Investigation of glycerol-based sol-gel synthesis of Li1Ni0.8Co0.1Mn0.1O2 layered transition metal oxide cathodes for lithium-ion batteries
Authors:   , Lohman Samuel David 
Published: October 2023

[EMBARGOED UNTIL 8/1/2024] Lithium-ion batteries (LIBs) are the predominant portable power source today, and much attention is being given to the cathode materials of LIBs. LiNi1-y-zCoyMnzO2 (NCM) materials utilize the advantages of cobalt's structural stabilization, thus enhancing capacity retention. Nickel also maintains reasonable structural stability, while being much cheaper than cobalt, and having the highest redox potential vs lithium. Finally, manganese increases the chemical stability of the material. Li1Ni0.8Co0.1Mn0.1O2 (NCM811) is of great interest to current research because it might be able to achieve working capacities in the range of 220-240 mAh/g, with initial discharge capacities possibly as high as 270 mAh/g. The sol-gel process for NCM811 synthesis typically uses H2O as a solvent. Water provides only 1 OH- group per molecule that can participate in the hydrolysis reactions. On the other hand, glycerol can provide 3 hydroxyl groups per molecule. This group has already demonstrated that the use of glycerol as a solvent is in fact a superior method of synthesis. The increased number of hydroxyl groups leads to faster crystal formation, which leads to more uniform and more stable structures. It was therefore the goal of this work to synthesize NCM811 cathode materials with improved crystalline structure, and therefore higher capacities and better capacity retention, using the glycerol-based sol-gel method of synthesis. This thesis details the results of this effort, as well as many supplementary studies undertaken along the way. Batteries are assembled from materials synthesized in this way, and potentiostatically cycled to evaluate performance related to a variety of relevant processing conditions. At charge rates of 0.1C, batteries achieve initial discharge capacities of ~210 mAh/g, with exceptional capacity retention of ~93 percent at 100 cycles, and > 80 percent up to 250 cycles. ICP-MS and XRD are used to show that the material was in fact NCM811, and that it has good, layered structure and cationic ordering. It is further shown that while powders lose their capacity during long storage times, they can be revived with heat treatment. It was discovered that the cathode materials must be annealed in small batches to have good performance. It is demonstrated that it matters very little, or not at all, whether the cells are rested for 3 days, rested 3 minutes, or sonicated 3 minutes. In fact, a larger improvement results from the changing of the cell assembly procedure to include better electrolyte distribution. It is discovered that polishing the cell holders resulted in better electrical contact, improving the smoothness of the discharge capacity curves. It is also demonstrated that NCM811 batteries made with cathode materials annealed in powder form performed as well or better than those annealed in pellet form. It is discovered that batteries made from NCM811 that have been synthesized with 10 percent excess lithium measured (actually 8.5 percent) have vastly superior performance to those from NCM811 that have been synthesized with 4 percent excess lithium measured. Finally, from the glycerolate studies, it is concluded that the transition metal precursors, nickel, cobalt, and manganese, do in fact form glycerolates under the experimental conditions designed to isolate the reaction intermediates of the glycerol-based sol-gel method of layered transition metal oxide cathode synthesis. However, it is also concluded that Lithium does not form glycerolates with the same molecule of glycerol as the other transition metals.