JP2005268127A - Positive electrode material for lithium secondary battery, its manufacturing method and lithium secondary battery using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new positive electrode material with large charging/discharging capacity at high voltage and a good cycle property to reduce the weight of a lithium secondary battery and its manufacturing method, and to provide the lithium secondary battery using the positive electrode material. <P>SOLUTION: This positive electrode material for the lithium secondary battery has a chemical composition represented by Li<SB>x</SB>Mn<SB>1-y</SB>Ti<SB>y</SB>O<SB>2</SB>(0.40< x<0.50, 0< y<0.56) and a crystal structure belonging to a rhombic system, and consists of lithium, manganese, titanium and oxygen which have a tunnel structure which lithium occupies. The sodium content is 0.05 or less in a molar ratio of Na/Li. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a positive electrode material for a lithium secondary battery, a manufacturing method thereof, and a lithium secondary battery containing the material as a positive electrode active material.

  Currently, in Japan, most of the secondary batteries installed in portable electronic devices such as mobile phones and notebook computers are lithium secondary batteries. In addition, lithium secondary batteries are expected to be put into practical use as large batteries for electric vehicles and power load leveling systems in the future, and their importance is increasing.

  This lithium secondary battery uses a positive electrode having a lithium-containing transition metal composite oxide as an active material and a material capable of inserting and extracting lithium, such as lithium metal, lithium alloy, metal oxide, or carbon. A main component is a negative electrode as a substance and a separator or solid electrolyte containing a non-aqueous electrolyte.

Among these components, those studied as positive electrode active materials include layered rock salt type lithium cobalt oxide (LiCoO ₂ ), layered rock salt type lithium nickel oxide (LiNiO ₂ ), spinel type lithium manganese oxide ( LiMn ₂ O ₄ ) and the like.

In particular, the layered rock salt type lithium cobalt oxide LiCoO ₂ has a battery operating voltage (difference between the redox potential of the transition metal in the positive electrode and the redox potential of the negative electrode element), charge / discharge capacity (can be desorbed and inserted from the positive electrode). It is expected that the future demand will increase further as a positive electrode constituent material of a lithium secondary battery.
However, since this compound contains cobalt, which is a rare metal, as a main component, it is one of the high-cost factors for lithium secondary batteries. Furthermore, considering that about 20% of the world's cobalt production is already used in the battery industry, it is unclear whether only the cathode material made of LiCoO ₂ can meet future demand growth. is there.

Further, the layered rock salt type lithium nickel oxide LiNiO ₂ using nickel cheaper than cobalt is advantageous in terms of cost and capacity, and is being developed as a promising alternative material for lithium cobalt oxide. . However, the lithium secondary battery using this lithium nickel oxide as the positive electrode active material has the risk of decomposition, heat generation, ignition, etc. if kept at a high temperature due to the instability of the positive electrode active material in the charged state. There are still many issues that need to be resolved regarding safety.

Furthermore, the spinel-type lithium manganese oxide LiMn ₂ O ₄ is used less expensive manganese than cobalt and nickel, and since it is excellent in safety in the charged state, some alternative to LiCoO ₂ Has been put to practical use.
However, there is a problem that the capacity is small compared to LiCoO ₂ and LiNiO ₂ . Further, since there is a problem of remarkable characteristic deterioration due to dissolution of manganese in an electrolytic solution at 50 ° C. or higher, substitution of LiCoO ₂ by this material has not progressed as expected.

On the other hand, as a feature of the crystal structure, research has been conducted to synthesize Li _0.44 MnO ₂ by an ion exchange method using Na _0.44 MnO ₂ having a one-dimensional tunnel structure as a starting material. (See Non-Patent Document 1)

M.M. M.M. Doeff, KT. Hwang, A .; Anapolsky, T .; J. et al. Richardson, Electrochem. Soc. Proceedings Volume, 99-24, 48-56 (2000).

