Alexandra Morscher, Alex R. Neale, Matthew S. Dyer, John B. Claridge, Michael W. Gaultois, Andrij Vasylenko, Benjamin B. Duff, Laurence J. Hardwick, Paul M. Sharp, Ruiyong Chen, J. Felix Shin, Frédéric Blanc, Marco Zanella, Luke M. Daniels, Jacinthe Gamon, Matthew J. Rosseinsky, Vladimir V. Gusev, Department of Chemistry University of Liverpool, University of Liverpool, Stephenson Institute for Renewable Energy, Leverhulme Research Centre for Functional Materials Design, and We thank the UK Engineering and Physical Sciences Research Council (EPSRC) for funding through grant number EP/N004884. We are grateful for computational support from the UK Materials and Molecular Modelling Hub, which is partially funded by EPSRC (EP/P020194). We acknowledge the ISCF Faraday Challenge project: 'SOLBAT The Solid-State (Li or Na) Metal-Anode Battery' [grant number FIRG007], including partial support of a studentship to B.B.D., also supported by the University of Liverpool. We thank Diamond Light Source for access to beamline I11 (proposal CY23666) and Prof. Chiu Tang and Dr. Claire Murray for assistance on the beamline. We thank STFC for access to Polaris at ISIS Spallation Neutron Source (Xpress proposal 1990189) and Dr. Ron Smith for running the measurement. M.W.G. and V.G. thank the Leverhulme Trust for funding via the Leverhulme Research Centre for Functional Materials Design. M.J.R. thanks the Royal Society for a Research Professorship.