JP5880996B2 - Lithium manganese silicate composite, positive electrode for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a lithium manganese silicate composite useful as a positive electrode material of a nonaqueous electrolyte typified by a lithium ion secondary battery.

As one type of nonaqueous electrolyte secondary battery, a lithium secondary battery and a lithium ion secondary battery are known. These non-aqueous electrolyte secondary batteries are small and have high energy density, and are widely used as power sources for portable electronic devices. Lithium cobaltate (LiCoO ₂ ) and lithium manganate (LiMn ₂ O ₄ ) are known as typical positive electrode active materials for non-aqueous electrolyte secondary batteries. However, this type of material also has various problems for further performance improvement, and the improvement is strongly demanded.

For example, when the charging voltage is made higher than 4.2 V in order to extract more LiCoO ₂ capacity, there is a problem that the cycle characteristics are remarkably deteriorated. This is reported to be caused by the unstable crystal structure of LiCoO ₂ . In order to solve this problem, surface coating of LiCoO ₂ with oxides (ZrO ₂ , Al ₂ O ₃ , TiO ₂ , B ₂ O _3, etc.) has been studied (for example, see Non-Patent Document 1). As a result, the structure was stabilized and the cycle characteristics were improved even when charged to 4.2 V or higher.

On the other hand, LiMn ₂ O ₄ has problems such as a decrease in crystallinity associated with elution of trivalent Mn during a high-temperature charge / discharge cycle and decomposition of the electrolyte. In order to suppress these, substitution of Mn with a divalent element or a trivalent element and surface coating of LiMn ₂ O ₄ with an oxide (Al ₂ O ₃ , MgO, ZnO, etc.) have been studied.

  Since these oxide-based positive electrode active materials have higher electronic conductivity (conductivity) than lithium-manganese-silicate-based positive electrode active materials, even when they are coated with oxides, conductive carbon is used when preparing a slurry. Is added, a nonaqueous electrolyte secondary battery having excellent charge / discharge characteristics can be obtained.

  In recent years, lithium silicate compounds have attracted attention as positive electrode active materials from the viewpoint of increasing the capacity of battery materials, high safety, and the amount of resources. Lithium silicate compounds are inexpensive, low in environmental impact because they consist of abundant constituent metal elements, have a high theoretical charge / discharge capacity of lithium ions, and do not release oxygen at high temperatures. It attracts attention as a positive electrode material for next-generation lithium ion secondary batteries (see, for example, Patent Documents 1 to 5).

As one type of lithium silicate compound, one containing manganese (referred to as lithium manganese silicate compound) can be given. Lithium manganese silicate (Li ₂ MnSiO ₄ ) has a high theoretical charge / discharge capacity (theoretical capacity 333.2 mAh / g), and is considered preferable as a positive electrode active material for a lithium ion secondary battery. On the other hand, however, there is a problem that the electronic conductivity is low. Furthermore, the lithium ion secondary battery using lithium manganese silicate as the positive electrode active material has a problem inferior in cycle characteristics. For this reason, when used as a positive electrode active material of a non-aqueous electrolyte secondary battery typified by a lithium ion secondary battery, a lithium manganese silicate compound that can suppress a decrease in cycle characteristics is desired.

JP 2008-218303 A JP 2007-335325 A JP 2001-266882 A JP 2008-293661 A International Publication No. 2010/089931 J. et al. Cho, Y. et al. J. et al. Kim, J. et al. T.A. Kim, B.B. Park, Angew. Chem. Int. Ed. 40 (18) (2001) 3367. R. Dominko, M.M. Bele, a. Kokalj, M.M. Gabersek, J.A. Jamnik, Journal of Power Sources 174 (2007) 457-461 Wengang Liu, Yunhua Xu, Long Yang, Journal of Alloys and Compounds 480 (2009) L1-L4

  The present invention relates to a material containing a lithium manganese silicate compound and capable of suppressing deterioration in cycle characteristics of a non-aqueous electrolyte secondary battery when used as a positive electrode material, and a non-aqueous electrolyte secondary battery using this material An object is to provide a positive electrode and a non-aqueous electrolyte secondary battery using this material.

Lithium manganese silicate compounds have not been studied because they have low electrical conductivity and, when coated with the same oxide as LiCoO ₂ or LiMn ₂ O ₄ , leads to an increase in resistance. Furthermore, the reason why the lithium ion secondary battery using the lithium manganese silicate compound as the positive electrode active material is inferior in cycle characteristics is that the crystal structure of manganese in the lithium manganese silicate compound is broken during charging ( (See Non-Patent Documents 2 and 3 above). The inventors of the present invention have found that there is a lithium ion secondary battery having excellent cycle characteristics despite using a lithium manganese silicate compound containing manganese with low crystallinity. Furthermore, it discovered that manganese contained in a lithium manganese silicate type compound eluted at the time of charge. Then, the inventors of the present invention provide a lithium manganese silicate composite in which an oxide or element such as a carbon composite, a carbon coating, a hydrofluoric acid resistant compound, and a fluorine absorbing compound is combined with a lithium manganese silicate compound. It was found that the deterioration of the cycle characteristics of the lithium ion secondary battery can be suppressed by using this as a positive electrode active material.

  That is, the lithium manganese silicate composite of the present invention that solves the above problems includes a core composed of a lithium manganese silicate compound containing lithium (Li), manganese (Mn), silicon (Si), and oxygen (O), and a resistance to lithium. And at least one selected from a hydrofluoric acid compound and a fluorine absorbing compound and a coating layer made of a carbon-based material and covering the core.

  The hydrofluoric acid resistant compound is scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, titanium oxide, zirconium oxide, hafnium oxide, vanadium oxide, niobium oxide, tantalum oxide, molybdenum oxide. Preferably, it is made of at least one selected from tungsten oxide and aluminum oxide.

  The fluorine absorbing compound is preferably composed of at least one selected from magnesium, calcium, strontium, barium, silicon, lanthanum, cerium and oxides thereof.

  The carbon-based material is preferably made of a carbon material having a carbon skeleton obtained by carbonization of an organic substance and / or conductive carbon.

  The positive electrode for a non-aqueous electrolyte secondary battery of the present invention that solves the above-mentioned problems is characterized by including the lithium manganese silicate composite of the present invention. The nonaqueous electrolyte secondary battery of the present invention includes the positive electrode for a nonaqueous electrolyte secondary battery of the present invention as a constituent element.

  When the lithium manganese silicate composite of the present invention is used as a positive electrode active material of a non-aqueous electrolyte secondary battery, it can suppress a decrease in cycle characteristics.

