Self-supported binder-free hard carbon electrodes for sodium-ion batteries: insights into their sodium storage mechanisms

Hard carbons are some of the most promising negative electrode materials for sodium-ion batteries (NIBs). In contrast to most of the published studies employing powder-like electrodes containing binders, additives and solvents, we report herein an innovative way to prepare binder-free electrodes by simple impregnation of cellulose and cotton filter papers with a phenolic resin solution. The latter enables improvement of the poor mechanical properties and thermal stability observed for pristine hard carbon self-standing electrodes (SSEs) along with the carbon yield. A high reversible specific capacity and long-term stability were observed for cellulose compared to those of cotton-based SSEs in NIBs, i.e., 240 mA h g−1 vs. 140 mA h g−1, respectively, for a C/10 rate and high mass loading (∼5.2 mg cm−2). This could be ascribed to the larger number of defects on cellulose than on cotton as quantified by temperature programmed desorption coupled with mass-spectrometry (TPD-MS), the structure and porosity being similar for both materials. Furthermore, the addition of a conductive sputter coating on the cellulose SSE surface improved the reversible specific capacity (to ∼300 mA h g−1) and initial coulombic efficiency (ICE) (to 85%). Operando X-ray diffraction (XRD) was performed to provide additional insights into the Na storage mechanisms. Although no shift was noticed for the graphite (002) diffraction peak, clear evidence of sodium intercalation was observed in the plateau region with the appearance of a new diffraction peak (∼28.0° 2θ) likely associated with a sodium intercalation compound. Consequently, the sloping region could be related to the Na+ adsorption on hard carbon defects and pores.