JP7243406B2 - Electrolyte and lithium ion secondary battery - Google Patents

Description

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

Lithium-ion secondary batteries generally include 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 electrolytic solution, a mixed solvent using LiPF ₆ as an electrolyte and a chain carbonate such as ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate and a cyclic carbonate such as ethylene carbonate as a non-aqueous solvent is used. A liquid is generally used.

Actually, in Patent Documents 1 to 3, LiPF ₆ is adopted as an electrolyte, and chain carbonates such as ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate and ethylene carbonate, which is a cyclic carbonate, are used as non-aqueous solvents. Lithium ion secondary batteries with solvent-based electrolytes are specifically described.

Electrolyte solutions employing (FSO ₂ ) ₂ NLi as the electrolyte are also known. Patent Document 4 discloses a lithium ion secondary battery equipped with an electrolytic solution employing a mixed solvent in which (FSO ₂ ) ₂ NLi and LiPF ₆ are used together as an electrolyte, and ethylene carbonate, dimethyl carbonate and fluoroethylene carbonate are used together as a non-aqueous solvent. Batteries are specifically mentioned.

JP 2015-185509 A JP 2015-179625 A WO2014/080608 JP 2011-150958 A

As described above, researches on electrolyte solutions used in lithium-ion secondary batteries are being vigorously conducted. Further, the industry is demanding a lithium-ion 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 lithium ion secondary battery having excellent battery characteristics.

The inventors have studied techniques for further improving the battery characteristics of lithium-ion secondary batteries. Specifically, the relationship between the negative electrode and the electrolytic solution was examined.

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 lithium ion 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 groups in the SEI coating oxidizes the Si-containing negative electrode active material. be. 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 present inventor aimed to use an ether-based solvent as a 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.

When the present inventor conducted a study using an ether-based solvent, it was found that the ether-based solvent is not compatible with LiPF ₆ , which is commonly used as an electrolyte, and that the ether-based solvent is not compatible with (FSO ₂ ) ₂ NLi. It was found to be superior to

Therefore, the present inventors have made intensive studies on electrolytic solutions employing (FSO ₂ ) ₂ NLi as the electrolyte and ether-based solvents as the non-aqueous solvent. By adding, it was found that the performance of the lithium ion secondary battery provided with the electrolytic solution is improved.

Based on these findings, the present invention was completed.

The electrolytic solution of the present invention contains (FSO ₂ ) ₂ NLi, a quaternary ammonium salt, and an ether solvent,
It is characterized by containing the quaternary ammonium salt at a ratio of 10 mol % or less with respect to the (FSO ₂ ) ₂ NLi.

A lithium ion secondary battery comprising the electrolyte solution of the present invention has excellent battery characteristics.

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 contains (FSO ₂ ) ₂ NLi, a quaternary ammonium salt, and an ether solvent,
It is characterized by containing the quaternary ammonium salt at a ratio of 10 mol % or less with respect to the (FSO ₂ ) ₂ NLi.

In the electrolytic solution of the present invention, the concentration of ( _FSO2 ) _2NLi is not limited. Preferred concentrations of (FSO ₂ ) ₂ NLi are in the range of 0.5 to 5 mol/L, 0.7 to 4 mol/L, 0.9 to 3 mol/L, and 1 to 2.5 mol/L. Within the range of L can be exemplified.

A quaternary ammonium salt is represented by the following general formula (1).
General formula (1) R ⁴ N ⁺ X ⁻
Each R is independently selected from a saturated alkyl group optionally substituted with a substituent, an unsaturated alkyl group optionally substituted with a substituent, and an aromatic group optionally substituted with a substituent be. The R's may combine with each other to form a ring, and a heteroatom selected from O, S and N may be present in the skeleton of the ring.
X is a monovalent anion.

Examples of the number of carbon atoms in R include 1 to 12, 1 to 8, 1 to 6, and 1 to 4.
Saturated alkyl groups include chain saturated alkyl groups and cyclic saturated alkyl groups, and unsaturated alkyl groups include chain unsaturated alkyl groups and cyclic unsaturated alkyl groups. A phenyl group and a naphthyl group can be illustrated as an aromatic group.
Examples of substituents for R include halogens, alkoxy groups, alkyl groups, and aromatic groups.

