JP2014063660A - Sulfate, positive electrode material for lithium secondary battery, positive electrode for lithium secondary battery using the same, and lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: novel sulfate; a positive electrode material for a lithium secondary battery, using the sulfate and capable of providing a lithium secondary battery that achieves both high safety and high energy density; a positive electrode for a lithium secondary battery; and a lithium secondary battery using them.SOLUTION: Sulfate is represented by the following general formula (1). Preferably, the sulfate is powder including primary particles having an average particle size of less than 1 μm. Q is selected from the group consisting of Fe, Co, Ni, V, Mg, Ca, Sr, Cu and Zn. A positive electrode material for a lithium secondary battery comprises the sulfate. Li(MnQ)(SO)...(1) (Q represents at least one dopant element, an index (a) is selected so as to secure the electrical neutrality in the general formula (1), satisfies a relation of 0.5>a≥0, and is a quantity maintaining a crystal structure at a=0.)

Description

  The present invention relates to a sulfate and a positive electrode material for a lithium secondary battery, a positive electrode for a lithium secondary battery using the sulfate, and a lithium secondary battery including the positive electrode for a lithium secondary battery.

  Lithium secondary batteries are excellent in energy density and output density, and are effective in reducing the size and weight. Therefore, the demand for power supplies for portable devices such as notebook computers, mobile phones, and handy video cameras is growing rapidly. ing. Lithium secondary batteries are also attracting attention as power sources for electric vehicles and power load leveling.

Currently, as a positive electrode active material for a lithium secondary battery, an oxide typified by layered LiMO ₂ (where M is a transition metal such as Ni, Mn, Co, etc.) and a polyanion typified by LiFePO ₄ are used. It is roughly divided into The oxide-based positive electrode active material requires rare and expensive Ni and Co in order to increase the discharge potential, and further tends to release oxygen in a charged state, and thus has a potential safety problem. On the other hand, the polyanionic positive electrode active material can achieve a higher discharge potential than the oxide even with Fe and Mn, which are abundant and inexpensive due to the inductive effect due to the polyanion, and oxygen is covalently bound within the polyanion. Therefore, it is difficult to release oxygen even in a charged state, and has an inherently high safety.

However, since the charge / discharge potential of LiFePO ₄ is lower than that of a 4V class positive electrode active material represented by about 3.4 V (vs. Li ^/ Li ⁺ ) and LiCoO ₂ , the energy density is low. Have

Here, it is known that the discharge potential can be increased by making the phosphate group a sulfate group having a higher electronegativity.
Actually, in Non-Patent Document 1, the charge / discharge potential of Li ₂ Fe (SO ₄ ) ₂ is improved to about 3.8 V (vs. Li / Li ⁺ ), but the oxidation-reduction of Fe in this compound is divalent. It is described that the theoretical capacity is 102 mAh / g, which is smaller than the theoretical capacity 170 mAh / g of LiFePO ₄ , since it occurs only during trivalent.

Non-Patent Document 2 describes that Li ₂ Mn ₂ (SO ₄ ) ₃ is electrochemically inactive in a potential window of 3.0 to 4.3 V (vs. Li / Li ⁺ ). ing.

Marine Reynaud, Mohamed Ati, Brent C. Melot, Moulay T. Sougrati, Gwenaelle Rousse, Jean-Noel Chotard and Jean-Marie Tarascon, Electrochemistry Communications, 21 (2012) 77-80 J. Isasi, C.I. Train, S. Jaulmes, A. Elfakir and M. Quarton, Journal of Solid State Chemistry, 158 (2001) 148-153

  With the need for higher capacity and higher safety for batteries in recent years, development of a polyanionic positive electrode active material that exhibits intrinsically high safety and high energy density is desired. Sufficient materials have not been developed from the viewpoint. Therefore, for example, development of a novel polyanionic positive electrode active material capable of reversibly utilizing a plurality of lithium per transition metal is strongly demanded.

As a sulfate containing abundant raw materials and containing lithium composed of Fe and Mn, the substances known so far are two compounds of Li ₂ Fe (SO ₄ ) ₂ and Li ₂ Mn ₂ (SO ₄ ) ₃ described above. Only. However, it is described in Non-Patent Document 1 that Li ₂ Fe (SO ₄ ) ₂ can use only 0.86 lithium per transition metal.
Non-Patent Document 2 describes that Li ₂ Mn ₂ (SO ₄ ) ₃ is not capable of expressing electrochemical activity.

The present invention has been made in view of the above problems.
That is, the present invention uses a novel sulfate and a positive electrode material for a lithium secondary battery and a positive electrode for a lithium secondary battery that can provide a secondary battery having both high safety and high energy density using the same. An object is to provide a lithium secondary battery.

As a result of intensive studies in view of the above problems, the present inventors have found a novel sulfate represented by the following general formula (1). Further, the inventors have found that the electrochemical activity is improved by setting the size of the primary particles to less than 1 μm, thereby completing the present invention.
That is, the gist of the present invention is as follows.

[1] A sulfate represented by the following general formula (1).
_{Li 2 (Mn 1-a Q} a) (SO 4) 2 ··· (1)
(Where Q represents at least one dopant element, and the index a is selected so as to ensure the electrical neutrality of the sulfate represented by the general formula (1), and 0.5> a ≧ 0) Yes, the amount maintaining the crystal structure at a = 0.)

[2] The sulfate according to [1], wherein the primary particles have a mean particle size of less than 1 μm.

