JPH0773053B2 - Positive electrode for lithium secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a lithium secondary battery using lithium or a lithium alloy as a negative electrode, and is particularly intended to improve a positive electrode active material.

2. Description of the Related Art Chromium oxide and manganese dioxide are known as positive electrode active materials for lithium secondary batteries. Chromium oxide has a problem in terms of pollution, and recently manganese dioxide has been actively researched.

The lithium secondary battery using manganese dioxide as the positive electrode active material has a problem that the capacity gradually decreases when the charge / discharge cycle is repeated. It was considered that this is because the crystallinity of manganese dioxide gradually decreased.

In order to improve this problem, a composite oxide of manganese and lithium, LiMn ₂ O _4, has been proposed (for example, JP-A-63-1).
14065 Publication, Tuckley, etc.
Research Bulletin Vol. 18, 1983, pp. 461-472). As a method for synthesizing this LiMn ₂ O ₄ , Mn ₂ O ₃ and Li ₂ CO ₃
There is known a method of heating the same, or a method of heating electrolytic manganese dioxide or chemical manganese and LiOH or Li ₂ CO ₃ . However, when it is applied to a battery by the synthetic method, its cycle characteristics are greatly different, and chemical manganese (MnO ₂ ) and Li
LiMn ₂ O ₄ synthesized from OH is considered good.

However, even if LiMn ₂ O ₄ synthesized from these manganese dioxide and LiOH or Li ₂ CO ₃ is used as the active material, the capacity and cycle characteristics of the lithium secondary battery are still insufficient. It was

In view of the above problems, it is an object of the present invention to increase the capacity of the positive electrode active material during charge / discharge cycles, especially the capacity per volume, and to improve the cycle characteristics.

Means for Solving the Problems In a lithium secondary battery including a lithium or lithium alloy as a negative electrode, a non-aqueous electrolyte in which a lithium salt is dissolved, and a positive electrode, manganese nitrate and a lithium salt in the presence of oxygen as a positive electrode active material. It is characterized by using the product obtained by heating at.

Action Conventional methods for synthesizing LiMn ₂ O ₄ have all been carried out by reacting manganese oxide with a lithium salt. Therefore, the properties of the product also changed significantly depending on the properties of the starting material manganese oxide. For example, when electrolytic manganese dioxide is used, the original bulk density is high, and therefore the bulk density of the product is also high. However, since the reactivity with the lithium salt is poor, there are drawbacks in both discharge capacity and cycle characteristics even when the heating temperature and time are changed. On the other hand, chemical manganese has a low bulk density, and the product has a low bulk density. However, the reactivity with a lithium salt at the time of heating is good, and when the product is an active material, the capacity per unit weight is larger than that when electrolytic manganese dioxide is used as a raw material. When converted per unit volume, the bulk density was larger when electrolytic manganese dioxide was used as the raw material, so there was no difference compared to when chemical manganese was used.

The present invention is characterized in that manganese nitrate that is liquid at room temperature is used without using manganese oxide that undergoes a solid-phase reaction as a raw material. That is, when a lithium salt is dissolved in water and manganese nitrate is added thereto, a uniform solution state is obtained. That is, in this state, manganese ions and lithium ions are uniformly dissolved. It was considered that by heating and reacting this, uniform and high bulk density LiMn ₂ O ₄ was produced. Also, because the conventional solid-phase reaction is not used,
There is also an advantage that the product can be obtained even at a low temperature. LiMn ₂ O ₄
It had a spinel crystal structure unlike conventional γ-manganese dioxide, and it was thought that the crystalline state did not change much even if the charge / discharge cycle was repeated. Therefore, it is considered that LiMn ₂ O _{4 having} lower crystallinity facilitates diffusion of lithium ions in the lattice and improves battery characteristics. In the conventional synthesis method using a solid-phase reaction, a temperature of 400 ° C. or more is indispensable to cause a sufficient reaction, and the crystallinity of the product is increased due to this high temperature. On the other hand, in the present invention, since the solid phase reaction is not used, the reaction proceeds even at a low temperature of 200 ° C., and when it is used as a positive electrode active material having low crystallinity, it becomes an active material having a large capacity per unit weight. In summary, the positive electrode active material of the present invention is LiMn ₂ O _{4 having} a large bulk density and low crystallinity, and when used in a battery, a battery having a large capacity per unit volume and good cycle characteristics.

Example (Example 1) 0.1 mol of lithium nitrate was dissolved in 50 cc of water, and 0.2 mol of manganese nitrate was added thereto. Heating to 250 ° C. in air gave the product. This is designated as the active material 1 of the present invention.

As the active material of the conventional example, 0.1 mol of LiOH was added to 0.2 mol of chemical manganese (MnO ₂ ), and the powder was mixed well and heated at 470 ° C. to obtain LiMn ₂ O ₄ . Further, as an active material of Comparative Example, 0.2 mol of electrolytic manganese dioxide and 0.1 mol of LiOH were added and heated at 470 ° C. to obtain LiMn ₂ O ₄ .

Acetylene black 5 as a conductive agent on 10 g of each active material
g, add 10 g of polytetrafluoroethylene resin as a binder,
It was well kneaded to obtain a positive electrode mixture.

A battery as shown in FIG. 2 was constructed using these positive electrode mixtures. The positive electrode mixture is pressure-molded at 1 ton / cm ² and the diameter is 1
The height is 7.5 mm and the height is 0.5 mm. This is for comparing capacities per unit volume. The molded positive electrode 1 was put in a case 2. Directly connected, metal lithium 3 punched to a height of 17.5 mm and a height of 0.5 mm was pressure-bonded to the sealing plate 4 to obtain a negative electrode. A polypropylene non-woven fabric was interposed as a separator 5 between the positive electrode and lithium. As a non-aqueous electrolyte, a mixed solution of propylene carbonate and dimethoxyethyne with a volume ratio of 1: 1 in which 1 mol / liter of lithium perhydrochloride was dissolved was used, and after pouring the solution into the positive electrode and the negative electrode, the battery was sealed with a sealing material 6. .

