JPH06275276A - Manufacture of spinel limn2o4 with high surface area and its application to nonaqueous battery - Google Patents

Abstract

PURPOSE:To manufacture LiMn2O4 having a high specific surface area of 2m<2>/g or above and used as a positive electrode material for a high energy density type lithium secondary battery. CONSTITUTION:Lithium nitrate or lithium hydride and electrolytically or chemically synthesized manganese dioxide having the specific surface area of 80m<2>/g or above are baked at 450 deg.C or below. After LiMn2O4 having the amorphous distortion spinel structure is synthesized, it is again heat-treated at 600-750 deg.C. The crystalline LiMn2O4 having the specific surface area of 2m<2>/g or above is manufactured in two-stage heat treatment.

Description

Description: FIELD OF THE INVENTION The present invention is used as a positive electrode active material in a lithium secondary battery using a metallic lithium or lithium carbon (lithium-graphite) intercalation compound as a negative electrode active material. It relates to LiMn ₂ O _{4 having} a spinel structure. 2. Description of the Related Art LiNi is used as a positive electrode active material for a 4-volt high energy density type lithium secondary battery.
Other than O ₂ , LiCoO ₂ and LiMn ₂ O ₄ can be used. Batteries using LiCoO ₂ as the positive electrode active material are already on the market. However, since cobalt has a small amount of resources and is expensive, it is not suitable for mass production with the spread of batteries.
Manganese compounds are promising cathode materials in terms of resource amount and price. Manganese dioxide, which can be used as a raw material, is currently produced in large quantities as a dry battery material. Conventionally,
Crystalline spinel type LiMn ₂ O ₄ contains lithium carbonate and M
n ₂ O ₃ (Hunter; J. Solid State
Chem. , 39 142 (1981)) or heating lithium carbonate and manganese carbonate (Tackeray
Et al., Mat. Res. Bull. , 19 , 179 (19
84) and the like) were synthesized. Lithium carbonate and Mn
The specific surface area of spinel LiMn ₂ O ₄ obtained by firing ₂ O ₃ at 900 ° C. for 24 hours is 0.1 m ² / g or less (Reference Example 1). This sample is 0.01C as shown in FIG.
The discharge rate is 80 mAH / g. Further, there is a drawback that the capacity decreases with the cycle. When the discharge rate is 0.3 C, the capacity drops to 30 mAH / g.
Table 1 shows the discharge capacities when the sample was crushed to have a specific surface area of 0.4 m ² / g or 1.3 m ² / g and charged at 0.01 C and discharged at various discharge rates. This result shows that the specific surface area is an important factor for determining the discharge capacity in the discharge at a higher current density. However, LiMn ₂ O ₄ having an amorphous strain spinel structure, which has a large specific surface area and is generated by low temperature firing, has a small discharge capacity as a 4V class battery and also has poor cycle characteristics. Therefore, LiMn having a crystalline spinel structure is used as the positive electrode active material of the 4V class battery.
_It is necessary to use ₂ O ₄ . LiMn ₂ O ₄ for a lithium secondary battery capable of extracting a large current has a property to be embodied that it has a crystalline spinel structure and a large specific surface area.
As described above, the specific surface area of LiMn ₂ O ₄ can also be obtained by refining the particles by mechanical pulverization, but the manufacturing process increases and the cost increases, and a large amount of the conductive agent and the binder are used for manufacturing the electrode. Therefore, mechanical pulverization is not a solution because the discharge capacity per unit volume is reduced. In the conventional process for producing LiMn ₂ O ₄ by the one-step heat treatment at 700 to 900 ° C. described above, Mn ₂ O ₃ is used, or when manganese carbonate is used, Mn ₂ O ₃ is produced as an intermediate product. In both methods, Mn ₂ O ₃ and lithium carbonate react to generate LiMn ₂ O ₄ having a spinel structure. As shown in FIG. 2 Mn ₂