The material having a tunnel structure of Li _0.44 MnO ₂ uses manganese oxide as a raw material, which is less resource-constrained and inexpensive from the viewpoint of the cobalt-based oxide substitute material. It is a promising material from the viewpoint of structural stability that can withstand changes in the chemical bond between manganese and oxygen associated with the oxidation-reduction reaction of ³⁺ and Mn ⁴⁺ and that does not change into a spinel structure due to charge / discharge.

  However, in Non-Patent Document 1, the crystallinity of the sodium compound used as a raw material is low, and the reaction temperature of the ion exchange treatment cannot be increased to 200 ° C. or higher. It could not be completely removed and the expected performance could not be realized. The reported amount of residual sodium is about 0.057 in terms of Na / Li molar ratio. Significant residual sodium hinders lithium ion conduction in the tunnel and makes it difficult to realize the theoretical capacity. It is supposed to be.

Therefore, the present invention solves the above-mentioned problems as described above, has a Li _0.44 MnO ₂ type tunnel structure as described above, and further reduces the weight of the lithium secondary battery. And a lithium secondary battery using the positive electrode material, a method for producing the same, and a lithium secondary battery using the positive electrode material.

As a result of intensive studies on starting materials, synthesis conditions, etc., the present inventor can synthesize a highly crystalline raw crystal material by firing at 900 ° C. to 1400 ° C., which is higher than the firing temperature of Non-Patent Document 1, and , Na _0.44 Mn _1-y Ti _y O ₂ crystal material fired at high temperature can be used as a raw material, ion exchange treatment at higher temperature becomes possible, and as a result, the amount of residual sodium can be significantly reduced In addition, when the resulting lithium compound subjected to the ion exchange treatment is used as a positive electrode material for a lithium ion secondary battery, it is possible to charge and discharge stably even in the 4 V region that has not been developed so far. The present inventors have found that it is possible, has a large charge / discharge capacity, and good cycle characteristics, and has completed the present invention.

That is, this invention takes the structure of 1-6 shown below.
1. It is expressed as Li _x Mn _1-y Ti _y O ₂ (0.40 <x <0.50, 0 <y <0.56) as the chemical composition formula, belongs to the orthorhombic system as the crystal structure, and is occupied by lithium A lithium secondary battery positive electrode material comprising lithium, manganese, titanium, and oxygen having a tunnel structure that has a sodium content of 0.05 or less in terms of a Na / Li molar ratio.
2. The chemical composition is expressed as Na _x Mn _1-y Ti _y O ₂ (0.40 <x <0.50, 0 <y <0.56), and the crystal structure belongs to the orthorhombic system and is occupied by sodium. 2. The method for producing a positive electrode material according to 1, wherein the compound is produced by ion exchange treatment using a compound composed of sodium, manganese, titanium and oxygen having a tunnel structure as a starting material.
3. 3. The method for producing a positive electrode material according to 2, wherein the ion exchange treatment is performed in a molten salt containing a lithium compound.
4). The method for producing a positive electrode material according to claim 2, wherein the ion exchange treatment is performed in an organic solvent or an aqueous solution in which a lithium compound is dissolved.
5). A lithium secondary battery, wherein the positive electrode of the lithium secondary battery having a positive electrode, a negative electrode, and an electrolyte material is composed of the positive electrode material described in 1.
6). 6. The lithium secondary battery according to 5, wherein the battery can be stably charged and discharged in an area of 4 V or more as an operating voltage of the battery.

ADVANTAGE OF THE INVENTION According to this invention, the novel lithium manganese titanium oxide positive electrode material which can be charged / discharged stably in a high operating voltage area | region (4V or more) can be obtained using an inexpensive raw material.
The lithium secondary battery of the present invention using this positive electrode material has high voltage and high capacity, can exhibit excellent charge / discharge cycle characteristics, and is highly practical.

1) Sodium Transition Metal Oxide Crystal Material and Method for Producing the Same Na _x Mn _1-y Ti _y O ₂ (0.40 <x <0.50, 0 <y <0.56) used as a raw material for the positive electrode material of the present invention ) Compounds are prepared using sodium compounds, manganese compounds and titanium compounds as starting materials, and known Na _0.44 MnO ₂ or Na ₄ Mn ₄ Ti ₅ O ₁₈ (= Na _0.44 Mn _0.44 Ti _{0. 56} O ₂ ), a compound having a tunnel structure, and the ratio of manganese and titanium can be freely selected within the above composition range.

