Solid Electrolyte Interphase Thickness Research Articles

Effective Measures of Thickness Evolution of the Solid Electrolyte Interphase of Graphite Anodes for Li-Ion Batteries.

Upon forming, the intensity or thickness of the solid electrolyte interphase (SEI) in a Li-ion battery (LIB) evolves to various states depending on the cell materials and operation conditions. Despite a crucial role in comprehending the behaviors of an LIB, its quantitative measure is far from satisfactory mainly because of the undue complexity of the concentration profiles of the comprising chemical species. Here, we calculate the depth profiles of atomic mole fractions of C and F and their ratio as RC/F = C/F of graphite anodes for LIBs in comparison to an X-ray photoelectron spectroscopy (XPS) experiment. To this end, we take a differential equation approach to dC/dt*, where t* is the reduced XPS etching time for depth. As a result, the respective analytical expression derived for C, F, and RC/F(t*) is verified to accurately account for the experiment. Moreover, we show that RC/F(t*) in the j state can be practically expressed in , where γ is a constant for a given anode. Based on this, we suggest ξj* = (αi + βi - βj)/αj as a measure of the SEI thickness evolution from the i to j state in terms of the cycle number. As an intriguing finding, the SEI thickness evolves up to about 3 times that of its initial state, beyond which it does not appear to grow any more.

Enhancing bismuth-rich SEI on Li anode under high current density through the use of highly conductive additives

The forming of Li-Bi alloy can build a lithiophilic protective layer on the lithium anode, however, it is found that the lithium dendrites trend to grow on the surface of bismuth-rich solid electrolyte interphase (SEI) and there is an exfoliation of SEI from substrate caused by de-alloying process at a large current density. The failure cause of the bismuth-rich SEI is considered as the inadequate Li-ionic conductivity of the passivation layer, which cannot facilitate the lithium deposition on the substrate. Then the components with high Li-ionic conductivities such as Li3N, LiF and Li2S are introduced into the SEI to adjust the characteristic of the bismuth-rich layer by the addition of LiNO3, Bi2S3 and FEC. All of them enhance the SEI’s performances effectively and LiNO3 presents the best. With the addition of LiNO3, the SEI thickness is decreased and Li-ionic conductivity is enhanced while Li3N became one of the major components of the layer. Utilizing the improved bismuth-rich protective layer the Li/Li symmetric cell presents an overpotential at ∼10 mV in a long cycle over ∼1700 h at 4 mA cm−2, and the LiFePO4/Li cell with the cathode loading of ∼10 mg cm−2 exhibits a capacity retention rate 72.1% in 400 cycles. This work provides a new SEI design strategy for lithium metal anode operating at high current density and an inspiration for improving other functional interfacial layers in lithium batteries.

How Relevant Are SEI Studies from Half Cell Samples to Understand Aging Mechanisms in Potassium Batteries? An in-Depth Photoelectron Spectroscopy Study

Owing to a shared common technological basis, monovalent K-ion battery (KIB) systems follow in the footsteps of Li-ion batteries (LIBs) with the aim to develop a broader portfolio of available battery technologies and to put them on a more sustainable material basis. For example, both cations intercalate into a graphite host structure, which can be used in combination with carbonate-based electrolytes as negative electrode. Especially in half cells, the degradation of graphite electrodes in KIBs is significantly accelerated in comparison to LIBs. Previous surface studies demonstrated that the formation of a durable and protective solid electrolyte interphase (SEI), that is paramount for long cycle life in alkali-ion batteries, is a major bottleneck for KIBs because of less benign SEI properties[1][2].Recently reported crosstalk phenomena in half cell configurations due to the highly reactive potassium counter electrode make the analysis of surface layers a particular challenge [2][3]. In this presentation, a detailed characterization of the SEI formation on graphite was performed in both half and full cell configurations by in-house and synchrotron-based photoelectron spectroscopy (PES) in order to demonstrate fundamental differences in surface layer composition under the influence of a reactive counter electrode. While it is generally acknowledged that PES results show differences in SEI composition and thickness between both configurations, our results clearly demonstrate that in case of KIBs the changes become severe. This challenges interpretations of SEI layer properties made in half cells, as they are distinctly different from those in full cells. More importantly, this also affects interpretations on the major electrolyte degradation pathways in these systems. Using PES depth profiling and complementing results with gas chromatography, as well as DFT calculations, delivers new insights on the irreversible processes in KIBs. Reference s : [1] Naylor, et al., ACS Appl. Mater. Interfaces. 11 (2019) 45636–45645.[2] Allgayer et al., ACS Appl. Energy Mater. 5 (2022) 1136–1148.[3] Hosaka et al., ACS Energy Lett. 6 (2021) 3643–3649.

