Lithium Intercalation into ATi2(PS4)3 (A = Li, Na, Ag)

Comparison of the electrochemical insertion of lithium into ATi2(PS4)3 with A = Li, Na, Ag and ATi2(PO4)3 with Li, Ag is striking. Whereas only four lithium per formula unit (Li/f.u.) can be inserted reversibly into the phosphates, up to 7 and 10 Li/f.u. can be inserted reversibly in the thiophosphates with A = Li and Ag. Moreover, the Ag+ to Ag0 reduction in AgTi2(PO4)3 is not reversible, but in AgTi2(PS4)3 it is reversible. Strong hybridization of the Ag-5s and host antibonding bands stabilizes the formal valences Ag0, Ti+, and (PS4)4− in the discharged state of AgTi2(PS4)3; but only the formal valence Ti2+ is accessible in LiTi2(PS4)3. Unfortunately the large volume change associated with the lithium insertion renders the structure progressively more amorphous on cycling, which causes the capacity to fade quite dramatically on further cycling. The thiophosphates transform to the phosphates on heating in air.