In-Situ Confined MnO Quantum Dots within N-Doped Carbon Spheres as Long-Life Anodes for Sodium-Ion Batteries

We report ultrasmall MnO quantum dots (approximately 2 nm) uniformly confined within N-doped carbon spheres (MnOQD/NC) through a simple strategy involving in-situ spatial confinement followed by carbonization, and demonstrate its application as long-life anodes for Na storage. The quantum-dot-sized MnO nanoparticles are intimately embedded within nitrogen-doped carbon spheres (rather than surface-loaded), forming a rationally designed confined architecture which effectively accommodates the volume expansion of MnO, prevents aggregation of quantum dots, suppresses electrode pulverization, and provides highly efficient electron/ion transport pathways. Meanwhile, the combination of quantum-scale MnO (providing larger specific surface area) and N-doped carbon (offering abundant adsorption sites) leads to pseudocapacitance-dominated sodium storage behavior. Furthermore, ex-situ TEM and HRTEM characterizations demonstrate that this composite architecture exhibits remarkable structural reversibility and exceptional cycling stability. Hence, after 500 cycles at 0.2 A g −1 , the MnOQD/NC anode delivers high reversible specific capacity of 479.8 mAh g -1 at 0.2 A g −1 , outstanding rate capability of 229.2 mAh g -1 at 15 A g −1 and long life with a high capacity of 258.4 mAh g -1 after 4100 cycles at 10 A g −1 (capacity retention of 89%). Furthermore, Na full cell also display excellent electrochemical properties. The novel work on nanostructure and a simple method offers new perspectives for synthesizing other composites consisting of N-doped carbon and oxide quantum dot anodes for sodium-ion batteries.