JPH03127453A - Manufacture of positive electrode active material for lithium secondary battery - Google Patents

Abstract

PURPOSE:To produce a positive electrode active material which has both stable discharge capacity and high energy density by calcining MnO2 and LiNO3 by a specified method. CONSTITUTION:MnO2, if desirable, electrolyte manganese deoxides shall be used. MnO2 and LiNO3 are mixed with each other using water as a medium so as to be calcined, after LiNO3 has been dissolved into water in advance. MnO2 and LiN3 are mixed at a mole ratio, that is, Mn:Li=2.2:1.0 through 1.8:1.0 while they are clcined in air at the temperature range equal to or more than 880 deg.C but less than 1000 deg.C. By the use of a positive electrode active material produced as mentioned above, a 4V class battery with both cycle reversibility and high energy density can thereby be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an organic electrolyte lithium secondary battery having a high energy density using lithium as a negative electrode active material, and particularly to a method for producing its positive electrode active material.

BACKGROUND OF THE INVENTION A primary battery using MnO2 as a positive electrode has already been put into practical use as a lithium battery. In the case of lithium batteries, the presence of moisture has a negative effect on battery performance, so conventionally,
MnO2 was heat-treated at a temperature of 250°C to 400°C to remove attached water and bound water, and was used as a positive electrode of a lithium battery. As disclosed in Japanese Patent Publication No. 49-25571, the crystal structure of M n 02 is 250°C ~
γ-β type heat treated at a temperature of 350°C or 350°C as disclosed in U.S. Pat. No. 4,133,856.
It is considered to be the β type heat treated at a temperature of ~430°C. However, in subsequent studies, M
nO2 is also said to be γ-β type MnO2, and it is said that bound water cannot be completely removed.

However, if bound water is completely removed, the γ-β type cannot be maintained, and the β-type M n has extremely low activity as a battery active material.
It is said that it becomes O2.

Furthermore, it is known that even if the γ-β type is maintained, the capacity characteristics deteriorate as the heat treatment temperature increases. One of the reasons for this is that the surface of the active material has partially changed to the β type, but the main cause is said to be a decrease in surface activity such as partial reduction of the surface of the active material. In view of these considerations, currently the γ-β type ~ ino, which is heat-treated at a temperature of about 350°C to 400°C and retains a small amount of bound water:
is used in lithium batteries. However, when MnO2 having this crystal structure is used as a lithium secondary battery, the capacity decreases with cycles due to the collapse of the crystal structure during charging and discharging. Furthermore, as the crystal structure collapses, residual bound water flows out, which is said to have an adverse effect on battery performance, particularly cycle characteristics and storage characteristics. Therefore, from the viewpoint of positive electrode active materials for lithium secondary batteries that require cycle reversibility, manganese oxide, including improvements to MnO2, has been actively developed, and several proposals have been made.

One such attempt was to use manganese dioxide, which has a spinel-type structure, as a positive electrode active material.
or in combination with a solid electrolyte as shown in JP-A No. 63114065.
There is a positive electrode mainly composed of M n 204.

Problem to be Solved by the Invention Spinel type manganese dioxide has L i M n 20
As shown in FIG. 1, it is a two-stage discharge, and the discharge voltage on the high potential side is as high as 4.5 cm, making it possible to expect a high energy density battery. Furthermore, its structure is not destroyed by charge/discharge cycles, and it is in a crystalline form that does not contain any bound water, so it is considered to be a promising active material for lithium secondary batteries. LiM
The main manufacturing methods for n2O4 are lithium carbonate, Mn
203 or MnO2 at a molar ratio of Mn:Li=2+1 and heated at 800°C to 900°C (Japanese Unexamined Patent Publication No. 187569/1983), lithium carbonate and MnO:
are mixed at the above molar ratio and heated at 400°C in a nitrogen atmosphere, or lithium iodide and MnO2 are heated at 300°C in a nitrogen atmosphere (Japanese Patent Application Laid-Open No. 63-11).
No. 4065) has been reported. However, at present, batteries that perform two-stage discharge are difficult to use in practice even if they have excellent cycle reversibility; for example, if only the high potential side is used, the capacity will be small, and even if the voltage is high, the energy density will have a lower advantage.

