LiF-rich Solid Electrolyte Interphase Research Articles

An effective solid-electrolyte interphase for stable solid-state batteries

An effective solid-electrolyte interphase for stable solid-state batteries

Interplay between solid-electrolyte interphase and (in)active LixSi in silicon anode

Solid-electrolyte interphase (SEI) is regarded as the most important but the least understood part of lithium (Li)-ion batteries. A comprehensive understanding of the nature of the SEI and especially its interplay with active materials during cycling is crucial since it governs the charge transfer and Li+ transport. Herein, the dynamic interplay between SEI and silicon (Si) anode during cycling is revealed quantitatively and qualitatively by titration gas chromatography (TGC), cryogenic transmission electron microscopy (cryo-TEM), and other techniques to probe charge transfer, nanostructure, and equilibrium. The results show that it is difficult to construct an equilibrium interplay between the SEI and LixSi due to the intrinsic instability of some SEI components (e.g., Li2O and carbonates) and the pulverization of Si anode, resulting in the continuous formation of the SEI and inactive LixSi. The addition of fluoroethylene carbonate helps construct such equilibrium interplay through formation of a LiF-rich SEI, thus improving cyclability.

Lithiophilic NiF2 coating inducing LiF-rich solid electrolyte interphase by a novel NF3 plasma treatment for highly stable Li metal anode

A three-dimensional lithiophilic skeleton is an effective strategy that is widely studied due to its large specific surface area and the lithiophilicity for suppressing the growth of dendrites on Li metal anodes. However, there are few studies on the lithiophilicity of the metal fluoride. In this study, we employ a novel NF3 plasma fluoridized strategy to fabricate a NiF2 coating on Ni foams for the first time. Due to the lithiophilic NiF2 layer, the Li+ flux tends to deposit preferentially on the sites of NiF2 and homogeneously distributes around the surface of the Ni foam, leading to a smooth and dense Li deposition. Moreover, the LiF-rich solid electrolyte interphase layer generated from the reaction between Li and NiF2 significantly improves the interfacial stability. The fluoridized Ni foam exhibits a highly stable cycling performance of over 2000 cycles at 2 mA cm−2 and over 1600 h at 1 mA cm−2. In addition, the fluoridized Ni foam@Li || LiFePO4 full cell delivers a capacity retention of 93% after 250 cycles at 1C. This work provides a safe and simple plasma strategy to construct a lithiophilic metal fluoride coating for strengthening the solid electrolyte interphase and suppressing the growth of Li dendrites.

A high-performance, solution-processable polymer/ceramic/ionic liquid electrolyte for room temperature solid-state Li metal batteries

All-solid-state electrolytes provide a guarantee for the safe running of Li metal batteries (LMBs) with high energy density. Nevertheless, the low ionic conductivity and huge interfacial impedance between lithium anodes and electrolytes are the critical issues baffling their rapid development and practical application, particularly limiting their operation at room temperature. The introduction of ionic liquids (IL) is expected to solve the above problems. However, the effect of the IL-involved solid-state electrolytes on lithium dendrites suppression has not been clearly revealed and still necessitates in-depth evaluation. In this article, we report an in situ LiF-rich solid-electrolyte interphase (SEI) on the lithium anode triggered by reductive decomposition of IL and Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 -involved electrolyte. For the first time, the mechanism of SEI formed on Li metal based on IL-based solid-state electrolyte was unveiled. A combination of experimental and computational investigation manifests that the presence of Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 promotes the release of fluorine anion from IL, and a SEI layer with high content of LiF can be generated in situ through the reductive decomposition of wandering fluorine anion. Thanks to the high mechanical modulus from Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 , the symmetric Li｜Li batteries equipped with synthesized solid-state composite electrolyte (SSCE) exhibit extremely stable Li plating/stripping behavior for more than 2700 h with a small polarization voltage of 50 mV at 0.1 mA cm −2 . Moreover, the assembled solid-state Li｜LiFePO 4 batteries based on SSCE could operate steadily for 196 cycles at ambient temperature, with 90.7% capacity retention. These results provide a promising insight into the design of SSCE and realization of room temperature solid-state LMBs with high performance. • Solid-state electrolyte based on ionic liquid/polymer/ceramic composites for room temperature solid-state LMBs is fabricated. • The synergistic effect on the SEI formation with high content of LiF is unveiled by experimental and computational study. • The high ionic conductivity and excellent mechanical strength of the SSCE enable superior cyclical stability.