In situ repairing solid electrolyte interphase of faded graphite electrodes using a temperature–voltage coupling method

The thickening of the solid electrolyte interphase (SEI) increases battery polarization and severely impedes the long cycle life of graphite electrodes. We developed an in situ temperature–voltage coupling method that factors high temperature and high anode potential to decrease the SEI thickness of graphite anodes. A thin and inorganics-rich SEI film is formed upon graphite anode by the method, which is electrochemically and thermally stable. Electrochemical studies revealed that the impedance of the SEI film decreased when the Li/graphite–steel half cells were charged. X-ray photoelectron spectroscopy analysis verified that the proposed method promoted organic-component decomposition within the SEI film. Transmission electron microscope analysis of the morphology revealed SEI thickness reduction. The SEI thickness is plummeted using the proposed method. Thus, this research introduces an innovative method to reconstruct a compact and thin SEI film on the faded graphite anode surface.

Modelling of solid electrolyte interphase growth using neural ordinary differential equations

In this work, neural ordinary differential equations (NODE) are used to identify phenomenological growth rate functions to model the solid electrolyte interphase (SEI) growth during formation. To analyse the capabilities of this approach in a controlled setting, synthetic SEI thickness data is generated using a model that uses a mechanistic growth rate function. Several possible implementations and extensions of the NODE are investigated, including physical constraints and data augmentation. All the investigated variants agree well with the training data, but significant differences are observed for the validation data. The results show that the growth rate functions learnt by the baseline implementation without further constraints significantly differs from the growth rate function given by the mechanistic model. However, it is shown that the use of appropriate data augmentation or physical constraints provides a significant improvement, and low errors can be achieved within the validation data sets. It is concluded that NODE can reveal growth rate functions, but careful consideration is needed to achieve functions that are phenomenologically consistent with the underlying mechanisms.

A Diffuse-Interface Description of Electron Tunneling across Solid Electrolyte Interphases

Solid electrolyte interphase (SEI) is an electronically insulating layer on battery electrode due to electrolyte decomposition, which critically affects the battery performance. Electron tunneling is a key short-term electron transport mechanism that controls the SEI thickness and its growth behavior. The electron tunneling behavior is governed by the static Schrödinger equation and the tunneling barrier of SEIs, which shows an exponential decay of electron probability across the SEI layer. Here we develop a diffuse-interface description of electron tunneling behavior by formulating a phase-dependent tunneling barrier, so that the electrode/SEI and SEI/electrolyte interface positions do not need to be explicitly tracked when numerically solving the Schrödinger equation. We will show that this diffuse-interface description can accurately and efficiently predict the electron tunneling behaviors in 2D and 3D especially when the electrode/SEI and SEI/electrolyte interfaces are highly nonuniform. Furthermore, we will show that it can be seamlessly integrated with the phase-field simulation of electrodeposition and SEI growth in lithium-ion batteries, providing guidance for controlling the SEI morphology and improving the battery performance.

Interfacial Chemistry Effects in the Electrochemical Performance of Silicon Electrodes under Lithium-Ion Battery Conditions.

