Solid Electrolyte Interphase Formation Process Research Articles

Unexplored Reaction Pathways of Ethylene Carbonate during Solid Electrolyte Interphase Formation

The solid electrolyte interphase (SEI) keeps eluding our grasp. Fundamental understanding of Li-ion battery performance will not be complete until the major underlying reaction pathways of SEI formation and growth have been described.1 Although the SEI has been observed to be a complex and highly diverse matrix of in-/organic crystalline and amorphous aggregates, likely only few compounds are functional, i.e. passivate the electrode while at the same time sustaining the Li+ transport. The aim of the presentation is to provide experimental evidence for hitherto largely unexplored reaction pathways of several commonly observed SEI compounds (such as lithium ethyl dicarbonate, abbr. LEDC) and prove how the formation of the same can be altered by the choice of electrolyte additives. For instance, Electrochemical Quartz Crystal Microbalance with Dissipation monitoring complemented by Online Electrochemical Mass Spectrometry is demonstrated to separate electrochemical from chemical reactions in a multistep SEI formation process.2-3 Based on our experimental findings, the role and implication of each compound (be it LiOH, Li2O, LEDC, etc.) in the SEI formation process will be discussed.4

In-Situ Probing the Origin of Interfacial Instability of Na Metal Anode

The chemical-mechanical stability of solid–electrolyte interphase (SEI) is probably the most critical factor determining the performance of alkali metal anode (Li, Na, etc.) in secondary batteries. Although extensive advanced characterization methods have been carried out to study SEI layers of Na metal anode, including solid state nuclear magnetic resonance1, 2, cryogenic transmission electron microscopy3, etc., the structural/componential evolution of SEI is still an uncharted territory due to its transient formation process and complicated components. In this work, we systematically analyze the SEI formation and dissolution processes via jointly combining multiple in-situ characterization technologies. By revealing spatial-temporal resolved information of SEI evolution, the buried origin of chemical-mechanical instability of SEI in Na anode is further clarified, which provides valuable guidelines for SEI engineering. A dynamic SEI formation/dissolution model of Na metal anode is demonstrated as follow:Quantitative evaluation methods for the chemical instability (i.e., solubility) and mechanical instability (i.e., modulus) are designed. According to the mass variation in EQCM and the modulus measurement in in-situ AFM, we firstly quantitatively observe the chemical and mechanical stability evolution during SEI formation process.The dynamic evolution picture of SEI formation has been explicitly established. We discover the instantaneous electrochemical formation process of SEI is obviously divided into two stages based on the potential. It is revealed that the formation of efficient passivation layer anchored on Na surface during the 1st (passivating) stage (2.3 – 1 V vs Na/Na+) (Scheme 1 a-b) is the critical factor to construct stable SEI. In absence of passivation layer, the Na mental surface will trigger unrestricted electrolyte decomposition and homogenous components distribution during the subsequent (growing) stage.The dissolution model of SEI was revealed related to its spatial distribution of organics and inorganics. SEI with layered structure evolved from a compact passivation layer is found to have higher stability than that with homogenously distributed components. The inorganic species in the latter structure tend to detach from the SEI with the dissolution of organics, resulting in poor SEI chemical stability (Scheme 1 c and e). By contrast, SEIs with hierarchical structure growing based on the top of a passivation layer exhibits lower dissolution tendency (Scheme 1 d and f).The dynamic analysis of SEI evolution of Na anode presented in this work not only sheds light on how to construct a stable SEI, but also provides guiding significance in unveiling the seemingly complicated interfacial chemistry in batteries via a concerted characterization approach.References Gao, L.N., Chen, J.E., Chen, Q.L. et al. The chemical evolution of solid electrolyte interface in sodium metal batteries. Science Advances 8, 4606 (2022).Xiang, Y., Zheng, G., Liang, Z. et al. Visualizing the growth process of sodium microstructures in sodium batteries by in-situ 23Na MRI and NMR spectroscopy. Nat. Nanotechnol. 15, 883–890 (2020). Han, B., Zou, Y., Zhang, Z. et al. Probing the Na metal solid electrolyte interphase via cryo-transmission electron microscopy. Nat Commun 12, 3066 (2021). Figure 1

Macroscale Inhomogeneity of Electrochemical Lithium-Metal Plating Triggered by Gas Phase Evolution

