JP2012104240A - Method of producing lithium borate based compound - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of producing a lithium borate based material useful as the positive electrode material for a lithium ion secondary battery in which the cycle characteristics, capacity, and the like at around a room temperature are improved and excellent performance is ensured.SOLUTION: In the method of producing a lithium borate based compound, a lithium-containing molten salt raw material containing at least lithium nitrate, a transition metal-containing raw material containing pure iron, pure manganese and at least one kind selected from a group consisting of compounds containing iron and/or manganese, and boric acid are made to react under a mixture gas atmosphere containing carbon dioxide and a reductive gas, in a molten salt of a lithium-containing molten salt raw material at a temperature higher than the melting point thereof but lower than 900°C.

Description

  The present invention mainly relates to a method for producing a lithium borate compound useful as a positive electrode active material of a lithium ion secondary battery, and a lithium borate compound obtained by this method.

Lithium secondary batteries are small and have high energy density, and are widely used as power sources for portable electronic devices. Layered compounds such as LiCoO ₂ have mainly been used as positive electrode active materials. However, these compounds have a drawback that oxygen is easily desorbed at about 150 ° C. in a fully charged state, and this easily causes an oxidative exothermic reaction of the non-aqueous electrolyte.

In recent years, olivine phosphate compounds LiMPO ₄ (LiMnPO ₄ , LiFePO ₄ , LiCoPO _4, etc.) have been proposed as positive electrode active materials. This system improves thermal stability by using a bivalent / trivalent redox reaction instead of a trivalent / tetravalent redox reaction using an oxide such as LiCoO ₂ as a positive electrode active material, Furthermore, it has been attracting attention as a system capable of obtaining a high discharge voltage by disposing a polyanion of a heteroelement having a high electronegativity around a central metal.

However, the positive electrode material made of an olivine phosphate compound is limited to a theoretical capacity of about 170 mAh / g because the molecular weight of the phosphate polyanion is large. Furthermore, LiCoPO ₄ and LiNiPO ₄ have a problem that the operating voltage is too high and there is no electrolyte solution that can withstand the charging voltage.

Accordingly, LiFeBO ₃ (theoretical capacity 220 mAh / g), LiMnBO _{3 is a} cathode material that is inexpensive, has a large amount of resources, has a low environmental load, has a high theoretical charge / discharge capacity of lithium ions, and does not release oxygen at high temperatures. Lithium borate-based materials such as (theoretical capacity 222 mAh / g) are drawing attention. The lithium borate material is a material that can be expected to improve the energy density by using B, which is the lightest element in the polyanion unit, and the true density (3.46 g / cm ³ ) of the borate material is phosphorus. It is smaller than the true density (3.60 g / cm ³ ) of the acid olivine iron material, and weight reduction can also be expected.

  As a method for synthesizing a borate compound, a solid phase reaction method in which a raw material compound is reacted in a solid phase is known (see Non-Patent Documents 1 to 3 below). However, in the solid phase reaction method, it is necessary to react at a high temperature of 600 ° C. or higher for a long time, and it is possible to dissolve the dope element, but the crystal grains become as large as 10 μm or more, and the diffusion of ions It leads to the problem of being slow. In addition, since the reaction is performed at a high temperature, there is a problem that the doping element that cannot be completely dissolved in the cooling process is precipitated, impurities are generated, and the resistance is increased. Furthermore, since it is heated to a high temperature, a borate-based compound having lithium deficiency or oxygen deficiency is formed, and there is a problem that it is difficult to increase capacity and improve cycle characteristics.

  Therefore, the present inventors have conducted intensive research to overcome the above problems. As a result, using a transition metal compound containing an iron compound or a manganese compound, boric acid, and lithium hydroxide as raw materials, in a mixed molten salt of lithium carbonate and other alkali metal carbonate, in a reducing atmosphere, It has been found that a lithium borate-based compound containing iron or manganese can be obtained under a relatively mild condition by the method of reacting the above raw materials (molten salt method) (see Patent Document 1).

International Publication No. 2010/104137

Y. Z. Dong et.al Electrochim. Acta, 53, 2339-2345 (2008). Y. Z. Dong et.al J. Alloys Comp. 461, 585-590 (2008). V. Legagneur et.al Solid State Ionics, 139, 37-46 (2001).

  The lithium borate compound obtained by the method described in Patent Document 1 is a borate compound synthesized by a conventional method under relatively high temperature use conditions when used as a positive electrode material for a lithium ion secondary battery. Cycle characteristics, capacity, etc. improved. However, it was not evaluated near room temperature.

  The present invention has been made in view of the current state of the prior art described above, and its main purpose is to provide a cycle characteristic and capacity near room temperature for a lithium borate material useful as a positive electrode material for a lithium ion secondary battery. It is an object of the present invention to provide a method capable of producing a material having an improved performance and the like with a relatively simple means.

  As a result of the inventors' extensive research and trial and error, a lithium borate system that exhibits excellent battery performance even at room temperature when used as a positive electrode material by using nitrate as a molten salt instead of carbonate. It was newly found that a compound can be obtained.

  That is, the method for producing a lithium borate compound of the present invention includes at least one selected from the group consisting of a lithium-containing molten salt raw material containing at least lithium nitrate, and pure iron, pure manganese, and a compound containing iron and / or manganese. A transition metal-containing raw material and boric acid are reacted in a molten salt of the lithium-containing molten salt raw material in a mixed gas atmosphere containing carbon dioxide and a reducing gas, the melting point of the lithium-containing molten salt raw material being 900 ° C. or lower. It is characterized by making it.

