Amorphization-Driven Lithium Ion Storage Mechanism Change for Anomalous Capacity Enhancement

Capacity fading as a function of lithiation/delithiationcyclesis a major limitation of Li-ion batteries. Most Li storage materialsare susceptible to this phenomenon due to the degradation of the crystalstructure and particle integrity as a result of volume changes associatedwith lithiation/delithiation processes and/or irreversible redox reactions.However, some Li storage materials show an increase in capacity withan increase in cycles; this phenomenon has been termed "negativefading." Negative fading in Li host materials is usually associatedwith the additional charge storage at the particle/solid-electrolyteinterface (SEI) layer, decomposition/formation of the SEI layer, orredox reactions of various Li species at the interface. In this work,we report the observation of negative fading in a newly discoveredanode material, TiNbO4 (TNO), and reveal amorphizationas a new mechanism for negative fading in Li host materials. Thisassertion was confirmed via a close relationship between changes inthe crystal structure and the Li storage mechanism in TNO. Given thatother titanium niobium oxide analogues (e.g., TiNb2O7) suffer from capacity loss due to amorphization, this uniqueelectrochemical behavior of TNO may provide an interesting new directionto tune the titanium niobium oxides for high-performance, stable batteryanodes.