Progress and Perspectives on Lithium Metal Powder for Rechargeable Batteries

The increasing demand for batteries with high‐energy densities for applications such as electric vehicles necessitates a paradigm shift from the use of conventional graphite as anodes. Li metal is spotlighted as a replacement for graphite due to its ultrahigh theoretical capacity (3860 mAh g−1). However, Li metal foil is plagued with limited cycle life and safety concerns due to poor Coulombic efficiency and uncontrollable growth of Li dendrites. To overcome these challenges, utilizing Li metal in powder form instead of the conventional foil proves to be advantageous. The anode consisting of spherical‐shaped Li metal powders (LMPs) has a larger surface area than Li metal foil, resulting in a lower effective current density. Furthermore, using the powder‐based slurry process facilitates the fabrication of large‐area and thin‐film (≤20 μm) Li anodes. In this review, the various fabrication methods and surface stabilization techniques of LMPs are summarized with their associated patents. Also, research trends with regard to LMP‐based anodes toward high‐performance Li metal batteries (LMBs) are carefully presented. Additionally, the application of LMPs as prelithiation agents in electrode active materials for batteries and capacitors is outlined. Finally, perspectives are suggested regarding the future of LMPs to accelerate the commercialization of advanced LMBs.


Introduction
9][10][11][12] As the insatiable quest for batteries with augmented capacities intensifies, there is the need to investigate alternative anode materials, leading LIBs to achieve an increase in gravimetric energy density from 90 to 250 Wh kg À1 . [13,14][34][35][36][37][38][39] However, implementing Li metal anodes in commercial battery systems faces several challenges, including Li dendritic growth and the unpredictable formation of a solid electrolyte interphase (SEI) and dead Li layer (Figure 1a). [40]he Li metal anode stores Li ions based on an electrodeposition mechanism.It implies that Li nucleation and growth take place on the Li surface during the charging process.Also, a native layer with a thickness of tens of nanometers formed by the reaction between Li metal and the atmosphere covers the Li surface, [46][47][48][49] acting as a resistive element for electrochemical reactions.However, an important point to consider is that the composition and thickness of the native layer are not uniform across the Li metal surface.Specific areas with less resistive or thinner native layers could undergo Li electrodeposition The increasing demand for batteries with high-energy densities for applications such as electric vehicles necessitates a paradigm shift from the use of conventional graphite as anodes.Li metal is spotlighted as a replacement for graphite due to its ultrahigh theoretical capacity (3860 mAh g À1 ).However, Li metal foil is plagued with limited cycle life and safety concerns due to poor Coulombic efficiency and uncontrollable growth of Li dendrites.To overcome these challenges, utilizing Li metal in powder form instead of the conventional foil proves to be advantageous.The anode consisting of spherical-shaped Li metal powders (LMPs) has a larger surface area than Li metal foil, resulting in a lower effective current density.Furthermore, using the powder-based slurry process facilitates the fabrication of large-area and thin-film (≤20 μm) Li anodes.In this review, the various fabrication methods and surface stabilization techniques of LMPs are summarized with their associated patents.Also, research trends with regard to LMP-based anodes toward high-performance Li metal batteries (LMBs) are carefully presented.Additionally, the application of LMPs as prelithiation agents in electrode active materials for batteries and capacitors is outlined.Finally, perspectives are suggested regarding the future of LMPs to accelerate the commercialization of advanced LMBs.
[52][53] After the nucleation process, the Li growth may become predominant in areas where the native layer is disrupted, leading to the formation of dendritic Li structures at specific sites.These dendrites can pierce and penetrate the separator, leading to internal short circuits.60] As mentioned earlier, Li metal batteries (LMBs) store Li based on an electrodeposition mechanism.Specifically, the electrolyte salt is reduced at the Li anode surface through a reduction reaction, Li þ þ e À !Li.During the plating process, if the reduction kinetics are faster than the supply rate of the Li ions from the bulk electrolyte to the Li anode surface, the electrolyte salt concentration near the Li anode surface gradually decreases.According to Sand's time theory (Figure 1c), [42] from the point at which the salt concentration within the electrolyte on the Li anode surface reaches zero, electrodeposition occurs selectively toward areas on the Li anode with lower resistance, leading to the growth of a dendritic shape.Reproduced with permission. [40]Copyright 2017, Springer.b) Sand's time theory.Reproduced with permission. [41]Copyright 2017, The Royal Society of Chemistry.c) Porous and tortuous structure of dead Li layer on Li metal anodes.Reproduced with permission. [42]Copyright 2016, The Royal Society of Chemistry.Schematic illustration of Li ion distribution and scanning electron microscopy (SEM) images of d) Li metal foil and e) LMP anodes during electroplating.Reproduced with permission. [43,44]Copyright 2019, Elsevier and Copyright 2004, Elsevier.f ) Width comparison between commercial Li foil and LMP anode.Reproduced with permission. [45]Copyright 2021, Wiley.