[3] Q is one or more selected from the group consisting of Fe, Co, Ni, V, Mg, Ca, Sr, Cu, and Zn [1] or [2] The sulfate described in 1.

[4] A positive electrode material for a lithium secondary battery, comprising the sulfate according to any one of [1] to [3].

[5] A positive electrode for a lithium secondary battery, comprising a positive electrode active material layer containing the sulfate according to any one of [1] to [3] and a binder on a current collector.

[6] A lithium secondary battery comprising a negative electrode capable of inserting and extracting lithium, a non-aqueous electrolyte containing a lithium salt, and a positive electrode capable of inserting and extracting lithium, wherein the lithium according to [5] A lithium secondary battery using a positive electrode for a secondary battery.

  According to the present invention, a lithium secondary battery that achieves both high safety and high energy density can be realized at low cost.

Hereinafter, the present invention will be described in detail, but the present invention is not limited to the following description, and can be arbitrarily modified and implemented without departing from the gist of the present invention.

[Sulfate]
The sulfate of the present invention is represented by the following general formula (1).
_{Li 2 (Mn 1-a Q} a) (SO 4) 2 ··· (1)
(Where Q represents at least one dopant element, and the index a is selected so as to ensure the electrical neutrality of the sulfate represented by the general formula (1), and 0.5> a ≧ 0) Yes, the amount maintaining the crystal structure at a = 0.)

The sulfate of the present invention is not particularly limited as long as it is a composition satisfying the general formula (1), and a different element not included in the general formula (1) may be introduced. The different elements include B, Na, Al, K, Ti, V, Cr, Y, Zr, Nb, Ru, Rh, Pd, Ag, In, Sn, Sb, Te, Ba, Ta, Mo, W, Re, Os, Ir, Pt, Au, Pb, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Bi, N, F, Cl, Br, What is selected from any 1 or more types of I is mentioned. These foreign elements may be incorporated into the crystal structure of the sulfate, or may not be incorporated into the crystal structure of the sulfate, and may be unevenly distributed as a simple substance or a compound on the particle surface or grain boundary. Good.
In addition, although the sulfate of this invention can be utilized as an ion conductor, the positive electrode material of a lithium secondary battery, and a catalyst, the use of the sulfate of this invention is not limited to these.

<Dopant element Q>
In the sulfate of the present invention, the dopant element Q is preferably Fe, Co, Ni, V, Mg, Ca, Sr, Cu, Zn, and among these, in terms of having electrochemical activity, Fe and V are preferable, and Co, Ni, Mg, Ca, Sr, Cu, and Zn are preferable in terms of stabilization of the crystal structure.
The sulfate of the present invention may contain only one of the above dopant elements as the dopant element Q, or may contain two or more kinds.

<Crystal structure at a = 0>
The crystal structure at a = 0 in the general formula (1) is not particularly limited as long as it is a crystal structure possessed by a substance having a composition of Li ₂ Mn (SO ₄ ) _2, but the thermodynamic stable phase is monoclinic. Belonging to the space group P2 ₁ or P2 ₁ / c. Whether or not the sulfate represented by the general formula (1) maintains the crystal structure at a = 0 can be confirmed by powder X-ray diffraction measurement as shown in the Examples section described later. it can.

  In the present invention, the composition of sulfate is obtained by using Li and Mn with an inductively coupled plasma emission spectrometer (ICP-AES), and Mn, S and O with an energy dispersive X-ray spectrometer (EDX). And calculating the ratio of Li / Mn / S / O. The desirable a value greatly depends on the dopant element Q. If Q is an electrochemically inactive element, it is desirable that the a value be small, but Q is electrochemically active such as Fe or V. If it is an element, a larger value may be desirable within a range in which the crystal structure at a = 0 can be maintained.

  In the general formula (1), the index a is preferably 0.5> a ≧ 0 particularly when the dopant element Q is Fe or V, and the dopant element Q is Co, Ni, Mg, Ca, Sr, In the case of Cu and Zn, it is particularly preferable that 0.4 ≧ a ≧ 0.

<Average particle size of primary particles>
The average particle diameter of the primary particles of the sulfate of the present invention can be determined from the average value obtained by observing three or more different fields of view with an SEM. When the primary particles are fibrous, the fiber diameter direction is used as the average particle diameter, and when the primary particles are elliptical (rugby ball shape), the minor axis direction is used as the average particle diameter. The average particle diameter of the primary particles of the sulfate of the present invention is less than 1 μm, preferably 0.1 μm or less, more preferably 10 nm or less. In particular, in applications as a positive electrode material for lithium secondary batteries, it is easy to put in and take out lithium if the average particle size of the primary particles is smaller than the above upper limit, and it is possible to express a capacity close to the theoretical capacity. preferable. In addition, although there is no restriction | limiting in particular about the minimum of the average particle diameter of the primary particle of the sulfate of this invention, Usually, it is 1 nm at the point of the ease of handling.

<Method for producing sulfate>
The method for producing a sulfate of the present invention is not limited to a specific production method. For example, lithium sulfate and manganese sulfate may be mixed at a predetermined ratio, and the mixture may be baked and produced. Anhydrous lithium sulfate and manganese sulfate are mixed and dispersed in an ionic liquid such as 1-ethyl-3-methylimidazolium bis (trifluoromethanesulfonyl) imide or an organic solvent such as tetraethylene glycol, followed by heat treatment in an autoclave. You may manufacture by doing.