Promoting homogenous electrode reactions is essential to suppressing electrode degradation and thus realizing stable energy storage systems. In particular, lithium-metal anodes that undergo drastic morphological change during charging/discharging require strict homogeneity control, more so than conventional intercalation electrodes. Previous studies have mainly focused on the micrometer-scale homogeneity of lithium plating and its control by tailoring the electrolyte chemistry and following the solid-electrolyte interphase formation process. However, the larger-scale, electrode-level homogeneity issue recently emerging for conventional electrodes has received little attention. Here, for the first time, we observe clear electrode-level inhomogeneity during lithium plating that is independent from the micro-scale lithium dendritic growth. Ex-situ and in-situ experiments clearly reveal that this electrode-level inhomogeneity is triggered by gas-generating reactions and the resulting gas-phase (i.e., bubble) evolution that causes localized lithium growth. Experiments using Li|Cu half cells demonstrate that this macroscale inhomogeneity is accompanied by an overpotential increase mainly attributed to limited lithium-ion mass transport through bubbles, as revealed by continuum modeling and simulation. Full-cell experiments using lithium iron phosphate demonstrate that this bubble-inducing macroscale inhomogeneity significantly contributes to decay of the cell capacity and energy density. Our observation of this larger-scale lithium plating inhomogeneity suggests that homogeneity control and its implications for lithium-metal cell performance should be approached from a multi-scale perspective.

Role of Oxygen in the Formation of the Solid-Electrolyte Interphase Evaluated by Online Electrochemical Mass Spectrometry and Electrochemical Atomic Force Microscopy

Solid-electrolyte interphase (SEI) is a passive film formed on the anode surface of Li-ion batteries by decomposing organic electrolyte solutions during the initial charging process. It plays an essential role in the stability and cyclability of Li-ion batteries. We found that a small amount of oxygen can significantly influence the SEI formation process and its properties on the carbon anode surface of Li-ion batteries in ethylene carbonate (EC)-based electrolyte solution. Herein, a quantitative evaluation of the effect of oxygen concentration was conducted by detecting volatile product evolution during SEI formation by online mass spectrometry. In addition, electrochemical atomic force microscopy was employed to observe the morphological features of the electrode surface during the SEI formation process in argon and oxygen atmospheres. Furthermore, attenuated total reflection Fourier transform infrared spectroscopy was used to investigate the chemical composition of the SEI. Our results show that the presence of oxygen strongly suppressed the generation of ethylene (C2H4) molecules during SEI formation. We propose a novel mechanism for SEI formation in the oxygen-containing EC electrolyte. In the presence of oxygen, the oxygen reduction reaction (ORR) generates superoxide ions which can induce EC decomposition and form a loose SEI film on the electrode surface. As the potential becomes more negative, EC molecules can be electrochemically reduced. The decomposition products can deposit on the defects and voids of the loose SEI film formed during the ORR stage, making the SEI film thicker and denser.

Origin of dendrite-free lithium deposition in concentrated electrolytes

The electrolyte solvation structure and the solid-electrolyte interphase (SEI) formation are critical to dictate the morphology of lithium deposition in organic electrolytes. However, the link between the electrolyte solvation structure and SEI composition and its implications on lithium morphology evolution are poorly understood. Herein, we use a single-salt and single-solvent model electrolyte system to systematically study the correlation between the electrolyte solvation structure, SEI formation process and lithium deposition morphology. The mechanism of lithium deposition is thoroughly investigated using cryo-electron microscopy characterizations and computational simulations. It is observed that, in the high concentration electrolytes, concentrated Li+ and anion-dominated solvation structure initiate the uniform Li nucleation kinetically and favor the decomposition of anions rather than solvents, resulting in inorganic-rich amorphous SEI with high interface energy, which thermodynamically facilitates the formation of granular Li. On the contrary, solvent-dominated solvation structure in the low concentration electrolytes tends to exacerbate the solvolysis process, forming organic-rich mosaic SEI with low interface energy, which leads to aggregated whisker-like nucleation and growth. These results are helpful to tackle the long-standing question on the origin of lithium dendrite formation and guide the rational design of high-performance electrolytes for advanced lithium metal batteries.

Reactivity of VC Electrolyte at Li-Metal Electrode: New Insights on SEI Initial Formation from Density Functional Embedding Theory

