Solid Electrolyte Interphase Formation Research Articles

Tackling issues of lithium metal anodes with a novel polymeric lithicone coating

Lithium metal (Li) has been hindered from as anodes in commercial batteries for over 50 years, due to two main issues: continuous formation of solid electrolyte interphase (SEI) and lithium dendritic growth. In this work, we report a new strategy to tackle these issues, i.e., using molecular layer deposition (MLD) to grow an ionically conducting but electrically insulating polymeric lithicone coating, an Li-containing triethanolamine (LiTEA). Our electrochemical tests revealed that this LiTEA coating could serve as an exceptional protection layer over Li anodes. Consequently, the LiTEA-coated Li electrodes could achieve a superior cyclability of > 10000 Li stripping/plating cycles at a current density of 5 mA cm−2 and a long cyclability of > 5500 cycles at 2 mA cm−2 in Li||Li symmetric cells without failures, under a fixed areal capacity of 1 mAh cm−2. Characterizations using scanning electron microscopy and X-ray photoelectron microscopy verified that Li+ ions could be easily extracted through and deposited under the LiTEA coating during the stripping/plating processes. Consequently, this LiTEA coating significantly inhibited the formation of SEI and Li dendrites. This underlies the long cycling lifetime of the LiTEA-coated Li||Li cells. Coupling with the nickel-rich LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes, more encouragingly, the LiTEA-coated Li anodes could remarkably extend the cyclability and sustainable capacity of the resultant Li||NMC full cells. We also demonstrated that the performance of Li||NMC cells could be further improved through combining an LiTEA-coated Li anode with an Li2S-modified NMC811 via atomic layer deposition (ALD). Thus, this study is inspiring for developing high-energy Li||NMC lithium metal batteries.

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.

Operando Investigation of Solid Electrolyte Interphase Formation, Dynamic Evolution, and Degradation During Lithium Plating/Stripping.

The solid electrolyte interphase (SEI) dictates the stability and cycling performance of highly reactive battery electrodes. Characterization of the thin, dynamic, and environmentally sensitive nature of the SEI presents a formidable challenge, which calls for the use of microscopic, time-resolved operando methods. Herein, we employ scanning electrochemical microscopy (SECM) to directly probe the heterogeneous surface electronic conductivity during SEI formation and degradation. Complementary operando electrochemical quartz crystal microbalance (EQCM) and ex situ X-ray photoelectron spectroscopy (XPS) provide comprehensive analysis of the dynamic size and compositional evolution of the complex interfacial microstructure. We have found that stable anode passivation occurs at potentials of 0.5 V vs Li/Li+, even in cases where anion decomposition and interphase formation occur above 1.0 V. We investigated the bidirectional relationship between the SEI and lithium plating-stripping, finding that plating-stripping ruptures the SEI. The current efficiency of this reaction is correlated to the anodic stability of the SEI, highlighting the interdependent relationship between the two. We anticipate this work will provide critical insights on the rational design of stable and effective SEI layers for safe, fast-charging, and long-lifetime lithium metal batteries.

Interphasial Pre-lithiation and Reinforcement of Micro-Si Anode through Fluorine-free Electrolytes.

Micro-sized silicon (mSi) anodes offer advantages in cost and tap density over nanosized counterparts. However, its practical application still suffers from poor cyclability and low initial and later-cycle coulombic efficiency (CE), caused by the unstable solid electrolyte interphase (SEI) and irreversible lithiation of the surface oxide layer. Herein, a bifunctional fluorine (F)-free electrolyte was designed for the mSi anode to stabilize the interphase and improve the CE. A combined analysis revealed that this electrolyte can chemically pre-lithiate the native oxide layer by the reductive LiBH4 , and relieve SEI formation and accumulation to preserve the internal conductive network. The significance of this F-free electrolyte brings unprecedented F-free interphase that also enables the high-performance mSi electrode (80 wt % mSi), including high specific capacity of 2900 mAh/g, high initial CE of 94.7 % and excellent cyclability capacity retention of 94.3 % after 100 cycles at 0.2 C. This work confirms the feasibility of F-free interphase, thus opening up a new avenue toward cost-advantaged and environmentally friendly electrolytes for more emerging battery systems.

