Solid Electrolyte Interphase Formation Mechanism Research Articles

Spectroscopic and Computational Evaluation of Electrochemical Stability of Electrolyte Solutions; Solvents, Electrolytes and Their Concentration Dependence

Since the introduction of Lithium-ion batteries (LIBs) into commercial use, great improvements have been achieved in their performance such as energy density, durability, and safety. However, there still remain many technical issues to meet the increasing demands for a longer driving range of electric vehicles, a larger storage for renewable energy. Research and development of electrode active materials are extensively progressing to respond those market needs, and R&D for electrolyte solutions are also active in order to utilize the fullest extent of the newly developed electrode materials.Various kinds of characteristics are required for electrolyte solutions in LIBs. Ideally, the electrochemical stability in a wide potential range is needed, but almost all the electrolyte solutions reductively decompose on negative electrodes, and the decomposition products precipitate on the electrode to form a protective surface film, which is referred to as SEI (solid electrolyte interphase). Because the nature of SEI is influenced by chemical structures of electrolyte solutions, the solvents and lithium salts need to be combined properly to realize LIBs with better performance. Although the conventional electrolyte solutions show satisfactory performance to some extent, there still remain some disadvantages; instability against moisture and temperature, and volatility and flammability of carbonate ester solvents. Great efforts have been made to overcome those disadvantages in the conventional electrolyte solutions, for example, utilization of ionic liquid, polymer electrolytes and inorganic solid state electrolytes, and recently, highly concentrated electrolyte solution gathers a lot of attention.Based on these background, solid electrolyte interphase (SEI) formed on graphite in conventional and highly concentrated electrolyte solutions was thoroughly characterized by a combined experimental and computational study. The comprehensive understanding revealed that a chemical composition of SEI, as well as the solvation structures unstable to reductive potential, can be predicted by a profound understanding of density functional theory (DFT) calculation results of the solvates containing a counter anion. The solvation structures were determined by Raman spectroscopy to evaluate electron affinity (EA) and LUMO of the solvates containing an anion by DFT calculation. The chemical composition of SEI was quantitatively analyzed by X-ray photoelectron spectroscopy (XPS), and the results were compared with a prediction based on the calculation. The formation mechanism of SEI during charge and discharge process can be partly estimated by the combination of experiment and theoretical calculation. These results indicate that electrolyte solutions can be efficiently designed by predicting the physicochemical properties of SEI through the more effective utilization of DFT calculation. Thus, a combination of DFT prediction and experimental analysis is proved to be an effective approach to reveal SEI formation mechanisms, and can be utilized as a versatile approach to develop new solvents, electrolyte salts, and additives, and to design electrolyte solutions with appropriate concentration.It is expected that the oxidative stability and reaction mechanism of electrolyte solutions at the positive electrode can be estimated using similar approaches. Here, it is necessary to predict oxidative stability using ionization potentials (IP), rather than EA. In the same way, other solvents than carbonate, such as ethers and sulfones, which have been difficult to put into practical use in conventional electrolytes, can also be the target to be studied. By analyzing the solvation structure and estimating its chemical stability, we believe it will be possible to get many insights on the reactions that proceed at the interface between the electrode and the electrolyte. Figure 1

Effective Approach by Computational Chemical Prediction and Experimental Verification to Elucidate SEI Formation Mechanism in LiPF6-, LiFSI-, and LiBF4-Containing Electrolyte Solutions

Solvation structures of ethylene carbonate-based electrolyte solutions containing 1 mol/L LiPF6, LiN(SO2F)2 (LiFSI), or LiBF4 are experimentally determined, and their electron affinity and lowest unoccupied molecular orbital (LUMO) distribution are calculated by density functional theory (DFT). The stability of the electrolyte solutions against reduction can be evaluated using electron affinity of the solvates, and the formation mechanism of the solid electrolyte interphase (SEI) can be predicted by their LUMO distributions. The calculation results suggest that 1 mol/L LiPF6 and LiFSI electrolyte solutions should form SEI consisting of carbonate and carboxylate which derives from solvent decomposition and LiF originating from anion decomposition. These predictions are verified experimentally by scanning electron microscopy–energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy measurements. On the other hand, intriguingly, the reductive decomposition of 1 mol/L LiBF4 electrolyte solution does not simply proceed as theoretically predicted, and the subsequent reactions of SEI need to be considered: further decomposition of the organic SEI and formation of inorganic particles containing lithium, fluoride, oxygen, and boron. These results are consistent with an increase in the resistance of SEI and interfacial lithium ion transfer at the graphite negative electrode. Thus, a combination of DFT prediction and experimental analysis is an effective approach to reveal SEI formation mechanisms and can be utilized as a versatile approach to develop new solvents, electrolyte salts, and additives and to design electrolyte solutions with an appropriate concentration.

