Formation Of Solid Electrolyte Interphase Layer 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

Electrochemical Preparation of CNTs-Coated Cu Substrate for Si-O-C Composite Deposition and Characteristics of Si-O-C/CNTs/Cu As an Anode of Li Secondary Batteries

A silicon anode for Li secondary batteries has a critical problem with volume change up to 400 % during lithiation/delithiation process, causing cracks on surface of active material and formation of new solid electrolyte interphase (SEI) on the cracked surface. For this reason, stable cycle ability of battery is decreased. To solve the serious problem, in our previous studies, a novel Si-O-C composite prepared by electrodeposition from an organic solvent was reported for anode of Li secondary batteries [1-3]. In this research, CNTs were deposited on a Cu substrate by electrophoretic deposition (EPD) with cobalt (II) chloride at high voltage. The deposited CNTs on the Cu substrate is supposed to be as 3-dimension (3D) layer inside of the Si-O-C composite and it can reduce the internal resistance of the Si-O-C composite and increase the adhesion between Si-O-C with the Cu substrate because of its high electric conductivity. This research introduces the Si-O-C composite deposited on the CNTs/Cu substrate (Si-O-C/CNTs/Cu) with outstanding delithiation capacity compared to the Si-O-C composite on the conventional Cu substrate (Si-O-C/Cu composite). Figure 1 shows the schematic figure of Si-O-C composite deposition process on the CNTs/Cu substrate. In Fig. 1a, 2D-surface morphology of the conventional Cu substrate is obviously indicated. However, it is observed that the CNTs were homogeneously deposited on the Cu substrate forming 3D-structured morphology as shown in Fig. 1b. The Si-O-C composite was deposited on the 3D-structured CNT layer with good uniformity in Fig. 1c. The amount of Si deposited on the CNTs/Cu substrate increases in 33% compared to that of conventional Cu substrate. It is supposed that the electrodeposited Si-O-C composite were filled inside of CNT layer because of increased adhesion between Si-O-C with Cu substrate (see schematic figure in Fig. 1d). Electrochemical properties were examined with cycle ability for 100 cycles at 0.1 C-rate in Fig. 2. The discharge capacity of Si-O-C/CNTs/Cu delivered 2463.2 and 1340.6 mAh g-1 of Si at the 1st and 100th cycle, which was higher than that of Si-O-C/Cu (1340.9 and 840.0 mAh g-1 of Si at the 1st and the 100th cycle, respectively), as shown in Fig. 2. Coulombic efficiencies of the Si-O-C/CNTs/Cu and the Si-O-C/Cu anodes were compared. The coulombic efficiencies of both anodes at 1st cycle were decreased under the 24.1% because of formation of SEI layer on anode. However, after 2nd cycle the coulombic efficiencies were increased dramatically. Both anodes achieved high coulombic efficiencies over 86.9 and 99.7% at the 2nd and 100th of the cycle test. In addition, after 10th cycle, Si-O-C/CNTs/Cu and Si-O-C/Cu anode show the stable coulombic efficiencies over 99.8%. It is obviously indicates that the Si-O-C/CNTs/Cu was performed as anode with good cycle ability and high coulombic efficiencies for Li secondary batteries. Figure 1

Contribution of Tof-SIMS Ion Profiling to Understanding the Surface and Bulk Modifications of Si Anode As a Function of Electrolyte Composition and Additives

