X-Ray Photoelectron Study of the Binder/Electrode Interface on Silicon Anodes for Lithium Ion Batteries

Abstract

Polymeric binders are a critical component of all composite electrodes used in today's lithium ion batteries (LIB). Binders keep active material and conductive additive mixture together, while also adhering this mixture to the current collector. At the particle level the binder forms bridges between particles of active material. Failure of these bridges contributes to electrical isolation of active material and thus poor cyclic life. the mechanical strength of these binder bridges is of key interest to understand the failure mechanisms of LIBs. Failure of binder bridges is highlighted in LIBs that incorporate Li-alloy forming anode materials such as Si which are known to degrade due to expansion and contraction during cycling. Meanwhile, the formation of solid electrolyte interphase (SEI) can also exacerbate capacity loss in novel anode materials, however the contribution of the binder to SEI formation and stability of alloying anodes has not been emphasized. Favorable interface properties between binder and active material resulting in good adhesion is desirable for anodes which encounter larger stresses during cycling. The objective of this study is to understand the effect different polymer binders have on the nature, formation, and location of the SEI layer of silicon based anodes. In this study crystalline Si wafers coated with thin films of CMC and PVDF binders were utilized. No conductive additives were used. Half cells containing either bare crystal Si or a wafer with a thin film of CMC or PVDF were cycled in a standard lithium-ion electrolyte (1 molar LiPF6 in 1:1:1 vol. ratio of EC:DC:DMC) to form a stable SEI. After cycling, the cells were opened in an inert atmosphere and x-ray photoelectron spectroscopy (XPS) was carried out to analyze and compare the surface chemistries. Depth profiling was then conducted utilizing C-60 fullerene ions opposed to the conventional Ar ion milling probe due to the known issue of chemical degradation caused by the Ar ion milling. In addition, the morphology of the substrates and binder films were characterized by SEM. Preliminary results indicate that binder layers on the order of hundreds of nm thick do not impede the lithiation and delithiation of Si (Figure 1A) but do play a role in the SEI composition (Figure B-D). These results can help inform the optimization of current and advanced composite electrodes for commercial LIB. Figure 1