It is a SEM image of the negative electrode after the first charge in the coin battery which used lithium manganese silicate type compound for the positive electrode active material, and used SiO for the negative electrode. It is a graph showing the result of having analyzed the negative electrode after the first charge in the coin battery which used lithium manganese silicate type compound for the positive electrode active material, and used SiO for the negative electrode by EDX. It is a SEM image of the lithium manganese silicate composite of an Example. It is a graph showing the result of having analyzed the lithium manganese silicate composite of the Example by EDX. It is a SEM image of the lithium manganese silicate type compound of a comparative example. It is a graph showing the result of having analyzed the lithium manganese silicate type compound of the comparative example by EDX. It is a graph showing the charging / discharging curve of the lithium ion secondary battery of an Example. It is a graph showing the cycle characteristic of the lithium ion secondary battery of an Example. It is a graph showing the charging / discharging curve of the lithium ion secondary battery of a comparative example. It is a graph showing the cycle characteristic of the lithium ion secondary battery of a comparative example.

  As described above, the inventors of the present invention have found that manganese contained in a lithium manganese silicate compound is eluted during charging. This is supported by the following test.

<Confirmation of manganese elution>
A lithium manganese silicate compound was prepared as a positive electrode active material. The lithium manganese silicate compound was prepared by the following method.

[Synthesis of manganese-based precipitates]
Anhydrous lithium hydroxide (LiOH) (2.5 mol) was dissolved in 1000 mL of distilled water to prepare an aqueous lithium hydroxide solution. Further, 0.25 mol of manganese chloride tetrahydrate (MnCl ₂ .4H ₂ O) was dissolved in 500 mL of distilled water to prepare an aqueous manganese chloride solution. At room temperature (about 20 ° C.), an aqueous lithium hydroxide solution was gradually added dropwise to the aqueous manganese chloride solution over several hours. This operation produced a manganese-based precipitate. Thereafter, while stirring the reaction solution containing the manganese-based precipitate, air was blown into the reaction solution, and bubbling treatment was performed at room temperature for 1 day. The obtained manganese-based precipitate was filtered, washed with distilled water three times, and filtered. The washed manganese-based precipitate was dried at 40 ° C. overnight.

  The manganese-based precipitate after drying was analyzed using X-ray diffraction. As a result, it was found that the manganese-based precipitate after drying was a compound represented by the composition formula: MnOOH. That is, 1 mol of Mn is contained in 1 mol of this manganese-based precipitate. Moreover, it confirmed that the obtained deposit was porous by SEM.

[Synthesis of lithium manganese silicate compounds]
Lithium carbonate (Kishida Chemical Co., Ltd., purity 99.9%), sodium carbonate (Kishida Chemical Co., Ltd., purity 99.5%) and potassium carbonate (Kishida Chemical Co., Ltd., purity 99.5%) A carbonate mixture was prepared by mixing in lithium: sodium carbonate: potassium carbonate = 43.5: 31.5: 25 (molar ratio). This carbonate mixture, 0.03 mol of the above manganese-based precipitate, and 0.03 mol of lithium silicate (Li ₂ SiO ₃ , manufactured by Kishida Chemical Co., Ltd., purity 99.5%) are 100 masses of carbonate mixture. The total amount of manganese-based precipitates and lithium silicate was 160 parts by mass with respect to parts. A mixture obtained by adding 20 ml of acetone to this mixture was mixed in a zirconia ball mill at 500 rpm for 60 minutes and dried. The mixed powder after drying was put in a gold crucible, and this gold crucible was put in an electric furnace and heated in a mixed gas atmosphere of carbon dioxide (flow rate: 100 mL / min) and hydrogen (flow rate: 3 mL / min). The heating temperature at this time was 550 ° C., and the reaction time was 65 hours. During the reaction, the carbonate mixture maintained a molten state.

  After the reaction, the entire reactor core (including the gold crucible) as a reaction system was taken out of the electric furnace and rapidly cooled to room temperature while the mixed gas was circulated. Next, 20 mL of water was added to the solidified reaction product and crushed using a pestle and mortar to obtain a powder. In order to remove salt and the like from the powder, the powder was washed with water and filtered. The powder obtained after filtration and drying was used as a powder of lithium manganese silicate compound.

[Carbon coating treatment]
The lithium manganese silicate compound obtained by the above method was further coated with carbon. This treatment improves the conductivity of the positive electrode active material.

Specifically, the above lithium manganese silicate compound and acetylene black were mixed at a mass ratio of 5: 4. Ball milling was used for mixing, and the mixing conditions were 450 rpm and 5 hours. Thereafter, heat treatment was performed at 700 ° C. for 2 hours under a gas flow rate of CO ₂ : H ₂ = 100: 3 (cm ³ ) using a heat treatment furnace capable of controlling the atmosphere. In this step, a lithium manganese silicate composite composed of carbon and lithium manganese silicate was produced.

[Production of lithium ion secondary batteries]
The above lithium manganese silicate compound is used as a positive electrode active material, SiO (Osaka Titanium Technologies Co., Ltd., powder having a particle size distribution of 45 microns or less) is used as a negative electrode active material, and a case (CR2032 type coin battery case) is used. I made a coin battery.

  Specifically, a carbon-coated lithium manganese silicate compound: acetylene black (AB): polytetrafluoroethylene (PTFE) = 17.1: 4.7: 1 (mass ratio) is kneaded to form a film. Then, an electrode was manufactured by pressure bonding to an aluminum current collector, and vacuum-dried at 140 ° C. for 3 hours was used as the positive electrode.

  As the negative electrode, the above-described SiO was used as an electrode, and after charging and discharging using lithium as a counter electrode, the irreversible capacity was eliminated. In addition, as a negative electrode mixture for SiO negative electrode, SiO: KB: polyimide binder = 80: 5: 15 (mass ratio) was used.

As the electrolytic solution, a solution in which LiPF ₆ was dissolved in ethylene carbonate (EC): dimethylene carbonate (DMC) = 1: 1 to 1 mol / L was used.

  A coin battery was manufactured using the positive electrode and the negative electrode. Specifically, in a dry room, a separator (Celgard 2400 made by Celgard, a polypropylene microporous membrane having a thickness of 25 μm) and a glass nonwoven fabric filter (thickness 440 μm, made by ADVANTEC, GA100) are placed between the positive electrode and the negative electrode. To be an electrode body battery. This electrode body battery was accommodated in a battery case (CR2032-type coin battery member, manufactured by Hosen Co., Ltd.) made of a stainless steel container. The above electrolyte was injected into the battery case. The battery case was sealed with a caulking machine to obtain a lithium ion secondary battery.

The lithium ion secondary battery was charged at a current value corresponding to 0.05 mA per unit area (1 cm ² ) of the positive electrode active material. The temperature at this time was 30 ° C., and charging was a constant voltage (10 hours at 4.4 V).

