Due to the high price of lithium, it is an urgent task to investigate earth-abundant elements (Na, K, Mg, Al, Zn et al.) to provide a feasible alternative to Li ions as the charge carriers of rechargeable ion batteries (RIBs).

Until now, sodium-ion batteries (SIBs) have been intensively studied and recently industrialized. Polyanionic phosphates are here an attractive family of cathode materials that provides wide Na⁺ transport channels as well as relatively high working potential (> 3 V vs. Na⁺/Na). The most famous representative here is the NASICON (Na⁺ super ion conductor), namely Na₃V₂(PO₄)₃ (NVP), which has been proved to be an excellent cathode material for SIBs. Furthermore, mixed-pyrophosphates represent another class of interesting materials. For instance, a mixed phosphate compound Na₇V₄(P₂O₇)₄PO₄ (NVPP) can deliver higher working potentials (up to ~4 V vs. Na⁺/Na).

Compared with Na, Mg is a more promising option for RIBs with higher volumetric energy density, thanks to the double valence of Mg²⁺. Interestingly, the NVP is an active cathode material for MIBs as well. Our group investigated an NVP-NVPP material blend and revealed, for the first time, the capability of NVPP as a high-potential cathode material for MIBs. After further investigation on the synthetic routes of NVPP, a carbon-coated phase-pure NVPP was synthesized and investigated for potential MIBs application.