JP7155898B2 - Electrolyte and secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electrolytic solution used in a secondary battery and a secondary battery provided with the electrolytic solution.

Generally, a secondary battery includes a positive electrode, a negative electrode, and an electrolyte as main components. The positive electrode is equipped with a positive electrode active material that participates in charging and discharging, and the negative electrode is equipped with a negative electrode active material that participates in charging and discharging. As the electrolytic solution, a solution obtained by dissolving an electrolyte in a non-aqueous solvent is generally used.

Here, as the non-aqueous solvent, chain carbonates such as ethyl methyl carbonate, dimethyl carbonate and diethyl carbonate and cyclic carbonates such as ethylene carbonate are generally used together. In fact, in Patent Documents 1 to 3, a lithium ion secondary battery equipped with an electrolytic solution using both a linear carbonate such as ethylmethyl carbonate, dimethyl carbonate, and diethyl carbonate as a non-aqueous solvent and ethylene carbonate as a cyclic carbonate. are specifically described.

It is also known to employ fluorine-containing cyclic carbonates such as fluoroethylene carbonate and difluoroethylene carbonate as part of the non-aqueous solvent. In fact, Patent Document 4 describes that the capacity retention rate of the secondary battery is optimized by using a fluorine-containing cyclic carbonate such as fluoroethylene carbonate or difluoroethylene carbonate as a non-aqueous solvent for the electrolyte in the secondary battery. is described (see Table 17).

Furthermore, techniques for improving the performance of secondary batteries by adding additives to electrolytes are being actively studied.
For example, Patent Literature 5 specifically describes a secondary battery including an electrolytic solution containing lithium bisoxalate borate as an additive. Patent Document 6 specifically describes a secondary battery comprising an electrolytic solution containing 1,3-propanesultone as an additive.

JP 2015-185509 A JP 2015-179625 A WO2014/080608 Japanese Patent Application Laid-Open No. 2009-32492 JP 2016-100051 A WO2016/136210

As described above, researches on electrolytes used in secondary batteries and researches on additives for electrolytes are vigorously conducted. In addition, the industry is demanding a secondary battery with excellent battery characteristics.
The present invention has been made in view of such circumstances, and an object of the present invention is to provide a suitable electrolytic solution for providing a secondary battery having excellent battery characteristics.

The inventors have studied techniques for further improving the battery characteristics of secondary batteries, particularly the capacity retention rate. In order to improve the capacity retention rate of a secondary battery, it is important to suppress deterioration of the positive electrode, the negative electrode, and the electrolytic solution under respective charge/discharge conditions.
The inventors first examined the relationship between the negative electrode and the electrolytic solution.

Conventionally, commonly used carbonate-based solvents are known to form an SEI (Solid Electrolyte Interphase) film on the surface of the negative electrode by reductive decomposition on the surface of the negative electrode. The SEI coating can prevent direct contact between the negative electrode active material and the electrolytic solution, but it is believed that the SEI coating formed using a carbonate-based solvent as a raw material contains ₃ groups of CO 2 .

Here, for example, in the case of a secondary battery including a Si-containing negative electrode active material that has relatively low resistance to oxidation, it is presumed that the oxygen of the CO ₃ group in the SEI coating oxidizes the Si-containing negative electrode active material. In addition, since the Si-containing negative electrode active material expands and contracts greatly during charging and discharging, the SEI coating is destroyed and a new portion not covered with the SEI coating (hereinafter, such a portion is referred to as a “new surface”. ) can occur. Here, there is a concern that the Si-containing negative electrode active material on the new surface may be oxidized due to contact with the carbonate-based solvent.

Therefore, the inventors of the present invention aimed to use ether as the non-aqueous solvent for the electrolytic solution, which is unlikely to undergo reductive decomposition on the surface of the negative electrode and is considered to have low oxidizing ability. However, when tetrahydrofuran was used as a specific ether, the capacity retention rate of the lithium ion secondary battery deteriorated.

The present inventors suspected that the problems caused by tetrahydrofuran may be related to radical generation in tetrahydrofuran. In general, in _CH2 groups sandwiched between oxygen and alkyl chains, radicals are stabilized due to adjacent group interactions exerted by adjacent oxygen and adjacent alkyl chains, so it is believed that radicals are likely to occur. . In tetrahydrofuran, the _CH2 group at the 2-position is the most likely site for radical formation.

Accordingly, the present inventor searched for an ether that is considered to be less likely to generate radicals than tetrahydrofuran, and arrived at 4-methyltetrahydropyran, which is a 6-membered cyclic ether. Then, a lithium ion secondary battery employing 4-methyltetrahydropyran as a non-aqueous solvent for the electrolytic solution was manufactured and tested, and it was confirmed that the battery exhibited suitable battery characteristics.

Next, the inventors investigated the relationship between the positive electrode and the electrolytic solution.
In general, since ether has a high HOMO energy level, it is feared that it will be oxidatively decomposed by contact with the positive electrode under charging conditions at a high potential.
In fact, the present inventor manufactured a lithium ion secondary battery employing 4-methyltetrahydropyran, which is a six-membered cyclic ether, as a non-aqueous solvent for the electrolyte, and conducted a charge/discharge test under high potential conditions. As a result, it was confirmed that the capacity decreased rapidly.

Therefore, the present inventor added various additives to an electrolytic solution containing 4-methyltetrahydropyran, which is a 6-membered cyclic ether, and conducted tests. However, it was found that a remarkably excellent capacity retention effect was exhibited.
Based on these findings, the present invention was completed.

The electrolytic solution of the present invention comprises an electrolyte, a boron compound having a five-membered ring structure composed of B forming an O—B—O bond, two oxygen elements, and two carbon elements, and an alkyl group. It is characterized by containing a 6-membered cyclic ether that may be

A secondary battery comprising the electrolytic solution of the present invention is excellent in battery characteristics, particularly in capacity retention rate.

EMBODIMENT OF THE INVENTION Below, the form for implementing this invention is demonstrated. Unless otherwise specified, the numerical range "a to b" described in this specification includes the lower limit a and the upper limit b. Further, a numerical range can be configured by arbitrarily combining these upper and lower limits and the numerical values listed in the examples. Furthermore, a numerical value arbitrarily selected from within these numerical ranges can be used as the new upper and lower numerical values.

The electrolytic solution of the present invention comprises an electrolyte, a boron compound having a five-membered ring structure composed of B forming an O—B—O bond, two oxygen elements, and two carbon elements, and an alkyl group. 6-membered cyclic ether (hereinafter sometimes simply referred to as "6-membered cyclic ether").

As the electrolyte, those used in secondary batteries such as lithium ion secondary batteries may be adopted. Examples of electrolytes used in lithium ion secondary batteries include lithium salts such as lithium borate, lithium sulfonimide, lithium sulfonate, lithium phosphate, and lithium perchlorate. As the lithium salt, one containing fluorine in the molecule is preferable.

From the viewpoint of the chemical stability of the 6-membered cyclic ether, the lithium salt is preferably lithium borate, lithium sulfonimide, or lithium sulfonate.

As the lithium borate salt, the lithium borate salt of the general formula (1) can be exemplified.
General formula (1) LiBF _4-m R _m
In general formula (1), m is either 0, 1, 2, 3 or 4. _R is _{CnHaFb .} n, a, and b are each independently an integer of 0 or greater and satisfy 2n+1=a+b.

As the sulfonimide lithium salt, the sulfonimide lithium salt of general formula (2) can be exemplified.
General formula ( ₂ ) (RSO2) _2NLi
In general formula (2 ₎ , each R is _{independently CnHaFb} . n, a, and b are each independently an integer of 0 or greater and satisfy 2n+1=a+b. Also, two R's may combine with each other to form a ring, in which case 2n=a+b is satisfied.

As the lithium sulfonate, the lithium sulfonate of general formula (3) can be exemplified.
General formula ( ₃ ) RSO3Li
In general formula ₍ 3 ₎ , R is _CnHaFb . n, a, and b are each independently an integer of 0 or greater and satisfy 2n+1=a+b.

In general formula (1), general formula (2) and general formula (3), n is preferably 0, 1, 2, 3, 4, 5 or 6. Further, a is preferably 0, 1 or 2.

