Carbon Materials as Lithium-Free Anode for Hybrid Lithium-Ion/Metal Batteries: Mechanism, Design Strategy, In Situ Characterization, and Prospects.

Carbon based anodes that rely solely on Li-ion storage are inherently limited in energy density, whereas systems dominated by Li-metal storage suffer from poor reversibility and short cycle life. Hybrid Li-ion/Li-metal batteries (LIB/LMBs), enabled by carbon-based Li-free anodes, offer a promising pathway by integrating Li-ion intercalation and Li-metal plating/stripping within a single framework. Despite rapid progress, the practical implementation of hybrid LIB/LMBs remains hindered by low Coulombic efficiency, unstable solid electrolyte interphase, dendrite growth, and large volume fluctuations. These challenges are fundamentally associated with irreversible Li loss and interfacial instability, which limit long-term cycling and energy efficiency. In this review, we systematically summarize recent advances in carbon-based Li-free anodes for hybrid LIB/LMBs, with a focus on (i) the hybrid storage mechanism, (ii) rational design of carbon materials, (iii) interface engineering strategies, (iv) mechanistic insights from in situ characterization, and (v) critical perspectives toward practical deployment. Particular emphasis is placed on the relationship between Coulombic efficiency, Li inventory retention, and system-level performance. This review provides a unified framework for understanding hybrid storage chemistry and offers strategic guidance for the development of next-generation high-energy-density batteries.