JP2018147694A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery having superior characteristics.SOLUTION: A lithium ion secondary battery comprises: a positive electrode including a lithium metal composite oxide represented by the general formula of a lamellar rock-salt structure, LiNiCoMDO; a negative electrode including a silicon material having a structure in which a plurality of platelet silicon bodies are laminated in a thickness direction; and an electrolyte solution containing (FSO)NLi and a chain carbonate represented by the general formula (A) below, provided that the mole ratio of the chain carbonate to the (FSO)NLi is within a range of 2-6. The value of (Capacity of the negative electrode)/(Capacity of the positive electrode) is 1.07-1.13. ROCOOR(A).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a lithium ion secondary battery.

In general, a lithium ion secondary battery includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and an electrolytic solution as main components. An appropriate electrolyte is added to the electrolytic solution in an appropriate concentration range. For example, a lithium salt such as LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , CF ₃ SO ₃ Li, or (CF ₃ SO ₂ ) ₂ NLi is added as an electrolyte to the electrolyte solution of a lithium ion secondary battery. Here, the concentration of the lithium salt in the electrolytic solution is generally about 1 mol / L.

  In addition, in order to dissolve the electrolyte suitably, an organic solvent having a high relative dielectric constant and dipole moment, such as ethylene carbonate and propylene carbonate, is mixed and used at about 30% by volume or more for the organic solvent used in the electrolytic solution. It is common.

Actually, Patent Document 1 discloses a lithium ion secondary battery using a mixed organic solvent containing 33% by volume of ethylene carbonate and using an electrolytic solution containing LiPF ₆ at a concentration of 1 mol / L.

  Recently, Patent Document 2 and Patent Document 3 reported an electrolytic solution containing a metal salt as an electrolyte at a high concentration and a lithium ion secondary battery including the electrolytic solution.

Patent Document 2 specifically describes an electrolytic solution containing (FSO ₂ ) ₂ NLi as a metal salt and dimethyl carbonate as an organic solvent in a molar ratio of dimethyl carbonate to (FSO ₂ ) ₂ NLi of 2 or 3. A lithium ion secondary battery including the electrolytic solution is also specifically described.

  In Patent Document 3, an electrolyte solution having a specific organic solvent to specific metal salt molar ratio of 3 to 5 is described as having excellent physical properties, and a lithium ion secondary battery including the electrolyte solution is disclosed. It is specifically described.

JP 2013-149477 A International Publication No. 2015/045389 International Publication No. 2016/063468

Now, the lithium ion secondary battery of the outstanding characteristic is calculated | required from the industry.
The present invention has been made in view of such circumstances, and an object thereof is to provide a lithium ion secondary battery having excellent characteristics.

  The present inventor aimed to provide a high-capacity lithium ion secondary battery using the electrolytic solution described in Patent Document 3. As a means for that purpose, a lithium metal composite oxide having a layered rock salt structure containing lithium, nickel, cobalt, and aluminum or manganese was selected as a positive electrode active material because of its high capacity and excellent durability. In the lithium metal composite oxide having a layered rock salt structure, nickel, cobalt, aluminum, and manganese are considered to have the following roles.

It is most active during Ni: Li charge / discharge reaction. The capacity increases as the Ni content in the active material increases.
Al or Mn: Li is most inactive during charge / discharge reaction. The capacity decreases as the content of Al or Mn in the active material increases. On the other hand, the crystal structure of the active material is more stable as the content of Al or Mn in the active material increases.
The activity during the Co: Li charge / discharge reaction is intermediate between Ni and Al or Mn. The capacity and the degree of stability with respect to the content in the active material are also intermediate between Ni and Al or Mn.

The present inventors have, for the purpose of higher capacity of lithium ion secondary batteries, the general formula of higher layered rock salt structure is the nickel _{ratio: Li a Ni b Co c M} d D e O f (0.2 ≦ a ≦ 2, 0.7 ≦ b <1, 0 <c <0.3, 0 <d <0.3, 0 ≦ e <0.2, b + c + d + e = 1, M is Al and / or Mn. 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 And at least one element selected from Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, P, and V. Lithium metal composite oxidation represented by 1.7 ≦ f ≦ 3) The thing was selected.

  As the negative electrode active material, a silicon material described in International Publication No. 2014/080608 was selected from Si-containing materials having a large theoretical capacity. The silicon material has a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction.

  Usually, in a lithium ion secondary battery, the value of (negative electrode capacity) / (positive electrode capacity) (hereinafter sometimes referred to as “PN ratio”) is a viewpoint that suppresses the deposition of lithium metal on the negative electrode surface. Therefore, it is designed to exceed 1. Here, when the PN ratio exceeds 1, as the PN ratio approaches 1, the amount of the negative electrode active material becomes relatively small. Therefore, in the lithium ion secondary battery having a PN ratio close to 1, the negative electrode active material itself It can be said that the irreversible capacity derived from is small. Furthermore, in a lithium ion secondary battery having a PN ratio close to 1, the entire surface area of the negative electrode active material contained in the negative electrode is small, so that SEI (Solid Electrolyte Interface) generated on the surface of the negative electrode active material due to the decomposition of the electrolytic solution. ), The irreversible capacity derived from SEI is also small. Therefore, in general, it can be said that it is common technical knowledge that the initial efficiency, that is, (initial discharge capacity) / (initial charge capacity) in the lithium ion secondary battery increases as the PN ratio approaches 1.

