Formation Of Stable Solid Electrolyte Interphase Research Articles

Regulating Electric Double Layer via Self‐Assembled Monolayer for Stable Solid/Electrolyte Interphase on Mg Metal Anode

Modulating the cationic solvation structure with high donor‐number (DN) additive is an effective strategy to construct a stable solid electrolyte interphase (SEI) on the Mg anode, which necessitates meticulous consideration of electrolyte chemistry and substantial quantities of additives. Notably, the electric double layer (EDL) adjacent to anode is pivotal role in SEI formation yet remains understudied. In this study, we propose a novel self‐assembled monolayer (SAM) strategy by utilizing (trifluoromethyl)trimethylsilane (TFTMS, only 2 vol%, 1.4 mol%, 0.135 mol L‐1), a low DN but strong Mg metal absorbability through electron‐withdrawing groups(−CF3), to modulate the complex structure of the Helmholtz absorption plane. Through dipole−dipole interaction between the electronegative fluorine (−CF3 of TFTMS) and electropositive hydrogen (−CH3/−CH2 of solvents), TFTMS weakens solvent−Mg2+ coordination and thus enhances anion−Mg2+ interaction, inducing a stable and dense SEI with superior electronic insulation to realize horizontal Mg plating/stripping. Consequently, the designed electrolytes enable the symmetric cells to cycle stable for up to 1000 hours and reversible Mg plating/stripping under 5 mA cm‐2, they show favorable compatibility with various cathodes. More importantly, the SAM strategy has good generality in other electrolyte systems, demonstrating a simple and effective route to promote the practical application of rechargeable magnesium battery.

Synergy of Artificial SEI and Electrolyte Additive for Improved Performance of Silicon Electrodes in Li-Ion Batteries.

Maintaining the electrochemically and mechanically stable solid electrolyte interphase (SEI) is of highest importance for the performance of high-capacity anode materials such as silicon (Si). Applying flexible Li-ion permeable coatings to the electrode surface using molecular layer deposition (MLD) offers a strategy to improve the properties of the SEI and greatly contributes to an increase in the cycle life and capacity retention of Si electrodes. In this study, the long-term cycling of Si electrodes with an MLD alucone coating is investigated in the context of more stable SEI formation. When the joined strategy introducing both MLD coating and anFEC electrolyte additive was realized, high performance of Si anodes was achieved, capable of delivering more than 1500 mAh g-1 even after 400 cycles. The reason for the significantly improved longevity is the ability of the alucone layer to react with HF present in LiPF6-based electrolytes already under OCV-like conditions, fluorinating most of the available -OH groups in the alucone structure. This reaction not only partially scavenges hydrofluoric acid but also does not disturb the confining effect of alucone-like fluorinated artificial SEI. This study shows the significance of searching for synergetic solutions, such as a combination of electrode surface modification and electrolyte composition, for maximizing the capacity retention of Si as an active material or as a capacity-enhancing additive to graphite electrodes, and as well can be applied to other high-energy battery materials with large volume changes during cycling.

From Bulk to Nanosheet: How Tellurene Can Influence the Performance of Li‐S Batteries on Both Electrodes

AbstractIn the face of growing environmental challenges and the need for alternative energy storage, lithium‐sulfur (Li‐S) batteries stand out for their high theoretical energy density and sulfur's eco‐friendliness. However, the practical application of Li‐S batteries faces significant obstacles, particularly at the cathode and anode. This study explores the transformative impact of 2D tellurene, focusing on how its dimensional variations influence Li‐S battery performance. At the cathode, tellurene not only improves the conductivity and sulfur utilization but also accelerates the conversion of lithium polysulfides (LiPSs). Its reduced size, compared to the bulk counterpart, offers numerous active sites for efficient Li‐ion migration and promotes LiPSs decomposition. At the anode, the protective polytellurosulfides are formed, thereby establishing a stable solid‐electrolyte interphase (SEI) and enhancing reversible lithium deposition. Diminishing the dimensions of tellurene improves its catalytic interface, leading to faster polysulfide conversion kinetics at the cathode and robust SEI formation at the anode. Li‐S batteries equipped with ultra‐thin tellurene‐modified separators demonstrate exceptional performance, exhibiting high areal capacity and significantly reduced capacity decay. Computational simulations further validate the advantages of optimizing tellurene dimensions, confirming its essential role in LiPSs decomposition and overall battery efficiency.