The metallic lithium (Li) represents the most promising anode material among the next generation of solid-state lithium batteries [1]. An efficient strategy to achieve durable and effective Li-anode batteries is by engineering the solid-electrolyte interphase (SEI) with purposely designed molecules.To this aim, the vinylene carbonate (VC) is one of the most used additives in conventional electrolytes. Some recent experiments proved that the VC promotes the formation of a stable and protective SEI layer between Li metal and electrolyte [2, 3]. Unless the well-known SEI composition, it is difficult to control the VC reactivity, that involves dissociation and polymerization at the electrode surface. Therefore, to dissect these tangled processes, here we present new atomistic insights on VC-Lithium SEI formation via first-principles calculations by Density Functional Embedding Theory (DFET) [4, 5]. Such approach has potentialities for modeling complex reactions at hybrid interfaces in electrocatalysis: it is well suited to combine the best feasible approaches for molecular species (in this case, hybrid HF-DFT for VC molecules and derivatives) and for Li metal electrode (semi-local GGA density functional).Our results highlight different VC dissociation pathways, with formation of reactive radical species and localized cluster of Li2O and Li2CO3. The use of hybrid-DFT-in-DFT embedding is crucial for obtaning energy barriers and qualtitative results in agreement with experiments [3]. Overall, the energetics and structural features of these intermediates improve the current understanding of SEI formation process and can be exploited to drive the reactions toward the desired interfacial properties.[1] J. Janek, W. G. Zeier, Nat Energy 1 (2016) 16141.[2] A. L. Michan et al. Chem. Mater. 28 (2016) 8149.[3] Y. Kamikawa, K. Amezawa, K. Terada, J. Phys. Chem. C 124 (2020) 19937[4] C. Huang, M. Pavone, E. A. Carter, J. Chem. Phys. 134 (2011) 154110[5] C. Huang, A.B. Muñoz-García, M. Pavone, J. Chem. Phys. 145 (2016) 244103

Temperature Dependent Formation of the Graphite SEI with Vinylene Carbonate Electrolyte Additive

The charge/discharge efficiency and the cycle life of lithium-ion batteries (LiBs) depends on a large number of competing interfacial processes. Probably the best-known interface in LiBs is the solid-electrolyte-interphase (SEI), which is a passivating layer formed at the negative electrode by electrolyte decomposition during the first cycles.[1–3] Since the SEI stability is a deciding factor with regards to cycle life, understanding and improving the SEI formation process is crucial. In order to yield an effective SEI, commercial battery cells undergo an extensive so-called formation procedure after cell assembly, which generally consist of multiple voltage-holds and current-steps at various temperatures.[4,5] During the first cycles of a LiB, the SEI is generated by the reduction products of the commonly used carbonate-based electrolyte components, such as ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).[6,7] Additionally, electrolyte additives are commonly added to form an SEI layer with increased cycling stability and reduced irreversible capacity loss during the first cycle.[6,8–10] Among a variety of additives, vinylene carbonate (VC) is the most prominent. Since the reduction of VC initiates at higher reduction potentials compared to alkyl carbonate solvents (e.g., EC), it is reduced preferentially during the first charge and thus strongly affects the SEI composition.[6,11] In the present study we investigated the effect of the formation temperature in a range between 10-60 °C on the performance characteristics of full-cells with a graphite anode (MAGE) and an NCM831205 cathode as well as of MAGE/Li half-cells, both with LP572 (1 M LiPF6 in EC:EMC 3:7wt/wt + 2wt% VC) electrolyte. These include the first-cycle irreversible capacity loss, the negative electrode impedance build-up, the gas evolution during formation, the rate capability, and the cycle life.Employing electrochemical impedance spectroscopy using a µ-reference electrode allowed us to determine the graphite intercalation resistance (R Int), representing the sum of the SEI resistance (R SEI) and of the charge transfer resistance (R CT), after two 0.1C formation cycles at a given temperature. The impedance spectrum was recorded at 25 °C and 40% SOC referenced to 190 mAh/gNCM reversible capacity. As depicted in Figure 1, both R Int of the graphite electrode in the MAGE/NCM83125 full-cell and the first-cycle irreversible Li-loss determined a MAGE/Li half-cell increase with increasing formation temperature.Additionally, on-line electrochemical mass spectrometry (OEMS) was applied to identify and quantify the gaseous reduction products for LP572 in a MAGE/NCM831205 full-cell during one 0.1C formation cycle at different temperatures. As displayed in Figure 1, the relative increase of the total gas evolved, which is the sum of hydrogen, ethylene, carbon monoxide, and carbon dioxide, follows the relative increase of the intercalation resistance. Next to the overall gas amounts, changes in the quantity of the individual gases give insights into the highly temperature dependent reduction mechanisms of VC. Acknowledgement: The authors thank BMW AG for their financial support.

