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

•The effect of LiPF6 concentration on high voltage cycling has been evaluated.•Li[Ni0.4Mn0.4Co0.2]O2/graphite pouch cells were used as test vessels.•The impedance of the positive electrode increases during high voltage cycling.•High LiPF6 concentration lowers the impedance growth rate of cells operated at 4.5 V.•The SEI of the electrodes is affected by LiPF6 concentration variations.

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.