JP7164178B2 - lithium metal phosphate, positive electrode material for lithium ion secondary battery, lithium ion secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a lithium metal phosphate, a lithium metal phosphate, a positive electrode material for a lithium ion secondary battery, and a lithium ion secondary battery.

LiCoO ₂ (LCO) is known as a positive electrode material for lithium ion secondary batteries. As a more reliable material to replace LCO, olivine-type lithium metal phosphate represented by the general formula LiMPO ₄ is being developed. In addition, M is a transition metal element that takes a valence of +2 in the stoichiometric composition and can change its valence to +3 in order to maintain charge neutrality with the desorption of Li, and includes Fe, Co, Ni and Mn. is known (for example, Patent Document 1).

JP 2008-184346 A

By the way, as described in Patent Document 1, the electrical conductivity of olivine-type lithium metal phosphate is generally low. Lithium metal phosphate is required to have higher electrical conductivity in addition to reliability because it is used as a positive electrode material for secondary batteries.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing a lithium metal phosphate having excellent electrical conductivity. Another object of the present invention is to provide a lithium metal phosphate, a positive electrode material for a lithium ion secondary battery, and a lithium ion secondary battery obtained by the production method.

As a result of intensive studies by the inventors in order to solve the above problems, the adoption of a high-temperature flux method (flux method) and doping with boron are effective in producing a lithium metal phosphate with excellent electrical conductivity. We have found this to be important, and have completed the present invention. That is, the present invention includes a compound containing Li element, a compound containing metal element M (M is at least one selected from the group consisting of Fe, Co, Ni and Mn), a compound containing B element, and phosphate ions. Provided is a method for producing lithium metal phosphate, comprising a mixing step of obtaining a mixture of a compound and a flux, a melting step of obtaining a melt of the mixture, and a cooling step of cooling the melt to obtain a precipitate. do.

In the production method of the present invention, the melting temperature in the melting step may be 600° C. or higher.

In the production method of the present invention, the element ratio of B element to P element in the mixture may be 1/99 to 99/1.

Further, the present invention provides a compound represented by the general formula LiMPO ₄ (wherein M is at least one selected from the group consisting of Fe, Co, Ni and Mn), in which part of P in the compound is substituted with B , a lithium metal phosphate.

The lithium metal phosphate of the present invention has the general formula Li _1+α M(B _x P _1-x )O ₄ (wherein x is 0.01 to 0.99 and α is more than 0 and 2x or less). may have

The lithium metal phosphate of the present invention may be used as a positive electrode material for lithium ion secondary batteries.

The present invention also provides a positive electrode material for a lithium ion secondary battery comprising the above lithium metal phosphate.

Furthermore, the present invention provides a lithium ion secondary battery comprising a positive electrode containing the above positive electrode material.

ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of lithium metal phosphate excellent in electrical conductivity can be provided. Moreover, according to the present invention, it is possible to provide a lithium metal phosphate, a positive electrode material for a lithium ion secondary battery, and a lithium ion secondary battery obtained by the production method.

4 is a graph showing electrical conductivity evaluation results of lithium metal phosphates according to examples. 4 is a graph showing electrical conductivity evaluation results of lithium metal phosphates according to examples. 4 is a graph showing electrical conductivity evaluation results of lithium metal phosphates according to examples. 4 is a graph showing electrical conductivity evaluation results of lithium metal phosphates according to examples. 4 is a graph showing electrical conductivity evaluation results of lithium metal phosphates according to comparative examples.

Several embodiments of the invention are described in detail below. However, the present invention is not limited to the following embodiments.

<Method for Producing Lithium Metal Phosphate>
A method for producing a lithium metal phosphate comprises a compound containing a Li element, a compound containing a metal element M (M is at least one selected from the group consisting of Fe, Co, Ni and Mn), a compound containing a B element, and A mixing step of obtaining a mixture of a compound containing phosphate ions (PO ₄ ³⁻ ) and a flux, a melting step of obtaining a melt of the mixture, and a cooling step of cooling the melt to obtain a precipitate. . By the production method of the present embodiment, part of P in the compound represented by the general formula LiMPO ₄ (wherein M is at least one selected from the group consisting of Fe, Co, Ni and Mn) is replaced with B. a lithium metal phosphate can be obtained.

