Revealing sodium storage mechanism in lithium titanium phosphate: Combined experimental and theoretical study

Abstract We investigate a LiTi2(PO4)3–carbon composite (LTP-C) with a sodium superionic conductor (NASICON)-type structure as a potential electrode material for sodium storage. Operando X-ray diffraction and ex situ X-ray absorption spectroscopic analyses reveal that repetitive electrochemical reduction (discharge) and oxidation (charge) between 1.2 and 3.1 V results in a two-phase redox process associated with the Ti4+/3+ redox couple. The rearrangement of the alkali sites during discharge/charge is investigated using first-principles calculations and Rietveld refinement. Using first-principles calculations, we verify the possibility of ion exchange from Li+ to Na+ in LiTi2(PO4)3 in Na cells as well as various theoretical electrochemical properties of LiNa2Ti2(PO4)3 and Na3Ti2(PO4)3. Notably, the sodiated LTP-C exhibits a stable cycle life for over 300 cycles at 0.5C and for over 1000 cycles at 5C with capacity retention of 99% and 94%, respectively.