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Review

Improving Performance and Safety of Lithium Metal Batteries Through Surface Pretreatment Strategies

by
Gyuri Youk
,
Jeongmin Kim
and
Oh B. Chae
*
School of Chemical, Biological and Battery Engineering, Gachon University, Seongnam-si 13120, Republic of Korea
*
Author to whom correspondence should be addressed.
Energies 2025, 18(2), 261; https://doi.org/10.3390/en18020261
Submission received: 29 November 2024 / Revised: 31 December 2024 / Accepted: 7 January 2025 / Published: 9 January 2025
(This article belongs to the Special Issue Electrode Materials for Lithium-Ion and Sodium-Ion Batteries: Developments, Challenges, and Prospects)
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Versions Notes

Abstract

:
Lithium metal batteries (LMBs) are promising candidates for electric vehicles (EVs) and next-generation energy storage systems owing to their high energy densities. The solid electrolyte interphase (SEI) on the Li metal anode plays an important role in influencing the Li deposition form and the cycle life of the LMB. However, the SEI on Li metal differs from that for other anodes, such as graphite, owing to its instability and reactivity. In addition, dendrite growth has hindered the commercial application of Li metal batteries in regular portable electronics to EVs. This review summarizes SEI formation on Li metal, dendrite formation and growth, and their impact on battery performance. In addition, we reviewed the recent progress in pretreatment strategies using materials such as polymers, carbon materials, and inorganic compounds to suppress dendritic growth.

1. Introduction

Li-ion batteries have grown rapidly as major materials for the next generation 4th industry owing to the increase in demand for electric vehicles (EVs). They are also widely used in various fields, such as small electronic devices and energy storage systems, because they do not contain environmental regulatory materials, are lightweight, and have a high energy density compared to existing batteries [1,2]. However, graphite, which is mainly used in Li-ion battery anodes, has a theoretical capacity of about 372 mAh/g, which does not provide the high energy density required for application to long-distance EVs [3,4,5]. Li metal has emerged as an anode material to replace graphite. Li metal has a theoretical capacity of 3860 mAh/g, which is much higher than that of the existing graphite anode, and the redox potential is very low at −3.04 V (vs standard hydrogen electrode). Therefore, higher energy density can be achieved by replacing graphite [6,7]. In the graphite anode having a layered structure, Li ions are intercalated and deintercalated [8], whereas the Li metal anode is simply a form in which Li metal and Li ions are converted [9]. The Li ions moving to the anode material are immediately reduced, indicating a significant advantage in enabling fast charging owing to their high reactivity in the conversion to Li metal.
The inherent reactivity of the electrolyte and Li metal at low potentials causes fast consumption of the Li metal and promotes the formation of a solid electrolyte interface (SEI) [10]. The SEI acts as a physical barrier, effectively preventing direct contact between Li and the solvent, thereby stabilizing Li in a specific organic solvent [11]. In addition, the SEI can adjust the distribution of Li ions from the electrolyte to the anode [12]. However, unstable SEI formation causes low coulombic efficiency (CE), electrode polarization, and limited cycle life, resulting in uneven ion diffusion and initial Li nucleation, in turn leading to Li dendrite growth [13,14]. The presence of Li dendrites makes Li metal porous, which increases the volume change in the anode during cycling and causes side reactions between Li metal and the electrolyte [15]. This eventually causes pulverization of the anode and separation of Li powder from the bulk [16]. Moreover, as the charge and discharge cycles are repeated, the Li dendrites grow from the anode to the cathode. The grown dendrite penetrates the separator, contacts the cathode, and internally short-circuits the cell, resulting in thermal runaway and explosion risk [17] and irreversibly consumes Li from the electrolyte, causing loss of cell capacity and electrolyte degradation [18].
Various innovative strategies, such as modification of the liquid electrolyte recipe [19,20,21], development of solid electrolytes [22,23,24], surface modification, and protective layers [25,26,27], are being explored to address the safety issues of Li metal anodes and enhance battery performance. The focus of this review is limited to the surface pretreatment of Li metal in lithium metal batteries (LMBs). The pretreatment strategies generally provide deeper and more intrinsic stability than surface coating methods. In addition, several studies have explored three-dimensional (3D) Li anodes, but the high effective surface area can lead to serious consumption of Li and the electrolyte during repeated Li stripping/plating cycles. Therefore, strategies to protect interfacial regions such as pretreatment should be developed [28].
In this review, we first summarized the SEI formation, dendrite formation, and growth mechanisms in LMBs. Next, we reviewed the recent advances in pretreatment strategies, which are one of the methods used to suppress dendrite formation, by classifying them into materials such as polymers, carbon materials, and inorganic compounds (Figure 1). Finally, we highlighted the ongoing research efforts to control dendrite growth and discussed future research directions.

2. Solid Electrolyte Interface (SEI)

Li metal reacts with organic solvents during the initial charge/discharge process to form a surface film. In 1979, Peled first realized an electrically insulating and ionically conductive interface, which he named SEI [12]. The SEI aimed to alleviate the formation of dendrites by preventing direct contact between the Li metal and the electrolyte. However, achieving a uniform and durable layer remained challenging owing to the unstable and dynamic properties of Li-metal SEI. In particular, the use of Li metal in liquid electrolytes could cause safety risks owing to the formation of nonuniform Li dendrites. Lithium metal undergoes infinite volume changes due to its “host-less” nature, in contrast to the 10% and 300% volume changes observed in graphite and silicon anodes, respectively. (Table 1) The Li-metal exposed to the electrolyte due to the volume change formed an unstable SEI because its electrochemical potential was lower than the potential window of the electrolyte components [2,29,30]. Therefore, an SEI layer with high elasticity and adhesiveness was required to prevent Li deposited under the SEI from direct contact with the liquid electrolyte [31,32].

