Method for manufacturing lithium metal phosphate, lithium metal phosphate, positive electrode material for lithium-ion secondary battery, positive electrode of lithium-ion secondary battery, and lithium-ion secondary battery

Abstract

The purpose of the present invention is to provide a method for preparing a lithium metal phosphate having a stable crystal structure and excellent electric capacity and electric conductivity.　The present invention pertains to a method for preparing a lithium metal phosphate having an olivine-type crystal structure, the method comprising: a mixing step for obtaining a mixture of a solute raw material and a flux, the solute raw material containing a compound containing element Li, a compound containing metal element M (M represents at least one selected from the group consisting of Fe, Co, Ni, and Mn), at least one among a compound containing element B and a compound containing element Si, and a pyrophosphate ion-containing compound containing element P; a melting step for obtaining a melt of the mixture; and a cooling step for cooling the melt to obtain precipitates. A tetraborate ion-containing compound containing element B may be used instead of or in addition to the pyrophosphate ion-containing compound containing element P.

Description

Technical Field

[0001] 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, a positive electrode of a lithium ion secondary battery, and a lithium ion secondary battery.

Background Art

[0002] LiCoO2 (LCO) is known as a positive electrode material for a lithium ion secondary battery. LCO has a high energy density (electromotive force × capacitance), but has problems with stability and lifespan. As a more stable material to replace this LCO, an olivine-type lithium metal phosphate represented by the general formula LiFePO4 has been put into practical use. On the other hand, Li2FeSiO4, which is similar to the lithium metal phosphate, is expected to have twice the capacitance because it has two Li in the molecule, but cannot be used as a positive electrode material because it is an insulator.

[0003] <A By the way, as transition metal elements (hereinafter referred to as "M" or "M element") that take a divalent state in the stoichiometric composition and can change their valence to trivalent to maintain charge neutrality with the desorption of Li, Fe, Co, Ni, and Mn are known (for example, Patent Document 1). However, as described in Patent Document 1, generally, the electrical conductivity of olivine-type lithium metal phosphates is low. Therefore, olivine-type lithium metal phosphates other than LiFePO4 (for example, LiCoPO4 and LiMnPO4) are not widely used (Patent Document 2).

[0004] Therefore, attempts have been made to improve the conductivity of this material by substituting a part of the pentavalent element P with a trivalent element such as B or Al and compensating for the charge by adding Li or oxygen defects. However, when the ratio of B to P is increased, spinel (M3O4), fonticite (M3BO5), metal borate {M3(BO3)2}, metal lithium borate, silicon Li 3-2α M αHeterogeneous phases such as (P,B)O4 (where part of the Li in the Li3PO4 structure is replaced by a divalent metal element) are formed, resulting in deteriorated electrical conductivity. Also, Li 1+x Fe(P 1-x ,B x )O4 also has heterogeneous phases mixed into the olivine phase during powder sintering, leading to deteriorated electrical conductivity (Patent Document 3). This phenomenon also occurs during powder sintering of Li 1+x M(P 1-x ,B x )O4, and the B substitution amount is significantly restricted (Patent Document 4). In the above composition formula, M represents a transition metal.

[0005] The following shows some side reactions that are not preferable from the perspective of reducing the purity of crystals when manufacturing olivine-type lithium metal phosphates to increase electrical conductivity and capacitance. In each composition formula, M represents a transition metal. 3Li2O + 3MCl2 + O → M3O4 + 6LiCl (formation of spinel) 3 / 2Li2O + 3MCl2 + Li3BO3 + 1 / 2(O) → M3BO5 + 6LiCl (formation of foscenite) Li3PO4 + xMCl2 → Li 3-2x MxPO 4+2x LiCl (formation of LISICON) Li3BO3 + MCl2 → LiMBO3 + 2LiCl (formation of lithium metal borate salt) 2Li3BO3 + 3MCl2 → M3(BO3)2 + 6LiCl (formation of metal borate salt)

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

[0007] [Non-Patent Document 1] Uehara: Manufacturing and evaluation of raw material powders for electrodes of secondary batteries, "Crushing" No. 56 (2013) pp18-24. [Overview of the project] [Problems that the invention aims to solve]

[0008] Generally, materials with a high proportion of Li in their composition can be expected to have high electrical capacitance. Furthermore, by substituting some of the P with other doping elements, improvements in electrical conductivity can also be expected. Therefore, based on the findings of Patent Documents 3 and 4, in order to increase the proportion of Li that contributes to electrical capacitance in olivine-type lithium metal phosphate, P 5+ B has a smaller ionic radius. 3+ Ya Si 4+ We considered substituting it with to increase the substitution rate. However, our investigations revealed that increasing the substitution rate tends to destabilize the olivine-type structure.

[0009] The present invention has been made in view of the above circumstances, and aims to provide a method for producing a lithium metal phosphate that has a stable crystal structure and excellent electrical capacity and electrical conductivity. The present invention also aims to provide a lithium metal phosphate that has a stable crystal structure and excellent electrical capacity and electrical conductivity, a positive electrode material for a lithium-ion secondary battery using the lithium metal phosphate, a positive electrode for a lithium-ion secondary battery, and a lithium-ion secondary battery. [Means for solving the problem]

[0010] In order to solve the above problems, the inventors conducted diligent research and found that by employing a high-temperature flux method, doping with boron and silicon to substitute for a portion of the phosphorus, and using pyrophosphate as at least a portion of the phosphate used as a raw material, or using tetraborate as at least a portion of the boron-doped material, a stable olivine-type structure with excellent capacitance and electrical conductivity can be obtained, thus completing the present invention.

[0011] In other words, the present invention provides a method for producing lithium metal phosphate having an olivine-type crystalline structure, comprising a mixing step to obtain a mixture of a solute raw material containing a compound containing the element Li; a compound containing the metal element M (where M represents at least one selected from the group consisting of Fe, Co, Ni, and Mn); at least one of a compound containing the element B and a compound containing the element Si; and a compound containing the element P containing a pyrophosphate ion, and a flux; a melting step to obtain a molten product of the mixture; and a cooling step to cool the molten product to obtain a precipitate.

[0012] In this case, the compound containing element B may also contain tetraborate ions.

