Therefore, alternative anode materials are demanded. One family of candidates is nongraphitic carbon, which comes in the forms of “hard carbon” (HC) or “soft carbon” (SC). For both SC and HC the amount of sodium ions that can be inserted at positive voltages between the graphene layers is greatly increased compared to graphite. The specificity of HC is to present an additional capacity traduced by a low voltage plateau (LVP). Since the pioneer work of Stevens and Dahn in the early 2000’s, the sloping voltage below 1V and the low voltage plateau are usually ascribed to defect assisted intercalation and packing of sodium within the voids opened between cross-linked graphitic layers, respectively,  although recent results demonstrate that it might be more complicated than that. Due to this extra capacity due to the LVP, the effort of the community has been mainly focused in recent years on hard carbons and on improving their capacities by tuning their micro-porosity. Thus, there is tremendous need to understand deeply the sodium insertion/extraction mechanism into disordered carbons.
In the present work an optimized PVC-based SC as active material for NIB negative electrode will be underlined. A comparative study of the performance of PVC-C compared to Sugar-HC with similar particle size and electrode preparation will be presented. Although it does not present the typical low voltage plateau of hard carbons, PVC-SC reaches an initial reversible capacity of 230mAh/g and retains 225mAh/g after 150 cycles at 24.8mA/g. At high current density of 1C (372mA/g), the electrode still can achieve a capacity of 175mAh/g with a Coulombic efficiency near 100%.
The microstructure and the morphology of these carbons have been studied by coupling gas adsorption, powder X-ray diffraction (in-situ and ex-situ PXRD) and Small Angle X-ray Scattering (SAXS) measurements, which, by correlating with their electrochemical performance, had led us to identify the and identifying key-microstructural features at origin of the performance of PVC—SC and give new insights into the mechanism of sodium insertion into disordered soft and hard carbons. 
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