  The negative electrode surface after the first charge was analyzed. As the apparatus, a scanning electron microscope (SEM) and an energy dispersive X-ray spectrometer (EDX) attached thereto were used. For EDX, simple quantitative analysis was performed by the ZAF method. The measurement conditions are as follows: device name: 6390 (LA), acceleration voltage: 20.0 kV, irradiation current: 1.00000 nA, PHA mode: T2, elapsed time: 491.52 sec, effective time: 409.32 sec, dead time: 16% The counting rate was 2875 cps, and the energy range was 0 to 20 keV. FIG. 1 shows an SEM image of the negative electrode after initial charging in a coin battery using a lithium manganese silicate compound as the positive electrode active material and SiO as the negative electrode, and FIG. 2 and Table 1 show the results of analysis by EDX. Table 1 is obtained by converting the analysis result of FIG. 2 into a numerical value.

As a result of analyzing the surface of the negative electrode particle shown in FIG. 1 by EDX, C, O, F, Si, P, Ca and Mn were detected (FIG. 2). In the horizontal axis of FIG. 2, the presence of Mn is evidenced by the fact that the MnKα line is clearly seen near 6 keV.

  As shown in Table 1, manganese in an amount of 1.65 atomic% was present on the negative electrode surface after the initial charge when the entire negative electrode was taken as 100 atomic%. Since the negative electrode is made of silicon oxide (SiO), it is considered that manganese deposited on the surface of the negative electrode is derived from manganese contained in the positive electrode active material. That is, this result supports that manganese contained in the lithium manganese silicate is eluted during charging. In addition, when charging / discharging of the lithium ion secondary battery was repeated, the manganese precipitation amount (atomic%) on the negative electrode surface increased. After 50 cycles of charge / discharge, about 7 atomic% of manganese was deposited on the negative electrode surface. This fact confirms that manganese contained in the lithium manganese silicate was gradually eluted by repeating charging and discharging.

<Lithium manganese silicate compound>
The lithium manganese silicate composite of the present invention includes a core part and a coating layer. The core is made of a lithium manganese silicate compound. A lithium manganese silicate compound is a compound containing lithium (Li), manganese (Mn), silicon (Si), and oxygen (O), and lithium manganese silicate (Li ₂ MnSiO ₄ ) is well known. The lithium manganese silicate compound may be a compound containing lithium (Li), manganese (Mn), silicon (Si), and oxygen (O), and may contain other elements.

  The coating layer in the lithium manganese silicate composite of the present invention is composed of at least one selected from a hydrofluoric acid compound and a fluorine absorbing compound and a carbon-based material, and covers the core. Manganese contained in lithium manganese silicate is considered to elute into the electrolytic solution by the action of hydrofluoric acid generated by the decomposition of the electrolytic solution. Therefore, it is considered that elution of manganese can be suppressed by covering the lithium manganese silicate compound with at least one substance selected from hydrofluoric acid, that is, a hydrofluoric acid resistant compound and a fluorine absorbing compound.

  For example, the silicon-rich lithium manganese silicate compound represented by the following formula 1 is a material that can impart excellent battery characteristics to a lithium ion secondary battery when used as a positive electrode active material.

Composition formula: Li _{2 + ab} Ab Mn _1-x M _x Si _{1 + α} O _{4 + c} (formula 1)
In the formula, A is at least one element selected from the group consisting of Na, K, Rb and Cs, and M is Mg, Ca, Co, Al, Ni, Nb, Ti, Cr, Cu, It is at least one element selected from the group consisting of Zn, Zr, V, Mo and W. For each subscript, 0 ≦ x ≦ 0.5, −1 <a <1, 0 ≦ b <0.2, 0 ≦ c <1, and 0 <α ≦ 0.2.

  The silicon-excess lithium manganese silicate compound represented by Formula 1 is relatively excellent in cycle characteristics as compared with lithium manganese silicate, but the cycle characteristics can be further improved by covering with a coating layer.

  What is necessary is just to use what was prepared by the known method as a lithium manganese silicate type compound. For example, one synthesized by a hydrothermal synthesis method or one synthesized by a solid phase reaction method may be used, or another one may be used. For example, in a molten salt containing an alkali metal salt, an aqueous solution containing a lithium silicate compound and a compound containing manganese (hereinafter referred to as a manganese compound) is made alkaline in a mixed gas atmosphere containing carbon dioxide and a reducing gas. You may use what formed the precipitate and made it react at 300 to 600 degreeC. According to this method, the above-described silicon-rich lithium manganese silicate compound can be synthesized. Hereinafter, this method is referred to as a molten salt method.

<Composition of molten salt>
More specifically, in the molten salt method, a synthesis reaction of a lithium manganese silicate compound is performed in a molten salt containing at least one selected from alkali metal salts.

  Examples of the alkali metal salt include at least one selected from the group consisting of a lithium salt, a potassium salt, a sodium salt, a rubidium salt, and a cesium salt. Of these, lithium salts are desirable. When a molten salt containing a lithium salt is used, the formation of an impurity phase is small, and a lithium manganese silicate compound containing an excess of lithium atoms is likely to be formed.

Further, the type of alkali metal salt is not particularly limited, but it is preferable to include at least one of alkali metal carbonate, alkali metal nitrate and alkali metal hydroxide. Specifically, lithium carbonate (Li ₂ CO ₃ ), potassium carbonate (K ₂ CO ₃ ), sodium carbonate (Na ₂ CO ₃ ), rubidium carbonate (Rb ₂ CO ₃ ), cesium carbonate (Cs ₂ CO ₃ ), Lithium nitrate (LiNO ₃ ), potassium nitrate (KNO ₃ ), sodium nitrate (NaNO ₃ ), rubidium nitrate (RbNO ₃ ), cesium nitrate (CsNO ₃ ), lithium hydroxide (LiOH), potassium hydroxide (KOH), hydroxide Examples include sodium (NaOH), rubidium hydroxide (RbOH), and cesium hydroxide (CsOH). As the alkali metal salt, one of these may be used alone, or two or more of these may be mixed and used.

  For example, the melting temperature in the case of lithium carbonate alone is about 700 ° C., but when the molten salt is a mixture of lithium carbonate and another alkali metal salt, the melting temperature can be 600 ° C. or less. Therefore, the use of such an alkali metal salt makes it possible to synthesize the target lithium manganese silicate compound at a relatively low reaction temperature of 300 to 600 ° C.

