Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
1999-06-14
2002-02-12
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S331000, C429S326000
Reexamination Certificate
active
06346351
ABSTRACT:
This invention relates to an electrolyte system, a method for the preparation thereof, the use thereof, and a battery containing the electrolyte system, and particularly to an electrolyte composition and a rechargeable battery of high compatiblity towards positive electrode structures, high cyclability and low irreversible loss.
The use of non-aqueous electrolytes has allowed the development of electrochemical systems based on lithium-containing negative electrode structures and positive intercalation metal oxides, which have high energy density. For those systems, however, the limiting factor for their continuous performance has been their low electrolyte electrochemical stability, leading to poor cell cyclability.
Said cyclability is defined as the number of times a battery can be charged and discharged between 4.2V and 3.0V at a current density of 1 mA/cm
2
before the capacity is reduced to 80% of the discharge capacity of the first discharge of the battery.
Upon operation of such electrochemical cells, a capacity loss during initial charging of the cells is observed, as is a fading capacity upon extended cycling. Those capacity reduction phenomena can be ascribed to the instability of the electrolyte towards the negative and the positive electrode of the electrochemical cell. The instability towards the negative electrode leads to gassing and formation of a passivating film. The instability towards the positive electrode leads to corrosion of the electrode structure. Both phenomena result in loss of active material from the cell. Further, passivation and corrosion may lead to increased cell impedance, and reduced materials utilisation and rate capability.
The gassing and the formation of the passivating film is believed to take place during the initial charging, subsidiary during the young life of the electrochemical cell. Therefore, the capacity loss during the initial charging is mainly ascribed to the instability of the electrolyte-negative electrode structure-system. The corrosion is an ongoing process, which is believed mainly to take place towards the end of each charging cycle at maximum potential of the positive electrode structure. Therefore, the capacity loss during cycling is mainly ascribed to the instability of the electrolyte-positive electrode structure-system, i.e. lack of compatibility of the electrolyte towards the positive electrode structure.
The capacity loss during the first charging of the battery is referred to as the initial irreversible loss, and is defined as the loss of active lithium in mAh of charge vs. the weight of the negative electrode structure in grains. In this context the loss is defined as active material which reacts irreversibly with the negative electrode structure, and which cannot be deintercalated from the structure during the subsequent discharge of the battery.
The known electrolyte or electrolyte systems are mainly composed of one or more salts in a solvent or solvent mixture. Traditionally, a number of salts are commonly applied in non-aqueous electrolyte systems, including lithium perchlorate, lithium hexafluorarsenate, lithium trifluoromethylsulfonate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium bromide, lithium hexafluoroantimonate, LiC(CF
3
SO
2
)
3
and LiN(CF
3
SO
2
)
2
. The solvents used are e.g. ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, &ggr;-butyrolactone, &ggr;-valerolactone, tetrahydrofuran, 2-methyl tetrahydrofuran, dimethoxyethane, diethoxyethane, methyl acetate, methyl formate, 1.3-dioxolane, sulfolane, acetonitrile and butyronitrile.
A large number of electrolyte systems have been composed from these salts and solvents in order to improve inter alia the electrochemical stability of the systems.
It has now been found that the cyclability and stability of an electrochemical system such as a battery may be improved considerably, by using a electrolyte system having the composition of claim
1
.
By using salt mixtures of lithium tetrafluoroborate and lithium hexafluorophosphate, excessive in the borate, in batteries it has been observed, that high cyclability (low capacity loss during extended discharge-recharge) and a low initial irreversible loss can be obtained.
This excellent performance has been found for those salts compositions containing 60-90% by mole of lithium tetrafluoroborate and 10-40% by mole of lithium hexafluorophosphate. These salt compositions are believed to be dominated by a high compatibility of the borate against the positive electrode structure, supported by negative structure compatibility and contribution to the conductivity from the phosphate.
It has further been found that the performance is especially pronounced for the high stability solvent mixture of ethylene carbonate and dimethyl carbonate and particularly if these solvents are the only liquid solvents present. Further, the performance of the electrolyte system is especially pronounced, when combined with negative electrode structures of carbonaceous intercalation materials like coke and graphite and/or positive electrode structures of lithium manganese oxide spinels.
A number of patents describe the use of the lithium tetrafluoroborate/lithium hexafluorophosphate salts in a solvent mixture consisting of or comprising ethylene carbonate and dimethyl carbonate.
U.S. Pat. No. 5,192,629 and U.S. Pat. No. 5,422,203 of Bell Communication Research describe the use of an electrolyte system with a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as solvent and lithium hexafluorophosphate and mixtures of lithium hexafluorophosphate with up to about equal mole parts of lithium tetrafluoroborate as solute. The teaching in these patents is that this group of electrolytes provides an improved resistance against oxidation. The only example showing the effect of the electrolyte, is composed from 1M LiPF
6
in 95 DMC:5 EC. Though it is mentioned that the electrolyte may comprise lithium tetrafluoroborate, nothing is mentioned about the effect of lithium tetrafluoroborate.
U.S. Pat. No. 5,079,109 of Toshiba Battery Co., Ltd. describes an electrolytic salt consisting of one of lithium phosphate hexafluoride (LiPF
6
) and lithium borofluoride (LiBF
4
) in a solvent mixture consisting of ethylene carbonate, 2-methyltetrahydrofuran and at least one ester-based nonaqueous solvent selected from butylene carbonate, dimethyl carbonate, &ggr;-butyrolactone and sulfolane. In none of the numerous examples of this patent, however, the mixture of said electrolytic salts and the positive effect thereof are described.
U.S. Pat. No. 5,296,318 and U.S. Pat. No. 5,429,891 of Bell Communication Research, Inc. describe a solid electrolyte for a rechargeable lithium intercalation battery cell of a self-supporting film of a copolymer of vinylidene fluoride and hexafluoropropylene, the electrolyte thereof being selected from LiPF
6
, LiAsF
6
, LiCIO
4
, LiN(CF
3
SO
2
)
2
, LiBF
4
, LiCF
3
SO
3
and LiSbF
6
and the solvent thereof being selected from ethylene carbonate, propylene carbonate, dimethyl carbonate, diethoxyethane, diethyl carbonate, dimethoxyethane, dipropyl carbonate and mixtures thereof. The patent, however, describes no mixtures of said electrolytic salts and their performance.
G. Pistoia, A. Antonini, R. Rosati and D. Zane (Electrochemica Acta 41 2683-9 (1996)) describe cathodes for Li-ion batteries based on electrolyte compositions of either LiPF
6
or LiBF
4
in a mixture of ethylene carbonate and dimethyl carbonate. The authors do not describe, however, the salt mixtures of the present invention.
U.S. Pat. No. 5,370,949 of National Research Council of Canada describes a non-aqueous electrolyte of which the salt is selected from LiAsF
6
, LiPF
6
, LiBF
4
, LiCIO
4
, LiBr, LiAICI
4
, LiCF
3
SO
3
. KLiC(CF
3
SO
2
)
3
and LiN(CF
3
SO
2
)
2
and mixtures thereof a solvent thereof being selected from ethylene carbonate, 2-methyl tetrahydrofuran, tetrahydrofuran, dimethoxyethane, diethoxyethane, dimethyl carbonate, diethyl carbonate, methyl acetate, methyl formate, &ggr;-butyrolacto
Rao Ningling
Yde-Andersen Steen
Brouillette Gabrielle
Danionics A/S
Darby & Darby
Ruthkosky Mark
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