WO2012060084A1

WO2012060084A1 - Lithium borate compound and method for producing same

Info

Publication number: WO2012060084A1
Application number: PCT/JP2011/006089
Authority: WO
Inventors: 敏勝小島; 琢寛幸; 境　哲男; 晶小島; 淳一丹羽; 一仁川澄
Original assignee: 株式会社豊田自動織機; 独立行政法人産業技術総合研究所
Priority date: 2010-11-05
Filing date: 2011-10-31
Publication date: 2012-05-10
Also published as: JP2012104240A; JP5110664B2

Abstract

This method for producing a lithium borate compound reacts a lithium-containing-fused-salt starting material containing at least lithium nitrate, a transition-metal-containing starting material containing at least one component selected from the group consisting of pure iron, pure manganese, and a compound containing iron and/or manganese, and boric acid in the fused salt of the lithium-containing-fused-salt starting material in a mixed gas ambient containing carbon dioxide and a reducing gas and at a temperature that is between the melting point of the lithium-containing-fused-salt starting material and 900°C inclusive. As a result, it is possible to produce a material having superior performance, improved capacity and the like, and cycling characteristics near room temperature as a lithium borate material that is useful as a positive electrode material for a lithium ion secondary battery.

Description

Lithium borate compound and method for producing the same

The present invention relates mainly 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 ion secondary batteries are small in size and high in energy density, and are widely used as power sources for portable electronic devices. As positive electrode active materials, mainly layered compounds such as LiCoO ₂ have been used. However, these compounds have a disadvantage that oxygen is easily desorbed at around 150 ° C. in a fully charged state, which is likely to cause an oxidative exothermic reaction of the non-aqueous electrolyte.

In recent years, an olivine phosphate compound LiMPO ₄ (LiMnPO ₄ , LiFePO ₄ , LiCoPO ₄ or the like) has been proposed as a positive electrode active material. In this system, thermal stability is improved by using a divalent / multivalent redox reaction instead of a trivalent / multivalent redox reaction in which an oxide such as LiCoO ₂ is used as a positive electrode active material, Furthermore, it has been noted as a system capable of obtaining a high discharge voltage by arranging a polyanion of a heteroelement having a high electronegativity around a central metal.

However, the positive electrode material composed of a phosphate olivine compound has a theoretical capacity limited to 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 that can withstand the charging voltage.

Therefore, 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 impact, has a high theoretical charge-discharge capacity of lithium ions, and does not release oxygen at high temperatures. Lithium borate materials such as (theoretical capacity 222 mAh / g) have attracted attention. Lithium borate materials are materials that can be expected to improve energy density by using B, which is the lightest element among polyanion units, and the true density (3.46 g / cm ³ ) of borate materials 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 synthesis method of a borate compound, a solid phase reaction method in which a raw material compound is reacted in a solid phase state is known (see Non-Patent Documents 1 to 3 below). However, in the solid phase reaction method, it is necessary to react for a long time at a high temperature of 600 ° C. or more, and although it is possible to form a solid solution with the doping element, 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, the doping element which can not form a solid solution in the cooling process is precipitated to generate an impurity, which causes a problem that the resistance becomes high. Furthermore, in order to heat to high temperature, a borate compound of lithium deficiency or oxygen deficiency is formed, and there is also a problem that it is difficult to increase the capacity or to improve the cycle characteristics.

Therefore, the present inventors have intensively studied 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 another alkali metal carbonate under a reducing atmosphere, According to the method (molten salt method) in which the raw materials are reacted, it has been found that lithium borate compounds containing iron or manganese can be obtained under relatively mild conditions (see Patent Document 1).

International Publication 2010/104137

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

The present invention has been made in view of the above-mentioned current state of the prior art. The main object of the present invention is to provide a lithium borate based material useful as a positive electrode material for lithium ion secondary batteries etc., a material having excellent performance with relatively improved cycle characteristics, capacity etc. in the vicinity of room temperature. It is to provide a method that can be manufactured by means.

As a result of intensive research and trial and error conducted by the present inventors, lithium nitrate based lithium nitrate shows excellent battery performance even at room temperature when it is used as a positive electrode material by using nitrate instead of carbonate as molten salt. It has newly been found that compounds can be obtained.

