Insight to defects regulation on sugarcane waste-derived hard carbon anode for sodium-ion batteries

A great deal of attention has been paid on developing plant-derived hard carbon (HC) materials as anodes for sodium-ion batteries (SIBs). So far, the regulation of HC has been handicapped by the well-known ambiguity of Na⁺ storage mechanism, which fails to differentiate the Na⁺ adsorption and Na⁺ insertion, and their relationship with the size of d-interlayer spacing and structural porosity. Herein, bagasse-derived HC materials have been synthesized through a combination of pyrolysis treatment and microwave activation. The combined protocol has enabled to synergistically control the d-interlayer spacing and porosity. Specifically, the microwave activation has created slit pores into HC and these pores allow for an enhanced Na⁺ adsorption with an increased sloping capacity, establishing a strong correlation between the porosity and sloping capacity. Meanwhile, the pyrolysis treatment promotes the graphitization and it contributes to an intensified Na⁺ insertion with an increased plateau capacity, proving that the plateau capacity is largely contributed by the Na⁺ insertion between interlayers. Therefore, the structural regulation of bagasse-derived HC has provided a proof on positively explaining the Na⁺ storage with HC materials. The structural changes in the pore size distribution, specific surface area, d-interlayer spacing, and the electrochemical properties have been comprehensively characterized, all supporting our understanding of Na⁺ storage mechanism. As a result, the HC sample with an optimized d-interlayer spacing and porosity has delivered an improved reversible capacity of 323.6 mAh g⁻¹ at 50 mA g⁻¹. This work provides an understanding of Na⁺ storage mechanism and insights on enhancing the sloping/plateau capacity by rationally regulating the graphitization and porosity of HC materials for advanced SIBs.