JPH10284129A - Nonaqueous electrolyte battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide excellent charge and discharge cycle characteristics at a high voltage, a high capacity and a high density and enhanced safety, by using a salt containing carbon for the main component material of an electrolyte and silicon with an electron conductivity of not less than a specific value for a negative active material. SOLUTION: A salt containing carbon is used for the main component solute of an electrolyte in a battery. It is preferable that an electron conductivity of silicon which is the main component material for a negative active material has not less than 10<-5> S cm<-1> at 20 deg.C. For the salt containing carbon, a lithium salt expressed by (R1 SO2 )(R2 SO2 )NLi is used. In the formula, R1 , R2 are expressed by CnF2n+1 , where (n) is an integer from 1 to 4, and R1 =R2 or R1 ≠R2 is preferable. As for the crystal system of silicon, single crystal is preferable since excellent charge and discharge characteristics can be obtained. A salt containing carbon is hard to be decomposed and hydrogen fluoride gas and the like are seldom discharged when reaction with water. Accordingly, an increase in an electric resistance of surface coating of the negative active material and a decrease in the ion conductivity are suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte battery, and more particularly to a negative electrode active material and an electrolyte used in the battery.

[0002]

2. Description of the Related Art Conventionally, lithium has been typically used as a negative electrode active material for a nonaqueous electrolyte battery. However, dendritic deposition of lithium generated during charging (dendrite)
Therefore, there was a problem in terms of cycle life. In addition, the dendrite penetrates through the separator, causing an internal short circuit and causing ignition.

Further, lithium alloys have been used for the purpose of preventing dendrite generated at the time of charging as described above. However, when the charge amount is increased, there are problems such as fine powdering of the negative electrode and falling off of the negative electrode active material. .

At present, batteries using a carbon material for the negative electrode have been attracting attention for their long life and safety, and some of them have been put to practical use. However, the carbon material used for the negative electrode is
At the time of quick charging, there was a problem that an internal short circuit or a reduction in charging efficiency occurred. Since these carbon materials generally have a lithium doping potential of the carbon material close to 0 V, when rapid charging is performed, the potential becomes 0 V or less and lithium may be deposited on the electrode. For this reason, it causes internal short circuit of the cell and lowers the discharge efficiency. Also,
Although such carbon materials have been significantly improved in terms of cycle life, their relatively low density results in low capacity per volume. That is, this carbon material is still insufficient in terms of high energy density. In addition, in the case where a film needs to be formed on carbon, the initial charge / discharge efficiency decreases, and the amount of electricity used for forming the film is irreversible, which leads to a reduction in capacity corresponding to the amount of electricity. Therefore, development of a safe negative electrode material for a non-aqueous electrolyte battery with a higher capacity, a higher energy density, a longer cycle life, and the like is desired.

[0005]

As an alloy of lithium and silicon, Binary Alloy Pha has already been used.
As in se Diagrams (p2465),
It is known to alloy with compositions up to Li ₂₂ Si ₅ . Japanese Patent Application Laid-Open No. Hei 5-74463 reports that a single crystal of silicon is used for a negative electrode. However, when trying to dope lithium into silicon as a negative electrode material of a nonaqueous electrolyte battery for rapid charge / discharge, it was found that lithium was deposited with almost no doping. Then, the present inventor studied an extrinsic semiconductor already having an impurity (dopant), and found that the occlusion and release of lithium proceeded. This occlusion reaction proceeds at a potential very close to the lithium potential of about 0.1 V, and occlusion of lithium close to the theoretical capacity occurs.
It was found that there was also reversibility. However, it was found that when an electrolyte using LiBF ₄ as the solute was used, the charge / discharge efficiency in each cycle was low, and cycle deterioration occurred.

