Investigation of modified graphite as anode material for lithium ion batteries

The requirement for energy increases each passing day with the improving technology and therewith new systems for energy storage are required depending on increasing population, industrialisation and globalization. Due to the need for especially electrical energy, energy storage systems in lower volumes and lower weights are designed with the increase in portable electrical devices. These portable devices such as; laptops, tablet computers, cell phones, cameras and MP3 players are gradually becoming indispensable in the daily life. The most important parameter for these devices is longer charge/discharge cycle life. In other respects, due to similar reasons with electrical devices for hybrid/electrical vehicles which are produced to minimize the environmental damage, the improvement of systems which are capable to store energy and enables to be used longer is considered as a technological reform. Lithium ion battery technologies come into prominence by fulfilling the need of energy of electrical devices in long term and by providing efficient usage. The features such as; high energy storage capacity, high energy intensity (kWh/kg), long cycle life, high charge/discharge efficiency, low costs, low volume and low weight attract attention to the lithium ion batteries and constitutes the basis to the future researches and studies in this area. The main reason of lithium being used in lithium ion batteries is that it is one of the smallest atoms after hydrogen and helium in periodic table. It has low atomic weight of 6,941 g/mol and theoritical energy intensity of 3,86 Ah/g which is the highest comparing with the other materials. Due to these properties, lithium ions can easily transfer between positive and negative electrodes. A basic lithium ion battery consists of three main parts as negative electrode (anode), positive electrode (cathode) and electrolyte. These components generate a medium which provides the electron transition during the charge – discharge processes. Rechargable batteries have two processes as charge and discharge which occur in case of Li+ ions reciprocate between positive and negative electrodes. The process of Li+ ions enter to the electrodes is called as intercalation and the reverse is deintercalation. These reactions are reversible which enables the battery to charge and discharge repeatedly. During charge process, lithium metal oxide as positive material releases Li+ ion and this Li+ ion pass through the electrolyte and goes to the negative electrode to merge with carbon structure as negative material. Discharge process is exact opposite of charge process. The positive material is reduced in charge process where the negative material is oxidized. To sum up, during the charge process, reduction takes place in the positive material and oxidation takes place in the negative material. The main components of the batteries have many advantages and disadvantages due to their material and composition. It is quite significant for researchers to be able to use an optimum material with high performance and high capability. There are a number of materials for all of the components to be researched on and the researchers have being determined these circumstances over the years. For the anode materials used in lithium ion batteries, the most common material is graphite which is used commercially in many battery technologies. Metal materials such as; Mg, Ca, Al, Si, Ge, Sn, Pb, As, Sb, Bi, Pt, Ag, Au, Zn and Cd have higher specific capacities than graphite (372 mAh/g) but they exhibit an expansion at about 400 % during charge/discharge cycles which causes danger in usae of batteries, like; explosion or combustion originating from increase of temperature. Consequently, the researches in this area mainly focus on developing the specific capacity of graphite anode material by surface treatments or thermal processing. In lithium ion batteries which uses graphitic carbons as anode material, the morphology and chemistry of graphite surface have a significant influence on the formation of solid electrolyte interphase (SEI), irreversible charge loss and the overall electrochemical anode performance. The formation of SEI layer is the most important property of the lithium ion batteries which effects the performance and cyclic time of the battery directly. This layer occurs in the first charge step in the way of electrolyte and additives decompose and collect on graphite anode material. This formation process is irreversible, thus causes capacity loss in the first cycle. Therefore, it is possible to improve the battery performance with the modifications on the morphology and surface of graphite anode material. Graphitic carbons principally possess two different kinds of surfaces, basal plane and prismatic (edge) surfaces. Basal plane surfaces are homogeneous and smooth which consist only of carbon atoms. On the contrary, prismatic surfaces are heterogeneous and beside carbon, contain various surface groups, mostly oxygen containing. The transport of lithium ions during charge/discharge operation takes place via the prismatic surfaces rather than the basal plane surfaces. Therefore, oxidative modifications on the surface of graphite provide more inert basal plane which make the differentiation of anode performance by the differentiation of the structure and the chemistry of prismatic surfaces. Along with the surface modifications, Li+ intercalation is improved by stabilisation of graphite surfaces and the decomposition of electrolyte is prevented. In the scope of the thesis, the graphite anode material has been treated by oxygen, nitrogen and phosphorus groups modifications and in this way the surface properties have been improved. Modifications have been made via changing the weights and concentrations of the abovementioned groups by heat treatments. For this purpose, two kinds of heat treatments have been applied to the graphite anode material which have been taken place in tube furnace and vacuum furnace. Obtained samples have been characterized by surface electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The materials which properties are improved, thereafter have been transformed to the slurries with solvent, carbon black and binder to be able to coat on the copper foil. After drying the foils, the samples have been formed to lithium-ion coin cells by gathering all necessary components in previous specified line. Finally, all of the prepared cells have been taken to the galvanostatic charge – discharge test equipment to measure the effects of battery performances and cyclic life. When the obtained results had analyzed, it was evaluated that the oxygen groups have a 20 % burn-off rate at 580 °C. The highest efficiency of oxygen modified graphite was achieved at 540 °C as a value of 68 %. For the other graphite samples which are modified with melamine as nitrogen groups have high incidence of graphitize nitrogen layers. As a consequence, first discharge specific capacity of 517 mAh/g was achieved with 75 % melamine (by weight) modified graphite. During the modification process with phosporus groups, it has been observed that the structure of the samples decomposes at higher temperatures and phosphorus groups have been removed from the structure. Therefore, some failures have been determined in the battery analyzes results. Furthermore, in the majority of the results of measured batteries, it is seen that the sudden drop of specific capacity has been resulted from SEI formation in the first cycle.