(Invited) Towards Understanding of Initial Co-Intercalation of Solvated Lithium Ions Under Galvanic and Linear Sweeping Control via In-Situ/In-Operando Atomic Force Microscopy

The applications of lithium-ion batteries (LIBs) have rapidly embedded into almost every aspect of our life and society. In the LIB industry, a well-controlled and lengthy process is proceeded before the LIBs are brought into markets. The aim of this process is to form a “good” solid electrolyte interphase (SEI) on the graphite anode in a LIB to prevent continuous electrolyte decomposition and graphite exfoliation, which greatly influence the performance and cycle life of a LIB. The SEI formation process usually takes one or two weeks, is the major bottleneck and the most expensive component for mass production of LIBs. Efforts have been devoted to accelerating the SEI formation process by increasing charging rates or skipping the high state-of-charge region. However, the shortened SEI formation time usually comes with a large decrease in capacity retention of a LIB.[1-3] Improving our understanding on the structure and property evolution of SEI during the SEI formation process is urgently required for the development of faster formation protocols without sacrificing the high quality SEIs.[4-7]Aiming at better understanding of the initial stage of SEI formation process in LIB production, in-situ electrochemical atomic force microscopy (EC-AFM) has been employed to monitor the morphological evolution of the electrode materials in nano-scale levels. While different from most of the previous works, this work applied Galvanic constant current (CC) control to closely mimic the industrial SEI formation processes of different formation rates. For comparison, linear potential sweep (LS) controlled SEI formation of various potential sweeping rates, which is commonly applied in fundamental studies of SEI formation, was also performed in parallel. For the first time, it is revealed that the onset potentials for co-intercalation and electrochemical reduction of solvated lithium-ion complexes, the two competitive and/or sequential processes during SEI formation, vary as a function of potential sweeping rates. The onset potential for co-intercalation shifts positively as the potential changing rate increases, which is in high contrast to the onset potential of their electrochemical reduction. It has been well established that the onset potential of their electrochemical reduction is largely determined by the lowest unoccupied molecular orbital (LUMO) level of the solvent molecules in the solvated lithium ion complexes,[6] so that it may remain largely constant or shift negatively, due to electrode polarization.[2] As a result, the onset potential for co-intercalation becomes more positive than that of the electrochemical reduction as the potential sweeping rate increases. Our experimental results demonstrate that this relationship applies to both CC and LS-controlled SEI formation processes. However, the difference between these two onset potentials is much greater in CC than that of LS controlled processes, due to the associated faster potential changing rates at the beginning of CC controlled lithiation. The difference becomes even more prominent when high charging current is applied to shorten the SEI formation time and lower LIB production cost. This understanding combined with the clear visible evidence of different structural evolution under CC and LS controls should provide more practical guidance to develop protocols for faster formation of SEI with higher quality, which is intensively pursued in industry.[1] Lee, H. H.; Wang, Y. Y.; Wan, C. C.; Yang, M. H.; Wu, H. C.; Shieh, D. T., A fast formation process for lithium batteries. Journal of Power Sources 2004, 134 (1), 118-123.[2] Bhattacharya, S.; Alpas, A. T., Micromechanisms of solid electrolyte interphase formation on electrochemically cycled graphite electrodes in lithium-ion cells. Carbon 2012, 50 (15), 5359-5371.[3] An, S. J.; Li, J. L.; Du, Z. J.; Daniel, C.; Wood, D. L., Fast formation cycling for lithium ion batteries. Journal of Power Sources 2017, 342, 846-852.[4] Wang, L. N.; Menakath, A.; Han, F. D.; Wang, Y.; Zavalij, P. Y.; Gaskell, K. J.; Borodino, O.; Iuga, D.; Brown, S. P.; Wang, C. S.; Xu, K.; Eichhorn, B. W., Identifying the components of the solid-electrolyte interphase in Li-ion batteries. Nature Chemistry 2019, 11 (9), 789-796.[5] Cai, W. L.; Yao, Y. X.; Zhu, G. L.; Yan, C.; Jiang, L. L.; He, C. X.; Huang, J. Q.; Zhang, Q., A review on energy chemistry of fast-charging anodes. Chemical Society Reviews 2020, 49 (12), 3806-3833.[6] Xu, K., Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chemical Reviews 2004, 104 (10), 4303-4417.[7] Xu, K., Electrolytes and Interphases in Li-Ion Batteries and Beyond. Chemical Reviews 2014, 114 (23), 11503-11618. Figure 1

Impact of the Crystalline Li15Si4 Phase on the Self-Discharge Mechanism of Silicon Negative Electrodes in Organic Electrolytes.

