Progress in Modeling and Applications of Solid Electrolyte Interphase Layers for Lithium Metal Anodes.

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

Electronic devices have been advanced rapidly over the last few decades with improved performance through high-level integration. Lithium ion batteries (LIB), which is representative rechargeable energy storage system, is increasingly demanding to high-energy-density storage system for the stable operation of high performance devices like mobile, laptop, etc. In addition, a recent quantum leap in electronic vehicles (EVs) require to energy storage system with long cycling stability incomparable to previous electronic devices. To achieve high performances and long lifetime, the use of lithium metal is one of attractive anode for as LIBs, because Li metal has high theoretical specific capacity of 3860 mAh/g, low redox potential of -3.04V (verse the standard hydrogen electrode) and low gravimetric density of 0.59 g/cm3. Nevertheless, since Li metal anode has the lithiation-delithiation mechanism of the ‘hostless’ electrochemical plating/stripping unlike the conventional graphite anode, it is confronted with serious problems occurred from uncontrollable interfacial reaction between surface of lithium metal anode and electrolyte applying to traditional LIB system; that is (1) the uncontrollable formation of solid electrolyte interphase (SEI) layer limitless volume expansion of Li metal anode (2) the dendritic growth and isolation of Li interface derived from inhomogeneous deposition of Li+ ion flux (3) the Li surface fracture induced by heterogeneously plated Li+ ion. Through these process, repetitive formation of undesirable SEI layer at surface of Li anode make fresh electrolyte dissipation and concurrently the anode surface becomes more distorted. These side-phenomena strongly correlate with low coulombic efficiency and decreasing lifetime of lithium metal battery (LMB) system where lithium metal is used as anode. In this works, in a dimensional aspect, to solve such problem mentioned above through stabilizing of surface of lithium metal anode, we introduce Janus-faced protective layer (JPL) as gel polymer electrolyte. JPL was composed with dual layer of Ni (3~7μm) embedded poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) and bare PVdF-HFP through simple casting method. The process of preparing JPL membrane is so simple to instantly bring in existing roll-to-roll process, and it is also possible to make a low cost process considered relatively low nickel price. In terms of mechanical property, it is based on flexible polymer and shows the merit of being foldable enough to able origami. In addition, JPL play a role of protection of Li metal anode. JPL has two functionalities. The metal part of JPL directly contacted with anode, and this layer was frameworks for preventing irregular SEI layer formation through inward plating of Li ion, the polymer part is stable state enough to be used as a gel polymer electrolyte. When lithium metal anode is subjected to cycle-test while keeping in contact with the nickel side of JPL membrane, Scanning electron microscope (SEM) image showed that the surface of lithium metal anode was very clean without any fluctuation compared to the case where only polypropylene (PP) separator was used. (Figure. 1) Moreover, as shown in Figure 2., the LMB system applied JPL show high stability of tolerable overvoltage within 5mV, 15mV even at high-current of 10mA/cm2, 20mA/cm2, respectively, in Li//JPL/JPL//Li symmetric cell. These results are attributable to effective function of each layer as mentioned earlier. JPL consists of two layer, the tough and adhesive nature of PVdF-HFP prevents the polymer part from being damaged or torn by dendritic growth and simultaneously ensure that metal part functions as robust framework for SEI layer formation without escaped particle from the membrane. This frameworks induce the SEI layer, which can act as a resistive element, to grown in the metal part while surface of Li metal anode become get out of main subject from the SEI layer formation. By virtue of that property, JPL has not allow forming resistive elements on the surface of Li metal anode, preventing the voltage rising up due to the formation of resistive elements at surface, so tolerability of overvoltage with clean surface of anode is maintained. In conclusion, the model study of nickel micro-particle/PVdF-HFP composite suggests the straightforward direction of protective layer to apply lithium metal as anode in LIB system with alleviating the problem caused by the ‘hostless’ property of lithium metal anode. Thus, when JPL is introduced to a LIB system, it shows a fairly positive possibility for high performance Lithium metal battery. Figure 1

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

The property of the solid electrolyte interphase (SEI) layer is of prime importance for the performance of lithium metal anodes. Replacing the spontaneously formed inhomogeneous and unstable SEI layer with a high-performance artificial SEI is an effective strategy. Herein, a self-healing SEI layer with high lithium-ion conductivity and a stable framework to address the issues of poor performance of lithium metal anodes is achieved. C, Li2S, and LiI are uniformly distributed on the lithium surface via a "sauna" reaction between CS2-I2 mixed steam and metal lithium, which has the potential to be applied to large-scale preparation. The obtained SEI layer possesses high mechanical strength and facilitated lithium-ion transport capability, which are inherited from the amorphous C and lithium compounds (Li2S and LiI). Most importantly, the LiI component can migrate through the electrolyte and cover the exposed lithium caused by flaws and cracks, leading to a self-healing property. As a result, the C-Li2S-LiI@Li electrode exhibits excellent electrochemical performance with low overpotential and long lifespan.

