Design of Nanostructured Heterogeneous Solid Ionic Coatings through a Multiscale Defect Model.

Lithium metal is often deemed the “Holy Grail” of rechargeable battery technology, due to its high theoretical capacity and low electrochemical potential. However, the cascading heterogeneity from the formation of the solid-electrolyte interphase (SEI) up to the non-uniform current distribution leads to both poor efficiency via rapid consumption of active materials as well as safety issues from treacherous dendrite growth. An SEI that is ionically conductive while electrically resistive, viscoelastic, electrochemically stable, and possesses good mechanical properties is paramount for hindering these failure mechanisms. The discovery of novel approaches to producing an ideal SEI is a crucial piece to solving the lithium metal anode puzzle.One route of controlling the SEI is through interface engineering by producing a stable artificial SEI. In this work, a mixed artificial SEI was produced with aim toward improving ionic conductivity while retaining electrochemical stability and mechanical properties. This mixed SEI embraces the unavoidable surface heterogeneity, all the while preventing dendrite growth that plagues lithium metal batteries. A mixed artificial SEI, as well as a single-component artificial SEI, were produced through a simple and rapid deposition method and tested to observe their failure mechanisms compared to bare lithium.Symmetric cells (i.e., Li||Li) were utilized to isolate the failure mechanisms of these different anode treatments. The failure mechanism of the single-component artificial SEI cells shifted to failure via electrolyte consumption rather than short circuiting seen with bare lithium. This revealed that artificial interphases led to reduced dendrite propagation. Additionally, a reduction in cell potential range was observed with the mixed artificial SEI when compared to the single-component artificial SEI, as well as improved cyclability. Upon further investigation, it appears that the added component of the mixed artificial SEI, which was meant to improve the ionic conductivity, dissolved into the electrolyte upon initial exposure. This dissolution left behind a new SEI structure, theoretically producing easier ionic pathways and thus resulting in decreased cell overpotential. Dissolution was confirmed with high-resolution surface characterization techniques. Additionally, symmetric cell electrodes rinsed with solvent prior to testing retained superior cell performance. Though this dissolution should lead to increased available “hot spots”, the growth of dendrites was hindered. This work discusses the effect an artificial SEI has on failure mechanisms, as well as provides insight into the methods of producing stable, long-life lithium metal batteries. This includes a focus on how nanoscale heterogeneity effects cell-scale performance via high-resolution characterization and computational simulations.Different surface to sub-surface characterization techniques were utilized to measure properties of artificial SEIs and bare lithium. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS) provided elemental make-up and distribution, as well as morphological characteristics. X-ray photoelectron spectroscopy (XPS) provided the chemical state of surfaces, as well as sub-surfaces with the use of depth profiling. Different atomic force microscopy (AFM) techniques were used to spatially measure nanoscale properties such as work functions with the Kelvin probe technique (SKPFM) and quantifiable nanomechanical properties (QNM), as well as observe kinetic mechanisms with in-situ electrochemical methods (EC-AFM). Lastly, computational simulations from the atomistic scale via density functional theory (DFT) up to molecular dynamics (MD) were utilized to support assumptions on the reaction mechanisms derived from experimental observations. Figure 1

Dendrite-free Li-metal anode via a dual-function protective interphase layer for stable Li-metal pouch cell

