Entropy-stabilized titanium niobium oxides (TNOs) with crystallographic shear structures (e.g., TiNb2O7 and Ti2Nb10O29) are generally synthesized by high-temperature calcination in an air or an oxygen atmosphere to compensate for their positive enthalpies of formation. In this work, we demonstrate that changing the reaction atmosphere into a slightly reductive environment using in situ carbonization leads to the creation of a new class of TNO with a formula of TiNbO4. Unlike its predecessors, this new lithium reservoir is a rutile phase, and most strikingly, in situ X-ray diffraction analysis revealed that its lithium intercalation occurs via a purely solid-solution process. Since solid-electrolyte-interface-free, high capacity anode materials with long cyclic life are required to meet the stringent requirements of widespread lithium-ion battery utilization, this finding of a new electrode material with purely single-phase lithium intercalation is of great interest for the development of high-performance anode materials. Distinctive electrochemical behavior that is different from that of crystallographic shear structured TNO is revealed by in-depth electrochemical analyses, which is ascribed to the unique structural and electronic properties of TiNbO4. We believe this work opens a new avenue for the development of feasible oxide-based alternatives to graphite, which can be safer and suitable for high-power performance.

TiNbO4 is a recently discovered Li host material. Unlike most mixed oxides of Ti and Nb, TiNbO4 does not show a phase transformation for Li storage. It stores Li-ions via a purely single-phase reaction. In this work, we show that TiNbO4 stores Li-ions via intercalation pseudocapacitance in both bulk and nanostructured states. The rate performance and capacity of TiNbO4 are independent of nanostructuring, indicating that TiNbO4 stores charges through an intrinsic pseudocapacitance mechanism.

Capacity fading as a function of lithiation/ delithiation cycles is a major limitation of Li-ion batteries. Most Li storage materials are susceptible to this phenomenon due to the degradation of the crystal structure and particle integrity as a result of volume changes associated with lithiation/delithiation processes and/or irreversible redox reactions. However, some Li storage materials show an increase in capacity with an increase in cycles; this phenomenon has been termed "negative fading." Negative fading in Li host materials is usually associated with the additional charge storage at the particle/solid−electrolyte interface (SEI) layer, decomposition/formation of the SEI layer, or redox reactions of various Li species at the interface. In this work, we report the observation of negative fading in a newly discovered anode material, TiNbO 4 (TNO), and reveal amorphization as a new mechanism for negative fading in Li host materials. This assertion was confirmed via a close relationship between changes in the crystal structure and the Li storage mechanism in TNO. Given that other titanium niobium oxide analogues (e.g., TiNb 2 O 7 ) suffer from capacity loss due to amorphization, this unique electrochemical behavior of TNO may provide an interesting new direction to tune the titanium niobium oxides for high-performance, stable battery anodes.