Tungsten oxide, an electrochromic (EC) material with immense potential for technical applications such as in smart windows, has attracted much interest for its energy-saving qualities. However, after multiple cycles, such EC devices exhibit degradation in optical modulation and reversibility owing to lithium-ion trapping. Now, scientists have adopted a quantitative approach to understand the irreversibility of lithium intercalated tungsten oxide films, laying the groundwork for developing superior electrochromic materials and devices.