Differences in Interfacial Reactivity of Graphite and Lithium Metal Battery Electrodes Investigated Via Operando Gas Analysis

Abstract

Gases evolved from lithium batteries can drastically affect their performance and safety; for example, cell swelling is a serious safety issue. Here, we combine operando pressure measurements and online electrochemical mass spectrometry measurements to identify the nature and quantity of gases formed in batteries with graphite and lithium metal electrodes. We demonstrate that ethylene, a main gas evolved in SEI formation reactions, is quickly consumed at lithium metal electrodes unless they have been pretreated in the electrolyte. Polyolefins such as polyethylene are suggested as the possible reaction product from ethylene consumption, evidencing another pathway of SEI formation that had been previously overlooked because it does not produce any gas product.

Figure 1

Operando pressure measurements of a graphite vs lithium cell. Prior to the measurements, the lithium electrode had been soaked in electrolyte for 24 h, and additionally, the cell was left for equilibration with the graphite at 1.5 V vs Li⁺/Li for 48 h.

Figure 2

As in Figure 1, but with a graphite vs lithium cell in which the lithium electrode was not presoaked in the electrolyte and with a rest period for cell equilibration of only 6 h.

Figure 3

As in Figure 1, but with a graphite vs oversized LiFePO₄ cell, cycling at a C-rate of C/5 between potentials corresponding to 1.45 V and 5 mV vs Li⁺/Li, except for the 4th cycle, in which a lower potential limit of −50 mV vs Li⁺/Li was used.

Figure 4

Results of the analysis of gases evolved from a graphite vs LiFePO₄ cell using the OEMS system shown in Figure S7 and the experimental conditions in Figure 3.