Self-Healing Wide and Thin Li Metal Anodes Prepared Using

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Self-Healing Wide and Thin Li Metal Anodes Prepared Using Calendared Li Metal Powder for Improving Cycle Life and Rate Capability Dahee Jin,†,⊥ Jeonghun Oh,†,⊥ Alex Friesen,† Kyuman Kim,† Taejin Jo,‡ Yong Min Lee,*,§ and Myung-Hyun Ryou*,† †

Department of Chemical and Biological Engineering, Hanbat National University, 125 Dongseo-daero, Yuseong-gu, Daejeon, 34158, Republic of Korea ‡ Iljin Materials, 45, Mapo-daero, Mapo-gu, Seoul, 04167, Republic of Korea § Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), 333 Techno Jungang-Daero, Daegu 42988, Republic of Korea S Supporting Information *

ABSTRACT: The commercialization of Li metal electrodes is a long-standing objective in the battery community. To accomplish this goal, the formation of Li dendrites and mossy Li deposition, which cause poor cycle performance and safety issues, must be resolved. In addition, it is necessary to develop wide and thin Li metal anodes to increase not only the energy density, but also the design freedom of large-scale Li-metal-based batteries. We solved both issues by developing a novel approach involving the application of calendared stabilized Li metal powder (LiMP) electrodes as anodes. In this study, we fabricated a 21.5 cm wide and 40 μm thick compressed LiMP electrode and investigated the correlation between the compression level and electrochemical performance. A high level of compression (40% compression) physically activated the LiMP surface to suppress the dendritic and mossy Li metal formation at high current densities. Furthermore, as a result of the LiMP self-healing because of electrochemical activation, the 40% compressed LiMP electrode exhibited an excellent cycle performance (reaching 90% of the initial discharge capacity after the 360th cycle), which was improved by more than a factor of 2 compared to that of a flat Li metal foil with the same thickness (90% of the initial discharge capacity after the 150th cycle). KEYWORDS: stabilized Li metal powder, Li metal electrode, high power capability, high Coulombic efficiency, Li dendrite suppression

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INTRODUCTION

components. However, new active materials based on new chemistry should be developed. Along this line, Li metal has been considered to be a promising anode material candidate owing to its high theoretical specific capacity of 3860 mA h g−1 and lowest potential of −3.040 V versus standard H.4 However, several drawbacks are hindering the commercialization of Li metal as an anode material. The formation of Li dendrites and mossy Li deposition during plating, which provoke uncontrollable interfacial reactions with electrolytes, remain present.5

Li-ion batteries (LIBs) have been regarded as the main energy source for consumer electronics and the most promising energy source for upcoming large-scale applications such as electric vehicles (EVs) and energy storage systems (ESSs). Although the energy densities of commercialized LIBs based on intercalation chemistry (carbonaceous anode materials and transition metal oxide-based cathode materials) have improved over the past four decades, they are currently reaching the theoretical density limit of active materials. 1−3 As a considerably higher energy density is required for the successful implementation of EVs and ESSs, high-energy-density battery systems cannot be developed by optimizing the existing battery © XXXX American Chemical Society

