Correlating long-term lithium ion battery performance with solid electrolyte interphase (SEI) layer properties

This study was conducted to understand effects of some of key factors (i.e., anode surface properties, formation cycling conditions, and electrolyte conditions) on solid electrolyte interphase (SEI) formation in lithium ion batteries (LIBs) and the battery cycle life. The SEI layer passivates electrode surfaces and prevents electron transfer and electrolyte diffusion through it while allowing lithium ion diffusion, which is essential for stable reversible capacities. It also influences initial capacity loss, self-discharge, cycle life, rate capability and safety. Thus, SEI layer formation and electrochemical stability are primary topics in LIB development. This research involves experiments and discussions on key factors (graphite surface properties, electrolyte volume, and formation cycle) affecting SEI formation. For the graphite anode surface property study, ultraviolet (UV) light was applied to battery electrodes for the first time to improve the SEI and cycle life. UV treatment for 40 minutes resulted in the highest capacity retention and the lowest resistance after the cycle life testing. Anode analysis showed changes in surface chemistry and wetting after the UV treatment. It also showed increases in solvent products and decreases in salt products on the SEI surface when UV-treated anodes were used. XPS analysis showed that UV light decomposed polyvinylidene fluoride (binder) but helped to increase the oxygen level on graphite, which, resulted in a thin SEI layer, low resistance, and eventually high capacity retention. For the formation cycling condition study, a fast SEI formation protocol was proposed. The protocol involved more (shallow) charge-discharge cycles between 3.9 V and 4.2 V and fewer (full depth of discharge) cycles below 3.9 V. It improved SEI and capacity retention and shortened formation time by 6 times or more without compromising cell performance. To understand effects of electrolyte conditions, electrolyte volumes were controlled in full cells. A minimum electrolyte volume factor of 1.9 or 3 times the total pore volume of cell components (cathode, anode, and separator) was needed for long-term cyclability and low impedance of cells consisting of graphite anode or 15 weight percent Si-graphite anode, respectively. Less electrolyte resulted in an increase of the measured Ohmic resistances.