Solid Electrolyte Interphase Components Research Articles

Lithium Hydride in the Solid Electrolyte Interphase of Lithium‐Ion Batteries as a Pulverization Accelerator of Silicon

As a highly promising next‐generation high‐specific capacity anode, the industrial‐scale utilization of micron silicon has been hindered by the issue of pulverization during cycling. Although numerous studies have demonstrated the effectiveness of regulating the inorganic components of the solid electrolyte interphase (SEI) in improving pulverization, the evolution of most key inorganic components in the SEI and their correlation with silicon failure mechanisms remain ambiguous. This study provides a clear and direct correlation between the lithium hydride (LiH) in the SEI and the degree of micron silicon pulverization in the battery system. The reverse lithiation behavior of LiH on micron silicon during de‐lithiation exacerbates the localized stress in silicon particles and contributes to particle pulverization. This work successfully proposes a novel approach to decouple the SEI from electrochemical performance, which can be significant to decipher the evolution of critical SEI components at varied battery anode interfaces and analyze their corresponding failure mechanisms.

Lithium Hydride in the Solid Electrolyte Interphase of Lithium-Ion Batteries as aPulverization Accelerator of Silicon.

As a highly promising next-generation high-specific capacity anode, the industrial-scale utilization of micron silicon has been hindered by the issue of pulverization during cycling. Although numerous studies have demonstrated the effectiveness of regulating the inorganic components of the solid electrolyte interphase (SEI) in improving pulverization, the evolution of most key inorganic components in the SEI and their correlation with silicon failure mechanisms remain ambiguous. This study provides a clear and direct correlation between the lithium hydride (LiH) in the SEI and the degree of micron silicon pulverization in the battery system. The reverse lithiation behavior of LiH on micron silicon during de-lithiation exacerbates the localized stress in silicon particles and contributes to particle pulverization. This work successfully proposes a novel approach to decouple the SEI from electrochemical performance, which can be significant to decipher the evolution of critical SEI components at varied battery anode interfaces and analyze their corresponding failure mechanisms.

Thermal decomposition mechanism of lithium methyl carbonate in solid electrolyte interphase layer of lithium-ion battery

To predict the early stages of thermal runaway and improve the safety of lithium-ion batteries, it is necessary to examine the elementary reaction steps for the thermal decomposition of individual solid electrolyte interphase (SEI) components. This study elucidates a detailed decomposition mechanism of lithium methyl carbonate (LMC) which is one of SEI components and accounts for the majority of SEI components formed in commercial electrolytes. The detailed reaction mechanism for the thermal decomposition of LMC is proposed based on the in-situ/ex-situ experiments and density functional theory calculations. LMC underwent six reactions before finally converting to Li2CO3 at 300 °C. To supplement the reliability of the proposed reaction mechanism, the actual gas composition measured using mass spectrometry (MS) are compared with the chain reactions of radicals generated through the thermal decomposition reactions, which is calculated using GRI-MECH 3.0, a gas-phase reaction mechanism. The methodology proposed in this study can be used both for analyzing the reaction mechanisms of different SEI components and their coupling effects with the electrolyte in the future. Understanding these reaction mechanism sets will help us to understand the degradation reactions of real complex SEI and the initial self-heating stage during thermal runaway.

Inorganic Composition Modulation of Solid Electrolyte Interphase for Fast Charging Lithium Metal Batteries.

The solid electrolyte interphase (SEI) with lithium fluoride (LiF) is critical to the performance of lithium metal batteries (LMBs) due to its high stability and mechanical properties. However, the low Li ion conductivity of LiF impedes the rapid diffusion of Li ions in the SEI, which leads to localized Li ion oversaturation dendritic deposition and hinders the practical applications of LMBs at high-current regions (>3 C). To address this issue, a fluorophosphated SEI rich with fast ion-diffusing inorganic grain boundaries (LiF/Li3P) is introduced. By utilizing a sol electrolyte that contains highly dispersed porous LiF nanoparticles modified with phosphorus-containing functional groups, a fluorophosphated SEI is constructed and the presence of electrochemically active Li within these fast ion-diffusing grain boundaries (GBs-Li) that are non-nucleated is demonstrated, ensuring the stability of the Li || NCM811 cell for over 1000 cycles at fast-charging rates of 5 C (11mA cm-2). Additionally, a practical, long cycling, and intrinsically safe LMB pouch cell with high energy density (400Wh kg-1) is fabricated. The work reveals how SEI components and structure design can enable fast-charging LMBs.

