Problem, Status, and Possible Solutions for Lithium Metal Anode of

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Cite This: ACS Appl. Energy Mater. 2018, 1, 910−920

Problem, Status, and Possible Solutions for Lithium Metal Anode of Rechargeable Batteries Sheng S. Zhang* Electrochemistry Branch, RDRL-SED-C, Sensors and Electron Devices Directorate, U.S. Army Research Laboratory, Adelphi, Maryland 20783-1138, United States ABSTRACT: Lithium (Li) metal has been regarded as the ultimate anode material for high-energy-density rechargeable batteries due to its high specific capacity and low reduction potential. However, the application of Li metal in rechargeable batteries was hampered by two major problems: dendritic deposition and inferior cycling efficiency. In this minireview, the mechanisms relating to these two major problems and relative solutions are reviewed by starting with the intrinsic natures of Li metal. In addition, limitations of the electrochemical characterization methods currently used for evaluation of Li cycling performances are discussed, and possible directions for future research are suggested. KEYWORDS: rechargeable lithium battery, lithium anode, lithium deposition, dendrite-free, cycling efficiency important roles in affecting the morphology of Li nucleation.7,8 In general, Li is preferentially nucleated along cracks or metal stress lines because of the higher electric field (or overpotential in the other words) in these spots than in other smooth regions (Figure 1b).4 The case with the Cu substrate, a material widely used for studying of the Li cycling efficiency, is similar because the surface of Cu is imperfect, as indicated by a number of defects in Figure 1c, which consequently results in inhomogeneous Li nucleation (Figure 1d).9 The facts above reveal that, determined by the metallurgical nature, Li cannot homogeneously nucleate on the surface of Li and Cu substrates, which is an important cause for the growth of the next Li dendrites. 2.2. Mossy or Dendritic Li Growth. Influenced by the adhesion of Li nuclei to the electrode substrate, Li is grown in two modes, (1) root-growth mode and (2) surface-growth mode, as illustrated in Figure 2a. When the adhesion is weak, electric contact between the Li nuclei and electrode substrate is not sufficient to enable the next Li being plated onto the preformed Li nuclei. Instead, the next Li is plated onto the electrode substrate, leading to a root-growth mode (mode 1 in Figure 2a); i.e., the next Li sprouts from the root like volcanic eruptions. When the adhesion is strong, intimate electric contact can guarantee the next Li is plated on the surface of preformed Li nuclei beneath the solid electrolyte interphase (SEI), presenting a surface-growth mode (mode 2 in Figure 2a) that makes the Li granule bigger and taller. Morphology of the Li deposits plated by the surface-growth mode can be changed from whiskers through rods to rather flat flakes with an increase in the ratio of the horizontally to vertically growing rate. The modes of Li growth are strongly influenced by the current

1. INTRODUCTION Lithium metal has been recognized to be the ultimate anode material for the next generation of high-energy-density batteries due to its high theoretical specific capacity (3861 mAh g−1) and low electrochemical potential (−3.0401 V vs standard hydrogen electrode). However, commercializing rechargeable lithium batteries has been hampered by inferior reversibility and poor safety of the Li metal anode, which are mainly related to the extremely high reactivity of Li metal with electrolyte components as well as the uncontrollable growth of Li dendrites in the charging process. As solutions to these problems, previous efforts have been centered on the physical protection of Li metal electrode and modification of traditional electrolytes, which have been summarized in several excellent reviews.1−3 Not intending to repeat previous reviews, this minireview briefly summarizes the problem and status of current research on the subject and suggests several costacceptable and feasible approaches for practically viable rechargeable Li batteries. 2. PROBLEM OF LI METAL ELECTRODE 2.1. Inhomogeneous Li Nucleation. The surface of Li metal is covered by a native Li2O and Li2CO3 layer. Removal of this native layer shows that the surface of Li metal is not smooth and, instead, contains numerous irregular cracks (Figure 1a), thought to be determined by the metallurgical nature of Li metal.4 These cracks make it difficult to obtain homogeneous current distribution over the Li surface. As a result, needle-like Li dendrites are still formed even at low current density (0.2 mA cm−2).5 The previous models, such as the ion flux limited model,6 developed from aqueous solutions for prediction of the dendritic electrodeposition of other metals are not applicable to the Li deposition. Instead, it was recognized that the surface state of Li metal plays more This article not subject to U.S. Copyright. Published 2018 by the American Chemical Society

