A Facile Route to Synthesis LiMPO4/Graphene Freestanding Cathode Electrodes for Lithium Ion Batteries

Abstract

To globally solve the problem of an environmentally clean and secure distribution of energy is a major task for scientist worldwide. The use of clean energy will have a direct positive influence on, for example, the emission of greenhouse gases and our future climate. Today the rechargeable battery with the highest energy density is the Li-ion or Li-polymer battery. Commercially this battery is used for mobile phones, laptops and camcorders. The development of new batteries of this type has to go towards larger batteries for hybrid vehicles or towards further miniaturisation for smaller electronic devices. Both these routes will, however, require basic materials research within chemistry and physics; new materials must be synthesized (some materials used today are too expensive for up-scaling) and the complex chemistry occurring both in bulk electrode materials and at the interface between electrolyte and electrode most be better understood. Among different electrode materials, layered transition metal oxides (LMO) are the most successful cathode material in application at present time, but its further development is severely hindered due to the intrinsic safety limitation and the cost. Seeking for some other cost-effective cathode materials with better performance is a significant task. And therefore, olivine-structured phosphate LiMPO4 (M = Fe, Mn, Co, etc.) materials have attracted great attention due to many advantages, such as lower toxicity, lower cost, better thermal, chemical stability. Unfortunately, phosphate based cathode material possesses low intrinsic electronic and ionic conductivity. So, that it is difficult to prepare LiMPO4 (M: Fe, Mn, Co, etc) with a high performance, and many groups have explored various solutions to solve the problems, including enhancing the electronic conductivity among particles. In order to overcome these issues, studies are mostly focused on synthesis methods with a coating of an electronically conductive phase such as carbon. Another route to solve these problems could be done by using graphene. It has been reported that the two-dimensional graphene has unique properties such as excellent electrical and mechanical properties, unique physical properties such as high specific surface area (2630 m2/g), high electrical conductivity, a broad electrochemical window and good flexibility. Graphene oxide paper and graphene paper have been produced by vacuum filtration of graphene sheets. As a result, graphene free standing electrodes have been used directly as electrode materials for flexible energy storage devices without binder, conductive additives and current collectors. As compared with conventional electrodes, free-standing electrodes can increase the electrical conductivity of electrode materials meanwhile achieve higher active material-to-substrate mass ratios. In this study, a microwave hydrothermal synthesis process is used to produce novel, and facile strategy for the preparation of LiMPO4 (M=Mn, Fe, and Co) microstructures at a low temperature, using ethanol as the solvent, LiI as the lithium source, transitional metal salt as the M sources, H3PO4 as the phosphorous source, and poly(vinyl pyrrolidone) as the carbon source and template. As-synthesized LiMPO4 were then subjected to vacuum filtration techniques with graphene in order to obtain freestanding cathode electrodes for high capacity Li-ion batteries. Keywords: LiMPO4, graphene, freestanding electrode, cathode electrode, Li ion.