(Mixing process)
The compound containing the Li element is not particularly limited, and may be lithium carbonate, lithium acetate, lithium nitrate, lithium hydroxide, lithium chloride, lithium phosphate, lithium dihydrogen phosphate, lithium borate, anhydrides or hydrates thereof. things, etc.

The compound containing the metal element M is not particularly limited, and compounds containing Fe elements such as iron oxalate, iron chloride, iron sulfate, iron nitrate, iron oxide, iron hydroxide, anhydrides or hydrates thereof ( The valence of Fe in the compound may be divalent or trivalent); Co-containing compounds such as cobalt carbonate, cobalt oxalate, cobalt chloride, their anhydrides or hydrates; nickel carbonate, oxalic acid Ni element-containing compounds such as nickel, nickel chloride, anhydrides or hydrates thereof; compounds containing Mn element such as manganese carbonate, manganese oxalate, manganese chloride, anhydrides or hydrates thereof; be done.

Among these compounds, a compound containing a divalent transition metal element can be used as the compound containing the metal element M from the viewpoint of exhibiting excellent electrical conductivity. Although it is not clear why the use of a compound containing a divalent transition metal element in boron-doped lithium metal phosphate particularly improves the electrical conductivity, the inventors speculate as follows. That is, the transition metal is in a state of equilibrium with oxygen in the atmosphere in a high-temperature melt state, and in the presence of a small amount of oxygen, it is in a state of coexistence of divalent and trivalent metals. By substituting and forming a solid solution with B at the 4-coordinated P position in the crystal structure, Li ions are further increased at the 6-coordinated position for charge compensation, or a trivalent transition metal enters. It is thought that the ratio of 3 valences in the crystal increases, the crystal turns black, electrons move between the 2 and 3 valences of the transition metal, and electrical conductivity develops. In addition, compared to silicate, it is believed that Li ions, which correspond to the change in the valence of the transition metal when electrons move, move more easily, thus increasing the mobility of electrons. It is speculated that ionic conduction and electronic conduction complementarily contribute to the improvement of electrical conductivity.

The compound containing element B is not particularly limited, and boron oxide, boric acid, boric anhydride, lithium borate, and the like can be used.

Compounds containing phosphate ions are not particularly limited, and ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium phosphate, lithium dihydrogen phosphate, phosphoric acid, diphosphorus pentoxide, and these Anhydrides, hydrates, and the like can be mentioned.

Using a compound containing two or more of Li element, M element, B element and phosphate ions facilitates the mixing step. For example, lithium borate can be used as a compound containing Li and B elements. In addition, lithium phosphate and lithium dihydrogen phosphate can be used as compounds containing Li element and phosphate ions. In this way, a compound containing Li element, a compound containing metal element M, a compound containing B element, and a compound containing phosphate ions may be prepared and used. A compound containing M, a compound containing the metal element M, and a compound containing the B element may be prepared and used.

The element ratio of the M element, the P element and the B element in the weighed raw material of each compound can be 0.8 to 1.2:1. From the viewpoint of the stoichiometric ratio of lithium metal phosphate, it may be 1:1. On the other hand, the element ratio of the B element to the P element can be 1/99 to 99/1. However, from the viewpoint of obtaining a desired crystal structure and exhibiting better electrical conductivity, the element ratio can be 5/95 to 50/50, and may be 10/90 to 30/70. The element ratio of the Li element and the P and B elements must be adjusted so that the Li element is large. Although the elemental ratio is 1:1 from the stoichiometric ratio of lithium metal phosphate, this is because there is evaporation loss of Li during the melting process and the cooling process.

