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An ultrathin Al2O3 layer of varying thickness is deposited on biomass-derived hard carbon by atomic layer deposition (ALD). This hard carbon is used as an anode for sodium-ion batteries. The structures and morphologies of the hard carbon remain unchanged even with an increase in the cycle number of Al2O3 ALD. Improved electrochemical performance of the hard carbon is obtained when 20 cycles of Al2O3 ALD are applied. Compared with the pristine samples, the initial irreversible capacity loss is reduced from 108 to 97 mAh g–1, and columbic efficiency and plateau region capacity are improved from 67 to 72% and 150 to 172 mAh g–1, respectively. The samples also exhibit a higher capacity, 296 mAh g–1, than the pristine sample, 277 mAh g–1, after 100 charge–discharge cycles at 0.2 C current density (1 C = 250 mA g–1). Moreover, the discharge capacity of the pristine samples increases from 100 to 130 mAh g–1 at 4 C rate. The enhanced electrochemical performances arise from the complete protection of the ultrathin Al2O3 layer on the electrode to alleviate solid electrolyte interphase (SEI) layer formation. Consequently, the SEI layer and charge transfer resistance are reduced. The Na-ion diffusivity below 0.1 V is then improved, which dominates the high rate performance.

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