According to Equation (1), Sand's time (t Sand ) is a function of apparent diffusion coefficient (D app ), charge number of the cation (z c ), bulk salt concentration (c o ), Faraday constant (F), effective current density ( J), and transference number of anion species in salt (t a ). [42]To suppress the formation of dendritic Li, it is essential to increase Sand's time to ensure sufficient duration for selective Li plating.Indeed, increasing salt concentration in electrolytes for LMBs suppresses undesired growth of Li and also significantly improves the uniformity of Li plating.Furthermore, electrolyte design incorporating varying salt concentrations alters the solvation structure of Li ions, leading to the control of stable SEI and suppressing the Li dendritic growth.Ren et al. increased the salt concentration in the electrolyte, which modified the solvation structures and facilitated the supply of Li ions toward the Li metal anode, thereby extending Sand's time. [61]im et al. demonstrated efficient suppression of dendritic Li plating through convective Li ion transfer to maintain a high Li ion concentration at the Li surface using nanospinbar-dispersed colloidal electrolytes. [62]Also, Cheng et al. suggested a 3D fibrous matrix anode with a large active surface area that could provide enough space to accommodate the electrolyte and decrease the effective current density for stable Li-S battery operation. [63]owever, research into modifying the electrolyte system necessitates the optimization of electrodes, separators, and even internal battery components such as pouches.Introducing elements like nanospinbar and other structural enhancements to improve performance inevitably reduces the energy density of LMBs.Hence, to capitalize on the high theoretical capacity of the Li anode and optimize the energy density of LMBs, the anode should be exclusively structured using Li.In this context, Yoon et al. employed Li metal powders (LMPs) to fabricate a Li anode, which strategically nullified the need to introduce supplementary structural components.Additionally, BET analysis indicated that the active surface area of the devised anode was several times superior to that of traditional Li foil. [64]he effective current density (J, unit: mA cm À2 ) in Equation (1) represents the applied current divided by the active surface area.Given the high active surface area of the LMP anode, the effective current density decreases, resulting in a reduced amount of Li at a unit surface area.This effectively distributes the Li ion flux around the Li anode surface, promoting a more uniform deposition process (Figure 1d,e). [43,44]Consequently, the salt concentration on the Li anode surface remains relatively elevated.In summary of LMP anode properties, an increase in the Sand's time (t Sand ) driven by low effective current density with increased active surface area, subsequently could delay the starting time of Li dendrite formation.These characteristics depend on the LMP size with different active surface areas. [65]However, despite the advantages resulting from the structural features of LMP, further improvement of LMP anodes is essential for stable LMB operation.[68][69][70][71][72][73][74] Jin et al. demonstrated the fabrication of Li anodes as thin as 20 μm across an expansive 145 mm width through a slurry coating technique utilizing LMPs (Figure 1f ). [45]urthermore, it has been suggested that LMP anodes can efficiently create thinner anodes over larger widths, surpassing traditional extrusion/pressing and deposition/electrodeposition methods.
Transitioning to LMP as the active material of anode substrate emerges as a prospective solution to these technical impediments.In this manuscript, we endeavor to provide a rigorous analysis of the methodologies underpinning LMP, spanning its intricate fabrication processes and state-of-the-art strategies to prevent its atmospheric reaction.In this regard, we analyzed patents related to LMP fabrication, categorizing them by company, year, and technology to provide new insights.Furthermore, by interpreting a curated selection of schematic representations, this work conveys a holistic perspective of the current progress, challenges, and prospective research directions, underscoring the pivotal role of LMP in the genesis of next-generation rechargeable battery systems.