  This compound is produced, for example, by firing a mixture containing (1) at least one of sodium and a sodium compound, (2) at least one of manganese and a manganese compound, and (3) titanium and at least one of a titanium compound. can do.

As the sodium raw material, at least one of sodium (metallic sodium) and a sodium compound is used. The sodium compound is not particularly limited as long as it contains sodium. For example, oxides such as Na ₂ O and Na ₂ O ₂ , salts such as Na ₂ CO ₃ and NaNO ₃ , hydroxides such as NaOH, etc. Is mentioned. Among these, Na ₂ CO ₃ is particularly preferable.

As the manganese raw material, at least one of manganese (metallic manganese) and a manganese compound is used. The manganese compound is not particularly limited as long as it contains manganese. For example, oxides such as Mn ₃ O ₄ , Mn ₂ O ₃ and MnO ₂ , salts such as MnCO ₃ and MnCl ₂ , Mn (OH) ₂ And hydroxides such as MnOOH and the like. Among these, Mn ₂ O ₃ , MnO _{2 and the} like are particularly preferable.

As the titanium raw material, at least one of titanium (metallic titanium) and a titanium compound is used. The titanium compound is not particularly limited as long as it contains titanium, and examples thereof include oxides such as TiO, Ti ₂ O ₃ and TiO ₂ , salts such as TiCl _{4 and the} like. Among these, TiO ₂ is particularly preferable.

First, a mixture containing these is prepared. The mixing ratio of the sodium raw material, the manganese raw material, and the titanium raw material is preferably mixed so that the tunnel structure is generated. Specifically, the chemical composition formula may be Na _x Mn _1-y Ti _y O ₂ (0.40 <x <0.50, 0 <y <0.56). For example, mixing may be performed so that Na / (Mn + Ti) is about 0.4 to 0.7, preferably 0.43 to 0.55 in terms of molar ratio. Usually, when firing at a high temperature, the contained sodium is likely to volatilize, and the amount of sodium in the product is often less than the charged composition. It is preferable to increase the amount charged. Further, the ratio between manganese and titanium may be arbitrarily selected within the range of (0 <y <0.56).

  Moreover, a mixing method is not specifically limited as long as these can be mixed uniformly, For example, what is necessary is just to mix by a wet type or a dry type using well-known mixers, such as a mixer.

  The mixture is then fired. The firing temperature can be appropriately set according to the composition of the mixture, but is usually about 600 to 1400 ° C., preferably 900 to 1200 ° C. Also, the firing atmosphere is not particularly limited, but usually it may be carried out in an oxidizing atmosphere or air. The firing time can be appropriately changed according to the firing temperature and the like. Although the cooling method is not particularly limited, it may be naturally cooled (cooled in the furnace) or gradually cooled.

  After firing, the fired product may be pulverized by a known method as necessary, and the above firing step may be further performed. That is, in the method of the present invention, it is preferable that the mixture is repeatedly fired, gradually cooled and pulverized twice or more. Note that the degree of pulverization may be appropriately adjusted according to the firing temperature and the like.

Through the above-described steps, the target chemical composition formula Na _x Mn _1-y Ti _y O ₂ (0.40 <x <0.50, 0 <y <0.56), needle-like or fibrous form Crystal particles (typical size: 10 × 10 × 1 μm) or flaky crystal particles (typical size: 3 × 1 × 0.5 μm), in the case of needles, about 10 × 2 × 1 μm, flaky In this case, about 3 × 3 × 1 μm is preferable, and a more preferable crystal material having a size of about 20 × 3 × 1 μm can be obtained.

  Non-Patent Document 1 attempts to obtain a sodium transition metal oxide having as small a particle as possible, but produces a positive electrode material for a lithium secondary battery in which the obtained sodium compound has low crystallinity and has desired characteristics. As a raw material, it was unsuitable. On the other hand, according to the present invention, a sodium transition metal oxide crystal useful as a raw material for producing a positive electrode material for a lithium secondary battery having desired characteristics by using a crystal material having a large size. Get material.

