A current limitation to improving the volumetric energy density of Na-ion batteries is the low density of the hard carbon(HC) anode. This problem can be solved by using high-density, high-capacity materials like SnS, which reacts with Na over a combined conversion and alloying reaction that theoretically provides 1022 mAh g−1 and 5335 mAh cc−1(materials level). Here, composites containing SnS and thermally activated graphite(t-G) are prepared by ball-milling and tested with different electrolyte solutions. Adding 5 wt.% of t-G is sufficient to obtain significant improvements in capacity and cycle life, reaching 608 mAh g−1 initially and 439 mAh g−1 after 100 cycles. Even without calendaring, the obtained volumetric capacity of 283 mAh cc−1 (electrode level) is already on-par with commercial HC electrodes. Moreover, ether-based electrolytes are found to be superior to ester-based electrolytes, enabling high storage capacity and cycle life. The reaction is investigated by operando X-ray diffraction and operando dilatometry. The inferior performance in ester-based electrolytes is found to be due to a larger polarization that largely prevents the alloying reaction that occurs close to 0 V. Over cycling, the conversion reaction becomes gradually inactive while the alloying reaction shows a much better degree of reversibility.
Wang et al. (Wed,) studied this question.