Catalytic role of in-situ formed C-N species for enhanced Li2CO3 decomposition

Sluggish kinetics of the CO2 reduction/evolution reactions lead to the accumulation of Li2CO3 residuals and thus possible catalyst deactivation, which hinders the long-term cycling stability of Li-CO2 batteries. Apart from catalyst design, constructing a fluorinated solid-electrolyte interphase is a conventional strategy to minimize parasitic reactions and prolong cycle life. However, the catalytic effects of solid-electrolyte interphase components have been overlooked and remain unclear. Herein, we systematically regulate the compositions of solid-electrolyte interphase via tuning electrolyte solvation structures, anion coordination, and binding free energy between Li ion and anion. The cells exhibit distinct improvement in cycling performance with increasing content of C-N species in solid-electrolyte interphase layers. The enhancement originates from a catalytic effect towards accelerating the Li2CO3 formation/decomposition kinetics. Theoretical analysis reveals that C-N species provide strong adsorption sites and promote charge transfer from interface to *CO22− during discharge, and from Li2CO3 to C-N species during charge, thereby building a bidirectional fast-reacting bridge for CO2 reduction/evolution reactions. This finding enables us to design a C-N rich solid-electrolyte interphase via dual-salt electrolytes, improving cycle life of Li-CO2 batteries to twice that using traditional electrolytes. Our work provides an insight into interfacial design by tuning of catalytic properties towards CO2 reduction/evolution reactions.

Li3PO4-Enriched SEI on Graphite Anode Boosts Li+ De-Solvation Enabling Fast-Charging and Low-Temperature Lithium-Ion Batteries.

Li+ de-solvation at solid-electrolyte interphase (SEI)-electrolyte interface stands as a pivotal step that imposes limitations on the fast-charging capability and low-temperature performance of lithium-ion batteries (LIBs). Unraveling the contributions of key constituents in the SEI that facilitate Li+ de-solvation and deciphering their mechanisms, as a design principle for the interfacial structure of anode materials, is still a challenge. Herein, we conducted a systematic exploration of the influence exerted by various inorganic components (Li2CO3, LiF, Li3PO4) found in the SEI on their role in promoting the Li+ de-solvation. The findings highlight that Li3PO4-enriched SEI effectively reduces the de-solvation energy due to its ability to attenuate the Li+-solvent interaction, thereby expediting the de-solvation process. Building on this, we engineer Li3PO4 interphase on graphite (LPO-Gr) anode via a simple solid-phase coating, facilitating the Li+ de-solvation and building an inorganic-rich SEI, resulting in accelerated Li+ transport crossing the electrode interfaces and interphases. Full cells using the LPO-Gr anode can replenish its 80 % capacity in 6.5 minutes, while still retaining 70 % of the room temperature capacity even at -20 °C. Our strategy establishes connection between the de-solvation characteristics of the SEI components and the interfacial structure design of anode materials for high performance LIBs.

Li3PO4‐Enriched SEI on Graphite Anode Boosts Li+ De‐Solvation Enabling Fast‐Charging and Low‐Temperature Lithium‐Ion Batteries

AbstractLi+ de‐solvation at solid‐electrolyte interphase (SEI)‐electrolyte interface stands as a pivotal step that imposes limitations on the fast‐charging capability and low‐temperature performance of lithium‐ion batteries (LIBs). Unraveling the contributions of key constituents in the SEI that facilitate Li+ de‐solvation and deciphering their mechanisms, as a design principle for the interfacial structure of anode materials, is still a challenge. Herein, we conducted a systematic exploration of the influence exerted by various inorganic components (Li2CO3, LiF, Li3PO4) found in the SEI on their role in promoting the Li+ de‐solvation. The findings highlight that Li3PO4‐enriched SEI effectively reduces the de‐solvation energy due to its ability to attenuate the Li+‐solvent interaction, thereby expediting the de‐solvation process. Building on this, we engineer Li3PO4 interphase on graphite (LPO−Gr) anode via a simple solid‐phase coating, facilitating the Li+ de‐solvation and building an inorganic‐rich SEI, resulting in accelerated Li+ transport crossing the electrode interfaces and interphases. Full cells using the LPO−Gr anode can replenish its 80 % capacity in 6.5 minutes, while still retaining 70 % of the room temperature capacity even at −20 °C. Our strategy establishes connection between the de‐solvation characteristics of the SEI components and the interfacial structure design of anode materials for high performance LIBs.

