Solid Electrolyte Interphase Species Research Articles

Deconvoluting Effects of Lithium Morphology and SEI Stability through Interface Engineering

The widespread use of renewable energy resources and electrification is mandatory for transitioning into a fossil-fuel-free world. Currently, Li-ion battery (LIB) is the universal mode of electrochemical energy storage, but it is reaching the theoretical limit, creating the urgency of more efficient batteries for next-generation applications. Li-metal batteries (LMBs) are considered a key technology to overcome the current energy density limitations of LIBs. In the most energy-dense LMB configuration, the anode-free LMB, Li is directly plated on and stripped from the Cu current collector (CC), subjecting it to the most stringent cycling conditions and making performance regulation crucial. Therefore, engineering the Cu-electrolyte interface is both fundamentally and practically significant.The two most crucial performance modulators in LMBs are electrodeposited Li morphology and solid electrolyte interphase (SEI). The literature has shown that low surface area morphology and anion-derived SEIs are usually beneficial for improved performance. However, it is difficult to deconvolute the individual impact of low surface area morphology and anionic SEI species on performance as they coexist and are correlated. One reported approach to understand the decoupled effects of morphology and SEI is ultrafast electrodeposition of lithium outpacing SEI formation (>100 mA cm-2). Our work establishes a new approach applicable at regular plating and stripping conditions (1 mA cm-2) that uses interface engineering with atomic layer deposited (ALD) thin films to deconvolve Li morphology and SEI stability effects. First, we modify the Cu CC surface using two different thin films with distinct characteristics: resistive, acidic HfO2; and conductive, acidic ZnO.Leveraging ALD, we precisely control the thickness of the nanofilms and establish that increasing the film resistance results in improved performance due to resistance-derived Li morphology. Our approach of decoupling the impact of SEI species from morphology is to preform the SEI before cycling using a simple potential hold step for both these acidic films with opposite electrical properties at different thicknesses. We find that with increasing film thickness, generally, the preformed SEI has more anionic species due to the surface acidity of the thin films. Despite being anion-rich, the preformed SEI does not improve performance, which we attribute to its evolution into an organic-rich SEI during plating. Moreover, the impact of the preformed SEI is statistically insignificant during long-term cycling, whereas the role of resistance becomes more apparent. The effects are demonstrated through interface characterization and performance measurements in three different electrolytes, with at least a twofold increase in cycle life and improved capacity retention achieved in practical anode-free pouch cells. Our results thus indicate that morphological control is more effective for improved battery cyclability due to the inherent challenges with preformed SEI preservation, and as a result the resistance of the thin films is more important than surface acidity for CC modification.

Upcycling of Graphite Anodes via Tailored Solvent Treatment

To achieve a truly closed-loop direct recycling process for lithium-ion batteries, all component materials must be recovered. To date, direct recycling method development has primarily focused on the high-value transition-metal cathode materials, while the inherently lower-value graphite has been challenging to recover in a cost-effective manner. However, end-of-life graphite contains a unique engineered value due to the presence of the solid electrolyte interphase (SEI). Growth of the SEI during the cell’s active lifetime stabilizes the electronically reactive graphite surface through an irreversible consumption of Li, and thus necessitates both excess lithiation of the cathode and a costly and time-intensive formation procedure during manufacturing. An optimized pre-formed SEI that capitalizes on existing SEI components from end-of-life batteries has the potential to significantly reduce cathode lithiation requirements and eliminate the critical bottleneck of formation cycling during cell remanufacturing. Further, retaining Li at the anode obviates the need for a separate Li leaching and recovery step, improving the overall efficiency of the direct recycling line. In this work, we present a novel approach to “upcycling” spent graphite through use of tailored chemical treatment to remove adverse (i.e., highly resistive and/or poorly passivating) SEI species while retaining beneficially passivating components. We have explored a rational set of solvents spanning a range of polarity, proticity, and molecular size to evaluate structure-property-performance relationships between applied solvent(s), removed and remaining SEI species, and electrochemical response of the resulting graphite product. Further, we have developed and optimized a robust and holistic analysis procedure that couples symmetric-cell electrochemical testing, multi-modal materials characterization, and advanced electrochemical modeling. These analysis results inform a set of correlative metrics for graphite performance relative to both solvent properties and upcycled SEI composition. We demonstrate effective tunability in the residual SEI composition by varying solvent identity and concentration, and report on several promising solvent systems that achieve comparable performance to pristine graphite. Figure 1

Using Nanoscale Metal Oxide Films to Investigate the Influence of Anode Interfaces on Lithium Battery Performance