Synthesis and Stabilization of LMP
Several methods, such as the droplet emulsion technique (DET), electrochemical deposition, cryo-milling, and solvent processing techniques, have been reported in literature regarding the synthesis of LMP.Also, many protective layer coatings (Li 2 CO 3 , Li 3 PO 4 , LiF, wax, LiF/Li 2 O, ionic liquid, polypyrrole [PPy]) have been identified to enable easy and safe handling of LMPs owing to the very high reactivity of Li metal, even when handling under dry room conditions.As a result, many companies have been actively involved in producing LMP on a large scale.

Synthesis Methods of LMP
Yoon et al. first reported the DET [44,[75][76][77] to synthesize micrometer-sized LMPs.The process involved: 1) emulsion formation by mixing molten Li and silicone oil at a high temperature (above the melting point of Li: 180.5 °C) while stirring at ≈25000 rpm, 2) obtaining solidified LMPs by cooling down the emulsion to room temperature, and 3) the LMPs are then separated from the silicone oil followed by washing with hexane (Figure 2a). [78]The obtained LMP with a diameter ranging from 10 to 20 μm was then pressed into an anode, and its electrochemical performance was compared to bare Li metal foil.As expected, the LMP anodes outperform its Li metal foil counterpart.The DET process is by far the most widely used synthesis route for LMP and allows for easy modification of the LMP surface. [76]i et al. produced air-stable Li spheres (ASLSs; diameter: 0.5-3 μm) comprising a metallic Li core and a LiF/Li 2 O shell via a facile electrochemical plating technique.[79] The size of the ASLSs was easily controlled by adjusting the deposition time.Also, the applied current density (0.05-20 mA cm À2 ) was found to have a direct impact on the shape of the ASLSs (Figure 2b).The LiF/Li 2 O shell efficiently protects the ASLSs in the air.The uniform small size enables a high prelithiation performance of ASLSs for anodes such as graphite, SiO@C, and Sn@C.
Due to the high temperature required for the DET approach and the resultant large-size LMPs, there is a need to develop a facile and safe approach to prepare LMPs with smaller sizes.Pu et al. [80] synthesized nanoscaled LMPs (<500 nm) via a cryo-milling process.The process involved cryo-milling of Li foil with a high-boiling-point ionic liquid as a protection layer for Li under cryogenic temperature (Figure 2c).The prepared LMP anodes exhibited very low overpotentials, high areal capacity, long-term cyclability, and effectively suppressed dendrite formation.Furthermore, the nanoscaled LMP also functions as a favorable prelithiation reagent for Li-free anodes such as Si, SiO, and SnO 2 .However, there is a need to optimize the LMP and ionic liquid ratio to achieve a high active Li utilization.Furthermore, Li et al. [81] reported a convenient solventprocessing approach to prepare and extract high-purity LMP with particle size ≈30-60 μm.The process involved the addition of Li foils into a glass vial containing a polymer liquid consisting of poly(ethylene glycol) monomethyl ether (mPEG) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) followed by heating at 180 °C.The Li metal formed smaller pieces and started to melt, forming a grayish emulsion in the presence of the polymer liquid at a shearing speed of 800 rpm.The mPEG serves as a reactive .Reproduced with permission. [78]Copyright 2013, American Chemical Society.b) Schematic of the electrodeposition process and corresponding SEM images of the air stable Li spheres (ASLSs) obtained at different electrodeposition times.Reproduced with permission. [79]Copyright 2021, Elsevier.c) Schematic illustration of the cryo-milling process.Reproduced with permission. [80]Copyright 2019, Wiley.d) Scheme and optical images of the preparation procedure of Li microparticles via solvent-processing.Reproduced with permission. [81]Copyright 2019, American Chemical Society.
surfactant, and the presence of LiTFSI helps stabilize the emulsion.The mixture was then allowed to cool down to room temperature to form a uniform colloidal dispersion.Afterward, the mixture was dispersed in tetrahydrofuran (THF), followed by sonication overnight, resulting in LMP/THF suspension.The upper LMP layer was then extracted and dried under vacuum (Figure 2d).An LMP/carbon nanotube (CNT) composite anode (LMCA) was prepared, and its electrochemical performance was compared to conventional Li foil.The LMCA showed superior characteristics in both symmetric and full cells.

Protective Layer Coatings for LMP
Due to the very high reactivity of Li metal to atmospheric conditions during handling and experimental procedures, it is very important to form protective layers on the surface of the LMP.Lithium carbonate (Li 2 CO 3 ) has been used as a surface protection layer and commercialized by Livent (Incl.FMC Lithium Corporation, USA).Li 2 CO 3 , LiF, and wax protection layers are formed by introducing CO 2 , fluorides and wax, respectively, as surfactants during the emulsification step in the DET process. [76]i metal readily reacts with CO 2 to form a Li 2 CO 3 protection layer.[82] According to a study by Xiang et al. [83] the Li 2 CO 3 layer was estimated to have a thickness and electrical conductivity of about 350 nm and 10 À6 S cm À1 , respectively (Figure 3a).
Park et al. [84] used wax as an SEI control agent for LMP via DET (Figure 3c).The wax layer was found to play the role of the SEI, and very little change in the internal resistance of the cell was observed during charging/discharging.The wax-coated LMP anodes exhibited improved long-term cycle performance compared to the uncoated LMP and Li foil.Also, the wax layer prevented the LMP from reacting with the electrolytes, which mitigates the degree of electrolyte loss during the continuous charge/discharge process.
Hong et al. [44] formed a native LiF film on the surface of LMPs using LiPF 6 as a surfactant in the DET process (Figure 3d).The modified surface film was found to have a lower resistance than the bare LMP surface.This was attributed to the thinner surface film on the modified LMP than the bare LMP.Also, the LiFmodified LMP exhibited a greater tendency to suppress dendritic formation than the bare LMP.

LMP-Producing Companies and Associated Patents
Several companies, mainly from the USA, Japan, and China, have been actively involved in LMP research and development over decades and have filed many patents, as shown in Figure 4a,b.[127][128][129][130][131] These patents encompass the fabrication and protective layer coatings of LMPs.Among these companies, Livent (including FMC) Corporation [132][133][134][135][136][137][138][139][140][141][142] tops the chart with the greatest number of patents.Using DET, FMC established a mass production process for stabilized LMP (SLMP).DET is the most reported fabrication approach of LMPs, as shown in Figure 4c.Until 2015, there were LMP manufacturing patents using only DET.But from 2016, other manufacturing methods such as evaporation and condensation and ultrasonication were reported.Table 1 summarizes the pros and cons of each manufacturing method as reported in the patents.

