JPH07245106A - Manufacture of lixmn2o4 for lithium secondary battery and application thereof to nonaqueous battery - Google Patents

Abstract

PURPOSE:To provide a manufacturing method of LiMn2O4 which is used as a positive electrode material for a high energy density type lithium secondary battery and has the high specific surface area. CONSTITUTION:Lithium nitrate or litium hydroxide and electrolytically or chemically synthesized manganese dioxide are held (unnecessary in the case of the lithium hydroxide) at a prescribed temperature, and are baked within a temperature range of 250 to 470 deg.C, and LiMn2O4 having an amorphous strain spinel structure is synthesized. Afterwards, crystalline LiMn2O4 having the specific surface area not less than 2m<2>/g is obtained by two-stage heat treatment to again perform heat treatment at 500 to 800 deg.C.

Description

Description: FIELD OF THE INVENTION The present invention relates to a lithium secondary battery using an intercalation compound such as metallic lithium or lithium carbon (lithium-graphite) as a negative electrode active material, and as a positive electrode active material. It relates to the spinel structure LiMn ₂ O ₄ used. 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. SolidState
Chem. , 39 142 (1981)) or heating lithium carbonate and manganese carbonate (Tackeray
Et al., Mat. Res. Bull. , 19 , 179 (1
984) and the like). Lithium carbonate and M
The specific surface area of spinel LiMn ₂ O ₄ obtained by firing n ₂ O ₃ at 900 ° C. for 24 hours is 0.1 m ² / g or less. This sample has a discharge rate of 0.01C and is 80 mAH /
There is only g capacity. Further, there is a drawback that the capacity decreases with the cycle. If the discharge rate is 0.3C, the capacity is 30
It decreases to 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 having a large specific surface area generated by low temperature firing in air has a small discharge capacity as a 4V class battery. However, if this point can be improved, it can be used as a positive electrode active material for a 4V class battery. LiMn ₂ for 4V class lithium secondary battery capable of extracting large current
O ₄ should have a 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. However, mechanical pulverization is not a drastic solution because the discharge capacity per unit volume is reduced. In the conventional process of producing LiMn ₂ O ₄ by the above-described one-step heat treatment at 700 to 900 ° C., Mn ₂ O ₃ is used, or when manganese carbonate is used, Mn ₂ is used as an intermediate product.
Generates O ₃ . According to two methods when the _Mn 2 _{O 3} generated at the initial sintering process, the _Mn 2 _{O 3} reacts with lithium carbonate, finally LiMn ₂ O ₄ having a spinel structure is formed. As shown in FIG. 1, Mn ₂ O ₃ undergoes a sintering reaction even by heat treatment at a low temperature of 650 ° C., and the specific surface area is reduced to 1/10 or less of the raw material Mn ₂ O ₃ by heat treatment for 24 hours.
Therefore, according to the conventional method, Mn ₂ O ₃ and Li ₂ which are reaction intermediates are used.
Before CO ₃ reacts to form LiMn ₂ O ₄ , Mn ₂ O
The sintering of No. ₃ progresses and the specific surface area decreases. When the specific surface area becomes small, not only the discharge capacity at high current density decreases but also the cycle characteristics deteriorate. The present invention has been made in view of the above problems of the prior art, and was obtained as a result of proceeding with the research of acceptance number 29305501980, which has already been applied for. Use of LiMn ₂ O _{4 having} a crystalline spinel structure having a large specific surface area or LixMn ₂ O ₄ having a flat discharge curve as a positive electrode active material for a lithium secondary battery to improve discharge capacity at high discharge rate and cycle characteristics. And to synthesize a large-capacity spinel-structured LixMn ₂ O ₄ (1.04 <x <1.2) at 800 ° C. or lower. 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 and lithium hydroxide, which undergo a solid-liquid reaction, are suitable for the lithium salt. Further, the molten lithium nitrate is kept at 250 to 300 ° C. in order to penetrate into the manganese dioxide pores. At this time, amorphous Li _y MnO ₂ (y = 0.33) is generated. After that, if baked at 350 ° C. or higher, LixM having a spinel structure
n ₂ O ₄ is produced. High specific surface area spinel LiMn ₂ O
What is important in producing No. ₄ is that Mn ₂ O ₃ is not formed. Therefore, amorphous Li at 260-500 ° C
_y MnO ₂ (Y = 0.33) or Li having a strained spinel structure
A process that produces lithium-containing MnO ₂ such as Mn ₂ O ₄ and spinel LiMn ₂ O ₄ is desirable. The crystalline spinel LiMn ₂ O ₄ undergoes a structural change during charge and discharge, and as the cycle increases, the expansion and contraction of the crystal lattice causes the active material particles to break down and become finer, degrading the cycle characteristics. We have found a way to minimize this crystal structure change. As shown in FIG. 2 (7), the charge and discharge curves when the metallic lithium of spinel LiMn ₂ O ₄ having an Li / Mn atomic ratio of 0.5 of Example 2 was used as the negative electrode were around 3.95 V and around 4.1 V. It has a two-stage voltage flat portion. 3.8-
Up to 4.05 V is a uniform solid-phase reaction region in which the crystal structure expands and contracts without changing, and up to 4.0-4.2 V is a two-phase reaction region in which two crystal layers coexist. Li
When the / Mn atomic ratio is set to 0.56 as shown in Example 3, the charging / discharging voltage monotonously increases and decreases, and the two-stage flat portions disappear, resulting in a flat curve. The shape of this curve is clearly different from the conventional spinel LiMn ₂ O ₄ . This LixM