[0013] Furthermore, the present invention provides a method for producing lithium metal phosphate having an olivine-type crystalline structure, comprising a mixing step to obtain a mixture of a solute raw material containing a compound containing the element Li; a compound containing the metal element M (where M represents at least one selected from the group consisting of Fe, Co, Ni, and Mn); a compound containing the element B containing a tetraborate ion; and a compound containing the element P containing a phosphate ion, and a flux; a melting step to obtain a molten product of the mixture; and a cooling step to cool the molten product to obtain a precipitate.

[0014] In this case, the solute raw material may further contain a compound containing the element Si.

[0015] In one embodiment, it is preferable that the melting temperature in the melting process is 600°C or higher.

[0016] In one embodiment, the ratio of element B to element P in the mixture is preferably 1 / 99 to 99 / 1.

[0017] In one embodiment, the ratio of Si elements to P elements in the mixture is preferably 1 / 99 to 99 / 1.

[0018] The present invention has an olivine-type crystal structure represented by the general formula LiM(P,Q)O4, and has an electrical conductivity of 10 -8 We provide lithium metal phosphates with a density of S / cm or higher. (In the formula, M represents at least one element selected from the group consisting of Fe, Co, Ni, and Mn. (P, Q) indicates that a portion of P is substituted with Q, where Q is at least one of B and Si.)

[0019] Furthermore, the present invention relates to the general formula Li 1+α M(P 1-x-y B x Si y The present invention provides a lithium metal phosphate having an olivine-type crystal structure represented by O4. (In the formula, M represents at least one element selected from the group consisting of Fe, Co, Ni, and Mn. Also, x is between 0 and 0.6, y is between 0 and 0.6, x + y = greater than 0.2 and less than or equal to 0.8, and α is between 0.4 and 1.2.)

[0020] In this case, the electrical conductivity is 10 -8 It may be S / cm or higher.

[0021] The present invention provides a positive electrode material for a lithium-ion secondary battery, comprising the above-mentioned lithium metal phosphate.

[0022] The present invention provides a positive electrode for a lithium-ion secondary battery, comprising the above-described positive electrode material.

[0023] The present invention provides a lithium-ion secondary battery comprising the above-described positive electrode. [Effects of the Invention]

[0024] According to the present invention, it is possible to provide a method for producing a lithium metal phosphate that has a stable crystal structure and excellent electrical capacity and conductivity. According to the present invention, it is possible to provide a lithium metal phosphate with a stable crystal structure and excellent electrical capacity and conductivity, a positive electrode material for a lithium-ion secondary battery using the lithium metal phosphate, a positive electrode for a lithium-ion secondary battery, and a lithium-ion secondary battery.

[0025] The lithium metal phosphate according to the present invention can have a crystal structure with a high Li ratio, thus offering excellent electrical capacitance, and also has a stable olivine-type crystal structure (also called an olivine structure), thus offering the potential for extended cathode life. [Brief explanation of the drawing]

[0026] [Figure 1] This is the powder X-ray diffraction chart for Example 1. [Figure 2] This is the powder X-ray diffraction chart for Example 2. [Figure 3] This is the powder X-ray diffraction chart for Example 3. [Figure 4] This is the powder X-ray diffraction chart for Example 4. [Figure 5] This is the powder X-ray diffraction chart for Example 5. [Figure 6] This is the powder X-ray diffraction chart for Example 6. [Figure 7] This is the powder X-ray diffraction chart for Example 7. [Figure 8] This is the powder X-ray diffraction chart for Example 8. [Figure 9] This is the powder X-ray diffraction chart for Comparative Example 1. [Figure 10] This is the powder X-ray diffraction chart for Comparative Example 2. [Figure 11] This graph shows the results of the electrical conductivity evaluation of lithium metal phosphate in Example 1. [Figure 12] This graph shows the results of the electrical conductivity evaluation of lithium metal phosphate salts in Example 2. [Figure 13] This graph shows the results of the electrical conductivity evaluation of lithium metal phosphate in Example 3. [Figure 14] This graph shows the results of the electrical conductivity evaluation of lithium metal phosphate in Example 4. [Figure 15] This graph shows the results of the electrical conductivity evaluation of lithium metal phosphate in Example 5. [Figure 16] This graph shows the results of the electrical conductivity evaluation of lithium metal phosphate in Example 6. [Figure 17] This graph shows the results of the electrical conductivity evaluation of lithium metal phosphate in Example 7. [Figure 18] This graph shows the results of the electrical conductivity evaluation of lithium metal phosphate in Example 8. [Figure 19] This graph shows the results of the electrical conductivity evaluation of lithium metal phosphate in Comparative Example 1. [Modes for carrying out the invention]

[0027] Several embodiments of the present invention will be described in detail below. However, the present invention is not limited to the following embodiments.

[0028] <Method for producing lithium metal phosphate> A method for producing lithium metal phosphate comprises a mixing step of obtaining a mixture of a raw material compound and a flux, a melting step of obtaining a molten mixture, and a cooling step of cooling the molten mixture to obtain precipitates.

[0029] The above manufacturing method requires employing a high-temperature flux method, doping with boron or silicon to substitute for a portion of the phosphorus, and making at least a portion of the P-element compound a compound containing pyrophosphate ions, or making at least a portion of the B-element compound used for boron doping a compound containing tetraborate ions. This makes it possible to obtain an olivine structure with a stable crystal structure while maintaining excellent capacitance and electrical conductivity.

[0030] (Mixing process) The mixing process can be carried out in two ways, from the viewpoint of suppressing the generation of metal oxides as impurities due to side reactions and improving the yield of lithium metal phosphate having an olivine structure.

[0031] In other words, the mixing step may be a step of obtaining a mixture of a solute raw material containing a compound containing the element Li; a compound containing the metal element M (where M represents at least one selected from the group consisting of Fe, Co, Ni, and Mn); at least one of a compound containing the element B and a compound containing the element Si; and a compound containing the element P containing a pyrophosphate ion, and a flux. In this case, the solute raw material may contain a compound containing the element B if the compound containing the element Li does not contain the element B (borate ion), a compound containing the element Si if the compound containing the element Li does not contain the element Si (silicate ion), and a compound containing the element P if the compound containing the element Li does not contain the element P (pyrophosphate ion). The compound containing the element B may also contain a tetraborate ion.