  The molten salt is selected from the above alkali metal salts so that the melting temperature is 600 ° C. or lower, and if the alkali metal salts are mixed and used, the mixing ratio is adjusted so that the melting temperature of the mixture is 600 ° C. or lower. Thus, a mixed molten salt may be obtained. Since the mixing ratio varies depending on the type of salt, it is difficult to define it unconditionally.

  For example, if a carbonate mixture containing lithium carbonate and containing other carbonates is used as the molten salt, normally, when the total carbonate mixture is 100 mol%, lithium carbonate is 30 mol% or more, further 30 It is preferable to contain -70 mol%. Specific examples of the carbonate mixture include a mixture composed of 30 to 70 mol% lithium carbonate, 0 to 60 mol% sodium carbonate, and 0 to 50 mol% potassium carbonate. As a more preferred specific example of such a carbonate mixture, a mixture comprising 40 to 45 mol% lithium carbonate, 30 to 35 mol% sodium carbonate and 20 to 30 mol% potassium carbonate can be mentioned.

  In addition, since the melting temperature (melting point) of alkali metal nitrate and alkali metal hydroxide is at most 450 ° C. (lithium hydroxide), when using a molten salt containing one kind of nitrate or hydroxide alone. Also, a low reaction temperature can be realized.

<Raw compound>
As a lithium and silicon supply source in the molten salt method, a lithium silicate compound represented by Li ₂ SiO ₃ can be used. Moreover, as a manganese supply source, the precipitate formed by making aqueous solution containing a manganese compound alkaline can be used. A specific method for forming this precipitate will be described below.

  Any manganese compound can be used without particular limitation as long as it is a component capable of forming an aqueous solution containing a manganese compound (hereinafter sometimes referred to as “aqueous solution”). As the manganese compound, usually a water-soluble compound may be used. Specific examples of such water-soluble compounds include water-soluble salts such as chlorides, nitrates, sulfates, oxalates and acetates, hydroxides and the like. These water-soluble compounds may be either anhydrides or hydrates. Even when a water-insoluble compound such as an oxide or oxide hydroxide is selected as the manganese compound, it can be dissolved in an acid such as hydrochloric acid or nitric acid and used as an aqueous solution. Each of these manganese compounds may be used alone or in combination of two or more.

  The manganese compound-containing aqueous solution contains manganese as an essential component, but may further contain other metals. From the viewpoint of obtaining a precipitate in which a metal element is present at a valence of 2 or less, the valence of the metal is preferably present at a valence of 2 or less even in an aqueous solution. Therefore, specifically, manganese compounds include manganese chloride (II), manganese nitrate (II), manganese sulfate (II), manganese acetate (II), manganese acetate (III), manganese acetyl acetate (II), Examples thereof include potassium manganate (VII), manganese (III) acetyl acetate, and hydrates thereof. Further, if necessary, iron (II) chloride, iron (III) chloride, iron (III) nitrate, iron (II) sulfate, iron (III) sulfate, magnesium chloride, magnesium nitrate, magnesium oxalate, magnesium sulfate, acetic acid Magnesium, calcium chloride, calcium nitrate, calcium oxalate, calcium sulfate, calcium acetate, cobalt chloride (II), cobalt nitrate (II), cobalt oxalate (II), cobalt sulfate (II), cobalt acetate (II), chloride Aluminum (III), aluminum nitrate (III), aluminum oxalate (III), aluminum sulfate (III), aluminum acetate (III), nickel chloride (II), nickel nitrate (II), nickel oxalate (II), sulfuric acid Nickel (II), nickel acetate (II), niobium chloride Titanium chloride, titanium sulfate, chromium chloride (III), chromium nitrate (III), chromium sulfate (III), chromium acetate (III), copper chloride (II), copper nitrate (II), copper oxalate (II), sulfuric acid Copper (II), copper acetate (II), zinc chloride (II), zinc nitrate (II), zinc oxalate (II), zinc sulfate (II), zinc acetate (II), zirconium chloride, zirconium sulfate, vanadium chloride , Vanadium sulfate, molybdenum acetate, tungsten chloride, and hydrates thereof may be used to form a precipitate containing manganese and other metals.

  When it is desired to obtain a precipitate containing two or more metal elements, the mixing ratio of the above compounds in the aqueous solution may be set to an element ratio similar to the element ratio of each metal element in the target lithium manganese silicate compound. .

  The concentration of each compound in the aqueous solution is not particularly limited, and may be determined as appropriate so that a uniform aqueous solution can be formed and a precipitate can be smoothly formed. For example, the concentration of the manganese compound is preferably 0.01 to 5 mol / L, more preferably 0.1 to 2 mol / L.

  The manganese compound-containing aqueous solution may contain alcohol in addition to the manganese compound and water. That is, as a solvent, water may be used alone, or a water-alcohol mixed solvent containing a water-soluble alcohol such as methanol or ethanol may be used. By using a water-alcohol mixed solvent, the alcohol functions as an antifreeze and precipitates can be formed at temperatures below 0 ° C. The amount of alcohol used may be appropriately determined according to the target precipitation temperature, etc., but it is usually appropriate that the amount of alcohol used is 50 parts by weight or less with respect to 100 parts by weight of water. In the present specification, the case where alcohol is included is also regarded as an “aqueous solution”.

  Next, a precipitate (which may be a coprecipitate) is generated from the aqueous solution. In order to generate the precipitate, the aqueous solution may be alkaline. Conditions for forming a good precipitate vary depending on the type and concentration of each compound contained in the aqueous solution, and thus cannot be defined unconditionally. However, the pH is usually preferably 8 or more, more preferably 11 or more.

  There is no particular limitation on the method of making the aqueous solution containing the metal salt alkaline, and usually a precipitate can also be formed by a method of adding the aqueous solution to an aqueous solution containing an alkali. Further, an alkali or an aqueous solution containing an alkali may be added to the aqueous solution.

  Examples of the alkali used for making the aqueous solution alkaline include alkali metal hydroxides such as potassium hydroxide, sodium hydroxide and lithium hydroxide, hydrates thereof, and ammonia. Particularly preferred is lithium hydroxide. This is because by using lithium hydroxide, the impurities contained in the precipitate can be made only Li contained in the target lithium manganese silicate compound. In addition, when lithium hydroxide is used, there is an advantage that pH adjustment of the aqueous solution can be easily performed. When these alkalis are used as an aqueous solution, they can be used, for example, as an aqueous solution having a concentration of 0.1 to 20 mol / L, preferably 0.3 to 10 mol / L.

  Further, the alkali may be dissolved in a water-alcohol mixed solvent containing a water-soluble alcohol, similarly to the aqueous solution of the metal compound described above.

