| Summary: | It is difficult to obtain ultrathin two-dimensional (2D) tungsten trioxide (WO<sub>3</sub>) nanosheets through direct exfoliation from bulk WO<sub>3</sub> in solution due to the strong bonding between interlayers. Herein, WO<sub>3</sub> nanosheets with controllable sizes were synthesized via K<sup>+</sup> intercalation and the exfoliation of WO<sub>3</sub> powder using sonication and temperature. Because of the intercalation and expansion in the interlayer distance, the intercalated WO<sub>3</sub> could be successfully exfoliated to produce a large quantity of individual 2D WO<sub>3</sub> nanosheets in <i>N</i>-methyl-2-pyrrolidone under sonication. The exfoliated ultrathin WO<sub>3</sub> nanosheets exhibited better electrochromic performance in an electrochromic device than WO<sub>3</sub> powder and exfoliated WO<sub>3</sub> without intercalation. In particular, the prepared small WO<sub>3</sub> nanosheets exhibited excellent electrochromic properties with a large optical modulation of 41.78% at 700 nm and fast switching behavior times of 9.2 s for bleaching and 10.5 s for coloring. Furthermore, after 1000 cycles, the small WO<sub>3</sub> nanosheets still maintained 86% of their initial performance.
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