Specific examples of lithium borate salts include LiBF ₄ , LiBF ₃ (CHF ₂ ), LiBF ₃ (CF ₃ ), LiBF ₃ (C ₂ F ₅ ), LiBF ₃ (C ₃ F ₇ ), LiBF ₂ (CF ₃ ) ₂ can be exemplified.

Specific examples of sulfonimide lithium salts include (FSO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) ₂ NLi, FSO ₂ (CF ₃ SO ₂ )NLi, FSO _{2 ( C2F5SO2} ) _NLi , _{( SO2CF2CF2SO2} )NLi, _{( SO2CF2CF2CF2SO2} ) _{NLi , FSO2 ( CH3SO2} ) _NLi , _{FSO2 ( C 2} H ₅ SO ₂ )NLi can be exemplified.

Specific examples of sulfonic acid lithium salts include FSO ₃ Li, CF ₃ SO ₃ Li, C ₂ F ₅ SO ₃ Li, C ₃ F ₇ SO ₃ Li, C ₄ F ₉ SO ₃ Li, C ₅ F ₁₁ SO _{3Li , C6F13SO3Li , CH3SO3Li , C2H5SO3Li , C3H7SO3Li , CF3CH2SO3Li , CF3C2H4SO3Li _ _ _ _ _ _} can be exemplified.

The concentration of the entire electrolyte in the electrolytic solution of the present invention, or the concentration of lithium borate, lithium sulfonimide, or lithium sulfonate is 0.3 to 4 mol/L, 0.5 to 3 mol/L, 1 to 2 .5 mol/L, 1.5 to 2 mol/L, and 1.6 to 1.9 mol/L can be exemplified.

A boron compound having a five-membered ring structure composed of B and two oxygen atoms and two carbon atoms forming an O—B—O bond (hereinafter sometimes simply referred to as “boron compound”) has a secondary It is believed that it decomposes during charging and discharging of the battery to form a stable SEI film on the surface of the positive electrode and/or the negative electrode. Since the SEI film suppresses direct contact between the electrolytic solution and the positive electrode active material and/or the negative electrode active material, deterioration of the electrolytic solution, the positive electrode active material, or the negative electrode active material is considered to be suppressed. As a result, the secondary battery comprising the electrolyte solution of the present invention is considered to have excellent capacity retention rate.

Examples of boron compounds include those having the following chemical structures.

In the above chemical structures, R ¹ and R ² are each independently a group selected from hydrogen and alkyl groups, or R ¹ and R ² together represent =O. R ³ and R ⁴ are each independently a group selected from hydrogen and an alkyl group, or R ³ and R ⁴ together represent =O.

The alkyl group preferably has 1 to 6 carbon atoms, more preferably 1 to 4 carbon atoms, and even more preferably 1 to 2 carbon atoms.

In a boron compound that is not a salt, there are three B bonds. A specific example of a boron compound that is not a salt is a dimer having the above chemical structure having a BB bond. Further, compounds in which an alkoxy group is bonded to B in the above chemical structure can be exemplified. The number of carbon atoms in the alkoxy group is preferably 1-6, more preferably 1-4, even more preferably 1-2.

A boron compound may be a salt. The cation of the salt is preferably an alkali metal salt, and is preferably the same as the charge carrier of the secondary battery. The anion of the salt is obtained by binding two ligands or multidentate ligands to B in the above chemical structure. The two ligands or multidentate ligands are not limited as long as they can bond with B.

The two ligands may have the same chemical structure or different chemical structures. Suitable ligands include halogens such as fluorine and chlorine, and C _n H _a F _b (where n and a are each independently an integer of 0 or greater, b is an integer of 1 or greater, and 2n+1=a+b is satisfied. ) can be exemplified. In C _n H _a F _b , n is preferably 1-3, and a is preferably 0-1. Suitable polydentate ligands are OC(R ¹ )(R ² )-C(R ³ )(R ⁴ )-O.

One type of boron compound may be used, or a plurality of types may be used in combination.

The concentration of the boron compound in the electrolytic solution of the present invention is 0.001 to 2 mol/L, 0.005 to 1.5 mol/L, 0.01 to 1 mol/L, 0.05 to 0.5 mol/L, 0 Examples include 0.08 to 0.4 mol/L and 0.1 to 0.3 mol/L.
The concentration of the boron compound in the electrolytic solution of the present invention is preferably lower than that of the electrolyte.

The six-membered ring cyclic ether contained in the electrolytic solution of the present invention is less likely to generate radicals, and thus is considered to have high resistance to reductive decomposition at the negative electrode during charging and discharging. The reason is as follows.

The site where radicals are most likely to be generated in the 6-membered cyclic ether, in other words, the carbon on which the hydrogen radical is most likely to leave is the carbon adjacent to the oxygen of the ether among the carbons constituting the 6-membered cyclic ether. It can be said that there is.

Six-membered cyclic ethers have thermally stable conformations such as chair-form and boat-form. And each conformation can also be mutually converted. Such a conformation is formed by having an sp ³ -hybridized orbital at the carbons forming the ring.
However, the carbon from which the radical is generated is converted from the sp ³ hybridized orbital so far to the sp ² hybridized orbital. Ideally, the three bonds in the sp ² hybrid orbital lie on the same plane.

Then, when a radical is generated in a 6-membered cyclic ether, the carbon having the radical has an sp ² hybridized orbital, so that the steric configuration such as chair-form or boat-form that is thermally stable It cannot take a seat, so it has to have a three-dimensional structure that is thermally unstable.
It is considered that such a thermally unstable steric structure serves as an energy barrier for radicalizing the 6-membered cyclic ether.

As the 6-membered cyclic ether which may be substituted with an alkyl group, one in which five of the six elements constituting the ring are carbon and one is oxygen is preferred.
The alkyl group that can be bonded to the carbon atoms of the 6-membered ring preferably has 1 to 6 carbon atoms, more preferably 1 to 4 carbon atoms, and even more preferably 1 to 2 carbon atoms.

An alkyl group that can be attached to a carbon of a 6-membered ring is preferably attached to a carbon that is not adjacent to the oxygen of the ether.
When an alkyl group is attached to the carbon adjacent to the oxygen of the ether, the carbon has the structure O--CH ₍ alkyl group)--CH.sub.2. Here, in a CH group sandwiched between oxygen and an alkyl chain, radicals are stabilized due to adjacent group interactions between adjacent oxygen and adjacent alkyl chains, so it is believed that radicals are likely to occur. . Therefore, it is presumed that a six-membered cyclic ether in which an alkyl group is bonded to the carbon adjacent to the oxygen of the ether is relatively likely to generate radicals.

The electrolytic solution of the present invention may be used in combination with a known non-aqueous solvent other than the six-membered cyclic ether within the scope of the present invention. The ratio of the six-membered ring cyclic ether to the total non-aqueous solvent contained in the electrolytic solution of the present invention is 50 to 100% by volume, 70 to 100% by volume, 90 to 100% by volume, 50 to 100% by mass, 70 to 100 % by mass, 90 to 100% by mass, 50 to 100% by mol, 70 to 100% by mol, and 90 to 100% by mol.

The electrolytic solution of the present invention may contain known additives within the scope of the present invention.

A secondary battery provided with the electrolytic solution of the present invention is referred to as a secondary battery of the present invention, and a lithium ion secondary battery provided with the electrolytic solution of the present invention is referred to as a lithium ion secondary battery of the present invention. The lithium-ion secondary battery of the present invention will be described below as a typical example of the secondary battery of the present invention.

The lithium-ion secondary battery of the present invention specifically includes the electrolytic solution of the present invention, a positive electrode, a negative electrode, and a separator.

The positive electrode includes a current collector and a positive electrode active material layer formed on the surface of the current collector.

A current collector refers to a chemically inert electronic conductor for continuing current flow to an electrode during discharge or charge of a lithium ion secondary battery. The material of the current collector is not particularly limited as long as it is a metal that can withstand a voltage suitable for the active material used. The material of the current collector is at least one selected from silver, copper, gold, aluminum, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, molybdenum, and stainless steel. can be exemplified by metal materials such as The current collector may be covered with a known protective layer. A current collector whose surface has been treated by a known method may be used as the current collector.