Understanding the solid electrolyte interphase (SEI) formation and (de)lithiation phenomena at silicon (Si) electrodes is key to improving the performance and lifetime of Si-based lithium-ion batteries. However, these processes remain somewhat elusive, and, in particular, the role of Si surface termination merits further consideration. Here, scanning electrochemical cell microscopy (SECCM) is used in a glovebox, followed by secondary ion mass spectrometry (SIMS) at identical locations to study the local electrochemical behavior and associated SEI formation, comparing Si (100) with a native oxide layer (SiOx /Si) and etched with hydrofluoric acid (HF-Si). HF-Si shows greater spatial electrochemical heterogeneity and inferior lithiation reversibility than SiOx /Si. This is attributed to a weakly passivating SEI and irreversible lithium trapping at the Si surface. Combinatorial screening of charge/discharge cycling by SECCM with co-located SIMS reveals SEI chemistry as a function of depth. While the SEI thickness is relatively independent of the cycle number, the chemistry - particularly in the intermediate layers - depends on the number of cycles, revealing the SEI to be dynamic during cycling. This work serves as a foundation for the use of correlative SECCM/SIMS as a powerful approach to gain fundamental insights on complex battery processes at the nano- and microscales.

A Solution‐Mediated Pathway for the Growth of the Solid Electrolyte Interphase in Lithium‐Ion Batteries

AbstractLithium‐ion batteries (LIBs) are a widely used battery technology. During the initial LIB cycle, a passivation layer, called the solid electrolyte interphase (SEI), forms on the anode surface, which plays a crucial role in the performance and long‐term cyclability of LIBs. The overall mesoscale mechanisms of SEI formation and its composition remain elusive both in experimental and computational approaches. Here a multiscale approach to comprehensively characterize the growth and composition of the SEI based on a chemistry‐specific reaction network is presented. Generating an ensemble of over 50 000 simulations representing different reaction conditions, it is found that the organic SEI forms and grows in a solution‐mediated pathway by aggregation of SEI precursors far away from the surface via a nucleation process. The subsequent rapid growth of these nuclei leads to the formation of a porous layer that eventually covers the surface. This finding offers a solution to the paradoxical situation that SEI constituents can form only near the surface, where electrons are available, but does not stop growing when this narrow region is covered. The study is able to identify the key reaction parameters that determine SEI thickness, which pave the way to optimize battery performance and lifetime.

Mechanism of Improved Lithium-Sulfur Battery Performance by Oxidation Treatment to Microporous Carbon as Sulfur Matrix

