A rechargeable electrochromic energy storage device enabling effective energy recovery

Efficient energy recovery from electrochromic (EC) devices gives new insight into reducing the consumption of energy and facilitating the recycling of energy. However, one challenge is to realize the effective energy storage and conversion without sacrificing the electrochromic performance. Herein, spinel Li₄Ti₅O₁₂(LTO), a desirable anodic material for lithium ion batteries, was employed to develop a rechargeable electrochromic energy storage device (EESD), instead of utilizing conventional capacitive EC materials. Moreover, the LTO film exhibited a good electrochromic performance in terms of visible and near-infrared transmittance modulation (54.9% at 550 nm and 71.6% at 1000 nm), indicating that it can effectively block 90% of the thermal radiation energy in the sunlight. In addition to its high theoretical capacity and excellent structural stability, LTO displayed a large optical contrast during energy storage and conversion, and its EC mechanism was further revealed by analyzing its electronic structure evolution in response to lithiation based on density functional theory calculations. When assembled with a complementary EC NiO anode, the well-designed LTO-based EESD demonstrated a reversible transmittance variation over 50% between charged (colored) and discharged (bleached) states. For energy storage, the rechargeable EESD with a high operating voltage of 3.0 V could power a 1.7 V red light-emitting diode (LED) for more than 10 min and provide an energy density of 0.2 W h cm⁻³, which is superior to most state-of-the-art energy storage systems based on conventional EC materials. As a proof of concept, EESD is designed to be a prototype device of smart window to elucidate the feasibility for energy recovery and self-power applications. The energy used to drive the coloring of the EESD window could be stored on a sunny day and then power a household lamp (3 V/5 W) at night. It is envisioned that the rechargeable EESD in this work will provide an applicable platform for effective energy recovery in real-world applications involving advanced energy technology and smart window architecture.