Capacity and Electrochromism Decoupling Strategy for Real-Time State-of-Charge Monitoring of High-Areal-Capacity Electrochromic Batteries

State-of-charge (SOC) is a central parameter in battery management systems for electric vehicles, directly affecting driving range, power delivery, safety, and battery lifetime. However, real-time and high-precision SOC monitoring remains a challenge for practical batteries. Here, we report a capacity and electrochromism decoupling strategy for real-time SOC monitoring of high-areal-capacity electrochromic batteries. The bilayer cathode combines a pure electrochromic coloration layer with an underlying conductive, high-loading capacity layer, thereby spatially separating optical readout from high-capacity storage. The device delivers an areal capacity of 3.4 mAh cm–2 with 97.8% capacity retention after 300 cycles. The monotonic SOC–Hue relationship with 1% SOC intervals was established by converting in situ reflectance spectra into Hue values. The modulation of reflectance-peak wavelength shift retains 81.2% after 1000 coloration cycles. This architecture provides a route toward batteries with built-in optical state reporting for advanced battery management.