JPH10223220A - Non-aqueous electrolyte cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-aqueous electrolyte cell having highly voltage, capacity, and energy density and which indicates highly charging/discharging cyclic characteristic, and is highly safe by constitution the main component of the negative active material of the covalent bond crystal of the prescribed electric conductivity. SOLUTION: The concentration of impurities in a mixed manner is generally one donor atom or one acceptor atom for 10<7> -10<6> silicon atoms, but preferably, doping with high concentration is suitable, and the concentration of one donor atom or one acceptor atom for 10<4> silicon atoms or higher concentration is preferable. Electric conductivity σ to be obtained by doping these impurities is preferably >=10<-5> Scm<-1> at 20 deg.C, more preferably, >=10<-2> Scm<-1> and most preferably >=1Scm<-1> . The covalent bond crystal is preferably a powder material, whose means grain size is 0.1-500μm.

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 thereof.

[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.

[0003] Further, alloys with metallic lithium have been used for the purpose of preventing the dendrite generated during charging as described above. However, when the charged amount is large, problems such as fine powdering of the negative electrode and falling off of the negative electrode active material are caused. was there.

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
There has been a problem that an internal short circuit and a reduction in charging efficiency occur during rapid charging. 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. This may cause an internal short circuit of the cell or lower 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.

On the other hand, as a negative electrode active material other than metallic lithium, a lithium alloy or a carbon material, a composite oxide containing silicon and lithium, Li _x Si _{1- y My} O _z (Japanese Patent Laid-Open No.
-230800) and an amorphous chalcogen compound M ¹ M
² _p M ⁴ _q has been proposed to use a (JP-A-7-288123), a high capacity, has been improved in terms of high energy density.

[0006] However, the above-mentioned composite oxide has a problem in quick charging and load characteristics because the electric conductivity of the active material itself is low. Attempts have been made to add conductive agents to solve this problem, but in order to obtain satisfactory charge / discharge characteristics, it is necessary to use a low-density carbon material as a conductive agent in a sufficient amount, The capacity per unit will be reduced.

Further, since the material of the composite oxide or the like is itself an oxide, it is considered that the reaction with lithium proceeds through the reduction of the oxide. Efficiency was sometimes reduced. 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.

[0008]

That is, when lithium metal or an alloy of lithium and a metal is used as the negative electrode, there are advantages of high voltage, high capacity, and high energy density, but in terms of cycleability and safety. There is a problem, and when a carbon material is used, it is advantageous in terms of high voltage and safety, but insufficient in terms of high capacity and high energy density. Furthermore, when an oxide negative electrode is used, the points of high capacity and high energy density seem to be improved, but they are not satisfactory in terms of high voltage, charge / discharge efficiency characteristics, cycle life and safety. .

For this reason, at a high voltage and a 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.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and the main constituent material of the negative electrode active material is an ideal negative electrode active material used in a non-aqueous electrolyte battery. It is characterized by comprising a covalent crystal having a conductivity σ of 10 ⁻⁵ Scm ⁻¹ or more at 20 ° C.

Further, the main constituent material of the above-mentioned negative electrode active material is preferably made of a powdered silicon single crystal.

First, as an alloy of lithium and silicon, B
inary Alloy Phase Diagram
As in s (p2465), alloying with a composition up to Li ₂₂ Si ₅ is known. Further, Japanese Unexamined Patent Publication No.
No. 74463 reports that the cycle characteristics are improved by using a single crystal of silicon for the 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 inventors have studied the improvement of electron conductivity with respect to the covalent bond crystal, and as a result, have found that the occlusion and release of lithium smoothly proceed without the phenomenon of lithium precipitation. Further, it was found that this reaction proceeded at a potential very close to the lithium potential of about 0.1 V, a high capacity close to the theoretical capacity was obtained, and the reversibility was excellent.

In other words, although an alloy of lithium and silicon is known, silicon itself was originally an intrinsic semiconductor of a covalent bond crystal, and as it was, the electron conductivity was low, and the characteristics as a battery negative electrode material were poor. For this reason, it was a material that was difficult to be studied.However, by doping silicon with a potential donor or acceptor atom as a material to be incorporated into the battery, electron conductivity was improved and lithium was easily absorbed and released. Find out what happens. In particular, it was found that when silicon was used as a single crystal, phenomena such as crystal collapse, pulverization, and falling were not observed, and cycle characteristics were improved.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The covalent crystals referred to herein include Si, Ge, GaAs, GaP, InSb, and Ga.
Among them, P, SiC, BN and the like can be mentioned. Among them, silicon is particularly preferable because it has excellent charge / discharge characteristics, is abundant in resources, and has low toxicity, but is not limited thereto. In addition, about the crystal system,
Single crystal, polycrystal, amorphous and the like are preferable. Among them, single crystal is preferable because particularly excellent charge / discharge characteristics can be obtained, but is not limited thereto.