Interphasial Pre‐lithiation and Reinforcement of Micro‐Si Anode through Fluorine‐free Electrolytes

AbstractMicro‐sized silicon (mSi) anodes offer advantages in cost and tap density over nanosized counterparts. However, its practical application still suffers from poor cyclability and low initial and later‐cycle coulombic efficiency (CE), caused by the unstable solid electrolyte interphase (SEI) and irreversible lithiation of the surface oxide layer. Herein, a bifunctional fluorine (F)‐free electrolyte was designed for the mSi anode to stabilize the interphase and improve the CE. A combined analysis revealed that this electrolyte can chemically pre‐lithiate the native oxide layer by the reductive LiBH4, and relieve SEI formation and accumulation to preserve the internal conductive network. The significance of this F‐free electrolyte brings unprecedented F‐free interphase that also enables the high‐performance mSi electrode (80 wt % mSi), including high specific capacity of 2900 mAh/g, high initial CE of 94.7 % and excellent cyclability capacity retention of 94.3 % after 100 cycles at 0.2 C. This work confirms the feasibility of F‐free interphase, thus opening up a new avenue toward cost‐advantaged and environmentally friendly electrolytes for more emerging battery systems.

Engineering Ultrathin Carbon Layer on Porous Hard Carbon Boosts Sodium Storage with High Initial Coulombic Efficiency.

Hard carbon (HC) has been widely adopted as the anode material for sodium ion batteries (NIBs). However, it is troubled by a low initial Coulombic efficiency (ICE) due to its porous structure. Herein, a graphitized and ultrathin carbon layer coating on HC is proposed to solve this challenge. The as-prepared porous carbon material coated with an ultrathin carbon layer composite (PCS@V@C) exhibits a cavity structure, which is prepared by using bis(cyclopentadienyl) nickel (CP-Ni) as the carbon source for outer coating, glucose carbon spheres as porous carbon, and introducing a silica layer to facilitate the coating process. When utilized as the anode for NIBs, the material shows an ICE increase from 47.1% to 85.3%, and specific capacity enhancement at 0.1 A g-1 from 155.3 to 216.7 mA h g-1. Moreover, its rate capability and cycling performance are outstanding, demonstrating a capacity of 140.3 mA h g-1 at 10 A g-1, and a retaining capacity of 225.6 mA h g-1 after 300 cycles at 0.1 A g-1 with the Coulombic efficiency of 100% at the second cycle. The excellent electrochemical performance of the PCS@V@C is attributed to the ultrathin carbon layer, which is beneficial for the formation of a stable solid electrolyte interphase (SEI) film. Therefore, this study provides a feasible surface modification method for the preparation of anode materials for NIBs with high specific capacity and ICE.

Facile Synthesis of a SiOx-Graphite Composite toward Practically Accessible High-Energy-Density Lithium-Ion Battery Anodes.

SiOx-based material is a promising candidate for lithium-ion batteries (LIBs) owing to its high theoretical capacity. The inherent disadvantages of poor electronic conductivity and large volume variation can be solved by constructing the outermost carbon layer and reserving internal voids. However, the practical application of SiOx/C composites remains a great challenge due to the unsatisfactory energy density. Herein, we propose a facile synthetic approach for fabricating SNG/H-SiOx@C composites, which are constructed by amorphous carbon, hollow SiOx (H-SiOx), and spherical natural graphite (SNG). H-SiOx alleviates volume expansion, while amorphous carbon promotes Li+ migration and stable solid electrolyte interphase (SEI) formation. The as-prepared SNG/H-SiOx@C demonstrates a high reversible capacity (465 mAh g-1), excellent durability (93% capacity retention at 0.5C after 500 cycles), lower average delithiation potential than SNG (0.143 V after 500 cycles), and a 14% gravimetric energy density improvement at a loading level of 4.5 mg cm-2. Even at a compacted density of 1.5 g cm-3, the SNG/H-SiOx@C anode presents a modest volume deformation of 14.3% after 100 cycles at 0.1C.

