Production of lithium rich NCM cathode via sol-gel method & enhancing electrochemical properties with Al2O3 surface modification

Humanity need renewable energy resources because of limited conventional energy sources and need to store energy which is generated by clean energy resources. Moreover, high energy density systems are gained importance with developing technology. The main objective of high energy density systems is giving high performance. In recent years, people need not only high performance but also lower weight and smaller size as far as possible. These needs create opportunities to develop rechargable lithium ion battery technology. There are numerous studies which are aimed to produce lithium ion batteries with higher electrochemical performance and better physical properties. Lithium ion batteries are secondary batteries and that group of batteries are rechargable. Also, lithium ion batteries have high gravimetric and volumetric energy density which gives properties like high energy density with low volume and low weight. The other advantages of lithium ion batteries are; no memory effect, no need to maintenance, low self-discharge rate, high coulombic efficiency, low toxicity. On the other hand, lithium ion batteries have disadvantages like poor cycle life, safety risks if overcharged. Li-ion batteries provide lightweight, high energy density power sources for a variety of devices such as smart phones, electronic tablets, laptops, digital cameras and camcorders. Furthermore, lithium ion batteries are using in electronic vehicles. In next decade more lithium ion batteries can be seen at bicycles, cars and aircrafts. Also, lithium ion batteries have other usage areas like telecomunication, military, medical applications and storing energy which are generated from renewable sources. Lithium ion batteries components can be classified in 4 main categories; these are anode (negative electrode), cathode (positive electrode), separator and electrolyte. During charging, Li+ move from cathode to anode and electrons move from the outer circuit at same direction with Li+, current moves to opposite direction with electrons at the outer circuit. During discharge ions and electrons move vice versa. Generally, carbon, silicon and tin based materials are using as anode material, layered LiCoO2, spinel LiMn2O4, olivine LiFePO4, layered LiNi0,8Co0,15Al0,05O2 (NCA) and LiNiCoMnO2 (NCM) are using as cathode materials. Nowadays, LiNi0,8Co0,15Al0,05O2 (NCA) and LiNiCoMnO2 (NCM) high energy density cathode materials are very popular because these materials can be used in electicle vehicles. Also, lithium rich NCM cathode materials are new alternative cathode materials because of its high discharge capacity (220-300 mAh/g) and high working voltage range (2-4.8V). Lithium rich NCM cathode materials have two phases such as LiMO2 (M=Ni, Co, Mn) and Li2MnO3. Due to Li2MnO3 phase, lithium rich NCM cathode materials give higher capacity over 4.4 V. Therefore, lithium rich NCM cathode materials give higher energy densities than NCM and NCA cathode materials. On the other hand, lithium rich NCM cathode materials face some challenges; first, electrolyte oxidation at cathode-electrolyte interface and irreversible Li loss caused by Li2O release from Li2MnO3 to electrolyte at high voltages in first charge. Second, transition metal decomposition/dissolution from active material as a result of HF attacks to cathode. Third, transformation of the structure from layered to spinel. All of them cause capacity fade and that problem has to be solved to commercialize Li-rich NCM cathode materials. Metal oxide surface modifications are used to overcome these challenges and Al2O3 is the one of the most promising surface modification material in literature. In the scope of this thesis, lithium rich NCM (Li1,2Ni0,2Co0,08Mn0,52O2) powder is produced via sol-gel method at 850°C heat treatment temperature. Afterwards 0.5 wt.% Al2O3 surface modification is applied via sol-gel method to improve electrochemical performance by preventing the challenges of Li-rich NCM cathode materials. Getting lower capacity fade than the pristine sample with a low wt% surface modification is the purpose of applying 0.5 wt.% Al2O3 surface modification. Due to low wt% surface modification, high initial charge and discharge capacities can be achieved by increasing the active material ratio. Sol-gel method is used for producing both powders because sol-gel method offers sub-micron particle size, has relatively low cost and low toxicity. Powders produced are characterized with XRD, SEM and EDS analyses. After these characterization methods, powders are coated on the aluminium foil with lamination method. After battery assembly, half cell batteries are tested with electrochemical test methods. Galvanostatic test, C-rate test, cyclic voltammetry and impedence analyses are applied to both samples. Also, SEM analyses applied to both samples after 50 cycles at galvanostatic test. XRD and SEM characterization methods are used to understand the effect of phases and surface morphology to electrochemical performances. In XRD result, all necessary peaks are observed for LiMO2 (M=Ni, Mn, Co) and Li2MnO3 phases. Especially, existence of superlattice peaks (C/2m) shows that Li2MnO3 phase is occured. Li2MnO3 phase is activated above 4.4 V and provide higher capacity. On the other hand, I(003)/I(104) ratio is lower than 1.2. This result shows that the powder has high cation mixing and this can cause capacity fading. In SEM images, powder particle size vary between 50 - 200 nm and particles are sphere-like shaped. There is no difference observed on SEM images between Al2O3 surface modification applied sample and pristine sample. Also, EDS analysis is applied to samples to understand elemental ratios and all elements are existed nearly same with stoichiometric ratios. EDS analysis results are proved that Al2O3 surface modification applied as 0.5 wt.%. Electrochemical tests are applied to understand electrochemical performance of produced lithium rich NCM cathode material and effects of 0.5 wt.% Al2O3 surface modification. Galvanostatic test is applied between 2-4.8 V and 0.1C rate (25 mA/g). The oxidation of nickel is seen in 1st charge at 3.8 V and at 4.4 V cobalt oxidation is occurred. At 4.4 V, Li2MnO3 phase is activated. After that, charge capacities of pristine and Al2O3 surface modification applied samples are increased rapidly to 341.1 mAh/g and 323.3 mAh/g, respectively. In the second charge, both charge capacities are decreased but charge capacity of pristine sample decreased more than the surface modified sample. Main cause of that issue is Li2O release from Li2MnO3 to electrolyte after Li2MnO3 activation at high voltages in first charge and that problem is suppressed with Al2O3 surface modification. Initial discharge capacities of pristine and modified samples are found as 228.7 mAh/g and 235.2 mAh/g, respectively. Discharge capacity retentions are calculated as %68.9 ve %87.9 after 50 cycle. According to galvanostatic test results, capacity fade is decreased with 0.5 wt.% Al2O3 surface modification. Cyclic voltammetry test is applied between 2-4.8 V, anodic and cathodic peaks are occured at proper voltages. In impedence test, Al2O3 surface modification applied sample has lower charge-transfer resistance than the pristine sample. That result shows that, Al2O3 coating layer could act as a protective barrier which prevents the reactions between the cathode and electrolyte. Also, it could increase the structural stability of the cathode. C-rate test is applied at 0.1C, 0.2C, 0.5C, 1C, 2C and again 0.1C rates. Modified sample has higher resistance than pristine sample to increasing C-rates. That test result matched with impedence test results. After 50 cycle, SEM results show crack in the pristine sample but no crack is observed at the modified one. It can be said that, structural stability on the surface could be improved by Al2O3 surface modification. In conclusion, 0.5 wt.% Al2O3 surface modification has shown an improvement in lithium rich NCM cathode material as far as electrochemical performance is concerned. Al2O3 surface modification suppresses capacity fade and enhancement in capacity retention can be accomplished via low wt.% Al2O3 surface modification.