Initial Solid Electrolyte Interphase Research Articles

Insights into the SEI Formation Mechanism By in-Situ Scanning Tunneling Microscopy and Electrochemical Quartz Crystal Microbalance Studies

The strongly increasing worldwide energy demand threatens to lead to a critical shortage of fossil fuel resources as well as a strong climate change in the next decades. The changeover to renewable energy sources like sun and wind and a complete transition in the transport sector to electromobility are potential solutions for the existing problems and thus under intensive discussion nowadays [1]. For this reason, strong efforts have been undertaken for the further development and improvement of electric energy storage devices such are batteries [2,3]. Li+-ion batteries as the front-runners especially in the area of electromobility undergo intensive research for improving efficiency, lifetime and safety [4]. These properties are crucially affected by the properties of the formed SEI (Solid Electrolyte Interphase) on the battery electrodes [5]. In order to optimize the battery performance, a complete understanding of the formation mechanism, morphology and chemistry of this interphase is inevitable. Therefore, the formation of the SEI has been in the focus of recent research [6, 7]. In this study two powerful tools, namely In-Situ Scanning Tunneling Microscopy and the Electrochemical Quartz Crystal Microbalance (EQCM) Technique, have been applied to gain further insights into the SEI formation and its morphology. The STM studies were building on earlier work by Inaba et al. [8]. Highly oriented pyrolytic graphite (HOPG) has been chosen as the model electrode for intercalation of Li+-ions from commercially available and alternative electrolytes. Both in-situ and ex-situ imaging was carried out. It could be demonstrated that the initially formed SEI is reversible, i.e. it dissolves when increasing the potential, while at lower potentials it becomes irreversible. Also the insertion of solvated ions could be visualized. The STM studies were complemented by EQCM measurements. EQCM with bare Au electrode coated quartz resonators have been shown to provide information on SEI formation [7]. In our studies the electrodes were modified in order to be more representative of real battery anodes and to represent more closely the experimental conditions applied in STM. The conclusions from the results of these measurements for the SEI formation in different electrolytes are discussed in this contribution. . [1] Esbenshade J. L. et al., Effect of Mn and Cu addition on lithiation and SEI formation an model anode electrodes, Journal of The Electrochemical Society 161 (2014) A513 [2] Armand M. et al., Novel weakly coordinating heterocyclic anions for use in lithium batteries, Journal of Power Sources 178 (2008) 821 [3] Goodenough J. B. et al., Rechargeable batteries: challenges old and new, J. Solid State Electrochem. 16 (2012) 2019 [4] Amereller M. et al., Electrolytes for lithium and lithium ion batteries: From synthesis of novel lithium borates and ionic liquids to development of novel measurement methods, Progress in Solid State Chemistry 42 (2014) 39 [5] Balbuena P. B. et al., Lithium-Ion Batteries: Solide-Electrolyte Interphase, Imperial College Press (2004), ISBN 1-86094-362-4 [6] Gourdin G. et al., Spectroscopic Compositional Analysis of Electrolyte during Initial SEI Layer Formation, J. Phys. Chem. C 118 (2014) 17383 [7] Tavassol H. et al., Solvent Oligomerization during SEI Formation on Model Systems for Li-Ion Battery Anodes, J. Electrochem. Soc. 159 (2012) A730 [8] Inaba M. et al., Electrochemical Scanning Tunneling Microscopy Observation of Highly Oriented Pyrolytic Graphite Surface Reactions in an Ethylene Carbonate-Based Electrolyte Solution, Langmuir 12 (1996) 1535

Dendrite-free lithium deposition with self-aligned nanorod structure.

Suppressing lithium (Li) dendrite growth is one of the most critical challenges for the development of Li metal batteries. Here, we report for the first time the growth of dendrite-free lithium films with a self-aligned and highly compacted nanorod structure when the film was deposited in the electrolyte consisting of 1.0 M LiPF6 in propylene carbonate with 0.05 M CsPF6 as an additive. Evolution of both the surface and the cross-sectional morphologies of the Li films during repeated Li deposition/stripping processes were systematically investigated. It is found that the formation of the compact Li nanorod structure is preceded by a solid electrolyte interphase (SEI) layer formed on the surface of the substrate. Electrochemical analysis indicates that an initial reduction process occurred at ∼ 2.05 V vs Li/Li(+) before Li deposition is responsible for the formation of the initial SEI, while the X-ray photoelectron spectroscopy indicates that the presence of CsPF6 additive can largely enhance the formation of LiF in this initial SEI. Hence, the smooth Li deposition in Cs(+)-containing electrolyte is the result of a synergistic effect of Cs(+) additive and preformed SEI layer. A fundamental understanding on the composition, internal structure, and evolution of Li metal films may lead to new approaches to stabilize the long-term cycling stability of Li metal and other metal anodes for energy storage applications.

