Compositions were measured using a Thermo iCAP RQ ICP-MS with a 3% HNO3 (aq) diluting matrix. All electrodes were dissolved in 8 mL of aqua regia after rinsing at least 6 times with at least 10 mL of pure acetonitrile. The Li concentration measurements were performed by diluting the sampled acetonitrile solution 100 times (Supplementary 24). The selectivity is calculated by [ΔLi/ΔNa]/[Liinitial/Nainitial] where the change in Li + and Na + composition between intercalation and deintercalation states in the electrode is called ΔLi and ΔNa, respectively. Liinitial and Nainitial refer to the initial Li + and Na + composition in the solution.