Probing how Ti- and Nb-substitution affect the stability and improve the electrochemical performance of beta- and epsilon-LiVOPO4

LiVOPO4 is a promising next-generation multi-electron cathode material, boasting a theoretical capacity of 305 mA h g(-1), significantly higher than any commercially used Li-ion battery cathode material. However, the material still faces several limitations, including the difficulty in attaining the full theoretical capacity at higher rates and capacity fade over several cycles. In this paper, we show that Ti- and Nb-substitution can be used to improve the thermal stability and electrochemical performance of LiVOPO4. We show through in situ heating with XRD and a novel gradient heating technique that both Ti- and Nb-substitution cause beta-LiVOPO4 to be stabilized relative to epsilon-LiVOPO4. This is due to transition-metal substitution, which increases the O-vacancy formation energies, pushing the beta -> epsilon transition to higher temperatures. We show that it is still possible to synthesize pure-phase epsilon-LiVOPO4 through the use of high temperatures to generate these O-vacancies. We show that even 1% of Ti- or Nb-substitution can improve the initial capacity and long term cycling capability of LiVOPO4 by improving the high-voltage capacity and reducing the capacity fade in both the high- and low-voltage regions. This is due to the an overall improved Li+ ion diffusion which is caused by an improved charge-transfer resistance during cycling.