Recently, secondary batteries configured to store electric energy have been attracted attentions for application to hybrid cars, or electric cards. As an energy saving technology, moreover, there are interests in the energy harvesting technology that generates electricity from small environmental energy. The secondary batteries capable of storing and supplying generated electric energy have been attracted attentions because there is a limitless possibility for various applications of the secondary batteries.
Attentions have been drawn to particularly all-solid-state batteries, which are secondary batteries and do not use a liquid for an electrolyte, in view of safety. All-solid-state batteries currently available on the market have a structure where LiCoO2 is used for a cathode, LiPON (lithium phosphorus oxynitride) is used for an electrolyte, and Li is used for an anode. LiPON is a solid electrolyte having a high lithium ion conductivity.
In order to widen applicable fields, such batteries are always desired to increase an energy density. In order to improve an energy density of a battery, there are two ways, an increase in a capacity density and an increase in operating voltage.
When an increase in operating voltage is performed, there is a report that LiPON that is an electrolyte of an all-solid-state battery currently commercially available is decomposed with voltage of 5.6 V or higher (see, for example, Xiaohua Yu, J. B. Bates, G. E. Jellison, Jr. and F. X. Hart, J. Electrochem. Soc., Vol. 144, 524 (1997)). Considering safety, therefore, used voltage of an all-solid-state battery using LiPON as a solid electrolyte is preferably kept about 5.0 V.
Moreover, proposed as a solid electrolyte used for an all-solid-state battery is a solid electrolyte which uses LiPON as a base and is modified by adding various elements (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2005-38843 and 2011-511399).