Further, the covalent crystal may contain an impurity for the purpose of improving electron conductivity. The impurities referred to here are those which can be donor atoms and acceptor atoms among all elements in the periodic table, and are preferably P, Al, As, Sb, B, Ga, In, etc., but are not limited to these. It is not something to be done.

A method for obtaining a single crystal of silicon is as follows.
Z method (Czochralski method or lifting method), FZ
(Floating zone) method, smoke method, and the like, but are not limited thereto.

[0017] For the concentration of the mixed impurities, it is usually a proportion of silicon atoms 10 ⁷ 1 donor atoms or acceptor atoms to 10 ^6, preferably has a high concentration of doping suitable, silicon atoms 10 ⁴ It is desirable that the concentration be as high as one donor atom or one acceptor atom or more. The electric conductivity σ obtained by doping such impurities at 20 ° C. is preferably 10 ⁻⁵ Scm ⁻¹ or more, more preferably 10 ⁻² Scm ⁻¹ or more, and most preferably ¹ Scm ^−1.
That is all.

The covalent crystal used in the present invention is preferably a powder having an average particle size of 0.1 to 500 μm. A pulverizer or a classifier is used to obtain a powder having a predetermined shape. 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 contains lithium metal, lithium alloy, etc., 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 further inserted into the alloy of the covalent crystal and lithium used in the present invention inside the battery.

When the alloy of the covalent crystal and lithium according to 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. Usually, natural graphite (flaky graphite, flaky graphite, earthy 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, carbomethoxy cellulose And the like, a thermoplastic resin, a polymer having rubber elasticity, a polysaccharide and the like can be used as one kind or as a mixture of two or more kinds. 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 from 1 to 50% by weight, particularly preferably from 2 to 3% by weight.
0% by weight is preferred.

Any filler 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 addition amount of the filler is preferably 0 to 30% by weight.

The current collector of the electrode active material may be any electronic conductor that does not adversely affect the battery. For example, as the material of the positive electrode current collector, aluminum, titanium, stainless steel, nickel, calcined carbon,
In addition to conductive polymers, conductive glasses, etc., it is possible to use aluminum, copper, etc. whose surfaces are treated with carbon, nickel, titanium, silver, etc. for the purpose of improving adhesiveness, conductivity, and oxidation resistance. it can. Materials for the negative electrode current collector include copper,
In addition to stainless steel, nickel, aluminum, titanium, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., the surface of copper etc. is carbonized for the purpose of improving adhesion, conductivity and oxidation resistance. , Nickel, titanium, silver or the like can be used. These materials can be oxidized on the surface. As these shapes, in addition to the foil shape, a film shape, a sheet shape, a net shape, a punched or expanded shape, a lath body, a porous body, a foamed body, a formed body of a fiber group, and the like are used. The thickness is not particularly limited, but a thickness of 1 to 500 μm is used.

The alloy of the covalent crystal and lithium thus obtained can be used as a negative electrode active material. On the other hand, as the positive electrode active material, MnO ₂ , MoO ₃ , V ₂ O
₅ , Li _x CoO ₂ , Li _x NiO ₂ , Li _x Mn ₂ O
₄ , metal oxides such as TiS ₂ , MoS ₂ , NbSe
Metallic chalcogenides such as ₃ ; graphite intercalation compounds such as polyacene, polyparaphenylene, polypyrrole, and polyaniline; alkali metal ions such as conductive polymers; and various substances capable of absorbing and releasing anions can be used.

