with a current collecting lead, etc., this electrode body is impregnated with the electrolytic solution of the present invention and sealed in the battery case, A lithium ion secondary battery is completed.

  When using a lithium ion secondary battery, it charges first and activates a positive electrode active material. However, when a positive electrode active material made of a lithium manganese oxide belonging to a layered rock salt structure is used, lithium ions are released and oxygen is generated during the first charge. For this reason, it is desirable to charge the battery case before sealing it.

  The lithium ion secondary battery of the present invention described above can be suitably used in the field of automobiles in addition to the fields of communication devices such as mobile phones and personal computers and information-related devices. For example, if this lithium ion secondary battery is mounted on a vehicle, the lithium ion secondary battery can be used as a power source for an electric vehicle.

  Hereinafter, the present invention will be described specifically by way of examples.

<Preparation of positive electrode for lithium ion secondary battery>
Prepare a raw material mixture by mixing 0.20 mol of lithium hydroxide monohydrate LiOH · H ₂ O (8.4 g) as the molten salt raw material and 0.02 mol of manganese dioxide MnO ₂ (1.74 g) as the raw material of the metal compound did. At this time, since the target product is Li ₂ MnO ₃ , assuming that all Mn of manganese dioxide was supplied to Li ₂ MnO ₃ , (target product Li) / (molten salt raw material Li) is 0.04 mol / 0.2 mol = 0.2.

  The raw material mixture was put in a crucible, transferred into an electric furnace at 700 ° C., and heated in vacuum at 700 ° C. for 2 hours. At this time, the raw material mixture melted to form a molten salt, and a black product was precipitated.

  Next, the crucible containing the molten salt was cooled to room temperature in the electric furnace and then taken out from the electric furnace. After the molten salt was sufficiently cooled and solidified, the crucible was immersed in 200 mL of ion exchange water and stirred to dissolve the solidified molten salt in water. Since the black product was insoluble in water, the water became a black suspension. Filtration of the black suspension yielded a clear filtrate and a black solid residue on the filter paper. The obtained filtrate was further filtered while thoroughly washing with acetone. The washed black solid was vacuum-dried at 120 ° C. for 12 hours and then pulverized using a mortar and pestle.

The black powder obtained was subjected to X-ray diffraction (XRD) measurement using CuKα rays. According to XRD, the obtained black powder was found to have a layered rock salt structure. The composition of the obtained black powder was confirmed to be Li ₂ MnO ₃ from the average valence analysis of Mn by emission spectroscopic analysis (ICP) and oxidation-reduction titration.

In addition, the valence evaluation of Mn was performed as follows. A sample of 0.05 g was placed in an Erlenmeyer flask, and 40 mL of sodium oxalate solution (1%) was accurately added, and 50 mL of H ₂ SO ₄ was further added, and the sample was dissolved in a 90 ° C. water bath in a nitrogen gas atmosphere. To this solution, potassium permanganate (0.1N) was titrated, and the end point (titration amount: V ₁ ) was changed to a slight red color. In another flask, 20 mL of a sodium oxalate solution (1%) was accurately taken, and potassium permanganate (0.1 N) was titrated to the end point in the same manner as described above (titration amount: V ₂ ). From V ₁ and V ₂ , the amount of oxalic acid consumed when an expensive number of Mn was reduced to Mn ²⁺ was calculated as the amount of oxygen (active oxygen amount) by the following formula.

Active oxygen amount (%) = [(2 × V ₂ −V ₁ ) × 0.00080 / sample amount] × 100 And the average valence of Mn was calculated from the amount of Mn in the sample (ICP measurement value) and the amount of active oxygen .

  The obtained positive electrode active material, acetylene black as a conductive additive, and polytetrafluoroethylene (PTFE) as a binder resin were mixed at a mass ratio of 50:40:10. Subsequently, this mixture was crimped | bonded to the aluminum mesh which is a collector. Then, it vacuum-dried at 120 degreeC for 12 hours or more, and was set as the electrode (positive electrode: 30x25mm).

<Preparation of negative electrode for lithium ion secondary battery>
First, SiO powder (manufactured by Sigma-Aldrich Japan, average particle size 5 μm) was heat-treated at 900 ° C. for 2 hours to prepare SiO _x powder having an average particle size of 5 μm. With this heat treatment, if silicon monoxide SiO is a homogeneous solid having a ratio of Si to O of approximately 1: 1, it is separated into two phases of Si phase and SiO ₂ phase by solid internal reaction. The Si phase obtained by separation is very fine.

