Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
1999-04-13
2001-03-27
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S221000, C429S223000, C429S224000, C429S231100, C429S231200, C429S231300, C429S231500, C429S231950
Reexamination Certificate
active
06207327
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a totally-solid lithium secondary battery which uses a lithium ion conductive solid electrolyte as an electrolyte, and a transition metal chalcogenide or a lithium•transition metal chalcogenide as an active material for negative electrode.
2. Description of Related Art
With recent development of portable equipment such as personal computers, portable telephones, etc., demand for batteries as electric sources thereof much increases. Particularly, lithium batteries are intensively investigated in various fields as batteries capable of providing high energy density because lithium has a small atomic weight and has a large ionization energy.
As active materials for electrodes used in lithium batteries, for example, iron disulfide performs a four-electron reaction as shown in the following formula 1, and shows a high theoretical capacity density of 894 mAh/g.
FeS
2
&rlarr2;Li
3
Fe
2
S
4
&rlarr2;Li
2+x
Fe
1-x
S
2
+Fe
1-x
S&rlarr2;Li
2
FeS
2
&rlarr2;Li
2
S+F 1
Moreover, when the transition metal element is cobalt or nickel, the four-electron reactions shown in the following formulas 2 and 3 also take place, and a high theoretical capacity density is obtained.
CoS
2
&rlarr2;Co
3
S
4
&rlarr2;Co
x
S
9
&rlarr2;Co 2
NiS
2
&rlarr2;NiS&rlarr2;Ni
7
S
6
&rlarr2;Ni
3
S
2
&rlarr2;Ni 3
However, reversibility of these reactions is low, and is insufficient for practical secondary batteries.
BRIEF SUMMARY OF THE INVENTION
Concerning these problems, the inventors have found that when a lithium ion conductive solid electrolyte is used as an electrolyte for the electrode reaction of the transition metal chalcogenides, the reaction of the transition metal element being reduced to metal does not take place, and the electrode reaction takes place reversibly. The inventors have further found that precipitation of metals at the interface of the solid electrolyte can be inhibited by adding to negative electrode at least two compounds, namely, (a) at least one first transition metal chalcogenide or lithium•transition metal chalcogenide and (b) at least one second transition metal chalcogenide or lithium transition metal chalcogenide.
The object of the present invention will be explained below. For simplification of the explanation, Li
2
FeS
2
is taken as an example of the lithium•transition metal chalcogenide which is an active material for negative electrode.
The negative electrode reaction at the time of charging of a totally-solid lithium secondary battery which uses Li
2
FeS
2
as an active material for negative electrode is shown by the following formula 4.
Li
2
FeS
2
+xLi
+
xe
−
&rlarr2;Li
2+x
FeS
2
4
On the other hand, when the battery is operated at a current greater as compared with the reaction rate of the formula 4, the reaction of the formula 5 also takes place competitively.
Li
+
+xe
−
&rlarr2;Li 5
When this reaction predominantly takes place, metallic lithium is precipitated at the interface between Li
2
FeS
2
and the solid electrolyte. If metal is precipitated at the interface, bonding between Li
2
FeS
2
particles and the solid electrolyte is weakened. As a result, due to the decrease of the area of the reaction interface, the reaction of the formula 4 becomes further difficult to take place, and the reaction for precipitation of metallic lithium shown by the formula 5 takes place more predominantly. Finally, separation between Li
2
FeS
2
and the solid electrolyte occurs, and there is the possibility of no normal charge and discharge characteristics being exhibited.
The present invention solves the above problems, and the object of the present invention is to provide a rechargeable lithium secondary battery which uses a transition metal sulfide as an active material for negative electrode. Other objects of the present invention will be readily apparent from the following description of the invention.
REFERENCES:
patent: 4302518 (1981-11-01), Goodenough et al.
patent: 4983476 (1991-01-01), Slane et al.
patent: 5154990 (1992-10-01), Plichta et al.
patent: 5260147 (1993-11-01), Delmas et al.
patent: 5284721 (1994-02-01), Beard
Takada K. et al: “Lithium iron sulfide as an electrode material in a solid state lithium battery” Solid State Ionics, vol. 117, No. 3-4, Feb. 2, 1999, pp. 273-276 XP004154434.
European Search Report dated Jul. 29, 1999.
Brec, et al., “Chemical and Electrochemical Study of the LixFeS2Cathodic System (0<×≦2),” Mat. Res. Bull., Mar. 10, 1980, vol. 15, pp. 619-625.
Murphy, et al., “Low Voltage Behavior of Lithium/Metal Dichalcogenide Topochemical Cells,” Journal of the Electrochemical Society: Electrochemical Science and Technology, Mar. 1979, vol. 126, pp. 349-351.
Preto, et al., “Reactions of FeS2, CoS2, and NiS2Electrodes in Molten LiCl-KCL Electrolytes,” Journal of the Electrochemical Society: Electrochemical Science and Technology, Feb. 1983, vol. 130, pp. 264-273.
Iwamoto Kazuya
Kondo Shigeo
Takada Kazunori
Matsushita Electric - Industrial Co., Ltd.
Stevens Davis Miller & Mosher LLP
Weiner Laura
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