Received: February 18, 2018 Accepted: April 20, 2018

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DOI: 10.1021/acsami.8b02740 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Li metal is covered with a native film composed of numerous inorganic materials, such as Li2O, LiOH, and Li2CO3.6,7 Owing to the physical and chemical nonhomogeneity of this native film, Li+ ions prefer the locations at which the film resistance is low and concentrated.4,6,8,9 Consequently, the current distribution on the Li metal surface is nonuniform, which is the origin of uncontrolled Li deposition during plating. This uncontrolled deposition, including dendritic and mossy Li metal, causes safety risks associated with the explosive and flammable tendency of metallic nanopowders.10 In addition, the uncontrolled Li deposition causes severe morphological changes on the Li metal surface and exposes fresh Li metal to the electrolytes consisting of Li salt and organic solvents to form a protective passivation layer, that is, the so-called solid electrolyte interphase (SEI).11,12 The newly formed SEI consumes a large amount of electrolyte, resulting in poor cycle performance and low Coulombic efficiency (CE). Significant efforts have been made using various approaches to suppress the uncontrolled Li metal plating, and many improvements have been made, such as the development of (i) advanced electrolyte systems for liquid electrolytes13−15 as well as solid electrolytes;16−20 (ii) interfacial engineering, including that of Li metal protection layers;4,21−25 (iii) hierarchical host materials;24,26−30 and (iv) physical Li metal shape transformation techniques.31−36 Nevertheless, Li metal anode research still has a long way to go to meet the requirements of the new large-scale battery application market, including those of EVs and ESSs, which require high energy densities and large dimensions. First, the Li metal thickness should be minimized to increase the energy densities of Li-metal-based battery systems. This problem has been overlooked because the cycle performance improvement has been even more urgent. The Li metal cycle life, however, is largely dependent on the Li metal thickness. Thicker Li metal anodes provide better cycle performance because of the excess of Li source material,37 and thick Li metal anodes have been utilized in many studies, as listed in Table S1 (Supporting Information). Second, along with decreasing the Li metal thickness, the width of the Li metal should be increased to achieve the required large dimensions of EVs and ESSs. For instance, Honjo Metal (Japan), the major Li metal manufacturer, has reported that widths of up to 10 cm can be achieved with thick Li metal (>100 μm), whereas this width limitation is only 5.5 cm for thin Li metal (>20 μm).38 Using Li metal powder (LiMP) electrodes seems to be a promising means of solving both chronic issues. The LiMP electrodes can be prepared via slurry casting, and the widths of the LiMP electrodes are not limited. For instance, in this study, we successfully fabricated a 21.5 cm wide and 40 μm thick LiMP electrode. (Figure S1, Supporting Information). The width of the LiMP electrode was dependent on the maximum width of the doctor blade used in this study. Although a few LiMP anode studies have been reported, the main goal of these studies was to increase the active Li metal electrode surface area to decrease the effective current density (i.e., current density per reactive surface area) in comparison to those of Li metal foils to suppress the formation of the dendritic structures.36,39−44 As discussed above, from the viewpoint of increasing the energy density of a battery system, the thickness of the Li metal should be minimized. In other words, the porous structures of the LiMP electrodes are very disadvantageous in terms of achieving high-energy-density batteries. An alternative means of using the LiMP involves

compressing the LiMP through calendaring.34,45 However, the reason for the improved electrochemical performance of the compressed LiMP compared to that of a flat Li metal foil is unclear because the porous structure of the LiMP, which increased the surface area of the active material resulting in a low current density during plating/stripping, significantly reduced after compression.34−36,45 In this research, we applied a delicate calendaring technique to the LiMP electrodes to adjust their porous structures and adjusted the calendaring conditions to optimize the electrochemical performance. Furthermore, we analyzed the Li plating/stripping mechanism of the LiMP electrodes, which has been regarded as an unsolved and controversial problem in LiMP research.