As renewable energy and battery-powered technologies become more widespread, high energy-density and high-performing batteries are required to meet society’s needs. While lithium-ion batteries have been the staple, their energy density development has been slow, and at gravimetric energy densities of around 250 Wh/kg, they are approaching their theoretical limit.1 The lithium-metal battery (LMB) is considered a next-generation battery due to high theoretical energy densities (ex: Lithium-Sulfur, 2600 Wh/kg), and lithium metal’s high gravimetric capacity (3860 mAh/g) and low reduction potential (-3.04 V vs S.H.E.). However, commercialization of LMBs has been limited due to various issues.Two properties that dictate the performance of the LMBs are: 1. The composition of the solid electrolyte interphase (SEI), an electronically conducting, yet passivating, layer at the anode-electrolyte interface, and 2. The morphology of the lithium deposited at the anode. Morphological studies focus on creating dense, low surface area, deposits that limit SEI fracturing and the formation of electronically disconnected lithium. SEI quality is improved by incorporating inorganic or anion-rich SEIs, formed from the electrolyte salt, to promote SEIs that are mechanically robust, ionically conducting, and homogeneous. Our group’s previous work aimed to improve LMB performance via both routes by growing resistive Al2O3 coatings on the copper (Cu) current collector using atomic layer deposition (ALD). Defects in the thin film create areas of low resistance that behave like ultramicroelectrodes which encourage radial diffusion of lithium ions to the nuclei promoting lateral growth and producing low surface area, dense, planar lithium deposits. Simultaneously, the Al2O3 coating’s Lewis acidic properties promote the decomposition and subsequent incorporation of inorganic electrolyte species into the SEI.We present a study of how the different interfaces present at the anode in LMBs impact SEI composition and battery performance. To hold the Li nucleation morphology at the anode constant, we deposit metal oxide films of a fixed resistance onto the Cu current collector, generating planar, low surface area, lithium deposits. This experimental design allows us to decouple the effect of the metal oxide thin film-electrolyte interface on SEI quality and battery performance from that of the lithium morphology. First, as shown in Fig. 1a, different metal oxide films of equal resistance, but different thicknesses, are grown via ALD, confirmed by measuring the first-cycle nucleation overpotential. The resistive metal oxides investigated are Al2O3, HfO2, AlHfxOy, and ZrO2. Scanning electron microscopy results indicate that all cells modified with equal resistance metal oxide films exhibit the same low surface area, dense, planar Li nucleation morphology. However, long-term cycling tests show differences in cell performance and life cycle. Subsequently, the composition of the SEI is investigated using X-ray photoelectron spectroscopy (XPS). Through XPS, an elemental atomic ratio analysis is conducted to investigate the relative number of species derived from the anion of the electrolyte versus the solvent of the electrolyte to reflect the anion-derived nature of the SEI. Additionally, high-resolution peaks are used to determine the relative abundance of high-quality SEI species such as lithium fluoride. This analysis confirms that the metal oxide thin film tunes the SEI composition. Additionally, we identify trends present between the SEI quality of the ALD-modified cell and its performance.To further investigate the thin film-electrolyte interface, a set of cells modified with binary stacked metal oxide thin films with two different stack orders were created, as shown in Fig. 1b. Investigating SEI composition and LMB performance, with the morphology fixed by the nucleation overpotential, allowed for deconvolution of the impact of the Cu-thin film interface and thin film-electrolyte interface. Both interfaces can in principle impact the battery’s function because 1. The Cu-thin film interface controls the electron transport to form Lio from Li+ ion during electrodeposition, and 2. The thin film-electrolyte interface can modulate the SEI. Our results indicated that the SEI and performance were both primarily regulated by the thin film-electrolyte interface. Based on our fixed-resistance and binary stack experimental design, we propose that modulation of the SEI composition by the metal oxide coating controls LMB performance when the Li nucleation morphology is fixed. This result suggests the potential for tuning LMB performance by careful selection of the metal oxide interfacial coating. References (1) Cheng, X.-B., et al. Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review. Chem. Rev. 2017, 117 (15), 10403–10473. https://doi.org/10.1021/acs.chemrev.7b00115.(2) Oyakhire, S. T., et al. Electrical Resistance of the Current Collector Controls Lithium Morphology. Nature Communications 2022, 13 (1), 3986. https://doi.org/10.1038/s41467-022-31507-w. Figure 1

