JPH07105232B2 - Non-aqueous electrolyte secondary battery and method for producing positive electrode active material used therein - Google Patents

Description

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery in which lithium or a lithium alloy is used as a negative electrode and an organic electrolyte is used as an electrolyte, and an electrolytic manganese dioxide is improved. By using a positive electrode active material, a secondary battery having a high discharge voltage, a large energy density, and a long charge / discharge cycle life is provided.

2. Description of the Related Art Attempts to obtain a so-called non-aqueous electrolyte secondary battery that uses lithium or a lithium alloy as a negative electrode and an organic electrolytic solution as an electrolytic solution have been vigorously carried out, and as a positive electrode active material, Chalcogen compounds such as titanium disulfide and molybdenum disulfide have been mainly used. [Takehara Chemistry 37
168 (1982)] However, when a chalcogen compound is used for the positive electrode, the discharge voltage is low and therefore the energy density is low.
Also, many chalcogen compounds are difficult to synthesize and expensive.

In order to overcome these drawbacks, the use of various oxides as a positive electrode active material has been studied, and among them, electrolytic manganese dioxide has a high average discharge voltage of 2.8 V when combined with lithium, and industrial use. Since it can be produced and is inexpensive, it is expected to be applied to secondary batteries.

However, since the reaction when charging and discharging electrolytic manganese dioxide in the organic electrolytic solution is the movement of lithium ions into and out of the manganese dioxide crystal, the volume of manganese dioxide repeats expansion and contraction due to charging and discharging, and the crystal structure gradually increases. However, there is a problem in that the discharge capacity decreases with an increase in the number of cycles due to the collapse of the particles and the poor contact between the manganese dioxide crystal and the conductive material. [G.Pistoia J. Elec
trochem.Soc., 129 1861 (1982)] In order to solve this problem, attempts have been made to improve the charge / discharge characteristics by adding various compounds to manganese dioxide. Among them, as a method of using lithium hydroxide,
A method in which electrolytic manganese dioxide is placed in an aqueous solution of lithium hydroxide to irradiate a micro liquid to dope lithium into manganese dioxide and heat at 350 to 430 ° C. [JP-A-62-108455]
No. gazette] and that the LiOH-MnO ₂ fired body has excellent plasticity [Proceedings of the 28th Open Battery Symposium, 3B09, P203, 1987.11.18 ~
20] etc. have been reported.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention In the non-aqueous electrolyte secondary battery using electrolytic manganese dioxide as described above, by eliminating the drawback that the discharge capacity decreases with an increase in the number of cycles, the discharge voltage is high, It has been a subject to obtain a secondary battery having a large discharge capacity and a large energy density.

Means for Solving the Problems The present invention uses a non-aqueous electrolyte secondary battery using lithium or a lithium alloy for a negative electrode, in which lithium hydroxide is contained in the crystal of electrolytic manganese dioxide as a positive electrode active material. It is characterized by In addition, as a method for producing the positive electrode active material, 30% by weight or less of lithium hydroxide monohydrate is added to electrolytic manganese dioxide and heated at a temperature of 350 ° C. or less.

Action When using electrolytic manganese dioxide as an active material of a non-aqueous electrolyte battery, a method of heating and dehydrating at a constant temperature is used in order to remove water contained in crystals to some extent. In the present invention, when the electrolytic manganese dioxide is heated and dehydrated, a certain amount of lithium hydroxide monohydrate (LiOH.H ₂ O) is added to the electrolytic manganese dioxide in advance, and the lithium hydroxide monohydrate is decomposed by heating. The fact that the lithium hydroxide thus generated enters the crystal of manganese dioxide is utilized.

However, electrolytic manganese dioxide has a γ-type crystal structure containing water at room temperature and is dehydrated by heating, but at 250 ° C or higher, a mixture of γ-type and β-type crystal structures, and at 350 ° C or higher, β-type crystal structure. Becomes Β when charging / discharging the battery
Since the γ-type crystal structure is preferable to the − type, the heating temperature needs to be 350 ° C. or lower.

The positive electrode active material according to the present invention is synthesized under the condition that the release of water from electrolytic manganese dioxide and the decomposition of lithium hydroxide monohydrate occur at the same time. Therefore, it is considered that the product is a state in which lithium hydroxide is contained in the manganese dioxide crystal. Therefore, when manganese dioxide is charged and discharged, lithium ions are electrochemically moved in and out, but even when lithium hydroxide enters the manganese dioxide crystal in advance and the manganese dioxide crystal is not charged and discharged, it expands in advance. As a result, the volume change due to the inflow and outflow of lithium during charge and discharge is made smaller than that of electrolytic manganese dioxide alone, and as a result, the expansion and contraction due to charge and discharge is very small, and the contact between crystals and the conductive agent is good. And most of the manganese dioxide is used in the reaction.

Further, by setting the heating temperature to 350 ° C. or lower, the crystal structure of manganese dioxide does not change to β-type, and a γ-type or a mixture of γ-type and β-type, which is a crystal structure advantageous for battery charge / discharge. Is retained.

Examples The present invention will be described below with reference to preferred examples.

[1. Method for Synthesizing Positive Electrode Active Material] Electrolytic manganese dioxide (γ-type crystal structure) powder and lithium hydroxide monohydrate powder are uniformly mixed at a constant ratio, put in a crucible and placed in an electric furnace for 5 hours. Heated and positive electrode active material No. 1 ~
No. 10 was synthesized. The mixing ratio and heating temperature are as shown in Table 1.