That is, when lithium metal or an alloy of lithium and a metal is used as the negative electrode, there are advantages in terms of high voltage, high capacity, and high energy density, but there are problems in cycleability and safety. Is advantageous in terms of high voltage and safety, but insufficient in terms of high capacity and high energy density. When silicon, which is expected to have a high capacity and a high energy density, is used as a negative electrode active material, there is a problem that the charge and discharge efficiency of each cycle is low, leading to cycle deterioration.

For this reason, high voltage, high energy density,
In order to obtain a secondary battery with excellent charge-discharge cycle characteristics and high safety, there is little change in the crystal system or volume change during insertion and extraction of lithium during charging and discharging, and in an operating region as close to the lithium potential as possible. A conductive compound capable of reversibly inserting and extracting lithium has been desired.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and is characterized in that silicon and lithium are the main constituent materials of a negative electrode active material used in a non-aqueous electrolyte battery. Wherein a salt containing carbon is used as a main constituent solute of the electrolyte. Preferably, the electronic conductivity of silicon, which is a main constituent material of the negative electrode active material, is not less than 10 ^-5 Scm ^-1 at 20 ° C.

Further, the above-mentioned carbon-containing salt is characterized by being represented by the following general formula (1): (R ₁ SO ₂ ) (R ₂ SO ₂ ) NLi... . Preferably, R ₁ and R ₂ in the general formula (1) are represented by C _n F _{2n + 1} , n is a number from 1 to 4, and R ₁ = R ₂ or R ₁ ≠ R ₂ Is desirable.

Here, in a non-aqueous electrolyte battery, when LiBF ₄ or LiPF _6, which has been generally used in the past, is used as an electrolyte, the ion conductivity of the electrolyte itself is excellent, but when it is decomposed, it is not compatible with Lewis acid or water. It is known that the reaction produces hydrogen fluoride and the like. When silicon is used as a negative electrode active material by using these solutes, impurities generated from the solute react with the film existing on the silicon surface, and the surface film changes into a film having high electric resistance and poor ion conductivity. Do you get it. Therefore, it is conceivable that the electrode resistance increases each time charging / discharging is performed, which lowers the charging / discharging efficiency, thereby leading to cycle deterioration.

On the other hand, it has been found that salts containing carbon are hardly decomposed and hardly release hydrogen fluoride or the like even in the reaction with water. Therefore, when silicon is used as the negative electrode active material, it is conceivable that an increase in the electrical resistance of the surface coating and a decrease in ionic conductivity are suppressed, the charge / discharge efficiency is improved, and the cycle characteristics are improved.

It has also been found that silicon having excellent electron conductivity is suitable for alloying with lithium.
In particular, it has been found that a foreign semiconductor having an electron conductivity of 10 ^-5 Scm ^-1 or more at room temperature, preferably ¹ Scm ^-1 or more has excellent charge / discharge characteristics. That is, although an alloy of lithium and silicon is known, silicon itself was originally an intrinsic semiconductor, and as it was, the electron conductivity was low, and the characteristics as a battery negative electrode material were poor. For this reason, it is a material that is difficult to be studied, but a foreign semiconductor doped with impurities, particularly a p-type semiconductor, an n-type semiconductor, and a semiconductor having a pn junction, have a high impurity concentration and good electron conductivity. The present inventors have found that a material having a better charge-discharge characteristic can be obtained as a negative electrode active material, and have reached the present invention.

[0013]

Embodiments of the present invention will be described below.

As for the silicon used in the present invention, the crystal system includes single crystal, polycrystal, amorphous and the like. Among them, single crystal is preferable because particularly excellent charge / discharge characteristics can be obtained. However, the present invention is not limited to this.

Further, by adding an impurity to silicon, electron conductivity can be improved. The term “impurity” as used herein means any of the elements in the periodic table that can be a donor atom or an acceptor atom, but is preferably P, Al, As, Sb, B, Ga, In or the like, and most preferably. B, but is not limited thereto. In addition, it is considered that the presence of lattice defects found in polycrystals and the like also contributes to improving electron conduction.

