An Artificial Hybrid Solid Electrolyte Interface for Reviving Lithium Metal Battery

Li metal is an ideal anode material for next-generation energy storage devices due to its extremely high theoretical specific capacity and the lowest negative electrochemical potential [1,2]. However, uncontrolled Li dendrite growth and the side reaction between Li metal and electrolytes have prevented its practical application in rechargeable Li metal batteries. In our contributions, artificial solid electrolyte interphase (SEI) layer and 3D current collector have been developed to address these issues [3,4]. In this presentation, we will introduce an artificial Li3PO4 SEI layer [3] fabricated by in situ reaction of polyphosphoric acid (PPA) with Li metal and its native film (Li2CO3, LiOH, and Li2O). The partial dissolution of the Li native film and formation of the SEI layer lead to a porous structure of the Li surface, thereby promoting Li dendrite formation and growth. In the ingenious design, the native film of Li metal is replaced by the stable and uniform Li3PO4 SEI layer. The artificial Li3PO4 SEI layer with thickness of 50 nm exhibits a smooth surface and a high Young’s modulus (10-11 GPa). Furthermore, the artificial Li3PO4 SEI layer is demonstrated to be stable in the electrolyte and during cycling in a Li|LiFePO4 battery system. The Li-conducting Li3PO4SEI layer can achieve uniform Li deposition, and reduce the side reaction between the Li metal and the electrolyte. Thus, the artificial SEI strategy we described is a promising route to tackle the intrinsic problems of Li metal anodes and other metal based anodes. Furthermore, we will show that Li metal anode can be accommodated in the reserved pores of the 3D current collector [4] with a submicron skeleton and high surface area, thereby suppressing the Li dendrite growth and solving the associated problems of Li metal anodes. Because of the high electroactive area of the submicron 3D structure, the 3D current collector can improve the Li plating/stripping efficiency and reduce the resistance of the Li metal anode. The utilization of the 3D architecture to accommodate the Li metal anode will highlight the importance of rational design of current collectors and reveal a new avenue for developing Li metal anodes with a long lifespan. References Y.-X. Yin, S. Xin, Y.-G. Guo, L.-J. Wan, Angew. Chem. Int. Ed. 2013, 52, 13186.W. Xu, J. L. Wang, F. Ding, X. L. Chen, E. Nasybutin, Y. H. Zhang, J. G. Zhang, Energ. Environ. Sci. 2014, 7, 513.N.-W. Li, Y.-X. Yin, C.-P. Yang, Y.-G. Guo, Adv. Mater. 2015, DOI: 10.1002/adma.201504526.C.-P. Yang, Y.-X. Yin, S.-F. Zhang, N.-W. Li, Y.-G. Guo, Nat. Commun. 2015, 6, 8058