  In the case of a sulfate containing the dopant element Q, in addition to lithium sulfate and manganese sulfate, a sulfate of the dopant element Q can be used in a predetermined ratio and can be produced in the same manner as above, and each sulfuric acid used as a raw material By adjusting the use ratio of the salt, a sulfate having a desired composition can be obtained.

  There are no particular limitations on the firing conditions during firing of the above mixture, but the firing temperature is preferably 300 ° C or higher, particularly 400 ° C or higher, and 1000 ° C or lower, particularly 900 ° C or lower. If the calcination temperature is too low, unreacted raw materials remain or a long reaction time is required, and if it is too high, the sulfate anion may be decomposed. The firing time is usually 5 to 168 hours, preferably 24 to 96 hours, although it varies depending on the firing temperature and the raw materials used. If the firing time is shorter than this range, the reaction may not be completed, and if it is longer, it is uneconomical.

The firing atmosphere requires an inert atmosphere such as Ar or N ₂ when Fe or V is used as the dopant element Q. In other cases, an air atmosphere may be used, but an inert atmosphere may be used.

  In addition, what is necessary is just to grind | pulverize by a ball mill etc. after baking in order to obtain a sulfate whose average particle diameter of a primary particle is smaller than the said upper limit by manufacturing a sulfate in this way. In order to directly obtain a sulfate having an average primary particle size smaller than the above upper limit, an ionothermal method or a solvothermal method using the ionic liquid or organic solvent is preferable.

[Positive electrode material for lithium secondary batteries]
The positive electrode material for a lithium secondary battery of the present invention is composed of the sulfate of the present invention as described above. As described above, the average particle size of primary particles is less than 1 μm, particularly 0.1 μm or less, especially 10 nm or less. It is preferable that it is 1 nm or more.

[Positive electrode for lithium secondary battery]
The positive electrode for a lithium secondary battery of the present invention is characterized by having a positive electrode active material layer containing the sulfate of the present invention and a binder on the current collector.

<Positive electrode active material>
The positive electrode for a lithium secondary battery of the present invention uses the sulfate of the present invention which is a positive electrode material for a lithium secondary battery of the present invention as a positive electrode active material (positive electrode material) of a positive electrode active material layer. As the positive electrode active material, only the sulfate of the present invention may be used, and if necessary, other positive electrode active materials other than the sulfate of the present invention may be used in combination. In that case, as other positive electrode active materials, transition metal oxides such as lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide, transition metal polyanions such as lithium iron phosphate, lithium manganese phosphate, etc. Two or more kinds can be used, but the ratio of the sulfate of the present invention in the total positive electrode active material is 20 mass in order to effectively obtain the safety and energy density improvement effect by using the sulfate of the present invention. It is preferable to use another positive electrode active material so that it may become more than%, especially 40 mass% or more.

  The content of the positive electrode active material in the positive electrode active material layer is preferably 50 to 98% by mass, particularly 65 to 95% by mass. If the content of the positive electrode active material in the positive electrode active material layer is too small, the battery capacity and conductivity may not be sufficient, and if it is too large, the binder content will be relatively small and the mechanical strength of the positive electrode will be reduced. May be insufficient.

<Binder>
As the binder, a polymer that is stable against a liquid solvent described later is preferable. For example, resin polymers such as polyethylene, polypropylene, polyethylene terephthalate, aromatic polyamide, cellulose, rubbery polymers such as styrene / butadiene rubber, isoprene rubber, butadiene rubber or ethylene / propylene rubber, styrene / butadiene / styrene block Polymers, hydrogenated products thereof, styrene / ethylene / butadiene / styrene copolymers, styrene / isoprene / styrene block copolymers and thermoplastic elastomeric polymers such as hydrogenated products, syndiotactic 1,2-polybutadiene , Ethylene / vinyl acetate copolymer, or soft resinous polymer such as propylene / α-olefin (carbon number 2 to 12) copolymer, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), or polytetrafluoroethylene. Fluorine polymers such as Oroechiren-ethylene copolymer, a polymer composition such as having ion conductivity of an alkali metal ion (especially lithium ion) and the like. These may be used individually by 1 type and may be used in combination of 2 or more type.

  Examples of the polymer composition having ion conductivity include polyether polymer compounds such as polyethylene oxide and polypropylene oxide, crosslinked polymers of polyether compounds, polyepichlorohydrin, polyphosphazene, polysiloxane, and polyvinylpyrrolidone. , A polymer obtained by combining a lithium salt or an alkali metal salt mainly composed of lithium with a polymer compound such as polyvinylidene carbonate or polyacrylonitrile, or a high dielectric constant such as propylene carbonate, ethylene carbonate or γ-butyrolactone. Alternatively, a polymer in which an organic compound having an ion-dipole interaction force is mixed can be used.

  Specifically, resin polymers such as polyethylene, polypropylene, polyethylene terephthalate, aromatic polyamide, or cellulose and derivatives thereof (for example, carboxymethylcellulose), styrene / butadiene rubber, isoprene rubber, butadiene rubber, or ethylene / propylene are usually used. Rubber-like polymers such as rubber, fluoropolymers such as polyvinylidene fluoride, polytetrafluoroethylene, or polytetrafluoroethylene / ethylene copolymers, polyether polymer compounds such as polyethylene oxide and polypropylene oxide, polyethers Examples include crosslinked polymers of the compound, preferably polyethylene, polypropylene, styrene / butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, or polyethylene oxide. Is, more preferably, polyethylene, styrene-butadiene, polyvinylidene fluoride, or polytetrafluoroethylene. These are currently used industrially and are suitable because they are easy to handle.

