Initial Solid Electrolyte Interphase Research Articles

Electrochemical Interface Engineering on a Silicon‐Based Anode via Fluorinated‐Additive‐Assisted Interplay with the Electric Double Layer

AbstractIncorporating functional fluorinated additives into the electrolyte has demonstrated a promising strategy for improving the electrochemical and interfacial stability of silicon‐based anode materials. In previous studies, these additives are claimed responsible for formation of a fluorinated solid electrolyte interphase (SEI) owing to matched orbital energy level with the other electrolyte components. The electric double layer (EDL) created via ionic‐electronic coupling at a (sub)nanoscale shows potential influence on the initial SEI formation at the anode, yet the underlying relationship among electrolyte additive, EDL and SEI remains obscure. Here, it is shown that, introduction of 0.5 wt.% trimethylsilyl trifluoromethanesulfonate (TMSOTF) additive into a conventional LiPF6‐based electrolyte helps to refine the EDL configuration, allowing stronger participation of additive molecules and counter anions for building a fluoride‐rich layers during initial SEI formation. This dynamic maintenance of an inorganic‐rich matrix (LiF, LixPOyFz, and LixSy) throughout the electrochemical process results in a SEI with optimized chemical composition, enhancing Li+ transport, mechanical strength, and structural integrity. Consequently, a SiOx (x≈1) anode exhibits improved cycling and rate performance, and electrode conformality. This work helps to clarify the EDL‐SEI interplay and provide guidelines for rational design of kinetically‐stable SEI on a high‐capacity anode with substantial volume variations.

(Invited) Generation and Evolution of Materials in the Anode Solid Electrolyte Interphase (SEI) of Lithium Ion Batteries

A solid electrolyte interphase (SEI) is generated on the anode of lithium ion batteries during the first few charging cycles. The SEI provides a passivation layer on the anode surface which inhibits further electrolyte decomposition and affords the long calendar life required for many applications. However, despite thorough investigation of the SEI for the last few decades, the SEI remains poorly understood. Over the last several years additional investigations of the structure of the initial SEI formed on graphite electrodes along with changes which occur to the SEI upon additional cycling have been conducted. The investigations provide significant new insight into the structure and evolution of the anode SEI. The characterization of SEI components via ex-situ surface analysis of cycled electrodes has been complemented by investigations utilizing the one-electron reducing agent, lithium naphthalanide, to independently prepare and investigate the reduction products which constitute the SEI. The initial reduction products of ethylene carbonate (EC) are lithium ethylene dicarbonate (LEDC) and ethylene. However, the instability of LEDC generates an intricate mixture of compounds which greatly complicates the composition of the SEI. The reduction products and their subsequent decomposition products have been thoroughly investigated via a combination of NMR, XPS, IR-ATR, TGA, GCMS, and OEMS. Mechanisms for the generation of the complicated mixture of products are presented along with the differences in the SEI structure and function in the presence of electrolyte additives and the effect of crossover reactions related to electrolyte oxidation on the cathode will be discussed.

A Comparison of Standard SEI Growth Models in the Context of Battery Formation

Growth of the Solid Electrolyte Interphase (SEI) layer on negative electrode particles during the formation cycle is one of the most complex and least understood steps of lithium-ion battery manufacturing. This initial SEI formation significantly impacts battery performance, lifetime, and degradation. Zero-dimensional models, which reduce the complexity of SEI’s morphology, material, and structure, are commonly used to study long-term SEI growth rates and capacity fade. These models are derived based on limiting mechanisms. We aim to compare the most common SEI growth models, focusing on the first few cycles at low C-rates representing formation protocols. Using consistent parameters across models, we seek to understand if they can capture the dynamics of SEI formation. We conducted qualitative comparisons with experimental measurements of Coulombic efficiency in 2032-type coin cells at low C-rate. Our analysis shows that the models predict SEI growth in the first cycle to be higher than in subsequent cycles. However, the difference between cycles in these models is insufficient to explain the experimental results, which indicate a capacity fade during the first cycle that is two orders of magnitude higher than in later cycles. This suggests new models are needed to accurately describe the physics of the formation cycle.

Binary Cation Matrix Electrolyte and Its Effect on Solid Electrolyte Interphase Suppression and Evolution of Si Anode.

