In recent years, the solid electrolyte interphase (SEI) has been recognized as a critical component of the lithium-ion battery (LIB) system. As battery applications in electric vehicles and stationary storage drive demand for higher energy density LIBs, alternative anode materials are being investigated to replace the current graphite-based anode. In alloying anode materials such as silicon (Si), volumetric changes occur during lithiation and delithiation, inducing additional SEI instability at the Si interface.¹ The electronic properties of the SEI are of particular interest, as an electronically insulating SEI is necessary to maintaining passivation of the anode. Consequently, understanding the fundamental electronic structures of SEI that influence its performance is a necessary step to future implementation of next generation anodes in LIBs.

Three-dimensional resistivity vs. depth profiling has emerged as an advanced characterization approach for measuring the electronic resistivity and thickness of SEI formed on Si.² This technique utilizes scanning spreading resistance microscopy (SSRM) to mechanically profile away SEI with a conductive, wear resistant probe, enabling nanoscale mapping of electronic resistivity of SEI structures.

In this work, we investigate the evolution of SEI formed from a standard electrolyte (1.2 M LiPF₆ in EC:EMC (3:7 wt%)) through various points in cycling on several model Si surfaces. We also examine the solubility of SEI by resting the anode in the cell at varied temperatures then characterizing the residual SEI. Evolution of the SEI surface is also analyzed with atomic force microscopy (AFM) measurements. AFM and SSRM-derived results are presented in combination with chemical characterization, including time-of-flight secondary ion mass spectrometry (TOF-SIMS).

Additional studies into the changes to the electronic properties of the underlying Si anode through cycling are also presented. Conductivity of active Si in LIBs has been shown to decrease through cycling, but the specific causes and implications on battery performance are still poorly understood.³ Three-dimensional electronic resistivity vs. depth profiling is an effective technique for nanoscale investigation of the SEI as well as the active anode material beneath.

References

1. Cao, C.; Abate, I.I.; Sivonxay, E.; Shyam, B.; Jia, C.; Moritz, B.; Devereaux, T.P.; Persson, K.A.; Steinrück, H.G.; Toney, M.F., Solid Electrolyte Interphase on Native Oxide-Terminated Silicon Anodes for Li-Ion Batteries. Joule 2019, 3, 762-781.
2. Stetson, C.; Yoon, T.; Coyle, J.; Nemeth, W.; Young, M.; Norman, A.; Pylypenko, S.; Ban, C.; Jiang, C.-S.; Al-Jassim, M.; Burrell, A., Three-dimensional electronic resistivity mapping of solid electrolyte interphase on Si anode materials. Nano Energy 2019, 55, 477-485.
3. Kim, S.H.; Kim, Y.S.; Baek, W.J., Heo, S.; Yun, D.J.; Han, S.; Jung, H., Nanoscale Electrical Degradation of Silicon-Carbon Composite Anode Materials for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2018, 10, 24549-24553.