Characterization by NMR Spectroscopy of the SEI Layer Formed on Ti3C2 MXene Materials Prepared with Various Terminations

The nature and content of surface terminations are one of the key factors that define the electrochemical signature of the widely studied MXene materials. In this paper, the surface termination of molten salt synthesized Ti3C2Tx MXene with -O and -Cl terminations (T=Cl,O) are studied for the first time using solid state NMR technique, with respect to conventional HF synthesized Ti3C2Tx (T=F, O, OH). Both materials have been further used as negative electrode of Li-ion battery. The evolution of surface terminations during the Solid Electrolyte Interphase (SEI) layer formation was studied from the SEI components formed in both MXenes. Analysis of the NMR signal provided insights into the porous nature of SEI with LiF as main component in HF terminated MXenes. While a thick uniform formation of the SEI was observed for the molten salt synthesized Ti3C2Tx (T=Cl,O) with Li2CO3 as dominant component.

Dynamic and Reversible Blending Interface on Polyoxovanadate Electrode for High-Performance Lithium-Ion Batteries.

Solid electrolyte interphase (SEI) plays a critical role in the performance of lithium-ion batteries (LIBs). In contrast to the clear interface between the traditional consecutive electrode materials and SEI, ionic polyoxometalates (POMs) as electrode could bilaterally diffuse with SEI and form a blending interface for superior electrochemical performance. POMs have recently aroused much interest as electrode materials in LIBs due to their structural flexibility, high capacity, and cycling stability. However, the interface evolution between POM-based electrodes and SEI, which is critical for Li+ ion transportation, has rarely been explored. Herein, we choose Li10[V12B18O60H6] (LVB) as an example to investigate the formation and structural evolution of the electrode-electrolyte interface. Time-of-flight secondary ion mass spectrometry together with X-ray photoelectron spectroscopy demonstrates the evolution of a blending layer at the interface containing typical SEI components, a polyanion from LVB and a phosphate anion from decomposition products of LiPF6. In the blending layer, ion migration takes place between the P-related inorganic species and the polyanion during the Li+ insertion/extraction reaction. Such a compatible blending layer favors Li+ transportation and the reversibility of the redox reactions, as supported by a series of electrochemical analyses. This work provides detailed insights into understanding the interface evolution of the LVB electrode and demonstrates the importance of interfacial engineering to induce proper interface layers in the development of high-performance POM-based electrodes for LIBs.

Beyond LiF: Tailoring Li2O-Dominated Solid Electrolyte Interphase for Stable Lithium Metal Batteries.

The components and structures of the solid-electrolyte interphase (SEI) are critical for stable cycling of lithium metal batteries (LMBs). LiF has been widely studied as the dominant component of SEI, but Li2O, which has a much lower diffusion barrier for Li+, has rarely been investigated as the dominant component of SEI. The effect of Li2O-dominated SEI on electrochemical performance still remains elusive. Herein, an ultrastrong coordinated cosolvation diluent, 2,3-difluoroethoxybenzene (DFEB), is designed to modulate solvation structure and tailor Li2O-dominated SEI for stable LMBs. In the DFEB-based LHCE (DFEB-LHCE), DFEB intensively participates in the first solvation shell and synergizes with FSI- to tailor an Li2O-dominated inorganic-rich SEI which is different from the LiF-dominated SEI formed in conventional LHCE. Benefiting from this special SEI architecture, a high Coulombic efficiency (CE) of 99.58% in Li||Cu half cells, stable voltage profiles, and dense and uniform lithium deposition, as well as effective inhibition of Li dendrite formation in the symmetrical cell, are achieved. More importantly, the DFEB-LHCE can be matched with various cathodes such as LFP, NCM811, and S cathodes, and the Li||LFP full cell using DFEB-LHCE possesses 85% capacity retention after 650 stable cycles with 99.9% CE. Especially the 1.5 Ah practical lithium metal pouch cell achieves an excellent capacity retention of 89% after 250 cycles with a superb average CE of 99.93%. This work unravels the superiority of the Li2O-dominated SEI and the feasibility of tailoring SEI components through modulation of solvation structures.

