A molecular selenium iodide cathode and hybrid electrolyte for solid-state Na–Se2I2 batteries

Challenges such as polyselenide shuttling and poor reaction kinetics persist in sodium–selenium (Na–Se) batteries. While solid-state Na–Se batteries could potentially eliminate the shuttle effect, they have received limited attention due to poor solid–solid interfacial contact and intrinsically sluggish conversion kinetics. Herein, we report a solid-state Na–Se2I2 battery that employs a low-melting point Se2I2 molecular cathode and a tailored hybrid solid electrolyte. At an operating temperature above its melting point, liquid Se2I2 establishes favorable liquid–liquid interfaces and enables a facile liquid–solid conversion pathway. Furthermore, the sodium super ion conductor (NASICON)/poly(ethylene oxide) hybrid electrolyte simultaneously dissolves polyselenides to promote redox kinetics and physically blocks their shuttling to ensure cycling stability. As a result, a reversible six-electron conversion reaction is achieved, and the solid-state Na–Se2I2 battery delivers a high specific capacity of 336 mAh g−1 at 0.1 C and a stable cycling over 200 cycles with 88.2% capacity retention at 0.5 C. The electrochemically active Se2I2 molecular design opens an alternative chemistry for selenium-based electrodes and provides a promising direction for future solid-state battery research.