Flux is a compound used in the high-temperature flux method (flux method), which is a type of crystal production method, and is an inorganic compound that acts as a solvent for dissolving the desired crystal. The high-temperature flux method utilizes the property that the solubility of crystals, which are solutes, in a solvent changes with temperature. When using a flux whose solubility is high at high temperatures and low at low temperatures, the solubility in the flux exceeds with cooling, and the target substance in a supersaturated state precipitates as crystals. As the flux, a compound that melts at a temperature lower than that of the target crystal may be selected. Examples of the flux include lithium chloride, lithium fluoride, lithium vanadate, sodium dihydrogen phosphate, lead fluoride, lead oxide, bismuth oxide, molybdate, tungstate and the like. Or it can be used in combination of two or more. When there are multiple solute components, a specific solute component is increased and the component ratio is intentionally deviated from the desired component ratio (stoichiometric ratio) so that the solute component also functions as a flux. A self-flux method can also be used.

In the mixing step, a mixture is obtained by mixing the weighed raw materials of each compound to be the solute and the flux. The mixing method may be either a dry mixing method or a wet mixing method. Specifically, a method of mechanically mixing each raw material using a mortar, a ball mill, etc., a coprecipitation method of dissolving each raw material in water and then precipitating and mixing, and a method of gelling a sol in which each raw material is dissolved. A sol-gel method, etc., in which the materials are mixed together can be used.

(melting process)
In the melting step, the mixture obtained in the mixing step is melted. The mixture is placed in a predetermined container as required, put into a firing furnace, and melted in an inert atmosphere or an inert atmosphere containing a small amount of oxygen. Examples of the inert atmosphere include an atmosphere substituted with argon gas, nitrogen gas, helium gas, or the like. These inert gases may contain less than 1000 ppm of oxygen. The melt in the melting step means that the flux added as a solvent is completely melted, and most of the solutes (compounds containing the Li element, compounds containing the metal element M, compounds containing the B element, and phosphate ions are containing compound) dissolved in the flux. The melt may contain unmelted solutes.

The melting temperature in the melting process can be set to a temperature at which most or all of the solute melts in the molten flux. The melting temperature may be at least 600° C. or higher, may be 650° C. or higher, or may be 700° C. or higher, although it depends on the composition of the mixture and is not necessarily limited. The upper limit of the melting temperature can be 1000° C. or lower from the viewpoint of suppressing the decomposition and evaporation of the raw material and the lithium compound, and may be 900° C. or lower. The holding time at the melting temperature can be set to a time for a certain amount of solute to melt, for example, it can be at least 3 hours or more, may be 4.5 hours or more, or 6 hours or more. may be The upper limit of the holding time can be 24 hours or less from the viewpoint of suppressing decomposition and evaporation of the raw material and the lithium compound, and may be 12 hours or less.

If the flux is not completely melted during the melting process, or if most of the solute is not dissolved in the flux, impurities such as unreacted flux components will mix into the lithium metal phosphate obtained after cooling. There is a risk of being compounded. These impurities are compounds that are produced when the mixture is simply sintered, and are factors that reduce the electrical conductivity of the lithium metal phosphate. The sintering referred to here is a method that does not use flux, and heats the raw materials at a high temperature that does not completely melt the raw materials to cause the raw materials to react with each other, thereby obtaining a sintered product having a composition according to the raw materials. be. In sintering, the reaction progresses due to solid-phase diffusion of elements, so unreacted portions may remain, and the distribution of elements in the product tends to become non-uniform. Also, since calcination is based on a solid state reaction, it is practically impossible to produce crystals large enough to be separated. Therefore, it is necessary to repeat sintering, crushing and re-mixing many times in order to produce a homogeneous substance with a complicated structure and a variety of elements.

From the viewpoint of suppressing the generation of impurities, a calcination step may be performed prior to the melting step. In the calcination step, the mixture can be calcined at about 400 to 600° C. to obtain a calcined product. The calcined product can be subjected to a melting step.

