Electrochromism of Ni-deficient nickel oxide -- Theoretical justification

TL;DR

This paper uses DFT+U calculations to propose a new mechanism for electrochromism in Ni-deficient NiO, involving hole polarons in oxygen states rather than Ni ion transitions, which could impact smart window technology.

Contribution

It introduces a novel theoretical explanation for Ni-deficient NiO electrochromism based on hole polarons, diverging from the traditional Ni2+/Ni3+ transition model.

Findings

Hole polarons form at oxygens near Ni vacancies.

Li insertion fills holes, switching the material's color state.

The mechanism is based on oxygen p-states, not Ni ion oxidation states.

Abstract

The development of new electrochromic materials and devices, like smart windows, has an enormous impact on the energy efficiency of modern society. One of the crucial materials in this technology is nickel-oxide. Ni-deficient NiO shows anodic electrochromism whose mechanism is still under debate. Using DFT+U calculations, we show that Ni vacancy generation results in the formation of hole polarons localised at the two oxygens next to the vacancy. Upon Li insertion or injection of an extra electron into Ni-deficient NiO, one hole gets filled, and the hole bipolaron is converted into a hole polaron well-localized at one O atom. Furthermore, the calculated absorption coefficients demonstrate that Li insertion/extraction or rather the addition/removal of an extra electron into Ni-deficient NiO can lead to switching between the oxidized (colored) and the reduced (bleached) states. Hence, our results suggest a new mechanism of Ni-deficient NiO electrochromism not related to the Ni2+/Ni3+ transition but based on the formation and annihilation of hole polarons in oxygen p-states.