Li metal is an ideal anode material for next-generation energy storage devices due to its extremely high theoretical specific capacity and the lowest negative electrochemical potential [1,2]. However, uncontrolled Li dendrite growth and the side reaction between Li metal and electrolytes have prevented its practical application in rechargeable Li metal batteries. In our contributions, artificial solid electrolyte interphase (SEI) layer and 3D current collector have been developed to address these issues [3,4]. In this presentation, we will introduce an artificial Li3PO4 SEI layer [3] fabricated by in situ reaction of polyphosphoric acid (PPA) with Li metal and its native film (Li2CO3, LiOH, and Li2O). The partial dissolution of the Li native film and formation of the SEI layer lead to a porous structure of the Li surface, thereby promoting Li dendrite formation and growth. In the ingenious design, the native film of Li metal is replaced by the stable and uniform Li3PO4 SEI layer. The artificial Li3PO4 SEI layer with thickness of 50 nm exhibits a smooth surface and a high Young’s modulus (10-11 GPa). Furthermore, the artificial Li3PO4 SEI layer is demonstrated to be stable in the electrolyte and during cycling in a Li|LiFePO4 battery system. The Li-conducting Li3PO4SEI layer can achieve uniform Li deposition, and reduce the side reaction between the Li metal and the electrolyte. Thus, the artificial SEI strategy we described is a promising route to tackle the intrinsic problems of Li metal anodes and other metal based anodes. Furthermore, we will show that Li metal anode can be accommodated in the reserved pores of the 3D current collector [4] with a submicron skeleton and high surface area, thereby suppressing the Li dendrite growth and solving the associated problems of Li metal anodes. Because of the high electroactive area of the submicron 3D structure, the 3D current collector can improve the Li plating/stripping efficiency and reduce the resistance of the Li metal anode. The utilization of the 3D architecture to accommodate the Li metal anode will highlight the importance of rational design of current collectors and reveal a new avenue for developing Li metal anodes with a long lifespan. References Y.-X. Yin, S. Xin, Y.-G. Guo, L.-J. Wan, Angew. Chem. Int. Ed. 2013, 52, 13186.W. Xu, J. L. Wang, F. Ding, X. L. Chen, E. Nasybutin, Y. H. Zhang, J. G. Zhang, Energ. Environ. Sci. 2014, 7, 513.N.-W. Li, Y.-X. Yin, C.-P. Yang, Y.-G. Guo, Adv. Mater. 2015, DOI: 10.1002/adma.201504526.C.-P. Yang, Y.-X. Yin, S.-F. Zhang, N.-W. Li, Y.-G. Guo, Nat. Commun. 2015, 6, 8058

The cycling lifespan of Li-metal batteries is compromised by the unstable solid electrolyte interphase (SEI) and the continuous Li dendrites, restricting their practical implementations. Given these challenges, establishing an artificial SEI holds promise. Herein, a trinitarian gradient interphase is innovatively designed through composite coatings of magnesium fluoride (MgF2), N-hexadecyltrimethylammonium chloride (CTAC), and polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) on Li-metal anode (LMA). Specifically, the MgF2/CTAC/PVDF-HFP SEI spontaneously forms a lithium fluoride (LiF)-rich PVDF-HFP-based SEI, along with lithium-magnesium (Li-Mg) alloy substrate as lithiophilic electronic conductor and positively charged CTAC during plating. Noticeably, the Li-Mg alloy homogenizes the distribution of electric field and reduce the internal resistance, while the electronically insulated LiF/PVDF-HFP composite SEI offers fast ion-conducting and mechanical flexibility, accommodating the volumetric expansion and ensuring stable Li-ion flux. Additionally, CTAC at the dendritic tip is pivotal for mitigating dendrites through its electrostatic shield mechanism. Innovatively, this trinitarian synergistic mechanism, which facilitates colossal granular Li deposits, constructs a dendrite-free LMA, leading to stable cycling performances in practical Li||LFP, popular Li||NCM811, and promising Li||S full cells. This work demonstrates the design of multifunctional composite SEI for comprehensive Li protection, thereby inspiring further advancements in artificial SEI engineering for alkali-metal batteries.