The surface chemistry is one of the most important factor influencing the electrochemical performance of LiBs. The modification of the chemical composition of electrode materials is related to formation of a passive layer, named as a Solid Electrolyte Interphase (SEI) layer1, due to decomposition of electrolyte mostly during the first cycle of charge/discharge. The formation of the SEI layer is irreversible, consumes some amount of electrolyte, and leads to irreversible capacity loss, lower rate capability, cyclability, or electrode degradation. Although a multiple research activities in this domain, there are still a lot of ambiguities concerning the SEI layer, so the famous statement of Martin Winter “The Solid Electrolyte Interphase - The Most Important and the Least Understood”2 is still valid. Various spectroscopic techniques, such as XPS, AES, FTIR, IRAS, or Raman spectroscopy, are used for analysis a mechanism of the SEI layer formation and identification of the SEI components. We present here some recent studies on the surface chemistry of Si thin film electrode material (prepared by Plasma-Enhanced Chemical Vapor Deposition - PECVD) as a function of cycling and electrolyte composition by means of the Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). Depending on type of electrolytes and/or the electrolyte additives like vinylene carbonate (VC) and monofluoroethylene carbonate (FEC), having polymerizable features,3,4 it is possible to improve the mechanical properties of the SEI layer and the cyclability of the Si electrode undergoing big volume variations. ToF-SIMS presents several advantages: is a highly sensitive surface analytical technique where a pulsed primary ion beam (e.g. Bi+) is used to extract secondary ions that are analyzed by time–of–flight spectrometry. Interlaced with a sputtering ion beam (e.g. Cs+), elemental depth profiles with excellent depth resolution (monolayer) and high sensitivity (ppb) can be readily obtained. Although the big volume variations occurring during the 100 cycles of discharge/charge of the Si thin film electrode (100 nm), the higher specific capacity can be observed in PC/LiClO4 1M than in EC:DMC (1:1)/LiPF6. The ToF-SIMS ion depth profiles show no significant modifications (i.e. no significant volume increase) of the Si thin film electrode after 100 cycles in PC/LiPF6 1M when comparing to PC/LiClO4 1M. Exchanging LiPF6 for LiClO4 in EC/DMC electrolyte leads to important increase of Li- and CO3 - ion signal intensities at the surface and in the bulk of Si thin film electrode, which can be attributed to accumulation of Li-like components (lithiated silicon) and products of electrolyte decomposition as already observed in previous studies.5,6 The enhanced lithiation of Si electrode in a case of PC/LiClO4 1M can explain its higher capacity. The additives (VC or FEC) introduced to PC/LiClO4 1M electrolyte lead to better columbic efficiency and better cyclability of Si electrode. The ToF-SIMS ion profiles show more significant volume changes of Si electrode in a case of cycling in PC/LiClO4 1M electrolyte containing the FEC than the VC additive. The more intense ToF-SIMS Li- ion profile observed for Si electrode cycled in PC/LiClO4 1M with FEC additive can explain also the higher capacity than those obtained in electrolyte with VC addition. In a case of EC:DMC (1:1)/LiPF6 1M electrolyte much lower cycling stability can be observed than in PC/LiClO4 1M. The cycling stability is significantly improved after addition of FEC (2%) to EC:DMC (1:1)/LiPF6 1M electrolyte. The volume increase of Si electrode after 100 cycles estimated from ToF-SIMS sputtering time is much less important in EC:DMC (1:1)/LiPF6 1M than in PC/LiClO4 1M. The FC additive into the EC:DMC (1:1)/LiPF6 1M does not significantly influence the morphological modifications (volume increase) of Si thin film electrode.

The Surface Modification of Electrode with Solid Electrolyte Interphase for Hybrid Supercapacitor

A hybrid supercapacitor (HS) is an energy storage device used to enhance the low weight energy density (Wh/kg) of a supercapacitor. On the other hand, a sudden decrease in capacity has been pointed out as a reliability problem after many charge/discharge cycles. The reliability problem of a HS affects the early aging process. In this study, the capacity performance of a HS was observed after charge/discharge. For detailed analysis of the initial charge/discharge cycles, the charge and discharge curve was measured at a low current density. In addition, a solid electrolyte interphase (SEI) layer was confirmed after the charge/discharge. A HC composed of a lithium titanate (LTO) anode and active carbon cathode was used. The charge/discharge efficiency of the first cycle was lower than the late cycles and the charge/discharge rate was also lower. This behavior was induced by SEI layer formation, which consumed Li ions in the LTO lattice. The formation of a SEI layer after the charge/discharge cycles was confirmed using a range of analysis techniques.

Degradation of Cobalt Oxide Anode for Lithium Batteries at High Conducting Material Content

Cobalt oxide is considered as promising option for the anode material in next generation rechargeable lithium battery thanks to its large specific capacity (~900 mAh/g). However, its low electrical conductivity causes poor energy efficiency and decreased power density. In order to lower the electrode resistance, many researchers have performed their experiments using the cobalt oxide anode containing large amount of conducting material. On the other hand, the overgrowth of the solid electrolyte interphase (SEI) layer on the cobalt oxide surface and the resulting decrease in specific capacity and energy density are another key issue for its practical use. Specifically, it has been known that the active materials move away from its original position to the overgrown SEI layer during the repetitive charge-discharge cycling and is gradually lost. Accordingly, the larger is the quantity of the SEI layer, the greater is the decrease in the specific capacity. In this work, we report that the larger amount of conducting material in the cobalt oxide anode leads to its faster degradation during the operation. From the microscopic observation and porosity measurement of the pre- and post-cycled samples with different conducting material contents, it is proved that the free volume in the electrode decreases as the content of conducting material is raised. Furthermore, the analysis of the change in cumulative irreversible capacity loss during charge-discharge cycles shows that the high content of the conducting material promotes the growth of the SEI layer. In order to further investigate the character of the SEI layers formed at different conducting material contents, the SEI layer formation is mathematically modeled on the basis of the Butler-Volmer relation. By fitting the model to the experimental data, a few physical parameters of the SEI layer can be successfully determined. In particular, from the calculation it is confirmed that the resistivity of SEI layer is considerably changed with the conducting material content. This implies that the incorporation rate of metallic cobalt in the SEI layer during cycling might be critically dependent on the conducting material content. In this presentation, the optimal content of conducting material is discussed from the viewpoint of cycling stability and rate capability. Furthermore, the validity of the model for the systems used in this work is addressed by an analysis of fitting accuracy.