Unravelling the convoluted and dynamic interphasial mechanisms on Li metal anodes.

Accurate understanding of the chemistry of solid-electrolyte interphase (SEI) is key to developing new electrolytes for high-energy batteries using lithium metal (Li0) anodes1. SEI is generally believed to be formed by the reactions between Li0 and electrolyte2,3. However, our new study shows this is not the whole story. Through synchrotron-based X-ray diffraction and pair distribution function analysis, we reveal a much more convoluted formation mechanism of SEI, which receives considerable contributions from electrolyte, cathode, moisture and native surface species on Li0, with highly dynamic nature during cycling. Using isotope labelling, we traced the origin of LiH to electrolyte solvent, moisture and a new source: the native surface species (LiOH) on pristine Li0. When lithium accessibility is very limited as in the case of anode-free cells, LiOH develops into plate-shaped large crystals during cycling. Alternatively, when the lithium source is abundant, as in the case of Li||NMC811 cells, LiOH reacts with Li0 to form LiH and Li2O. While the desired anion-derived LiF-rich SEI is typically found in the concentrated electrolytes or their derivatives, we found it can also be formed in low-concentration electrolyte via the crosstalk effect, emphasizing the importance of formation cycle protocol and opening up opportunities for low-cost electrolyte development.

Switching Electrolyte Interfacial Model to Engineer Solid Electrolyte Interface for Fast Charging and Wide-Temperature Lithium-Ion Batteries.

Engineering the solid electrolyte interphase (SEI) that forms on the electrode is crucial for achieving high performance in metal‐ion batteries. However, the mechanism of SEI formation resulting from electrolyte decomposition is not fully understood at the molecular scale. Herein, a new strategy of switching electrolyte to tune SEI properties is presented, by which a unique and thinner SEI can be pre‐formed on the graphite electrode first in an ether‐based electrolyte, and then the as‐designed graphite electrode can demonstrate extremely high‐rate capabilities in a carbonate‐based electrolyte, enabling the design of fast‐charging and wide‐temperature lithium‐ion batteries (e.g., graphite | LiNi0.6Co0.2Mn0.2O2 (NCM622)). A molecular interfacial model involving the conformations and electrochemical stabilities of the Li+‐solvent‐anion complex is presented to elucidate the differences in SEI formation between ether‐based and carbonate‐based electrolytes, then interpreting the reason for the obtained higher rate performances. This innovative concept combines the advantages of different electrolytes into one battery system. It is believed that the switching strategy and understanding of the SEI formation mechanism opens a new avenue to design SEI, which is universal for pursuing more versatile battery systems with greater stability.

Elucidation of the Solid Electrolyte Interphase Formation Mechanism in Micro‐Mesoporous Hard‐Carbon Anodes

AbstractThe microstructure of hard carbons can be designed to maximize their performance as anodes for sodium‐ion batteries. However, the nature of the electrolyte is also decisive in the capacity and long‐term stability. Here, hard carbons with a tailored bimodal pore network of internal micropores interconnected through mesopores are studied as sodium‐ion battery anodes. The evolution of their solid electrolyte interphase (SEI) is analyzed in three different electrolytes (NaPF6 in an ether‐based solvent, and NaPF6 or NaClO4 in a carbonate‐based system). Combining experiments with density functional theory calculations, it is proposed that formation of the SEI is mainly controlled by the decomposition of the salt anion. This process occurs through the intermediate functionalization of the carbon surface by the decomposed anion fragments. It is suggested that the innermost SEI sub‐layer governs the performance and long‐term stability of the anode. While the presence of a fluorine‐containing salt appears to have a determining role in the SEI stability, the electrochemical decomposition of carbonate‐based solvents is detrimental for the long‐term stability as the interfacial resistance increases. In contrast, the ether‐based system enables stable long‐term cycling as the interphase remains almost intact once the first fluorine‐rich SEI layer is formed.

