In recent years, the application range of lithium secondary battery (hereinafter, also referred to as “lithium-ion secondary battery”) has been increasingly broadened not only in portable electronic devices such as mobile telephones and laptop computers, but also as a large power source for electric cars and electric power storage. Particularly recently, there is a strong demand for a battery that can be installed in hybrid cars and electronic cars and has high capacity, high output and high energy density.
Such lithium-ion secondary battery is mainly constituted by a negative electrode composed of lithium metal and/or a carbon material (such as graphite) having excellent lithium absorbing and releasing properties; a positive electrode composed of a complex oxide of lithium and a transition metal; and a non-aqueous electrolyte solution.
Examples of positive electrode active material used in such positive electrode include lithium metal oxides such as LiCoO2, LiMnO2, LiNiO2 and LiFePO4.
Furthermore, as the non-aqueous electrolyte solution, for example, a solution in which a lithium salt such as LiPF6, LiBF4, LiN(SO2CF3)2 or LiN(SO2C2F5)2 is added to a mixed solvent of a highly dielectric cyclic carbonate, such as propylene carbonate or ethylene carbonate, and a low-viscosity chain carbonate, such as diethyl carbonate, methylethyl carbonate or dimethyl carbonate, is generally used.
Meanwhile, as negative electrode active material used in the negative electrode, lithium metal, metal compounds (such as elemental metals, oxides and alloys formed with lithium) that are capable of absorbing and releasing lithium and carbon materials are known and, in particular, lithium-ion secondary batteries utilizing a coke, artificial graphite and/or natural graphite, which is capable of absorbing and releasing lithium, have been put into practical use.
In recent years, in terms of the battery performance, not only a high capacity but also a high output are desired; therefore, there is a demand for a method of reducing the battery resistance under a variety of conditions.
As a factor of increasing the battery resistance, film formation on the surface of the negative electrode by a degradation product of the solvent or an inorganic salt, which is caused by reductive decomposition reaction of the electrolyte solution, is considered. When such reductive reaction occurs continuously, the film amount is increased and the battery resistance is consequently increased, so that the charge-discharge efficiency is decreased and the energy which can be extracted from the battery is reduced.
Furthermore, other problems to be solved include deterioration of the battery performance in a high-temperature environment. Deterioration of a lithium-ion secondary battery in a high-temperature environment is caused by a variety of factors such as degradation of lithium transition metal oxide, degradation of the electrolyte solution and destruction of the film formed on the negative electrode surface. Therefore, there is also a demand for a method of inhibiting such deterioration of the battery performance in a high-temperature environment.
In order to solve these problems, it has been attempted to improve the storage properties and resistance of a battery by adding vinylene carbonate (VC) to a non-aqueous electrolyte solution (see, for example, Japanese Patent Application Laid-Open (JP-A) No. H5-13088).
Furthermore, there have been proposed techniques for allowing a non-aqueous electrolyte solution to contain a compound having phosphorus (P) as a constituent element. Examples of such compound include chain phosphonic acid esters (see, for example, JP-A No. 2009-224258, JP-A No. 2000-164251 and JP-A No. II11-219711), cyclic anhydrides of phosphonic acid (see, for example, JP-A No. 2008-66062), cyclic phosphonic acid esters (see, for example, JP-A No. 2001-351681) and phosphoric acid silyl esters (see, for example, JP-A No. 2001-319685).
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