  It is estimated that the lithium borate compound obtained by the production method of the present invention exhibits excellent battery performance even at room temperature for the following reason.

The lithium borate compound obtained by the production method of the present invention is presumed to have improved battery characteristics as a result of reduced generation of impurities compared to the case of using a carbonate molten salt. As a result of investigations by the present inventors, in order to obtain a lithium borate compound in a molten salt, oxide ions (O ²⁻ ) must exist together with lithium, boron, transition metal elements, etc. as dissolved species in the molten salt. Was found to be important. The lithium nitrate used as the molten salt has a low melting point and a low decomposition temperature (the melting point of lithium nitrate is 261 ° C., the decomposition temperature is about 550 ° C., the melting point of lithium carbonate is 735 ° C., and the decomposition temperature is about 950 ° C.). It is considered that O ²⁻ is easily released into the molten salt. In such a molten salt containing lithium nitrate, the reaction activity is high and the reaction proceeds promptly even at a low temperature, so that impurities are hardly generated.

  Further, when lithium nitrate and lithium carbonate are compared at the same temperature, lithium nitrate has a lower viscosity of the molten salt. Therefore, in the molten salt of lithium nitrate, the diffusion rate and thus the reaction rate is high, and it is considered that impurities are hardly generated.

The impurities whose generation is suppressed in the present invention are, for example, LiBO ₂ , Li ₅ Fe ₅ O ₈ , Fe ₃ (BO ₃ ) O ₂ , and Li ₂ Fe ₃ O ₄ , which are difficult to suppress the generation. In addition to these, unreacted materials such as MnO can be used. In addition, unreacted substances are suppressed by adjusting the amount of raw materials charged.

  Furthermore, since the melting point of lithium nitrate is 261 ° C., it is possible to stably synthesize a lithium borate compound at a low temperature even when used alone. As a result, grain growth is suppressed during the synthesis reaction, and a fine lithium borate compound is formed. In addition, since the molten salt contains a nitrate containing Li, a lithium borate-based compound containing excessive Li is easily formed. Such a lithium borate compound becomes a positive electrode material for a lithium ion battery having good cycle characteristics and high capacity.

The lithium borate compound of the present invention is obtained by the production method of the present invention,
Composition formula: Li _{1 + ab Ab} M _1-x M ′ _x BO _{3 + c}
(Wherein, A is at least one element selected from the group consisting of Na, K, Rb and Cs, M is at least one element selected from the group consisting of Fe and Mn, M ′ Is at least one element selected from the group consisting of Mg, Ca, Co, Al, Ni, Nb, Mo, W, Ti and Zr, where each subscript is as follows: 0 ≦ x ≦ 0. 5, 0 <a <1, 0 ≦ b <0.2, 0 <c <0.3, and a> b).
When used as a positive electrode active material for a lithium secondary battery, the initial constant voltage charge is performed at 0.1 C at a test temperature of 30 ° C. for 10 hours at 4.5 V, and after 50 cycles of charge and discharge at 4.5 to 1.5 V. The lithium secondary battery has a discharge capacity of 90% or more of the initial discharge capacity.

  According to the method for producing a lithium borate compound of the present invention, lithium ions having a high capacity near room temperature and excellent cycle characteristics are obtained by a relatively simple means using a molten salt method using lithium nitrate. The lithium borate compound of the present invention useful as a positive electrode material for a secondary battery can be obtained.

The X-ray-diffraction pattern of the product of Example 1 and Reference Example 1 is shown. 2 shows a scanning electron microscope (SEM) photograph of the product of Example 1. It is a graph which shows the charging / discharging characteristic of the lithium ion secondary battery which used the product of Example 1 as a positive electrode active material, Comprising: A test result when making it charge / discharge at 30 degreeC is shown. It is a graph which shows the charging / discharging characteristic of the lithium ion secondary battery which used the product of Example 1 as a positive electrode active material, Comprising: A test result when making it charge / discharge at 60 degreeC is shown. It is a graph which shows the charging / discharging characteristic of the lithium ion secondary battery which used the product of the reference example 1 as a positive electrode active material, Comprising: A test result when making it charge / discharge at 30 degreeC is shown.

  The best mode for carrying out the lithium borate compound of the present invention and the method for producing the same will be described below. Unless otherwise specified, the numerical range “mn” described in the present specification includes the lower limit m and the upper limit n in the range. In addition, the numerical range can be configured by arbitrarily combining the numerical values described in the present specification within the numerical range.

<Method for producing lithium borate compound>
The method for producing a lithium borate compound of the present invention comprises a lithium-containing molten salt raw material containing at least lithium nitrate, a transition metal-containing raw material containing at least one selected from the group consisting of iron, manganese, iron compounds, and manganese compounds, and boron. The acid is reacted in the molten salt of the lithium-containing molten salt raw material. Below, the raw material to be used is demonstrated in order.

