Modulating Na-ion Solvation in Carbonate-based Electrolytes by Nitrogen-doped Carbon Dots Enables Superior Na Metal Batteries

Abstract

Sodium (Na) anode for Na-metal batteries (SMBs) has gained significant attention for its high theoretical capacity and the lowest redox potential. However, challenges caused by random growth of Na dendrites and unstable solid-electrolyte-interphase (SEI) layers impede the practical applications for SMBs in carbonate-based electrolytes. In this study, we propose a novel additive, nitrogen-doped carbon quantum dots (NCQDs) in the carbonate-based electrolyte. NCQDs modify the solvation structures among Na-ion, ${\text{PF}}_6^{-}$ anion and solvent molecules. The negatively charged NCQDs exhibit a strong affinity toward Na-ions, impeding interactions between ${\text{PF}}_6^{-}$ anions and solvent molecules. This coordination between NCQDs and Na-ions slightly alters the solvation environment of the electrolyte, leading to a weak solvation structure. This electrolyte induces densely inorganic-rich SEI layers and thus improves the electrochemical performance. Our approach ensures a stable cycling of the Na metal anode for cycling up to 700 hrs at 0.5 mA cm^-2 in Na||Na batteries, along with an average Coulombic efficiency (91 %) at 1 mA cm^-2 in Na||Cu batteries. Moreover, practical SMBs with P2-Na_0.67Ni_0.33Mn_0.67O₂ demonstrate stable cycling stability (93 % for 110 cycles), even operating up to 4.3V. This study enables high-voltage SMBs and provides guiding principles in electrolyte design for Na-based batteries.