JPH07112929B2 - Synthesis method of lithium manganese oxide solid solution - Google Patents

Description

The present invention relates to a method for synthesizing a lithium manganese oxide solid solution.

[Conventional technology]

Recently, LiMnO _2, such as LiMnO ₂ (lithium manganese oxide)
(Lithium transition metal oxide) is receiving attention as a positive electrode active material for lithium batteries, which can be expected to have high energy density.

However, in the case of LiMnO ₂ , it had to be synthesized while limiting the amount of oxygen supplied to the amount required for the reaction. In the case of LiMnO ₂ , unlike LiCoO _2, etc., when heated in an open oxidizing atmosphere, Mn (manganese) is easily oxidized to Mn (IV), for example LiMO ₂ such as LiMn ₂ O _4. Since the crystal structure is different from that of
This is because it is necessary to prevent the generation of Mn (IV).

Under such circumstances, until now, Li ₂ O ₂ and MnO were sealed in a vacuum tube, and Mn was formed only by O ₂ generated by thermal decomposition of Li ₂ O _2.
(II) O was oxidized to Mn (III) to synthesize LiMnO ₂ (J. Am. Chem, Soc., 78 , 3255 (1956)).

However, this method has a problem that it takes a lot of time to synthesize it because it is a vacuum sealed tube, and it is dangerous because it uses a peroxide of Li ₂ O ₂ .

[Problems to be solved by the invention]

The present invention has hitherto been limited by the fact that the amount of oxygen supplied by a vacuum sealed tube during the synthesis of LiMnO ₂ must be limited, and in view of the fact that handling also involved danger, for example, a lithium battery with a positive electrode active The purpose of the present invention is to synthesize a useful lithium manganese oxide solid solution rather than LiMnO ₂ when used as a substance, without causing the above-mentioned restrictions and risks.

[Means for solving problems]

The present invention is a mixture of a manganese oxide or a salt which becomes an oxide upon heating, an oxide of cobalt or a salt which becomes an oxide upon heating, and an oxide of lithium or a salt which becomes an oxide upon heating to obtain oxygen. The formula (I) Li (Mn _1-y Co _y ) O ₂ (I) (I) (I) (I) (I) (I) (I), which is useful as a positive electrode active material for lithium batteries rather than LiMnO ₂ by heating in an open atmosphere without limiting the supply In the formula, y is 0.1 or more and less than 1), and a lithium manganese oxide solid solution represented by the formula can be obtained without limiting the amount of oxygen supplied.

In the present invention, manganese, cobalt, and lithium are each subjected to a reaction in the state of an oxide or a salt which becomes an oxide by heating. Examples of the salt which becomes an oxide by heating include carbonate and acetate. And so on. In particular, when the reaction is carried out in the carbonate state, it is decarboxylated by heating and atomized at that time, so that the mixed state of each reaction component becomes more uniform, and a preferable result is obtained. As the oxide, in the case of Mn, MnO, Mn ₂ O ₃ , Mn ₃ O _4, etc. are used, in the case of cobalt, CoO, Co ₂ O ₃ , Co ₃ O _4, etc. are used, and in the case of Li Is Li ₂ O or the like.

The reaction is carried out in an open atmosphere in which the oxygen supply amount is not restricted, but the open atmosphere in which the oxygen supply amount is not restricted may be an atmosphere in which oxygen is leaner than in the atmosphere, rather, it is a solid solution of cobalt. If the amount is small or the reaction temperature is low, the oxygen-diluted atmosphere is characterized by being able to obtain the desired solid solution more stably than in the atmosphere.

In the present invention, Mn, cobalt, and lithium,
Each of them is subjected to the reaction in the state of an oxide or a salt that becomes an oxide by heating, but by reacting in a three-component system containing cobalt in this way, Co becomes a solid solution with Mn, It is considered that even if the material is heated in an oxygen atmosphere whose supply amount is not limited, the valence of Mn, which is easily oxidized, does not increase under normal conditions, and the oxide of Mn (III) is synthesized.