The current collector can take the form of foil, sheet, film, wire, rod, mesh, and the like. Therefore, metal foils such as copper foil, nickel foil, aluminum foil, and stainless steel foil can be preferably used as the current collector. When the current collector is in the form of foil, sheet or film, the thickness is preferably in the range of 1 μm to 100 μm.

When the potential of the positive electrode is 4 V or more based on lithium, it is preferable to employ aluminum as the current collector for the positive electrode.

Specifically, it is preferable to use aluminum or an aluminum alloy as the positive electrode current collector. Here, aluminum refers to pure aluminum, and aluminum with a purity of 99.0% or more is called pure aluminum. An alloy obtained by adding various elements to pure aluminum is called an aluminum alloy. Aluminum alloys include Al--Cu, Al--Mn, Al--Fe, Al--Si, Al--Mg, Al--Mg--Si and Al--Zn--Mg.

Further, as aluminum or aluminum alloy, specifically, for example, A1000 series alloys (pure aluminum series) such as JIS A1085 and A1N30, A3000 series alloys (Al--Mn series) such as JIS A3003 and A3004, JIS A8079, A8021 and the like. A8000 series alloy (Al--Fe series).

The positive electrode active material layer contains a positive electrode active material capable of intercalating and deintercalating lithium ions, and optionally a binder and a conductive aid. The positive electrode active material layer preferably contains 60 to 99% by mass, more preferably 70 to 95% by mass, of the positive electrode active material with respect to the total mass of the positive electrode active material layer. As the binder and conductive aid, those described for the negative electrode may be used in appropriate amounts.

As the positive electrode active material, a general formula of layered rock salt structure _{: LiaNibCocMdDeOf (} M _is selected from _Mn and Al, _D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, Y, Bi, at least one element selected from S, Si, Na, K, P, and V. a, b, c, d, e, and f are 0.2≦a≦2, b+c+d+e=1, 0≦e<1 , 1.7≦f≦3.) and Li ₂ MnO ₃ . Further, as the positive electrode active material, a spinel-structured metal oxide such as LiMn ₂ O ₄ , a solid solution composed of a mixture of a spinel-structured metal oxide and a layered compound, LiMPO ₄ , LiMVO ₄ or Li ₂ MSiO ₄ (in the formula is selected from at least one of Co, Ni, Mn and Fe). Furthermore, examples of the positive electrode active material include taborite compounds represented by LiMPO ₄ F (M is a transition metal) such as LiFePO ₄ F, and borate compounds represented by LiMBO ₃ (M is a transition metal) such as LiFeBO ₃ . be able to. Any metal oxide used as a positive electrode active material may have the above compositional formula as a basic composition, and those in which the metal elements contained in the basic composition are replaced with other metal elements can also be used. Moreover, you may use the thing which does not contain a lithium ion as a positive electrode active material. Examples include elemental sulfur, compounds of sulfur and carbon, metal _sulfides such as _TiS2 , oxides such as _V2O5 and _MnO2 , polyaniline and anthraquinone, and compounds containing these aromatics in their chemical structures. Conjugated materials such as acetic acid-based organic substances and other known materials can also be used. Furthermore, compounds having stable radicals such as nitroxide, nitronyl nitroxide, galvinoxyl, and phenoxyl may be employed as positive electrode active materials. When using a positive electrode active material that does not contain lithium ions, it is preferable to previously add lithium to the positive electrode and/or the negative electrode by a known method.

From the viewpoint of high capacity and excellent durability, the positive electrode active material has a layered rock salt structure with a general formula: Li _a Ni _b Co _c M _{d De} Of (M _is selected from Mn and Al. D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, Ru, at least one element selected from Sc, Sn, In, Y, Bi, S, Si, Na, K, P, and V. a, b, c, d, e, and f are 0.2≦a≦2 , b+c+d+e=1, 0≦e<1, and 1.7≦f≦3.).

In the above general formula, the values of b, c, and d are not particularly limited as long as they satisfy the above conditions. Also, at least one of b, c, and d should be in the range of 0.1 ≤ b ≤ 0.95, 0.01 ≤ c ≤ 0.5, and 0.01 ≤ d ≤ 0.5. preferably 0.3≤b≤0.9, 0.03≤c≤0.3, more preferably 0.03≤d≤0.3, 0.5≤b≤0.9, More preferably, the ranges are 0.05≦c≦0.2 and 0.05≦d≦0.2.

The values of a, e, and f may be within the ranges defined by the above general formulas, preferably 0.5≤a≤1.5, 0≤e<0.2, 1.8≤f≤2. .5, more preferably 0.8≦a≦1.3, 0≦e<0.1, and 1.9≦f≦2.1.

Li _x Mn _2-y A _y O ₄ (A is Ca, Mg, S, Si, Na, K, Al, P, Ga , Ge, and at least one metal element selected from transition metal elements such as Ni, 0<x≦2.2, 0≦y≦1). Examples of the range of values of x are 0.5≦x≦1.8, 0.7≦x≦1.5, and 0.9≦x≦1.2, and the range of values of y is 0. ≤y≤0.8 and 0≤y≤0.6 can be exemplified. Specific spinel structure compounds include LiMn ₂ O ₄ and LiMn _1.5 Ni _0.5 O ₄ .

_LiFePO4 , _{Li2FeSiO4 , LiCoPO4} , _Li2CoPO4 , _Li2MnPO4 , _Li2MnSiO4 , _{and Li2CoPO4F can} be _exemplified as specific _positive electrode active materials. Another specific positive electrode active material is Li ₂ MnO ₃ —LiCoO ₂ .

In view of the results of Evaluation Example 2, which will be described later, it is preferable that the positive electrode active material is charged and discharged at a potential of 4 V or higher with respect to lithium. As such a positive electrode active material, the lithium mixed metal oxide represented _by the general formula of the _layered rock salt structure _{: LiaNibCocMdDeOf} , _{LixMn2 - yAy} with a _{spinel structure} , O ₄ and the like can be exemplified.

The negative electrode includes a current collector and a negative electrode active material layer formed on the surface of the current collector.
As the current collector, the one described for the positive electrode may be appropriately employed.

The negative electrode active material layer contains a negative electrode active material capable of intercalating and deintercalating lithium ions, and optionally a binder and a conductive aid.

As the negative electrode active material, a material that can be used in a lithium ion secondary battery may be adopted. In view of the favorable durability under reducing conditions of the six-membered cyclic ether shown in the results of Evaluation Example 1 described later, graphite and Si-containing negative electrode active materials can be exemplified as suitable negative electrode active materials.

As the Si-containing negative electrode active material, a Si-containing material capable of intercalating and deintercalating lithium ions can be used.

Specific examples of the Si-containing material include simple Si and SiO _x (0.3≦x≦1.6). When x in SiO 2 _x is less than the lower limit, the ratio of Si becomes excessive, so that the volume change during charging and discharging becomes too large, and the cycle characteristics of the lithium ion secondary battery may deteriorate. On the other hand, if x exceeds the upper limit, the Si ratio becomes too small and the energy density decreases. The range of x is more preferably 0.5≤x≤1.5, more preferably 0.7≤x≤1.2.

Specific examples of Si-containing materials include silicon materials (hereinafter simply referred to as “silicon materials”) disclosed in International Publication No. 2014/080608 and the like.

The silicon material has a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction. The silicon material is prepared, for example, by reacting CaSi ₂ with an acid to synthesize a layered silicon compound containing polysilane as a main component, and further by heating the layered silicon compound at 300° C. or higher to release hydrogen. It is manufactured.