The lithium-containing molten salt raw material plays a role as a supply source of lithium (Li) as well as a role of dispersing other raw materials as a flux in the production method of the present invention. As the lithium-containing molten salt raw material, only lithium nitrate may be used, or a mixture with other nitrates may be used. Specifically, it is at least one alkali metal nitrate selected from the group consisting of potassium nitrate (KNO ₃ ), sodium nitrate (NaNO ₃ ), rubium nitrate (RbNO ₃ ), and cesium nitrate (CsNO ₃ ). By using one or more of these alkali metal nitrates mixed with lithium nitrate, the melting point of the lithium-containing molten salt raw material is lowered, so that a stable lithium borate compound can be synthesized even at low temperatures. That is, although lithium nitrate melts at 270 ° C. or higher, a melting temperature lower than 270 ° C. can be achieved by using a mixed molten salt with other alkali metal nitrates. As a result, even if the synthesis temperature is lowered, the viscosity of the molten salt is low, which suppresses the generation of impurities and is suitable for the synthesis of fine lithium borate compounds.

  Of course, since the melting point of lithium nitrate is originally low, even if lithium nitrate is used alone as the lithium-containing molten salt raw material, the same effect as when the mixed molten salt is used can be obtained. Moreover, it can avoid that alkali metal elements other than lithium remain in the lithium borate type compound obtained by using lithium nitrate alone. Therefore, the obtained lithium borate compound is suitable as a positive electrode material for a lithium ion secondary battery.

  The ratio of lithium nitrate in the lithium-containing molten salt raw material is not particularly limited, but is preferably 60 to 100 mol%, more preferably 80 to 100 mol%, when the entire lithium-containing molten salt raw material is 100 mol%.

Further, the lithium-containing molten salt raw material may contain a lithium salt other than lithium nitrate as a lithium supply source at a ratio that does not significantly increase the melting point of the molten salt. For example, lithium carbonate (Li ₂ CO ₃ ), lithium hydroxide (LiOH, etc.), lithium metaborate (LiBO ₂ etc.), etc. are reacted even if one or more of these are contained in the lithium-containing molten salt raw material. As a result, only oxide ions (O ²⁻ ) and borate ions (BO ₃ ⁻ ) are generated.

  The transition metal-containing raw material is mainly a source of iron (Fe) and / or manganese (Mn), and includes at least one selected from the group consisting of pure iron, pure manganese, and a compound containing iron and / or manganese. Examples of the compound containing iron and / or manganese include an iron compound, a manganese compound, and a composite compound containing iron and / or manganese and optionally containing other metal elements. Since both Fe and Mn are stable when they are present in the lithium borate compound that is the target product of the production method of the present invention, the transition metal-containing raw material contains Fe and / Alternatively, Mn may be included. Therefore, examples of the transition metal-containing raw material include pure iron (zero valent), pure manganese (zero valent), a divalent iron compound, and a divalent manganese compound. Examples of the divalent compound include oxalates such as iron oxalate and manganese oxalate. One of these can be used alone or in combination of two or more.

  The transition metal element-containing raw material used in the present invention contains iron and / or manganese as an essential component, but may further contain other metal elements as necessary. Examples of other metal elements include at least one selected from the group consisting of Mg, Ca, Co, Al, Ni, Nb, Mo, W, Ti, and Zr. These metal elements may be in a metallic state such as pure magnesium, or a compound containing a metal element having a valence of up to 2, for example, sulfate, carbonate, hydroxide, etc. Also good. The transition metal element-containing raw material may contain only one of the metal elements listed above, or may contain two or more metal elements at the same time. The transition metal element-containing raw material may be a single compound or a mixture of two or more compounds. That is, the transition metal element-containing raw material specifically requires a raw material containing iron and / or manganese, and if necessary, cobalt oxide, magnesium oxide, calcium carbonate, calcium oxide, aluminum oxide, nickel oxide, oxidation One or more of niobium, lithium titanate, chromium (III) oxide, copper (II) acetate, zinc oxide, zirconium oxide, vanadium carbide, lithium molybdate and lithium tungstate may be included.

  In the transition metal element-containing raw material, the content of at least one transition metal element selected from the group consisting of iron and manganese is 50 mol% or more, where the total amount of metal elements contained in the transition metal element-containing raw material is 100 mol%. It is necessary to be. That is, the amount of at least one metal element selected from the group consisting of Mg, Ca, Co, Al, Ni, Nb, Mo, W, Ti and Zr is the total amount of metal elements contained in the transition metal element-containing raw material Can be 0 to 50 mol%, further 10 to 30 mol%.

  Boric acid is a source of boron (B). There is no particular limitation on the blending ratio of the transition metal-containing raw material to boric acid, but it is more preferably 0.9 to 1.2, and more preferably 0.95 to 1.1 in terms of molar ratio. Further, the transition metal-containing raw material and boric acid may be used in a uniformly dispersed ratio in the molten salt of the lithium-containing molten salt raw material. For example, it is preferable that the total amount of the transition metal-containing raw material and boric acid be in the range of 50 to 100 parts by mass with respect to 100 parts by mass of the total amount of the lithium-containing molten salt raw material, Is more preferably 90 to 95 parts by mass.

  Although the specific reaction method is not particularly limited, usually, the above-described lithium-containing molten salt raw material, transition metal-containing raw material and boric acid are weighed, uniformly mixed using a ball mill or the like, and then heated. The lithium-containing molten salt raw material may be melted. Thereby, in the molten salt of a lithium containing molten salt raw material, reaction of a lithium containing molten salt raw material, a transition metal containing raw material, and boric acid advances, and the target lithium borate type compound can be obtained.

