In situ pore sealing of cellulose derived hard carbon via coal tar cross linking for high capacity sodium ion batteries

The plateau capacity of hard carbon (HC) dictates the energy density of sodium ion batteries, but is severely limited by open pore defects generated during the pyrolysis of precursors. Herein, we propose an in-situ pore sealing strategy to successfully construct a closed pore enriched HC via the synergistic co-pyrolysis of sodium carboxymethyl cellulose (CMC) and coal tar (CT). During the pyrolysis process, the low softening point CT uniformly coats the surface of the CMC skeleton and constructs a dense cross-linked network with the radicals generated from CMC pyrolysis, effectively retarding the rapid escape of volatiles. Concurrently, leveraging the thermal shrinkage effect of CT at elevated temperatures, the nascent open pores are seamlessly sealed in situ. This interfacial synergy endows the material with an exceptional closed pore volume of 0.1280 cm³ g⁻¹ and an average pore diameter of 2.99 nm. Furthermore, the induced long range pseudo graphitic nanoribbons provide ultrafast Na⁺ diffusion channels, facilitating the dense accumulation of quasi metallic sodium clusters within these closed pores. Consequently, the CMC-CT anode has 343.98 mAh g⁻¹ high capacity and retains 84.26% capacity after 1000 cycles at 1500 mA g⁻¹. Validated by a stable Na₃V₂(PO₄)₃‖CMC-CT full cell, this scalable pore engineering paradigm holds great promise for next generation energy storage.