The battery was discharged at a constant current of 2 mA until the voltage reached 2 V, and charged at 2 mA until the voltage reached 3.8 V. A battery using the active material 1 of the present invention is designated as A, a battery using the conventional active material is designated as B, and a battery using the comparative active material is designated as C.
FIG. 1 shows the discharge curve of each battery in the 10th cycle. It can be seen that the battery A of the present invention has a large capacity. Further, FIG. 3 shows cycle characteristics in which the discharge capacity when the charge / discharge cycle is repeated is plotted. In the battery of the present invention, deterioration with cycle is small.

When the X-ray diffraction of the active material 1 of the present invention is examined, a broad peak appears in a place roughly corresponding to LiMn ₂ O ₄ .
However, it was too broad to identify the exact peak position, and it seemed difficult to clearly identify it as LiMn ₂ O ₄ . However, as will be described later, the peak is closer to LiMn ₂ O ₄ of the conventional example and the comparative example as it is synthesized at a higher temperature, so that the active material 1 of the present invention is closer to LiMn ₂ O _4. Conceivable.

(Example 2) 0.1 mol of lithium nitrate was dissolved in 50 cc of water, and 0.2 mol of manganese nitrate was added thereto. The product was obtained by heating at different temperatures in the atmosphere. The product was made into a battery in the same manner as in Example 1 and the same test was conducted. Figure 4 shows the heating temperature,
It is a plot of the discharge capacity at the 10th cycle of the battery. From this, it was found that the temperature was good between 200 ° C and 550 ° C. When the product was examined by X-ray diffraction, it was found that when the product was heated at a high temperature, especially 450 ° C or higher, L
It is close to iMn ₂ O ₄ . However, the product of the present invention has a larger discharge capacity. The reason is that the weight of the positive electrode molded to the same volume is large when the product of the present invention is used, and it is considered that the product of the present invention has a higher bulk density. To be

The curve reaches its maximum around 300-400 ° C, because it has low crystallinity at low temperature but low bulk density and high bulk density at high temperature, but it also has high crystallinity.

(Example 3) Various lithium salts were weighed so that lithium ion was 0.1 mol, dissolved in 50 cc of water, 0.2 mol of manganese nitrate was further added, and the mixture was heated at 470 ° C in the atmosphere. A battery was constructed using each product in the same manner as in Example 1 and tested. The state was different depending on the lithium salt used. The types of lithium salts used and the discharge capacities at the 10th cycle are shown in the table. Thereby, as the lithium salt, lithium nitrate>
The order was lithium sulfate> lithium hydroxide> lithium carbonate. The reason for this is unknown, but it is considered that the type of the lithium salt has some influence on the bulk density of the product because the bulk densities of the products are in the above order.

Example 4 The number of moles of lithium nitrate was changed, this was dissolved in water, and 0.2 mole of manganese nitrate was added. The product was obtained by heating at 300 ° C. in air. The same battery as in Example 1 was constructed and the same test was conducted. The ratio of the number of moles of manganese in manganese nitrate and the number of moles of lithium in lithium nitrate was represented by x: 1, and the discharge capacity at the 10th cycle of each battery was plotted in FIG. From this curve, x = 2, that is, the composition ratio corresponding to LiMn ₂ O ₄ does not reach the maximum, but rather becomes maximum at around 1.5. The x-ray diffraction peak is broad and it is difficult to find a clear peak position. I think that LiMn ₂ O ₄ may be a compound with a non-stoichiometric ratio as well as a compound with a clear composition ratio.

In all of the above examples, the active material was synthesized by the method of heating in the atmosphere. This is because manganese in the raw material manganese nitrate is in a divalent state, whereas manganese in the product is
This is because it is in a mixed state of trivalent and tetravalent, and atmospheric oxygen is also involved in the reaction. When the heating shown in the examples was performed in a nitrogen atmosphere, the capacity was small even when the product was charged and discharged.

In addition to LiClO ₄ , the non-aqueous electrolyte used was LiPF ₆ or LiA.
The active material of the present invention is also effective when a polymer electrolyte such as polyethylene oxide in which LiCF ₃ SO ₃ is dissolved is not limited to the organic electrolyte in which sF _{6 and the} like are dissolved.

EFFECTS OF THE INVENTION As is apparent from the above description, the lithium salt and the manganese nitrate of the present invention are heated in the presence of an enzyme to synthesize a product as a positive electrode active material, so that the capacity per volume is large,
In addition, a lithium secondary battery having excellent cycle characteristics can be constructed, which is of great industrial significance.

[Brief description of drawings]

FIG. 1 is a discharge curve of a lithium secondary battery according to an embodiment of the present invention at the 10th cycle, FIG. 2 is a longitudinal sectional view of the lithium secondary battery, and FIG. 3 is cycle characteristics of the lithium secondary battery. Fig. 4 shows the relationship between the heating temperature during synthesis and the discharge capacity of the battery when the product was used as a battery. Fig. 5 shows the ratio when the molar ratio of manganese to lithium was expressed as: 1. FIG. 3 is a relational diagram of discharge capacity when a product is used as a battery.

Claims (1)

[Claims] 1. In a lithium secondary battery comprising a positive electrode, a negative electrode and an electrolyte, manganese nitrate and a lithium salt having a molar ratio of manganese element: lithium element of 1.2: 1 to 2.2: 1 are used as a positive electrode active material in the presence of oxygen. A positive electrode for a lithium secondary battery, characterized by using a product obtained by heating below.