A quasi-solid state polymer electrolyte for high-rate and long-life sodium-metal batteries

High-energy rechargeable sodium metal batteries, featuring quasi-solid polymer electrolytes (QSSPEs), hold the promise of improved safety. However, the low ionic conductivity and limited cyclability of QSSPEs at room temperature have paralyzed the efforts to develop energy-dense metal anode batteries. Herein, we report a QSSPE comprising a blend of polyvinyl pyrrolidone (PVP) and polyethylene glycol (PEG). Notably, the porosity and the electrolyte uptake capacity of the QSSPE are highly responsive to the concentration of PEG. The optimized PVP-based QSSPE (PVP-PEG 70 30) shows a high ionic conductivity of 1.06 mS cm−1 at room temperature, along with a high transference number of approximately 0.81. A sodium symmetric cell using PVP-PEG 70 30 demonstrates reversible stripping/plating of sodium for over 700 h at a current density of 1 mA cm−2. Furthermore, PVP-PEG 70 30 alters the localized chemistry of the solid electrolyte interphases (SEIs), facilitating the formation of fluorine-rich SEIs. When coupled with a Prussian blue cathode and sodium metal anode, a full cell employing PVP-PEG 70 30 exhibits an initial discharge capacity of 168 mAh g−1, with an average discharge capacity of 125 mAh g−1 for over 600 cycles at 0.5C. Remarkably, this cell can be cycled stably and reversibly at a high rate of 6.5C. These results suggest that PVP-based QSSPE holds great promise as an electrolyte candidate for high-performance sodium metal batteries.

Perception of insight in the formation of solid electrolyte interphase

While the solid electrolyte interphase (SEI) film in carbonate-based lithium-ion batteries (LIBs) has been extensively studied, less is known about the formation of SEI due to the complexity and instability of interfacial chemical reactions between Li+ and solvent. This study presents new insights into the formation mechanism of SEI film on graphite anode in ethylene carbonate (EC) and dimethyl carbonate (DMC) co-solvent-based electrolytes. The potential-resolved in-situ electrochemical impedance spectroscopy technology and density functional theory calculations combined with multi-sweep cyclic voltammetry are utilized to confirm that EC prefers to solvate with Li+. Then, a large amount of Li+-EC and a small amount of Li+-DMC solvent molecules undergo solvent migration process and de-solvation at the electrode/electrolyte interface. It was found that Li+-DMC is preferentially de-solvation and reduced to form an unstable SEI film, while Li+-EC inhibits the reduction of DMC and turns the reduction process to be diffusion-controlled. This work provides a fresh perspective on the formation of SEI film by Li+-solvent interaction, which could guide the construction of a stable interface by regulating the Li+-solvation kinetic for high-performance LIBs.

Nanowire sulfide/carbon composite with high electrochemical performance in potassium-ion batteries

Transition metal chalcogenides (TMCs) have demonstrated great potential in energy storage devices due to their versatile structures and composite functionalities. However, the application of TMCs in potassium-ion batteries (PIBs) suffers from the issues of large volume expansion, polysulfide dissolution, and sluggish kinetics. To overcome these challenges, this work develops nano-flower-like MnS-Co3S4 confined in poly-pyrrole (PPY) carbon nanotube (denoted as MS-CS-PPY) as an excellent anode in PIBs. The nitrogen-doped PPY framework facilitates the interface electron transfer, confines active materials MS-CS effectively, and mitigates the volume change, thus resulting in boosted reaction kinetics and exceptional cycling stability. TMCs induce the surface capacitance and enable the chemical anchoring of the charge/discharge products during the potassium/de-potassium process. Moreover, this work reveals the potassium/de-potassium reaction mechanism, redox kinetics, and solid electrolyte interphase formation of MS-CS-PPY in different electrolytes through theoretical calculations and experimental studies. The solvation ability of electrolytes plays a vital role in manipulating the redox kinetics of the MS-CS-PPY anode material. This study offers feasible strategies for electrode design and electrolyte selection for developing TMCs negative electrodes in future PIBs.

Unveiling LiTFSI Precipitation as a Key Factor in Solid Electrolyte Interphase Formation in Li-Based Water-in-Salt Electrolytes.