As the method of doping the impurities, C
Z method (Czochralski method or lifting method), FZ
(Floating zone method), alloy method, diffusion method, ion implantation method, epitaxial method and the like, but are not limited thereto.

The concentration of the impurity added is usually from 10 ⁷ to 10 ⁶ silicon atoms with one donor atom or one acceptor atom, but high-concentration doping is preferable, and 10 ⁴ silicon atoms is suitable. It is desirable that the concentration be as high as one donor atom or one acceptor atom or more.

The silicon used in the present invention has excellent electron conductivity and is suitable for alloying with lithium. The silicon has an electron conductivity of at least 10 ^-5 Scm ^{-1 at} room temperature, preferably ^{1 -5} Scm ^-1.
An extrinsic semiconductor having Scm ^-1 or more has excellent charge / discharge characteristics.

The silicon used in the present invention is desirably a single wafer-shaped plate having a thickness of 0.1 to 500 μm or a powder having an average particle size of 0.1 to 100 μm. In obtaining a predetermined shape, a diamond cutter is used to obtain a wafer-like veneer, and a pulverizer or a classifier is used to obtain powder. When powder is obtained, for example, a mortar, a ball mill, a sand mill, a vibration ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling air jet mill, a sieve, and the like are used.
At the time of pulverization, wet pulverization in which an organic solvent such as water or hexane coexists can be used. The classification method is not particularly limited, and a sieve, an air classifier, or the like is used as needed in both dry and wet methods.

The negative electrode material that can be used in conjunction with the present invention includes lithium metal, lithium alloy, and the like, calcined carbonaceous compounds capable of inserting and extracting lithium ions or lithium metal, chalcogen compounds, and lithium such as methyllithium. Organic compounds and the like can be mentioned. In addition, by using lithium metal, a lithium alloy, and an organic compound containing lithium in combination, lithium can be inserted into the foreign semiconductor used in the present invention inside the battery.

When the foreign semiconductor of the present invention is used as a powder, a conductive agent, a binder, a filler or the like can be added as an electrode mixture. Any conductive material may be used as long as it does not adversely affect battery performance. Normally, natural graphite (scale graphite, flake graphite, earth graphite, etc.), artificial graphite, carbon black, acetylene black, Ketjen black, carbon whiskers, carbon fibers and metals (copper, nickel, aluminum, silver, gold, etc.)
A conductive material such as powder, metal fiber, metal deposition, and conductive ceramic material can be included as one type or a mixture thereof. Among these, the combined use of graphite, acetylene black and Ketjen black is desirable. The addition amount is preferably 1 to 50% by weight, particularly preferably 2 to 30% by weight.

As the binder, usually, tetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, ethylene-propylene diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluoro rubber, carboxymethyl cellulose, etc. Such as a thermoplastic resin, a polymer having rubber elasticity, a polysaccharide and the like can be used alone or as a mixture of two or more. Further, it is desirable that a binder having a functional group that reacts with lithium, such as a polysaccharide, be deactivated by, for example, methylation. The addition amount is preferably 1 to 50% by weight, particularly 2 to 50% by weight.
30% by weight is preferred.

As the filler, any material may be used as long as it does not adversely affect battery performance. Usually, olefin polymers such as polypropylene and polyethylene, aerosil, zeolite, glass, carbon and the like are used. The added amount of the filler is preferably 30% by weight or less.

