JP2020043000A - Electrolyte and lithium ion secondary battery - Google Patents

Abstract

To provide a new lithium ion secondary battery with excellent battery characteristics.SOLUTION: A lithium ion secondary battery includes a negative electrode including a Si-containing negative electrode active material, and an electrolyte solution including an electrolyte and a 6-membered cyclic ether that is optionally substituted with an alkyl group, and a second ether having a higher relative dielectric constant than the 6-membered ring cyclic ether in a range in which (volume of the second ether)/(volume of 6-membered cyclic ether)≤3/7 is satisfied.SELECTED DRAWING: None

Description

  The present invention relates to an electrolytic solution used for a power storage device and a lithium ion secondary battery including the electrolytic solution.

  In general, a power storage device such as a secondary battery includes a positive electrode, a negative electrode, and an electrolyte as main components. The negative electrode is provided with a negative electrode active material involved in charge and discharge. The industry has been demanding an increase in the capacity of a power storage device, and various technologies are being studied to meet the demand. As one of specific technologies for increasing the capacity, it has been known to employ a Si-containing negative electrode active material containing Si, which has an excellent ability to occlude charge carriers such as lithium, as a negative electrode active material of a power storage device.

  For example, Patent Literature 1 and Patent Literature 2 describe a lithium ion secondary battery in which a negative electrode active material is SiO.

Patent Document 3 discloses a silicon obtained by synthesizing a layered silicon compound containing a layered polysilane whose main component is Ca by removing Ca by reacting CaSi ₂ with an acid and heating the layered silicon compound at 300 ° C. or more to release hydrogen. It describes that the material was manufactured, and a lithium ion secondary battery including the silicon material as a negative electrode active material.

A common electrolytic solution used for a power storage device contains an electrolyte and a non-aqueous solvent. Here, LiPF ₆ is generally used as the electrolyte, and a chain carbonate such as ethyl methyl carbonate, dimethyl carbonate and diethyl carbonate and a cyclic carbonate such as ethylene carbonate are generally used in combination as the non-aqueous solvent. It is a target. In fact, Patent Literatures 1 to 3 employ LiPF ₆ as an electrolyte and use a non-aqueous solvent in combination with a chain carbonate such as ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate, and ethylene carbonate that is a cyclic carbonate. A lithium ion secondary battery comprising a liquid is specifically described.

  It is also known that a fluorine-containing cyclic carbonate such as fluoroethylene carbonate and difluoroethylene carbonate is used as a part of the non-aqueous solvent. In fact, Patent Document 4 discloses that the capacity of a secondary battery is increased by using a fluorine-containing cyclic carbonate such as fluoroethylene carbonate or difluoroethylene carbonate as a nonaqueous solvent for an electrolyte in a secondary battery including a Si-containing negative electrode active material. It is stated that the retention was optimized (see Table 17).

JP 2015-185509 A JP 2015-179625 A International Publication No. 2014/080608 JP 2009-32492 A

The industry demands a lithium ion secondary battery having excellent battery characteristics.
The present invention has been made in view of such circumstances, and has as its object to provide a new lithium ion secondary battery having excellent battery characteristics.

  The present inventors have studied a technique for further improving the battery characteristics, particularly the capacity retention of a lithium ion secondary battery including a Si-containing negative electrode active material.

  The present inventor has considered that it is necessary to prevent the deterioration of the Si-containing negative electrode active material in order to improve the capacity retention rate of the lithium ion secondary battery including the Si-containing negative electrode active material. Then, it was considered that a carbonate-based solvent generally used in the past was one of the causes of deterioration of the Si-containing negative electrode active material. The reason is as follows.

It is known that a carbonate-based solvent forms a SEI (Solid Electrolyte Interphase) film on the surface of the negative electrode by reductive decomposition on the surface of the negative electrode. Although the SEI film can prevent direct contact between the negative electrode active material and the electrolytic solution, it is considered that the SEI film generated from a carbonate-based solvent as a raw material has a CO ₃ group. Then, it is estimated that the CO ₃ -based oxygen oxidizes the Si-containing negative electrode active material. In addition, since the Si-containing negative electrode active material has a large degree of expansion and contraction during charge and discharge, the SEI film is broken, and a new portion not covered with the SEI film (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 by contact with the carbonate-based solvent.

  Then, the present inventor aimed to adopt, as the non-aqueous solvent of the electrolytic solution, an ether which is unlikely to undergo reductive decomposition on the negative electrode surface and is considered to have low oxidizing ability. However, when tetrahydrofuran was employed as a specific ether, the capacity retention of the lithium ion secondary battery deteriorated.

The present inventor thought that the problem caused by tetrahydrofuran might be related to radical generation in tetrahydrofuran. Generally, in a CH ₂ group sandwiched between oxygen and an alkyl chain, radicals are considered to be easily generated because radicals are stabilized due to adjacent group interaction exerted by adjacent oxygen and adjacent alkyl chains. . In tetrahydrofuran, the CH ₂ group at the 2-position is where the radical is most likely to be generated.

Then, the present inventor searched for an ether which is considered to be less likely to generate a radical 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 electrolyte was manufactured and tested, and it was confirmed that the battery exhibited suitable battery characteristics.
However, it has also been found that when LiPF ₆ is employed as the electrolyte, a problem occurs.

Here the present inventors have defects caused in the case of adopting the LiPF ₆ as electrolyte, because is the cyclic ether 6-membered ring 4-methyl tetrahydropyran low dielectric constant, sufficiently lithium ion LiPF ₆ It was thought that coordination could not be performed, and as a result, lithium ions could not be moved smoothly in the electrolyte.

Therefore, the present inventor has proposed a mixed solvent using 4-methyltetrahydropyran which is a 6-membered cyclic ether as a main solvent and 1,2-dimethoxyethane having a higher dielectric constant than 4-methyltetrahydropyran as an auxiliary solvent. Then, an electrolytic solution in which LiPF ₆ was dissolved was produced, and a lithium ion secondary battery including the electrolytic solution was tested. As a result, it was found that the lithium ion secondary battery exhibited suitable battery characteristics.