  The above reaction is performed in a molten salt of a lithium-containing molten salt raw material having a melting point of 900 ° C. or lower and a melting point of the lithium-containing molten salt raw material in a mixed gas atmosphere containing carbon dioxide and a reducing gas.

The temperature of the molten salt, that is, the temperature at which the lithium-containing molten salt raw material is melted corresponds to the reaction temperature and is not lower than the melting point of the lithium-containing molten salt raw material and not higher than 900 ° C. When the reaction temperature exceeds 900 ° C., Li evaporates and a lithium borate compound lacking Li is generated. On the other hand, if the reaction temperature is less than 200 ° C., O ²⁻ is not easily released into the molten salt, and it takes a long time to synthesize a lithium borate compound, which is not practical. Therefore, desirable reaction temperature is 300-700 degreeC, 500-700 degreeC, and also 600-700 degreeC. However, since the reaction temperature needs to exceed the melting point of the lithium-containing molten salt raw material, it is necessary to prepare the composition of the lithium-containing molten salt raw material. At this time, the reaction time may be 1 to 20 hours, and further 5 to 13 hours.

In the reaction described above, during the reaction, in order to allow a metal element such as Fe contained in the transition metal-containing raw material to exist stably as a divalent ion in the molten salt, in a mixed gas atmosphere containing carbon dioxide and a reducing gas. Do. Under this atmosphere, even a metal element whose oxidation number before reaction is divalent or less 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 the reduction of lithium nitrate is promoted and the reaction rate is increased. However, if the reducing gas is excessive, Fe ²⁺ of the lithium borate compound may be reduced due to too high reducing property, and the reaction product may be destroyed. Therefore, it is preferable that the mixing ratio of the mixed gas is a volume ratio of carbon dioxide: reducing gas = 100: 3 to 60:40, or 75:25 to 65:35. 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 it may be under pressure or under reduced pressure.

  After performing the above reaction, the target lithium borate compound can be obtained by cooling and removing the solidified lithium-containing molten salt. The cooling rate is not particularly limited, but it is preferable to rapidly cool from the reaction temperature to room temperature (for example, at a cooling rate of 50 to 200 ° C./min). By quenching, a finer powdery product can be obtained.

  As a method for removing the lithium-containing molten salt, the lithium-containing molten salt may be dissolved and removed by washing the product using a solvent that can dissolve the cooled and solidified lithium-containing molten salt. For example, although water can be used as the solvent, it is preferable to use a nonaqueous solvent such as alcohol or acetone in order to prevent oxidation of the transition metal contained in the lithium borate compound. In particular, it is preferable to use acetic anhydride and acetic acid in a mass ratio of 2: 1 to 1: 1. This mixed solvent is excellent in the action of dissolving and removing the lithium-containing molten salt, and when acetic acid reacts with the lithium-containing molten salt to produce water, acetic anhydride takes in water to produce acetic acid. Therefore, it is possible to suppress the separation of water. When acetic anhydride and acetic acid are used, first, acetic anhydride is mixed with the product, ground with a mortar or the like to make particles fine, and then acetic anhydride is added in a state where acetic anhydride is intimately mixed with the particles. It is preferable. According to this method, since the water produced by the reaction of acetic acid and the lithium-containing molten salt reacts quickly with acetic anhydride and the product and water come into contact with each other, the oxidation and decomposition of the target product is effective. Can be suppressed.

<Lithium borate compounds>
The lithium borate compound obtained by the above-described method is
Composition formula: Li _{1 + ab Ab} M _1-x M ′ _x BO _{3 + c}
(Wherein, A is at least one element selected from the group consisting of Na, K, Rb and Cs, M is at least one element selected from the group consisting of Fe and Mn, M ′ Is at least one element selected from the group consisting of Mg, Ca, Co, Al, Ni, Nb, Mo, W, Ti and Zr, where each subscript is as follows: 0 ≦ x ≦ 0. 5, 0 <a <1, 0 ≦ b <0.2, 0 <c <0.3, and a> b).

  The lithium borate compound obtained by the production method of the present invention exhibits excellent cycle characteristics when used as a positive electrode active material of a lithium secondary battery. Specifically, when the initial constant voltage charge is performed at 4.5 V for 10 hours at a test temperature of 30 ° C. and the charge / discharge test is performed at 4.5 to 1.5 V, 50 cycle charge / discharge The later discharge capacity is 90% or more of the initial discharge capacity. More preferable ranges of a, b and c in the above composition formula are 0.01 ≦ ab ≦ 0.1 and 0.01 ≦ c ≦ 0.1.

  The compound used a molten salt of lithium nitrate, so that the lithium ions in the molten salt penetrated into the Li ion site of the lithium borate compound, resulting in excessive Li ions compared to the stoichiometric amount. It becomes a compound containing. Moreover, if it is a molten salt containing lithium nitrate, it becomes possible to perform the reaction at a relatively low temperature, the growth of crystal grains is suppressed, and the amount of the impurity phase is greatly reduced. As a result, when used as a positive electrode active material of a lithium ion secondary battery, the material has good cycle characteristics and high capacity. The lithium borate compound obtained by the above-described method preferably has an average particle size in the range of 500 nm to 50 μm, more preferably 600 nm to 20 μm. In this specification, the average particle diameter is the maximum diameter of a plurality of particles (maximum value of the distance between two parallel lines sandwiching the particles) from an image obtained by observation with a scanning electron microscope (SEM). It is a value calculated by actual measurement.

