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

The relationship between the propertiesof hard carbon(HC) andits performance as an anode in sodium-ion batteries (NIBs) is notwell understood. To address this issue, five HCs were synthesizedfrom different biopolymer precursors by direct pyrolysis at 1500 & DEG;C.The reversible capacity was found to increase with increasing graphiticinterlayer spacing (d (002)) and active surfacearea (ASA). The capacity coming from the plateau region predominantlycontributes to the reversible capacity and linearly correlates tothe interlayer spacing. A relationship between the reversible andplateau capacity with the closed porosity was established as well,i.e., a higher capacity is obtained when the fraction of closed poresis lower. These insightful correlations suggest an "adsorption-insertion"Na-ion storage mechanism. The initial Coulombic efficiency (iCE) provedmore challenging to correlate with HC features. This can be linkedto the distinct properties of the materials, known to affect theiCE (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-derivedHC exhibits an iCE of & SIM;87%, a reversible capacity of & SIM;309mA h(-1), and good retention after 50 cycles (& SIM;95%).The starch-rice and starch-potato HCs have performance comparableto that of the cellulose HC, while lignin and chitosan HCs deliverslightly lower performance. Rate capability tests at high C-ratesdemonstrates very robust materials, with high capacity retentionwhen increasing the C-rate from C/10 to 5C (& SIM;90%).