2) Positive electrode material for lithium secondary battery and method for producing the same In the present invention, the chemical composition formula Na _x Mn _1-y Ti _y O ₂ (0.40 <x <0.50, 0 <y <0) obtained above. .56) is subjected to an ion exchange treatment to form Li _x Mn _1-y Ti _y O ₂ (0.40 <x <0.50, 0 <0) having a tunnel structure similar to that of the crystalline material. y <0.56) A positive electrode material can be produced.

This manufacturing method is characterized in that sodium regularly arranged in a tunnel can be almost completely replaced with lithium by ion exchange treatment due to the characteristics of the crystal structure.
That is, the positive electrode material represented by the chemical composition formula Li _x Mn _1-y Ti _y O ₂ (0.40 <x <0.50, 0 <y <0.56) obtained in the present invention is lithium, Although manganese, titanium, and oxygen are the main constituent elements, a part of impurity elements within a range that does not hinder the effects of the present invention may be included.

This Li _x Mn _1-y Ti _y O ₂ (0.40 <x <0.50, 0 <y <0.56) positive electrode material has a Li _0.44 MnO ₂ type tunnel structure as a whole. However, a part of the other crystal structure may be included within a range not hindering the effect.

(Method for producing positive electrode material)
In Na _x Mn _1-y Ti _y O ₂ (0.40 <x <0.50, 0 <y <0.56) obtained in 1) above, (1) in a molten salt containing a lithium compound By performing ion exchange treatment or (2) performing ion exchange treatment in an organic solvent or aqueous solution, it has a crystal structure of Li _0.44 MnO ₂ type and has a chemical composition formula Li _x Mn _1-y Ti _y A compound represented by O ₂ (0.40 <x <0.50, 0 <y <0.56) is obtained.

In this case, (1) pulverized Na _x Mn _1-y Ti _y O ₂ (0.40 <x <0.50, 0 <y <0.56) in a molten salt containing a lithium-containing compound. It is preferable to perform ion exchange treatment while dispersing. As the molten salt, a molten salt containing any one or more of salts that melt at a low temperature such as lithium nitrate, lithium chloride, lithium bromide, and lithium iodide can be used. As a preferred method, a lithium compound and a powder of Na _x Mn _1-y Ti _y O ₂ fired product are mixed well. The mixing ratio is usually ₂ to 40, preferably 10 to 30, in terms of the molar ratio of Na in Li / Na _x Mn _1-y Ti _y O ₂ in the molten salt.

The ion exchange temperature is 260 ° C to 330 ° C. When the ion exchange temperature is lower than 260 ° C., sodium in Na _x Mn _1-y Ti _y O ₂ (0.40 <x <0.50, 0 <y <0.56) is completely converted into lithium. A substantial amount of sodium remains in the product without being exchanged. On the other hand, when the ion exchange temperature is higher than 330 ° C., a part of the ion exchange temperature changes to a spinel structure, so that a uniform crystal structure cannot be obtained. The treatment time is usually 2 to 20 hours, preferably 5 to 15 hours.

Further, as a method of ion exchange treatment, (2) a method of treating in an organic solvent or aqueous solution in which a lithium compound is dissolved is also suitable. In this case, pulverized Na _x Mn _1-y Ti _y O ₂ (0.40 <x <0.50, 0 <y <0.56) is put into the organic solvent in which the lithium-containing compound is dissolved. And processing at a temperature below the boiling point of the organic solvent. In order to increase the ion exchange rate, it is preferable to perform ion exchange while refluxing the solvent near the boiling point of the organic solvent. The treatment temperature is usually 100 ° C to 200 ° C, preferably 140 ° C to 180 ° C. Further, the treatment time is not particularly limited, but it is usually 5 to 50 hours, preferably 10 to 20 hours, because a reaction time is required at a low temperature.