The current collector of the electrode active material may be any collector as long as it does not adversely affect the constructed battery. For example, as a positive electrode material, in addition to aluminum, titanium, stainless steel, nickel, calcined carbon, conductive polymer, conductive glass, and the like, for the purpose of improving adhesiveness, conductivity, and oxidation resistance, aluminum and copper, etc. Surface carbon,
Those treated with nickel, titanium, silver, or the like can be used. As the negative electrode material, copper, stainless steel, nickel, aluminum, titanium, calcined carbon, conductive polymer,
In addition to conductive glass, Al-Cd alloy, and the like, those obtained by treating the surface of copper or the like with carbon, nickel, titanium, silver, or the like for the purpose of improving adhesion, conductivity, and oxidation resistance can be used. These materials can be oxidized on the surface. For these shapes, in addition to foils, films, sheets, nets, punches, expanded ones, laths, porous bodies, foams, formed bodies of fibers, and the like are used. The thickness is not particularly limited, but is 1 to 50
One having a thickness of 0 μm is used. On the other hand, as the positive electrode active material, MnO ₂ , MoO ₃ , V ₂ O ₅ , Li _x CoO ₂ ,
Metal oxides such as Li _x NiO ₂ and Li _x Mn ₂ O ₄ ; metal chalcogenides such as TiS ₂ , MoS ₂ and NbSe ₃ ; graphite intercalation compounds such as polyacene, polyparaphenylene, polypyrrole and polyaniline; Various substances capable of absorbing and releasing an alkali metal ion such as a conductive polymer and an anion can be used.

In particular, when the silicon of the present invention is used as a negative electrode active material, V
₂ O ₅ , MnO ₂ , Li _x CoO ₂ , Li _x NiO ₂ ,
Those having an electrode potential of 3 to ₄ V, such as Li _x Mn ₂ O _4, are desirable. In particular, Li _x CoO ₂ , Li _x NiO ₂ , L
i _x Mn lithium-containing transition metal oxides such as ₂ O ₄ are preferred.

As the electrolyte, for example, an organic electrolyte, a polymer solid electrolyte, an inorganic solid electrolyte, a molten salt, or the like can be used, and among them, an organic electrolyte is preferable. As the organic solvent of the organic electrolyte, propylene carbonate, ethylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, esters such as γ-butyrolactone, tetrahydrofuran, substituted tetrahydrofuran such as 2-methyltetrahydrofuran, dioxolane, Ethers such as diethyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, dimethylsulfoxide, sulfolane, methylsulfolane,
Acetonitrile, methyl formate, methyl acetate, N-methylpyrrolidone, dimethylformamide and the like can be mentioned, and these can be used alone or as a mixed solvent.

[0027] The main constituent solute of the electrolyte used in the present invention may be a salt containing carbon. For example, JP
Formula used in JP-A-8-225045: (C _n X
_{2n + 1} Y) ₂ N ⁻ , M ⁺ , and the following general formulas (2) and (3): (RSO ₂ ) ₃ C ⁻ , M ⁺ ... General formula (2) (RSO ₂ ) O ^- , M ⁺ ... It is preferable to use a compound represented by the general formula (3). More preferably, a compound represented by the following general formula (1) (R ₁ SO ₂ ) (R ₂ SO ₂ ) NLi...

In the above formula, Y is SO ₂ or CO, and R is C _n
F _{2n + 1} , R ₁ , R ₂ are C _n F _{2n + 1} , where n is 1 to 4
R ₁ = R ₂ or R ₁ ≠ R ₂ . Most preferably, R ₁ = R ₂ = CF ₃ , R ₁ = R ₂ = C ₂ F
₅ or R ₁ = CF ₃ , R ₂ = C ₄ F ₉ .

On the other hand, as the solid electrolyte, for example, an inorganic solid electrolyte, an organic solid electrolyte, an inorganic organic solid electrolyte, a molten salt, or the like can be used. Well known inorganic solid electrolytes include lithium nitrides, halides, oxyacid salts, phosphorus sulfide compounds, and the like, and one or more of these can be used in combination. Among them, Li ₃ N,
_{LiI, Li 5 NI 2, Li} 3 N-LiI-LiOH,
Li ₄ SiO ₄ , Li ₄ SiO ₄ -LiI-LiOH,
xLi ₃ PO _{4- (1-x)} Li ₄ SiO ₄ , Li ₂ SiS ₃
Etc. are effective. On the other hand, in an organic solid electrolyte, a polyethylene oxide derivative, a polymer containing at least the derivative, a polypropylene oxide derivative, a polymer containing at least the derivative, polyphosphazene or the derivative, a polymer containing an ion dissociating group, a phosphate ester polymer derivative Further, a polymer matrix material (gel electrolyte) in which a non-aqueous electrolyte is contained in a polyvinyl pyridine derivative, a bisphenol A derivative, polyacrylonitrile, polyvinylidene fluoride, fluorine rubber, or the like is effective.