Lithium metal is often deemed the “Holy Grail” of rechargeable battery technology, due to its high theoretical capacity and low electrochemical potential. However, the cascading heterogeneity from the formation of the solid-electrolyte interphase (SEI) up to the non-uniform current distribution leads to both poor efficiency via rapid consumption of active materials as well as safety issues from treacherous dendrite growth. An SEI that is ionically conductive while electrically resistive, viscoelastic, electrochemically stable, and possesses good mechanical properties is paramount for hindering these failure mechanisms. The discovery of novel approaches to producing an ideal SEI is a crucial piece to solving the lithium metal anode puzzle.One route of controlling the SEI is through interface engineering by producing a stable artificial SEI. In this work, a mixed artificial SEI was produced with aim toward improving ionic conductivity while retaining electrochemical stability and mechanical properties. This mixed SEI embraces the unavoidable surface heterogeneity, all the while preventing dendrite growth that plagues lithium metal batteries. A mixed artificial SEI, as well as a single-component artificial SEI, were produced through a simple and rapid deposition method and tested to observe their failure mechanisms compared to bare lithium.Symmetric cells (i.e., Li||Li) were utilized to isolate the failure mechanisms of these different anode treatments. The failure mechanism of the single-component artificial SEI cells shifted to failure via electrolyte consumption rather than short circuiting seen with bare lithium. This revealed that artificial interphases led to reduced dendrite propagation. Additionally, a reduction in cell potential range was observed with the mixed artificial SEI when compared to the single-component artificial SEI, as well as improved cyclability. Upon further investigation, it appears that the added component of the mixed artificial SEI, which was meant to improve the ionic conductivity, dissolved into the electrolyte upon initial exposure. This dissolution left behind a new SEI structure, theoretically producing easier ionic pathways and thus resulting in decreased cell overpotential. Dissolution was confirmed with high-resolution surface characterization techniques. Additionally, symmetric cell electrodes rinsed with solvent prior to testing retained superior cell performance. Though this dissolution should lead to increased available “hot spots”, the growth of dendrites was hindered. This work discusses the effect an artificial SEI has on failure mechanisms, as well as provides insight into the methods of producing stable, long-life lithium metal batteries. This includes a focus on how nanoscale heterogeneity effects cell-scale performance via high-resolution characterization and computational simulations.Different surface to sub-surface characterization techniques were utilized to measure properties of artificial SEIs and bare lithium. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS) provided elemental make-up and distribution, as well as morphological characteristics. X-ray photoelectron spectroscopy (XPS) provided the chemical state of surfaces, as well as sub-surfaces with the use of depth profiling. Different atomic force microscopy (AFM) techniques were used to spatially measure nanoscale properties such as work functions with the Kelvin probe technique (SKPFM) and quantifiable nanomechanical properties (QNM), as well as observe kinetic mechanisms with in-situ electrochemical methods (EC-AFM). Lastly, computational simulations from the atomistic scale via density functional theory (DFT) up to molecular dynamics (MD) were utilized to support assumptions on the reaction mechanisms derived from experimental observations. Figure 1

Dendrite-free Li-metal anode via a dual-function protective interphase layer for stable Li-metal pouch cell

The increasing demand for high-specific-energy lithium batteries has stimulated extensive research on the lithium metal anode owing to its high specific capacity and low electrode potential. However, the lithium metal will irreversibly react with the electrolyte during the first cycling process, forming an uneven and unstable solid electrolyte interphase (SEI) layer, which results in the non-uniform deposition of Li ions and thus the formation of lithium dendrites. This could cause a battery short circuit, resulting in safety hazards such as thermal runaway. In addition, the continuous rupture and repair of the SEIs during the repeated charge/discharge processes will constantly consume the active lithium, which leads to a significant decrease in battery capacity. An effective strategy to address these challenges is to design and construct ideal artificial SEIs on the surface of the lithium metal anode. This review analyzes and summarizes the mathematical modeling of SEI, the functional characteristics of SEIs with different components, and finally discusses the challenges faced by artificial SEIs in practical applications of lithium metal batteries and future development directions.

The cycling lifespan of Li-metal batteries is compromised by the unstable solid electrolyte interphase (SEI) and the continuous Li dendrites, restricting their practical implementations. Given these challenges, establishing an artificial SEI holds promise. Herein, a trinitarian gradient interphase is innovatively designed through composite coatings of magnesium fluoride (MgF2), N-hexadecyltrimethylammonium chloride (CTAC), and polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) on Li-metal anode (LMA). Specifically, the MgF2/CTAC/PVDF-HFP SEI spontaneously forms a lithium fluoride (LiF)-rich PVDF-HFP-based SEI, along with lithium-magnesium (Li-Mg) alloy substrate as lithiophilic electronic conductor and positively charged CTAC during plating. Noticeably, the Li-Mg alloy homogenizes the distribution of electric field and reduce the internal resistance, while the electronically insulated LiF/PVDF-HFP composite SEI offers fast ion-conducting and mechanical flexibility, accommodating the volumetric expansion and ensuring stable Li-ion flux. Additionally, CTAC at the dendritic tip is pivotal for mitigating dendrites through its electrostatic shield mechanism. Innovatively, this trinitarian synergistic mechanism, which facilitates colossal granular Li deposits, constructs a dendrite-free LMA, leading to stable cycling performances in practical Li||LFP, popular Li||NCM811, and promising Li||S full cells. This work demonstrates the design of multifunctional composite SEI for comprehensive Li protection, thereby inspiring further advancements in artificial SEI engineering for alkali-metal batteries.

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.