2.1. Mechanism of SEI Formation

In general, the SEI formation on Li consists largely of chemical and electrochemical reactions (Figure 2a). The chemical reaction occurs spontaneously and instantaneously between the Li metal and the electrolyte, forming insoluble products on the surface of the Li metal and a passivation layer. This passivation layer is called the “primitive SEI”. However, the primitive layer is not effective in preventing electrochemical decomposition of the electrolyte. Looking at the electrochemical reaction mechanism for SEI formation from the perspective of molecular orbital theory, the anode is negatively charged during charging and discharging, and its electrochemical potential (μA) is higher than the lowest unoccupied molecular orbital (LUMO) of the electrolyte. Therefore, electrons are transferred from the anode to the electrolyte, and a reduction reaction occurs in the electrolyte to form an SEI. Likewise, the electrochemical potential of the cathode (μC) is lower than the highest occupied molecular orbital (HOMO) of the electrolyte. Therefore, electrons are transferred from the electrolyte to the cathode, and the electrolyte is oxidized (Figure 2d,e). This process lasts until the primitive SEI is completely formed, and SEI formation consumes the electrolyte and Li cathode, leading to low CE [33]. In addition, because the Li electrode changes morphologically when an electrochemical reaction occurs, the pristine SEI formed by the chemical reaction may be destroyed, and pure Li may be exposed to the electrolyte. However, because graphite and Si are chemically stable in the electrolyte, only the electrochemical reduction decomposition of the electrolyte on the electrode is involved in the formation of the SEI (Figure 2b,c) [31]. Owing to this reason, although the SEI formed on Li metal exhibits functions similar to those of the SEI formed on graphite and Si electrodes, different battery characteristics appear.

2.2. Structure Models of SEI

Various SEI formation mechanisms have been proposed; however, a consensus has not yet been reached. This problem was caused by the limitation of structural observation due to the thin SEI thickness of a few to tens of nanometers, as well as the sensitivity to the environment and the medium used in the characterization of SEI and Li [31]. Therefore, in this section, several existing mechanisms are summarized and discussed to explain the SEI formation in Li metal anodes.
Peled proposed the first SEI model (Figure 3a) [12,35]. In this model, the layer that was formed of insoluble products by contact of the metal with the solution was named the SEI. The SEI was assumed to be similar to the single crystal of a Li-ion conductor. Later, Urban et al. found that SEI was a mixture of organic and inorganic species and comprised two or more layers. The double-layer model comprised a thin and dense inorganic-rich inner layer (in contact with Li) and an organic-rich outer layer (in contact with the electrolyte), which restricted the mass transport of ions in the electrolyte [36,37,38,39].
Subsequently, a mosaic model was proposed (Figure 3b) [40]. This model was proposed based on the research results showing that the SEI comprised an inorganic-rich layer and an organic-rich layer. The reduction in salt anions and solvents proceeded simultaneously on the negatively charged anode surface, and a mixture of insoluble multiphase products was deposited on the Li metal anode surface. The pure microphase of each component was assumed to form a SEI in the mosaic phase [41]. Li ions were allowed to migrate through the boundaries of the multiphase product.
With the development of technology, SEI structures have been continuously studied, and the long-standing knowledge of SEI structures has been challenged by recent observations obtained from cryogenic transmission electron microscopy (cryo-TEM). In 2017, Li et al. observed two completely different SEI nanostructures using cryo-TEM [42]. The SEI formed in a carbonate-based electrolyte, ethylene carbonate-diethyl carbonate, was observed to have inorganic crystalline grains dispersed in an amorphous organic polymer matrix, similar to the mosaic structure proposed by Peled et al [40]. The SEI formed in the carbonate-based electrolytes with fluoroethylene carbonate (FEC) additives had an amorphous polymer matrix inner layer and a large-grain (~15 nm) outer layer of Li oxide with a clear lattice pattern. This had an orderly multilayer structure similar to the multilayer system proposed by Aurbach et al. [36]. The SEI was mainly composed of amorphous phases, regardless of the electrolyte in the battery [43,44]. The crystalline microphases were not concentrated at the surface of metallic Li but were distributed randomly in the SEI or concentrated on the outer layer [45]. The SEI structure obtained from cryo-TEM observation was similar to the “plum pudding model”, where the amorphous phase was similar to the pudding, and the embedded crystalline microphases were similar to plums (Figure 3c) [31].
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Figure 3. Model diagram of SEI formation mechanism. (a) Peled model, (b) Mosaic model (Reprinted with permission from Ref. [46]; Copyright © 2023 The Authors. Published by Elsevier B.V.); (c) Plum-pudding model (Reprinted with permission from Ref. [31]; Copyright © 2020 Wiley-VCH GmbH).
Figure 3. Model diagram of SEI formation mechanism. (a) Peled model, (b) Mosaic model (Reprinted with permission from Ref. [46]; Copyright © 2023 The Authors. Published by Elsevier B.V.); (c) Plum-pudding model (Reprinted with permission from Ref. [31]; Copyright © 2020 Wiley-VCH GmbH).
Energies 18 00261 g003

3. Dendrite

LMBs were used in the 1970s but were soon discarded owing to the ramified and branched morphology of Li deposition, called dendrites, in liquid electrolytes, which caused short-circuiting of the cell and posed a safety hazard [18,47,48]. Dendritic formation becomes more severe at high current densities and during long-term cycle operations. Therefore, efforts should be made to suppress and eliminate dendrite formation during Li plating and stripping. Prominent results from previous investigations demonstrate that a strong correlation existed between the surface morphology and the electrochemical performance of Li anodes in secondary Li batteries. When the morphologies of the electrodeposited Li were flat and not fiber-like, the performance and stable cycling of Li cells could be expected for a long life. However, the needle or dendritic morphologies of plated Li caused the battery system to collapse [49].