  The present invention has been completed based on these findings.

The lithium ion secondary battery of the present invention,
A negative electrode comprising a Si-containing negative electrode active material;
An electrolyte, a 6-membered cyclic ether which may be substituted with an alkyl group, and a second ether having a higher relative dielectric constant than the 6-membered cyclic ether are obtained by (volume of the second ether) / (Volume of the 6-membered cyclic ether) ≦ 3/7.

  The lithium ion secondary battery of the present invention is excellent in battery characteristics, particularly, capacity retention.

  Hereinafter, embodiments for implementing the present invention will be described. Unless otherwise specified, the numerical range “ab” described in this specification includes the lower limit a and the upper limit b. A numerical range can be formed by arbitrarily combining these upper and lower limits and the numerical values listed in the examples. Further, numerical values arbitrarily selected from within these numerical ranges can be set as new upper and lower numerical values.

The lithium ion secondary battery of the present invention,
A negative electrode comprising a Si-containing negative electrode active material;
An electrolyte, a 6-membered cyclic ether which may be substituted with an alkyl group, and a second ether having a higher relative dielectric constant than the 6-membered cyclic ether are obtained by (volume of the second ether) / (Volume of the 6-membered cyclic ether) ≦ 3/7 (hereinafter referred to as the “electrolyte of the present invention”).

First, the electrolytic solution of the present invention will be described.
The electrolyte is not limited as long as it can be used as an electrolyte of a lithium ion secondary battery. One type of electrolyte may be used, or a plurality of types may be used in combination.
Specific examples of the electrolyte include LiClO ₄ , LiAsF ₆ , lithium borate, lithium sulfonimide, lithium sulfonate, and lithium phosphate. As the lithium borate, lithium sulfonimide, lithium sulfonate, and lithium phosphate, those containing fluorine in the molecule are preferable.

Examples of the lithium borate salt include a lithium borate salt of the general formula (1).
General formula (1) LiBF _{4-m-2 × l} R _m (C ₂ O ₄ ) ₁
In the general formula (1), m is 0, 1, 2, 3, or 4. l is 0, 1, or 2. R is _C n _H a _{F b.} n, a, and b are each independently an integer of 0 or more, and satisfy 2n + 1 = a + b.

As the sulfonimide lithium salt, a sulfonimide lithium salt of the general formula (2) can be exemplified.
General formula (2) (RSO ₂ ) ₂ NLi
In the general formula (2), R is each _{independently} a C _n H a _{F b.} n, a, and b are each independently an integer of 0 or more, and satisfy 2n + 1 = a + b. Two Rs may be bonded to each other to form a ring, in which case 2n = a + b is satisfied.

As the lithium sulfonate, a lithium sulfonate of the general formula (3) can be exemplified.
General formula (3) RSO ₃ Li
In the general formula (3), R is _C n _H a _{F b.} n, a, and b are each independently an integer of 0 or more, and satisfy 2n + 1 = a + b.

As the lithium phosphate, a lithium phosphate of the general formula (4) can be exemplified.
General formula (4) LiPF _{6-m-2 × l} R _m O _l
In the general formula (1), m is any one of 0, 1, 2, 3, 4, 5, and 6. l is 0, 1, or 2. R is _C n _H a _{F b.} n, a, and b are each independently an integer of 0 or more, and satisfy 2n + 1 = a + b.

  In the general formulas (1), (2), (3) and (4), n is preferably 0, 1, 2, 3, 4, 5, or 6. A is preferably 0, 1, or 2.

As the lithium borate salt, specifically, 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 , LiB (C ₂ O ₄ ) ₂ , and LiBF ₂ (C ₂ O ₄ ).

Specific examples of the sulfonimide lithium salt 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, and FSO ₂ (C ₂ F ₅ SO ₂ ) NLi, (SO ₂ CF ₂ CF ₂ SO ₂ ) NLi, (SO ₂ CF ₂ CF ₂ CF ₂ SO ₂ ) NLi, FSO ₂ (CH ₃ SO ₂ ) NLi, FSO ₂ (C ₂ H ₅ SO ₂₎ NLi it can be exemplified.

Specific examples of the lithium sulfonate 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, and C ₅ F ₁₁ SO. _{3 Li, C 6 F 13 SO} 3 Li, CH 3 SO 3 Li, C 2 H 5 SO 3 Li, C 3 H 7 SO 3 Li, CF 3 CH 2 SO 3 Li, CF 3 C 2 H 4 SO 3 Li Can be exemplified.

Specific examples of the lithium phosphate include LiPF ₆ , LiPF ₅ (CF ₃ ), LiPF ₄ (CF ₃ ) ₂ , LiPF ₃ (CF ₃ ) ₃ , LiPOF ₄ , and LiPO ₂ F ₂ .

  The concentration range of the entire electrolyte or the concentration range of the individual electrolyte in the electrolytic solution of the present invention is 0.3 to 4 mol / L, 0.5 to 3 mol / L, 1 to 2.5 mol / L, 1.3 to 2 mol. / L, 1.4 to 1.6 mol / L.

  It is considered that the 6-membered cyclic ether contained in the electrolytic solution of the present invention hardly generates a radical. The reason is as follows.

  In the 6-membered ring cyclic ether, the site where a radical is most likely to be generated, in other words, the carbon from which the hydrogen radical is most likely to be eliminated is the carbon adjacent to the ether oxygen among the carbons constituting the 6-membered ring cyclic ether. It can be said that there is.

The 6-membered cyclic ether has a conformation such as a chair-form or a boat-form that is stable in terms of thermal energy. And each conformation can also be converted mutually. Such a conformation is formed when the carbons constituting the ring have sp ³ hybrid orbitals.
However, the radical-produced carbon is converted from the sp ³ hybrid orbital to the sp ² hybrid orbital. Ideally, the three bonds in the sp ² hybrid orbital lie on the same plane.

