The discovery of conductive ionic bonds in NiO/Ni transparent counter electrodes for electrochromic smart windows with an ultra-long cycling life

Developing a highly stable electrode has been a research focus for electrochromic smart windows (ECSWs) with an ultralong cycling life. This paper reports a NiO/Ni counter electrode and the discovery of conductive ionic bonds at the NiO/Ni interface. The formation of these conductive ionic bonds between NiO and Ni is verified by electron localization function (ELF) and Bader charge analyses. Furthermore, the density of states (DOS) results reveal the metallic character of these ionic bonds. Such conductive ionic bonds stabilize the NiO lattice in the process of ion intercalation/deintercalation and weaken the electrostatic interaction between Ni and O. The experimental results show that the NiO/Ni electrode shows high visible transparency in the bleached state (90.1%), a fast switching speed (4.5 s for coloring and 5 s for bleaching) and an ultralong cycling life (87.0% retention of the original optical contrast at 550 nm after 10 000 cycles). Additionally, a 10 cm × 10 cm ECSW assembled with this NiO/Ni counter electrode and a WO3 electrode shows excellent EC performances such as large optical contrast (67.1%@550 nm, −2 V), very small transparency (lower than 1% in the wavelength range of 1600–800 nm), and a high coloration efficiency of 175.7 cm2 C−1. This work on an all-solution preparation of NiO/Ni counter electrodes represents an important step toward the development of cheap ECSWs.