3.1. Formation Mechanism of Different Morphologies of Li Dendrites

Mossy, dendritic, and granular morphologies were deposited during the charging and discharging of the cell [50,51,52,53]. Osaka et al. used scanning electron microscopy (SEM) to observe that dendrites were deposited on the surface of a Li anode, which was consistent with the results reported by Aurbach [54,55]. Aurbach and Yair also observed the nonuniform deposition of Li on the surface of a Li anode using atomic force microscopy (AFM). They found that the Li deposition morphology was highly dependent on the composition of the solution [56].
Matsui and Takeyama studied the morphology of the electrodeposition and dissolution processes of Li using the Monte Carlo technique with a two-dimensional (2D) lattice scheme. The 2D lattice scheme of the Monte Carlo technique was based on the fact that Li deposition occurred on the surface of a product that has already been deposited, and no mass transfer of Li in the Li electrode occurred. At a high current density, rod-shaped Li with attached branches was formed because deposition occurred at the tip of the initially deposited product. In addition, under low current density conditions, the shape of Li was leaf-like or flat, owing to activated ion diffusion at the surface of the electrode. If only a small part of the electrode was activated, the deposition was a temple bell-shaped deposition. Branched rod-shaped deposition products were separated from the electrode, and various deposited products remained during the stripping process; however, temple-bell-shaped deposition products could be stripped off without isolation [50].
Morigaki et al. measured the surface morphology of deposited Li using AFM and reported that the size of the grain was about 1–2 μm and that the grains contained thin lines of 100–300 nm and flat parts on the native Li surface [50]. Arakawa et al. studied the effect of current density on the cycle life of Li electrodes and morphology and found that when the discharge current density decreases or the charge current density increases, the needle-like deposition of Li increases, which leads to the “dead Li” formation. The cycle life was significantly affected by the morphological fluctuations of the Lit anode, depending on the charge/discharge current density [57].
Brissot et al. observed the dendrite morphology at various current densities. The dendrite growth rate was similar to that measured by the Chazalviel model, which depends on the electric field and anion mobility [58]. They observed dendritic growth under two current-density regimes. At a high current density, dendritic growth was observed as the ion concentration dropped to zero at the anode. At a low current density, local inhomogeneities played a major role. The dendrites exhibited an arborescent-like morphology at high current densities and a needle-like morphology at low current densities (Figure 4) [59].
Isamu reported that needle- and particle-like deposition of Li occurred during cycling in Li electrodes. All particle-like Li was consumed during discharge, but needle-like Li remained to form dead Li, reducing the cycling efficiency [49]. The microstructure and morphology of Li deposition have been studied using various techniques, including SEM and AFM [60,61,62], and several models have been proposed to prove the morphology of dendrites based on their shape, size, and growth mechanism [63,64]. Bhattacharyya et al. provided quantitative and time-resolved information on the deposition properties of Li metal using in situ nuclear magnetic resonance spectroscopy and stated that monitoring dendrite growth at the initial stage of Li battery cycling was possible [65,66].
The metallic Li anode was repeatedly plated and stripped during cycling. In this process, the initial nucleation sites and positions played important roles in the subsequent deposition nature/behavior of Li. The mechanism of Li dendritic growth was self-enhanced, and according to theoretical studies and experimental investigations, protrusions with large curvature had a higher electric field on their tip sites and attracted more Li ions to grow in the form of dendrites. In addition, the protrusions with the hemispherical tip promoted the 3D diffusion of Li ions, such that Li on the tips was rapidly deposited [17]. The mechanism of dendritic formation was also affected by several factors, such as the properties of the electrolyte, current density, charging method, temperature, and design.

3.2. Li Dendrites Growth Patterns

Li dendrites have various morphologies and are mainly classified into three growth patterns: needle-like Li, moss-like Li, and branched Li (Figure 5). Dendrites can cause various problems, including short circuits, dead Li, and electrolyte consumption.
Needle-like Li has no branched morphology and retains its structural characteristics along one dimension. These Li dendrites frequently grow at the existing nucleation sites in Li. They are positioned for growth under the influence of amorphous regions/grain boundaries, very thin portions of the SEI layer, and chemical inhomogeneities of previously deposited Li [68], and have a larger and more complete crystal structure of Li metal than other patterns of dendritic growth. Needle-like Li dendrites are the main cause of short circuits in LMBs.
The moss-like Li dendrites consume more Li to form SEI films owing to their small diameter and high specific surface area. They grow under certain conditions with multiple defects and branching. One-dimensional (1D) needle-like Li can also grow into 3D moss-like, and this process occurs through the branching and broadening of filament growth. According to the cosmological model of Steiger et al. [69], moss-like growth has no center, the branches expand in random directions, and each part grows independently. This growth occurs not only at the tips but also frequently at the growth points distributed throughout the moss. This is a random and nonlinear growth process that is not controlled by the direction of the electric field.
Branched Li dendrites have been studied extensively. They grow in length, width, and branches in all directions, similar to the neat hierarchical structure. Park et al. studied these dendrites both theoretically and experimentally [70]. The branched morphology was investigated in the electroplating of different metals, such as Au [71] and Pb [72]. The branched Li deposition pattern during the electrochemical deposition pattern of Li ions was evaluated by applying the nonlinear phase field model using the Butler–Volmer electrochemical reaction kinetics [73]. Branched dendrites depended on the interfacial morphology and applied voltage and could be transformed into moss-like forms under certain conditions [64]. This made distinguishing between them difficult because all three dendrite patterns could coexist. Therefore, understanding the morphology of the Li deposit and properly simplifying the morphology for efficient electrode surface customization and safe battery design was necessary.
All three morphologies of Li dendrites shared the basic structure of Li deposits in the form of tubes. Although these dendrites have different morphologies, the basic shapes and structures of the formed Li deposits were similar.

4. Pretreatment Strategies According to Materials

SEI plays a crucial role in LMBs by isolating the electrolyte from direct contact with the Li metal. This isolation helps prevent parasitic reactions and regulates the Li plating and stripping behavior [25,74]. Li ions dissolve in the electrolyte, diffuse through the SEI layer, and finally reach the surface of the Li anode, where they accept electrons and are deposited. However, the uneven deposition of Li can lead to dendrite formation, which disrupts the SEI layer and results in performance degradation and safety issues [75,76]. The growth of Li dendrites begins with nonuniform deposition, causing volumetric expansion and stress on the SEI layer, leading to cracks. The fresh Li exposed at these cracks continues to react with the electrolyte, forming a new SEI layer. This process causes preferential deposition of Li in thinner SEI areas, leading to dendrite formation [77]. Some dendrites become electrically isolated by the SEI layer, contributing to Li loss and reduced CE during stripping. Repeated deposition and peeling of Li result in the accumulation of “dead Li”, which further deteriorates battery performance and increases the risk of short circuits if dendrites puncture the separator. Therefore, modifying the fragile and unstable natural SEI layer is essential to create a stable SEI layer and enhance the performance and safety of LMBs.
Pretreatment is proposed as a technique for forming a stable SEI layer and suppressing dendrite growth. The pretreatment strategy for LMBs improves the interaction between the Li metal anode and the electrolyte using various materials. In general, materials such as polymers, carbon materials, metal oxides, and inorganic compounds are used and are designed to increase the electrochemical stability of the battery and, in particular, to prevent dendrite growth and decomposition of the electrolyte in Li metal. Each material has clear features. In this regard, it is introduced as follows.

