Small but mighty: Empowering sodium/potassium-ion battery performance with S-doped SnO2 quantum dots embedded in N, S codoped carbon fiber network

SnO2 has been extensively investigated as an anode material for sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) due to its high Na/K storage capacity, high abundance, and low toxicity. However, the sluggish reaction kinetics, low electronic conductivity, and large volume changes during charge and discharge hinder the practical applications of SnO2-based electrodes for SIBs and PIBs. Engineering rational structures with fast charge/ion transfer and robust stability is important to overcoming these challenges. Herein, S-doped SnO2 (S-SnO2) quantum dots (QDs) (approximate to 3 nm) encapsulated in an N, S codoped carbon fiber networks (S-SnO2-CFN) are rationally fabricated using a sequential freeze-drying, calcination, and S-doping strategy. Experimental analysis and density functional theory calculations reveal that the integration of S-SnO2 QDs with N, S codoped carbon fiber network remarkably decreases the adsorption energies of Na/K atoms in the interlayer of SnO2-CFN, and the S doping can increase the conductivity of SnO2, thereby enhancing the ion transfer kinetics. The synergistic interaction between S-SnO2 QDs and N, S codoped carbon fiber network results in a composite with fast Na+/K+ storage and extraordinary long-term cyclability. Specifically, the S-SnO2-CFN delivers high rate capacities of 141.0 mAh g(-1) at 20 A g(-1) in SIBs and 102.8 mAh g(-1) at 10 A g(-1) in PIBs. Impressively, it delivers ultra-stable sodium storage up to 10,000 cycles at 5 A g(-1) and potassium storage up to 5000 cycles at 2 A g(-1). This study provides insights into constructing metal oxide-based carbon fiber network structures for high-performance electrochemical energy storage and conversion devices.