  There is no particular limitation on the temperature of the aqueous solution, and the precipitate may be formed at room temperature (20 to 35 ° C.). good. By keeping at a low temperature, the formation of impurity phase (for example, spinel ferrite, a kind of iron oxide) accompanying the generation of heat of neutralization during the reaction is suppressed, and a uniform precipitate is easily formed. Become.

  After the aqueous solution is made alkaline, the precipitate is oxidized and aged while blowing air into the reaction solution at 0 to 150 ° C., preferably 10 to 100 ° C., for 1 hour to 7 days, preferably half a day to 4 days. It is preferable to carry out the treatment. The oxidation / aging process may be performed at room temperature. The precipitate can be purified by washing the resulting precipitate with distilled water or the like to remove excess alkali components, residual raw materials, and the like, followed by filtration.

  Manganese contained in the obtained precipitate is preferably divalent to tetravalent. Further, the precipitate may further contain other metal elements as necessary. Examples of other metal elements include at least one transition element selected from the group consisting of Mg, Ca, Co, Al, Ni, Nb, Ti, Cr, Cu, Zn, Zr, V, Mo, and W.

  The manganese content in the manganese compound needs to be 50 mol% or more, with the total amount of metal elements being 100 mol%. That is, the amount of at least one transition metal element selected from the group consisting of Mg, Ca, Co, Al, Ni, Nb, Ti, Cr, Cu, Zn, Zr, V, Mo, and W is the amount of the transition metal element. The total amount can be 0 to 50 mol%, with 100 mol%.

Regarding the mixing ratio of the lithium silicate compound represented by Li ₂ SiO ₃ and the manganese compound, the total amount of the metal elements contained in the manganese compound is usually 0.9 to 1. with respect to 1 mol of the lithium silicate compound. The amount is preferably 2 mol, and more preferably 0.95 to 1.1 mol.

<Method for producing lithium manganese silicate compound>
In the molten salt method, it is necessary to react the raw material compound at 300 to 600 ° C. in a mixed gas atmosphere containing carbon dioxide and a reducing gas in the molten salt.

  The specific reaction method is not particularly limited, but usually, a molten salt raw material containing at least one selected from the above alkali metal salts, a lithium silicate compound and the above manganese compound are mixed, and a ball mill or the like is used. Then, the molten salt raw material may be melted by heating to a temperature equal to or higher than the melting point of the molten salt raw material. Thereby, in the molten salt, the dissolution-precipitation reaction of lithium, silicon, manganese, and other additive metals proceeds, and the target lithium manganese silicate compound can be obtained. At this time, the mixing ratio of the lithium silicate compound and the manganese compound and the molten salt raw material is not particularly limited as long as the raw material can be uniformly dispersed in the molten salt. For example, the lithium silicate compound and manganese It is preferable that the total amount of the molten salt raw material is in the range of 20 to 300 parts by mass with respect to the total amount of 100 parts by mass with the compound, More preferably, the amount is

  The reaction temperature of the lithium silicate compound and the manganese compound in the molten salt may be 300 to 600 ° C., further 400 to 560 ° C. If the temperature is lower than 300 ° C., the reactivity is poor because the salt is difficult to melt because the temperature is low, and it takes a long time to synthesize the lithium manganese silicate compound, which is not practical. On the other hand, when the reaction temperature exceeds 600 ° C., the resulting lithium manganese silicate compound particles are likely to be coarsened, which is not preferable.

When the lithium manganese silicate compound synthesized by the molten salt method is used as the positive electrode active material of a lithium ion secondary battery, the battery characteristic that is remarkably improved is the discharge average voltage. Also, the initial discharge capacity is increased and the irreversible capacity is reduced. Although the absolute value of the temperature varies depending on the composition of the lithium manganese silicate compound to be synthesized, it tends to grow into plate-like particles as the reaction temperature increases. For example, when the reaction temperature is 470 ° C. or higher, Li ₂ MnSiO ₄ powder having a needle-like or plate-like particle shape can be obtained. In particular, by reacting at 470 to 510 ° C., Li ₂ MnSiO ₄ is likely to grow into acicular particles. Further, by reacting at 520~560 _℃, Li 2 MnSiO ₄ tends to grow into a plate-like particles.

  In the reaction described above, during the reaction, in order to allow manganese contained in the manganese compound (manganese and other metal elements in the case of containing other metal elements) to be stably present in the molten salt as divalent ions, It is performed in a mixed gas atmosphere containing carbon dioxide and a reducing gas. Under this atmosphere, even if the oxidation number before the reaction is a metal element other than divalent, it can be stably maintained in a divalent state. The ratio of carbon dioxide and reducing gas is not particularly limited. However, when a large amount of reducing gas is used, carbon dioxide for controlling the oxidizing atmosphere is reduced, so that decomposition of the molten salt raw material is promoted and the reaction rate is increased. However, if the reducing gas is excessive, the reducing property becomes too high, and the divalent metal element of the lithium manganese silicate compound may be reduced and the reaction product may be decomposed. Therefore, the preferable mixing ratio of the mixed gas is preferably 1 to 40 parts by volume, more preferably 3 to 20 parts by volume of the reducing gas with respect to 100 parts by volume of carbon dioxide in volume ratio. As the reducing gas, for example, hydrogen, carbon monoxide and the like can be used, and hydrogen is particularly preferable. There is no particular limitation on the pressure of the mixed gas of carbon dioxide and reducing gas, and it may be usually atmospheric pressure, but may be under pressure or under reduced pressure.

  The reaction time between the lithium silicate compound and the manganese compound is usually 10 minutes to 70 hours, preferably 5 to 25 hours, and more preferably 10 to 20 hours.

  After completion of the above reaction, the lithium manganese silicate compound can be obtained by cooling and removing the alkali metal salt used as the flux. In general, the alkali metal salt is solidified by cooling after the reaction. For this reason, the alkali metal salt can be dissolved and removed by washing the product using a solvent capable of dissolving the solidified alkali metal salt. For example, water may be used as the solvent.

<Lithium manganese silicate compound by molten salt method>
The lithium manganese silicate compound obtained by the above-described method is represented by the following composition formula.

Composition formula: Li _{2 + ab} Ab Mn _1-x M _x Si _{1 + α} O _{4 + c} (formula 1)
In the formula, A is at least one element selected from the group consisting of Na, K, Rb and Cs, and M is Mg, Ca, Co, Al, Ni, Nb, Ti, Cr, Cu, It is at least one element selected from the group consisting of Zn, Zr, V, Mo and W. The subscripts are 0 ≦ x ≦ 0.5, −1 <a <1, 0 ≦ b <0.2, 0 ≦ c <1, and 0 <α ≦ 0.2. Preferably, −0.5 ≦ a ≦ 0.5, further −0.1 ≦ a ≦ 0.1, 0 ≦ b ≦ 0.1, further 0 ≦ b ≦ 0.05, 0 <α ≦ 0.1. Furthermore, 0.01 ≦ α ≦ 0.05.