Application of LMP as an Anode for LMBs
][145][146][147][148][149][150][151][152][153][154][155] Problems such as the loss of physical contact between the individual LMPs and the current collector, nonuniform and unstable LMP surface, and poor active material utilization have been associated with the LMP-based anodes.Researchers have focused on overcoming these intrinsic limitations of the LMP anodes, with many breakthroughs attained.

Fabrication Process of the LMP-Based Anodes
Figure 5 represents the fabrication process of the LMP anodes.The process involves thoroughly mixing a slurry consisting of LMP, polymeric binder, and solvent in appropriate proportions.The uniform slurry is then cast onto a copper foil with the help of a doctor blade, followed by drying in the vacuum chamber of the glove box in the lab.Then, the anode is calendered into the desired thickness using a gap-controlled roll-press machine.The entire fabrication process is carried out in a glove box in the lab.The slurry coating approach allows for easily adding additives and modifying the LMP electrode.Hereafter, the electrode is punched into discs and paired with appropriate cathodes to fabricate LMBs.

Research Efforts on LMP as an Anode for LMBs
A typical LMP anode consists of several spherical LMPs (with protective layer coatings) of varying diameters, resulting in voids within the anode (Figure 6a).According to the Kepler conjecture, the maximum density of the packing of congruent spheres in Euclidean three-space is ≈ 74%. [157]This suggests that ≈ 26% of the empty volume inside the anode could be occupied by the liquid electrolyte.Corrosion of Li metal is inevitable when  .Fabrication process of LMP anodes.A slurry comprising LMP, binder, and solvent is cast onto the current collector using a doctor blade.After sufficient drying and calendering, LMP anodes are produced.Reproduced with permission. [156]Copyright 2023, Elsevier.
in contact with Cu foil in the presence of a liquid electrolyte, resulting in the loss of contact between the LMPs and the Cu current collector.Thus, anode porosity is a critical parameter that must be controlled since it influences the accessibility of the electrolyte to the current collector.The calendering process is an efficient way to reduce the porosity, as will be discussed later.
The corrosion process negatively affects the practical capacity of the LMP anodes, increases the overpotential of Li stripping, and leads to the continuous decomposition of the electrolyte, as demonstrated by Koleskinov et al. [158]
Jin et al. [162] carried out a comparative study between PVdF, PIB, and PI as binders for LMP anodes, as shown in Figure 6b.It was found that the PI-binder-based LMP anodes showed better electrochemical performance than the other counterparts.This was attributed to the unique scaffold structure formed by PI within the anode.Furthermore, the PI binder exhibited a higher adhesive strength than the other candidates.
To realize the full potential of LMP anodes, it is essential to reduce the porosity of the anodes and also break the highly resistive Li 2 CO 3 layer to activate the LMPs.Jin et al. [159] investigated the effect of calendering on the electrochemical performance of LMP anodes.This study compared non-calendered (0%) and anodes with 20% and 40% compression rates, as shown in Figure 6c.The 40% compressed anodes showed the best electrochemical performance, followed by the 20% and 0% compressed anodes.A compression rate of more than 40% was found to be detrimental to the Cu current collector.Copyright 2020, Elsevier.b) Improvement of mechanical adhesion/cohesion (F inter /F mid ) of LMP anodes with different binders.Reproduced with permission. [162]opyright 2020, Elsevier.c) Illustration of physically compressing LMP anodes through calendering to remove the native layer, enhance the contact between LMPs, and reduce porosity.Reproduced with permission. [159]Copyright 2018, Elsevier.d) Introduction of carbon interlayer on the current collector and corresponding nucleation overpotential.Reproduced with permission. [164]Copyright 2021, Elsevier.
To overcome the limitation of the detachment of the LMPs from the Cu foil due to corrosion, Jin et al. [164] introduced a submicron carbon layer on the surface of Cu foil prior to the LMP anode fabrication (Figure 6d).The carbon-coated Cu foil exhibited higher adhesive strength between the LMP composite layer than the pristine Cu foil under conditions where corrosion is inevitable (electrolyte impregnation and cycling) from surface and interfacial cutting analysis system.Furthermore, this carbon interlayer was found to ensure Li nucleation at lower overpotentials and homogeneous Li distribution by acting as Li seeding sites.Consequently, the LMP anodes with the carbon-coated interlayer showed superior electrochemical performance in Li/Li symmetric and Li/LiMn 2 O 4 full cell tests.

