Thermoresistant Defects Preserve Fast Ion Conduction in LiBH₄-ZrO₂ Conductor-Insulator Nanocomposites

Highly conducting electrolytes with superior thermal and electrochemical stabilities are urgently needed for many areas of energy storage. While clever doping strategies and crystal-chemical engineering can be used to modify bulk dynamic properties, in interface-dominated nanomaterials, another degree of freedom to manipulate through-going ion transport is given by controlling the properties of their interfacial regions. In these regions, the local defect structure, which greatly affects ionic conductivity, sensitively depends on the complex interplay of size effects, the kind of foreign phases, and thermal history. However, a clear-cut understanding of how to preserve these advantageous interfacial properties is still missing. We hypothesize that the right preparation conditions and the correct choice of the foreign insulating phase in so-called dispersed nanostructured conductor-insulator composites can indeed stabilize defects at such interfaces and thus arrest improved properties. Here, we show how the interfacial regions in LiBH₄:ZrO₂ composites need to be manipulated to guarantee a high, thermostable-defect-mediated conductivity. As an example, for the LiBH₄:ZrO₂ nanocomposite, the highest conductivity achieved was on the order of 1.6 × 10^-5 S cm^-1 (298 K). Compared to single-phase nanocrystalline LiBH₄, this value refers to an increase of the overall conductivity by almost 3 orders of magnitude. The composite exhibits excellent thermal stability with virtually no loss in conductivity if subjected to elevated temperatures up to 473 K. We think that our findings will guide the implementation of nanoceramics in all-solid-state batteries, where boundary regions are often considered as the crucial bottlenecks that block long-range transport. Here, we turn the tables and use these interfaces to generate a network of fast interfacial pathways for the Li⁺ ions.