1. IntroductionLithium-sulfur (Li-S) batteries are rechargeable devices assembled with a sulfur cathode and a lithium metal anode. Li-S batteries have twice the volumetric energy density and 5 times the gravimetric energy density of lithium-ion batteries (LIB). Hence, Li-S batteries are expected to be applied to stationary power sources and EV vehicles [1]. However, Li-S batteries have the following issues:・Sulfur and the final discharge product (Li2S) are insulators.・In the discharge process, sulfur expands up to 1.8 times, so the structure of batteries is unstable.・Intermediate products (Li2Sx (x = 4 – 8)) dissolve in an electrolyte; Li2Sx (x = 4 – 8) diffused to an anode to provide an insulating layer at the anode surface.・In the charge process, Li2Sx (x = 4 – 8) causes redox shuttling.As a result, Li-S batteries cannot charge and discharge stably. In order to deal with this problem, Nazar et al. reported the method that sulfur is confined in porous carbon [2]. This approach provides a cathode realizing good electronic conductivity, restriction of sulfur expansion, and suppression of Li2Sx (x = 4 – 8) dissolution. Although these improved characteristics allow Li-S batteries to operate, The discharge capacity of Li-S batteries is still not high enough and this needs to be addressed.In our previous study, we reported that oxidation treatment to microporous carbon (MC) with HNO3 improves Li-S batteries' discharge capacity [3]. Moreover, we clarified that the discharge capacity of Li-S batteries has an approximate proportional relation with the amount of oxygen-containing functional groups on the MC surface [4].This work attempts to elucidate the mechanism of improved Li-S battery performance by oxidation treatment to MC. Our report would lead to the proposal of a novel strategy to improve the performance of Li-S batteries.2. Method2.1 Preparation of Oxidized MC-Sulfur composite (Ox MC-S)MC was added into 69 wt.% HNO3 and refluxed at 120ºC for 2 h. By vacuum filtration and washing with deionized water, Ox MC was obtained. Ox MC dried in vacuum at 80ºC overnight was mixed with sulfur at a weight ratio of Ox MC: S = 48: 52. The mixture was thermally annealed at 155ºC for 5 h (Ox MC-S). Untreated MC was also composited with sulfur by the same method (MC-S).2.2 Assembling of CellsEach MC-S cathode was prepared by mixing the MC-S, acetylene black, carboxymethyl cellulose, and styrene butadiene rubber at a respective weight ratio of 89: 5: 3: 3 and coating the resulting aqueous slurry on an Al foil current collector. The cells with the MC-S electrode and Li metal foil as an anode were assembled in a glove box filled with Ar. Lithium bis(trifluorosulfonyl)imide (LiTFSI): tetraglyme (G4): hydrofluoroether (HFE) = 10: 8: 40 (by mol) was used as the electrolyte.2.3 Electrochemical Impedance Spectroscopy (EIS)To elucidate the effect of oxidation treatment on the internal resistance of Li-S batteries, EIS was carried out at various potentials (Discharge 2.0 – 1.0 V and Charge 1.0 – 3.0 V). The obtained Nyquist plots were used for the evaluation of solid electrolyte interphase (SEI) resistance (Rsei), charge-transfer resistance (Rct), and Warburg impedance (Rw). Rw was investigated with the calculation of Warburg coefficient (σ).3. Major results and conclusionSince oxidation treatment to MC significantly increased the discharge capacity of Li-S batteries [3][4], it was expected that oxidation treatment would lower the internal resistance of Li-S batteries. EIS of MC-S and Ox MC-S at various potentials showed that oxidation treatment reduced Rsei by an average of 12.2 Ω. This indicates that the SEI thickness was reduced, or the SEI was composed of highly ion-conductive components by the oxidation treatment. Rct decreased only at lower potentials, and the Warburg coefficient decreased except at the end of charge and discharge potential. These results suggest that the oxidation treatment decreases overall resistance, but especially SEI resistance and Warburg impedance, which may improve the discharge capacity of Li-S batteries.We will also report the activation energy of Rsei and Rct and mechanism analysis of decreasing Rsei by oxidation to MC. This work was supported by “Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING [JPMJAL1301])” from JST.[1] Y. Guo et al., Angew. Chem. Int. Ed., 52 (2013) 13186.[2] X. Ji et al., Nat. Mater., 8 (2009) 500.[3] S. Okabe et al., Electrochemistry, 85 (2017) 671.[4] L. Yoshida et al., ECS 238th PRiME Meeting s (2020).

From Additive to Cosolvent: How Fluoroethylene Carbonate Concentrations Influence Solid–Electrolyte Interphase Properties and Electrochemical Performance of Si/Gr Anodes

Silicon-containing anodes perform best when the solid–electrolyte interphase (SEI) accommodates the high volume changes of silicon particles, as this reduces side reactions and extends the cell lifetime. With this work, we investigate the influence of different fluoroethylene carbonate (FEC) electrolyte concentrations on the SEI composition and thickness and correlate these SEI properties to the electrochemical performance. Three electrolytes (i.e., 2FEC:98LP30, 20FEC:80DMC, and 50FEC:50DMC) are cycled with 9% Si/Gr anodes, and their SEIs are characterized postmortem using photoelectron spectroscopy (XPS). We propose a fitting model for the XPS results in which FEC decomposition yields −C–O, DO (1,3-dioxolan-2-one), −CO2Li, Li2CO3, and LiF. −C–O, DO, and −CO2Li are most probably incorporated in a cross-linked polymeric network. Due to its distinct chemical environments, detecting DO can be unambiguously linked to the presence of FEC decomposition products in the SEI. The presence of DO-type species in the C 1s spectra is correlated to the electrochemical performance: A higher retention in silicon activity was observed for the 20 and 50 vol % FEC-containing electrolytes, where FEC decomposition products (i.e., DO) were present even after 100 cycles. By contrast, when cycling in the 2FEC:98LP30 electrolyte, the silicon activity cannot be retained, and FEC decomposition products are barely detected after 100 cycles. We suggest that the presence of the −C–O-, DO-, and −CO2Li-containing polymeric network positively influences the SEI during silicon volume changes. Additionally, we show that the interaction of FEC and LiPF6 plays an important role in the formation of SiOxFy species.

