During typical lithium-ion cell usage over application lifetimes (e.g. automotive) they form a film, which is nanometres thick, on the surface of the negative electrode which helps to passivate against further reaction. Its chemical composition and thickness affect capacity and power capabilities of a cell through 1. Consumption of lithium inventory during the surface film forming reactions and 2. More surface film which increases surface resistance. Over time and with further cycling, this film thickens. Understanding and controlling surface film thickness and composition as well as its relationship to cell operating conditions could provide a pathway to extend cell and device service life.

As a first step toward achieving this goal we focused on characterising surface film thickness as a function of cell age and operating conditions. Currently, there is no reliable method for determining surface film thickness at the electrode surface. We tested the capacity and power of commercial 18650-sized cells cycled at different C-rates or stored at different temperatures and states of charge. We then present a method of XPS analysis to determine the relative surface film thicknesses within each of these three conditions. XPS analysis was completed at two different institutions on two different systems to determine reproducibility.

The novel XPS method reliably showed that the surface film thickness increased with temperature, state of charge and higher C-rates.

We found that the chemical composition of the surface film changed as a consequence of temperature, State of charge and charge rate. We also found that the thickness of the surface film followed traditional 1-dimensional models of surface film growth, that is, a sharp initial growth followed by a reduced rate after ‘X’ months / C-rate.

These findings indicate that, using our method, it is possible to determine the relative thickness of the negative electrode surface film. It also showed that surface film thickness increased with cell charge- rate, temperature and SoC. This is invaluable in determining the relationship between cell conditions and its effect on internal chemical composition of the cell.

The work at Argonne National Laboratory was performed under the auspices of the U.S. Department of Energy (DOE), Office of Vehicle Technologies, under Contract No.
DE-AC02-06CH11357.

The submitted issue has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) under Contract No. W-31-109-Eng-38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, non-exclusive, irrevocable, worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.