A Bi-Layer Dense/Porous Solid Electrolyte Interphase for Enhanced Lithium-Metal Stability

Due to its high theoretical capacity and low electrochemical potential, lithium metal is a highly investigated anode for next-generation high-energy batteries. However, the unstable chemical and topographical heterogeneous surface of lithium gives rise to safety and efficiency concerns that prevent it from being utilized in practical applications. Exposure to electrolyte leads to non-uniform, mixed organic-inorganic solid-electrolyte interphase (SEI) formation, starting a cascading to non-uniform flux distribution, consumption of active materials, and dendrite growth. The SEI is the key feature that will lead to harnessing the capabilities possible with lithium metal.In this work, the formation of a closed-host bi-layer solid electrolyte interphase (SEI) improves the stability of lithium anode. This is successfully realized by forming an interconnected porous LiF-rich artificial SEI in contact with Li metal, and a dense, stable in-situ formed upper layer SEI. The porous layer increases the number of Li/LiF interfaces, which reduces local volume fluctuations and improves Li+ diffusion along these interfaces. Additionally, the tortuous porous structure guides uniform Li+ flux distribution and mechanically suppresses dendrite propagation. The dense upper layer of the SEI accomplishes a closed-host design through reaction with LiNO3 additive and prevents continuous consumption of active materials seen without the additive included in the electrolyte. The duality of a dense top layer with porous bottom layer led to extended cycle life and improved rate performance, evidenced with symmetric cell testing, as well as full cell testing paired with sulfur and LiFePO4 (LFP) cathodes. This work is a good example of a rational design of the SEI, based on comprehensive consideration of various critical factors to improve Li-metal anode stability, and highlights a new pathway to improve cycling and rate performances of Li metal batteries. Figure 1

Lithium Cyano Tris(2,2,2-trifluoroethyl) Borate as a Multifunctional Electrolyte Additive for High-Performance Lithium Metal Batteries

Lithium cyano tris(2,2,2-trifluoroethyl) borate (LCTFEB) has been synthesized and investigated as a new electrolyte additive for high-performance lithium metal batteries. LCTFEB is prepared by the reaction of tris(2,2,2-trifluoroethyl) borate with lithium cyanide. The incorporation of LCTFEB into a carbonate-based electrolyte has been investigated. The electrochemical performance of NCM523/Li cells and symmetric Li/Li cells is significantly improved upon the incorporation of LCTFEB (5 wt %) into the electrolyte. Various characterizations of the lithium metal morphology and the solid electrolyte interphase (SEI) on the lithium metal anode have been conducted using field-emission scanning electron microscopy, cryogenic transmission electron microscopy, and X-ray photoelectron spectroscopy, suggesting the generation of a thin (∼10 nm) LiF-rich SEI with low concentrations of B and N compounds. The different SEI structures are formed by the preferential reductive decomposition of LCTFEB, resulting in improved electrochemical performance. The new electrolyte additive provides insight into innovative approaches for multifunctional electrolyte additive design.

Design of a LiF-Rich Solid Electrolyte Interphase Layer through Highly Concentrated LiFSI-THF Electrolyte for Stable Lithium Metal Batteries.

Lithium metal is a promising anode material for lithium metal batteries (LMBs). However, dendrite growth and limited Coulombic efficiency (CE) during cycling have prevented its practical application in rechargeable batteries. Herein, a highly concentrated electrolyte composed of an ether solvent and lithium bis(fluorosulfonyl)imide (LiFSI) salt is introduced, which enables the cycling of a lithium metal anode at a high CE (up to ≈99%) without dendrite growth, even at high current densities. Using 3.85 m LiFSI in tetrahydrofuran (THF) as the electrolyte, a Li||Li symmetric cell can be cycled at 1.0 mA cm-2 for more than 1000 h with stable polarization of ≈0.1 V, and Li||LFP cells can be cycled at 2 C (1 C = 170 mA g-1 ) for more than 1000 cycles with a capacity retention of 94.5%. These excellent performances are observed to be attributed to the increased cation-anion associated complexes, such as contact ion pairs and aggregate in the highly concentrated electrolyte; revealed by Raman spectroscopy and theoretical calculations. These results demonstrate the benefits of a high-concentration LiFSI-THF electrolyte system, generating new possibilities for high-energy-density rechargeable LMBs.