Glyoxylic acetals as electrolytes for Si/Graphite anodes in lithium-ion batteries

Using silicon-containing anodes in lithium-ion batteries is mainly impeded by undesired side reactions at the electrode/electrolyte interface leading to the gradual loss of active lithium. Therefore, electrolyte formulations are needed, which form a solid electrolyte interphase (SEI) that can accommodate to the volume changes of the silicon particles. In this work, we analyze the influence of two glyoxylic acetals on the cycling stability of silicon-containing graphite anodes, namely TMG (1 M LiTFSI in 1,1,2,2-tetramethoxyethane) and TEG (1 M LiTFSI in 1,1,2,2-tetraethoxyethane). The choice of these two electrolyte formulations was motivated by their positive impact on the thermal stability of LIBs. We investigate solid electrolyte decomposition products employing x-ray photoelectron spectroscopy (XPS). The cycling stability of Si/Gr anodes in each electrolyte is correlated to changes in SEI thickness, composition, and morphology upon formation and aging. This evaluation is completed by comparing the performance of TMG and TEG to two carbonate-based reference electrolytes (1 M LiTFSI in 1:1 ethylene carbonate: dimethyl carbonate and 1 M LiPF6 in the same solvent mixture). Cells cycled in TMG display inferior electrochemical performance to the two reference electrolytes. By contrast, cells cycled in TEG exhibit the best capacity retention with overall higher capacities. We can correlate this to better film-forming properties of the TEG solvent as it forms a smoother and more interconnected SEI, which can better adapt to the volume changes of the silicon. Therefore, TEG appears to be a promising electrolyte solvent for silicon-containing anodes.

Solid electrolyte interface formation between lithium and PEO-based electrolyte

One of the approaches to improve the performance and ensure safe operation of lithium-metal batteries is the use of solid polymer electrolytes (SPE) that demonstrate relatively low reactivity towards metallic lithium. However, when lithium comes into contact with SPE, a solid electrolyte interphase (SEI) film is formed at the interface, although its composition, properties, and formation mechanism have not yet been sufficiently investigated. The present paper focuses on the issue of the lithium metal - polymer electrolyte interface stability. We used cyclic voltammetry and impedance spectroscopy to show that a passivating SEI layer forms at the working electrode in contact with SPE (PEO20LiTFSI) both during the cell cycling and at an open circuit potential, with the SEI thickness increasing to a certain point depending on the temperature. The chemical reaction that corresponds to the SEI formation was studied by XPS and NEXAFS spectroscopy for model experiments, including lithium deposition on polyethylene oxide (PEO) under ultrahigh vacuum. Based on the spectroscopy data, we propose a reaction mechanism that features reductive cleavage of a PEO molecule by lithium atoms, which yields not only lithium alkoxides (ROLi) but also polyethylene fragments. Although such kind of SEI should generally reduce the ionic conductivity of SPE, it adds stability to the system at the same time.

Microstructure-resolved degradation simulation of lithium-ion batteries in space applications

In-orbit satellite REIMEI, developed by the Japan Aerospace Exploration Agency, has been relying on off-the-shelf Li-ion batteries since its launch in 2005. The performance and durability of Li-ion batteries is impacted by various degradation mechanisms, one of which is the growth of the solid-electrolyte interphase (SEI). In this article, we analyse the REIMEI battery and parameterize a full-cell model with electrochemical cycling data, computer tomography images, and capacity fading experiments using image processing and surrogate optimization. We integrate a recent model for SEI growth into a full-cell model and simulate the degradation of batteries during cycling. To validate our model, we use experimental and in-flight data of the satellite batteries. Our combination of SEI growth model and microstructure-resolved 3D simulation shows, for the first time, experimentally observed inhomogeneities in the SEI thickness throughout the negative electrode for the degraded cells.

