Investigating the Solid Electrolyte Interphase Formed by Additive Reduction Using Physics-Based Modeling

Abstract

In lithium-ion batteries, chemical additives are used as co-solvents to primary electrolytes to improve capacity and power retention. These additives facilitate the formation of a passivation layer, the solid electrolyte interphase (SEI), on the electrode surface. In this work, SEIs are formed in neat electrolyte and in electrolyte containing fluoroethylene carbonate and vinylene carbonate. The formed SEIs are then compared using a redox couple to probe their physical properties. For passivated samples, the impedance response of the redox couple shows the presence of multiple time constants, with processes at longer time scales corresponding to redox couple transport in the porous layer of the SEI. Samples passivated with additive-containing electrolyte versus neat electrolyte exhibit less redox-couple kinetic and mass-transport impedance. Simulations of the electrode-electrolyte interface indicate that compact and porous layer growth lead to slowed redox kinetics and mass transport. Model results suggest that SEI thicknesses are found to be at least an order of magnitude larger than expected compared to graphite electrodes. SEM cross sections of SEIs formed by neat and additive-containing electrolyte support the model findings. Experimentally measured formation charges, coupled with FTIR measurements of SEI composition, suggest that polymerization reactions are causing the unexpected film growth. In lithium-ion batteries, formation and growth of the solid electrolyte interphase (SEI) is a well-established mode of capacity and power fade. 1,2 Cell characteristics, including electrode chemistry and primary electrolyte composition, influence the chemical and physical properties of SEIs formed. 3 In addition to the primary electrolyte, chemical additives have been incorporated as co-solvents to address SEI formation and growth. When included as co-solvents, both fluoroethylene carbonate (FEC) and vinylene carbonate (VC) electrolyte additives preferentially react with the electrode during the SEI formation process instead of the primary electrolyte. Improved capacity retention, calendar life, and thermal stability are observed with additive inclusion in the electrolyte mixture. Additives such as FEC and VC have been prescribed as co-solvents to primary electrolytes for a variety of electrode chemistries. [4][5][6][7][8] Several groups have investigated the efficacy of these co-solvents to enhance cell performance. In these studies, the additives are limited to less than or equal to ten weight percent of the total electrolyte. For example, Ryou et al. found improved capacity retention at 60 • C with the addition of FEC, for a graphite negative electrode and LiMn 2 O 4 positive electrode full cell. 7 Bordes et al. found that, for a silicon-graphene composite negative electrode, FEC enhanced electrochemical performance by forming a less resistive SEI with smaller thickness using impedance spectroscopy and electron microscopy. 9 Aurbach et al. observed improved capacity with the addition of VC to a graphite negative ele...

Formation of the Solid Electrolyte Interphase at Constant Potentials: A Model Study on Highly Oriented Pyrolytic Graphite

“…Beyond that, we analyzed the potential‐dependent composition of the SEI by ex‐situ X‐ray photoelectron spectroscopy (XPS), and we broke down the EIS data into a Distribution of Relaxation Times (DRT). We discuss the findings of our multi‐method study in the context of previous electrochemical model studies and existing SEI models,, and suggest how our findings may inform more efficient SEI formation protocols in industry.…”

“…Particularly the SEI on Glassy Carbon (GC) was studied extensively by Tang et al ,,. and others ,. For commercial electrolyte, Fc/Fc + as an electrochemical probe for the effect of the SEI on ion transport and electrochemical kinetics revealed that SEI formation can be broken down to two key steps at ∼700 and ∼400 mV vs. Li/Li + .…”

“…Hence, surface reactions will be dominated by the availability of reactants and the electrochemical potential. In analogy to studies on glassy carbon,, the low surface area of HOPG allows applying potential steps followed by constant‐potential periods, which on technical high‐surface area electrodes would cause high current peaks, long relaxation times, and inhomogeneous potential distribution. In the electrochemical part of our experiments, we followed the protocols that Tang et al.…”

Influence of the Formation Current Density on the Transport Properties of Galvanostatically Formed Model‐Type Solid Electrolyte Interphases

“…Due to these contradictory experimental results on graphite SEIs and due to the high interest in faster SEI formation in commercial battery production, we carried out a systematic study on galvanostatic formation of model‐SEIs on planar glassy carbon electrodes. In previous studies, it was shown that such model‐type planar electrodes are well suited for studying the transport of ions and molecules across the SEI . This is in contrast to graphite composite electrodes, which exhibit a very complex morphology strongly complicating the understanding of transport processes.…”

Formation of the Solid Electrolyte Interphase at Constant Potentials: a Model Study on Highly Oriented Pyrolytic Graphite

“…Hence, in analogy to fundamental research in the electrocatalysis community 5 [25,26], deeper insights require studies at well-defined model surfaces under potentiostatic conditions, and involving both electrochemical techniques as well as high resolution surface microscopy and spectroscopy [19,21,[27][28][29][30][31][32][33][34][35]. Particularly the SEI on Glassy Carbon (GC) was studied extensively by Tang et al [23,36,37] and others [38,39]. For commercial electrolyte, Fc/Fc + as an electrochemical probe for the effect of the SEI on ion transport and electrochemical kinetics revealed that SEI formation can be broken down to two key steps at ~700 and ~400 mV vs. Li/Li + [35].…”