  As the lithium-containing compound used in the present invention, carbonates, acetates, nitrates, oxalates, halides, butyllithium and the like are preferable, and these may be used alone or in combination of two or more as required. Further, as the organic solvent used in the present invention, higher alcohols such as hexanol and ethoxyethanol, ethers such as diethylene glycol monoethyl ether, or organic solvents having a boiling point of 140 ° C. or more are good in workability. preferable. These may be used alone or in combination of two or more as required.

The density | concentration of the lithium containing compound in an organic solvent or aqueous solution is 3-10 mol% normally, Preferably it is 4-6 mol%. The dispersion concentration of Na _x Mn _1-y Ti _y O _{2 in} the organic solvent or aqueous solution is not particularly limited, but is preferably 1 to 20% by weight from the viewpoints of operability and economy.

After the ion exchange treatment, the obtained product is thoroughly washed with distilled water, then washed with methanol and ethanol, and dried to obtain the target Li _x Mn _1-y Ti _y O ₂ (0.40). <X <0.50, 0 <y <0.56). The washing method and the drying method are not particularly limited, and a normal method may be used, or natural drying in a desiccator may be used.

3) Lithium secondary battery The lithium secondary battery of the present invention is characterized by using the positive electrode material for a lithium secondary battery. That is, a battery element of a known lithium secondary battery (coin type, button type, cylindrical type, etc.) can be used as it is except that the lithium manganese titanium oxide of the present invention is used as the positive electrode material.

  Therefore, for example, a positive electrode mixture obtained by blending a lithium manganese titanium oxide of the present invention with a conductive agent, a binder and the like as necessary can be prepared, and a positive electrode can be produced by pressure-bonding it to a current collector. As the current collector, a stainless mesh, aluminum foil or the like can be preferably used. As the conductive agent, acetylene black, ketjen black or the like can be preferably used. As the binder, tetrafluoroethylene, polyvinylidene fluoride, or the like can be preferably used.

  The composition of the lithium manganese titanium oxide, the conductive agent, the binder and the like in the positive electrode mixture is not particularly limited, and may be a known composition ratio.

  In the lithium secondary battery of the present invention, as the counter electrode with respect to the positive electrode, known ones such as metallic lithium, a carbon-based material, an alloy-based material can be employed. Moreover, a well-known battery element should just be employ | adopted for a separator, a battery container, etc.

  Moreover, a well-known thing is applicable also as electrolyte solution. For example, an electrolyte such as lithium perchlorate or lithium hexafluorophosphate dissolved in a solvent such as ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC), or diethyl carbonate (DEC) It can be used as an electrolyte.

  EXAMPLES Examples will be shown below to further clarify the features of the present invention, but the present invention is not limited to these examples.

(Example 1)
[Production of sodium transition metal oxide]
Sodium carbonate (Na ₂ CO ₃ ), manganese oxide (Mn ₂ O ₃ ), and titanium oxide (TO ₂ ) were uniformly mixed at a molar ratio of 1: 1: 2. The mixture was calcined in air at 1000 ° C. for 15 hours and then gradually cooled in a furnace. A series of operations (calcination, slow cooling and pulverization) of pulverizing the obtained fired body were repeated again to obtain a starting material having a target Li _0.44 MnO ₂ type crystal structure of a substantially single phase. The X-ray powder diffraction pattern of the obtained product is shown in FIG. Moreover, the schematic diagram which shows the cross section of this crystal | crystallization is shown in FIG. In FIG. 3, black circles represent lithium. A large S-shaped tunnel structure in the direction perpendicular to the paper surface is observed at the center of the portion surrounded by the rectangle, and a distorted hexagonal small tunnel structure is observed above and below the tunnel structure.
Assuming a Li _0.44 MnO ₂ type crystal structure, the orthorhombic lattice constant is obtained, and a = 9.2329 (6) A (the horizontal length of the portion surrounded by the rectangle in FIG. 3) ), B = 26.3938 (19) A (the vertical length of the portion surrounded by the rectangle in FIG. 3), and c = 2.8828 (3) A. As a result of analyzing the chemical composition of this sample by ICP emission analysis, it was confirmed that Na _0.44 Mn _0.5 Ti _0.5 O ₂ was appropriate.

