Optimizing Disorder: Pretreatment Strategies for Saccharide-Based Hard Carbon Anodes in Na-Ion Batteries

The need for an alternative to lithium-ion batteries motivates the development of sodium-ion batteries, specifically focusing on biomass-derived hard carbon (HC) as anode material. However, the compositional variability in biomass leads to a lack of correlation between the microstructure and properties of HC. In this study, we used four simple carbohydrates present in all biomass-derived precursors as model objects to establish a reliable relationship between pretreatment temperature, the carbonization mechanism, defectiveness, and the electrochemical characteristics of HCs. The statistical analysis of Raman spectra from 25 HC samples enabled us not only to determine the degree of defectiveness in HCs but also, for the first time, to compare materials based on their inhomogeneity—that is, the variation in defectiveness within each HC material. The initial Coulombic efficiency of hard carbons was improved by 40%, reaching 89%, through the simultaneous reduction of surface area, defectiveness, and inhomogeneity. Additionally, the full cell with the HC and Na 3 V 2 (PO 4 ) 3 /С cathode material demonstrated 80% capacity retention after 300 cycles.