That is, the method for producing a lithium borate compound according to the present invention comprises at least one lithium 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. The transition metal-containing raw material and boric acid are reacted in the molten salt of the lithium-containing molten salt raw material of the lithium-containing molten salt raw material and not less than the melting point of 900 ° C. in a mixed gas atmosphere containing carbon dioxide and a reducing gas It is characterized by

The reason why the lithium borate compound obtained by the production method of the present invention exhibits excellent battery performance even at room temperature is presumed to be as follows.

The lithium borate compound obtained by the production method of the present invention is estimated to have improved battery characteristics as a result of the reduction in the generation of impurities as compared with the case where a molten salt of carbonate is used. As a result of investigations by the present inventors, in order to obtain a lithium borate compound in the molten salt, it is necessary that an oxide ion (O ²⁻ ) exists together with lithium, boron, a transition metal element, etc. as a dissolved species in the molten salt. Was found to be important. 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., 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 rapidly even at low temperatures, so that impurities are hardly generated.

When lithium nitrate and lithium carbonate are compared at the same temperature, lithium nitrate has a lower viscosity of the molten salt. Therefore, it is also conceivable that in the molten salt of lithium nitrate, the diffusion rate, and hence the reaction rate, is fast, and it is difficult to form impurities.

In the present invention, the impurities whose formation is suppressed include, 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 formation. Besides, etc., unreacted substances such as MnO can be mentioned. In addition, unreacted material is suppressed by adjusting the preparation amount of a raw material.

Furthermore, since lithium nitrate has a melting point of 261 ° C., lithium borate compounds can be stably synthesized at low temperature even if used alone. As a result, grain growth is suppressed during the synthesis reaction to form a fine lithium borate compound. In addition, since the molten salt contains a nitrate containing Li, a lithium borate compound containing a large amount of Li is easily formed. Such lithium borate compounds serve as positive electrode materials for lithium ion batteries having good cycle characteristics and high capacity.

The lithium borate compound of the present invention is obtained by the above-mentioned production method of the present invention,
Composition formula: Li _{1 + a−b} A _b 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, and 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, 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 of a lithium secondary battery, after performing initial constant voltage charging at 4.5 V for 10 hours at 0.1 C at a test temperature of 30 ° C. after 50 cycles of charge and discharge at 4.5 to 1.5 V The discharge capacity of the lithium secondary battery is 90% or more of the initial discharge capacity.

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

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

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

<Method of 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 member selected from the group consisting of iron, manganese, iron compounds and manganese compounds, An 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 source of lithium (Li) together with the role of dispersing other raw materials as a flux in the production method of the present invention. The lithium-containing molten salt raw material may use only lithium nitrate, but may be used in combination with other nitrates. Specifically, it is at least one alkali metal nitrate selected from the group consisting of potassium nitrate (KNO ₃ ), sodium nitrate (NaNO ₃ ), rubidium nitrate (RbNO ₃ ) and cesium nitrate (CsNO ₃ ). The melting point of the lithium-containing molten salt raw material is lowered by mixing and using one or more of these alkali metal nitrates with lithium nitrate, so that a stable lithium borate compound can be synthesized even at a low temperature. 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 low, the viscosity of the molten salt is low, the formation of impurities is suppressed, and it is suitable for the synthesis of fine lithium borate compounds.

Of course, since the melting point of lithium nitrate is originally low, the use of lithium nitrate alone as the lithium-containing molten salt raw material provides the same effect as the mixed molten salt. Moreover, it is avoidable that alkali metal elements other than lithium remain | survive in the lithium borate type-compound obtained by using lithium nitrate independently. Therefore, the obtained lithium borate type compound is suitable as a positive electrode material of a lithium ion secondary battery.

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

In addition, the lithium-containing molten salt raw material may contain a lithium salt other than lithium nitrate as a lithium source at such a rate that the melting point of the molten salt is not greatly increased. 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 It is desirable because only oxide ions (O ²⁻ ) and borate ions (BO ₃ ⁻ ) are generated.