Comparative X-ray Photoelectron Spectroscopy Study of the SEI and CEI in Three Different Lithium Ion Cell Formats

The solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) of three lithium ion cell formats, i.e., coin, lab-scale pouch and multi-layer pouch, are compared. Half the cells are additionally dried prior to electrolyte filling and cycling. The highest water content per cell, determined by Karl Fischer titration, is found for lab-scale pouch cells due to their disadvantageous ratio of cell housing area to electrode area. The water content influences the performance during electrochemical formation as well as the impedance. This is linked to increasing lithium fluoride concentration, as determined by X-ray photoelectron spectroscopy. For dried cells, this is not the case because there is less conducting salt hydrolysis. The CEI thickness decreases for dried pouch cells, while the organic SEI thickness increases in all cell formats for dried cells. It is concluded that the initial thickness of the porous organic SEI depends on the insulation of the dense inorganic SEI close to the electrode surface. Organic species are more likely to contribute to negative electrode passivation when the extent of conducting salt hydrolysis is low. For coin cells, the presence of atmospheric gases during formation results in thicker SEI and CEI, no matter whether cells are additionally dried.

Solid-State Transport Limitations within the SEI Cause Lithium Dendrite Initiation While Charging Li-Metal Batteries

High-energy density batteries are essential for electric aircraft and energy-hungry portable devices. Li metal promises the highest theoretical specific capacity among the available anode materials, yet it has not been used successfully in secondary batteries due to safety concerns and short cycling life. The persistent barrier to the development of secondary Li-metal batteries is dendrite growth that plagues the Li anode during charging. In this presentation, we show that the onset of dendritic Li growth is due to solid-state transport limitations. Using electrochemical techniques and optical imaging, we observe that Li dendrites initiate at a time when the surface overpotential during galvanostatic electrodeposition reaches a maximum value. The dendrite onset time (τonset) is shown to increase with increasing temperature and decrease with increasing current density and initial solid electrolyte interphase (SEI) thickness. In brief, reducing transport barriers to solid-state diffusion leads to slower dendrite initiation. These observations form the basis of an analytical transport model. We propose that the availability of Li+ decreases gradually within the SEI as this layer becomes thicker during electrodeposition, and at lower temperatures. At τonset, the mobile Li+ concentration at the Li–SEI interface approaches zero. Due to this Li+ concentration depletion, the roughness of the Li electrode amplifies, eventually producing dendrites and fracturing the SEI. Model predictions of how τonset varies with current density i, initial SEI thickness L 0, and temperature are found to be in qualitative agreement with experimental observations. The analytical model provides a mechanistic picture of how mass transport limitations within the SEI control the onset time of Li dendrites. The model highlights the critical material parameters that could suppress or eliminate the solid-state Li+ concentration depletion. Such SEI materials, if properly optimized, may hold the key to controlling the formation of dendrites during Li-metal battery charging. Figure 1

