Understanding (electro)chemical reactions at the electrode-electrolyte interface (EEI) is crucial to promote the cycle life of lithium-ion batteries. In situ studies of EEI can provide new insights into reaction intermediates and soluble species not accessible by ex situ characterization of electrode surfaces. In this study, we developed an in situ Fourier-transform infrared spectroscopy (FTIR) method to investigate the (electro)chemical reactions at the interface between the electrolyte and composite positive electrode surface during charging. While ethyl methyl carbonate (EMC) and ethylene carbonate (EC) were stable against (electro)chemical oxidation on Pt up to 4.8 VLi, dehydrogenation of both carbonates on the surface of LiNi0.8Co0.1Mn0.1O2 (NMC811) electrodes was revealed by in situ FTIR spectra and density functional theory (DFT). Both solvents can dehydrogenate and form de-H EC and de-H EMC, respectively, with carbon atom binding to lattice oxygen. De-H EC can further remove another hydrogen atom to form vinylene carbonate (VC), or bind together to form oligomers, both of which are soluble and cannot be accessed through previous ex-situ methods. The protic species generated from dehydrogenation process were suggested in previous works to further attack salt, and the decomposed solvents and salt form a passivating layer on the surface, leading to greater interfacial impedance demonstrated by electrochemical impedance spectroscopy (EIS). This in situ FTIR method tracked the pathway of electrolyte decomposition on electrode surface that accounts for battery degrading, which is oxide-dependent and solvation structure-dependent. We finally proposed multiple strategies to further improve the cycling stability of high-energy density positive electrode in Li-ion batteries.

Understanding electrode-electrolyte interface in Li-ion batteries by in situ Fourier transform infrared spectroscopy (FTIR)

2019 Fall National Meeting

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