SOL-JEL YÖNTEMİ UYGULANARAK ZrO2 İLE YÜZEY MODİFİKASYONU YAPILMIŞ LiMn2O4 YAPISININ SENTEZLENMESİ VE KATOT AKTİF MALZEMESİ OLARAK İNCELENMESİ

At present time, the most of the individual and massive requirements are met by using advanced energy storage devices. There is an increasing demand on portable electrical and electronic equipment in markets due to these requirements. The basic terms to sustain the functionality and high efficiency of these devices are having a safe energy storage with high energy density, long lasting, easily maintained, enviromentally benign and charged at short time. Thus, in recent years in order to meet the individual and massive requirements, research and development activities relative to lithium ion batteries have become crucial. A lithium ion cell basiclly consists of anode, cathode and electrolyte. The working principle of lithium-ion batteries is the movement of lithium ions between anode and cathode through an electrolyte. Lithium-ion is a low maintenance battery, an advantage that most other chemistries can not claim. There is no memory effect and no scheduled cycling is required to prolong the battery's life, performs high specific and volumetric energy density. In addition, the self-discharge is less than half compared to nickel-cadmium. Because of its advantages lithium ion batteries has become primary energy sources for consumer electronics and EVs. With the increasing demand in those applications R&D studies for LIBs has been accelarated to meet the requirements. LiMn2O4 is ideal as a high-capacity Li-ion battery cathode material by virtue of its low toxicity, low cost, and the high natural abundance of Mn. The architecture forms a three-dimensional spinel structure that improves ion flow on the electrode, which results in lower internal resistance and improved current handling. A further advantage of spinel is high thermal stability and enhanced safety, but the cycle and calendar life are limited. Initial capacity lost and low rate performance are most crucial drawbacks for LIBs. There are lots of researches continues to commercialize new active materials and also improve the electrochemical performance of commercial active materials. Surface modification, doping using different additives are some of these techniques for enhancement. Anionic and cationic dopings and surface modifications are one of the most important methods to improve the electrochemical performance of cathodes. By doping structural stability can be increased and with surface modifications active material can be protected from the highly reactive hydrofluoric acid which improves the cyclability. As the solid electrolyte interface (SEI) of cathode is far thinner than anode SEI there should be an extra passivation film at the surface of cathodes to prevent the irreversible reactions that causes loss of active material. Furthermore, surface modification can preserve the electrode integrity and it can also decrase the charge transfer resistance that enhances the rate performance of electrode. LiMn2O4 can be synthesized by different methods like solid-state reactions, pechini and sol-gel methods. Among these methods sol-gel offers low cost, less toxicity and sub-micron particle size. This method not only for material production but also for surface modifications. To this respect in this study that production method was chosen. In the scope of this thesis LiMn2O4 active material is synthesized and it was modified by nucleating the ZrO2 particles at surface of LiMn2O4. Gel precursors is dissolved in distilled water and stirred at 200 rpm and 80 oC temperature for 4 hours until the gel structure attained. After that, obtained gel dried at 100 oC for 24 hours and than heat treatment is applied. First treatment was for removing organic compounds from gel (500 oC) and the second heat treatment for crystallization of LiMn2O4 (850 oC) in order to obtain fine LiMn2O4 powders. Surface modification of LiMn2O4 is started by adding LiMn2O4 powders in to zirconia sol. Than, sonication was started and three different sonication powers were tested (10%, 30%, 50%). After the second gel is obtained, last heat treatment for ZrO2 crystallization was done at 500 oC for 5 hours. For surface modification ZrO2 is chosen due to its excellent corrosive resistance against reactive compounds and the reason to test different sonication parameters was to observe the distribution of ZrO2 particles at LiMn2O4 surface and its effects at electrochemical performance of LiMn2O4. After surface modification, powder mix is obtained (which contains 80% of active powder, %10 carbon black, %10 binder) by adding binder polymer and carbon black. Than this powder mix is laminated on 12 µm aluminium foil. Each coating thickness is adjusted to 150 µm by Doctor Blade gauge. XRD, SEM techniques are used to observe the effect of surface morfology to charge/discharge performances. XRD values show that active powders do not contain any impurity phases which have negative effect on capacity values, instead powders contain LiMn2O4 spinel phase primarily. In SEM images, powder particle size vary between 750 nm-1500 nm. Particles are sharp edged and have smooth surface. After ZrO2 coating particles have soft edges with rough surface. For galvanostatic and potentiostatic analyzes electrodes are punched and tested with different current densities. After 100 cycle 46.66 % capacity retention is obtained from bare LiMn2O4. ZrO2 coated sample with magnetic stirring is showed 45.9 % capacity retention and this ratio is increased by using ultrasonic stirring. The sample that has the highest sonication power achieved 57.06 % capacity retention. This shows that with the increase of sonication power ZrO2 particles distributed at LiMn2O4 surface more homogenously and by this way Mn dissolution is restrained. CV and EIS analyzes are performed for further information of cathode structure. In CV graphs, polarization at the surface of LiMn2O4 can be seen by comparing bare LiMn2O4 and ZrO2 coated LiMn2O4 due to shift at cathodic and anodic peaks. However, in EIS results this polarization that occurs beacause of the low conductivity of ZrO2 did not effect the overall empedance and the charge transfer resistance is decreased with the increase of sonication power. C-rate tests also proofed the EIS results. At 2C charge/discharge rate best performance is achieved from the highest sonication speed performed sample. SEM analyses performed again after 100 cycles. There is no crack observed at the surface of samples. This shows there is a low Jahn-Teller effect inside the structure due to low spin Mn+3 ions.