<Carbon coating treatment>
The lithium borate compound represented by the composition formula: Li _{1 + ab Ab} M _1-x M ′ _x BO _{3 + c} obtained by the above-described method may be further subjected to a coating treatment with carbon to improve conductivity. preferable.

The specific method of the carbon coating treatment is not particularly limited. In addition to a vapor phase method in which heat treatment is performed in an atmosphere containing a carbon-containing gas such as methane gas, ethane gas, or butane gas, an organic substance that is a carbon source and a lithium borate system A thermal decomposition method in which an organic substance is carbonized by heat treatment after the compound is uniformly mixed is also applicable. In particular, it is preferable to apply a ball milling method in which a carbon material and Li ₂ CO ₃ are added to the lithium borate-based compound, and the lithium borate-based compound is uniformly mixed by a ball mill until it becomes amorphous, followed by heat treatment. According to this method, the lithium borate compound, which is a positive electrode active material, is made amorphous by ball milling, and is uniformly mixed with carbon to increase adhesion. Further, by heat treatment, the lithium borate compound is recrystallized. At the same time, carbon is uniformly deposited around the lithium borate compound. At this time, the presence of Li ₂ CO ₃ does not cause the lithium excess borate compound to be deficient in lithium, and exhibits a high charge / discharge capacity.

Regarding the degree of amorphization, in the X-ray diffraction measurement using Cu Kα 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 represented by B (011) _Crystal , there the half width of the peak of the sample obtained by ball milling the case of the _{B (011) mill, B (} 011) Crystal / B (011) the ratio of _mill is in the range of about 0.1 to 0.5 That's fine.

  In this method, acetylene black (AB), ketjen black (KB), graphite or the like can be used as the carbon material.

Regarding the mixing ratio of the lithium borate compound, the carbon material, and Li ₂ CO ₃ , the carbon material is 20 to 40 parts by mass and the Li ₂ CO ₃ is 20 to 40 parts by mass with respect to 100 parts by mass of the lithium borate compound. And it is sufficient.

  Ball milling is performed until the lithium borate-based compound becomes amorphous by the above-described method, and then 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 borate compound divalent. As the reducing atmosphere in this case, as in the synthesis reaction of the lithium borate compound in the molten salt, in order to suppress the reduction of the divalent transition metal ion to the metal state, nitrogen and carbon dioxide are used. It is preferably in a mixed gas atmosphere of at least one gas selected from the group consisting of a reducing gas and a reducing gas. The mixing ratio of the reducing gas and at least one gas selected from the group consisting of nitrogen and carbon dioxide may be the same as in the synthesis reaction of the lithium borate 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 borate compound, while if the heat treatment temperature is too high, decomposition of the lithium borate compound or lithium deficiency may occur. This is not preferable because the charge / discharge capacity decreases. The heat treatment time may normally be 1 to 10 hours.

In addition, as another carbon coating treatment method, a carbon material and LiF are added to the lithium borate compound, and the mixture is uniformly mixed by a ball mill until the lithium borate compound becomes amorphous in the same manner as described above, followed by heat treatment. May be performed. In this case, as in the case described above, carbon is uniformly deposited around the lithium borate compound simultaneously with recrystallization of the lithium borate compound, and the conductivity is improved. Further, the lithium borate compound is further improved. A part of the oxygen atom of
Composition formula: Li _{1 + ab Ab} M _1-x M ′ _x BO _{3 + cy} F _2y
Wherein A is at least one element selected from the group consisting of Na, K, Rb and Cs, M is Fe or Mn, and M ′ is Mg, Ca, Co, Al, Ni And at least one element selected from the group consisting of Nb, Mo, W, Ti and Zr, where the subscripts are as follows: 0 ≦ x ≦ 0.5, 0 <a <1, 0 ≦ b <0.2, 0 <c <0.3, 0 <y <1, and a> b) are formed.

  When this compound is used as a positive electrode due to the addition of F, the average voltage increases from 2.6 V to 2.8 V, and a positive electrode material having better performance is obtained. At this time, the presence of LiF does not cause the lithium excess borate compound to be deficient in lithium and exhibits a high charge / discharge capacity.

In this method, the mixing ratio of the lithium borate 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 borate compound. Good. Furthermore, Li ₂ CO ₃ may be included as necessary. The conditions for ball milling and heat treatment may be the same as described above.

<Positive electrode for lithium ion secondary battery>
The lithium borate compound obtained by synthesis in the above molten salt, the lithium borate compound subjected to carbon coating treatment, and the lithium borate compound added with fluorine are all effective as an active material for a lithium secondary battery positive electrode. Can be used for The positive electrode using these lithium borate compounds can have the same structure as a normal positive electrode for a lithium ion secondary battery.

  For example, the lithium borate compound may be added to acetylene black (AB), ketjen black (KB), a vapor grown carbon fiber (Vapor Carbon Carbon Fiber: VGCF), or the like, a polyvinylidene fluoride (Polyvinylidene Fluoride: PVdF), a polytetrafluorocarbon. A positive electrode is prepared 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 the conductive aid used, it can be, for example, 5 to 20 parts by mass with respect to 100 parts by mass of the lithium borate compound. The amount of the binder used is not particularly limited, but can be 5 to 20 parts by mass with respect to 100 parts by mass of the lithium borate compound, for example. In addition, as another method, a mixture of a lithium borate compound, the above conductive additive and a binder is kneaded with a mortar or a press to form a film, and this is crimped to a current collector with a press. The positive electrode can be manufactured also by the method to do.