Determining the Composition and Structure of a High-Performing SEI Layer for Lithium-Mediated Nitrogen Reduction to Ammonia

The industrial synthesis of ammonia (NH3) via the Haber-Bosch process has enabled mass production of fertilizer and the human population growth to 8 billion.1 NH3 also shows promise as a long-term, energy-dense fuel amenable to widespread distribution.2 However, the Haber-Bosch process operates at high temperatures and pressures in centralized facilities and accounts for 1.3% of global carbon emissions.1,2 To ensure a food-secure future and realize NH3 as a truly carbon-free fuel, we must develop an NH3 synthesis alternative that operates at ambient conditions and can be driven by renewables.One of the most promising NH3 synthesis method under mild conditions is the electrochemical, Li-mediated process (Li-NRR). In this process, Li is electrodeposited from solution and reacted with N2 to form Li3N. A proton shuttle, e.g., ethanol, protonates Li3N to form NH3.3 The electrodeposited Li forms a solid-electrolyte interphase (SEI) layer that controls N2, Li+, and H+ transport and is key to system performance. The use of fluorinated Li salts, e.g., LiBF4, have recently advanced Li-NRR system performance. Improved selectivity has been attributed to changes in the SEI layer, but evaluating SEI structure and composition remains a significant challenge.3 Neutron-based characterization methods, complementary to X-ray methods, offer unique benefits to study SEI layers containing light elements such as Li- and H; for example, an element’s neutron scattering cross section does not depend on its atomic number.4 In this work, we studied the formation of the SEI layer in-situ before and after applying -0.3 mA/cm2 in galvanostatic experiments (chronopotentiometry, CP) on a copper (Cu) working electrode. We utilized a combination of deuterated and non-deuterated solvents and proton donors to distinguish their contributions to the SEI. We found that the SEI forms two layers: a compact, lithium rich layer, and a diffuse layer with carbonaceous species (Figure 1).4 We also identified the presence of boron in the layers, indicated by neutron absorption. Finally, we observed more proton-rich species in experiments containing the non-deuterated H+ donor, corroborating previous studies that indicate that Li-ethoxide may be a major component of the diffuse SEI (Figure 1b-c).5 Compared to LiClO4-based electrolytes, the structure of these LiBF4-derived layers is consistent even after repeated current cycling, indicating a more stable SEI structure and suggesting a relationship between SEI stability and increased Li-NRR performance.References Nørskov, J. et al. DOE Roundtable Report (2016).Chang, F. et al. Advanced Materials 33, 2005721 (2021).Li, S. et al. Joule 6, 2083–2101 (2022).Blair, S.J., Doucet, M. et al. ACS Energy Lett., 7, 6, 1939-1946 (2022).Steinburg, K. et al. Nature Energy, 8, 138-148 (2023). Figure 1: (a) reflectivity curve before and after 4 mins constant applied current at -0.3 mA/cm2 (chronopoteniometry, CP) to plate Li using 1 M LiBF4 with 1 vol% ethanol, (b) resulting scattering length density (SLD) profile, and (c) schematic of layers indicated in SLD profile Figure 1

Effect of Electrolyte on Sodium-Ion Storage Behavior into Non-Graphitizable Carbon Negative Electrode

