JP2005259362A - Positive electrode material for lithium secondary battery, manufacturing method of the same, and lithium secondary battery using the material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new positive material for stably charging and discharging in the same operation voltage area (almost 4 V) as that of the existing lithium cobalt oxide based positive electrode material, a manufacturing method of the same, and a lithium secondary battery including the material as a positive activator. <P>SOLUTION: The positive electrode material for lithium secondary battery, made of lithium-manganese oxide, represented by chemical composition formula: Li<SB>x</SB>MnO<SB>2</SB>(0.40<x<0.50), belonging to a rhombic crystal system as a crystal structure, having a tunnel structure occupied by lithium, is composed of lithium, manganese and oxygen, and of which the content of sodium is ≤0.05 at a molar ratio of Na/Li. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a lithium secondary battery positive electrode material, a method for producing the same, and a lithium secondary battery containing the material as a positive electrode active material. It is related with the technique which provides the lithium secondary battery which can discharge stably also in the area | region, and is large in discharge capacity.

  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.

In the spinel-type lithium manganese oxide (LiMn ₂ O _4), with the insertion and extraction reactions of lithium ions, 3V (vs.Li ⁺ / Li) and 4V (vs.Li ⁺ / Li) 2 near TsunoTakashi It is known to have a voltage flat region accompanying discharge. In general, since charging / discharging is performed using only the 4 V region, LiMn ₂ O ₄ having a stoichiometric composition composed only of Li and Mn can only provide a capacity of about 130 mAh / g. On the other hand, if charging / discharging is performed using both 3 V and 4 V regions, a capacity of 200 mAh / g or more can be obtained, but the problem is that the cycle characteristics are very poor due to the change in crystal structure. .

On the other hand, research on lithium manganese oxide (Li _x MnO ₂ ; 0 <x <1) having a crystal structure different from that of the conventional spinel type lithium manganese oxide (LiMn ₂ O ₄ ) has been actively conducted.

Layered lithium manganese oxide (LiMnO ₂ ) having a crystal structure similar to LiCoO ₂ or LiNiO ₂ has two known crystal structures depending on the synthesis method. When a charge / discharge cycle is performed in a potential range of 2.0 V to 4.5 V (vs. Li ⁺ / Li) in a battery using these layered lithium manganese oxides, a high initial capacity of 200 mAh / g or more can be obtained. .
However, with the charge / discharge cycle, the crystal structure gradually changes to a spinel structure, eventually resulting in charge / discharge as spinel type LiMn ₂ O ₄ , and the capacity increases as the number of cycles increases. It will decrease.

On the other hand, as a feature of the crystal structure, many studies have been conducted on a high-capacity lithium manganese oxide using an alkali manganese oxide (for example, hollandite type) having a one-dimensional tunnel structure as a starting material. These crystal structures are considered to be easily diffused by ions because they have large tunnels, and are attracting attention as high-capacity electrode materials because they are inexpensive.
However, it is often difficult to completely remove alkali ions, hydrate ions, crystallization water, etc. contained in the starting material, which degrade the electrolyte and negative electrode, and replace them with lithium ions. However, it is lower than the discharge voltage of LiCoO ₂ by about 1 V or more, and it is difficult to use it as a substitute material for cobalt.

Research is also being conducted on the synthesis of Li _0.44 MnO ₂ by ion exchange using Na _0.44 MnO ₂ having a tunnel structure as a starting material. Since this compound has two types of tunnels having different sizes, it is considered that ion diffusion is easy. For example, the compound has attracted attention as a positive electrode material having high output (capable of rapid charge / discharge). (See Non-Patent Documents 1 and 2)

A. R. Armstrong, H .; Huang, R.A. A. Jennings, P.M. G. Bruce, J. et al. Mater. Chem. 8, 255-259 (1998) M.M. M.M. Doeff, A.M. Anapolsky, L.A. Edman, T .; J. et al. Richardson, L.M. C. De Jonghe, J.A. Electrochem. Soc. , 148, A230-A236 (2001)

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, since Li _0.44 MnO ₂ known so far has a discharge voltage of about 3 V and it has not been possible to completely desorb lithium, it has a high charge as expected from the structure. The discharge capacity was not obtained. This is because the sodium of the sodium compound used as the starting material is not completely exchanged with lithium by the ion exchange treatment and remains in a considerable amount, and the remaining sodium contributes to the battery reaction. This is thought to interfere with diffusion and insertion / desorption reactions.