Frontiers in Theoretical Analysis of Solid Electrolyte Interphase Formation Mechanism.

Solid electrolyte interphase (SEI) is an ion conductive yet electron-insulating layer on battery electrodes, which is formed by the reductive decomposition of electrolytes during the initial charge. The nature of the SEI significantly impacts the safety, power, and lifetime of the batteries. Hence, elucidating the formation mechanism of the SEI layer has become a top priority. Conventional theoretical calculations reveal initial elementary steps of electrolyte reductive decomposition, whereas experimental approaches mainly focus on the characterization of the formed SEI in the final form. Moreover, both theoretical and experimental methodologies could not approach intermediate or transient steps of SEI growth. A major breakthrough has recently been achieved through a novel multiscale simulation method, which has enriched the understanding of how the reduction products are aggregated near the electrode and influence the SEI morphologies. This review highlights recent theoretical achievements to reveal the growth mechanism and provides a clear guideline for designing a stable SEI layer for advanced batteries.

Unveiling the Role of Li+ Solvation Structures with Commercial Carbonates in the Formation of Solid Electrolyte Interphase for Lithium Metal Batteries

Solid electrolyte interphase (SEI), determined by the components of electrolytes, can endow batteries with the ability to repress the growth of Li dendrites. Nevertheless, the mechanism of commercial carbonates on in situ-generated SEI and the consequential effect on cycling performance is not well understood yet, although some carbonates are well used in electrolytes. In this work, quantum chemical calculations and molecular dynamics are used to reveal the formation mechanisms of SEI with carbonate-based electrolyte additives on the atomic level. It is confirmed that the Li-coordinated carbonate species are the leading participant of SEI formation and their impact on battery performance is clarified. Fluoroethylene carbonate (FEC) exhibits a completely different behavior from vinyl ethylene carbonate (VEC), ethylene carbonate (EC), and vinylene carbonate (VC). High reduction potential Li+ -coordinated additives, e.g. FEC and VEC can dominate the formation of SEI by excluding propylene carbonate (PC) and LiPF6 from the decomposition, and the corresponding Li||Li symmetric cells show enhanced long-term performance compared with those with pure PC electrolyte, while the low reduction priority additives (e.g., EC and VC) cannot form a uniform SEI by winning the competitive reaction.

Reversible Cycling of Graphite Electrodes in Propylene Carbonate Electrolytes Enabled by Ethyl Isothiocyanate.

As one of the greatest inventions in the history of electrochemistry, the lithium-ion battery (LIB) has radically transformed human beings' daily life by powering portable electronics and electric vehicles. When we look back upon the long and arduous effort devoted to the development of the LIB technology, it is found that the birth of LIBs could have been even earlier if reversible cycling of the graphite electrode had been realized in the propylene carbonate (PC) electrolyte, one of the few dominating electrolytes extensively used in nonaqueous electrochemistry long before the concept of LIBs. In this work, a functional electrolyte additive, that is, ethyl isothiocyanate, has been identified to enable the reversible Li+ ion intercalation/de-intercalation into/out of the graphite electrode in PC electrolyte by forming a high-quality solid electrolyte interphase (SEI) on the graphite electrode. A wide range of advanced in situ and ex situ spectroscopic characterization techniques coupled with theoretical calculations have been employed to understand the SEI formation mechanism. The results reported here rejuvenate the promise of PC as the primary electrolyte solvent for LIBs by artificially rectifying the interfacial electrochemical processes.

