Electrochemical lithiation/delithiation kinetics and capacity of phosphate tungsten bronze and its chemically pre-lithiated derivatives in aqueous solutions

Monophosphate bronze (PW8O26, WPB) was synthesized by low-temperature (650 °C) heating of 12-tungstophosphoric acid (WPA). Its lithiated derivative with a low lithium content (Li3-WPB) was synthesized by heating lithium salt of WPA at 650 °C. Its lithiated derivative with a high lithium content was synthesized by mechanochemical lithiation of Li3WPB (Lin-WPB) (n > 3), followed by heat treatment at 650 °C. X-ray powder diffraction analysis confirmed that the bronze structure changed with the increase of the incorporated lithium content. These bronzes were investigated from the aspect of lithiation/delithiation kinetics in an aqueous saturated LiNO3 electrolyte solution. By cyclic voltammetry, both the kinetics of intercalation/deintercalation and the coulombic capacity were found to decrease with the increase of initial lithium content, in agreement with literature data for electrochemically lithiated tungstophosphate bronzes. Monoclinic lithium-free bronze PW8O26, as the most promising material, was subjected to a further detailed galvanostatic investigation coupled with the LiFe0.95V0.05PO4/C composite as a cathode material in an aqueous battery. In comparison to its behavior in an organic electrolyte, a considerably lower initial capacity of the bronze electrode was measured. However, its cyclic stability was much better in an aqueous than in an organic electrolyte.