Promoting the reversibility of lithium ion/lithium metal hybrid graphite anode by regulating solid electrolyte interface

Highlights: • The influence of SEI on hybrid graphite anode were studied by in situ AFM, in situ Raman, XPS and TEM. • An ideal SEI with high reversibility and stability was formed by regulating structural uniformity in LiFSI electrolyte. • The designed SEI enables superior lithium-ion (de)intercalation and lithium plating/striping behaviors during cycling. Recent research revealed graphite anode can not only be used to intercalate lithium ions but also plate lithium metal on them, leading to the development of graphite-based lithium ion/metal hybrid anode with capacity more than twice that of the lithium ion battery system. Currently, there is limited understanding on this system, consequently suitable strategies to further enhance and stabilize the capacity are still lacked. In this work, the influence of electrolytes, i.e. lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF₆) in the carbonate solvents, on the evolution of solid electrolyte interface (SEI) and electrochemical performance were studied via in situ electrochemical atomic force microscopy (EC-AFM). An ideal SEI with high reversibility and mechanical/chemical stability was finally achieved by regulating structural uniformity in the LiFSI-based electrolyte. The graphite anode with the as-designed SEI exhibited a high initial capacity of 1531 mAh g⁻¹ and columbic efficiency higher than 90% after 120 cycles, much higher than the reported graphite-based lithium ion or metal anode. The capacity degradation mainly originates from the irreversible lithium (de)intercalation processes. In addition, the full cells of graphite anode pre-deposited with limited Li capacity coupling with LiFePO₄ cathode achieved stable cycling performance. This work highlights the reversible capacity of the hybrid graphite anode can be greatly improved via suitable electrolyte design and the corresponding SEI structural regulation.