Hydrated V₂O₅ with an expanded bilayer structure for high-performance zinc-ion electrochromic energy storage devices

Zinc-ion electrochromic energy storage devices (ZECDs) that integrate dynamic optical modulation with rechargeable battery behavior represent a transformative paradigm for net-zero energy consumption smart windows. However, the development of cathode materials that simultaneously achieve rapid ion diffusion kinetics, high optical contrast, substantial charge storage capacity, and long-term cycling stability remains a formidable challenge. Here, an in situ annealing spray-coating process is conducted to prepare crystallized hydrated vanadium oxide (V₂O₅·1.6H₂O) cathode thin films. The V₂O₅·1.6H₂O films exhibit a well-ordered bilayer structure with an expanded interlayer spacing. This unique crystallized hydrated architecture, combined with interlayer water molecules that act as both structural "pillars" and Zn²⁺ transport facilitators, endows the material with enhanced and reversible Zn²⁺ intercalation/deintercalation kinetics, high optical modulation, and reversibility. As a result, the optimized device (ZECDs-2) shows a good balance among optical contrast (66.69% at 650 nm), switching speed (t_c = 2.3 s, t_b = 6.7 s), coloration efficiency (38.81 cm² C^-1), and areal capacity (12.1 μAh cm^-2 at 0.5 mA cm^-2). The device also exhibits good energy retrieval capability for energy saving. Furthermore, large-area (10 cm × 10 cm) and patterned devices are successfully fabricated, demonstrating electrochromic switching and customizable display functions. This work provides a practical route toward scalable, energy-recycling smart windows and displays.