LiF-Rich Solid Electrolyte Interphase Formation by Establishing Sacrificial Layer on the Separator.

The formation of a stable solid electrolyte interphase (SEI) layer is crucial for enhancing the safety and lifespan of Li metal batteries. Fundamentally, a homogeneous Li+ behavior by controlling the chemical reaction at the anode/electrolyte interface is the key to establishing a stable SEI layer. However, due to the highly reactive nature of Li metal anodes (LMAs), controlling the movement of Li+ at the anode/electrolyte interface remains challenging. Here, an advanced approach is proposed for coating a sacrificial layer called fluorinated self-assembled monolayer (FSL) on a boehmite-coated polyethylene (BPE) separator to form a stable SEI layer. By leveraging the strong affinity between the fluorine functional group and Li+, the rapid formation of a LiF-rich SEI layer in the cell production and early cycling stage is facilitated. This initial stable SEI formation promotes the subsequent homogeneous Li+ flux, thereby improving the LMA stability and yielding an enhanced battery lifespan. Further, the mechanism behind the stable SEI layer generation by controlling the Li+ dynamics through the FSL-treated BPE separator is comprehensively verified. Overall, this research offers significant contributions to the energy storage field by addressing challenges associated with LMAs, thus highlighting the importance of interfacial control in achieving a stable SEI layer.

Thermodynamic Understanding of Formation and Evolution of Solid Electrolyte Interface in Li‐Ion Batteries

AbstractFormation is an essential process in the manufacturing of lithium‐ion batteries and determines the performance of batteries, including life time, rate capability and so forth. The core of formation is to form a stable solid electrolyte interphase (SEI), which profoundly influences interfacial reactions and ion transport within Li‐ion batteries, on the surface of electrode. It is crucial to fully and deeply understand the intricate structure and evolution of SEI, which remains largely uncharted. This paper aims to systematically analyze SEI thermodynamics, encompassing energy distribution, entropy dynamics during SEI formation, and enthalpy changes. SEI evolves towards an equilibrium state, maximizing entropy. The intricate relationship between SEI thermodynamics and battery performance is established by dissecting the impacts of temperature and current density on SEI thermodynamic parameters and battery characteristics. Elevated temperatures and higher current densities foster the development of a more porous and high‐entropy outer SEI layer, rendering organic SEI constituents prone to decompose into high‐energy gases and low‐energy inorganic compounds. Notably, a fast formation strategy within a high state‐of‐charge range offers a potential solution to high quality SEI. This review strives to bridge the divide between fundamental thermodynamics and practical battery performance, ultimately contributing to the advancement of battery manufacturing.

Design of dual carbon encapsulated porous micron silicon composite with compact surface for enhanced reaction kinetics of lithium-ion battery anodes

Developing high-performance composites with fast charging and superior cycle life is paramount for lithium-ion batteries (LIBs). Herein, we synthesized a double-shell carbon-coated porous structure composite with a compact surface (P-Si@rGO@C) using low-cost commercial micron-sized silicon (Si) instead of nanoscale silicon. Results reveal that the unique P-Si@rGO@C features high adaptability to volume expansion, accelerates electron/ion transmission rate, and forms a stable solid electrolyte interphase (SEI) film. This phenomenon arises from the synergistic effect of abundant internal voids and an external double-layer carbon shell with a dense surface. Specifically, the P-Si@rGO@C anode exhibits a high initial coulombic efficiency (ICE) (88.0 %), impressive rate-capability (612.1 mAh/g at 2C), and exceptional long-term cyclability (972.2 mAh/g over 500 cycles at 0.5C). Further kinetic studies elucidate the diffusion-capacitance hybrid energy storage mechanism and reveal an improved Li+ diffusion coefficient (from 3.47 × 10-11 to 2.85 × 10-9 cm2 s-1). Ex-situ characterization confirms the crystal phase change of micron-sized Si and the formation of a stable LiF-rich SEI. Theoretical calculations support these findings by demonstrating an enhancement in the adsorption ability of Si to Li+ (from -0.89 to -0.97 eV) and a reduction in the energy migration barrier (from 0.35 to 0.18 eV). Additionally, practical LixSi powder is shown to increase the ICE of full cells from 67.4 % to 87.9 %. Furthermore, a pouch cell utilizing the prelithiated P-Si@rGO@C anode paired with LiNi1/3Co1/3Mn1/3O2 (NCM111) cathode delivers a high initial reversible capacity of 7.2 mAh and 76.8 % capacity retention after 100 cycles. This work provides insights into the application of commercial silicon-aluminum alloy powder in the anode of high-energy LIBs.