A water-in-salt electrolyte is a highly concentrated aqueous solution (i.e., 21mol LiTFSI in 1kg H2 O) that reduces the number of water molecules surrounding salt ions, thereby decreasing the water activity responsible for decomposition. This electrolyte widens the electrochemical stability window via the formation of a solid electrolyte interphase (SEI) at the electrode surface. However, using high concentration electrolytes in Li-ion battery technology to enhance energy density and increase cycling stability remains challenging. A parasitic reaction, called the hydrogen evolution reaction, occurs when the reaction operates at a lower voltage. It is demonstrated here that a micrometric white layer is indeed a component of the SEI layer, not just on the nanoscale, through the utilization of an operando high-resolution optical microscope. The results indicate that LiTFSI precipitation is the primary species present in the SEI layer. Furthermore, the passivation layer is found to be dynamic since it dissolves back into the electrolyte during open circuit voltage.

In situ construction of fluorine-containing modified gel polymer electrolyte with high interfacial stability for high-rate lithium metal battery

Gel polymer electrolytes (GPEs) play a crucial role in promoting the development of lithium metal batteries, as they combine the high ionic conductivity of liquid electrolytes with the enhanced safety provided by solid electrolytes. However, traditional polymer electrolytes are typically poured using non-in-situ methods, which results in poor electrode-electrolyte contact and compromises their performance. In this paper, in situ thermal polymerization is utilized to incorporate di-pentaerythritol pentaacrylate (DPPA) and 3,3,4,4,5,5,6,6,7,7,8,8-tridecafluorooctyl acrylate (TFOA) into PAN spinning porous membranes. Application of TFOA reduces the crystallinity of GPEs and thus enhancing the Li+ transport capability, which enables the newly constructed GPEs to exhibit an ionic conductivity of 2.52 × 10−3 S cm−1 and a lithium-ion transport number of 0.61 at 30 °C. Simultaneously, fluoro-containing groups actively contribute to the formation of a negative solid electrolyte interphase (SEI) film full of LiF, ensuring the stable operation of Li symmetrical batteries for over 2000 h at a current density of 0.40 mA cm−2. The capacity retention rate of lithium metal batteries (LMBs) with commercial NCM811 cathodes reaches 88.2% after 300 cycles. We firmly believe that our newly designed gel polymer electrolytes (GPEs) can offer a viable solution for high-voltage and high-rate LMBs.

Rationally Designed Fluorinated Polycation Electrolytes for High-Rate, Dendrite-Free Lithium Metal Batteries.

The investigation of high-performance polymer-based electrolytes holds significant importance for advancing the development of next-generation lithium metal batteries (LMBs). In this work, a quasi-solid-state electrolyte (EFA-G) comprising pyrrolidinium type polymeric ionic liquids and fluoropolymers was synthesized through a photoinitiated free radical copolymerization process in the presence of solvate ionic liquids. EFA-G not only exhibited high ionic conductivity (9.87 × 10-4 S cm-1) but also had a wide electrochemical stability window (0-5.0 V vs Li+/Li). The improvement in Li+ transport number (tLi+ = 0.33) of EFA-G was attributed to the enhancement of the Li+ migration ability and the hindrance of anion mobility. Due to the shielding effect of the polymeric ionic liquid on the lithium electrode and the formation of a LiF-rich solid electrolyte interphase (SEI), EFA-G supported stable long-term plating/stripping cycling (>1000 h) of lithium symmetric cells. Li/LFP cells assembled with EFA-G at 30 °C exhibited excellent battery performance with a discharge specific capacity of 78.1 mA h g-1 at 8 C and long cycling life (>600 cycles) with high discharge specific capacity (127.8 mA h g-1 after 600 cycles). EFA-G also enabled decent performance for high-voltage cathode batteries. This work provides insights into the design of high-performance polymer-based electrolytes for LMBs.

Continuum-level modeling of Li-ion battery SEI by upscaling atomistically informed reaction mechanisms

Understanding and controlling solid-electrolyte interphase (SEI) formation to stabilize cell performance is a significant challenge for next-generation Li-ion battery technologies. In recent years, computational modeling has become an essential tool in providing fundamental insights into SEI properties and dynamics. However, neither atomistic nor continuum-level approaches alone can capture the complexities of SEI chemistry across all relevant length and time scales. In this work, a continuum-level model is developed that is informed by reaction mechanisms obtained from first-principle calculations. The atomistically informed continuum-level model is used to understand electrolyte degradation, including the decomposition of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and fluoroethylene carbonate (FEC). The model presented here is the most chemically complex continuum-level SEI model in the literature to date. The SEI model is calibrated against experimental irreversible leakage currents and shows qualitative agreement with expected SEI growth trends. The model framework is expected to accelerate fundamental understanding of SEI formation, facilitate mechanism development feedback, and dynamically interact with experimental insights.