Examples of the ring formed by bonding R together include a pyrrolidine ring, a piperidine ring, a morpholine ring, a pyrrole ring, an imidazole ring, a pyrazole ring, an oxazole ring, a thiazole ring, a triazole ring, a pyridine ring, a pyrimidine ring, and a pyrazine ring. .

Specific chemical structures of quaternary ammonium salt cations include trimethylpropylammonium, trimethylhexylammonium, trimethyloctylammonium, tetraethylammonium, triethylmethylammonium, trimethylallylammonium, methoxymethyltrimethylammonium, and ethylmethoxymethyldimethylammonium. , 1-ethyl-1-methylpyrrolidinium, 1-ethyl-1-propylpyrrolidinium, 1-butyl-1-methylpyrrolidinium, 5-azoniaspiro[4.4]nonane, 1-propyl-1- Examples include methylpiperidinium, 1-ethyl-3-methylimidazolium, n-butyl-pyridinium.

The chemical structure of X includes halogen, ClO ₄ , carboxylate, phosphate, sulfonate, bisoxalate borate, AlCl ₄ , NO ₂ , NO ₃ , AsF ₆ , BF _4-x (C _n F _2n+1 ) _x (where , n is an integer of 1 to 4. x is an integer of 0 to 4.), PF _6−x (C _n F _2n+1 ) _x (where n is an integer of 1 to 4, x is 0 is an integer of ~6), N( _{SO2CnF2n +1} ) ₂ , (where n is each _{independently} an integer of 0 _to 6), N( _SO2 ) ₂ ( _CnF2n ) , (where n is an integer of 2 to 4).

From the viewpoint of price and charge/discharge characteristics, the chemical structure of X includes halogen, ClO ₄ , carboxylate, phosphate, sulfonate, bisoxalate borate, AlCl ₄ , NO ₂ , NO ₃ , AsF _{6 .} , BF _4-x (C _n F _2n+1 ) _x (where n is an integer of 1 to 4 and x is an integer of 0 to 4), PF _6-x (C _n F _2n+1 ) _x (where , n is an integer from 1 to 4, and x is an integer from 0 to 6.).

The mixing ratio of the quaternary ammonium salt in the electrolytic solution of the present invention is 10 mol % or less with respect to (FSO ₂ ) ₂ NLi. If the proportion of quaternary ammonium salt is too high, it is wasteful, and there is also the possibility that the proportion of quaternary ammonium salt that is too high can be detrimental.

The ratio of the quaternary ammonium salt to (FSO ₂ ) ₂ NLi is 0.005 to 5 mol%, 0.01 to 4 mol%, 0.05 to 3 mol%, 0.1 to 2 mol%, 0.5 to 5 mol%. 1.5 mol % can be exemplified.
As the quaternary ammonium salt, one type may be used, or multiple types may be used in combination.

The ether solvent is not limited as long as it can be used as a non-aqueous solvent for lithium ion secondary batteries. As the ether-based solvent, one type may be adopted, or a plurality of types may be used in combination.
Ether-based solvents include chain ethers and cyclic ethers.

As the chain ether, one having 1 or 2 oxygen atoms is preferable.
A chain ether having one oxygen is represented by R ¹ OR ² . Here, R ¹ and R ² are each independently a chain alkyl group or a cyclic alkyl group which may be substituted with fluorine. Examples of the number of carbon atoms in the chain alkyl group include 1-6, 2-5, and 3-4. Examples of the number of carbon atoms in the cyclic alkyl group include 3-8, 4-7 and 5-6.

Chain ethers include diethyl ether, di-n-propyl ether, di-i-propyl ether, ethyl n-propyl ether, ethyl i-propyl ether, di-n-butyl ether, di-iso-butyl ether, di-sec -butyl ether, di-tert-butyl ether, n-butyl methyl ether, iso-butyl methyl ether, sec-butyl methyl ether, tert-butyl methyl ether, n-butyl ethyl ether, iso-butyl ethyl ether, sec-butyl ethyl ether , tert-butyl ethyl ether, n-butyl n-propyl ether, iso-butyl n-propyl ether, sec-butyl n-propyl ether, tert-butyl n-propyl ether, di-n-pentyl ether, diisopentyl ether , methylneopentyl ether, ethylneopentyl ether, di-n-hexyl ether, methylcyclopentyl ether, methylcyclohexyl ether, ethylcyclopentyl ether, and ethylcyclohexyl ether.

