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 ReO3 structure. The homologous series TiNbxO2+2.5x 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. TiNb2O7, TiNb24O62, and TiNb5O14.5 (often called Ti2Nb10O29) all have positive enthalpies of formation from binary oxides (TiO2 and Nb2O5), 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. TiNb24O62 has a high entropy consistent with extensive disorder, whereas TiNb2O7 and TiNb5O14.5 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. TiNb2O7 and TiNb5O14.5 can be relatively stable intercalating anode materials, while TiNb24O62 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.