Positive electrode material for lithium secondary battery and its manufacture and lithium secondary battery using it

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Industrial application field]

The present invention relates to a positive electrode material used for a lithium secondary battery, particularly a non-aqueous electrolyte secondary battery, a method for producing the same, and a lithium secondary battery using the same.

【0002】

[Conventional technology]

In recent years, portable and cordless electronic devices such as AV devices and personal computers have been rapidly advanced, and there is a high demand for a secondary battery that is compact, lightweight, and has a high energy density as a power source for driving these devices.

【0003】

In this respect, non-aqueous secondary batteries, particularly lithium secondary batteries, are highly expected as batteries having a particularly high voltage and high energy density. LiCoO capable of inserting and removing lithium as a positive electrode active material material that meets the above requirements₂, LiNiO₂And spinel structure LiMn₂O₄Research and development such as is being actively carried out.

【0004】

But Li_XCoO₂Since the crystal structure changes significantly during the process of lithium intercalation and deintercalation, the reversibility of the active material itself decreases, and the active material and the conductive agent are repeatedly expanded and contracted due to repeated expansion and contraction of the electrodes. There is a problem that the contact becomes insufficient, and as a result, the capacity deteriorates with the charge / discharge cycle. In addition, although it is not a technical issue, cobalt is expensive and there is a concern about supply.

【0005】

[Problems to be Solved by the Invention]

The present invention provides a positive electrode material for a lithium secondary battery capable of solving these problems of the prior art and improving charge / discharge cycle characteristics, a method for producing the same, and a lithium secondary battery using the same. The purpose.

【0006】

[Means for solving problems]

In order to solve the above problems, the present invention introduces tetravalent manganese ions to stabilize the host structure and improve the charge / discharge cycle characteristics.

【0007】

As a result of diligent research, the present inventors have LiCoO belonging to the space group R-3m.₂However, Li belonging to the space group C2 / m₂MnO₃(Rewrite Li (Li_1/3Mn_2/3) O₂) And 3Co³⁺ Li⁺+ 2Mn⁴⁺Can be replaced with Li (Li)_xMn_2xCo_1-3x) O₂It was found that it is possible to form the present invention, and the present invention has been completed.

【0008】

That is, in the present invention, the cobalt of lithium cobalt oxide having a layered structure is 3Co.³⁺ 2Mn⁴⁺+ Li⁺It is replaced with manganese and lithium according to the chemical formula of Li (Li)._xMn_2xCo_1-3x) O₂It is found in a positive electrode material for a lithium secondary battery, which is represented by (0 <x <1/3).

【0009】

So far, LiCoO as shown below₂Although it has been reported that manganese is added to improve the cycle characteristics, these are merely intended to replace cobalt with manganese, and the material of the present invention is clearly an atomic arrangement in a crystal. Are different and are clearly different from the materials of the present invention.

【0010】

For example, according to Japanese Patent Application Laid-Open No. 4-267053, the general formula Li_xM_yN_zO₂(Here, M is at least one selected from the group of Fe, CO, and Ni, and N is at least one selected from the group of Ti, V, Cr, and Mn). It has been proposed that the cycle characteristics be improved. Further, according to JP-A-6-44973, the general formula Li_y(Co_yMn_x) O₂It has been proposed that batteries using materials that can be represented by (x + y = 1) improve cycle characteristics. However, these are related to materials belonging to the space group R3m or have different chemical compositions, and are clearly different from the materials of the present invention.

【0011】

In order to produce the positive electrode material of the present invention, lithium salts such as lithium carbonate, lithium hydroxide and lithium nitrate, manganese compounds such as manganese oxide, manganese carbonate and manganese oxyhydroxide, cobalt oxide, cobalt carbonate, cobalt hydroxide and the like can be produced. Cobalt compound can be used as a raw material. The desired solid solution can be obtained by mixing these raw materials in a predetermined mixing ratio with only the lithium compound in excess and firing at 800 to 1000 ° C. in an air or oxygen atmosphere. Adding excess lithium salt is Li (Li_xMn_2xCo_1-3x) O₂If the raw materials were mixed at the ratio of the metal elements according to the chemical formula and fired, Li₂MnO₃Etc. are generated and a single phase cannot be obtained. The cause of this is not clear, but it is presumed that the liquid phase helps the diffusion of each metal element because the excess lithium salt melts during firing.

【0012】

After the reaction, excess lithium remains in the form of lithium hydroxide, lithium oxide, etc., but it can be used as it is as the positive electrode material of the battery, or it can be removed by washing with water and used as the positive electrode material of the battery. Is also possible.

