TiP2O7 exhibiting reversible interaction with sodium ions in aqueous electrolytes

The reversible electrochemical cycling of sodium ions in several variants of low cost, easy to produce TiP2O7 as the negative electrode material in an aqueous battery is demonstrated for the first time. TiP2O7 is synthesized using a high temperature solid-state route and a low temperature sol gel route. The materials are electrochemically characterized in an aqueous 1 M Na2SO4 electrolyte with a Na0.44MnO2 counter electrode material. While little capacity for sodium ion cycling is reflected in cyclic voltammetry, all observed samples exhibited reversible cycling of sodium ions in galvanostatic cycling with potential limitation. To our knowledge, this is the first report of solid-state synthesized TiP2O7 reversibly cycling sodium ions, and the first report of TiP2O7 reversibly cycling sodium ions in aqueous electrolytes. Discharge specific capacity is found to be positively correlated with crystallite size and low specific currents.