(A04 Best Poster Award Winner) Formation Mechanism and Stability of Solid Electrolyte Interphases in Linear Carbonate-Based Electrolytes

Broad application of lithium batteries, necessitates improvements in their safety, cycle life, durability and energy density. Clear understanding of the electrode/electrolyte interfaces is one of the important factors in designing batteries with enhanced performance. Linear organic solvents are extensively used in electrolytes of lithium batteries to reduce viscosity and melting temperature of cyclic carbonates. However, these organic compounds are not stable in contact with anode, leading to decomposition of the electrolyte and formation of Solid Electrolyte Interphase (SEI). Although SEI is proven to be one of the most important parts of batteries, its structural complexity, mechanism of formation, and properties are not completely understood yet. In this work, we consider mechanism of formation and stability of SEIs composed of redox products of linear carbonate solvents. So far, the main focus of research was on decomposition products of cyclic carbonates. In case of linear carbonates, the SEI stability consideration is limited to the systems being in DMC solvent and to the best of our knowledge, there are no theoretical investigations on linear carbonate-based SEIs formation mechanism and properties.In this work, we study Lithium Methyl Carbonate (LMC), Lithium Ethyl Carbonate (LEC) and Lithium Propyl Carbonate (LPC), which are single-electron reduction products of carbonate solvents in contact with the electrode, in their corresponding solvents being Dilithium Methyl Carbonate (DMC), Dilithium Ethyl Carbonate (DEC) and Dilithium Propyl Carbonate (DPC), respectively (Figure 1). Our aim is to chemically, kinetically, and thermodynamically study the aggregation, formation and stability of SEI layer in these systems from experimental, Molecular Dynamics (MD) simulation and Density Functional Theory (DFT) calculation points of view. Our study indicates that in LMC/DMC mixture LMC molecules have strong tendency to self-assemble and have low barriers to undergo second-electron reduction to form stable Li2CO3. Stable SEI was also observed in LPC/DPC as well, which is believed to be a result of slow dynamics of larger molecules and therefore increased possibility of the second-electron reduction near electrode surface. In the LEC/DEC system, however, we did not observe a stable SEI formation due to low tendency of LEC molecules to self-assemble, low electronic affinity coupled with high energy barrier for possible second electron reduction reaction. Therefore, our work illustrates that the selection of the linear carbonate as a blending partner for cyclic carbonates, might have an important impact on the ability of electrolyte to form stable and compact SEI layer. Figure 1

The Protection of Lithium Metal Enabled by LiNO3 for Lithium-Sulfur Batteries

An insight into the role of LiNO3 additive in an ether-based electrolyte for lithium sulfur battery has been presented. Herein, we proposed a formation mechanism of solid-electrolyte interphase on the surface of lithium metal anode by using the theoretical reactive force field (ReaxFF) simulation method. The interaction between the reactive lithium metal and nitrate ions results in the formation of LixNOy clusters, distributed homogeneously in both the SEI layer and electrolyte phase. Not only can these clusters be an efficient surface protection layer of lithium metal anode but also, they can suppress the lithium polysulfides shuttle effect by adsorbing liquid lithium polysulfide intermediates.

Determination of solid electrolyte interphase formation mechanism on negative electrode surface in Li-O2 battery electrolyte by operando electrochemical atomic force microscopy observation

Solid electrolyte interphase (SEI) on the negative electrode surface affect the electrochemical operation and cycle performance of the Li-O2 batteries. Two-type of SEI formation processes involving the O2 molecule on negative electrode surface have been suggested: (i) direct electrochemical reaction and (ii) indirect multi-step chemical reaction. In this research, the operando electrochemical atomic force microscopy observation was conducted to determine the proper model. Well-polished cupper substrate was used as model working electrode and was observed during the cyclic-voltammetry operation in the tetraglyme (G4) based electrolyte with and without dissolved O2. In the pristine electrolyte, in which O2 is not dissolved, no solid formation was observed. In the O2-dissolved electrolyte, the solid formation was suddenly observed from 1.3 V vs. Li counter electrode (LiCE) in the negative scan, which voltage is remarkably lower than the onset voltage of reduction current at 1.8 V vs. LiCE. This significant delay of solid formation against the electron transfer reaction strongly supported the multi-step solid formation model. The direct observation of electrode surface coupled with electrochemical operation determine the SEI formation mechanism in the Li-O2 battery.