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EXPERIMENTAL SECTION

Material. LiMn2O4 (LMO, Iljin Materials, Korea) and stabilized LiMP (SLMP, FMC, USA) were respectively used as the cathode and anode materials. A mixture of 1.15 M Li hexafluorophosphate (LiPF6) in ethylene carbonate/ethyl methyl carbonate (EC/EMC = 3/7, v/v) containing 2 wt % fluoroethylene carbonate was used without further purification (Enchem, Korea). Microporous polyethylene (PE) membranes (thickness = 20 μm, ND420, Asahi Kasei E-materials, Japan) were used as separators. Electrode Preparation. Each cathode was prepared with an Nmethyl-2-pyrrolidone (NMP)-based slurry containing 90 wt % LMO (Iljin Materials, Korea), 5 wt % conductive C (Super-P Li, Imerys, Switzerland), and 5 wt % polyvinylidene fluoride (PVdF) as binder material (KF-1300, Kureha, Japan, Mw = 350 000), which was cast onto an Al foil (15 μm, Sam-A Aluminum, Korea) using a gapcontrolled doctor blade. The cathode was dried in an oven at 130 °C for 1 h and roll-pressed with a gap-control-type roll-pressing machine (CLP-2025, CIS, Korea). The cathode loading level and thickness were 7.8 mg cm−2 and 45 μm, respectively. Each LiMP anode was prepared in a glovebox in an Ar atmosphere. An NMP-based slurry containing 97 wt % SLMP (FMC, USA) and 3 wt % PVdF binder (KF-1300, Kureha, Japan, Mw = 350 000) was mixed for 1 min. The slurry was cast onto a Cu foil (8 μm, Iljin Materials, Korea) and dried in a vacuum (25 °C, 12 h). The prepared LiMP anodes were roll-pressed with a gap-control-type roll-pressing machine for 0, 20, or 40% in comparison to the initial thickness, resulting in electrode thicknesses of approximately 70, 55, or 40 μm, respectively (Figure S5, Supporting Information). Preparation of Electrochemical Cells. To evaluate the effects of the electrochemical performance, 2032-coin-type Li cells with LMO as cathodes (diameter = 12 mm) and Li metal as anodes were assembled in a glovebox in an Ar atmosphere. The anodes consisted of either Li metal (40 μm, diameter = 15 mm, Honjo Metal Co, Japan) or LiMP electrodes (diameter = 15 mm). Furthermore, symmetrical LiMP/ LiMP cells (diameter = 12/15 mm) were assembled with the Li metal reference electrodes. Electrochemical Analysis. The Li plating and stripping experiments were conducted at a current density of 0.5 mA cm−2 with symmetrical LiMP/LiMP cells. Each cycle consisted of 30 min of plating, 10 min of rest, 30 min of stripping, and 10 min of rest to reduce the influences of the concentration gradients. The potential changes were determined based on the reference Li electrode. The cycling procedure for the LMO/Li cells consisted of one formation cycle and three stabilization cycles (precycling), followed by the cycle aging procedure. All of the cycles were performed in the voltage window of 3.0−4.4 V. The formation step consisted of constant current (CC) charging and discharging at C/10 (0.087 mA cm−2). In the stabilization step, the cells were charged and discharged at a rate of C/5 (0.174 mA cm−2) in a CC/constant voltage (CC/CV) mode and a CC mode, respectively, for three consecutive cycles. The cycle performance of each unit cell was evaluated in a CC/CV mode with a current density of C/2 (0.435 mA cm−2) at 25 °C using a charge/discharge cycle tester (PNE Solution, Korea). The rate B


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ACS Applied Materials & Interfaces

Figure 1. (a) SEM images of the prepared LiMP electrodes after (a,b) 0%, (d,e) 20%, and (g,h) 40% compression, and (c,f,i) the corresponding EDS mapping analysis results of elemental O. capability of each cell was evaluated at the discharge current densities ranging from C/2 (0.435 mA cm−2) to 15 C (13.05 mA cm−2) and a fixed charge current density of C/2 (0.435 mA cm−2) in the range of 3.0−4.4 V at 25 °C. The AC impedance of each cell was determined using an impedance analyzer (VSP, Bio-Logic SAS, France) over the frequency range of 1 MHz to 50 mHz, with the potentiostatic mode at a voltage amplitude of 10 mV. Postmortem Analysis. After the electrochemical investigations had been performed, the cells were carefully disassembled in a dry Arfilled glovebox. The Li samples were gently washed with dimethyl carbonate several times and fully dried overnight in a vacuum. The samples were analyzed by performing field-emission scanning-electron microscopy with energy-dispersive X-ray analysis (FE-SEM/EDX, S4800, Hitachi, Japan).

line structure with grain boundaries (Figure 1g,h). In this case, the LiMP electrodes were no longer porous. Rather, they exhibit flattened surfaces with pits and pores up to several micrometers in size (Figure 1h). If the compressions were more than 40%, the Cu substrate would be torn or wrinkled; hence, the compression rate was limited to 40%. To visualize the effects of compression on the LiMP electrodes, EDX electroscope analysis was performed. The results for elemental O are shown in Figure 1c,f,i, providing an indirect evidence that Li2CO3 protectively coated the LiMP. The 0% compressed LiMP electrodes show homogenous distributions of the coating on the particle surfaces (Figure 1c). Meanwhile, the O distribution change is significant for the compressed surfaces (20 and 40% compressed LiMP electrodes). The 20% compressed LiMP electrodes exhibit O signal increases at the edges of the crushed particle regions, which is believed to be due to the agglomeration of the Li2CO3 coating during the compression process (Figure 1f). The crushed particle regions still show O signals even though they are smaller than those of the 0% compressed LiMP electrodes. The 40% compressed LiMP electrodes show stronger O signals at the grain boundaries, but the O signals are weak in the crushed particle regions (Figure 1i). The uneven chemical distributions of the compressed LiMP electrodes are mainly attributable to the differences between the material properties of the Li metal and Li2CO3 coating.48 The compression stress causes fracturing of the brittle Li2CO3 and plastic deformation of the ductile and soft Li metal. As the LiMP electrodes were calendared, the Li2CO3 coating cracked into several pieces, and space for the newly exposed Li metal opened. At the edges, the Li2CO3 coating agglomerated because of the compression, creating boundaries between the individual particles. Consequently, the crushed particle regions consisted of small Li metal islands surrounded by Li2CO3. The Li plating and stripping mechanisms of the LiMP electrodes were investigated in a CC charging and CC discharging mode (CC/CC mode, 0.5 mA cm−2, 30 min per charge and discharge) by observing the evolution of the Li overvoltage in symmetrical Li/Li cells, as shown in Figure 2.