Then, when a radical is generated in the 6-membered cyclic ether, the carbon having the radical has an sp ² hybridized orbital, and thus a steric configuration such as a heat-energy stable carbon-form or a boat-form. It cannot take a seat and must be in a three-dimensional structure that is unstable in terms of thermal energy.
It is considered that such a three-dimensional structure unstable in terms of thermal energy is an energy barrier for radicalizing a six-membered cyclic ether.

As the cyclic ether of a 6-membered ring 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 capable of bonding to the carbon of the 6-membered ring preferably has 1 to 6 carbon atoms, more preferably has 1 to 4 carbon atoms, and still more preferably has 1 to 2 carbon atoms.

The alkyl group that can be bonded to the carbon of the 6-membered ring is preferably bonded to a carbon that is not adjacent to the oxygen of the ether.
If the alkyl group in the carbon adjacent to the oxygen of the ether are attached are the carbon resulting in the structure of the O-CH (alkyl) -CH _2. Here, in a CH group sandwiched between oxygen and an alkyl chain, radicals are considered to be easily generated because radicals are stabilized due to adjacent group interactions exerted by adjacent oxygen and adjacent alkyl chains. . Therefore, it is presumed that a 6-membered cyclic ether in which an alkyl group is bonded to carbon adjacent to the oxygen of the ether is relatively easy to generate a radical.

  The second ether is not limited as long as it has a higher dielectric constant than the cyclic ether of the 6-membered ring which may be substituted with the alkyl group used. Since the second ether has a high relative permittivity and a high polarity, it is presumed that the second ether can suitably coordinate with the electrolyte to promote the movement of lithium ions.

  Table 1 shows the relative dielectric constants of the main ethers.

  When a cyclic ether having a tetrahydropyran skeleton is employed as the 6-membered cyclic ether, a 1,2-dialkoxyethane, an ether having a tetrahydrofuran skeleton, or an ether having a 1,3-dioxolane skeleton is used as the second ether. It can be said that it is preferable to employ.

In the electrolytic solution of the present invention, a 6-membered cyclic ether which may be substituted with an alkyl group, and a second ether having a higher relative dielectric constant than the 6-membered cyclic ether may be used (the second ether). ) / (Volume of the 6-membered cyclic ether) ≦ 3/7.
If the value of (volume of the second ether) / (volume of the 6-membered cyclic ether) exceeds 3/7, the battery characteristics may be deteriorated.
The preferable range of (volume of the second ether) / (volume of the cyclic ether having the 6-membered ring) = r is 0.5 / 9.5 ≦ r ≦ 2.5 / 7.5 or 1/9. ≦ r ≦ 2/8.

  A known non-aqueous solvent other than the 6-membered cyclic ether and the second ether may be used in combination with the electrolytic solution of the present invention without departing from the spirit of the present invention. The total ratio of the 6-membered cyclic ether and the second 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 volume. Mass%, 70 to 100 mass%, 90 to 100 mass%, 50 to 100 mol%, 70 to 100 mol%, 90 to 100 mol%.

  Known additives may be added to the electrolytic solution of the present invention without departing from the spirit of the present invention.

  Next, matters other than the electrolytic solution of the present invention, which specifically specify the lithium ion secondary battery of the present invention, will be described. Specifically, the lithium ion secondary battery of the present invention includes the electrolytic solution of the present invention, a negative electrode including the Si-containing negative electrode active material, a positive electrode, and a separator. A specific embodiment of the negative electrode includes a current collector and a negative electrode active material layer formed on a surface of the current collector. The negative electrode active material layer contains a Si-containing negative electrode active material.

  The current collector refers to a chemically inert electronic conductor for continuously supplying current to the electrodes during discharging or charging of the lithium ion secondary battery. The material of the current collector is not particularly limited as long as the metal can withstand a voltage suitable for the active material to be used. As the material of the current collector, 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 And other metal materials. 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 may be in the form of a foil, a sheet, a film, a line, a bar, a mesh, or the like. Therefore, for example, a metal foil such as a copper foil, a nickel foil, an aluminum foil, and a stainless steel foil can be suitably used as the current collector. When the current collector is in the form of a foil, sheet, or film, the thickness is preferably in the range of 1 μm to 100 μm.

  The negative electrode active material layer contains a Si-containing negative electrode active material and, if necessary, a binder and a conductive auxiliary. The negative electrode active material layer may include a known negative electrode active material other than the Si-containing negative electrode active material.

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

Specific examples of the Si-containing material include Si alone and SiO _x (0.3 ≦ x ≦ 1.6). If _x of SiO _x is less than the lower limit, the ratio of Si becomes excessively large, so that the volume change during charging and discharging becomes too large, and the cycle characteristics of the lithium ion secondary battery may be degraded. 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, and even more preferably 0.7 ≦ x ≦ 1.2.

  Specific examples of the Si-containing material include a silicon material disclosed in WO 2014/080608 (hereinafter, simply referred to as “silicon material”).

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

An ideal reaction formula for a method of manufacturing a silicon material using hydrogen chloride as an acid is as follows.
3CaSi ₂ + 6HCl → Si ₆ H ₆ + 3CaCl ₂
Si ₆ H ₆ → 6Si + 3H ₂ ↑

However, in the upper reaction of synthesizing Si ₆ H ₆ as polysilane, water is usually used as a reaction solvent from the viewpoint of removing by-products and impurities. Since Si ₆ H ₆ can react with water, in the step of synthesizing the layered silicon compound including the upper reaction, the layered silicon compound is hardly 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 anion of an acid, s + t + u = 6, 0 <s <6, 0 <t <6, 0 <u <6). Manufactured as such. In the above chemical formula, inevitable impurities such as Ca that may remain are not considered. The silicon material obtained by heating the layered silicon compound also contains elements derived from oxygen and anions of acids.