(Cooling process)
The resulting melt is cooled to room temperature by this step. Cooling is preferably carried out at a rate of 5° C./hour or less at the fastest. If the cooling rate is too fast, defects may occur in the crystal.

The recovered product (cooled product) obtained after the cooling step may contain other components such as flux components, unreacted substances, and side-reacted substances in addition to the desired lithium metal phosphate precipitates. For example, only the desired lithium metal phosphate can be obtained from the recovered material by washing the recovered material with hot water (pure water at 60° C. or the like) and filtering only the hardly soluble crystals. From this point of view, the manufacturing method of the present embodiment may include a washing step of washing the recovered material obtained in the cooling step. It should be noted that it can be confirmed by X-ray diffraction method or the like that the obtained crystal is a lithium metal phosphate.

<Lithium metal phosphate>
In the lithium metal phosphate of the present embodiment, part of P in the compound represented by the general formula LiMPO ₄ (wherein M is at least one selected from the group consisting of Fe, Co, Ni and Mn) is replaced with Lithium metal phosphate has an olivine structure. In general, the olivine structure has a hexagonal close-packed oxygen skeleton, with P ions at the tetracoordinated tetrahedral sites and Li ions and transition metal ions M at the hexacoordinated octahedral sites. In the lithium metal phosphate of this embodiment, part of the pentavalent P ions are replaced with trivalent B ions. The valence of M is +2 in the stoichiometric composition, and may change to +3 in order to maintain charge neutrality as Li is desorbed. Such a lithium metal phosphate can be called a boron-doped lithium metal phosphate (LiM(B,P)O ₄ ), and can be represented, for example, by the following general formula (X).

Li _1+α M(B _x P _1-x )O ₄ (X)
In the formula, x is 0.01 to 0.99, and α is more than 0 and 2x or less. If x is too small, electrical conductivity tends to be insufficient, and if x is too large, heterogeneous phases tend to occur and the yield tends to decrease. From the viewpoint of efficiently obtaining a desired crystal structure and exhibiting better electrical conductivity, x can be 0.05 to 0.5, and may be 0.1 to 0.3.

The reason why the lithium metal phosphate of the present embodiment has excellent electrical conductivity is not necessarily clear, but the inventors speculate as follows. In general, in lithium metal phosphate having an olivine structure, the charging and discharging process of Li is represented by Li _1-y (M ²⁺ _1-y , M ³⁺ _y )PO ₄ . At the same time that Li enters the crystal by y due to discharge, y equivalent electrons flow into the transition metal M, which was 3+, and the valence of M changes to 2+ by y. On the other hand, in the lithium metal phosphate doped with B, when α=2x, the charging and discharging process of Li is Li _1+2x−y (M ²⁺ _1−y , M ³⁺ _y )( B _x P _1-x )O ₄ (x ranges from 0.01 to 0.99, y ranges from 0.01 to 1). First, Li enters the crystal by y due to discharge, and at the same time, y equivalent electrons flow into the transition metal M, which was 3+, and the valence of M changes to 2+ by y. Next, when α=2x by charging, one Li ion is released to become 3+ M, and when further charged, 2x Li ions may be released to become 4+ or 5+ M. Electron hopping conduction due to valence mixing of transition metals is thought to improve electrical conductivity. It is believed that this is because the solid solution of P and B loosens the oxygen lattice, making it easier for Li ions to move, so that charge compensation is quickly performed. Thus, B-doped lithium metal phosphate is believed to have excellent electrical conductivity. It is surprising that such excellent electrical conductivity as well as reliability can be obtained in olivine-type lithium metal phosphate, which has a more robust structure compared to layered rocksalt _- type LiCoO2, The boron-doped lithium metal phosphate in this embodiment is extremely suitable as a positive electrode material (positive electrode active material) for lithium ion secondary batteries.