4.1. Polymers

Polymers have flexibility and elasticity and can respond well to the volume changes that occur during the charge and discharge cycles of Li metal. They provide chemical stability to reduce the reaction between Li metal and the electrolyte and suppress the formation of dendrites, thereby improving the cycle life and safety of batteries. In addition, polymers can be applied through various coating techniques, which can be adjusted to suit the production scale and requirements [78]. At this time, the polymer must be stable so as not to react with Li metal and have ionic conductivity so that Li ions can move freely. The polymer layer deposited on the Li surface should be similar to that of a solid polymer electrolyte (SPE) and should have a low glass transition temperature (Tg) to maintain rubber at room temperature in order to maintain Li+ conductivity similar to that of a liquid electrolyte system. The polymer must have excellent flexibility and a high Young’s modulus to accommodate Li deformation during cycling and prevent Li dendrite formation [79].
Lithium polyacrylic acid (LiPAA) is a polymer with a significantly high elasticity (strain of 582% relative to its initial length) and the ability to form a uniform ion-conductive surface coating. Li et al. introduced a highly elastic smart SEI layer using LiPAA to address the substantial volume changes during Li plating/stripping (Figure 6a) [80]. This 20 nm-thick LiPAA layer, formed by drop-casting on the Li surface, demonstrated excellent long-term stability, maintaining a flat voltage plateau for up to 700 h in symmetric Li/Li cells at 0.5 mA cm−2, significantly outperforming uncoated Li anodes (Figure 6b). The self-adaptive SEI design adjusts to volume fluctuations, reducing side reactions and suppressing dendrite formation. This flexible interface also enhanced the stability of Li metal anodes in full cells, demonstrating their potential to improve the performance and safety of Li metal batteries.
Polymer coating layers with excellent mechanical deformation and low densities are promising candidates for the stabilization of Li metal anodes. For example, a poly(dimethylsiloxane) (PDMS) layer with a nanopore structure has a flexible and high modulus, which can accommodate the volumetric change in Li deposition without fracturing during cycling and improve cycling stability [81]. As shown in Figure 6c, Li was deposited on the Cu foil through the nanopores of the thin PDMS film. Therefore, the modified PDMS film could protect the surface of the deposited Li from the electrolyte, which could significantly suppress the formation of dendrites and improve stability. A PDMS film of approximately 500 nm thickness was obtained through the spin-coating process (Figure 6d), and nanopores of 40–100 nm were clearly observed in the SEM images (Figure 6e). The nanopores fabricated by the HF acid etching process served as Li-ion passages, and the pore size was controlled by varying the etching time. Because it exhibited excellent mechanical and chemical stability when in contact with Li metal, improving the transport of Li ions and inhibiting dendrite growth was effectively possible.
Hao et al. developed a novel SPE protective layer based on Li3PO4 and polyvinyl alcohol (PVA) to stabilize Li metal anodes (Figure 6f) [82]. A thin protective layer of the PP-Li anode was formed directly on the surface by an in situ reaction of the treatment solution (0.4% P2O5 and 0.5% PVA, dissolved in dimethyl sulfoxide (DMSO)) with Li metal to protect the anode. Owing to the synergy between Li3PO4 and PVA, the protective layer was characterized by excellent mechanical properties, flexibility, and uniform channels for Li-ion transfer, effectively regulating the behavior of Li deposition and inhibiting the growth of Li dendrites. When the PP-Li anode was used in the Li|Li symmetric cell, the cell showed an ultra-long-term steady-state cycle of more than 1000 h with a capacity of 1 mA h cm−2 at a current of 2 mA cm−2 and stably maintained the cycle for 800 h, even at a high current of 5 mA cm−2 and a high capacity of 2.5 mA h cm−2 (Figure 6g,h). Likewise, the lithium titanium oxide (LTO) cells with the PP-Li anode at a high rate of 5 C showed high CE and high capacity retention even after 300 cycles.
Polymers can also be fabricated using various techniques. Layer-by-layer (LBL) self-assembly is a method for sequentially depositing polymers with opposite charges on a planar surface, and a soft and multilayered film is formed on metallic Li anodes [83]. Creating an “open space” on the substrate is possible by modifying the ionic properties of pH-sensitive polymers. When the polymer is in an ionic form (positively charged –NH3(+)), it strongly adheres to the substrate; however, when it is deprotonated to a neutral form (–NH2), it detaches from the substrate, leaving gaps in the structure. These gaps help Li to be deposited in a non-dendritic form in the LBL film, as shown in Figure 7. Fourteen multilayers with different ratios of PDAD/PEDOT: PSS, PADD/Nafion, or PPy/PEDOT: PSS were bonded to the surface of a Cu substrate using a pH-sensitive polymer (PAH layer). In addition, many studies have been conducted to suppress dendrite growth and improve battery performance using Nafion and other polymer-based protective coatings on Li metal anode [84,85,86].
However, because commonly used polymers are not good conductors for the transport of Li ions, polymers with organized channels that are capable of conducting Li ions well are required. When the polymer layer is formed on the Li metal surface, it maintains a high aspect ratio with the unique design of a vertical nanoscale channel [87]. As shown in Figure 8a, the volume change during Li deposition/dissolution of a bare electrode can easily penetrate the SEI layer, which can lead to a concentration of Li+ flux near cracks, pits, and crevices, resulting in the growth of Li dendrites and rapid consumption of the electrolyte. By contrast, uniform Li deposition without a moss-like morphology is expected for the modified electrode coated with a thin polymer layer with vertically aligned nanochannels, as shown in Figure 8b. The relatively uniform Li+ flux in each nanochannel with a large aspect ratio is expected to contribute to the uniform distribution and growth of Li nuclei, which can stabilize the Li metal anode. However, the conclusion of this strategy suggests that the micro-sized confinement structure is not practically effective in suppressing Li dendrites, requiring further development for stable Li anodes. The conductivity of these polymer layers can be improved by functionalizing them with inorganic components. For example, when the interface layer of the poly(methyl methacrylate) (PMMA) polymer is replaced with a SiO2@PMMA core–shell nanospherical film, the ionic conductivity can be increased while maintaining the dendrite prevention ability [88].

