Impact of a Titanium-Based Surface Coating Applied to Li[Ni0.5Mn0.3Co0.2]O2 on Lithium-Ion Cell Performance

The effect of a Ti-based surface coating on Li­[Ni0.5Mn0.3Co0.2]­O2 (NMC532) positive electrode material in NMC532/graphite Li-ion pouch cells has been investigated using high temperature storage testing (60 °C), ultrahigh precision coulometry, electrochemical impedance spectroscopy, accelerating rate calorimetry, and long-term cycling. Several superior electrolyte additive combinations were selected for this study. Comparing data from cells containing coated and uncoated NMC532 showed that the surface coating generally contributed to improved cell performance from many perspectives; however, for one electrolyte additive combination, cells containing coated and uncoated NMC532 had virtually identical excellent performance. In an effort to understand why the coating was effective, X-ray photoelectron spectroscopy was used to study the solid electrolyte interphases on both coated and uncoated NMC532. X-ray fluorescence studies of negative electrodes harvested from aged cells showed that the coating helped to prevent transition metal dissolution, although the amounts of metal dissolved were very small from both coated and uncoated NMC532. The “pouch bag” method was also used to study the effect of interactions between delithiated NMC532 (coated or uncoated) and lithiated graphite on gas evolution and impedance growth.