Thickness Of Solid Electrolyte Interphase Research Articles

Structural Restoration of Degraded LiFePO 4 Cathode with Enhanced Kinetics Using Residual Lithium in Spent Graphite Anodes

Structural Restoration of Degraded LiFePO ₄ Cathode with Enhanced Kinetics Using Residual Lithium in Spent Graphite Anodes

Unraveling the mechanical origin of stable solid electrolyte interphase

Summary The capability of a solid electrolyte interphase (SEI) in accommodating deformation is critical to the electrode integrity. However, the nanoscale thickness and environmental sensitivity of SEI make it challenging to characterize multiple mechanical parameters and identify an appropriate predictor for such capability. Here, we develop a feasible atomic force microscopy (AFM)-based nanoindentation test that circumvents the interference of the anode substrate and allows accurate probing of Young's modulus and elastic strain limit of SEI. The "maximum elastic deformation energy" (U) that an SEI can absorb is proposed to predict the stability of the SEI, as successfully demonstrated in Li/K metal anodes. We show that another asset of U is to provide a rapid and effective means to screen proper electrolytes for the stabilization of Sn and Sb microparticle anodes through building high-U SEIs. Overall, this new indicator, U, offers future directions toward rational design of robust SEIs for advanced anodes.

Evaluating Solid-Electrolyte Interphases for Lithium and Lithium-free Anodes from Nanoindentation Features

Summary The morphology and mechanical property of SEIs for lithium anodes crucially affect the electrochemical cycling performance of Li-metal batteries such as Li-S and Li-O2 batteries. Herein, we establish the standards for rapidly assessing SEIs for Li and Li-free type of anodes by AFM nanoindentation features toward prediction of corresponding electrochemical performances. A series of single-layered and multi-layered SEIs with known composition and structures are purposely constructed. A set of unique nanoindentation features representative of mechanical properties of the basic SEIs are formed, serving as the standards for rapidly assessing the mechanical properties and electrochemical performances of unknown SEIs together with their Young’s modulus, smoothness, and thickness. To validate the applicability of the method, SEIs formed by electrochemical aging and formed on Cu substrates are further evaluated. Our work not only contributes to understanding the influence of SEIs on cycling performances but also provides a rapid way for screening SEIs.

Sweeping potential regulated structural and chemical evolution of solid-electrolyte interphase on Cu and Li as revealed by cryo-TEM

A fundamental understanding of solid-electrolyte interphase (SEI) is paramount importance for controlling the cycling performance of rechargeable lithium metal batteries. The structural and chemical evolution of SEI with respect to electrochemical operating condition remains barely established. Here we develop a unique method for imaging the evolution of SEI formed on the Cu foil under sweeping electrochemical potential. By using cryogenic TEM imaging combined with energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy electronic structure analyses, we reveal that, for the vinylene carbonate (VC)-free electrolyte, the SEI formed at 1.0 V is a monolithic amorphous structure, which evolves to amorphous matrix embedded with Li 2 O particles as the voltage decreases to 0 V. In the case of VC-containing electrolyte, the SEI is featured by an amorphous matrix with Li 2 O particles from 1.0 V to 0 V. The thickness of SEI formed on Cu foil increases with decreasing voltage. Associated with the localized charge modulation by the surface topographic feature and defects in the Cu foil, the SEI layer shows direct spatial correlation with these structural defects in the Cu. In addition, upon Li deposition, the SEI formed on the Li metal has similar thickness with, but different composition from the SEI formed on the Cu foil at 0 V. Those results provide insight toward SEI engineering for enhanced cycling stability of Li metal. A fundamental understanding of solid-electrolyte interphase is paramount importance for controlling the cycling performance of rechargeable lithium metal batteries. Here we develop a unique method for imaging the evolution of SEI formed on the Cu foil under sweeping electrochemical potential. By using cryogenic TEM imaging combined with energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy electronic structure analyses to directly probe the morphology, atomic structure, and chemical bonding of solid-electrolyte interphase formed on Cu and Li in the electrolyte without/with vinylene carbonate additive. We reveal that the vinylene carbonate additive is shown to significantly affect the solid electrolyte interphase on Cu at different potentials. Solid electrolyte interphases formed on Cu and Li show significantly different in chemical composition. • Developing a unique method for the formation of solid electrolyte interphase on Cu to facilitate cryo-TEM imaging. • Structural and chemical evolution of SEI formed on Cu with sweeping potentials were revealed by Cryo-TEM and XPS. • Vinylene carbonate additive is shown to significantly affect the solid electrolyte interphase on Cu at different potentials. • Solid electrolyte interphases formed on Cu and Li show significance difference in chemical composition.

