Electrochemical energy storage beyond lithium is highly relevant for sustainable energy technology. New electrode concepts are needed for the intercalation of larger monovalent (Na+, K+) or multivalent ions (Mg2+).
One example for a promising Na-ion battery is presented based on symmetrical NASICON-structured Na2VTi(PO4)3 electrodes [1].  operando synchrotron diffraction and absorption spectroscopy unravel the underlying sodium storage and charge compensation mechanisms.
Model systems for multivalent-ion insertion are also hybrid batteries with two mobile metal ions in the electrolyte, where Mg is plated at the negative electrode, while Li- or Na-ions are inserted at the positive electrode [2,3]. Recent results on the working mechanisms in such hybrid batteries are revealed by operando synchrotron diffraction and ex situ XPS.
Appropriate material combinations for Mg-batteries with insertion-type positive electrodes and sufficiently high cell voltages are still lacking. For example, V2O5 works only with unstable electrolytes in contact with Mg-metal and steel housing. The Mg-insertion mechanism was therefore investigated for a full cell with MgxMo6S8 as a suitable negative electrode [4].

This work contributes to research performed at CELEST (Center for Electrochemical Energy Storage Ulm-Karlsruhe) and was funded by the German Research Foundation (DFG) under Project ID 390874152 (POLiS Cluster of Excellence)

1. Wang, D. et al., Nature Communications 2017, 8, 15888.
2. Bian, X. et al., Mater. Chem. A 2017, 5, 600.
3. Fu, Q. et al., Electrochim. Acta 2018, 277, 20.
4. Fu, Q. et al., J. Am. Chem. Soc. 2019, 141, 2305.