Effect of LiPF6 concentration in Li[Ni0.4Mn0.4Co0.2]O2/graphite pouch cells operated at 4.5 v

The effect of LiPF6 concentration in the 1 M-2.5 M range was studied in Li[Ni0.4Mn0.4Co0.2O]2 (NMC(442))/graphite and LaPO4 coated-NMC(442)/graphite pouch cells cycled to high voltage. Electrochemical impedance spectroscopy on symmetric cells revealed that the dramatic impedance growth observed in NMC(442)/graphite cells cycled to high voltage comes from the interface impedance of the positive electrode. The use of high LiPF6 concentrations in the 2-2.5 M range dramatically slowed down the impedance growth of both coated and uncoated NMC(442)/graphite cells containing certain electrolyte additive blends and cycled to high voltage. However no beneficial effect was observed in control cells containing no electrolyte additive. X-ray photoelectron spectra of cycled electrodes of coated-NMC(442)/graphite cells showed that LiPF6 concentration greatly affected the composition of the solid electrolyte interphase of both the positive and negative electrodes of cells containing additives.