Modeling fracture of solid electrolyte interphase in lithium-ion batteries during cycling

In lithium-ion batteries, the volume change of anode materials will result in fracture of solid electrolyte interphase (SEI) during continuous lithiation and delithiation. Herein, an analytical model has been developed to determine the fracture mechanism of the SEI and the fatigue in lithium-ion batteries. The evolution of diffusion-induced stresses and concentration have been evaluated. In addition, surface effects are found to effectively reduce the stresses and the crack propagation in SEI during lithiation. With combined energy release rate, the critical thickness of SEI is also established to prevent crack propagation. Finally, the capacity fade of lithium-ion batteries during cycling may be predicted using fatigue model. It is found that the depths of discharge (DODs) are closely related to the capacity fade of lithium-ion batteries. Overall, this work may provide physical understanding for optimized structural design to alleviate the SEI fracture and the fatigue of the lithium-ion batteries.

Three-Dimensional Resistivity Vs. Depth Profiling for Advanced Characterization of the Lithium-Ion Battery Solid Electrolyte Interphase

In recent years, the solid electrolyte interphase (SEI) has been recognized as a critical component of the lithium-ion battery (LIB) system. As battery applications in electric vehicles and stationary storage drive demand for higher energy density LIBs, alternative anode materials are being investigated to replace the current graphite-based anode. In alloying anode materials such as silicon (Si), volumetric changes occur during lithiation and delithiation, inducing additional SEI instability at the Si interface.1 The electronic properties of the SEI are of particular interest, as an electronically insulating SEI is necessary to maintaining passivation of the anode. Consequently, understanding the fundamental electronic structures of SEI that influence its performance is a necessary step to future implementation of next generation anodes in LIBs. Three-dimensional resistivity vs. depth profiling has emerged as an advanced characterization approach for measuring the electronic resistivity and thickness of SEI formed on Si.2 This technique utilizes scanning spreading resistance microscopy (SSRM) to mechanically profile away SEI with a conductive, wear resistant probe, enabling nanoscale mapping of electronic resistivity of SEI structures. In this work, we investigate the evolution of SEI formed from a standard electrolyte (1.2 M LiPF6 in EC:EMC (3:7 wt%)) through various points in cycling on several model Si surfaces. We also examine the solubility of SEI by resting the anode in the cell at varied temperatures then characterizing the residual SEI. Evolution of the SEI surface is also analyzed with atomic force microscopy (AFM) measurements. AFM and SSRM-derived results are presented in combination with chemical characterization, including time-of-flight secondary ion mass spectrometry (TOF-SIMS). Additional studies into the changes to the electronic properties of the underlying Si anode through cycling are also presented. Conductivity of active Si in LIBs has been shown to decrease through cycling, but the specific causes and implications on battery performance are still poorly understood.3 Three-dimensional electronic resistivity vs. depth profiling is an effective technique for nanoscale investigation of the SEI as well as the active anode material beneath. References Cao, C.; Abate, I.I.; Sivonxay, E.; Shyam, B.; Jia, C.; Moritz, B.; Devereaux, T.P.; Persson, K.A.; Steinrück, H.G.; Toney, M.F., Solid Electrolyte Interphase on Native Oxide-Terminated Silicon Anodes for Li-Ion Batteries. Joule 2019, 3, 762-781.Stetson, C.; Yoon, T.; Coyle, J.; Nemeth, W.; Young, M.; Norman, A.; Pylypenko, S.; Ban, C.; Jiang, C.-S.; Al-Jassim, M.; Burrell, A., Three-dimensional electronic resistivity mapping of solid electrolyte interphase on Si anode materials. Nano Energy 2019, 55, 477-485.Kim, S.H.; Kim, Y.S.; Baek, W.J., Heo, S.; Yun, D.J.; Han, S.; Jung, H., Nanoscale Electrical Degradation of Silicon-Carbon Composite Anode Materials for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2018, 10, 24549-24553.

