Macromolecule-Enriched Solvation Enabling High-Voltage Sodium-Ion Batteries

Abstract

Sodium-ion batteries (SIBs) are emerging as a viable alternative for sustainable and cost-effective energy storage, yet their energy density is curtailed by relatively low voltage outputs (< 4 V) due to the lack of high-voltage electrolytes. Here, for the first time, we describe a high-voltage Na⁺ electrolyte featuring a macromolecule-enriched solvation architecture. The vulnerable small molecules in the Na⁺ solvation shell are replaced by macro polyamide (PA) molecules with high thermodynamic resilience, ensuring a wide electrochemical stability window for the electrolytes with suppressed oxidative/reductive decomposition. Concomitantly, the anions engage in H-bonding with the amido groups of PA, which not only stabilizes the anions against hydrolysis, but also delivers a high Na⁺ transference number of 0.93. Importantly, the nitrogen-rich composition of the macromolecule-enriched electrolyte (MEE) fosters the formation of robust nitride interphases that impart enduring stability to both the cathode and anode. As a result, the hard carbon (HC) || NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) full cells demonstrate significant rechargeability even with an ultrahigh cutoff voltage of 4.4 V. Our approach distinctively avoids the use of fluorinated molecules typically found in (localized-) high-concentration electrolytes, presenting a novel principle that could revolutionize high-voltage electrolyte design.