Among the many challenges for Li-ion batteries (LIBs) to achieve long cycle life, formation of high quality solid electrolyte interphase (SEI) is of critical importance. SEI forms on LIB anode during the initial cycles, and its properties directly impact battery life and power performance. In-depth understanding of SEI growth mechanism can provide critical parameter input for modeling and simulation, set foundation for the SEI properties and performance correlation, and lead to the ultimate effective artificial SEI or coating design to achieve satisfactory cell cycle efficiency. However, due to the nm thickness and air/moisture sensitivity of SEI, designing and performing in-situ diagnostics are technically challenging. To date, most studies on SEI properties are performed on electrodes harvested after SEI formation, and the time-dependent growth mechanism of SEI during its formation remains to be elucidated. In this talk, we will present an approximation of in-situ diagnostic method, by utilizing isotope-assisted time-of-flight secondary ion mass spectrometry (TOF-SIMS). This method is capable of obtaining time-dependent information during SEI growth, even after the growth process has completed. The model battery design includes a Cu electrode, 6Li labeled electrolyte (1M 6LiBF4 in EC:DEC 1:2) and a 7Li metal counter electrode. As SEI grows on the Cu electrode, the 6Li:7Li isotope ratio in the electrolyte decreases due to 6LiBF4 consumption. The time scale of SEI growth thus can be represented by the 6Li:7Li ratio in the SEI, and each 6Li:7Li ratio corresponds to a snapshot during SEI growth. The isotope ratio are measured with TOF-SIMS after harvesting the SEI. With this method, we have found that the SEI portion formed first (higher 6Li:7Li ratio) presents at the electrolyte/SEI interface, and the SEI formed last (lower 6Li:7Li ratio) exists at the SEI/electrode interface. These results also indicate that new SEI forms at the SEI/electrode interface. Further chemistry analyses with TOF-SIMS and XPS show that the portion formed first are mostly organic compounds while the portion formed last are mostly inorganic compounds. This method could also be applied in other fields where growth dynamics are of interest, such as investigating Li dendrite growth. Figure 1

Alkali and alkaline earth metal anodes are gaining increasing research attention because they could provide high energy densities and theoretical specific capacities. Among them, lithium metal is the most investigated anode candidate for next generation batteries. However, due to some challenges, the commercialization of lithium metal anodes is still delayed. Reactions between lithium metal and the electrolyte lead to the formation of an inhomogeneous solid electrolyte interphase (SEI). Consequently, electrodissolution/-deposition of lithium is favored where the SEI is less resistive or cracked, resulting in high surface area lithium and dendrite growth. This does not only lower the coulombic efficiency (CE) and cell specific capacity but also causes safety issues due to an increased risk of short-circuits and thermal runaway.1,2 To enable increased safety in high energy batteries with lithium metal anodes an effective SEI is required to limit lithium dendrite growth. There is a variety of approaches to grow an effective SEI, such as the use of electrolyte additives, mechanical methods and chemical modification.3-5 Recently, alkaline earth metal anodes, such as magnesium or calcium, are also of considerable interest for next generation anodes. Their main advantage is the significantly lower susceptibility to dendrite formation, while still offering high theoretical specific capacities. However, their oxide passivation layer does not allow significant mobility for their divalent ions, leading to large overvoltage and slow reaction kinetics which hinders the cycling. Therefore, electrolytes which enable a non-passivating SEI have to be applied and additives are investigated on their ability to remove the oxide layers.6,7 Herein, we present a novel mechanochemical approach to form an effective SEI on the lithium metal surface by combining mechanical and chemical modification utilizing ionic liquids (ILs) prior to cell assembly. This method suppresses dendrite growth even at high current densities of 10 mA cm-2 in lithium metal batteries with liquid electrolytes. To further highlight the importance of surface treatment and SEI formation, we investigate electrolytes for calcium metal batteries as well as mechanical modification of calcium metal anodes. Acknowledgements: The authors would like to acknowledge financial support from the European Union through the Horizon 2020 framework program for research and innovation within the project “VIDICAT” (829145).

