Synthesis and development of electrochemical properties of graphene reinforced spinel LiMn2O4 nanorods cathode materials for Li-ion batteries

Keywords: Li-ion battery, LiMn2O4; cathode, nanorod, graphene, Ni and Cr doping, freestanding electrode. As the worldwide demand for energy and environmental applications increases, efforts to develop efficient energy storage devices with high energy density for portable electronics, hybrid electric vehicles/electric vehicles and grid storage applications are increasing day by day. In this context, lithium-ion batteries are considered as a potential candidate due to their high gravimetric and volumetric energy densities. One of the areas that has been extensively studied in lithium-ion batteries is to identify and design cathode materials currently used, which include improved capacity, stability, speed capability, and long cycle life. Among the many cathode candidates, spinel LiMn2O4 is recognized as one of the best candidates for high-power cathode materials due to its environmental friendliness advantages, abundant manganese source, and 3-dimensional lithium diffusion paths. However, LiMn2O4 electrode has disadvantages such as dissolution of manganese, Jahn Teller distortion and formation of oxygen vacancies. In the literature, different nanostructures such as nanoparticles, nanorods, nanolayers, porous structure, hollow structures and LiMn2O4-carbon composites and methods such as metal ion doping by reducing the amount of Mn in the LiMn2O4 structure for improved electrochemical properties at high current have been proposed to improve the electrochemical properties by solving the aforementioned problems. In this thesis, LiMn2O4 nanorod structures were produced in order to provide an effective electron transfer path. -MnO2 nanowires produced by microwave hyrothermal synthesis method were used as the starting material. LiMn2O4 nanorod structures were obtained by using solid state synthesis method and hydrothermal method. LiNi0.2Mn1.8O4 and LiCr0.2Mn1.8O4 structures were produced by doping Cr and Ni to the LiMn2O4 structure to eliminate problems such as Jahn Teller distortion and Mn dissolution. Free standing electrodes were produced by forming each powder composite with graphene that produced using the modified Hummers method. The electrochemical properties of each electrode were investigated. At the end of the thesis study, the graphene reinforced LiNi0.2Mn1.8O4 electrode produced with this new approach showed the best electrochemical performance with a high capacity of 139 mAh/g at 0,5 charge/discharge rate.