Hard carbon has been extensively investigated as a promising anode material for sodium-ion
batteries; however, its use as an active anode material for lithium-ion batteries (LIBs) has
received comparatively little attention. This neglect largely stems from its early replacement
by graphite more than three decades ago and the limited progress since then in developing a
fundamental understanding of hard carbon. Owing to its highly complex microstructure and
the diverse microstructural motifs that emerge depending on the synthesis route, elucidating
lithium and sodium storage mechanisms in hard carbon remains a major challenge. Recently,
our group demonstrated that hard carbon possesses exceptional potential as an LIB anode,
exhibiting electrochemical performance that can surpass the reversible capacity and overall
lithium-storage characteristics of conventional Li–graphite intercalation compounds. Herein,
we systematically investigate the lithium storage mechanism of hard carbon and further
delineate the similarities and differences relative to sodium storage behaviour. On this basis,
we establish practical design guidelines for high-performance hard carbon anodes not only
for sodium-ion batteries but also for LIBs. By precisely tailoring the microstructure, we
realize a high-capacity hard carbon anode delivering approximately twice the reversible
lithium capacity of graphite, while simultaneously achieving excellent rate capability and
stable cycling performance.