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 Na_0.44MnO₂ 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.

References

[1] W. Wu, S. Shanbhag, A. Wise, J. Chiang, A. Rutt, J. Whitacre, “High Performance TiP 2 O 7 Based Intercalation Negative Electrode for Aqueous Lithium-Ion Batteries via a Facile Synthetic Route,” Journal of The Electrochemical Society, 162 (2015).

[2] H. Wang, K. Huang, Y. Zeng, S. Yang, L. Chen, “Electrochemical properties of TiP 2 O 7 and LiTi 2 (PO 4 ) 3 as anode material for lithium ion battery with aqueous solution electrolyte,” Electrochemica Acta, 52 (2007) 3280-3285.

[3] L. Chen, Q. Gu, X. Zhou, S. Lee, Y. Xia, Z. Liu, “New-concept batteries based on aqueous Li + /Na + mixed-ion electrolytes,” Scientific Reports, 3 (2013).

[4] P. Senguttuvan, G. Rousse, J. Oro-Sole, J.M. Tarascon, M.R. Palacin, “A low temperature TiP 2 O 7 polymorph exhibiting reversible insertion of lithium and sodium ions,” Journal of Materials Chemistry A, 1 (2013).