Ion coordination and migration mechanisms in alkali metal complex borohydride-based solid electrolytes

Solid electrolytes (SEs) typically consist of a static framework of anions (FA) and a sublattice of mobile cations (M), with non-covalent dispersion interactions (E_disp) playing a key role in structural stability. However, the impact of these interactions on M-ion migration-whether they assist or hinder it-remains unclear. In this study, we investigate the diffusion barriers of M-ions in SE frameworks, focusing on the M₂B₁₂H₁₂ family (M = Li, Na, K), and clarify the role of non-covalent interactions. Our computational analyses reveal that moving from Li to Na to K analogues in M₂B₁₂H₁₂ results in significant changes in many-body dispersion (MBD) interactions. The MBD interactions act as springs embedded within the M₂B₁₂H₁₂ framework, regulating the effective coordination number (ECN) of M-ions and shaping the potential energy landscape for their migration. A linear correlation is observed between ECN and E_disp, consistent across different alkali ions. This correlation reveals a unified mechanism in which dispersion interactions regulate M-ion migration barrier and identifies a critical bottleneck for beyond-Li diffusion in M₂B₁₂H₁₂ SEs. Intriguingly, MBD interactions further influence the prefactor in the diffusivity equation, driving anomalous diffusion behavior in SEs.