An unstable solid electrolyte interphase (SEI) has been recognized as one of the biggest challenges to commercializing silicon (Si) anodes for high-energy-density batteries. This work thoroughly investigates a binary cation matrix of Mg2++Li+ electrolyte and its role in SEI development, suppression, and evolution of a Si anode. Findings demonstrate that introducing Mg ions dramatically reduces the SEI growth before lithiation occurs, primarily due to the suppression of solvent reduction, particularly ethylene carbonate (EC) reduction. The Mg2+ alters the Li+ cation solvation environment as EC preferably participates in the oxophyllic Mg2+ solvation sheath, thereby altering the solvent reduction process, resulting in a distinct SEI formation mechanism. The initial SEI formation before lithiation is reduced by 70% in the electrolyte with the presence of Mg2+ cations. While the SEI continues to develop in the postlithiation, the inclusion of Mg ions results in an approximately 80% reduction in the postlithiation SEI growth. Continuous electrochemical cycling reveals that Mg2+ plays a crucial role in stabilizing the deep-lithiated Si phases, which effectively mitigates side reactions, resulting in controlled SEI growth and stable interphase while eliminating complex LixSiy formation. Mg ions promote the development of a notably more rigid and homogeneous SEI, characterized by a reduced dissipation (ΔD) in the Mg2++Li+ ion matrix compared to the solely Li+ system. This report reveals how the Mg2++Li+ ion matrix affects the SEI evolution, viscoelastic properties, and electrochemical behavior at the Si interface in real time, laying the groundwork for devising strategies to enhance the performance and longevity of Si-based next-generation battery systems.

Enhancing Charging Efficiency with Lithium Silicate in Silicon Composite Anode Materials Through Lithiothermic Reduction Reaction Synthesis

AbstractThis study synthesizes silicon (Si) powders with lithium silicates including lithium orthosilicate (Li4SiO4), lithium metasilicate (Li2SiO3), and lithium disilicate (Li2Si2O5), creating a composite structure of crystalline Si within a LixSiyOz matrix through the lithiothermic reduction reaction (LTRR) process. The reduction of Li‐ion consumption of the anode is investigated by 1) initial solid electrolyte interphase (SEI) layer formation, 2) SEI layer formation in response to Si expansion‐induced damage, 3) trapping of Li ions at Si defects, and 4) side reactions during initial charge and discharge cycles. Si/LixSiyOz electrode exhibits a specific capacity of 1522.2 mAh g−1 and an initial coulombic efficiency of 83.5%. The effect of the calendering process is observed, and a pressurization condition of 5000 kgf cm−2 or less is set, and the ICE is improved to 93.4%–96%. Si/LixSiyOz electrodes outperform pure crystalline Si electrodes in specific capacity (7.3%), ICE (42%), and retention characteristics (17%). The integration of the LixSiyOz matrix into Si anodes enhances Li‐ion transport and partially suppresses Si expansion. Additionally, the Si/LixSiyOz electrode exhibits superior rate capability in the 0.2–1.6 A g−1 range.

Methods–Temperature-Dependent Gassing Analysis by On-Line Electrochemical Mass Spectrometry of Lithium-Ion Battery Cells with Commercial Electrolytes

The evolution of gases is often associated with the decomposition of the electrolyte or active materials. Thus, its detection can be powerful for understanding degradation mechanisms in Li-ion batteries (LIBs). Here, we present an evaluation method for gas detection and quantification by on-line electrochemical mass spectrometry (OEMS) when using volatile electrolytes (e.g., electrolytes with linear alkyl carbonates) and a new OEMS cell design for improved leak tightness. With a significant fraction of the gases in the cell head-space being electrolyte vapor, we observe a pressure/time-dependency of the electrolyte background in the mass spectrometer, for which we here developed a correction method. We apply this method for the temperature-dependent gas analysis of a graphite/NCM831205 full-cell with an LP57 (1 M LiPF6 in EC:EMC 3:7 wt:wt) electrolyte. We conclude that the activation energy of the gas evolution associated with the formation of the solid-electrolyte interphase (SEI) is ∼15–20 kJ mol−1. Furthermore, we identify a significant temperature dependence of the lithium alkoxide triggered trans-esterification of EMC with an activation energy of ∼70 kJ mol−1. Lastly, the temperature-dependent analysis reveals the relation between the evolution of hydrogen related to water and HF impurities during the initial SEI formation and in situ generated protons.