(Invited) Investigation of Solid Electrolyte Interphase on Amorphous Siox/Si Films Using Tip Enhanced Raman Spectroscopy

The composition, morphology and thickness of solid electrolyte interphase (SEI) on silicon anodes are dictated by multiple factors such as surface chemistry, electrolyte composition, chemical passivation effects etc. Further the SEI constantly evolves, as it is electrochemically lithiated (delithiated) during the enormous volume changes. SEI on silicon typically is reported to be in the range of tens of nanometers and can be compositionally heterogeneous. Normal vibrational spectroscopy techniques such as micro-Raman and FT-IR while pretty useful, provide chemical information at a relatively bulk scale, typically much > 100 nm from the sample surface.This makes it harder for getting information from the real SEI, which can be only order of tens of nanometer. Tip Enhanced Raman Spectroscopy (TERS) is based on the field enhancement of Raman active signal locally at regions close to the tip. The Raman cross section is enhanced at least several orders of magnitude due to resonance coupling between surface plasmon modes (of the tip) and the electromagnetic field of the excitation laser. TERS enables simultaneous chemical mapping and topography at a nanoscale spatial resolution. TERS mapping of cycled amorphous silicon show the SEI composition and morphology changing as a function of electrochemical cycles. These observations will be compared with results from other techniques such as X-ray photoelectron spectroscopy

Probing the Nanoscale Heterogeneity of SEI on Silicon Anode Using Tip Enhanced Raman Spectroscopy (TERS)

The composition, morphology and thickness of solid electrolyte interphase (SEI) on silicon anodes are dictated by multiple factors, ranging from the chemistry of silicon surfaces, electrolyte solution structure and chemical passivation on Si surface. In addition, the SEI constantly evolves, as it is electrochemically lithiated (delithiated) due to the enormous volume changes. SEI on silicon typically is reported to be in the range of tens of nanometers and can be compositionally heterogeneous. Normal vibrational spectroscopy techniques such as confocal micro-Raman and FT-IR are useful, but they only provide chemical information at a relatively bulk scale, typically in micron-distance from the solid surface. This makes it extremely hard for getting information from the real SEI, which can be only on the order of 10 nm. Tip Enhanced Raman Spectroscopy (TERS) combines surface plasmon-enhanced Raman spectroscopy with atomic force microscopy, capable of providing the chemical vibrational information and topography of the sample in the nanometer spatial resolution simultaneously. In this report we demonstrate the first TERS study of SEI on cycled amorphous silicon. Amorphous silicon anode (20 nm in thickness on copper current collector) was cycled in a commercial lithium-ion battery electrolyte (1M LiPF6 in EC:DEC, 1:1 vol). TERS analysis on cycled amorphous silicon sample indicates that the nanometer scale SEI “islands” are unevenly distributed on the Si anode surface. Even for the same SEI “island”, the composition is different from point to point with inter-point distance smaller than 2 nm. Acknowledgment This work is funded by Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy

Modulation of solid electrolyte interphase of lithium-ion batteries by LiDFOB and LiBOB electrolyte additives

Solid-electrolyte interphase (SEI) layer is an organic–inorganic composite layer that allows Li+ transport across but blocks electron flow across and prevents solvent diffusing to electrode surface. Morphology, thickness, mechanical and chemical properties of SEI are important for safety and cycling performance of lithium-ion batteries. Herein, we employ a combination of in-situ AFM and XPS to investigate the effects of two electrolyte additives namely lithium difluoro(oxalate)borate (LiDFOB) and lithium bis(oxalato)borate (LiBOB) on SEI layer. LiDFOB is found to result in a thin but hard SEI layer containing more inorganic species (LiF and LiCO3); meanwhile LiBOB promotes formation of a thick but soft SEI layer containing more organic species such as ROCO2Li. Findings from present study will help development of electrolyte additives that promote formation of good SEI layer.