Nano-Scale Lithium Titanium Oxide As Anode Material for Nanoelectrofuel Flow Batteries

Nanoelectrofuel (NEF) flow battery is new transformational technology that merges the advantages of high energy density solid materials and the convenience of flowable battery format. Traditional flow batteries rely on solubility of redox salts and suffer from the low energy densities. New nanoelectrofuel format resolves this issue by using stable suspensions of solid battery-active nanoparticles in the electrolyte at high loading that can be charged and discharged multiple times when passing through customized flow battery cells. The flowable format of this technology provides a key advantage over conventional batteries: its energy storing material - nanoelectrofuel - can be separated from its charging device, the flow cell. The latter allows for highly flexible battery system designs and a wide range of applications form fueling the electric vehicles to supplying electricity from renewable sources to homes and more. This presentation will focus on the development of nanoelectrofuel anodes with Li-ion battery chemistries. Amongst the known Li-ion battery chemistries we chose nanosized Li4Ti5O12 (LTO) for nanoelectrofuel anode development. Its theoretical capacity is 175 mAh/g and typical experimental capacity values are up to 150 mAh/g [1]. The main reason for selection of this material for NEF anode however is its relatively high and flat lithiation/delithiation voltage (around 1.55 V vs. Li/Li+), which can minimize the formation of solid-electrolyte interphase layer (SEI) affecting the anode performance and its excellent capacity reversibility due to its minimal decrease in unit cell volume of 0.2% as transitions between Li4Ti5O12 and Li7Ti5O12 [2]. It was previously demonstrated that electrochemical performance of electrode material could be effectively improved by changing particle sizes and microstructures because decreasing grain size to nano-scale can shorten the Li+diffusion pathway [3,4]. In our work we compare performance of LTO nanoparticles in solid casted electrodes to the electrochemical performance of the same nanoparticles in form of suspension electrodes. For solid casted electrodes LTO nanoparticles are deposited on Cu substrate for electrochemical study in coin cells vs. Li-metal foil (half-cell configuration). Cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy are used in characterization of the coin cells. The LTO nanomaterials show good cycling performance and the initial discharge capacity of 150 mAh/g. After charge/discharge 100 cycles at C/4 rate the discharge capacity is 96% of initial value. For comparison we tested commercial pre-deposited LTO electrode with micron-sized LTO particles in the same conditions and it showed LTO nanoparticles exhibit higher reversible capacity and better cycling performance. These effects are most likely related to the advantages of nano-scale materials. Next we investigate suspensions of LTO nanoparticles dispersed in commercial Li-ion electrolyte. The capacities are studied by measuring charge/discharged curves at different C-rates. Various weight percentage active materials are studies as well as the ratio required between active material and the current collectors, the materials of the current collectors and effects of the different additives and surface modifications on the suspension electrochemical performance. The LTO nanoparticles and the current collectors are separated from suspension after the electrochemical tests and examined with X-ray diffraction and scanning electron microscopy to study the phase changes, agglomeration and residue on the current collectors after multiple charge/discharge cycles. Additionally this presentation will discuss characterization of solid casted and suspension electrodes with X-ray absorption fine structure (XAFS) that reveals local structure changes around Ti atoms in charged and discharged LTO nanoparticles.

Current State of Understanding of the Solid-Electrolyte Interphase (SEI) in Lithium-Ion Cells and Its Relationship to Formation Cycling

The process of formation cycling refers to the electrochemical side reactions involved with creating a “passive” film on the anode active material, for instance graphite, known as the solid electrolyte interface (SEI) layer. This interfacial layer is formed during the first several charge-discharge cycles primarily by the reaction of electrolyte components with graphite at reducing potentials near the equilibrium potential of lithium metal (-3.045 V vs. SHE), and it plays a protective role by preventing the graphite from undergoing subsequent reaction with the electrolyte solvent and salt. [1] An ideal SEI layer should be thin, electrochemically inert, electronically insulating, and conductive to lithium ions. The commonly accepted hypothesis of the structure and composition of the SEI layer is an inner (interface with the graphite surface) inorganic layer of compounds such as LiF, Li2CO3, LiOH, LiO2, etc. and an outer (interface with the electrolyte solvent) organic layer primarily consisting of alkyl carbonates such as lithium ethylene dicarbonate (LiEDC). However, obtaining direct experimental evidence of SEI-layer composition and structure is extremely challenging. Information about thicknesses, constituent mass fractions, crystallinity, reactivity, and how these characteristics change from early cell life through thousands of charge-discharge cycles has remained elusive. Formation cycling is performed immediately after a lithium ion cell has been constructed and filled with electrolyte, and it has a profound economic impact on lithium ion battery manufacturing. The formation process requires that battery producers install many cycling stations to complete the process, which results in a heavy capital equipment investment and a much larger plant size. The irreversible capacity loss associated with anode SEI formation involves the consumption of lithium from the fresh cathode, resulting in a diminished battery lifetime. The formation process occurs in two successive stages, with the first stage involving formation of a highly resistive SEI layer at higher anode potentials (>0.25 V vs. Li/Li+). The second stage involves simultaneous intercalation of lithium into the graphite at potentials <0.25 V vs. Li/Li+ where the SEI layer is converted to a highly conductive film. [2] Fong et al. showed that the second stage involving the initial intercalation of lithium into graphite does not occur without the proper passivating electrolyte solvent and a sufficient coverage of SEI film. [3] There are three primary methods of formation cycling, which consist of a two-step current-charge formation, [4] pulse formation, [5] or the ageing process at elevated temperature. [6] In industry, combinations of these three approaches are often used. Electrode wettability plays a critical role in SEI layer formation. [7] After electrodes are coated and dried, they are usually calendered at high pressure to compact the composite structure, thus improving the energy density of the electrode layer. However, the electrode porosity is correspondingly reduced to only 30-35%, which has a significant impact on the pore-size distribution and the related wetting of the electrolyte.8 Wettability of the electrolyte to the electrode pores can be enhanced in two ways: 1) by supplying an additive to the electrolyte to lower its composite surface tension; and 2) by increasing the composite surface energy of the electrode. There is evidence that both of these approaches are effective, but the latter will be the focus of this presentation, especially as it relates to the SEI layer formation chemistry, growth in early cell life, and degradation in aged cells. Acknowledgment This research at Oak Ridge National Laboratory (ORNL), managed by UT Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO) Applied Battery Research (ABR) subprogram (Program Managers: Peter Faguy and David Howell).