Among chain ethers having one _oxygen number, _{examples of fluorine - substituted} ethers _{include CF3CH2OCF2CH3 , CF3CH2OCF2CHF2 , CHF2CF2CH2OCF2CHF2 ,} and _{CF3CF2 . CH2OCF2CHF2 , CF3CHFCF2OCH2CF3 , C3F7OCH3 , C4F9OCH3 , C6F13OCH3 , C2F5CF ( OCH3 ) C3F7 _ _ _ _ _ _ , C4F9OC2H5 , C3HF6CH ( CH3 ) OC3HF6 .}

Examples of chain ethers having two oxygen atoms include 1,2-dialkoxyethane and 1,3-dialkoxypropane. The alkoxy groups in these ethers each independently preferably have 1 to 6 carbon atoms, more preferably 1 to 4 carbon atoms, and even more preferably 1 to 2 carbon atoms.

As the cyclic ether, one having 1 or 2 oxygen atoms is preferable. The cyclic ether is preferably a 4- to 8-membered ring ether optionally substituted by an alkyl group, more preferably a 5- to 7-membered ring ether optionally substituted by an alkyl group, and is substituted by an alkyl group. 6-membered ring ethers are more preferred.
Examples of the cyclic ether having one oxygen include tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 2-methyltetrahydropyran, 3-methyltetrahydropyran, 4-methyltetrahydropyran, and oxepane. As the cyclic ether having 2 oxygen atoms, 1,4-dioxane can be exemplified.

The reason why a 6-membered ring optionally substituted with an alkyl group is preferable as the cyclic ether will be specifically explained.
A six-membered cyclic ether 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.

Electrolytes other than (FSO ₂ ) ₂ NLi and non-aqueous solvents other than ether solvents may be added to the electrolytic solution of the present invention within the scope of the present invention. Moreover, various additives may be added to the electrolytic solution of the present invention.

60 to 100 mol%, 70 to 99 mol%, 80 to 98 mol%, 90 to 98 mol% as a ratio of the total amount of (FSO ₂ ) _2NLi and quaternary ammonium salt to the total electrolyte contained in the electrolytic solution of the present invention 95 mol % can be exemplified.

Examples of the ratio of the ether solvent to the total nonaqueous solvent contained in the electrolytic solution of the present invention are 60 to 100% by volume, 70 to 98% by volume, 80 to 95% by volume, and 85 to 90% by volume.

A lithium ion secondary battery comprising 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 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 specific evaluation results to be described later, it is preferable that the positive electrode active material is charged/discharged at a potential of 4 V or more 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. Preferred lithium-based potentials at which the positive electrode active material charges and discharges include 4 to 5.5V, 4.1 to 5V, 4.2 to 4.8V, and 4.3 to 4.5V.

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. Graphite and Si-containing negative electrode active materials can be exemplified as suitable negative electrode active materials in view of specific evaluation results described later.

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, 3,5-diaminobenzoic acid, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, bis(4-aminophenyl)sulfone, benzidine, aromatic diamines such as o-tolidine, 2,4-tolylenediamine, 2,6-tolylenediamine, xylylenediamine and naphthalenediamine;

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, 4.2 to 4.8 V, and 4.3 to 4.5 V based on lithium.

Also, the lithium ion secondary battery of the present invention can operate under high temperature conditions. Examples of high temperature conditions include 40 to 80°C and 50 to 70°C.

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.

(Example 1)
(FSO ₂ ) ₂ NLi and 1-butyl-1-methylpyrrolidinium bromide, which is a quaternary ammonium salt, were dissolved in 4-methyltetrahydropyran, which is an ether solvent, so that the concentration of (FSO ₂ ) ₂ NLi was 1.01 mol/L and the concentration of the quaternary ammonium salt was 0.01 mol/L.
In the electrolytic solution of Example 1, the ratio of the quaternary ammonium salt to (FSO ₂ ) ₂ NLi is 1 mol %.