AFM Investigation of Solid Electrolyte Interphase on HOPG Anode in Sodium Ion Battery

Chemical and morphological structures of solid electrolyte interphase (SEI) play a vital role in sodium- ion battery (NIB). Up to date, SEI remains the least understood component in NIB due to its trace presence, delicate chemical nature, heterogeneity in morphology, elusive formation mechanism, and lack of reliable in situ quantitative characterization tools. SEI morphological evolution during the first cyclic voltammetry and the thickness of SEI after first discharging-charging cycle on highly oriented pyrolytic graphite (HOPG) surface were investigated by using in situ electrochemical atomic force microscopy (EC-AFM). Complemented by an ex situ XPS analysis, the differences in interfacial features and fundamental constitution of SEI formation in ethylene carbonate (EC) and fluoroethylene carbonate (FEC)-based electrolytes on HOPG electrode surface were revealed. In 1 mol/ L NaClO4/EC/DMC electrolyte, dense films were formed at the step edges, while thinner films were observed on the basal plane. In 1 mol/L NaClO4/EC/DMC electrolyte, stable films with doubled layer structures were formed on HOPG electrode after initial electrochemical cycling. The upper layer was composed of large particles, which was soft and easy to be scrapped off by AFM tip while the under layer was composed of dense small particles.

Mechanical insights into the stability of heterogeneous solid electrolyte interphase on an electrode particle

Solid electrolyte interphase (SEI) is known to be heterogeneous which comprises inorganic and organic decomposition products. To assess the effects of heterogeneity on the stability of the SEI, we establish a mathematical model in simulating the stress of heterogeneous SEI. Comparing with the analytical solution of stress in the homogeneous film, the heterogeneity of SEI is identified to be essential in the stress calculation of the inorganic layer, which is proven decisive for the stability of SEI. Further, the peak tensile stress within the inorganic layer is found in the bilayer SEI. It generates at the interface between the active material and the inorganic layer when the battery is fully charged. In addition, the impacts of the interface parameter, modulus, Poisson’s ratio, thickness of SEI and lithiation properties of active material on SEI stress are systematically investigated. Based on the simulation results, this work provides insights into the stress analysis of the heterogeneous SEI, as well as suggestions in pursuing a well-designed SEI, i.e., a functional gradient heterogeneous SEI in which the inner layer close to the active material provides sufficient mechanical performance while the outer layer near the electrolyte performs good chemical and electrochemical behaviors, to enhance the battery performance.

Population Balance Model of Formation and Growth of Solid Electrode Interphase in Lithium Ion Batteries

We develop a physical-statistical model for formation and growth of the solid electrolyte interphase (SEI) in the negative electrode of Li ion batteries. During charging/discharging cycles, the SEI layer forms via a reaction between lithium ions, electrons and solvent molecules. The growth of the SEI layer leads to capacity fade and an increase of the ion transport resistance. In addition, SEI growth decreases the total porosity. Our statistical modeling framework employs the Fokker-Planck theory and it uses a statistical particle density distribution function as input. Structure-changing processes at the particle level transform this statistical distribution, causing changes in battery performance, e.g. capacity fade and power fade. The present study focuses on the impact of SEI formation, explored within this modeling framework. To this end, we have formulated a set of kinetic and transport equations. The solution of the statistical model reveals a broadening of the initial statistical distribution of SEI thicknesses and a growth of the average thickness of SEI increases from 47 nm to 80 nm at 25 oC and C/2 charge rate after 1000 cycles. This results in a 5% reduction of the battery capacity as shown in Fig.1. The statistical model of SEI formation is used to perform a multi-objective optimization to find a set of optimized battery design parameters, which lead to minimizing the capacity loss and maximizing the power density. In this optimization, the weight between capacity loss minimization and power density maximization can be adjusted as desired, as shown in Fig. 2. Figure 1

Polyglutamate Binder for Silicon-Graphite Negative Electrode: Effects of Cut-Off Voltage and Electrolyte Additive