Lithium metal batteries, directly utilizing lithium metal as an anode, are promising as new-generation energy storage devices, and the characteristics of the solid electrolyte interphase (SEI) layer are critical to battery operation. However, the roles of electrolyte compositions on the structure and thickness of the passivating SEI film at the atomistic level have been previously overlooked, resulting in poor understanding of SEI characteristics. In this study, the formation of the native SEI layer on lithium metal electrode and various electrolyte composition is scrutinised using ReaxFF molecular dynamics (MD) simulations. The study successfully identifies different layers of the SEI film using charge distribution as a tool. The decrease in lithium density and the increase in the charge states of oxidised lithium atoms contribute to the identification of an inner layer and a transition layer within the previously known interface layer, as well as the sparse layer. Additionally, different reaction schemes are observed through the analysis of ratio between consumed lithium atoms and electrolyte molecules. Furthermore, results show the lithium salt concentration effects in suppressing the growth of the SEI between the lithium anode and the liquid electrolyte, and inhibiting the formation of key SEI components. This work provides new insights into the nature of SEI surface chemistry and potentially facilitates the design of artificial SEI layers and optimal electrolyte solutions.

Artificial solid electrolyte interphase (SEI) layers have been widely regarded as an effective protection for lithium (Li) metal anodes. In this work, an artificial SEI film consisting of dense Li6.4La3Zr1.4Ta0.6O12 (LLZTO) nanoparticles and polymerized styrene butadiene rubber is designed, which has good mechanical and chemical stability to effectively prevent Li anode corrosion by the electrolyte. The LLZTO-based SEI film can not only guide Li to uniformly deposit at the interface but also accelerate the electrochemical reaction kinetics due to its high Li+ conductivity. In particular, the high Young's modulus of the LLZTO-based SEI will regulate e- distribution in the continuous Li plating/stripping process and achieve uniform deposition of Li. As a consequence, the Li anode with LLZTO-based SEI (Li@LLZTO) enables symmetric cells to demonstrate a stable overpotential of 25 mV for 600 h at a current density of 1 mA cm-2 for 1 mA h cm-2. The Li@LLZTO||LFP (LiFePO4) full cell exhibits a capacity of 106 mA h g-1 after 800 cycles at 5 C with retention as high as 90%. Our strategy here suggests that the artificial SEI with high Young's modulus effectively inhibits the formation of Li dendrites and provides some guidance for the design of higher performance Li metal batteries.

A polymer brush-based robust and flexible single-ion conducting artificial SEI film for fast charging lithium metal batteries

To meet the rapidly growing market in electrical vehicles (EVs) and portable electronic devices, a battery with high energy density is required. In order to reach this goal, Li metal, is considered as the most promising candidate as anode. However, the dendrite forming, low Coulombic efficiency (CE), and unstable solid electrolyte interphase (SEI) pose big challenges in applying Li metal in batteries. Here we introduce a dual-layer coated Li metal surface, which consists a lithium carboxylate organic layer and a doped MXene layer. The porous organic layer increases the wetting of electrolyte and forms a thin solid electrolyte interface (SEI). While the doped MXene leads to about 50% reduced thickness of inorganic-rich components among the SEI layer. A low overpotential of less than 30 mV over 1000 h cycling in symmetric cells was received. The functional groups along with the good electronic conductivity and smooth surface of the MXene greatly reduce the lithium plating/stripping energy barrier, enabling a dendrite-free lithium metal anode. The battery paired with NMC811 exhibited a high initial CE of 82.1%. A high capacity-retention of 175.4 mAh/g at 1.0 C was achieved after 350 cycles. After long 500 cycles, a reliable capacity was still remained while the cell with un-modified Li metal quickly dropped to below 74.2 mAh/g. In a 475 mAh pouch cell, the battery still exhibited a high discharge capacity of 165.6 mAh/g with a capacity retention of 90.2% after 200 cycles, while the cell with pure Li metal decreased to 80.5%. Keywords: Lithium metal; Battery; Anode; MXene; Doping

Lithium (Li) metal anode (LMA) is highly considered as a desirable anode material for next-generation rechargeable batteries because of its high specific capacity and the lowest reduction potential. However, uncontrollable growth of Li dendrites, large volume change, and unstable interfaces between LMA and electrolyte hinder its practical application. Herein, a novel in situ formed artificial gradient composite solid electrolyte interphase (GCSEI) layer for highly stable LMAs is proposed. The inner rigid inorganics (Li2 S and LiF) with high Li+ ion affinity and high electron tunneling barrier are beneficial to achieve homogeneous Li plating, while the flexible polymers (poly(ethylene oxide) and poly(vinylidene fluoride)) on the surface of GCSEI layer can accommodate the volume change. Furthermore, the GCSEI layer demonstrates fast Li+ ion transport capability and increased Li+ ion diffusion kinetics. Accordingly, the modified LMA enables excellent cycling stability (over 1000h at 3mA cm-2 ) in the symmetric cell using carbonate electrolyte, and the corresponding Li-GCSEI||LiNi0.8 Co0.1 Mn0.1 O2 full cell demonstrates 83.4% capacity retention after 500 cycles. This work offers a new strategy for the design of dendrite-free LMAs for practical applications.