Nonflammable highly-fluorinated polymer electrolytes with enhanced interfacial compatibility for dendrite-free lithium metal batteries

Conventional liquid electrolytes applied in lithium-ion batteries suffer from high flammability, lithium dendrites and leakage. Gel polymer electrolytes (GPEs) are promising alternative electrolytes owing to their high ionic conductivities and enhanced safeties. Nevertheless, the flammable plasticizers of GPEs still bring safety hazards for the batteries. To address these issues, nonflammable highly-fluorinated quasi-solid-state polymer electrolytes (ED@PVDF) with ultrahigh ionic conductivities, excellent flame-retardant properties and superior cycling stability are developed. ED@PVDF possesses three-dimensional polymeric network within the framework of electrospun PVDF fibers, which is fabricated by thiol-ene click reaction. Highly-fluorinated carbonates (fluoroethylene carbonate and methyl 2,2,2-trifluoroethyl carbonate) are employed as plasticizers for ED@PVDF to improve flame-retardant property and enhance compatibility with anodes. Nonflammable ED@PVDF exhibits remarkable flame resistance, the highest ionic conductivity (4.41 mS cm−1 at 30 °C) among the nonflammable polymer electrolytes and wide electrochemical windows (5.6 V). ED@PVDF can promote the generation of LiF-rich solid electrolyte interphase and effectively restrain dendrite growth on Li metal anodes. ED@PVDF endows LiFePO4 cells with outstanding rate capability (specific capacity of 123.8 mAh g−1 at 5C rate) and superior cycling stability (capacity retention of 81.4% over 1000 cycles).

Dendrite-free lithium anode achieved under lean-electrolyte condition through the modification of separators with F-functionalized Ti3C2 nanosheets

An unstable solid electrolyte interphase (SEI) and chaotic lithium ion flux are key impediments to commercial high-energy-density lithium batteries because of the uncontrolled growth of rigid lithium dendrites, which would pierce through the conventional polypropylene (PP) separator, causing short circuit and safety issues. Herein, the homogenization of lithium ion flux and the generation of stable SEI layers on lithium anodes were achieved via coating a fluorine-functionalized Ti3C2 (F-Ti3C2) nanosheets on PP separator (F-Ti3C2@PP). F-Ti3C2 nanosheets provide abundant ions pathways to homogeneously manipulate lithium ion flux and increase the Young’s modulus and electrolyte wettability of the separators. In addition, F species derived from the F-Ti3C2 nanosheets would promote the formation of LiF-rich SEI film. The synergistic effect contribute to the uniform lithium deposition. Symmetric Li|Li, asymmetric Li|Cu and full Li|LiFePO4 cells incorporated with the modified separators exhibit improved electrochemical performance even under lean electrolyte conditions. This work provides a feasible strategy to improve the performance of lithium batteries through both fluoridized SEI formation and lithium ion flux manipulation.

New Insights into the Mechanism of LiDFBOP for Improving the Low-Temperature Performance via the Rational Design of an Interphase on a Graphite Anode.

The high impedance of the solid electrolyte interphase (SEI) is one of the important factors that deteriorate the charge behavior of lithium-ion batteries (LIBs) at low temperatures, which hinders their practical application in portable electronic products and electric vehicles under extreme conditions. Based on this consideration, a LiF-rich SEI film with low impedance, using lithium difluorobis(oxalato)phosphate (LiDFBOP) as an electrolyte additive and a blank electrolyte without commercial additives, is constructed on a graphite surface. The decomposition mechanism of LiDFBOP is further deduced by density functional theory calculations. This additive inhibits the decomposition of the electrolyte and then forms a thin SEI film with more LiF. LiF, possessing high Young's modulus, makes the SEI film dense and stable. At the same time, more LiF/Li2CO3 interfaces are formed to increase the ionic conductivity. Benefiting from the components and the structure of the SEI, the graphite/Li cells exhibit excellent cycling stability (ca. 85.5% initial capacity retention for 200 cycles at 1 C) and an impressive low-temperature performance (ca. 200% capacity for electrolytes without LiDFBOP at -20 °C). This work presents an effective strategy for developing a functional electrolyte to meet the requirement of LIBs with enhanced low-temperature performance.

