Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
1999-02-02
2001-11-27
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S317000, C429S223000, C429S224000, C429S231100
Reexamination Certificate
active
06322929
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a lithium secondary battery.
Recent years have witnessed the demand for batteries growing tremendously with development and spread of portable appliances such as personal computers and cellar phones. Especially in the lithium battery, researches have been actively conducted in various quarters as a power source which has a high energy density, because lithium is an element small in molcular weight yet capable of releasing a large amount of ionized energy.
Hitherto, the positive electrode active materials used for such a lithium battery were MnO
2
, V
2
O
5
and the like which could generate an electromotive force of the order of three volts. In recent years, a lithium battery of the order of four volts based on LiCoO
2
has been commercialized.
In seeking to further raise the battery power, researches have been carried out in active materials for a positive electrode which generate a higher electromotive force. The following substances have now been reported as ones with an electromotive force of 4.8 volts versus metallic lithium: oxides of lithium manganese with a spinel structure such as LiNiVO
4
(G. T. Fey, W. Li, and J. R. Dahn, J. Electrochem. Soc., vol. 141, 2279 (1994) of Reference 1); LiCr
y
Mn
2−y
O
4
(C. Sigala, D, Guymard, A. Verbaere, Y. Piffard, and M. Tournoux, Solid State Ionics, vol. 81, 167 (1995) of Reference 2); LiNi
x
Mn
2−x
O
4
(Q. Zhong, A. Banakdarpor, M. Zhang, Y. Gao, and J. R. Dahn, J. Electrochem. Soc., vol. 144, 205 (1997) of Reference 3); and LiMn
2−x−y
Ni
x
Cr
y
O
4
(Y. Todorov, C. Wang, B. I. Banov, and M. Yoshio, Electrochemical Society Proceedings, vol. 97 of 18, 176 (1997) of Reference 4); and phosphates with an olivine structure such as LiCoPO
4
(Japanese Laid-Open Patent Publication Hei 9-134724) of Reference 5).
In lithium batteries made with those positive electrode active materials of References 1 to 5, however, the electrolyte is exposed to a strong oxidation environment in the area where it comes in contact with the active materials, because those positive electrode active materials generate a high electromotive force. As a result, the lithium secondary batteries of that kind obtained present a number of problems.
In Reference 1, it is reported that since the oxidation reaction of the electrolyte takes place as secondary reaction, the discharge capacity decreases. References 2 and 3 indicate that because of a similar oxidation reaction of the electrolyte, the charge and discharge efficiency is low. Reference 4 points out that because of unstable electrolyte, the self-discharge of the battery is large.
Those observations are said of the compounds of the spinel structure. Lithium batteries made with compounds of the olivine structure, too, are small in discharged quantity of electricity as compared with charged quantity of electricity, that is, low in charge-discharge efficiency as shown in FIG. 5 of Reference 5.
As described, lithium secondary batteries made with those high voltage generating active materials for the positive electrode tended to self-discharge with the decomposition of the electrolyte and were low in charge-discharge efficiency. Furthermore, while not mentioned in the references, the decomposition of the electrolyte can deteriorate the long-term reliability resulting in shortened charge-discharge cycle life and other problems.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to provide a lithium secondary battery with a high charge-discharge efficiency and a low self-discharging tendency by keeping down the decomposition of the electrolyte—the problem encountered with the lithium secondary batteries constructed using the positive electrode active materials with a high electromotive force as described above.
The present invention provides a lithium secondary battery comprising a positive electrode which contains a transition metal compound exhibiting a potential of not lower than 4.5 volts versus Li, a negative electrode and a sulfide based lithium ion conductive solid electrolyte. According to the present invention, it is possible to keep charge-discharge efficiency of the battery from dropping and control the self-discharge tendency caused by decomposition of the electrolyte in spite of the positive electrode active material generating a high voltage being used in the battery.
The sulfide based lithium ion conductive solid electroly used in the present invention comprises a glass network former sulfide and a glass network modifier sulfide.
In a preferred mode of the present invention, the glass network former sulfide is selected from the group consisting of SiS
2
, B
2
S
3
and P
2
S
5
, and the glass network modifier sulfide is LiS
2
.
In another preferred mode of the present invention, the sulfide based lithium ion conductive solid electrolyte contains bridging oxygen and silicons bonded to the bridging oxygen.
The sulfide based solid electrolyte containing the above-mentioned bridging oxygen can be produced by melting and quenching a mixture of materials including a glass network former sulfide, a glass network modifier sulfide and an oxygen source such as an oxide and oxysalt of lithium. In the resultant solid electrolyte, the bridging oxygen is one generated from the above-mentioned oxygen source.
REFERENCES:
patent: 5631104 (1997-05-01), Zhong et al.
patent: 0656667 (1995-06-01), None
patent: 0802575 (1997-10-01), None
patent: 0829913 (1998-03-01), None
patent: 08096836 (1996-04-01), None
patent: 008138725 (1996-05-01), None
patent: 08162151 (1996-06-01), None
patent: 08217452 (1996-08-01), None
patent: 09134724 (1997-05-01), None
patent: 09293516 (1997-11-01), None
patent: 98/04010 (1998-01-01), None
J. Electrochem, Soc., vol. 144, No. 1, Jan. 1997, pp. 205-213.
Journal of Materials Chemistry, 1998, 8(4), pp. 837-839.
J. Electrochem. Soc., vol. 141, No. 9, Sep. 1994, pp. 2279-2282.
Solid State Ionics 81 (1995), pp. 167-170.
Database WPI Section Ch, Wook 9918, Derwent Publications, Ltd., London, GB; AN 99-207800 & JP 11025983.
A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough: “Phospho-olivines as Positive-Electrode Materials for Rechargeable Lithium Batteries” Journal of Electromechanical Society, vol. 144, No. 4, Apr. 1997, pp. 1188-1194.
Kawai H, et al.: “A New Lithium Cathode Licomn04: Toward Practical 5 V Lithium Batteries” Electromechanical and Solid-State Letters, vol. 1, No. 5, Nov. 1, 1998, pp. 212-214.
Fujino Makoto
Iwamoto Kazuya
Kondo Shigeo
Takada Kazunori
Matsushita Electric - Industrial Co., Ltd.
McDermott & Will & Emery
Weiner Laura
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