  However, the present inventor conducted a study on a lithium ion secondary battery in which the specific positive electrode active material, the specific negative electrode active material, and the electrolyte solution described in Patent Document 3 selected as described above were combined. However, the results were different from the common sense in the previous paragraph. Specifically, it has been found that the initial efficiency in the lithium ion secondary battery increases as the PN ratio approaches 1, but the initial efficiency decreases from a certain point.

  As a result of many experiments conducted by the inventor, an optimum PN that expresses a suitable initial efficiency in a lithium ion secondary battery in which a specific positive electrode active material, a specific negative electrode active material, and a specific electrolyte are combined. I found the ratio. Based on this discovery, the present inventor has completed the present invention.

The lithium ion secondary battery of the present invention is
Formula of the layered rock salt _{structure: Li a Ni b Co c M} d D e O f (0.2 ≦ a ≦ 2,0.7 ≦ b <1,0 <c <0.3,0 <d <0. 3, 0 ≦ e <0.2, b + c + d + e = 1, M is Al and / or Mn, D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Nb, From Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, P, V A positive electrode comprising a lithium metal composite oxide represented by 1.7 ≦ f ≦ 3), which is at least one element selected;
A negative electrode comprising a silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction;
Electrolysis including a chain carbonate represented by (FSO ₂ ) ₂ NLi and the following general formula (A) and having a molar ratio of the chain carbonate to (FSO ₂ ) ₂ NLi in the range of 2 to 6. A liquid,
The value of (capacity of the negative electrode) / (capacity of the positive electrode) is 1.07 to 1.13.
R ¹ OCOOR ² general formula (A)
(R ¹ and R ² are each independently C _n H _a F _b Cl _c Br _d I _e which is a chain alkyl, or C _m H _f F _g Cl _h Br _i I containing a cyclic alkyl in the chemical structure. selected from _j , n is an integer of 1 or more, m is an integer of 3 or more, a, b, c, d, e, f, g, h, i, j are each independently an integer of 0 or more 2n + 1 = a + b + c + d + e, 2m-1 = f + g + h + i + j is satisfied.)

  The lithium ion secondary battery of the present invention exhibits excellent initial efficiency.

It is a graph which shows the relationship between the PN ratio about each lithium ion secondary battery in evaluation example 1, and initial stage efficiency.

  Below, the form for implementing this invention is demonstrated. Unless otherwise specified, the numerical range “ab” described herein includes the lower limit “a” and the upper limit “b”. The numerical range can be configured by arbitrarily combining these upper limit value and lower limit value and the numerical values listed in the examples. Furthermore, numerical values arbitrarily selected from these numerical ranges can be used as new upper and lower numerical values.

The lithium ion secondary battery of the present invention is
Formula of the layered rock salt _{structure: Li a Ni b Co c M} d D e O f (0.2 ≦ a ≦ 2,0.7 ≦ b <1,0 <c <0.3,0 <d <0. 3, 0 ≦ e <0.2, b + c + d + e = 1, M is Al and / or Mn, D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Nb, From Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, P, V A positive electrode comprising a lithium metal composite oxide represented by 1.7 ≦ f ≦ 3), which is at least one element selected;
A negative electrode comprising a silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction;
Electrolysis comprising a chain carbonate represented by (FSO ₂ ) ₂ NLi and the following general formula (A) and having a molar ratio of the chain carbonate to (FSO ₂ ) ₂ NLi in the range of 2 to 6. A liquid,
The value of (capacity of the negative electrode) / (capacity of the positive electrode) is 1.07 to 1.13.
R ¹ OCOOR ² general formula (A)
(R ¹ and R ² are each independently C _n H _a F _b Cl _c Br _d I _e which is a chain alkyl, or C _m H _f F _g Cl _h Br _i I containing a cyclic alkyl in the chemical structure. selected from _j , n is an integer of 1 or more, m is an integer of 3 or more, a, b, c, d, e, f, g, h, i, j are each independently an integer of 0 or more 2n + 1 = a + b + c + d + e, 2m-1 = f + g + h + i + j is satisfied.)

  First, the positive electrode will be described.

  In the above general formula of the lithium metal composite oxide, the values of b, c and d are not particularly limited as long as the above conditions are satisfied, but with respect to b, 0.75 ≦ b ≦ 0.95, 0.8. 8 ≦ b ≦ 0.93, 0.03 ≦ c ≦ 0.2, 0.05 ≦ c ≦ 0.15 for c, 0.01 ≦ d ≦ 0.2, 0.02 ≦ d ≦ for d 0.1 can be illustrated as a suitable range, respectively.

  In the above general formula of the lithium metal composite oxide, the values of b, c and d limited to the case where M is Al are not particularly limited as long as the above conditions are satisfied. b ≦ 0.95, 0.8 ≦ b ≦ 0.93, 0.85 ≦ b ≦ 0.93, 0.03 ≦ c ≦ 0.2, 0.05 ≦ c ≦ 0.15, 0 with respect to c 0.05 ≦ c ≦ 0.1, and 0.01 ≦ d ≦ 0.2, 0.02 ≦ d ≦ 0.1, and 0.02 ≦ d ≦ 0.05 with respect to d can be exemplified as preferable ranges, respectively. .