Nanoscale IR Near-Field Imaging of the SEI Layer on an HOPG Electrode

The solid electrolyte interphase (SEI) layer [1] is essential to Li-ion battery functionality as it ideally forms an ionically conductive passivation layer, which protects the electrolyte from further reduction at the electrode surface. Although the SEI has been studied extensively in a number of electrode/electrolyte systems, its chemical composition, structure, and operation mechanism still remain a subject of considerable debate. Among the principal obstacles to detailed SEI characterization is the fact that the signal spatial resolution of traditional optical techniques such as Fourier transform infrared (FTIR) and Raman spectroscopy/microscopy is limited by the diffraction limit c.a. 500 nm. The standard optical response signal from the SEI layer convolves spectral features from many different compounds, which makes characterization of the inherently nanoscale chemical building blocks of the SEI layer nearly impossible. Infrared apertureless near-field scanning optical microscopy (IR aNSOM) [2] can easily overcome the diffraction limit. In aNSOM, light is focused onto a metal-coated atomic force microscope (AFM) tip, which causes a strongly enhanced field at the tip apex. Since the tip is held in close proximity to the sample surface, the microscope is capable of imaging at a resolution only limited by the tip’s radius of curvature (~20 nm). The scattering from the tip is modulated by the dielectric function of the sample surface, allowing for a direct surface-selective measurement of local absorption at the nanoscale. This capability is valuable in characterization of Li-ion electrode materials [3]. In this work we apply IR aNSOM to image the surface film on highly oriented pyrolitic graphite (HOPG). Linear sweep voltammetry (LSV) was conducted on ZYH-grade HOPG electrode in 1 M LiPF6 in EC:DEC 1:2 (w:w) electrolyte from open-circuit potential (OCP ~ 2.9 V vs. Li/Li+) to several potentials that correspond to the SEI layer formation. The HOPG electrodes were removed from the cells and imaged ex situ under a dry nitrogen atmosphere with the IR aNSOM at a variety of wavelengths within the IR laser tuning range 927-1094 cm-1. IR aNSOM images of the HOPG electrode are shown in Figure 1. The image intensity Asinϕis proportional to the IR absorption at the tip location. The variation in IR contrast with wavenumber demonstrates the nanoscale chemical inhomogeneity of the layer. Variation in wavenumber produces near-complete contrast reversal, highlighting the locations of different compounds with non-overlapping absorption bands within the laser tuning range. These results show the capability of near-field microscopy to explore the composition and structure of interfaces in electrochemical systems. Correlating near-field absorption data with spectra of reference compounds will enable full identification of the SEI’s chemical constituents at the nanoscale. Acknowledgement This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 under the Batteries for Advanced Transportation Technologies (BATT) Program. References E. Peled, J. Electrochem. Soc., 126, 2047 (1979).L. Novotny and B. Hecht, Principles of Nano-Optics, Cambridge University Press, Cambridge (2006).I. Lucas et al. Nano Lett., DOI: 10.1021/nl5010898. Figure 1: Ex situ IR aNSOM images of the HOPG electrode at (a) 1.36 V, 940 cm-1, (b) 1.36 V, 1070 cm-1, (c) 0.9 V, 1030 cm-1 (d) 0.9 V, 1088 cm-1 Figure 1

Investigation of SEI Layer Formation in Conversion Iron Fluoride Cathodes by Combined STEM/EELS and XPS

Li-ion cathodes based on conversion reactions such as iron fluoride (FeF2) can achieve in principle high specific capacity. However, significant capacity fading is observed upon cycling. This has been attributed in part to the formation and continuous growth of a solid electrolyte interphase (SEI) layer at the cathode/electrolyte interface. In this work, scanning transmission electron microscopy, electron energy loss spectroscopy, selected area electron diffraction, and X-ray photoelectron spectroscopy were used in combination to study both the structural changes of the FeF2/C active material and the growth and evolution of the SEI layer upon cycling. Two main sources of capacity loss have been found. An increasing amount of Fe2+ appeared trapped inside the SEI layer with increasing cycle number, thus resulting in the loss of active material. In addition, reconversion is strongly impeded with increasing cycle number, leaving untransformed LiF and Fe0 upon delithiation. This correlates with the irreversibl...

