Preparation And Performance Research Of Graphene/Carbon Nanotubes Modified Cathode Materials For Lithium Ion Batteries

Along with the rapid development of industry and technology, energy depletion, environment pollution, energy and environmental issues have become increasingly prominent, which put forward to higher requirements for the development of new energy and using existing energy effectively. Lithium-ion batteries have become one of the most promising energy storage and power sources due to their high charge/discharge voltage, high energy density, long cycle life, low self-discharge, no memory effect and non-pollution. At present, the main focus on the development and improvement of lithium ion batteries are improving battery performance and reducing the cost of electrode materials. Lithium iron phosphate (LiFePO4), which has many advantages such as low cost, nontoxic, high theoretical capacity and stable voltage platform, has become the most promising lithium-ion batteries cathode materials for next-generation. However, the low lithium ion diffusion coefficient and poor electrical conductivity of LiFePO4hindered its application in high rate commercial batteries. Lithium manganese oxide (LiMn2O4) with rich resources, low cost, non-pollution and high magnification performance is expected to become another ideal cathode material for lithium-ion batteries. However, LiMn2O4itself is not stable.It can be easily decomposed and produced gas. Other disadvanteges suche as fast cycle life decay, poor high temperature performance and Jahn-Teller defect during charge/discharge processe prevent it from commercial application.In order to overcome the shortcomings of LiFePO4and LiMn2O4, researchers have done a lot of studies, such as carbon-coating, optimization the particle size and shape of LiFePO4and LiMn2O4, cationic or oxide doping have been done. Among various methods, LiFePO4and LiMn2O4modified with high conductivity carbon materials (carbon black, graphene, carbon nanotubes etc.) are considered to be an effective way to improve the electrochemical properties of lithium-ion batteries. In this paper, the olivine LiFePO4and spinel LiMn2O4were selected as cathode materials for study. In order to prepare cathode electrode materials with good electrochemical performance, a detailed study on the synthesis and modification of cathode materials for lithium ion batteries was carried out. The main contents of the paper are as follows:1) LiFePO4/CNTs/graphene cathode material was prepared by ball milling and hydrothermal methods combined with heat treatment, respectively. LiMn2O4/CNCNTs cathode material was synthesis via hydrothermal method. The structure and electrochemical performance of LiFePO4and LiMn2O4modified with carbon nanotubes (CNTs) and graphene were studied. The crystal structures of the samples were characterized by X-ray diffraction (XRD) and Raman scattering (Raman) analyzes the structure of the synthetic crystal. Scanning electron microscopy (SEM) and transmission electron microscope (TEM) were conducted to investigate the morphology of the prepared cathode materials. Take galvanostatic charge-discharge, cyclic voltammetry and electrochemical impedance spectroscopy were taken to investigate the electrochemical properties of LiFePO4/CNTs/graphene and LiMnO4/CNTs cathode materials.2) LiFePO4modified with CNTs and graphene (LiFePO4/CNTs/graphene, LFP-CNT-G) was prepared by ball milling followed by heat treatment. The graphene nanosheets, carbon nanotubes and lithium iron phosphates constructed a three dimensional conductive network, which accelerats electron transfer in lithium-ion reversible process and reduces resistance, shortening the lithium ion diffusion channels and improving lithium-ion diffusion rate. The synergies of carbon nanotubes and graphene nanosheets improve the rate performance and cycle stability of lithium iron phosphate cathode. The LFP-CNT-G composite electrode delivered a discharge capacity of168.9mAh-g-1at0.2C rate and capacity of115.8mAh-g-1at20C rate. From EIS measure, the Rct of LFP-CNT-G composite electrode was only50.17Ω, the lithium ion diffusion (D) was1.040×10-12cm2s-1and the exchange current densityi was5.127×10-4mA cm-2.3) Graphene and carbon nanotubes modified lithium iron phosphate composites (LiFePO4/graphene/CNTs, LFP-G-CNT) was prepared by hydrothermal method followed by heat treatment. The first charge and discharge capacity lithium iron phosphate was significantly improved. LFP-G-CNT composite electrode delivered a discharge capacity of168.4mAh-g-1at0.1C rate, which was about99%of the LiFePO4theory capacity. In addition, the lithium ion diffusion rate and exchange current density of LFP-G-CNT composite electrode were also improved greatly, which were5.744×10-11cm2s-1and1.226×10-3mA cm-2, respectively. These excellent performances can be attributed to the fact that the one dimensional CNTs along with two dimensional graphene can form a three dimensional conductive network structure, which is beneficial for electrolyte absorption and improves the surface electrical conductivity of LiFePO4particles, thus reducing the polarization resistance of the electrode. This conductive network also can shorten the lithium ion diffusion channels and improve the lithium-ion diffusion rate, resulting in high rate performance of lithium iron phosphate.4) LiMn2O4/CNTs composite was synthesized via hydrothermal method. The high charge-discharge capacity and coulombic efficiency of LiMn2O4/CNCNTs was owed to the good lattice structure and conductivity of CNTs. LiMn2O4/CNTs composite electrode delivered a discharge capacitor of101.6mAh-g-1at0.1C rate. After more than40cycle times, the LiMn2O4/CNTs electrode keeps a retention capacity of100.4mAh-g-1, which is96.9%of the capacity at0.1C.