Therefore, the present invention solves the above-mentioned current problems, has the Li _0.44 MnO ₂ type tunnel structure as described above, and operates in the same manner as an existing lithium cobalt oxide-based positive electrode material. It is an object to provide a positive electrode material of a novel lithium secondary battery that can be stably charged and discharged in a voltage region (about 4 V), a manufacturing method thereof, and a lithium secondary battery containing the material as a positive electrode active material. .

As a result of intensive studies on starting materials and synthesis conditions, the present inventor has found a sodium compound having a stable structure with respect to an ion exchange reaction at a higher temperature as compared with Non-Patent Document 1 described above, and uses it as a starting material. As a result, the amount of residual sodium in the positive electrode material having the Li _0.44 MnO ₂ type tunnel structure is greatly reduced, and when used as a positive electrode material of a lithium secondary battery, charging and discharging is stable even in the 4 V region. The present invention has been completed by confirming the expected charge / discharge capacity.

That is, this invention relates to the positive electrode material for lithium secondary batteries which consists of a lithium manganese oxide shown below, its manufacturing method, and the lithium secondary battery using this positive electrode material.
1. It is represented by the chemical composition formula Li _x MnO ₂ (0.40 <x <0.50), belongs to the orthorhombic system as a crystal structure, and has a tunnel structure occupied by lithium, and is composed of lithium, manganese, and oxygen, A positive electrode material for a lithium secondary battery, wherein the sodium content is 0.05 or less, preferably 0.01 or less, in terms of a Na / Li molar ratio.
2. _2. Lithium according to 1, characterized in that an ion exchange treatment is performed with a lithium compound using Na _x MnO ₂ (0.40 <x <0.50) prepared by mixing and baking a sodium compound and a manganese compound as a starting material. A method for producing a positive electrode material for a secondary battery.
3. 3. The method for producing a positive electrode material for a lithium secondary battery according to 2, wherein the ion exchange treatment is performed in a molten salt containing a lithium compound.
4). 4. The method for producing a positive electrode material for a lithium secondary battery according to 3, wherein the ion exchange treatment is performed in a temperature range of 280 to 330 ° C.
5). 2. A lithium secondary battery, wherein the positive electrode of a lithium secondary battery having a positive electrode, a negative electrode, and an electrolyte substance is composed of the positive electrode material for a lithium secondary battery 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.

  According to the present invention, a novel lithium manganese oxide positive electrode material that can be stably charged and discharged in an operating voltage range (about 4 V) equivalent to that of an existing lithium cobalt oxide-based positive electrode material using an inexpensive raw material Can be obtained.

1) Positive electrode material for lithium secondary battery and method for producing the same Li _x MnO ₂ (0.40 <x <0.50) positive electrode material of the present invention is Na _x MnO ₂ (0.40 <x < 0.50) is prepared by ion exchange treatment, and the amount of sodium contained is optimized to be not more than 0.05 in terms of Na / Li molar ratio by optimizing the ion exchange treatment conditions.
In the positive electrode material disclosed in the above-mentioned non-patent document, if the ion exchange reaction is incomplete and the amount of sodium is more than the above, charging and discharging cannot be performed in a high voltage region of 4 V or more, and sufficient characteristics are obtained. I can't. Similarly, when the alkali is desorbed and changes to a composition of Li _0.40 MnO ₂ during the ion exchange process, charging / discharging at 4 V or more is similarly difficult.

The Li _x MnO ₂ positive electrode material of the present invention desirably has a Li _0.44 MnO ₂ type tunnel structure as a whole, but some other crystal structures are included within a range not impeding the effects of the present invention. May be.

Hereinafter, the present invention will be described in more detail.
The positive electrode material represented by the chemical composition formula Li _x MnO ₂ (0.40 <x <0.50) of the present invention ionizes Na _x MnO ₂ (0.40 <x <0.50) as a starting material. It is produced by an exchange process. First, a method for producing Na _x MnO ₂ as a starting material will be described.

