Non-aqueous electrolyte secondary cells, especially lithium secondary cells, are characterized by having a high voltage and a high energy density, and are excellent in terms of storage performance and output performance; therefore, they are used in a variety of electrical products. Recently, sodium secondary cells using sodium ions as carrier ions instead of lithium ions have been attracting attention as low-cost non-aqueous electrolyte secondary cells.
An organic solvent, such as a carbonate ester, which is used in an electrolytic solution of a non-aqueous electrolyte secondary cell has a low flash point ranging from 10 to 160° C. With this in mind, various approaches have been taken to ensure the safety of such a cell even when a foreign matter is accidentally present in the cell or when the cell falls into an abnormal condition such as an overcharge condition due to malfunction of a cell controller or a charger.
As examples of the product of an attempt aimed at improving the safety of a cell in terms of an electrolytic solution, there have been disclosed lithium-ion cells using an electrolytic solution including an organic solvent such as a carbonate ester and a liquid phosphazene flame retardant added to the organic solvent (see Patent Literature 1 and 2).
Patent Literature 1 discloses a lithium-ion cell using an electrolytic solution including: two organic solvents, ethylene carbonate and diethyl carbonate; and a liquid phosphazene flame retardant added to the organic solvents, the liquid phosphazene flame retardant having an ethoxy group which is the same functional group as that of diethyl carbonate. Specifically, it is disclosed that the liquid phosphazene flame retardant can be added in an amount of 5 to 35 mass % to a non-aqueous electrolytic solution containing carbonates as main components. It is also disclosed that the use of this electrolytic solution makes it possible, when the lithium-ion cell is left for a long period of time, to keep the phosphazene from causing a functional group substitution reaction with the organic solvents and thus maintain the properties of the phosphazene as a flame retardant, so that the flame retardancy of the non-aqueous electrolytic solution can be exhibited sufficiently to ensure the safety of the cell when the cell is in an abnormal condition.
Patent Literature 2 discloses a cell using a non-aqueous electrolytic solution in which are mixed a phosphazene flame retardant and a fluorine-substituted ether whose flash point is undetectable. Specifically, it is disclosed that 0 to 10 weight % of the phosphazene flame retardant and 0 to 50 weight % of the fluorine-substituted ether can be added to the non-aqueous electrolytic solution containing a carbonate as a main component. It is also disclosed that the fluorine-substituted ether is a linear ether compound represented by R1-O—R2 (R1 and R2 are each an alkyl group having 1 to 10 carbon atoms), at least some of whose hydrogen atoms are substituted with fluorine atoms. Thanks to the mixing of the phosphazene flame retardant, the electrolytic solution can prevent burning of the cell caused by abnormal increase in temperature of the cell and stabilize the behavior of the cell, thereby ensuring the safety of the cell. It is further disclosed that the mixing of the fluorine-substituted ether limits the increase in viscosity of the non-aqueous electrolytic solution caused by the mixing of the phosphazene flame retardant, and hence ensures ion mobility in the non-aqueous electrolytic solution, thereby making it possible to reduce the deterioration in high-rate discharge characteristics.