The transition metal-containing raw material is a source of mainly 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 compounds containing iron and / or manganese include complex compounds containing iron compounds, manganese compounds, iron and / or manganese and optionally other metal elements. Since both Fe and Mn are present in the lithium borate compound, which is the target product of the production method of the present invention, in the case of being divalent, they are stable, so the transition metal-containing raw material is Fe and / or divalent oxidation number. Or Mn may be included. Therefore, as a transition metal containing raw material, pure iron (0 value), pure manganese (0 value), a bivalent iron compound, a bivalent manganese compound, etc. are mentioned. Examples of divalent compounds include oxalate such as iron oxalate and manganese oxalate. One of these may be used alone or in combination of two or more.

The transition metal element-containing raw material used in the present invention essentially contains iron and / or manganese, but may further contain other metal elements as required. As another metal element, at least one selected from the group consisting of Mg, Ca, Co, Al, Ni, Nb, Mo, W, Ti and Zr can be exemplified. These metal elements may be in a metal state such as pure magnesium, or a compound containing a metal element having a valence of up to two, such as sulfates, carbonates, hydroxides, etc. It is 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 simultaneously. The transition metal element-containing raw material can be used alone or in combination of two or more compounds. Specifically, 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, oxide 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 contained.

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, assuming that the total amount of metal elements contained in the transition metal element-containing raw material is 100 mol%. It is necessary to be there. 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 made 0 to 50 mol%, and further 10 to 30 mol%, where 100 mol% is

Boric acid is a source of boron (B). There are no particular limitations on the blending ratio of the transition metal-containing raw material to boric acid, but the molar ratio of the metal element to boron is preferably 0.9 to 1.2, more preferably 0.95 to 1.1. Moreover, the transition metal-containing raw material and the boric acid may be used in a ratio uniformly dispersed in the molten salt of the lithium-containing molten salt raw material. For example, the total amount of the transition metal-containing raw material and the boric acid is preferably in the range of 50 to 100 parts by mass, more preferably 80 to 95 parts by mass, with respect to 100 parts by mass of the total lithium-containing molten salt raw material. More preferably, the amount is in the range of 90 to 95 parts by mass.

Although a 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. Thus, in the molten salt of the lithium-containing molten salt raw material, the reaction of the lithium-containing molten salt raw material, the transition metal-containing raw material and the boric acid proceeds to obtain the target lithium borate compound.

The above reaction is carried out in a molten salt of a lithium-containing molten salt raw material which is higher than the melting point of the lithium-containing molten salt raw material and not higher than 900 ° C. in a mixed gas atmosphere containing carbon dioxide and a reducing gas.

The temperature of the molten salt, that is, the temperature for melting the lithium-containing molten salt raw material corresponds to the reaction temperature, and is not less than the melting point of the lithium-containing molten salt raw material and 900 ° C. or less. When the reaction temperature exceeds 900 ° C., Li evaporates to form a lithium deficient lithium borate compound. Further, if the reaction temperature is less than 200 ° C., O ²⁻ is hardly released into the molten salt, and it takes a long time to synthesize the lithium borate compound, which is not practical. Thus, desirable reaction temperatures are 300-700 ° C., 500-700 ° C. and even 600-700 ° C. However, since the reaction temperature needs to be higher than 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 set to 1 to 20 hours and further 5 to 13 hours.

The reaction described above is performed under a mixed gas atmosphere containing carbon dioxide and a reducing gas in order to cause a metal element such as Fe contained in the transition metal-containing raw material to stably exist in the molten salt as divalent ions during the reaction. Do. Under this atmosphere, it is possible to stably maintain the metal element in a divalent state even if the oxidation number before the reaction is a divalent or less metal. The ratio of carbon dioxide to reducing gas is not particularly limited, but when a large amount of reducing gas is used, carbon dioxide for controlling the oxidizing atmosphere is reduced, so reduction of lithium nitrate is promoted to accelerate the reaction rate. However, if the reducing gas is excessive, the Fe ²⁺ of the lithium borate compound may be reduced due to too high reducibility, and the reaction product may be destroyed. Therefore, a preferable mixing ratio of the mixed gas is, in volume ratio, carbon dioxide: reducing gas = 100: 3 to 60:40, and further preferably 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.