(Invited) Correlating Electrolyte Structure with Interphasial Chemistry of Lithium Metal Anode

Properties of the solid-electrolyte interphase (SEI) strongly effects the morphology, features and overall performance of the Lithium metal anode (LMA). It is well accepted but least understood that the formation mechanism of SEI is correlated with the electrolyte solvation structure. Here, synchrotron X-ray diffraction (XRD), pair distribution function (PDF) analysis, and molecular dynamic (MD) simulation are employed to obtain direct insight to the correlation between electrolyte solvation structure and interphasial chemistry. Systems of lithium bis(fluorosulfonyl)imide in propylene carbonate, dimethyl carbonate, and dimethoxyethane at both low (1M) and high (5 M) salt concentrations are studied. While solvents dominate in the solvation structure for low concentration electrolytes (LCE), a large number of anions enter the solvation shell in high concentration electrolytes (HCE). Quantification of crystalline phases in the bulk SEI reveal that while dendrites are the dominant phases for SEI of LCE, their formation is significantly suppressed in SEI of HCE. Highly nano-sized LiF, Li2O, and LiOH are dominant crystalline phases in SEI of HCE. The SEI of HCE is also very rich in oligomers resulting from anion decomposition. The small crystallites and amorphous oligomers promise fast Li ion conduction in the SEI and therefore a superior electrochemical performance of LMA.

Formation Mechanism and Stability of Solid Electrolyte Interphases in Linear Carbonate-Based Electrolytes

Broad application of lithium batteries, necessitates improvements in their safety, cycle life, durability and energy density. Clear understanding of the electrode/electrolyte interfaces is one of the important factors in designing batteries with enhanced performance. Linear organic solvents are extensively used in electrolytes of lithium batteries to reduce viscosity and melting temperature of cyclic carbonates. However, these organic compounds are not stable in contact with anode, leading to decomposition of the electrolyte and formation of Solid Electrolyte Interphase (SEI). Although SEI is proven to be one of the most important parts of batteries, its structural complexity, mechanism of formation, and properties are not completely understood yet. In this work, we consider mechanism of formation and stability of SEIs composed of redox products of linear carbonate solvents. So far, the main focus of research was on decomposition products of cyclic carbonates. In case of linear carbonates, the SEI stability consideration is limited to the systems being in DMC solvent and to the best of our knowledge, there are no theoretical investigations on linear carbonate-based SEIs formation mechanism and properties.In this work, we study Lithium Methyl Carbonate (LMC), Lithium Ethyl Carbonate (LEC) and Lithium Propyl Carbonate (LPC), which are single-electron reduction products of carbonate solvents in contact with the electrode, in their corresponding solvents being Dilithium Methyl Carbonate (DMC), Dilithium Ethyl Carbonate (DEC) and Dilithium Propyl Carbonate (DPC), respectively (Figure 1). Our aim is to chemically, kinetically, and thermodynamically study the aggregation, formation and stability of SEI layer in these systems from experimental, Molecular Dynamics (MD) simulation and Density Functional Theory (DFT) calculation points of view. Our study indicates that in LMC/DMC mixture LMC molecules have strong tendency to self-assemble and have low barriers to undergo second-electron reduction to form stable Li2CO3. Stable SEI was also observed in LPC/DPC as well, which is believed to be a result of slow dynamics of larger molecules and therefore increased possibility of the second-electron reduction near electrode surface. In the LEC/DEC system, however, we did not observe a stable SEI formation due to low tendency of LEC molecules to self-assemble, low electronic affinity coupled with high energy barrier for possible second electron reduction reaction. Therefore, our work illustrates that the selection of the linear carbonate as a blending partner for cyclic carbonates, might have an important impact on the ability of electrolyte to form stable and compact SEI layer. Figure 1. Carbonate solvents and components studied in this work. Figure 1

Compositions and Formation Mechanisms of Solid-Electrolyte Interphase on Microporous Carbon/Sulfur Cathodes

We report the formation mechanism and compositions of a solid-electrolyte interphase (SEI) on a microporous carbon/sulfur (MC/S) cathode in Li–S batteries using a carbonate-based electrolyte (1 M L...