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RESULTS AND DISCUSSION The SLMP obtained from FMC (USA) consists of spherical particles coated with a thin Li2CO3 layer (min 0.5−max 2.5 wt %)46 with a specific capacity of ∼3073 mA h g−1 (Figure S4, Supporting Information).44 This Li2CO3 coating layer stabilizes the LiMP against air or solvents, which enables its handling in a dry room atmosphere.46,47 As shown in Figure 1, we prepared three types of LiMP electrodes with various degrees of compression: LiMP electrodes without calendaring (0% compression) and LiMP electrodes compressed to 20 and 40% through calendaring. The 0% compressed LiMP electrodes show nondeformed Li powder particles of various sizes (Figure 1a,b), which implies that the electrode fabrication processes, such as slurry coating and drying, do not damage the Li powder morphology. The 20% compressed LiMP electrodes consist of partially deformed particles, with the flat surfaces in direct contact with the rollpress machine (Figure 1d,e). On the other hand, the 20% compressed LiMP electrodes exhibit much smaller porous structures than the 0% compressed LiMP electrodes, which implies that the porous structures of the LiMP electrodes efficiently buffer changes in physical structure during calendaring. Finally, the 40% compressed LiMP electrodes show flat and dense structures similar to a typical polycrystalC


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polarization from the beginning. The 20% compressed LiMP electrodes exhibit lower polarization in the first few cycles than the 0% compressed LiMP electrodes, but the polarization increases sharply to the same level as that of the 0% compressed LiMP electrodes after 30 h. In contrast, the 40% compressed LiMP electrodes show the smallest and the most stable polarization behavior up to 400 h. The polarization decreases with the cycling time and remains stable for an extended period. Considering the fact that 0% compressed LiMP electrodes are covered by highly resistive Li2CO3 with low Li-ion diffusion (∼10−8 S cm−1)49 and low electronic conductivity (∼10−6 S cm−1),48 and that the 20 and 40% compressed LiMP electrodes experienced morphological changes because of calendaring, as shown in Figure 1, the polarization tendency seems reasonable. The voltage profiles of symmetrical Li/Li cells during charging and discharging were then investigated in more detail. Figure 2c presents the voltage profiles up to 2.0 h. Although the voltages differ in degree, all the three samples (0, 20, and 40% compressed LiMP electrodes) show the general shapes of the voltage traces of plating and stripping for the Li metal symmetric cells.50 An initial decrease in voltage, which is related to dendritic nucleation and SEI formation, is observable in the first charging region (A). In the subsequent first discharging region (B), the voltage (i) decreases immediately upon switching polarity, (ii) reaches a minimum, (iii) rises to a local maximum, and (iv) decreases again.50 Step (i) correlates with the dendritic nucleation formation on the counter electrodes. Step (ii) corresponds to a transition in the reaction pathways from nucleation to the growth of dendrites. In step (iii), active Li in the form of dendrites is depleted, and Li is electrodissolved from the bulk surface. Finally, in step (iv), the reaction pathways change, resulting in preferential Li electrodissolution from the pits rather than the bulk surface. Moreover, the voltage profiles in region C, which are repeated during subsequent charging, are associated with the new dendrite growth on the Li surface.50 The surfaces of the LiMP and bare Li metal foil electrodes after plating with low and high plating currents were observed using SEM (Figure 3). The symmetrical Li/Li cells were fabricated and Li ions were deposited. By controlling the time as well as the current density, we kept the total amount of Li ion migration constant at 0.17 mA h cm−2 in both cases (low current density = 0.1 mA cm−2 for 100 min; high current density = 2.0 mA cm−2 for 5 min).