Efficient Chemical Prelithiation with Modificatory Li+ Solvation Structure Enabling Spatially Homogeneous SEI toward High Performance SiOx Anode†

Comprehensive SummaryChemical prelithiation is widely proven to be an effective strategy to address the low initial coulombic efficiency (ICE) of promising SiOx anode. Though the reagent composition has been widely explored, the Li+ solvation structure, which practically plays the cornerstone role in the prelithiation ability, rate, uniformility, has rarely been explored. A novel environmentally‐friendly reagent with weak solvent cyclopentyl methyl ether (CPME) is proposed that enables both improved ICE and spatial homogeneous solid electrolyte interphase (SEI). And the prelithiation behavior and mechanism were explored focused on the Li+ solvation structure. Both theoretical investigation and spectroscopic results suggest that weak solvent feature of CPME reduces the solvent coordination number and decreases the Li+ desolvation energy. The optimized Li+ solvation structure enables high‐efficiency prelithiation that ensures the horizontal homogenization and mechanical properties of SEI. Moreover, the accompanied CPME molecules preferentially occupy positions in initial SEI, reducing the likelihood of LiPF6 decomposition and promoting longitudinal homogenization of SEI. Consequently, the efficient and homogenous prelithiation enables impressive ICE of 109.52% and improved cycling performance with 80.77% retained after 300 cycles via just 5 min soaking. Furthermore, the full cells with LiNi0.83Co0.12Mn0.05O2 (NCM831205) cathode display an enhancement in the energy density of 179.74% and up to 648.35 Wh·kg–1.

Suppressing the damage of deposited Mn(II) ions to graphite anode in lithium‐ion batteries by electrolyte additive agent and positive material coating

AbstractSEM and EDS techniques are carried out to demonstrate the variation of morphology and chemical compound on the surface of graphite anode, which suggest a well‐accepted concept that the manganese ions have serious influence on the reversible capacity fade of graphite anode in lithium ion batteries. Based the main chemical compounds of the inorganic layer on the graphite surface, the evolution steps of graphite structure damaged by Mn ions are derived. Although the amount of deposited manganese ions is small, these play an important role in the catalytic decomposition of the electrolyte. Moreover, Raman analysis shows that the structure of the graphite anode becomes irregular at initial SEI formation cycles and tends to be stable at subsequent cycles. This structure variation is probably generated from the manganese ion deposition and the solid electrolyte interphase (SEI) film formation. According to the capacity tests, the cycling performance of NCM811/graphite lithium‐ion batteries could be improved 50% by FEC additive and B2O3 surface coating. FEC additive maybe benefit graphite forming a stable SEI film in the early stages of cycling to suppress the damage of Mn2+ ions, then improving the cycling performance.

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 ).

Unraveling propylene oxide formation in alkali metal batteries.

The increasing need for electrochemical energy storage drives the development of post-lithium battery systems. Among the most promising battery types are sodium-based battery systems. However, like its lithium predecessor, sodium batteries suffer from various issues like parasitic side reactions, which lead to a loss of active sodium inventory, thus reducing the capacity over time. Some problems in sodium batteries arise from an unstable solid electrolyte interphase (SEI) reducing its protective power. While it is known that the electrolyte affects the SEI structure, the exact formation mechanism of the SEI is not yet fully understood. Here we follow the initial SEI formation on sodium metal submerged in propylene carbonate with and without the electrolyte salt sodium perchlorate. We combine X-ray photoelectron spectroscopy, gas chromatography, and density functional theory to unravel the sudden emergence of propylene oxide after adding sodium perchlorate to the solvent. We identify the formation of a sodium chloride layer as a crucial step in forming propylene oxide by enabling precursors formed from propylene carbonate on the sodium metal surface to undergo a ring-closing reaction. We identify changes in the electrolyte decomposition process, propose a reaction mechanism to form propylene oxide and discuss alternatives based on known synthesis routes.