  In addition, when this lithium manganese silicate type compound contains iron in addition to manganese as a transition metal, the lithium manganese silicate type compound is represented by the following composition formula. In this case, in the formula, M ′ is Fe and Mn. Otherwise, it is the same as Equation 1.

Composition formula: Li _{2 + ab Ab} M ′ _1-x M _x Si _{1 + α} O _{4 + c} (formula 2)
When a lithium salt is contained in the molten salt, lithium ions in the molten salt enter the Li ion site of the lithium manganese silicate compound. For this reason, this compound turns into a compound which contains Li ion excessively compared with stoichiometric amount. That is, the subscript “a” in the composition formula is a positive value. In addition, by performing the reaction at a low temperature of 600 ° C. or less in the molten salt, the growth of crystal grains of the lithium manganese silicate compound is suppressed, and the average particle diameter becomes fine particles of several μm or less. The amount of is greatly reduced. As a result, a lithium ion secondary battery using this lithium manganese silicate compound as a positive electrode active material can exhibit good cycle characteristics and rate characteristics, as well as high capacity. The shape of the lithium manganese silicate compound is not particularly limited, such as a plate shape or a needle shape. The cycle characteristics of a lithium manganese silicate compound vary depending on its shape. However, in any case, the cycle characteristics can be improved by covering the core made of the lithium manganese silicate compound with the coating layer.

<Coating layer>
The coating layer may be formed by a known method, and a known coating method such as lamination, film formation, support, etc. by a general coating method such as vapor deposition or sputtering, sol-gel method, chemical vapor deposition method, solution method, etc. This method can be used. The coating layer is made of at least one selected from A: a hydrofluoric acid compound, B: a fluorine absorbing compound, and C: a carbon-based material.

  Of these, the hydrofluoric acid resistant compound of A is scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, titanium oxide, zirconia oxide, hafnium oxide, vanadium oxide, niobium oxide, tantalum oxide, It consists of at least one selected from molybdenum oxide, tungsten oxide, and aluminum oxide. When the coating layer is composed of these materials, the lithium manganese silicate compound in the core can be protected because it does not easily react with fluorine.

  The fluorine-absorbing compound of B consists of at least one selected from magnesium, calcium, strontium, barium, silicon, lanthanum, cerium (metal), or an oxide thereof. When the coating layer is made of these materials, it easily reacts with fluorine, so that fluorine can be taken in and the amount of fluorine can be reduced. As a result, the lithium manganese silicate compound in the core can be protected.

  The carbon-based material of C is composed of a carbon material having a carbon skeleton obtained by carbonization of an organic substance and / or conductive carbon. Carbonization of the organic substance is preferably performed in the range of 500 to 800 ° C. As the organic material, sucrose, glucose, citric acid, phenol resin, ascorbic acid, oxalic acid, adipic acid and the like are preferably used. As the conductive carbon, acetylene black, ketjen black, carbon black or the like is preferably used. As the organic substance, any material such as plant origin or animal origin may be used. When the coating layer is composed of these materials, the conductivity of the lithium manganese silicate compound in the core can be improved. Further, it also serves to join the above-mentioned hydrofluoric acid-resistant compound or fluorine absorbing compound and the lithium manganese silicate compound in the core.

  In addition to the coating layer, a layer other than the coating layer may be formed on the core portion. Examples of the layer other than the coating layer include a fluorine-treated layer. Specifically, a coating layer formed with fluorine gas is exemplified. Further, a conductive layer made of a carbon material may be formed. For example, a conductive layer derived from these gases can be formed by carbonizing butane gas, ethane gas, methane gas, acetylene gas or the like by gas decomposition or the like.

  Furthermore, in addition to the process which forms a coating layer, you may give another process to a core part. Furthermore, the lithium manganese silicate composite of the present invention including the core portion and the coating layer may be subjected to an amorphization treatment by classification, heating, milling, or the like.

The method for forming the coating layer using the carbon-based material C is not particularly limited, but it is preferable to use, for example, a gas phase method, a thermal decomposition method, a ball milling method, or the like. The gas phase method is a method in which heat treatment is performed in an atmosphere containing a carbon-containing gas such as methane gas, ethane gas, or butane gas. The thermal decomposition method is a method in which an organic substance that is a carbon source and a lithium manganese silicate compound are uniformly mixed, and then the organic substance is carbonized by heat treatment. The ball milling method is a method in which a carbon material and Li ₂ CO ₃ are added to a lithium manganese silicate compound, and the mixture is uniformly mixed by a ball mill until the lithium manganese silicate compound becomes amorphous, followed by heat treatment. Of these, the ball milling method is particularly preferred. According to this method, the lithium manganese silicate compound, which is a positive electrode active material, is made amorphous by ball milling, and is uniformly mixed with carbon to increase adhesion. At the same time, since carbon is uniformly deposited around the lithium manganese silicate compound, the lithium manganese silicate compound can be coated with carbon substantially uniformly and with high adhesion. At this time, due to the presence of Li ₂ CO ₃ , the lithium-excess silicate compound does not become lithium deficient and maintains a high charge / discharge capacity.

Regarding the degree of amorphization, in the X-ray diffraction measurement using CuKα ray as the light source, the half-value width of the diffraction peak derived from the (011) plane of the sample having crystallinity before ball milling is B (011) _crystal , ball milling. by when the half width of the peak of the obtained sample was _{B (011) mill, B (} 011) crystal / B (011) ratio of the _mill may be in the range of about 0.1 to 0.5 .

  When the coating layer made of a carbon material is formed by the ball milling method, acetylene black (AB), ketjen black (KB), activated carbon, graphite, or the like can be used as the carbon material.

Lithium manganese silicate-based compounds, for the mixing ratio of the carbon material and _Li 2 CO _3, the lithium manganese silicate compound 100 parts by weight of 5 to 40 parts by weight of the carbon-based material, 20-40 wt _Li 2 CO ₃ Part.

  In the ball milling method, a ball milling process is performed until the lithium manganese silicate compound becomes amorphous, and then a heat treatment is performed. The heat treatment is performed in a reducing atmosphere in order to keep the transition metal ions contained in the lithium manganese silicate compound divalent. In this case, the reducing atmosphere is preferably a mixed gas atmosphere of carbon dioxide and reducing gas. This is because, similarly to the synthesis reaction of the lithium manganese silicate compound in the molten salt, the divalent transition metal ion is prevented from being reduced to the metal state. The mixing ratio of carbon dioxide and reducing gas may be the same as in the synthesis reaction of the lithium manganese silicate compound.

