Nanoporous hard carbon anodes for improved electrochemical performance in sodium ion batteries

• Nanoporous hard carbon was synthesized via a thermal decomposition of bicarbonates.
• Particle size and pore structure were sensitive to the amount of bicarbonates.
• Hard carbon with optimal porosity showed 1.4-fold increase in Na+-storage capability.
• The rate performance was also superior when compared with unmodified hard carbon.
• Enhancement in kinetics resulted from both decrease in RCT and increase in DNa.

The porosity and morphology of sucrose-based hard carbon (SHC) was regulated by varying the amount of bicarbonate salts added during a simple two-stage sintering process. During the first–stage thermal treatment of sugar at 200 °C, CO2 liberated from bicarbonate contributed to the pulverization of particles and to the formation of submicron-sized pores. Na2CO3 entrapped in a precursor matrix also released CO2 during the second–stage sintering at 850 °C, producing nanometric pores (ca. 10 nm in diameter). The excessively high content of bicarbonates, however, resulted in paper-thin graphitic layers with no submicron-sized pores. These dual roles of bicarbonates produced nanoporous SHC (NSHC) with the submicron-to-nano-sized pores and the largest surface area that was possible for a specific bicarbonate concentration. The optimal nanoporosity of NSHC lent itself to a sharp increase in reversible capacity. Reversible capacity of 324 and 289 mA h g−1 were obtained for the first and 100th cycles at 20 mA g−1, in contrast to 251 and 213 mA h g−1, respectively, for SHC. The rate capability of NSHC also was enhanced due to a substantial decrease in the charge transfer resistance and a 5-fold increase in the Na+ diffusion coefficient.

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