Received: January 14, 2018 Accepted: February 26, 2018 Published: February 26, 2018 910

DOI: 10.1021/acsaem.8b00055 ACS Appl. Energy Mater. 2018, 1, 910−920

ACS Applied Energy Materials

Review

Figure 1. Metallurgical nature of Li and Cu substrates and inhomogeneous Li nucleation. (a) Cleaned Li surface and (b) Li nuclei on Li surface. Reproduced with permission from ref 4. Copyright 2006 Elsevier. (c) Cu surface and (d) Li nuclei on Cu surface. Reproduced with permission from ref 9. Copyright 2017 American Chemical Society.

Figure 2. (a) Two modes of Li growth and (b) morphology of Li deposits at different Li plating stages. Reproduced with permission from ref 18. Copyright 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Schematic Li stripping processes.

mode (mode 1) should be minimized. It is worth clarifying that the Li growth mode and SEI formation are two different issues. The former relates to morphology of Li deposits while the latter to Coulombic efficiency of Li cycling. Ideally, Li is expected to be deposited beneath the dense and highly ionconductive SEI. 2.3. Inefficient Li Stripping. Due to the extremely high reactivity of Li metal with electrolyte components (especially with the solvents), Li whiskers upon formation are immediately encapsulated by an electron-insulating SEI, and the SEI is uncontrollably grown over time. Therefore, the Li whiskers formed by Mode 1 cannot be effectively stripped, instead, they are successively reacted with the electrolyte solvents and

density (also referred to as overpotential by many authors) and the pressure between two electrodes. Without exception, mossy deposition (i.e., mode 1) is dominated at high current densities,10−12 or/and in pressure-free cells such as those used for the in situ or in operando observation of Li deposition.13−17 In actual electrochemical systems, two modes of Li deposition are coexistent as suggested by the fact that flocculent Li and dendritic Li whiskers are, respectively, predominant in different Li plating stages (Figure 2b).18 It is observed that dendritic Li whiskers dominate at the stages of 0.5 and 2 mAh cm−2, whereas both the flocculent Li and dendritic Li coexist in the stage of 4 mAh cm−2. In order to obtain high Li cycling efficiency, Li growth by the root-growth 911



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cycling efficiency (Figure 3a) is measured by using a Li/Cu cell to galvanostatically plate a certain amount of Li onto the Cu substrate, followed by galvanostatically stripping the plated Li out of the Cu substrate, in which the same current densities are usually used for the Li plating and stripping steps. Impedance growth (Figure 3b) is indirectly evaluated by recording the change of the cell’s polarization with successive Li plating and stripping time. In this testing, the same current densities and times for Li plating and stripping are commonly used. The short time (Figure 3c) is defined as the time before the cell is shortened by Li dendrites, which is measured by galvanostatically plating Li until the cell is shortened by resultant Li dendrites or Li in the counter electrode is completely depleted, as indicated by an increase in the cell’s voltage. Since the Cu substrate can be eventually covered by Li deposits, both a Li/ Cu cell and a Li/Li symmetric cell can be used for the measurement of the impedance growth and short time. When a Li/Cu cell is used to measure the impedance growth, the Cu electrode is first plated for a little larger amount of Li than that needed for the process of Li stripping, and then the cell is cycled between the plated Li and the Li electrode. The Li cycling efficiency measured from a Li/Cu cell cannot reflect the true state of the Li metal anode in the battery. As shown in Figure 4a,b,21 first, distribution of Li deposits on the Cu substrate is not uniform, and second, there is a large gap between the Li deposits and Cu substrate. It was observed that dead Li residues in gray-to-black color are often left in the Cu substrate even after the first stripping test. In other words, though the Li ions can be effectively electrodeposited to Li metal, the resultant Li metal cannot be fully stripped, leading to low Coulombic efficiency. Therefore, the formation of dead Li in relation to the poor adhesion of Li deposits to the Cu substrate unavoidably affects the accuracy of Li cycling efficiency. The results of impedance growth are influenced by unexpected electric shorting. The shorting may be due to the penetration of Li dendrites through the separator or due to the crossover of Li dendrites along the edge of the separator. Figure 4c shows two typical voltage patterns for the normal and failed cells, respectively.22 The normal cell (upper panel of Figure 4c) shows that the cell’s polarization remains constant or slowly increases with testing time, whereas the failed cell (bottom panel) shows that the cell’s polarization unusually decreases