(Invited) A Study of the SEI Layer Cross-Talk in Si/C Composite Li-Ion Anodes

Composite electrodes, especially silicon/carbon (Si/C) anodes, present significant opportunities for advancing the energy density of lithium-ion batteries (LIBs). However, challenges emerge, particularly in performance metrics when a high weight percentage of Si beyond 8-10 wt.% is employed 1. A nuanced understanding of the solid electrolyte interphase (SEI) in Si/C composites is imperative to overcome these limitations and enhance energy density. Conventional techniques such as ex-situ X-ray photoelectron spectroscopy and cryogenic electron microscopy face inherent drawbacks, including exposure to ultrahigh vacuum and the need for solvent washing, potentially altering the SEI layer 2. In contrast, nano-FTIR, utilizing Fourier transform infrared near-field spectroscopy, overcomes these challenges, enabling non-destructive probing of the SEI layer's chemistry and structure at room temperature within an inert atmosphere 3–5.In this study, we engineer custom-patterned electrodes featuring amorphous Si on atomically flat highly ordered pyrolytic graphite (HOPG) to emulate model composite electrodes. Leveraging nano-FTIR spectroscopy, near-field nanoscale white-light imaging, and atomic force microscopy measurements, our investigation reveals that the SEI on HOPG is enriched in organic lithium ethylene decarbonate (LiEDC), while the SEI on lithiated silicon unveils the presence of inorganic Li2CO3. Notably, we unveil a unique "mixed" SEI layer zone at the Si/HOPG interface, comprising a mixture of dominant SEI species from the surfaces of both the lithiated Si and the HOPG. The coexistence of these species suggests the formation of an SEI layer with presumably unique physicochemical properties, forming along Si-C interfaces in Si/C composite electrodes. These findings provide a comprehensive approach to studying model composite electrodes and offer valuable insights into optimizing the surface passivation of Si/C composite electrodes for high-performance LIBs. Acknowledgments This research was supported by the US Department of Energy (DOE)’s Vehicle Technologies Office under the Silicon Consortium Project directed by Brian Cunningham and managed by Anthony Burrell. References V. G. Khomenko and V. Z. Barsukov, Electrochim Acta, 52, 2829–2840 (2007).J. Wu, M. Ihsan-Ul-Haq, Y. Chen, and J. K. Kim, Nano Energy, 89, 106489 (2021).X. He, J. M. Larson, H. A. Bechtel, and R. Kostecki, Nat Commun, 13, 1398 (2022).I. Yoon, J. M. Larson, and R. Kostecki, ACS Nano, 17, 6943–6954 (2023).A. Dopilka, Y. Gu, J. M. Larson, V. Zorba, and R. Kostecki, ACS Appl Mater Interfaces, 15, 6755–6767 (2023).

Operando X-Ray Absorption Spectroscopy of Solid Electrolyte Interphase Formation on Silicon Anodes

Lithium-ion batteries (LIBs) are key to the transition from fossil fuels towards increased use of renewable energy sources. However, more widespread deployment requires improvements in energy density, cost and cycle-lifetime. Various cathode and anode materials are under consideration for next-generation LIBs, and the interfacial stability of these materials in contact with the electrolyte is a critical consideration. Interface-sensitive operando characterization techniques are thus urgently needed to reveal the reactions occurring in working batteries.1,2 The solid electrolyte interphase (SEI) that forms on Li-ion battery anodes is critical to their long-term performance, however observing SEI formation processes at the buried electrode-electrolyte interface is a significant challenge. Here we show that operando soft X-ray absorption spectroscopy in total electron yield mode can resolve the chemical evolution of the SEI during electrochemical formation in a Li-ion cell, with nm-scale interface sensitivity. O, F, and Si K-edge spectra, acquired as a function of potential, reveal when key reactions occur on high-capacity amorphous Si anodes cycled with and without fluoroethylene carbonate (FEC).3 Cross-referencing to cycling data, complementary bulk sensitive fluoresecent yield (FY) XAS measurements, and density functional theory (DFT) calculated spectra enables identification of the electrolyte and SEI species, and the dominant mechanisms of SEI formation. Without FEC present, LiF formation is detected at 0.6 V vs. Li/Li+ prior to significant lithiation of the a-Si, whilst at lower potentials the SEI grows in thickness with an increased contribution from organic components containing -C(=O)O- species. The observed sequential formation of inorganic and organic components is implicated in layering of the SEI. With FEC as an additive we see the onset of SEI formation at much higher potentials (1.0 V vs. Li/Li+), and attribute the improved cycle life seen with this additive to the rapid healing of SEI defects formed during delithiation. Operando TEY-XAS offers new insights into the formation mechanisms of electrode-electrolyte interphases and their stability for a wide variety of electrode materials and electrolyte formulations. References Wu et al. Phys. Chem. Chem. Phys. 2015, 17, 30229.Weatherup et al. Top. Catal. 2018, 61, 2085.Swallow et al. Nature Commun. 2022, 13, 6070.