As described above, the silicon material has a structure in which a plurality of plate-like silicon bodies are stacked in the thickness direction. In order to efficiently store and release lithium ions, the plate-like silicon body preferably has a thickness in the range of 10 nm to 100 nm, more preferably 20 nm to 50 nm. The length in the longitudinal direction of the plate-like silicon body is preferably in the range of 0.1 μm to 50 μm. Moreover, it is preferable that (length in the longitudinal direction) / (thickness) of the plate-shaped silicon body is in the range of 2 to 1,000. The laminated structure of the plate-like silicon body can be confirmed by observation with a scanning electron microscope or the like. This laminated structure is considered to be a remnant of the Si layer in the raw material CaSi ₂ .

  Preferably, the silicon material includes amorphous silicon and / or silicon crystallites. In particular, in the plate-like silicon body, it is preferable that amorphous silicon is used as a matrix and silicon crystallites are scattered in the matrix. The size of the silicon crystallite is preferably in the range of 0.5 nm to 300 nm, more preferably in the range of 1 nm to 100 nm, still 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 obtained X-ray diffraction chart by performing X-ray diffraction measurement on the silicon material. .

  The abundance and size of the silicon plate, amorphous silicon, and silicon crystallite contained in the silicon material mainly depend on the heating temperature and the heating time. The heating temperature is preferably in the range of 350C to 950C, more preferably in the range of 400C to 900C.

The Si-containing negative electrode active material is preferably in the form of a powder, which is an aggregate of particles. The average particle diameter of the Si-containing negative electrode active material is preferably in the range of 1 to 30 μm, and more preferably in the range of 2 to 20 μm. When a Si-containing negative electrode active material having an average particle diameter that is too small is used, the manufacturing operation may be difficult. On the other hand, a lithium ion secondary battery provided with a negative electrode using a Si-containing negative electrode active material having an average particle diameter that is too large may not be able to suitably charge and discharge.
The average particle diameter herein means a D ₅₀ in the case of measuring a sample in a conventional laser diffraction particle size distribution analyzer.

  As the Si-containing negative electrode active material, a carbon layer-coated Si-containing negative electrode active material in which a Si-containing negative electrode active material is coated 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 coating-Si-containing negative electrode active material, a mechanical milling method of applying a strong pressure to a mixture of a Si-containing negative electrode active material and carbon powder and stirring and integrating the mixture, 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 preferred because the surface of the Si-containing negative electrode active material can be uniformly coated with a thin carbon layer. Among the CVD methods, a thermal CVD method in which a gaseous organic substance as a carbon source is decomposed by heat to generate carbon is preferable.

  The thermal CVD process for producing the carbon layer-coated Si-containing negative electrode active material using the thermal CVD method will be specifically described. More 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 to form a carbon layer formed by carbonizing the organic substance on the surface of the Si-containing negative electrode active material. Is a step of forming When performing the thermal CVD process, a known CVD device such as a fluidized-bed reaction furnace, a rotary furnace, a tunnel furnace, a batch-type firing furnace, or a rotary kiln of a type such as a hot wall type, a cold wall type, a horizontal type, or a vertical type is used. It may be used.

  As the organic substance, those which can be thermally decomposed and carbonized by heating under a non-oxidizing atmosphere are used, for example, methane, ethane, propane, butane, isobutane, pentane, saturated aliphatic hydrocarbons such as 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 And a mixture or a mixture selected from aromatic hydrocarbons such as pyridine, esters such as ethyl acetate, butyl acetate, and amyl acetate, and fatty acids.

  The processing temperature in the thermal CVD step varies depending on the type of the organic substance, but is preferably set to a temperature higher than the temperature at which the organic substance is thermally decomposed by 50 ° C. or more. However, if the heating temperature is excessively high, free carbon (soot) may be generated in the system. Therefore, it is preferable to select a condition under which free carbon (soot) is not generated. The thickness of the formed carbon layer can be controlled by the processing time.

  It is desirable that the thermal CVD process be performed with the Si-containing negative electrode active material 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 substance, and a more uniform carbon layer can be formed. There are various methods for bringing the Si-containing negative electrode active material into a fluidized state, for example, using a fluidized bed. However, it is preferable to bring the Si-containing negative electrode active material into contact with an organic substance while stirring. For example, if a rotary furnace having a baffle plate inside is used, the Si-containing negative electrode active material remaining on the baffle plate is agitated by falling from a predetermined height with the rotation of the rotary furnace, and at that time comes into contact with organic matter. As a result, 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 preferably 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 anode active material layer preferably contains the anode active material in an amount of 60 to 99% by mass, more preferably 70 to 95% by mass, based on the mass of the entire anode active material layer.

  Examples of the binder include polyvinylidene fluoride, polytetrafluoroethylene, fluorine-containing resins such as fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide-based resins such as polyimide and polyamideimide, resins containing an alkoxysilyl group, and carboxymethylcellulose. A known material such as styrene-butadiene rubber may be used.

  Further, a cross-linked polymer disclosed in WO 2016/063882 in which a carboxyl group-containing polymer such as polyacrylic acid or polymethacrylic acid is cross-linked with a polyamine such as diamine may be used as the binder.

  Examples of the diamine used for the crosslinked polymer include alkylenediamines such as ethylenediamine, propylenediamine, and hexamethylenediamine, 1,4-diaminocyclohexane, 1,3-diaminocyclohexane, isophoronediamine, and bis (4-aminocyclohexyl) methane. Saturated carbocyclic diamine, m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylether, bis (4-aminophenyl) sulfone, benzidine, o-tolidine, 2,4- Aromatic diamines such as tolylenediamine, 2,6-tolylenediamine, xylylenediamine, and naphthalenediamine are exemplified.

  The mixing ratio of the binder in the negative electrode active material layer is preferably from 1: 0.005 to 1: 0.3 in terms of mass ratio. If the amount of the binder is too small, the moldability of the electrode is reduced, and if the amount of the binder is too large, the energy density of the electrode is reduced.