Lithium-sulfur (Li-S) batteries promise the next-generation energy storage. However, one of the issues facing Li-S batteries is the high reactivity of the Li metal, leading to serious interfacial side reactions; the other is the dendritic Li growth that can usually breach the solid electrolyte interphase (SEI) applied for mitigating side reactions. To address these issues, we propose the indium fluoride-based artificial solid electrolyte interphase with tunable surface properties. Over the initial charging, InF3 nanoparticles coated on the Li anode surface can be lithiated to form a layer of nanoporous lithium/indium alloy (LixIn) encapsulating lithium fluoride (LiF) nanoparticles (3Li++3e-+InF3→3LiF+In) as shown in Figs. 1(a, b). The tightly anchored LixIn layer well attaches LiF on the anode surface during Li stripping/plating, while LiF with a higher Young’s modulus physically suppresses dendritic Li growth. Both LixIn and LiF provide facile adsorption towards Li atoms as well as high Li+ conductivity, inducing uniform Li plating [1-4]. In addition, both materials are stable with corrosive electrolyte containing polysulfides and shield Li anode from side reactions. Tuning the SEI surface properties via adding other metal fluorides such as BiF3 and AlF3, we further show the possibility to further enhance the Young’s modulus and flexibility for the SEI. Applying the rationally designed artificial SEI, substantial enhancement in terms of Li stripping/plating stability (Fig. 1c) and Li-S battery cyclability is achieved even using LiNO3-free electrolyte and the gas evolution arising from interfacial electrolyte decomposition is suppressed [5]. With enhanced mechanical strength and chemical stability, the metal fluoride-based artificial SEI paves a new way in developing durable and safer Li-S batteries. Key words: Li metal; artificial solid electrolyte interphase; dendrite; side reaction; metal fluoride. Acknowledgement The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R) Reference[1] Y. Ren, T. Zhao, M. Liu, Y. Zeng, H. Jiang, Journal of Power Sources, 361 (2017) 203-210.[2] X.Q. Zhang, X. Chen, R. Xu, X.B. Cheng, H.J. Peng, R. Zhang, J.Q. Huang, Q. Zhang, Angewandte Chemie, 129 (2017) 14395-14399.[3] M. Liu, Y. Ren, H. Jiang, C. Luo, F. Kang, T. Zhao, Nano Energy, 40 (2017) 240-247.[4] Y. Ren, T. Zhao, H. Jiang, M. Wu, M. Liu, Journal of Power Sources, 347 (2017) 136-144.[5] A. Jozwiuk, B.B. Berkes, T. Weiß, H. Sommer, J. Janek, T. Brezesinski, Energy & Environmental Science, 9 (2016) 2603-2608. Figure 1

In this study, the artificial solid electrolyte interphase (SEI) formed on lithium metal when treated in ZnCl2 solutions is thoroughly investigated. The artificial SEI on lithium metal electrodes substantially decreases the interfacial resistance by ca. 80% and improves cycling stability in comparison to untreated lithium. The presence of a native SEI negatively affects the morphology and interfacial resistance of the artificial SEI. Increasing the ZnCl2 concentration in tetrahydrofuran (THF) (precursor solution) results in higher homogeneity of the surface morphology. Independent of the ZnCl2 concentrations, the artificial SEI is composed of Cx, CO, LiCl, Li2CO3, ZnCl2, and LixZny alloys. ZnCl2 (1 M) produces the most homogenous surface and additional surface species with carbonyl side groups. Nonetheless, the ZnCl2 concentration only has a small effect on the interfacial resistance or cycling stability. Using ethyl methyl carbonate (EMC) as the solvent significantly reduces the interfacial resistance to 7 Ω cm2, in comparison to 25 Ω cm2 for THF. The composition of the artificial SEIs varies depending on the solvent. Either way, the SEI consists of Cx LixC, LiCl, Li2CO3, ZnCl2, and LiZn alloys. The THF-based SEI additionally features ether and carbonyl groups, LiZnO, and Zn metal. For the artificial SEI formed with both solvents, the atomic percentage of the LiZn alloy increases close to the Li surface.

The solid electrolyte interphase (SEI) has been identified as a key challenge for Li metal anodes. The brittle and inhomogeneous native SEI generated by parasitic reactions between Li and liquid electrolytes can devastate battery performance; therefore, artificial SEIs (ASEIs) have been proposed as an effective strategy to replace native SEIs. Herein, as a collaboration between academia and industrial R&D teams, a multifunctional (crystalline, high modulus, and robust, Li+ ion conductive, electrolyte‐blocking, and solution processable) ASEI material, LiAl‐FBD (where “FBD” refers to 2,2,3,3‐tetrafluoro‐1,4‐butanediol), for improving Li metal battery performance is designed and synthesized. The LiAl‐FBD crystal structure consists of Al3+ ions bridged by FBD2– ligands to form anion clusters while Li+ ions are loosely bound at the periphery, enabling an Li+ ion conductivity of 9.4 × 10–6 S cm–1. The fluorinated, short ligands endow LiAl‐FBD with electrolyte phobicity and high modulus. The ASEI is found to prevent side reactions and extend the cycle life of Li metal electrodes. Specifically, pairing LiAl‐FBD coated 50 µm thick Li with industrial 3.5 mAh cm–2 NMC811 cathode and 2.8 µL mAh–1 lean electrolyte, the Li metal full cells show superior cycle life compared to bare ones, achieving 250 cycles at 1 mA cm–2.

