Recently, a lithium rechargeable battery tends to increase in size from a compact rechargeable battery used in a notebook computer and a cellular phone to a rechargeable battery for an electric vehicle and a hybrid electric vehicle. Such a battery for an electric vehicle is used outdoors and thus is desired to be less sensitive to the temperature change. In particular, an output at a high temperature and/or a low temperature is important. Such an output at a high temperature and/or a low temperature depends on the physical properties of the electrolytic solution in many cases, and this is because the viscosity of the solvent rapidly increases when the temperature decreases to near a certain point of the solvent of electrolyte and thus the mobility of the ions significantly decreases.
The rechargeable batteries including a lithium rechargeable battery are composed of a cathode, an anode, an electrolyte and a separator. During discharging, the lithium ion is removed to cause an oxidation reaction in the anode and the lithium ion is intercalated to cause a reduction reaction in the cathode. During charging, the lithium ion is removed to cause an oxidation reaction in the cathode and the lithium ion is intercalated to cause a reduction reaction in the anode. The electrolyte does not exhibit conductivity for an electron but exhibits only ionic conductivity and plays a role to deliver the lithium ion between the cathode and the anode.
Although the primary performance such as the operating voltage and the energy density of the lithium rechargeable battery is determined by the materials constituting the cathode and the anode, the electrolyte is required to be equipped with high ionic conductivity, electrochemical stability, thermal stability and the like in order to obtain excellent battery performance. An electrolytic salt and an organic solvent are used as the components constituting the electrolyte. The electrolyte is required to be electrochemically stable in the potential region corresponding to the reduction potential and the oxidation potential in consideration of the reduction reaction with the anode and the oxidation reaction with the cathode.
The organic solvent is required to be equipped with low reactivity with lithium, a minimized internal resistance for smooth transfer of the lithium ion, thermal stability in a wide range of temperature, high compatibility with the anode active material and a dielectric constant that is high enough to dissolve a great amount of a lithium salt. Hitherto, as such an organic solvent, a cyclic carbonate such as propylene carbonate (PC) and ethylene carbonate (EC); or a linear carbonate such as dimethyl carbonate (DMC) and diethyl carbonate (DEC) has been mainly used, in addition a hydrocarbon-based solvent such as 1,2-dimethoxyethane and diethoxyethane has been used.
However, PC is highly viscous and has a problem that it is intercalated between the carbon layers of the anode and decomposes to produce propylene gas and lithium carbonate during charging when it is used together with a crystalline carbon-based anode active material such as graphite, and the battery capacity decreases and the irreversible capacity of battery increases as a result. In addition, the linear carbonate such as DMC and DEC is less viscous, easily intercalated between the anode active materials to decrease the irreversible capacity of battery, and less reactive with lithium, but has a disadvantage that it is not able to dissolve a great amount of a lithium salt since it has a low dielectric constant. In particular, DMC is expected to be used in a high-current and high-voltage battery since it exhibits high electrical conductivity, but it is poor in low temperature characteristics since it has a high melting point (4.6° C.). In addition, an organic solvent such as dimethylformamide and acetonitrile has a high dielectric constant but is highly reactive with lithium, and thus there is a problem that it is practically difficult to use an organic solvent.
Meanwhile, the characteristics required to the non-aqueous electrolytic solution for a lithium rechargeable battery have been stricter year after year. As one of such properties required, it is required to solve the problem of securing stability at a high temperature and stability at the time of being overcharged (for example, nonflammability or fracture resistance). This is because a lithium rechargeable battery for an electric vehicle and power storage is often exposed to an external high-temperature environment, the temperature of the battery may increase by instantaneous charge and discharge, and thus the lifespan of the battery may be shortened in such a high-temperature and high-voltage environment and the quantity of energy stored may decrease.