Chemical and Structural Insights into Solid Electrolyte Interphase Evolution for Sodium Metal Electrodes

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

Sodium metal is a promising anode material for room-temperature sodium sulfur batteries. Due to its high reactivity, typical liquid electrolytes (e.g. carbonate-based solvents and a Na salt) can undergo reduction to form a solid electrolyte interphase (SEI) layer, with inorganic components such as Na2CO3, Na2O, and NaOH, covering the anode surface along with other SEI organic products. One of the challenges is to understand the effect of the SEI film on the decomposition of soluble sodium polysulfide molecules (e.g., Na2S8) upon shuttling from the cathode to anode during battery cycling. Here, we use ab initio molecular dynamics (AIMD) simulations to study the role of an inorganic SEI used as a model passivation layer in polysulfide decomposition. Compared to other film chemistries, it is found that the Na2CO3 film can suppress decomposition with the slowest reduction rate and the smallest amount of charge transfer towards Na2S8. The Na2CO3 film can maintain its structural properties during the simulations. In contrast, Na2O and NaOH allow some decomposed polysulfide fragments to be inserted into the SEI layer. Moreover, the decomposition of Na2S8 on both Na2O and NaOH SEI layers is more reactive with more charge transfer to Na2S8 when compared to that of Na2CO3. Thus, the ability of the SEI to suppress polysulfide decomposition is in the order: Na2CO3 > NaOH ∼ Na2O. Analyses of the density of states reveal that the Na2S8 molecule receives electrons from the Na metal directly in the presence of n-type semiconductor films of Na2CO3 and NaOH, while the charge migration behavior is different in a p-type semiconductor Na2O with the SEI film donating its electrons to the polysulfide solely. Thus, this work adds new insights into charge transfer behavior of inorganic thin film SEIs that could be present at the initial stages of SEI formation.

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

The performance of lithium metal batteries can be significantly enhanced by incorporating fluorinated ether-based electrolytes, yet the solid electrolyte interphase (SEI) formation mechanism on lithium metal surfaces remains elusive. This study employs classical and ab initio molecular dynamics simulations to investigate the decomposition mechanisms of lithium bis(fluoromethanesulfonyl)imide (LiFSI) in 1,2-diethoxyethane (DEE) and its fluorinated analogues, F5DEE and F2DEE, when in contact with lithium metal. Our findings indicate that F5DEE-based electrolytes favor the formation of a FSI-rich primary solvation shell around Li+, while F2DEE-based electrolytes yield a solvent-rich environment. The normalized number density at the Li/electrolyte/Li interface shows a depletion of FSI anions in the electrochemical double layer (EDL) structure near the Li anode upon charging, with the distance between the first main peak of the FSI anion and Li anode following the order F5DEE < DEE < F2DEE. Analysis of the electronic projected density of states and charge transfer dynamics unveils the reductive dissociation pathways of FSI anions and fluorinated DEE solvents on the lithium metal surface, taking into account the influence of the EDL structure. DEE is identified as the most reduction-stable solvent, leading to the selective dissociation of FSI anions and the formation of an entirely inorganic SEI. In contrast, F2DEE displays a pronounced reduction tendency, forming an organic-rich SEI due to the solvent-dominated lowest unoccupied molecular orbital at the interface. F5DEE, competing with FSI anions for reduction, results in the formation of an inorganic-rich hybrid SEI with the highest LiF content. The simulation results correlate well with experimental observations and underscore the pivotal role of various fluorinated functional groups in the formation of EDL and SEI near the lithium metal surface.

The solid electrolyte interphase (SEI) is regarded as the most important yet least understood component in Li-ion batteries. Considerable effort has been devoted to unravelling its chemistry, structure, and ion-transport mechanism; however, the nucleation and growth mode of SEI, which underlies all these properties, remains the missing piece. We quantify the growth mode of two representative SEIs on carbonaceous anodes based on classical nucleation theories and in situ atomic force microscopy imaging. The formation of inorganic SEI obeys the mixed 2D/3D growth model and is highly dependent on overpotential, whereby large overpotential favors 2D growth. Organic SEI strictly follows the 2D instantaneous nucleation and growth model regardless of overpotential and enables perfect epitaxial passivation of electrodes. We further demonstrate the use of large current pulses during battery formation to promote 2D inorganic SEI growth and improve capacity retention. These insights offer the potential to tailor desired interphases at the nanoscale for future electrochemical devices.