The method for producing a silicon material is represented by an ideal reaction formula using hydrogen chloride as an acid as follows.
_{3CaSi2 +} 6HCl→ _Si6H6 + _3CaCl2
Si ₆ H ₆ → 6Si+3H ₂ ↑

However, in the upper reaction for synthesizing polysilane Si ₆ H ₆ , water is usually used as a reaction solvent from the viewpoint of removing by-products and impurities. In addition, since Si ₆ H ₆ can react with water, in the process of synthesizing the layered silicon compound including the reaction in the upper stage, the layered silicon compound is rarely produced as containing only Si ₆ H ₆ . The silicon compound is represented by Si ₆ H _s (OH) _t X _u (X is an element or group derived from an acid anion, s+t+u=6, 0<s<6, 0<t<6, 0<u<6). manufactured as In the above chemical formula, unavoidable impurities such as Ca that may remain are not taken into consideration. A silicon material obtained by heating the layered silicon compound also contains elements derived from oxygen and acid anions.

As described above, the silicon material has a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction. For efficient absorption and release of lithium ions, the thickness of the plate-shaped silicon body is preferably in the range of 10 nm to 100 nm, more preferably in the range of 20 nm to 50 nm. The longitudinal length of the plate-shaped silicon body is preferably within the range of 0.1 μm to 50 μm. In addition, it is preferable that (longitudinal length)/(thickness) of the plate-like silicon body is within the range of 2 to 1,000. The lamination structure of the plate-shaped silicon bodies can be confirmed by observation with a scanning electron microscope or the like. It is also believed that this layered structure is a remnant of the Si layer in the CaSi ₂ raw material.

Preferably, the silicon material includes amorphous silicon and/or silicon crystallites. In particular, in the plate-like silicon body, a state in which amorphous silicon is used as a matrix and silicon crystallites are scattered in the matrix is preferable. The size of the silicon crystallites is preferably in the range of 0.5 nm to 300 nm, more preferably in the range of 1 nm to 100 nm, even more preferably in the range of 1 nm to 50 nm, and particularly preferably in the range of 1 nm to 10 nm. The size of the silicon crystallite is calculated from Scherrer's formula using the half width of the diffraction peak of the Si (111) plane of the X-ray diffraction chart obtained by performing X-ray diffraction measurement on the silicon material. .

The amount and size of the plate-shaped silicon bodies, amorphous silicon and silicon crystallites contained in the silicon material are mainly affected by the heating temperature and heating time. The heating temperature is preferably within the range of 350°C to 950°C, more preferably within the range of 400°C to 900°C.

The Si-containing negative electrode active material is preferably in the form of powder, which is an aggregate of particles. The average particle size of the Si-containing negative electrode active material is preferably in the range of 1-30 μm, more preferably in the range of 2-20 μm. If a Si-containing negative electrode active material with too small an average particle size is used, the manufacturing operation may become difficult. On the other hand, a lithium ion secondary battery having a negative electrode using a Si-containing negative electrode active material with an excessively large average particle size may not be able to be charged and discharged appropriately.
The average particle size in this specification means _D50 when a sample is measured with a general laser diffraction particle size distribution analyzer.

As the Si-containing negative electrode active material, a carbon layer-coated Si-containing negative electrode active material obtained by coating the Si-containing negative electrode active material with a carbon layer may be employed. The carbon layer-coated-Si-containing negative electrode active material is preferably one in which the carbon layer and the Si-containing negative electrode active material are integrated. As a method for producing such a carbon layer-coated Si-containing negative electrode active material, there is a mechanical milling method in which a mixture of a Si-containing negative electrode active material and carbon powder is stirred and integrated after applying strong pressure, A CVD (chemical vapor deposition) method in which carbon generated from a carbon source is deposited on a Si-containing negative electrode active material can be exemplified.

The CVD method is preferable because the surface of the Si-containing negative electrode active material can be uniformly coated with a thin carbon layer. Of the CVD methods, a thermal CVD method is preferred in which gaseous organic matter, which is a carbon source, is thermally decomposed to generate carbon.

A thermal CVD process for producing a carbon layer-coated Si-containing negative electrode active material using a thermal CVD method will be specifically described. Specifically, in the thermal CVD process, the Si-containing negative electrode active material is brought into contact with an organic substance under a non-oxidizing atmosphere and under heating conditions, and a carbon layer formed by carbonizing the organic substance on the surface of the Si-containing negative electrode active material. It is a step of forming When performing a thermal CVD process, a known CVD apparatus such as a hot wall type, cold wall type, horizontal type, vertical type, fluidized bed reactor, rotary furnace, tunnel furnace, batch firing furnace, rotary kiln, etc. is used. You can use it.

As the organic matter, those that can be thermally decomposed and carbonized by heating in a non-oxidizing atmosphere are used. Examples include saturated aliphatic hydrocarbons such as methane, ethane, propane, butane, isobutane, pentane, hexane, ethylene, Unsaturated aliphatic hydrocarbons such as propylene and acetylene, alcohols such as methanol, ethanol, propanol, and butanol, benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, and benzofuran. , aromatic hydrocarbons such as pyridine, esters such as ethyl acetate, butyl acetate, and amyl acetate, fatty acids, and the like, or a mixture thereof.

The processing temperature in the thermal CVD process varies depending on the type of organic substance, but is preferably 50° C. or more higher than the temperature at which the organic substance thermally decomposes. However, if the heating temperature is excessively high, free carbon (soot) may be generated in the system, so it is preferable to select conditions that do not generate free carbon (soot). The thickness of the carbon layer formed can be controlled by the treatment time.

The thermal CVD process is desirably performed while the Si-containing negative electrode active material is in a fluid state. By doing so, the entire surface of the Si-containing negative electrode active material can be brought into contact with the organic matter, and a more uniform carbon layer can be formed. There are various methods for making the Si-containing negative electrode active material fluid, such as using a fluidized bed, but it is preferable to bring the Si-containing negative electrode active material into contact with the organic substance while stirring. For example, if a rotary furnace with a baffle inside is used, the Si-containing negative electrode active material remaining on the baffle is stirred by falling from a predetermined height as the rotary furnace rotates, and at that time comes into contact with the organic matter. Since the carbon layer is formed in the Si-containing negative electrode active material, a more uniform carbon layer can be formed over the entire Si-containing negative electrode active material.

Carbon layer coating--The carbon layer of the Si-containing negative electrode active material is amorphous and/or crystalline, and the carbon layer preferably covers the entire surface of the Si-containing negative electrode active material particles. The thickness of the carbon layer is preferably in the range of 1 nm to 100 nm, more preferably in the range of 5 to 50 nm, and even more preferably in the range of 10 to 30 nm.

The negative electrode active material layer preferably contains 60 to 99% by mass, more preferably 70 to 95% by mass, of the negative electrode active material with respect to the total mass of the negative electrode active material layer.

Binders include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, alkoxysilyl group-containing resins, and carboxymethyl cellulose. , styrene-butadiene rubber, etc. may be employed.

Also, a crosslinked polymer obtained by crosslinking a carboxyl group-containing polymer such as polyacrylic acid or polymethacrylic acid with a polyamine such as diamine disclosed in International Publication No. 2016/063882 may be used as a binder.

Diamines used in the crosslinked polymer include alkylenediamines such as ethylenediamine, propylenediamine and hexamethylenediamine, 1,4-diaminocyclohexane, 1,3-diaminocyclohexane, isophoronediamine, bis(4-aminocyclohexyl)methane and the like. saturated carbocyclic diamine, m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, bis(4-aminophenyl)sulfone, benzidine, o-tolidine, 2,4- Aromatic diamines such as tolylenediamine, 2,6-tolylenediamine, xylylenediamine and naphthalenediamine are included.

The mixing ratio of the binder in the negative electrode active material layer is preferably negative electrode active material:binder=1:0.005 to 1:0.3 in mass ratio. This is because if the amount of the binder is too small, the formability of the electrode will deteriorate, and if the amount of the binder is too large, the energy density of the electrode will be low.

A conductive aid is added to increase the conductivity of the electrode. Therefore, the conductive aid may be added arbitrarily when the conductivity of the electrode is insufficient, and may not be added when the conductivity of the electrode is sufficiently excellent. The conductive aid may be a chemically inert high electron conductor, and examples include carbon black, graphite, vapor grown carbon fiber, and various metal particles, which are carbonaceous fine particles. be. Examples of carbon black include acetylene black, Ketjenblack (registered trademark), furnace black, and channel black. These conductive aids can be added to the active material layer singly or in combination of two or more.