  Content of the binder in a positive electrode active material layer is 1-15 mass% normally, Preferably it is 1-10 mass%. If the content of the binder in the positive electrode active material layer is too small, the positive electrode active material cannot be sufficiently retained and the positive electrode has insufficient mechanical strength, which may deteriorate battery performance such as cycle characteristics. If the amount is too large, the battery capacity and conductivity may be reduced.

<Conductive agent>
The positive electrode active material layer according to the present invention may contain a conductive agent. The conductive agent may be any electron conductive material that does not cause a chemical change at the charge / discharge potential of the positive electrode active material to be used. For example, carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black and thermal black, carbon fibers, vapor grown carbon fibers (VGCF), conductive fibers such as metal fibers, fluorine Metal powders such as carbonized carbon and copper can be contained alone or as a mixture thereof. Of these conductive agents, acetylene black and VGCF are particularly preferable. These may be used individually by 1 type and may be used in combination of 2 or more type.

  Although content of the electrically conductive agent in a positive electrode active material layer is not specifically limited, 1-30 mass% is preferable with respect to a positive electrode active material, and 1-15 mass% is especially preferable. If the content of the conductive agent in the positive electrode active material layer is too small, the conductivity may be insufficient. Conversely, if the content is too large, the battery capacity may be reduced.

<Current collector>
As the current collector, for example, a metal cylinder, a metal coil, a metal plate, a metal foil film, a carbon plate, a carbon thin film, a carbon cylinder, or the like is used. Among these, a metal foil film is particularly preferable because it is currently used in industrialized products. The metal thin film may be used in a suitable mesh shape.

  The thickness of the metal foil film used as a current collector is not particularly limited, but is usually 1 μm or more, preferably 5 μm or more, more preferably 10 μm or more, and usually 100 μm or less, preferably 50 μm or less, more preferably 20 μm or less. It is. In the case of a metal foil film thinner than the above range, the strength required as a current collector may be insufficient. On the other hand, if it is thicker than the above range, the handleability may be impaired.

  Specific examples of the metal used for the current collector include copper, nickel, stainless steel, iron, titanium, aluminum, and an aluminum alloy, preferably aluminum.

<Manufacturing method>
The manufacturing method of the positive electrode for lithium secondary batteries of this invention is not specifically limited. The positive electrode for a lithium secondary battery is, for example, a slurry (positive electrode active material) in which the sulfate of the present invention as a positive electrode active material, a binder, and a conductive agent used as necessary are dispersed in a dispersion. Slurry) is manufactured by a thin coating and drying process on a current collector, followed by a pressing process for compacting to a predetermined thickness and density.
An organic solvent is used as a dispersion medium for preparing a positive electrode active material slurry when a positive electrode active material and a binder are mixed with a conductive agent used as necessary and applied onto a current collector.

  As the organic solvent, usually, cyclic amides such as N-methylpyrrolidone, linear amides such as N, N-dimethylformamide and N, N-dimethylacetamide, and aromatic hydrocarbons such as anisole, toluene and xylene And alcohols such as butanol and cyclohexanol. Among them, cyclic amides such as N-methylpyrrolidone, linear amides such as N, N-dimethylformamide and N, N-dimethylacetamide are preferable. These organic solvents may be used individually by 1 type, and may be used in combination of 2 or more type.

  A positive electrode active material, a binder, and a conductive agent blended as necessary are mixed with these organic solvents to prepare a positive electrode active material slurry, and this is applied to a positive electrode current collector to a predetermined thickness. By doing so, a positive electrode active material layer is formed.

The upper limit of the concentration of the positive electrode active material in the positive electrode active material slurry is usually 70% by mass or less, preferably 55% by mass or less, and the lower limit is usually 30% by mass or more, preferably 40% by mass or more. If the concentration of the positive electrode active material exceeds this upper limit, the positive electrode active material in the positive electrode active material slurry tends to aggregate, and if it falls below the lower limit, the positive electrode active material tends to settle during storage of the positive electrode active material slurry.
Further, the upper limit of the concentration of the binder in the positive electrode active material slurry is usually 30% by mass or less, preferably 10% by mass or less, and the lower limit is usually 0.1% by mass or more, preferably 0.5% or more. . When the concentration of the binder exceeds this upper limit, the internal resistance of the positive electrode obtained increases, and when it falls below the lower limit, the binding property of the positive electrode active material layer becomes poor.

  The positive electrode active material layer obtained by coating and drying is preferably consolidated by a roller press or the like in order to increase the packing density of the positive electrode active material.

  The positive electrode for a lithium secondary battery is also paste-like by mixing the sulfate of the present invention as a positive electrode active material, a binder, and a conductive agent used as necessary, as shown in the examples described later. It is also possible to manufacture the film by forming a film.

  Thus, the thickness of the positive electrode active material layer formed is about 10-200 micrometers normally.

<Filling density>
The packing density of the positive electrode active material layer of the positive electrode for a lithium secondary battery of the present invention is not particularly limited, but is usually 0.1 g / cm ³ or more, preferably 0.5 g / cm ³ or more, and usually 5.0 g / cm. ³ or less, preferably 4.0 g / cm ³ or less. If the packing density of the positive electrode is below this range, it may be difficult to obtain a high-capacity battery. On the other hand, if it exceeds this range, the amount of pores in the positive electrode may decrease, and it may be difficult to obtain favorable battery characteristics. As the packing density of the positive electrode, a value obtained by dividing the weight of the positive electrode excluding the current collector by the positive electrode area and the positive electrode thickness is used.

