Electrochemical Lithium and Sodium Insertion Studies on Tungsten Oxyphosphate

Electrochemical Li insertion/de-insertion studies were performed on monoclinic W₂O₃(PO₄)₂in the voltage window of 3.5-1.8 V vs. Li⁺/Li. At C/5 rate, 2.0 Li⁺ions/f.u (90 mAh g⁻¹) are inserted in the lattice during first discharge and 1.75 Li⁺ions (75 mAh g⁻¹) are de-inserted during first charge with a reversible two phase process. A stable discharge capacity of 65 and 55 mAh g⁻¹were obtained in the voltage window of 3.5-1.8 V and 3.5-2.25 V at C/5 rate, respectively. In case of discharge to 1.4 V, the initial discharge capacity of 200 mAh g⁻¹was obtained with insertion of 4.5 Li⁺ions. The irreversible phase change was observed on charge with 11^thcycle discharge capacity of 50 mAh g⁻¹. The X-ray photo electron spectroscopy analysis shows the feasibility of W⁶⁺/ W⁵⁺and W⁵⁺/W⁴⁺redox couples vs. Li⁺/Li. The sodium ion insertion into the crystal lattice leads to poor reversibility with delivered discharge capacity of 30 mAh g⁻¹at the end of 25 cycles at C/5 rate in the voltage window of 1.2-3.0 V vs. Na⁺/Na. Ex-situ X-ray diffraction patterns indicate that Li insertion/de-insertion was possible in the parent phase. Formation of a new phase was observed which was also amenable for facile insertion/de-insertion of Li. Both the parent as well as the new phase undergoes reversible phase transitions during the process of insertion/de-insertion of Li in the voltage window of 1.8-3.5 V.