A facile route to plastic inorganic electrolytes for all-solid-state batteries based on molecular design

Lithium-ion batteries for electric vehicles and energy storage systems are an indispensable part of a clean-energy future, and included in these is the prospect of all-solid-state designs. The demand for high performance solid-state electrolytes (SSEs), which are an essential component of such devices, has drawn much attention owing to their non-flammability and mechanical toughness. Unlike the widely used sulfide-based SSEs, the excellent anodic stability of chloride-based SSEs over 4.3 V (versus Li⁺/Li) allows for electrochemical compatibility with high energy density cathodes, such as NCM. More recently, glassy-state oxychloride electrolytes have emerged that are superior to chloride-based SSE; however, the lack of a reliable structural model has prohibited the rational design of these materials.

Here, we report a novel approach for designing glassy-state electrolyte design based on well-defined lithium aluminum oxychloride (LAOC) linear oligomers [Li⁺ + Al₃O₂Cl_8-x^(3-x)- (0≤x≤2)]. The glassy solid model is constructed by packing these oligomers, as demonstrated by ab-initio molecular dynamics (AIMD). With this model, we define two types of ion dynamics. First, Li-migration is enhanced (> 1 mS/cm at 30 °C) by the large geometrical frustration of the Li-ion environment, promoted by mixed coordination of Cl and O anions. Secondly, a rotational motion of the terminal Cl groups is observed, arising from the conformational dynamics of the oligomer backbone (rotational and bending motions). This motion leads to exhibit mechanical plasticity as well as glass transition (T_g at -15 °C) through the creation of transient local free volume, like what is observed in organic polymers. Although it is composed of “hard” anions (O and Cl), which are favorable for excellent anodic stability and low cost, the molecular nature of LAOC enables it to exhibit mechanical softness alongside high ion conductivity. Our all-solid-state battery based on this solid electrolyte shows long term electrochemical stability with a high-nickel NCM cathode. This fundamental understanding of ion motions (vibrational, rotational and translational) in LAOC, based on both theorical and experimental studies, will pave the way for the development of oligomeric (molecular) solid electrolytes as a new type of inorganic SSE.