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

LiNi _0.866 Co _0.096 Al _0.038 O ₂ showing a layered rock salt structure as a positive electrode active material, polyvinylidene fluoride as a binder, acetylene black as a conductive aid, and LiNi _0.866 Co _0.096 Al _{0 .038} O ₂ , polyvinylidene fluoride, and acetylene black were mixed in a mass ratio of 95.7:2:2.3, and N-methyl-2-pyrrolidone was added as a solvent to prepare a slurry. An aluminum foil was prepared as a positive electrode current collector. A doctor blade was used to apply the slurry to the surface of the aluminum foil in a film form. The solvent was removed by drying the aluminum foil coated with the slurry. Through the above procedure, a positive electrode in which a positive electrode active material layer was formed on the surface of an aluminum foil was manufactured.

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) 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 Example 1 was further injected to obtain a coin-shaped battery. This was used as the lithium ion secondary battery of Example 1-P.

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

Polyacrylic acid having a weight average molecular weight of 1,500,000, 3,5-diaminobenzoic acid, and water were mixed to form a mixed aqueous solution. Here, the molar ratio of acrylic acid monomers constituting polyacrylic acid and 3,5-diaminobenzoic acid was set to 16:1.
A binder solution was produced by stirring the mixed aqueous solution at 80° C. for 2 hours in a nitrogen gas atmosphere. The water content in the binder solution was 85% by mass.

A Si-containing negative electrode active material was prepared by coating crystalline silicon powder with carbon.
80 parts by mass of the Si-containing negative electrode active material, 10 parts by mass of acetylene black as a conductive aid, the above binder solution in an amount such that the solid content is 10 parts by mass as a binder, and an appropriate amount of water are mixed to form a slurry. manufactured. A copper foil was prepared as a current collector for the negative electrode. A doctor blade was used to apply the slurry to the surface of this copper foil in the form of a film. Water was removed by drying the copper foil coated with the slurry. After that, a negative electrode having a negative electrode active material layer formed on the surface of the copper foil was manufactured by a procedure of pressing and heating at 230°C.

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 Example 1 was further injected to obtain a coin-shaped battery. This was used as the lithium ion secondary battery of Example 1-N.

(Example 2)
(FSO ₂ ) ₂ NLi and 1-butyl-1-methylpyrrolidinium bromide, which is a quaternary ammonium salt, were dissolved in 4-methyltetrahydropyran, which is an ether solvent, so that the concentration of (FSO ₂ ) ₂ NLi was 1.01 mol/L and the electrolyte of Example 2 with a concentration of quaternary ammonium salt of 0.05 mol/L was prepared.
In the electrolyte solution of Example 2, the ratio of the quaternary ammonium salt to (FSO ₂ ) ₂ NLi is 5 mol %.

A lithium ion secondary battery of Example 2-P was manufactured in the same manner as in Example 1, except that the electrolyte solution of Example 2 was used.

(Comparative example 1)
In the same manner as in Example 1, except that the quaternary ammonium salt was not used, the electrolyte solution of Comparative Example 1, the lithium ion secondary battery of Comparative Example 1-P, and the lithium ion of Comparative Example 1-N A secondary battery was manufactured.

(Evaluation example 1)
The lithium ion secondary batteries of Example 1-P, Example 2-P, and Comparative Example 1-P were charged at 0.2 mA to 4.35 V under the condition of 60° C., and charged at 0.2 mA to 3.5V. Initial charge/discharge was performed by discharging to 0V. Further, each lithium ion secondary battery was charged and discharged at 0.5 mA to 4.35 V and discharged at 0.5 mA to 3.0 V under the condition of 60° C., which was repeated 20 times. The capacity retention rate was calculated by the following formula.
Capacity retention rate (%) = 100 × (discharge capacity at 20th cycle) / (discharge capacity at 1st cycle)
Table 1 shows the above results.

Table 1 shows that the addition of the quaternary ammonium salt to the electrolytic solution improved the capacity retention rate of the lithium ion secondary battery. Also, from the results of Examples 1-P and 2-P, it can be understood that a small amount of the quaternary ammonium salt is sufficient.

(Evaluation example 2)
When the lithium ion secondary batteries of Example 1-N and Comparative Example 1-N were charged to 0.01 V at 0.2 mA and discharged to 1.0 V at 0.2 mA under the condition of 60 ° C. was charged and discharged. The initial efficiency was calculated by the following formula.
Initial efficiency (%) = 100 x (discharge capacity) / (charge capacity)
Table 2 shows the above results.

From Table 2, it can be seen that the lithium ion secondary battery of Example 1-N is superior to the lithium ion secondary battery of Comparative Example 1-N in both parameters of initial discharge capacity and initial efficiency.

