Molecular organic solid-state electrolytes (MOSSEs): a review of an overlooked class of solid electrolytes

Lithium-ion batteries are crucial for applications like consumer electronics, electric vehicles, and renewable energy storage. Despite their importance, challenges persist in enhancing electrode materials for higher energy density, faster kinetics, and cycle life. Improving the safety and reliability of electrolytes is also vital, as traditional liquid electrolytes pose flammability and leakage risks. While carbonate-based liquid electrolytes are commonly used, they face limitations such as narrow electrochemical stability windows and compatibility issues with high-capacity or high-voltage electrodes. Solid electrolytes offer a promising alternative, providing improved safety and enabling the use of lithium metal anodes for higher energy density. The main types are solid polymer electrolytes (SPEs), inorganic solid electrolytes (ISEs), and their composites. This review introduces and examines molecular organic solid-state electrolytes (MOSSEs), a sub-class of molecular solid electrolytes (MSEs) formed by combining salts of alkali metal cations (e.g., Li⁺, Na⁺ and K⁺) with weakly coordinating organic molecules. These systems give rise to solids with unique crystalline structures that exhibit high ionic conductivity reaching 10^-3 S cm^-1 at ambient temperatures. MOSSEs also possess a low Young's modulus, improving electrode-electrolyte contact and lowering interfacial resistance, while facilitating processing and cell fabrication. We analyze the synthesis methods, structural characteristics, ion transport mechanisms, electrochemical stability windows, and battery performance of various MOSSE systems. Challenges such as moderate thermal stability and limited scalability are addressed, alongside strategies to improve interfacial compatibility and expansion of their electrochemical stability window. By overcoming key limitations of conventional solid electrolytes, such as brittleness, poor electrode-electrolyte contact, and high-temperature processing, MOSSEs offer a promising path toward safer, high-performance solid-state batteries for lithium and beyond-lithium systems.