[0032] Furthermore, the mixing step may be a mixing step to obtain a mixture of a solute raw material containing a compound containing Li element; a compound containing a metal element M (where M represents at least one selected from the group consisting of Fe, Co, Ni, and Mn); a compound containing an element B containing a tetraborate ion; and a compound containing an element P containing a phosphate ion, and a flux. In this case, the solute raw material may contain a compound containing an element B containing a tetraborate ion if the compound containing Li element does not contain a tetraborate ion, and may contain a compound containing an element P containing a phosphate ion if the compound containing Li element does not contain a phosphate ion. The solute raw material may further contain a compound containing an element Si, and in this case, it may further contain a compound containing an element Si if the compound containing Li element does not contain an element Si (silicate ion).

[0033] The compounds containing the element Li are not particularly limited and include lithium carbonate, lithium acetate, lithium nitrate, lithium hydroxide, lithium phosphate, lithium pyrophosphate, lithium dihydrogen phosphate, lithium borate, lithium tetraborate, lithium silicate, lithium metasilicate, and their anhydrous or hydrated forms. Lithium pyrophosphate and lithium tetraborate are particularly preferred from the viewpoint of suppressing the generation of metal oxides as impurities due to side reactions and improving the yield of olivine.

[0034] Compounds containing the metallic element M are not particularly limited and include compounds containing the element Fe, such as iron oxalate, iron chloride, iron sulfate, iron nitrate, iron oxide, iron hydroxide, and their anhydrous or hydrate forms (the valency of Fe in the compound may be divalent or trivalent); compounds containing the element Co, such as cobalt carbonate, cobalt oxalate, cobalt chloride, and their anhydrous or hydrate forms; compounds containing the element Ni, such as nickel carbonate, nickel oxalate, nickel chloride, and their anhydrous or hydrate forms; and compounds containing the element Mn, such as manganese carbonate, manganese oxalate, manganese chloride, and their anhydrous or hydrate forms.

[0035] Compounds containing element B are not particularly limited and include boron oxide; compounds containing borate ions such as boric acid, boric anhydride, and lithium borate; and compounds containing tetraborate ions such as lithium tetraborate. In particular, from the viewpoint of suppressing the generation of metal oxides as impurities due to side reactions and improving the yield of olivine, it is preferable that at least a portion of the compound containing element B be a compound containing tetraborate ions, and lithium tetraborate is preferred as such a compound.

[0036] The compound containing the element Si is not particularly limited, and silicon dioxide, silica gel, lithium silicate, lithium metasilicate, etc., can be used. Of these, lithium silicate is preferred from the viewpoint of purity and reactivity.

[0037] Compounds containing element P include phosphate ions (PO4 3- Compounds containing ) and pyrophosphate ions (P2O7 4-Compounds containing phosphate ions are examples. Compounds containing phosphate ions are not particularly limited and include ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium phosphate, lithium dihydrogen phosphate, phosphoric acid, phosphorus pentoxide, and their anhydrides or hydrates. Compounds containing pyrophosphate ions include pyrophosphate, lithium pyrophosphate, and their anhydrides or hydrates. In particular, from the viewpoint of suppressing the generation of metal oxides as impurities due to side reactions and improving the yield of olivine, it is preferable that at least a portion of the compound containing element P be a compound containing pyrophosphate ions, and lithium pyrophosphate is preferred as such a compound.

[0038] Using compounds containing two or more elements from among Li, M, B, Si, and P faculties facilitates the mixing process. For example, lithium borate can be used as a compound containing Li and B. Lithium phosphate and lithium dihydrogen phosphate can also be used as compounds containing Li and phosphate ions. Thus, compounds containing Li, compounds containing metal element M, compounds containing B, compounds containing Si, and compounds containing P can be prepared and used separately. For example, a compound containing Li and P, and compounds containing metal element M, compounds containing B, and compounds containing Si can be prepared.

[0039] This section describes the advantages of converting some or all of the phosphate ions into pyrophosphate ions in compounds containing element P. For example, when pyrophosphate is used as a compound containing element P, pyrophosphate (H4P2O7) reacts with an equivalent amount of Li2O to produce Li4P2O7 and water. However, if an excess of Li is added beyond the amount required to produce a pure phosphate, the pyrophosphate (H4P2O7) reacts with the excess Li2O to produce 2Li3PO4. In this case, the chemical reactions of the active state Li3PO4, the divalent transition metal salt, and the excess lithium are accelerated because the activation energy decreases. 1+αThis results in the formation of M(P,Q)O4. This suppresses the formation of metal oxides such as spinel, phonsenite, and lithicone as impurities due to side reactions, thereby improving the yield of olivine.

[0040] In compounds containing element B, the advantages of converting some or all of the borate ions to tetraborate ions are similar to the advantages of converting some or all of the phosphate ions to pyrophosphate ions. That is, for example, when lithium tetraborate is used as the compound containing element B, lithium tetraborate (Li2B4O7) reacts with excess Li2O to produce 4LiBO2. This active LiBO2 suppresses the formation of metal oxides as impurities through side reactions and improves the yield of olivine.

[0041] From the viewpoint of more easily suppressing the formation of metal oxides as impurities due to side reactions, it is also possible to use a combination of converting some or all of the phosphate ions to pyrophosphate ions and converting some or all of the borate ions to tetraborate ions.

[0042] The elemental ratio (molar ratio) of element M to elements P, B, and Si in the weighing of the raw materials for each of the above compounds can be 0.8 to 1.2:1. From the viewpoint of the theoretical ratio of lithium metal phosphates having an olivine-type crystal structure, this ratio may also be 1:1.

[0043] The elemental ratio of element B to element P (element B / element P) can be 1 / 99 to 99 / 1 or 10 / 90 to 99 / 1. However, from the viewpoint of obtaining a desired crystal structure and exhibiting better electrical conductivity and capacitance, the ratio can be 15 / 85 to 50 / 50, or 10 / 90 to 30 / 70. The elemental ratio of Si to P (Si / P) can be 1 / 99 to 99 / 1 or 10 / 90 to 99 / 1. However, from the viewpoint of obtaining a desired crystal structure and exhibiting better electrical conductivity and capacitance, the ratio can be 15 / 85 to 50 / 50, or even 10 / 90 to 30 / 70.