Moreover, although Evaluation Example 1 and Evaluation Example 2 were tests under a high temperature condition of 60° C., the lithium ion secondary battery provided with the electrolytic solution of the present invention exhibited favorable battery characteristics. A lithium ion secondary battery comprising the electrolyte solution of the present invention is considered to be resistant to charging and discharging under high temperature conditions.

Claims (9)

(FSO ₂ ) ₂ NLi, a quaternary ammonium salt, and an ether solvent,
containing the quaternary ammonium salt at a ratio of 10 mol% or less with respect to the (FSO ₂ ) ₂ NLi ,
The ratio of the total amount of (FSO ₂ ) ₂ NLi and the quaternary ammonium salt to the total electrolyte contained in the electrolytic solution is 60 to 100 mol%,
The ratio of the ether solvent to the total non-aqueous solvent contained in the electrolytic solution is 60 to 100% by volume,
When the ether-based solvent contains a 6-membered ring ether optionally substituted with an alkyl group, and the 6-membered ring ether is substituted with an alkyl group, an alkyl group that can bond to the carbon of the 6-membered ring is bound to carbons not adjacent to the oxygen of the ether . (FSO ₂ ) ₂ NLi, a quaternary ammonium salt, and an ether solvent only ,
containing the quaternary ammonium salt at a ratio of 10 mol% or less with respect to the (FSO ₂ ) ₂ NLi ,
When the ether-based solvent contains a 6-membered ring ether optionally substituted with an alkyl group, and the 6-membered ring ether is substituted with an alkyl group, an alkyl group that can bond to the carbon of the 6-membered ring is bound to carbons not adjacent to the oxygen of the ether . (FSO ₂ ) ₂ NLi, a quaternary ammonium salt, and an ether solvent,
containing the quaternary ammonium salt at a ratio of 10 mol% or less with respect to the (FSO ₂ ) ₂ NLi ,
The ratio of the total amount of (FSO ₂ ) ₂ NLi and the quaternary ammonium salt to the total electrolyte contained in the electrolytic solution is 60 to 100 mol%,
The ratio of the ether solvent to the total non-aqueous solvent contained in the electrolytic solution is 60 to 100% by volume,
When the ether-based solvent contains a 6-membered ring ether optionally substituted with an alkyl group, and the 6-membered ring ether is substituted with an alkyl group, an alkyl group that can bond to the carbon of the 6-membered ring is bonded to a carbon not adjacent to an ether oxygen (except when said electrolyte is a semi-solid electrolyte for a semi-solid electrolyte) . (FSO ₂ ) ₂ NLi, a quaternary ammonium salt, and an ether solvent only ,
containing the quaternary ammonium salt at a ratio of 10 mol% or less with respect to the (FSO ₂ ) ₂ NLi ,
When the ether-based solvent contains a 6-membered ring ether optionally substituted with an alkyl group, and the 6-membered ring ether is substituted with an alkyl group, an alkyl group that can bond to the carbon of the 6-membered ring is bonded to a carbon not adjacent to an ether oxygen (except when said electrolyte is a semi-solid electrolyte for a semi-solid electrolyte) . The electrolytic solution according to any one of claims 1 to 4, wherein the 6-membered ring ether optionally substituted with an alkyl group is 4-methyltetrahydropyran. The anion of said quaternary ammonium salt is halogen, ClO ₄ , carboxylate, phosphate, sulfonate, bisoxalateborate, AlCl ₄ , NO ₂ , NO ₃ , AsF ₆ , BF _4-x (C _n F _2n+1 ) _x (where n is an integer of 1 to 4; x is an integer of 0 to 4), PF _6-x (C _n F _2n+1 ) _x (where n is an integer of 1 to 4; The electrolyte solution according to any one of claims 1 to 5 , wherein x is an integer of 0 to 6.). 7. The electrolytic solution according to any one of claims 1 to 6 , wherein the quaternary ammonium salt is contained in a ratio of 0.01 to 4 mol% with respect to the (FSO ₂ ) ₂ NLi. The electrolytic solution according to any one of claims 1 to 7, wherein the concentration of (FSO ₂ ) ₂ NLi is within the range of 0.5 to 5 mol/L. A lithium ion secondary battery comprising a positive electrode, a negative electrode, and the electrolytic solution according to any one of claims 1 to 8 .