4.2. Carbon Materials

Carbon is widely used as an inorganic material to protect metallic Li anodes. Carbon-based materials, including carbon nanotubes (CNTs), graphene, and their derivatives, can satisfy the requirements of artificial SEI owing to their excellent mechanical properties [89,90]. Carbon, particularly graphitic carbon (e.g., graphite), can store Li+ ions and artificially increase the surface area of the Li foil. Moreover, the layered structure can accommodate volume changes during repeated stripping/plating cycles. The dense structure of graphitic carbon prevents direct contact between the electrolyte and Li, thereby limiting side reactions and electrolyte consumption. Additionally, the mechanical flexibility of the carbon sheets is believed to reduce the growth of Li dendrites. Consequently, graphitic carbon can be useful not only in preventing dendrite penetration through the electrolyte but also in favoring uniform Li electrodeposition by increasing the active surface area and reducing the actual current density [79].
Zhang et al. deposited amorphous carbon on the surface of metal Li foil using the magnetron sputtering technique [91]. The a-C coating on the Li surface can effectively prevent contact between metallic Li and the electrolyte, suppress the formation of dendritic Li, and greatly improve the electrochemical performance. However, to achieve excellent performance for the Li/C electrode, controlling the sputtering time is necessary to obtain an appropriate thickness of the a-C coating. Zhang et al. deposited a nano-sized, thick, nitrogen-doped, amorphous carbon (a-CNx) film on a Li anode using the same technique (magnetron sputtering) [92]. The excellent mechanical strength and high flexibility of the a-CNx film provided a stable and strong artificial SEI layer on the Li electrode, which suppressed the formation of dendritic Li and significantly improved its electrochemical performance.
Graphene sheets, which are 2D crystals of sp2-bonded carbon atoms, have high Li ion conductivity and excellent mechanical strength [93] and thus are one of the promising materials as protective layers for Li metal anodes. The graphene oxide (GO) sheets are stacked in an orderly manner after suction filtration and can essentially suppress the formation of dendritic Li when complexed with Li [94,95,96]. Chen et al. coated GO layers onto Li metal anodes using drop-casting and spray-coating methods [97]. With these various coating procedures and the synergistic effect of Al2O3 nanoparticles, the GO was self-assembled in three forms: compact, mesoporous, and macroporous structures (Figure 9a). The GO coatings, enhanced by the addition of Al2O3 nanoparticles, were tested in Li|Li symmetrical cells at a current density of 5 mA cm–2 for 2000 cycles, demonstrating effective suppression of dendrite formation without short circuits or significant polarization increases (Figure 9b). The GO layer transformed into reduced GO during operation, improving conductivity and reducing local current density, which helped mitigate dendrite growth. Yao et al. demonstrated the use of a 2D reduced graphene oxide (rGO) membrane, prepared via solvent-evaporation-assisted self-assembly, as a passivation layer for Li metal anodes to reduce interfacial resistance and suppress dendrite growth [89]. During lithiation/delithiation, the rGO coating formed mosaic-like flakes with interconnected gaps that facilitated uniform Li plating/stripping. The rGO-coated anodes exhibited superior cycling stability, maintaining a low overpotential of 87.9 mV over 600 cycles at 5 mA cm–2 in symmetric Li‖Li cells. In half-cell tests, the Cu/Li/rGO electrode showed enhanced Li usage and CE compared to bare Li. The high affinity between lithiated rGO and Li promoted fast and homogeneous Li deposition, leading to improved electrochemical performance in both Li-ion and Li-S batteries and achieving a stabilized capacity of 1200 mAh g−1 at 1 C for nearly 1000 cycles in Li-S batteries.
“Defect-free” graphene is synthesized via a special flow-aided sonication exfoliation method, allowing direct comparison of Li deposition behavior and electrochemical performance with general rGOs. Liu et al. highlighted the critical role of graphene structure and chemistry in promoting stable Li plating and stripping in Li metal anodes for LMBs [90]. This demonstrated that graphene defects, particularly in rGO, led to excessive SEI growth, rapid consumption of the FEC electrolyte additive, and the formation of Li dendrites, all of which degraded cycling performance. A novel flow-aided sonication-exfoliation method was used to create defect-free graphene (df-G) for direct comparison with r-GO (Figure 9c). Although r-GO initially exhibited better wettability and lithiophilicity, over time, df-G exhibited superior electrochemical performance with a lower nucleation overpotential, smoother post-cycled surfaces, and dendrite-free Li deposition. By contrast, r-GO displayed rapid CE decay and severe SEI growth, as confirmed by electrochemical impedance spectroscopy (EIS) (Figure 9d,e). This study emphasizes that an ideal graphene host for Li metal anodes must be non-catalytic for SEI formation, offering a new design rule for stable LMBs.
Zhang et al. developed a lithiophilic–lithiophobic gradient interfacial layer strategy to address the challenge of Li dendrite growth in Li metal anodes [98]. This approach featured a lithiophilic ZnO/CNT sublayer at the bottom that tightly adhered to the Li foil, promoting stable SEI formation and preventing mossy Li corrosion. The top lithiophobic CNT sublayer complemented this effect by effectively suppressing dendrite formation. This configuration enabled ultralong-term stable Li stripping/plating, even at high current densities (10 mA cm−2), and demonstrated significantly improved cycling performance in various systems, including copper current collectors, 10 cm2 pouch cells, and Li-S batteries.