electrically isolated from the electrode substrate by resultant SEI, resulting to the formation of dead Li (Figure 2c). This could be one of the important causes for inferior Li cycling efficiency and electrolyte depletion in rechargeable Li batteries.19,20 The Li whiskers formed by mode 2 have good contact with the electrode substrate and relatively lower areato-volume ratio, and therefore, they can be better stripped. For high Li cycling efficiency, the Li whiskers are ideally stripped in a meltdown mode as illustrated in Figure 2c; that is, the Li whiskers are gradually shrunk without fracturing. However, this is not true because, during stripping, the Li whiskers are easily broken and the resultant Li fragments are quickly encapsulated by the subsequently formed SEI to form dead Li.

3. LIMITATIONS OF ELECTROCHEMICAL CHARACTERIZATION Cycling efficiency, impedance growth rate, and short time are three important electrochemical parameters that have been widely used for evaluation of the cycling performances of the Li metal electrode, as schematically illustrated in Figure 3. Li

Figure 3. Three electrochemical characterizations for evaluation of Li cycling performances: (a) Li cycling efficiency, (b) impedance growth of a Li/Li symmetric cell, and (c) short time of Li dendrites.

Figure 4. Morphology of Li deposits at 0.5 mAh cm−2 on the Cu substrate: (a) top view and (b) cross-sectional view. Reproduced with permission from ref 21. Copyright 2017 Elsevier. (c) Voltage profile of a normal Li/Li symmetric cell vs a shortened Li/Li cell. Reproduced with permission from ref 22. Copyright 2017 Elsevier. 912



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Figure 5. Morphology of Li electrodes after plating at 1.1 mA cm−2 for 4 h in a LiPF6 ethylene carbonate/dimethyl carbonate electrolyte without and with extra pressure, respectively: (a, b) top view and (c, d) cross-sectional view. Reproduced with permission from ref 26. Copyright 2015 American Chemical Society.

with the testing time.22 Since the shorting is unpredictable, a major challenge for this technique is the difficulty in finding how a local shorting affects the result even for the normal voltage patterns. In addition, the amount of liquid electrolyte in the testing cells must be strictly consistent; otherwise, the depletion of liquid electrolyte will be a predominant origin resulting in the growth of the cell’s impedance (to be discussed later). Short time is the best measure for the growth of Li dendrites though this technique has been the least reported in recent publications.23−25 A shortcoming of this technique is poor repetition, as determined by the nonpredictable nature of Li dendrite growth. A small variation in the pressure between two electrodes may significantly affect the result of short time, and so does the type of separator.

inorganic materials; Polyolefin,32 poly(dimethylsiloxane),33 oxidized polyacrylonitrile nanofibers,34 polymeric grafted skin,35 hydrogen-bond-forming viscoelastic polymer,36 and poly(styrene-co-divinylbenzene) microspheres37 were used as the organic polymer materials; The surface layers formed by reacting native Li2O and/or Li2CO3 on the Li surface or Li metal itself with tetraethoxysilane,38 chlorosilanes,39 or trimethylsilyl chloride40 were used as the inorganic−organic hybrid materials; Li3N41 and garnet42 were used as the solid state electrolytes; and Poly(3,4-ethylenedioxythiophene)-copoly(ethylene glycol) copolymer43 and poly(vinylidene-cohexafluoropro-pylene)44 were used as the gel polymer electrolytes. Depending on the property of individual materials, the protective layer can be applied to the Li surface by physical vapor or atomic layer deposition,28 slurry-coating,30,37 or chemical reaction of Li surface with precursor materials.29,39,40,45 Of particular interest in the Li-ion and Li-ion capacitor technologies, the stabilized lithium metal powders (SLMP, a trademark of FMC Lithium) are prepared by reacting molten Li with the layer precursors, such as H3PO4, in a 200 °C mineral oil bath under vigorous stirring.29 Regardless of the starting materials and preparing methods, the protective layers are more or less porous except for a few made by using viscoelastic polymers36 due to their very thin thickness, as representatively indicated in Figure 6. A protective layer prevents penetration of Li dendrites generally through two functions of (1) serving as an additional separator to physically prevent the penetration of Li dendrites, and (2) forcing Li deposits firmly adhering to the Li bulk. Improvement in the adhesion of Li deposits to the Li bulk can be easily validated by a decrease of the contact resistance in the ac-impedance spectra.21,46 As stated above, the protective layers are either porous so that they are capable of absorbing liquid electrolyte for rigid materials, or capable of forming a gel polymer electrolyte with the liquid electrolyte for viscoelastic polymers. In this regard, the protective layers are unable to completely protect Li deposits from coming into contact with the liquid electrolytes. For example, the SLMP can afford the short-time slurry-coating process of the electrodes, but are unable to provide long-term protection of Li metal particles from reaction with the solvents or moisture and O2 from air. Therefore, the improvements by the physical protection are considered to be temporary.