In-situ EC-AFM and ex-situ XPS characterization to investigate the mechanism of SEI formation in highly concentrated aqueous electrolyte for Li-ion batteries

The structure and chemical composition of solid electrolyte interphase (SEI) play a critical role in the performance of Li-ion batteries. However, the complexity of SEI layer and lack of relevant and efficient characterization techniques limit understanding of SEI formation mechanism and associated properties. Moreover, compared with organic electrolyte-based SEI, few studies have focused on aqueous electrolyte-based solid-electrolyte interphase (A-SEI). Herein, we present in-situ AFM images to demonstrate the morphological evolution of SEI in the presence of 21 M lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) aqueous electrolyte. Moreover, a combination of electrochemical-controlled atomic force microscopy (EC-AFM) and ex-situ X-ray photoelectron spectroscopy (XPS) is utilized to unveil the mechanism of SEI formation and identify its components. The results reveal that A-SEI layer is unevenly distributed and the inner and outer layers are mainly composed of Li2CO3 and LiF, respectively. The Young’s modulus of 4–6 nm thick SEI layer is found to be 30 ± 10 GPa. The current study provides novel insights into the formation of A-SEI by using in-situ AFM and provides a baseline for the future development of aqueous electrolytes for Li-ion batteries.

(Invited) X-Ray Reflectivity Studies of Interfaces in Lithium-Ion Batteries

In lithium-ion batteries (LIBs), the arrangement of electrolyte molecules directly at the electrode interface, and the corresponding electric double layer (EDL) are expected to govern the interfacial ion transport during charge/discharge and the electrochemical stability, i.e. the origin and properties of the solid electrolyte interphase (SEI). The SEI is formed on anode surfaces due to electrolyte decomposition at low potentials outside the electrolyte's electrochemical stability window, and dictates cell chemistry, cycle life, and electrochemical reversibility. Towards this end, the first topic of this talk covers an Ångstrom-resolution combined experimental and theoretical structural determination of solid–liquid interfaces relevant to LIBs, specifically a baseline organic electrolyte containing various concentrations of lithium hexafluorophosphate (LiPF6) salt at a metal oxide interface. We find good agreement between our x-ray reflectivity (XRR) and molecular dynamics (MD) simulations. These show well defined interfacial layering of electrolyte molecules and that solvent molecules in the first interfacial layer are aligned surface-parallel. Furthermore, we observe reorientation of solvent molecules in the presence of ions [1,2]. The second portion of this talk describes a multi-property characterization study of the physical and chemical properties as well as the nucleation and growth of the SEI on silicon anodes relevant to LIBs. We combined in situ XRR, linear sweep voltammetry (LSV), and ex situ x-ray photoelectron spectroscopy (XPS) to develop a holistic understanding of the SEI formation mechanism and properties on native oxide terminated single crystalline Si electrodes. Our self-consistent findings show the formation of two distinct inorganic SEI layers, and reveal the critical role of the surface oxide in the surface electrochemistry of silicon anodes. These novel insights will be discussed with regard to the performance of LIB in general [3,4,5]. Finally, we will present recent results from combining in situ XRR and precision electrochemical measurements to understand origins of coulombic efficiencies, capacity fading, and charging rate capabilities in thin film silicon batteries. ­­­­References H.-G. Steinrück, C. Cao, Y. Tsao, C. J. Takacs, O. Konovalov, J. Vatamanu, O. Borodin, M. F. Toney, Energy Environ. Sci., 11, 594-602 (2018)Horowitz, H.-G. Steinrück, H.-L. Han, C. Cao, I. I. Abate, Y. Tsao, M. F. Toney, G. A. Somorjai, Nano Lett. 18, 2105-2111 (2018)Cao, H.-G. Steinrück, B. Shyam, K. H. Stone, M. F. Toney, Nano Lett.16, 7394-7401 (2016)Cao, H.-G. Steinrück, B. Shyam, M. F. Toney, Adv. Mater. Interfaces 4, 1700771 (2017)C. Cao, I. I. Abate, E. Sivonxay, B. Shyam, C. Jia, B. Moritz, T. P. Devereaux, K. A. Persson, H.-G. Steinrück, M. F. Toney, Joule (accepted 2018)

Exploration of New Electrolyte Additive for Solid Electrolyte Interphase (SEI) Modification