Elucidating the role of polar functional groups in fluorinated polymer artificial interphase for stable lithium anodes

The existence of passivating layers at the interfaces is a major factor enabling modern lithium-ion (Li-ion) batteries. The properties of the passivation layers determine the cycle life, performance, and safety of batteries. One critical passivation layer is the solid electrolyte interphase (SEI), a heterogeneous multicomponent film formed due to the decomposition of the electrolyte at the surface of the anode. The multicomponent nature is critical for its functioning as the interfaces between these components play a critical role in determining performance and safety. In this work, we use first-principles simulations to investigate the thermodynamic, kinetic, and electronic properties of the interface between lithium fluoride (LiF) and lithium carbonate (Li2CO3), two common SEI components present in Li-ion batteries with organic liquid electrolytes. We construct a coherent interface between these components that restricts the strain in each of them to below 3%. We find that the interfacial structure has a formation energy of the Frenkel defect higher than bulk calculations and similar to pristine Li2CO3, generating Li vacancies in LiF and Li interstitials in Li2CO3 responsible for transport. On the other hand, the Li interstitial hopping barrier is reduced from 0.3 eV in bulk Li2CO3 to 0.10 or 0.22 eV in the interfacial structure considered, demonstrating the favorable role of the interface. Controlling these two effects in a heterogeneous SEI is crucial for maintaining fast ion transport in the SEI. We further perform Car–Parrinello molecular dynamics simulations to explore Li-ion conduction in our interfacial structure, which reveal an enhanced Li-ion diffusion in the vicinity of the interface. Understanding the interfacial properties of the multiphase SEI represents an important frontier to enable next-generation batteries.

Lithium (Li)-metal anode has attracted extensive attention owing to its high theoretical specific capacity (3860 mAh/g), which is more than ten times to that of graphite anode (372 mAh/g). However, the notorious reactivity of Li-metal prevents their practical applications due to undesired Li dendrite growth and unstable solid electrolyte interphase (SEI) formation. Here, we developed a facile, cost-effective and one-step approach for creating an artificial Li-metal/electrolyte interface by treating Li metal anode with an electrolyte containing SnF2. As a result, the artificial SEI composed of LiF, Sn and Sn-Li alloy was formed with higher ionic conductivity and increased Young’s modulus. The higher ionic conductivity and larger Young’s modulus of the artificial layer facilitate fast Li-ion diffusion and suppress Li dendrite growth, respectively. The hybrid artificial SEI showed a synergistic effect of storing Li by a reversible Sn-Li alloy formation and Li plating underneath the SEI. The artificial SEI treated Li symmetrical cells showed outstanding plating/stripping cycles (~2325 hrs) with reduced overpotential compared to the bare Li. Coupled with a high-loading (11.88 mg.cm-2) LiNi1/3Co1/3Mn1/3O2 (NMC111) cathode, the SEI protected Li full cells exhibited remarkable performance such as longer stable cycling, higher capacity retention and higher capacity at higher rates compared to the bare Li anode.