The mixing ratio of the conductive aid in the negative electrode active material layer is preferably negative electrode active material:conductive aid=1:0.01 to 1:0.5 in mass ratio. This is because if the amount of the conductive aid is too small, an efficient conductive path cannot be formed, and if the amount of the conductive aid is too large, the formability of the active material layer is deteriorated and the energy density of the electrode is lowered.

Conventionally known methods such as a roll coating method, a die coating method, a dip coating method, a doctor blade method, a spray coating method, and a curtain coating method are used to form the positive electrode active material layer or the negative electrode active material layer on the surface of the current collector. can be used to apply the positive electrode active material or the negative electrode active material to the surface of the current collector. Specifically, a positive electrode active material or negative electrode active material, a binder, a solvent, and, if necessary, a conductive aid are mixed to form a slurry, which is then applied to the surface of a current collector and then dried. . Examples of solvents include N-methyl-2-pyrrolidone, methanol, methyl isobutyl ketone, and water. In order to increase the electrode density, it may be compressed after drying.

The separator separates the positive electrode from the negative electrode and allows lithium ions to pass through while preventing short circuits due to contact between the two electrodes. As the separator, a known one may be adopted, and synthetic resins such as polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamide, polyaramid (aromatic polyamide), polyester, polyacrylonitrile, polysaccharides such as cellulose and amylose, and fibroin. , natural polymers such as keratin, lignin and suberin, and porous bodies, non-woven fabrics, and woven fabrics using one or a plurality of electrically insulating materials such as ceramics. Also, the separator may have a multilayer structure.

A specific method for manufacturing the lithium ion secondary battery of the present invention will be described.
For example, an electrode body is formed by sandwiching a separator between a positive electrode and a negative electrode. The electrode body may be of either a laminated type in which a positive electrode, a separator and a negative electrode are laminated, or a wound type in which a laminated body of a positive electrode, a separator and a negative electrode is wound. After connecting the current collector of the positive electrode and the current collector of the negative electrode to the positive electrode terminal and the negative electrode terminal leading to the outside using a current collecting lead or the like, the electrolyte solution of the present invention is added to the electrode body to form a lithium ion secondary. A battery may be used.

The shape of the lithium-ion secondary battery of the present invention is not particularly limited, and various shapes such as cylindrical, square, coin-shaped, and laminate-shaped can be employed.

The lithium ion secondary battery of the present invention is excellent in durability against charging and discharging under high potential conditions. Therefore, the lithium ion secondary battery of the present invention is preferably charged to a high potential. A high potential here means a potential of 4 V or higher with respect to lithium. Examples of the high potential range include 4 to 5.5 V, 4.1 to 5 V, 4.15 to 4.8 V, and 4.2 to 4.7 V relative to lithium.

The lithium ion secondary battery of the present invention may be mounted on a vehicle. The vehicle may be any vehicle that uses electrical energy from a lithium-ion secondary battery as a power source in whole or in part, and may be, for example, an electric vehicle or a hybrid vehicle. When a lithium ion secondary battery is mounted on a vehicle, it is preferable to connect a plurality of lithium ion secondary batteries in series to form an assembled battery. Devices equipped with lithium ion secondary batteries include, in addition to vehicles, personal computers, mobile communication devices, various home electric appliances driven by batteries, office equipment, industrial equipment, and the like. Furthermore, the lithium ion secondary battery of the present invention is used for wind power generation, solar power generation, hydraulic power generation, and other power storage devices and power smoothing devices for power systems, power sources for ships and/or auxiliary equipment, aircraft, power source for spacecraft and/or auxiliary equipment, auxiliary power source for vehicles that do not use electricity as a power source, power source for mobile home robots, power source for system backup, power source for uninterruptible power supply, It may be used as a power storage device that temporarily stores electric power required for charging in a charging station for an electric vehicle.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. Various modifications, improvements, etc. that can be made by those skilled in the art can be implemented without departing from the scope of the present invention.

EXAMPLES The present invention will be described more specifically below with reference to examples, comparative examples, and the like. It should be noted that the present invention is not limited by these examples.

(Reference example 1)
LiBF ₄ was dissolved in 4-methyltetrahydropyran to prepare an electrolytic solution of Reference Example 1 having a LiBF ₄ concentration of 1.5 mol/L.

Using the electrolytic solution of Reference Example 1, a lithium ion secondary battery of Reference Example 1 was manufactured as follows.

CaSi ₂ was added to a concentrated hydrochloric acid solution at 0° C. under stirring conditions and allowed to react for 1 hour. Water was added to the reaction solution and the mixture was filtered to collect a yellow powder. The yellow powder was washed with water, further washed with ethanol, and dried under reduced pressure to obtain a layered silicon compound containing a layered polysilane. Next, the layered silicon compound was heated at 800° C. in an argon atmosphere to desorb hydrogen to produce a silicon material. By heating the silicon material at 880° C. in a propane gas atmosphere, a carbon-coated silicon material, which is a carbon layer-coated-Si-containing negative electrode active material, was produced.

A polyacrylic acid solution containing 10% by weight of polyacrylic acid was prepared by dissolving polyacrylic acid having a weight average molecular weight of 800,000 in N-methyl-2-pyrrolidone. Also, 0.2 g (1.0 mmol) of 4,4'-diaminodiphenylmethane was dissolved in 0.4 mL of N-methyl-2-pyrrolidone to prepare a 4,4'-diaminodiphenylmethane solution. Under stirring conditions, the total amount of the 4,4′-diaminodiphenylmethane solution was added dropwise to 7 mL of the polyacrylic acid solution (corresponding to 9.5 mmol in terms of acrylic acid monomer), and the resulting mixture was allowed to stand at room temperature for 30 minutes. Stirred. Then, using a Dean-Stark apparatus, the mixture was stirred at 130° C. for 3 hours to allow the dehydration reaction to proceed, thereby producing a binder solution.

72.5 parts by mass of a carbon-coated silicon material as a Si-containing negative electrode active material, 13.5 parts by mass of acetylene black as a conductive aid, the binder solution in an amount such that the solid content is 14 parts by mass as a binder, and A suitable amount of N-methyl-2-pyrrolidone was mixed to prepare a slurry. A copper foil having a thickness of 10 μm was prepared as a negative electrode current collector. A doctor blade was used to apply the slurry to the surface of this copper foil in the form of a film. The copper foil coated with the slurry was dried at 80° C. for 15 minutes to remove N-methyl-2-pyrrolidone. Then, by pressing, a negative electrode having a negative electrode active material layer with a thickness of 25 μm was manufactured.

The negative electrode was cut to a diameter of 11 mm and used as an evaluation electrode. A metallic lithium foil having a thickness of 500 μm was cut into a diameter of 13 mm to form a counter electrode. A glass filter (Hoechst Celanese) and celgard 2400 (Polypore Co., Ltd.), which is a single-layer polypropylene, were prepared as separators. Two types of separators were sandwiched between the counter electrode and the evaluation electrode in the order of the counter electrode, the glass filter, celgard2400, and the evaluation electrode to form an electrode assembly. This electrode body was housed in a coin-shaped battery case CR2032 (Hosen Co., Ltd.), and the electrolytic solution of Reference Example 1 was further injected to obtain a coin-shaped battery. This was used as a lithium ion secondary battery of Reference Example 1.

(Reference example 2)
(FSO ₂ ) ₂ NLi was dissolved in 4-methyltetrahydropyran to prepare an electrolytic solution of Reference Example 2 having a concentration of (FSO ₂ ) ₂ NLi of 1.5 mol/L.
A lithium ion secondary battery of Reference Example 2 was manufactured in the same manner as in Reference Example 1, except that the electrolytic solution of Reference Example 2 was used.

(Reference example 3)
(CF ₃ SO ₂ ) ₂ NLi was dissolved in 4-methyltetrahydropyran to prepare an electrolytic solution of Reference Example 3 having a (CF ₃ SO ₂ ) ₂ NLi concentration of 1.5 mol/L.
A lithium-ion secondary battery of Reference Example 3 was manufactured in the same manner as in Reference Example 1, except that the electrolytic solution of Reference Example 3 was used.

