TiP2O7 Exhibiting Reversible Insertion of Sodium Ions in an Aqueous Environment

Abstract

Aqueous sodium ion batteries are attractive for large-scale energy storage due to their low cost, intrinsic safety and environmental friendliness [1,2]. A key challenge in designing aqueous batteries is creating electrode pairs that operate within the stable potential window of water, with anode material selection being particularly challenging. Furthermore, good materials for sodium ion cycling are limited [2,3]. TiP2O7 presents a promising anode material due to its lower intercalation/deintercalation potentials, allowing for more optimized use of an aqueous electrolyte’s stability window [1-3]. TiP2O7 is typically reported to exclusively cycle lithium ions, showing no capacity in sodium aqueous electrolytes in cyclic voltammetry tests [3]. One report demonstrated that a low temperature polymorph of TiP2O7 exhibits reversible cycling of sodium ions in an organic electrolyte [4]. In this work, the reversible insertion of sodium ions in TiP2O7 as the anode material in an aqueous battery is demonstrated. Several polymorphs of TiP2O7 are synthesized using a high temperature solid-state route and a low temperature sol gel route. In addition to physical characterization, the materials are electrochemically characterized in an aqueous 1M Na2SO4 electrolyte against Na0.44MnO2 cathode material. While no capacity for sodium ion cycling is reflected in cyclic voltammetry tests of these TiP2O7, all polymorphs exhibited reversible cycling of sodium ions in constant current galvanostatic cycling with potential limitation. To our knowledge, this is the first report of solid-state synthesized TiP2O7 reversibly cycling sodium ions. Specific capacity is found to be positively correlated with both crystallite size and lattice parameter. A full cell rate study with three electrode data in aqueous Na2SO4 electrolyte is presented. Acknowledgments This work is supported by Carnegie Mellon University and Aquion Energy Inc.