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 (Li)-metal anode has attracted extensive attention owing to its high theoretical specific capacity (3860 mAh/g), which is more than ten times to that of graphite anode (372 mAh/g). However, the notorious reactivity of Li-metal prevents their practical applications due to undesired Li dendrite growth and unstable solid electrolyte interphase (SEI) formation. Here, we developed a facile, cost-effective and one-step approach for creating an artificial Li-metal/electrolyte interface by treating Li metal anode with an electrolyte containing SnF2. As a result, the artificial SEI composed of LiF, Sn and Sn-Li alloy was formed with higher ionic conductivity and increased Young’s modulus. The higher ionic conductivity and larger Young’s modulus of the artificial layer facilitate fast Li-ion diffusion and suppress Li dendrite growth, respectively. The hybrid artificial SEI showed a synergistic effect of storing Li by a reversible Sn-Li alloy formation and Li plating underneath the SEI. The artificial SEI treated Li symmetrical cells showed outstanding plating/stripping cycles (~2325 hrs) with reduced overpotential compared to the bare Li. Coupled with a high-loading (11.88 mg.cm-2) LiNi1/3Co1/3Mn1/3O2 (NMC111) cathode, the SEI protected Li full cells exhibited remarkable performance such as longer stable cycling, higher capacity retention and higher capacity at higher rates compared to the bare Li anode.

The lithium metal battery is strongly considered to be one of the most promising candidates for high-energy-density energy storage devices in our modern and technology-based society. However, uncontrollable lithium dendrite growth induces poor cycling efficiency and severe safety concerns, dragging lithium metal batteries out of practical applications. This review presents a comprehensive overview of the lithium metal anode and its dendritic lithium growth. First, the working principles and technical challenges of a lithium metal anode are underscored. Specific attention is paid to the mechanistic understandings and quantitative models for solid electrolyte interphase (SEI) formation, lithium dendrite nucleation, and growth. On the basis of previous theoretical understanding and analysis, recently proposed strategies to suppress dendrite growth of lithium metal anode and some other metal anodes are reviewed. A section dedicated to the potential of full-cell lithium metal batteries for practical applications is included. A general conclusion and a perspective on the current limitations and recommended future research directions of lithium metal batteries are presented. The review concludes with an attempt at summarizing the theoretical and experimental achievements in lithium metal anodes and endeavors to realize the practical applications of lithium metal batteries.

Owing to its lowest standard redox potential, low density, and high theoretical specific capacity, lithium metal has been considered to be the ideal anode material for secondary lithium batteries. However, lithium metal is thermodynamically unstable in liquid organic electrolytes (LOEs). When lithium metal comes in contact with an LOE, it reacts easily with it to form the solid electrolyte interphase (SEI) layer. Once the stable and robust SEI layer forms, it can inhibit the direct contact between lithium metal and LOE and the further decomposition of the electrolyte. Nevertheless, the inhomogeneity in chemical composition or thickness of the SEI layer can cause the growth of lithium dendrites, which lead to short-circuits in batteries. In this study, we suggested the use of urea as a new electrolyte additive to restrain the growth of lithium dendrites via the formation of a uniform and robust SEI layer on the lithium surface. The Li symmetric cell with 0.5 M urea electrolyte additive exhibited better cyclability over 415 cycles at 1 mA cm–2; this number of cycles was >40 times larger than that of the Li symmetric cell without urea additive. Further, the Li–O2 cell with electrolyte additive was cycled for more than 200 cycles at 0.1 mA cm–2 under the limited capacity mode of 1000 mA h g–1.

The practical application of lithium (Li) metal battery is impeded by the Li dendrite growth and unstable solid electrolyte interphase (SEI) layer. Herein, an ultra‐stretchable and ionic conducting chemically crosslinked pressure‐sensitive adhesive (cPSA) synthesized via the copolymerization of 2‐ethylhexyl acrylate and acrylic acid with poly(ethyleneglycol)dimethacrylate as crosslinker (short for 70cPSA), is developed as both artificial SEI layer and solid polymer electrolyte (SPE) for stable Li‐metal electrode, enabling all‐solid‐state Li metal batteries with excellent cycling performance. As an artificial SEI layer, the 70cPSA‐modified electrodes exhibit excellent electrochemical performance in Li|70cPSA@Cu half cells and 70cPSA@Li|70cPSA@Li symmetric cells. In full cells with LiFePO4 (LFP) as cathode, the 70cPSA@Li|LFP cell exhibits stable cycling performance over 250 cycles. Utilized as SPE, the all‐solid‐state Li|SPE|LFP cell delivers excellent cycling stability with a capacity retention of 86% over 500 cycles. With high‐voltage LiNi0.8Mn0.1Co0.1O2 (NMC811) as cathode, the Li|SPE|NMC811 cell exhibits a discharge capacity of 124.3 mAh g−1 with a capacity retention of 71% after 200 cycles. The rational design of PSAs and investigation of their dual role for stable and safe Li‐metal batteries may shed a light on adhesive polymers for battery applications.

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.