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

Abstract 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) (≈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.