Modification And Electrochemical Properties Of Mesocarbon Microbeads For Lithium-Ion Battery

The development of lithium ion batteries and carbon anode materials, especially mesocarbon microbeads(MCMBs), are reviewed in detail. MCMB were researched with different modified treatment in order to increase properties, reduce cost promote industrialization. The preparation parameters, structures and electrochemical behaviors of modified MCMB and their relations were investigated in order to develop a novel carbon anode with higher properties, and to prompt the research and commercialization of lithium ion batteries.In this thesis, Polyaniline(PANI)/MCMBs negative composite were obtained through encapsulating or inserting MCMBs with PANI;Oxidized-MCMB were obtained through oxidizing MCMBs with HNO3and (NH4)2S2O8,respectively. Based on the modifying mentioned above, Oxidized the MCMBs first, then further modified it by inserting PANI. at last, obtained the PANI/MCMBs negative composite. The structure characteristics and the surface functional group information of these anode materials and their lithium insertion performance were studied by X-RayDiffraction(XRD), X-Ray Photoelectron Spectroscopy(XPS), cyclic voltammetry(CV) and charge-discharge measurements. The mainresearch results of this dissertation are summarized as follows:1MCMB was encapsulated and intercalated with PANI respectively for surface modification treatment. The influence of PANI modification on MCMB surface functional group, structure and electrochemical performance was discussed. Results showed:(1) Amorphous PANI was unevenly accumulated on the surface of MCMB after it was modified through encapsulated with PANI, so the micro hole or channel for lithium ion diffusion was obstructed. With the charging/discharging cycle, PANI membrane gradually fall off duo to the loosely linkage between PANI and MCMB. These factors impede the intercalated into or de-intercalated from MCMB, leading to MCMB’s electrochemistry performance decline.When the weight ratio between MCMB and PANI was16to1and soaking treatment for24hours can produce PANI/MCMB composite material with the best electrochemistry performance. The first charge capacity, discharge capacity and charge efficiency was169.4mAh/g,150.2mAh/g and88.7%respectively.(2) Crystalline PANI was evenly distributed on the surface of MCMB after it was modified through intercalated with PANI. The PANI membrane on the surface of MCMB has electrochemistry activity and can play a role of SEI membrane. In addition, excessive ammonium persulfate has a surface oxidation effect on MCMB. These factors favor the intercalated into or de-intercalated from MCMB and constrain the decomposition of electrolyte, leading to MCMB’s electrochemistry performance improvement.When the weight ratio between MCMB and PANI was1.5to1and soaking treatment for24hours can produce PANI/MCMB composite material which was used as negative material for lithium ion battery. The first charge and discharge capacity was361.8mAh/g and338.1mAh/g respectively. Charge/discharge capacity wasn’t obvious decay after26cycles.2Oxidation treatment was respectively imposed on MCMB with strong nitric acid and ammonium persulfate as oxidizer. The effect of depth of oxide treatment on MCMB structure and electrochemical performance was discussed. Results showed:(1) Oxidation modification take place on the surface or between layers of MCMB, but no damage to layer structure of graphite.(2) After oxidation treatment with nitric acid and ammonium persulfate respectively,accordingly MCMB’s first charge capacity,discharge capacity and charge efficiency was363.3mAh/g,328.3mAh/g and90.3%(3) The reason of MCMB electrochemical performance improvement may lie in:MCMB’s stability improved for a number of high active structural deficiencies was removed through oxidation, the increment of the number of nano-level micro hole and channel, increase in the number of crossings; Furthermore, the oxide layer can be formed as the passivation film prevent the damage to graphite structural.3Oxidation treatment was respectively imposed on MCMB with strong nitric acid and ammonium persulfate as oxidizer. Further modification was made to the oxidized MCMB through PANI intercalation treatment. Results showed:(1) Oxidation treatment with nitric acid or ammonium persulfate together with PANI intercalation modification, resulted in reduced surface deficiencies structure and increased number of nano-level micro hole and channel. But at the same time due to loosely linkage between oxide layer and PANI membrane. These factors favor the intercalated into or de-intercalated from MCMB and constrain the decomposition of electrolyte. But with the proceeding of charge/discharge cycle, MCMB’s structure damage can’t prevent. These factors led to increase in the first charge/discharge capacity after MCMB was modified, but cycle performance has not improved.(2) After PANI interaction modification to Oxidized MCMB (oxidized by nitric acid), it’s first charge capacity, discharge capacity and charge efficiency was365.3mAh/g,327.6mAh/g and89.7%respectively.(3) After PANI interaction modification to Oxidized MCMB (oxidized by ammonium persulfate), it’s first charge capacity, discharge capacity and charge efficiency was356.9mAh/g,334.2mAh/g and93.6%respectively.