A precursor composition of a gel electrolyte is provided, which includes (1) meta-stable nitrogen-containing polymer, (2) gelling promoter, (3) carbonate compound, and (4) metal salt. The (1) meta-stable nitrogen-containing polymer is formed by reacting (a) nitrogen-containing heterocyclic compound with (b) maleimide compound, wherein (a) nitrogen-containing heterocyclic compound and (b) maleimide compound have a molar ratio of 1:0.1 to 1:10.