A soft co-crystalline solid electrolyte for lithium-ion batteries

Alternative solid electrolytes are the next key step in advancing lithium batteries with better thermal and chemical stability. A soft solid electrolyte, (Adpn)₂LiPF₆ (Adpn, adiponitrile), is synthesized and characterized that exhibits high thermal and electrochemical stability and good ionic conductivity, overcoming several limitations of conventional organic and ceramic materials. The surface of the electrolyte possesses a liquid nano-layer of Adpn that links grains for a facile ionic conduction without high pressure/temperature treatments. Further, the material can quickly self-heal if fractured and provides liquid-like conduction paths via the grain boundaries. A substantially high ion conductivity (~10^-4 S cm^-1) and lithium-ion transference number (0.54) are obtained due to weak interactions between 'hard' (charge dense) Li⁺ ions and the 'soft' (electronically polarizable) -C≡N group of Adpn. Molecular simulations predict that Li⁺ ions migrate at the co-crystal grain boundaries with a (preferentially) lower activation energy E_a and within the interstitial regions between the co-crystals with higher E_a values, where the bulk conductivity is a smaller but extant contribution. These co-crystals establish a special concept of crystal design to increase the thermal stability of LiPF₆ by separating ions in the Adpn solvent matrix, and also exhibit a unique mechanism of ion conduction via low-resistance grain boundaries, which contrasts with ceramics or gel electrolytes.