Improvement of Active Material Utilization of LMP Anodes
Kolesnikov et al. [161] reported that LMP-based anodes suffer from poor active material utilization as a result of the electronic isolation between individual LMPs during the Li stripping process.LMP-based anodes undergo severe morphological changes as it is difficult to maintain the pristine particle shape during continuous Li plating/stripping processes, resulting in the formation of hemispherical LMPs (Figure 7a).It is essential to improve the contact between the individual LMPs as well as contact with the current collector.To increase the active material utilization, Kolesnikov et al. [161] carried out a facile chemical modification of the LMPbased anodes using a solution of ZnI 2 in THF.A Li-Zn intermetallic alloy was formed between the LMPs and the current collector by a cementation reaction with ZnI 2 .This resulted in an improved active material utilization due to suppressed side reactions during Li stripping, as shown in Figure 7a.Consequently, the ZnI 2modified LMP anodes demonstrated a more stable cycling behavior with low polarization and longer cycle life in Li/Li symmetric and Li/LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM622) cells.
Also, Kolesnikov et al. [165] prepared a composite LMP anode by adding a conductive carbon additive (Super C65).Super C65 addition resulted in an improved electronic contact at the Figure 7. Improvement of electronic contact within LMP anodes.a) SEM image after Li stripping and schematic of the effect of ZnI 2 cementation reaction.Reproduced with permission. [161]Copyright 2019, Elsevier.b) Schematic illustration of the improved electronic contact between LMPs and the Cu current collector.Reproduced with permission. [165]Copyright 2022, Elsevier.c) Schematic of CNT functioning as nucleation sites and electronic connection bridge in LMP anodes.Reproduced with permission. [163]Copyright 2022, Elsevier.
Li|Li and Li|Cu interfaces during cycling, leading to superior active material utilization (Figure 7b).As a result, the composite anode showed improved capacity in Li/Cu cells and superior electrochemical performance in Li/NCM622 full cells compared to the pure LMP anode.
Furthermore, Dzakpasu et al. [163] investigated the role of multi-walled CNTs as conductive agents to improve the active material utilization of LMP anodes.The CNTs were reported to form a continuous 3D electronic network within the LMP anodes and acted as Li nucleation sites, as shown in Figure 7c.The CNT-composite anode demonstrated a lower overpotential and superior electrochemical performance in Li/Cu, Li/Li and Li/NCM622 cells.The 1D structure of the CNT resulted in a long-range conductive network within the LMP anode compared to the short-range observed for 0D Super C65.

Surface Modification of LMPs
During manufacturing, even though the Li 2 CO 3 layer passivates commercial LMPs, nonuniformity of the passivation layer can cause localized and uneven Li plating/stripping where the interfacial resistance is relatively low.Severe Li loss may result from a sequence of LMP surface failures that lead to poor SEI development and an accumulation of "dead" Li.Furthermore, excessive Li loss at individual LMPs during cycling might cause the spherical shape to be fully destroyed, causing nonhomogeneous Li plating/stripping.Therefore, stabilization of the LMP surface is crucial for LMB applications.Lithium nitrate (LiNO 3 ) salt has been extensively studied as an electrolyte additive for stabilizing Li metal anodes.
Jin et al. [45] proposed a facile method for modifying the interfacial environment of the LMP anode by adding LiNO 3 as an additive to the LMP slurry.The addition of LiNO 3 to the LMP slurry process allowed chemically induced nitration of the LMP surface by uniformly enriching with Li 3 N and LiN x O y compounds (Figure 8a).Uniform surface nitration in the composite anode enabled the spatial even distribution of Li deposits over the entire anode surface, resulting in highly reversible Li plating/ stripping.During prolonged cycling, preplanted LiNO 3 within the LMP composite anode was steadily released into the electrolyte, leading to sustainable SEI stabilization.This approach is useful to overcome the limited solubility of LiNO 3 in carbonate electrolytes.As a result, the ultrathin (20 μm) composite anode exhibited excellent electrochemical performance in Li/Li symmetric and Li/LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM611) cells.However, it is necessary to simultaneously provide uniform Li nucleation sites to achieve charging at faster rates.
As a follow-up, Kang et al. [156] successfully incorporated silver nitrate (AgNO 3 ) as an additive to the LMP slurry.The addition of AgNO 3 into the slurry not only functionalized the LMP surface with N-rich SEI comprising Li 3 N and LiN x O y but also formed lithiophilic metallic silver (Ag) driven by a chemical reaction between LMP and AgNO 3 via a physical contact, which serves as favorable Li nucleation sites (Figure 8b).As a result, preplanting AgNO 3 into a 40 μm thick LMP anode reinforced the cycling stability up to 500 cycles with 86.8% capacity retention at 1C/3C charging/discharging rates and allowed superior rate capability up to 3 °C.Furthermore, Choi et al. [166] deposited a LiCl/Li 13 In 3 composite film on the surface of LMPs using a facile liquid treatment method, as shown in Figure 8c.The coated LMP was mixed with Cu powder to produce a Li-Cu composite electrode (LCE) for LMBs.The coated LCE exhibited improved electrochemical properties in both Li/Li symmetric and full cell tests.These approaches have successfully improved the electrochemical performance of LMP-based anodes.