  In the above general formula of the lithium metal composite oxide, the values of b, c and d limited to the case where M is Mn are not particularly limited as long as the above conditions are satisfied. b ≦ 0.95, 0.8 ≦ b ≦ 0.93, 0.8 ≦ b ≦ 0.83, 0.03 ≦ c ≦ 0.2, 0.05 ≦ c ≦ 0.15, 0 with respect to c 0.09 ≦ c ≦ 0.12 and 0.01 ≦ d ≦ 0.2, 0.02 ≦ d ≦ 0.1, and 0.06 ≦ d ≦ 0.1 with respect to d can be exemplified as preferable ranges, respectively. .

  In the above general formula of the lithium metal composite oxide, a, e, and f may be numerical values within the range defined by the general formula, and a is preferably within the range of 0.5 ≦ a ≦ 1.5, The range of 0.7 ≦ a ≦ 1.3 is more preferable, and the range of 0.9 ≦ a ≦ 1.2 is more preferable. For e and f, 0 ≦ e ≦ 0.1, 0 ≦ e ≦ 0.05, e = 0, 1.8 ≦ f ≦ 2.5, 1.9 ≦ f ≦ 2.2, and f = 2. It can be illustrated.

The shape of the lithium metal composite oxide is not particularly limited, but a powder form is preferable. Examples of the average particle size of the lithium metal composite oxide include ranges of 0.5 to 20 μm, 1 to 15 μm, and 3 to 10 μm. In the present specification, the average particle diameter means D ₅₀ when a sample is measured with a general laser diffraction particle size distribution meter.

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

  The current collector refers to a chemically inert electronic conductor that keeps a current flowing through an electrode during discharge or charging of a lithium ion secondary battery. As the current collector, at least one selected from silver, copper, gold, aluminum, magnesium, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, molybdenum, and stainless steel Examples of such a metal material can be given.

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

  Specifically, it is preferable to use a positive electrode current collector made of aluminum or an aluminum alloy. 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 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, etc. A8000-based alloy (Al-Fe-based).

  The current collector may be covered with a known protective layer. Moreover, you may use what processed the surface of the electrical power collector by the well-known method as an electrical power collector.

  The current collector can take the form of a foil, a sheet, a film, a linear shape, a rod shape, 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 foil, sheet or film, the thickness is preferably in the range of 1 μm to 100 μm.

  The positive electrode active material layer includes the lithium metal composite oxide described above as the positive electrode active material, and may further include additives such as a conductive additive, a binder, and a dispersant. In addition, you may use together a well-known thing as a positive electrode active material within the range which does not deviate from the meaning of this invention.

  The amount of the lithium metal composite oxide with respect to the whole positive electrode active material layer is preferably in the range of 70 to 99% by mass, more preferably in the range of 80 to 98% by mass, and particularly preferably in the range of 90 to 97% by mass.

  The conductive assistant is added to increase the conductivity of the electrode. Therefore, the conductive auxiliary agent may be added arbitrarily when the electrode conductivity is insufficient, and may not be added when the electrode conductivity is sufficiently excellent.

  The conductive auxiliary agent may be a chemically inert electronic conductor, and examples thereof include carbon black fine particles, graphite, vapor grown carbon fiber, and various metal particles. Examples of the carbon black include acetylene black, ketjen black (registered trademark), furnace black, and channel black. These conductive assistants can be added to the positive electrode active material layer alone or in combination of two or more.

  The shape of the conductive auxiliary agent is not particularly limited, but it is preferable that the average particle diameter of the conductive auxiliary agent is small in view of its role. A preferable average particle size of the conductive auxiliary is 10 μm or less, and a more preferable average particle size is in the range of 0.01 to 1 μm.

  Although the compounding quantity of a conductive support agent is not specifically limited, If it dares to mention the compounding quantity of the conductive support agent in a positive electrode active material layer, the inside of the range of 0.5-10 mass% is preferable, and the inside of the range of 1-7 mass% is preferable. More preferably, it is in the range of 1.5 to 5% by mass.

  The binder plays a role of connecting the positive electrode active material and the conductive additive to the surface of the current collector. Examples of the binder 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, and alkoxysilyl group-containing resins. be able to. Moreover, you may employ | adopt the polymer which has a hydrophilic group as a binder. Examples of the hydrophilic group of the polymer having a hydrophilic group include a carboxyl group, a sulfo group, a silanol group, an amino group, a hydroxyl group, and a phosphate group. Specific examples of the polymer having a hydrophilic group include polyacrylic acid, carboxymethylcellulose, polymethacrylic acid, and poly (p-styrenesulfonic acid).

  The amount of the binder in the positive electrode active material layer is preferably in the range of 0.5 to 10% by mass, more preferably in the range of 1 to 7% by mass, and particularly in the range of 1.5 to 5% by mass. preferable. If the blending amount of the binder is too small, the moldability of the positive electrode active material layer may be lowered. Moreover, when there are too many compounding quantities of a binder, since the quantity of the positive electrode active material in a positive electrode active material layer reduces relatively, it is unpreferable.

  Known additives can be employed as additives such as a dispersing agent other than the conductive assistant and the binder.

  Next, the negative electrode will be described.

The silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction is, for example, a step of synthesizing a layered silicon compound mainly composed of polysilane by reacting CaSi ₂ and an acid, The layered silicon compound is manufactured through a step of heating hydrogen at 300 ° C. or higher to release hydrogen.

The production method of the silicon material is represented by the following ideal reaction formula when hydrogen chloride is used as the acid.
3CaSi ₂ + 6HCl → Si ₆ H ₆ + 3CaCl ₂
Si ₆ H ₆ → 6Si + 3H ₂ ↑

However, in the upper reaction for synthesizing Si ₆ H ₆ which is 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 manufactured as containing only Si ₆ H ₆ , and the layered 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 a product. In the above chemical formula, inevitable impurities such as Ca that can remain are not taken into consideration. A silicon material obtained by heating the layered silicon compound also contains an element derived from an anion of oxygen or acid.

As described above, the silicon material has a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction. In order to efficiently store and release charge carriers such as lithium ions, the plate-like silicon body preferably has a thickness in the range of 10 nm to 100 nm, more preferably in the range of 20 nm to 50 nm. The length in the longitudinal direction of the plate-like silicon body is preferably within a range of 0.1 μm to 50 μm. The plate-like silicon body preferably has (length in the longitudinal direction) / (thickness) in the range of 2 to 1000. 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 .

  The silicon material preferably includes amorphous silicon and / or silicon crystallites. In particular, the above plate-like silicon body is preferably in a state where 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 the Scherrer equation using X-ray diffraction measurement on the silicon material and using the half-value width of the diffraction peak of the Si (111) plane of the obtained X-ray diffraction chart. .

  The abundance and size of the plate-like silicon body, 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 within the range of 350 ° C to 950 ° C, more preferably within the range of 400 ° C to 900 ° C, and even more preferably within the range of 500 ° C to 800 ° C.

  A silicon material coated with carbon may be employed as the silicon material. By covering the silicon material with carbon, the conductivity is improved.

  The negative electrode has a current collector and a negative electrode active material layer formed on the surface of the current collector. The negative electrode active material layer includes the silicon material described above as the negative electrode active material, and may further include additives such as a conductive additive, a binder, and a dispersant. In addition, a well-known thing may be used together as a negative electrode active material within the range which does not deviate from the meaning of the present invention. What is necessary is just to employ | adopt suitably what was demonstrated with the positive electrode as a collector. A well-known thing can be employ | adopted for a dispersing agent.

  The amount of the negative electrode active material with respect to the whole negative electrode active material layer is preferably in the range of 60 to 90% by mass, and more preferably in the range of 70 to 80% by mass, in relation to the amounts of the conductive additive and the binder.

  Since the above-described silicon material is a semiconductor, a relatively large amount of conductive additive is required for the negative electrode active material layer in order to smoothly transfer electrons between the silicon material and the negative electrode current collector. The amount of the conductive additive in the negative electrode active material layer is preferably in the range of 5 to 20% by mass, more preferably in the range of 7 to 15% by mass, and still more preferably in the range of 10 to 15% by mass. What is necessary is just to employ | adopt suitably what was demonstrated with the positive electrode as a conductive support agent.

  Further, the above-described silicon material has a large degree of expansion and contraction associated with charge / discharge. Therefore, in order to prevent the silicon material from falling off the negative electrode due to expansion and contraction, a relatively large amount of binder is required for the negative electrode active material layer. The blending amount of the binder in the negative electrode active material layer is preferably in the range of 5 to 20% by mass, and more preferably in the range of 10 to 15% by mass.

  As the binder for the negative electrode, one or a plurality of known binders described in the positive electrode or a styrene butadiene rubber may be employed.

  A cross-linked polymer obtained by cross-linking 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. Due to the use of the crosslinked polymer as a binder, it can be expected that the characteristics of the lithium ion secondary battery of the present invention are more suitable.

  Examples of the diamine used in the crosslinked polymer include alkylene diamines such as ethylene diamine, propylene diamine, and hexamethylene diamine, 1,4-diaminocyclohexane, 1,3-diaminocyclohexane, isophorone diamine, and bis (4-aminocyclohexyl) methane. Saturated carbocyclic diamine, m-phenylenediamine, p-phenylenediamine, 4,4′-diaminodiphenylmethane, 4,4′-diaminodiphenyl ether, bis (4-aminophenyl) sulfone, benzidine, o-tolidine, 2,4- Aromatic diamines such as tolylenediamine, 2,6-tolylenediamine, xylylenediamine, and naphthalenediamine are exemplified.

  In order to form the 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, or a curtain coating method may be used. Specifically, an active material, a solvent, and a binder and a conductive aid as necessary are mixed to form a slurry, and 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. In order to increase the electrode density, the dried product may be compressed.

Next, the electrolytic solution will be described. The electrolytic solution contains (FSO ₂ ) ₂ NLi at a relatively high concentration as an electrolyte. By using an electrolytic solution containing (FSO ₂ ) ₂ NLi at a relatively high concentration, a unique SEI derived from (FSO ₂ ) ₂ NLi is formed on the negative electrode surface of the lithium ion secondary battery of the present invention. .

