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

Abstract 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||Na 3 V 2 (PO 4 ) 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.