Solvated Sodium Storage via a Coadsorptive Mechanism in Microcrystalline Graphite Fiber

Abstract Graphite has been widely accepted for its reversible solvated sodium cointercalation mechanism into the graphite layers in ether‐based electrolytes. However, the cointercalation suffers from insufficient Coulombic efficiency with high redox potentials, which significantly limits its energy output. Herein, instead of the conventional solvated Na⁺ cointercalation into the graphite, a new coadsorptive mechanism is proposed through the microcrystalline graphite fiber (MCGF), which can reversibly store the solvated Na⁺ at the ribboned grain boundaries and in the mesopores of the MCGF. The mechanism is manifested by various advanced spectroscopy techniques, including in situ synchrotron small‐angle X‐ray scattering to track the long‐periodic structure evolution and ex situ synchrotron X‐ray absorption fine structure to verify the interaction of solvated Na and graphite layers. The origin of the boosted rate‐capability and reversibility is further revealed by density functional theory simulations and aberration‐corrected transmission electron microscopy. As a proof‐of‐concept, the MCGF electrode exhibits high initial coulombic efficiency (92.5%), fast‐charging (within 4 min), and enhanced cycling stability (≈98% retention after 800 cycles). The results provide a new understanding of the sodium storage mechanism in graphite‐based materials, which may inspire further exploration of carbon electrodes for Na‐ion batteries.