  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 according to 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 an ordinary 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 manufacturing method of the lithium borate type compound of this invention 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.

  The present invention will be specifically described below with reference to examples of the method for producing a lithium borate compound of the present invention.

[Example 1] Synthesis of iron-containing lithium borate compound by molten salt method 0.01 mol of iron (manufactured by High-Purity Chemical Co., Ltd., purity 99.9%), boric acid H ₃ BO ₃ (manufactured by Kishida Chemical Co., Ltd.) , Purity 99%) 0.01 mol and lithium nitrate (Kishida Chemical Co., Ltd., purity 99%) 0.01 mol were mixed. The mixing ratio was such that the total amount of iron and boric acid was 100 parts by mass with respect to 100 parts by mass of lithium nitrate.

  2 mL of water was added thereto, mixed using a pestle and mortar, heated to 100 ° C., further mixed, and dried at 100 ° C. Thereafter, the obtained powder was heated in a gold crucible and heated to 650 ° C. in a mixed gas atmosphere of carbon dioxide (flow rate: 70 mL / min) and hydrogen (flow rate: 30 mL / min) to obtain lithium nitrate. Was reacted for 13 hours in a molten state.

  After the reaction, the entire reactor core as a reaction system was taken out of an electric furnace as a heater, and rapidly cooled to room temperature while passing gas. The cooling rate at this time was 51 ° C./min. Thereafter, the product was ground to obtain a powder of an iron-containing lithium borate compound.

The obtained product was subjected to X-ray diffraction measurement using a CuKα ray by a powder X-ray diffractometer. The XDR pattern is shown in FIG. This XDR pattern almost coincided with the reported pattern of monoclinic LiFeBO _{3 in} the space group C2 / c.

  A scanning electron microscope (SEM) photograph of the product is shown in FIG. When the average particle size was calculated from FIG. 2, it was confirmed that the powder was composed of fine crystal grains of 6 μm.

Further, as a result of elemental analysis of the product by an inductively coupled plasma (ICP) method, the composition formula is Li _1.05 FeBO _3.08 , and it can be confirmed that it is a lithium-excess LiFeBO ₃ -based lithium borate compound. It was.

[Comparative Example 1] Synthesis of iron-containing lithium borate compound by solid phase method Lithium carbonate Li ₂ CO ₃ , iron oxalate FeC ₂ O ₄ .2H ₂ O, and boric acid H ₃ BO ₃ in a molar ratio of 1: 1: After ball milling the mixed powder mixed so as to be 1, an iron-containing lithium borate compound was obtained by heat treatment at 650 ° C. for 10 hours.

As a starting material for the synthesis of Reference Example 1 Iron-containing lithium borate compound by molten salt method, iron oxalate _{FeC 2 O 4 · 2H 2 O} ( Sigma-Aldrich, 99.99% purity), lithium hydroxide (anhydride) LiOH ( 0.005 mol each of Kishida Chemical Co., Ltd., purity 98%) and boric acid H ₃ BO ₃ (Kishida Chemical Co., Ltd., purity 99.5%) were used, and this was used as a carbonate mixture (lithium carbonate (Kishida Chemical Co., Ltd.). Product, purity 99.9%), sodium carbonate (produced by Kishida Chemical Co., Ltd., purity 99.5%), and potassium carbonate (produced by Kishida Chemical Co., Ltd., purity 99.5%) in a molar ratio of 0.435: 0. 315: 0.25). The mixing ratio was such that the total amount of iron oxalate, lithium hydroxide and boric acid was 225 parts by mass with respect to 100 parts by mass of the carbonate mixture.

  Acetone (20 ml) was added thereto, mixed in a zirconia ball mill at 500 rpm for 60 minutes, and dried. Thereafter, the obtained powder was heated in a gold crucible and heated to 400 ° C. in a mixed gas atmosphere of carbon dioxide (flow rate: 100 mL / min) and hydrogen (flow rate: 3 mL / min) to form carbonate. The mixture was allowed to react for 15 hours in the molten state.

  After the reaction, when the temperature was lowered to 100 ° C., the entire reactor core as a reaction system was taken out from the electric furnace as a heater and cooled to room temperature while passing gas.

  Next, acetic anhydride (20 ml) was added to the product and ground in a mortar, and acetic acid (10 ml) was added to react and remove carbonates and the like, followed by filtration to obtain a powder of an iron-containing lithium borate compound.

The obtained product was subjected to X-ray diffraction measurement using a CuKα ray by a powder X-ray diffractometer. The XDR pattern is shown in FIG. The XDR pattern almost matched the reported pattern of LiFeBO _{3 in} space group C2 / c. Moreover, as a result of observing a product by SEM, it has confirmed that it was a powder which consists of a crystal grain of several micrometers or less. Furthermore, as a result of elemental analysis of the product by the ICP method, the composition formula was Li _1.04 FeBO _3.10 . It was confirmed that the product was a lithium-excess LiFeBO ₃ -based lithium borate compound.

[Production of lithium ion secondary battery]
A lithium ion secondary battery using any one of the lithium borate compounds obtained in Example 1, Comparative Example 1 and Reference Example 1 as a positive electrode active material was produced.

