High-vacancy-type titanium oxycarbide for large-capacity lithium-ion storage

Electrochemical processes involving the ion insertion/desertion are usually accompanied by composition variation and structural evolution of electrode materials. Here we propose a meaningful lattice regulation by inserting lithium ions to unlock an active crystalline plane from which high energy storage performance can be obtained. A rock-salt titanium oxycarbide featuring 12% titanium vacancies (Ti0.88□0.12C0.63O0.37) in high active (011) crystalline plane bears excellent electrochemical activity that enables additional reversible lithium insertion, providing a high initial specific capacity of 390 mAh g−1 at 0.05 A g−1. EPR, XAS, PDF and TEM measurements confirm abundant titanium vacancies, electrochemical studies combined with computational calculations demonstrate high activity of (011) crystalline plane as domain channels for lithium-ion insertion, and ex-situ XRD investigations disclose structural evolution during lithium-ion insertion/desertion. Notably, the repeated lithium-ion insertion/desertion promotes new exposure of (011) crystalline planes, leading to a capacity "negative fading" from the initial 120 to 325 mAh g−1 after 1100 cycles at 0.5 A g−1. The study presents the inner relationship between material microstructural transformation and electrochemical properties, providing a new perspective for mechanism re-understanding and structure development of ion-storage electrode materials.