<Porosity>
The porosity of the positive electrode active material layer of the positive electrode for a lithium secondary battery of the present invention is not particularly limited, but is usually 10% or more, preferably 20% or more, and usually 50% or less, preferably 40% or less. When the porosity of the positive electrode is less than this range, there are few pores in the positive electrode, and the electrolyte does not easily penetrate, and it may be difficult to obtain preferable battery characteristics. On the other hand, if this range is exceeded, there may be many pores in the positive electrode and the positive electrode strength becomes too weak, making it difficult to obtain favorable battery characteristics. As the porosity of the positive electrode, a percentage obtained by dividing the total pore volume obtained by measuring the pore distribution with a mercury porosimeter of the positive electrode by the apparent volume of the positive electrode active material layer excluding the current collector is used.

[Lithium secondary battery]
The lithium secondary battery of the present invention includes a positive electrode and a negative electrode capable of inserting and extracting lithium ions, and a non-aqueous electrolyte containing a lithium salt, and uses the positive electrode for a lithium secondary battery of the present invention as a positive electrode. .

  The selection of members other than the positive electrode necessary for the battery configuration such as the negative electrode and the nonaqueous electrolyte constituting the lithium secondary battery of the present invention is not particularly limited. In the following, materials of members other than the positive electrode constituting the lithium secondary battery of the present invention using the positive electrode for lithium secondary battery of the present invention will be exemplified, but usable materials are limited to these specific examples. It is not a thing.

<Negative electrode>
The negative electrode is formed by forming an active material layer containing a negative electrode active material and an organic substance (binder) having a binding and thickening effect on a current collector substrate. Usually, the negative electrode active material and the binder are used. Is formed by a step of thinly applying and drying a slurry in which water is dispersed in water or an organic solvent on a current collector, followed by a pressing step of compacting to a predetermined thickness and density.

<Negative electrode active material>
The negative electrode active material is not particularly limited as long as it has a function capable of occluding and releasing lithium. For example, natural graphite (scaly graphite, spheroidized graphite, etc.), artificial graphite (mesocarbon microbeads, etc.) Graphite, amorphous carbon obtained by firing pitch, resin, etc., multi-phase structural material obtained by combining graphite and amorphous carbon, metals such as aluminum and tin, SiO ₂ , lithium titanate, TiO _2, etc. These oxides are mentioned. These may be used individually by 1 type and may be used in combination of 2 or more type.

  The content of the negative electrode active material in the negative electrode active material layer is preferably 50 to 99% by mass, particularly preferably 65 to 95% by mass. If the content of the negative electrode active material in the negative electrode active material layer is too small, the battery capacity and conductivity may not be sufficient. If the content is too large, the binder content will be relatively small, and the mechanical strength of the negative electrode will be reduced. May be insufficient.

<Conducting agent>
A negative electrode conductive agent can be used for the negative electrode active material layer. The negative electrode conductive agent may be any electron conductive material that does not cause a chemical change at the charge / discharge potential of the negative electrode active material to be used. For example, graphite such as natural graphite (flaky graphite, etc.), artificial graphite, etc .; carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, etc .; conductive properties such as carbon fiber, metal fiber, etc. Conductive fibers; Carbon fluorides; Metal powders such as aluminum; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Organic conductive materials such as polyphenylene derivatives; It can be included alone or as a mixture thereof. Among these conductive agents, artificial graphite, acetylene black and the like are particularly preferable.
These may be used individually by 1 type and may be used in combination of 2 or more type.

  Although the addition amount of a electrically conductive agent is not specifically limited, 1-50 mass% is preferable with respect to a negative electrode active material, and 1-30 mass% is especially preferable. In the case of carbon black or graphite, 2 to 15% by mass is particularly preferable.

<Binder>
There is no restriction | limiting in particular as a binder used for formation of a negative electrode active material layer, Any of a thermoplastic resin and a thermosetting resin may be sufficient. For example, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene butadiene rubber, tetrafluoroethylene-hexafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), Tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer (ETFE resin) , Polychlorotrifluoroethylene (PCTFE), vinylidene fluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylene copolymer, ethylene-chlorotrif Oroethylene copolymer (ECTFE), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene copolymer, ethylene-acrylic acid copolymer or (Na ⁺ ) ion crosslinked product, ethylene-methacrylic acid copolymer or (Na ⁺ ) ion crosslinked product of the material, ethylene-methyl acrylate copolymer, or (Na ⁺ ) ion crosslinked product of the material, ethylene-methacrylic acid Examples thereof include an acid methyl copolymer or a (Na ⁺ ) ion-crosslinked product of the above materials, and these materials can be used alone or as a mixture. Among these materials, more preferable materials are polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

  The content of the binder in the negative electrode active material layer is preferably 1 to 30% by mass, and particularly preferably 1 to 10% by mass.

<Other additives>
In addition to the conductive agent described above, the negative electrode active material layer may further contain a filler, a dispersant, an ionic conductor, a pressure enhancer, and other various additives. Any filler can be used as long as it is a fibrous material that does not cause a chemical change in the constructed battery. Usually, olefin polymers such as polypropylene and polyethylene, and fibers such as glass and carbon are used. Although the addition amount of a filler is not specifically limited, 0-30 mass% is preferable as content in a negative electrode active material layer.