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.

First, a mixture containing these is prepared. The mixing ratio of the sodium raw material and the manganese raw material is preferably mixed at such a ratio that a Na _0.44 MnO ₂ type structure is generated. Specifically, the chemical composition formula may be Na _x MnO ₂ (0.40 <x <0.50). For example, mixing may be performed so that the molar ratio of Na / Mn is about 0.4 to 0.7, preferably 0.43 to 0.55.

  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 1200 ° C., preferably 850 to 1050 ° C. Above 850 ° C., grain growth is remarkable, and it becomes possible to obtain a fired product having a large particle size that is stable at higher temperatures.
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. The cooling method is not particularly limited, but 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.

Next, the calcined Na _x MnO ₂ (0.40 <x <0.50) was subjected to an ion exchange treatment, whereby the molar ratio Na / Li with lithium exchanged by the remaining sodium was changed to 0.00. A compound represented by the chemical composition formula Li _x MnO ₂ (0.40 <x <0.50) of 05 or less is obtained.

In this case, it is preferable to perform the ion exchange treatment while dispersing the pulverized Na _x MnO ₂ in the molten salt containing the lithium-containing compound. 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 MnO 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 MnO ₂ in the molten salt.

The temperature of ion exchange is 280 ° C to 330 ° C. When the ion exchange temperature is lower than 280 ° C., sodium in Na _x MnO ₂ is not completely exchanged for lithium and remains in the tunnel structure. 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.

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 MnO ₂ . The washing method and the drying method are not particularly limited, and a normal method may be used, or natural drying may be used.

2) Lithium secondary battery The lithium secondary battery of the present invention uses 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 oxide of the present invention is used as the positive electrode material.

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

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

  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.

  Next, examples and comparative examples will be shown to further clarify the features of the present invention, but the present invention is not limited to these specific examples.

(Example 1)
Sodium carbonate (Na ₂ CO ₃ ) and manganese oxide (MnO ₂ ) were uniformly mixed at a molar ratio of 1: 4. The mixture was calcined in air at 900 ° C. for 10 hours and then gradually cooled in a furnace. A series of operations (calcination, slow cooling and pulverization) of pulverizing the obtained fired body was repeated again to obtain a starting material having a substantially single phase Na _0.44 MnO ₂ type crystal structure. 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 the crystal structure of Na _0.44 MnO ₂ , the orthorhombic lattice constant is obtained, and a = 9.0888 (8) A (the horizontal length of the portion surrounded by the rectangle in FIG. 3) B = 26.4872 (19) A (vertical length of the portion surrounded by the rectangle in FIG. 3), c = 2.8250 (5) A.

Next, this sample 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 MnO _{2 in} the molten salt was a molar ratio: Li in the molten salt: Na in the sample = 20: 1, and the temperature of the molten salt was 300 ° C. 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 MnO ₂ is reasonable, and the remaining sodium content was 0.007 in terms of a Na / Li molar ratio. It was. Furthermore, an X-ray powder diffraction pattern is shown in FIG. Assuming that it has the same structure as Na _0.44 MnO _{2 used} as a starting material, the orthorhombic lattice constants (a = 8.9079 (5) A, b = 24.2532 (11) A, c = 2.82781 (19) A) was confirmed to be nearly single phase.
Further, the lattice constant of Li _0.40 MnO ₂ described in Non-Patent Document 1 (a = 8.9316 (2) A, b = 24.4350 (5) A, c = 2.83072 (5) A) ), The a-axis length and the b-axis length were clearly shorter, and it was judged that the difference in chemical composition such as the amount of residual sodium was markedly exhibited.

(Example 2)
The obtained Li _0.44 MnO ₂ sample was used as the positive electrode material 1, 1M in which lithium metal was dissolved in a negative electrode material 3, and 6 hexafluorophosphate was dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC). A lithium secondary battery (electrochemical cell) as shown in FIG. 4 was prepared using the solution as the electrolytic 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 cutoff potential of 4.8 V-2.5 V, the average discharge voltage was 3.57 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 157 mAh / g, and the cycle characteristics were also good. FIG. 5 shows the charge / discharge characteristics up to 10 cycles, and FIG. 6 shows the charge / discharge capacity.
In FIG. 5, 1-10c represents a charge curve, and 1-10d represents a discharge curve.