4.3. Inorganic Compounds

Inorganic compounds with higher mechanical strength and stability against Li metal than organic layers are more effective at physically suppressing dendrites. Typically, metal oxides such as Al2O3, TiO2, and ZrO2 are used, and among them, Al2O3 is frequently used to construct protective coatings for Li anodes. Jing et al. fabricated a porous Al2O3 layer on the surface of a metallic Li anode using a spin-coating method [99]. The porous Al2O3 protective layer acted as a stable intermediate layer and inhibited side reactions between the electrolyte and the Li metal. In addition, the Li-ion flux was uniformly distributed and exhibited improved electrochemical performance, along with relatively uniform Li deposition on the surface of the anode. Peng et al. developed a unique methodology to confine plated Li metal inside a ceramic porous layer and isolate the trapped Li from the electrolyte solvent through a reinforced skin layer (embedded Al2O3 particles) [100]. The CE of the Li-metal electrode reached an average of 97.5% over 50 cycles. Wang et al. fabricated a nanometer-thick amorphous Al2O3 thin film on the surface of Li metal by the magnetron sputtering technique to form an artificial SEI layer [101]. The Al2O3 thin film acted as a “surfactant” with a high Li diffusion rate, ultimately promoting uniform Li nucleation and growth in a smooth LBL film method. EIS measurements show that initially, the total resistance of the Li/Al2O3-20 nm cell (652 Ω) is higher than that of the pristine Li cell (426 Ω) due to the insulating property of Al2O3 (Figure 10a). However, the total impedance of the pristine Li cell significantly increased from 141 Ω at the 100th cycle to 283 Ω at the 200th cycle. In contrast, the impedance of the Li/Al2O3-20 nm cell remains almost unchanged from 106 Ω at the 100th cycle to 114 Ω at the 200th cycle, and the nearly stable impedance ensures a long cycle life (Figure 10b,c).
Among the coating materials used to mitigate the growth of Li dendrites, LiF is one of the most suitable candidates for battery applications owing to its high chemical stability and high mechanical strength. Fan et al. introduced a new strategy for achieving uniform Li electrodeposition by creating an artificial SEI layer on a Li anode through magnetron sputtering deposition [102]. An artificial SEI enriched with LiF was designed based on density functional theory simulations, which showed that LiF facilitated lower energy barriers for Li-ion diffusion compared to other SEI components such as Li2CO3 and Li2O. Electron localization function analysis revealed that Li atoms experienced smooth diffusion on the LiF surface, contributing to improved cycling stability. Experimentally, LiF-coated Li electrodes demonstrated enhanced Li deposition and stability during galvanostatic cycling at high current densities, with significant reductions in dendrite formation. Lang et al. developed a simple solution method for applying a LiF coating to a Li metal anode via an in situ reaction between metallic Li and polyvinylidene fluoride (PVDF)-dimethyl formamide solution (Figure 10d) [103]. The uniform and stable LiF coating not only effectively minimized side reactions between metallic Li and the liquid electrolyte but also inhibited the growth of dendrites. At various current densities, the symmetric cell with LiF-coated Li anodes exhibited better performance than the bare Li symmetric cell. In addition, when the Li|LiCoO2 batteries with an areal capacity of 1 mAh cm−2 were tested, the full cell with a LiF-coated Li anode showed a capacity retention of 85.7% after 200 cycles. By contrast, the capacity of the bare Li full cell decreased significantly after 200 cycles (Figure 10e). To further investigate the stability of LiF-coated Li anodes, the LiF-coated symmetric cell was disassembled after 100 cycles at a current density of 1 mA cm−2 and observed by SEM. As shown in Figure 10f, the surface of the LiF-coated Li electrode remains flat without obvious dendrites, indicating that the LiF coating is mechanically and electrochemically stable during Li plating/stripping cycles. In contrast, the bare Li electrode has a rough surface full of cracks (Figure 10g). Therefore, this artificial SEI layer improved the cycling stability of Li metal electrodes in both symmetric and full cells.
Generally, organic materials have flexibility while having a low mechanical modulus. Inorganic compounds have higher mechanical rigidity and higher ionic conductivity but are brittle. Therefore, a hybrid material with the advantages of flexibility and rigidity can be used by mixing two materials [104]. The organic–inorganic composite layer simultaneously provides fast Li+ ion diffusion, high modulus, and excellent shape suitability [105]. Huang et al. applied an effective and efficient artificial protective layer (APL) consisting of PVDF-co-hexafluoropropylene (PVDF-HFP) and LiF to a Li metal anode to improve the interfacial stability (Figure 10h) [106]. The protective layer with soft-rigid properties suppressed the random deposition of Li, prevented dendrite growth, and solved the problem of depletion of the electrolyte and Li metal. In addition, this protective layer ensured adequate ionic conductivity by interacting with soft and rigid structural elements. Recently, a double-layer artificial interface was fabricated using Al-doped Li6.75La3Zr1.75Ta0.25O12 (LLZTO)-based garnet as the bottom layer and elastically lithiated Li-Nafion as the top layer [107]. The Al-doped LLZTO-based garnet bottom layer provided fast Li-ion conductivity and mechanical strength, and the Li-Nafion top layer supported the uniformity of Li deposition through elasticity, which controlled volume changes. The unique double-layer LLZTO/Li-Nafion (denoted as LLN) artificial film modulated the homogeneous and high-efficiency Li+ diffusion method on the surface of the Li anode, thus preferring small and dense Li plating patterns. Consequently, a shape that suppressed dendrite formation was induced, as shown in Figure 10i.
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Figure 10. Electrochemical impedance spectra of pristine and 20 nm Al2O3-modified Li electrodes obtained at an open circuit voltage after charge–discharge cycles at a current density of 0.5 mA/cm2. (a) Fresh cell state, (b) after 100 cycles, and (c) after 200 cycles. (Reprinted with permission from Ref. [101]; Copyright © 2017 Elsevier B.V. (Amsterdam, The Netherlands) All rights reserved); (d) Illustration of the process for fabricating LiF-coated Li. (e) Cycling performance of the LiF-coated Li|LiCoO2 cell. (f) LiF-coated Li electrode and (g) bare Li electrode after 100 cycles at a current density of 1 mA cm−2. (Reprinted with permission from Ref. [103]; Copyright © 2018 Published by Elsevier B.V. (Amsterdam, The Netherlands)); (h) Schematics of Li deposition on bare Li anode without protection, with a pure PVDF-HFP layer and with APL composed of organic PVDF-HFP and inorganic LiF protective layer Li anode. (Reprinted with permission from Ref. [106]; Copyright © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany); (i) Schematics of Li deposition patterns for the single-ion-conducting LLN coating with rigid LLZTO and elastic Li-Nafion protected Li metal anode. (Reprinted with permission from Ref. [107]; Copyright © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany).
Figure 10. Electrochemical impedance spectra of pristine and 20 nm Al2O3-modified Li electrodes obtained at an open circuit voltage after charge–discharge cycles at a current density of 0.5 mA/cm2. (a) Fresh cell state, (b) after 100 cycles, and (c) after 200 cycles. (Reprinted with permission from Ref. [101]; Copyright © 2017 Elsevier B.V. (Amsterdam, The Netherlands) All rights reserved); (d) Illustration of the process for fabricating LiF-coated Li. (e) Cycling performance of the LiF-coated Li|LiCoO2 cell. (f) LiF-coated Li electrode and (g) bare Li electrode after 100 cycles at a current density of 1 mA cm−2. (Reprinted with permission from Ref. [103]; Copyright © 2018 Published by Elsevier B.V. (Amsterdam, The Netherlands)); (h) Schematics of Li deposition on bare Li anode without protection, with a pure PVDF-HFP layer and with APL composed of organic PVDF-HFP and inorganic LiF protective layer Li anode. (Reprinted with permission from Ref. [106]; Copyright © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany); (i) Schematics of Li deposition patterns for the single-ion-conducting LLN coating with rigid LLZTO and elastic Li-Nafion protected Li metal anode. (Reprinted with permission from Ref. [107]; Copyright © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany).
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5. Conclusions

