Non-aqueous liquid electrolyte, electricity storage device using same, and phosphorus compound used therein

The purpose of this invention is to provide a secondary battery, the electrochemical properties of which can be improved over a wide temperature range. In this invention, in general formula (X), R 10 and R 20 each independently represent either a lithium atom or an organic group selected from among C 1-8 alkyl groups, C 2-6 alkenyl groups, C 3-6 alkynyl groups, and C 6-12 aryl groups. Also, X represents: a polar group (i) containing the -C(=O) group, the -P(=O) group, or the -S(=O) 2 group; a polar group (ii) containing the -CN group or a C 1-6 alkyl group in which fluorine atoms are substituted in place of some of the hydrogen atoms; or a polar group (iii) consisting of a four-to-seven-membered ring containing the -C(=O)-N- group or the -C(=O)-O- group. If X represents a polar group (iii) consisting of a four-to-seven-membered ring containing the -C(=O)-N- group, at least one of R 10 and R 20 represents a lithium atom.