Introduction Sodium-ion batteries (SIBs) are expected to be an alternative power source to lithium-ion batteries (LIBs) because their abundant resources reduce the cost of SIBs. As the negative electrode of SIBs, non-graphitizable carbon recieves a lot of attention because it can store ions not only in the graphene interlayer but also in its internal pores.[1] By controlling the pores, it is possible to achieve larger reversible capacity of more than 400 mAh g−1.[2] While many researchers have studied non-graphitizable carbon from the viewpoint of thermodynamics, there are a few reports that pay attention to kinetic viewpoint. The kinetic viewpoint is a very important factor because it affects the rate performance of the battery.We have focused on the interfacial reactions of non-graphitizable carbon electrodes and found that the charge-transfer resistance and the activation energy of the charge-transfer reaction in SIBs were larger than those in LIBs.[3] This result and the Lewis acidity indicated that the desolvation process was not identified as the rate determining step in the charge-transfer reaction. One of the significant factors responsible for the increased resistance is the effect of the solid electrolyte interphase (SEI). Objective In this work, we used six types of electrolytes and formed SEI on the surface of non-graphitizable carbon. The interfacial reaction of non-graphitizable carbon with SEI derived from these electrolytes was investigated.In this paper, we report about the effects of SEI on the interfacial reaction between electrolyte and non-graphitizable carbon electrode. Experimental Non-graphitizable carbon heat-treated at 2273 K (HC-2000) was used as the negative electrode material. A three-electrode cell was used for electrochemical measurements. HC-2000 composite electrode, natural graphite composite electrode, and sodium metal were used as the working electrode, counter electrode, and reference electrode, respectively. The electrolytes were prepared using sodium bis(trifluoromethanesulfonyl) amide (NaTFSA), sodium bis(fluorosulfonyl) amide (NaFSA), and NaPF6 as sodium salts and ethylene carbonate (EC) + diethyl carbonate (DEC) (1:1 by volume) and fluoroethylene carbonate (FEC) as solvents.Cyclic voltammetry (CV) was performed to form SEI on HC-2000. The scan rate and range were 0.1 mV s−1 and 0–2.5 V, respectively. After CV, electrochemical impedance spectroscopy (EIS) was performed with an electrode potential of 0.2 V and an AC amplitude of 10 mV in the frequency range of 100 kHz–10 mHz. In addition, X-ray photoelectron spectroscopy (XPS) was used to analyze the electrode after CV measurements with respect to SEI. Results and discussion Figure 1 shows the Nyquist plots obtained from the EIS analyses. The semicircles in the low frequency region were attributed to the charge-transfer resistances. Among EC+DEC solvents, the system of NaFSA had the largest charge-transfer resistance. For NaTFSA and NaPF6, the charge-transfer resistances increased in the FEC solvent, while no significant differences in the charge-transfer resistance were observed for the NaFSA system in the EC+DEC and FEC solvents.The SEI components of the HC-2000 composite electrode in each electrolyte were determined by XPS analysis after CV measurements. In NaFSA/EC+DEC system, the amount of NaF was remarkably large, while the amount of NaF was relatively small in NaTFSA/EC+DEC and NaPF6/EC+DEC systems. On the other hand, in FEC solvents, the amount of NaF increased in NaTFSA and NaPF6 systems. The systems with higher NaF contents had the larger charge-transfer resistances. This result indicates that NaF was involved in the charge-transfer resistance.Finally, EIS analyses were performed at different temperatures. The activation energies were calculated using Arrhenius plots and the Arrhenius equation. The values of the activation energies were in the range of 60–70 kJ mol−1. There was a slight variation in the values, but they were within the error range and were not significantly different for all systems. These results showed that NaF affects the frequency factor rather than the activation energy. Conclusion The effects of SEI on the interfacial reaction between electrolyte and non-graphitizable carbon were investigated. The systems with the large charge-transfer resistance had a large amount of NaF. NaF did not affect the activation energy of charge-transfer resistance but did affect the frequency factor.

Amorphization of Inorganic Solid Electrolyte Interphase Components and Its Impact on Enhancing Their Transport and Mechanical Properties.

The safety and cycle life of lithium-ion batteries (LIBs) are largely determined by the solid electrolyte interphase (SEI) formed on the surface of the anode. However, there is still a lack of understanding regarding the structure and properties of the individual SEI components. Among others, lithium oxide (Li2O), lithium carbonate (Li2CO3), and lithium fluoride (LiF) are known to be the main components of the inorganic SEI layer in conventional LIBs, but their intrinsic protective roles remain controversial. Herein, we present the transformational effects of their amorphous phase on the mechanical and transport characteristics, based on first-principles calculations. Our studies clearly demonstrate that their amorphous phases exhibit significantly improved Li-ion conductivity when compared to the crystalline structures. Additionally, among them, amorphous LiF emerges as a frontrunner for fast Li+ ion transportation, reversing the conventionally understood hierarchy. Under ambient conditions, the amorphous phases of LiF, Li2O, and Li2CO3 are thermodynamically unstable and tend to undergo recrystallization. However, this work highlights that exceptionally ductile and resilient amorphous phases can form if SEI formation and growth would involve some admixing of lithiophilic impurities like nitrogen (N) within the host matrices.

