Modulation of Crystal Surface and Lattice by Doping: Achieving Ultrafast Metal-Ion Insertion in Anatase TiO₂

We report that an ultrafast kinetics of reversible metal-ion insertion can be realized in anatase titanium dioxide (TiO₂). Niobium ions (Nb⁵⁺) were carefully chosen to dope and drive anatase TiO₂ into very thin nanosheets standing perpendicularly onto transparent conductive electrode (TCE) and simultaneously construct TiO₂ with an ion-conducting surface together with expanded ion diffusion channels, which enabled ultrafast metal ions to diffuse across the electrolyte/solid interface and into the bulk of TiO₂. To demonstrate the superior metal-ion insertion rate, the electrochromic features induced by ion intercalation were examined, which exhibited the best color switching speed of 4.82 s for coloration and 0.91 s for bleaching among all reported nanosized TiO₂ devices. When performed as the anode for the secondary battery, the modified TiO₂ was capable to deliver a highly reversible capacity of 61.2 mAh/g at an ultrahigh specific current rate of 60 C (10.2 A/g). This fast metal-ion insertion behavior was systematically investigated by the well-controlled electrochemical approaches, which quantitatively revealed both the enhanced surface kinetics and bulk ion diffusion rate. Our study could provide a facile methodology to modulate the ion diffusion kinetics for metal oxides.