Recently, Li metal has been refocused on as an anode material for high-energy-density batteries. However, significant challenges existed regarding the safety issues caused by dendrite growth. Therefore, protecting the surface of Li metal was very important for stabilizing the anode and extending battery life. Pretreatment, one of the methods for the surface protection of Li metal anodes, played an important role in improving the performance and stability of Li metal anodes. Various materials such as polymers, carbon materials, and inorganic compounds were used in the pretreatment method, and each material contributed to reducing the side effects between the Li metal and the electrolyte and suppressing dendrite growth. Polymers were commonly used in the pretreatment process because they provided flexibility and elasticity, which allowed them to accommodate the significant volume changes experienced by Li metal during repeated charge and discharge cycles. In addition, polymers could provide chemical stability, which could reduce the possibility of side reactions between Li metal and the electrolyte. This stability helped suppress the formation of dendrites, which were essential for maintaining the integrity of the Li metal anode and ensuring long-term performance. Carbon-based materials exhibited excellent mechanical strength and conductivity, which contributed to uniform Li deposition. With its dense structure, side reactions were minimized by limiting direct contact between the electrolyte and Li metal. This dense structure also controlled the Li-ion flux on the surface, further reducing the risk of dendritic growth and promoting a more stable and even Li plating/stripping process. Inorganic compounds such as Al2O3 and LiF have also shown potential as protective layers in Li metal anodes. These materials have particularly high Li+ ion conductivity and high Young’s modulus, which provided a rigid barrier that physically suppressed dendritic growth. The high mechanical strength of these inorganic layers prevented the formation of sharp Li dendrites, ensuring smoother and more uniform Li deposition. The inorganic layer effectively reduced side reactions and improved the long-term cycle stability of LMBs by creating a stable SEI. The development of these protective layers was very important not only for improving safety and stability but also for achieving a longer battery life. By reducing unwanted side reactions between the Li metal and the electrolyte, these layers served as barriers to prevent electrolyte decomposition and Li corrosion. These barrier functions were key to maintaining consistent battery performance because they limited the continuous loss of active Li.
In summary, solving the problems related to SEI stability and dendritic growth to develop LMBs is necessary. Continuous research and development of pretreatment strategies and coating materials aim to improve the safety, efficiency, and lifespan of Li metal anodes. Future research will focus on finding new materials and innovative methods to further suppress dendritic growth and improve the overall performance of the protective layers. In addition, practical approaches, such as minimizing manufacturing costs and simplifying processing technology, are required to increase the commercial feasibility of LMBs. As research progresses, the combination of material innovation and process optimization will be key to realizing the full potential of Li metal anodes in next-generation energy storage solutions.