Stoichiometric Ti3C2Tx Coating for Inhibiting Dendrite Growth in Anode‐Free Lithium Metal Batteries

Lithium metal batteries (LMBs) and anode‐free LMBs (AFLMBs) present a solution to the need for batteries with a significantly superior theoretical energy density. However, their adoption is hindered by low Coulombic efficiency (CE) and rapid capacity fading, primarily due to the formation of unstable solid electrolyte interphase (SEI) layer and Li dendrite growth as a result of uneven Li plating. Here, we report on the use of a stoichiometric Ti3C2Tx (S‐Ti3C2Tx) MXene coating on the copper current collector to enhance the cyclic stability of an anode‐free lithium metal battery. The S‐Ti3C2Tx coating provides abundant nucleation sites, thereby lowering the overpotential for Li nucleation, and promoting uniform Li plating. Additionally, the fluorine (−F) termination of S‐Ti3C2Tx participates in the SEI formation, producing a LiF‐rich SEI layer, vital for stabilizing the SEI and improving cycle life. Batteries equipped with S‐Ti3C2Tx@Cu current collectors displayed reduced Li consumption during stable SEI formation, resulting in a significant decrease in capacity loss. AFLMBs with S‐Ti3C2Tx@Cu current collectors achieved a high initial capacity density of 4.2 mAh cm−2, 70.9% capacity retention after 50 cycles, and an average CE of 98.19% in 100 cycles. This innovative application of MXenes in the energy field offers a promising strategy to enhance the performance of AFLMBs and could potentially accelerate their commercial adoption.

Correlation between surface/bulk properties and electrochemical cycling performance of pure silicon anode materials

State-of-the-art lithium-ion batteries (LIBs) employ silicon (Si)-graphite composite anode with just a few percentages of Si active material, whose cycle-life relies on a stable solid electrolyte interphase (SEI) layer. As a number of various types of industrial Si active materials are currently available, an informative guideline based on the material properties and resultant cycling performance of different pure Si (i.e., without graphite) anode materials is strongly needed for material selection. The present study provides for the first time the correlation among material's surface and bulk properties, the SEI formation behavior, and cycling behavior of highly loaded pure Si anodes (2.3 mAh cm−2) prepared from four different industrial active materials, which are SiOx, carbon-coated Si, and bare Si with submicron and micron sizes. Commercial electrolyte with 10 wt% fluoroethylene carbonate additive is used for electrochemical performance evaluation. Based on solid-state NMR, XPS, and electrochemical analysis results, SiOx is determined to be relatively beneficial for practical applications beyond Si materials, in the aspects of high structural and particle morphology stability, cycling stability and suppressed interfacial resistance. This is mainly because of more effective accommodation of volume change of Si and oxygen-enriched surface (SEI) stability. The data give insight into the rational design of well-working Si-dominant or pure Si anodes for high energy density LIBs.

(Invited) Modeling of the Electric Double Layer (EDL) at Li/SEI/Electrolyte Interfaces

