Biopolymer-Based Hard Carbons: Correlations between Properties and Performance as a Na-Ion Battery Anode

The relationship between the properties of hard carbon (HC) and its performance as an anode in sodium-ion batteries (NIBs) is not well understood. To address this issue, five HCs were synthesized from different biopolymer precursors by direct pyrolysis at 1500 °C. The reversible capacity was found to increase with increasing graphitic interlayer spacing (d002) and active surface area (ASA). The capacity coming from the plateau region predominantly contributes to the reversible capacity and linearly correlates to the interlayer spacing. A relationship between the reversible and plateau capacity with the closed porosity was established as well, i.e., a higher capacity is obtained when the fraction of closed pores is lower. These insightful correlations suggest an "adsorption–insertion" Na-ion storage mechanism. The initial Coulombic efficiency (iCE) proved more challenging to correlate with HC features. This can be linked to the distinct properties of the materials, known to affect the iCE (i.e., surface surface area, chemical composition, defects, etc.), thus leading to very similar iCE values (86–89% for most materials). Moreover, the HCs proposed herein deliver high performance. Cellulose-derived HC exhibits an iCE of ∼87%, a reversible capacity of ∼309 mA h–1, and good retention after 50 cycles (∼95%). The starch-rice and starch-potato HCs have performance comparable to that of the cellulose HC, while lignin and chitosan HCs deliver slightly lower performance. Rate capability tests at high C-rates demonstrates very robust materials, with high capacity retention when increasing the C-rate from C/10 to 5C (∼90%).