Long cycle life and high rate sodium-ion chemistry for hard carbon anodes

As the most promising anode material for Na-ion batteries, hard carbons suffer both poor rate capability and cycling stability, which restrict its practical application. These challenges come not only from the carbonaceous electrode structure, but also from how to tailor-design an electrolyte that can support high sodium ion transport while simultaneously providing a protective solid-electrolyte-interphase (SEI). Here we develop a new strategy to decouple the "bulk ion transport" and "interphasial" requirements for electrolytes. By pre-engineering a "foreign SEI" from an ester-based electrolyte, we successfully stabilized the hard carbon anodes in ether-based electrolyte and realized significant improvement in electrochemical performance. At a high rate of 500 mA g(-1), a capacity of 200 mA h g(-1) was retained for over 1000 cycles without detectable capacity fading. Such an outstanding performance is attributed to the "foreign SEI" approach, which makes it possible for a synergistic cooperation between the rapid diffusion of sodium ions in ether-based electrolytes and a protective ester-originated interphase.