Spectroscopic Compositional Analysis of Electrolyte during Initial SEI Layer Formation

The energy density of an electrochemical capacitor can be significantly improved by utilizing a lithiated negative electrode and a high surface area positive electrode. During lithiation of the negative carbon electrode, the electrolyte reacts with the electrode surface and undergoes decomposition to form a solid electrolyte interphase (SEI) layer that passivates the surface of the carbon electrode from further reactions between Li and the electrolyte. The reduction reactions that the solvent undergoes not only form insoluble and gaseous byproducts but also electrolyte-soluble products. In this work, those liquid-phase products generated by reductive decomposition of a carbonate-based electrolyte, 1.2 M LiPF6 in EC/PC/DEC (3:1:4), were analyzed at different stages during the lithiation process of an amorphous carbon electrode. An LCMS analysis of the electrolyte during the initial lithiation process was correlated to a DRIFTS analysis of the carbon electrode surface and the results from the EIS and gas-phase analysis previously reported. It was concluded that the formation reactions of the electrolyte-soluble reduction products are dependent on electrochemical processes but are independent of those reactions that directly lead to the generation of the decomposition gases. This is the first time that the composition of the electrolyte during the initial stages of formation of the SEI layer was systematically determined through the use of an in situ electrochemical-MS analysis. The results presented in this work are focused on the initial analysis, elucidation of the empirical formulas, and determining possible structures for these compounds through a detailed analysis of the spectra from each of these unknowns.

Additive Effect on Reductive Decomposition and Binding of Carbonate-Based Solvent toward Solid Electrolyte Interphase Formation in Lithium-Ion Battery

The solid-electrolyte interphase (SEI) formed through the reductive decomposition of solvent molecules plays a crucial role in the stability and capability of a lithium-ion battery (LIB). Here we investigated the effects of adding vinylene carbonate (VC) to ethylene carbonate (EC) solvent, a typical electrolyte in LIBs, on the reductive decomposition. We focused on both thermodynamics and kinetics of the possible processes and used density functional theory-based molecular dynamics with explicit solvent and Blue-moon ensemble technique for the free energy change. We considered Li(+) in only EC solvent (EC system) and in EC solvent with a VC additive (EC/VC system) to elucidate the additive effects. In addition to clarifying the equilibrium properties, we evaluated the free energy changes along several EC or VC decomposition pathways under one-electron (1e) reduction condition. Two-electron (2e) reduction and attacks of anion radicals to intact molecules were also examined. The present results completely reproduce the gaseous products observed in the experiments. We also found a new mechanism involving the VC additive: the VC additive preferentially reacts with the EC anion radical to suppress the 2e reduction of EC and enhance the initial SEI formation, contrary to the conventional scenario in which VC additive is sacrificially reduced and its radical oligomerization becomes the source of SEI. Because our mechanism needs only 1e reduction, the irreversible capacity at the SEI formation will decrease, which is also consistent with the experimental observations. These results reveal the primary role of VC additive in the EC solvent.

New Insight into the Solid Electrolyte Interphase with Use of a Focused Ion Beam

The formation and evolution of the solid electrolyte interphase (SEI) film on the surface of natural graphite spheres in the electrolyte of 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio 1:1) were investigated with use of focused ion beam (FIB) technology. Secondary electron FIB images clearly show the surface and cross-section morphology of the SEI film. The composition variation along the surface and cross section of the SEI film was also explored by the elemental line scan analysis (ELSA). The initial SEI film with an apparent thickness range of approximately 450 to approximately 980 nm is rough in morphology and nonuniform in composition, and contains small splits. After certain electrochemical cycles, the thickened SEI film displays microscale holes and obvious cracks on the surface, and the content of organic compounds increases. In addition, the concept of "internal SEI film" is first proposed based on the characterization of the cross section of the natural graphite spheres with the aid of FIB. Finally, the capacity fading mechanisms of the natural graphite spheres corresponding to different electrochemical stages are discussed.