New Plastic Crystal Composite Electrodes Employing Delocalized Transition Metal Salts for Low-Cost, High-Safety All-Solid-State Salt Batteries

We demonstrate a novel approach to creating soft interphases that can comprise both the cathode and electrolyte structures for advanced all-solid-state Li ion batteries. Organic ionic plastic crystals (OIPCs) with promising properties, such as soft and plastic texture, nonflammability, high electrochemical, thermal stability, and high ion conductivity, were employed in both the cathode and solid-state electrolyte (SSE). By incorporating a delocalized transition metal salt active material (exemplified by Fe(BF4)2·6H2O in this work) and graphene, the OIPC composite provides high ion conductivity to the electrode and forms a new phase that supports redox activity when doped with Li or Na salts. The 2.8 V OIPC-Fe(BF4)2·6H2O|Li solid-state battery achieves an initial discharge capacity of 150.6 mAh g–1 at 0.05C and 50 °C, retaining 78 mAh g–1 after 50 cycles. The rate performance of the OIPC electrode is superior to that of an equivalent cathode using a commercial CMC binder, highlighting the role of the OIPC in enhancing the Fe(BF4)2·6H2O utilization. This approach has also been extended to Na-based batteries as a proof of concept, demonstrating stable cycling with an average discharge capacity of 102 mAh g–1 at 0.1C and 50 °C for over 100 cycles. This discovery opens opportunities to explore the vast scope of OIPC and salt design to create new low-cost and safe solid-state batteries.