Core-Shell: Resolving the Dilemma of Hard Carbon Anodes by Sealing Nanoporous Particles With Semi-Permeable Coatings

A core-shell strategy is introduced to overcome the dilemma of common non-graphitic hard carbon anodes, linking high reversible storage capacity to practically unacceptable irreversible losses in the first cycle(s). Just as graphite homogeneously combines effective lithium storage with an electrolyte solvent-sieving function, we show that both of these functions could be strategically integrated into non-graphitic carbons in a heterogeneous structure. Highly porous activated carbons are sealed by kinetically tuned gas-phase deposition of non-graphitic carbon to form a functional core-shell structure. Gas sorption porosimetry on core, shell, core-shell, and cracked core-shell particles confirms preserved core porosity and a semi-permeable shell. Diethyl carbonate sorption analysis is introduced as a more suitable probe than N₂ or CO₂ sorption, linking first-cycle losses to the liquid-solid interface of carbon anodes. The functional core-shell particles with much reduced diethyl carbonate uptake allow for high storage capacity and reduced first cycle losses. Delivering 400 ± 24 mAh g^{- 1} with 82 ± 2% first-cycle reversibility, it is shown that three-stage Na storage in designed core-shell anodes can compensate for the larger size of sodium compared to lithium stored in graphite anodes (372 mAh g^-1). The designed core-shell anodes show state-of-the-art performance with commercial promise.