Hard Carbon Structural Engineering Enables Sodium Cluster Capture with Enhanced Adsorption-Insertion-Filling Storage...

The development of low-potential plateau capacity in hard carbon (HC) negative electrodes is a key route toward enhancing the energy density of sodium-ion batteries. However, owing to the intrinsic structural complexity of HC, the microstructural origin of sodium storage in the low-potential region remains elusive. Here, a direct correlation between microstructure and electrochemical behavior is established, revealing that sodium storage proceeds via the formation of quasi-metallic Na⁺ clusters accommodated within closed pores and regulated by surface mesoporous architectures, thereby resolving longstanding mechanistic ambiguities associated with the low-potential plateau. In contrast, microporous-dominated surfaces fail to effectively activate plateau capacity, whereas mesoporous-dominated structures construct efficient Na⁺ diffusion pathways that enable rapid access to closed-pore storage sites. As a result, the plateau capacity is enhanced by 82.5% relative to microporous structures, accompanied by a pronounced increase in the initial discharge capacity from 168.4 to 347.7 mAh g^-1. In situ electrochemical impedance spectroscopy combined with relaxation time distribution analysis further confirms the irreversible evolution of the solid electrolyte interphase and its role in stabilizing interfacial kinetics. Moreover, in situ Raman spectroscopy, integrated with multimodal structural characterizations, unambiguously verifies a multistep "adsorption-intercalation/filling" sodium storage mechanism. This work provides fundamental insights into the role of surface pore structures in HC, thereby guiding the rational design of high-performance HC negative electrodes.