An Optimized PVC-Based Soft Carbon As the Negative Electrode of Sodium-Ion Batteries
Authors: Orayech Brahim , Saurel Damien 
Published: February 2020

The critical role of energy storage is by now self-evident: they are indispensable for an increased share of renewable energy and electric vehicles. Considering the abundance of sodium resources, sodium-ion batteries (NIBs) are poised as an alternative to Li-ion batteries as sustainable electrochemical energy storage (EES) solution.[1−6] However, several technical challenges still remain to be addressed before the commercialization of NIBs, with the anode being one of them. Graphite has been employed as the anode for commercial lithium-ion batteries (LIBs) since 1991.[7] However, for NIB, a high degree of disorder of the carbon structure is needed to allow sodium insertion at positive voltages.
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).[8] 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, [9] although recent results demonstrate that it might be more complicated than that.[10] 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. [11]
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[8] Na-ion batteries for large scale applications: a review on anode materials and solid electrolyte interphase formation; Muñoz-Márquez, M. Á.; Saurel, D.; Gómez-Cámer, J. L.; Casas-Cabanas, M; Castillo-Martínez, E.; Rojo, T.; accepted to Advanced Energy Materials (2017).
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[11] Ultra-microporous soft carbon as high performance active material for the negative electrode of sodium-ion batteries; Orayech, B.; Clarke, C.; Acebedo, B. and Saurel, D.; submitted.