ALD-Coated HfO ₂ Artificial SEI Improves Wettability and Cycling Stability of Hard Carbon Anodes

Ultrathin oxide coatings are a promising route to stabilize hard-carbon (HC) anodes in sodium-ion batteries, yet the role of high dielectric constant HfO 2 interlayers remains unstudied. Here, we employ atomic layer deposition (ALD) to conformally coat HC with 5 and 10 nm HfO 2 (HC−HfO 2 −5 and HC−HfO 2 −10) and systematically compare them with uncoated HC. XRD and SEM show that the coatings do not alter the turbostratic carbon framework, while EDS mapping confirms homogeneous HfO 2 coverage. Contact-angle measurements reveal greatly enhanced wettability (13° for uncoated HC vs. 7° and 0° for 5 and 10 nm HfO 2 ), and EIS demonstrates markedly lower interfacial resistance for coated electrodes. Both HC−HfO 2 −5 and HC−HfO 2 −10 retain the typical sloping/plateau Na-storage profile and deliver initial reversible capacities of ∼300 mAh g −1 with ∼80% initial Coulombic efficiency. Long-term cycling at 0.2 A g −1 affords capacity retentions of 66.6 and 67.9% over 200 cycles for 5 and 10 nm coatings, respectively, whereas uncoated HC suffers pronounced capacity decay after ∼90 cycles, indicative of unstable SEI reconstruction. Analysis of the slope/plateau contributions reveals that HfO 2 preserves electrochemically active nanopores and stabilizes high-energy-density plateau-type Na storage, rather than merely maintaining overall capacity. ALD HfO 2 functions as an artificial interphase that moderates interfacial reactions and helps preserve nanopore-associated plateau storage. It improves wettability, moderates SEI growth, and maintains plateau-type Na storage, with 5 nm identified as an optimal thickness that balances interfacial thickness and ion-transport resistance.