context: Moreover, cycling stability is a very important parameter of the electrochromic films in practical applications. We have measured the CV curves for more than 200 cycles from −0.6 to 0.9 V. In each cycle, the reversible process of coloring and bleaching can be observed in the annealed WO3−x film. Fig. 4d shows the CV curves for the first, 50th, 100th, 150th and 200th cycle. The CV curves in the first 50 cycles show slight promotion, and remain almost stable without any degradation even after 200 cycles. Such a high cycling stability may originate from the solid nanostructure of the annealed WO3−x film and the prevention of the harmful side reaction at the WO3−x film/electrolyte interface.