DEVELOPMENT OF NOVEL ELECTRODE/ELECTROLYTE SYSTEMS FOR A LI-ION CELL WITH HIGHER ENERGY DENSITY AND LONGER LIFETIME
The energy density of lithium ion cells can increase when the charge cut-off potential increases or high Ni content positive electrode materials are used. However, this normally decreases the lifetime of cells because of the parasitic reactions that occur between the electrolyte and the charged electrodes. This thesis project focuses on developing high voltage lithium ion cells having high energy density and long lifetime. Ethylene carbonate (EC) is normally used as an electrolyte solvent in all Li-ion cells due to its stability and most lithium ion battery scientists believe EC is essential for good Li-ion battery operation. Surprisingly, with an appropriate amount of electrolyte additive to passivate the graphite electrode, cells with only 1M LiPF6 plus ethyl methyl carbonate (EMC) functioned well up to 4.4 V vs. graphite. In order to optimize the amount of electrolyte additive used in the cells, gas chromatography (GC) coupled with mass spectrometry (MS) was used to track the consumption of several additives and the trans-esterification of EMC during formation, cycling and storage. Some other advanced analysis methods were also used to obtain a better understanding of EC-free electrolytes. The effect of lithium difluorophosphate (LiPO2F2) as an electrolyte additive on single crystal Li[Ni0.5Mn0.3Co0.2]O2 (NMC532)/graphite pouch cells was examined. The combination of coated single crystal NMC532 and LiPO2F2 is extremely useful for extending cell lifetime, especially when some other suitable electrolyte additives (e.g. fluoroethylene carbonate (FEC)) are used at the same time. The effect of a surface coating on single crystal NMC532 was also investigated systematically.