The pressure of the mixed gas of carbon dioxide and reducing gas is not particularly limited, and may be atmospheric pressure in general, but may be under pressure or under pressure.

After the above reaction, the reaction product is cooled and the solidified lithium-containing molten salt is removed to obtain the target lithium borate compound. Although the cooling rate is not particularly limited, it is preferable to quench from the reaction temperature to room temperature (eg, 50 to 200 ° C./min at the cooling rate). By quenching, a finer powdery product is obtained.

As a method of removing the lithium-containing molten salt, the lithium-containing molten salt may be dissolved and removed by washing the product using a solvent capable of dissolving the cooled and solidified lithium-containing molten salt. For example, water may be used as the solvent, but in order to prevent oxidation of the transition metal contained in the lithium borate compound, it is preferable to use non-aqueous solvents such as alcohol and acetone. 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 dissolving and removing the lithium-containing molten salt, and when acetic acid reacts with the lithium-containing molten salt to form water, acetic anhydride takes in water to form acetic acid. Thus, the separation of water can be suppressed. In addition, when using acetic anhydride and acetic acid, first, acetic anhydride is mixed with a product, and after grinding using a mortar etc. and pulverizing particles, acetic acid is added in the state to which acetic anhydride is made to adapt to particles. Is preferred. According to this method, the water formed by the reaction of acetic acid and the lithium-containing molten salt can be quickly reacted with acetic anhydride to reduce the chance of contact between the product and water, so the oxidation and decomposition of the target substance are effective. Can be suppressed.

<Lithium borate compounds>
The lithium borate compound obtained by the method described above is
Composition formula: Li _{1 + a−b} A _b 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, and 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, 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 manufacturing method of the present invention exhibits excellent cycle characteristics when used as a positive electrode active material of a lithium secondary battery. Specifically, when metal lithium is used for the negative electrode, and the initial constant voltage charging is performed for 10 hours at 4.5 V at 0.1 C at a test temperature of 30 ° C., the charge and discharge test is performed at 4.5 to 1.5 V The discharge capacity after 50 cycles of charge and discharge 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.

In this compound, the lithium ion in the molten salt infiltrates into the lithium ion site of the lithium borate compound by using the molten salt of lithium nitrate, and the lithium ion is excessive compared to the stoichiometric amount. It becomes a compound to contain. Moreover, in the case of a molten salt containing lithium nitrate, the reaction can be performed 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, it becomes a material having good cycle characteristics and high capacity. The lithium borate compounds obtained by the above-mentioned method preferably have an average particle diameter in the range of 500 nm to 50 μm, more preferably 600 nm to 20 μm. In the present specification, the average particle diameter refers to the maximum diameter of the plurality of particles (maximum value of the distance between two parallel lines sandwiching the particles) from the image obtained by observation with a scanning electron microscope (SEM). It is a value measured and calculated.

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

The specific method of the carbon coating treatment is not particularly limited, and in addition to the vapor phase method in which the heat treatment is performed in an atmosphere containing a carbon-containing gas such as methane gas, ethane gas and butane gas, organic substances as a carbon source and lithium borate A thermal decomposition method is also applicable by carbonizing the organic substance by heat treatment after uniformly mixing the compound. In particular, it is preferable to apply a ball milling method in which a heat treatment is performed after a carbon material and Li ₂ CO ₃ are added to the lithium borate compound and uniformly mixed until the lithium borate compound is amorphized by a ball mill. According to this method, the lithium borate compound, which is a positive electrode active material, is made amorphous by ball milling, uniformly mixed with carbon and adhesion is increased, and recrystallization of the lithium borate compound is further performed by heat treatment. At the same time, carbon is uniformly deposited around the lithium borate compound. At this time, due to the presence of Li ₂ CO ₃ , the lithium excess borate type compound does not become lithium deficient and exhibits high charge and discharge capacity.

As for the degree of amorphization, in the X-ray diffraction measurement using the Kα ray of Cu as a light source, the half value width of the diffraction peak derived from the (011) plane of the sample having crystallinity before ball milling is B (011) _Crystal , 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 0.1-0.5 Just do it.