Investigating Solid Electrolyte Interphase Layer Dynamics in the Electrochemical Li-Mediated Ammonia Synthesis

Due to its important role in the production of fertilizers ammonia is one of the most produced chemicals worldwide. In recent years there has been increasing interest in the electrochemical lithium mediated nitrogen reduction reaction (LMNRR), as a possible alternative to the Haber-Bosch process. The LMNRR is advantageous as it can be carried out under ambient temperatures and pressures [1].The LMNRR process involves electrochemical plating of metallic Li from an organic electrolyte. This plated Li reacts with elemental nitrogen forming an intermediate nitride species, which is subsequently protonated to form ammonia. To ensure high selectivity towards ammonia production the competing hydrogen evolution reaction has to be limited [2]. The solid electrolyte interphase (SEI), formed from electrolyte decomposition at the working electrode plays a crucial role in this as it influences mass transport to the electrode [3]. Studies using neutron reflectometry with one-minute time resolution investigated the initial stages of SEI formation in LiClO4 containing electrolytes [4]. Recent investigations show improved performance using LiBF4 containing electrolytes [5].In this study formation and dynamics of the SEI layer derived from LiClO4 and LiBF4 containing electrolytes were examined in operando using grazing incidence wide-angle X-ray scattering (GI WAXS) in a synchrotron setup. The use of GI WAXS provides a time resolution in the seconds range, enabling the identification of transient species and fluctuations of deposits on the electrode surface.Different SEI species and their dynamics have been observed and correlated with electrolyte composition and faradaic efficiency towards ammonia. Figure 1a) shows the dynamic behavior of diffraction features attributed to Li and LiF in an experiment using LiBF4 as electrolyte salt. In Figure 1b) the development of the corresponding peak intensities is shown and correlated with the start and end of chronopotentiometry (CP), used to study the cell. LiF was formed from the decomposition of LiBF4 and accumulated on the working electrode, showing that LiF is a significant component of the SEI in fluoride-containing electrolytes. Furthermore, metallic Li was found to be more stable when LiBF4 was utilized as electrolyte salt, in contrast to LiClO4. This finding suggests that the SEI derived from LiF has different mass transport characteristics and is more robust than that derived from LiClO4. This may explain the previously reported superior performance of electrolytes containing LiBF4 [5].

Operando NMR Characterization of Cycled and Calendar Aged Si Anodes

Replacing graphite anodes with Si anodes can greatly increase the capacity of current Li-ion batteries. Detailed characterization of Si lithiation reactions, solid electrolyte interphase (SEI) formation, and reversibility are therefore active areas of research. Solid-state nuclear magnetic resonance (NMR) spectroscopy is useful for characterizing different local environments within Si anodes as well as differentiating surface and bulk environments. Furthermore, non-invasive and non-destructive NMR methods can reveal metastable LixSi phases or SEI species forming on (dis)charge that may be too reactive to detect via ex situ methods. Here, we use an in situ and in operando NMR methodology to characterize full pouch cells comprising of Si anodes and NMC cathodes. We can observe changes in the relative amounts of Li in different environments within the same Si anode before/during/after charge & discharge and with various amounts of cycling or calendar aging using in situ 7Li NMR. The pouch cells used in our in situ NMR study are comparable to pouch cells made using mid- to large-scale fabrication methods as opposed to laboratory scale; in other words, challenges of cell-to-cell variability are minimized. We hypothesize “trapped” lithiated phases formed in the Si anode no longer contribute to electrochemical cycling and thus limit capacity in subsequent cycles/aging. We also characterized the SEI/surface species in situ with aging via 13C NMR measurements, identifying reactions of lithium silicides with electrolyte solvents and their subsequent decomposition.

Fundamental Understanding and Quantification of Capacity Losses Involving the Negative Electrode in Sodium-Ion Batteries.

Knowledge about capacity losses related to the solid electrolyte interphase (SEI) in sodium-ion batteries (SIBs) is still limited. One major challenge in SIBs is that the solubility of SEI species in liquid electrolytes is comparatively higher than the corresponding species formed in Li-ion batteries. This study sheds new light on the associated capacity losses due to initial SEI formation, SEI dissolution and subsequent SEI reformation, charge leakage via SEI and subsequent SEI growth, and diffusion-controlled sodium trapping in electrode particles. By using a variety of electrochemical cycling protocols, synchrotron-based X-ray photoelectron spectroscopy (XPS), gas chromatography coupled with mass spectrometry (GC-MS), and proton nuclear magnetic resonance (1 H-NMR) spectroscopy, capacity losses due to changes in the SEI layer during different open circuit pause times are investigated in nine different electrolyte solutions. It is shown that the amount of capacity lost depends on the interplay between the electrolyte chemistry and the thickness and stability of the SEI layer. The highest capacity loss is measured in NaPF6 in ethylene carboante mixed with diethylene carbonate electrolyte(i.e., 5 µAhh-1/2 pause or 2.78 mAhg·h-1/2 pause ) while the lowest value is found in NaTFSI in ethylene carbonate mixed with dimethoxyethance electrolyte (i.e., 1.3 µAhh-1/2 pause or 0.72 mAhg·h-1/2 pause ).