An object of the present invention is to provide a lithium secondary battery that has a discharge capacity that is always stable over the course of cycles and has a high energy density. The object of the present invention is to improve the high-potential side capacity of spinel-type manganese dioxide, and to improve not only cycle reversibility but also excellent energy density.
battery.

Means for Solving the Problems The present invention relates to the improvement of spinel type manganese dioxide, in which MnO2 and T, i N O3 are combined into M n : L
An oxide consisting of lithium and manganese obtained by mixing at a molar ratio of i = 2.2:1.0 to 1.8:1.0 and firing in air at a temperature range of 880°C or higher and 1000°C or lower. It is used as an active material.

Further, MnO2 is preferably electrolytic manganese dioxide (EMD), and it is preferable to use water as a medium during one test of LiNO3 and MnO2, and to dissolve LiNO3 in water in advance, and then to sinter it at the above-mentioned predetermined temperature. By using the manufacturing method of the present invention, a positive electrode having not only cycle reversibility but also excellent discharge gffi on the high potential side can be obtained, and a high energy density battery of 4■ class can be achieved.

In addition, in a battery to which the positive electrode active material of the present invention is applied, the negative electrode may be made of metal Li, an alloy with Li, a polymer material doped with Li, or a carbon material capable of occluding Li.

Function Conventionally, spinel type M' n O2 is synthesized at a much higher temperature than the heat treatment temperature of γ-β type MnO2 or in a reducing atmosphere such as in nitrogen gas. That is, γ-β type M
It is expected that the surface of the active material is partially reduced, as is the case when nO2 is heat-treated at a high temperature, and becomes extremely inert to battery reactions. This is probably why the active material utilization rate was low and the capacity was also small. On the other hand, when LiN0+ was used as the Li source as in the present invention, the capacity was increased compared to the conventional one despite the high temperature treatment.

Therefore, since there is a possibility that some kind of change in the crystal structure has occurred, the X Linear diffraction analysis was performed. As a result, it was found that all of them were basically spinel type, but the one of the present invention had an unidentified peak that was hardly observed in the conventional one. Therefore, it seems that some kind of different crystal phase is formed, but the details are not clear.

Furthermore, although the decomposition temperature of Li NO:l is 600°C, when the temperature is increased in the firing process of the present invention,
Significant generation of NO8 was observed from around 00°C, which was thought to be due to decomposition of LiNC)+. From this, Mn
It is expected that a decomposition reaction of LiNO3 using 0= as a catalyst occurs simultaneously.

Furthermore, since NO:t, NO□, etc. generated during this decomposition process act as strong oxidizing agents, at least MnO,
It is thought that the surface of is in an extremely active state. Presumably, these act effectively to bring about some changes in the crystal structure, and also contribute to the maintenance of Mnoz surface activity. This is thought to be the reason why the active material has a superior capacity compared to conventional materials. Therefore, the technique of the present invention using LiN0:i is extremely interesting.

Example Examples of the present invention will be shown below.

Example 1 A spinel-type oxide comprising L and Mn of the present invention was prepared as follows. First, a predetermined amount of LiNO3 is dissolved in water, and LiNO. Make something close to a saturated aqueous solution of A predetermined amount of MnO2 powder is added thereto, thoroughly stirred and mixed, and a portion of the water is evaporated to form a mud-like mass, which is then fired at a predetermined temperature for 4 to 5 hours using an electric furnace.

Alternatively, the same product could be prepared by mixing MnO2 and LiNO3 in powder form in advance and then adding water and kneading. However, when the mixture of powders was directly fired without using water, the reaction did not occur uniformly, resulting in large variations in performance.

Therefore, it is preferable to involve water in the mixing.

Example 2 Using EMD as Mn0z, LiN0:i was
: Mixed at a molar ratio of L f = 2.0 + 1.00, 9
A conventional active material was synthesized by mixing the active material of the present invention fired at 00°C and EMD with lithium carbonate at the same molar ratio as above, and firing at 900°C.