In situ SEM observation of the Si negative electrode reaction in an ionic-liquid-based lithium-ion secondary battery.

By exploiting characteristics such as negligible vapour pressure and ion-conductive nature of an ionic liquid (IL), we established an in situ scanning electron microscope (SEM) method to observe the electrode reaction in the IL-based Li-ion secondary battery (LIB). When 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide ([C2mim][FSA]) with lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) was used as the electrolyte, the Si negative electrode exhibited a clear morphology change during the charge process, without any solid electrolyte interphase (SEI) layer formation, while in the discharge process, the appearance was slightly changed, suggesting that a morphology change is irreversible in the charge-discharge process. On the other hand, the use of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mim][TFSA]) with Li[TFSA] did not induce a change in the Si negative electrode. It is interesting to note this distinct contrast, which could be attributed to SEI layer formation from the electrochemical breakdown of [C2mim](+) at the Si negative electrode|separator interface in the [C2mim][TFSA]-based LIB. This in situ SEM observation technique could reveal the effect of the IL species electron-microscopically on the Si negative electrode reaction.

Damage Evaluation in Lithium Cobalt Oxide/Carbon Electrodes of Secondary Battery by Acoustic Emission Monitoring

Acoustic Emission (AE) technique was employed for evaluating charge/discharge damage in a lithium-ion battery. A coin-type battery of lithium cobalt oxide/carbon electrodes was used for acoustic monitoring during accelerated charge/discharge cycle test. A number of AE signals were successfully detected during charge/discharge. Microstructural observation of the electrodes after the cycle test revealed mechanical damage such as micro-cracking of the cathode and chemical damage such as solid electrolyte interphase (SEI) layer formed on the anode. The detected AE signals were classified into two distinct types (i.e., type 1 and type 2) based on the AE waveform parameters (i.e., duration and amplitude). The main frequency component of the type 1 signal with short duration and high amplitude was in the range of 121­160 kHz, whereas the frequency of type 2 signals with long duration and low amplitude was between 81 and 120 kHz. Active AE source of type 1 and type 2 signal was attributed to micro-cracking in cathode and gas bubble accompanied by SEI layer formation on anode, respectively. These results demonstrate the feasibility of the AE technique for the evaluation of charge/discharge degradation of secondary battery. [doi:10.2320/matertrans.M2014396]

Li Containing Binder for Improved the First Coulombic Efficiency and Cycleability of Li-Ion Batteries

Li containing binder for improved the first coulombic efficiency and cycleability of Li-ion BatteriesJun-Hwan Ku, a Seung Sik Hwang, a Min-Sang Song, a Jeong-Kuk Shon, a Sang-Min Ji, a Jae-Man Choi, a a Energy Lab, Samsung Advanced Institute of Technology, Electronic Materials Research Complex 130 Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do, Republic of KoreaLithium-ion batteries occupy a large and increasing share of the energy storage device market as a result of their excellent performance in terms of long-life, energy density and high safety. Among the various electrode materials for LIBs, graphites which are representative of carbon materials have been the most commonly used negative electrode materials due to their low working potential close to lithium metal anode, and remarkable cycling performance. As lithium-ion cells typically operate beyond the thermodynamic stability of organic electrolytes, the reduction products arising from chemical reactions during the first few cycles form passivating films on the carbon anode surface, as we call it 'solid electrolyte interphase (SEI)'.[1] Such SEI, which is comprised of electrochemically insulating layer, plays important roles of preventing the further electrochemical reactions between electrode surface and electrolyte and enabling only lithium ions tunnel through the layer. It is well known that the formation of SEI layers is a determinant factor on the performance of LIBs, affecting cycle-life, life time, power capability and even safety.[2]While the SEI plays an essential part in delivering the best performance in cells, the development of SEI layers are caused by the irreversible reaction accompanying an electrolyte decomposition, which makes the Coulombic efficiency to decrease during the first few cycles. When the stable SEI is not formed, any accidental misuse such as overcharge, high temperature exposure, and mechanic impact might damage the already formed SEI, resulting in more irreversible reaction during charging. The new anode surface, exposed to the electrolyte, immediately reacts with it to form a fresh thin protective film, which eventually leads to a poor cycleability and other undesirable properties in LIBs. Thus, to improve the performance of the cell, not only minimization of the electrolyte degradation but also thinner SEI film which provides an excellent passivating roll is required because thick and resistive SEI film is not favorable to battery operation.Many research studies are thus focused on the improvement of the chemical nature and morphology of SEI. For the modification to SEI films having superior properties, several previous papers reported the polymer binder including the SEI ingredients and their electrochemical effects of LIBs.[3,4]The effects of the surface modification of the graphite electrode by the functional polymer binder, such as poly(acrylic acid), have been reported in several papers. Concretely, it has been demonstrated that the polyion complex layer, which have oxygen species as functional groups, could play the role like artificial SEI to assist the facile penetration of Li ions. Obviously, the stable and efficient operation of LIBs is closely connected with ingredients of surface films, morphology, and coverage feature.Here we report that artificial pre-SEI, which is driven by Li ions containing polymer binder with functional group (-COOLi), can enhance the Coulombic efficiency and cycleability (Fig. 1). Furthermore, electrochemical performance is compared for SEI films that are produced from some different binders, in which different degree of lithium quantity is included in ISOBAM polymer. Fig. 1. (a) 1H-NMR spectra substituted ISOBAM polymer by 20, 50, 80, 100 amounts of Li ions, respectively. (b) Cycleability of SFG6 graphites with all investigated binders. Reference[1] E. Peled, J. Electrochem. Soc., 1979, 1 26, 2047.[2] E. Peled, in: J. P. Gabano (ED.), Lithium Batteries, AP, 1983, 43[3] K. Xu, Chem. Rev. (Washington, D.C.), 2004, 104, 4303.[4] S. Komaba, N. Yabuuchi, T. Ozeki, K. Okushi, H. Yui, K. Konno, Y. Katayama, and T. Miura, J. Power Sources, 2010, 195, 6069.

