Pre-sodiation strategies for high reversibility of hard carbon anodes: a review

Sodium-ion batteries (SIBs) represent a promising next-generation energy storage technology, yet their commercialization is impeded by low initial Coulombic efficiency, which severely limits energy density. This issue stems from irreversible sodium consumption during cycling, primarily caused by solid electrolyte interphase (SEI) formation and parasitic side reactions. Such sodium loss not only reduces energy output but also accelerates electrode degradation, thereby shortening cycle life. Pre-sodiation has emerged as an essential strategy to mitigate these challenges by introducing an external sodium source to compensate for irreversible sodium loss, thereby improving both energy density and long-term cyclability. However, developing efficient, scalable, and economically viable pre-sodiation methods remains a critical hurdle. This review is motivated by the urgent need to systematize and evaluate emerging pre-sodiation strategies specifically for hard carbon anodes—a leading candidate in SIB applications—and to provide strategic guidance for future research. We comprehensively summarize and analyze both conventional and novel pre-sodiation methods, correlating their mechanisms with the fundamental sources of sodium loss. Furthermore, we compare the practicality and scalability of recent advances, identify key technical barriers, and propose a holistic framework combining complementary sodium supplementation and storage approaches. Finally, we offer forward-looking perspectives on the development of industrially applicable pre-sodiation techniques, underscoring their vital role in achieving high-energy–density and long-life sodium-ion batteries.