4. STATUS OF CURRENT RESEARCH 4.1. Applying Extra Pressure. Li deposits formed by electrodeposition are highly porous, and their surfaces are inevitably encapsulated by electron-insulating solvent reaction products. Therefore, electric contact between Li deposits is rather poor, and the Li cycling efficiency is not satisfactory. It was shown that in a pressure-free glass capillary cell, flocculent Li can be easily formed.15 Applying extra pressure to the electrodes not only reduces the porosity of Li deposits but also enhances the electric contact between Li deposits. By studying a pouch cell, Change et al.26 found that applying a 5.8 ± 0.9 atm of extra pressure not only dramatically increased the compactness of Li deposits (see Figure 5b vs Figure 5a) but also considerably reduced the thickness of Li deposits (see Figure 5d vs Figure 5c), as compared with the pristine cell that had only 1.9 atm of initial pressure. It has been proven that applying extra pressure is one of the simplest and most effective solutions to inferior Li cycling efficiency and cycle life.4,26,27 4.2. Physical Protection. Physical protection has been most employed for prevention of Li dendrites from penetrating into the separator by applying a protective layer to the surface of the Li anode. All the materials that are chemically stable against Li metal are suitable for this purpose. A number of materials have been studied for the protective layer, which range from inorganic through organic to inorganic−organic hybrid materials. For example, Al2O3 particles,28 Li3PO4,29 ceramic powder,30 and hybrid silicate31 were used as the 913



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types of separators are substantially different. Under the same porosity and thickness conditions, the separators made by the wet process are superior in preventing the penetration of Li dendrites due to their much more tortuous and interconnected pore structures. However, high tortuosity meanwhile increases the path of ion movement, which adversely affects the battery’s rate capability. On the other hand, the same strategies as those used in the physical protection are also applicable to the separator by coating a protective layer onto the side facing the Li anode. For example, all these separators, coated with a polymer formed by the ultraviolet polymerization of polysilsesquioxane,48 boronnitride nanosheets,49 polymer-modified Li7La3Zr2O12 ceramic,50 functionalized nanocarbon,51 or Cu metal particles,23 show to different degrees improvement on the morphology and cycling efficiency of Li electrode as compared with those of the pristine ones. Of interest, the protective layers made of conductive materials, such as graphene,52 carbon,51 or Cu metal,23 offer extra benefits for improving the morphology of Li electrode because of their ability to homogenize local current densities over the Li surface. Alternatively, the protective layer can be placed between the Li anode and separator as an interlayer. This method was adopted particularly for the conductive protective layer, such as a carbon paper53 and a carbon nanofiber layer.54 In addition to physically preventing the penetration of Li dendrites, the conductive protective layer also functions as the second electrode substrate, on which Li dendrites are grown toward the Li anode. However, such benefits cannot last long since Li can be equally grown on the other side of the conductive protective layer, i.e., the side facing the cathode. 4.4. Electrolyte. In Li batteries, the morphology and cycling efficiency of Li electrode are most crucially affected by the electrolyte. A post-mortem analysis on many cycled Li/ LixMnO2 AA batteries showed that most failures of the Li batteries are related to the depletion of liquid electrolyte, i.e., the uncontrollable reactions between Li deposits and electrolyte solvents.10,19,20 Typically, the cycled Li shows rough and mossy structures composed of three regions: (1) dendritic Li layer on top, (2) porous layer in intermediate, and (3) residual Li in the bulk, as shown in Figure 8,55 of which the porous layer is mainly the dead Li encapsulated by the reaction products of Li and electrolyte solvents. Due to successive reactions between the dead Li and electrolyte solvents, thickness of the porous layer is progressively increased over time regardless of the battery being in operation or in storage. To date, there are no suitable electrolytes that can support long-term operation of a Li electrode because of their inability to form a robust and durable SEI on the Li surface. Solvents are mainly focused on two categories of ethers and esters. In general, ether solvents favor forming compact and flat Li deposits with relatively higher cycling efficiency whereas carbonate solvents lead to porous and dendritic Li deposits with lower cycling efficiency.56,57 However, carbonate solvents have better oxidative stability, which is essential for the highvoltage Li batteries.58 Therefore, moderate performances of the Li cycling efficiency and electrochemical window can be obtained by adjusting the ratio of ether and carbonate solvents in an electrolyte.57 Among ether solvents, cyclic ethers such as 1,3-dioxolane, tetrahydrofuran, and their derivatives are more effective than the linear ones in promoting the formation of robust SEI probably because in the presence of Li+ ions their reduction produces more solid products (i.e., insoluble Li salts),