Liquid electrolytes, consisting of salts, solvents, and additives, must form a stable solid electrolyte interphase (SEI) to ensure the performance and durability of lithium(Li)-ion batteries. However, the electric double layer (EDL) structure near charged surfaces is still unsolved, despite its importance in dictating the species being reduced for SEI formation near a negative electrode. Recently, we have developed an interactive Molecular Dynamics -Density Functional Theory -data statistics (MD-DFT-data) model to investigate the reduction reactions of multicomponent electrolytes within the EDL. We will illustrate the effect of EDL on SEI formation in two essential electrolytes, the carbonate-based electrolyte for Li-ion batteries and the ether-based electrolyte for batteries with Li-metal anodes. Our results reveal that the role of fluoroethylene carbonate (FEC) additive differs drastically in the two electrolytes as an SEI modifier to form the beneficial F-containing SEI component (e.g., LiF). The competition among the cations, anions, and various species in the solvents with a charged surface at different temperatures can all jointly determine the EDL structure and therefore the SEI compositions. [1]While the classical Poisson–Boltzmann EDL model developed for fully solvated ions face new challenges in high-concentration liquid electrolytes (HCE), localized high-concentration liquid electrolytes (LHCE) as well as solid electrolytes (SE), new theoretical developments are required. We will introduce a new DEL model in SE and SEI by solving the DFT-informed Poisson–Fermi–Dirac equation and demonstrate how it can be used for interlayer thickness design. [2][1] Wu, Q.S, McDowell, M.T., & Qi, Y., Journal of the American Chemical Society 145 (4), 2473-2484 (2023)[2] Swift, M.W., Swift, J.W. & Qi, Y. Modeling the electrical double layer at solid-state electrochemical interfaces. Nat Comput Sci 1, 212–220 (2021)

(Invited) Modeling of the Bulk and Electrical Double-Layer Structures of Localized High-Concentration Electrolytes

A new class of electrolytes, localized high-concentration electrolytes (LHCE), have shown many benefits to high-capacity electrodes (lithium (Li)-metal, silicon, sodium, zinc, and potassium).[1] These electrolytes combine high-concentration electrolytes (HCE) with salt and solvent with low-viscosity diluents. The dilutes were added to increase ionic conductivity while the locally highly concentrated salt-solvent clusters will facilitate stable solid-electrolyte interphase (SEI) formation while preventing metallic-dendrite formation. For example, it was demonstrated that an LHCE based on lithium bis(fluorosulfonyl)imide (LiFSI) salt, Dimethoxyethane (DME) solvent, and tris(2,2,2-trifluoroethyl)orthoformate (TFEO) diluent (e.g., LiFSI-1.2DME-2TFEO by mol) forms inorganic-rich, monolithic SEI and shows excellent cycling performance for Li-metal batteries under practical conditions.[2] Here, the interactions among cations, anions, solvent molecules, and diluent molecules create a complex design space, making the trial-and-error empirical approach less efficient.Using density functional theory and classical molecular dynamics simulations, we reveal several important design principles:a) The dielectric constant of the pure solvent or diluent is not a good descriptor to describe the solubility of the salt in solvent and diluent.b) The single ion–solvent (diluent) binding energy is insufficient to predict the solubility, which also depends on the molecular structures and the corresponding solvation shell structure.c) The salt-solvent, salt-diluent, and diluent-solvent solubilities jointly determine the solvation structures in LHCE. For example, at certain concentrations, the Li+ solvation shells contain both anions and solvent molecules with minimal participation of the diluent molecules. The local salt concentration can be even higher than its counterpart in the HCE. These simulated solvation structures in bulk electrolytes compare well with Raman spectrum and SAXS data.d) Moreover, the solvation structures are temperature-sensitive. We revealed why the local salt concentration is higher at a certain temperature by MD simulations, which is further validated through Raman deconvolution analyses.e) The SEI formation is sensitive to the electrolyte structure near a charged surface. Therefore, the electrical double-layer (EDL) structure of LHCE needs to be resolved. Here we give a statistical representation of the LHCE in the EDL and show how it differs from the bulk structure and its impact on SEI formation.Reference:[1] J. Electrochem. Soc., 2021, 168, 010522[2] Nat. Energy 2019, 4, 796-805

Investigation of Gas Evolution by Operando GC/MS from SEI Forming Additives in Lithium-Ion-Batteries

