Elucidation of the sodiation/desodiation mechanism in Ca0.5Ti2(PO4)3/C as promising electrode for sodium batteries: New insights into the phase transitions

The structure evolution and electrochemical performance of NaSICON-type Ca0.5Ti2(PO4)3 for sodium batteries are presented. This phosphate was synthesized by a solid-state method, and the obtained particles were coated with carbon using sucrose. This compound crystallizes in the rhombohedral system with space group R-3. The presence of carbon in the Ca0.5Ti2(PO4)3/C composite was confirmed by Raman and Thermogravimetric analysis. The electrochemical performance of Ca0.5Ti2(PO4)3/C was investigated in the potential window 1.5–3.0 V vs. sodium metal at different scan rates. The compound is able to initially intercalate/deintercalate 1.6/1.15 Na per formula unit, respectively. In operando synchrotron diffraction was done in the potential window 0.02–3.0 V vs. Na|Na+ and revealed the occurrence of several reaction regions upon first discharge. Up to 4 Na+ ion per formula unit can be inserted during the first discharge. An intensive refinement of the synchrotron X-ray diffraction (SXRD) patterns of discharged NaxCa0.5Ti2(PO4)3 evidenced the existence of five regions depending on the sodium content while the crystal structures of new phases were elucidated for the first time where sodium insertion occurs in the unusual M3 and M’3 sites of the NaSICON structure.