Exploring Route for Pyrophosphate‐based Electrode Materials: Interplay between Synthesis and Structure

Abstract

New sodium vanadium(III) hydrogenphosphate hydrate NaV(HPO4)2(H2O)0.5 and sodium vanadium(III) hydrogenphosphate β‐NaV(HPO4)2 were prepared in mild hydrothermal conditions. The crystal structures of NaV(HPO4)2(H2O)0.5 [space group Cc, Z = 4, a = 8.46174(19) Å, b = 9.52583(19) Å, c = 8.69376(15) Å, β = 110.9553(11) °, V = 654.41(2) Å3] and β‐NaV(HPO4)2 [space group C2/c, Z = 4, a = 7.8681(3) Å, b = 9.8451(3) Å, c = 8.5180(2) Å, β = 107.626(2) °, V = 628.85(3) Å3] were solved and refined from X‐ray powder diffraction data. Both compounds were used as precursors in a new route for the preparation of attractive cathode material β‐NaVP2O7. The formation of the structure motive providing fast sodium ion diffusion at the first synthesis stage and its further conservation upon stepwise dehydration was revealed. The oversized Na+‐embedding channels are stabilized by site‐coordinated water in NaV(HPO4)2(H2O)0.5 structure. The topology resemblance and difference in known sodium vanadium(III) complex phosphates are discussed.