C/Li2MnSiO4 katot malzemesinin kimyasal deinterkalasyonu ve farklı elektrolitler ile kararlılığının incelenmesi

Rechargeable lithium ion (Li-ion) batteries are commercially used for portable electronics and light electrical devices since 1991. Despite of the wide variety of applications, most commonly used Li-ion batteries have important problems related to their safety, environmental impact and high cost of materials. Thus, a research interest in production of alternative cathode materials have arisen, where dilithium orthosilicates (Li₂MSiO₄, M = Co, Ni, Mn, Fe) are mostly preferred for being safe, environmentally friendly, cheaper and thermally and chemically stable. Besides these, Li₂MSiO₄ have a very important advantage of theoretical possibility for reversible exchange of up to two lithium ions per formula unit that leads to high capacities up to 333 mAh/g. In this study, Li₂MnSiO₄ nanoparticles were synthesized via sol-gel Pechini type synthesis and coated with different amounts of conductive carbon layer (10, 15, 20, 25 and 30 wt.%) by water mediated impregnation process to improve the electrical conductivity. Afterwards, R2032 coin-cell type batteries were assembled by using C/Li₂MnSiO₄ nanocomposites as cathode material and 9 different electrolytes. Besides these, pristine Li₂MnSiO₄ materials were tried to be chemically delithated by using different oxidizing agents in order to observe if delithiation of both lithium atoms is possible. XRD was used to observe structure of the synthesized and modified samples while TGA was used to determine the actual carbon content in composite materials. XPS was used to determine oxidation state of Mn in both chemically and electrochemically delithated samples. DSC was used to examine the reactivity of pristine sample with electrolyte solutions and EC measurements were done to compare the effect of carbon coating loading on conductivity of the cathode materials. Finally, galvanostatic charge-discharge tests were performed to observe electrochemical performance and practical capacities of prepared materials. It was observed from the XRD results that, nanosized Li₂MnSiO₄ particles are successfully synthesized and the structure of the material was maintained after water mediated impregnation processes. TGA measurements showed that the water mediated impregnation process is accurate to coat Li₂MnSiO₄ with the desired amount of carbon. Studies proved that applied technique was successful for the synthesis of pristine Li₂MnSiO₄ nanoparticles having Pmn21 configuration in the range of 35 – 50 nm. It is also seen that amount of carbon loading could be precisely controlled during synthesis of C/Li₂MnSiO₄ nanocomposites using water impregnation process. According to the conductivity measurements, electrical conductivity of the C/Li₂MnSiO₄ nanopowders could be increased by optimizing, in this case increasing, the carbon loading in the composites. It is seen that formation reaction of a passivation layer affects the measured cell capacity during the first cycle of galvanostatic charge-discharge tests. Thus, capacity values measured for the second charge-discharge cycles are used to compare the battery capacities. Even though highest stability (least reactivity) is observed for LiClO4(TMS:EMC) between the studied electrolyte solutions; highest battery capacity, based on the charge discharge tests, is observed for the organic electrolytes containing LiPF6 salts. The highest battery capacity observed throughout the study was 177.0 mAh/g, for the second charge-discharge cycle, and it was measured for the battery prepared using C/Li₂MnSiO₄ nanocomposite containing 30 wt.% of carbon and 1M LiPF₆ in EC:DMC electrolyte solution. XPS analyses of the C/Li₂MnSiO₄ nanocomposite, taken out of the same type of battery after charging process, showed that complete delithiation of the cathode materials could be done electrochemically. Neither of the applied chemical delithiation techniques were successful for complete delithiation of the pristine samples, among which only 3 of them could chemically delithiated pristine Li₂MnSiO₄ nanoparticles.