[2. Manufacturing method of positive electrode plate] The above positive electrode active material, acetylene black (conductive agent), and dispersion Teflon were mixed at a weight ratio of 90: 8: 2 to form a paste, and a nickel lead wire was attached. It was coated on a 10 mm x 10 mm expanded nickel grid. The amount of the positive electrode mixture applied was about 50 mg per electrode plate. This was pressed to form a uniform surface, and then vacuum dried at 200 ° C. for 20 hours to dehydrate excess water.

[3. Battery Trial Production and Test Conditions] The battery is composed of one positive electrode plate and one negative electrode plate. The negative electrode plate is a 10 mm x 10 mm lithium plate with nickel lead wires attached by pressure bonding. A polypropylene sheet having fine pores was used as the separator, and a non-aqueous electrolytic solution in which 1.5 mol / mol of lithium hexafluoroarsenate (LiAsF ₆ ) was dissolved in 2-methyltetrahydrofuran was used as the electrolytic solution.

This electrode group was put in a Teflon case, and the whole was sealed in a separable flask in an argon atmosphere, and a charge / discharge test was conducted. The charge / discharge test conditions are as follows.

Temperature: 25 ℃ ± 2 ℃ Current: 1.0mA / cell constant current for both charge and discharge Initial voltage: (Charge) 3.50V, (Discharge) 2.00V [4. Charge and discharge test result] Active materials No.1 to No.6 Regarding the batteries used, the amount of lithium hydroxide monohydrate added during the synthesis of the positive electrode active material and the positive electrode active material 1k
The relationship with the discharge capacity per g is shown in FIG. However, since the discharge capacity changes with the number of cycles, the values at the 10th cycle are all compared below. The discharge capacity becomes maximum when the amount of lithium hydroxide monohydrate added is 10 wt% and decreases when the amount added is increased. When the amount of lithium hydroxide monohydrate added is 30 wt% or less, the discharge capacity becomes larger than that when no lithium hydroxide is added.

FIG. 2 shows the relationship between the heating temperature and the discharge capacity when 10 wt% of lithium hydroxide monohydrate was added to the batteries using the active materials No. 3 and No. 7 to No. 10. The discharge capacity becomes maximum when heated at 300 ° C, and becomes smaller when the heating temperature becomes lower or higher. Although manganese dioxide has a γ-type crystal structure at room temperature, it is dehydrated by heating to a γ / β-type crystal structure in the range of 250 to 350 ° C.
It is considered that the β-type crystal structure is formed in the range of 0 to 450 ° C., and the heating at 350 ° C. or higher results in a crystal structure not suitable for charging and discharging. Therefore, the heating temperature of the positive electrode active material is suitably 350 ° C. or lower.

Next, Fig. 3 shows the change in discharge capacity depending on the number of charge / discharge cycles of the battery using the active material No. 1 (no addition) and the battery using No. 3 (adding 10 wt% lithium hydroxide monohydrate). . It can be seen from the figure that the discharge capacity decreases drastically with the number of cycles unless lithium hydroxide monohydrate is added, but the change in discharge capacity when lithium hydroxide monohydrate is added is very small.

EFFECTS OF THE INVENTION When the positive electrode active material according to the present invention is used, most of manganese dioxide contained in the electrode plate participates in the reaction in the charge / discharge reaction, so that the same weight of electrolytic manganese dioxide is used as compared with the case of using it alone. The discharge capacity becomes large. Moreover, since the discharge voltage is an average of 2.8 V as in the case of manganese dioxide alone, the discharge energy density of the battery is extremely large.

In addition, since lithium hydroxide enters into the manganese dioxide crystal, the volume change of the manganese dioxide crystal due to charging and discharging is small, and the contact between the crystals is maintained in a good state. Is small, and a secondary battery having an extremely long cycle life can be obtained.

In the examples, the negative electrode is lithium and the electrolyte is 2-
Methyl tetrahydrofun-lithium hexafluoroarsenate was used, but the negative electrode was an alloy containing lithium, such as lithium-
Aluminum alloys and the like can be used, and various organic electrolytes that do not directly react with lithium can be used as the electrolyte. In any case, the effect of the present invention can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the content of lithium hydroxide monohydrate at the time of synthesizing a positive electrode active material and the discharge capacity of a battery using the active material. FIG. 2 is a diagram showing the relationship between the heating temperature during the synthesis of the positive electrode active material and the discharge capacity of the battery. FIG. 3 is a diagram showing the relationship between the number of charge / discharge cycles and the discharge capacity of the battery according to the present invention and the conventional battery.

Claims (2)

[Claims] 1. A non-aqueous electrolyte secondary battery using lithium or a lithium alloy as a negative electrode, wherein an electrolytic manganese dioxide crystal containing lithium hydroxide is used as a positive electrode active material. Electrolyte secondary battery. 2. A method for producing a positive electrode active material used in the non-aqueous electrolyte secondary battery according to claim 1, wherein the electrolytic manganese dioxide contains 30% by weight or less of lithium hydroxide monohydrate (LiOH.H ₂ O). )
And a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, which comprises heating at 350 ° C. or lower.