Because of their high specific capacity and rather low operating potential, silicon-based negative electrode materials for lithium-ion batteries have been the subject of extensive research over the past 2 decades. Although the understanding of the (de)lithiation behavior of silicon has significantly increased, several major challenges have not been solved yet, hindering its broad commercial application. One major issue is the low initial Coulombic efficiency and the ever-present self-discharge of silicon electrodes. Self-discharge itself affects the long-term stability of electrochemical storage systems and, additionally, must be taken into consideration for inevitable prelithiation approaches. The impact of the crystalline Li15Si4 phase is of great interest as the phase transformation between crystalline (c) and amorphous (a) phases not only increases the specific surface area but also causes huge polarization. Moreover, there is the possibility for electrochemical over-lithiation toward the Li15+aSi4 phase because of the electron-deficient Li15Si4 phase, which can be highly reactive toward the electrolyte. This poses the question about the impact of the c-Li15Si4 phase on the self-discharge behavior in comparison to its amorphous counterpart. Here, silicon thin films used as model electrodes are lithiated to cut-off potentials of 10 mV and 50 mV versus Li|Li+ (U10mV and U50mV) in order to systematically investigate their self-discharge mechanism via open-circuit potential (UOCP) measurements and to visualize the solid electrolyte interphase (SEI) growth by means of scanning electrochemical microscopy. We show that the c-Li15Si4 phase is formed for the U10mV electrode, while it is not found for the U50mV electrode. In turn, the U50mV electrode displays an almost linear self-discharge behavior, whereas the U10mV electrode reaches a UOCP plateau at ca. 380 mV versus Li|Li+, which is due to the phase transition from c-Li15Si4 to the a-LixSi phase. At this plateau potential, the phase transformation at the Si|electrolyte interface results in an electronically more insulating and more uniform SEI (U10mV electrode), while the U50mV electrode displays a less uniform SEI layer. In summary, the self-discharge mechanism of silicon electrodes and, hence, the irreversible decomposition of the electrolyte and the corresponding SEI formation process heavily depend on the structural nature of the underlying lithium-silicon phase.

Nickel Impurities in the Solid-Electrolyte Interphase of Lithium-Metal Anodes Revealed by Cryogenic Electron Microscopy

Summary Dissolution of transition metals from high-voltage cathodes and their incorporation into the solid-electrolyte interphase (SEI) of carbonaceous anodes drastically reduces the lifetime of Li-ion batteries. The effects of dissolved transition metals on the performance of carbonaceous anodes are well characterized; however, the impact on Li-metal anode performance and the SEI is rarely considered. Here, we use cryogenic electron microscopy to reveal the impact of dissolved Ni on the SEI formation process on Li-metal. A link between Ni incorporation into the SEI and the failure of Li-metal batteries is established. We find that Ni is reduced into its metallic state and incorporates as small clusters into the SEI, locally changing the chemistry and nanostructure of the SEI. These chemical and nanostructure changes locally modify the Li-ion and electron transport properties of the SEI, accelerating electrolyte decomposition, increasing formation of “dead” Li, and ultimately causing failure.

Solid-electrolyte interphase formation process on Li2TiSiO5 anode in LiPF6-based carbonate electrolyte

Electrolyte decomposition in lithium-ion batteries results in the formation of a solid-electrolyte interphase (SEI), and this layer plays an essential role in determining the stability and cyclability of the battery. Besides electrolyte, electrode also affects the SEI products. In this work, the SEI formation processes on Li2TiSiO5 electrode is investigated and compared with nickel electrode using ex situ X-ray photoelectron spectroscopy. Li2TiSiO5 is a recently discovered Li-ion battery anode that works at an optimum voltage (~0.28 V vs. Li+/Li) but suffers from low coulombic efficiency. The electrolyte selected is 1 M LiPF6 in ethylene carbonate:diethyl carbonate:dimethyl carbonate (1:1:1, volumetric). The SEI layer formed on the surface of Li2TiSiO5 is shown to consist of lithium carbonate (Li2CO3), lithium alkyl carbonates (ROCOOLi), and lithium fluoride (LiF), with Li2CO3 being the dominant component. Small amounts of ROCOOLi and LiF are mostly formed in the deep lithiation stages. The SEI layer formed on nickel electrode, in comparison, only contains inorganic Li2CO3. These results suggest that Li2TiSiO5 either catalyzes or participates in the SEI formation reaction. A proper coating or a change in composition of Li2TiSiO5 can modify the property of SEI.

Interphase Formation of PEO20:LiTFSI-Li6PS5Cl Composite Electrolytes with Lithium Metal.