The performance of aqueous zinc‐ion batteries is limited by parasitic reactions and dendrite growth, originating from a H 2 O‐rich electric double layer (EDL) and the disordered diffusion of Zn 2+ at the Zn anode/aqueous interface. This study proposes a steric‐hindrance‐driven interfacial engineering strategy, using sulfonate‐based additives with various molecular sizes in ZnSO 4 electrolytes. Combined experimental and theoretical analyses reveal that sodium diphenylamine‐4‐sulfonate (SDPS) with large steric hindrance effect spontaneously adsorbs onto the Zn anode, establishing a H 2 O‐poor EDL while reconstructing the anion‐rich solvation structure and the hydrogen‐bond network. This reconfigured interface further facilitates the in situ formation of an organic–inorganic hybrid solid electrolyte interphase (SEI) through the SDPS and SO 4 2− decomposition. Consequently, an interface, comprising an SDPS adsorption shield, a carbon‐rich organic phase, and a sulfur‐rich (ZnS‐dominated) inorganic phase, is achieved, which collectively suppresses H 2 O‐induced parasitic reactions and inhibits dendrite growth. The Zn||Zn symmetric cell with the SDPS‐containing ZnSO 4 electrolyte exhibits prolonged cycling capabilities exceeding 1400 h at 20 mA cm −2 , and maintains stable operation for 570 h under a high depth of discharge of 95.7%. By correlating molecular steric hinderance with the co‐evolution of the EDL and SEI, this work provides new insights for designing electrolyte additives for advanced aqueous batteries.

We explore the interrelation between external pressure (0.1, 1, 10 MPa), solid electrolyte interphase (SEI) structure/morphology, and lithium metal plating/stripping behavior. To simulate anode-free lithium metal batteries (AF-LMBs) analysis was performed on “empty” Cu current collectors in standard carbonate electrolyte. Lower pressure promotes organic-rich SEI and macroscopically heterogeneous, filament-like Li electrodeposits interspersed with pores. Higher pressure promotes inorganic F-rich SEI with more uniform and denser Li film. A “seeding layer” of lithiated pristine graphene (pG@Cu) favors an anion-derived F-rich SEI and promotes uniform metal electrodeposition, enabling extended electrochemical stability at a lower pressure. State-of-the-art electrochemical performance is achieved at 1MPa: pG-enabled half-cell is stable after 300 hrs (50 cycles) at 1 mA cm-2 rate - 3 mAh cm-2 capacity (17.5 mm plated/stripped), with cycling Coulombic efficiency (CE) of 99.8%. AF-LMB cells with high mass loading NMC622 cathode (21 mg cm-2) undergo 200 cycles with a CE of 99.4% at C/5-charge and C/2-discharge (1C = 178 mAh/g). Density functional theory (DFT) highlights the differences in the adsorption energy of solvated-Li+ onto various crystal planes of Cu (100), (110), and (111), versus lithiated/delithiated (0001) graphene, giving insight regarding the role of support surface energetics in promoting SEI heterogeneity.

Lithium metal batteries (LMBs) hold great promise for next-generation energy storage due to the ultrahigh theoretical capacity of lithium metal. However, an unstable solid electrolyte interphase (SEI) remains a critical challenge as these interfaces govern lithium diffusion during plating and stripping and prevent electrolyte degradation during cycling. Understanding the SEI's chemical composition and evolution is crucial for improving LMB performance.This study investigates the impact of different lithium salts on SEI formation in ether-based electrolytes using in-operando Fourier Transform Infrared (FTIR) spectroscopy. Three electrolytes were studied: 1M LiTFSI, 1M LiClO4, and 1M LiDFOB in DME: DOL. In-operando FTIR measurements were conducted on Li ||Li symmetric cells during plating and stripping at a current density of 3 mA/cm2. The results reveal distinct differences in SEI kinetics, composition, formation, and evolution depending on the salt used. In-operando FTIR in tandem with post-mortem X-ray Photoelectron Spectroscopy (XPS) reveals different compounds forming on the lithium surface. LiTFSI-based electrolytes showed an instantaneous interaction with the Li surface forming both organic and inorganic SEI components. In contrast, LiClO4 showed a delayed Li-salt interaction started after minutes into plating. The LiDFOB electrolyte resulted in an organic dominated SEI with limited inorganic compounds leading to an unstable SEI. These differences in SEI chemistry correlate with electrochemical performance, with LiTFSI exhibiting superior performance.This study provides direct spectroscopic evidence of SEI formation and evolution in LMBs, highlighting the critical role of electrolyte salt chemistry in determining interfacial properties and battery performance.

