Assessment on the Use of High Capacity “Sn₄P₃”/NHC Composite Electrodes for Sodium‐Ion Batteries with Ether and Carbonate Electrolytes

Abstract This work reports the facile synthesis of a Sn–P composite combined with nitrogen doped hard carbon (NHC) obtained by ball‐milling and its use as electrode material for sodium ion batteries (SIBs). The “Sn₄P₃”/NHC electrode (with nominal composition “Sn₄P₃”:NHC = 75:25 wt%) when coupled with a diglyme‐based electrolyte rather than the most commonly employed carbonate‐based systems, exhibits a reversible capacity of 550 mAh g_electrode⁻¹ at 50 mA g⁻¹ and 440 mAh g_electrode⁻¹ over 500 cycles (83% capacity retention). Morphology and solid electrolyte interphase formation of cycled “Sn₄P₃”/NHC electrodes is studied via electron microscopy and X‐ray photoelectron spectroscopy. The expansion of the electrode upon sodiation (300 mAh g_electrode⁻¹) is only about 12–14% as determined by in situ electrochemical dilatometry, giving a reasonable explanation for the excellent cycle life despite the conversion‐type storage mechanism. In situ X‐ray diffraction shows that the discharge product is Na₁₅Sn₄. The formation of mostly amorphous Na₃P is derived from the overall (electro)chemical reactions. Upon charge the formation of Sn is observed while amorphous P is derived, which are reversibly alloying with Na in the subsequent cycles. However, the formation of Sn₄P₃ can be certainly excluded.