High-Iodine Li(BH4)1-x I x Thin Films Enabled by Multilayer Interdiffusion and Exhibiting Stress-Assisted Conductivity Retention.

We report a vacuum-compatible multilayer interdiffusion strategy that enables controlled iodine incorporation into Li-(BH4)1-x I x thin films. Alternating deposition of LiI and LiBH4 at room temperature, followed by mild annealing at 423 K yields flat films with suppressed impurity formation and large-area compositional homogeneity. Raman spectroscopy indicates progressive stabilization of the hexagonal phase with increasing iodide content, while SEM/EDS indicates extensive interdiffusion and disappearance of multilayer boundaries, enabling high iodine compositions (x = 0.20-0.40) that are difficult to achieve by conventional thin-film routes. AC impedance measurements reveal enhanced ionic transport below the phase-transition temperature, with a maximum room-temperature conductivity of 4.9 × 10-5 S cm -1 for x = 0.20 during coolingcomparable to or exceeding that of widely used thin-film electrolytes such as LiPON. In addition, a thin-film-specific conductivity retention ("memory") effect is observed, whereby the high-conductivity state partially persists upon cooling, likely associated with interfacial constraints. This multilayer interdiffusion approach provides a general pathway for fabricating compositionally tunable hydride solid-electrolyte thin films and highlights the additional design opportunities that arise in confined thin-film geometries.