Study of the influence of the formation protocol on the SEI layer formed at the graphite electrode surface of a non-aqueous potassium-ion hybrid supercapacitor (KIC) through STEM and XPS analyses

Abstract

The non-aqueous potassium-ion hybrid supercapacitor (KIC) represents a strategic material-free and low-cost technology with increased safety and higher energy densities compared to conventional supercapacitors. It has been shown previously that the stability and reproducibility of cycling results are very different depending on the formation protocol applied to KIC cells. The formation protocol is known to be a key manufacturing step of Li-ion batteries to create a stable and compact solid electrolyte interphase (SEI) on the negative electrode surface. This study is therefore focused on the role played by the formation protocol in the SEI layer nature within KIC technology. First, the combination of STEM images and XPS analysis allowed us to analyze the appearance and composition of the SEI after two distinct formation protocols. Then, a follow-up of the composition of the SEI by XPS analysis enabled the study of the relationship between the stability of the SEI, especially regarding the potassium fluoride (KF) content, and the electrochemical performance. We concluded that the constant voltage formation leads to the creation of a homogeneous and continuous SEI with a low KF content. Moreover, a correlation between the evolution of the KF content in the SEI and the evolution of cycling performance was observed.