In addition to (FSO ₂ ) ₂ NLi, the electrolyte solution may include other electrolytes that can be used for the electrolyte solution of the lithium ion secondary battery. The electrolyte solution preferably contains (FSO ₂ ) ₂ NLi at 30% by mass or more, more preferably 40% by mass or more, and 50% by mass or more with respect to the total electrolyte contained. Is more preferable. All electrolyte contained in the electrolytic solution may be a _{(FSO 2) 2} NLi.

As other electrolytes, LiPF ₆ , LiBF ₄ , LiAsF ₆ , Li ₂ SiF ₆ , (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) ₂ NLi, FSO ₂ (CF ₃ 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 F 5 SO 2) NLi, or FSO ₂ (C ₂ H ₅ SO ₂ ) NLi, (OCOCO ₂ ) ₂ BLi, (OCOCO ₂ ) BF ₂ Li can be exemplified.

  The electrolytic solution contains a chain carbonate represented by the general formula (A) as an organic solvent (hereinafter sometimes simply referred to as “chain carbonate”).

  In the chain carbonate represented by the general formula (A), n is preferably an integer of 1 to 6, more preferably an integer of 1 to 4, and particularly preferably an integer of 1 to 2. m is preferably an integer of 3 to 8, more preferably an integer of 4 to 7, and particularly preferably an integer of 5 to 6.

  Of the chain carbonates, those represented by the following general formula (A-1) are particularly preferred.

R ³ OCOOR ⁴ general formula (A-1)
(R ³ and R ⁴ are each independently selected from C _n H _a F _b , which is a chain alkyl, or C _m H _f F _g , which includes a cyclic alkyl in the chemical structure. N is 1 (The above integers, m is an integer of 3 or more, a, b, f, and g are each independently an integer of 0 or more, and satisfy 2n + 1 = a + b and 2m-1 = f + g.)

  In the chain carbonate represented by the general formula (A-1), n is preferably an integer of 1 to 6, more preferably an integer of 1 to 4, and particularly preferably an integer of 1 to 2. m is preferably an integer of 3 to 8, more preferably an integer of 4 to 7, and particularly preferably an integer of 5 to 6.

  Among the chain carbonates, dimethyl carbonate (hereinafter sometimes referred to as “DMC”), diethyl carbonate (hereinafter sometimes referred to as “DEC”), and ethyl methyl carbonate (hereinafter sometimes referred to as “EMC”). ), Fluoromethyl methyl carbonate, difluoromethyl methyl carbonate, trifluoromethyl methyl carbonate, bis (fluoromethyl) carbonate, bis (difluoromethyl) carbonate, bis (trifluoromethyl) carbonate, fluoromethyl difluoromethyl carbonate, 2,2, 2-trifluoroethyl methyl carbonate, pentafluoroethyl methyl carbonate, ethyl trifluoromethyl carbonate, bis (2,2,2-trifluoroethyl) carbonate are particularly preferred.

  One type of chain carbonate may be used for the electrolytic solution, or a plurality of types may be used in combination. By using a plurality of chain carbonates in combination, it is possible to suitably ensure the low temperature fluidity of the electrolyte and the lithium ion transportability at low temperatures. As a combination example of the chain carbonate, two or three types selected from DMC, DEC and EMC can be used. In the combined use of DMC and DEC or EMC, these molar ratios are preferably within the range of DMC: DEC or EMC = 70: 30 to 95: 5.

  In addition to the chain carbonate, the organic solvent in the electrolytic solution includes other organic solvents that can be used in an electrolytic solution such as a lithium ion secondary battery (hereinafter, simply referred to as “other organic solvent”). It may be included.

  The electrolytic solution preferably contains 70% by volume, 70% by mass or more, or 70% by mol or more of the chain carbonate with respect to the total organic solvent contained, and is 80% by volume, 80% by mass or more, or 80% by mol. %, More preferably 90% by volume, 90% by mass or more, or 90% by mole or more. All the organic solvents contained in the electrolytic solution may be the chain carbonate.

  Note that an electrolytic solution containing another organic solvent may have a higher viscosity or lower ionic conductivity than an electrolytic solution not containing another organic solvent. Furthermore, the reaction resistance of a secondary battery using an electrolytic solution containing another organic solvent may increase.

  Specific examples of other organic solvents include nitriles such as acetonitrile, propionitrile, acrylonitrile, malononitrile, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,2-dioxane, 1, Ethers such as 3-dioxane, 1,4-dioxane, 2,2-dimethyl-1,3-dioxolane, 2-methyltetrahydropyran, 2-methyltetrahydrofuran, crown ether, fluoroethylene carbonate, ethylene carbonate, propylene carbonate, etc. Cyclic carbonates, amides such as formamide, N, N-dimethylformamide, N, N-dimethylacetamide, N-methylpyrrolidone, isopropyl isocyanate, n-propyl isocyanate, chloromethyl isocyanate Isocyanates such as methyl acetate, ethyl acetate, butyl acetate, propyl acetate, methyl propionate, methyl formate, ethyl formate, vinyl acetate, methyl acrylate, methyl methacrylate, esters, glycidyl methyl ether, epoxy butane, 2-ethyl Epoxies such as oxirane, oxazoles such as oxazole, 2-ethyloxazole, oxazoline and 2-methyl-2-oxazoline, ketones such as acetone, methyl ethyl ketone and methyl isobutyl ketone, acid anhydrides such as acetic anhydride and propionic anhydride , Sulfones such as dimethyl sulfone and sulfolane, sulfoxides such as dimethyl sulfoxide, nitros such as 1-nitropropane and 2-nitropropane, furans such as furan and furfural, γ-butyrolactone, γ-vale Cyclic esters such as lactone and δ-valerolactone, aromatic heterocycles such as thiophene and pyridine, heterocycles such as tetrahydro-4-pyrone, 1-methylpyrrolidine and N-methylmorpholine, trimethyl phosphate, phosphorus And phosphoric acid esters such as triethyl acid.