Knowledge-driven design of solid-electrolyte interphases on lithium metal via multiscale modelling

Due to its high energy density, lithium metal is a promising electrode for future energy storage. However, its practical capacity, cyclability and safety heavily depend on controlling its reactivity in contact with liquid electrolytes, which leads to the formation of a solid electrolyte interphase (SEI). In particular, there is a lack of fundamental mechanistic understanding of how the electrolyte composition impacts the SEI formation and its governing processes. Here, we present an in-depth model-based analysis of the initial SEI formation on lithium metal in a carbonate-based electrolyte. Thereby we reach for significantly larger length and time scales than comparable molecular dynamic studies. Our multiscale kinetic Monte Carlo/continuum model shows a layered, mostly inorganic SEI consisting of LiF on top of Li2CO3 and Li after 1 µs. Its formation is traced back to a complex interplay of various electrolyte and salt decomposition processes. We further reveal that low local Li+ concentrations result in a more mosaic-like, partly organic SEI and that a faster passivation of the lithium metal surface can be achieved by increasing the salt concentration. Based on this we suggest design strategies for SEI on lithium metal and make an important step towards knowledge-driven SEI engineering.

Interface Engineering Regulation by Improving Self‐Decomposition of Lithium Salt‐Type Additive using Ultrasound

AbstractLithium salt‐type additives have triggered widespread concern because of their excellent film‐forming properties to construct a solid electrolyte interphase (SEI) with high stability and low impedance. However, little attention has been paid to enhancing the utilization efficiency of the expensive lithium salt‐type additives. Herein, the main factor limiting the decomposition of lithium difluoro bis(oxalate) phosphate (LiDFBOP) during the initial SEI formation is clarified. Combining the electrochemical analysis results and quantum chemistry calculations, it is inferred that LiDFBOP preferentially decomposes and generates soluble products accumulating on the graphite surface due to diffusion control kinetics. The excessive aggregation of these products on the electrode inhibits the subsequent continuous decomposition of LiDFBOP. An ultrasound auxiliary method is developed to accelerate the soluble product diffusion and promote the reduction decomposition of LiDFBOP increasing the inorganics content of Li2C2O4 and LiF in the formed SEI. Accordingly, the cell performance is greatly improved compared with that without ultrasound: Li/graphite cells with ultrasound can retain 60.60% of initial capacity after 500 cycles at 1C. This work not only pioneers the study of improving the utilization efficiency of additives contributing to reducing the electrolytes cost but also has direct guiding significance in the targeted regulation of interface properties.

(Invited) Dendrite Formation in Lithium Electrodeposition – Insights from Experiments and Modeling

High-energy density batteries are essential for powering future electric vehicles and electric aircrafts. The Li-metal anode offers the highest theoretical specific energy among practically available anode materials; however, a major hurdle in the progress towards rechargeable Li-metal batteries is the dendritic electrodeposition of Li metal during the battery charging process. The physicochemical mechanisms behind Li dendrite formation have been a topic of rich and fervent research, but many questions remain presently unanswered. Among these, a key question of significance to battery charging is: at what time instant is Li dendritic growth initiated? In the present talk, this question will be addressed through a combination of experiments and modeling. Chronopotentiometry during galvanostatic Li electrodeposition coupled with in situ optical microscopy reveals that the first emergence of Li dendrites corresponds to the time instant when the surface overpotential reaches a maximum.1 Electrochemical impedance spectroscopy shows that the increase in the surface overpotential preceding this maximum is due to gradual growth of the solid electrolyte interphase (SEI). This SEI layer growth induces Li+ concentration depletion at the electrode-SEI interface, promoting amplification of surface asperities and thus dendrites that rupture the SEI. A transport model incorporating Li+ diffusion across a growing SEI layer is developed, and shown to predict Li dendrite onset times that are in good agreement with experimental observations over a wide range of current densities, temperature, and initial SEI thicknesses.2 A key conclusion of our work is that liquid-phase mass transport limitations, often characterized by the Sand’s equation, do not reliably predict the onset of Li dendrites.3 Rather, transport characteristics and associated instabilities within solid-phase surface films on Li are predominantly responsible for dendrite formation. Our work provides clues for how dendrite formation could be arrested for enabling rechargeable Li-metal anodes for use in next-generation batteries.

