Disordered Carbons As Negative Electrode Materials for Na-Ion Batteries
Authors: Saurel Damien , Clarke Skye , Ségalini Julie 
Published: February 2020

With the recent renewal of interest for Na-ion batteries as a potential low cost alternative to Li-ion technology, the study of sodium insertion into carbonaceous materials experiences a revival in recent years.[1] Although many candidates for positive electrode active materials have been discovered with fairly good performance,[1] major improvements remain to be made on the negative electrode side.[2-3] Mimicking the Li-ion technology shows its limit in this domain as graphite presents very limited Na insertion.[4-6] Although promising efforts have been recently dedicated to circumvent this issue,[6-7] it does not match the best reports made on disordered carbons.[4,8-12]
Disordered carbons, being “soft” or “hard”, present a typical sloping voltage-composition curve below 1V related to the insertion of Na between the disordered graphene layers. The specificity of hard carbons is to present an additional low voltage plateau ascribed to the packing of sodium within the micro-pores formed by cross-linked graphitic layers.[13] Due to this extra capacity, the efforts of the community have been mainly focused on hard carbons. To improve the capacity efforts have been put on increasing the micro-porosity of hard carbons obtained by the pyrolysis of sucrose, via ball milling or physical activation.[9-10] However, it resulted in lower capacities than the pristine material after pyrolysis,[9-10] demonstrating that the correlation between the microstructure of these carbons and their electrochemical performance is not yet fully understood.
Moreover, as the plateau of hard carbons is observed at very low voltage (< 50mV) the use of this extra capacity may induce safety issues for being very close to metal plating, as pointed out for its use in Li-ion batteries.[14-15] This is even more critical in the case of sodium due to its higher reactivity, lower fusion point and tendency to form dendrites.[2] Should we avoid this potential risk, the actual practical capacity of hard carbons would be limited to the sloping region. This later extends from 120 to 170 mAh/g for best reports,[8,10-12] which is similar to the best capacities reported for soft carbons or carbon spheres with no low voltage plateau.[13,16-18] This suggests that, although presenting lower overall capacity, soft carbons are also worth considering as good candidates for negative electrodes.
Within this perspective, we underwent a comparative electrochemical and microstructural study of various soft and hard carbons. By coupling gas adsorption, powder X-ray diffraction (in-situ and ex-situ PXRD) and Small Angle X-ray Scattering (SAXS) we were able to depict the microstructure and morphology of these carbons. It allowed us to get new insights into the mechanism of sodium insertion into disordered soft and hard carbons and identify key microstructural features to be considered for best electrochemical performance. We will also report on the electrochemical performance of a disordered carbon presenting no low voltage plateau, with a reversible capacity of 170mAh/g at low rate and 76% coulombic efficiency at first cycle, and presenting a better rate capability than sugar hard carbon prepared and tested in similar conditions.
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