Hard carbon derived from cellulose as anode for sodium ion batteries: Dependence of electrochemical properties on structure

Cellulose, the most abundant organic polymer on Earth, is a sustainable source of carbon to use as a negative electrode for sodium ion batteries. Here, hard carbons (HC) prepared by cellulose pyrolysis were investigated with varying pyrolysis temperature from 700 °C to 1600 °C. Characterisation methods such as Small Angle X-ray Scattering (SAXS) measurements and N2 adsorption were performed to analyse porosity differences between the samples. The graphene sheet arrangements were observed by transmission electron microscopy (TEM): an ordering of the graphene sheets is observed at temperatures above 1150 °C and small crystalline domains appear over 1400 °C. As the graphene sheets start to align, the BET surface area decreases and the micropore size increases. To correlate hard carbon structures and electrochemical performances, different tests in Na//HC cells with 1 M NaPF6 ethylene carbonate/dimethyl carbonate (EC/DMC) were performed. Samples pyrolysed from 1300 °C to 1600 °C showed a 300 mAh/g reversible capacity at C/10 rate (where C = 372 mA/g) with an excellent stability in cycling and a very good initial Coulombic efficiency of up to 84%. Furthermore, hard carbons showed an excellent rate capability where sodium extraction rate varies from C/10 to 5C. At 5C more than 80% of reversible capacity remains stable for hard carbons synthesized from 1000 °C to 1600 °C.