Current State of Understanding of the Solid-Electrolyte Interphase (SEI) in Lithium-Ion Cells and Its Relationship to Formation Cycling

The process of formation cycling refers to the electrochemical side reactions involved with creating a “passive” film on the anode active material, for instance graphite, known as the solid electrolyte interface (SEI) layer. This interfacial layer is formed during the first several charge-discharge cycles primarily by the reaction of electrolyte components with graphite at reducing potentials near the equilibrium potential of lithium metal (-3.045 V vs. SHE), and it plays a protective role by preventing the graphite from undergoing subsequent reaction with the electrolyte solvent and salt. [1] An ideal SEI layer should be thin, electrochemically inert, electronically conductive, and conductive to lithium ions. The commonly accepted hypothesis of the structure and composition of the SEI layer is an inner (interface with the graphite surface) inorganic layer of compounds such as LiF, Li2CO3, LiOH, LiO2, etc. and an outer (interface with the electrolyte solvent) organic layer primarily consisting of alkyl carbonates such as lithium ethylene dicarbonate (LiEDC). However, obtaining direct experimental evidence of SEI-layer composition and structure is extremely challenging. Information about thicknesses, constituent mass fractions, crystallinity, reactivity, and how these characteristics change from early cell life through thousands of charge-discharge cycles has remained elusive. Formation cycling is performed immediately after a lithium ion cell has been constructed and filled with electrolyte, and it has a profound economic impact on lithium ion battery manufacturing. The formation process requires that battery producers install many cycling stations to complete the process, which results in a heavy capital equipment investment and a much larger plant size. The irreversible capacity loss associated with anode SEI formation involves the consumption of lithium from the fresh cathode, resulting in a diminished battery lifetime. The formation process occurs in two successive stages, with the first stage involving formation of a highly resistive SEI layer at higher anode potentials (>0.25 V vs. Li/Li+). The second stage involves simultaneous intercalation of lithium into the graphite at potentials <0.25 V vs. Li/Li+ where the SEI layer is converted to a highly conductive film. [2] Fong et al. showed that the second stage involving the initial intercalation of lithium into graphite does not occur without the proper passivating electrolyte solvent and a sufficient coverage of SEI film. [3] There are three primary methods of formation cycling, which consist of a two-step current-charge formation, [4] pulse formation, [5] or the ageing process at elevated temperature. [6] In industry, combinations of these three approaches are often used. Electrode wettability plays a critical role in SEI layer formation. [7] After electrodes are coated and dried, they are usually calendered at high pressure to compact the composite structure, thus improving the energy density of the electrode layer. However, the electrode porosity is correspondingly reduced to only 30-35%, which has a significant impact on the pore-size distribution and the related wetting of the electrolyte.8 Wettability of the electrolyte to the electrode pores can be enhanced in two ways: 1) by supplying an additive to the electrolyte to lower its composite surface tension; and 2) by increasing the composite surface energy of the electrode. There is evidence that both of these approaches are effective, but the latter will be the focus of this presentation, especially as it relates to the SEI layer formation chemistry, growth in early cell life, and degradation in aged cells. Acknowledgment This research at Oak Ridge National Laboratory (ORNL), managed by UT Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO) Applied Battery Research (ABR) subprogram (Program Managers: Peter Faguy and David Howell).

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.

Silicon nanowire degradation and stabilization during lithium cycling by SEI layer formation.

Silicon anodes are of great interest for advanced lithium-ion battery applications due to their order of magnitude higher energy capacity than graphite. Below a critical diameter, silicon nanowires enable the ∼300% volume expansion during lithiation without pulverization. However, their high surface-to-volume ratio is believed to contribute to fading of their capacity retention during cycling due to solid-electrolyte-interphase (SEI) growth on surfaces. To better understand this issue, previous studies have examined the composition and morphology of the SEI layers. Here we report direct measurements of the reduction in silicon nanowire diameter with number of cycles due to SEI formation. The results reveal significantly greater Si loss near the nanowire base. From the change in silicon volume we can accurately predict the measured specific capacity reduction for silicon nanowire half cells. The enhanced Si loss near the nanowire/metal current collector interface suggests new strategies for stabilizing nanowires for long cycle life performance.