Silicon is one of the promising active materials for negative electrodes of Li-ion batteries (LIB) because of its larger reversible capacity (~3600 mAh g-1) through lithiation/delithiation process, compared with that of conventional graphite active material (~370 mAh g-1). However, it is difficult to maintain its superior discharge capacity for long-term cycles because large volume change of Si occurs during cycles, which triggers intensive electrode degradation. To overcome this drawback, binder polymer is thought to be a key factor for improvement of the performance. In particular, polyacrylate binder1,2) and natural polymer binder3) effectively suppress the degradation due to their superior mechanical strength, coatability for active materials, and effects of pre-formed solid electrolyte interphase (SEI)4). Recently, we have focused on polyglutamate (PGlu) as an efficent binder for the Si/graphite composite electrode of LIB. PGlu is known to be contained in Japanese traditional food “Natto,” which is fermented soy beans. In this study, effects of discharge cut-off voltage in the cycles and electrolyte additive on the performance of Si/graphite electrodes with PGlu binder are investigated from the viewpoint of surface chemistry. Figure 1 shows cycle performance of the Si/graphite electrodes with different discharge cut-off voltage. For additive-free cells, capacity retention was highly improved under the cycles with lower discharge cut-off voltage (1.0 V vs. Li/Li+). Furthermore, addition of fluoroethylene carbonate (FEC) to the electrolyte effectively suppressed deterioration of the performance and brought the best capacity retention, leading to discharge capacity of ~800 mAh g-1 even after 100 cycles. To clarify details of the effects, chemical components and thickness of the SEI formed on the electrode surface were examined by hard X-ray photoelectron spectroscopy (HAXPES, hν = 7.94 keV). Figure 2 shows the HAXPES C 1s and Si 1s spectra of Si/graphite electrodes before and after the first cycle with different discharge cut-off voltage. For the electrode with cut-off voltage of 1.0 V, the peak intensity at 284.6 eV is relatively lower than that of the 2.0 V cut-off electrode, which is assigned to sp 2 carbon bonding in graphite of the composite. Similarly, the peaks at 1839.5 eV and 1844 eV in the Si 1s spectra, which are assigned to Si0 and silicon oxide, respectively, are smaller for the case of lower cut-off voltage. These results indicate that thickness of SEI is affected by the discharge cut-off voltage. We presume that moderately-low discharge cut-off voltage of 1.0 V effectively brings thicker and stable SEI on the electrode surface by suppression of oxidative decomposition and dissolution of SEI, which probably occur at higher potential condition in delithiation process. In this presentation, other electrochemical and morphological characterizations of the electrode and superior effects of FEC for the cyclability will be also discussed.

Impact of Libob Additive on Structure and Property of Solid Electrolyte Interphase Formed on Negative Electrodes Surface