It is believed that the crystals of lithium metal phosphate of the present embodiment precisely deviate from the conventional definition of the olivine structure. The olivine structure is a crystal having the same crystal structure as the natural silicate mineral olivine X ₂ SiO ₄ (in the formula, X contains the bivalent metals Mg and Fe in a ratio of about 9:1). In X ₂ SiO ₄ , oxygen is packed in a substantially hexagonal close-packed manner, and there are 8 gaps per 4 oxygen atoms surrounded by 4 oxygen atoms, and Si occupies ⅛ of these gaps. There are also 4 gaps surrounded by 6 oxygen atoms per 4 oxygen atoms, and X occupies 1/2 of them. On the other hand, in LiMPO ₄ (M=Fe, Co, Ni, Mn) in which B is not added, P is substituted for Si, and Li and M are substituted for X. Li ₃ PO ₄ containing no M at all is the ultimate in excess Li, but this crystal has a γ-Li ₃ PO ₄ type structure different from that of olivine. In γ-Li ₃ PO ₄ , oxygen also forms an almost hexagonally close-packed skeleton, and in addition to Li occupying half of the space surrounded by 6 oxygen atoms, Li is surrounded by 4 oxygen atoms. It also enters the gap and occupies 1/8 of it. When B is added to _LiMPO4 , B replaces a part of P, and it is speculated that the lack of positive charge due to the substitution of trivalent B for pentavalent P is compensated by excess Li. be done. From this, it is speculated that the lithium metal phosphate crystal of the present embodiment may have a novel crystal structure intermediate between the olivine type and the γ-Li ₃ PO ₄ type.

<Lithium ion secondary battery>
A lithium-ion secondary battery includes a positive electrode that includes the above-described lithium metal phosphate positive electrode material. More specifically, a lithium ion secondary battery includes the positive electrode, negative electrode, electrolyte, and the like.

(positive electrode)
The positive electrode can contain, in addition to the above positive electrode material, a conductive aid, a binder, and the like.

The conductive aid is not particularly limited, and includes acetylene black, carbon black, graphite, carbon fiber, metal fiber, aluminum powder, fluorocarbon, zinc oxide, potassium titanate, titanium oxide, polyphenylene derivatives and the like. These can be used individually by 1 type or in combination of 2 or more types.

The binder is not particularly limited, and includes polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, ethylene propylene diene polymer, and the like. These can be used individually by 1 type or in combination of 2 or more types.

(negative electrode)
The negative electrode may consist of the negative electrode active material itself, or may contain the negative electrode active material and a binder. That is, the negative electrode may be made of metallic lithium, lithium-aluminum alloy, lithium-tin alloy, etc., and may contain graphite, carbon fiber, coke, mesocarbon microbeads (MCMB), etc. and a binder. There may be.

(Electrolytes)
The electrolyte may be liquid or solid.

When the electrolyte is liquid (that is, an electrolytic solution is used), one obtained by dissolving the supporting electrolyte in an organic solvent can be used. The organic solvent is not particularly limited and includes carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, oxolane compounds and the like. These can be used individually by 1 type or in combination of 2 or more types.

The supporting electrolyte is not particularly limited, and inorganic salts such as LiPF ₆ , LiBF ₄ , LiClO ₄ and LiAsF ₆ , LiSO ₃ CF ₃ , LiC(SO ₃ CF ₃ ) ₂ , LiN(SO ₃ CF ₃ ) ₂ , LiN( SO ₂ C ₂ F ₅ ) ₂ , LiN(SO ₂ CF ₃ )(SO ₂ C ₄ F ₉ ) and other organic salts, and derivatives of these salts.

When an electrolytic solution is used, a porous synthetic resin film (particularly a polyolefin polymer porous film) may be used as the separator.

When the electrolyte is solid (that is, a solid electrolyte is used), compounds such as oxides and sulfides can be used. Examples of oxide-based compounds include La _0.51 Li _0.34 TiO _2.94 , NASICON-type Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ , and garnet-type Li ₇ La ₃ Zr. ₂ O ₁₂ and the like, and sulfide-based compounds include two-component systems such as Li ₂ S—SiS ₂ systems, and ternary system materials in which LiI, Li ₃ PO ₄ , etc. are added thereto. mentioned.

