Study On Preparation And Electrochemical Performance Of Natural Graphite Anode Materials With Mild Expanded Interlayer By Oxidation

Natural flake graphite is considered as the most commonly used commercial lithium-ion battery anode materials. However, the adhesion of graphite intercalation is mainly van der Waals force. In the charge-discharge process, exfoliation and pulverization appear on graphite sheet due to the volume expansion. This phenomenon can lead to the poor cycling performance. Thus natural graphite has other defects such as low reversible capacity (372 mAh/g) and not ideal rate capacity. So natural flake graphite needs to be modified, before used as anode materials for LIBs.The mild expansion treatment by oxidation is a relatively simple and low cost method for modifying graphite. It expands graphite sheet slightly through a certain process. The cycling performance of graphite anode can be improved by easing the volume expansion in the insertion and extraction process of lithium-ion. The rate performance can also be improved by widening the diffusion path of lithium-ion.This paper focuses on the graphite preparation process by mild expansion of oxidation. The impact of oxidants and reaction time on the structure and electrochemical properties of different materials of graphite are investigated in this paper. By characterizing the crystal structure, surface morphology, physical properties (specific surface area, pore size distribution and tap density) and the electrochemical properties of modified graphite to study the influence of mild expansion treatment process on structure and electrochemical properties of natural graphite.Average interlayer distance d002 of oxidation and mild expanded flake graphite is slightly increased and the grain size is significantly reduced. When the reaction time is increased, exfoliation appears on the surface of graphite sheet. When the reaction time is 3 h by weak oxidizer-organic acid, its initial reversible capacity and coulombic efficiency increase from 331.9 mAh·g-1 and 86.5% to 366.7 mAh·g-1 and 89%. After 1 h treatment by strong oxidants-organic acid, the specific surface area is reduced from 3.45 m2/g to 2.69 m2/g, while the amount of nanoscale pore is also increased about 2?4 nm. Its initial reversible capacity and coulombic efficiency are 364.7 mAh·g-1 and 88.8% respectively.After spherical graphite mild expanded, the average interlayer distance of doo2 is expanded slightly, while the average crystallite size is decreased significantly. The treatment of long time also causes etching on particle surface and flaking of the graphite sheet. When the reaction time is 2 h by weak oxidizer-organic acid, morphology of spherical graphite isn't changed significantly. But the tap density is decreased from 1.160 g/cm3 to 0.945 g/cm3 and the specific surface area is increased from 4.69 m2/g to 5.15 m2/g. The reaction barrier of lithium's insertion and extraction lowers, while initial reversible capacity increases from 363.0 mAh·g-1 to 392.7 mAh-g'1. In the current density of IC, the reversible capacity is improved by 46.3% than the spherical graphite material and the capacity retention is 86.4% of 0.1 C. When the reaction is going on 1 h by strong oxidants-organic acid, the initial reversible capacity increases to 381.5 mAh·g-1. After the 30th cycle, the capacity retention of the raw material is 99.20%, while the modified graphite's capacity retention is 99.71%. The capacity retention is 84.3% of 0.1C.The results have shown that oxidizing agent and reaction time are the important parameters to control the structure and electrochemical performance in the process of mild expansion treatment. The treatment can slightly increase the average interlayer distance of graphite, reduce its reaction barrier of lithium's insertion and extraction. Then it significantly improves the electrochemical performance of natural graphite.