Electrochemistry of Sodium Nonatitanates in Lithium and Sodium Ion Batteries

Sodium nonatitanate, NaTi₃O₆(OH)·2H₂O, readily undergoes electrochemical sodium and lithium insertion reactions at unusually low potentials.[1] The lower average insertion potential and higher theoretical capacity (~300 mAh/g) suggest that it could be a higher energy density alternative to the well-known anode material Li₄Ti₅O₁₂.[2-3] Titanates are also denser materials than graphite, so full utilization of titanates in a lithium-ion cell should lead to a somewhat higher energy density. This presents an intriguing possibility of using high-energy anodes based on titanate materials for lithium and sodium ion batteries, provided they could be developed further.

The presence of mobile sodium ions in the lithium ion system is undesirable from a performance and safety point of view. A recent NMR study [4] on composite electrodes derived from sodium nonatitante cycled in lithium cells showed evidence of in situ sodium plating during the discharge process. Our ex situ and in situ synchrotron XRD diffraction patterns [5] suggest that mobile sodium ions undergo ion-exchange with the electrolytic solution and/or are de-intercalate upon recharge, then subsequently are reduced to metallic sodium at the very low potentials encountered during later discharges. We show that a simple ion-exchange process prior to incorporation in electrochemical cells removes all sodium ions, producing the lithiated form of the material. The lithiated material performs similarly to sodium nonatitanate in lithium cells, although coulombic inefficiencies are somewhat higher (Figure 1). A comparison is made between the structure and electrochemical behavior of sodium nonatitanate and the lithiated analog in lithium cells.

References

1. M. Shirpour, J. Cabana, and M. Doeff, Energy & Environmental Science, 2013, 6(8), 2538-2547.

2. Z. Yang, D. Choi, S. Kerisit, K. M. Rosso, D. Wang, J. Zhang, G. Graff and J. Liu, Journal of Power Sources, 2009, 192, 588–598.

3. D. Deng, M. G. Kim, J. Y. Lee and J. Cho, Energy & Environmental Science, 2009, 2, 818–837.

4. Mallory Gobet, PhD thesis, Department of Physics and Astronomy, Hunter College of CUNY, New York, 2014.

5. D. Seshadri, M. Shirpour, M. Doeff, Journal of The Electrochemical Society, 2014, 162 (1) A1-A8.