The reason why Mn is less likely to be oxidized as described above is not always clear at present, but when Co is dissolved in Mn, Mn and Co are in a trivalent state, Li (Mn _1-y Co _y ) It is considered that the crystal structure of O ₂ is stabilized and Mn also stays in the trivalent state.

In the present invention, in the formula (I), that is, Li (Mn _1-y Co _y ) O ₂
The value of y is 0.1 or more and less than 1 (that is, 0.1 ≦ y <1)
However, when y is smaller than 0.1, the solid solution of Mn and cobalt does not sufficiently occur, and therefore, in an open oxygen atmosphere where the amount of oxygen supplied is not limited, not only LiMnO ₂ but also lithium manganese oxide is oxidized. This is because a solid solution cannot be obtained. Then, since LiCoO ₂ can be synthesized in an open oxygen atmosphere, if y is less than 1, it is considered that the upper limit value does not exist.

In the present invention, as described above, Mn, cobalt,
Lithium and carbonate are preferably used in the reaction in the form of a carbonate. At this time, Mn and cobalt are preferably mixed by coprecipitating them as a carbonate in advance. This is because the Mn carbonate and the cobalt carbonate are uniformly mixed by coprecipitation.

When coprecipitated as described above, the obtained carbonate is likely to be a basic carbonate unless under particularly strictly controlled conditions. Therefore, the carbonate in the present invention is a concept including a basic carbonate.

As described above, for coprecipitating Mn and cobalt as carbonate in an aqueous solution, for example, a water-soluble salt of Mn (for example, chloride of Mn) and a water-soluble salt of cobalt (for example, chloride of cobalt) are used. It is carried out by dissolving carbon dioxide gas in saturated pure water, adding soluble carbonate such as NaHCO ₃ or Na ₂ CO ₃ to this solution, and reacting them.

As the heating temperature, it is preferable to adopt 800 to 1200 ° C, but when it is less than 800 ° C, solid solution of Mn and cobalt hardly occurs, while when it exceeds 1200 ° C, loss due to volatilization of Li increases. This is because it becomes difficult to obtain a quantitative reaction product. When the amount of cobalt such as Co added to Mn increases, the reaction can be reliably performed even on the low temperature side within the above range.

Cooling after heating is not particularly limited, but rapid cooling is preferable because there is no risk of unplanned phase separation (separated solid solution is separated) during cooling.

In the present invention, the obtained lithium-manganese oxide solid solution is represented by Li (Mn _1-y Co _y ) O _2. However, the (Mn _1-y Co _y ) portion is slightly excessive and the amount is equal to or more than Li. (Usually in the range of 5% or less), or when oxygen deficiency occurs
O ₂ portions _{O 2-[delta],} i.e. _{Li (Mn 1-y Co y} ) O
_2-δ (δ ≦ 0.3) may occur, but the lithium-manganese oxide solid solution represented by the formula (I) in the present invention includes those cases.

When the lithium manganese oxide solid solution represented by the formula (I) synthesized by the present invention is used as a positive electrode active material of a lithium battery, the battery voltage is 3.9 to 4.6 V (Li varies in the range of 0 to 1). Battery voltage also fluctuates accordingly) and batteries using sulfide-based active materials such as titanium disulfide and molybdenum disulfide (usually battery voltage is 3 V or less)
Higher and higher energy density can be expected. Also, LiMn
For example, Li (Mn _1-y Co _y ) O ₂ is more useful than O ₂ as a positive electrode active material because it has less polarization during charging / discharging and a smaller change in battery voltage during charging / discharging. The decrease in polarization is due to the solid solution of Co, which causes Li ⁺
It is considered that this is because the diffusion of ions becomes faster.

〔Example〕

 Next, the present invention will be described in more detail with reference to examples.

Example 1 A Li (MnCo) O ₂ solid solution was synthesized using Li, Mn, and Co. Co
The molar ratio of Mn to Mn is 10:90 (that is, Co / Mn = 10/90 (molar ratio)).