LMP as Prelithiation Agents
The increasing demand for high-end portable electronics and longer-range EVs has necessitated extensive research for advanced LIBs with high-energy density.High-capacity-alloyand conversion-type anodes have been explored to replace the conventional low-specific-capacity graphite anode.However, one common issue plaguing these anodes is the large initial capacity loss caused by the formation of SEI and other irreversible parasitic reactions, resulting in an energy density decrease.Prelithiation (pre-doping an appropriate amount of active Li ion prior to the battery assembly) becomes indispensable to compensate for the initial capacity loss, enhance the full cell cycling performance, and bridge the gap between laboratory studies and the practical requirements of advanced LIBs. [167][195][196][197][198][199][200][201] The highsurface area of LMP ensures extended contact with the electrode active materials, enabling efficient prelithiation.Prelithiation can be achieved by coating, spraying, and compaction/pressure activation. Apart fro compensating for the initial capacity loss and enhancement in cycling performance, prelithiation with LMP has other benefits: 1) easy to control the amount of LMP, 2) uniform prelithiation can be achieved, 3) prelithiation can be carried out under dry air conditions, and 4) little or no Li residues after prelithiation.[202] However, the multistep prelithiation process involved is costly and time-consuming.Also, the high cost of LMP makes it unsuitable for large-scale applications.Therefore, it is essential to cut the cost to realize their practical application.

Prelithiation of Electrode Active Materials in Li Batteries
Jeon et al. [203] prelithiated a graphite anode by mechanically pressing LMP onto its surface.The exertion of pressure is critical to breaking the Li 2 CO 3 shell, allowing a short circuit to be developed between Li metal and graphite.As a result, electrons are directly transferred from Li metal to graphite due to the reduction potential difference.Li ions generated because of the electron donation of Li 0 are simultaneously intercalated into the reduced graphite (Figure 9a).By utilizing the prelithiated graphite created with LMP, the unoccupied octahedral sites of the LMO cathode were effectively lithiated at 3 V.Consequently, by using both the n-type and p-type redox sites of LMO, the study realized a total capacity of 200 mAh g À1 in full cells.
Lee et al. [204] reported the dry prelithiation of graphite/ silicon-based diffusion-dependent electrodes (DDEs) upon introduction of LMP, as shown in Figure 9b.DDEs are considered promising electrode designs to realize high-energy density for  [45] Copyright 2021, Elsevier.b) Metallic silver and nitrate-anion-rich SEI is formed through the reaction between AgNO 3 and LMPs during slurry mixing, which function as Li nucleation sites and ensure uniform Li ion flux, respectively.Reproduced with permission. [156]Copyright 2023, Elsevier.c) Schematic illustration of the process of chemical polishing and LMP surface coating.Reproduced with permission. [166]Copyright 2023, Elsevier.
all-solid-state batteries (ASSBs) beyond conventional compositetype electrode design.However, the graphite/silicon-based electrode also suffers from large initial irreversible capacity loss and capacity fade caused by significant volume changes during cycling, limiting the advantages of the DDEs in ASSBs.Since the LMP supplied Li ions to graphite and silicon even in a dry state, the lithiation states of the electrode active materials were increased.Moreover, the residual Li within the prelithiated DDE further serves as an activator and a reservoir for promoting the lithiation reaction of the electrode active materials and compensating for the active Li loss upon cycling, respectively.As a result, ASSBs with the prelithiated-graphite/silicon DDE demonstrated excellent electrochemical performance.
Furthermore, Huang et al. [205] developed an efficient and convenient prelithiation strategy for silicon monoxide (SiO) electrodes using a mixture of LMP, styrene butadiene rubber (SBR), and toluene.The SBR ensures a uniform dispersion of LMP in toluene.The SiO electrode was prelithiated by dropwise addition of the mixture, followed by pressure activation.The degree of prelithiation was conveniently adjusted within limits by controlling the volume of the mixture, leading to improved initial CE within 75-120%.However, the prelithiation process had just a little influence on the cycling and rate tests.This was attributed to the incomplete usage of the added LMP and the inhomogeneity of the prelithiation as a result of the large size of the SiO particles.There is a need to improve the usage efficiency of the LMP and the homogeneity of the prelithiation.
Zheng et al. [206] fabricated a stable sulfur/microporous carbon (S/MC) cathode in a low-cost carbonate-based electrolyte by infusing small sulfur molecule (S 2 ) vapor into MC under vacuum at 600 °C.Prelithiation of the fabricated S/MC composite cathode was then carried out by spraying LMP onto the S/MC cathode film, followed by compression (Figure 9c).The S/MC cathode showed almost 100% Coulombic efficiency and good capacity retention in carbonate-based electrolytes.Notably, the in situ prelithiated Li 2 S/MC cathode demonstrated better electrochemical performance than the pristine S/MC.Furthermore, the Li 2 S/MC cathodes can be paired with Li-free anodes like graphite, carbon/tin alloys, and silicon nanowires by manipulating the first delithiation capacity.Copyright 2017, Elsevier.b) Schematic illustration of the fabrication process of prelithiated graphite/silicon composite electrodes.Reproduced with permission. [204]Copyright 2023, Elsevier.c) Formation of lithium sulfide/microporous carbon cathodes for Li ion batteries.Reproduced with permission. [206]Copyright 2013, Elsevier.d) Illustration of prelithiation of hard carbon cathode for Li ion capacitor.Reproduced with permission. [207]opyright 2014, Elsevier.