Artificial polymer solid electrolyte interphases (SEIs) with microphase-separated structures provide promising solutions to the inhomogeneity and cracking issues of natural SEIs in lithium metal batteries (LMBs). However, achieving homogeneous ionic conductivity, excellent mechanical properties, and superior interfacial stability remains challenging due to interference from hard-phase domains in ion transport and solid-solid interface issues with lithium metal. Herein, we present a dynamic supramolecular ion-conducting poly (urethane-urea) interphase (DSIPI) that achieves these three properties through modulating the hard-phase domains and constructing a composite SEI in situ. The soft-phase polytetrahydrofuran backbone, featuring loose Li+-O coordinating interactions, ensures uniform Li+ transport. Concurrently, sextuple hydrogen bonds in the hard phase dissipate strain energy through sequential bond cleavage, thereby imparting exceptional mechanical properties. Moreover, enriched bis (trifluoromethanesulfonyl) imide anion (TFSI-) in DSIPI promotes the in situ formation of a stable polymer-inorganic composite SEI during cycling. Consequently, the DSIPI-protected lithium anode (DSIPI@Li) enables symmetric cells with exceptional cyclability exceeding 4,000 hours at an ultra-high current density of 20 mA cm-2, thereby demonstrating excellent cycling stability. Furthermore, DSIPI@Li facilitates stable operation of the pouch cells under the constraints of a high-loading LiNi0.8Co0.1Mn0.1O2 cathode and low negative/positive capacity (N/P) ratio. This work presents a powerful strategy for designing artificial SEIs and high-performance LMBs.

In this study, the artificial solid electrolyte interphase (SEI) formed on lithium metal when treated in ZnCl2 solutions is thoroughly investigated. The artificial SEI on lithium metal electrodes substantially decreases the interfacial resistance by ca. 80% and improves cycling stability in comparison to untreated lithium. The presence of a native SEI negatively affects the morphology and interfacial resistance of the artificial SEI. Increasing the ZnCl2 concentration in tetrahydrofuran (THF) (precursor solution) results in higher homogeneity of the surface morphology. Independent of the ZnCl2 concentrations, the artificial SEI is composed of Cx, CO, LiCl, Li2CO3, ZnCl2, and LixZny alloys. ZnCl2 (1 M) produces the most homogenous surface and additional surface species with carbonyl side groups. Nonetheless, the ZnCl2 concentration only has a small effect on the interfacial resistance or cycling stability. Using ethyl methyl carbonate (EMC) as the solvent significantly reduces the interfacial resistance to 7 Ω cm2, in comparison to 25 Ω cm2 for THF. The composition of the artificial SEIs varies depending on the solvent. Either way, the SEI consists of Cx LixC, LiCl, Li2CO3, ZnCl2, and LiZn alloys. The THF-based SEI additionally features ether and carbonyl groups, LiZnO, and Zn metal. For the artificial SEI formed with both solvents, the atomic percentage of the LiZn alloy increases close to the Li surface.

The lithium metal anode is the holy grail of the battery field due to its lowest reduction potential and high specific capacity; however, its application is hindered by severe safety hazards and inferior cyclic stability due to dendrites and unstable solid electrolyte interphase (SEI). Aiming at these problems, a coiled Li anode with a unique upright structure is proposed. The upright structure endows coiled Li anode with abundant inner reaction interface/space/mass for lithium deposit/storage/transport, which can induce the inner growth of Li dendrites and SEI. The Li+ transport/deposit behavior and mechanism of coiled Li anode are clarified via in situ observation and numerical simulation. Benefiting from the small volume expansion and sufficient Li+ transport, the coiled Li anodes combined with Li4Ti5O12 cathodes achieve a long life of over 2000 cycles at 5C with a reversible capacity of 129 mAh g−1 and 100% Coulombic efficiency.