<Current collector>
As the negative electrode current collector, for example, a valve metal or an alloy thereof that forms a passive film on the surface by anodic oxidation in an electrolytic solution is preferably used. Examples of the valve metal include metals belonging to Groups 4, 5, and 13 of the periodic table and alloys thereof. Specifically, Al, Ti, Zr, Hf, Nb, Ta and alloys containing these metals can be exemplified, and Al, Ti, Ta and alloys containing these metals can be preferably used. . In particular, Al and its alloys are desirable because of their light weight and high energy density. The thickness of the negative electrode current collector is not particularly limited, but is usually about 1 to 50 μm.

<Manufacturing method>
A negative electrode active material slurry is prepared by mixing the above-described negative electrode active material, a binder, a negative electrode conductive agent blended as necessary, and other fillers with these solvents, and this is used as a negative electrode current collector. The negative electrode active material layer is formed by coating so that the thickness becomes.

  For the preparation of the negative electrode active material slurry, an aqueous solvent or an organic solvent is used as a dispersion medium. As the aqueous solvent, water is usually used, and additives such as alcohols such as ethanol and cyclic amides such as N-methylpyrrolidone may be added to water up to about 30% by mass or less. it can. As the organic solvent, usually, cyclic amides such as N-methylpyrrolidone, linear amides such as N, N-dimethylformamide and N, N-dimethylacetamide, and aromatic carbonization such as anisole, toluene and xylene Examples thereof include alcohols such as hydrogens, butanol and cyclohexanol, among which cyclic amides such as N-methylpyrrolidone, linear amides such as N, N-dimethylformamide and N, N-dimethylacetamide are preferable. . These may be used individually by 1 type and may be used in combination of 2 or more type.

The upper limit of the concentration of the negative electrode active material in the negative electrode active material slurry is usually 70% by mass or less, preferably 55% by mass or less, and the lower limit is usually 30% by mass or more, preferably 40% by mass or more. When the concentration of the negative electrode active material exceeds this upper limit, the negative electrode active material in the negative electrode active material slurry tends to aggregate, and when the concentration is lower than the lower limit, the negative electrode active material tends to settle during storage of the negative electrode active material slurry.
The upper limit of the binder concentration in the negative electrode active material slurry is usually 30% by mass or less, preferably 10% by mass or less, and the lower limit is usually 0.1% by mass or more, preferably 0.5% or more. . When the concentration of the binder exceeds the upper limit, the internal resistance of the negative electrode obtained increases, and when the concentration is lower than the lower limit, the binding property of the negative electrode active material layer may be inferior.

  The thickness of the negative electrode active material layer thus formed is usually about 5 to 200 μm.

<Nonaqueous electrolyte>
As the non-aqueous electrolyte, any non-aqueous electrolyte such as a non-aqueous electrolyte solution or a solid electrolyte can be used. Here, the electrolyte refers to all ionic conductors, and both the electrolytic solution and the solid electrolyte are included in the electrolyte.
As the non-aqueous electrolyte, for example, a solution obtained by dissolving a solute in a non-aqueous solvent can be used. A lithium salt is used as the solute. Specifically, LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) _{3 and the} like are preferably used. One kind of these solutes may be selected and used, or two or more kinds may be mixed and used. The content of these solutes in the electrolytic solution is preferably 0.2 mol / L or more, particularly 0.5 mol / L or more, and 2 mol / L or less, particularly 1.5 mol / L or less.

Examples of the non-aqueous solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate and vinylene carbonate, cyclic ester compounds such as γ-butyrolactone; chain ethers such as 1,2-dimethoxyethane; crown ethers, 2- Cyclic ethers such as methyltetrahydrofuran, 1,2-dimethyltetrahydrofuran, 1,3-dioxolane and tetrahydrofuran; chain carbonates such as diethyl carbonate, ethylmethyl carbonate and dimethyl carbonate; sulfones such as tetramethylene sulfone and the like can be used. . Among these, a nonaqueous solvent containing a cyclic carbonate and a chain carbonate is preferable.
One type of these solvents may be selected and used, or two or more types may be mixed and used.

  The nonaqueous electrolytic solution according to the present invention may contain various auxiliary agents such as a cyclic carbonate having an unsaturated bond in the molecule, a conventionally known overcharge inhibitor, a deoxidizer, and a dehydrator.

  Examples of the cyclic carbonate having an unsaturated bond in the molecule include vinylene carbonate compounds, vinyl ethylene carbonate compounds, methylene ethylene carbonate compounds, and the like.

  Examples of the vinylene carbonate compounds include vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, 4,5-dimethyl vinylene carbonate, 4,5-diethyl vinylene carbonate, fluoro vinylene carbonate, trifluoromethyl vinylene carbonate, and the like.

  Examples of the vinyl ethylene carbonate compound include vinyl ethylene carbonate, 4-methyl-4-vinyl ethylene carbonate, 4-ethyl-4-vinyl ethylene carbonate, 4-n-propyl-4-vinyl ethylene carbonate, 5-methyl- Examples include 4-vinylethylene carbonate, 4,4-divinylethylene carbonate, 4,5-divinylethylene carbonate, and the like.

  Examples of the methylene ethylene carbonate compound include methylene ethylene carbonate, 4,4-dimethyl-5-methylene ethylene carbonate, 4,4-diethyl-5-methylene ethylene carbonate, and the like.

Of these, vinylene carbonate and vinyl ethylene carbonate are preferable, and vinylene carbonate is particularly preferable.
These may be used individually by 1 type, or may use 2 or more types together.