In situ formation of polymer-inorganic solid-electrolyte interphase for stable polymeric solid-state lithium-metal batteries

<h2>Summary</h2> Composite polymer electrolytes (CPEs) for solid-state Li-metal batteries (SSLBs) still suffer from gradually increased interface resistance and unconstrained Li-dendrite growth. Herein, we addressed the challenges by designing a LiF-rich inorganic solid-electrolyte interphase (SEI) through introducing a fluoride-salt-concentrated interlayer on CPE film. The rigid but flexible CPE helps accommodate the volume change of electrodes, while the polymeric highly concentrated electrolyte (PHCE) surface-layer regulates Li-ion flux due to the formation of a stable LiF-rich SEI via anion reduction. The designed CPE-PHCE presents enhanced ionic conductivity and high oxidation stability of >5.0 V (versus Li/Li⁺). Furthermore, it dramatically reduces the interfacial resistance and achieves a high critical current density of 4.5 mA cm⁻². The SSLBs, fabricated with thin CPE-PHCE membranes (<100 μm) and Co-free LiNiO₂ cathodes, exhibit exceptional electrochemical performance and long cycling stability. This approach of SEI design can also be applied to other types of batteries.

Doubling the cyclic stability of 3D hierarchically structured composites of 1T-MoS2/polyaniline/graphene through the formation of LiF-rich solid electrolyte interphase

MoS2 has been viewed as a unique anode material for lithium-ion batteries; however, its electrochemical performance is usually restrained by poor conductivity of 2H phase and an uneven solid-electrolyte interphase (SEI). To tackle the above issues, the ternary composite is synthesized by self-assembling MoS2 2D sheets with higher percentage of metallic 1T-phase on polyaniline functionalized reduced graphene oxide (PANI-rGO) through an electrostatic interaction. The flower-like aggregate of 1T-MoS2 having increased interlayer distances is beneficial for diffusion and intercalation of lithium ions. Furthermore, the graphene substrate facilitates the electron transportation. At the current density of 0.1 and 2 A g−1, the ternary composite has the specific capacities of 812 and 333 mAh g−1 respectively. On the other hand, the cyclic stability of ternary composite can be further improved when 10 wt% fluoroethylene carbonate (FEC) is added into the traditional carbonate-based electrolyte. The capacity retention of ternary composite cycled in a FEC-mixed electrolyte is two times higher than that in a traditional electrolyte after 400 charge–discharge cycles. This is attributed to the fact that the use of FEC enables the formation of active LiF-rich SEI.

Can an Inorganic Coating Serve as Stable SEI for Aqueous Superconcentrated Electrolytes?

Developing a stable, conformal solid electrolyte interphase (SEI) for aqueous-based Li-ion batteries has been a long-awaited dream to support the development of nontoxic and eco-friendly energy storage technologies. Toward that goal, aqueous superconcentrated electrolytes were recently introduced as their unique solvation structure allows for forming a LiF-rich SEI layer at the negative electrode, imparting the stability to the interface. However, the intrinsic stability of such LiF-rich SEI was never measured, despite growing evidence of poor passivation properties and water reduction upon operation. In this work, LiF conformal layers were coated onto lithium electrodes, and their reactivity toward superconcentrated aqueous electrolytes was studied by combining solubility measurements, in situ microscopy, and gas chromatography. We demonstrate that the use of superconcentrated electrolytes drastically reduces the solubility of LiF. However, such layer is intrinsically unstable in aqueous environments, but stable in organic electrolytes, owing to the absence of self-passivation. Comparing different interfaces, we conclude that an artificial SEI made of an inorganic coating is not suitable for preventing water reactivity in aqueous systems.

Design Rules for Selecting Fluorinated Linear Organic Solvents for Li Metal Batteries.

