Electrochemistry of Sodium Nonatitanates in Lithium and Sodium Ion Batteries
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  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  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.
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