(Reference example 4)
(C ₂ F ₅ SO ₂ ) ₂ NLi was dissolved in 4-methyltetrahydropyran to prepare an electrolytic solution of Reference Example 4 having a concentration of (C ₂ F ₅ SO ₂ ) ₂ NLi of 1.5 mol/L. .
A lithium ion secondary battery of Reference Example 4 was manufactured in the same manner as in Reference Example 1, except that the electrolytic solution of Reference Example 4 was used.

(Reference example 5)
CF ₃ SO ₃ Li was dissolved in 4-methyltetrahydropyran to prepare an electrolytic solution of Reference Example 5 having a CF ₃ SO ₃ Li concentration of 1.5 mol/L.
A lithium-ion secondary battery of Reference Example 5 was manufactured in the same manner as in Reference Example 1, except that the electrolytic solution of Reference Example 5 was used.

(Reference example 6)
C ₄ F ₉ SO ₃ Li was dissolved in 4-methyltetrahydropyran to prepare an electrolytic solution of Reference Example 6 having a C ₄ F ₉ SO ₃ Li concentration of 1.5 mol/L.
A lithium ion secondary battery of Reference Example 6 was manufactured in the same manner as in Reference Example 1, except that the electrolytic solution of Reference Example 6 was used.

(Reference example 7)
To the electrolytic solution of Reference Example 1 containing LiBF ₄ , (difluoromethyl)trimethylsilane in an equimolar amount to LiBF ₄ was added to prepare a reaction solution, which was stirred at room temperature for 12 hours and then at 50° C. for 8 hours. did. The reaction solution was cooled to obtain an electrolytic solution of Reference Example 7 containing LiBF ₃ (CHF ₂ ) at a concentration of 1.5 mol/L.
A lithium ion secondary battery of Reference Example 7 was manufactured in the same manner as in Reference Example 1, except that the electrolytic solution of Reference Example 7 was used.
In addition, the reaction formula in said reaction is as follows. FSiMe ₃ , which is a by-product, has a low boiling point and moves out of the reaction system.
LiBF ₄ + (CHF ₂ )SiMe ₃ → LiBF ₃ (CHF ₂ ) + FSiMe ₃ ↑

(Reference example 8)
To the electrolytic solution of Reference Example 1 containing LiBF ₄ , an equimolar amount of (trifluoromethyl)trimethylsilane was added to LiBF ₄ to prepare a reaction solution, which was stirred at room temperature for 12 hours and then at 50° C. for 8 hours. Stirred. The reaction solution was cooled to obtain an electrolytic solution of Reference Example 8 containing LiBF ₃ (CF ₃ ) at a concentration of 1.5 mol/L.
A lithium-ion secondary battery of Reference Example 8 was manufactured in the same manner as in Reference Example 1, except that the electrolytic solution of Reference Example 8 was used.
In addition, the reaction formula in said reaction is as follows. FSiMe ₃ , which is a by-product, has a low boiling point and moves out of the reaction system.
LiBF ₄ + (CF ₃ )SiMe ₃ → LiBF ₃ (CF ₃ ) + FSiMe ₃ ↑

(Reference example 9)
To the electrolytic solution of Reference Example 1 containing LiBF ₄ , an equimolar amount of (pentafluoroethyl)trimethylsilane was added to LiBF ₄ to obtain a reaction solution, which was stirred at room temperature for 6 hours and then at 60° C. for 8 hours. Stirred. The reaction solution was cooled to obtain an electrolytic solution of Reference Example 9 containing LiBF ₃ (C ₂ F ₅ ) at a concentration of 1.5 mol/L.
A lithium ion secondary battery of Reference Example 9 was manufactured in the same manner as in Reference Example 1, except that the electrolytic solution of Reference Example 9 was used.
In addition, the reaction formula in said reaction is as follows. FSiMe ₃ , which is a by-product, has a low boiling point and moves out of the reaction system.
LiBF ₄ + (C ₂ F ₅ )SiMe ₃ → LiBF ₃ (C ₂ F ₅ ) + FSiMe ₃ ↑

(Reference example 10)
To the electrolytic solution of Reference Example 1 containing LiBF ₄ , (heptafluoropropyl)trimethylsilane in an equimolar amount to LiBF ₄ was added to prepare a reaction solution, which was stirred at room temperature for 6 hours and then at 60° C. for 8 hours. Stirred. The reaction solution was cooled to obtain an electrolytic solution of Reference Example 10 containing LiBF ₃ (C ₃ F ₇ ) at a concentration of 1.5 mol/L.
A lithium ion secondary battery of Reference Example 10 was manufactured in the same manner as in Reference Example 1, except that the electrolytic solution of Reference Example 10 was used.
In addition, the reaction formula in said reaction is as follows. FSiMe ₃ , which is a by-product, has a low boiling point and moves out of the reaction system.
LiBF ₄ + (C ₃ F ₇ )SiMe ₃ → LiBF ₃ (C ₃ F ₇ ) + FSiMe ₃ ↑

(Reference example 11)
To the electrolytic solution of Reference Example 1 containing LiBF ₄ , (trifluoromethyl)trimethylsilane corresponding to twice the molar amount of LiBF ₄ was added to prepare a reaction solution, which was stirred at room temperature for 12 hours, and then heated to 50°C. and stirred for 8 hours. The reaction solution was cooled to obtain an electrolytic solution of Reference Example 11 containing LiBF ₂ (CF ₃ ) ₂ at a concentration of 1.5 mol/L.
A lithium ion secondary battery of Reference Example 11 was manufactured in the same manner as in Reference Example 1, except that the electrolytic solution of Reference Example 11 was used.
In addition, the reaction formula in said reaction is as follows. FSiMe ₃ , which is a by-product, has a low boiling point and moves out of the reaction system.
LiBF ₄ +2(CF ₃ )SiMe ₃ →LiBF ₂ (CF ₃ ) ₂ +2FSiMe ₃ ↑

(Reference Comparative Example 1)
A mixed solvent was prepared by mixing fluoroethylene carbonate and diethyl carbonate at a volume ratio of 1:9. LiPF ₆ was mixed with the mixed solvent to prepare an electrolytic solution of Reference Comparative Example 1 in which the concentration of LiPF ₆ was 2 mol/L.
A lithium ion secondary battery of Reference Comparative Example 1 was manufactured in the same manner as in Reference Example 1, except that the electrolytic solution of Reference Comparative Example 1 was used.

(Reference Comparative Example 2)
LiBF ₄ was dissolved in tetrahydrofuran to prepare an electrolytic solution of Reference Comparative Example 2 having a LiBF ₄ concentration of 1.5 mol/L.
A lithium ion secondary battery of Reference Comparative Example 2 was manufactured in the same manner as in Reference Example 1 except that the electrolytic solution of Reference Comparative Example 2 was used.

(Reference Comparative Example 3)
LiBF ₄ was dissolved in cyclopentyl methyl ether to prepare an electrolytic solution of Reference Comparative Example 3 having a LiBF ₄ concentration of 2 mol/L.
A lithium ion secondary battery of Reference Comparative Example 3 was manufactured in the same manner as in Reference Example 1 except that the electrolytic solution of Reference Comparative Example 3 was used.

(Reference Comparative Example 4)
(FSO ₂ ) ₂ NLi was dissolved in tetrahydrofuran to prepare an electrolytic solution of Reference Comparative Example 4 having a concentration of (FSO ₂ ) ₂ NLi of 2 mol/L.
A lithium ion secondary battery of Reference Comparative Example 4 was manufactured in the same manner as in Reference Example 1 except that the electrolytic solution of Reference Comparative Example 4 was used.

(Reference Comparative Example 5)
(FSO ₂ ) ₂ NLi was dissolved in 1,2-dimethoxyethane to prepare an electrolytic solution of Reference Comparative Example 5 having a concentration of (FSO ₂ ) ₂ NLi of 2 mol/L.
A lithium ion secondary battery of Reference Comparative Example 5 was manufactured in the same manner as in Reference Example 1 except that the electrolytic solution of Reference Comparative Example 5 was used.

