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Hard carbon (HC) is an attractive anode material for grid-level sodium-ion batteries (NIBs) due to the widespread availability of carbon, its high specific capacity, and low electrochemical working potential. However, the issues of low first cycle Coulombic efficiency and poor rate performance of HC need to be addressed for it to become a practical long-life solution for NIBs. These drawbacks appear to be electrolyte dependent, since ether-based electrolytes can largely improve the performance compared with carbonate electrolytes. An explanation for the mechanism behind these performance differences is critical for the rational design of highly reversible sodium storage. Combining gas chromatography, Raman spectroscopy, cryogenic transmission electron microscopy, and X-ray photoelectron spectroscopy, this work demonstrates that the solid electrolyte interphase (SEI) is the key difference between ether- and carbonated-based electrolyte, which determines the charge transfer kinetics and the extent of parasitic reactions. Although both electrolytes show no residual sodium stored in the HC bulk structure, the uniform and conformal SEI formed by the ether-based electrolyte enables improved cycle efficiency and rate performance. These findings highlight a pathway to achieve long-life grid-level NIBs using HC anodes through interfacial engineering.