SnS Anodes with High Volumetric Capacity for Na‐ion Batteries and Their Characterization in Ether and Ester Electrolytes

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.