Figure 6. Morphology of representative protective layers on Li surface. (a) Stabilized lithium metal powder. Provided with the courtesy of FMC Lithium. (b) A 2.5 nm thickness of inorganic Al2O3 layer coated by atomic layer deposition. Reproduced with permission from ref 28. Copyright 2017 The Royal Society of Chemistry. (c) An inorganic− organic hybrid layer coated by vapor deposition. Reproduced with permission from ref 31. Copyright 2011 The Royal Society of Chemistry. (d) An inorganic−organic hybrid solid state electrolyte made by self-healing electrochemical polymerization and atomic layer deposition, (e) a protection layer made by reacting (CH3)3SiCl with native Li2O on Li surface, and (f) a grafted polymeric skin prepared by ring-opening polymerization of a functional monomer. Reproduced with permission from refs 45, 40, and 35, respectively. Copyright 2017, 2016, and 2017, respectively, American Chemical Society.

4.3. Separator. In a manner similar to that of the protective layers, the separator plays an essential role in preventing penetration of Li dendrites. In the battery, Li dendrites are grown preferentially along with the direction of current. In other words, the Li dendrites penetrate into the separator along the path of Li+ ion flux, i.e., the orientation of pores in the separator, rather than mechanically penetrating into the separator. There are two types of separators available in the current markets: one is made by the dry process that creates pores by mechanical stretching, and the other is made by the wet process that creates pores by solvent-extracting.47 As shown in Figure 7, the orientation and tortuosity of pores in these two

Figure 7. Pore structure of two types of separators made by (a) dry process and (b) wet process. Reproduced with permission from ref 47. Copyright 2007 Elsevier. 914



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Figure 8. Morphology of Li anode after 250 cycles at 2.0 mA cm−2 with an initial capacity of 1 mAh cm−2. (a) Cross-sectional overview, (b) dendritic layer on top, (c) needle-like dendrites in the dendritic layer, (d) porous layer in intermediate, and (e) interphase between porous layer and Li bulk. The scale bars are 50 μm in part a; 10 μm in parts b, d, and e; and 2 μm in part c. Reproduced with permission from ref 55. Copyright 2009 The Electrochemical Society.

LiN(SO2 F)2 + 2Li+ + 2e− → LiNSO + LiSO3F + LiF

as described by eqs 1 and 2, respectively. However, the cyclic ethers are poorly compatible with LiPF6 and LiAsF6, which easily dissociate into Lewis acids (PF5 and AsF5) that consequently initiate the ring-opening polymerization of cyclic ethers. Since Li reacts with all electrolyte solvents, the morphology and cycling efficiency of Li electrode are determined mainly by the ability of resultant reaction products in constituting a robust SEI on the Li surface. In this regard, fluorinated solvents such as fluoroethylene carbonate (FEC) are of particular interest because of their ability to react with native Li2O on the Li surface forming a LiF-rich SEI, which is representatively described by eq 3 for FEC.