The solid solution obtained by the synthesis is represented by the formula (I) and is Li (Mn _0.9 Co _0.1 ) O ₂ .

The synthetic method is as follows.

First, at the ratio of Mn and Co such that Co / Mn = 10/90 (molar ratio)
MnCl · 4H ₂ O and CoCl ₂ · 6H ₂ O were dissolved in pure water saturated with carbon dioxide gas, a NaHCO ₃ aqueous solution with a concentration of 4 wt% was added, and the mixture was allowed to stand for 2 hours to coprecipitate Mn and Co as carbonates. To form a uniform mixture, and the resulting precipitate is filtered and washed with water, then in argon.
It was dried at 140 ° C for 5 days.

Next, the coprecipitate of Mn and Co and Li ₂ CO ₃ are mixed at a molar ratio of 1: 1.
Mix in the ratio of 3 and in air (N ₂ / O ₂ = 80/20) at 1200 ℃
After heating for a period of time, synthesis was performed by air quenching (a method in which a heated sample was taken out into the atmosphere at 25 ° C. and rapidly cooled). The port used for accommodating the sample during heating is mainly composed of Al ₂ O ₃ . Also, the synthesis was carried out under the same conditions as above except that the synthesis temperature was changed to 1000 ° C.

Furthermore, in the above, the synthesis was performed using Li ₂ CO ₃ in an equivalent amount,
To compensate for the loss due to volatilization of lithium during heating,
A synthesis was also performed in which an equivalent amount of Li ₂ CO ₃ was charged.

Example 2 A Li (MnCo) O ₂ solid solution was synthesized in the same manner as in Example 1 except that the Co / Mn ratio was 30/70 (molar ratio). Displaying the synthesized solid solution according to the formula (I), Li (Mn _0.7 Co
_0.3 ) O ₂ .

Example 3 A Li (MnCo) O ₂ solid solution was synthesized in the same manner as in Example 1 except that the Co / Mn ratio was changed to 50/50 (molar ratio). Displaying the synthesized solid solution according to the formula (I), Li (Mn _0.5 Co
_0.5 ) O ₂ .

Example 4 A Li (MnCo) O ₂ solid solution was synthesized in the same manner as in Example 1 except that the Co / Mn ratio was 70/30 (molar ratio). Displaying the synthesized solid solution according to the formula (I), Li (Mn _0.3 Co
_0.7 ) O ₂ .

Example 5 A Li (MnCo) O ₂ solid solution was synthesized in the same manner as in Example 1 except that the Co / Mn ratio was 80/20 (molar ratio). Displaying the synthesized solid solution according to the formula (I), Li (Mn _0.2 Co
_0.8 ) O ₂ .

Comparative Example 1 Example 1 except that Co / Mn ratio was 0/100 without solid solution of Co
Synthesis was performed in the same manner as in.

Comparative Example 2 Synthesis was performed in the same manner as in Example 1 except that Co was 100 mol% and the Co / Mn ratio was 100/0.

Examples 1-5 and Comparative Examples 1-2 synthesized as described above
An X-ray diffraction pattern was measured for the substance (1) to identify the synthesized substance. The measurement was carried out using an X-ray source of 50 kV, 150 mA, Cu target, 2θ = 7-100 at a scan rate of 4 ° / min.
This was done by measuring the range of °. The results are shown in Table 1.

As shown in Table 1, Comparative Example 1 (Co
/ Mn = 0/100), the substance LiMnO ₂ was not synthesized,
_It became ₂ O ₄ , and Mn was oxidized to a high valence.

On the other hand, Example 1 in which a small amount of Co was added (Co / Mn = 10/9
In (0), it became a Li (MnCo) O ₂ solid solution, and the valence of Mn was trivalent and did not increase.

