Heterostructured Solid Electrolyte Interphase Enables Facilitated Kinetics for Low-Temperature Sodium-Ion Batteries

Ether-based electrolytes demonstrate competitiveness in sodium-ion batteries (SIBs) operating at low temperatures due to their high ionic conductivities and low desolvation energies. However, the accumulation of inorganic components in the inner solid electrolyte interphase (SEI), caused by substantial sodium salt decomposition, impedes the kinetics of Na⁺ interphasial transport. Here, a NaF/Na₂CO₃-rich heterostructured SEI, with significantly improved Na⁺ diffusion kinetics, is proposed in a dilute ether-based electrolyte. The heterostructure provides additional Na⁺ transport pathways through Na₂CO₃ and along the NaF/Na₂CO₃ interface. The interfacial synergy effect between Na₂CO₃ and NaF effectively reduces the Na⁺ diffusion energy barrier and improves the mechanical robustness of SEI, ensuring facilitated interphasial kinetics. Consequently, the hard carbon (HC) anode delivers a high reversible specific capacity of 238 mAh g⁻¹ at −40 °C and an impressive cycling stability at −20 °C with a capacity retention of 89.67% after 1800 cycles at 1 C. Additionally, the Na_0.85Li_0.12Ni_0.22Mn_0.66O₂||HC full cells exhibit a high discharge capacity of 85 mAh g⁻¹ at −20 °C (85% of its room-temperature capacity). This work underscores the critical role of engineering SEI with fast Na⁺ transport kinetics for SIBs operating under low temperatures.