  The conductive additive is added to increase the conductivity of the electrode. Therefore, the conductive assistant may be arbitrarily added when the conductivity of the electrode is insufficient, and may not be added when the conductivity of the electrode is sufficiently excellent. The conductive auxiliary agent may be any chemically inert high electron conductor, and examples thereof include carbon black, graphite, vapor grown carbon fiber (Vapor Grown Carbon Fiber), and various metal particles. You. Examples of the carbon black include acetylene black, Ketjen Black (registered trademark), furnace black, and channel black. These conductive assistants can be used alone or in combination of two or more.

  It is preferable that the compounding ratio of the conductive auxiliary in the negative electrode active material layer is negative electrode active material: conductive auxiliary = 1: 0.01 to 1: 0.5 by mass ratio. If the amount of the conductive assistant is too small, an efficient conductive path cannot be formed, and if the amount of the conductive assistant is too large, the moldability of the active material layer deteriorates and the energy density of the electrode decreases.

  To form the negative electrode active material layer on the surface of the current collector, a conventionally known method 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 is used. What is necessary is just to apply the negative electrode active material to the surface of the electric conductor. Specifically, a negative electrode active material, a binder, a solvent, and, if necessary, a conductive auxiliary are mixed to form a slurry, and then the slurry is applied to the surface of the current collector and then dried. Examples of the solvent include N-methyl-2-pyrrolidone, methanol, methyl isobutyl ketone, and water. The dried product may be compressed to increase the electrode density.

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

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

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

  Specific examples of aluminum or aluminum alloy include, 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-based alloy (Al-Fe-based).

  The positive electrode active material layer contains a positive electrode active material capable of occluding and releasing lithium ions, and, if necessary, a binder and a conductive assistant. The positive electrode active material layer preferably contains the positive electrode active material in an amount of 60 to 99% by mass, and more preferably 70 to 95% by mass, based on the total mass of the positive electrode active material layer. What is necessary is just to employ | adopt what was demonstrated by the negative electrode in a suitable quantity suitably as a binder and a conductive auxiliary agent.

As the positive electrode active material, the general formula of the layered rock salt _{structure: Li a Ni b Co c M} d D e O f (M is .D selected from Mn and Al 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, and 0 ≦ e <1. Satisfying 1.7 ≦ f ≦ 3), and a lithium composite metal oxide represented by the formula: Li ₂ MnO ₃ . As the positive electrode active material, a metal oxide having a spinel structure such as LiMn ₂ O ₄ , a solid solution composed of a mixture of a metal oxide having a spinel structure and a layered compound, LiMPO ₄ , LiMVO _4, or Li ₂ MSiO ₄ (wherein M is selected from at least one of Co, Ni, Mn, and Fe). Furthermore, as the positive electrode active material, tavorite compound (the M a transition metal) LiMPO ₄ F, such as LiFePO ₄ F represented by, Limbo ₃ such LiFeBO ₃ (M is a transition metal) include borate-based compound represented by be able to. Any of the metal oxides used as the positive electrode active material may have the above composition formula as the basic composition, and those obtained by replacing the metal element contained in the basic composition with another metal element can also be used. Alternatively, a material containing no lithium ion may be used as the positive electrode active material. For example, elemental sulfur, a compound of sulfur and carbon, a metal sulfide such as TiS ₂ , an oxide such as V ₂ O ₅ and MnO ₂ , a compound containing polyaniline and anthraquinone and their aromatics in the chemical structure, a conjugated compound Conjugated materials such as acetic acid-based organic materials and other known materials can also be used. Further, a compound having a stable radical such as nitroxide, nitronyl nitroxide, galvinoxyl, and phenoxyl may be employed as the positive electrode active material. When a positive electrode active material containing no lithium ions is used, it is preferable to add lithium to the positive electrode and / or the negative electrode in advance by a known method.

From the viewpoint of excellent and high capacity, and durability, as the positive electrode active material, the general formula of the layered rock salt _{structure: Li a Ni b Co c M} d D e O f (M is .D selected from Mn and Al 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, 1.7 ≦ f ≦ 3.) It is preferable to employ a lithium composite metal oxide represented by the following formula:

  In the above general formula, the values of b, c, and d are not particularly limited as long as they satisfy the above conditions, but those satisfying 0 <b <1, 0 <c <1, and 0 <d <1. In addition, at least one of b, c, and d may be in the range of 10/100 <b <90/100, 10/100 <c <90/100, and 5/100 <d <70/100. More preferably, it is more preferably in the range of 20/100 <b <80/100, 12/100 <c <70/100, 10/100 <d <60/100, and 30/100 <b <70/100, It is more preferable that the range of 15/100 <c <50/100 and the range of 12/100 <d <50/100 are satisfied.

  a, e, and f may be numerical values within the range defined by the above general formula, and are preferably 0.5 ≦ a ≦ 1.5, 0 ≦ e <0.2, and 1.8 ≦ f ≦ 2. 0.5, more preferably 0.8 ≦ a ≦ 1.3, 0 ≦ e <0.1, and 1.9 ≦ f ≦ 2.1.

From the viewpoint of excellent and high capacity, and durability, as a positive electrode active material, the _{Li x Mn 2-y A y} O 4 (A spinel structure, Ca, Mg, S, Si , Na, K, Al, P, Ga , Ge, and at least one metal element selected from transition metal elements such as Ni, and 0 <x ≦ 2.2, 0 ≦ y ≦ 1). The range of the value of x can be exemplified by 0.5 ≦ x ≦ 1.8, 0.7 ≦ x ≦ 1.5, 0.9 ≦ x ≦ 1.2, and the range of the value of y is 0 ≦ y ≦ 0.8 and 0 ≦ y ≦ 0.6. Specific examples of the compound having a spinel structure include LiMn ₂ O ₄ and LiMn _1.5 Ni _0.5 O ₄ .

Specific positive electrode active material _can be exemplified by _{LiFePO 4, Li 2 FeSiO 4,} LiCoPO 4, Li 2 CoPO 4, Li 2 MnPO 4, Li 2 MnSiO 4, Li 2 CoPO 4 F. As another specific positive electrode active material _can be exemplified by Li ₂ MnO 3 -LiCoO _2.