Judging from the above results, solid solution occurs and Li (Mn
The lower limit of Co for _1-y Co _y ) O ₂ is Co / Mn = 0/100 to
Between 10/90 and at least Co / Mn 10/90 (molar ratio)
If so, it is considered that the desired solid solution can be obtained even in an oxygen atmosphere.

In Examples 2 to 5 in which the amount of Co added was increased, Li was
(Mn _1-y Co _y ) O ₂ was obtained, and also in Comparative Example 2 in which Co was 100 mol%, LiCoO ₂ was obtained. Therefore, there is an upper limit of the amount of Co that can maintain the trivalence of Mn. It is thought not to.

The results shown in Table 1 above were obtained when the charged amount of Li ₂ CO ₃ was increased to compensate for the loss due to volatilization of Li during heating. When the equivalent amount of Li ₂ CO ₃ is charged, the Mn is 50% from the Co / Mn ratio of 50/50.
In the large amount of Examples 1, 2, and 3, a small amount of LiMn ₂ O ₄ was generated. This is probably because Li volatilizes during heating. Therefore, in order to stably obtain a Li (Mn _1-y Co) O ₂ -based solid solution, it can be said that it is preferable to use Li ₂ CO ₃ in excess of the equivalent amount. In addition, it is considered that even if the amount of Li ₂ CO ₃ increases, no particular adverse effect occurs on the reaction.

Further, although the lower limit of the synthesis temperature is not clear, in Examples 3 to 5 in which Co is equal to or more than Mn, it is possible to stably synthesize the Li (MnCo) O ₂ solid solution even if the synthesis temperature is lowered to 800 ° C. I was able to.

Next, the lattice constants c ₀ and a ₀ of the Li (MnCo) O ₂ solid solution belonging to the hexagonal system were calculated from the peak values of the (006) and (110) planes, respectively. The results are shown in FIG. The sample used for this was synthesized at a synthesis temperature of 1200 ° C.

As shown in FIG. 1, it can be seen that c ₀ and a ₀ decrease linearly as the amount of Co increases, and that the solid solution uniformly occurs. This is because the ionic radius of Co ³⁺ is 0.05 compared to that of Mn ^3+.
Since it is as small as A, it can be explained quantitatively.

〔The invention's effect〕

As described above, in the present invention, by solid solution of Co in Mn, the reaction in an open oxygen atmosphere does not limit the amount of oxygen supplied, Li (Mn _1-y Co _y ) O ₂ The lithium manganese oxide solid solution shown can now be easily synthesized. As a result, problems such as the fact that the amount of oxygen supplied by a vacuum sealed tube had to be limited when synthesizing LiMnO ₂ in the past, or it was accompanied by danger, were solved, and LiMnO ₂ was used as a positive electrode active material for lithium batteries rather than LiMnO _2. Rather, useful LiMnO ₂ analogues can be easily synthesized.

[Brief description of drawings]

FIG. 1 is a diagram showing the lattice constants a ₀ and c _{0 of} the Li (Mn _1-y Co _y ) O ₂ type solid solution synthesized according to the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshikatsu Manabe 1-88, 1-Tora, Ibaraki-shi, Osaka Prefecture Hitachi Maxell Co., Ltd. No. (72) Inventor Nanao Kawai 5-47-20 Senriyama Nishi, Suita City, Osaka Prefecture

Claims (2)

[Claims] 1. A mixture of an oxide of manganese or a salt which becomes an oxide by heating, an oxide of cobalt or a salt which becomes an oxide by heating, and an oxide of lithium or a salt which becomes an oxide by heating, It is represented by the formula (I) Li (Mn _1-y CO _y ) O ₂ (I) (wherein y is 0.1 or more and less than ₁ ) by heating in an open atmosphere without limiting the oxygen supply amount. A method for synthesizing a lithium manganese oxide solid solution, which comprises synthesizing a lithium manganese oxide solid solution. 2. The method for synthesizing a lithium-manganese oxide solid solution according to claim 1, wherein the salt which becomes an oxide by heating is a carbonate.