Electrochemical Profile of LiTi2(PO4)3and NaTi2(PO4)3in Lithium, Sodium or Mixed Ion Aqueous Solutions

Carbon-coated LiTi 2 (PO 4 ) 3 and NaTi 2 (PO 4 ) 3 were prepared using the sol-gel method, followed by solid-state reaction and thermal decomposition vapor deposition process. The electrochemical profile of both compounds were examined in three different aqueous electrolytes, including 1 M Li 2 SO 4 , 1 M Na 2 SO 4 and 1 M Li 2 SO 4 /Na 2 SO 4 mixed solutions (1:1 in the volume ratio). Both Li and Na ions can successfully inserted into these NASCION materials, and the two materials deliver a similar higher discharge potential in the Li 2 SO 4 solution (−0.76 V vs. Ag/AgCl) compared to that in the Na 2 SO 4 solution (−0.8 V vs. Ag/AgCl) after several charge/discharge cycles. The XRD analysis demonstrate that LiTi 2 (PO 4 ) 3 is converted to NaTi 2 (PO 4 ) 3 in the Na 2 SO 4 and mixed solutions, and NaTi 2 (PO 4 ) 3 was transformed to LiTi 2 (PO 4 ) 3 in the Li 2 SO 4 solution after cycling. It suggests that the Li or Na ions in the M1 sites of the NASCION materials are replaced by the ions from the electrolyte, but Na ions prefer to occupy the M1 sites compared with Li ions.