Precisely Tunable Instantaneous Carbon Rearrangement Enables Low‐Working‐Potential Hard Carbon Toward Sodium‐Ion Batteries with Enhanced Energy Density

As the preferred anode material for sodium-ion batteries, hard carbon (HC) confronts significant obstacles in providing a long and dominant low-voltage plateau to boost the output energy density of full batteries. The critical challenge lies in precisely enhancing the local graphitization degree to minimize Na⁺ ad-/chemisorption, while effectively controlling the growth of internal closed nanopores to maximize Na⁺ filling. Unfortunately, traditional high-temperature preparation methods struggle to achieve both objectives simultaneously. Herein, a transient sintering-involved kinetically-controlled synthesis strategy is proposed that enables the creation of metastable HCs with precisely tunable carbon phases and low discharge/charge voltage plateaus. By optimizing the temperature and width of thermal pulses, the high-throughput screened HCs are characterized by short-range ordered graphitic micro-domains that possess accurate crystallite width and height, as well as appropriately-sized closed nanopores. This advancement realizes HC anodes with significantly prolonged low-voltage plateaus below 0.1 V, with the best sample exhibiting a high plateau capacity of up to 325 mAh g^-1. The energy density of the HC||Na₃V₂(PO₄)₃ full battery can therefore be increased by 20.7%. Machine learning study explicitly unveils the "carbon phase evolution-electrochemistry" relationship. This work promises disruptive changes to the synthesis, optimization, and commercialization of HC anodes for high-energy-density sodium-ion batteries.