Fluorinated linear organic solvents have great potential in improving the safety and lifetime of next-generation Li metal batteries. However, this group of solvents is underexplored. Here, we investigate the molecular and interfacial reactivity properties of seven partially and fully fluorinated linear carbonates designed based on conventional solvents. Using density functional theory, we find the highest occupied molecular orbital levels decrease with increasing substitution of the fluorinated functional groups, implying that fluorination, to a first approximation, improves the stability toward high voltage cathodes. On the basis of the simulated decomposition mechanisms and statistical analyses, we find that a fluorinated linear carbonate with partial fluorination at the methyl component is more accessible in terms of degradation and LiF nascence formation, leading to a potentially LiF-rich solid electrolyte interphase (SEI). The molecular design concepts and the computational techniques presented are transferable to ester and ether systems, facilitating the navigation in a large chemical design space.

Stabilizing Li-metal host anode with LiF-rich solid electrolyte interphase

The development of lithium (Li)-metal anode is high priority research to initiate next-generation Li batteries. Applying Li-metal in practical applications as anode still has many hurdles to clear away, such as low Coulombic efficiency and capacity degradation by the continuous formation of dead Li. We demonstrate that cobalt (Co) nanoparticle incorporation in a porous carbon host anode can play a critical role in the formation of a thick lithium fluoride dominated solid-electrolyte interphase in ether-based electrolyte. As a result, the host anode containing Co nanoparticles shows excellent electrochemical performance with high Li-metal reversible capacity and even stable long-term cyclability with no dead Li formation.

Local Solvation Modulates Activity of Covalent C−F Bonds in Fluorinated Electrolyte Additives

Fluorinated reactants that enable fluoride bond cleavage, either in electrolytes or cathodes, are receiving increased research attention because of the formation of highly stable decomposition products (e.g. LiF), which also allows large Gibbs free energy change. Electrolytes using fluorinated solvents or additives (e.g. fluoroethylene carbonate (FEC), methyl 2,2,2-trifluoroethyl carbonate (FEMC), and liquefied CH3F) were shown to be able to effectively increase Li cyclability by forming LiF rich solid electrolyte interphase (SEI) on Li. In addition, conversion type Li battery cathodes, such as transition-metal fluorides (MFy, M=Fe, Co, Ni, Cu, y=2, 3) and CFx, can deliver high energy densities through fluoride bond reduction reactions. However, the strong electronegativity of fluorine making most of the fluoride bonds highly stable (i.e. hard to break), which limits their applications in electrochemical systems. Therefore, fundamental understanding of how to modulate fluoride bond strength to unlock activity is of vital importance. Herein, we use perfluoroalkyl group (RF)-containing liquid reactants as model systems to demonstrate that the covalent C−F bonds in RF, which were generally believed to be extremely difficult to reduce, can be unlocked through combined reactant-electrolyte design, with the electrolyte playing a central role in governing activity. The interaction between solvent and fluorinated reactants, such as solvation effects and formation of hydrogen bonds, can alter the electron density distribution along the carbon-fluorine skeleton and weaken fluoride bond strengths as indicated by nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) calculations. As a result, the reduction potential (from galvanostatic measurements for Li cells) can be dramatically increased from < 2.0 V to > 2.9 V vs. Li/Li+ through modulation of the solvent. In addition to activating electrochemically-active pathways, solvent molecules also need to exhibit proper polarities to allow both high miscibility (requiring low polarity), and Li salt solubility (requiring high polarity). Other electrolyte design aspects that can direct the reduction activity of highly fluorinated reactants will also be discussed, including fluoridation of solvent molecules, Li salt dissociation energy, and reactant/salt ratio. These understandings on tuning fluoride bond strength via electrolyte−reactant interactions will open up new design opportunities for fluorinated electrolyte components, either solvents or additives, for Li cells with high cycle stabilities, as well as high-energy Li primary battery with liquid cathodes.