The solid electrolyte interphase (SEI) is a solid that forms between the anode and electrolyte in batteries. It is made up electrolyte species that have decomposed due to reduction by the anode, but its growth is self-limited as its very presence inhibits further electrolyte decomposition. The SEI is made up of numerous inorganic and organic species, and its composition and properties can vary drastically. For the proper operation of the battery, it needs to be ionically conducting while being electronically insulating. These two properties are often correlated, so it is important to understand how to decouple them to achieve desired battery performance.In this presentation, we will discuss results from our group on studying electron transport through simplified models of the SEI from first-principles atomistic modeling. Our approach uses the non-equilibrium Green's function technique in combination with density functional theory (NEGF-DFT) to calculate electron transport through SEI structures. We first applied this method to compare the electronic conductivity of single-crystal versus polycrystalline LiF, a common inorganic SEI component. The grain boundaries in polycrystalline LiF are known to be essential for Li ion transport, but they also make for leakage pathways for electrons to pass through the material. Next, we apply the same method to investigate the SEI in Ca ion batteries, with a study of electron transport through CaF2. In this study, we considered single-crystal CaF2, as well as several grain boundaries and an amorphous version of CaF2. We found the amorphous version of this SEI component to be orders of magnitude more electronically conductive than the crystalline counterparts.Finally, we used ab initio molecular dynamics (AIMD) to contrast the reductive decomposition of two electrolyte solvents: tetrahydrofuran (THF) and ethylene carbonate (EC) at a Ca anode. The former does not decompose in the timescales of our simulations (ca. 50 ps), while the latter undergoes rapid decomposition. We extracted structural atomistic snapshots from our AIMD trajectories and fed them through the NEGF-DFT code to obtain time-dependent electron transport for the THF and EC solvents near the Ca anode, while decompositions are occurring in the EC system. While THF is more conductive than EC, its conductivity has relatively small variation in time. EC has drastic fluctuations in its conductivity, part of which can be attributed to individual molecular decomposition events, which may be useful to develop diagnostic or characterization techniques.The SEI plays an essential role in maintaining long-term battery stability. Understanding how electron transport occurs through this battery component will help develop SEIs with desired properties for ultimate performance in both Li-ion batteries and beyond Li-ion batteries such as Ca ion and other multivalent systems.

The formation of the solid electrolyte interphase (SEI) on Si is examined in detail using several in situ techniques. The results show that employing different conditions during the first lithiation cycle produces SEI films with substantially different properties. Longer time at higher potentials produces softer SEI, whereas inorganic phases produced at lower potentials have higher elastic moduli. The SEI thickness stabilizes during the first cycle; however, the SEI resistance decreases during the first 20 cycles (in sharp contrast to typical surface passivation processes, where resistance is expected to increase with time). This behavior is consistent with the slow growth of inorganic constituents at lower potentials, inside of a mesoporous soft SEI that initially forms at higher potentials. This interpretation is based on the premise that these inorganic phases have a lower resistivity than that associated with electrolyte transport through the mesoporous organic phase. Based on this set of observations, the multiphase structure that evolves during initial cycling determines critical electrochemical and mechanical properties of the SEI. A basic model of these tradeoffs is proposed to provide guidelines for creating more stable interfacial films.

The SiO electrode interface is passivated with a SiO2 layer, which hinders the deposition of an inorganic solid electrolyte interphase (SEI) due to its high surface work function and low exchange current density of electrolyte decomposition. Consequently, a thermally vulnerable, organic-based SEI formed on the SiO electrode, leading to poor cycling performance at elevated temperatures. To address this issue, the SEI formation process is thermoelectrochemically activated. Increasing the formation temperature lowers the work function by shifting the electron energy levels and increases the exchange current density for SEI formation. Higher temperatures promote the incorporation of inorganic Li2CO3 into the SEI film, resulting from the two-electron reduction of ethylene carbonate, and hence the thermally stable SEI film leads to stable cycleability. However, excessively high temperatures cause the SEI layer to become thick and resistive, significantly increasing the polarization of the SiO electrode, which leads to a deficient improvement of cycle performance. Therefore, moderate temperature exposure is required to convert the organic SEI into less resistive, inorganic components. The implementation of a mechanism-assisted SEI formation process in pouch cells using identical materials significantly improves the cycling performance, with a 20% enhancement by the 300th cycle. Additionally, the thermoelectrochemical activation of SEI formation reduces cathodic side reactions on SiO electrodes, which helps in preventing coupled failure of the NCM electrode by mitigating intergranular cracking and preserving its structure.