A lithium ion secondary battery is manufactured, for example, as follows.

A coating liquid is prepared by dispersing a negative electrode active material and a binder in a solvent. The obtained coating liquid is uniformly applied on the negative electrode current collector and dried to obtain a laminate composed of the negative electrode current collector and the negative electrode active material layer. This laminate is housed in a negative electrode member so that the negative electrode current collector and the inner surface of the negative electrode member are in contact with each other to obtain a negative electrode. In addition, when metallic lithium foil or the like is used, the itself may be used as the negative electrode.

Next, the positive electrode active material, conductive aid and binder are dispersed in a solvent to prepare a coating liquid. The obtained coating liquid is uniformly applied on the positive electrode current collector and dried to obtain a laminate composed of the positive electrode current collector and the positive electrode active material layer. A positive electrode is obtained by housing this laminate in a positive electrode member so that the positive electrode current collector and the inner surface of the positive electrode member are in contact with each other.

When using an electrolytic solution, the negative electrode and the positive electrode manufactured as described above are superimposed so that a separator is interposed between the negative electrode active material layer and the positive electrode active material layer, and the electrolytic solution is filled and sealed. A lithium ion secondary battery is completed by sealing the inside of the battery with the material.

On the other hand, when a solid electrolyte is used, for example, the raw material powder for the negative electrode is deposited to a uniform thickness to form a powder layer for the negative electrode, and on the powder layer for the negative electrode, a solid electrolyte layer containing the solid electrolyte powder is formed. The raw material powder is deposited to a uniform thickness to form a solid electrolyte layer powder layer, and on the solid electrolyte layer powder layer, the positive electrode raw material powder is deposited to a uniform thickness to form a positive electrode powder layer. After molding, these three layers are compression molded to obtain a powder laminate. A lithium ion secondary battery can be obtained using the obtained powder laminate. In addition, the solid electrolyte layer, the negative electrode, and the positive electrode can also be formed separately and laminated to obtain a lithium ion secondary battery.

The shape of the lithium ion secondary battery is not particularly limited, and may be cylindrical, square, coin, button, or the like.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.

(Example 1)
As the solute powder, lithium phosphate _Li3PO4 as a compound containing Li element and phosphate ion, cobalt chloride _CoCl2 as a compound containing Co element ₍ metal element M), and boron oxide B2 as a compound containing _B element. _O3 was prepared. These were weighed to give an elemental ratio of Li:Co:B:P=24:10:2:8. Lithium carbonate Li ₂ CO ₃ and lithium chloride LiCl were prepared as fluxes. The mixing ratio of the solute powder and the flux was adjusted to a weight ratio of 5:1. The weighed solute powder and flux were well mixed using a mortar and pestle to obtain a mixed powder.

The mixed powder was placed in a platinum crucible and placed in an atmosphere-controlled electric furnace. Then, the temperature was raised to 890° C. while general nitrogen was circulated in the electric furnace, and the temperature was maintained for 3 hours. Thus, the mixed powder was melted to obtain a melt. The oxygen concentration at the outlet of the furnace was several tens of ppm. The melt was then slowly cooled at 1°C/hr.

After cooling the melt to room temperature, the platinum crucible was removed. The content of the platinum crucible was washed with hot water to remove flux and the like, and only the sparingly soluble precipitate was collected by filtration.

When the crystal structure of the collected precipitate was confirmed by X-ray diffraction, the precipitate was identified as single crystal Li _1+α Co(B _x P _1-x )O ₄ .

(Example 2)
An experiment was conducted in the same manner as in Example 1, except that manganese chloride _MnCl2 was used as a compound containing Mn element instead of cobalt chloride, and a sparingly soluble precipitate was recovered.

When the crystal structure of the collected precipitate was confirmed by X-ray diffraction, the precipitate was identified as single crystal Li _1+α Mn(B _x P _1-x )O ₄ .

