Structural Analysis of Sucrose-Derived Hard Carbon and Correlation with the Electrochemical Properties for Lithium, Sodium, and Potassium Insertion

Hard carbon possesses the ability to store Li, Na, and K ions between stacked sp(2) carbon layers and voids (micropores). We have explored hard carbon as a candidate for negative electrode materials for Li-ion, Na-ion, and K-ion batteries. Hard carbon samples have been prepared by carbonizing sucrose at different heat treatment temperatures (HTTs) in the range of 700-2000 degrees C to make them structurally suitable for reversible Li, Na, and K insertion. Structures and particle morphology of the hard carbon samples synthesized at different HTTs were systematically characterized using X-ray diffraction, small-angle X-ray scattering, pair distribution function analysis, electron microscopy, Raman spectroscopy, and electron spin resonance spectroscopy. All these characterizations of hard carbon samples have revealed advanced ordering of carbons and reduction of carbon defects with increasing HTT. Thus, the average stacked carbon interlayer distance decreases, the number of the stacking layers increases, the layered domains grow in the in-plane direction, and interstitial voids enlarge. Electrochemical properties of the hard carbons were examined in nonaqueous Li, Na, and K cells. Potential profiles and reversible capacities upon galvanostatic charge/discharge processes in nonaqueous cells are significantly different depending on HTTs and different alkali metal ions. On the basis of these findings, strategies to design high-capacity hard carbon negative electrodes for high-energy-density Li-ion, Na-ion, and K-ion batteries are discussed.