Due to the chemical reactivity of the electrolyte, electrolyte decomposition often occurs at the electrode-electrolyte interface during cycling, which can lead to a loss in LIB performance. On the anode side, however, the formation of a stable Solid Electrolyte Interphase (SEI) is essential for the safe operation of Lithium-ion batteries (LIBs). The SEI layer, which is permeable for Li+ ions, protects the electroactive materials from direct contact with the electrolyte, thereby inhibiting further electrolyte decomposition. In the formation cycles of fresh cells, electrolyte degradation is primarily attributed to the SEI formation processes at the anode. Several literature reports have confirmed that the newly formed interphase consists of organic and inorganic decomposition products of the electrolyte.1 To increase battery performance, cycle life and safety, it is essential to form a stable SEI.2 A common strategy to stabilize the SEI is to use additives in the electrolyte composition. The additives should exhibit higher reduction potentials than other electrolyte components to enable preferential reduction and incorporation into the SEI. The most common SEI forming additives are fluoroethylene carbonate (FEC) and vinylene carbonate (VC). They both decompose during the formation cycle to form a polymeric layer at the electrode surface.The electrolyte decomposition reactions which lead to SEI formation are usually accompanied by gaseous side products. The evolving gas species form a complex gas mixture which can be separated by gas chromatography (GC) into their individual components and subsequently identified by mass spectrometry (MS). While operando GC/MS gives information on the volatile side products of the electrolyte decomposition reactions, X-ray photoelectron spectroscopy (XPS) can be used to identify the degradation products that form on the surface of the electrode and are incorporated into the SEI. The combination of the two techniques allows a more comprehensive picture of SEI formation processes to be drawn.In this work, operando GC/MS of Lithium-ion full cells with NMC 811 cathodes and graphite anodes was used to compare the gas species evolving during the formation reactions in an electrolyte with the composition of 1M LiPF6 in EC/DEC 1:1 with FEC and VC electrolyte additives in a ratio of 1w%, respectively. In a comparative study we present the decomposition pathways of the electrolytes with and without SEI forming additives by analysing the composition of the gas phase as a function of cell potential. We observe that the complex gas mixture consists of carbonate components originating from transesterification reactions and evaporation of the volatile electrolyte. In addition, inorganic components such as carbon monoxide and carbon dioxide are produced, which mainly stem from the degradation of cyclic carbonates. Furthermore, the gas phase consists of saturated and unsaturated hydrocarbons which can be ascribed to the linear carbonate components as well as ether and carbonyl components. We have also observed that it is possible to distinguish between volatile compounds which form during the charge and discharge processes, respectively. The increased gas evolution at the beginning of the formation cycle confirms that a substantial part of the passivation layer is formed during charge. Finally, operando GC/MS was also used to monitor the evolution of the decomposition products during battery operation at overcharge conditions.Acknowledgement:The author gratefully acknowledges the FFG (Austrian Research Promotion Agency) for funding this research within project No. 879613

Recent Advances in Rechargeable Metal-CO2 Batteries with Nonaqueous Electrolytes.

This review article discusses the recent advances in rechargeable metal-CO2 batteries (MCBs), which include the Li, Na, K, Mg, and Al-based rechargeable CO2 batteries, mainly with nonaqueous electrolytes. MCBs capture CO2 during discharge by the CO2 reduction reaction and release it during charging by the CO2 evolution reaction. MCBs are recognized as one of the most sophisticated artificial modes for CO2 fixation by electrical energy generation. However, extensive research and substantial developments are required before MCBs appear as reliable, sustainable, and safe energy storage systems. The rechargeable MCBs suffer from the hindrances like huge charging-discharging overpotential and poor cyclability due to the incomplete decomposition and piling of the insulating and chemically stable compounds, mainly carbonates. Efficient cathode catalysts and a suitable architectural design of the cathode catalysts are essential to address this issue. Besides, electrolytes also play a vital role in safety, ionic transportation, stable solid-electrolyte interphase formation, gas dissolution, leakage, corrosion, operational voltage window, etc. The highly electrochemically active metals like Li, Na, and K anodes severely suffer from parasitic reactions and dendrite formation. Recent research works on the aforementioned secondary MCBs have been categorically reviewed here, portraying the latest findings on the key aspects governing secondary MCB performances.

Preferential decomposition of the major anion in a dual-salt electrolyte facilitates the formation of organic-inorganic composite solid electrolyte interphase.

