Ni0.5TiOPO4 phosphate: Sodium insertion mechanism and electrochemical performance in sodium-ion batteries

Ni0.5TiOPO4 oxyphosphate anode material was prepared via sol-gel method and coated with sucrose. This compound crystallizes in the monoclinic system (S.G. P2(1)/c) with a 3D framework providing convenient pathways for sodium ion diffusion and is able to accommodate 1.5 Na+/f.u. on the available sites (theoretical capacity of 256 mAh.g(-1)). The electrochemical performance of C-coated Ni(0.5)TioPO(4) (NiTP-C) was investigated for the first time vs. Na+/Na in the potential window 0.2-3 V. The C-coated Ni0.5TiOPO4 electrode material can exhibit an initial discharge capacity of 520 mAh.g(-1) at C/10, with a long plateau at 0.75 V in the half cell against metallic Na. The subsequent charge profile clearly differs form that recorded during the discharge revealing an irreversibility in the electrochemical process in the first cycle. Rate capability tests have demonstrated satisfactory performances. The specific capacities delivered at 2C and 5C were 185 mAh.g(-1) and 135 mAh.g(-1), respectively. In situ diffraction using synchrotron radiation revealed that C-coated Ni0.5TiOPO4 exhibits a conversion mechanism starting from 0.64 V in discharge, and at 0.49 V the compound shows a complete amorphization characterized by the disappearance of the Bragg reflections from crystalline phase.