Optimization of the molecular weight range of coating pitch and its effect on graphite anodes for lithium-ion batteries

This study aimed to optimize the molecular weight range of coating pitch to enhance the electrochemical performance of graphite-based anodes used in lithium-ion batteries by understanding the characteristics of the coating pitch. The coating pitch was divided into four fractions based on its solubility in hexane, acetone, toluene, and n-methyl-2-pyrrolidone (NMP). These four fractions were estimated based on the thickness and homogeneity of the coated surfaces. The lighter fractions of pitch, such as hexane and acetone, assisted in forming a homogeneous surface by decreasing the viscosity during carbonization. Heavy fractions, such as toluene and NMP, were the main components of the coating. They improved the rate performance of the anode by forming an isotropic layer, which increased the number of lithium-ion intercalation sites. However, thick surfaces increased the charge-transfer resistance because of the increased diffusion path lengths of lithium ions. The pitch molecular weight fractions of 128–768, 768–1152, and 1,152–1,480 m/z should be controlled to 70–84.49, 11.20–18.21, and 3.35–5.15%, respectively. Furthermore, the results of this study can be applied to optimize the coating properties for other anode materials, such as silicon, at a controllable pitch coating thickness according to the molecular weight.