Origin of electrochromism in high-performing nanocomposite nickel oxide

Electrochromic effects of transition metal oxides provide a great platform for studying lithium intercalation chemistry in solids. Herein, we report on an electronically modified nanocomposite nickel oxide (i.e., Li _2.34NiZr_0.28O_x) that exhibits significantly improved electrochromic performance relative to the state-of-the-art inorganic electrochromic metal oxides in terms of charge/discharge kinetics, bleached-state transparency, and optical modulation. The knowledge obtained from O K-edge X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) suggests that the internally grown lithium peroxide (i.e., Li₂O₂) species plays a major role in facilitating charge transfer thus enabling optimal electrochromic performance. This understanding is relevant to recent theoretical studies concerning conductivity in Li ₂O₂ for lithium-air batteries (as cited in the main text). Furthermore, we elucidate the electrochromism in modified nickel oxide in lithium ion electrolyte with the aid of Ni K-edge XAS and Ni L-edge XAS studies. The electrochromism in the nickel oxide materials arises from the reversible formation of hole states on the NiO₆ units, which then impacts the Ni oxidation state through the Ni3d-O2p hybridization states. This study sheds light on the lithium intercalation chemistry for general energy storage and semiconductor applications.