Understanding Solid Electrolyte Interphase (SEI) Species at the Electrode-Electrolyte Interface in Liquid Lithium Metal Batteries Via in-Situ Fourier Transform Infrared (FTIR) Spectroscopy

Lithium metal is the ideal candidate to replace conventional carbonaceous anodes due to its high theoretical specific capacity of 3860 mAh/g and low negative thermodynamic potential of -3.040 V vs. SHE [1]. Fine tuning the solid-electrolyte interphase (SEI) is critical for the practical implementation of liquid lithium metal batteries [2]. The presence of particular SEI species are hypothesized to improve the coulombic efficiency and mitigate dendrite formation for enhanced lifetime and safety, respectively [2]. Recently, Zhang et al. developed a novel in-situ Fourier transform infrared (FTIR) spectroscopy method to reveal electrolyte oxidation via carbonate dehydrogenation on Ni-based oxides in Li-ion batteries [3]. We leverage this technique and apply it to the Cu side in a Li || Cu battery to probe SEI growth and evolution. In this study, we systematically change the salt, solvent and additive components of the electrolyte and monitor the organic and inorganic SEI species that form under different electrochemical conditions. Furthermore, we directly correlate the chemical composition of the SEI to cell-level coulombic efficiency [4]. Complimentary characterization techniques will be used to further resolve the chemical structure of the SEI. Finally, results from this study will help generate descriptors for the rationale design of new electrolytes.

Experimental and Computational Study on SEI Composition and Electrochemical Performance of Lithium-Ion Battery with Silicon Oxide-Graphite Composite Electrode

Demand on batteries with high energy density is surging. Conventionally, carbon materials have been used as an anode active material, but recently silicon materials are likely to be incorporated to form a composite anode in lithium-ion batteries (LIBs) for higher energy density. However, LIBs which include silicon materials in anode tend to show a low cycle performance especially the ratio of silicon materials in the anode increases. Commercial LIBs still have difficulty in incorporating large amount of silicon materials especially under the requirements where multiple aspects like safety, charging rate and energy density should be totally met.One potential pathway to improve the cycle performance in LIBs with silicon materials is to tune the nature of solid electrolyte interphase (SEI), a thin film which forms at the interface between anode active material and electrolyte. The nature of SEI is particularly important for LIBs with silicon materials because such silicon active materials show large volume changes during battery operation and SEI is dynamically reconstructed. It has been qualitatively reported so far that LiF is an important component for better battery performance. Our work suggests that the ratio of LiF in initially formed SEI is a crucial factor to realize improved cycle performance in such LIBs.In our work, anode with silicon oxide (SiOx)-graphite composite is considered. We investigated the relationship between electrochemical performance of the cell with such composite electrode and the nature of the film estimated by characterization and molecular simulation. We systematically changed the composition of liquid electrolyte to form various types of SEI on anode surface. For characterization, both fresh and aged cells with different electrolytes were dismantled, and SEI components were extracted by designated solvents, then NMR (nuclear magnetic resonance) was applied to identify the SEI species and quantify the amounts of major SEI components. Comparison between the evolution of SEI amount and an impedance element in electrochemical impedance spectroscopy during aging indicates that degradation of the cells is closely related to the growth of SEI. To further deepen understanding the role of LiF, molecular dynamics simulation was carried out, where the mixture of LiF and typical organic SEI was modelled. Correlation between calculated mechanical property and measured electrochemical performance of the cells were revealed. In our presentation, the requirements that SEI should satisfy will be discussed from the insight of electrochemical measurements, NMR characterization and modeling approaches.