Prelithiation of Electrode Active Materials in LICs
The LIC, also called a hybrid or asymmetric supercapacitor, consists of a supercapacitor-type cathode (ex., activated carbon) and a LIB-type anode (ex., graphite and hard carbon [HC]) and is considered a high-performance energy storage device.LICs possess higher energy density than supercapacitors as well as higher power density and longer cycle life than LIBs due to the electrode materials combination. [208]Nevertheless, limitations such as the large initial capacity loss due to SEI formation and the imbalanced electrode kinetics (i.e., the sluggish intercalation/ deintercalation kinetics of the anode vs the fast physical adsorption/desorption kinetics of the cathode) must be overcome to achieve high-performance LICs. [167,209]Thus, prelithiation is also very essential in LICs.First, prelithiation supplies the primary Li source, which shuttles between the anode and cathode during the charge/discharge cycles.Also, prelithiation acts as the active Li þ donor to compensate for the initial capacity loss, enabling the fabrication of high-performance LICs.212][213][214][215][216][217] Cao et al. [207] developed high-energy density prototype LIC pouch cells [218] with lab-scale equipment using activated carbon and HC/LMP as the cathode and anode, respectively.The prelithiated HC anode was achieved by coating the surface of the HC with LMP using a doctor blade, followed by roll-pressing (Figure 9d).The cells demonstrated high-energy and power densities coupled with high-capacitance retention.
Hao et al. [219] prepared a LiF-coated LMP using perfluororesin as a fluorine source under nitrogen gas protection at a high temperature.The LiF-coated LMP was then pre-embedded into a mesocarbon microbeads cathode and paired with an activated carbon anode to form an LIC.The fluorination step was crucial to prevent the formation of Li dendrites and SEI films, the corrosion of LMP by the electrolyte, and improve the electrochemical properties of the LIC.As a result, the LIC with the LiF-coated LMP prelithiated electrode demonstrated superior specific capacitance, cyclic stability, and energy density than its pristine LMP counterpart.
Furthermore, Cao et al. [220] investigated the effect of the amount of LMP loadings in the anode on the voltage drop during the charge and discharge of LICs.The LICs were fabricated using an activated carbon cathode and HC anode with different LMP loadings.It was deduced that LICs with high LMP loadings showed smaller voltage drops than those with small LMP loadings.Also, it was found that the IR voltage drops at high cell voltages were smaller than those at low cell voltages for low LMP loadings.In contrast, at high LMP loadings, small IR voltage drops were obtained for both low and high cell voltages.Therefore, it is crucial to carefully control the amount of LMP added during prelithiation to achieve high-performance LICs.

Conclusion and Perspectives
LMPs synthesized through various techniques, notably the DET, hold remarkable promise as new anode materials for LMBs.LMP anodes with large reactive surface area induce a lower effective current density on the surface, leading to a more uniform Li plating/stripping and an extended life span of LMBs.Also, the slurry process for fabrication of the LMP anodes ensures easy control of the anode thickness and width compared to the extrusion/ roll-pressing technique used for Li foil.
Several researchers have dedicated tremendous efforts to addressing the challenges associated with LMP-based anodes (such as adhesion, porosity, active material utilization, and nonuniform surfaces), leading to enhanced battery performance.Additionally, the role of LMPs as prelithiation agents has been underscored in various applications, such as batteries and LICs.By compensating for the initial irreversible capacity, improving Coulombic efficiency, and fostering a stable SEI, LMP is pivotal in advancing energy density and battery life span, positioning it at the forefront of modern battery research and development.
However, like many advanced materials, LMP is not without its challenges.One of its most significant hurdles is its high reactivity with moisture and air, leading several companies to implement methods to stabilize LMPs (like SLMP by Livent).This characteristic not only poses challenges in the handling and storage of LMPs but also escalates the costs associated with their production and use, owing to the need for specialized equipment and environments to mitigate these reactive properties.Moreover, the lightweight of LMP can cause airborne particles during processing, which poses potential health risks to operators.Furthermore, another critical concern is the fire hazard related to residual LMPs that can ignite spontaneously.Thus, ensuring strict monitoring and control throughout the processing chain is imperative to prevent these risks of fire or explosion, safeguarding both the product and personnel involved.The industry needs to continuously develop and implement innovative strategies to safely handle, store, and utilize LMPs while minimizing their environmental impact.
As research continues and techniques advance, it's conceivable that cost-effective methods to control its reactivity and enhance its safety will be developed, opening up even more opportunities for its widespread application.In essence, while there are clear challenges to be addressed, the path forward offers ample opportunities to leverage the strengths of LMPs, ensuring their place in the future of energy storage.To the best of our knowledge, this article is the first comprehensive review of research trends on LMPs encompassing fabrication methods, state-of-the-art development of LMP-based anodes and applications of LMP.

