Soft and Highly-Conductive Inorganic Lithum Metal Oxychloride Solid-State Electrolytes Based on 1-Dimensional Polymeric Structures

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 oxychloride-based SSEs over 4.3 V (versus Li+/Li) allows for electrochemical compatibility with high energy density cathodes, such as high-nickel NCM, without the need for a coating layer.
This talk will present a holistic understanding of the underlying nature of oxychloride SSEs, particularly - Li-Nb-O-Cl and Li-Al-O-Cl - which are comprised of 1-dimensional polymeric structures. LiNbOCl4 is reported to be one of the most highly conducting materials, exhibiting an ionic conductivity of ∼11 mS^●cm−1. Here, isolated 1-D [NbOCl4]− polymeric chains are aligned to form a semi-crystalline solid, and exhibit energetically favorable orientational disorder that is - in turn - correlated to multiple, disordered, and equi-energetic Li+ sites in the lattice. The activation energy barrier for Li migration through the frustrated energy landscape is reduced by the elastic nature of the NbO2Cl4 octahedra evident from very widely dispersed Cl−Nb−Cl bond angles in AIMD simulations. On the other hand, glassy Li-Al-O-Cl (LAOC) is constructed by the packing of Li+ cations and anionic Al-O-Clx- oligomers. Physicochemical properties (i.e. conductivity, plasticity, etc) of our LAOC solid model can be generalized from the local structure despite the lack of crystalline periodicity. Re-orientational motion of the terminal Cl groups is observed even at 300 K based on AIMD simulations. This arises from the conformational dynamics of the oligomer backbone, leading to mechanical plasticity through the creation of transient local free volume, as observed in organic polymers.
Although those oxychloride SSEs are composed of “hard” anions (O and Cl), which are favorable for excellent electrochemical stability and low cost, they also show mechanical softness and high ion conductivity. This is a unique advantage for all-solid-state-batteries (ASSB). Understanding the local free volume in semi-crystalline solids—possibly introduced by large amplitudes of librational motion—will be a key concept in designing future SSEs beyond hard crystalline solids. Our study from theory to experiment will hep pave the way for exploring the future material compositions based on “flex-ion” inorganic solids and open new directions for the design of high-conductivity, soft solid electrolytes for ASSBs.