Na3V2O2x(PO4)2F3-2x As a High Voltage Cathode Material for Rechargeable Aqueous Sodium-Ion Batteries

As an alternative to current lithium ion batteries, sodium-ion batteries (SIBs) are potential energy storage systems due to the high natural abundance and low cost of Na. However, safety issues for SIBs based on flammable organic solvents still hinder the practical applications of SIBs [1]. In this view, rechargeable aqueous SIBs can overcome  safety concerns as well as environmental toxicity [2]. Recently, we reported the reversible electrochemical reaction of Na₂FeP₂O₇, which is a promising polyanion compound for SIBs, in a neutral aqueous solution.  But, Na₂FeP₂O₇ in aqueous electrolytes exhibits a low energy density due to the low redox potential of Fe^2+/3+ [3]. Therefore, we have studied several polyanion compounds in aqueous electrolytes to achieve high energy density SIBs. Here, the electrochemical activity of the Na₃V₂O_2x(PO₄)₂F_3-2x in aqueous electrolytes is reported for the first time. Na₃V₂O_2x(PO₄)₂F_3-2x with multi-walled carbon nanotubes (MWCNTs) exhibits a long-term stability by 1100 cycles in aqueous electrolytes. Two different types of Na-ion full-cells demonstrate the feasibility of the Na₃V₂O_2x(PO₄)₂F_3-2x/MWCNT composite as a cathode for aqueous sodium-ion batteries. A high full-cell voltage of 1.7 V and a high energy density of 84 Wh kg⁻¹were achieved using Zn metal as an anode.

References:

[1]   Z. Chang,   Y. Yang,   M. Li,   X. Wang and Y. Wu, J. Mater. Chem. A, 2, 10739 (2014).

[2]   H. Kim, J. Hong, K.Y. Park, H. Kim, S.W. Kim and K. Kang, Chem. Rev., 114 11788 (2014)

[3]   Y. H. Jung,  C. H. Lim,   J. H. Kim   and D. K. Kim,  RSC Adv., 4, 9799 (2014)