Stress-Assisted Conductivity Retention in High-Iodine LiBH₄ Thin Films

Researchers develop a multilayer method that enables high-iodine LiBH₄ thin films while improving room-temperature ionic conductivity

Solid electrolytes are promising materials for safer, more compact all-solid-state batteries, but producing smooth thin films with high ionic conductivity remains challenging. In a recent study, researchers from Shibaura Institute of Technology, Japan, developed a multilayer method using lithium borohydride and lithium iodide to create high-iodine thin films. Their approach improved iodine incorporation, maintained relatively smooth surfaces, and produced good room-temperature ionic conductivity, opening new possibilities for thin-film solid electrolytes.

All-solid-state lithium batteries require solid materials that allow lithium ions to move efficiently. Lithium borohydride (LiBH₄) is a promising material for this purpose because its high-temperature hexagonal phase supports rapid lithium-ion transport. Introducing iodide into LiBH₄ can stabilize this conductive phase at lower temperatures, but producing smooth films with enough iodine has been difficult. Earlier methods added only about 8% iodine and often produced rough surfaces, limiting their usefulness in thin-film devices.

To address this challenge, a research team led by Ph.D. student Erika Fukushi and Professor Hiroyuki Oguchi from Shibaura Institute of Technology in Tokyo, Japan, developed a multilayer method. They deposited thin layers of lithium iodide and LiBH₄ one after another at room temperature, then heated them so the layers could mix through interdiffusion. This approach allowed the researchers to add more iodine while keeping the films relatively smooth. Their findings were made available online on May 27, 2026, and were published in Volume 11, Issue 22 of the journal ACS Omega on June 09, 2026.

“By depositing LiBH₄ and lithium iodide separately, we can overcome the different vapor pressures that previously limited iodine incorporation,” says Ms. Fukushi. “The multilayer design also lets us control composition through individual layer thicknesses while keeping deposition at room temperature, which helps suppress island-like growth seen in films deposited at higher temperatures.”

After heating, the separate layers had largely mixed together, with SEM/EDS indicating extensive interdiffusion and large-area compositional homogeneity, although some local iodine-rich regions remained. The new films contained iodide substitution levels of 20–40% (x = 0.20–0.40), a substantial increase from the previous limit of about 8%. They also remained relatively flat compared with films made using higher-temperature methods, which often developed uneven, island-like surfaces. This combination of composition control and surface smoothness could facilitate systematic studies of electrode/solid-electrolyte interfaces.

The films exhibited enhanced lithium-ion conductivity, with the best room-temperature conductivity reaching 4.9 × 10⁻⁵ S cm⁻¹. This value is comparable to or higher than that of widely used thin-film solid electrolytes such as LiPON. However, adding too much iodine reduced conductivity, suggesting that there is an optimal iodine level. At excessive concentrations, iodide can obstruct lithium-ion migration pathways through site blocking and iodine-induced lattice strain or expansion.

The researchers also found that some films continued to exhibit high ionic conductivity as they cooled after heating. This behavior gradually weakened over about one week, suggesting that the films temporarily retained a high-conductivity, metastable state. The effect may result from stresses at the film–substrate interface, which could hinder the structural changes normally associated with cooling.

“The conductivity retention is especially interesting because it shows that thin films can behave differently from bulk materials,” says Ms. Fukushi. “Interfacial constraints may provide a way to tune how a conductive state is retained, and future experiments varying the substrate and film thickness could help clarify this effect.”

Overall, the study presents a practical way to make smooth, iodine-rich LiBH₄ films with useful room-temperature conductivity. These films may eventually be applicable to thin-film solid-state batteries and small electrochemical devices. Further research is needed to test their performance with battery electrodes, evaluate long-term stability, and determine how their interfaces affect charge and discharge behavior.