Universal construction of ultrafine metal oxides coupled in N-enriched 3D carbon nanofibers for high-performance lithium/sodium storage

To develop high-performance lithium/sodium-ion batteries, new method for the mass production of distinctive nanomaterials with ultrafine transition metal oxides intimately coupled in 3D carbon matrix still remains challenging. Herein, a universal strategy for synthesizing ultrafine metal oxide quantum dots (3-6 nm) tightly embedded in nitrogen-enriched 3D carbon nanofiber networks {MO@NCFs (MO = Co3O4, Mn3O4, Fe3O4)} is reported in a scalable manner. The MO@NCFs are constructed by coordinating nanofibrous carboxymethyl chitosan (CMCh) hydmgels and metal ions to form a stable metal-polysaccharide framework, and then by pyrolyzing process to generate hierarchically porous structure. The in situ and confined coordination in the CMCh framework can not only generate the metal oxides quantum dots with ultrafine size, but also endow the quantum dots to be robustly embedded in N-enriched 3D carbon nanofiber networks, leading to fast electron and ion transport, and excellent structural stability. As expected, the Co3O4@ NCFs exhibit a high Li+ storage capacity of 1199 mAh g(-1) at 200 mA g(-1), and a good cycling lifespan with capacity retention of 721 mAh g(-1) at 1000 mA g(-1) after 400 discharge/charge cycles. For sodium storage, the Co3O4@NCFs display superior capacities of 645 mAh g(-1) at 100 mA g(-1), good rate capability (191 mAh g(-1) at 4000 mA g(-1)), and remarkable capacity retention of 301 mAh g(-1) at 1000 mA g(-1) after 400 cycles. This work provides a facile pathway to construct firmly coupled carbon hybrids by utilizing sustainable polymers derived from seafood waste for enhancing energy storage.