Chlorine Doping of Amorphous TiO₂ for Increased\nCapacity and Faster Li⁺‑Ion Storage

Abstract

Titania (TiO₂) offers a high theoretical capacity of\n336 mAh g^–1 with the insertion of one Li per Ti\nunit. Unfortunately, the poor ionic and electronic conductivity of\nbulk TiO₂ electrodes limits its practical implementation.\nNanosizing titania below ∼20 nm has shown to increase the rate\nperformance and accessible capacity but still not more than 75% of\nthe theoretical capacity at 1 C. In this work, we discovered that\nchlorine doping of amorphous TiO₂ (TiO_2–<i>x</i>Cl_2<i>x</i>) can achieve a high\ncapacity without the need for nanosizing. By in situ doping during\natomic layer deposition, an unprecedented 90% of the theoretical capacity\nwas achieved at 1 C for 100 nm thick films. Even at a charging rate\nof 20 C, 40% of the maximum capacity was accessible for the film with\nhighest Cl-content (<i>x</i> = 0.088). The capacity was\nfound linearly dependent on the chloride content for a Cl/Ti atomic\nratio from 0.06 to 0.09. The enhanced insertion kinetics are ascribed\nto enhanced electronic conductivity and facilitated Li⁺-ion diffusion as a result of Cl-doping. Furthermore, the potential\nof TiO_2–<i>x</i>Cl_2<i>x</i> films as high rate anode were demonstrated on micropillar\nelectrodes in a half-cell configuration using a liquid electrolyte\nsolution, showing 10 times higher capacity at 10 C compared to the\nliterature.