High Initial Coulombic Efficiency Hard Carbon Anodes Enabled by Facile Surface Annealing Engineering

Hard carbon (HC) anode shows great potential due to its high capacity and excellent rate performance. However, state-of-the-art HC anode still suffers insufficient initial Coulomb efficiency (ICE) due to the abundant Li-trapping sites. Herein, we demonstrate a facile annealing engineering for HC anodes to improve the ICE and the mechanism is systematically studied. Accordingly, during the annealing process, metastable O- and N-containing functional groups are pyrolyzed, which cause the microstructure reconstruction of HC. Therefore, irreversible lithium ions adsorption is reduced significantly and the conversion of sp³ to sp² C contributes to the localized graphitization of HC. Consequently, the optimized HC achieves ultra-high ICE of 90% from initial 61%. It is demonstrated that HC will adsorb H₂ O and some organic species from environment gradually, causing conversion of some electrochemical stable functional groups to the irreversible Li-trapping sites. This work provides facile strategy and novel insight for high ICE HC anodes.