Unraveling the heterogeneity of solid electrolyte interphase kinetically affecting lithium electrodeposition on lithium metal anode

The stability and uniformity of solid electrolyte interphase (SEI) are critical for clarifying the origin of capacity fade and safety issues for lithium metal anodes (LMA). However, understanding the interplay of SEI heterogeneity and Li electrodeposition is limited by the coupling of complex electrochemistry and mechanics processes. Herein, the correlation between the SEI failure behavior and Li deposition morphology is investigated through a quantitative electrochemical-mechanical model. The local deformation and stress of SEI during Li electrodeposition identify that the heterogeneous interface between different components first fails. Compared with the well-known mechanical strength, component uniformity plays the most important role in the initial SEI failure and uneven Li deposition, and a relative component uniformity (p> 0.01) represents a proper balance to ensure the stability of the naturally heterogeneous SEI. Furthermore, the component regulation of SEI via the designed electrolyte experimentally demonstrates that improving component uniformity benefits SEI stability and the uniform Li electrodeposition for LMA, thereby increasing the capacity by ∼20% after 300 cycles. These fundamental understandings and proposed strategy can be not only used to guide the SEI optimization via the electrolyte regulation, but also extended to the rational designs of artificial SEI for high-performance LMA.

In Situ Prelithiation by Direct Integration of Lithium Mesh into Battery Cells.

Silicon (Si)-based anodes are promising for next-generation lithium (Li)-ion batteries due to their high theoretical capacity (∼3600 mAh/g). However, they suffer quantities of capacity loss in the first cycle from initial solid electrolyte interphase (SEI) formation. Here, we present an in situ prelithiation method to directly integrate a Li metal mesh into the cell assembly. A series of Li meshes are designed as prelithiation reagents, which are applied to the Si anode in battery fabrication and spontaneously prelithiate Si with electrolyte addition. Various porosities of Li meshes tune prelithiation amounts to control the degree of prelithiation precisely. Besides, the patterned mesh design enhances the uniformity of prelithiation. With an optimized prelithiation amount, the in situ prelithiated Si-based full cell shows a constant >30% capacity improvement in 150 cycles. This work presents a facile prelithiation approach to improve battery performance.

Interfacial Chemistry and Electrolyte Approaches for Enabling Metal Anode Batteries

Continuous solid electrolyte interphase (SEI) and dendrite growth, as well as formation of ion blocking interfaces are some of the crucial issues preventing the commercialization of batteries depending on the implementation of Li/Na/K/Mg/Ca metal anodes.[1,2] In the first part of my talk, I will focus on the chemistry of formed interfaces, including growth and ion transport control. Second part of my talk deals with bulk liquid, polymer and hybrid electrolyte molecular structure approaches for circumventing dendrite formation.Understanding of the SEI growth on alkali and alkaline earth metals in contact with liquid electrolytes under open circuit potential conditions important for shelf aging is still in its infancy. I will show that the SEI formed on Li in contact with glyme-based electrolyte grows via reaction-controlled mechanism that is rapidly substituted by the diffusion-controlled one.[3] The ionic transport and stability of the complex composite SEI material under current is highly dependent on the salt chemistry, with detrimental effects of LixSy and positive effects of the Li3N constituents. Most recent ion transport and morphological studies show that the initial SEI formed in contact with liquid electrolytes on Li, Na and Mg is nanoporous.[4,5] Based on the electrochemical impedance spectroscopy measurements in symmetric cells and related equivalent circuit models, I will discuss potential growth mechanism via SEI densification. Finally, I will present a possibility of using gas/solid synthesis for preparation of sulfide-based artificial SEIs for both liquid-based and solid-state batteries.[6]Dendrite formation on alkali and alkaline earth metal anodes is closely related to the ion concentration gradients formed near the electrodes in contact with the electrolytes. I will discuss the method to circumvent this problem by using electrolytes with high cationic transference number. In soft matter electrolytes this is possible when (i) interfacial effect enables preferential anionic adsorption in liquid/solid electrolytes,[7] (ii) the size and steric effects control the association and mobility of the anion in the liquid electrolytes,[8] (iii) ion transport proceeds through percolating amorphous clusters in polymer-in-salt electrolytes. [9]