Solid electrolyte interphase (SEI), which is generated from the decomposition of the electrolyte at low potentials, plays an important role in repeated reversible ion transport and thus dictates the cycle life of electrochemical cells. [1] A stable SEI layer effectively suppresses the continuous decomposition of the electrolyte and enhances the fast ion diffusion during cycling. [2] As a critical component of SEI, LiF could effectively inhibit electron tunneling and improve the quality of SEI formed on anode materials. A LiF-rich SEI layer is believed to improve the uniformity of the electrode surface morphology and reduce the uneven deposition of Li metal. [3] However, how to enrich the LiF component in SEI layers is challenging. In this work, a new electrolyte additive has been designed and synthesized to produce more inorganic LiF during SEI formation. The electrolyte additive has been applied to both Si and Li anodes to understand the SEI formation mechanism in the presence of the new additive. References Collins, J., Gourdin, G., Foster, M., & Qu, D. (2015). Carbon surface functionalities and SEI formation during Li intercalation. Carbon, 92, 193-244.Zhang, S. S. (2006). A review on electrolyte additives for lithium-ion batteries. Journal of Power Sources, 162(2), 1379-1394.Zhang, X. Q., Cheng, X. B., Chen, X., Yan, C., & Zhang, Q. (2017). Fluoroethylene carbonate additives to render uniform Li deposits in lithium metal batteries. Advanced Functional Materials, 27(10), 1605989.

Hybrid Quantum-Classical Simulation of Li Ion Dynamics and the Decomposition Reaction of Electrolyte Liquid at a Negative-Electrode/Electrolyte Interface

Solid–electrolyte interphase (SEI) films play a key role in the operation of Li ion batteries; however, the formation mechanism of SEI remains unclear. In this study, the dynamics of Li ions insert...

Existence of Solid Electrolyte Interphase in Mg Batteries: Mg/S Chemistry as an Example.

Magnesium redox chemistry is a very appealing "beyond Li ion chemistry" for realizing high energy density batteries due to the high capacity, low reduction potential, and most importantly, highly reversible and dendrite-free Mg metal anode. However, the progress of rechargeable Mg batteries has been greatly hindered by shortage of electrolytes with wide stability window, high ionic conductivity, and good compatibility with cathode materials. Unlike solid electrolyte interphase on Li metal anode, surface film formed by electrolyte decomposition in Mg batteries was considered to block Mg ion transport and passivate Mg electrode. For this reason, the attention of the community has been mainly focusing on surface layer free electrolytes, while reductively unstable salts/solvents are barely considered, despite many of them possessing all the necessary properties for good electrolytes. Here, for the first time, we demonstrate that the surface film formed by electrolyte decomposition can function as a solid electrolyte interphase (SEI). Using Mg/S chemistry as a model system, the SEI formation mechanism on Mg metal anode was thoroughly examined using electrochemical methods and surface chemistry characterization techniques such as EDX and XPS. On the basis of these results, a comprehensive view of the Mg/electrolyte interface that unifies both the SEI mechanism and the passivation layer mechanism is proposed. This new picture of surface layer on Mg metal anode in Mg batteries not only revolutionizes current understanding of Mg/electrolyte interface but also opens new avenues for electrolyte development by uncovering the potential of those reductively unstable candidates through interface design.

Elucidating the Surface Reactions of an Amorphous Si Thin Film as a Model Electrode for Li-Ion Batteries.

We investigated during the first lithiation/delithiation process the electrochemical reaction mechanisms at the surface of 30 nm n-doped amorphous silicon (a-Si) thin film used as a negative model electrode for Li-ion batteries. Usage of thin film allowed us to accurately discern the different reaction mechanisms occurring at the surface by avoiding interference from carbon and binder components. The potential dependency of the evolution of the solid electrolyte interphase (SEI) and the reactions on the a-Si and on the copper current collector were elucidated by coupling galvanostatic cycling with postmortem X-ray photoemission spectroscopy and scanning electron microscopy analyses. Our approach revealed the clear reversibility of lithiation/delithiation in the a-Si and native SiO2 layers; such a reaction for SiO2 has not been previously detected and was considered to be an irreversible process. Quantitative and qualitative analyses of the potential-dependent surface evolution revealed the decomposition products of both the salt (LiPF6) and solvent (dimethyl carbonate/ethylene carbonate), giving insight into the complex SEI formation mechanism on the a-Si film but also underlining the strong influence of "inert" materials such as the role of the current collector in the irreversible charge loss. A model mechanism describing the evolutionary complexity of the a-Si surface during the first galvanostatic cycle is proposed and discussed.