A New Perspective on Interphase Formation in Li-ion Batteries by Combined EIS & EQCM-D

Abstract

The Li-ion battery is widely acknowledged as one of the key energy storage technologies enabling the transition towards sustainable transport and power generation. Its high specific energy is based on the formation of the so-called solid electrolyte interphase (SEI) which passivates the anode predominately during initial battery charging. Due to the complex dynamics of the SEI formation process, its nm-sized dimensions, as well as the intermediacy and instability of many interphase species, operando analytical techniques of high surface sensitivity are preferred to study this phenomenon. In this work, a new method was developed for investigating interphase formation in Li-ion cells by combining operando electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) with simultaneous in situ electrochemical impedance spectroscopy (EIS). Hereby, the evolution of SEI mass and its mechanical properties can be directly correlated to the concurrent electrode impedance change during cycling. For this purpose, a new Li-ion cell was designed which enables simultaneous EIS and EQCM-D measurements. An additional high surface area working electrode implemented in the setup reduces the critical electrolyte volume to electrode surface area ratio (flooding factor) and facilitates detection of diffusion processes between different types of electrodes. Experimental data and EIS numerical simulations suggest that the SEI forming on anodes in Li-ion cells impedes ion diffusion between the electrode surface and the electrolyte volume. Based on these observations a new equivalent circuit was derived for describing the anode impedance spectrum. Diffusion limitation in the thin electrode-electrolyte interphase was modeled using a finite length Warburg element with transmissive boundary condition. In this work, combined EIS/EQCM-D was employed along with additional analytical techniques such as XPS and OEMS to study the SEI formation process on carbon based anodes and their copper current collectors in standard LiPF6 based electrolyte. The findings suggested that the electrolyte decomposition mechanism and the interphase properties differ significantly between both types of electrodes. On copper trace water in the electrolyte causes the catalytically enabled reduction of HF to LiF which forms an early rigid interphase on the current collector. Afterwards interactions with the porous carbon active material and reduction of the electrolyte solvent generate a second organic layer on top. On carbon V surfaces water in the electrolyte is reduced around 2 V vs. Li+/Li. OEMS together with doubly labeled heavy water D2 18O revealed the complicated electrolyte decomposition mechanism triggered by this reaction. While some reaction products precipitate on the anode surface, others dissolve and lead to an overall increase of electrolyte viscosity and cell ohmic resistance during cycling as detected by EIS/EQCM-D. Carbon anode passivation in pure LiPF6/EC+DEC occurs at potentials negative to 0.9 V due to a one electron EC reduction process. The electrolyte additives VC and FEC are reduced positive to 1 V, suppress subsequent EC decomposition, and significantly reduce the overall SEI mass. In particular the additive VC significantly improves anode passivation. The viscoelastic SEI parameters that were measured in this work by EQCM-D are much lower than one might expect from the individual interphase components, which indicates that the electrolyte solvent partially penetrates the SEI. Water, VC, and FEC are reduced at the anode to form (among others) rigid inorganic salts and therefore significantly increase the SEI G’ modulus when added to the electrolyte. In all investigated cells the SEI properties were found to change with electrode potential. In general the SEI becomes more rigid and at the same time more conductive with decreasing electrode potential which could be caused by Li+/EC concentration changes at the anode surface. These findings highlight the importance of operando analytical techniques for interphase research. This work demonstrates that the combination of EIS with EQCM-D provides unique insights into the evolution and properties of interphases in Li-ion cells. The technique can help to further the fundamental understanding of passivation processes in today’s batteries.