[0044] The amounts of P, B, and Si are adjusted so that Li is greater. While the stoichiometric ratio of lithium metal phosphate represented by LiFePO4 is 1:1 for Li to P, adjusting the amounts of P, B, and Si allows the ratio of Li to (P+B+Si) to be increased to approximately 2:1 during the subsequent melting and cooling processes. Adjusting the amount of Li is also necessary due to the evaporation loss of Li and the complete decomposition of pyrophosphate ions into phosphate ions. Excess lithium promotes olivine formation and suppresses the generation of other phases. Excess lithium is removed during the washing process to separate the single crystals. For example, the reaction equation when lithium pyrophosphate is used is as follows, and since pyrophosphate becomes the active phosphate, the reaction that produces olivine becomes dominant. Li4P2O7 + Li2O → 2Li3PO4 1 / 2Li3PO4+1 / 2Li3BO3+MCl2+1 / 2Li2O→Li2M(P0 .5 ,B 0.5 )O4+2LiCl

[0045] Regarding the use of excess lithium, specifically, when mixing a compound containing Li, a compound containing metal element M, a compound containing B, a compound containing Si, and a compound containing P, the elemental ratio of Li to B, Si, and P (Li / (B + Si + P)) is preferably 1.4 to 2.2, and more preferably 1.5 to 2.0, from the viewpoint of maintaining electrical neutrality and easily obtaining a stable olivine structure.

[0046] Flux is an inorganic compound used in the high-temperature flux method, a type of crystallization method, and acts as a solvent to dissolve the target crystal. The high-temperature flux method utilizes the property that the solubility of the solute crystal in a solvent changes with temperature. By using a flux that has high solubility at high temperatures and decreases solubility as the temperature decreases, the target substance will exceed the solubility in the flux as it cools, resulting in a supersaturated state where the target substance precipitates as crystals. As a flux, a compound that melts at a lower temperature than the target crystal should be selected. Examples of fluxes include lithium chloride, lithium carbonate, lithium fluoride, lithium vanadate, sodium dihydrogen phosphate, lead fluoride, lead oxide, bismuth oxide, molybdate, and tungstate, which can be used individually or in combination of two or more. Furthermore, when there are multiple solute components, a self-fluxing method can be used, in which a specific solute component is increased to intentionally shift its component ratio from the desired ratio (stoichiometric ratio), thereby allowing that solute component to also function as a flux. In other words, compounds containing excess Li element can also be used as self-fluxes.

[0047] In the mixing step, each solute compound is mixed with the flux to obtain a mixture. Each compound is weighed appropriately to obtain a lithium metal phosphate of the desired composition. The mixing ratio of each compound to the flux is preferably 1:0.1 to 1:1, and more preferably 1:0.2 to 1:0.5, from the viewpoint of the dissolution temperature and solubility of the raw materials. The mixing method may be either a dry mixing method or a wet mixing method. Specifically, methods such as mechanically mixing each raw material using a mortar and pestle or ball mill, a coprecipitation method in which each raw material is dissolved in water and then precipitated and mixed, and a sol-gel method in which the sols obtained by dissolving each raw material are gelled and mixed can be used.

[0048] (Melting process) In the melting process, the mixture obtained in the mixing process is melted. The mixture is placed in a designated container, such as a platinum crucible, as needed, and introduced into a firing furnace, where it is melted in an inert atmosphere or an inert atmosphere containing a small amount of oxygen. Examples of an inert atmosphere include an atmosphere purged with argon gas, nitrogen gas, helium gas, etc. These inert gases may contain less than 100 ppm of oxygen. In the melting process, the molten material refers to a state in which the flux added as a solvent has completely melted, and most of the solute (compounds containing Li, compounds containing the metal element M, compounds containing the element B, compounds containing the element Si, and compounds containing the element P, including phosphate ions and pyrophosphate ions) has melted in the flux. The molten material may contain unmelted solute.

[0049] The melting temperature in the melting process can be set to a temperature at which most of the solute melts in the molten flux, or at which the entire amount of solute melts. For example, melting a mixture containing SiO2 without using flux requires a high temperature of 1000°C or higher, but using flux allows for melting at a lower temperature. The melting temperature is not necessarily limited as it depends on the composition of the mixture, but since the melting point of lithium chloride, the main flux component, is 605°C, it can be at least 600°C, may be 650°C or higher, or may be 700°C or higher. The upper limit of the melting temperature can be 1000°C or lower, or may be 900°C or lower, from the viewpoint of suppressing the decomposition and evaporation of the raw materials and lithium compounds. The holding time at the melting temperature can be set to the time required for a certain amount of solute to melt, for example, it can be at least 3 hours, may be 4.5 hours or higher, or may be 6 hours or higher. The upper limit of the holding time can be 24 hours or lower, or may be 12 hours or lower, from the viewpoint of suppressing the decomposition and evaporation of the raw materials and lithium compounds.

[0050] If the flux is not completely melted during the melting process, or if most of the solute does not dissolve in the flux, there is a risk that unreacted raw materials and impurities such as flux components will be mixed into the lithium metal phosphate with an orthorhombic olivine-type crystal structure obtained after cooling, leading to compound formation. These impurities are compounds that are produced when a simple calcination is performed on the mixture, and they can reduce the electrical conductivity of the lithium metal phosphate with an olivine-type crystal structure. Here, calcination refers to a method that does not use flux, where the raw materials are heated to a high temperature that does not completely melt them to cause a reaction between the raw materials and obtain a calcined product with a composition corresponding to the raw materials. In calcination, the reaction proceeds more easily due to solid-phase diffusion, which can leave unreacted parts, and the elemental distribution in the product tends to become non-uniform. Also, since calcination is a solid-phase reaction, it is practically impossible to produce crystals large enough to be separated. Therefore, when calcining without flux, it is necessary to repeat calcination and disintegration / remixing many times in order to create a homogeneous material with a complex structure and many different elements.

[0051] From the viewpoint of suppressing the generation of impurities such as unreacted raw materials and flux components, a calcination step may be performed prior to the melting step. In the calcination step, the mixture is calcined at approximately 400 to 600°C to obtain a calcined product. This calcined product can then be used in the melting step.

[0052] (cooling process) The resulting molten material is cooled to room temperature in this process. While the cooling rate is fast, it is preferable to maintain a rate of 5°C / hour or less until solidification occurs. Cooling too quickly may cause defects to form within the crystal.