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

Lithium borate-based compound, for the mixing ratio of the carbon material, and _Li 2 CO _3, the lithium borate-based compound to 100 parts by mass, 20 to 40 parts by weight of carbon-based material, the _Li 2 CO ₃ 20 to 40 parts by weight And it is sufficient.

After the ball milling treatment is performed until the lithium borate compound is amorphized by the method described above, a heat treatment is performed. The heat treatment is performed in a reducing atmosphere to keep the transition metal ion contained in the lithium borate compound at a divalent value. As the reducing atmosphere in this case, nitrogen and carbon dioxide are used to suppress reduction of the divalent transition metal ion to the metal state, as in the synthesis reaction of the lithium borate compound in the molten salt. It is preferable to be in a mixed gas atmosphere of at least one gas selected from the group consisting of: and a reducing gas. The mixing ratio of the reducing gas to the 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.degree. 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. It is not preferable because the charge and discharge capacity decreases. The heat treatment time may be usually 1 to 10 hours.

Further, as another carbon coating treatment method, a carbon material and LiF are added to the above lithium borate compound, and uniformly mixed until the lithium borate compound is amorphized by a ball mill in the same manner as the above method, followed by heat treatment You may In this case, carbon is uniformly deposited around the lithium borate compound simultaneously with the recrystallization of the lithium borate compound in the same manner as described above, the conductivity is improved, and the lithium borate compound is further obtained. Part of the oxygen atoms of is substituted with fluorine atoms,
Composition formula: Li _{1 + a−b} A _b M _1−x M ′ _x BO _{3 + c−y} 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 At least one element selected from the group consisting of Nb, Mo, W, Ti and Zr, each subscript being as follows: 0 ≦ x ≦ 0.5, 0 <a <1, 0 ≦ b A fluorine-containing lithium borate compound represented by <0.2, 0 <c <0.3, 0 <y <1 and a> b) is formed.

When this compound is used as a positive electrode due to the addition of F, the average voltage rises from 2.6 V to 2.8 V, and becomes a positive electrode material having more excellent performance. At this time, due to the presence of LiF, the lithium excess borate-based compound does not become lithium deficient and exhibits high charge and discharge capacity.

In this method, with respect to the mixing ratio of the lithium borate compound, the carbon material and LiF, 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, if necessary, Li ₂ CO ₃ may be contained. The conditions for ball milling and heat treatment may be the same as those described above.

<Positive electrode for secondary battery>
The lithium borate compounds obtained by synthesizing in the molten salt, the lithium borate compounds subjected to the carbon coating treatment, and the lithium borate compounds added with fluorine are all actives for positive electrodes such as lithium ion secondary batteries It can be used effectively as a substance. The positive electrode using these lithium borate compounds can have the same structure as that of a normal lithium ion secondary battery positive electrode.

For example, to the above-mentioned lithium borate compounds, conductive aids such as acetylene black (AB), ketjen black (KB), vapor grown carbon fiber (vapor grown carbon fiber (VGCF), etc. A binder such as ethylene oxide (PTFE), styrene butadiene rubber (SBR), or a solvent such as N-methyl-2-pyrrolidone (NMP) is added to form a paste, and this is applied to the current collector to produce a positive electrode. can do. The amount of the conductive aid used is not particularly limited, but 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, for example, 5 to 20 parts by mass with respect to 100 parts by mass of the lithium borate compound. Further, as another method, a mixture of a lithium borate compound, the above-mentioned conductive additive and a binder is kneaded using a mortar or a press to form a film, which is crimped to a current collector with a press. The positive electrode can also be produced by the following method.

The current collector is not particularly limited, and materials conventionally used as a positive electrode for lithium ion secondary batteries, such as aluminum foil, aluminum mesh, stainless steel mesh and the like can be used. Furthermore, carbon non-woven fabric, carbon woven fabric and the like can also be used as the current collector.

The positive electrode for a lithium ion secondary battery according to the present invention is not particularly limited in its shape, thickness and the like, but for example, the active material is filled and then compressed to a thickness of 10 to 200 μm, more preferably Is preferably 20 to 100 μm. Therefore, the loading amount of the active material may be appropriately determined according to the type, structure, and the like of the current collector to be used so as to obtain the above-mentioned thickness after compression.