[Manufacture of positive electrode materials]
Next, the sample obtained above was subjected to ion exchange treatment in a molten salt in which lithium nitrate and lithium chloride were mixed at a molar ratio of 88:12. The amount of Na _0.44 Mn _0.5 Ti _0.5 O _{2 in} the molten salt is a molar ratio, Li in the molten salt: Na in the sample = 20: 1, and the temperature of the molten salt is 300 ° C. did. An ion exchange treatment was performed for a treatment time of 10 hours, and the obtained solid was washed with distilled water, methanol, ethanol, etc. and dried to obtain a sample. As a result of analyzing the chemical composition of this sample by ICP emission spectrometry, the chemical formula of Li _0.44 Mn _0.5 Ti _0.5 O ₂ is reasonable, and the remaining sodium content is Na in terms of molar ratio. The molar ratio of / Li was 0.005. Moreover, the X-ray powder diffraction pattern of the obtained sample is shown in FIG. Assuming the same structure as the starting material Na _0.44 Mn _0.5 Ti _0.5 O ₂ , the orthorhombic lattice constant (a = 9.0313 (10) A, b = 24. 681 (4) A, c = 2.8863 (5) A) was confirmed to be nearly single phase.

[Lithium secondary battery]
The obtained Li _0.44 Mn _0.5 Ti _0.5 O ₂ sample was used as the positive electrode material 1, metal lithium as the negative electrode material 3, lithium hexafluorophosphate as ethylene carbonate (EC) and diethyl carbonate (DEC). A lithium secondary battery (electrochemical cell) as shown in FIG. 4 was prepared using the 1M solution dissolved in the above mixed solvent as the electrolyte solution 2, and its charge / discharge characteristics were measured. The battery was produced according to a known electrochemical cell configuration / assembly method.

When this lithium secondary battery was subjected to a charge / discharge test at room temperature with a current density of 0.057 mA / cm ² and a cut-off potential of 4.8 V-2.5 V, the average discharge voltage was 3.48 V and the initial discharge capacity was 163 mAh. It was found that charging / discharging can be stably performed at / g. The discharge capacity after 10 cycles was maintained at about 150 mAh / g, and the cycle characteristics were also good. The charge / discharge characteristics and the charge / discharge capacity up to 10 cycles are shown in FIGS.

(Example 2)
[Production of sodium transition metal oxide]
Na obtained by treating in the same manner as in Example 1 while changing the molar ratio of manganese oxide (Mn ₂ O ₃ ) and titanium oxide (TiO ₂ ) as starting materials under the same conditions as in Example 1 above. _When the lattice constant of _0.44 Mn _1-y Ti _y O ₂ fired body (y = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.56) was determined, As shown in FIG. 7, a systematic change in lattice volume depending on the amount of titanium y was confirmed, confirming that the system had a solid solution composition.

[Manufacture of positive electrode materials]
In the same manner as in Example 1, each sample was subjected to ion exchange treatment to produce a Li _0.44 Mn _1-y Ti _y O ₂ sample. As shown in FIG. 8, each lattice constant was confirmed to show a systematic change depending on the amount of titanium y as in the case of the sodium compound.

[Lithium secondary battery]
Li _0.44 Mn _1-y Ti _y O ₂ fired body thus obtained (y = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.56) A lithium secondary battery was prepared in the same manner as in the previous section using each of the above samples as a positive electrode material, and a charge / discharge test was performed under the same conditions. In each case, the average discharge voltage was about 3.5 V and the initial discharge capacity was 150 to 180 mAh / g. It was confirmed that charging / discharging was possible stably. FIG. 9 shows changes in the initial discharge capacity depending on the chemical composition (titanium content).

According to the present invention, as a chemical composition formula, Na _x Mn _1-y Ti _y O useful as a raw material of a lithium manganese titanium oxide positive electrode material that can be stably charged and discharged in a high operating voltage region (4 V or higher). ₂ (0.40 <x <0.50, 0 <y <0.56), a crystal material belonging to the orthorhombic system as a crystal structure and having a tunnel structure occupied by sodium can be obtained.