[0053] The recovered material (cooled material) obtained after the cooling step may contain not only precipitates of lithium metal phosphate having the desired olivine-type crystal structure, but also other components such as flux components, unreacted materials, and by-reactants. For example, by washing the recovered material with warm water (such as pure water at 60°C) and filtering out only the sparingly soluble crystals, it is possible to visually select only the lithium metal phosphate having the desired olivine-type crystal structure from the remaining recovered material. From this viewpoint, the manufacturing method of this embodiment may include a washing step to wash the recovered material obtained in the cooling step. The fact that the obtained crystals are lithium metal phosphate having an olivine-type crystal structure can be confirmed by X-ray diffraction or the like.

[0054] <Lithium metal phosphate> The lithium metal phosphate of this embodiment is obtained by substituting a portion of the P in a compound represented by the general formula Li2MPO4 (wherein M is at least one selected from the group consisting of Fe, Co, Ni, and Mn) with B, Si, or B and Si. The lithium metal phosphate has an olivine structure.

[0055] Generally, the olivine structure has a hexagonal, close-packed oxygen skeleton, with P ions at the tetrahedral sites and Li ions and transition metal ions M at the octahedral sites. In the lithium metal phosphate of this embodiment, some of the pentavalent P ions are replaced by trivalent B ions and tetravalent Si ions. The valency of M is +2 in the stoichiometric composition, and can change to +2 to +4 in order to maintain neutral charge as Li is removed.

[0056] Such lithium metal phosphates can be called boron-silicon doped lithium metal phosphates, or boron-phosphorus doped lithium silicate Li2M(P,B,Si)O4.

[0057] The lithium metal phosphate of this embodiment has the general formula Li 1+α M(P 1-x-y B x Si y It has an olivine-type crystal structure represented by O4. In the above formula, M represents at least one selected from the group consisting of Fe, Co, Ni, and Mn. Also, x is from 0 to 0.6, y is from 0 to 0.6, x + y > 0.2 and ≤ 0.8, and α is from 0.4 to 1.2.

[0058] x may be 0, but when x > 0, excellent capacitance and electrical conductivity can be exhibited. From this perspective, x is preferably 0.3 or more, and more preferably 0.4 or more. When x ≤ 0.6, it is difficult for a heterogeneous phase to occur and the yield is improved. From this perspective, x is preferably 0.5 or less.

[0059] y may be 0, but when y > 0, excellent capacitance and electrical conductivity can be exhibited. From this perspective, y is preferably 0.2 or more, and more preferably 0.3 or more. When y ≤ 0.6, the electrical conductivity is improved. From this perspective, y is preferably 0.5 or less. In addition, from the perspective of being easily able to suppress the occurrence of a heterogeneous phase, it is preferable that x < y.

[0060] When x + y > 0.2, excellent capacitance and electrical conductivity can be exhibited. From this perspective, x + y is preferably 0.3 or more, and more preferably 0.4 or more. When x + y ≤ 0.8, it is difficult for a heterogeneous phase to occur and the yield is improved. From this perspective, x + y is preferably 0.7 or less.

[0061] When α ≥ 0.4, excellent capacitance and electrical conductivity can be exhibited. From this perspective, α is preferably 0.5 or more. When α ≤ 1.2, it is difficult for a heterogeneous phase to occur and the yield is improved. From this perspective, α is preferably 1 or less.

[0062] The above lithium metal phosphate has excellent electrical conductivity and therefore has an olivine-type crystal structure represented by the general formula LiM(P,Q)O4, with an electrical conductivity of 10 -8 It can also be described as a lithium metal phosphate with an electrical conductivity of S / cm or higher. Here, the electrical conductivity of LiFePO4 is 10 -8 This value is measured in accordance with the standard of / cm. In the formula, M represents at least one element selected from the group consisting of Fe, Co, Ni, and Mn. (P, Q) indicates that a portion of P is substituted with Q, where Q is B or B and Si.

[0063] The reason why lithium metal phosphates doped with boron or silicon and having an olivine-type crystal structure exhibit improved electrical conductivity is unclear, but the inventor speculates as follows.

[0064] First, transition metals exist in a high-temperature molten state in equilibrium with oxygen in the atmosphere, and in the presence of trace amounts of oxygen, divalent and trivalent transition metal ions coexist. In the crystal structure, the crystal radius of the four-coordinate site is Si 4+ >P 5+ >B 3+ The largest value is Si at 0.40, and the crystal radius of the 6-coordinate site of the transition metal is 0.75-0.83, so it is not usually conceivable that B or Si would occupy the 6-coordinate site occupied by the transition metal. If Si or B substitutes into the position of P, which is 4-coordinate in the crystal structure, and the atmosphere is reducing with an excess of Li, then more Li ions will be added to the 6-coordinate position for charge compensation. Alternatively, if the atmosphere is somewhat oxidizing, a trivalent transition metal will be present. If a trivalent transition metal ion is present in the crystal, it is thought that electron transfer occurs between the divalent and trivalent transition metal ions, resulting in the emergence of electronic conductivity.

[0065] Furthermore, Li2FeSiO4, in which all four coordination sites are Si, is known to have very poor electrical conductivity. This is because Si is strongly and rigidly bonded to the surrounding oxygen, resulting in a lack of flexibility in the crystal structure. +This is presumed to be because ions are less likely to move. Generally, the melting point of a solid solution is lower than that of a pure substance because the distortion of the crystal lattice weakens the bonding force between the constituent cations and oxygen ions. In solid solutions where elements other than Si are mixed in the 4-coordinate sites, the bonding force with oxygen is weaker compared to a pure silicate, and it is thought that Li ions corresponding to the change in the valence of the transition metal move more easily. This is thought to be the same for lithium metal phosphate salts, and therefore the electron mobility is thought to increase. It is presumed that ionic conduction and electron conduction complementarily contribute to the improvement of electrical conductivity.

[0066] Furthermore, when some of the Si in the insulator Li2SiO4 is replaced with P, electrical conductivity appears, and when all of it is replaced with P, it becomes LiFePO4. Replacing some of the P in LiFePO4 with B further increases electrical conductivity, but the crystal structure becomes unstable and it becomes difficult to maintain the olivine structure. When replacing P in LiFePO4 with Si, the crystal structure is not as distorted as in the case of B, and when some of the P is replaced with Si and B, the solid solution region of B increases, so the electrical conductivity increases even though it is olivine. In other words, when the types of elements in the 4-coordination site are mixed in a solid solution, the bond with oxygen weakens and Li + This is thought to make it easier for ions to move.