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

Lithium metal batteries are one of the promising technologies for future energy storage. One open challenge is the generation of a stable and well performing Solid Electrolyte Interphase (SEI) between lithium metal and electrolyte. Understanding the complex interaction of reactions at the lithium surface and the resulting SEI is crucial for knowledge-driven improvement of the SEI. This study reveals the internal species distribution and geometrical aspects of the native SEI during formation by model-based analysis. To achieve this, a combination of molecular dynamics, density functional theory, and stand-alone 3D-kinetic Monte Carlo simulations is used. The kinetic Monte Carlo model determines the SEI growth features over a long time and length scale so that the SEI can be analyzed quantitatively. The simulation confirms the frequently postulated layered SEI structure arising from the decomposition of an ethylene carbonate/lithium hexafluorophosphate (2 M) electrolyte with lithium metal. These layers are not clearly separated, which is contrary to what is often reported. The gradient distribution of the species within the SEI therefore corresponds to a partly mosaic structured SEI at the borders of the layers. At the lithium surface, an inorganic layer of lithium fluoride and then lithium carbonate is observed, followed by an organic, more porous SEI layer consisting of lithium ethylene dicarbonate. Simulations further reveal the strong prevalence of corrosion processes of the metal, which provide more than 99% of the lithium for the SEI reaction processes. The salt contributes less than 1% to the SEI formation. Additionally, SEI formation below and above the initial interface was observable. The here presented novel modeling approach allows an unprecedented in-depth analysis of processes during native SEI formation that can be used to improve design for high battery performance and durability.

Enabling high-performance all-solid-state lithium batteries with high ionic conductive sulfide-based composite solid electrolyte and ex-situ artificial SEI film

A nanometer thick passivation layer will spontaneously form on Li-metal in battery applications due to electrolyte reduction reactions. This passivation layer in rechargeable batteries must have "selective" transport properties: blocking electrons from attacking the electrolytes, while allowing Li+ ion to pass through so the electrochemical reactions can continue. The classical description of the electrochemical reaction, Li+ + e → Li0, occurring at the Li-metal|electrolyte interface is now complicated by the passivation layer and will reply on the coupling of electronic and ionic degrees of freedom in the layer. This passivation layer is called "solid electrolyte interphase (SEI)" and is considered as "the most important but the least understood in rechargeable Li-ion batteries," partly due to the lack of understanding of its structure-property relationship. Predictive modeling, starting from the ab initio level, becomes an important tool to understand the nanoscale processes and materials properties governing the interfacial charge transfer reaction at the Li-metal|SEI|electrolyte interface. Here, we demonstrate pristine Li-metal surfaces indeed dissolve in organic carbonate electrolytes without the SEI layer. Based on joint modeling and experimental results, we point out that the well-known two-layer structure of SEI also exhibits two different Li+ ion transport mechanisms. The SEI has a porous (organic) outer layer permeable to both Li+ and anions (dissolved in electrolyte), and a dense (inorganic) inner layer facilitate only Li+ transport. This two-layer/two-mechanism diffusion model suggests only the dense inorganic layer is effective at protecting Li-metal in electrolytes. This model suggests a strategy to deconvolute the structure-property relationships of the SEI by analyzing an idealized SEI composed of major components, such as Li2CO3, LiF, Li2O, and their mixtures. After sorting out the Li+ ion diffusion carriers and their diffusion pathways, we design methods to accelerate the Li+ ion conductivity by doping and by using heterogonous structure designs. We will predict the electron tunneling barriers and connect them with measurable first cycle irreversible capacity loss. Finally, we note that the SEI not only affects Li+ and e- transport, but it can also impose a potential drop near the Li-metal|SEI interface. Our challenge is to fully describe the electrochemical reactions at the Li-metal|SEI|electrolyte interface. This will be the subject of ongoing efforts.

