The disclosed method includes: a step (I) in which a dispersion liquid (S) including an aluminum compound (A) is prepared; a step (II) in which the dispersion liquid (S) and a prescribed phosphorus compound (B) are mixed and a coating liquid (U) is prepared; a step (III) in which the coating liquid (U) is coated on a base material (X) and a precursor layer for a layer (Y) is formed; and a step (IV) in which the precursor layer is heat treated at a temperature of at least 110 DEG C and the layer (Y) is formed. The aluminum compound (A) can be formed by adding acid to a solution including an aluminate. The number of moles (NM) of aluminum atoms derived from the aluminum compound (A) and the number of moles (NP) of phosphorus atoms derived from the phosphorus compound (B) fulfill 0.8<=(NM)/(NP)<=4.5.