Understanding the Electrolyte Chemistry Induced Enhanced Stability of Si Anodes in Li-Ion Batteries based on Physico-Chemical Changes, Impedance, and Stress Evolution during SEI Formation

The volume expansion/contraction of Si-based anodes during electrochemical lithiation/delithiation cycles causes a loss in mechanical integrity and accrued instability of the solid electrolyte interphase (SEI) layer, culminating into capacity fade. Electrolyte additives like fluoroethylene carbonate (FEC) improve SEI stability, but the associated causes still under debate. This work reveals some of the roles of FEC via post-mortem observations/analyses, operando stress measurements and a comprehensive study of the impedance associated with the formation/evolution of SEI during lithiation/delithiation. Usage of 10 vol.% FEC as electrolyte additive leads to significant improvements in cyclic stability, Coulombic efficiency and facilitates smoother/compact/crack-free surface/SEI, in contrast to the cracked/pitted/uneven surface upon non-usage of FEC. Operando stress measurements during SEI formation reveal compressive stress development, followed by loss in mechanical integrity, upon non-usage of electrolyte additive, in contrast to insignificant stress development associated with SEI formation upon usage of FEC. The EIS model proposed here facilitates good fit with the impedance data at all states-of-charges, with the SEI resistance and capacitance exhibiting expected variations with cycling and the SEI resistance progressively decreasing with cycle number in the presence of FEC. By contrast, in the absence of FEC, severe fluctuations observed with the SEI resistance and capacitance indicate instability.

Interphase Engineering Enhanced Electro-chemical Stability of Prelithiated Anode.

Prelithiation is an essential technology to compensate for the initial lithium loss of lithium-ion batteries due to the formation of solid electrolyte interphase (SEI) and irreversible structure change. However, the prelithiated materials/electrodes become more reactive with air and electrolyte resulting in unwanted side reactions and contaminations, which makes it difficult for the practical application of prelithiation technology. To address this problem, herein, interphase engineering through a simple solution treatment after chemical prelithiation is proposed to protect the prelithiated electrode. The used solutions are carefully selected, and the composition and nanostructure of the as-formed artificial SEIs are revealed by cryogenic electron microscopy and X-ray photoelectron spectroscopy. The electrochemical evaluation demonstrates the unique merits of this artificial SEI, especially for the fluorinated interphase, which not only enhances the interfacial ion transport but also increases the tolerance of the prelithiated electrode to the air. The treated graphite electrode shows an initial Coulombic efficiency of 129.4%, a high capacity of 170 mAh g-1 at 3 C, and negligible capacity decay after 200 cycles at 1 C. These findings not only provide a facile, universal, and controllable method to construct an artificial SEI but also enlighten the upgrade of battery fabrication and the alternative use of advanced electrolytes.

Bio-Inspired Polyanionic Electrolytes for Highly Stable Zinc-Ion Batteries.

For zinc-ion batteries (ZIBs), the non-uniform Zn plating/stripping results in a high polarization and low Coulombic efficiency (CE), hindering the large-scale application of ZIBs. Here, inspired by biomass seaweed plants, an anionic polyelectrolyte alginate acid (SA) was used to initiate the in situ formation of the high-performance solid electrolyte interphase (SEI) layer on the Zn anode. Attribute to the anionic groups of -COO- , the affinity of Zn2+ ions to alginate acid induces a well-aligned accelerating channel for uniform plating. This SEI regulates the desolvation structure of Zn2+ and facilitates the formation of compact Zn (002) crystal planes. Even under high depth of discharge conditions (DOD), the SA-coated Zn anode still maintains a stable Zn stripping/plating behavior with a low potential difference (0.114 V). According to the classical nucleation theory, the nucleation energy for SA-coated Zn is 97 % less than that of bare Zn, resulting in a faster nucleation rate. The Zn||Cu cell assembled with the SA-coated electrode exhibits an outstanding average CE of 99.8 % over 1,400 cycles. The design is successfully demonstrated in pouch cells, where the SA-coated Zn exhibits capacity retention of 96.9 % compared to 59.1 % for bare Zn anode, even under the high cathode mass loading (>10 mg/cm2 ).

