With the recent rapid spread of portable and cordless electronic devices, the demand for small, lightweight, and high-energy density secondary batteries used as power sources for driving these electronic devices is increasing. As the applications of secondary batteries are broadened from small consumer electronic devices to electric power storage and electric vehicles, technological development of large secondary batteries that are required to have high capacity and high durability is accelerated.
From the above point of view, nonaqueous electrolyte secondary batteries, particularly lithium secondary batteries, are expected to be used for electrical devices, power storage, and power sources of electric vehicles because these secondary batteries offer high voltage and have high energy density.
The above-described nonaqueous electrolyte secondary battery includes a positive electrode, a negative electrode, and a separator interposed therebetween, and lithium cobalt oxide (e.g., LiCoO2) that has a high potential with respect to lithium and can be easily synthesized is used as a positive electrode active material.
In recent years, for the purpose of achieving an increase in capacity, layered active materials composed mainly of nickel and three-component layered compounds including nickel, cobalt, and manganese are used as positive electrode active materials. Various carbon materials such as graphite are used as negative electrode active materials, and a polyolefin-made microporous film is mainly used as the separator. A nonaqueous electrolyte solution prepared by dissolving a lithium salt such as LiBF4 or LiPF6 in an aprotic organic solvent is used as the nonaqueous electrolyte.
When a charge-discharge cycle is repeated for a long time, side reaction products of the electrolyte solution with the positive electrode active material and the negative electrode active material are accumulated. This causes a reduction in discharge capacity, and deterioration in durability occurs disadvantageously. In view of the above, PTL 1 proposes the use of cyclic carbonate in which at least 60% by mass thereof is fluoroethylene carbonate (hereinafter may be referred to as FEC) having a fluorine atom directly bonded to the carbonate ring. In this case, a coating containing a reduction product of the FEC is formed on the surface of the negative electrode active material during charge and discharge, and this allows an improvement in cycle characteristics. However, even when the technique proposed in PTL 1 is used, sufficiently good cycle characteristics are not obtained.
USPTO Backgrounds
USPTO Backgrounds