Covalency modulation doping enables durable high-voltage operation in NiO-based all-solid-state electrochromic devices

Inorganic all-solid-state electrochromic devices (ECDs) are promising for smart windows and adaptive optoelectronics, but they often suffer from insufficient optical contrast, slow switching kinetics and poor cycling stability. High-voltage operation enhances optical modulation and redox kinetics, yet it accelerates metastable phase transitions and structural degradation. Here, we reveal that the degradation of NiO-based ECDs under high-voltage cycling originates from strengthened Ni-O covalency and the accumulation of metastable H1-3 phases, during the O3-O1 transition, which suppresses Ni regeneration and stress-buffering heterojunctions formation. To address this, we propose a covalency modulation strategy via Mo⁶⁺ doping. In-situ characterizations and theoretical calculations reveal that Mo incorporation weakens Ni–O bonding, enabling the in-situ formation of regenerable Ni/Mo_xNi_1-xO_y heterojunctions. The resulting ECD achieves exceptional durability over 17,000 cycles without performance degradation, together with high optical modulation (82.09%) and superior coloration efficiency (236.51 cm² C⁻¹), providing a general strategy toward durable high-voltage electrochromic and energy devices.