Molecular-level identification of organic species of ether-based solid-electrolyte interphase in sodium-ion batteries

The lack of precise chemical information of solid-electrolyte interphase (SEI) is a huge obstacle for battery technology breakthroughs. Herein we identify the organic component of diglyme-based SEI by artificially synthesizing bulk crystals and reveal its intrinsic properties. The organic component of SEI shows a layered structure and it actually is formed by the co-solvation of Na+, triflate ion and diglyme (Na3T3D), rather than by the electrochemical reduction of diglyme. Low-current-beam transmission electron microscopy revealed that the organic component is crystalline and soluble in most solvents, which upend humanity’s understanding of its thin and amorphous features. Further, results disclosed that Na3T3D is the only organic SEI component and it is a super ionic conductor with rational migration pathways and facile energy barrier for Na+ migrating. Our work has overturned some existing misconceptions about the diglyme-based SEI and disclosed its essential characteristics.

The influence and mechanism of transition metal ions dissolution in lithium-ion batteries

Lithium-ion battery (LIB) has been extensively applied in the field of power vehicles and energy storage. Extending the service life is the research priority. Transition metal (TM) ions dissolved from the cathode are considered a key factor affecting battery electrochemical properties. To reveal comprehensively the mechanisms of influence of TM ions at the full-cell level, this paper reviews the recent efforts of the exploration of TM ions affecting lithium-ion batteries using conventional metal oxide as cathode matched with graphite, silicon-containing, or Li metal as anode, respectively. The existence of TM ions in electrolyte or solid electrolyte interphase (SEI) films can change the morphology, the structure as well as the component concentration of the SEI films and accelerate the side reaction as a catalyst. In the comparison of different TM ions, the failure mechanism could be diverse, and it is proved that Mn ion is the most detrimental and extensively studied. However, in some special lithium-ion battery systems, TM ions are even in favour of electrochemical performance by forming LiF-rich SEI components.

Constructing Solid Electrolyte Interphase for Aqueous Zinc Batteries

AbstractProblems of zinc anode including dendrite and hydrogen evolution seriously degrade the performance of zinc batteries. Solid electrolyte interphase (SEI), which plays a key role in achieving high reversibility of lithium anode in aprotic organic solvent, is also beneficial to performance improvement of zinc anode in aqueous electrolyte. However, various studies about interphase for zinc electrode is quite fragmented, and lack of deep understanding on root causes or general design rules for SEI construction. And water molecules with high reactivity brings serious challenge to the effective SEI construction. Here, we reviewed the brief development history of zinc batteries firstly, then summarized the approaches to construct SEI in aqueous electrolyte. Furthermore, the formation mechanisms behind approaches are systematically analyzed, together with discussion on the SEI components and evaluation on electrochemical performance of zinc anode with various types of SEI. Meanwhile, the challenge between lab and industrialization are also discussed.

Constructing Solid Electrolyte Interphase for Aqueous Zinc Batteries.

Problems of zinc anode including dendrite and hydrogen evolution seriously degrade the performance of zinc batteries. Solid electrolyte interphase (SEI), which plays a key role in achieving high reversibility of lithium anode in aprotic organic solvent, is also beneficial to performance improvement of zinc anode in aqueous electrolyte. However, various studies about interphase for zinc electrode is quite fragmented, and lack of deep understanding on root causes or general design rules for SEI construction. And water molecules with high reactivity brings serious challenge to the effective SEI construction. Here, we reviewed the brief development history of zinc batteries firstly, then summarized the approaches to construct SEI in aqueous electrolyte. Furthermore, the formation mechanisms behind approaches are systematically analyzed, together with discussion on the SEI components and evaluation on electrochemical performance of zinc anode with various types of SEI. Meanwhile, the challenge between lab and industrialization are also discussed.