(Evaluation example 1)
Each lithium ion secondary battery was charged to 0.01 V at 0.2 mA and discharged to 1.0 V at 0.2 mA for the initial charge/discharge. Further, each lithium ion secondary battery was charged to 0.01 V at 0.5 mA and discharged to 1.0 V at 0.5 mA, and this charge/discharge cycle was repeated 20 times.
Note that in this evaluation example, the application in which the Si-containing negative electrode active material absorbs lithium is called charging, and the application in which the Si-containing negative electrode active material releases lithium is called discharging.

The initial efficiency and the capacity retention rate were calculated by the following formulas. Table 1 shows the results.
Initial efficiency (%) = 100 x (initial discharge capacity) / (initial charge capacity)
Capacity retention rate (%) = 100 × (discharge capacity at 20th cycle) / (discharge capacity at 1st cycle)
In the tables below, MTHP is an abbreviation for 4-methyltetrahydropyran, FEC is an abbreviation for fluoroethylene carbonate, DEC is an abbreviation for diethyl carbonate, THF is an abbreviation for tetrahydrofuran, CPME is an abbreviation for cyclopentyl methyl ether, and DME is an abbreviation for 1,2-dimethoxyethane.

From the results of the capacity retention rate of the lithium ion secondary battery of Reference Comparative Example 1 and the results of other Reference Comparative Examples, it can be seen that the lithium ion secondary battery of Reference Comparative Example 1 is formed mainly of decomposition products of fluoroethylene carbonate. It is considered that the presence of the SEI coating on the secondary battery exhibited a better capacity retention rate than the lithium ion secondary batteries of the other reference comparative examples.
However, since the lithium ion secondary battery of Reference Example showed a better capacity retention rate than the lithium ion secondary battery of Reference Comparative Example 1, in the lithium ion secondary battery of Reference Comparative Example 1, SEI Although the film prevented direct contact between the Si-containing negative electrode active material and the electrolyte, it is assumed that the Si-containing negative electrode active material was oxidized and deteriorated due to oxidizing components such as CO ₃ groups contained in the SEI film. be done.

From the results of Reference Example 1, Reference Comparative Example 2 and Reference Comparative Example 3 in which the electrolyte is LiBF ₄ and Reference Example 2, Reference Comparative Example 4 and Reference Comparative Example 5 in which the electrolyte is (FSO ₂ ) ₂ NLi, It can be seen that even ether has a great effect on battery performance due to the difference in its chemical structure.

From the above results, it can be said that the six-membered cyclic ether has excellent durability under reducing conditions.

(Example 1)
By dissolving (FSO ₂ ) ₂ NLi and LiBF ₂ (C ₂ O ₄ ) in 4-methyltetrahydropyran, the electrolyte (FSO ₂ ) ₂ NLi concentration is 1.8 mol/L, and the boron compound LiBF ₂ An electrolytic solution of Example 1 having a (C ₂ O ₄ ) concentration of 0.1 mol/L was prepared.
LiBF ₂ (C ₂ O ₄ ) is represented by R ¹ and R ² together representing =O, and R ³ and R ⁴ together representing =O in the chemical structure described herein. It has the chemical structure shown.

Using the electrolytic solution of Example 1, a lithium ion secondary battery of Example 1-1 and a lithium ion secondary battery of Example 1-2 were produced as follows.

(Example 1-1)
LiNi _0.8 Co _0.1 Al _0.1 O ₂ as a positive electrode active material, polyvinylidene fluoride as a binder, and acetylene black as a conductive aid were combined into LiNi _0.8 Co _0.1 Al _0.1 O ₂ , polyvinylidene fluoride and acetylene black were mixed at a mass ratio of 94:3:3, and N-methyl-2-pyrrolidone was added as a solvent to prepare a slurry. This slurry was applied to the surface of an aluminum foil as a current collector using a doctor blade and dried at 80° C. for 20 minutes to remove the solvent by volatilization to form a positive electrode active material layer. The aluminum foil having the positive electrode active material layer formed on its surface was compressed using a roll press to obtain a bonded product in which the aluminum foil and the positive electrode active material layer were brought into close contact with each other. The bonded product was heated at 120° C. for 6 hours in a vacuum dryer to produce a positive electrode.

The positive electrode was used as an evaluation electrode. A metallic lithium foil having a thickness of 500 μm was used as the counter electrode. A glass filter (Hoechst Celanese) was prepared as a separator. A separator was sandwiched between a counter electrode and an evaluation electrode to form an electrode body. This electrode body was housed in a coin-shaped battery case CR2032 (Hosen Co., Ltd.), and the electrolytic solution of Example 1 was further injected to obtain a coin-shaped battery. This was used as the lithium ion secondary battery of Example 1-1.

(Example 1-2)
A polypropylene porous membrane was prepared as a separator.
The positive electrode, the separator, and the negative electrode manufactured in the same manner as in Reference Example 1 were laminated in this order to form a laminate. This laminate and the electrolytic solution of Example 1 were placed in a laminated film bag, and the bag was sealed to produce a lithium ion secondary battery of Example 1-2.

(Example 2)
By dissolving (FSO ₂ ) ₂ NLi and LiBF ₂ (C ₂ O ₄ ) in 4-methyltetrahydropyran, the electrolyte (FSO ₂ ) ₂ NLi concentration is 1.8 mol/L, and the boron compound LiBF ₂ An electrolytic solution of Example 2 having a (C ₂ O ₄ ) concentration of 0.2 mol/L was prepared.
A lithium ion secondary battery of Example 2-1 was manufactured in the same manner as in Example 1-1, except that the electrolyte solution of Example 2 was used.

(Example 3)
By dissolving (FSO ₂ ) ₂ NLi and LiBF ₂ (C ₂ O ₄ ) in 4-methyltetrahydropyran, the electrolyte (FSO ₂ ) ₂ NLi concentration is 1 mol/L, the boron compound LiBF ₂ (C ₂ O ₄ ) concentration was 1 mol/L.
A lithium ion secondary battery of Example 3-1 was manufactured in the same manner as in Example 1-1, except that the electrolyte solution of Example 3 was used.

(Example 4)
By dissolving (FSO ₂ ) ₂ NLi and LiBF ₂ (C ₂ O ₄ ) in 4-methyltetrahydropyran, the electrolyte (FSO ₂ ) ₂ NLi concentration is 1 mol/L, the boron compound LiBF ₂ (C ₂ O ₄ ) concentration of 1.5 mol/L, the electrolytic solution of Example 4 was prepared.
A lithium ion secondary battery of Example 4-1 was manufactured in the same manner as in Example 1-1, except that the electrolyte solution of Example 4 was used.

(Example 5)
A mixed solvent was prepared by mixing 4-methyltetrahydropyran and 1,2-dimethoxyethane at a volume ratio of 8:2. (FSO ₂ ) ₂ NLi and LiBF ₂ (C ₂ O ₄ ) are dissolved in a mixed solvent, and the electrolyte (FSO ₂ ) ₂ NLi concentration is 1.8 mol/L, and the boron compound LiBF ₂ (C ₂ O ₄ ) concentration was 0.1 mol/L, and the electrolytic solution of Example 5 was prepared.
A lithium ion secondary battery of Example 5-1 was manufactured in the same manner as in Example 1-1, except that the electrolyte solution of Example 5 was used.

(Example 6)
(FSO ₂ ) ₂ NLi and lithium bisoxalate borate were dissolved in 4-methyltetrahydropyran to obtain a concentration of (FSO ₂ ) ₂ NLi as an electrolyte of 1.8 mol/L and lithium bis oxalate borate as a boron compound. concentration was 0.02 mol/L.
It should be noted that lithium bisoxalate borate has a chemical structure in which R ¹ and R ² together represent =O, and R ³ and R ⁴ together represent =O in the chemical structure described herein. have
A lithium ion secondary battery of Example 6-1 was manufactured in the same manner as in Example 1-1, except that the electrolyte solution of Example 6 was used.