  The separator separates the positive electrode and the negative electrode, and prevents lithium ions from passing through while preventing a short circuit due to contact between the two electrodes. Known separators may be used as the separator, and synthetic resins such as polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamide, polyaramid (Aromatic polyamide), polyester, and polyacrylonitrile; polysaccharides such as cellulose and amylose; and fibroin. , Natural polymers such as keratin, lignin, suberin, and the like, porous bodies, nonwoven fabrics, and woven fabrics using one or more electrically insulating materials such as ceramics. Further, 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 any of a stacked type in which a positive electrode, a separator, and a negative electrode are stacked, or a wound type in which a stacked 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 electrolytic solution of the present invention is added to the electrode body, and lithium ion secondary Use a battery.

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

  The lithium ion secondary battery of the present invention may be mounted on a vehicle. The vehicle may be any vehicle that uses electric energy from a lithium ion secondary battery for all or a part of its power source, such as an electric vehicle or a hybrid vehicle. When a lithium ion secondary battery is mounted on a vehicle, a plurality of lithium ion secondary batteries may be connected in series to form an assembled battery. Examples of devices equipped with a lithium ion secondary battery include various home electric appliances, office equipment, industrial equipment, and the like, other than vehicles, such as personal computers and portable communication devices, which are driven by batteries. Further, the lithium ion secondary battery of the present invention is a power storage device and a power smoothing device for wind power generation, photovoltaic power generation, hydroelectric power generation and other power systems, power sources for ships and the like and / or power supply sources for auxiliary equipment, aircraft, Power supply for spacecraft and other power supplies and / or auxiliary equipment, auxiliary power supply for vehicles that do not use electricity as power source, power supply for mobile home robots, power supply for system backup, power supply for uninterruptible power supply, The present invention may be applied to a power storage device for temporarily storing power required for charging at a charging station for an electric vehicle or the like.

  The embodiment of the present invention has been described above, but the present invention is not limited to the above embodiment. The present invention can be implemented in various forms with modifications, improvements, and the like that can be made by those skilled in the art without departing from the gist of the present invention.

  Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples. Note that the present invention is not limited by these examples.

(Reference Example 1)
LiBF ₄ was dissolved in 4-methyltetrahydropyran to prepare an electrolyte solution of Reference Example 1 in which the concentration of LiBF ₄ was 1.5 mol / L.

  Using the electrolyte 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 reacted for 1 hour. Water was added to the reaction solution, the mixture was filtered, and a yellow powder was collected by filtration. 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 release hydrogen, thereby producing a silicon material. By heating the silicon material at 880 ° C. in a propane gas atmosphere, a carbon-coated silicon material as a carbon layer-coated negative electrode active material containing Si was produced.

  Polyacrylic acid having a weight average molecular weight of 800,000 was dissolved in N-methyl-2-pyrrolidone to prepare a polyacrylic acid solution containing 10% by mass of polyacrylic acid. Further, 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 entire amount of the 4,4′-diaminodiphenylmethane solution was added dropwise to 7 mL of the polyacrylic acid solution (equivalent 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. Thereafter, using a Dean-Stark apparatus, the mixture was stirred at 130 ° C. for 3 hours to advance a dehydration reaction, 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 additive, an amount of the binder solution in which the solid content becomes 14 parts by mass as a binder, and An appropriate amount of N-methyl-2-pyrrolidone was mixed to produce a slurry. A 10-μm-thick copper foil was prepared as a negative electrode current collector. The slurry was applied to the surface of the copper foil in the form of a film using a doctor blade. The copper foil coated with the slurry was dried at 80 ° C. for 15 minutes to remove N-methyl-2-pyrrolidone. Thereafter, by pressing, a negative electrode on which a 25 μm-thick negative electrode active material layer was formed was manufactured.

  The negative electrode was cut into a diameter of 11 mm and used as an evaluation electrode. A metal lithium foil having a thickness of 500 μm was cut into a diameter of 13 mm to serve as a counter electrode. As a separator, a glass filter (Hoechst Celanese) and celgard 2400 (Polypore), which is a single-layer polypropylene, were prepared. Two types of separators were sandwiched between the counter electrode and the evaluation electrode in the order of the counter electrode, the glass filter, celgard 2400, and the evaluation electrode to form an electrode body. The electrode body was housed in a coin-type battery case CR2032 (Hosen Co., Ltd.), and the electrolyte of Reference Example 1 was further injected to obtain a coin-type battery. This was designated 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 electrolyte solution of Reference Example 2 in which the concentration of (FSO ₂ ) ₂ NLi was 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 produce an electrolyte solution of Reference Example 3 in which the concentration of (CF ₃ SO ₂ ) ₂ NLi was 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 electrolyte solution of Reference Example 4 in which the concentration of (C ₂ F ₅ SO ₂ ) ₂ NLi was 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 electrolyte solution of Reference Example 5 in which the concentration of CF ₃ SO ₃ Li was 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 electrolyte solution of Reference Example 6 in which the concentration of C ₄ F ₉ SO ₃ Li was 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 electrolyte solution of Reference Example 6 was used.

