Variation of coulombic efficiency versus upper cutoff potential of Li-ion cells tested with aggressive protocols

• Coulombic efficiency depends on the cycling protocol and upper cutoff potential.
• Longer exposure, per cycle, to high V lowers coulombic efficiency.
• Longer exposure, per cycle, to high V does not increase short-term capacity fade.
• Oxidation products migrating to the negative electrode do not consume active Li.
• High V exposure causes an increase in cathode/electrolyte interface impedance.

Three different cycling protocols including “continuous-cycling”, “barn-charge” and “cycle-store” were applied with an ultra high precision charger to Li[Ni0.42Mn0.42Co0.16]O2/graphite and/or Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cells tested using different upper cutoff potentials. The barn-charge and cycle-store protocols were designed so that cells stay at high potential for a larger fraction of their testing time compared to continuous cycling. For cells tested to 4.2, 4.4 or 4.5 V, the greater the fraction of testing time spent at high potential, the lower the coulombic efficiency and the greater the charge endpoint capacity slippage rate, with the effects being more severe at higher potential. These results confirm that Li[Ni0.42Mn0.42Co0.16]O2/graphite and Li[Ni1/3Mn1/3Co1/3]O2/graphite Li-ion cells which are charged and then left at high potential (>4.4 V) for extended periods of time will have much shorter calendar and cycle life compared to those that are continuously cycled as has been recently reported in long-term test results.