An in-Situ FTIR Study of Lithium-Mediated Electrochemical Nitrogen Reduction

Electrochemical ammonia synthesis under ambient conditions is a promising sustainable alternative to the highly carbon-intensive Haber-Bosch process. The lithium-mediated system1 is to date the only rigorously verified method of reducing dinitrogen that has been reproduced by several laboratories; nonetheless, its mechanism is not yet understood. It is generally accepted that lithium is key to the success of this system. Metallic lithium binds strongly to dinitrogen and can directly reduce it under ambient conditions. However, it is possible that the apparently unique ability of this system to reduce dinitrogen under ambient conditions lies in its formation of a Solid Electrolyte Interphase (SEI)2 on the working electrode, like those formed in lithium-ion batteries. This likely provides a kinetic barrier to the competing hydrogen evolution reaction by moderating, but not blocking, proton mobility, thus favouring nitrogen reduction.While some post-mortem characterisation of the SEI for this system has been published3, specific SEI forming reactions and the roles of SEI species in the success of the lithium-mediated system are not yet understood. Optimising this system requires optimisation of the properties of the SEI. It is therefore highly desirable to identify key components which govern its properties and develop a mechanistic understanding of their formation. Just as lithium-ion battery electrolytes are tailored to optimise SEI properties, a deeper understanding of the formation of SEI species from bulk electrolyte components in this system would then inform specific tailoring of the electrolyte to optimise selectivity towards nitrogen reduction. Some reports thus far1,4,5 have proposed that electrodeposited metallic lithium reduces dinitrogen1; others that metallic Li or a LiH/Li3N layer provides a catalytic surface for direct protonation5. However, to the best of our knowledge, no proposed mechanism is yet supported by operando spectroscopic evidence, and no operando characterisation of the SEI has been published.In this work, we present a series of studies using operando surface-enhanced infrared spectroscopy to investigate the mechanisms of the two key processes using a bespoke ATR cell. Firstly, we investigate mechanisms of SEI formation reactions in different electrolyte formulations at different applied potentials, thus identifying the key SEI components that may govern the system’s stability and selectivity, and the potentials at which they form. Secondly, we exploit the surface-enhancement effect to directly probe nitrogen reduction reaction intermediates on the working electrode. By detecting potential NH intermediates, we identify the mechanism of nitrogen reduction and the active sites, potentially informing the design of novel electrocatalysts with greater activity and selectivity. A. Tsuneto, A. Kudo and T. Sakata. Chemistry Letters 22, 851-854 (1993).O. Westhead, R. Jervis and I.E.L. Stephens. Science 372, 1149-1150 (2021)K. Li et al. Science 1597, 1593-1597 (2021).A. Bagger, H. Wan, I.E.L. Stephens and J. Rossmeisl. ACS Catalysis 11, 6596-6601 (2021)J. Schwalbe et al. ChemElectroChem 7, 1-9 (2020).

A Dual Functional Artificial SEI Layer Based on a Facile Surface Chemistry for Stable Lithium Metal Anode.

Solid electrolyte interphase (SEI) on a Li anode is critical to the interface stability and cycle life of Li metal batteries. On the one hand, components of SEI with the passivation effect can effectively hinder the interfacial side reactions to promote long-term cycling stability. On the other hand, SEI species that exhibit the active site effect can reduce the Li nucleation barrier and guide Li deposition homogeneously. However, strategies that only focus on a separated effect make it difficult to realize an ideal overall performance of a Li anode. Herein, a dual functional artificial SEI layer simultaneously combining the passivation effect and the active site effect is proposed and constructed via a facial surface chemistry method. Simultaneously, the formed LiF component effectively passivates the anode/electrolyte interface and contributes to the long-term stable cycling performance, while the Li-Mg solid solution alloy with the active site effect promotes the transmission of Li+ and guides homogeneous Li deposition with a low energy barrier. Benefiting from these advantages, the Li||Li cell with the modified anode performs with a lower nucleation overpotential of 2.3 mV, and an ultralong cycling lifetime of over 2000 h at the current density of 1 mA cm−2, while the Li||LiFePO4 full battery maintains a capacity retention of 84.6% at rate of 1 C after 300 cycles.

Multiscale Modeling of Solid Electrolyte Interphase Growth in Silicon Anodes

Solid electrolyte interphase (SEI) growth on silicon results in rapid calendar fade as well as thickness variations during cycling, termed as “SEI breathing”. This is primarily attributed to large volume expansion of the silicon anode resulting in SEI fracture and reformation. However, capacity fade and breathing characteristics are also exhibited in thin film/nano sized Si anodes obviating mechanics considerations and pointing to a chemical mechanism origin. In this work, we develop a multispecies, multiphase SEI model capable of capturing Si SEI characteristics via an electrochemical mechanism. Thermodynamic, reaction kinetics and transport property data for silicon and SEI species (Li+, EC, DMC, Li2EDC, Li2O, Li2CO3 etc.) in our model have been tabulated from atomistic calculations as well as experimental datasets for calendar and cycling are used for model validation. The model is used to investigate what contributes to the instability of Si SEI and better understand why Si SEI behaves distinctly different from SEI formed on graphite. Furthermore, reversibility of the electrolyte decomposition and solid phase conversion reactions can help unravel the SEI breathing behavior of silicon anodes. Figure 1

Unravel SEI Formation on Li Metal Interfaces By Mechanistic Multi-Scale Modelling