As the separator, an insulating thin film having excellent ion permeability and mechanical strength can be used. Olefin polymers such as polypropylene and polyethylene, glass fiber, and organic solvent resistant and hydrophobic
Sheets, microporous membranes, and nonwoven fabrics made of polyvinylidene fluoride, polytetrafluoroethylene, or the like are used.
The pore size of the separator is in a range generally used for a battery, and is, for example, 0.01 to 10 μm. The same applies to the thickness, which is in the range generally used for batteries, for example, 5 to 300 μm.

When the foreign semiconductor of the present invention is used as a wafer-shaped plate, a conductive adhesive layer may be provided between the current collector and the active material for the purpose of collecting current. Usually, silver paste and carbon paste are used as the conductive adhesive. Further, by plating a part of the crystal with nickel, it is possible to join with a molten metal such as solder or silver brazing. The shape can be freely processed by a diamond cutter or an etching process.

As described above, according to the present invention, in a non-aqueous electrolyte battery in which the main constituent material of the negative electrode active material is silicon, by using a salt containing carbon as the main constituent solute of the electrolyte, at least 0 Lithium ions can be inserted and extracted in the range of up to 2 V, and since silicon is strong, fine powdering during charge and discharge and partial isolation of the negative electrode active material, which are observed in ordinary alloys, are suppressed. By using such a solute as a non-aqueous electrolyte, it can be used as a negative electrode of a secondary battery having excellent charge-discharge efficiency, good cycle characteristics, and excellent charge-discharge characteristics. In particular, by doping a high concentration of impurities, the electron conductivity inside the crystal is improved, the alloying of silicon and lithium is smoothly performed, and the charge / discharge rate characteristics are improved. Further, since the negative electrode potential is close to the lithium potential and low, the voltage of the battery becomes high, and a high energy density is achieved due to its large capacity.

[0033]

Embodiments of the present invention will be described below.

(Invention) A silicon single crystal, which is a p-type semiconductor doped with 10 ⁴ silicon atoms at a ratio of 1 B atom, is (a). As a p-type semiconductor, 10 ⁴ silicon atoms have 1 B atom. Silicon polycrystal (b) doped in a proportion,
(C) A commercially available silicon polycrystal having a purity of 99.99% was implanted into amorphous silicon by ion implantation.
10 ⁴ silicon atoms in those injected with a ratio of one B atom and (d), by grinding each in a mortar, prototype A coin type nonaqueous electrolyte battery as follows by using the negative electrode active material did. The active material, acetylene black and polytetrafluoroethylene powder were mixed at a weight ratio of 85: 10: 5, and toluene was added and kneaded sufficiently. This was formed into a sheet having a thickness of 0.1 mm by a roller press. Next, this was punched into a circle having a diameter of 16 mm,
After drying at 15 ° C. for 15 hours, negative electrode 2 was obtained. The negative electrode 2 was used by being pressed against a negative electrode can 5 provided with a negative electrode current collector 7. The positive electrode 1 was prepared by mixing LiCoO ₂ , acetylene black and polytetrafluoroethylene powder as a positive electrode active material in a weight ratio of 85: 1.
The mixture was mixed at 0: 5, and toluene was added and kneaded well. This was formed into a 0.8 mm thick sheet by a roller press. Next, this was punched out into a circle having a diameter of 16 mm and dried under reduced pressure at 200 ° C. for 15 hours to obtain a positive electrode 1. Positive electrode 1
Was press-bonded to a positive electrode can 4 having a positive electrode current collector 6. 1 m of (C ₂ F ₅ SO ₂ ) ₂ NLi was added to a mixed solvent of ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1.
ol / liter of an electrolytic solution was used, and a polypropylene microporous membrane was used for the separator 3. A coin-type lithium battery having a diameter of 20 mm and a thickness of 1.6 mm was manufactured using the above-mentioned positive electrode, negative electrode, electrolyte and separator. Batteries using silicon (a) to (d) are referred to as batteries (A1) to (D1), respectively.