The performance of a lithium metal battery (LMB) with liquid electrolytes depends on the realization of a stable solid electrolyte interphase (SEI) on the Li anode surface. According to a recent experiment, a high-concentrated (HC) dual-salt electrolyte is effective in modulating the SEI formation and improving the battery performance. However, the underlying reaction mechanism between this HC dual-salt electrolyte and the lithium metal anode surface remains unknown. To understand the SEI formation mechanism, we first performed 95ps ab initio Molecular Dynamics (AIMD) simulation and then extend this AIMD simulation to another 1ns by using Hybrid ab Initio and Reactive Molecular Dynamics (HAIR) to investigate the deep reactions of such dual-salt electrolytes consists of lithium difluorophosphate and lithium bis(trifluoromethanesulfonyl)imide in dimethoxyethane (DME) solvent at lithium metal anode surface. We observed the detailed reductive decomposition processes of DFP- and TFSI-, which include the formation pathway of CF3 fragments, LiF, and LixPOFy, the three main SEI components observed experimentally. Furthermore, after extending the simulation to 1.1ns via the HAIR scheme, the decomposition reactions of DME solvent molecules were also observed, producing LiOCH3, C2H4, and precursors of organic oligomers. These microscopic insights provide important guidance in designing the advanced dual-salt electrolytes for developing high-performance LMB.

A Nonflammable High-Voltage 4.7 V Anode-Free Lithium Battery.

Anode-free lithium-metal batteries employ in situ lithium-plated current collectors as negative electrodes to afford optimal mass and volumetric energy densities. The main challenges to such batteries include their poor cycling stability and the safety issues of the flammable organic electrolytes. Here, a high-voltage 4.7V anode-free lithium-metal battery is reported,which usesa Cu foil coated with a layer (≈950nm) of silicon-polyacrylonitrile (Si-PAN, 25.5µg cm-2 ) as the negative electrode, a high-voltage cobalt-free LiNi0.5 Mn1.5 O4 (LNMO) as the positive electrode and a safe, nonflammable ionic liquid electrolyte composed of 4.5m lithium bis(fluorosulfonyl)imide (LiFSI) salt in N-methyl-N-propyl pyrrolidiniumbis(fluorosulfonyl)imide (Py13 FSI) with 1 wt% lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as additive. The Si-PAN coating is found to seed the growth of lithium during charging, and reversibly expand/shrink during lithium plating/stripping over battery cycling. The wide-voltage-window electrolyte containing a high concentration of FSI- and TFSI- facilitates the formation of stable solid-electrolyte interphase, affording a 4.7V anode-free Cu@Si-PAN/LiNi0.5 Mn1.5 O4 battery with a reversible specific capacity of ≈120 mAh g-1 and high cycling stability (80% capacity retention after 120 cycles). These results represent the first anode-free Li battery with a high 4.7V discharge voltage and high safety.

Towards a Better Understanding of Redox Shuttle Generation in Lfp/Graphite and NMC811/Graphite Cells By Systematic Investigation of Different Electrolyte Additives

Recent observations by our group show the creation of a reversible shuttle species in LFP/graphite and NMC811/graphite cells with 3:7 ethylene carbonate:dimethyl carbonate (EC:DMC) based electrolytes. This is indicated by a high reversible self-discharge of these cells in the absence of electrolyte additives. Electrolyte extraction from pouch cells after formation allowed to directly investigate the electrolytes for redox shuttle currents. For this purpose, the extracted electrolytes were inserted into coin cells with an Al foil as the working electrode (WE) and a Li foil as the counter electrode (CE). The measured cyclic voltammetry (CV) of the coin cells show a clear relationship between high formation temperature and high shuttle currents. Interestingly, the addition of vinylene carbonate (VC) to the electrolyte completely prevents the shuttle current, even at elevated formation temperatures. [1]In this study, we systematically investigate the effect of various electrolyte additives on the generation of shuttle molecules. LFP/graphite and NMC811/graphite pouch cells were filled with electrolyte consisting of 3:7 EC:DMC with 1.5 M lithium hexafluorophosphate (LiPF6) and different additives. The pouch cells were formed at different temperatures, TF. The electrolytes were then extracted and inserted into the aforementioned coin cell setup for CV measurements.We have found that additives such as VC, fluoroethylene carbonate (FEC), ethylene sulfate (DTD), prop-1-ene-1,3-sultone (PES), and triallyl phosphate (TAP), which are known to create a stable solid electrolyte interphase (SEI), [2-4] prevent shuttle current in the CV. On the other hand, additives such as succinonitrile (SN) and trimethylsilyl isothiocyanate (TMSNCS), which do not contribute to the formation of a better SEI, [5,6] cannot prevent the shuttle current. This suggests that the formation of the shuttles is due to a poor SEI and therefore occurs at the interface between electrolyte and graphite anode.Analogue experiments with DMC as only solvent instead of 3:7 EC:DMC show similar shuttle currents in CVs, which suggests that linear carbonates such as DMC are required to form the shuttle.Figure 1 shows CVs for 1.5 M LiPF6 DMC electrolyte. The shuttle current appears to be the same for electrolyte extracted from LFP/graphite and NMC811/graphite cells ranging up to 6 μA in both cases. This indicates that the shuttle is formed independently of the cathode material, and therefore gives rise to the hypothesis that it is formed at the anode-electrolyte interface. Figure 1 also shows that the shuttle current increases with higher formation temperatures TF.