In Situ Multimodal Optical Imaging Platforms to Study Battery Reactions

Increasingly complex and heterogeneous chemical reactions on the battery and catalytic electrode surfaces require the characterization methods to provide a complete picture of molecular interactions across the interface in situ. For example, the biggest challenge in accelerating the Li metal batteries towards higher energy density is the lack of fundamental understanding of interfacial chemical reaction on the electrode surface. During the initial lithiation cycles, a solid electrolyte interphase (SEI) forms on the electrode surface due to the electrochemical instability of the electrolyte. Inhomogeneity in reaction activity/deposition rate, chemical compositions, and ionic and electrical conductivity on the battery electrode will cause the non-uniform Li-ion diffusion, and lead to inhomogeneous nucleation and dendrite formation. The traditional imaging and measurement techniques have experienced challenges to characterize these complicated interfacial chemical reactions. For example, most of the methods only provide rich information at a certain time point in the dynamic process or measure an average result over a large area during the reaction. On the other hand, these interfacial reactions are highly dynamic and spatially varied, and the signals at different time points or different locations could be totally different.We have developed a multimodal optical imaging platform to image the battery electrode reaction dynamics across the interfaces throughout the entire reaction process. The platform includes a 3D optical microscope, surface plasmon microscope, optical reflection microscope, and Raman spectrometry. The 3D optical microscope allows us to image the morphology changes during the battery reaction dynamics. The Zn deposition process on 3D Zn-Mn alloy electrode has been studied, and the 3D morphology and the Zn nucleation have been imaged in situ. We have also used the surface plasmon microscope, optical reflection microscope, and Raman spectrometry to study the SEI formation on Li metal electrodes. The platform provides us important interfacial chemical information, including SEI formation and chemical compositions: 1) The plasmonic imaging technique provides us information on localized SEI thickness and localized charging status during the reactions; 2) optical reflection microscope allow us to study the SEI and Li nucleation dynamics; and 3) Raman spectrometry provide us chemical composition information. The integration of all the information obtained from this multimodal imaging platform will allow us fundamentally understand SEI formation and electrochemical reaction on the anode electrode.

(Invited) Developing in Situ Optical Imaging Techniques to Understand Battery Solid Electrolyte Interface Formation

Developing in situ Optical Imaging Techniques to Understand Battery Solid Electrolyte Interface FormationXiaonan Shan. Xu Yang, Guangxia Feng, Yaping Shi, ECE, University of Houston Increasingly complex and heterogeneous chemical reactions on the battery and catalytic electrode surfaces require the characterization methods to provide a complete picture of molecular interactions across the interface in situ. For example, the biggest challenge in accelerating the Li metal batteries towards higher energy density is the lack of fundamental understanding of interfacial chemical reaction on the electrode surface. During the initial lithiation cycles, a solid electrolyte interphase (SEI) forms on the electrode surface due to the electrochemical instability of the electrolyte. Inhomogeneity in reaction activity/deposition rate, chemical compositions, and ionic and electrical conductivity on the battery electrode will cause the non-uniform Li-ion diffusion, and lead to inhomogeneous nucleation and dendrite formation. The traditional imaging and measurement techniques have experienced challenges to characterize these complicated interfacial chemical reactions. For example, most of the methods only provide rich information at a certain time point in the dynamic process, or measure an average result over a large area during the reaction. On the other hand, these interfacial reactions are highly dynamic and spatially varied, and the signals at different time points or different locations could be totally different. We have developed a multimodal optical imaging platforms to image the battery electrode reaction dynamics across the interfaces throughout the entire reaction process. The platform includes surface plasmon microscope, optical reflection microscope, and Raman spectrometry. The platform provides us important interfacial chemical information including SEI formation and chemical compositions: 1) The plasmonic imaging technique provide us information of localized SEI thickness and localized charging status during the reactions; 2) optical reflection microscope allow us to study the SEI and Li nucleation dynamics; and 3) Raman spectrometry provide us chemical composition information. The integration of all the information obtained from this multimodal imaging platform will allow us fundamentally understand SEI formation and electrochemical reaction on the anode electrode.

