LiMn2-xMxO4 (M = Li, Co) cathode production via sol-gel method and characterization for li-ion batteries

Today, the usage of rechargeable storage systems (secondary lithium ion batteries) have been increased significantly. The importance of battery systems have been increased due to extensive usage of mobile electronic devices and demand for electric cars in all over the world. The basic reasons to prefer these systems are being light, having the best electrochemical potential and high energy density. Secondary lithium ion batteries are preferred more than other battery systems due to high energy density and rechargeable property. Today, developments on current secondary lithium ion battery technology still continue. To increase energy density, cycle life and to eliminate safety issues are the key parameters that researchers work on. Mobile electronic device and eletric car technology exist with lithium ion battery technology however, the performance expectations are excessive from lithium ion batteries depending on rapid development of device technology. Researchers mainly work on battery components (cathode, anode, seperator, electrolyte) to overcome these expectations. Main components of the system are negative electrode (anode), positive electrode (cathode), electrolyte that contains lithium salts and seperator that seperates positive and negative electrodes from each other. Lithium ions delithiate from electrodes and travel between positive and negative electrodes when lithium ion battery in operation. In state of charge, lithium ions delithiate from positive electrode structure, passes through seperator that shows permeability property to lithium ions and lithiate into negative electrode structure. In state of discharge, lithium ions delithiate from negative electrode structure, passes through seperator and lithiate into positive electrode structure as a reverse of charge condition. At the same time, free electrons move through outer circuit and supply energy to the device in which lithium ion battery is used. Lithiation and delithiation mechanisms damage the electrodes structure in time. To minimize this negative effect is the most critical subject of concern. Only in this way long cycle life can be achieved. Basicly, researchers deal with this negative effect of lithiation and delithiation mechanisms to increase battery cycle life. Mainly cathodes consist of spinel and layered structure materials. Cathodes are source of lithium ions that moves in electrolyte during lithium ion battery cycles. So that, lithium ion battery performance mostly depends on cathode performance. Commercially, LiCoO2, LiMn2O and LiFePO4 cathodes are used. Researches are based on to decrease cost and degree of environmental hazard, increase capacity and cycle life. There are several methods to improve stated properties of cathode materials. These methods are to obtain small particle size, composite material, doping, morphology control, coating, electrolyte modification. Active cathode material with smaller particle size icreases cathode surface area and capacity values accordingly. Composite cathodes are manufactured to obtain two or more positive property in the same structure. Doping method is used to minimize negative behaviour of main element in cathode by integrating different elements into the system with small amounts. Coating process is used to prevent cathode structure deformation after long cycles by forming thin film layer on main structure. Morphology control is used to control particle shape and homogenization. New electrolyte researches are made to invent an electrolyte which does not react with cathode active material. There are wide range of researches regarding preparation technology and electrochemical performance of cathode materials. Mostly, Sol-Gel method is preferred as a preparation route. The fundemantal reasons to use Sol-Gel method are to obtain homogeneous chemical composition and particle size, to process at low temperatures and durations. In addition to that preperation technology, gathering positive properties of the elements, which are used as a cation element, in one cathode material is a key technique to obtain better electrochemical results. In this study, seven active material with different stoichiometries are produced via Sol-Gel method: LiMn2O4, LiCo0.2Mn1.8O4, LiCo0.4Mn1.6O4, LiCo0.6Mn1.4O4, LiMn1.9Ni0.1O4, LiMn1.7Ni0.3O4, LiMn1.5Ni0.5O4. 100 ml Sol-Gel solutions are prepared for each powder, after 4 hours of process gel structure is obtained with 250 rpm stirring speed, pH value of 7 and 80 °C solution temperature. Obtained gel sctucture is dried at 100 °C for 24 hours. After drying process, the sample is calcined and crystallized at 800-850 °C with a certain heating ramp to obtain final crystallographic structure. Under 800-850 °C, the calcination and crystallization process is not enough to obtain exact powder stoichiometry and crystallographic structure. The reason to choose seven spesific compositions are to investigate different cation effect (cobalt and nickel) and cation amount on electrochemical capacity and cycle life. XRD, SEM, EDS techniques are used to understand the effect of active material amount to the charge/discharge performances. XRD graphs show that active powders do not contain any impurity phases which have negative effect on capacity values, instead powders contain LiMn2-xMxO4 spinel phases primarily. In SEM images, powder particle size varies between 100 nm-1000 nm. Particles are not spherical or sharp edged instead have soft edges with facet structure. EDS analysis show that desired chemical compositions are obtained in accordance with theoretical stoichiometric calculations. Before lamination process, powder mix was obtained (which contains 80% of active powder, %10 carbon black, %10 binder) by addition of binder powder and carbon black and this powder mix was laminated on 19 µm aluminium foil. Doctor Blade method was used for lamination process. Each coating thickness was adjusted to 500 µm by Doctor Blade gauge. When binder powder is used less than a certain amount, powder mix does not stick properly on the aluminium substrate. In order to get proper sticking behaviour 10% binder powder was used for powder mix. Besides, multiple rolling of the coating favors the sticking behaviour. In addition to that, rolling operation will contribute to cycle life of batteries during high current density experiments. Stereo microscope was used to obtain images of the coatings. Stereo microscope images represent that all coatings have homogenious distributed cracks. Luckily, after cycle EDS analysis do not show any aluminium (substrate) peaks. Coatings were cut according to the lithium ion cell dimensions and half lithium ion cells were assembled. Charge/Discharge performance of cathode materials were studied with 25, 50, 100 mA/g current density respectively. Charge/Discharge graphs show that, the active material with Ni additive has the highest charge capacity value. Active material with stoichiometry of LiMn1.9Ni0.1O4 exhibits 126 mAh/g specific charge capacity value at 25 mA/g current density. This capacity value is 85% of LiMn2O4 theoritical charge capacity (148 mAh/g). Additionally, 85% capacity retention was obtained after 30 cycles. Active material of same composition was tested with 50 mA/g and 100 mA/g current densities. This composition was choosed due to highest capacity value among the other compositions. Charge/discharge electrochemical graphs show that 118 and 68 mAsa/g first charge specific capacity were obtained respectively and %89, %99 capacity retentions were obtained after 50 cycles. The capacity retention values tend to increase with increasing current density (which also means high cycle speed). This behaviour is promising to have stable cell structure after many charge/discharge cycles. SEM, EDS, CV characterization techniques were used to investigate charge/discharge performances in detail. Coatings with low capacity performances have more and oversized cracks in SEM images. EDS graphs show that composition of the structure are stable generally. However, the powder with composition of LiNi0.1Mn1.9O4 does not have homogenious composition in all the surface of coating. This condition explains that sol-gel parameters were not constant during all process for this powder. Even so best electrochemical performances were obtained with that powder. CV results show that two stage reactions do not occur and capacity values get lower with increasing doping amount.