(Invited, Digital Presentation) Detailed Chemical Modeling of Solid Electrolyte Interphase Growth and Evolution

The solid electrolyte interphase (SEI) is a layer that forms at the anode-electrolyte interphase in lithium ion batteries. The layer forms due to voltage instability of the electrolyte at low anode potentials, but serves to passivate the electrolyte to protect against further uncontrolled decomposition. In theory, the SEI is self-limiting, but in reality, continued growth over the battery’s lifetime leads to capacity fade, poor rate capability, and eventually cell death. Though significant progress in recent years has improved the SEI’s function and stability, poor understanding of its most basic chemistry impedes “rational design” of SEI layers for advanced battery applications. Understanding and quantifying the elementary chemistry of the SEI is made challenging by the layer’s thickness (10s to ~100 nm), chemical sensitivity, mechanical fragility, and complex chemistry (upwards of 100 reactions have been proposed). For these reasons, These factors combine to make studying the fundamental chemistry of SEI growth and evolution a significant challenge. Both the computational tools to model the SEI’s complex chemistry and the experimental data required to validate such models are all too rare.This talk will provide an overview of recent efforts to model the fundamental electrochemistry of SEI growth and evolution. The simulations are based on a continuum-scale, finite-volume framework, and leverage the open-source software package Cantera to enable efficient simulation of arbitrarily complex electrochemical mechanisms. Simulation results, as in Figure 1, are validated against two operando measurements of the SEI grown on a non-intercalating tungsten anode: depth profiling via neutron reflectometry (NR), and SEI mass uptake data via quartz crystal microbalance with dissipation (QCM-D) taken during cyclic voltammetry cycling. Interrogation of the detailed model results provide insights into the complex phenomena controlling initial SEI growth, and validation against NR and QCM-D data identify prominent reaction pathways and key chemical species present in the SEI. Finally, we will conclude by discussing future steps, includuing transferring mechanistic insights to new scales, geometries, and chemistries, and coupling models with inputs from atomistic simulations. Figure 1

Analysis of Lithium‐Ion Battery State and Degradation via Physicochemical Cell and SEI Modeling

AbstractThe quality of lithium‐ion batteries is affected by the formation of the solid electrolyte interphase (SEI). For a better understanding of its effect on cell performance and aging, fast and economically scalable SEI diagnostics are indispensable. Battery models promise to extract hardly accessible interfacial and bulk properties of the SEI from electrochemical impedance spectra and discharge data. The common analysis of only one measurement, often with empirical models, impedes a precise localization of degradation‐related and performance‐limiting processes. This work offers a solution by combining physicochemical SEI and cell modeling for the joint analysis of both measurement types. Our analysis highlights the minor importance of the SEI ionic conductivity for cell performance along with a significant improvement and notable effect of its interfacial properties along aging. Such a detailed understanding of the initial SEI and its evolution over time could enable, e. g., a knowledge‐based optimization of the cell formation process.

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

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

Mechanistic Insight on the Solid Electrolyte Interphase (SEI) Formed By a Superconcentrated [Li][TFSI] in Acetonitrile Electrolyte Near Lithium Metal

Superconcentrated salt solutions are promising alternatives to conventional electrolytes in Li-ion batteries because of their enhanced redox stability and tendency to form a stable and rigid solid electrolyte interphase (SEI). Herein, we performed density functional theory-based molecular dynamics (DFT-MD) simulations to understand the initial electrode reactions between the Li metal and superconcentrated electrolyte solution of Li bis(trifluoromethanesulfonyl)imide ([Li][TFSI]) in acetonitrile (AN). We thoroughly analyzed the structure and composition of the SEI formed near the Li metal. We show that reductive decomposition of the TFSI anions contributes majorly in the initial SEI formation and the AN molecules are relatively stable against the redox processes occurring near the Li metal. The atomized C, O, F, and S atoms contributed by the TFSI anions forms the stable inorganic layer leading to highly structured and inhomogeneous SEI. The AN molecules and undissociated N–SO2–CF3 fragments are mostly aligned away from the Li layers. Figure 1