Hollow Carbon Spheres from Nano to Micron Sizes Derived from MOFs and Polystyrene for Sodium-Ion Storage

Hard carbons are considered the most viable electrodes for sodium-ion batteries (SIBs), but the plating at lower voltages (<0.1 V vs Na+/Na) has become the key issue in their practical implementation. The sodiation at lower voltage leads to dendritic growth in hard carbons and raises safety concerns, which drives the attention of scientists toward the sloping storage regions of batteries. The development of micro- and nanostructurally controlled carbonaceous materials operating at >0.1 V vs Na+/Na with a capacity comparable to hard carbons becomes necessary for next-generation SIBs. Herein, for the first time, we propose a powerful physicochemical reconfiguration tactic to achieve nanoporous hollow carbon spheres (HCS) of different sizes by coating zeolitic imidazolate framework-67 nanoparticles over the growth kinetics-controlled polystyrene spheres. The robust hollow structure exhibits the complete pseudocapacitive Na+ storage mechanism in the sloped voltage region with a capacity comparable to that of hard carbons. The optimized HCS of 900 nm size with a 403.35 m2 g–1 surface area gives 309 mAh g–1 capacity at 0.03 A g–1 and exhibits the ability to serve for 1000 cycles at 1 A g–1. The unified ex situ analysis elucidated the charge storage mechanism and dendrite-free reversible Na+ ion dynamics within the amorphous carbon matrix. These comparative electrochemical investigations illuminate the critical role of size-controlled hollow-porous design in enhancing ionic transport and mechanical resilience, positioning HCS as a forefront anode candidate for SIBs.