(5)

Increasing the salt concentration of the electrolyte has been proven to be a simple and effective means for improving the morphology and cycling efficiency of a Li electrode.67,68 The relative improvement is attributed to two merits of the concentrated electrolyte solutions: (1) a decrease in the number of free solvent molecules as a result of strong solvation with the Li+ ions, and (2) an increase in the size of Li+ ion and solvent clusters. It is believed that the first merit favors increasing the Li cycling efficiency by reducing chemical reactions between Li deposits and solvent molecules, and that the second merit is beneficial to smooth and flat deposition of Li metal by moderately raising the reduction potential of Li+ ions, serving the same function as does a complexing agent in the electrodeposition of transition metals from aqueous solutions.69 By a similar principle, improvement can be alternatively made by using a solvate ionic liquid70 or an ionic liquid71 as the solvent. The high concentration is unique in improving Li deposition through the bulk properties of electrolyte, which could be one of the best approaches capable of providing long-term improvement for the Li electrode. The weaknesses of high-concentration electrolytes are the high viscosity, poor wettability to nonpolar polyolefin separators, and high cost of Li salts. Additives have been intensively studied as the most costeffective route for improving the cycling performance of Li electrode. A crucial requirement for additives is that their participation improves the cycling performance of Li electrode without adversely affecting performances of the cathode. According to the improving mechanism and function, the additives can be roughly classified into three categories: (1) By chemical reactions with the native Li2O or Li2CO3 on the Li surface or Li itself, such as H2O,72,73 HF,74,75 CO2,76 SO2,77 triacetoxyvinylsilane,78 as well as salts LiPF6,62 LiAsF620 (note that a recent work showed AsF6− anion can be reduced and combined with Li to form a Li−As alloy79), LiBOB, and LiDFOB.66 (2) By electrochemical or chemical (with metal Li) reduction on the Li surface, such as O2,7,80 Li2Sn,46 LiNO3,81 nitromethane,77 methyl viologen,82 lithium azide,83 dimethyl sulfate,84 as well as LiNO3−Li2Sn85 and P2S5− Li2Sn86 hybrids.

Therefore, fluorinated solvents have been widely used as the cosolvent or additive for improving the cycling performances of the Li electrode.59,60 Salts affect Li deposition though their anions, and appear to have more influence on the cycling efficiency than on the Li morphology.61−63 Particular interest was placed on such salts that are capable of actively participating in the formation of robust SEI, for example, LiPF6, LiBF4, LiAsF6, lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalato)borate (LiBOB), and lithium difluoro(oxalate)borate (LiDFOB). This is because fluorine-containing anions participate in the formation of LiF-rich SEI,64,65 whereas LiBOB and LiDFOB react with native Li2O to produce substantially insoluble B−O and oxalate molecular species that are consequently constituted to robust SEI.66 In particular, LiFSI was called a “magic salt” due to its unique ability to either react with native Li2O or electrochemically reduce to form LiF-rich SEI, as briefly described by eqs 4 and 5.65 LiN(SO2 F)2 + 2Li 2O → LiN(SO3Li)2 + 2LiF

(4) 915



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(3) By alloying with Li metal, such as Na+,87 K+,88 Cs+,89,90 Mg2+,91 Al3+,92 and Sn2+.92 Most of the additives are either chemically reactive to the cathode materials or electrochemically instable at high potentials, which adversely affect the performances of the cathode. Therefore, their amounts in the electrolytes must be strictly limited (normally not more than 5% by either weight or volume66). On the other hand, all additives are irreversibly consumed in the first cycle or in the initial several cycles, and resultant SEIs or alloys are inevitably broken with repeated plating and stripping of the Li electrode. Therefore, the improvement of Li cycling performances by additives is considered to be temporary. Among all additives, the Cs+ ion is unique in its lower reduction potentials at low concentrations, for example −3.144 V at 0.01 M versus standard hydrogen electrode,89 than the standard electrode potential (−3.040 V) of the Li+ ion, which makes it nonconsumable. It was reported that the role of the Cs+ ion additive is to improve the morphology of Li deposits especially at high current densities,89,90 which can be alternatively explained by eqs 6 and 7 in addition to the self-healing electrostatic shield mechanism as proposed by the original authors. xCs+ + y Li+ + (x + y)e− → CsxLi y

(6)

CsxLi y + x Li+ → (x + y)Li + xCs+

(7)