Study of Prelithiated Silicon As Anode in Lithium-Ion Cells

NASA is developing Ultra High Energy (UHE) lithium ion (Li-ion) cell designs and batteries for future exploration mission under NASA Advanced Space Power System project [1]. Development of silicon (Si) as an advanced anode is part of the effort for UHE Li-ion cell designs since Si is regarded as one of the most promising anode due to its high theoretical capacity (~10 times of graphite [2,3]) and its abundance on earth. However, there are challenges associated with using Si as a Li-ion anode mainly due to the significant volume change in Si particles upon Li alloying/de-alloying process. The volume change during the cycling results in degradation of the solid-electrolyte interphase (SEI) layer and/or disintegration of the Si anode electrode, causing significant irreversible capacity loss at initial few cycles and subsequent rapid capacity fade [4].The formation of SEI layer on Si is slow and difficult to form during the initial few cycles [5], and SEI formation process consumes lithium, which results in partial capacity loss irreversibly. As Si anode is incorporated into the Li-ion full-cell, the irreversible capacity loss at the initial few cycles is significantly higher, and subsequent capacity fade is much faster than those in the corresponding half-cell, which is due to the limited Li in the full-cell. In addition, the Si anode voltage rises quickly at the end of discharge because of the depletion of Li, as shown in Figure 1, which causes the higher cathode voltage cut-off, results in lower utilization of cathode and reduces the overall cell specific capacity. It is expected that the prelithiation of Si anode can help to overcome the initial irreversible capacity loss, lower the voltage rise at the end of discharge, reduce the capacity fade and extend the cycling life. In this report, the impact of a prelithiated Si anode on SEI formation, the first cycle irreversible capacity loss and the capacity fade has been studied. Various prelithiation methods, including to apply stabilized lithium metal powder (SLMP) on top of Si anode by spray method has been investigated. Electrochemical techniques such as cyclic voltammetry and electrochemical impedance spectroscopy in combination with SEM and other spectroscopy techniques are used to study the SEI formation of prelithiated Si anode vs. non-prelithiated Si anode. Full-cell with reference electrode is constructed to monitor Si anode voltage profile during the cycling and help to understand how prelithiated Si anode help to reduce the irreversible Li loss and extend the cycling life.

Reduction Mechanism of Fluoroethylene Carbonate for Stable Solid Electrolyte Interphase Film on Silicon Anode

Silicon (Si) is one of the most promising anode materials for Li-ion batteries because of its very high specific capacity (4212 mAh/g for Li21Si5).1 However, the high specific capacity of silicon is associated with large volume changes (more than 300 percent) when alloyed with lithium. These extreme volume changes can cause severe cracking and disintegration of the electrode and lead to capacity loss. The degradation of the carbonate solvents or the continuous growth of the Solid Electrolyte Interphase (SEI) film during cycling also results in significant irreversible capacity loss because it decreases electrochemical reactivity of electrodes.Fluoroethylene carbonate (FEC) is a well-known electrolyte additive for silicon anodes which can significantly enhance the stability and flexibility of SEI film and improve the stability and Coulombic efficiency of silicon-based anodes.2, 3 Although FEC has demonstrated significant effect on the electrochemical performance of silicon-based anodes, the fundamental mechanisms of this effect are still controversial in the literature. One possible reduction mechanism of FEC on silicon-based anodes is that the FEC first converts into vinylene carbonate (VC) through defluorination and then the resultant VC polymerizes.2, 4, 5 The main products from this mechanism are LiF and poly(vinylene carbonate). Another possible reduction mechanism is that the FEC is reduced through opening the five-member ring to form a (CH2CHFOCO2Li)2 dimer.6 According to these two mechanisms, the FEC reduction products formed in the SEI film should be LiF and poly(vinylene carbonate) or (CH2CHFOCO2Li)2.2, 6 In both cases, the lithium-to-fluorine ratio of the reaction products is close to 1. The majority of the fluorine should exist in either LiF or organic products,4-7but not in both forms. However, our recent analysis found that the ratio of lithium to fluorine is ≥1.5 for the SEI film formed on a silicon-based anode which was cycled in an electrolyte with FEC as additive or pure FEC as solvent (table 1).Our X-ray Photoelectron Spectroscopy (XPS) study also reveals that fluorine exists in both LiF and organic compounds (Figure 1), which cannot be explained by the previous mechanisms proposed in the literature. Therefore, a new mechanism has been proposed (Figure 2) and verified for the reduction and polymerization of FEC and the formation of a stable SEI layer on anode materials. According to this mechanism, FEC is reduced on the anode surface through the opening of the five-member ring, leading to the formation of lithium poly(vinyl carbonate), LiF, and some dimers. Table 1 The percentage of Li, C, O and F in the SEI films in electrolytes (a) 1M LiPF6 in EC/DMC (1:2 in vol), (b) 1M LiPF6 in EC/DMC (1:2 in vol) with 10% FEC and (c) 1M LiClO4in pure FECElements samples%Li%C%O%FLi/F ratioElectrolyte (a), 2 cycles27.723.324.124.91.11Electrolyte (a), 35 cycles22.731.337.68.42.70Electrolyte (b), 2 cycles22.932.532.112.51.83Electrolyte (b), 100 cycles21.033.831.913.31.58Electrolyte (c), 2 cycles21.134.732.212.01.76 Figure 1 XPS spectra of F1s for the SEI film formed in LiClO4/FEC electrolyte Figure 2Proposed possible reduction reaction of FEC Acknowledgment This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract No. 18769 under the Batteries for Advanced Transportation Technologies (BATT) program. A portion of the research was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Reference 1. B. A. Boukamp, G. C. Lesh, and R. A. Huggins, Journal of The Electrochemical Society, 128(4), 725-729 (1981).2. V. Etacheri, O. Haik, Y. Goffer, G. A. Roberts, I. C. Stefan, R. Fasching, and D. Aurbach, Langmuir : the ACS journal of surfaces and colloids, 28(1), 965-976 (2012).3. N.-S. Choi, K. H. Yew, K. Y. Lee, M. Sung, H. Kim, and S.-S. Kim, Journal of Power Sources, 161(2), 1254-1259 (2006).4. D. Aurbach, K. Gamolsky, B. Markovsky, Y. Gofer, M. Schmidt, and U. Heider, Electrochimica Acta, 47(9), 1423-1439 (2002).5. S. S. Zhang, Journal of Power Sources, 162(2), 1379-1394 (2006).6. Z.-C. Wang, J. Xu, W.-H. Yao, Y.-W. Yao, and Y. Yang, ECS Transactions, 41(41), 29-40 (2012).7. H. Wu, G. Chan, J. W. Choi, I. Ryu, Y. Yao, M. T. McDowell, S. W. Lee, A. Jackson, Y. Yang, L. Hu, and Y. Cui, Nat Nano, 7 (5), 310-315 (2012).