First, 50 parts by mass of acetylene black (AB) is added to 100 parts by mass of a lithium borate compound, and milling is performed at 450 rpm for 5 hours using a planetary ball mill (5 mm zirconia ball). Heat treatment was performed at 700 ° C. for 2 hours in an atmosphere of a mixed gas (CO ₂ : H ₂ (molar ratio) = 100: 3).

  25 parts by mass of a mixture of acetylene black (AB) and polytetrafluoroethylene (PTFE) (a mixture of AB: PTFE (mass ratio) = 2: 1) is added to 100 parts by mass of the obtained powder, and the sheet method Thus, three types of electrodes (positive electrode) were prepared and vacuum-dried at 140 ° C. for 3 hours.

Thereafter, a solution made by dissolving LiPF ₆ in a predetermined solvent to 1 mol / L is used as an electrolyte, a polypropylene film (Celgard 2400, manufactured by Celgard) as a separator, a lithium metal foil as a negative electrode, and the three types already described as a positive electrode Coin batteries (# E1, # C1, and # 01) using any of the electrodes were prepared.

  In addition, according to the test temperature of a charging / discharging test, the electrolyte solution prepared two types from which a solvent differs, and used it for said coin battery. When the test temperature was 30 ° C., a solvent in which ethylene carbonate (EC): dimethylene carbonate (DMC) was mixed at a volume ratio of EC: DMC = 1: 1 was used. When the test temperature was 60 ° C., a solvent in which ethylene carbonate (EC): dimethyl ether carbonate (DEC) was mixed at a volume ratio of EC: DEC = 1: 1 was used.

(Charge / discharge test)
The manufactured coin battery was subjected to a charge / discharge test at 30 ° C. or 60 ° C. The test conditions are as follows: the test temperature is 30 ° C. and the voltage is 4.5 to 1.5 V at 0.1 C (the first constant voltage charge is 4.5 V for 10 hours), and the test temperature is 60 ° C. and the voltage is 4.2 at 0.1 C. To 1.5V (initial constant voltage charging is 4.2V for 10 hours). 3 and 4 show the charge / discharge characteristics of battery # E1, and FIG. 5 shows the charge / discharge characteristics of battery # 01. Table 1 shows the discharge capacity after 5 cycles of each battery, the average voltage after 5 cycles, and the number of cycles that can maintain 90% of the initial discharge capacity.

From FIG. 3 and FIG. 4, the battery # E1 using the lithium borate compound synthesized in Example 1 as the positive electrode active material exhibits sufficient battery characteristics at 30 ° C. (near room temperature) and 60 ° C. I understood it. On the other hand, Battery # 01 using the lithium borate compound synthesized in Reference Example 1 as the positive electrode active material had a high average voltage after 5 cycles measured at 60 ° C., but was very low when measured at 30 ° C. It was. Moreover, from the charge / discharge test results at 30 ° C. shown in Table 1, it was found that the battery # E1 was superior to the battery # 01 in all items. In Example 1, the generation of impurities was suppressed as compared to Reference Example 1 (FIG. 1), and the particles were fine even when the synthesis temperature was relatively high at 650 ° C. (FIG. 2). It is presumed that the characteristics were excellent and the capacity was high.

Since the lithium borate compound synthesized in Comparative Example 1 used the solid phase reaction method, the particles grew greatly. Although not shown, the presence of LiBO ₂ and Fe ₃ O ₄ was confirmed from the XRD pattern. Therefore, battery # C1 had a small capacity and insufficient cycle characteristics.

  In Reference Example 1, a lithium borate compound was synthesized using the molten salt method in the same manner as in Example 1. In Reference Example 1, since the synthesis temperature was 400 ° C. and low temperature, it was estimated that the product was obtained with fine particles. However, the capacity and cycle characteristics of the battery # 01 at 30 ° C. were lower in both capacity and cycle characteristics than the battery # E1.

  Therefore, it was found that the lithium borate compound synthesized by the molten salt method using lithium nitrate brings high capacity and excellent cycle characteristics even when used at room temperature when used as a positive electrode active material. Further, since it is possible to synthesize at a lower temperature than in Example 1 by utilizing the fact that lithium nitrate has a low melting point, it is possible to further refine the particles and to improve the battery characteristics.

Lithium ion secondary batteries are small and have high energy density, and are widely used as power sources for portable electronic devices, and layered compounds such as LiCoO ₂ have mainly been used as positive electrode active materials. However, these compounds have a drawback that oxygen is easily desorbed at about 150 ° C. in a fully charged state, and this easily causes an oxidative exothermic reaction of the non-aqueous electrolyte.

Furthermore, since the melting point of lithium nitrate is 261 ° C., it is possible to stably synthesize a lithium borate compound at a low temperature even when used alone. As a result, grain growth is suppressed during the synthesis reaction, and a fine lithium borate compound is formed. In addition, since the molten salt contains a nitrate containing Li, a lithium borate-based compound containing excessive Li is easily formed. Such a lithium borate compound becomes a positive electrode material for a lithium ion secondary battery having good cycle characteristics and high capacity.