In the electrolytic solution, the chain carbonate is contained in a molar ratio of 2 to 6 with respect to (FSO ₂ ) ₂ NLi. In view of the technical contents disclosed in Patent Document 3, it can be said that the molar ratio is preferably in the range of 3 to 5. As discussed in detail in Patent Document 3, when the molar ratio is too low or too high, the capacity retention rate of the lithium ion secondary battery may be reduced. Further, from the viewpoint of satisfying both ionic conductivity and low-temperature stability in a balanced manner, the molar ratio in the electrolytic solution is preferably in the range of 3-6.

Examples of the concentration of (FSO ₂ ) ₂ NLi in the electrolyte include 1.5 to 3.5 mol / L and 2.0 to 3.0 mol / L.

  The electrolytic solution may contain an organic solvent made of hydrocarbon. The electrolytic solution of the present invention containing an organic solvent made of hydrocarbon can be expected to have an effect that its viscosity is lowered.

  Specific examples of the organic solvent composed of the hydrocarbon include benzene, toluene, ethylbenzene, o-xylene, m-xylene, p-xylene, 1-methylnaphthalene, hexane, heptane, and cyclohexane.

  In addition, a flame retardant solvent can be added to the electrolytic solution. By adding a flame retardant solvent to the electrolyte, the safety of the electrolyte can be further increased. Examples of the flame retardant solvent include halogen solvents such as carbon tetrachloride, tetrachloroethane, and hydrofluoroether, and phosphoric acid derivatives such as trimethyl phosphate and triethyl phosphate.

  When the electrolytic solution is mixed with a polymer or an inorganic filler to form a mixture, the mixture contains the electrolytic solution and becomes a pseudo solid electrolyte. By using the pseudo-solid electrolyte as the battery electrolyte, leakage of the electrolyte in the battery can be suppressed.

  Moreover, you may add a well-known additive to electrolyte solution in the range which does not deviate from the meaning of this invention. Examples of known additives include carbonate compounds represented by phenylethylene carbonate and erythritan carbonate; succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride , Cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, carboxylic anhydrides represented by phenylsuccinic anhydride; γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-valerolactone, δ- Caprolactone, lactone represented by ε-caprolactone; cyclic ether represented by 1,4-dioxane; ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfolane, sulfo , Sulfur compounds represented by dimethylsulfone, tetramethylthiuram monosulfide; 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2- Nitrogen compounds represented by imidazolidinone and N-methylsuccinimide; phosphates represented by monofluorophosphate and difluorophosphate; saturated hydrocarbon compounds represented by heptane, octane and cycloheptane; biphenyl , Alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, unsaturated hydrocarbon compounds represented by dibenzofuran, vinylene carbonate, fluorovinylene carbonate, methyl vinylene -Unsaturated cyclic carbonates typified by boronate, fluoromethyl vinylene carbonate, ethyl vinylene carbonate, propyl vinylene carbonate, butyl vinylene carbonate, dimethyl vinylene carbonate, diethyl vinylene carbonate, dipropyl vinylene carbonate, trifluoromethyl vinylene carbonate, vinyl ethylene carbonate Can be mentioned.

  The lithium ion secondary battery of the present invention is a combination of a positive electrode including a specific positive electrode active material, a negative electrode including a specific negative electrode active material, and a specific electrolyte solution, and has a PN ratio of 1.07 to 1. It is 13, It is characterized by the above-mentioned. Due to the PN ratio being 1.07-1.13, the lithium ion secondary battery of the present invention is excellent in initial efficiency. From the viewpoint of initial efficiency, the PN ratio is preferably 1.07 to 1.12, more preferably 1.08 to 1.11, still more preferably 1.08 to 1.10, and particularly preferably 1.09.

  It can be said that the preferred range of the PN ratio varies depending on the combination of the positive electrode active material, the negative electrode active material, and the electrolytic solution. In particular, the lithium ion secondary battery of the present invention has a lithium metal composite oxide with a high nickel ratio, a silicon material that has a unique structure and contains impurities such as oxygen, and is decomposed on the surface of the silicon material at the time of initial charge. Since it is a combination with an electrolytic solution capable of forming SEI, numerous experiments with trial and error were indispensable for setting an appropriate PN ratio. The factor is that the irreversible capacity of the silicon material itself and the irreversible capacity caused by SEI derived from the electrolyte are unique. In addition, aiming for a high-capacity lithium-ion secondary battery with a large capacity per unit volume, a lithium metal composite oxide with a high nickel ratio was used and the PN ratio was designed to be close to 1. The amount of the material is relatively reduced, and as a result, the depth of charge with respect to the silicon material is increased, and even when charging and discharging is performed once, the silicon material can be separated from the negative electrode current collector. .

