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, TiNbO4 (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., TiNb2O7) 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.
Keywords: amorphization; charge storage mechanism; lithium-ion batteries; negative fading; titanium niobium oxide.