Demonstration of an Electrochemical Liquid Cell for Operando Transmission Electron Microscopy Observation of the Lithiation/Delithiation Behavior of Si Nanowire Battery Anodes

Over the past few years, in situ transmission electron microscopy (TEM) studies of lithium ion batteries using an open-cell configuration have helped us to gain fundamental insights into the structural and chemical evolution of the electrode materials in real time. In the standard open-cell configuration, the electrolyte is either solid lithium oxide or an ionic liquid, which is point-contacted with the electrode. This cell design is inherently different from a real battery, where liquid electrolyte forms conformal contact with electrode materials. The knowledge learnt from open cells can deviate significantly from the real battery, calling for operando TEM technique with conformal liquid electrolyte contact. In this paper, we developed an operando TEM electrochemical liquid cell to meet this need, providing the configuration of a real battery and in a relevant liquid electrolyte. To demonstrate this novel technique, we studied the lithiation/delithiation behavior of single Si nanowires. Some of lithiation/delithation behaviors of Si obtained using the liquid cell are consistent with the results from the open-cell studies. However, we also discovered new insights different from the open cell configuration-the dynamics of the electrolyte and, potentially, a future quantitative characterization of the solid electrolyte interphase layer formation and structural and chemical evolution.

Si nanotubes array sheathed with SiN/SiOxNy layer as an anode material for lithium ion batteries

Silicon has received high interest as an anode material for lithium ion batteries due to its large theoretical Li storage capacity. However, poor cyclability and low coulombic efficiency of the Si based electrode, caused by the pulverization of the active material and the continuous formation of unstable solid electrolyte interphase (SEI) due to large volume change associated with Li, limits its practical use as an anode material. We have developed a Si nanotube array sheathed with silicon nitride compound to improve the mechanical integrity, resulting in improved electrochemical performance. The SiN/SiOxNy outer shell has excellent mechanical properties, such as a high elastic modulus and hardness. This guides the volume expansion of the Si into the hollow inner space of the tubular structure during charge, which prevents both the pulverization of the Si active material, as well as continuous SEI layer formation by protecting the exposure of fresh Si surface to the electrolyte. Si nanotube array sheathed with silicon nitride electrode compound exhibits improved electrochemical performance, including stable capacity retention and high coulombic efficiencies, over the analogous homogeneous Si nanotube system.

Measurement and modeling of the mechanical and electrochemical response of amorphous Si thin film electrodes during cyclic lithiation

A combination of experimental measurements and numerical simulations are used to characterize the mechanical and electrochemical response of thin film amorphous Si electrodes during cyclic lithiation. Parameters extracted from the experiment include the variation of elastic modulus and the flow stress as functions of Li concentration; the strain rate sensitivity; the diffusion coefficient for Li transport in the electrode; the free energy of mixing as a function of Li concentration in the electrode; the exchange current density for the Lithium insertion reaction; as well as reaction rates and diffusion coefficients characterizing the rate of formation of solid-electrolyte interphase layer at the electrode surface. Model predictions are compared with experimental measurements; and the implications for practical Si based electrodes are discussed.