Composite polymer electrolytes (CPEs), consisting of solid electrolyte particles embedded within a solid polymer electrolyte matrix, are promising materials for all-solid-state batteries because of their mechanical properties and scalable production processes. In this study, CPEs consisting of PEO20:LiTFSI blended with 1, 10, and 40 wt % (CPE40) of the Li6PS5Cl electrolyte filler are prepared by a slurry-based process. The incorporation of Li6PS5Cl improves the lithium-ion conductivity from 0.84 mS cm-1 (PEO20:LiTFSI) to 3.6 mS cm-1 (CPE40) at 80 °C. Surface-sensitive X-ray photoelectron spectroscopy (XPS) reveals LiF, polysulfides, and Li3PO4 on the CPE surface, originating from decomposition reactions between PEO20:LiTFSI and Li6PS5Cl. The decomposition products influence the formation of the solid electrolyte interphase (SEI) at the lithium metal | CPE interface, resulting in a reduced SEI resistance of 3.3 Ω cm2 (CPE40) compared to 5.8 Ω cm2 (PEO20:LiTFSI) at 80 °C. The SEI growth follows a parabolic rate law and the growth rate declines from 1.2 Ω cm2 h-0.5 (PEO20:LiTFSI) to 0.57 Ω cm2 h-0.5 (CPE40) during thermal aging at 80 °C. By substituting CPEs for PEO20:LiTFSI in lithium plating and stripping experiments, the increase in SEI resistance was reduced by more than 75%. In order to get a deeper understanding of the SEI formation process, in situ XPS measurements were carried out where the lithium metal is successively deposited on the CPE sample and XPS is measured after each deposition step. On the basis of these measurements, a multistep decomposition mechanism is postulated, including the formation of LiF and Li2S as key components of the SEI.

Highly Reversible Sodiation of Tin in Glyme Electrolytes: The Critical Role of the Solid Electrolyte Interphase and Its Formation Mechanism.

Utilization of high-capacity alloying anodes is a promising yet extremely challenging strategy in building high energy density alkali-ion batteries (AIBs). Excitingly, it was very recently found that the (de-)sodiation of tin (Sn) can be a highly reversible process in specific glyme electrolytes, enabling high specific capacities close to the theoretical value of 847 mA h g-1. The unique solid electrolyte interphase (SEI) formed on Sn electrodes, which allows highly reversible sodiation regardless of the huge volume expansion, is herein demonstrated according to a series of in situ and ex situ characterization techniques. The SEI formation process mainly involves NaPF6 decomposition and the polymerization/oligomerization of the glyme solvent, which is induced by the catalytic effect of tin, specifically. This work provides a paradigm showing how solvent, salt, and electrode materials synergistically mediate the SEI formation process and obtains new insights into the unique interfacial chemistry between Na-alloying electrodes and glyme electrolytes, which is highly enlightening in building high energy density AIBs.

The influence of FEC on the solvation structure and reduction reaction of LiPF6/EC electrolytes and its implication for solid electrolyte interphase formation

Fluoroethylene carbonate (FEC) has been proposed as an effective electrolyte additive that enhances the stability and elasticity of the solid electrolyte interphase (SEI) of emerging Si and Li metal anodes. However, uncertainties still remain on the exact mechanism through which FEC alters the electrolyte decomposition and SEI formation process. Herein, the influence of FEC on LiPF6/ethylene carbonate (EC) electrolytes for Si anodes is investigated through classical molecular dynamics, Fourier-transform infrared spectroscopy, and quantum chemical calculations. Albeit a minority species, FEC is found to significantly modify the solvation structure and reduction behavior of the electrolyte while being innocuous to transport properties. Even with limited 10% of FEC, the Li+ solvation structure exhibits a notably higher contact-ion pair ratio (14%) than the parent EC electrolyte (6%). Moreover, FEC itself, as a new fluorine-containing species, appears in 1/5 of the Li+ solvation shells. The Li+-coordinated FEC is found to reduce prior to EC and uncoordinated FEC which will passivate the anode surface at an early onset (ca. 0.3 V higher than EC) by forming LiF. The critical role of FEC in tailoring the Li+ solvation structure and as-formed protective SEI composition provides mechanistic insight that will aid in the rational design of novel electrolytes.

Investigations on the Aging-Related Lithium Loss in Lithium Ion Batteries by Using 6Li-Isotopically-Enriched Cathode Material and Post-Mortem Analysis by Means of Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry

In the last decades, the lithium ion battery (LIB) became the technology of choice for a huge variety of power storage applications. Nevertheless, the loss of performance caused by complex aging phenomena remains challenging. In the first electrochemical cycles, the liquid organic electrolyte is decomposed at the electrodes surfaces leading to the formation of an electrically insulating passivation layer on the anode, the solid electrolyte interphase (SEI). Although the SEI formation involves an irreversible loss of electrolyte and active lithium, it is an essential process to provide sufficient cycling stability, as it prevents the remaining electrolyte from further decomposition. In order to improve the cycle performance of LIBs a deeper understanding of the aging processes is crucial. Focusing on the SEI formation process, common techniques for lateral and/or depth resolved isotopic surface analysis are time of flight-secondary ion mass spectrometry (ToF-SIMS) and glow discharge-mass spectrometry (GD-MS), respectively. However, lateral and depth resolved isotopic imaging of whole electrodes at a reasonable time scale is only possible with laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). This work features the post-mortem analysis of graphitic SEI formed in LIBs, including a 6Li-isotopically-enriched LiNi1/3Co1/3Mn1/3O2 (NMC111) cathode material, to investigate the SEI formation process and the lithium distribution in the first electrochemical cycles. Therefore, lithium isotope deposition pattern of aged anodes are investigated in lateral and depth dimension by means of LA-ICP-MS.