Author Contributions

Conceptualization, G.Y., J.K. and O.B.C.; writing—original draft preparation, G.Y.; writing—review and editing, G.Y., J.K. and O.B.C.; supervision, O.B.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Gachon University Research Fund of 2023 (GCU-202400930001).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic of pretreatment strategies according to materials.
Figure 1. Schematic of pretreatment strategies according to materials.
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Figure 2. Schematic of SEI formation processes on (a) Li, (b) Graphite, and (c) Si-negative electrodes. (Reprinted with permission from Ref. [31]; Copyright © 2020 Wiley-VCH GmbH); (d) Schematic of the formation process of SEI film on Li surfaces. (e) Schematic of the positive and negative potential limits of electrolyte stability and the energy levels of LUMO and HOMO. (Reprinted with permission from Ref. [34]; Copyright © 2019 American Chemical Society).
Figure 2. Schematic of SEI formation processes on (a) Li, (b) Graphite, and (c) Si-negative electrodes. (Reprinted with permission from Ref. [31]; Copyright © 2020 Wiley-VCH GmbH); (d) Schematic of the formation process of SEI film on Li surfaces. (e) Schematic of the positive and negative potential limits of electrolyte stability and the energy levels of LUMO and HOMO. (Reprinted with permission from Ref. [34]; Copyright © 2019 American Chemical Society).
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Figure 4. Photograph of a dendrite obtained in the (a) high current densities (J = 0.7 mA cm−2) regime, (b) low current densities (J = 0.1 mA cm−2) regime. (Reprinted with permission from Ref. [59]; Copyright © 1999 Elsevier Science S.A. All rights reserved).
Figure 4. Photograph of a dendrite obtained in the (a) high current densities (J = 0.7 mA cm−2) regime, (b) low current densities (J = 0.1 mA cm−2) regime. (Reprinted with permission from Ref. [59]; Copyright © 1999 Elsevier Science S.A. All rights reserved).
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Figure 5. Morphology of dendrites. (a) Needle-like dendrites. (b) Branched dendrites. (c) Moss-like dendrites. (Reprinted with permission from Ref. [67]; Copyright © 2019 Elsevier Ltd. (Amsterdam, The Netherlands) All rights reserved).
Figure 5. Morphology of dendrites. (a) Needle-like dendrites. (b) Branched dendrites. (c) Moss-like dendrites. (Reprinted with permission from Ref. [67]; Copyright © 2019 Elsevier Ltd. (Amsterdam, The Netherlands) All rights reserved).
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Figure 6. (a) Design of the flexible SEI. (b) Comparison of the cycling stability of pristine Li and LiPAA-Li in a symmetrical cell at 0.5 mA cm−2 (Reprinted with permission from Ref. [80]; Copyright © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim); (c) Schematics of Li deposition on bare Cu foil and the Cu foil coated with PDMS thin film. (d) Cross-section SEM image of the PDMS film, and (e) top view SEM image of PDMS film after HF acid treatment (red circles: nanopores). (Reprinted with permission from Ref. [81]; Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim); (f) Li3PO4/PVA protective layer on the Li foil. Voltage profiles of Li metal plating/stripping in a Li|Li symmetrical cell under different current densities: (g) 2 mA cm–2 with 1 mA h cm–2 and (h) 5 mA cm–2 with 2.5 mA h cm–2 (Reprinted with permission from Ref. [82]; Copyright © 2020 American Chemical Society).
Figure 6. (a) Design of the flexible SEI. (b) Comparison of the cycling stability of pristine Li and LiPAA-Li in a symmetrical cell at 0.5 mA cm−2 (Reprinted with permission from Ref. [80]; Copyright © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim); (c) Schematics of Li deposition on bare Cu foil and the Cu foil coated with PDMS thin film. (d) Cross-section SEM image of the PDMS film, and (e) top view SEM image of PDMS film after HF acid treatment (red circles: nanopores). (Reprinted with permission from Ref. [81]; Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim); (f) Li3PO4/PVA protective layer on the Li foil. Voltage profiles of Li metal plating/stripping in a Li|Li symmetrical cell under different current densities: (g) 2 mA cm–2 with 1 mA h cm–2 and (h) 5 mA cm–2 with 2.5 mA h cm–2 (Reprinted with permission from Ref. [82]; Copyright © 2020 American Chemical Society).
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Figure 7. (a) Scheme depicting LBL deposition of oppositely charged polymer layers. (b) “Open spaces” that are cleared by modifying the ionic character of post-assembly of a pH-sensitive polymer can be observed to accommodate the non-dendritic deposition of Li metal.
Figure 7. (a) Scheme depicting LBL deposition of oppositely charged polymer layers. (b) “Open spaces” that are cleared by modifying the ionic character of post-assembly of a pH-sensitive polymer can be observed to accommodate the non-dendritic deposition of Li metal.
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Figure 8. (a) Modified electrode (stainless steel (SS)) with a nanochannel layer coating. (b) Simulation performed for the bare electrode. (Reprinted with permission from Ref. [87]; Copyright © 2016 American Chemical Society).
Figure 8. (a) Modified electrode (stainless steel (SS)) with a nanochannel layer coating. (b) Simulation performed for the bare electrode. (Reprinted with permission from Ref. [87]; Copyright © 2016 American Chemical Society).
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Figure 9. (a) Schematics of different coating methods and their effects on Li deposition in Li anodes. (b) Overpotential plotted with respect to cycle number for B-Li, D-GO-Li, D-GOAl-Li, and S-GO-Li symmetrical cells tested at a current density of 5 mA cm–2 and a deposition capacity of 1 mA h cm–2 for 2000 cycles. (Reprinted with permission from Ref. [97]; Copyright © 2018 American Chemical Society); (c) Schematic of the exfoliation process for df-G. EIS Nyquist plots (d) df-G, (e) r-GO. (Reprinted with permission from Ref. [90]; Copyright © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany).
Figure 9. (a) Schematics of different coating methods and their effects on Li deposition in Li anodes. (b) Overpotential plotted with respect to cycle number for B-Li, D-GO-Li, D-GOAl-Li, and S-GO-Li symmetrical cells tested at a current density of 5 mA cm–2 and a deposition capacity of 1 mA h cm–2 for 2000 cycles. (Reprinted with permission from Ref. [97]; Copyright © 2018 American Chemical Society); (c) Schematic of the exfoliation process for df-G. EIS Nyquist plots (d) df-G, (e) r-GO. (Reprinted with permission from Ref. [90]; Copyright © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany).
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Table 1. Comparison of volume change ratio and SEI formation process on different negative electrodes. (Reprinted with permission from Ref. [31]; Copyright © 2020 Wiley-VCH GmbH).
Table 1. Comparison of volume change ratio and SEI formation process on different negative electrodes. (Reprinted with permission from Ref. [31]; Copyright © 2020 Wiley-VCH GmbH).
ElectrodeVolume Change RatioSEI Formation Process
GraphiteMinimal (<10%)Electrochemical decompositions
SiliconSignificant (>300%)Electrochemical decompositions
LithiumInfiniteChemical and electrochemical decompositions
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Youk, G.; Kim, J.; Chae, O.B. Improving Performance and Safety of Lithium Metal Batteries Through Surface Pretreatment Strategies. Energies 2025, 18, 261. https://doi.org/10.3390/en18020261

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Youk G, Kim J, Chae OB. Improving Performance and Safety of Lithium Metal Batteries Through Surface Pretreatment Strategies. Energies. 2025; 18(2):261. https://doi.org/10.3390/en18020261

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Youk, Gyuri, Jeongmin Kim, and Oh B. Chae. 2025. "Improving Performance and Safety of Lithium Metal Batteries Through Surface Pretreatment Strategies" Energies 18, no. 2: 261. https://doi.org/10.3390/en18020261

APA Style

Youk, G., Kim, J., & Chae, O. B. (2025). Improving Performance and Safety of Lithium Metal Batteries Through Surface Pretreatment Strategies. Energies, 18(2), 261. https://doi.org/10.3390/en18020261

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