A slurry was prepared by mixing 48 parts by mass of the obtained SiO _x powder with 34.4 parts by mass of graphite powder as a conductive additive, 2.6 parts by mass of ketjen black (KB) powder, and polyacrylic acid as a binder resin. . The composition ratio of each component in the slurry is SiO _x powder: graphite powder: ketchen black: polyacrylic acid = 48: 34.4: 2.6: 15 as a solid content. This slurry was applied to the surface of an electrolytic copper foil (current collector) having a thickness of 20 μm using a doctor blade to form a negative electrode active material layer on the copper foil.

  Then, it dried at 80 degreeC for 20 minute (s), and the organic solvent was volatilized and removed from the negative electrode active material layer. After drying, the current collector and the negative electrode active material layer were firmly and closely joined with a roll press. This was heat-cured at 200 ° C. for 2 hours to form an electrode (negative electrode: 31 × 26 mm) having an active material layer thickness of about 15 μm.

  Note that a negative electrode doped with lithium may be used as the negative electrode.

<Production of lithium ion secondary battery>
LiPF ₆ was dissolved at a concentration of 1M in a mixed solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a ratio of 3: 7 (volume ratio), and thiophene was added to a concentration of 0.1% by weight. A water electrolyte was prepared.

  A microporous polyethylene film having a thickness of 20 μm was sandwiched between the positive electrode and the negative electrode as an electrode body. The electrode body was wrapped with a laminate film, and the periphery was thermally fused to produce a film-clad battery. Before the last side was heat-sealed and sealed, the non-aqueous electrolyte was injected and impregnated into the electrode body. Then, CCCV charge (constant current constant voltage charge) to 4.5V was performed at 0.2C, and the positive electrode active material was activated.

<Test>
(Calculation of capacity recovery rate)
Perform a high temperature storage test to store the above lithium ion secondary battery at 80 ° C for 5 days, and measure 1C discharge capacity before high temperature storage test and 1C discharge capacity after SOC 100% charge after discharging after high temperature storage. The capacity recovery rate was calculated from the following equation.
Capacity recovery rate = 100 × (1C discharge capacity after discharging after storage and SOC 100% charge) / (1C discharge capacity before storage)
The results are shown in Table 1.

[Comparative Example 1]
A lithium ion secondary battery was produced in the same manner as in Example 1 except that thiophene was not added to the nonaqueous electrolytic solution. The capacity recovery rate was calculated in the same manner as in Example 1 except that this lithium ion secondary battery was used. The results are shown in Table 2.

<Evaluation>

  As is clear from Table 2, the capacity recovery rate of the lithium ion secondary battery of Example 1 is higher than that of the lithium ion secondary battery of Comparative Example 1. This is clearly the effect of including thiophene in the non-aqueous electrolyte.

Claims (7)

An electrolyte used for a lithium ion secondary battery having a positive electrode having a positive electrode active material comprising a lithium manganese-based oxide containing a lithium (Li) element and a tetravalent manganese (Mn) element and having a crystal structure belonging to a layered rock salt structure. There,
An electrolyte solution comprising: a solvent or a liquid dispersion medium; an electrolyte dissolved or dispersed in the solvent or liquid dispersion medium; and an additive selected from thiophene and a thiophene derivative dissolved in the solvent or liquid dispersion medium. .   2. The electrolytic solution according to claim 1, wherein the additive is thiophene, and is added to the electrolytic solution in an amount of less than 2.0% by mass. _3. The electrolytic solution according to claim 1, wherein the lithium manganese oxide is Li ₂ MnO ₃ .   A positive electrode active material comprising the electrolytic solution according to any one of claims 1 to 3, and a lithium manganese-based oxide that includes a lithium (Li) element and a tetravalent manganese (Mn) element and whose crystal structure belongs to a layered rock salt structure. A lithium ion secondary battery comprising a positive electrode having a substance and a negative electrode.   5. The lithium ion secondary battery according to claim 4, wherein the additive is thiophene, and is added in an amount of less than 2.0% by mass in the electrolytic solution. 6. The lithium ion secondary battery according to claim 4, wherein the lithium manganese oxide is Li ₂ MnO ₃ . 7. The lithium ion secondary battery according to claim 4, wherein the negative electrode includes a negative electrode active material made of silicon oxide represented by SiO _x (0.3 ≦ x ≦ 1.6).