<Secondary battery>
A secondary battery using the above-described positive electrode for secondary battery can be manufactured by a known method. That is, as the positive electrode material, the above-described positive electrode is used, and as the negative electrode material, known metal lithium, carbon based material such as graphite, silicon based material such as silicon thin film, alloy based material such as copper-tin or cobalt-tin, An oxide material such as lithium titanate may be used. In addition, as an electrolytic solution, 0.5 mol / liter of lithium salt such as lithium perchlorate, LiPF ₆ , LiBF ₄ , and LiCF ₃ SO _{3 in} a known non-aqueous solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, and dimethyl carbonate. A solution dissolved at a concentration of L to 1.7 mol / L may be used. Still other known battery components may be used to assemble a lithium ion secondary battery (or lithium secondary battery) according to a conventional method.

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. In the range which does not deviate from the summary of the present invention, it can carry out with various forms which gave change, improvement, etc. which a person skilled in the art can make.

Hereinafter, the present invention will be specifically described by way of 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 As a raw material, 0.01 mol of iron (manufactured by High Purity Chemical Co., Ltd., purity 99.9%), H ₃ BO ₃ boric acid (manufactured by Kishida Chemical Co., Ltd.) , Purity 99%) 0.01 mol and lithium nitrate (Kishida Chemical Co., Ltd. make, 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.

To this was added 2 mL of water, mixed using a pestle and mortar, heated to 100 ° C., mixed further, and dried at 100 ° C. Thereafter, the obtained powder is 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 The mixture was allowed to react for 13 hours in the molten state.

After the reaction, the entire reactor core, which is a reaction system, was removed from the electric furnace, which is a heater, and quenched 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 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 was approximately consistent with the reported pattern of monoclinic LiFeBO _{3 in} the space group C2 / c.

Also, a scanning electron microscope (SEM) photograph of the product is shown in FIG. When the average particle diameter was calculated from FIG. 2, it could be confirmed that it was a powder consisting of fine crystal grains of 6 μm.

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

Comparative Example 1 Synthesis of Iron-Containing Lithium Borate Compound by Solid State 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: The mixed powder mixed to become 1 was ball milled, and then heat treated at 650 ° C. for 10 hours to obtain an iron-containing lithium borate compound.

Reference Example 1 Synthesis of Iron-Containing Lithium Borate Compound by Molten Salt Method As a raw material, iron oxalate FeC ₂ O ₄ .2H ₂ O (manufactured by Sigma Aldrich, purity 99.99%), lithium hydroxide (anhydrous) LiOH (anhydrous) Carbonate mixture (lithium carbonate (Kishida Chemical Co., Ltd.) using 0.005 mol of each of Kisida Chemical Co., Ltd. (purity 98%) and boric acid H ₃ BO ₃ (Kishida Chemical Co., Ltd., purity 99.5%) Product, purity 99.9%), sodium carbonate (Kishida Chemical Co., Ltd., purity 99.5%), and potassium carbonate (Kishida Chemical Co., Ltd., purity 99.5%) at a molar ratio of 0.435: 0. Mixed at 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.

To this was added 20 ml of acetone, mixed with a zirconia ball mill at 500 rpm for 60 minutes, and dried. Thereafter, the obtained powder is 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 obtain carbonate The mixture was allowed to react in the molten state for 15 hours.

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

Then, acetic anhydride (20 ml) was added to the product and the mixture was triturated with a mortar, acetic acid (10 ml) was added, carbonates etc were reacted, removed, and filtered to obtain a powder of 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 pattern of LiFeBO _{3 in} the reported space group C2 / c. Moreover, as a result of observing a product by SEM, it could be confirmed that it was a powder composed of crystal grains of several μm or less. Furthermore, as a result of elemental analysis of the product by ICP method, the composition formula was Li _1.04 FeBO _3.10 , and it was confirmed that the composition was a lithium excess LiFeBO ₃ -based lithium borate compound.