  The heat treatment temperature is preferably 500 to 800 ° C. If the heat treatment temperature is too low, it is difficult to deposit carbon uniformly around the lithium manganese silicate compound. On the other hand, if the heat treatment temperature is too high, decomposition of the lithium manganese silicate compound or lithium deficiency may occur, and the charge / discharge capacity decreases, which is not preferable. The heat treatment time is usually 1 to 10 hours.

  In addition, when applying the ball milling method, after adding a carbon material and LiF to the lithium manganese silicate compound, and after mixing uniformly until the lithium manganese silicate compound becomes amorphous by a ball mill in the same manner as described above A heat treatment may be performed. In this case, as in the case described above, carbon is uniformly deposited around the lithium manganese silicate compound simultaneously with recrystallization of the lithium manganese silicate compound, and the conductivity is improved. Furthermore, some of the oxygen atoms of the lithium manganese silicate compound are substituted with fluorine atoms to form a fluorine-containing lithium manganese silicate compound.

  When this compound is used as a positive electrode due to the addition of F, the average voltage increases, and a positive electrode material having better performance is obtained. At this time, the presence of LiF does not cause the lithium-excess manganese silicate compound to be deficient in lithium, and exhibits a high charge / discharge capacity.

In this method, the blending ratio of the lithium manganese silicate compound, the carbon material, and LiF is 20 to 40 parts by mass of the carbon material and 10 to 40 parts by mass of LiF with respect to 100 parts by mass of the lithium manganese silicate compound. do it. Furthermore, if necessary, Li ₂ CO ₃ may be included. The conditions for ball milling and heat treatment may be the same as described above.

<Positive electrode for lithium ion secondary battery>
The lithium manganese silicate composite of the present invention including a core part and a coating layer can be effectively used as an active material for a lithium secondary battery positive electrode. The positive electrode using the lithium manganese silicate composite of the present invention can have the same structure as a normal positive electrode for a lithium ion secondary battery.

  For example, a lithium manganese silicate composite may be added to a conductive aid such as acetylene black (AB), ketjen black (KB), vapor grown carbon fiber (VGCF), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PVdF), A positive electrode is manufactured by adding a binder such as ethylene fluoride (PTFE) or styrene-butadiene rubber (SBR), or a solvent such as N-methyl-2-pyrrolidone (NMP), and applying this to a current collector. can do. Although there are no particular limitations on the amount of conductive aid used, for example, it can be 5 to 20 parts by mass with respect to 100 parts by mass of the lithium manganese silicate composite. The amount of the binder used is not particularly limited, but may be 5 to 20 parts by mass with respect to 100 parts by mass of the lithium manganese silicate composite, for example. As another method, a mixture of the lithium manganese silicate composite, the above-mentioned conductive aid and binder is kneaded using a mortar or a press to form a film, which is then pressed into a current collector with a press. The positive electrode can also be produced by a method of pressure bonding.

  The current collector is not particularly limited, and materials conventionally used as positive electrodes for lithium ion secondary batteries, such as aluminum foil, aluminum mesh, and stainless steel mesh, can be used. Furthermore, a carbon nonwoven fabric, a carbon woven fabric, etc. can be used as a collector.

  The positive electrode for a lithium ion secondary battery of the present invention is not particularly limited with respect to its shape, thickness, etc. For example, the thickness is preferably 10 to 200 μm, more preferably by compression after filling with an active material. Is preferably 20 to 100 μm. Therefore, the filling amount of the active material may be appropriately determined so as to have the above-described thickness after compression, depending on the type and structure of the current collector to be used.

<Lithium ion secondary battery>
A lithium ion secondary battery using the above-described positive electrode for a lithium ion secondary battery can be produced by a known method. That is, the positive electrode described above is used as a positive electrode material, and as a negative electrode material, a known carbon-based material such as lithium, graphite, a silicon-based material such as a silicon thin film, an alloy-based material such as copper-tin or cobalt-tin, An oxide material such as lithium titanate is used, and as an electrolytic solution, a known nonaqueous solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, dimethyl carbonate, lithium perchlorate, LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ A lithium ion secondary battery according to a conventional method using a solution in which a lithium salt such as 0.5 mol / L to 1.7 mol / L is dissolved, and using other known battery components. Just assemble.

  As mentioned above, although embodiment of the lithium manganese silicate composite_body | complex of this invention and its manufacturing method was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

  Hereinafter, the lithium manganese silicate composite of the present invention will be specifically described with reference to examples.

(Example)
<Formation of coating layer>
The same lithium manganese silicate compound synthesized in the above <Confirmation of manganese elution> was used as the core. A first coating layer made of zirconia (zirconium oxide) which is a kind of hydrofluoric acid compound was formed on the upper layer of the core. Furthermore, the 2nd coating layer which consists of acetylene black which is 1 type of carbon type material was formed in the upper layer of the 1st coating layer. That is, the lithium manganese silicate composite of the example has two coating layers.

[First coating layer]
First, 100 mg of lithium manganese silicate compound and 20 mg of ZrO ₂ (first rare element chemical industry, ZSL-10T) were weighed and 10 ml of distilled water was added to prepare a lithium manganese silicate mixture. The mixture was sonicated with an ultrasonic cleaner for 10 minutes. The mixture after sonication was stirred with a hot stirrer at 800 rpm and 27 ° C. for 2 hours. After 2 hours, the stirring speed was changed to 400 rpm, and the mixture was further stirred at 27 ° C. for 15 hours. The mixture after stirring was separated into a liquid and a solid (precipitate) using a centrifuge (5000 rpm, 10 minutes). The separated precipitate was dried at 80 ° C. for 2-3 hours. In this step, the first covering layer was formed on the upper layer of the core portion.

[Second coating layer]
The precipitate after drying and acetylene black were mixed at a mass ratio of 5: 4. Ball milling was used for mixing, and the mixing conditions were 450 rpm and 5 hours. Thereafter, heat treatment was performed at 700 ° C. for 2 hours under a gas flow rate of CO ₂ : H ₂ = 100: 3 (cm ³ ) using a heat treatment furnace capable of controlling the atmosphere. In this step, the second coating layer was formed on the first coating layer covering the core. Through the above steps, the lithium manganese silicate composite of the example was obtained.

<Production of lithium ion secondary battery>
A lithium ion secondary battery for evaluation was manufactured using the lithium manganese silicate composite produced in the above process as a positive electrode active material.

