Abstract
Hard carbons are considered among the most promising anode materials for Na-ion batteries. Understanding their Na storage capabilities is of great importance for optimizing their structure and therefore achieving high performance. Herein carbon nanofibers (CNFs) were prepared by electrospinning and their microstructure, texture and surface functionality were tailored through carbonization at various temperatures ranging from 650 to 2800 o C. Stepwise carbonization gradually removes the heteroatoms and increases the graphitization degree, enabling us to monitor the corresponding electrochemical performance for establishing a correlation between the CNFs characteristics and Na storage behavior in hard carbons. Outstandingly, we found for CNFs carbonized above 2000 o C, a single voltage Na uptake plateau at ~ 0.1V with a capacity of ~200 mAh/g. This specific performance may be nested in the higher degree of graphitization, lower active surface area (ASA) and different porous texture of the CNFs at such temperatures. We demonstrate via the assembly of a full CNF/Na2Fe(SO4)3 cell the benefit of such CNFs electrode for enhancing the energy density of 2 full Na-ion cells. This finding sheds new insights in the quest for high performance carbon based anode materials.