Chlorine Doping of Amorphous TiO₂ for Increased Capacity and Faster Li⁺-Ion Storage

Abstract

Titania (TiO2) offers a high theoretical capacity of 336 mAh g–1 with the insertion of one Li per Ti unit. Unfortunately, the poor ionic and electronic conductivity of bulk TiO2 electrodes limits its practical implementation. Nanosizing titania below ∼20 nm has shown to increase the rate performance and accessible capacity but still not more than 75% of the theoretical capacity at 1 C. In this work, we discovered that chlorine doping of amorphous TiO2 (TiO2–xCl2x) can achieve a high capacity without the need for nanosizing. By in situ doping during atomic layer deposition, an unprecedented 90% of the theoretical capacity was achieved at 1 C for 100 nm thick films. Even at a charging rate of 20 C, 40% of the maximum capacity was accessible for the film with highest Cl-content (x = 0.088). The capacity was found linearly dependent on the chloride content for a Cl/Ti atomic ratio from 0.06 to 0.09. The enhanced insertion kinetics are ascribed to enhanced electronic conductivity and facilitated Li+-ion diffusion as a result of Cl-doping. Furthermore, the potential of TiO2–xCl2x films as high rate anode were demonstrated on micropillar electrodes in a half-cell configuration using a liquid electrolyte solution, showing 10 times higher capacity at 10 C compared to the literature.