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

Abstract

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+2.5x 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. 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.

Wadsley–Roth Crystallographic Shear Structure Niobium‐Based Oxides: Promising Anode Materials for High‐Safety Lithium‐Ion Batteries

“…It should be noticed that the recent study about the entropy stabilization of TiO 2 -Nb 2 O 5 Wadsley-Roth shear phases indicated that TiNb 24 O 62 may convert into energetically more stable phases during repeated cycling in batteries, which will affect the long-term cycling stability. [51] Tetragonal PNb 9 O 25 with the space group of I4/m belongs to Wadsley-Roth phases with (3 × 3) ∞ ReO 3 structure blocks. As shown in Figure 9c, the crystalline structure of PNb 9 O 25 is built up from nine NbO 6 octahedra corner-share to form a 3 × 3 block in the ab plane, and these blocks are held together by cornersharing PO 4 tetrahedra.…”

“…It should be noticed that the recent study about the entropy stabilization of TiO 2 ‐Nb 2 O 5 Wadsley–Roth shear phases indicated that TiNb 24 O 62 may convert into energetically more stable phases during repeated cycling in batteries, which will affect the long‐term cycling stability. [ 51 ]…”

A Comparison of Order-Disorder in Several Families of Cubic Oxides

“…This suggests that the ordering of defects is thermodynamically favorable. Recently, Voskanyan and Navrotsky showed that TiO 2 -Nb 2 O 5 Wadsley-Roth CS phases are energetically metastable and stabilized via configurational entropy arising from the cation disorder at elevated temperatures ( Voskanyan et al, 2020 ). In many oxides, defect clustering at the short-range scale is energetically more favorable and long-range ordering of vacancies does not take place.…”

A Low‐Voltage Layered Na2TiGeO5 Anode for Lithium‐Ion Battery

“…[12][13][14][15] The commercialized Li 4 Ti 5 O 12 is a typical representation, which possesses a high working potential of 1.55 V. However, its low capacity is still the main problem at present. Other Ti-based oxides, such as TiO 2 , [16] TiNb 2 O 7 , [17] Li 2 MgTi 2 O 5 , [18] etc., also operate at relatively high average potentials, possessing relatively low energy density as anode materials for LIBs.…”