【0013】

[Action]

General formula Li (Li_xMn_2xCo_1-3x) O₂By substituting trivalent cobalt with tetravalent manganese, the positive electrode material for lithium secondary batteries obtained in the present invention represented by is able to intercalate lithium and expand and contract the lattice during deintercalation. It is suppressed and shows good cycle characteristics.

【0014】

[Example]

Hereinafter, the present invention will be specifically described based on examples.

【0015】

Example 1 Using cobalt carbonate, manganese carbonate, and lithium carbonate as raw materials, Li (Li)_xMn_2xCo_1-3x) O₂In, x = 1/6, and the excess lithium amount was changed to 0, 1/6, 2/6, 3/6, 4/6, 5/6, 6/6 in terms of molar ratio to mix, 900. After firing at ° C for 24 hours, it was rapidly cooled. Figure 1 shows the X-ray rotation pattern of the product. LiCoO when excess lithium is 0, 1/6₂, Li₂MnO₃However, peak separation is not observed above 2/6, indicating that the phase is single. That is, the solid solution Li (Li) at 900 ° C._xMn_2xCo_1-3x) O₂To synthesize Li (Li_xMn_2xCo_1-3x) O₂2/6 or more excess lithium salt is required.

【0016】

Example 2 The synthesis was carried out in exactly the same manner as in Example 1 except that the firing temperature was set to 1000 ° C. Figure 2 shows the X-ray rotation pattern of the product. At a calcination temperature of 1000 ° C, the excess lithium amount was 1/6 or more, no peak separation was observed, and a single phase was formed. It is presumed that the effect of excess lithium is small because the firing temperature is high and atoms are easily diffused.

【0017】

Example 3 The synthesis was carried out in exactly the same manner as in Example 1 except that the lithium compound was lithium hydroxide. Figure 3 shows the X-ray rotation pattern of the product. As in Example 1, when the excess lithium amount is 0 and 1/6, LiCoO₂, Li₂MnO₃However, no peak separation was observed above 2/6, resulting in a single phase.

【0018】

Example 4 The excess lithium amount was 0.75 × (1 + x), the manganese: cobalt ratio was changed in 9 steps from 1: 9 to 9: 1, and the mixture was calcined at 900 ° C for 24 hours and then rapidly cooled. Figure 4 shows the X-ray rotation pattern of the product. LiCoO in any of the rounding patterns₂, Li₂MnO₃Coexistence is not recognized, and it can be said that they are in a single phase.

【0019】

Example 5 Of the solid solutions obtained in Example 4, a sample having a manganese: cobalt ratio of 1: 9 was washed with water to remove excess lithium salt, and then the charge / discharge cycle characteristics of the battery were evaluated using the model cell shown in FIG. The electrolyte used was 1 mol of lithium tetrafluoride dissolved in a mixed solvent of propylene carbonate and 1,2-dimethoxyethane, with a cut voltage of 3 to 4.3 V and a current density of 1 mA / cm.²And said. Table 1 shows the evaluation results. In FIG. 5, 1 is a negative electrode terminal, 2 is an insulator, 3 is a negative electrode current collector plate, 4 is a negative electrode material, 5 is a separator, 6 is a positive electrode mixture, and 7 is a positive electrode terminal.

【0020】

Comparative example 1 LiCoO under the same conditions as in Example 4 with no manganese added and lithium: cobalt = 1: 1.₂Was prepared, and the charge / discharge cycle characteristics of the battery were evaluated in the same manner as in Example 5.

【0021】

[table 1]

<img file="JPP3506397B2_D0001.tif" />【0022】

As is clear from the results in Table 1, the discharge capacity of the positive electrode material that is not replaced up to 50 cycles is large, but after 50 cycles, the discharge capacity of the positive electrode material of the present invention is large, and the positive electrode material of the present invention was used. It can be seen that the lithium secondary battery has excellent charge / discharge cycle characteristics.

【0023】

[Effect of the invention]

As described above, the lithium secondary battery using the positive electrode material of the present invention is excellent in charge / discharge cycle characteristics. It also has the advantage that the amount of expensive cobalt used can be reduced.

[Simple explanation of drawings]

[Figure 1]

The graph which shows the X-ray diffraction pattern of the product in Example 1.

[Figure 2]

The graph which shows the X-ray diffraction pattern of the product in Example 2.

[Fig. 3]

The graph which shows the X-ray diffraction pattern of the product in Example 3.

[Fig. 4]

The graph which shows the X-ray diffraction pattern of the product in Example 4.

[Fig. 5]

Schematic cross-sectional view of the model cell of the lithium secondary battery used in Example 5.

[Explanation of symbols]

1: Negative electrode terminal, 2: Insulation, 3: Negative electrode current collector plate, 4: Negative electrode material, 5: Separator, 6: Positive electrode mixture, 7: Positive electrode terminal.