Revised Solvent Rule Unlocks Non-Conventional Solvents for Na+-Solvent Co-Intercalation in Graphite.

Solvent selection for electrochemical Na+-solvent co-intercalation in graphite has been constrained by an empirical rule requiring both high reductive stability and strong solvating power, thereby historically confining viable solvent candidates primarily to glyme-based ethers. Here, we re-examine this solvent selection rule and reveal that the cyclic consumption of Na+-solvent complexes at the graphite electrode and their regeneration at the counter electrode leads to a net cancellation of solvation energy in coupled electrode systems, thereby decoupling solvation power from co-intercalation feasibility. To validate this decoupling, we designed a fluorinated ether with deliberately attenuated solvating power but high reductive stability, as a proof-of-concept solvent. Successful co-intercalation was observed with this weakly solvating fluorinated ether, whereas typical carbonate solvents with superior solvation capability failed due to reductive decomposition above the intercalation threshold, confirming that solvation strength is not the determining factor. Leveraging this insight, we further designed and identified aminated ethers as a previously unexplored subclass of ethers capable of co-intercalation. This work revises the empirical dual-factor rule and eliminates solvation capability as a constraint, which broadens the range of viable solvents for graphite-based sodium-ion batteries.