Determining the Origin of Lithium Inventory Loss in NMC622||Graphite Lithium Ion Cells Using an LiPF6-Based Electrolyte

Abstract

X-ray photoelectron spectroscopy (XPS) is often used in interphase investigations of lithium ion batteries (LIBs). Yet, it is unclear, if its results convey an accurate picture of the lithium loss in aged LIBs. Herein, electrochemical and surface analytical approaches were used to separately quantify the contribution of interphase growth to lithium loss in LIBs. For this, LIB pouch cells (NMC622||graphite, 5 Ah) were aged for 400 full cycles at 20 °C or 60 °C. Electrodes were harvested post mortem and subsequent investigations in lithium metal battery cells showed notably higher reversible and irreversible lithium loss after 60 °C than after 20 °C cycling. While the interphases did not notably increase in thickness with aging, the surface area of both electrodes increased, leading to more electrolyte decomposition and larger lithium loss. Along with the surface area increase, more heterogeneous electrolyte decomposition product residues on the negative electrode surface and higher cathode|electrolyte charge-transfer resistances were observed. In conclusion, the applied combination of XPS and nitrogen adsorption can quantify homogenously distributed electrolyte decomposition layers of thicknesses <10 nm, but not thick and heterogeneous decomposition product residues arising with 60 °C cycling. For this, the need for an alternative quantification method is highlighted.