Confined interfacial microenvironment design of hard carbon anodes for wide-temperature sodium-ion batteries

Low-concentration electrolytes hold significant potential for the development of cost-effective sodium-ion batteries (SIBs), whereas they face persistent challenges in sustaining the cycling stability of hard carbon anodes, especially under wide-temperature operation. This issue arises from the poorly controlled solid electrolyte interphase (SEI) formed in low-concentration electrolytes, typically featured by an inhomogeneous, fragile, and kinetically sluggish inorganic inner layer. Herein, we report a confined interfacial microenvironment design strategy by reconstructing a low-concentration ether electrolyte using a trace amount of anionic surfactant (sodium dodecyl sulfate, SDS). SDS is proposed to spontaneously adsorb and enrich at the hard carbon-electrolyte interface, creating a locally concentrated and confined interfacial microenvironment. Spectroscopic and electrochemical analyses indicate that this interfacial enrichment reshapes the local coordination/association environment of Na⁺ during interfacial transport and desolvation, thereby promoting anion-involved interphase formation. This generates a thin SEI composed of an inner-layer rich in inorganic NaF and Na₂S and an organic out-layer, achieving a rigid-flexible integrated interphase to mitigate high-temperature interfacial instability and low-temperature sluggish kinetics. Consequently, the assembled SIBs show an improved cycling stability from a low capacity retention of 47.61% after 250 cycles to a high value of 91.44% after 350 cycles and demonstrate wide-temperature adaptability ranging from −15 to 50 °C. This study provides a promising interfacial microenvironment design strategy to address the instability challenge of hard carbon for high-performance wide-temperature SIBs.