(Reference Example 7)
The electrolyte solution of Example 1 containing LiBF _4, and the reaction solution was added with LiBF ₄ in equimolar (difluoromethyl) trimethylsilane, which after stirring for 12 hours at room temperature, 8 hours at 50 ° C. did. The reaction solution was cooled to obtain an electrolyte 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 electrolyte solution of Reference Example 7 was used.
In addition, the reaction formula in the above reaction is as follows. Since the by-produced FSiMe ₃ has a low boiling point, it moves out of the reaction system.
LiBF ₄ + (CHF ₂ ) SiMe ₃ → LiBF ₃ (CHF ₂ ) + FSiMe ₃ ↑

(Reference Example 8)
LiBF ₄ was added to the electrolytic solution of Reference Example 1 to obtain a reaction solution obtained by adding an equimolar amount of (trifluoromethyl) trimethylsilane to LiBF _4, and the mixture 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 electrolyte 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 electrolyte solution of Reference Example 8 was used.
In addition, the reaction formula in the above reaction is as follows. Since the by-produced FSiMe ₃ has a low boiling point, it moves out of the reaction system.
LiBF ₄ + (CF ₃ ) SiMe ₃ → LiBF ₃ (CF ₃ ) + FSiMe ₃ ↑

(Reference Example 9)
To the electrolyte solution of Reference Example 1 containing LiBF ₄ , an equimolar amount of (pentafluoroethyl) trimethylsilane was added to LiBF ₄ to form 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 electrolyte 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 the above reaction is as follows. Since the by-produced FSiMe ₃ has a low boiling point, it 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)
The electrolyte solution of Example 1 containing LiBF _4, and the reaction solution was added with LiBF ₄ in equimolar (heptafluoropropyl) trimethylsilane, which After stirring at room temperature for 6 hours, 8 hours at 60 ° C. Stirred. The reaction solution was cooled to obtain an electrolyte 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 electrolyte solution of Reference Example 10 was used.
In addition, the reaction formula in the above reaction is as follows. Since the by-produced FSiMe ₃ has a low boiling point, it 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 electrolyte solution of Example 1 containing LiBF ₄ , (trifluoromethyl) trimethylsilane corresponding to twice the mole of LiBF ₄ was added to form a reaction solution, which was stirred at room temperature for 12 hours and then heated to 50 ° C. For 8 hours. The reaction solution was cooled to obtain an electrolyte 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 the above reaction is as follows. Since the by-produced FSiMe ₃ has a low boiling point, it moves out of the reaction system.
LiBF ₄ +2 (CF ₃ ) SiMe ₃ → LiBF ₂ (CF ₃ ) ₂ + 2FSiMe ₃ ↑

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

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

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

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

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

(Comparative Example 6)
LiPF ₆ was dissolved in 4-methyltetrahydropyran to produce an electrolyte solution of Comparative Example 6 in which the concentration of LiPF ₆ was 1.5 mol / L.
A lithium ion secondary battery of Comparative Example 6 was manufactured in the same manner as in Reference Example 1, except that the electrolytic solution of Comparative Example 6 was used.

(Evaluation Example 1)
Each lithium ion secondary battery was charged / discharged at 0.2 mA to 0.01 V and then discharged at 0.2 mA to 1.0 V. Further, a charge / discharge cycle of charging each lithium ion secondary battery to 0.01 V at 0.5 mA and discharging it to 1.0 V at 0.5 mA was repeated 20 times.
In this evaluation example, the application in which the Si-containing negative electrode active material absorbs lithium is referred to as charging, and the application in which the Si-containing negative electrode active material releases lithium is referred to as discharging.

The initial efficiency and the capacity retention were calculated by the following equations. Table 2 shows the results.
Initial efficiency (%) = 100 × (initial discharge capacity) / (initial charge capacity)
Capacity retention rate (%) = 100 × (discharge capacity at 20th cycle) / (discharge capacity at 1st cycle)
In the following table, 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 in Table 2, it can be seen that the lithium ion secondary battery of the reference example including the 6-membered cyclic ether as the non-aqueous solvent has excellent initial efficiency and capacity retention.
In view of the results of the capacity retention ratio of the lithium ion secondary battery of Comparative Example 1 and the results of other comparative examples, the lithium ion secondary battery of Comparative Example 1 is mainly composed of an SEI film formed of a decomposition product of fluoroethylene carbonate. It is considered that due to the presence of, the capacity retention ratio was superior to that of the lithium ion secondary batteries of other comparative examples. However, since the lithium ion secondary battery of the reference example exhibited a better capacity retention ratio than the lithium ion secondary battery of Comparative Example 1, the SEI coating of the lithium ion secondary battery of Comparative Example 1 was Although direct contact between the Si-containing negative electrode active material and the electrolytic solution was prevented, it is estimated 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 coating. .

From the results of Reference Example 1, Comparative Example 2 and Comparative Example 3 in which the electrolyte is LiBF ₄ , and Reference Example 2, Comparative Example 4 and Comparative Example 5 in which the electrolyte is (FSO ₂ ) ₂ NLi, it is ether. It can also be seen that the difference in the chemical structure greatly affects the battery performance.

As supported by Reference Examples 1 to 11, the electrolyte in the case of using 4-methyltetrahydropyran as the non-aqueous solvent and lithium borate, lithium sulfonimide or lithium sulfonate as the electrolyte is lithium ion It can be said that the secondary battery has suitable characteristics. However, as evidenced by Comparative Example 6, the electrolyte using 4-methyltetrahydropyran as the non-aqueous solvent and LiPF ₆ as the electrolyte can be said to be inappropriate.

(Example 1)
4-Methyltetrahydropyran and 1,2-dimethoxyethane as a second ether are mixed at (volume of 1,2-dimethoxyethane) / (volume of 4-methyltetrahydropyran) = 1/9, and mixed with a mixed solvent. did. LiPF ₆ was dissolved in the mixed solvent to produce the electrolyte solution of Example 1 in which the concentration of LiPF ₆ was 1 mol / L.
A lithium ion secondary battery of Example 1 was manufactured in the same manner as in Reference Example 1, except that the electrolytic solution of Example 1 was used.

(Example 2)
4-Methyltetrahydropyran and 1,2-dimethoxyethane as the second ether are mixed in a ratio of (volume of 1,2-dimethoxyethane) / (volume of 4-methyltetrahydropyran) = 2/8 and a mixed solvent. did. LiPF ₆ was dissolved in the mixed solvent to produce an electrolyte solution of Example 2 in which the concentration of LiPF ₆ was 1 mol / L.
A lithium ion secondary battery of Example 2 was manufactured in the same manner as in Reference Example 1 except that the electrolytic solution of Example 2 was used.

