A coating liquid comprising: a polar solvent; a hydrophilic polymer; and polyvinylidene fluoride, wherein the polar solvent is at least one selected from the group consisting of N,N-dimethylformamide, N,N-dimethylacetoamide, N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, and β-alkoxypropionamides, wherein the hydrophilic polymer is at least one selected from the group consisting of chitosan derivatives, cellulose derivatives, starch derivatives, unmodified polyvinyl alcohols, and modified polyvinyl alcohols, wherein: the chitosan derivative is at least one hydroxyalkyl chitosan selected from the group consisting of hydroxyethyl chitosan, hydroxypropyl chitosan, hydroxybutyl chitosan, and dihydroxypropyl chitosan; the cellulose derivative is at least one selected from the group consisting of hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, hydroxyethyl methyl cellulose, and hydroxypropyl methyl cellulose; the starch derivative is at least one selected from the group consisting of hydroxyethyl starch, hydroxypropyl starch, and hydroxybutyl starch; the unmodified polyvinyl alcohol has a degree of saponification of 98 mol% or less; and the modified polyvinyl alcohol is at least one selected from the group consisting of carboxy group-modified polyvinyl alcohols, carbonyl group-modified polyvinyl alcohols, silanol group-modified polyvinyl alcohols, amino group-modified polyvinyl alcohols, cationically modified polyvinyl alcohols, sulfonate group-modified polyvinyl alcohols, and acetoacetyl group-modified polyvinyl alcohols, and wherein the coating liquid further comprises a polybasic acid. a polar solvent; a hydrophilic polymer; and polyvinylidene fluoride, wherein the polar solvent is at least one selected from the group consisting of N,N-dimethylformamide, N,N-dimethylacetoamide, N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, and β-alkoxypropionamides, wherein the hydrophilic polymer is at least one selected from the group consisting of chitosan derivatives, cellulose derivatives, starch derivatives, unmodified polyvinyl alcohols, and modified polyvinyl alcohols, wherein: the chitosan derivative is at least one hydroxyalkyl chitosan selected from the group consisting of hydroxyethyl chitosan, hydroxypropyl chitosan, hydroxybutyl chitosan, and dihydroxypropyl chitosan; the cellulose derivative is at least one selected from the group consisting of hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, hydroxyethyl methyl cellulose, and hydroxypropyl methyl cellulose; the starch derivative is at least one selected from the group consisting of hydroxyethyl starch, hydroxypropyl starch, and hydroxybutyl starch; the unmodified polyvinyl alcohol has a degree of saponification of 98 mol% or less; and the modified polyvinyl alcohol is at least one selected from the group consisting of carboxy group-modified polyvinyl alcohols, carbonyl group-modified polyvinyl alcohols, silanol group-modified polyvinyl alcohols, amino group-modified polyvinyl alcohols, cationically modified polyvinyl alcohols, sulfonate group-modified polyvinyl alcohols, and acetoacetyl group-modified polyvinyl alcohols, and wherein the coating liquid further comprises a polybasic acid. the chitosan derivative is at least one hydroxyalkyl chitosan selected from the group consisting of hydroxyethyl chitosan, hydroxypropyl chitosan, hydroxybutyl chitosan, and dihydroxypropyl chitosan; the cellulose derivative is at least one selected from the group consisting of hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, hydroxyethyl methyl cellulose, and hydroxypropyl methyl cellulose; the starch derivative is at least one selected from the group consisting of hydroxyethyl starch, hydroxypropyl starch, and hydroxybutyl starch; the unmodified polyvinyl alcohol has a degree of saponification of 98 mol% or less; and the modified polyvinyl alcohol is at least one selected from the group consisting of carboxy group-modified polyvinyl alcohols, carbonyl group-modified polyvinyl alcohols, silanol group-modified polyvinyl alcohols, amino group-modified polyvinyl alcohols, cationically modified polyvinyl alcohols, sulfonate group-modified polyvinyl alcohols, and acetoacetyl group-modified polyvinyl alcohols, and wherein the coating liquid further comprises a polybasic acid.