Ultrafine 3D Nanoporous Carbon Coated Multicomponent Sulfide Heterojunction for Enhanced Na Ion Storage

Transition metal sulfides are promising sodium-ion battery anodes. Herein, a multicomponent MnNiFe-based sulfide heterojunction with ultrafine nanoporous structure and inherent 3D carbon coating is synthesized using commercially available MnNiFeCO₃ as the precursor. It is found that the multicomponent metal interaction greatly inhibits the surface diffusion during carbonate decomposition process, resulting in ultrafine nanoporous MnNi(Fe)O₃ (≤ 10 nm) with a much larger surface area when compared with a single metal-based counterpart. To enhance the conductivity and structure stability of the heterojunction, a thin layer of N, S-doped 3D porous carbon is inherently coated on the nanoporous MnNiFeS_x (NS-C@MnNiFeS) by first forming a 3D polyaniline layer on the nanoporous MnNi(Fe)O₃, which is then transformed into doped carbon during the vulcanization process. Owing to the multicomponent heterojunction design (which may provide more active sites for effective Na⁺ adsorption), much increased specific surface area, and 3D inherent carbon coating, the NS-C@MnNiFeS exhibits significantly enhanced performance for Na⁺ storage. After 90 cycles at 0.1 A g⁻¹, a record-high reversible capacity of 859.2 mAh g⁻¹ is achieved, and it can be stably cycled for 1500 cycles at a current density of 10.0 A g⁻¹. DFT calculation reveals the modified electronic structure and enhanced Na⁺ adsorption by constructing the heterojunction.