Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer or spacer insulating structure
Reexamination Certificate
2001-04-10
2003-07-08
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer or spacer insulating structure
C429S332000, C429S342000, C429S343000, C029S623100
Reexamination Certificate
active
06589689
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte secondary battery. To be specific, the invention relates to a non-aqueous electrolyte secondary battery having an excellent stability in the charged state under a high temperature condition.
In recent years, there has been a rapid advancement in the realization of small, lightweight and cordless electronic appliances such as personal computers and portable telephones. As a power source for these appliances, secondary batteries having a high energy density have been required. In particular, non-aqueous electrolyte secondary batteries using lithium as an active material have been attracting a great deal of attention as batteries having a high voltage and a high energy density.
In order to obtain a non-aqueous electrolyte secondary battery having a high energy density and excellent characteristics, the characteristics of the non-aqueous electrolyte which transports lithium ions is critical. The non-aqueous solvent in the non-aqueous electrolyte usually comprises a solvent having a large dielectric constant which means facilitating dissolution of solutes, and a solvent having a low viscosity which is highly capable of transporting ions.
Usable solvents having a large dielectric constant are, for example, cyclic carbonic acid esters such as ethylene carbonate and propylene carbonate. Also, usable solvents having a low viscosity are non-cyclic carbonic acid esters such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.
The non-aqueous electrolyte prepared by dissolving a solute such as LiPF
6
in such a mixed solvent has a high ionic conductivity and can be adapted to a discharge under a low temperature of around 0° C., for example.
However, when the battery is stored in the charged state, there arise such problems as a decrease in the battery capacity due to the self-discharging and an increase in the inner pressure of the battery due to generation of gas. To be specific, on the negative electrode, the non-aqueous solvent is decomposed by the cathodic reduction reaction to generate gas. At this time, since lithium in the negative electrode is also reacted, the self discharge is accelerated and the battery capacity is decreased. In the case where a carbon material such as graphite is used as the negative electrode active material, the self-discharging is increased compared to the case where coke and the like is used, and the decrease in the capacity becomes explicit.
It is reported that, when ethylene carbonate is used as the non-aqueous solvent, ethylene carbonate is subjected to ring-opening dimerization, and it forms a film in the passive state on the surface of the negative electrode active material (J. Electrochem. Soc., 138, Aurbach et al., p. 3529). This film serves as a barrier which suppresses the decomposition of the non-aqueous solvent during storage of the battery in the charged state at around room temperature. However, since the film derived from ethylene carbonate is thermally unstable, it decomposes under a high temperature condition. As a consequence, when the battery in the charged state is stored under a high temperature condition, the decomposition reaction of the non-aqueous solvent in the battery cannot be suppressed.
Also studied is addition of vinylene carbonate having one carbon—carbon double bond in the molecule to the non-aqueous electrolyte. Vinylene carbonate has a good solubility with ethylene carbonate, and further, it is considered that vinylene carbonate preferentially forms a film on the surface of the negative electrode material (Japanese Laid-Open Patent Publication No. Hei 8-45545). Nevertheless, the film derived from vinylene carbonate is not sufficiently stable with heat and it also decomposes when the battery in the charged state is stored under a high temperature condition of 60 to 80° C. For such a reason, it is difficult to sufficiently suppress the decomposition reaction of the non-aqueous solvent by conventional techniques.
BRIEF SUMMARY OF THE INVENTION
In view of such circumstances as above, the present invention provides a non-aqueous electrolyte secondary battery having an excellent stability even when it is stored in the charged state under a high temperature condition.
The present invention relates to a non-aqueous electrolyte secondary battery comprising: a positive electrode; a negative electrode; and a non-aqueous electrolyte comprising a non-aqueous solvent and a solute dissolved therein, wherein the negative electrode material has on its surface a film derived from divinylethylene carbonate. The film derived from divinylethylene carbonate has an excellent thermal stability and allows lithium ions to permeate therethrough readily.
Also, the present invention relates to a non-aqueous electrolyte secondary battery comprising: a positive electrode; a negative electrode; and a non-aqueous electrolyte comprising a non-aqueous solvent and a solute dissolved therein, wherein the non-aqueous solvent contains divinylethylene carbonate.
It is preferable that the non-aqueous solvent comprises at least one selected from the group consisting of a carbonic acid ester and a carboxylic acid ester. Herein, the carbonic acid ester means those except divinylethylene carbonate.
The carbonic acid ester is preferably at least one selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, ethyl methyl carbonate, diethyl carbonate and dimethyl carbonate.
The carboxylic acid ester is preferably at least one selected from the group consisting of &ggr;-butyrolactone, &ggr;-valerolactone, &agr;-acetyl-&ggr;-butyrolactone, &agr;-methyl-&ggr;-butyrolactone, &agr;-bromo-&ggr;-butyrolactone, methyl acetate, ethyl acetate, methyl propionate, ethyl butylate, butyl acetate, n-propyl acetate, iso-butyl propionate and benzyl acetate.
The amount of divinylethylene carbonate is preferably 0.5 to 20 parts by volume per 100 parts by volume of the non-aqueous solvent.
A preferable non-aqueous solvent comprises 100 parts by volume of at least one selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate and vinylene carbonate, and 250 to 350 parts by volume of at least one selected from the group consisting of ethyl methyl carbonate, diethyl carbonate and dimethyl carbonate.
Another preferable non-aqueous solvent comprises 100 parts by volume of at least one selected from the group consisting of &ggr;-butyrolactone, &ggr;-valerolactone, &agr;-acetyl-&ggr;-butyrolactone, &agr;-methyl-&ggr;-butyrolactone and &agr;-bromo-&ggr;-butyrolactone, and 250 to 350 parts by volume of at least one selected from the group consisting of methyl acetate, ethyl acetate, methyl propionate, ethyl butylate, butyl acetate, n-propyl acetate, iso-butyl propionate and benzyl acetate.
It is also preferable that 90% by volume or more of the non-aqueous solvent consists of at least one selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate and vinylene carbonate.
It is still also preferable that 90% by volume or more of the non-aqueous solvent consists of at least one selected from the group consisting of &ggr;-butyrolactone, &ggr;-valerolactone, &agr;-acetyl-&ggr;-butyrolactone, &agr;-methyl-&ggr;-butyrolactone and &agr;-bromo-&ggr;-butyrolactone.
It is preferable that the positive electrode material contained in the positive electrode comprises a transition metal oxide containing lithium and that the negative electrode material contained in the negative electrode comprises a carbon material. The carbon material preferably comprises graphite.
Also, the present invention relates to a method for producing a non-aqueous electrolyte secondary battery comprising the steps of:
preparing a positive electrode material mixture containing a positive electrode material and applying the positive electrode material mixture to a core member to form a positive electrode plate;
preparing a negative electrode material mixture containing 100 parts by weight of a negative
Iwamoto Kazuya
Oura Takafumi
Ueda Atsushi
Yoshizawa Hiroshi
Cantelmo Gregg
Matsushita Electric - Industrial Co., Ltd.
McDermott & Will & Emery
Ryan Patrick
LandOfFree
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