Influence of Ti/V Cation-Exchange in Na₂Ti₃O₇ on Na-Ion Negative Electrode Performance: An Insight from First-Principles Study

Sodium titanate Na₂Ti₃O₇ (NTO) is regarded as a highly promising anode material with a very low voltage for Na-ion batteries and capacitors but suffers from relatively low specific capacity and poor electron conductivity. Here, we report a first-principles study of electrochemical properties of NTO and its vanadium-modified compounds, Na₂Ti₂VO₇ and Na₂TiV₂O₇ (NTVO), offering an insight into their detailed working mechanism and an evidence of enhancing anode performance by Ti/V cation exchange. Our calculations reveal that the specific capacity can increase from 177 mA h g^–1 in NTO to over 280 mA h g^–1 in NTVO when using NaTi_3–xV_xO₇ (x = 1, 2) as a starting material for Na insertion due to higher oxidation state of V⁺⁵, together with lower voltages and small volume expansion rates below 3%. With Ti/V exchange, we obtain slightly higher activation energies for Na-ion migrations along the two different pathways but find an obvious improvement of electronic transport in NTVO.