Preferential Coordination As a Means to Investigate Ca2+ Speciation and SEI Formation

Given concerns about the low earth abundance of Li in Li-ion batteries, there is growing interest in developing a beyond-Li materials basis for rechargeable batteries. Divalent batteries based on calcium (Ca) have received attention due to their ~2000-fold higher concentration in the earth’s crust along with attractive theoretical metrics if Ca is used as the metallic anode. When most common battery electrolytes come into contact with Ca, however, a passivating solid electrolyte interphase (SEI) forms that prevents Ca2+ transport.1 At present, only a small number of nonaqueous electrolytes have been identified that form sufficiently Ca2+-conductive SEIs for Ca electrodeposition, limiting the design space for Ca battery development. Thus, learning how to tailor Ca SEI formation is of central importance to identify handles for next-generation battery design. Several methods of modifying SEI to improve Ca2+ transport have been reported, including synthesizing an ex situ artificial layer,2 or the introduction of boron-containing electrolyte additives that form beneficial SEI compounds.3 Ca electrolyte modification faces a unique challenge, however, as the large size and divalent nature of Ca2+ leads to large coordination environments (up to 7 solvating molecules) in solution.4 Recent studies have begun to identify that co-reduction of these coordinating molecules has a significant impact on Ca electrochemistry. For example, strong coordination between Ca2+ and the anion TFSI- has been demonstrated to deactivate Ca plating behavior via TFSI- decomposition.5,6 However, competition between coordinating molecules can be exploited to favor specific Ca2+ speciation (i.e. BH4 - displaces bidentate TFSI---Ca2+ in favor of BH4 ---Ca2+ interactions) and unlock Ca plating behavior.6 Further mechanistic understanding is needed to identify the specific connections between Ca2+ coordination, SEI formation, and anode performance (beyond just a binary toggling of plating behavior). To address this, we have systematically introduced chelating glyme-based solvents into a baseline electrolyte, investigating the subsequent changes in SEI composition and electrochemical behavior.This study investigates the preferential coordination of Ca2+ by glymes in the baseline electrolyte 1 M Ca(BH4)2 in tetrahydrofuran (THF). THF is systematically replaced by 1,2-dimethoxyethane (G1) or bis(2-methoxyethyl) ether (G2) over a range of Gx/Ca2+ molar ratios. Microcalorimetry measurements demonstrate that the introduction of both G1 and G2 to the electrolyte is enthalpically favorable, as each glyme displaces THF from the Ca2+ coordination sphere and modulates Ca2+--BH4 - speciation, as indicated by NMR and Raman spectroscopy. Addition of a small amount of each glyme (1:1 Gx/Ca2+) increases electrolyte ionic conductivity by nearly half; however, glyme addition systematically increases the overpotentials of both Ca plating and stripping while decreasing Coulombic efficiency. The presence of glymes increases the amount of carbon present in both chemically- and electrochemically-formed SEI as indicated through XPS. We further find increased evidence of fragmentation of strongly Ca2+-coordinating molecules (G2, BH4 -) within the SEI formed during Ca deposition as compared to formed chemically. Calcium-based olefins as well as ionic phases (CaCO3, CaC2) are detected in electrochemically-formed SEI through titration gas chromatography. This work reveals factors that modulate both Ca2+ coordination and subsequent SEI formation, highlighting preferential coordination as a novel approach for Ca-based electrolyte design.1 Aurbach, D., Skaletsky, R. & Gofer, Y. The Electrochemical Behavior of Calcium Electrodes in a Few Organic Electrolytes. Journal of The Electrochemical Society 138, 3536-3545 (1991).2 Hou, Z., Zhou, R., Min, Z., Lu, Z. & Zhang, B. Realizing Wide-Temperature Reversible Ca Metal Anodes through a Ca2+-Conducting Artificial Layer. ACS Energy Letters, 274-279 (2022).3 Bodin, C. et al. Boron-Based Functional Additives Enable Solid Electrolyte Interphase Engineering in Calcium Metal Battery. Batteries & Supercaps n/a, e202200433 (2022).4 Tchitchekova, D. S. et al. On the Reliability of Half-Cell Tests for Monovalent (Li+, Na+) and Divalent (Mg2+, Ca2+) Cation Based Batteries. Journal of The Electrochemical Society 164, A1384-A1392 (2017).5 Hahn, N. T. et al. Influence of Ether Solvent and Anion Coordination on Electrochemical Behavior in Calcium Battery Electrolytes. ACS Applied Energy Materials 3, 8437-8447 (2020).6 Melemed, A. M., Skiba, D. A. & Gallant, B. M. Toggling Calcium Plating Activity and Reversibility through Modulation of Ca2+ Speciation in Borohydride-Based Electrolytes. The Journal of Physical Chemistry C 126, 892-902 (2022).