Towards an Atomic-Level Understanding of Solid Electrolyte Interphase Formation through Molecular Simulation

Lithium ion batteries (LIBs) have been one of the greatest technological achievements in recent history, allowing for leaps forward in consumer technologies. However, growing demand for electric vehicles worldwide is fueling demand for LIBs with larger capacity, faster charging, and longer cycle lives without compromising on safety. This has prompted a resurgence in the study of lithium metal as a next-generation anode, despite challenges with stabilization of the anode-electrolyte interface and the formation of dendrites. In order to realize the large theoretical capacity offered by lithium metal as an anode material, the interface must be engineered to allow for stable cycling through the in situ formation of an ionic conducting and electrically insulating surface layer known as the solid electrolyte interphase (SEI) by reductive decomposition of the electrolyte. While formation of a stable SEI remains an issue in the advancement of next-generation batteries, a fundamental understanding of the process at the graphite electrode/electrolyte interface remains elusive despite its ubiquity in commercial LIBs. The time- and size-scales at which the SEI is formed makes its study experimentally difficult at the appropriate resolution for the underlying phenomena which are critical to determining its composition, morphology, and properties. In order to build an atomic-level understanding of the SEI formation process, we have utilized molecular dynamics (MD) simulations with both classical force fields and density functional theory (DFT) to explore the reaction and transport processes which occur at the anode/electrolyte interface including reduction of ethylene carbonate (EC), intermediate diffusion near the electrode surface, and subsequent chemical reaction of the intermediate species. We have explored the effects of acyclic carbonates on reactions kinetics and interfacial structure as well the effects of some common additives. In this talk, we will present some of our recent findings on how local structure of the electrolyte influences reaction kinetics and transport processes which lead to the aggregation and precipitation of reduced carbonate solvents to form the SEI. This improved understanding of the underlying processes which contribute to the formation of the SEI on graphite electrodes may not only improve engineering of such interfaces in commercial applications, but also generate new ideas in the pursuit of a stable lithium/electrolyte interface.

Morphological Changes of Si Thin Film Electrode in Lifsi/ and Litfsi/Tetraglyme Electrolyte Solutions

Si is a promising negative-electrode material for next-generation lithium-ion batteries due to its high gravimetric and volumetric capacities. A solid electrolyte interphase (SEI) forms on the Si negative electrode owing to reductive decomposition of an electrolyte solution in the initial charging process. Although the SEI layer strongly affects the charge/discharge performance, details of its formation process have not been clarified yet. Therefore, understanding of the SEI formation process is important to improve the cycle performance of Si negative electrodes. In recent years, glyme-based concentrated electrolyte solutions have attracted attentions because of their striking features; dissolution of polysulfide anions is suppressed in Li-S batteries, and dendrite formation is suppressed in Li metal batteries [1, 2]. In this study, the SEI formation process on Si-thin-film model electrodes was observed by in situ atomic force microscopy (AFM) coupled with cyclic voltammetry (CV) in lithium bis(fluorosulfonyl)imide (LiFSI)/tetraglyme (G4) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/G4 electrolyte solutions. Si thin films (100 nm) were deposited on mirror polished Cu substrates by RF magnetron sputtering. In situ AFM coupled with CV was performed using a liquid immersion sample stage designed for electrochemical measurements, consisting of the Si-film working electrode, and Li-wire counter and reference electrodes, at a sweep rate of 0.5 mV s-1 between 1.9 and 0.02 V (vs. Li+/Li). The electrolyte solution used was 4.5 M LiFSI or LiTFSI dissolved in G4. AFM scan was conducted in the contact mode at room temperature, and typical scan area was 5 × 5 mm2. In situ AFM observation was conducted in an Ar-filled glovebox with a dew point of about -60°C. Small cathodic currents were observed at potentials below about 1.8 V during the initial potential sweep in both LiFSI/G4 and LiTFSI/G4. The cathodic currents suddenly increased at around 1 V due to the reductive decomposition of the electrolyte solutions. The cathodic current in LiFSI/G4 was larger than that in LiTFSI/G4, indicating that the LiFSI/G4 is more vulnerable to reduction. In fact, in situ AFM observation revealed that some precipitates appeared on the Si surface at around 1 V in LiFSI/G4, which suggests the formation of SEI. In contrast, no morphological change was seen in LiTFSI/G4 during the initial charging process. These results suggest that the decomposition products of LiTFSI/G4 can hardly form a stable SEI on the Si electrode in the initial cycle. The surface cracked after the 2nd potential cycle in both the electrolyte solutions. The crack progressed with repeated potential cycling, which was more significant in LiTFSI/G4. The redox currents due to the alloying/dealloying reaction of Si with Li decreased more rapidly with potential cycling in LiTFSI/G4. Based on the above results, the formation of a stable SEI in the initial cycling in LiFSI/G4 largely contributes to the suppression of crack formation and the superior performance of charge/discharge cycles. The effect of a fluoroethylene carbonate (FEC)-derived SEI on morphological changes of the Si film electrode will also be presented in our poster [3]. Reference [1] H. Wang et al. , ChemElctroChem., 2, 1144-1151 (2015) [2] K. Yoshida et al., J. Am. Chem, Soc., 133, 13121-13129 (2011). [3] M. Haruta et al., J. Electrochem. Soc., 165, A1874-A1879 (2018)