A Solid electrolyte interphase (SEI) that is formed on both anode and cathode surfaces during the early charge process, plays an important role stabilizing lithium ion battery (LiB) operation, by preventing further electrolyte decomposition during charge and discharge. Generally, electrolyte additives are used in order to effectively stabilize SEIs. Lithium bis-oxalato borate (LiBOB) is one example of such an effective additive that forms stable SEI on electrodes to inhibit the degradation of the graphite anode, improving the capacity retention during the cycling test at high temperature. Detailed mechanism on the impact of LiBOB additive, however, remains unknown. We, therefore, analyzed the SEI of graphite negative electrode with and without LiBOB by using Hard X-ray photoelectron spectroscopy (HAX-PES) which is suitable to analyze over-all structure of deposited SEIs with deep probing length. In addition, we performed NMR to confirm chemical composition of D2O-extracted components of SEIs. An electrolyte that consists of standard 1 M LiPF6 salt dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate (EC / DEC) in a 3:7 volume ratio was used. 3 wt% of LiBOB additive (Sigma-Aldrich Co. LLC) was added into the electrolyte. The coin-type cell, which is composed of a cathode electrode LiNi1/3Co1/3Mn1/3O2, a graphite anode and polyethylene separator, were assembled with different electrolytes in an argon-filled glove box. Charge-discharge tests were repeated for 50 cycles in the potential rage of 3.0-4.2 V at 50 °C at a charge/discharge rate of 1C. Then, the cell was disassembled and the anode was retrieved, and repeatedly washed with DEC in an argon-filled glove box. The HAXPES measurements were carried out at the BL46XU beamline at SPring-8. Figure shows C 1s HAXPES spectra for the graphite anodes after 1st and 50th charge and discharge cycles. The spectra were fitted with six peaks centered at 282.2eV (Peak 1), 284.2 eV (Peak 2), 285.6eV (Peak3), 287.9 eV (Peak 4), 289.2 eV (Peak 5), 290.1 eV (Peak 6). Peak 1 can be assigned to graphite. Peak2 can be assigned to chemical components for lithium alkyl carbonate, the organic materials of SEI. Peak 5 can be associated with a carbonate component for the Li2CO3, the inorganic material of SEI. Peak 3 and Peak 6 include the hydrocarbon and organofluorine components for the PVdF binder. Peak4 can be assigned oxalate originate in LiBOB decomposition. First, we note that the graphite peak (P1) in the C 1s HAXPES was still observed spectra even after 50th charge and discharge cycles. This result indicates that the depth of HAXPES (hν = 8 keV) is larger than thickness of SEI, thus HAXPES detected whole SEI and a part of the active materials exists underneath SEI film in the depth direction. After 50th cycles, the peak intensity of P1 decreased and the increase with remarkable peak intensity of P5 was observed without LiBOB additive. This indicates that thickness of inorganic SEI on graphite anode without LiBOB additive is larger than that of SEI with LiBOB additive. The similar tends were observed for LiF and POx components on graphite (the F 1s and P 1sXPS spectra were not shown). More detailed mechanism will be discussed at the Meeting combined with NMR data. Figure 1

Impact of FEC Additive on SEI Structure Formed on a Carbon Negative Electrode Studied By Hard X-Ray Photoelectron Spectroscopy

Solid electrolyte interphase (SEI) formed on a carbon negative electrode plays an important role in supporting the reversible Li ion intercalation phenomena during charge and discharge treatments. Since the chemical and electrochemical conditions of the electrolyte/electrode interface change during electrochemical lithiation and delithiation reactions, the structure of the SEI might change even after the formation of the SEI by the initial charge and discharge treatments, although details of the reaction mechanism remain unknown. We recently successfully analyzed the behavior of the SEI of a carbon negative electrode during electrochemical lithiation and delithiation reactions by using Hard X-ray photoelectron spectroscopy (HAX-PES) technique and indicated HAX-PES was useful for analyzing SEI structure. Additives can control the structure and components of SEI, leading to the improvement of battery performance. In the present study, we applied this method to understanding of the influence of the SEI forming additives. Lithiated and delithiated carbon anodes were prepared by using a coin-type cell with Li metal. The carbon anode was a composite graphite (active agents, carbon black, PVDF) coated on Cu foil. The electrolyte was 1 M LiPF6 in 90wt% ethylene carbonate / diethyl carbonate (EC / DEC) in a 1:1 volume ratio + 10 wt% fluoro ethylene carbonate (FEC). After charge and discharge, the cell was disassembled and the anode was then retrieved, washed with dimethyl carbonate repeatedly in an argon-filled glovebox (<1 ppm H2O, <1 ppm O2). The anodes were transferred from the glovebox to UHV measurement chamber without exposing to the air by using special transfer systems. The HAXPES measurements were carried out at the BL46XU beamline at SPring-8. The photon energy of incident x-rays was 8 keV. Figure 1 shows C 1s HAXPES spectra for the lithiated graphite anodes with the state of charge (SOC) 0%, 50%, 100% after twice charge and discharge cycles. The spectra were fitted with seven singlets centered at 283.5eV (Peak 1), 284.6 eV (Peak 2), 286.0eV (Peak3), 286.5 eV (Peak 4), 288.6 eV(Peak 5), 289.6 eV(Peak 6) and 290.5 eV(Peak 7). Peak 1 can be assigned to lithiated carbon LixC. Peak2 – Peak6 can be assigned to chemical components for lithium alkyl carbonate (LAC), the organic materials of SEI. Peak 7 can be associated with a carbonate component for the Li2CO3, a inorganic material of SEI. Peak 2 and Peak 7 include the hydrocarbon (Peak2) and organofluorine (Peak7) components for the PVDF binder. LixC was observed in the C 1s HAXPES spectra with all SOC conditions, whereas this peak wasn’t observed in the C 1s XPS spectra with Al K-α incident x-rays for the lithiated graphite anodes with SOC 50%, 100% after twice charge and discharge cycles (the C 1s XPS spectra were not shown). This result indicates that the probing depth of HAXPES (hν = 8keV) is larger than thickness of SEI, thus HAXPES detected whole SEI and a part of the LixC exists underneath SEI formed on all the lithiated graphite anodes in the depth direction. HAXPES technique allows us to analyze the variation of the chemical composition of SEI depending on electrochemical lithiation and delithiation reactions. Figure 2 shows the concentration ratio of LixC, LAC, and Li2CO3 depending on SOC determined by the C 1s HAXPES spectra. The ratio of LixC decreases up to SOC 100%, while those of LAC, and Li2CO3 increases up to SOC 100%. This variation shows the structural behavior of the SEI during electrochemical lithiation reactions after the formation of the SEI by the initial charge and cycles: SEI formed in the electrolyte with FEC additives continues growth and dissolution even after stable state is established. Figure 1

