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] The development of fast lithium-ion conductors is key to realising commercially-viable ASSBs, however significant challenges remain with the leading candidate materials. Complex metal hydrides, in particular BH₄^- derived materials, have gained recent attention due to their high ambient temperature lithium ionic conductivities and good stability with lithium metal.^[2] 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.^[3,4] 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 (Li₂NH), lithium amide (LiNH₂) and lithium nitride hydride (Li₄NH), all dominated by a disordered anti-fluorite phase. 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 solid electrolyte materials. We herein exhibit the first demonstration of a Li-N-H material in an ASSB, with the materials displaying high lithium ion conductivities, wide electrochemical stability windows and potential for further tuning for this application.

Ultimately, these results identify Li-N-H based complex hydrides as an interesting avenue of research for solid electrolyte application that is yet to be fully explored. We hope this presentation will highlight these materials and spark further development such as has been seen for oxide and sulphide based fast-ion conductors.

1) Z. Zhang, Y. Shao, B. Lotsch, Y. Hu, H. Li, J. Janek, L F. Nazar, C. Nan, J. Maier, M. Armand and L. Chen, Energy Environ. Sci., 2018, 11, 1945-1976

2) A. Unemoto, M. Matsuo, and S. Orimo, Adv. Funct. Mater., 2014, 24, 2267-2279

3) W. Li, G. Wu, C. M. Araujo, R. H. Scheicher, A. Blomqvist, R. Ahuja, Z. Xiong, Y. Fengb and P. Chen, Energy Environ. Sci., 2010, 3, 1524-1530

4) J. W. Makepeace, J. M. Brittain, A. S. Manghnani, C. A. Murray, T. J. Wood and W. I. F. David, Phys. Chem. Chem. Phys., 2021, 23, 15091