Figure 1 .
Figure 1.Schemes of the degradation behavior of Li metal and advantages of LMP anodes.a) Schematic illustration depicting dendrite and dead Li formation.Reproduced with permission.[40]Copyright 2017, Springer.b) Sand's time theory.Reproduced with permission.[41]Copyright 2017, The Royal Society of Chemistry.c) Porous and tortuous structure of dead Li layer on Li metal anodes.Reproduced with permission.[42]Copyright 2016, The Royal Society of Chemistry.Schematic illustration of Li ion distribution and scanning electron microscopy (SEM) images of d) Li metal foil and e) LMP anodes during electroplating.Reproduced with permission.[43,44]Copyright 2019, Elsevier and Copyright 2004, Elsevier.f ) Width comparison between commercial Li foil and LMP anode.Reproduced with permission.[45]Copyright 2021, Wiley.

Figure 2 .
Figure 2. Fabrication processes of LMP.a) Schematic of droplet emulsion technique (DET).Reproduced with permission.[78]Copyright 2013, American Chemical Society.b) Schematic of the electrodeposition process and corresponding SEM images of the air stable Li spheres (ASLSs) obtained at different electrodeposition times.Reproduced with permission.[79]Copyright 2021, Elsevier.c) Schematic illustration of the cryo-milling process.Reproduced with permission.[80]Copyright 2019, Wiley.d) Scheme and optical images of the preparation procedure of Li microparticles via solvent-processing.Reproduced with permission.[81]Copyright 2019, American Chemical Society.

Figure 3 .
Figure 3. Introduction of surface protection layers on the LMP to suppress side reactions with the atmosphere.a) SEM and transmission electron microscopy (TEM) images of LMP covered by Li 2 CO 3 as core/shell structure.Reproduced with permission.[83]Copyright 2013, Institute of Physics.b) Cross-sectional SEM image of wax-coated LMP.Reproduced with permission.[84]Copyright 2009, Elsevier.c) Schematic illustration of the surface films on the as-compacted LMP and LiF-modified LMP.Reproduced with permission.[44]Copyright 2004, Elsevier.d) TEM image of ionic-liquid-protected nanoscale LMPs.Reproduced with permission.[80]Copyright 2019, Wiley.

Figure 4 .
Figure 4. Schematic reflecting LMP fabrication patents in applied, granted, and expired status.a) Pie chart showing the status of patent applications by various companies.b) A graph showing the number of patents filed yearly (blue color dot-line graph) and the accumulated number of patents over the period (green color bar chart).c) Pie chart showing the status of patent applications by fabrication methods.

Figure 5
Figure 5. Fabrication process of LMP anodes.A slurry comprising LMP, binder, and solvent is cast onto the current collector using a doctor blade.After sufficient drying and calendering, LMP anodes are produced.Reproduced with permission.[156]Copyright 2023, Elsevier.

Figure 6 .
Figure 6.Improvement of interfacial connectivity of LMP anodes.a) SEM image of pristine LMP anodes, schematic representation of the important properties of LMP anodes and SEM images of LMPs isolated and having poor contact with the Cu foil.Reproduced with permission.[158]Copyright 2020, Elsevier.b) Improvement of mechanical adhesion/cohesion (F inter /F mid ) of LMP anodes with different binders.Reproduced with permission.[162]Copyright 2020, Elsevier.c) Illustration of physically compressing LMP anodes through calendering to remove the native layer, enhance the contact between LMPs, and reduce porosity.Reproduced with permission.[159]Copyright 2018, Elsevier.d) Introduction of carbon interlayer on the current collector and corresponding nucleation overpotential.Reproduced with permission.[164]Copyright 2021, Elsevier.

Figure 8 .
Figure 8. LMP surface modification.a) LiNO 3 within the LMP composite anode ensures nitrate-anion-rich SEI on the surface of LMPs.Reproduced with permission.[45]Copyright 2021, Elsevier.b) Metallic silver and nitrate-anion-rich SEI is formed through the reaction between AgNO 3 and LMPs during slurry mixing, which function as Li nucleation sites and ensure uniform Li ion flux, respectively.Reproduced with permission.[156]Copyright 2023, Elsevier.c) Schematic illustration of the process of chemical polishing and LMP surface coating.Reproduced with permission.[166]Copyright 2023, Elsevier.

Table 1 .
Summary of the pros and cons of the various LMP fabrication methods.Easy to form and modify coating layer using additives melted in silicone oil High-temperature operation above Li melting point Easy to control LMP size by controlling the stirring rate Unfavorable side reactions with silicone oil