A nano-SiO2 embedded HPC/P(VDF-HFP) composite gel membrane for fast-charging sodium metal batteries with long span life

Sodium metal batteries (SMBs), leveraging the natural abundance of sodium and high theoretical capacity, emerge as promising candidates for next-generation energy storage. However, the generation of the unstable solid electrolyte interphase (SEI) layer and uncontrolled Na dendrite growth in SMBs with carbonate-based electrolytes leads to poor Coulombic efficiency and security risks. Herein, a multifunctional separator (N-U@PE) for SMBs is fabricated through an innovative strategy involving the in situ growth of zirconium-based metal-organic frameworks (Zr-MOFs) on NASICON-type fast-ion conductor to prepare a dual-phase composite (N-U) as PE separator modification. Benefiting from abundant open metal sites (OMSs) and 3D ion transport channels, the N-U@PE separator can effectively immobilize anions and induce the formation of NaF-rich SEI layer, resulting in Na+ flux homogenization and Na+ transport kinetics enhancement. Satisfactorily, the Na|N-U@PE|Na symmetric cell achieves stable cycling for 3000h at 0.5mA cm-2 and 0.5 mAh cm-2. Remarkably, the Na3V2(PO4)3|N-U@PE|Na pouch cell exhibits 95.2% capacity retention after 1000 cycles at 2 C. The design of N-U@PE integrates ion transport regulation and interfacial stability, giving a practical solution for high-performance SMBs.

Sodium (Na) metal batteries (SMBs) have attracted many attentions because of high specific capacity and low redox potential. However, SMBs encounter several crucial challenges including Na dendrites growth, severe interfacial side reactions and safe issues from liquid organic electrolyte. In this study, a non‐flammable 1,2‐bis (1,1,2,2‐tetrafluoroethoxy) ethane (TFEE) aided ionic liquid (IL) electrolyte (TILE) is reported with sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) salt and N‐methyl‐N‐propylpyrrolidinium bis(fluorosulfonyl)imide ([Py13][FSI]) IL solvent. The IL solvent contains plentiful organic cations and anions, which helps to provide a cationic shielding effect by [Py13]+ cations and regulate the Na+ solvation structure with more FSI− and TFSI− anions. The unique solvation structure of Na+ facilitates a stable inorganic‐rich solid electrolyte interphase (SEI), effectively inhibiting unstable Na deposition and suppressing side reactions between the electrolyte and Na‐metal in SMBs. Therefore, the Na3V2(PO4)3 (NVP)||Na cells with TILE can deliver good rate capability and superior cycling performance with capacity retention of 95.5% and Coulombic efficiency of ≈99.9% after 2000 cycles at 5C rate and 25 °C. The high‐voltage NaNi0.33Fe0.33Mn0.33O2(NFM)||Na cells with TILE can also deliver good cycling performance with capacity retention of 92% after 150 cycles in the voltage range of 2.0–4.1 V at 0.5C rate and 25 °C.

Sodium metal batteries (SMBs), which possess abundant sodium resources and high energy density, have attracted widespread attention. However, the continuous reaction between the electrolyte and the sodium metal anode, along with the formation of an unstable solid electrolyte interphase (SEI), leads to rapid capacity decay and the safety hazard of potential ignition. In this work, designing a low-cost and flame-retardant electrolyte with solvent-solvent interactions is achieved by introducing sodium-difluoro(oxalato)borate (NaDFOB) as a single salt into the ester-based electrolyte on the basis of trimethyl phosphate. Theoretical research combined with experimental study disclose through the solvent-solvent interactions, an ion-aggregate-rich solvation structure is formed at low concentrations, leading to the formation of a gradient SEI enriched with inorganic compounds such as B and F on the anode. This effectively suppresses interfacial reactions and sodium dendrite growth, significantly improving the cycling stability along with the optimizing the safety of SMBs. The Na||Na3V2(PO4)3 battery using this electrolyte maintains a high-capacity retention of 93% after 5000 cycles (320 days) at 1C. This approach provides a reliable solution for the application of flame-retardant electrolytes in SMBs, which also sheds light on the designing principle of advanced battery systems.