A Haxpes Study on Solid Electrolyte Interphase Formed on a Carbon Negative Electrode

Solid electrolyte interphase (SEI) formed on a carbon negative electrode plays an important role in supporting the reversible Li ion intercalation phenomena during charge and discharge treatments. Since the chemical and electrochemical conditions of the electro-lyte/electrode interface change during electrochemical lithiation and delithiation reactions, the structure of the SEI might change even after the formation of the SEI by the initial charge and discharge treatments, although details of the reaction mechanism remain unknown. We, therefore, investigated the behavior of the SEI of a carbon negative electrode during electrochemical lithiation and delithiation reactions by using Hard X-ray photoelectron spectroscopy (HAX-PES) technique. Lithiated and delithiated carbon anodes were prepared by using a coin-type cell with Li metal. The carbon anode was a composite graphite (active agents, carbon black, PVDF) coated on Cu foil. The electrolyte was 1 M LiPF6 in ethylene carbonate / diethyl carbonate (EC / DEC) in a 1:1 volume ratio. After charge and discharge, the cell was disassembled and the anode was then retrieved, washed with dimethyl carbonate repeatedly in an argon-filled glovebox (<1 ppm H2O, <1 ppm O2). The anodes were transferred from the glovebox to UHV measurement chamber without exposing to the air by using special transfer systems. The HAXPES measurements were carried out at the BL46XU beamline at SPring-8. The photon energy of incident x-rays was 8 keV. Figure 1 shows C 1s HAXPES spectra for the lithiated graphite anodes with the state of charge (SOC) 0%, 50%, 100% after twice charge and discharge cycles. The spectra were fitted with six singlets centered at 283.5eV (Peak 1), 284.6 eV (Peak 2), 285.5 eV (Peak 3), 287 eV(Peak 4), 289 eV(Peak 5) and 290.5 eV(Peak 6). Peak 1 can be assigned to lithiated carbon LixC. Peak 2 can be assigned to hydrocarbons for lithium alkyl carbonate (LAC), the organic materials of SEI and PVDF binder. The carbonyl components for the LAC are observed as Peak 3, Peak 4, and Peak 5. Peak 6 can be associated with a organofluorine component for the PVDF binder and a carbonate component for the Li2CO3, a inorganic material of SEI. LixC was observed in the C 1s HAXPES spectra with all SOC conditions, whereas this peak wasn’t observed in the C 1s XPS spectra with Al K-α incident x-rays for the lithiated graphite anodes with SOC 50%, 100% after twice charge and discharge cycles (the C 1s XPS spectra were not shown). This result indicates that the probing depth of HAXPES (hν = 8keV) is larger than thickness of SEI, thus HAXPES detected whole SEI and a part of the LixC exists underneath SEI formed on all the lithiated graphite anodes in the depth direction. HAXPES technique allows us to analyze the variation of the chemical composition of SEI depending on electrochemical lithiation and delithiation reactions. Table 1 shows the concentration ratio of LixC, LAC, and Li2CO3 depending on SOC determined by the C 1s HAXPES spectra. The ratio of LixC decreases up to SOC 100%, while those of LAC, and Li2CO3 increases up to SOC 100%. This variation shows the structural behavior of the SEI during electrochemical lithiation reactions after the formation of the SEI by the initial charge and cycles: SEI continues growth and dissolution even after stable state is established. Acknowledgement The synchrotron experiments were carried out on beamline BL46XU at Spring-8 with approvals from Japan synchrotron radiation institute (JASRI) (Proposal nos. 2013A1234)[1] H. Hori, M. Shikano, H. Kobayashi, S. Koike, H. Sakaebe, Y. Saito, K. Tatsumi, H. Yoshikawa, and E. Ikenaga J. Power. Sources, 242, 844 (2013).[2] P. Verma, P. Maire1, and P. Novak, Electrochim. Acta., 55, 6332 (2010). [3] K. Xu and Arthur V. Cresce, J. Mater. Chem., 21, 9849 (2011).