Comparing the Lithiation and Sodiation of Hard-Carbon Using Online Electrochemical Mass Spectrometry

Sodium-ion batteries (SIBs) are a promising alternative to lithium-ion batteries (LIBs), especially for applications in which weight and size play a subordinate role.1 In contrast to LIBs, graphite is not a viable anode active material for SIBs since sodium cannot be inserted into graphite. So-called hard-carbons are promising anode active materials for SIBs because of their easy synthesis, high capacity, and high cycling stability.2 Major challenges are the high irreversible capacity in the first cycle and the stability of the solid electrolyte interphase (SEI).3 - 4 Online electrochemical mass spectrometry (OEMS) is a powerful tool for identifying and quantifying evolved gasses during SEI formation, giving insight into the mechanism of SEI formation. For LIB research, our group had developed a two-compartment OEMS cell design that enables the investigation of evolved gases from one of the electrodes of a battery cell without interference from the gases evolved from the other electrode, which is particularly useful for the mechanistic study of electrolyte additives or impurities without influences of the counter electrode.5–8 In this study, we investigate the gas evolution during the first lithiation/delithiation cycles of a hard-carbon anode active material using our existing two-compartment cell design. This will then be compared to the gas evolution during the first sodiation/desodiation cycles of the same hard-carbon material using a newly developed two-compartment OEMS-cell design for the study of sodium-ion batteries, which is based on a gas impermeable solid electrolyte separator composed of sodium ion conducting β-alumina. We compare the quantities of evolved gasses and look at the onset potentials of gas evolution during the first alkaline metal insertion. On the basis of gassing and impedance data,9,10 the differences in SEI formation and its properties will be discussed.Acknowledgments: We gratefully acknowledge funding by the BMBF (Federal Ministry of Education and Research, Germany) for its financial support under the auspices of the ExZellTUM III project, grant number 03XP0255

Insights into Electrolyte Reactivity and Solid Electrolyte Interphase Formation at the Li Metal Anode Surface from DFT Simulations

Solid electrolyte interphase (SEI) is a critical component of lithium ion batteries, yet is poorly characterized and not well understood. We use plane wave density functional theory simulations to perform a comprehensive study of the degradation reaction mechanisms of four electrolyte solvents and additives, ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC), at the Li (001) metal anode surface. The calculations provide fundamental insights into electrolyte decomposition and spontaneous SEI formation. Utilizing automated workflows implemented in the Schrödinger Materials Science Suite, we compute the adsorption energies and calculate the reaction energies for each decomposition pathway. Furthermore, we employ nudged elastic band calculations to compute the decomposition reaction barriers and provide mechanistic insights into the onset of SEI formation. We analyze trends in decomposition reaction energies and the relationships between the reaction energies and the activation barriers for the various electrolyte molecules as a function of applied bias potential. We find that the preferred decomposition pathways are different for the solvent molecules (EC and PC) than for the additives (FEC and VC). Calculations also suggest that applied bias reduces decomposition reaction barriers thus enhancing SEI formation. Such a comprehensive dataset of reaction energies and activation barriers provides useful benchmarks for the development and validation of reactive machine learned force fields for modeling advanced battery chemistries at a larger scale.