NOVELTY - A lithium ion secondary cell (10) comprises electrolyte liquid (30) in which positive electrode formed using a mixture containing a lithium-containing metal oxide (i.e. lithium manganate compound) and phosphoric acid alkali metal salt (i.e. pyrophosphate, tripolyphosphoric acid salt, tetra polyphosphate, and hexametaphosphate of an alkali metal), and lithium hexafluorophosphate and lithium bis (perfluoro alkane sulfonyl) imide (i.e. lithium bis (pentafluoroethane sulfonyl) imide) are dissolved.    USE - The lithium ion secondary cell (e.g., a square-shaped, cylindrical-shaped, or coin-shaped lithium ion secondary cell) is used for an electronic device, vehicles (e.g., a hybrid vehicle, a fuel cell electric vehicle, and an electric vehicle), and an electric-power reservoir.    ADVANTAGE - Capacitance deterioration in a high temperature state can be suppressed sufficiently. Decline in the capacitance maintenance factor after specifically performing repeatedly charging/discharging in a fall and high temperature state of the capacitance maintenance factor after storing in the high temperature state (e.g. greater than or equal to50degreesC) can fully be suppressed.    DETAILED DESCRIPTION - A lithium ion secondary cell comprises electrolyte liquid in which positive electrode formed using a mixture containing a lithium-containing metal oxide (i.e., lithium manganate compound) and phosphoric acid alkali metal salt (i.e., pyrophosphate, tripolyphosphoric acid salt, tetra polyphosphate, and hexametaphosphate of an alkali metal), and lithium hexafluorophosphate and lithium bis (perfluoro alkane sulfonyl) imide (i.e., lithium bis (pentafluoroethane sulfonyl) imide) are dissolved. The positive electrode is formed using the mixture, which contains 2-20 wt.% of a phosphoric acid alkali metal compound with respect to the lithium-containing metal oxide. The electrolyte liquid dissolves lithium bis (perfluoro alkane sulfonyl) imide and lithium hexafluorophosphate in the concentration of 0.5-1.5 and 0.01-0.2 M, respectively.    DESCRIPTION OF DRAWING(S) - The figure is a sectional drawing of a coin-shaped lithium ion secondary cell.    Lithium ion secondary cell (10)    Positive-electrode case (12)    Negative-electrode case (14)    Gasket (16)    Electrolyte liquid (30)