The solid electrolyte interphase (SEI) governs transport and reactivity at lithium interfaces, so its structure and composition are essential factors in improving the cyclability of next-generation lithium-metal anodes (1). An ideal SEI should passivate Li against continuous reactions with electrolyte while promoting facile transport of Li+ ions. However, achieving these properties is challenging, in part because our understanding of the relative desirability of different SEI materials is often based on qualitative relationships between characterization and cell performance; quantitative experimental conductivity (2, 3) and stability (4) measurements are limited. As a further complication, the highly reductive conditions (-3.04 V vs SHE) and the complex nanoscale structure of the SEI can lead to markedly different behavior in practical contexts compared to bulk environments (2, 5, 6). In previous work, our group developed techniques to synthesize single-component, nanoscale films of LiF and Li2O on Li, enabling direct measurements of their transport properties and reactivity in relevant electrolytes (6-8). Here, we turn to Li2CO3, which has a mixed reputation as an SEI material. While many papers assert its desirability based on high ionic conductivity (9-12), others raise concerns related to reductive instability (13-15).In this work, we synthesized Li2CO3 films via sequential reactions of O2 and CO2 on polished lithium surfaces at slightly elevated temperature (175-200°C). Scanning electron microscopy (SEM) and air exposure tests showed that the prepared films are conformal, tens of nanometers thick, and relatively pinhole-free. Fourier transform infrared spectroscopy (FTIR) was used to confirm the speciation of the films, and titration-based quantification yielded insights into their composition. We found that the formation of Li2CO3 is associated with generation of Li2O and Li2C2, confirming that the reductive instability of Li2CO3 results in the evolution of a more reduced inner SEI layer at the Li | Li2CO3 interface. We also studied the stability of films at the SEI | electrolyte interface, performing electrolyte soak tests then assessing native SEI evolution using FTIR and titration-based quantification of LiF. We found that Li2CO3 is poorly-passivating in fluorinated electrolytes, leading to continuous formation of native SEI. However, in 1M LiClO4 PC electrolyte, the Li2CO3 film remains intact, enabling the use of electrochemical impedance spectroscopy (EIS) to study film transport properties. This analysis reveals an ionic conductivity of ~4-8 nS/cm, which is substantially greater than ionic conductivities previously measured in Li2O (~1 nS/cm) and LiF (~0.5 nS/cm) (6). Together, these results show that though Li2CO3 has some stability limitations, it could promote facile Li+ ion transport as a stable meso-SEI layer in less-fluorinated electrolytes.This work was funded by the 2022-2023 ECS Toyota Young Investigator Fellowship award. References E. Peled and S. Menkin, Journal of The Electrochemical Society, 164, A1703 (2017).S. Lorger, K. Narita, R. Usiskin and J. Maier, Chemical Communications, 57, 6503 (2021).P. Lu and S. J. Harris, Electrochemistry Communications, 13, 1035 (2011).B. S. Parimalam, A. D. MacIntosh, R. Kadam and B. L. Lucht, Journal of Physical Chemistry C, 121, 22733 (2017).S. Shi, Y. Qi, H. Li and L. G. Hector, (2013).R. Guo and B. M. Gallant, Chemistry of Materials, 32, 5525 (2020).R. Guo, D. N. Wang, L. Zuin and B. M. Gallant, Acs Energy Letters, 6, 877 (2021).M. F. He, R. Guo, G. M. Hobold, H. N. Gao and B. M. Gallant, Proceedings of the National Academy of Sciences of the United States of America, 117, 73 (2020).E. Plichta, S. Slane, M. Uchiyama, M. Salomon, D. Chua, W. B. Ebner and H. W. Lin, J. Electrochem. Soc., 136, 1865 (1989).D. Aurbach, A. Zaban, Y. Gofer, E. Ely, I. Weissman, O. Chusid and O. Abramson, Recent studies of the lithium-liquid electrolyte interface Electrochemical, morphological and spectral studies of a few important systems, in Journal of Power Sources, p. 76 (1995).T. Osaka, T. Momma, Y. Matsumoto and Y. Uchida, Journal of Power Sources, 68, 497 (1997).J. Besenhard, M. W. Wagner, M. Winter, A. D. Jannakoudakis, P. D. Jannakoudakis and E. Theodoridou, Journal of Power Sources, 413 (1993).K. Leung, F. Soto, K. Hankins, P. B. Balbuena and K. L. Harrison, Journal of Physical Chemistry C, 120, 6302 (2016).N. Tian, C. Hua, Z. Wang and L. Chen, Journal of Materials Chemistry A, 3, 14173 (2015).B. Han, Z. Zhang, Y. C. Zou, K. Xu, G. Y. Xu, H. Wang, H. Meng, Y. H. Deng, J. Li and M. Gu, Advanced Materials, 33 (2021). Figure 1