areas of research as seen in recent publications. This subject is of particular significance for making Li-free rechargeable batteries (which was also called in situ enabled Li metal battery99). The Li-free batteries are typically made of a bare current collector as the anode and a Li-rich material as the cathode with the Li metal being in situ plated onto the anode current collector in the first charging process.21,99,100 For this purpose, two concepts have been proposed: One is to coat a lithiophilic material as the Li nucleation seeds onto the surface of Cu substrate to guide uniform and compact Li nucleation, and the other is to apply a robust physical layer onto the surface of Cu substrate to force Li to be plated and stripped underneath. The lithiophilic materials must be highly electron-conductive and have strong binding to the Li atoms. The most studied lithiophilic materials are a variety of carbon materials, including graphene,101,102 graphene−carbon hybrid,103 graphite,104 glass fiber,105 and various carbon spheres/fibers/nanotubes106,107 or carbon/silica core−sheath microcages,108 followed by metals, particularly those with the ability to form an alloy with Li such as Ag, Pt, Au, Zn, and Sn.21,109 The carbon materials are generally applied to the Cu substrate by slurry-coating, whereas the metals can be deposited by a variety of techniques, such as physical vapor deposition and magnetron sputtering,109 chemical vapor deposition,109 and electroless deposition.21 Due to the limited amount of Li+ ions available from the cathode, the Coulombic efficiency in the first Li plating and stripping cycle is considered to be the most critical challenge for the lithiophilic materials, especially for the graphene-based materials that contain numerous oxygen functional groups and irreversibly combine with the Li+ ions. However, only a few publications addressed this crucial challenge.21,110 In addition, it should be noted that, upon the uniform and compact nucleation, the next Li is grown on the Li nuclei, and hence the same problems as those observed in the Li bulk electrode, such as dendritic Li growth and uncontrollable reactions between Li deposits and electrolyte solvents, are still present. The materials used for the physical layer may be any of ionconductive solid state electrolytes, nonconductive inorganic ceramics, and organic polymers. However, the physical layer must allow Li+ ions to move freely, which is realized either through the porous structure of the inorganic physical layer to absorb liquid electrolyte or through plasticization of the polymeric film by liquid electrolyte to form a gel polymer electrolyte. The most studied inorganic materials are hexagonal boron nitride used together with graphene111 or LiF.112 The polymers studied include high-polarity β-phase polyvinylidene fluoride,18 polyether sulfone,113 poly(dimethylsiloxane),114 and silica-poly(methyl methacrylate) composite.115 Since Li is plated and stripped beneath the physical layer, the adhesion of the physical layer to the Cu substrate will be ultimately lost with repeated cycling. Therefore, such protections are considered to be temporary.

On uneven surfaces, the protuberant tips have higher overpotentials than other smooth regions, i.e., corresponding to more negative potentials with respect to the Li plating process. On these protuberant tips where otherwise dendritic Li would be formed due to the more negative potentials, the Cs+ ions at low concentrations can be reduced together with Li+ ions to form a Li−Cs alloy (eq 6). Since the Li−Cs alloys have more negative reduction potentials than Li metal, replacement between the resulting Cs−Li alloys and Li+ ions in the solution immediately takes place (eq 7), which leads to dendrite-free Li deposition without consumption of Cs+ ions. 4.5. Structured Li Electrode. A variety of three-dimensional (3D) structured Li electrodes have been designed to enable dendrite-free Li deposition. The concept is based on the hypothesis that 3D-scaffolds on one hand reduce local current density in the Li electrode by their large specific surface area, and on the other hand offer sufficient volume to accommodate large volumetric expansion and contraction for Li plating and stripping. The materials used for this purpose mainly are metals such as Cu93 and Ni,94 carbon materials,95 and their hybrids such as Cu−Ni core−shell nanowires,96 graphene−Cu nanowires,97 and graphene−Ni foams.98 The forms of 3D structures range from nanowires,96,97 to fibers,95 to foams.98 All 3D electrodes show some degree of improvement in promoting dendrite-free Li deposition. However, their applications in practical batteries are highly challenged by many concerns, such as the following: (1) the 3D-scaffolds by themselves do not contain Li, which requires an extra Li preloading process; (2) high surface area of the 3D structures inevitably intensifies parasitic reactions between Li and electrolyte solvents; (3) Li will uncontrollably deposit beyond the 3D-scaffolds, especially at high current densities; and (4) dendritic Li deposition is still present even at very low current densities. 4.6. Modification of Current Collector. Modification of the existing current collector has been one of the most active

5. CONCLUSIONS AND POSSIBLE SOLUTIONS As stated above, most problems with Li metal electrode can be attributed to three intrinsic properties of the Li metal: (1) inhomogeneous nucleation, which is the origin of dendritic deposition; (2) poor adhesion of Li deposits to Li bulk (also Cu substrate), which results in the formation of dead Li and further inferior cycling efficiency; and (3) extremely high reactivity with electrolyte solvents, which leads to low cycling efficiency and uncontrollable impedance growth. Because the 916