First, a button-type battery as shown in Figure 2 was assembled using these two active materials, and their characteristics were compared. In Figure 2, the positive electrode 1 is a mixture of carbon powder as a conductive agent (5% by weight based on the active material) and polytetrafluoroethylene resin powder as a binder (7% by weight based on the active material). It is press-formed onto a titanium net 2 fixed to the inside of the positive electrode case by spot welding. Further, the amount of active material was 100 mg in each case. And a polypropylene separator 3. The metallic lithium negative electrode 5 and the electrolytic solution 6 (1 mol/l LiAsF6 dissolved in a mixed solvent of propylene carbonate and ethylene carbonate) are sealed together via a polypropylene gasket 7, which is press-bonded to the sealing plate 4. The battery is 20m1W1 and 1.6cm high. In addition, this battery was prototyped for the purpose of comparing the characteristics of the positive electrode.
The capacity of the negative electrode is filled to about four times the capacity of the positive electrode, so that the charging and discharging characteristics are not affected by the lack of the negative electrode. The charge/discharge test was performed by constant current charging/discharging at 1.0 mA with the end-of-charge voltage set at 4.5 V and the end-of-discharge voltage set at 2.OV. FIG. 3 shows the discharge curve of the battery using the above two types of active materials at the 5th cycle. Third
In the figure, curve 8 is the characteristic of the conventional spinel type Mn0=, and curve 9 is the characteristic of the active material of the present invention. Both are 4
This is a so-called typical spinel-type MnO2 two-stage discharge with voltage plateaus near (1) and 3V. However, when comparing the two, it is clear that the active material of the present invention has a much larger discharge capacity near the above 4V than the conventional one. Also, the capacitance near the bottom 3V is also large, but the difference in capacitance is small compared to the top. For example, in the conventional case, the upper discharge capacity and the lower discharge capacity are almost equal, but in the case of the present invention, the upper and lower ratio is about 3:2. The increase in the total capacity (the sum of the upper and lower discharge capacities) is thought to be due to the surface activity of the active material, but it is presumed that the change in the upper and lower capacity balance is caused by a change in the crystal structure.

Therefore, since it is impossible to use both stages in actual use, we conducted a charge/discharge test using only the upper part, which is expected to have particularly high energy density.

The charging and discharging test was performed by constant current charging and discharging at 2 mA, with the end-of-charge voltage set at 4.5 V and the end-of-discharge voltage set at 3.5 V. FIG. 4 shows the capacity-cycle characteristics, and compares the characteristics of the conventional active material (curve 10) and the characteristics of the active material of the present invention (curve 11). As is clear from FIG. 4, the active material of the present invention has a capacity that is at least 25% higher than that of the conventional active material, and it seems that the cycle reversibility is also slightly improved. I also measured the bulk density of both, and they were almost the same.
The active material of the present invention is also excellent in volumetric efficiency.

Next, an active material prepared by using chemically synthesized manganese dioxide (CMD) as a raw material and firing it together with LiNO3 under the same conditions as above was also investigated. As a result, a button type battery using the same weight of active material had almost the same discharge characteristics as an EMD, and the capacity remained unchanged. However, as a result of measuring the bulk density, it was found to be nearly 20% bulkier than EMD, and had no superiority in volumetric efficiency over conventional active materials using EMD.

That is, when using positive electrodes of the same shape and size (generally, there are size restrictions in practical batteries), CMD has little advantage. Therefore, in order to achieve high energy density,
The raw material MnO is preferably EMD.

Example 3 As mentioned above, it was found that the spinel-type M n O2 of the present invention synthesized by combining EMD and [,1NO3 is an excellent active material. Next, we investigated the firing temperature during its synthesis. did.

EMD and LiNO3 are Mn: Li=2.0 +
Mixed at a molar ratio of 1.0 and fired at a temperature of 800°C and 850°C.
C., 900.degree. C., 950.degree. C., 1000.degree. C., 1050.degree. C. and 1100.degree. C., respectively.Batteries were constructed under the same conditions as in Example 2, and a charge/discharge test was conducted. The charge/discharge test was conducted at a constant current charge/discharge of 1.0 mA, and the end-of-charge voltage was set to 4.
．． 5 V, and the discharge end voltage was set to 3.5 V.

Figure 5 shows the capacity-cycle characteristics of each active material.
Both have a capacity larger than that of the conventional active material used in Example 2 (curve shown by the broken line in the figure), but as far as cycle reversibility is concerned, the capacity of the active material at 800°C (curve 12) and the active material at 850°C (curve 12)
℃ (curve 13) is inferior to the conventional one. Also, 9
00° C. to 1000° C. (curves 14 to 16) are superior to conventional ones in terms of cycle reversibility and capacity. However, those with a firing temperature of 1050°C or higher (curves 17 to 1)
8), the cycle reversibility is excellent, but the capacity is low. Therefore, as a result of discharging the lower stage by setting the discharge lower limit voltage to 2V, it was found that the lower capacity was also lowered.