In the X-ray diffraction diagram (using FeKα line) of n ₂ O ₄ (x = 1.12) and LiMn ₂ O ₄ , both 2θ = 23.3-2.
It has characteristic peaks of LiMn ₂ O _{4 of} spinel at 3.8, 45.8-46.2, and 56.0-56.5 ° (FIG. 3 (9)). Focusing on the line width of the peak, if the discharge curve is flat, it is 45.8-46.2 ° ((311)
Diffraction line), 56.0-56.5 ° ((400) diffraction line)
Becomes wider, and the (311) diffraction line width shows 0.5 ° or more. That is, by setting x = 1.12, a new LixMn ₂ O ₄ having excellent cycle characteristics was found. From MnO ₂ and LiNO ₃ to LiMn ₂ O ₄
The reduction process accompanying the release of oxygen from manganese dioxide is indispensable for the production of methane. The reduction reaction is difficult to proceed by firing in air in which about 20% oxygen is present. When firing under nitrogen gas, oxygen is forcibly released, and this reduction process proceeds smoothly. Therefore, it becomes possible to generate LiMn ₂ O ₄ having a spinel structure at a low temperature, and the sintering reaction is suppressed, so that the specific surface area becomes large. 4 shown in Example 4
FIG. 4 shows the charge / discharge curve of the sample at the firing stage of 60 ° C. for 48 hours.
It was shown to. As shown in FIG. 3 (10), the peak position of this sample coincides with the spinel structure, and the peak width is relatively narrow, but the peak strength is slightly inferior and the degree of crystal growth is inferior to that of the high temperature fired product. However, the peak intensity is higher and the peak width is narrower than that of the compound having a strained spinel structure (FIG. 3 (11)) which is fired in air at 460 ° C. for 96 hours, and high capacity LiMn ₂ O ₄ is fired at a low temperature around 500 ° C. And LixMn ₂ O ₄
Can be manufactured. As shown in Example 5, LiMn ₂ O ₄ obtained by firing at 750 ° C. for 24 hours in a nitrogen stream has a charge capacity of 130 mAH / g and a discharge capacity of 11 as well.
It shows 5 mAH / g or more. By firing under nitrogen gas, it became possible to produce high performance LiMn ₂ O ₄ with excellent cycle characteristics. 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 (Fardizer M, Sedema, International Common Sample No.
12) 8.69 g and lithium nitrate 3.438 g are ground and mixed. After holding at 270 ° C for 3 hours, 400 ° C, 48
Preliminary firing is performed in an air atmosphere for an hour to obtain amorphous LiMn ₂ O ₄ having a strained spinel structure. This again at 700 ℃ 12
LiMn ₂ O ₄ having a crystalline spinel structure after firing for 0 hours
And Spinel L obtained by firing at 700 ° C for 120 hours
A mixture composed of iMn ₂ O ₄ (3.2 m ² / g) and a conductive binder (composite of acetylene black and polytetrafluoroethylene: 20 Wt%) was used as a positive electrode, and lithium metal was used as a negative electrode at a rate of 0.2 C. The discharge curves when charging at (5 hours) and then discharging at various rates (0.2-5C) are shown. 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 130m even at a high discharge rate of 1C.
AH / g or more, and polarization is as small as 100 mV or less. [Example 2] Electrolytic manganese dioxide (International common sample N
o. 17) 3.00 g and 1.19 g of lithium nitrate are pulverized and mixed, and kept at 270 ° C. for 5 hours. 460 this sample
Li of the strained spinel structure after firing in air at 116 ° C for 116 hours
Mn ₂ O ₄ (5 m ² / g) is obtained. Further 650 ° C, 5
LiM with crystalline spinel structure after firing in air for 0 hours
obtain n ₂ O _4. The (311) diffraction line width of this sample is 0.
It was 43 °. The discharge curve of this sample is a curve having two voltage flat portions near 3.95 V and 4.1 V as shown in FIG. The charge / discharge current density was 0.3 C, and a propylene carbonate-diethyl carbonate mixed solution in which LiPF ₆ was dissolved was used as the electrolytic solution. Example 3 Electrolytic Manganese Dioxide (International Common Sample N
o. 17) 3.00 g and 1.32 g of lithium nitrate are pulverized and mixed, and kept at 270 ° C. for 5 hours. After baking in air at 460 ° C for 96 hours, it is further baked in air for 48 hours for 65 hours.
It was fired at 0 ° C. and XRD was crystalline spinel structure Li.
xMn ₂ O ₄ (x = 1.12) is obtained. As shown in FIG. 2 (5), the charge / discharge curve of this compound is a flat curve with two flat portions disappearing, which is different from the charge / discharge curve of LiMn ₂ O _{4 having} a conventional crystalline spinel structure. . Therefore, the cycle characteristics are significantly improved. This compound is considered to be a novel battery active material in view of the discharge curve. Also, (3
11) The diffraction line width was 0.56 °. Example 4 Electrolytic Manganese Dioxide (International Common Sample N
o. 17) 3.00 g and 1.32 g of lithium nitrate are pulverized and mixed, and kept in an alumina crucible at 270 ° C. for 5 hours.
It was baked at 460 ° C. for 48 hours in a nitrogen stream. This sample
Further, it was baked at 650 ° C. for 24 times in a nitrogen stream. The charge / discharge curve of this compound was an S-shaped curve as shown in FIG. 2 (6), and the capacity was 10% larger than that of Example 3. That is, by firing in a nitrogen stream, LixMn ₂ O ₄ having a spinel structure showing a flat discharge curve is easily generated. The (311) diffraction line width was 0.51 °.
This compound is easily obtained even if the atomic ratio of Li / Mn is increased, but the capacity decreases when the ratio is increased. [Example 5] Electrolytic manganese dioxide (International common sample N
o. 17) 3.00 g and 1.19 g of lithium nitrate are pulverized and mixed, and kept in an alumina crucible at 260 ° C. for 12 hours. It was baked at 750 ° C. for 24 hours in a nitrogen stream. [0011]