Trans-Difluoroethylene Carbonate as an Electrolyte Additive for Microsized SiOx@C Anodes

Microsized SiOx has been vigorously investigated as an advanced anode material for next-generation lithium-ion batteries. However, its practical application is seriously hampered by its huge volume variation during the repeated (de)lithiation process, which destroys the microparticle structure and results in rapid capacity fading. Herein, we propose the usage of trans-difluoroethylene carbonate (DFEC) as an electrolyte additive to maintain the structural integrity of microsized SiOx with a uniform carbon layer (SiOx@C). Compared with ethylene carbonate and fluoroethylene carbonate, DFEC has lower lowest unoccupied molecular orbital energy and higher reduction potential, which is easily reduced and promotes the in situ formation of a more stable LiF-rich solid electrolyte interphase (SEI) on the surface of anode materials. The LiF-rich SEI exhibits enhanced mechanical rigidity and ionic conductivity, thus enabling the microsized SiOx@C anodes' excellent lithium storage stability and high average Coulombic efficiency.

A closed-host bi-layer dense/porous solid electrolyte interphase for enhanced lithium-metal anode stability

Thanks to its high specific capacity and low electrochemical potential, lithium metal is an ideal anode for next-generation high-energy batteries. However, the unstable heterogeneous surface of lithium gives rise to safety and efficiency concerns that prevent it from being utilized in practical applications. In this work, the formation of a closed-host bi-layer solid electrolyte interphase (SEI) improves the stability of lithium metal anode. This is successfully realized by forming an interconnected porous LiF-rich artificial SEI in contact with Li metal, and a dense, stable in-situ formed upper layer SEI. The porous layer increases the number of Li/LiF interfaces, which reduces local volume fluctuations and improves Li+ diffusion along these interfaces. Additionally, the tortuous porous structure guides uniform Li+ flux distribution and mechanically suppresses dendrite propagation. The dense upper layer of the SEI accomplishes a closed-host design, preventing continuous consumption of active materials. The duality of a dense top layer with porous bottom layer led to extended cycle life and improved rate performance, evidenced with symmetric cell testing, as well as full cell testing paired with sulfur and LiFePO4 (LFP) cathodes. This work is a good example of a rational design of the SEI, based on comprehensive consideration of various critical factors to improve Li-metal anode stability, and highlights a new pathway to improve cycling and rate performances of Li metal batteries.

Polar interaction of polymer host–solvent enables stable solid electrolyte interphase in composite lithium metal anodes

The lithium (Li) metal anode is an integral component in an emerging high-energy-density rechargeable battery. A composite Li anode with a three-dimensional (3D) host exhibits unique advantages in suppressing Li dendrites and maintaining dimensional stability. However, the fundamental understanding and regulation of solid electrolyte interphase (SEI), which directly dictates the behavior of Li plating/stripping, are rarely researched in composite Li metal anodes. Herein, the interaction between a polar polymer host and solvent molecules was proposed as an emerging but effective strategy to enable a stable SEI and a uniform Li deposition in a working battery. Fluoroethylene carbonate molecules in electrolytes are enriched in the vicinity of a polar polyacrylonitrile (PAN) host due to a strong dipole–dipole interaction, resulting in a LiF-rich SEI on Li metal to improve the uniformity of Li deposition. A composite Li anode with a PAN host delivers 145 cycles compared with 90 cycles when a non-polar host is employed. Moreover, 60 cycles are demonstrated in a 1.0 Ah pouch cell without external pressure. This work provides a fresh guidance for designing practical composite Li anodes by unraveling the vital role of the synergy between a 3D host and solvent molecules for regulating a robust SEI.

Controlling a lithium surface with an alkyl halide nucleophile exchange

Despite its high theoretical energy density, lithium metal faces huge challenges in its implementation as an anode for Li secondary batteries because of its uncontrollable dendritic growth and large volume change during plating/stripping processes. These geometric changes cause degradation in a cell’s cycle life and performance and can lead to short-circuits and explosions. Here, we report a new approach to producing a LiF-rich phase on the lithium anode surface by employing a “bi-phase” separator. We fabricated it by coating polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) onto a cellulose separator. The coverage of coating was adjusted by varying its concentration in the coating solution. The combination of cellulose and PVDF-HFP produced a LiF-rich solid-electrolyte interphase layer on the Li metal surface via an alkyl halide nucleophile exchange in contact with the bi-phase separator during plating. Symmetric cell tests show that the bi-phase separator extends the cycle life by more than 300 h, with overpotentials of less than 100 mV under plating/stripping at 2 mA cm−2 for a capacity of 2 mAh cm−2. The formation mechanism of the LiF-rich phase is suggested from spectroscopic analyses.