(Example 7)
By dissolving (FSO ₂ ) ₂ NLi and bis(pinacolato)diboron in 4-methyltetrahydropyran, the concentration of (FSO ₂ ) ₂ NLi as an electrolyte is 1.8 mol/L, and bis(pinacolato)diboron as a boron compound. concentration was 0.05 mol/L.
Bis(pinacolato)diboron has the chemical structure described herein in which R ¹ , R ² , R ³ and R ⁴ are methyl groups.
A lithium ion secondary battery of Example 7-1 was manufactured in the same manner as in Example 1-1, except that the electrolyte solution of Example 7 was used.

(Example 8)
(FSO ₂ ) ₂ NLi and 2-methoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane were dissolved in 4-methyltetrahydropyran to obtain (FSO ₂ ) ₂ NLi as an electrolyte. The electrolytic solution of Example 8 having a concentration of 1.8 mol/L and a concentration of 2-methoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, which is a boron compound, of 0.1 mol/L manufactured.
2-Methoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane has the chemical structure described herein wherein R ¹ , R ² , R ³ and R ⁴ are methyl groups. has a chemical structure of
A lithium ion secondary battery of Example 8-1 was manufactured in the same manner as in Example 1-1, except that the electrolyte solution of Example 8 was used.

(Comparative example 1)
(FSO ₂ ) ₂ NLi was dissolved in 4-methyltetrahydropyran to prepare an electrolytic solution of Comparative Example 1 having a concentration of (FSO ₂ ) ₂ NLi as an electrolyte of 1.8 mol/L.
A lithium ion secondary battery of Comparative Example 1-1 was manufactured in the same manner as in Example 1-1, except that the electrolytic solution of Comparative Example 1 was used.
A lithium ion secondary battery of Comparative Example 1-2 was manufactured in the same manner as in Example 1-2, except that the electrolytic solution of Comparative Example 1 was used.

(Comparative example 2)
(FSO ₂ ) ₂ NLi and triisopropyl borate were dissolved in 4-methyltetrahydropyran, and the concentration of (FSO ₂ ) ₂ NLi and the concentration of triisopropyl borate were 1.8 mol/L and 0.1 mol, respectively. /L of the electrolytic solution of Comparative Example 2 was produced.
Although triisopropyl borate has an O—B—O bond, it is not cyclic.
A lithium ion secondary battery of Comparative Example 2-1 was manufactured in the same manner as in Example 1-1, except that the electrolytic solution of Comparative Example 2 was used.

(Comparative Example 3)
(FSO ₂ ) ₂ NLi and trimethyl borate were dissolved in 4-methyltetrahydropyran, and the concentration of (FSO ₂ ) ₂ NLi and the concentration of trimethyl borate were 1.8 mol/L and 0.1 mol/L, respectively. The electrolyte solution of Comparative Example 3 was produced.
Although trimethyl borate has an O—B—O bond, it is not cyclic.
A lithium ion secondary battery of Comparative Example 3-1 was manufactured in the same manner as in Example 1-1, except that the electrolytic solution of Comparative Example 3 was used.

(Comparative Example 4)
By dissolving (FSO ₂ ) ₂ NLi and 2,4,6-trimethoxyboroxine in 4-methyltetrahydropyran, the electrolyte (FSO ₂ ) ₂ NLi concentration is 1.8 mol/L, 2,4, An electrolytic solution of Comparative Example 4 having a 6-trimethoxyboroxine concentration of 0.1 mol/L was prepared.
Although 2,4,6-trimethoxyboroxine has an O—B—O bond, it has a 6-membered ring structure and does not have carbon as a constituent element of the ring.
A lithium ion secondary battery of Comparative Example 4-1 was manufactured in the same manner as in Example 1-1, except that the electrolytic solution of Comparative Example 4 was used.

(Comparative Example 5)
(FSO ₂ ) ₂ NLi and trimethyl phosphate were dissolved in 4-methyltetrahydropyran, and the concentration of (FSO ₂ ) ₂ NLi and trimethyl phosphate as electrolytes was 1.8 mol/L and 0.1 mol/L, respectively. The electrolyte solution of Comparative Example 5 was produced.
A lithium ion secondary battery of Comparative Example 5-1 was manufactured in the same manner as in Example 1-1, except that the electrolytic solution of Comparative Example 5 was used.

(Comparative Example 6)
(FSO ₂ ) ₂ NLi and 1,3-propanesultone were dissolved in 4-methyltetrahydropyran, and the concentration of (FSO ₂ ) ₂ NLi as an electrolyte was 1.8 mol/L, and the concentration of 1,3-propanesultone was was 0.1 mol/L.
A lithium ion secondary battery of Comparative Example 6-1 was manufactured in the same manner as in Example 1-1, except that the electrolytic solution of Comparative Example 6 was used.

(Evaluation example 2)
The lithium ion secondary batteries of Examples 1-1 to 8-1 and Comparative Examples 1-1 to 6-1 were charged to 4.2 V at a current of 0.5 mA and rested for 10 minutes. After that, the battery was discharged to 3.0 V at a current of 0.5 mA, and the charge-discharge cycle was repeated 20 times.
The capacity retention rate was calculated by the following formula. Table 2 shows the results.
Capacity retention rate (%) = 100 × (discharge capacity at 20th cycle) / (discharge capacity at 1st cycle)

The boron compounds added to the electrolytic solutions of the examples all have a five-membered ring structure composed of B, two oxygen elements, and two carbon elements forming an O--B--O bond. From the results of Examples 1-1 to 8-1 and Comparative Example 1-1, a 5-membered ring structure composed of B and two oxygen elements and two carbon atoms forming an O—B—O bond was It can be seen that the addition of a boron compound containing a compound remarkably improves the capacity retention rate of a secondary battery that is charged and discharged at a high potential.

On the other hand, from the results of Comparative Examples 2-1 to 4-1, the addition of an acyclic boron compound having an OB-O bond in the molecule, and the addition of three Bs forming an OB-O bond and It can be seen that the addition of a 6-membered ring structure boroxine derivative composed of three oxygen elements cannot improve the capacity retention rate of a secondary battery that is charged and discharged at a high potential.
In addition, from the results of Comparative Examples 5-1 to 6-1, addition of a phosphate ester having a PO bond in the molecule, S and oxygen elements that form an OS bond, and three carbon elements It can be seen that even the addition of the sultone having a five-membered ring structure cannot improve the capacity retention rate of a secondary battery that is charged and discharged at a high potential.

From the above results, the addition of a boron compound having a five-membered ring structure composed of B, two oxygen elements, and two carbon elements forming an O—B—O bond is a secondary It can be seen that the capacity retention rate of the battery is specifically improved.

(Evaluation example 3)
The lithium ion secondary batteries of Example 1-2 and Comparative Example 1-2 were charged with a current of 0.3 C to 3.9 V and stored at 60 ° C. for 20 hours. A charge/discharge cycle of charging to 4.07 V with a current and discharging to 3 V at a current of 1 C was repeated 100 times.
The capacity retention rate was calculated by the following formula. Table 3 shows the results.
Capacity retention rate (%) = 100 × (discharge capacity at 100th cycle) / (discharge capacity at 1st cycle)

From the results of Table 3, addition of a boron compound having a five-membered ring structure composed of B and two oxygen elements and two carbon elements forming an OBO bond is charged and discharged at a high potential. It can be seen that the capacity retention rate of the secondary battery is significantly improved.

Claims (4)

An electrolyte, a boron compound having a five-membered ring structure composed of B and two oxygen elements and two carbon atoms forming an OBO bond, and a six-membered ring optionally substituted with an alkyl group a non- aqueous solvent comprising a cyclic ether,
The electrolytic solution, wherein the ratio of the six-membered ring cyclic ether to the entire non- aqueous solvent is 50% by volume or more. The electrolytic solution according to claim 1, wherein the boron compound is a boron compound having the following chemical structure.

In the above chemical structures, R ¹ and R ² are each independently a group selected from hydrogen and alkyl groups, or R ¹ and R ² together represent =O. R ³ and R ⁴ are each independently a group selected from hydrogen and an alkyl group, or R ³ and R ⁴ together represent =O. A secondary battery comprising a positive electrode, a negative electrode, and the electrolytic solution according to claim 1 or 2. 4. The secondary battery according to claim 3, wherein the positive electrode active material provided in the positive electrode is charged/discharged at a potential of 4 V or higher with respect to lithium.