Effect of Anion Species in Early Stage of SEI Formation Process

We investigate the early stage of the solid electrolyte interphase (SEI) layer formation process in four electrolyte solutions, using electrochemical quartz crystal microbalance (EQCM) and X-ray photoelectron spectroscopy (XPS). The EQCM result proves that the LiBH4 in THF solution form a negligible amount of SEI layer. The SEI formation process of the LiPF6 solution occurs at 2.3 V vs. Li, and the SEI layer continuously grows even during the anodic scan. The electrolyte solutions containing LiTFSA form SEI layer around 1.5 V vs. Li. The XPS study shows that the SEI layer formed at high electrode potential, in the electrolyte containing LiPF6 forms a dense LiF layer. The LiTFSA solution forms a thick SEI layer containing organic components such as lithium alkyl carbonate or polyether.

SEI Formation Vs. Lithium Intercalation: In-Situ AFM Experiments on Graphite and Silicon Electrodes

The formation of the solid-electrolyte interphase (SEI) passivation layer on anode materials in lithium ion batteries (LIB) is not yet fully understood at the micro- and nanometer scale. Therefore, experiments under realistic conditions with high spatial resolution are of primary importance. Hence, atomic force microscopy (AFM) is an ideal technique to study in-situ the mechanisms at the electrode-electrolyte interface down to the (sub-) nanometer level. Within this contribution we present a combined AFM and electrochemical (de-) lithiation study of the electrode-interface for two types of anode materials. Highly oriented pyrolytic graphite (HOPG) as a model system has been investigated in order to observe the topographical and electrochemical properties simultaneously. We find a clear influence of defect structures on the SEI formation process. Furthermore, the structural as well as the mechanical behavior of silicon anodes during (de-) lithiation will be presented. Sputtered silicon thin film electrodes [1] have been used to avoid strain and crack formation [2] on the silicon electrode during lithiation. We will introduce AFM measurements that enable a differentiation between SEI formation and expansion of the silicon electrode upon lithiation. We observed significant variations in the elastic modulus during the (de-) lithiation processes on the silicon electrode that can be directly attributed to the formation of an SEI layer as well as the expansion of the electrode due to the lithium insertion. Additionally, the analysis of the surface roughness will be discussed with respect to (ir-) reversible processes on the electrode surface.

Formation of the Solid Electrolyte Interphase at Constant Potentials: A Model Study on Highly Oriented Pyrolytic Graphite

AbstractThe solid electrolyte interphase (SEI) on graphite anodes is a key enabler for rechargeable lithium‐ion batteries (LIBs). It ensures that only Li+ ions and no damaging electrolyte components enter the anode and hinders electrolyte decomposition. Its growth should be confined to the initial SEI formation process and stop once the battery is in operation to avoid capacity/power loss. In technical LIB cells, the SEI is formed at constant current, with the potential of the graphite anode slowly drifting from higher to lower voltages. SEI formation rate, composition, and structure depend on the potential and on the chemical properties of the anode surface. Here, we characterize SEIs formed at constant potentials on the chemically inactive basal plane of highly oriented pyrolytic graphite (HOPG). X‐ray photoemission spectroscopy (XPS) detects carbonate species only at lower formation potentials. Cyclic voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) with Fc/Fc+ as an electrochemical probe demonstrate how the formation potential influences ion transport and electrochemical kinetics to and at the anode surface, respectively. Breaking the EIS data down to a Distribution of Relaxation Times (DRT) reveals distinct kinetics and transport related peaks with varying Arrhenius‐type temperature dependencies. We discuss our findings in the context of previous electrochemical studies and existing SEI models and of SEI formation protocols suitable for industry.