  When the non-aqueous electrolyte contains a cyclic carbonate having an unsaturated bond in the molecule, the proportion in the non-aqueous electrolyte is usually 0.01% by mass or more, preferably 0.1% by mass or more, particularly preferably. Is 0.3% by mass or more, most preferably 0.5% by mass or more, usually 8% by mass or less, preferably 4% by mass or less, and particularly preferably 3% by mass or less.

  By including in the electrolyte a cyclic carbonate having an unsaturated bond in the molecule, the cycle characteristics of the battery can be improved. Although the reason is not clear, it is presumed that a stable protective film can be formed on the surface of the negative electrode. However, when the content is small, this property is not sufficiently improved. However, if the content is too large, the amount of gas generation tends to increase during high-temperature storage, so the content in the electrolyte is preferably in the above range.

Examples of the overcharge inhibitor include aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; 2-fluoro Partially fluorinated products of the above aromatic compounds such as biphenyl, o-cyclohexylfluorobenzene, p-cyclohexylfluorobenzene; fluorine-containing compounds such as 2,4-difluoroanisole and 2,5-difluoroanisole; Anisole compounds and the like can be mentioned.
These may be used alone or in combination of two or more.

  The ratio of the overcharge inhibitor in the non-aqueous electrolyte is usually 0.1 to 5% by mass. By containing an overcharge preventing agent, rupture / ignition of the battery can be suppressed during overcharge or the like.

Examples of other auxiliaries include carbonate compounds such as fluoroethylene carbonate, trifluoropropylene carbonate, phenylethylene carbonate, erythritan carbonate, spiro-bis-dimethylene carbonate, methoxyethyl-methyl carbonate; succinic anhydride, anhydrous glutar Carboxylic acid anhydrides such as acid, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, phenylsuccinic anhydride; Ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfolane, sulfolene, dimethyl sulfone, tetramethylthiuram monosulfide Sulfur-containing compounds such as N, N-dimethylmethanesulfonamide and N, N-diethylmethanesulfonamide; 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3 -Nitrogen-containing compounds such as dimethyl-2-imidazolidinone and N-methylsuccinimide; hydrocarbon compounds such as heptane, octane and cycloheptane; fluorine-containing compounds such as fluorobenzene, difluorobenzene, hexafluorobenzene and benzotrifluoride An aromatic compound etc. are mentioned.
These may be used alone or in combination of two or more.

  The ratio of these auxiliaries in the nonaqueous electrolytic solution is usually 0.1 to 5% by mass. By containing these auxiliaries, capacity maintenance characteristics and cycle characteristics after high-temperature storage can be improved.

Further, the non-aqueous electrolyte solution may include an organic polymer compound in the electrolyte solution, and may be a solid electrolyte such as a gel, rubber, or solid sheet. In this case, specific examples of the organic polymer compound include polyether polymer compounds such as polyethylene oxide and polypropylene oxide; crosslinked polymers of polyether polymer compounds; vinyl alcohol polymers such as polyvinyl alcohol and polyvinyl butyral. Compound: insolubilized product of vinyl alcohol polymer compound; polyepichlorohydrin; polyphosphazene; polysiloxane; vinyl polymer compound such as polyvinylpyrrolidone, polyvinylidene carbonate, polyacrylonitrile; poly (ω-methoxyoligooxyethylene methacrylate), Examples thereof include polymer copolymers such as poly (ω-methoxyoligooxyethylene methacrylate-co-methyl methacrylate).
These may be used alone or in combination of two or more.

<Other components>
In addition to the non-aqueous electrolyte, the negative electrode, and the positive electrode, an outer can, a separator, a gasket, a sealing plate, a cell case, and the like can also be used for the lithium secondary battery of the present invention as necessary.

  The material and shape of the separator are not particularly limited. The separator is for separating the positive electrode and the negative electrode so that they do not come into physical contact, and preferably has a high ion permeability and a low electric resistance. The separator is preferably selected from materials that are stable with respect to the electrolyte and excellent in liquid retention. Specific examples include porous sheets or nonwoven fabrics made from polyolefins such as polyethylene and polypropylene.

<Shape of lithium secondary battery>
The shape of the lithium secondary battery of the present invention is not particularly limited. For example, a cylinder type in which a sheet electrode and a separator are spiral, a cylinder type having an inside-out structure in which a pellet electrode and a separator are combined, a pellet electrode and a separator are stacked. It can be a coin type or the like.

<Method for producing lithium secondary battery>
The method for producing the lithium secondary battery of the present invention having at least a nonaqueous electrolyte, a negative electrode, and a positive electrode is not particularly limited, and can be appropriately selected from commonly employed methods. An example of a method for producing a lithium secondary battery of the present invention is as follows. A negative electrode is placed on an outer can, a non-aqueous electrolyte and a separator are provided thereon, a positive electrode is placed so as to face the negative electrode, a gasket, A method of assembling the battery by caulking with a sealing plate is mentioned.

  EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited at all by these Examples, unless the summary is exceeded.

[Measurement method of physical properties]
The physical properties and the like of sulfates produced in each of Examples and Comparative Examples described later were measured as follows.

<Primary particle size>
Three different visual fields were observed with SEM, and the average particle size of primary particles was determined from the average value.