In particular, when the alloy of the covalent bond crystal and lithium of the present invention is used as a negative electrode active material, V ₂ O ₅ , MnO ₂ , Li _x CoO ₂ , L
_{i x NiO 2, Li x Mn} 2 O 4, Li x Fe 2 (SO
₄ ) ₃ , Li _x FePO ₄ , Li _{1 + x} Ti ₂ (PO ₄ )
₃ , Li _{3 + x} Fe ₂ (PO ₄ ) ₃ or the like having an electrode potential of _{2 to 4} V is desirable. In particular, Li _x CoO ₂ , Li
_{x NiO 2, Li x Mn 2} O lithium-containing transition metal oxides such as 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. Examples of the organic solvent of the organic electrolyte include propylene carbonate, ethylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, esters such as γ-butyrolactone, and substituted tetrahydrofurans such as tetrahydrofuran and 2-methyltetrahydrofuran, and dioxolane. , Diethyl ether, dimethoxyethane, diethoxyethane, ethers such as methoxyethoxyethane,
Dimethyl sulfoxide, sulfolane, methyl sulfolane, acetonitrile, methyl formate, methyl acetate, N-methylpyrrolidone, dimethylformamide and the like can be used, and these can be used alone or as a mixed solvent. LiClO ₄ , LiP
F ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ ,
LiN (CF ₃ SO ₂ ) _{2 and the} like. On the other hand, as the polymer solid electrolyte, those obtained by dissolving the above-described supporting electrolyte salt in a polymer such as polyethylene oxide or a crosslinked product thereof, or polyphosphazene or a crosslinked product thereof can be used. Further, inorganic solid electrolytes such as Li ₃ N and LiI can be used. That is, any non-aqueous electrolyte having lithium ion conductivity may be used.

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. Also,
The same applies to the thickness, which is in the range generally used for batteries, for example, 5 to 300 μm.

When the covalent crystal of the present invention is used as a powder, it is possible to modify at least the surface layer portion of the powder. For example, gold, silver, carbon, nickel,
Coating a material with good electron conductivity, such as copper, or a material with good ion conductivity, such as lithium carbonate, boron glass, and solid electrolyte, by applying techniques such as plating, sintering, mechanofusion, and vapor deposition. .

The reason why such excellent charge / discharge characteristics are obtained is not necessarily clear, but is considered as follows. That is, it can be seen that the crystal having a covalent bond can be alloyed with lithium, and the abundance ratio of lithium in the alloy is large. However, a crystal having a covalent bond is an intrinsic semiconductor although it is a semiconductor, and its electric conductivity at room temperature is low and its polarization during charging and discharging is relatively large. On the other hand, when impurities that can be donor atoms and acceptor atoms are doped into the covalent bond crystal, electron conductivity is improved, polarization during charge / discharge is reduced, and electrons are easily given to lithium ions, so that The occluded lithium alloy and the occluded lithium alloy easily release electrons and are released as lithium ions. In other words, it is assumed that the flow of electrons inside the crystal becomes smoother by obtaining the mechanism by which the covalent bond crystal allows electrons to flow, thereby facilitating insertion and extraction of lithium ions. In addition, since the crystal structure of silicon and gallium is the same face-centered cubic structure as diamond, the bonding of crystals is very strong and follows the expansion and contraction related to the insertion and extraction of lithium. This is not seen, and it is considered that the reversibility of charge and discharge is improved. Furthermore, when a single crystal is used, there is no grain boundary inside the crystal, so that stress does not accumulate at the grain boundary even when the crystal expands and contracts during insertion and extraction of lithium. No dropout was observed, and it is considered that the reversibility of charge and discharge was improved.

The negative electrode active material of the present invention comprising a covalent crystal as a main constituent can occlude and release lithium ions in a nonaqueous electrolyte in a range of at least 0 to 2 V with respect to metallic lithium. In addition, since the covalent bond crystal is strong, it is possible to suppress fine powdering at the time of charging and discharging and partial electrical isolation of the negative electrode active material, which are observed in ordinary alloys. In addition, if the covalent crystal is doped in advance with an impurity that can be a donor atom or an acceptor atom, the electron conductivity is improved, and the alloying of the covalent crystal and lithium is smoothly performed. In addition, by treating such a covalent crystal as a powder, it becomes possible to coat it on a current collector, and it becomes possible to design various battery shapes such as a cylinder, a square, a flat, a coin, and a film. In addition, it can be used in combination with a conductive agent, and the charge / discharge rate characteristics are also improved. Further, since the negative electrode potential is close to the lithium potential and low, the voltage of the battery becomes high. In addition, a large amount of lithium can be stored, so that a high energy density is achieved. In particular, when a single crystal is used as a covalent crystal, even if the crystal expands and contracts during insertion and extraction of lithium because no grain boundary exists in the crystal, stress does not accumulate in the grain boundary, and as a result, the active material itself It is considered that the reversibility of charge / discharge was improved without miniaturization or falling off of the particles. In addition, using silicon as the negative electrode material is particularly excellent because silicon itself has low toxicity and is a material rich in resources. By using such a negative electrode active material as an electrode material, a high voltage, a high energy density, and excellent charge / discharge cycle characteristics are exhibited.
A highly safe nonaqueous electrolyte battery can be obtained.