Figure 2. Potential profiles of Li/Li symmetrical cells during galvanostatic cycling [+0.5 mA cm−2 (30 min) → rest (10 min) → −0.5 mA cm−2 (30 min) → rest (10 min)] drawn at different time scales up to (a) 400 h, (b) 20 h, and (c) 2.0 h.

The highly compressed LiMP electrodes exhibited smaller polarization properties during plating and stripping. In Figure 2a,b, the 0% compressed LiMP electrodes show much higher

Figure 3. SEM images of Li electrodes after plating at a low current density (0.1 mA cm−2 for 100 min) for (a) an Li foil electrode and (c) 0%, (e) 20%, and (g) 40% compressed LiMP electrodes. SEM images for high-current-density (2.0 mA cm−2 for 5 min) plating are shown in (b,d,f,h) in the same order. D


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Figure 4. SEM images of (a,b) 0%, (c,d) 20%, and (e,f) 40% compressed LiMP electrodes after plating and stripping with a current density of 0.17 mA cm−2.

Li dendrite formation.42 Consequently, the only means of observing the morphological changes of the LiMP electrodes during Li plating is an empirical observation; however, this work, which requires substantial time and effort, is beyond the scope of this study. Nevertheless, compared to the Li metal foil electrodes, the LiMP electrodes successfully suppressed the Li metal dendrite formation. The surface morphological changes of the Li metal electrodes after precycling were then observed using SEM (Figure 4). To obtain authentic results, the LiMP electrodes were plated prior to stripping. As shown in Figure 4a, the LiMPs located on the surfaces of the 0% compressed LiMP electrodes were partially ruptured, exposing the fresh Li surfaces to the electrolytes. As the deposited Li is not completely reversible, Li ions must be provided by the LiMP electrodes to compensate for the irreversible Li loss. To achieve this goal, the 0% compressed LiMP electrodes activated stable Li2CO3 coating, causing crack formation in the coating rather than diffusion of Li ions through it, which would have been difficult because of the high resistivity of the coating. Consequently, during the first activation, the LiMP electrodes caused a high polarization, which decreased in the subsequent cycles (Figure 2b). The 20% compressed LiMP electrodes show almost the same morphological changes as the 0% compressed LiMP electrodes, implying that the Li2CO3 coating layer breaking occurred. On the other hand, it is obvious that the polarization level was relaxed compared to that in the 0% compressed LiMP electrodes (Figure 2b). This characteristic implies that the Li2CO3 coating physically cleaved by calendaring facilitates Li ion migration by exposing a fresh Li metal surface. In particular, the 40% compressed LiMP electrodes show smoother surface morphologies and the lowest polarization (Figure 2b) because most of the LiMP was physically cracked during calendaring. In conclusion, the fresh Li metal surface exposure of the LiMP during cycling and calendaring were classified as electrochemical and physical activation, respectively. The Nyquist plots of the impedances of the unit cells (Li metal/ LMO) containing different types of the LiMP electrodes with 0, 20, and 40% compression after the formation cycles are presented in Figure 5a. The compression appears to have changed the impedance behavior of the cells significantly.

For the bare Li foil electrodes, small particles homogenously distributed over the entire surface are observable at a low current density (Figure 3a). On the other hand, nonhomogeneously deposited Li forms with dendritic features are evident in the localized portions of the Li metal surface at a high current density (Figure 3b). The morphological features of the Li metal deposited on the LiMP electrodes largely depend on the degree of compression and current density. Similar to the bare Li metal foil electrodes, the 0% compressed LiMP electrodes show a grain structure on the Li powder surface at a low current density (Figure 3c). In contrast, no dendritic Li structure is observable at a high current density for the 0% compressed LiMP electrodes (Figure 3d). These experimental trends are also evident for the other LiMP electrode systems. At a low current density for the 20% compressed LiMP electrodes, aggregated Li grains are observable on the fractured edges and flattened/cracked surface formed during calendaring, whereas the noncompressed LiMP inside the LiMP electrode remains clean (Figure 3e). On the other hand, the original morphological features of the 20% compressed LiMP electrodes are well maintained after the highcurrent-density Li plating (Figure 3f). Again, similarly, the 40% compressed LiMP electrodes show a much smoother surface morphology for Li plating with a high current density (Figure 3h) than that with a low current density (Figure 3g). In general, it has been strongly believed that the morphological characteristics of the Li metal depend on the current density and that high current density is the main cause of the Li dendrite formation and mossy Li deposition.9 Given this fact, we cannot clearly explain the observed morphological changes of the Li metal because the trends were opposite to the existing ones. Specifically, a low current density resulted in a more uncontrolled Li deposition. We believe that this exceptional result is associated with the inherent morphological characteristics of the LiMP. The morphological characteristics of the LiMP during Li plating are determined not only by the current density, but also by the total amount of plated Li ions.42 For instance, in contrast to the conventional bare Li metal foil electrodes, the LiMP electrodes have been observed to hinder the Li dendrite formation at a higher current density in specific Li plating conditions, whereas a low current density caused the E


