|M.Sc Student||Alon Shapira|
|Subject||Study and Understanding of Particularized Protective Metal|
Fluorides Films Developed on High Potential Li-Ion
Battery Cathode Materials
|Department||Department of Materials Science and Engineering||Supervisor||Full Professor Ein-Eli Yair|
|Full Thesis text|
Electric vehicles are the future of land transportation. In order to have less expensive cars with longer driving distance, less expensive batteries with higher specific energy and with higher volumetric energy are needed. In order to achieve those goals, research on LIBs is focused on the different battery components: switching graphite to silicon to get higher energy, adding additives to the electrolyte to increase the battery’s stability and moving towards 5V cathode materials.
Capacity fading is a major drawback in lithium ion battery (LIB) based on the 5V cathode material LiMn1.5Ni0.5O4 (LMNO). LMNO is a very promising high voltage cathode material with the theoretical capacity of 147mAh g-1. Even though it’s been researched extensively in recent years, no solution has been found to mitigate the capacity fading which is prominent especially at elevated temperatures when cycled against graphite. The focus of this research is on preventing manganese dissolution from the cathode which is responsible for most of the capacity fading causing the battery to fail.
In this study, a novel approach of coating the cathode material was implemented. Atomic layer deposition (ALD) of AlF3 was used to coat LMNO in its powder form, in order to protect the cathode material from interacting with the electrolyte and to prevent or slow down transition metal dissolution into the electrolyte, in particular preventing manganese dissolution.
Stability tests to the pristine and AlF?3 coated powder were performed in the electrolyte without applying voltage. The high resolution scanning electron microscope (HRSEM) images showed corrosion damage for the pristine powder which was enhanced at 45⁰C and intact LMNO particles when coated with 2 ALD cycles at 150⁰C of AlF3.
Electrochemical analysis in half-cell configuration showed better capacity retention with a better coulombic efficiency for AlF?3 coated cathode. AlF?3 coated cathode exhibited better capacity retention and better coulombic efficiency at elevated temperatures of 45⁰C at different C-rates experiments.
At first, electrochemical analysis in a full-cell configuration displayed no advantage for AlF3 coated cathodes, but after cells were composed with anodes that underwent treatment, the capacity retention was much better for coated cells than for pristine cathodes.
Coated cells showed lower initial capacity which was recovered to the pristine cells' values during cycling. A new formation cycling procedure was implemented, which bumped the initial capacity of AlF3 coated cells to their stable values after several cycles.
Fluorination or lithiation of the coating layer were suggested as mechanisms which caused the capacity recovery phenomenon in the coated cells.
Electrochemical results show equal or better capacity retention for all coted cells. Full-cells composed of 2 ALD cycles at 240⁰C were cycled for 180 cycles (150 of them were at 45⁰C). This result equals to ~600 cycles at RT.
With finer tuning of the ALD process (changing the deposition temperature/number of cycles etc.), an optimize coating layer of AlF3 will provide the best results, which may mitigate the capacity fade of the LMNO/graphite system.
In addition, metal fluorides coating using ALD method can be adapted to other LIB cathode material.