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binding of Li deposits to Li bulk and Cu substrate is substantially different, the results of Li cycling efficiency measured on the Cu electrode cannot reflect the true state of the Li electrode in the Li batteries. Determined by the nature of large volumetric expansion and contraction of Li cycling, most of the approaches currently attempted, such as physical protection, electrolyte additives, and structured Li electrodes, cannot provide long-term improvement. To date, there is not a single approach that can satisfactorily solve all the problems of the Li electrode. Facing three intrinsic properties of the Li metal, possible directions for the future research and development of practical Li or Li-free batteries may be in the following aspects: 1. Battery engineering: Mechanical pressure over the Li electrode is one of the most important factors affecting the adhesion of Li deposits to Li bulk and the compactness of Li deposits. Engineering capable of increasing and homogenizing the pressure over the Li electrode is urgently needed for suppressing the development of Li dendrites and increasing the Li cycling efficiency. In this regard, the flatness and uniformity of the counter electrode (i.e., the cathode) contribute a lot to the homogenization of pressure and further the distribution of current density over the Li anode. 2. Separator: Penetration of Li dendrites into the separator is more because Li dendrites grow along the path of Li+ ion flux, i.e., the orientation of pores in the separator, rather than because the Li dendrites mechanically penetrate through the separator. Therefore, development of the separators that not only are mechanically strong but also have highly tortuous pore structure is urgent for effectively suppressing electric shorting by the Li dendrites. Alternatively, composite solid or gel polymer electrolytes with functional filler capable of killing Li dendrites are effective to prevent penetration of Li dendrites. For this purpose, the functional fillers are required not only to mildly react with Li metal but also to be substantially insoluble and electronically insulating in both states before and after reacting with Li, for example, eigenstate conducting polymers and less stable inorganic solid state electrolytes are two kinds of potential candidates. 3. Electrolyte: Electrolyte is the most important element that affects the morphology of Li deposits and the cycling efficiency of the Li electrode. Most battery failures are due to the depletion of liquid electrolyte, rather than electric shorting by the Li dendrites. Due to the large volumetric expansion and contraction taking place during the cycling of the Li electrode, no SEI can provide longterm protection of the Li electrode. Therefore, improving the bulk properties of the electrolyte through the solvent and salt would be more effective than other methods, as suggested by several successful examples, such as highconcentration electrolytes,67,68 LiFSI behaving as a magic Li salt,65 and ether solvents being better than carbonates in promoting flat and compact Li deposition.56,57 Regarding the electrolyte additive, most additives are consumed during the initial several cycles, and the resulting SEI cannot provide long-term protection. Furthermore, in many cases the additives adversely affect the performances of the cathode. In consideration of the

above concerns, future research on the additives should be targeted to the nonconsumable additives. In this aspect, the Cs+ ion is a successful example. 4. Li-free battery: As long as a Li foil is used as the anode, the capacity of Li anode in the Li metal batteries must be extremely excessive compared with that of the cathode due to the lack of technology capable of making and handling very thin Li foils that match the capacity of the cathode. Excessive Li not only reduces the battery’s energy density, but also increases the battery’s safety hazard because the excessive Li may immediately become a strong fuel upon exposure to air or moisture in an accident. Li-free design seems to be the best solution for minimizing the excessive Li. In order to make the Li-free batteries operable, an anode current collector capable of enabling dendrite-free and efficient Li cycling is essential. Modification on the existing Cu current collector would be a direction for developing the Li-free rechargeable batteries. Meanwhile, equal efforts should be given to the development of Li-rich Li+ ion source materials that are mixed with the cathode material to one time provide extra Li+ ions for making up initial capacity loss of the anode or forming a Li primer layer to enable efficient cycling of next Li.99 In summary, dramatic progress has been recently made in suppressing Li dendritic deposition and increasing Li cycling efficiency. However, no single strategy can solve all the problems of the Li electrode. Most improvements are made temporarily, and some strategies are realized at high cost. In order to make Li batteries practically viable, breakthroughs must be made in solving the problems of dendritic deposition and inferior cycling efficiency of the Li electrode by a costacceptable and feasible means, which calls for more efforts in material development and battery engineering.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. ORCID

Sheng S. Zhang: 0000-0003-4435-4110 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS The author is grateful to Dr. C. Lundgren for her critical reading of the manuscript and valuable comments.

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