Ab Initio Molecular Dynamics of Li-Metal Anode in a Phosphate-Based Electrolyte: Solid Electrolyte Interphase Evolution

An interfacial study is performed using ab initio molecular dynamics simulations to elucidate the solid-electrolyte interphase (SEI) evolution between an electrolyte, based on trimethyl phosphate (TMP) and lithium bis(fluorosulfonyl)imide (Li+FSI−), and a Li-metal electrode (anode). We found a variety of products due to FSI-counterion dissociation such as Li2O, Li2S, Li3N and LiF, followed by Li2S, Li2O, LiF, Li3NSO2, during the second FSI dissociation, and Li2SO2NSO2 and LiF during the third one; then, the SEI prevents complete dissociation of the FSI as more counter-ions arrive to the interface. We report the electrochemical window stability to identify the energy barriers the electron must overcome to be transferred from the anode to the counter-ions to initiate the FSI dissociation. Using a nanobattery as a symmetrical cell, The anode open-circuit potential increases from −0.39, when there is not SEI, to 1.3 V when there is a SEI formed (in the anode) due to FSI dissociations. A method is presented to scale-up the nano-battery results into a real macro-battery; we identify the number of counter-ions that need to be consumed to obtain a stable SEI in a CR2032 coin battery and a linear extrapolation is performed to scale the SEI thickness as the counterions dissociate.

Solid Electrolyte Interphase on Li/Na Anodes in Contact with Liquid Electrolytes

Electrolyte consumption and continuous solid electrolyte interphase (SEI) growth are some of the crucial issues preventing the commercialization of battery systems depending on implementation of Li/Na metal anodes.1 In this work, recent study of SEI growth on Li metal in contact with glyme-based liquid/solid electrolyte under galvanostatic and OCV conditions by electrochemical impedance spectroscopy (EIS), ex situ X-ray photoelectron spectroscopy (XPS), and focused ion beam scanning electron microscopy (FIB-SEM) is presented.2,3 Under OCV conditions, reaction-controlled mechanism is substituted by diffusion-controlled mechanism at longer growth times. Upon galvanostatic cycling, both the SEI thickness and its room temperature ionic conductivity increase. Additionally, the transport properties of SEI are strongly influenced by the type of lithium salt and the concentration used. In the last part, a comparison of SEI growth in Li vs. Na cells in contact with glymes, carbonates, and water-containing electrolytes is given.4 Zhang, Zuo, Popovic, Lim, Yin, Maier, Guo, Towards better Li metal anodes: Challenges and strategies, Materials Today, 33, 56 (2020)Nojabaee, Popovic, Maier, Glyme-based liquid-solid electrolytes for lithium metal batteries, J. Mater. Chem. A, 7, 13331 (2019)Nojabaee, Küster, Starke, Popovic, Maier, Solid electrolyte interphase evolution on lithium metal in contact with glyme-based electrolytes, Small, 16(23), 2000756 (2020)Lim, Popovic, Maier, in preparation

Glyoxales As New Electrolytes for Si/Graphite-Anodes in Lithium-Ion Batteries: Analysis of the Solid-Electrolyte Interphase

Electrolytes play a crucial role when considering the safety aspects of a Lithium-ion battery. Most commonly, linear and cyclic carbonate-based electrolytes are used commercially. Their advantage lies in the combination of low viscosity and high dielectric constant. However, they display various safety issues such as a limited temperature stability window and high toxicity of the used Li-salt[1]. A new promising alternative are glyoxal based electrolytes using solvents such as tetraethoxy glyoxal (TEG) and tetramethoxy glyoxal (TMG)[2]. Both solvents display low viscosity, low toxicity, high flash points, and very low melting points. Previous works have shown that TEG containing electrolytes display good electrochemical performance in graphite and lithium iron phosphate half-cells as well as in NMC||graphite full-cells[2],[3],[4].With this work, we report the first use of TEG and TMG based electrolytes for silicon-graphite anodes. Our results show that in terms of capacity retention the TEG-based electrolyte outperforms TMG as well as the 2 reference electrolytes, namely LP30 (LiPF6 in EC/DMC) and L-E/D (LiTFSI-EC/DMC) (see figure). In this presentation, we focus on is the analysis of the solid electrolyte interphase (SEI) formed by these electrolytes. The interphase was studied by photoelectron spectroscopy (XPS) at specific potentials during SEI formation in the first cycle as well as after 100 cycles. We correlate SEI composition and thickness with the electrochemical performance depending on electrolyte composition, suggesting that TEG shows a thinner yet more stable SEI.