Integrating SEI into Layered Conductive Polymer Coatings for Ultrastable Silicon Anodes.

Tackling the huge volume expansion of silicon (Si) anode desires a stable solid electrolyte interphase (SEI) to prohibit the interfacial side reactions. Here, a layered conductive polyaniline (LCP) coating is built on Si nanoparticles to achieve high areal capacity and long lifespan. The conformal LCP coating stores electrolyte in interlamination spaces and directs an in situ formation of LCP-integrated hybrid SEI skin with uniform distribution of organic and inorganic components, enhancing the flexibility of the SEI to buffer the volume changes and maintaining homogeneous ion transport during cycling. As a result, the Si anode shows a remarkable cycling stability under high areal capacity (≈3mAhcm-2 ) after 150 cycles and good rate performance of 942mAhg-1 at 5 A g-1 . This work demonstrates the great potential of regulating the SEI properties by a layered polymer-directing SEI formation for the mechanical and electrochemical stabilization of Si anodes.

Multiscale Simulation of Solid Electrolyte Interface Formation in Fluorinated Diluted Electrolytes with Lithium Anodes.

Lithium metal batteries (LMBs) hold great promise in facilitating high-energy batteries due to their merits such as high specific capacity, low reduction potential, and so forth. However, the realizations of practical LMBs are hindered by severe problems such as undesirable dendrite growth, poor Coulombic efficiency, and so forth. A recently proposed fluorinated electrolyte based on 1 M lithium bis(fluorosulfonyl)imide (LiFSI) dissolved in designed fluorinated 1,4-dimethoxybutane (FDMB) solvent has attracted significant attention because of its excellent electrochemical performance that origins from its superior physical and chemical properties, especially its unique ability in forming a robust, stable solid electrolyte interphase (SEI). However, the detailed structure and reaction mechanism of the SEI formation in such a novel electrolyte remains unclear. In this work, we carry out a hybrid ab initio and reactive molecular dynamics (HAIR) simulation to investigate the elementary reactions that regulate the formation of the primitive SEI, paying special attention to the process that involves FDMB, the fluorinated solvent. HAIR simulation reveals that both FSI- anion and FDMB provide F that is adequate to form a uniformed LiF layer that resembles the inorganic inner layer (IIL) of the SEI. N and S radicals from the FSI- anion, which do not deposit on the electrode interface to form lithium-containing inorganic substances, promote the polymerization reaction of unsaturated carbon chains produced by FDMB defluorination, forming the organic outer layer (OOL) of the SEI. The combination of the LiF-rich IIL and polymer-rich organic OOL explains the superior performance of the FDMB-based electrolyte in the device. The detailed reaction mechanism and SEI observed in this work provide insights into the atomic scale for the rational design of F-rich electrolytes in the near future.

Engineering and characterization of interphases for lithium metal anodes.