[Fabrication of lithium secondary battery]
A lithium secondary battery using any 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, followed by milling for 5 hours at 450 rpm using a planetary ball mill (5 mm of zirconia balls) to obtain carbon dioxide and hydrogen mixed gas _(CO 2: _{H 2} (molar ratio) = 100: 3) in an atmosphere of, and was heated at 700 ° C..

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 a sheet method Thus, three types of electrodes (positive electrode) were produced and vacuum dried at 140 ° C. for 3 hours.

Thereafter, a solution of 1 mol / L in which LiPF ₆ is dissolved in a predetermined solvent is used as an electrolyte, and a polypropylene film (made by Celgard, Celgard 2400) as a separator, a lithium metal foil as a negative electrode, and the three types already described as a positive electrode. A coin battery (# E1, # C1 and # 01) was manufactured using any of the above electrodes.

In addition, according to the test temperature of a charging / discharging test, electrolyte solution prepared two types from which a solvent differs, and it used 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): diethylene carbonate (DEC) was mixed at a volume ratio of EC: DEC = 1: 1 was used.

[Charge and discharge test]
About the produced coin battery, the charging / discharging test was done at 30 degreeC or 60 degreeC. The test conditions are: voltage 4.5 to 1.5 V at 0.1 C at a test temperature of 30 ° C. (10 hours at 4.5 V for initial constant voltage charging), voltage 4.2 at a temperature of 60 ° C. at 0.1 C The voltage was set to 1.5 V (the first constant voltage charge was 10 hours at 4.2 V). The charge and discharge characteristics of the battery # E1 are shown in FIGS. 3 and 4, and the charge and discharge characteristics of the battery # 01 are shown in FIG. Also, Table 1 shows the discharge capacity after 5 cycles of each battery, the average voltage after 5 cycles, and the number of cycles in which the initial discharge capacity can be maintained at 90%.

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

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

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

Therefore, it was found that the lithium borate compounds synthesized by the molten salt method using lithium nitrate, when used as a positive electrode active material, provide high capacity and excellent cycle characteristics even when used at room temperature. In addition, since synthesis at a lower temperature than Example 1 is also possible by utilizing the fact that lithium nitrate has a low melting point, it is possible to further refine the particles, and it is expected that the battery characteristics will be improved.

Claims

Composition formula: Li 1 + a−b A b 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, and 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, 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 of a lithium secondary battery, after performing initial constant voltage charging at 4.5 V for 10 hours at 0.1 C at a test temperature of 30 ° C. after 50 cycles of charge and discharge at 4.5 to 1.5 V The 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 comprising at least one selected from the group consisting of pure iron, pure manganese and a compound containing iron and / or manganese;
It is characterized in that boric acid is reacted in a molten salt of the lithium-containing molten salt raw material which is higher than the melting point of the lithium-containing molten salt raw material and 900 ° C. or less in a mixed gas atmosphere containing carbon dioxide and a reducing gas. Method for producing lithium borate compounds.
The method for producing a lithium borate compound according to claim 2, wherein the temperature of the molten salt is 200 ° C or more and 900 ° C or less.
The method for producing a lithium borate compound according to claim 3, wherein the temperature of the molten salt is 300 ° C or more and 700 ° C or less.
The method according to claim 2, wherein the mixed gas contains hydrogen gas as the reducing gas.
The method for producing a lithium borate compound according to claim 2, wherein the mixed gas contains carbon dioxide and reducing gas in a volume ratio of carbon dioxide: reducing gas = 100: 3 to 60:40.
The method for producing a lithium borate compound according to claim 2, wherein the transition metal-containing raw material contains iron and / or manganese having a valence of 2 or less.
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 claim 2, wherein the lithium-containing molten salt raw material is a mixed molten salt containing lithium nitrate as the 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 method for producing a lithium borate compound according to claim 2, which contains 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. .
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 of Claim 2.
The positive electrode active material for lithium ion secondary batteries which consists of a lithium borate type compound obtained by the method of Claim 2.
The positive electrode for lithium ion secondary batteries containing the positive electrode active material of Claim 12.
A lithium ion secondary battery comprising the positive electrode according to claim 13 as a component.