In addition, Li _x Mn _1-y Ti _y O ₂ (0.40 <x <0.50, 0 <y <0.56) expressed as a chemical composition formula of the present invention obtained from this crystalline material is lithium manganese titanium. The oxide is useful as a positive electrode material for a lithium secondary battery having a high voltage, a high capacity, little cycle deterioration, low cost, and light weight.
Moreover, the lithium secondary battery of the present invention using the above lithium manganese titanium oxide material as a positive electrode material (positive electrode active material) can exhibit high charge and discharge cycle characteristics at high voltage and high capacity, and can be practically used. It is highly probable.

  The present invention is the result of “Development of lithium battery technology for fuel cell vehicles” (NEDO contract research), “Development of high-performance lithium battery element technology“ Development of new positive electrode material using base metal element ”in 2003.

The lithium manganese titanium oxide obtained by the present invention is useful as a positive electrode material for a low-cost lithium secondary battery having a high voltage, a high capacity, and little cycle deterioration.
Moreover, the lithium secondary battery of the present invention using the lithium manganese oxide material as a positive electrode material (positive electrode active material) can exhibit excellent charge / discharge cycle characteristics at a high voltage and a high capacity, and is practical. Is high.

_{2 is} an X-ray powder diffractogram of Na _0.44 Mn _0.5 Ti _0.5 O ₂ obtained in Example 1. FIG. 1 is an X-ray powder diffraction diagram of Li _0.44 Mn _0.5 Ti _0.5 O ₂ obtained in Example 1. FIG. Of the crystal material of the present _invention, it is a schematic diagram for explaining a _Li 0.44 MnO ₂ type tunnel structure. It is a schematic diagram which shows one example of the lithium secondary battery (electrochemical cell) of this invention. It is a figure which shows the charging / discharging characteristic to 10 cycles of the lithium secondary battery of Example 1. FIG. FIG. 3 is a diagram showing a charge / discharge capacity of up to 10 cycles of the lithium secondary battery of Example 1. Is a graph showing changes in resulting _Na 0.44 _Mn 1-y _Ti y lattice volume that depends on the amount of titanium y of _{O 2} in Example 2. Is a graph showing changes in the obtained _Li 0.44 _Mn 1-y _Ti y lattice volume that depends on the amount of titanium y of _{O 2} in Example 2. The _{Li 0.44 Mn 1-y Ti y} O 2 obtained in Example 2 as a positive electrode material is a diagram showing the amount of titanium dependence of the initial discharge capacity of the lithium secondary battery in which the metal lithium as a negative electrode material.

Explanation of symbols

1 Positive electrode material 2 Electrolyte 3 Negative electrode material

Claims (6)

It is expressed as Li _x Mn _1-y Ti _y O ₂ (0.40 <x <0.50, 0 <y <0.56) as the chemical composition formula, belongs to the orthorhombic system as the crystal structure, and is occupied by lithium A lithium secondary battery positive electrode material comprising lithium, manganese, titanium, and oxygen having a tunnel structure that has a sodium content of 0.05 or less in terms of a Na / Li molar ratio. The chemical composition is expressed as Na _x Mn _1-y Ti _y O ₂ (0.40 <x <0.50, 0 <y <0.56), and the crystal structure belongs to the orthorhombic system and is occupied by sodium. 2. The method for producing a positive electrode material according to claim 1, wherein the compound is produced by ion exchange treatment using a compound composed of sodium, manganese, titanium, and oxygen having a tunnel structure as a starting material.   The method for producing a positive electrode material according to claim 2, wherein the ion exchange treatment is performed in a molten salt containing a lithium compound.   The method for producing a positive electrode material according to claim 2, wherein the ion exchange treatment is performed in an organic solvent or an aqueous solution in which a lithium compound is dissolved.   A lithium secondary battery comprising the positive electrode material according to claim 1, wherein a positive electrode of a lithium secondary battery having a positive electrode, a negative electrode, and an electrolyte substance. The lithium secondary battery according to claim 5, wherein the battery can be stably charged and discharged in an area of 4 V or more as an operating voltage of the battery.

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