A Comparative Study of Pyridine Containing Lewis Acid-Base Adducts as Additives for Li[Ni0.5Mn0.3Co0.2]O2/graphite Pouch Cells

Abstract

Three Lewis acid-base adducts including pyridine boron trifluoride (PBF), pyridine phosphorus pentafluoride (PPF) and pyridine sulfur trioxide (PSO) were used as electrolyte additives in Li[Ni0.5Mn0.3Co0.2]O2/graphite pouch cells (NMC532/graphite). PBF, PPF and PSO have a common Lewis base (pyridine) but have different Lewis acids. Experiments included ultra-high precision coulometry (UHPC), electrochemical impedance spectroscopy (EIS), gas evolution, long-term continuous charge-discharge cycling and charge-hold-discharge cycling. The results showed that cells containing PBF and PPF had the smallest voltage drop during storage at 4.5 V and at 60°C compared to cells with PSO and other additives such as triallyl phosphate (TAP) and vinylene carbonate (VC). UHPC results showed that the coulombic efficiency (CE) of the cells could be improved by using PBF or PPF either singly or in combination with other additives. The charge-hold-discharge cycling protocol can be used to distinguish the difference between additives relatively quickly compared to continuous cycling. Cells with 2% PBF had the best capacity retention compared to cells with the other additives at 40°C.