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.