Morphology and modulus evolution of graphite anode in lithium ion battery: An in situ AFM investigation

We investigated the interfacial electrochemical processes on graphite anode of lithium ion battery by using highly oriented pyrolytic graphite (HOPG) as a model system. In situ electrochemical atomic force microscopy experiments were performed in 1 M lithium bis(trifluoromethanesulfonyl)imide/ethylene carbonate/diethyl carbonate to reveal the formation process of solid electrolyte interphase (SEI) on HOPG basal plane during potential variation. At 1.45 V, the initial deposition of SEI began at the defects of HOPG surface. After that, direct solvent decomposition took place at about 1.3 V, and the whole surface was covered with SEI. The thickness of SEI was 10.4 ± 0.2 nm after one cycle, and increased to 13.8 ± 0.2 nm in the second cycle, which is due to the insufficient electron blocking ability of the surface film. The Young’s modulus of SEI was measured by a peak force quantitative nanomechanical mapping (QNM). The Young’s modulus of SEI is inhomogeneous. The statistic value is 45 ± 22 MPa, which is in agreement with the organic property of SEI on basal plane of HOPG.

A new approach to surface properties of solid electrolyte interphase on a graphite negative electrode

A new method for observing the outermost surface morphology and the thickness of solid electrolyte interphase (SEI) are presented. Low acceleration voltage in extreme high resolution scanning electron microscopy (XHR-SEM) provides detailed information on the SEI surface morphology due to secondary electron contrast mechanism. Also, XHR-SEM under low acceleration voltage with low energy induced ion etching in x-ray photoelectron spectroscopy (XPS) studies provides valuable information on the SEI layer thickness. More detailed properties of the SEI layer were investigated by transmission electron microscopy (TEM).

An Artificial Solid Electrolyte Interphase Enables the Use of a LiNi0.5 Mn1.5 O4 5 V Cathode with Conventional Electrolytes

An artificial solid electrolyte interphase (SEI) consisting of a lithium-ion conducting material enables the use of a LiNi0.5Mn1.5O4 cathode with conventional carbonate electrolytes. The coulombic loss and electrolyte oxidation are largely remedied by the nanometer-scale artificial SEI. The thickness of the artificial SEI is optimized by balancing the protection and additional resistance.

Comparative study of the solid electrolyte interphase on graphite in full Li-ion battery cells using X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and electron microscopy

Commercial lithium-ion cells with LiMn1/3Ni1/3Co1/3O2 as the positive electrode, graphite as the negative electrode and a LiPF6-EC:PC:DEC electrolyte were cycled under several conditions, and the solid electrolyte interphase (SEI) on the graphite electrode was studied. Nuclear magnetic resonance (NMR) spectroscopy confirmed that LiPF6-EC:PC:DEC electrolyte was used in the cell and the relative volume ratio between solvents was acquired via quantitative NMR. Secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and a dual-beam focused ion beam/scanning electron microscope have been used to characterize the thickness, morphology and chemical composition of complex SEI on graphite anodes. The advantages and limitations of the characterization techniques are discussed and the results compared to provide a more comprehensive analysis of the SEI.