BRIEF DESCRIPTION OF THE DRAWINGS [FIG. 1] Relationship between sintering temperature and specific surface area of Mn ₂ O ₃ and LiMn ₂ O ₄ [FIG. 2] Various L synthesized under nitrogen stream and in air
Discharge curve of ixMn ₂ O ₄ . FIG. 3 Various L synthesized under nitrogen stream and in air
XRD diagram of ixMn ₂ O ₄ [FIG. 4] L obtained by firing at 460 ° C. under a nitrogen stream
Discharge curve of ixMn ₂ O ₄ . FIG. 5 is a discharge curve of LiMn ₂ O ₄ synthesized by this method. [Explanation of Codes] 1 LiMn ₂ O ₄ : 650 ° C. heat treatment 2 LiMn ₂ O ₄ : 750 ° C. heat treatment 3 Mn ₂ O ₃ : 650 ° C. heat treatment 4 Mn ₂ O ₃ : 750 ° C. heat treatment 5 spinel LixMn ₂ O ₄ 6 nitrogen stream Spinel LixMn ₂ O ₄ 7 crystalline spinel LiMn ₂ O ₄ 8 crystalline spinel LiMn ₂ O ₄ 9 spinel LixMn ₂ O ₄ 10 spinel Lix synthesized under nitrogen flow at 460 ° C.
Mn ₂ O ₄ 11 450 ℃ in air, spinel LiMn ₂ O4 obtained by firing 96 hours

Claims (1)

[Claims] 1. Method for producing crystalline LiMn ₂ O _{4 characterized} by a two-step firing method in which lithium nitrate and electrolytically or chemically synthesized manganese dioxide are calcined at 250-470 ° C. to synthesize lithium-containing manganese dioxide, and then calcined at 500-800 ° C. .
Lithium hydroxide may be used instead of lithium nitrate. 2. A method for producing crystalline LiMn ₂ O ₄ having a specific surface area of 2 m ² / g or more. 3. A method for producing LiMn ₂ O ₄ obtained by firing lithium nitrate and electrolytically or chemically synthesized manganese dioxide in a temperature range of 300 to 800 ° C. under a nitrogen gas stream. 4. The value of x is 1.0 in Li _x Mn ₂ O ₄ having a spinel structure.
Compounds in the range 4-1.2. 5. Peak of 2θ = 46 ° of XRD measured using FeKα with Li _x Mn ₂ O ₄ having a spinel structure ((311)
A compound having a half-width of the surface diffraction line of 0.5 ° or more. 6. A lithium secondary battery using LiMn ₂ O ₄ as a positive electrode active material and a positive electrode for a rocking chair type lithium ion battery having a negative electrode of an intercalation compound such as carbon. [0002]