Experimental and Computational Study on SEI Composition and Electrochemical Performance of Lithium-Ion Battery with Silicon Oxide-Graphite Composite Electrode

Demand on batteries with high energy density is surging. Conventionally, carbon materials have been used as an anode active material, but recently silicon materials are likely to be incorporated to form a composite anode in lithium-ion batteries (LIBs) for higher energy density. However, LIBs which include silicon materials in anode tend to show a low cycle performance especially the ratio of silicon materials in the anode increases. Commercial LIBs still have difficulty in incorporating large amount of silicon materials especially under the requirements where multiple aspects like safety, charging rate and energy density should be totally met.One potential pathway to improve the cycle performance in LIBs with silicon materials is to tune the nature of solid electrolyte interphase (SEI), a thin film which forms at the interface between anode active material and electrolyte. The nature of SEI is particularly important for LIBs with silicon materials because such silicon active materials show large volume changes during battery operation and SEI is dynamically reconstructed. It has been qualitatively reported so far that LiF is an important component for better battery performance. Our work suggests that the ratio of LiF in initially formed SEI is a crucial factor to realize improved cycle performance in such LIBs.In our work, anode with silicon oxide (SiOx)-graphite composite is considered. We investigated the relationship between electrochemical performance of the cell with such composite electrode and the nature of the film estimated by characterization and molecular simulation. We systematically changed the composition of liquid electrolyte to form various types of SEI on anode surface. For characterization, both fresh and aged cells with different electrolytes were dismantled, and SEI components were extracted by designated solvents, then NMR (nuclear magnetic resonance) was applied to identify the SEI species and quantify the amounts of major SEI components. Comparison between the evolution of SEI amount and an impedance element in electrochemical impedance spectroscopy during aging indicates that degradation of the cells is closely related to the growth of SEI. To further deepen understanding the role of LiF, molecular dynamics simulation was carried out, where the mixture of LiF and typical organic SEI was modelled. Correlation between calculated mechanical property and measured electrochemical performance of the cells were revealed. In our presentation, the requirements that SEI should satisfy will be discussed from the insight of electrochemical measurements, NMR characterization and modeling approaches.

Reactive Force Field (ReaxFF) and Universal Force Field Molecular Dynamic Simulation of Solid Electrolyte Interphase Components in Lithium-Ion Batteries

Abstract Understanding solid electrolyte interphase (SEI) is essential for the diagnosis of lithium-ion batteries because many aspects of battery performance such as safety and efficiency depend on these characteristics. LiF, Li2O, and Li2CO3 are important inorganic components of SEI. This electrode–electrolyte surface forms during the battery’s first charging/discharging cycle, preventing electrons’ movement through the electrolyte and stabilizing the lithium-ion battery. However, the concern is inorganic SEI components cause rate limitation of lithium-ion diffusivity through the SEI layer. Lithium-ion diffusivity through the SEI layer depends on many factors such as temperature, the width of the SEI layer, and the concentration/density of the layer. Lithium-ion diffusivity dependence on temperature, at working temperatures of lithium-ion batteries was observed at temperatures from 250 K to 400 K and diffusion coefficient data at higher temperatures have also been observed. Lithium-ion diffusivity at varying concentration/density was also observed in this paper using the reactive force field (ReaxFF) molecular dynamic simulation. To improve the lithium-ion diffusivity, vacancy defects were created in the inorganic components of the SEI layer LiF, Li2O, and Li2CO3 and the diffusion coefficient was obtained using the ReaxFF molecular dynamic simulations. Another approach to improve the lithium-ion diffusivity is doping alkali metal ions such Na, Ca, K, and Mg in the inorganic components of SEI layers of LiF, Li2O, and Li2CO3 and simulated using the universal force field (UFF), and the diffusion coefficient was observed.

Reforming the Uniformity of Solid Electrolyte Interphase by Nanoscale Structure Regulation for Stable Lithium Metal Batteries.

The stability of high-energy-density lithium metal batteries depends on the uniformity of solid electrolyte interphase (SEI) on lithium metal anodes. Rationally improving SEI uniformity is hindered by poorly understanding the effect of structure and components of SEI on its uniformity. Herein, a bilayer structure of SEI formed by isosorbide dinitrate (ISDN) additives in localized high-concentration electrolytes was demonstrated to improve SEI uniformity. In the bilayer SEI, LiNx Oy generated by ISDN occupies top layer and LiF dominates bottom layer next to anode. The uniformity of lithium deposition is remarkably improved with the bilayer SEI, mitigating the consumption rate of active lithium and electrolytes. The cycle life of lithium metal batteries with bilayer SEI is three times as that with common anion-derived SEI under practical conditions. A prototype lithium metal pouch cell of 430 Wh kg-1 undergoes 173 cycles. This work demonstrates the effect of a reasonable structure of SEI on reforming SEI uniformity.