Investigation of Na7V3(P2O7)4/carbon composite as cathode material for sodium-ion battery: Influences of carbon additives and voltage windows

Abstract

A sequentially processed method is developed to prepare a high-voltage cathode material sodium vanadium diphosphate Na7V3(P2O7)4 for Na-ion battery. We find that the growth of Na7V3(P2O7)4 generally introduces growth orientation due to their hybrid ionic and covalent bonding. The crystal growth of the diphosphate prefers to grow perpendicular to [002] direction, leading to an increase of the diffusion length of Na+ ion. To regulate the growth of crystal, three kinds of carbon additives, graphene oxide, Cabot carbon black and Ketjen carbon black are adopted to tune the morphologies of Na7V3(P2O7)4 to further improve its electrochemical performances. The morphologies and physicochemical properties of carbon additives and corresponding prepared Na7V3(P2O7)4 are both well characterized to clarify the effects of carbon additives on the growth process. We find that the Cabot carbon black can dramatically weaken the oriented growth direction and produce fine morphology. By adding Cabot carbon black to the sequential process, the reversible capacity of Na7V3(P2O7)4 can reach to 76 mAh g−1 with good cyclability within the voltage window of 2–4.5 V. An average voltage of 3.8 V can be obtained due to the strong inductive effect of diphosphate group.