Defect Chemistry and Mixed Conduction in Lithium Lanthanum Titanate During the Transition from Electrolyte to Anode Material

Lithium lanthanum titanate Li0.29+delta La0.57TiO3 (LLTO) is a promising material in Li ion battery application, due to its ambient stability and high ionic conductivity. When it is subjected to a high Li chemical potential, additional Li ions intercalate into vacant A sites, which is balanced by the reduction of Ti4+ ions to Ti3+. At this point, LLTO becomes a mixed ion and electron conductor, which means that it undergoes a transition from an electrolyte to a high rate capable electrode material in the potential range below ca 1.7 V vs Li metal. However, the exact voltage of the transition from electrolyte to the electrode, as well as the electronic conductivity of reduced LLTO were still unknown. Here, we investigate the thermodynamics of lithium insertion as well as ion and electron conductivity of reduced LLTO by employing a galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS). We can show that LLTO gradually changes from electrolyte material to a mixed conductor, with an ion transference number that depends on the Li chemical potential. Lastly, we present a defect chemical model that fits excellently to the U(delta) curves and the conductivity data.