(Example 3)
An experiment was conducted in the same manner as in Example 1 except that iron chloride FeCl ₂ was used as the compound containing Fe element instead of cobalt chloride, and a sparingly soluble precipitate was recovered.

When the crystal structure of the collected precipitate was confirmed by X-ray diffraction, the precipitate was identified as single crystal Li _1+α Fe(B _x P _1-x )O ₄ .

(Example 4)
An experiment was conducted in the same manner as in Example 1, except that nickel chloride NiCl ₂ was used as the compound containing Ni element instead of cobalt chloride, and a sparingly soluble precipitate was recovered.

When the crystal structure of the recovered precipitate was confirmed by X-ray diffraction, the precipitate was identified as single crystal Li _1+α Ni(B _x P _1-x )O ₄ .

(Comparative example 1)
As the solute powder, lithium phosphate Li ₃ PO ₄ as a compound containing Li elements and phosphate ions, and cobalt chloride CoCl ₂ as a compound containing Co elements were prepared. These were weighed so that the element ratio was Li:Co:P=3:1:1. Lithium carbonate Li ₂ CO ₃ and lithium chloride LiCl were prepared as fluxes. A mixing ratio of the solute powder and the flux was adjusted to a weight ratio of 5:1. The weighed solute powder and flux were well mixed using a mortar and pestle to obtain a mixed powder.

An experiment was conducted in the same manner as in Example 1, except that the mixed powder thus prepared was used, and a poorly soluble precipitate was recovered.

When the crystal structure of the recovered precipitate was confirmed by X-ray diffraction, the precipitate was identified as single crystal LiCoPO ₄ .

(Evaluation of electrical conductivity)
The electrical conductivity of the two opposing surfaces of the single crystal (thickness 0.5-2 mm) obtained in each example was measured using a DC voltage generator (1 V-2.5 kV), a current measuring instrument (1-110 μA), and a control It was evaluated at room temperature using a system equipped with a commercial PC. As terminals for measurement, a platinum wire was used for the anode and an aluminum foil was used for the cathode. The results are shown in the figure. 1 to 4 are graphs showing the electrical conductivity evaluation results of the lithium metal phosphates according to Examples 1 to 4, respectively, and FIG. 5 is the electrical conductivity evaluation of the lithium metal phosphate according to Comparative Example 1. It is a graph which shows a result. In the figure, the dashed line indicates the applied voltage (V), and the solid line indicates the detected current (μA).

As shown in the figure, a constant current was observed in the example, whereas no current was observed in the comparative example. The electrical conductivity of the examples was overwhelmingly superior to that of the comparative examples.

As can be seen from the figure, the current due to ionic conduction appears when the pressure rises and then rapidly attenuates (Fig. 2). On the other hand, in the case of electron conduction, it changes according to the voltage according to Ohm's law (Fig. 3). Although ionic conduction and electronic conduction are mixed immediately after the application, it becomes constant in about 3 minutes after energization, and it is considered that the electric current of only electronic conduction is measured. If the electronic conductivity is higher than the ionic conductivity, it is assumed that ionic conduction does not occur and obeys Ohm's law from the beginning. It should be noted that when heat is generated by electron conduction and the temperature of the crystal rises, the electric resistance decreases and the current value rises, contrary to metals (Fig. 1).

Claims (5)

A lithium metal phosphate obtained by substituting a portion of P in a compound represented by the general formula LiMPO ₄ (wherein M is at least one selected from the group consisting of Fe, Co, Ni and Mn) with B . Lithium according to claim 1 , having the general formula Li _1+α M(B _x P _1-x )O ₄ , where x is 0.01-0.99 and α is greater than 0 and less than or equal to 2x. metal phosphate. 3. The lithium metal phosphate according to claim 1, which is a positive electrode material for lithium ion secondary batteries. A positive electrode material for a lithium ion secondary battery comprising the lithium metal phosphate according to claim 3 . A lithium ion secondary battery comprising a positive electrode comprising the positive electrode material according to claim 4 .

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