[0067] The olivine structure originates from the naturally occurring silicate mineral olivine X2SiO4 structure (where X represents divalent metals Mg and Fe in a ratio of approximately 9:1). In X2SiO4, oxygen is packed almost hexagonally, with eight gaps for every four oxygen atoms, and Si occupies 1 / 8 of these gaps. Similarly, there are four gaps for every six oxygen atoms, and X occupies 1 / 2 of these gaps. On the other hand, in Li2MSiO4 (M=Fe,Co,Ni,Mn) with added P and B, P and B replace some of the Si, and X is replaced by two Li atoms and M. When equivalent amounts of P and B are substituted into Li2MSiO4, the ratio of Li in the molecule remains unchanged because tetravalent Si is replaced by pentavalent P and trivalent B.

[0068] <Lithium-ion rechargeable battery> A lithium-ion secondary battery comprises a positive electrode containing a positive electrode material made of the lithium metal phosphate mentioned above. More specifically, a lithium-ion secondary battery comprises the positive electrode, a negative electrode, an electrolyte, and the like.

[0069] (positive electrode) The positive electrode may include, in addition to the positive electrode material described above, a conductive additive, a binder, and the like.

[0070] The conductive additive is not particularly limited and includes acetylene black, carbon black, graphite, carbon fiber, metal fiber, aluminum powder, carbon fluoride, zinc oxide, potassium titanate, titanium dioxide, polyphenylene derivatives, etc. These can be used individually or in combination of two or more.

[0071] The binder is not particularly limited and examples include polytetrafluoroethylene, polyvinylidene plu-olide, polyvinyl chloride, and ethylene propylene diene polymer. These can be used individually or in combination of two or more.

[0072] (Negative electrode) The negative electrode may consist of the negative electrode active material itself, or it may contain the negative electrode active material and a binder. That is, the negative electrode may consist of metallic lithium, lithium-aluminum alloy, lithium-tin alloy, or it may contain graphite, carbon fiber, coke, mesocarbon microbeads (MCMB), etc., and a binder.

[0073] (electrolyte) The electrolyte may be in liquid or solid form.

[0074] When the electrolyte is in liquid form (i.e., an electrolyte solution is used), a solution of the supporting electrolyte dissolved in an organic solvent can be used. The organic solvent is not limited, but examples include carbonates, halogenated carbohydrates, ethers, ketones, nitriles, lactones, and oxolane compounds. These can be used individually or in combination of two or more.

[0075] The supporting electrolyte is not particularly limited and includes inorganic salts such as LiPF6, LiBF4, LiClO4, and LiAsF6, organic salts such as LiSO3CF3, LiC(SO3CF3)2, and LiN(SO2CF3)(SO2C4F9), and their derivatives.

[0076] When using an electrolyte solution, a porous synthetic resin membrane (especially a porous membrane of polyolefin molecules) can be used as a separator.

[0077] When the electrolyte is in solid form (i.e., a solid electrolyte is used), compounds such as oxide-based and sulfide-based compounds can be used. Examples of oxide-based compounds include La 0.5 1Li0 .34 TiO 2.94 , NASICON type Li 1.3 Al 0.3 Ti 1.7 (PO4), Garnet-type LI7La3Zr2O 12 Examples of such materials include sulfide-based compounds such as two-component systems like Li2S-SiS2, and three-component systems to which LiI, LI3PO4, etc., are added.

[0078] Lithium-ion rechargeable batteries are manufactured, for example, as follows:

[0079] A coating solution is prepared by dispersing the negative electrode active material and binder in a solvent. The obtained coating solution is uniformly applied to the negative electrode current collector and dried to obtain a laminate consisting of the negative electrode current collector and the negative electrode active material layer. This laminate is placed inside the negative electrode member so that the negative electrode current collector and the inner surface of the negative electrode member are in contact to obtain the negative electrode. If metallic lithium foil or the like is used, it can be used as the negative electrode itself.

[0080] Next, a coating solution is prepared by dispersing the positive electrode active material, conductive additive, and binder in a solvent. The obtained coating solution is uniformly applied to the positive electrode current collector and dried to obtain a laminate consisting of the positive electrode current collector and the positive electrode active material layer. This laminate is then placed inside a positive electrode member so that the positive electrode current collector and the inner surface of the positive electrode member are in contact to obtain the positive electrode.

[0081] When using an electrolyte, the negative electrode and positive electrode manufactured as described above are stacked with a separator in between the negative electrode active material layer and the positive electrode active material layer, the electrolyte is filled, and the inside of the battery is sealed with a sealing material to complete the lithium-ion secondary battery.

[0082] On the other hand, when using a solid electrolyte, for example, raw material powder for the negative electrode is deposited to a uniform thickness to form a negative electrode powder layer, raw material powder for the solid charge layer containing solid electrolyte powder is deposited to a uniform thickness on the negative electrode powder layer to form a solid electrolyte powder layer, and raw material powder for the positive electrode is deposited to a uniform thickness on the solid electrolyte powder layer to form a positive electrode powder layer. These three layers are then compressed and molded to obtain a powder laminate. A lithium-ion battery can be obtained using the obtained powder laminate. Alternatively, the solid electrolyte, negative electrode, and positive electrode can be molded separately and then laminated to obtain a lithium-ion secondary battery.

[0083] There are no particular restrictions on the shape of lithium-ion secondary batteries, and examples include cylindrical, prismatic, coin-shaped, and button-shaped batteries. [Examples]

[0084] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples.

[0085] (Example 1: Li2(Fe 0.5、 Mn 0.5 )(P 0.33、 B 0.33、 Si 0.3 3) Preparation of O4) As compounds containing elements Li and P, lithium pyrophosphate Li4P2O7 (a mixture with the excess lithium carbonate Li2CO3) was prepared by reacting commercially available pyrophosphate with an excess amount of lithium carbonate Li2CO3; as compounds containing the metal element M, iron chloride FeCl2 and manganese chloride MnCl2; as a compound containing element B, lithium borate Li4B2O5; and as a compound containing element Si, Li2SiO3 were prepared as solute powders. These were weighed to achieve an elemental ratio (molar ratio) of Li:Fe:Mn:P:B:Si = 40:9:9:6:6:6. Lithium carbonate and lithium chloride were also prepared as fluxes. The mixing ratio of solute powders to fluxes was adjusted to a mass ratio of 5:1. The weighed solute powders and fluxes were thoroughly mixed using a mortar and pestle to obtain a mixed powder.

