New perspectives of functional metal borohydrides

•Metal borohydrides are a diverse group of materials with many different properties.•Borohydrides’ high hydrogen content make them interesting for hydrogen storage.•Neutral ligands can expand the structurally diverse metal and bimetallic borohydrides.•Metal borohydrides are of interest as electrolytes in all solid-state batteries.•Introduction of neutral ligands and nanoparticles can improve dynamics. [Display omitted] The chemistry of metal borohydrides and their derivatives has expanded signficantly during the past decade involving new compositions, structures, and the diversity of associated properties. Here we provide an overview of interesting results mainly from the past few years, discussed relative to previously published results. A range of new synthesis strategies has been developed to obtain pure samples, which has allowed very detailed structural, physical, and chemical investigations. A short overview of mono- and dimetallic borohydrides is provided, including a description of the complete series of rare-earth metal borohydrides and the recently discovered ammonium metal borohydrides, where the latter has attracted interest due to an extreme hydrogen content. Metal borohydrides appear to be the most promising class of materials to achieve high cationic conductivity of divalent metals, and particularly derivatives of metal borohydrides with neutral molecules show promise as future electrolytes for new types of solid-state batteries. Furthermore, metal borohydrides display a wide range of other properties, including optical, magnetic, semi conduction and possibly superconducting properties, and are also used as a new approach for carbon capture and conversion. The aim of the present review is to highlight new trends in properties and provide an outlook with possible future applications. Here, we focus on the more recently discovered materials.