That is, this seems to be due to a decrease in the surface activity of the active material.

As mentioned above, since it was found that a firing temperature of around 900°C to 1000°C is good, the firing temperature was further examined in detail around this temperature range. As a result, 880℃
In the range from 900°C to 1000°C, almost the same good properties as those obtained at 900°C to 1000°C were obtained. Therefore, LiN0:+
The firing temperature of the active material of the present invention synthesized using Li as a Li source is preferably 880°C to 1000°C.

Example 4 Next, the molar ratio of Li to Mn (LiMn) in the active material was investigated. First, as a preliminary study, EMD and L
iN0. As a result of investigating the relationship between the mixing ratio of + and the ratio of Li to Mn in the active material by chemical analysis of the active material, it was confirmed that they matched with each other regardless of the firing temperature.

The method for preparing the active material was as shown in Example 1, the firing temperature was fixed at 900°C, and Mn:Li = 2.41.0.2.2.
: 1.0. 2.0 + 1.0. 1.8:1
．． 0. 1.6: The five types of active materials with 1.0 reached the bottom. Next, batteries were constructed under the same conditions as in Example 2 for each, and a charge/discharge test was conducted. The charge/discharge test is as follows: 1.
The charging and discharging were performed at a constant current of 0 mA, with the charging end voltage set at 4.5 V and the discharging end voltage set at 3.5 V. FIG. 6 compares the capacity-cycle characteristics of batteries using each of the above active materials. As is clear from this figure, Mn
The one with +Li=2.4·1.0 (curve 19) has a small capacity. Moreover, cycle reversibility is also poor. Therefore, when the lower discharge limit voltage was set to 2V and the lower stage was discharged, the lower capacity was increased. In other words, due to the lack of Li,
It is expected to have a spinel-type crystal structure with a low degree of perfection. Next, Mn+Li=2.2: 1.0 (curve 20), 2.0: 1.0 (curve 21), 1
．． 8:1.0 (curve 22) had excellent cycle reversibility and large capacity.

Also, as we found by comparing these three, Mn
As the content of Li increases, the capacity increases, and Li
The cycle reversibility improves as the content increases. However, in the case of Mn:Li-1,6: 1.0 (curve 23), the cycle reversibility was almost the same as that of 1.8:1.0, and only the capacity was decreased. Furthermore, when compared from the viewpoint of volumetric efficiency, the bulk density of 1.6:1.0 is significantly lower, and the difference in capacity becomes even larger. Therefore, Mn:Li=2.2:1.
0-1.8:1. Preferably it is O.

Effects of the Invention According to the present invention, a 4V class lithium secondary battery having cycle reversibility and high energy density can be provided.

[Brief explanation of the drawing]

Figures 1 and 3 are discharge characteristic diagrams, Figure 2 is a longitudinal cross-sectional view of a battery used in an example of the present invention, and Figures 4, 5, and 6 are capacity-cycle characteristics. FIG. 1...Positive electrode, 2...Titanium net, 3
... Separator, 4... Sealing plate, 5.
... Lithium negative electrode, 6 ... Electrolyte, 7.
·····gasket.

Claims (3)

[Claims] (1) Manganese dioxide (MnO_2) and lithium nitrate (
LiNO_3) Mn:Li=2.2:1.0~1.8
: A positive electrode active material for a lithium secondary battery, characterized in that it is mixed at a molar ratio of 1.0 and fired in air at a temperature range of 880°C or higher and 1000°C or lower to obtain an oxide consisting of lithium and manganese. manufacturing method. (2) Manganese dioxide is electrolytic manganese dioxide (EMD)
A method for producing a positive electrode active material for a lithium secondary battery according to claim (1). (3) Lithium according to claim (1), characterized in that water is used as a medium when lithium nitrate and manganese dioxide are mixed, and the lithium nitrate is dissolved in water in advance and then fired at the predetermined temperature. A method for producing a positive electrode active material for secondary batteries.