<Confirmation of crystal phase>
It calculated | required by the powder X-ray-diffraction measurement using the CuK (alpha) ray described below.
(Powder X-ray diffraction measurement device specifications)
Device name: X'Pert Pro, manufactured by PANallytical, Netherlands
Optical system: Concentrated optical system (optical system specification)
Incident side: Enclosed X-ray tube (CuKα)
Soller Slit (0.04 rad)
Divergence Slit (Fixed Slit)
Sample stage: Rotating sample stage (Spinner)
Light receiving side: Semiconductor array detector (PIXcel-1D)
Ni-filter
Soller Slit (0.04 rad)
Gonio radius: 240mm
(Measurement condition)
X-ray output (CuKα): 40 kV, 45 mA
Scanning axis: θ / 2θ
Scanning range (2θ): 10.0-110.0 °
Measurement mode: Continuous
Reading width: 0.013 °
Counting time: 298.095 sec

[Example 1]
<Manufacture of sulfate>
Li ₂ SO ₄ .H ₂ O and MnSO ₄ .H ₂ O are weighed and mixed at a molar ratio of 0.98: 1.00, and then heat-treated at 550 ° C. for 4 days in an air atmosphere. Thus, a sulfate having a composition of Li ₂ Mn (SO ₄ ) ₂ was obtained.
Table 1 shows the evaluation results of the physical properties of the obtained sulfate.

<Preparation of positive electrode for lithium secondary battery>
15% by mass of carbon black (manufactured by Alfa Aesar) as a conductive agent was added to the sulfate produced by the above method, and the resultant was treated with a ball mill for 2 hours. 20 mg of carbon black was further added as a conductive agent to 160 mg of this powder (containing 21 mg of carbon black), and mixed in a mortar. 20 mg of PTFE (“6-J” manufactured by Mitsui DuPont Fluorochemical Co., Ltd.) as a binder was added to this mixture, and further mixed in a mortar. The obtained mixture containing the binder was stretched and then punched out to a diameter of 12 mmφ to obtain a positive electrode. Here, the weight of the positive electrode was adjusted to be about 10 mg.
This positive electrode was dried at 60 ° C. overnight to obtain a positive electrode for evaluation.

<Production of lithium secondary battery>
The obtained positive electrode was transferred to a glove box under an Ar atmosphere, 1 mol / L-LiClO ₄ electrolytic solution using tetramethylene sulfone as a solvent, borosilicate glass fiber sheet (Whatman GF / D) as a separator, and lithium metal as a counter electrode. Using the foil, a swagelok cell (lithium secondary battery) was produced.

<Battery evaluation>
Charge the lithium counter electrode to 5.0 V at a current density of 1/20 C (calculated assuming a theoretical capacity of 1 C), and further charge until the current value reaches 1/100 C or the theoretical capacity at a constant voltage of 5.0 V. After desorbing lithium from the positive electrode, the lithium counter electrode was discharged to 2.0 V at a current density of 1/20 C, and the discharge capacity was examined. The results are shown in Table 1.

[Example 2]
In Example 1, as a raw material sulfate, Li ₂ SO ₄ .H ₂ O, MnSO ₄ .H ₂ O, CoSO ₄ .7H ₂ O were used in a molar ratio of 1.00: 0.80: 0.20. The production of sulfate, the production of the positive electrode and the battery, and the evaluation were carried out in the same manner except that they were used in Table 1. The results are shown in Table 1.

[Example 3]
In Example 1, as a raw material sulfate, Li ₂ SO ₄ .H ₂ O, MnSO ₄ .H ₂ O, CoSO ₄ .7H ₂ O were used in a molar ratio of 1.00: 0.60: 0.40. The production of sulfate, the production of the positive electrode and the battery, and the evaluation were carried out in the same manner except that they were used in Table 1. The results are shown in Table 1.

[Example 4]
In Example 1, except that Li ₂ SO ₄ .H ₂ O, MnSO ₄ .H ₂ O, and ZnSO ₄ were used as raw material sulfates in a molar ratio of 1.00: 0.80: 0.20. In the same manner, production of sulfate, production of positive electrode and battery, and evaluation were performed, and the results are shown in Table 1.

From these examples, when the sulfate of the present invention is electrochemically active and the electrochemically inactive element Q has a low doping amount, the initial charge capacity is 170 mAh / liter of LiFePO ₄ theoretical capacity. It can be seen that it exceeds g.

Claims (6)

A sulfate represented by the following general formula (1).
_{Li 2 (Mn 1-a Q} a) (SO 4) 2 ··· (1)
(Where Q represents at least one dopant element, and the index a is selected so as to ensure the electrical neutrality of the sulfate represented by the general formula (1), and 0.5> a ≧ 0) Yes, the amount maintaining the crystal structure at a = 0.)   The sulfate according to claim 1, wherein the average particle size of the primary particles is a powder having a particle size of less than 1 µm.   The sulfate according to claim 1 or 2, wherein Q is one or more selected from the group consisting of Fe, Co, Ni, V, Mg, Ca, Sr, Cu, and Zn. .   The positive electrode material for lithium secondary batteries which consists of a sulfate of any one of Claim 1 thru | or 3.   A positive electrode for a lithium secondary battery, comprising a positive electrode active material layer containing the sulfate according to any one of claims 1 to 3 and a binder on a current collector.   A lithium secondary battery comprising a negative electrode capable of inserting and extracting lithium, a nonaqueous electrolyte containing a lithium salt, and a positive electrode capable of inserting and extracting lithium, wherein the lithium secondary battery according to claim 5 is used as the positive electrode. A lithium secondary battery using a positive electrode for a battery.