Activating Free Volume of Polymeric Aggregates toward Advanced Hard Carbon for Sodium Storage

Constructing closed pore structure through precursor modification has been regarded as an effective route for boosting plateau capacity in hard carbon (HC) anodes for sodium-ion batteries (SIBs). However, the mechanistic role of polymer aggregation behavior in the closed pore formation in polymer-derived HCs remains poorly understood. Herein, flexible chain conformation has been incorporated into phenolic resin network to weaken the aggregated state and promote closed pores formation in the polymer-derived HC for the first time. During the stepwise immersion in an ethanol/water solvent, the extension of crystalline domains in polyethylene glycol segments can reduce the multichain aggregation as well as activate more free volumes within the polymer backbone. More free volumes can mitigate pyrolytic cross-linking reactions and facilitate the multiple releasing of volatile byproducts, which construct plentiful closed pore structure with ultra-small pore size during pyrolysis. As a result, the as-obtained HC anode demonstrates a high reversible capacity (357.9 mAh g ^− 1 at 0.1 C), enhanced rate performance (168.9 mAh g ^− 1 at 5 C) as well as excellent cycling stability over 2000 cycles at 4 C. This work provides a valuable insight into aggregate chemistry toward the development of high-performance HC anodes for advanced SIBs.