_{Conventionally} known Li _0.44 MnO ₂ could not be charged / discharged at a high potential of 4 V or higher due to the influence of remaining sodium, but in this sample, as shown in FIG. The initial discharge capacity was about 60 mAh / g, and stable charging / discharging was possible at about 53 mAh / g even after 10 cycles.

(Comparative Example 1)
The ion exchange treatment conditions for the Na _0.44 MnO ₂ sample obtained in Example 1 were changed. That is, in Example 1, the composition of the molten salt, the mixing ratio, the processing time, the subsequent washing and drying steps were all the same, and only the processing temperature was lowered to 260 ° C.
As a result of analyzing the chemical composition of the obtained sample by ICP emission spectrometry, the chemical formula of Li _0.42 Na _0.03 MnO ₂ is reasonable, and the residual ratio of Na / Li is about 0.07. Containing sodium was confirmed. When this sample was used as a positive electrode material, a lithium secondary battery similar to that of Example 2 was produced, and a charge / discharge test was performed under the same conditions. As a result, stable charge / discharge in the 4 V region was not observed.

(Comparative Example 2)
Furthermore, the Na _0.44 MnO ₂ sample obtained in Example 1 was also examined for an ion exchange treatment method in an organic solvent in which a lithium compound was dissolved. That is, lithium bromide was dissolved in diethylene glycol monoethyl ether, and Na _0.44 MnO ₂ was added to this solution, followed by ion exchange treatment with stirring at 150 ° C. for 24 hours. The obtained sample was washed with methanol and then suction filtered using a membrane filter. In the washing, a cycle in which the sample was stirred in methanol for about 1 day and then suction filtered was repeated several times until the filtrate became transparent. Finally, it was washed with ethanol, and after suction filtration, it was naturally dried in a desiccator.
As a result of analyzing the chemical composition of the obtained sample by ICP emission spectrometry, the chemical formula of Li _0.37 Na _0.16 MnO ₂ was obtained, and sodium contained in a residual ratio of about 0.43 in terms of a Na / Li molar ratio. Was confirmed. Using this sample as a positive electrode material, a lithium secondary battery similar to that in Example 2 was produced and a charge / discharge test was performed. However, as in Comparative Example 1, stable charge / discharge in the 4 V region was not observed.

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

_{2 is} an X-ray powder diffractogram of Na _0.44 MnO ₂ obtained in Example 1. FIG. _{2 is} an X-ray powder diffraction pattern of Li _0.44 MnO ₂ obtained in Example 1. FIG. It is a schematic diagram for explaining a _Li 0.44 MnO ₂ type tunnel structure of the present invention. 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 2. FIG. FIG. 4 is a diagram showing a charge / discharge capacity of up to 10 cycles of the lithium secondary battery of Example 2.

Explanation of symbols

1 Positive electrode material 2 Electrolyte 3 Negative electrode material

Claims (6)

It is represented by the chemical composition formula Li _x MnO ₂ (0.40 <x <0.50), belongs to the orthorhombic system as a crystal structure, and has a tunnel structure occupied by lithium, and is composed of lithium, manganese, and oxygen, A positive electrode material for a lithium secondary battery, wherein the sodium content is 0.05 or less in terms of a Na / Li molar ratio. _{2. The} ion exchange treatment with a lithium compound using Na _x MnO ₂ (0.40 <x <0.50) prepared by mixing and baking a sodium compound and a manganese compound as a starting material. Manufacturing method of positive electrode material for lithium secondary battery.   The method for producing a positive electrode material for a lithium secondary battery 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 for a lithium secondary battery according to claim 3, wherein the ion exchange treatment is performed in a temperature range of 280 to 330 ° C.   A lithium secondary battery comprising the positive electrode material for a lithium secondary battery according to claim 1, wherein the positive electrode of the 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.

Images