  In order to make the PN ratio within the range of 1.07 to 1.13, the usage amount of the lithium metal composite oxide and the usage amount of the silicon material used in the lithium ion secondary battery of the present invention should be appropriately adjusted. That's fine. Specifically, in view of the capacity per unit mass of each material, the amount of the active material-containing slurry applied to the current collector may be appropriately determined in the stage of forming each active material layer.

  The capacities per unit mass of the lithium metal composite oxide and silicon material are determined by the following method.

A half cell is manufactured using an electrode having a lithium metal composite oxide as a working electrode and metal Li as a counter electrode. Lithium metal composite oxide when the half cell is charged / discharged at a constant current of 0.05C rate (1C means a current value required to fully charge or fully discharge the battery in one hour at a constant current). The capacity per unit mass of the lithium metal composite oxide is calculated from the result.
The range of charging / discharging shall be the range of the voltage assumed to be actually usable. For example, it is assumed that the voltage with respect to the counter electrode of the working electrode is charged / discharged in a range of 4.3V to 2.5V, a range of 4.3V to 2.8V, a range of 4.2V to 3V, and the like. In Examples described later, the capacity per unit mass of the lithium metal composite oxide was determined using the initial discharge capacity when charging / discharging in the range of 4.3 V to 2.8 V.

  A half cell is manufactured using an electrode including a silicon material as a working electrode and metal Li as a counter electrode. The capacity of the silicon material was measured when the half cell was charged at a constant current of 0.05 C rate until the voltage with respect to the counter electrode of the working electrode became 0.01 V. From the result, the capacity per unit mass of the silicon material was determined. calculate. In the examples described later, the capacity per unit mass of the silicon material was determined using the initial charge capacity when the half cell after manufacture was charged to 0.01V.

  A separator is used in the lithium ion secondary battery of the present invention as necessary. The separator separates the positive electrode and the negative electrode and allows lithium ions to pass while preventing a short circuit due to contact between the two electrodes. A known separator may be employed, such as polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamide, polyaramid (Aromatic polyamide), polyester, polyacrylonitrile or other synthetic resin, cellulose, amylose or other polysaccharide, fibroin. And porous materials, nonwoven fabrics, woven fabrics, and the like using one or more electrical insulating materials such as natural polymers such as keratin, lignin, and suberin, and ceramics. The separator may have a multilayer structure.

A specific method for producing the lithium ion secondary battery of the present invention will be described.
If necessary, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body. The electrode body may be either a stacked type in which the positive electrode, the separator and the negative electrode are stacked, or a wound type in which the positive electrode, the separator and the negative electrode are sandwiched. 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 that communicate with the outside using a current collecting lead or the like, the electrolyte solution of the present invention is added to the electrode body and the lithium ion secondary Use batteries. Moreover, the lithium ion secondary battery of this invention should just be charged / discharged in the voltage range suitable for the kind of active material contained in an electrode.

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

  The lithium ion secondary battery of the present invention may be mounted on a vehicle. The vehicle may be a vehicle that uses electric energy generated by a lithium ion secondary battery for all or a part of its power source. For example, the vehicle may be 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 lithium ion secondary batteries include various home appliances driven by batteries such as personal computers and portable communication devices, office devices, and industrial devices in addition to vehicles. Furthermore, the lithium ion secondary battery of the present invention includes wind power generation, solar power generation, hydroelectric power generation and other power system power storage devices and power smoothing devices, power supplies for ships and / or auxiliary power supply sources, aircraft, Power supply for spacecraft and / or auxiliary equipment, auxiliary power supply for vehicles that do not use electricity as a power source, power supply for mobile home robots, power supply for system backup, power supply for uninterruptible power supply, You may use for the electrical storage apparatus which stores temporarily the electric power required for charge in the charging station for electric vehicles.

  As mentioned above, although embodiment of the electrolyte solution of this invention was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

  Hereinafter, the present invention will be specifically described with reference to production examples and evaluation examples. The present invention is not limited to these specific examples.

(Manufacture of electrolyte)
The electrolytic solution of Production Example 1 was produced by adding (FSO ₂ ) ₂ NLi and dissolving it in a mixed solvent in which dimethyl carbonate and ethyl methyl carbonate, which are chain carbonates, were mixed at a molar ratio of 72:28. In the electrolytic solution of Production Example 1, the molar ratio of the chain carbonate to (FSO ₂ ) ₂ NLi was 5.

(Manufacture of lithium ion secondary batteries)
Using the electrolytic solution of Production Example 1, using each lithium metal composite oxide shown in Table 1, and adjusting the PN ratio as shown in Table 1, the lithium ion secondary battery of each Production Example was prepared according to the following procedure. Manufactured. In the lithium ion secondary battery of each production example, the amount of the positive electrode active material layer per unit area of the positive electrode current collector is in the range of 22 to 23.5 mg / cm ² , and the unit area of the negative electrode current collector The amount of the negative electrode active material layer was in the range of 3.4 to 3.5 mg / cm ² .

  94 parts by mass of a lithium metal composite oxide, 3 parts by mass of acetylene black as a conductive additive, 3 parts by mass of polyvinylidene fluoride as a binder, and an appropriate amount of N-methyl-2-pyrrolidone are mixed to produce a slurry. did. An aluminum foil was prepared as a positive electrode current collector. The slurry was applied to the surface of the aluminum foil using a doctor blade so as to form a film. The aluminum foil coated with the slurry was dried to remove N-methyl-2-pyrrolidone. Thereafter, the aluminum foil was pressed to obtain a bonded product. The obtained joined product was heated and dried with a vacuum dryer to produce a positive electrode made of an aluminum foil on which a positive electrode active material layer was formed.

