Charge storage in oxygen deficient phases of TiO$_2$: defect Physics without defects

TL;DR

This paper explores oxygen-deficient TiO₂ Magnéli phases as natural charge storage materials, revealing their intermediate bands and pseudo-defect behavior that enable high-capacity charge storage comparable to supercapacitors.

Contribution

It demonstrates that Magnéli phases of TiO₂ can store charge through their intermediate bands without traditional point defects, offering a new perspective on defect physics in these materials.

Findings

Magnéli phases exhibit intermediate bands similar to defect levels in TiO₂.

These phases behave as if they contain a high pseudo-defect density.

They have a charge storage capacity comparable to top supercapacitors.

Abstract

Defects in semiconductors can exhibit multiple charge states, which can be used for charge storage applications. Here we consider such charge storage in a series of oxygen deficient phases of TiO$_2$, known as Magn\'eli phases. These Ti$_n$O$_{2n-1}$ Magn\'eli phases present well-defined crystalline structures, i. e., their deviation from stoichiometry is accommodated by changes in space group as opposed to point defects. We show that these phases exhibit intermediate bands with the same electronic quadruple donor transitions akin to interstitial Ti defect levels in TiO$_2$-rutile. Thus, the Magn\'eli phases behave as if they contained a very large pseudo-defect density: $\frac{1}{2}$ per formula unit Ti$_n$O$_{2n-1}$. Depending on the Fermi Energy the whole material will become charged. These crystals are natural charge storage materials with a storage capacity that rivals the best known supercapacitors.