Nowadays, lithium metal anodes are often referred to as the ‘holy grail’ of next-generation battery technology. Compared to graphite anodes used in state-of-the-art lithium-ion batteries they promise several times higher energy densities [1]. However, Li metal shows a high reactivity with liquid electrolytes and uncontrolled dendrite growth which hinders a safe and efficient cyclability and therefore the commercial application in rechargeable batteries. Thus, stabilization of the solid-electrolyte interphase (SEI) between liquid electrolyte and the lithium surface is the key challenge to be solved before bringing this promising battery technology to market. In order to suppress unwanted effects such as dendrite formation, persistent loss of active material and high kinetic losses, this layer of electrolyte decomposition products needs to be electrically insulating and should prevent direct contact of metal with electrolyte while ensuring a good ionic conductivity and mechanical stability.Such a beneficial SEI design requires detailed understanding of the fundamental formation mechanism and the resulting chemical SEI composition and structure depending on electrolyte composition. In literature, a large range of experimental methods are applied to characterize the SEI under various conditions [2]. These approaches are mainly descriptive and struggle to identify and understand the underlying mechanisms. Theoretical approaches like density functional theory (DFT), ab-initio molecular dynamics (MD) or classical MD are well suited to determine reaction mechanisms and to model the very beginning of SEI formation in the ns-range. However, they are too computationally expensive to model technical relevant length- and time scales [3].Therefore, this work focuses on overcoming these restrictions by developing an efficient multiscale-model with input from ab-initio calculations. The acceleration originates from focusing on rare events such as reaction or diffusion processes in a 3-dimensional kinetic Monte Carlo instead of considering the vibrational motions of single atoms like in molecular dynamics. Thereby, every process is represented by a probability derived from ab-initio calculations to ensure physical correctness. An additional coupling with efficient ordinary differential equations as shown in [4] ensures electroneutrality in the simulation box.Implementation of the degradation reactions for a 1.2M LiPF6/EC electrolyte obtained from molecular simulations (see Figure 1, appendix), allows to unravel the initial SEI formation, the emerging structure and its composition within the first µs. The simulation suggests an almost instantaneous formation of SEI species on the pristine Li metal within the first nanoseconds after immersion. Due to this early passivation layer the formation process slows down continuously until electron tunneling is blocked by SEI species and a first passivation of the lithium surface is reached. Interestingly, the formed passivation layer mainly consists of inorganic species such as LiF close to the lithium metal surface and Li2CO3 throughout the whole layer but only of minor organic (CH2OCO2Li)2-molecules, suggesting that organic species are mainly formed at larger time scales which is the reason for the typical layering structure of the SEI.In summary this novel approach allows unprecedented insight of initial SEI-formation and thereby extends the accessible insight, time- and length-scales compared to molecular simulations.

Quantum chemical calculations of lithium-ion battery electrolyte and interphase species

Lithium-ion batteries (LIBs) represent the state of the art in high-density energy storage. To further advance LIB technology, a fundamental understanding of the underlying chemical processes is required. In particular, the decomposition of electrolyte species and associated formation of the solid electrolyte interphase (SEI) is critical for LIB performance. However, SEI formation is poorly understood, in part due to insufficient exploration of the vast reactive space. The Lithium-Ion Battery Electrolyte (LIBE) dataset reported here aims to provide accurate first-principles data to improve the understanding of SEI species and associated reactions. The dataset was generated by fragmenting a set of principal molecules, including solvents, salts, and SEI products, and then selectively recombining a subset of the fragments. All candidate molecules were analyzed at the ωB97X-V/def2-TZVPPD/SMD level of theory at various charges and spin multiplicities. In total, LIBE contains structural, thermodynamic, and vibrational information on over 17,000 unique species. In addition to studies of reactivity in LIBs, this dataset may prove useful for machine learning of molecular and reaction properties.

SEI Formation on Sodium Metal Electrodes in Superconcentrated Ionic Liquid Electrolytes and the Effect of Additive Water.

We have previously reported that water addition (∼1000 ppm) to an N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (C3mpyrFSI) superconcentrated ionic liquid electrolyte (50 mol % NaFSI) promoted the formation of a favorable solid electrolyte interphase (SEI) and resulted in enhanced cycling stability. This study reports the characterization of Na-metal anode surfaces cycled with these electrolytes containing different water concentrations (up to 5000 ppm). Morphological and spectroscopic characterization showed that water addition greatly influences the formation of the SEI and that ∼1000 ppm of water promoted the formation of an active and more uniform deposit, with larger quantities of SEI species (S, O, F, and N) present. Water addition to the electrolyte system is also proposed to promote the formation of a new complex between the FSI anions, water molecules, and sodium cations as components of the SEI. For both dry and wet (∼1000 ppm) electrolytes, the SEIs were mainly composed of NaF, metal oxide (i.e., Na2O), and the complex, suggested to be Na2[SO3-N-SO2F]·nH2O (n = 0-2). Postcycling SEM analysis of the Na-metal electrodes after extensive cycling (500 cycles, 1.0 mA·cm-2, 1.0 mA·.cm-2) was used to estimate the minimal average cycling efficiency (ACE), which was enhanced by water addition: up to ∼99% for the 1000 ppm cell compared to ∼98% for the dry cell. Two distinct deposit morphologies, a microporous and a compact layer deposit, were evident after extended cycling in the wet and dry electrolytes. The presence of both the microporous and compact layer deposits on Na-metal surfaces cycled with the wet electrolyte, along with the distinct chemistry and morphology of the SEI, all contributed to a more stable symmetric cell voltage profile and lower cell polarization. In contrast, a higher fraction of microporous deposits and the absence of compact layer formation in the dry electrolyte were associated with higher cell polarization potentials and the occurrence of dendrites.