[0086] The mixed powder was placed in a platinum crucible and left to stand in an atmosphere-controlled electric furnace. The furnace was then heated to 890°C while circulating general nitrogen, and maintained at that temperature for 3 hours. This melted the mixed powder, yielding a molten material. The oxygen concentration at the furnace outlet was several tens of ppm. The molten material was then slowly cooled at 1°C / hr.

[0087] After the molten material was cooled to room temperature, the platinum crucible was removed. The contents of the platinum crucible were washed with warm water to remove flux and other contaminants, and sparingly soluble crystals were filtered out to isolate and recover only granular, dark brown crystals with facets larger than 1 mm in size. This allowed for the pure isolation of the target substance.

[0088] (Example 2: Li2(Fe 0.5、 Co 0.5 )(P 0.33、 B 0.33、 Si 0.3 3) Preparation of O4) As compounds containing the metal element M, iron chloride (FeCl2) and cobalt chloride (CoCl2) were prepared. Each compound was then weighed so that the elemental ratio was Li:Fe:Co:P:B:Si = 40:9:9:6:6:6. The experiment was carried out in the same manner as in Example 1, except for this difference, and black, sparingly soluble granular crystals were recovered.

[0089] (Example 3: Li2(Fe 0.33、 Mn 0.33、 Co 0.33 )(P 0.33、 B0 .33、 Si 0.33 (Preparation of O4) As compounds containing the metal element M, iron chloride (FeCl2), manganese chloride (MnCl2), and cobalt chloride (CoCl2) were prepared. Each compound was then weighed so that the elemental ratio was Li:Fe:Co:Mn:P:B:Si = 40:6:6:6:6:6:6. The experiment was carried out in the same manner as in Example 1, except for this difference, and black, sparingly soluble granular crystals were recovered.

[0090] (Example 4: Li2Co(P 0.5、 B 0.5 (Preparation of O4) Cobalt chloride (CoCl2) was prepared as a compound containing the metal element M. Li2SiO3 was not used as a compound containing the element Si. Each compound was weighed so that the elemental ratio was Li:Co:P:B = 27:10:5:5. The experiment was carried out in the same manner as in Example 1, except for this difference, and black, sparingly soluble granular crystals were recovered.

[0091] (Example 5: Li2(Fe 0.5、 Mn 0.5) (P 0.5、 B 0.5 (Preparation of O4) As compounds containing the metal element M, iron chloride (FeCl2) and manganese chloride (MnCl2) were prepared. Li2SiO3 was not used as a compound containing the element Si. Each compound was weighed so that the elemental ratio was Li:Fe:Mn:P:B = 27:5:5:5:5. The experiment was carried out in the same manner as in Example 1, except for this difference, and black, sparingly soluble granular crystals were recovered.

[0092] (Example 6: Li2(Fe 0.5、 Mn 0.5 )(P 0.5、 B 0.5 (Preparation of O4) Lithium phosphate (Li3PO4) containing Li and P elements, iron chloride (FeCl2) and manganese chloride (MnCl2) containing the metallic element M, and lithium tetraborate (Li2B4O7) containing the element B were prepared as solute powders. These were weighed to achieve an elemental ratio of Li:Fe:Mn:P:B = 24:5:5:5:5 for the crystal. Lithium chloride (LiCl) was prepared as the flux. The mixing ratio of the solute powder to the flux was adjusted to a mass ratio of 5:1. The weighed solute powder and flux were thoroughly mixed using a mortar and pestle to obtain a mixed powder. The experiment was carried out in the same manner as in Example 1, except for this step, and black, sparingly soluble granular crystals were recovered.

[0093] (Example 7: Li 1.5 Co(P 0.5、 Si 0.5 (Preparation of O4) Lithium pyrophosphate (Li4P2O7), a compound containing Li and P, cobalt chloride (CoCl2), a compound containing the metallic element M, and SiO2, a compound containing Si, were prepared as solute powders. These were weighed to achieve the elemental ratio of Li:Co:P:Si = 22:10:5:5 for the crystal. Lithium chloride (LiCl) was prepared as the flux. The mixing ratio of the solute powder to the flux was adjusted to a weight ratio of 5:1. The weighed solute powder and flux were thoroughly mixed using a mortar and pestle to obtain a mixed powder. The experiment was carried out in the same manner as in Example 1, except for this step, and black, sparingly soluble granular crystals were recovered.

[0094] (Example 8: Li 1.5 Mn(P 0.75、 B 0.25 (Preparation of O4) Lithium pyrophosphate (Li4P2O5) containing Li and P elements, manganese chloride (MnCl2) containing the metallic element M, and boron oxide (B2O3) containing the element B were prepared as solute powders. These were weighed to achieve the elemental ratio of Li:Mn:P:B = 10:4:3:1 for the crystal. Lithium carbonate (Li2CO3) and lithium chloride (LiCl) were prepared as fluxes. The mixing ratio of solute powders to fluxes was adjusted to a mass ratio of 5:1. The weighed solute powders and fluxes were thoroughly mixed using a mortar and pestle to obtain a mixed powder. The experiment was carried out in the same manner as in Example 1, except for this step, and black, sparingly soluble granular crystals were recovered.

[0095] (Comparative Example 1: Preparation of LiFePO4) Lithium phosphate (Li3PO4), a compound containing the element Li and phosphate ions, and iron chloride (FeCl2), a compound containing the metal element M, were prepared as solute powders. These were weighed to achieve an elemental ratio of Li:Fe:P = 1:1:1, which is the ratio of crystals. Lithium carbonate (Li2CO3) and lithium chloride (LiCl) were prepared as fluxes. The mixing ratio of solute powders to fluxes was adjusted to a mass ratio of 5:1. The weighed solute powders and fluxes were thoroughly mixed using a mortar and pestle to obtain a mixed powder. The experiment was carried out in the same manner as in Example 1, except for this step, and sparingly soluble granular crystals were obtained.

[0096] (Comparative Example 2) Lithium phosphate was prepared instead of lithium pyrophosphate. Each compound was then weighed so that the elemental ratio was Li:Co:B:P = 27:10:5:5. The experiment was carried out in the same manner as in Example 1, except for this difference, and bluish-black, sparingly soluble granular crystals were recovered.