Specifically, a mixture of lithium manganese silicate composite: acetylene black (AB): polytetrafluoroethylene (PTFE) = 17.1: 4.7: 1 (mass ratio) was kneaded to form a film, which was made of aluminum. An electrode was manufactured by pressure bonding to a current collector, and vacuum-dried at 140 ° C. for 3 hours was used as the positive electrode. As the negative electrode, metallic lithium was used. As the electrolytic solution, a solution in which LiPF ₆ was dissolved in ethylene carbonate (EC): dimethylene carbonate (DMC) = 1: 1 to 1 mol / L was used.

  A coin battery was manufactured using the positive electrode and the negative electrode. Specifically, in a dry room, a separator (Celgard 2400 made by Celgard, a polypropylene microporous membrane having a thickness of 25 μm) and a glass nonwoven fabric filter (thickness 440 μm, made by ADVANTEC, GA100) are placed between the positive electrode and the negative electrode. To be an electrode body battery. This electrode body battery was accommodated in a battery case (CR2032-type coin battery member, manufactured by Hosen Co., Ltd.) made of a stainless steel container. The above electrolyte was injected into the battery case. The battery case was sealed with a caulking machine to obtain a lithium ion secondary battery.

(Comparative example)
The positive electrode active material of the comparative example is the same lithium manganese silicate compound as that used as the core of the example. No zirconia layer was formed on this lithium manganese silicate compound. The lithium ion secondary battery of the comparative example is the same as the lithium ion secondary battery of the example except for the positive electrode active material.

<Analysis of Lithium Manganese Silicate Complex>
Component analysis was performed on the positive electrode active material of the example (that is, lithium manganese silicate complex) and the positive electrode active material of the comparative example (lithium manganese silicate compound). As the apparatus, SEM and EDX were used. For EDX, simple quantitative analysis was performed by the same ZAF method as described above. As a result, regarding the positive electrode active material of the example, the counting rate was 2791 cps and the fitting coefficient was 0.3342. Regarding the positive electrode active material of the comparative example, the counting rate was 2875 cps. The SEM image of the lithium manganese silicate complex is shown in FIG. 3, and the analysis results of the lithium manganese silicate complex by EDX are shown in FIG. FIG. 5 shows an SEM image of the lithium manganese silicate compound, and FIG. 6 and Table 3 show the analysis results of the lithium manganese silicate compound by EDX.

  Comparing the SEM image of the lithium manganese silicate complex shown in FIG. 3 with the SEM image of the lithium manganese silicate compound shown in FIG. 5, the lithium manganese silicate complex has rounded particles compared to the lithium manganese silicate compound. I was tinged.

As shown in Table 2, about 1% by mass of Zr was detected from the surface of the lithium manganese silicate composite. On the other hand, as shown in Table 3, Zr was not detected from the surface of the lithium manganese silicate compound. From this result, it is understood that a coating layer (specifically, a zirconia layer) is formed on the surface of the lithium manganese silicate composite of the example. It can also be seen from the EDX analysis that the presence or absence of the coating layer can be confirmed and whether or not the lithium manganese silicate composite of the present invention can be confirmed.

<Cycle test>
The lithium ion secondary batteries of Examples and Comparative Examples were repeatedly charged and discharged at 30 ° C. Each battery was charged and discharged at a current value corresponding to 0.05 mA per unit area (1 cm ² ) of the positive electrode active material. At that time, the final discharge voltage was 1.5V, and the final charge voltage was 4.5V. In addition, it hold | maintained at 4.5V for 10 hours only at the time of first charge. A charge / discharge curve of the lithium ion secondary battery of the example is shown in FIG. 7, and a cycle characteristic is shown in FIG. FIG. 9 shows a charge / discharge curve of the lithium ion secondary battery of the comparative example, and FIG. 10 shows the cycle characteristics. As shown in FIGS. 7 to 10, the lithium ion secondary battery of the example is superior to the lithium ion secondary battery of the comparative example in charge and discharge efficiency at the first cycle, and up to 10 cycles except at the initial charge. The charge / discharge capacity is large, and the capacity reduction accompanying charge / discharge is small. Comparing the discharge capacity retention after 50 cycles, the lithium ion secondary battery of the comparative example is 29%, while the lithium ion secondary battery of the example is greatly improved to 50%. From this result, it can be seen that covering the lithium manganese silicate compound with the coating layer can suppress a decrease in charge / discharge characteristics of the lithium ion secondary battery containing the lithium manganese silicate compound as the positive electrode active material. In other words, the lithium manganese silicate composite of the present invention including a core portion made of a lithium manganese silicate compound and a coating layer covering the core portion can suppress a decrease in cycle characteristics, and can be used for a lithium ion secondary battery. It is useful as a positive electrode active material.

Claims (9)

From a core composed of a lithium manganese silicate compound containing lithium (Li), manganese (Mn), silicon (Si) and oxygen (O), at least one selected from a hydrofluoric acid compound and a fluorine absorbing compound , and a carbon material And a covering layer covering the core portion,
The lithium manganese silicate composite, wherein the coating layer contains at least one fluorine absorbing compound selected from magnesium, calcium, strontium, barium, silicon, lanthanum, cerium, and oxides thereof.   The hydrofluoric acid resistant compound is scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, titanium oxide, zirconium oxide, hafnium oxide, vanadium oxide, niobium oxide, tantalum oxide, molybdenum oxide. The lithium manganese silicate composite according to claim 1, comprising at least one selected from the group consisting of tungsten oxide and aluminum oxide. From a core composed of a lithium manganese silicate compound containing lithium (Li), manganese (Mn), silicon (Si) and oxygen (O), at least one selected from a hydrofluoric acid compound and a fluorine absorbing compound, and a carbon material And a covering layer covering the core portion,
The lithium manganese silicate composite , wherein the coating layer contains the hydrofluoric acid compound made of zirconium oxide . The lithium manganese silicate composite according to any one of claims 1 to 3, wherein the carbon-based material includes a carbon material having a carbon skeleton obtained by carbonization of an organic substance and / or conductive carbon.   The lithium manganese silicate composite according to any one of claims 1 to 4, wherein a mass of the hydrofluoric acid resistant compound is 5% or less of a mass of the entire lithium manganese silicate composite.   The lithium manganese silicate composite according to any one of claims 1 to 5, wherein a mass of the fluorine absorbing compound is 5% or less of a total mass of the lithium manganese silicate composite.   7. The lithium manganese silicate composite according to claim 1, wherein the mass of the carbon-based material is 1 to 40% of the total mass of the lithium manganese silicate composite.   A positive electrode for a non-aqueous electrolyte secondary battery comprising the lithium manganese silicate composite according to any one of claims 1 to 7.   A nonaqueous electrolyte secondary battery comprising the positive electrode for a nonaqueous electrolyte secondary battery according to claim 8 as a constituent element.