(Example 3)
4-methyltetrahydropyran and 1,2-dimethoxyethane as the second ether are mixed in a ratio of (volume of 1,2-dimethoxyethane) / (volume of 4-methyltetrahydropyran) = 3/7, and mixed with a mixed solvent. did. LiPF ₆ was dissolved in the mixed solvent to produce an electrolyte of Example 3 in which the concentration of LiPF ₆ was 1 mol / L.
A lithium ion secondary battery of Example 3 was manufactured in the same manner as in Reference Example 1 except that the electrolytic solution of Example 3 was used.

(Comparative Example 7)
4-Methyltetrahydropyran and 1,2-dimethoxyethane as the second ether are mixed in a ratio of (volume of 1,2-dimethoxyethane) / (volume of 4-methyltetrahydropyran) = 5/5, and mixed with a mixed solvent. did. LiPF ₆ was dissolved in the mixed solvent to produce an electrolyte solution of Comparative Example 7 in which the concentration of LiPF ₆ was 1 mol / L.
A lithium ion secondary battery of Comparative Example 7 was manufactured in the same manner as in Reference Example 1, except that the electrolytic solution of Comparative Example 7 was used.

(Comparative Example 8)
4-Methyltetrahydropyran and 1,2-dimethoxyethane as the second ether are mixed in a ratio of (volume of 1,2-dimethoxyethane) / (volume of 4-methyltetrahydropyran) = 7/3 and a mixed solvent. did. LiPF ₆ was dissolved in the mixed solvent to produce an electrolyte of Comparative Example 8 in which the concentration of LiPF ₆ was 1 mol / L.
A lithium ion secondary battery of Comparative Example 8 was manufactured in the same manner as in Reference Example 1 except that the electrolytic solution of Comparative Example 8 was used.

(Example 4)
4-Methyltetrahydropyran and 1,2-dimethoxyethane as a second ether are mixed at (volume of 1,2-dimethoxyethane) / (volume of 4-methyltetrahydropyran) = 1/9, and mixed with a mixed solvent. did. (FSO ₂ ) ₂ NLi was dissolved in the mixed solvent to prepare an electrolyte solution of Example 4 in which the concentration of (FSO ₂ ) ₂ NLi was 1.5 mol / L.
A lithium ion secondary battery of Example 4 was manufactured in the same manner as in Reference Example 1 except that the electrolytic solution of Example 4 was used.

(Example 5)
4-Methyltetrahydropyran and 1,2-dimethoxyethane as the second ether are mixed in a ratio of (volume of 1,2-dimethoxyethane) / (volume of 4-methyltetrahydropyran) = 2/8 and a mixed solvent. did. (FSO ₂ ) ₂ NLi was dissolved in the mixed solvent to prepare an electrolyte solution of Example 5 in which the concentration of (FSO ₂ ) ₂ NLi was 1.5 mol / L.
A lithium ion secondary battery of Example 5 was manufactured in the same manner as in Reference Example 1 except that the electrolytic solution of Example 5 was used.

(Example 6)
4-methyltetrahydropyran and 1,2-dimethoxyethane as the second ether are mixed in a ratio of (volume of 1,2-dimethoxyethane) / (volume of 4-methyltetrahydropyran) = 3/7, and mixed with a mixed solvent. did. (FSO ₂ ) ₂ NLi was dissolved in the mixed solvent to prepare an electrolyte solution of Example 6 in which the concentration of (FSO ₂ ) ₂ NLi was 1.5 mol / L.
A lithium ion secondary battery of Example 6 was manufactured in the same manner as in Reference Example 1, except that the electrolytic solution of Example 6 was used.

(Comparative Example 9)
4-Methyltetrahydropyran and 1,2-dimethoxyethane as the second ether are mixed in a ratio of (volume of 1,2-dimethoxyethane) / (volume of 4-methyltetrahydropyran) = 5/5, and mixed with a mixed solvent. did. (FSO ₂ ) ₂ NLi was dissolved in the mixed solvent to produce an electrolyte solution of Comparative Example 9 in which the concentration of (FSO ₂ ) ₂ NLi was 1.5 mol / L.
A lithium ion secondary battery of Comparative Example 9 was manufactured in the same manner as in Reference Example 1 except that the electrolytic solution of Comparative Example 9 was used.

(Evaluation example 2)
The same tests as in Evaluation Example 1 were performed on the lithium ion secondary batteries of Examples 1 to 6 and Comparative Examples 7 to 9. Table 3 shows the results obtained when LiPF ₆ was used as the electrolyte, and Table 4 shows the results obtained when (FSO ₂ ) ₂ NLi was used as the electrolyte.
In the following table, r means (volume of 1,2-dimethoxyethane) / (volume of 4-methyltetrahydropyran).

From the results in Table 3, it can be seen that when LiPF ₆ is used as the electrolyte, there is an optimum r near ／. When r is larger than 3/7, it can be said that the capacity retention ratio sharply decreases.

From the results in Table 4, it can be seen that, even when (FSO ₂ ) ₂ NLi is used as the electrolyte, the capacity retention rate sharply decreases when r is larger than 3/7.

Claims (4)

A negative electrode comprising a Si-containing negative electrode active material;
An electrolyte, a 6-membered cyclic ether which may be substituted with an alkyl group, and a second ether having a higher relative dielectric constant than the 6-membered cyclic ether are obtained by (volume of the second ether) / (Volume of the 6-membered cyclic ether) ≦ 3/7, an electrolytic solution contained within a range satisfying;
A lithium ion secondary battery comprising:   The lithium ion secondary battery according to claim 1, wherein the electrolyte contains a lithium phosphate salt.   An electrolyte, a 6-membered cyclic ether which may be substituted with an alkyl group, and a second ether having a higher relative dielectric constant than the 6-membered cyclic ether are obtained by (volume of the second ether) / An electrolytic solution characterized by containing (volume of the 6-membered cyclic ether) ≦ 3/7.   The electrolytic solution according to claim 3, wherein the electrolytic solution contains a lithium phosphate.