[0097] (Powder X-ray diffraction analysis) The precipitates recovered in Examples 1-8 were examined using X-ray diffraction (characteristic X-ray: CuKα) to confirm their crystal structure, and the precipitates (granular crystals) were identified as single crystals with an olivine structure. The precipitate recovered in Comparative Example 1 was identified as a single crystal of LiFePO4 by X-ray diffraction, which confirmed its crystal structure. X-ray diffraction was used to confirm the crystal structure of the precipitate recovered in Comparative Example 2, and it was identified as a mixture of cobalt borate and olivine. Since compounds containing pyrophosphate ions or tetraborate ions were not used, it is thought that the olivine structure became unstable as the proportion of B increased, and other phases besides olivine were formed.

[0098] Figures 1-10 show the powder X-ray diffraction charts for Examples 1-8 and Comparative Examples 1-2, respectively.

[0099] (Evaluation of electrical conductivity) The electrical conductivity of two opposing faces of the crystals (thickness 0.5-2 mm) obtained in each example was evaluated at room temperature using a system equipped with a DC voltage generator (1V-2.5KV), a current meter (1-110μA), and a control PC. A φ0.5 mm platinum wire was used as the anode and an aluminum foil as the cathode for measurement. The results are shown in the figures. Figures 11-19 are graphs showing the electrical conductivity evaluation results of lithium metal phosphates for Examples 1-8 and Comparative Example 1, respectively. In the figures, the dashed line represents the applied electric field (vertical axis left: V / mm), and the solid line represents the detected current (vertical axis right: μA). The horizontal axis of the graph represents elapsed time (×0.1 seconds).

[0100] In Examples 1-8, M in the lithium metal phosphate is at least one of Fe, Mn, and Co, and B or Si is solid-dissolved in P. As a result, electrical conductivity comparable to or better than that of the LiFePO4 single crystal in Comparative Example 1 was obtained while maintaining an olivine structure. Comparing the minimum electric field strength at which a current of 1 μA or more flows after 5 minutes, it was 600 V / mm in Comparative Example 1, and in the Examples ranged from 300 to 1200 V / mm. According to Non-Patent Literature 1, the electrical conductivity of LiFePO4 is 10 -8 Considering that it is on the order of S / cm, if the minimum electric field strength at which a current of 1 μA or more flows after 5 minutes is within the above range, then it is equivalent to LiFePO4. -8It can be concluded that electrical conductivity on the order of S / cm is achieved.

[0101] Table 1 summarizes the compositions of Examples 1-8 and Comparative Example 1, as well as the applied electric field (electric field strength) and detected current values.

[0102] [Table 1]

[0103] LiCoPO4 and LiMnPO4 without B or Si substitution could not detect any current even at the upper limit of the DC voltage generator's applied voltage of 2500V. On the other hand, the lithium metal phosphates of the above examples, which detected current at voltages equivalent to or lower than LFePO4, had electrical conductivity equivalent to LiFePO4 and approximately 10 -8 It is thought to have a density on the order of S / cm, and like LiFePO4, it is considered suitable for use as a positive electrode material in lithium-ion secondary batteries. Furthermore, the lithium metal phosphate in the above example contains more B and Si, resulting in a higher Li content than LiMPO4. This is expected to increase the electrical capacity (if equal amounts of P and B are present, the Li content is twice that of LiFePO4). Furthermore, the lithium metal phosphate in the above embodiment contains Co or Mn instead of Fe as the metal element, and in this case as well, the electrical conductivity is equivalent to that of LiFePO4. As a result, the output voltage increases, and overall the energy density (output voltage per gram × capacitance) far exceeds that of LiFePO4, and may even surpass that of LCO. Furthermore, since the lithium metal phosphate in the above embodiment can have an olivine structure, its crystal structure is less likely to break down even after repeated charging and discharging, and its lifespan and stability are considered to be superior to that of LCO. [Industrial applicability]

[0104] According to the present invention, a lithium metal phosphate having an olivine-type crystal structure with excellent electrical conductivity can be obtained. The lithium metal phosphate of the present invention has a stable olivine structure and therefore has a long lifespan, and because the P in the crystal is substituted with Si or B, the ratio of Li is high, and a larger electrical capacity can be expected compared to LiFePO4.

[0105] Furthermore, the manufacturing method of the present invention makes it possible to produce lithium metal phosphate having an olivine-type crystal structure. Moreover, the lithium metal phosphate having an olivine-type crystal structure according to the present invention can be used as a positive electrode material for lithium-ion secondary batteries, and it is possible to provide a lithium-ion secondary battery that has a higher energy density and electrical capacity than LCO, and has the same safety and lifespan as LFP.

Claims

1. A mixing step to obtain a mixture of a solute raw material containing a compound containing the element Li; a compound containing the metal element M (where M represents at least one selected from the group consisting of Fe, Co, Ni, and Mn); at least one of a compound containing the element B and a compound containing the element Si; and a compound containing the element P containing a pyrophosphate ion, and a flux, A melting step to obtain a molten product of the mixture, A cooling step in which the molten material is cooled to obtain precipitates, A method for producing lithium metal phosphate having an olivine-type crystal structure.

2. A mixing step to obtain a mixture of a solute raw material containing a compound containing the element Li; a compound containing the metal element M (where M represents at least one selected from the group consisting of Fe, Co, Ni, and Mn); a compound containing the element B containing a tetraborate ion; and a compound containing the element P containing a phosphate ion, and a flux, A melting step to obtain a molten product of the mixture, A cooling step in which the molten material is cooled to obtain precipitates, A method for producing lithium metal phosphate having an olivine-type crystal structure.

3. The method for producing the material according to claim 1, wherein the compound containing element B contains a tetraborate ion.

4. The manufacturing method according to claim 2, wherein the solute raw material further comprises a compound containing the element Si.

5. The manufacturing method according to any one of claims 1 to 4, wherein the melting temperature in the melting step is 600°C or higher.

6. The manufacturing method according to any one of claims 1 to 5, wherein the ratio of element B to element P in the mixture is 1 / 99 to 99 / 1.

7. The manufacturing method according to claim 1 or 4, wherein the ratio of Si element to P element in the mixture is 1 / 99 to 99 / 1.

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