Long Cycle Life Lithium Metal Batteries Enabled with Upright Lithium Anode

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.

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).

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.

Lithium ion batteries (LIBs) with higher capacity, longer life span, and improved safety have been demanded for many applications such as electric vehicles and grid-level energy storage systems. Lithium metal anode, among various electrode materials, is often regarded as the “Holy grail” electrode because of its high theoretical capacity (3860 mAhg-1) and the lowest electrochemical potential (-3.040V vs. standard hydrogen electrode).Nevertheless, there are difficulty in using lithium metal as anode due to intrinsic problems such as unstable solid electrolyte interphase (SEI) and uncontrolled lithium dendrite growth which induce poor safety, low coulombic efficiency, and short span life of batteries. Recently, it was reported that micro-patterned lithium metal anodes can suppress the dendrite growth during lithium plating/stripping. In this work, we fabricated different shapes of pattern on lithium metal anode. We also investigated the behavior of lithium dendrite growth from different pattern morphology during lithium plating and stripping process.

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

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

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

Nowadays, due to rapid development in technology, regardless of automobile industry, portable devices and energy storage system, the requirement of energy density, capacity and operating voltage becomes higher and higher. Traditionally, graphite anode with low capacity is limited to develop advanced lithium ion battery(LIB) and is hard to achieve the high-tech product specification; therefore, battery industries have started to focus on silicon-carbon composite materials in substitution. However, it also faces its limitation to achieve energy density of 700 Wh L-1. Using lithium metal as the anode material seems to be the only route to achieve 1000 Wh L-1 due to its extremely high capacity (3860 mAh g-1), lightness (0.59 g cm-3), and low reduction potential (-3.04 V). Consequently, more and more battery experts and researchers have devoted themselves to lithium metal battery (LMB) research. Nevertheless, lithium as active metal easily reacts with electrolyte producing poor solid electrolyte interphase (SEI) with high internal resistance leading to low coulombic efficiency and short cycle life. During charge and discharge process, lithium plating and stripping leads to non-even deposition. As a further matter, under extreme environment, such as high current density and charge density, dendritic dendrite easily forms resulting in safety issue including failure of separator and short circuit. Based on the above issues, lithium metal battery could be hardly commercialized. Therefore, battery design should be aimed at lithium metal anode instead of traditional LIB anode. First of all, innovative electrolyte system which benefits the formation of highly-stable SEI should be established. Secondly, the artificial SEI on lithium anode is served as the first line of defense against dendrite due to excellent mechanical strength and elasticity. Finally, it is important to develop a structured anode for improving lithiophilic property, uniform electric field and solving volume expansion issue.

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.

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

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.

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

Recently, Lithium(Li) metal has been considered as a “ideal” anode material due to the extra-high capacity (3,860 mAh g-1), the lowest electrochemical equilibrium potential (-3.040 VSHE) and low density (0.534 g cm-3). Despite its ideal characteristics, Li metal anode(LMA) has a critical drawback that Li deposits grow in the form of dendrite due to its ununiform plating/stripping behaviors. The undesired dendritic growth of Li results in the detrimental effects on the cell performance, such as cell short-circuit, low coulombic efficiency(CE), and dead lithium, which have hampered widespread exploits of lithium metal-based batteries. Therefore, in recent years, numerous strategies to prevent or mitigate the problematic dendrite growth, including construction of artificial solid electrolyte interphase (SEI), lithiophilicity/morphology control of current collector, modification of electrolyte, and so forth [1,2].Meanwhile, by virtue of quite successful improvement in performance of LMAs, the battery system using in-situ formed LMAs, called anode-free lithium metal batteries (AFLMBs) have gained much interests [3]. In AFLMBs, the anode compartment has only a current collector at the initial state and then lithium metal anode is formed by electroplating with lithium ions provided from the cathode through initial charging step. The absence of corrosive/reactive Li metal in assembly process decreases the production cost and the energy density of a full-cell can be increased due to additional loading of cathode active materials. However, the dendritic growth of lithium could become more problematic because the cyclable lithium ions are fully limited to those provided from the cathode active materials. For solving notorious dendrite problem in AFLMBs, many strategies such as a use of highly concentrated salt electrolytes [3], the addition of functional electrolyte additives [4] and the introduction of lithiophilic elements on the current collector have been reported [5]. Unfortunately, however, it still seems difficult to nurture all strengths of AFLMBs related to the cost-effectiveness, simple cell design and high-energy density.One of the effective ways to regulate electroplating behavior of lithium is to introduce the surface-roughed metal current collector. Using such the structures, the charge accumulation sites on the microscopic structure of current collector can be randomly distributed, resulting in uniform electric field on the surface of current collector. Additionally, the localized current density can be reduced, and problematic volume changes of lithium can be buffered [2].In this context, we introduced the surface-roughed metal current collector as a substrate of AFLMBs to improve their stability. The surface roughness was controlled by an electrochemical method using a pulse-reverse polarization [6]. In particular, we focused on studying the effect of various pulse parameters on the morphological evolution of a metal current collector. To investigate their effects on the stability of AFLMBs, the CR 2032-type coin cells composed of the roughly surface-treated metal foil as a current collector and high-loading Li-Ni0.5Co0.2Mn0.3 (≈ 12.1 mg/cm2) as a cathode were assembled in an Ar-filled glove box. Figure 1-(a) presents an example of pulse signal used to modify roughness of metal surfaces. First, to investigate effect of pulse current density(iox=ire=i), we fixed the amount of metals, which were electrochemically oxidized and reduced by giving different pulse duration time (tox = tre = t,). Figure 1-(b) presents the effect of pulse current on the morphology of metal foils. After pulse with 0.5 mA/cm2, the surface of metal foil is covered by the clusters of metal/metal oxide particles. With increasing current density, the diameter of the clusters decreased and the number of clusters increased. Figure 2 shows the effect of surface-roughed metal surface on the electroplating behavior of lithium metals. As can be seen in Figure 2-(a), the morphology of electrodeposited lithium on the surface-treated metal foil has a denser and flatter Li deposit than that on pristine metal foil, where a needle-like dendritic structure is shown. Figure 2-(b) presents the CE of the repeated plating/stripping process of lithium metal. All the cells adopting the surface-treated foils showed a higher cycling stability compared to a pristine metal foil.Additionally, in this presentation, the correlation between pulse-reverse parameters and morphological evolution of a metal current collector, and the effect on the electrochemical long-term stability of AFLMBs will be discussed in more detail References [1] X.-B. Cheng et al., Chem. Rev, 117, 10403, 2017.[2] J. Zheng et al., Chem. Soc. Rev., 49, 2701, 2020.[3] J. Qian et al., Adv. Funct. Mater., 26, 7094, 2016.[4] N. A. Sahalie et al., J. Power Sources, 437, 226912, 2019.[5] S. S. Zhang et al., Electrochimica Acta, 258, 1201, 2017.[6]M. S. Chandrasekar and M. Pushpavanam, Eletrochimica Acta, 53, 3313, 2008 Figure 1

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.

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.