(Comparative Example) As a solute of the electrolytic solution, (C ₂ F
_A battery was prepared in the same manner as in the present invention except that LiBF ₄ was used instead of ₅ SO ₂ ) ₂ NLi. The obtained batteries are referred to as comparative batteries (A2) to (D2).

The battery of the present invention (A
A charge / discharge cycle test was performed using 1) to (D1) and comparative batteries (A2) to (D2). The test conditions were charging current 3
mA, a charge end voltage of 4.1 V, a discharge current of 3 mA, and a discharge end voltage of 3.0 V. Table 1 shows the results of the charge / discharge test of these batteries.

[0037]

[Table 1]

As can be seen from Table 1, batteries (A1) to (D) using a salt containing carbon as a solute of the electrolytic solution according to the present invention.
1) has better charge / discharge characteristics than the comparative batteries (A2) to (D2) using LiBF ₄ as a solute of the electrolytic solution,
The decrease after 10 cycles was small. The battery (A
From comparisons of 1) to (D1), it can be seen that the cycle characteristics of single crystal silicon are superior to those of polycrystalline semiconductors.
The reason for this is not clear, but is considered as follows. That is, a polycrystalline semiconductor is a mass of many small crystals, and a grain boundary exists between crystals. When these materials occlude and release lithium, the volume of the crystals changes. That is, it is considered that a crack is formed in the grain boundary portion with the change in volume, the active material is isolated, and the active material is pulverized, thereby causing cycle deterioration. With respect to (C1) using a negative electrode not doped with boron as an impurity, reversibility was recognized, although the capacity was slightly inferior. Further, it can be seen that the cycle characteristics of (D1) using amorphous silicon were excellent although the capacity was slightly reduced.

In the above description, (C
_Although 2F ₅ SO ₂ ) ₂ NLi was mentioned, the same effect was confirmed for other carbon-containing salts. The present invention is not limited to the starting materials, the production method, the positive electrode, the negative electrode, the electrolyte, the separator, the shape of the battery, and the like of the active material described in the above-described embodiment.

[0040]

Since the present invention is configured as described above, it is possible to provide a non-aqueous electrolyte battery having high voltage, high capacity, high energy density, excellent charge / discharge cycle characteristics, and high safety.

[Brief description of the drawings]

FIG. 1 is a sectional view of a coin-type nonaqueous electrolyte battery according to the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 positive electrode 2 negative electrode 3 separator 4 positive electrode can 5 negative electrode can 6 positive electrode current collector 7 negative electrode current collector 8 insulating packing

Claims (4)

[Claims] 1. A non-aqueous electrolyte battery wherein the main constituent material of the negative electrode active material is silicon, wherein a salt containing carbon is used as a main constituent solute of the electrolyte. 2. The carbon-containing salt is represented by the following general formula (1): (R ₁ SO ₂ ) (R ₂ SO ₂ ) NLi... General formula (1) The nonaqueous electrolyte battery according to any one of the preceding claims. 3. The compound of the formula (1) wherein R
_{3. The method according} to claim ₂ , wherein ₁ and R ₂ are represented by C _n F _{2n + 1} , n is a number from 1 to 4, and R ₁ = R ₂ or R ₁ ≠ R _2. Water electrolyte battery. 4. The non-aqueous electrolyte battery according to claim 1, wherein the electron conductivity of silicon as a main constituent material of the negative electrode active material is 10 ⁻⁵ Scm ⁻¹ or more at 20 ° C. .