O ₃ undergoes a sintering reaction even when heat-treated at a low temperature of 650 ° C., and the specific surface area is 1% of that of the raw material Mn ₂ O _{3 after} heat-treatment for 24 hours.
/ 10 or less. Therefore, in the conventional method, the reaction intermediate Mn ₂ O ₃ and Li ₂ CO ₃ are reacted to form LiMn ₂ O ₄
Before the formation of Mn ₂ O ₃ , the specific surface area decreases. When the specific surface area is small, there is a drawback that not only the discharge capacity at high current density is lowered but also the cycle characteristics are deteriorated. SUMMARY OF THE INVENTION The present invention has been made in view of the above problems of the prior art. Lithium having a crystalline spinel structure having a large specific surface area is used as a positive electrode active material for a lithium secondary battery.
The use of Mn ₂ O ₄ aims to improve the discharge capacity at a high discharge rate and to improve the cycle characteristics. Means for Solving the Problems As described above, Mn ₂
In the method of producing LiMn ₂ O ₄ via O ₃ , L
Before the formation of iMn ₂ O ₄ , the reaction intermediate Mn ₂ O
₃ progresses, and LiMn ₂ O ₄ having a small specific surface area is generated. When manganese dioxide is heated to 550 ° C. or higher, Mn ₂ O ₃ is produced. Therefore, manganese dioxide and a lithium salt are reacted at a temperature lower than 550 ° C.
Manufacture Mn ₂ O ₄ . By heat treatment again at 650 to 750 ° C., LiMn ₂ O _{4 having} an amorphous strain spinel structure is changed to LiMn ₂ O ₄ having a crystalline spinel structure.
This structural change is accompanied by a decrease in specific surface area, but once LiMn ₂ O ₄ having a crystalline spinel structure is formed, the sintering reaction of this substance is slow at 750 ° C. or lower as shown in FIG. Therefore, the heat treatment at 700 ° C. can suppress the decrease in the specific surface area. Lithium nitrate is suitable for the lithium salt, as the reaction proceeds in a solid-liquid reaction. Lithium carbonate, which has a high decomposition temperature, is unsuitable because of the low rate of amorphous spinel formation. The LiMn ₂ O ₄ produced by this method is shown in FIG.
As shown in Table 2, it has a discharge capacity of 130 mAH / g or more at a discharge rate of 1 C, has excellent cycle characteristics as shown in Table 2, and is used as a positive electrode active material for a lithium secondary battery used in equipment requiring a large current. It is useful. EXAMPLES Example 1 Chemically Synthesized Manganese Dioxide (Faradizer M, Sedema, International Common Sample No. 1)
2) 8.69 g and 3.438 g of lithium nitrate are pulverized and mixed, and put into an alumina crucible. Preliminary firing at 400 ° C for 48 hours in air atmosphere, amorphous LiM with strain spinel structure
obtain n ₂ O _4. This is fired again at 700 ° C. for 120 hours to obtain crystalline LiMn ₂ O ₄ having a spinel structure. In the X-ray diffraction diagram (using CuKα ray) of both, both are 2
θ = 18.6-18.8, 36.1-36.5, 44.
It has a characteristic peak of LiMn ₂ O ₄ of spinel at 0-44.4 degrees. It shows that the crystallization did not proceed sufficiently at the 400 ° C. firing stage, the peak intensity was weak, the line width was wide, and the crystallization did not proceed. A mixture of spinel LiMn ₂ O ₄ (3.2 m ² / g) obtained by firing at 700 ° C. for 120 hours and a conductive binder (20 Wt%) was used as a positive electrode,
The discharge curves are shown when the lithium negative electrode was charged at a rate of 0.2 C (5 hours) and then discharged at various rates (0.2-5 C). As the electrolytic solution, a mixed solution of ethylene carbonate-propylene carbonate in which LiAsF ₆ was dissolved was used. The charge / discharge voltage range was 4.5-3.0V. As shown in FIG. 4, the capacity is 130 mAH / g or more even at a high discharge rate of 1 C, and the polarization is 100 mV or less, which is small for a non-aqueous battery. Example 2 Electrosynthesis Manganese Dioxide (Hoeches)