Impacts of fluorinated phosphate additive on interface stabilization of 4.6 V battery cathode

Elevating the charge cut-off voltage of lithium-ion battery above 4.2 V vs. Li/Li+ can increase the capacity of cathode and energy density of the battery but requires higher anodic stability of electrolyte and higher interface stability between charged cathode and electrolyte than those of the state-of-the-art commercial electrolyte. Utilization of a small fraction of high-voltage electrolyte additive is a promising and economic approach to mitigate the high-voltage stability issue. Fluorinated ethyl phosphate (FEP) is known as a flame-retardant but we examine it as a high-voltage additive of a commercial electrolyte with 1 M LiPF6 in EC:EMC (3:7 vol ratio). Herein, we report FEP-assisted performance improvement of a lithium-ion cell under high charge cut-off voltage of 4.6 V vs. Li/Li+, with reduced impedance rise. Our surface analysis results reveal that FEP additive in a graphite‖LiNi0.5Co0.2Mn0.3O2 full-cell is effective preferably on cathode over anode by forming a surface-passivating organics-rich and fluorine-rich solid electrolyte interphase (SEI) layer. We propose that the anodic reaction of FEP begins by a single electron transfer from the O atom of P − O − C to the cathode surface and forms FEP-derived SEI species. The SEI layer enables the inhibition of metal-dissolution phenomena from the cathode, evidenced by elemental analysis results on the metal species at graphite anode, leading to superior cycling performance to the full-cell with commercial electrolyte only. A basic understanding of interface stabilization pathway of the cathode via FEP additive in the lithium-ion full-cell is clearly demonstrated.

A computational approach to characterize formation of a passivation layer in lithium metal anodes

AbstractLi metal anode is the “Holy Grail” material of advanced Lithium‐ion‐batteries (LIBs). However, it is plagued by uncontrollable dendrite growth resulting in poor cycling efficiency and short‐circuiting of batteries. This has spurred a plethora of research to understand the underlying mechanism of dendrite formation. While experimental studies suggest that there are complex physical and chemical interactions between heterogeneous solid‐electrolyte interphase (SEI) and dendrite growth, most of the studies do not reveal the mechanisms triggering these interactions. To deal with this knowledge gap, we propose a multiscale modeling framework which couples kinetic Monte Carlo and Molecular Dynamics simulations. Specifically, the model has been developed to account for (a) heterogeneous SEI, (b) dendrite‐SEI interactions, and (c) effect of electrolyte on Li electrodeposition and potential dendrite formation. This allows the proposed computational model to be extended to various electrolytes and SEI species and generate results consistent with previous experimental studies.

Quantification of Inactive Lithium and Solid–Electrolyte Interphase Species on Graphite Electrodes after Fast Charging

Rapid charging of Li-ion batteries is limited by lithium plating on graphite anodes, whereby Li+ ions are reduced to Li metal on the graphite particle surface instead of inserting between graphitic...

Probing the in-depth distribution of organic/inorganic molecular species within the SEI of LTO/NMC and LTO/LMO batteries: A complementary ToF-SIMS and XPS study

Spinel Li4Ti5O12 (LTO) is an attractive candidate for negative electrode materials of Li-ion batteries because of its outstanding safety characteristics. In this paper, the influence of common high voltage cathodes, LiNi3/5Co1/5Mn1/5O2 (NMC) and LiMn2O4 (LMO), upon the electrochemical performances of LTO/LMO and LTO/NMC cells, in relation with the Solid Electrolyte Interphase (SEI) properties formed over the LTO surface, is studied. After cycling, the electrodes were analyzed by ToF-SIMS and XPS with two X-ray sources (Ag and Al) to investigate both the chemical composition and the in-depth distribution of specific SEI species at the electrode surface. Facing both counter-electrodes, LTO electrodes are covered by surface layers due to the degradation of electrolyte components inducing an irreversible capacity loss, more important for LTO/LMO cells. The chemical composition of both layers is similar: organic and inorganic species but the SEI formed on LTO electrode cycled facing LMO electrode is thicker and contains small amounts of manganese compounds from the positive electrode. Moreover, 3D mappings reconstructed from ToF-SIMS depth-profile experiments, display different in-depth spatial distributions of SEI species, which are in agreement with XPS results. Consequently, the impact of interactions between electrodes on the formation of surface films is discussed.