Moderately concentrated electrolyte improves solid–electrolyte interphase and sodium storage performance of hard carbon

Hard carbon (HC) is a promising anode for sodium-ion batteries. The current hurdles for the HC electrodes are insufficient coulombic efficiency (CE), rate capability, and cyclic stability. This study reveals that an intelligent electrolyte design can effectively overcome these limitations. The sodium salt, concentration, and solvent of the electrolytes are systematically investigated. Incorporation of ethylene carbonate (EC) in propylene carbonate (PC) electrolyte can promote the formation of contact ion pairs and ion aggregates between Na⁺ and FSI^–. At a moderate concentration, the 3 mol dm⁻³ NaFSI in PC:EC electrolyte with reasonable conductivity and viscosity can lead to the formation of a robust organic–inorganic balanced solid–electrolyte interphase, which is thoroughly examined by electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. With this, the first-cycle and steady-state CE of the HC electrode is increased to 85% and>99.9%, respectively, and the reversible sodiation/desodiation capacities at high rates are markedly improved. In addition, 95% of the initial capacity can be retained after 500 charge–discharge cycles. The proposed electrolyte represents a huge step towards HC electrodes with high effectiveness and durability for electrochemical Na⁺ storage.