The lithium borate-based compound that is obtained by the method for producing a lithium borate-based compound of the present invention,
Composition formula: Li _{1 + ab Ab} M _1-x M ′ _x BO _{3 + c}
(Wherein, A is at least one element selected from the group consisting of Na, K, Rb and Cs, M is at least one element selected from the group consisting of Fe and Mn, M ′ Is at least one element selected from the group consisting of Mg, Ca, Co, Al, Ni, Nb, Mo, W, Ti and Zr, where each subscript is as follows: 0 ≦ x ≦ 0. 5,0 <a a <1,0 ≦ b <0.2,0 < c <0.3, and you express by a> b a is). Further, when this lithium borate compound is used as a positive electrode active material of a lithium secondary battery, initial constant voltage charging is performed at 4.5 V for 10 hours at 0.1 C at a test temperature of 30 ° C. discharge capacity of the lithium secondary battery after 50 cycles of charge and discharge Ru der 90% of the initial discharge capacity .5V.

<Positive electrode for lithium ion secondary battery>
The lithium borate compound obtained by synthesis in the above molten salt, the lithium borate compound subjected to carbon coating treatment, and the lithium borate compound added with fluorine are all active materials for positive electrodes of lithium ion secondary batteries. Can be used effectively. The positive electrode using these lithium borate compounds can have the same structure as a normal positive electrode for a lithium ion secondary battery.

<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 the positive electrode material, and as the negative electrode material, a known metal lithium (in this case, called “lithium secondary battery”) , a carbon-based material such as graphite, a silicon-based material such as a silicon thin film, Using an alloy material such as copper-tin or cobalt-tin, or an oxide material such as lithium titanate, the electrolyte is perchloric in a known non-aqueous solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, or dimethyl carbonate. Use a solution prepared by dissolving lithium salt such as lithium acid, LiPF ₆ , LiBF ₄ , LiCF ₃ SO _{3 at} a concentration of 0.5 mol / L to 1.7 mol / L, and other known battery components And what is necessary is just to assemble a lithium ion secondary battery according to a conventional method.

[Production of coin battery for charge / discharge test ]
A coin battery was fabricated using any of the lithium borate compounds obtained in Example 1, Comparative Example 1 and Reference Example 1 as the positive electrode active material.

Claims (14)

Composition formula: Li _{1 + ab Ab} M _1-x M ′ _x BO _{3 + c}
(Wherein, A is at least one element selected from the group consisting of Na, K, Rb and Cs, M is at least one element selected from the group consisting of Fe and Mn, M ′ Is at least one element selected from the group consisting of Mg, Ca, Co, Al, Ni, Nb, Mo, W, Ti and Zr, where each subscript is as follows: 0 ≦ x ≦ 0. 5, 0 <a <1, 0 ≦ b <0.2, 0 <c <0.3, and a> b).
When used as a positive electrode active material for a lithium secondary battery, the initial constant voltage charge is performed at 0.1 C at a test temperature of 30 ° C. for 10 hours at 4.5 V, and after 50 cycles of charge and discharge at 4.5 to 1.5 V. A lithium borate compound wherein the discharge capacity of the lithium secondary battery is 90% or more of the initial discharge capacity. A lithium-containing molten salt raw material containing at least lithium nitrate;
A transition metal-containing raw material containing at least one selected from the group consisting of pure iron, pure manganese and a compound containing iron and / or manganese;
Boric acid is reacted in the molten salt of the lithium-containing molten salt raw material in a mixed gas atmosphere containing carbon dioxide and a reducing gas at a melting point of the lithium-containing molten salt raw material of 900 ° C. or lower. A method for producing a lithium borate compound.   The method for producing a lithium borate compound according to claim 2, wherein the temperature of the molten salt is 200 ° C or higher and 900 ° C or lower.   The method for producing a lithium borate compound according to claim 3, wherein the temperature of the molten salt is 300 ° C or higher and 700 ° C or lower.   The method for producing a lithium borate compound according to claim 2, wherein the mixed gas contains hydrogen gas as the reducing gas.   6. The lithium borate-based compound according to claim 2, wherein the mixed gas contains carbon dioxide and a reducing gas in a volume ratio of carbon dioxide: reducing gas = 100: 3 to 60:40. Production method.   The said transition metal containing raw material is a manufacturing method of the lithium borate type compound in any one of Claims 2-6 containing iron and / or manganese below bivalent.   The method for producing a lithium borate compound according to claim 7, wherein the transition metal-containing raw material contains at least one selected from the group consisting of pure iron, pure manganese, iron oxalate, and manganese oxalate.   The method for producing a lithium borate compound according to any one of claims 2 to 8, wherein the lithium-containing molten salt raw material comprises a mixed molten salt containing lithium nitrate as an essential component and further containing another alkali metal nitrate.   The transition metal element-containing raw material contains 50 to 100 mol% of at least one transition metal element selected from the group consisting of iron and manganese, with the total amount of metal elements contained in the transition metal element-containing raw material being 100 mol%, The lithium borate according to any one of claims 2 to 9, comprising 0 to 50 mol% of at least one metal element selected from the group consisting of Mg, Ca, Co, Al, Ni, Nb, Mo, W, Ti and Zr. Of the production of the compound.   The manufacturing method of a lithium borate type compound including the process of removing the said lithium containing molten salt raw material with a solvent, after manufacturing a lithium borate type compound by the method in any one of Claims 2-10.   The positive electrode active material for lithium ion secondary batteries which consists of a lithium borate type compound obtained by the method in any one of Claims 2-11.   A positive electrode for a lithium secondary battery, comprising the positive electrode active material according to claim 12.   A lithium secondary battery comprising the positive electrode according to claim 13 as a constituent element.