Engineering homotype heterojunctions in hard carbon to induce stable solid electrolyte interfaces for sodium-ion batteries

Developing effective strategies to improve the initial Coulombic efficiency (ICE) and cycling stability of hard carbon (HC) anodes for sodium-ion batteries is the key to promoting the commercial application of HC. In this paper, homotype heterojunctions are designed on HC to induce the generation of stable solid electrolyte interfaces, which can effectively increase the ICE of HC from 64.7% to 81.1%. The results show that using a simple surface engineering strategy to construct a homotypic amorphous Al₂O₃ layer on the HC could shield the active sites, and further inhibit electrolyte decomposition and side effects occurrence. Particularly, due to the suppression of continuous decomposition of NaPF₆ in ester-based electrolytes, the accumulation of NaF could be reduced, leading to the formation of thinner and denser solid electrolyte interface films and a decrease in the interface resistance. The HC anode can not only improve the ICE but elevate its sodium storage performance based on this homotype heterojunction composed of HC and Al₂O₃. The optimized HC anode exhibits an outstanding reversible capacity of 321.5 mAh g⁻¹ at 50 mA g⁻¹. The cycling stability is also improved effectively, and the capacity retention rate is 86.9% after 2000 cycles at 1 A g⁻¹ while that of the untreated HC is only 52.6%. More importantly, the improved sodium storage behaviors are explained by electrochemical kinetic analysis.