  72.5 parts by mass of carbon-coated silicon material, 13.5 parts by mass of acetylene black as a conductive auxiliary agent, 14 parts by mass of a mixture of polyacrylic acid and 4,4′-diaminodiphenylmethane as a binder, and An appropriate amount of N-methyl-2-pyrrolidone was mixed to prepare a slurry. A copper foil was prepared as a negative electrode current collector. The slurry was applied in a film form on the surface of the copper foil using a doctor blade. The copper foil coated with the slurry was dried to remove N-methyl-2-pyrrolidone to produce a negative electrode on which a negative electrode active material layer was formed. The mixture of polyacrylic acid and 4,4′-diaminodiphenylmethane used as the binder is a crosslinked polymer in which the dehydration reaction proceeds by drying and the polyacrylic acid is crosslinked with 4,4′-diaminodiphenylmethane. To change.

  A polyolefin porous membrane was prepared as a separator. A separator was sandwiched between the positive electrode and the negative electrode to form an electrode plate group. The electrode plate group was covered with a set of two laminated films, and the three sides were sealed. Then, the electrolytic solution of Production Example 1 was injected into the laminated film in a bag shape. Thereafter, the remaining one side was sealed to produce a lithium ion secondary battery in which the four sides were hermetically sealed and the electrode plate group and the electrolyte were sealed.

(Evaluation example 1)
Each lithium ion secondary battery was charged to 4.2 V at 25 ° C. and a low current of 0.4 C or less, and then discharged to 2.8 V at 0.33 C. And the initial efficiency was computed with the following formula | equation. The results are shown in Table 2 and FIG. FIG. 1 also shows a cubic polynomial approximate curve calculated from the result.
Initial efficiency = 100 × (discharge capacity from 4.2V to 2.8V) / (charge capacity up to 4.2V)

  As shown in FIG. 1, the initial efficiency tends to increase as the PN ratio decreases from 1.17 to 1.1. However, when the PN ratio becomes smaller than 1.09, the initial efficiency tends to decrease. It was done. When the PN ratio is smaller than 1.09, the depth of charge for the silicon material increases, and as a result, the degree of expansion and contraction of the silicon material increases, and even from a single charge / discharge, the negative electrode current collector of the silicon material increases. It is considered that the initial efficiency was lowered due to the occurrence of peeling.

The cubic polynomial approximate curve shown in FIG. 1 shows a maximum value when the PN ratio is around 1.09. In view of the relationship between the PN ratio of the cubic polynomial approximate curve and the initial efficiency, it can be said that the lithium ion secondary battery having a PN ratio in the range of 1.07 to 1.13 is excellent in the initial efficiency. The correlation coefficient r of the cubic polynomial approximate curve shown in FIG. 1 is 0.91, and it can be statistically confirmed that there is a strong correlation between the PN ratio and the initial efficiency. Regarding the results, the correlation coefficient r when the approximate curve is linear is 0.61, and the correlation coefficient r when the approximate curve is a quadratic polynomial approximate curve is 0.87. Judgment based on a polynomial approximation curve is reasonable.
It was confirmed that the lithium ion secondary battery of the present invention has a suitable initial efficiency.

Claims (3)

Formula of the layered rock salt _{structure: Li a Ni b Co c M} d D e O f (0.2 ≦ a ≦ 2,0.7 ≦ b <1,0 <c <0.3,0 <d <0. 3, 0 ≦ e <0.2, b + c + d + e = 1, M is Al and / or Mn, D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Nb, From Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, P, V A positive electrode comprising a lithium metal composite oxide represented by 1.7 ≦ f ≦ 3), which is at least one element selected;
A negative electrode comprising a silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction;
Electrolysis including a chain carbonate represented by (FSO ₂ ) ₂ NLi and the following general formula (A) and having a molar ratio of the chain carbonate to (FSO ₂ ) ₂ NLi in the range of 2 to 6. A liquid,
A lithium ion secondary battery having a value of (capacity of the negative electrode) / (capacity of the positive electrode) of 1.07 to 1.13.
R ¹ OCOOR ² general formula (A)
(R ¹ and R ² are each independently C _n H _a F _b Cl _c Br _d I _e which is a chain alkyl, or C _m H _f F _g Cl _h Br _i I containing a cyclic alkyl in the chemical structure. selected from _j , n is an integer of 1 or more, m is an integer of 3 or more, a, b, c, d, e, f, g, h, i, j are each independently an integer of 0 or more 2n + 1 = a + b + c + d + e, 2m-1 = f + g + h + i + j is satisfied.)   2. The lithium ion secondary battery according to claim 1, wherein in the general formula, M is Al, and 0.75 ≦ b ≦ 0.95, 0 <c ≦ 0.2, and 0 <d ≦ 0.05 are satisfied. .   2. The lithium ion secondary battery according to claim 1, wherein in the general formula, M is Mn, and 0.75 ≦ b ≦ 0.9, 0 <c ≦ 0.2, and 0 <d ≦ 0.1 are satisfied. .