Knapsack (t. company) 8.69 g and lithium nitrate 3.438 g are pulverized and mixed, and put into an alumina crucible.
After firing at 400 ° C for 48 hours in an air atmosphere, 7
It was baked at 00 ° C. for 120 hours to obtain LiMn ₂ O ₄ . The BET surface area of this sample is 1.2 m ² / g, which is 40% or less of the case where chemically synthesized manganese dioxide is used as a raw material. LiMn ₂ O ₄ having a larger specific surface area when chemically synthesized manganese dioxide having a large specific surface area is used as a raw material than when electrolytically synthesized manganese dioxide having a small specific surface area is used as a raw material.
Is obtained. Even when electrolytic manganese dioxide having a large specific surface area is used, the obtained LiMn ₂ O ₄ has a large specific surface area, and exhibits excellent charge / discharge performance as in the case of using chemically synthesized manganese dioxide having a large specific surface area. Example 3 Chemically Synthesized Manganese Dioxide (Faradizer M, Sedema, International Common Sample No. 1)
2) 8.69 g and lithium hydroxide (LiOH.H ₂ O)
2.098 g is pulverized and mixed, and put in an alumina crucible.
After firing at 400 ° C for 48 hours in an air atmosphere, 7
Baking for 120 hours at 00 ° C. Obtained LiMn ₂ O ₄
Had a specific surface area of 2.2 m ² / g. As a result of the charge / discharge test at 0.3 C (Table 2), the capacity is inferior by about 3% to the one synthesized from LiNO _3, but no significant difference is seen in the cycle characteristics. Reference Example 1 7.89 g of Mn ₂ O ₃ and lithium carbonate (L
1.848 g of i ₂ CO ₃ ) is pulverized and mixed, and put into an alumina crucible. This is baked in air at 900 ° C. for 24 hours. The obtained LiMn ₂ O ₄ has a specific surface area of 0.1 m ² / g
It was below. This was crushed with a ball mill for 24 hours (0.4 m ² / g) and further crushed for 24 hours (1.3
m ² / g). In the battery test of these samples, 2 of the active materials
A positive electrode mixture was prepared using a double weight of the conductive binder. [0010]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 Relationship between discharge rate and discharge capacity FIG. 2 Relationship between sintering temperature and specific surface area of Mn ₂ O ₃ and LiMn ₂ O ₄ FIG. 3 Amorphous strain spinel XRD diagrams of LiMn ₂ O ₄ and crystalline spinel LiMn ₂ O ₄ [FIG. 4] Discharge curves of LiMn ₂ O ₄ synthesized by this method at various discharge rates [Description of symbols] 1 LiMn ₂ O ₄ : 650 ℃ heat treatment 2 LiMn ₂ O ₄ : 750 ℃ heat treatment 3 Mn ₂ O ₃ : 650 ℃ heat treatment 4 Mn ₂ O ₃ : 750 ℃ heat treatment 5 crystalline spinel LiMn ₂ O ₄ 6 amorphous strain spinel LiMn ₂ O ₄ 7 discharge rate 1C 8 Discharge rate: 2C 9 Discharge rate: 5C

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Bumanef             Gibonche, Sofia city, Bulgaria             Hubbard 10 (72) Inventor Hideyuki Noguchi             1684 Tokutomi, Morotomi-cho, Saga-gun, Saga

Claims (1)

[Claims] 1. Lithium nitrate and electrolytically or chemically synthesized manganese dioxide having a specific surface area of 80 m ² / g or more are fired at 450 ° C. or less to synthesize LiMn ₂ O ₄ having an amorphous strain spinel structure, and then heat treated again (600 to 750 ° C.). A method for producing crystalline LiMn ₂ O ₄ having a specific surface area of 2 m ² / g or more, characterized by a two-step heat treatment. Lithium hydroxide may be used instead of lithium nitrate. 2. A positive electrode for a lithium secondary battery, which uses the aforementioned LiMn ₂ O ₄ as a positive electrode active material. [0002]