[0031]

Embodiments of the present invention will be described below. (Example 1) By diffusion method, 10 ⁴ silicon atoms were added with P
The silicon single crystal is doped n-type semiconductor in a ratio of one atom (a), the polycrystalline silicon is n-type semiconductor doped at a ratio of one P atom 10 ⁴ silicon atoms (b), a silicon atom 10 ⁴ to the silicon single crystal is a p-type semiconductor doped in a proportion of 1 B atoms (c), doped to 10 ⁴ silicon atoms in B atomic ratio of one p
The silicon polycrystal, which is a type semiconductor, is designated as (d). The specific resistance was 20 ° C., the n-type semiconductor was 33 Scm ⁻¹ , and the p-type semiconductor was 20 Scm ⁻¹ . Each covalent crystal was ground in a mortar, and a coin-type lithium secondary battery was prototyped using the negative electrode active materials as follows. The negative electrode 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 0.3 mm by a roller press. Next, this was punched out into a circle having a diameter of 16 mm, and heat-treated under reduced pressure at 200 ° C. for 15 hours to obtain a negative electrode 2. 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 is made of LiCoO ₂ as a positive electrode active material.
And 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 0.8 mm thick sheet by a roller press. Next, this was punched out into a circle having a diameter of 16 mm,
Heat treatment was performed for 5 hours to obtain a positive electrode 1. The positive electrode 1 is a positive electrode current collector 6
The positive electrode can 4 was pressed and used. An electrolytic solution obtained by dissolving LiPF ₆ at a concentration of 1 mol / liter in a mixed solvent of ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1 was used, and a microporous polypropylene membrane was used as 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. These covalent crystals (a) to
The batteries using (d) are referred to as (A) to (D), respectively.

Comparative Example 1 A single crystal (e) of gallium arsenide was used as a covalent crystal containing no impurities. This specific resistance was 10 ⁻⁸ Scm ⁻¹ at 20 ° C. Except for this, the coin-type lithium battery was manufactured in the same manner as in Example 1 above. The obtained battery is designated as (E).

Comparative Example 2 A coin-type lithium battery was manufactured in the same manner as in Example 1 except that aluminum powder was used instead of the covalent crystal. Let the obtained battery be (F). A capacity test of the coin-type lithium battery manufactured as described above was performed. (A) using covalent crystal
To (E) and (F) using a metal crystal, insertion and extraction of lithium were confirmed, but almost no cell (D) using a covalent bond crystal lower than 10 ⁻⁵ Scm ⁻¹ at 20 ° C. Lithium could not be released. Table 1 shows the initial capacity and the capacity at the tenth cycle at this time. Cell (E) using a covalent crystal lower than 10 ⁻⁵ Scm ⁻¹ at 20 ° C.
It can be said that the resistance was large and that the absorption and release of lithium into silicon was difficult to occur. In addition, it is apparent from the results that (F) using a metal crystal has poor reversibility of charge and discharge. This is probably because the pulverization that occurs during alloying of the metal crystal and lithium increases the number of electrically isolated active materials. It can be seen that the negative electrode of the present invention using the covalent crystal doped with impurities and having improved electron conductivity has excellent charge / discharge cycle characteristics and high capacity. Further, in the covalent bonding crystals (A) to (D) in which the same impurity is doped and the electron conductivity is improved, a single crystal is used (A),
In (C), it can be seen that the cycle characteristics are improved as compared with (B) and (D) using polycrystal. That is, since the single crystal has no grain boundary inside the crystal, the occlusion of lithium,
This is because, even if the crystal expands or shrinks upon release, stress does not accumulate at the grain boundaries, and the active material itself can be suppressed from being pulverized or dropped, and as a result, cycle characteristics are considered to be improved.

[0035]

[Table 1]

In the embodiment, the electric conductivity σ is 20 ° C.
In the above, silicon was mentioned as a covalent crystal of 10 ⁻⁵ Scm ⁻¹ or more, but the same effect was confirmed for other covalent crystals. 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.

[0037]

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 a second embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 negative electrode 2 positive electrode 3 separator 4 negative electrode can 5 positive electrode can 6 negative electrode current collector 7 positive 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 a covalent crystal having an electric conductivity σ of 10 ⁻⁵ Scm ⁻¹ or more at 20 ° C. 2. The non-aqueous electrolyte battery according to claim 1, wherein the negative electrode active material is in a powder form. 3. The non-aqueous electrolyte battery according to claim 1, wherein the covalent crystal of the negative electrode active material is a single crystal. 4. The non-aqueous electrolyte battery according to claim 3, wherein said single crystal is made of silicon.