Facet-controlled bifunctional WO3 photocathodes for high-performance photo-assisted Li–O2 batteries

Abstract

Photocatalytic materials have been recently incorporated into Li–O₂ batteries to mitigate the challenge of high overpotential. However, the structure–activity relationship between photocatalytic materials and charge–discharge mechanisms in photo-assisted Li–O₂ batteries has rarely been experimentally revealed. Herein, we take facet-engineered WO₃ as a prototype to investigate the formation processes of discharge products and photocatalytic reaction activities. A combination of theoretical and experimental results demonstrates that a controllable transition of the generated Li₂O₂ from the solution growth route to the surface growth route with faster redox kinetics can be realized by increasing the exposed (002)/(020) facet ratio of WO₃ photocathodes. Furthermore, the (002) facet is revealed to possess higher oxidation ability for Li₂O₂ decomposition, thus resulting in an ultralow polarization overpotential of 0.07 V over the (002) facet dominant WO₃ photocathode. Importantly, the continuous growth of the Li₂O₂ film up to ∼130 nm through the surface growth mode is observed on the photocathode. In-depth studies show that a Z-type heterojunction tends to be formed between the WO₃ and formed Li₂O₂, facilitating continuous growth of the Li₂O₂ film under illumination. As a result, a high discharge capacity of up to 10500 mA h g⁻¹ is also achieved over the (002) facet dominated WO₃ photocathode. These new discoveries break through the limitation of premature battery death caused by the surface growth route and enable high capacities and sustained photo-assisted discharge.