Designing a Li-N-H based solid electrolyte for all-solid-state batteries Jeremy Lowen and Joshua Makepeace School of Chemistry, University of Birmingham, Birmingham B15 2TT, UK All-solid-state-batteries (ASSBs) have long promised to be the next-generation of high-performance energy storage devices, with a step-change in energy density, stability and cell safety touted as potential advantages compared to conventional Li-ion battery cells. [1] To realise commercially viable ASSBs, suitable solid electrolyte materials must first be found that adhere to a number of strict performance requirements. This includes possessing a high ionic conductivity (> 10 -4 Scm -1 ), wide electrochemical stability window (>3 V), and the material forming low resistance interfaces in contact with lithium metal that are stable over long term battery cycling conditions. [1,2] To date, numerous materials with different structure types have been proposed for this application, however significant challenges remain with the leading candidates. Complex metal hydrides, in particular BH 4 - -derived materials, have gained recent attention due to their high Li-ion conductivities and wide voltage stability windows with respect to lithium metal. [3] Nitrogen-based complex hydrides, such as materials within the Li-N-H system, have largely been studied as solid-state hydrogen stores. However, a key characteristic that links the impressive properties of the Li-N-H system in this application is the high Frenkel defect-based Li-ion conductivity, which facilitates the formation of amide-imide and imide-nitride-hydride antifluorite structured solid solutions. [4,5] These solid solutions represent an opportunity through careful stoichiometry control to tailor the properties of a Li-N-H based material for application as a solid electrolyte. Careful characterisation using X-ray diffraction and Raman measurements have allowed us to evaluate the structure of the solid solutions between lithium imide (Li 2 NH), lithium amide (LiNH 2 ) and lithium nitride hydride (Li 4 NH), all dominated by a disordered anti-fluorite phase (see Fig.1). We demonstrate the ability to tailor the conductivity of these materials between an ionic insulator and a superionic conductor by simple stoichiometry control. Lithium imide, and stoichiometries close to this compound, in particular display favourable properties as a solid electrolyte material. We herein exhibit the first demonstration of a Li-N-H material as a functioning solid electrolyte, with the materials displaying high lithium-ion conductivities, wide electrochemical stability windows and potential for further tuning for this application.
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