Lithium metal is a very promising anode material for achieving high energy density for next generation battery systems due to its low redox potential and high theoretical specific capacity of 3860 mA h g−1. However, dendrite formation and low coulombic efficiency during cycling greatly hindered its practical applications. The formation of a stable solid electrolyte interphase (SEI) on the lithium metal anode (LMA) holds the key to resolving these problems. A lot of techniques such as electrolyte modification, electrolyte additive introduction, and artificial SEI layer coating have been developed to form a stable SEI with capability to facilitate fast Li+ transportation and to suppress Li dendrite formation and undesired side reactions. It is well accepted that the chemical and physical properties of the SEI on the LMA are closely related to the kinetics of Li+ transport across the electrolyte–electrode interface and Li deposition behavior, which in turn affect the overall performance of the cell. Unfortunately, the chemical and structural complexity of the SEI makes it the least understood component of the battery cell. Recently various advanced in situ and ex situ characterization techniques have been developed to study the SEI and the results are quite interesting. Therefore, an overview about these new findings and development of SEI engineering and characterization is quite valuable to the battery research community. In this perspective, different strategies of SEI engineering are summarized, including electrolyte modification, electrolyte additive application, and artificial SEI construction. In addition, various advanced characterization techniques for investigating the SEI formation mechanism are discussed, including in situ visualization of the lithium deposition behavior, the quantification of inactive lithium, and using X-rays, neutrons and electrons as probing beams for both imaging and spectroscopy techniques with typical examples.

Rechargeable magnesium batteries enabled by conventional electrolytes with multifunctional organic chloride additives

The development of a conventional electrolyte, based on commercially available magnesium (Mg) salts in organic solvents, remains one of the most challenging quests for rechargeable Mg batteries. Conventional electrolytes typically form a passivation layer on Mg metal anode, necessitating the extensive use of inorganic chloride additives such as MgCl2. Herein, for the first time, we introduce an organic chloride, tetrabutylammonium chloride (TBAC), as a multifunctional electrolyte additive for conventional magnesium triflate (Mg(OTf)2)-based electrolytes. TBAC was found to perform three major roles: (1) enhance Mg(OTf)2 dissociation in aprotic solvent, (2) inhibit the reduction of triflate anions through surface-adsorbed TBA+, and (3) act as a chloride source to form Mg complexes and stabilize the Mg anode-electrolyte interface. The novel electrolyte combination of TBAC and Mg(OTf)2 in 1,2-dimethoxyethane exhibits excellent Mg plating/stripping with Coulombic efficiency of 97.7% over 200 cycles at 0.5 mA cm−2 and 0.5 mAh cm-2. Postmortem analysis unveils uniform Mg deposition and stable solid electrolyte interphase formation on Mg anode, which enables reversible cycling with Mo6S8 cathode. Together with our findings on another organic chloride additive, 1-ethyl-3-methylimidazolium chloride, and in combination with different electrolyte salts and solvents, we showcase the general promise of high-performance conventional electrolytes for rechargeable Mg batteries.

Revisiting the Ethylene Carbonate–Propylene Carbonate Mystery with Operando Characterization

AbstractThe “ethylene carbonate (EC)–propylene carbonate (PC) mystery” has puzzled electrochemists for decades. Surprisingly, the minor structural difference between PC and EC, a methyl vis‐à‐vis a proton, prevents PC unlike EC to form a stable solid electrolyte interphase (SEI) on carbon (C), which along with the popularity of PC has impeded the development of Li‐ion batteries with many years. Despite several hypotheses, the fundamental reason remains debated largely due to the lack of sufficient experimental evidence. Herein, SEI formed as a result of EC and PC reductions are analyzed by two state‐of‐the‐art operando techniques, online electrochemical mass spectrometry and electrochemical quartz crystal microbalance with dissipation monitoring. Although both EC‐ and PC‐based electrolytes appear to have virtually identical reaction pathways, PC is reduced much more extensively than EC and forms a much thicker SEI. However, while the SEI derived from EC remains on the electrode, PC reduction products redissolve in the electrolyte leaving the bare C electrode behind. The presented study illustrates the complex scheme of competing electro‐/chemical reactions behind SEI formation and provides further scientific details needed to eventually form a consensus of the processes governing electrode/electrolyte interphases in Li‐ion batteries.