Development and chracterization of dual active material cathodes for secondary lithium ion batteries

Cathode active materials, one of the main components of lithium ion batteries, are still unable to deliver the performance (energy intensity, power intensity, security and price) required by many applications. For this reason, numerous studies are carried out to improve the performance of each of the cathode active material groups with techniques such as nano configuration, doping and surface modification. One of the relatively new strategies for performance improvement is to blend two or three materials from different groups with physical mixing and produce superior electrodes that balance the avanlages of each of these material groups. Through blending, the deficiencies of the main materials can be minimized and the resulting blend can be tailored to have higher energy or power density with higher stability at a lower cost. In this dissertation, prepared with this motivation, composite cathodes were produced by blending layered oxide (Li[Ni0,5Mn0,3Co0,2]O2, NMC) and phospho-olivine (LiFePO4, LFP) active materials in different proportions and the electrochemical performance of the CR2032 type batteries having these composite cathodes were investigated. To determine the electrochamical properties depending on the cathode composition rate performance, cycle performance, cyclic voltmetry and electrochemical impedance spectroscopy measurements were performed. It was observed that the blended electrode containing 20% LFP shows significantly higher discharge capacity at high discharge rates (> 2C) compared to other electrodes. It was also observed that the capacity retention of the blended electrodes increased with increased amount of LFP after 100 cycles at 1C. In cyclic voltmetry, it was observed that, at about 20% LFP the redox peak intensities of iron and nickel ions have similar values. When the Nyquist curves were examined, lowering of internal resistance up to 20% LFP addition was observed in freshly batteries, while an increase in interface resistance was observed with the increase in the number of cycles for all electrodes. It was concluded that 80%NMC+20%LFP blended electrode is the best composition for the applications that require high energy density with high cycle life. It should be noted that this value may vary for different size and shape distributions of the particles that make up the porous electrode.