The electrolytic solution 22 is not specifically limited, and commercially available electrolytic solutions for lithium batteries such as a solution in which a lithium salt is dissolved in a non-aqueous solvent such as an organic solvent may be used. In particular, an electrolytic solution having excellent heat resistance is preferable, and such an electrolytic solution preferably contains lithium borofluoride (LiBF₄) in a non-aqueous solvent. In this case, the non-aqueous solvent is preferably at least one selected from the group consisting of γ-butyrolactone (GBL), ethylene carbonate (EC) and propylene carbonate (PC), more preferably a mixed solvent composed of EC and GBL, a single solvent composed of PC, a mixed solvent composed of PC and GBL, or a single solvent composed of GBL, particularly preferably a mixed solvent composed of EC and GBL or a single solvent composed of GBL. The non-aqueous solvent has an increased boiling point by containing γ-butyrolactone (GBL), which considerably improves the heat resistance. From such a viewpoint, the volume ratio of EC:GBL in the EC and/or GBL containing non-aqueous solvent is preferably 0:1 to 1:1 (GBL ratio: 50 to 100% by volume), more preferably 0:1 to 1:1.5 (GBL ratio: 60 to 100% by volume), further preferably 0:1 to 1:2 (GBL ratio: 66.6 to 100% by volume), particularly preferably 0:1 to 1:3 (GBL ratio: 75 to 100% by volume). The lithium borofluoride (LiBF₄) to be dissolved in the non-aqueous solvent is an electrolyte having a high decomposition temperature, which also considerably improves the heat resistance. The LiBF₄ concentration in the electrolytic solution 22 is preferably 0.5 to 2 mol/L, more preferably 0.6 to 1.9 mol/L, further preferably 0.7 to 1.7 mol/L, particularly preferably 0.8 to 1.5 mol/L.