With recent circulation of portable electronic devices such as mobile personal computers and handy video cameras, there have been widely used nonaqueous electrolyte solution secondary batteries with high voltage and high energy density as power sources. Further, battery cars and hybrid cars using electric power as part of motive power have been put in practical use from an environmental viewpoint.
Nonaqueous electrolyte solution secondary batteries, however, suffer from decrease of the capacity or increase of internal resistance in storage at high temperatures or in repetitive charge/discharge cycles, and hence they were not sufficiently reliable as stable power sources.
For improving stability and electric properties of nonaqueous electrolyte solution secondary batteries, various additives have been proposed. For example, in secondary batteries using lithium as negative electrodes, Patent Document 1 proposes an electrolyte solution containing 1,3-propanesultone, Patent Document 2 proposes an electrolyte solution containing vinylethylene carbonate, and Patent Document 3 proposes an electrolyte solution containing vinylene carbonate. In secondary batteries using negative electrodes made of carbon material, Patent Document 4 proposes an electrolyte solution containing 1,3-propanesultone and butanesultone. In secondary batteries using negative electrode made of graphite with high crystallinity, Patent Document 5 and Patent Document 6 propose electrolyte solutions containing vinylene carbonate, vinylethylene carbonate, and others.
Electrolyte solutions containing a sultone such as 1,3-propanesultone and butanesultone together with an unsaturated group-containing cyclic carbonate such as vinylene carbonate and vinylethylene carbonate exert a certain level of effects when used for any negative electrode of lithium metal, natural graphite, artificial graphite, easily-graphitizable carbon, non-graphitizable carbon, carbon-coated natural graphite, polyacenes, and the like. This is because such compounds form a stable coating film suppressing reductive decomposition of the electrolyte solution, so-called SEI (solid electrolyte interface) on the surface of the negative electrode. That is, the improving effect results from suppression of side reactions on the surface of the negative electrode such as decomposition of solvents by coating the surface of the negative electrode, thereby mitigating initial irreversible decrease in capacity or other problems. For this reason, vinylene carbonate and the like are widely used as additives for electrolyte solutions in particular. The effect, however, has been insufficient. Namely, since lithium ion is not easily transmitted in a coating film formed from a sultone such as 1,3-propanesultone and butanesultone, an unsaturated group-containing cyclic carbonate such as vinylene carbonate and vinylethylene carbonate, and the like, such a film has only a small effect on reduction of internal resistance. Further, this film is decomposed in long-term use of the battery or at high temperatures because of low durability, and hence the surface of the negative electrode is again exposed after decomposition of the coating film, which has a drawback that the battery is deteriorated in long-term use or at high temperatures. When these compounds are added excessively to electrolyte solutions as an attempt to compensate this drawback, on the contrary, there happens a problem that the resistance increasing rate becomes higher because of the high resistance of coating film formed, deteriorating the performances of batteries. Thus, the long-term performances or high-temperature characteristics of batteries has not been fundamentally improved by adding a sultone such as 1,3-propanesultone and butanesultone and an unsaturated group-containing cyclic carbonate such as vinylene carbonate and vinylethylene carbonate to electrolyte solutions.
Patent Document 7 proposes that the high-temperature stability of electrolyte solutions is improved by adding a silane to electrolyte solutions. Patent Documents 8, 9, and 10 propose batteries in which high capacity can be maintained based on reduced variation of internal resistance and reduced increase of internal resistance by adding a silane to the electrolyte solution. The effect was, however, still unsatisfactory.    [Patent Document 1]: Japanese Patent Laid-Open Publication No. S63-102173    [Patent Document 2]: Japanese Patent Laid-Open Publication No. H04-087156    [Patent Document 3]: Japanese Patent Laid-Open Publication No. H05-074486    [Patent Document 4]: Japanese Patent Laid-Open Publication No. H10-050342    [Patent Document 5]: Japanese Patent Laid-Open Publication No. H08-045545    [Patent Document 6]: Japanese Patent Laid-Open Publication No. 2001-006729    [Patent Document 7]: Japanese Patent Laid-Open Publication No. 2001-307772    [Patent Document 8]: Japanese Patent Laid-Open Publication No. 2002-134169    [Patent Document 9]: Japanese Patent Laid-Open Publication No. 2004-087459    [Patent Document 10]: Japanese Patent Laid-Open Publication No. 2004-039510
USPTO Backgrounds
USPTO Backgrounds