Entropy Stabilization of TiO2–Nb2O5 Wadsley–Roth Shear Phases and Their Prospects for Lithium-Ion Battery Anode Materials

Wadsley–Roth phases accommodate variable cation charge by crystallographic shear planes delineating blocks of the parent ReO 3 structure. The homologous series TiNb x O 2+2.5 x provides possible new anode materials for lithium-ion batteries. The thermodynamic stability of three of these shear phases was determined by high-temperature oxide melt solution calorimetry. TiNb 2 O 7, TiNb 24 O 62, and TiNb 5 O 14.5 (often called Ti 2 Nb 10 O 29 ) all have positive enthalpies of formation from binary oxides (TiO 2 and Nb 2 O 5 ), implying that they are entropy-stabilized and only stable above some minimum temperature. Hence, shear phases may represent a new and extensive class of “entropy-stabilized oxides”. Their thermodynamic stability decreases with the increasing Nb content. Entropies of formation were calculated using the measured enthalpy of formation and assuming that their synthesis temperature is their lowest temperature of stability and using calculated configurational entropies arising from cation disorder. TiNb 24 O 62 has a high entropy consistent with extensive disorder, whereas TiNb 2 O 7 and TiNb 5 O 14.5 appear to be substantially more ordered. These entropy values are further constrained by considering the stability of the Wadsley–Roth phases with respect to each other. TiNb 2 O 7 and TiNb 5 O 14.5 can be relatively stable intercalating anode materials, while TiNb 24 O 62 is likely to decompose near room temperature during extended battery cycling. This work accentuates the underlying role of thermodynamic studies in engineering electrochemically active materials with enhanced stability for next-generation lithium-ion batteries and beyond.