Lithium-sulfur (Li-S) batteries promise the next-generation energy storage. However, one of the issues facing Li-S batteries is the high reactivity of the Li metal, leading to serious interfacial side reactions; the other is the dendritic Li growth that can usually breach the solid electrolyte interphase (SEI) applied for mitigating side reactions. To address these issues, we propose the indium fluoride-based artificial solid electrolyte interphase with tunable surface properties. Over the initial charging, InF3 nanoparticles coated on the Li anode surface can be lithiated to form a layer of nanoporous lithium/indium alloy (LixIn) encapsulating lithium fluoride (LiF) nanoparticles (3Li++3e-+InF3→3LiF+In) as shown in Figs. 1(a, b). The tightly anchored LixIn layer well attaches LiF on the anode surface during Li stripping/plating, while LiF with a higher Young’s modulus physically suppresses dendritic Li growth. Both LixIn and LiF provide facile adsorption towards Li atoms as well as high Li+ conductivity, inducing uniform Li plating [1-4]. In addition, both materials are stable with corrosive electrolyte containing polysulfides and shield Li anode from side reactions. Tuning the SEI surface properties via adding other metal fluorides such as BiF3 and AlF3, we further show the possibility to further enhance the Young’s modulus and flexibility for the SEI. Applying the rationally designed artificial SEI, substantial enhancement in terms of Li stripping/plating stability (Fig. 1c) and Li-S battery cyclability is achieved even using LiNO3-free electrolyte and the gas evolution arising from interfacial electrolyte decomposition is suppressed [5]. With enhanced mechanical strength and chemical stability, the metal fluoride-based artificial SEI paves a new way in developing durable and safer Li-S batteries. Key words: Li metal; artificial solid electrolyte interphase; dendrite; side reaction; metal fluoride. Acknowledgement The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R) Reference[1] Y. Ren, T. Zhao, M. Liu, Y. Zeng, H. Jiang, Journal of Power Sources, 361 (2017) 203-210.[2] X.Q. Zhang, X. Chen, R. Xu, X.B. Cheng, H.J. Peng, R. Zhang, J.Q. Huang, Q. Zhang, Angewandte Chemie, 129 (2017) 14395-14399.[3] M. Liu, Y. Ren, H. Jiang, C. Luo, F. Kang, T. Zhao, Nano Energy, 40 (2017) 240-247.[4] Y. Ren, T. Zhao, H. Jiang, M. Wu, M. Liu, Journal of Power Sources, 347 (2017) 136-144.[5] A. Jozwiuk, B.B. Berkes, T. Weiß, H. Sommer, J. Janek, T. Brezesinski, Energy & Environmental Science, 9 (2016) 2603-2608. Figure 1

Enabling high-performance all-solid-state lithium batteries with high ionic conductive sulfide-based composite solid electrolyte and ex-situ artificial SEI film

The practical application of lithium-metal batteries is hindered by insufficient lithium Coulombic efficiency and uncontrolled dendrite growth, bringing a challenge concerning how to create robust solid electrolyte interphases (SEIs) that can regulate Li+ transport and protect reactive lithium-metal. Here, we present the rational construction of a multi-component jigsaw-like artificial SEI by the integration of fluorine-containing silane and polyether-containing silane. The fluorine-donating group prevents the parasitic reaction and yields a dense structure for homogeneous lithium deposition. Additionally, the lithophilicity of ethylene glycol backbone facilitates the rapid transport of Li+ and the cross-linked network increases mechanical robustness of the SEI. With this artificial SEI, we show highly reversible lithium plating and stripping cycling for more than 500 hours. Moreover, we also demonstrate stable operation of high-voltage LiNi0.8Co0.1Mn0.1O2 cathode in both coin and pouch cells under high loading, limiting excess lithium, and lean electrolyte conditions, holding great prospects for the practical application of high-voltage lithium-metal batteries.

Sodium metal anodes are ideal candidates for advanced high energy density Na metal batteries. Nevertheless, the unstable solid electrolyte interphase (SEI), the uncontrollable dendrite growth, and low Coulombic efficiency during cycling have prevented their applications. Herein, a high‐performance Na anode is achieved by introduction of an ex situ artificial Na3P layer on the surface via a simple red phosphorus pretreatment method. The artificial SEI layer possesses high ionic conductivity and high Young's modulus, which regulates uniform deposition of ions and prevents the dendrite growth. Benefiting from these merits, the Na||Na cells with the protected layers demonstrate excellent electrochemical performance (780 h at 1.0 mA cm–2, 1.0 mAh cm–2). When assembled into a full battery with a Na3V2(PO4)3 cathode, the Na metal battery exhibits a long lifespan of 400 cycles at 15 C and a high rate capacity of ≈53.2 mAh g–1 at 30 C. In addition the red P pretreatment method can be applied to potassium metal anodes. Outstanding performance is also achieved in K||K cells with the formation of a KxPy protecting layer (550 h at 0.5 mA cm–2, 0.5 mAh cm–2). The artificial P‐derived protection approach can also be extended to solid‐state alkali metal batteries with high power density and energy density.