Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-02-10
2002-11-05
Langel, Wayne A. (Department: 1754)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C423S598000
Reexamination Certificate
active
06475673
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a process for producing lithium titanate which is suitable for electrodes for lithium ion batteries used, for example, as backup power supplies for personal computers and for portable types of equipment and the like, and to lithium ion batteries using the same.
2. Background Art
The recent rapid development of technology in the field of electronics has allowed compact design and weight reduction in electronic equipment. Secondary batteries as driving or backup power supplies for these equipment are strongly required to be of compact design and light weight, and to have high energy density. Recently, development of larger electricity storage systems are also urgently required for electric automobiles and off-peak domestic electricity storage systems in order to reduce CO
2
emissions. As new types of secondary batteries which can meet these requirements, secondary lithium batteries which have high energy density to volume have attracted attention.
Lithium titanate, presented by the formula, Li
4
Ti
5
O
12
(termed Li
4/3
Ti
5/3
O
4
bellow), is used as a material for secondary lithium batteries. As processes for production of these compounds, wet methods and dry methods are known (for example, Japanese Patent Application, First Publication, No. 309727/97, and Journal of Low Temperature Physics, Vol. 25, p. 145, 1976). Although wet methods can produce lithium titanate having good crystallinity, these methods require complicated processes, waste water treatment and the like, which poses problems of economic efficiency. To contrast, although the conventional dry methods are simple in process, lithium titanate by-products with formulas other than the above are produced. Furthermore, control of the atomic ratio of titanium and lithium (hereinafter referred to as the Li/Ti ratio) is difficult due to vaporization loss of elemental lithium and lithium compounds, and titanium dioxide as a raw material remains in the products. As a result, the method poses problem in that lithium titanate, Li
4
Ti
5
O
12
, cannot be efficiently produced.
SUMMARY OF THE INVENTION
The present invention was made to solve the above-mentioned problems. Objects of the invention follow. The lithium titanate in the invention refers to compounds represented by the formula Li
4
Ti
5
O
12
, which refers to the “desired compound” in one case.
(1) Providing a production process for efficiently producing lithium titanate using a dry method.
(2) Providing a process for producing lithium titanate, in which vaporization loss of lithium compounds in the sintering reaction can be suppressed, so that the Li/Ti ratio can be freely controlled in the range of 0.78 to 0.82(i.e., around 0.80), and preferably in the range of 0.79 to 0.80.
(3) Providing a process for producing lithium titanate, in which persistence of titanium dioxide as a raw material can be suppressed.
According to research by the inventors, it was discovered that vaporization loss of lithium compounds can be extremely decreased, the Li/Ti ratio can be easily controlled, titanium dioxide as a raw material did not remain, and therefore lithium titanate compounds can be efficiently produced by employing a process in which specific sintering conditions are combined.
The invention was completed based on the above research. The invention provides a process for producing lithium titanate by presintering a mixture of titanium dioxide and at least one lithium compound selected from the group consisting of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium oxide, yielding at least one of a composition comprising TiO
2
and Li
2
TiO
3
and a composition comprising TiO
2
, Li
2
TiO
3
and Li
4
Ti
5
O
12
, and sintering the composition.
The method according to the invention will be explained in detail hereinafter.
DETAILED DESCRIPTION OF THE INVENTION
The lithium titanate produced by the invention is represented by the general formula Li
X
Ti
Y
O
12
, in which the Li/Ti ratio is in the range of 0.78 to 0.82, X is in the range of 3 to 5, and Y is in the range of 4 to 6. In particular, the invention is directed to mono-phasic lithium titanate with a spinel crystal structure presented by Li
4
Ti
5
O
12
, or a mixture or mixed crystal structure of Li
4
Ti
5
O
12
, Li
2
TiO
3
, and TiO
2
.
1. Raw Materials
The lithium compounds as raw materials for in the invention are one or more lithium compounds selected from the group consisting of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium oxide. Among these compounds, lithium carbonate and lithium hydroxide are preferably used. These lithium compounds used for as materials preferably have high purity, normally of 99.0 weight % or more. For example, when lithium hydroxide is used as a raw material, Li
2
CO
3
is preferably included at 99.0 weight % or more, and is more preferably 99.5 weigh % or more; the content of Na, Ca, and Mg, etc., as impurities, is preferably 100 ppm or less, and is more preferably 10 ppm or less; and the content of Cl and SO
4
is preferably 100 ppm or less, and is more preferably 50 ppm or less. The water component should be sufficiently removed, and the content thereof is preferably 0.1 weight % or less. Moreover, the average particle size is preferably in the range of 0.01 to 100 &mgr;m, and in the case of lithium hydroxide, the particle size is preferably in the range of 1 to 50 &mgr;m, and is more preferably in the range of 5 to 20 &mgr;m.
With respect to titanium dioxide (TiO
2
), it should also have high purity. In particular, the purity is preferably 99.0 weight % or more, and is more preferably 99.5 weight % or more; the content of Fe, Al, Si, and Na included in fine particles of titanium dioxide as impurities is preferably less than 20 ppm respectively; and the content of Cl is preferably less than 200 ppm. More preferably, the content of Fe, Al, Si, and Na included in fine particles of titanium dioxide are less than 10 ppm, respectively; and the content of Cl is less than 100 ppm, and is more preferably less than 50 ppm. The average particle size is preferably in the range of 0.05 to 30 &mgr;m, and is more preferably in the range of 0.1 to 10 &mgr;m.
2. Preparation of Materials for Presintering
Lithium titanate and titanium dioxide as materials for presintering are sufficiently mixed, and are provided for presintering. The mixing ratio of the materials may be chosen to coincide with the desired value of the Li/Ti ratio of the lithium titanate in the desired final compound product. The term “Li/Ti ratio” refers to the atomic ratio unless otherwise specified. For example, when the Li/Ti ratio of the desired product is 0.78 or 0.80, the materials are mixed in a proportion of 0.78 or 0.80. In this case, a crushing mixer such as a vibrating mill or a ball mill, a mixer with an agitator, a rotating mixer, or the like is used. The moisture absorption during mixing should be monitored. The mixing is preferably performed in dry air with an absolute humidity of 5 g/m
3
or less in air; alternatively, it may be performed in an inert gas. The mixture of the raw materials is provided to a presintering process in bulk or as a green compact which is formed by compression at a pressure of 0.5 ton/cm
2
.
3. Presintering Process
In the invention, first, the mixture of raw materials is presintered under the conditions indicated below. That is, the mixture of the raw materials is heated and sintered in an oxidizing atmosphere at a temperature in the range of 600 to 800° C., preferably in the range of 670 to 800° C., and more preferably in the range of 700 to 780° C. The duration of the heating and presintering is adequate at 30 minutes to 4 hours. The rate at which the temperature is increased is adequate in the range of 0.5 to 10° C./min. By this presintering process, a presintered product (hereinafter referred to as the “intermediate product”) is obtained.
The chemical composition of the intermediate product is two components substantially consisting of TiO
2
and Li
2
TiO
3
, or three components substan
Eto Kiyoshi
Sakai Hideki
Yamawaki Tetsuya
Langel Wayne A.
Oliff & Berridg,e PLC
Strickland Jonas
Toho Titanium Co., Ltd.
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