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the interfacial effects of the cell, may increase because of the thick SEI, although it may also decrease simultaneously because of a surface area increase resulting from uncontrolled Li plating.63−65 Consequently, the EIS analysis is difficult because such electrodes undergo severe morphological changes during cycling. The cycling performances of the LiMP electrodes show strong dependency on the compression level (Figure 6a).

Figure 5. EIS analysis of LMO/Li cells after precycling at room temperature and in the discharged state. (a) Measured impedance shown as a Nyquist plot, and the calculated values of (b) R1 (= Rb) and R2 (= RSEI + Rct).

In general, the impedance spectra of LIBs can be described as follows: in most of the voltage range, the electrochemical impedance spectroscopy (EIS) of the LIB cells is typically composed of two partially overlapping semicircles and a straight sloping line at the low frequency end.51,52 The total resistance (Rcell) of a battery cell is mainly composed of bulk resistance (Rb), SEI resistance (RSEI), and charge-transfer resistance (Rct). RSEI corresponds to the resistance due to the Li-ion migration through the electrode surface layer,51,53,54 and Rct is related to the charge-transfer resistance between the electrodes and electrolytes. To explore the origins of the electrochemical properties of the LIB cells in practice, the total cell resistance (Rcell = Rb + RSEI + Rct) should be closely monitored because LIB cells having smaller Rcell generally exhibit improved rate capabilities and cycle performances.52,55−60 We used a simple equivalent-circuit model approach to fit the EIS results, as shown in the insets of Figure 5b, with a proper understanding of the complicated Li battery system.61,62 The unique features of this model are the separation of (1) all ohmic resistant components lumping into R1 and (2) the faradic nonlinear components into R2. In other words, R1 and R2 represent Rb and RSEI + Rct, respectively. Using this model, R1 and R2 were calculated and are presented in Figure 5b. As shown in Figure 5b, the lower the compression level, the higher is the R1 value. In contrast, the higher the compression level, the higher is the R2 value. As identical cathodes, electrolytes, and separators were employed, the differences must have been caused by the LiMP electrodes. R1 becomes larger as excess Li ions and electrolytes are consumed during the formation of the thick SEI. R2, which corresponds to all of

Figure 6. Electrochemical measurements of LMO/Li cells employing LiMP electrodes after 0, 20, and 40% compression in comparison to a Li foil electrode. (a) Cycling performance measured at a rate of 0.5 C (0.435 mA cm−2) between 3.0 and 4.4 V (vs Li/Li+) (b) Coulombic efficiencies of unit cells relevant to a. (c) Comparison of the discharge capacities of the cells at different discharge rates while keeping the charge rate constant at 0.5 C (0.435 mA cm−2).

Increasing the compression considerably improves the cycling performance at the charge and discharge rates of C/2 (0.435 mA cm−2). The 0 and 20% compressed LiMP electrodes reach 80% of their initial discharge capacities only after the 8th and 75th cycles, respectively. Both cells exhibit cycling performances significantly worse than those of the bare Li metal foil electrode (130th cycle). The 40% compressed LiMP electrode displays the best cycling performance (the cell maintains ≥95% of its initial discharge capacity after the 350th cycle). Consequently, the 40% compressed LiMP electrode shows a F


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As the 0% compressed LiMP electrodes did not experience any physical activation, this voltage plateau can be closely related to the electrochemical activation of the nonactive LiMPs, as discussed above with regard to the SEM images in Figure 4. On the other hand, the 20 and 40% compressed LiMP electrodes, which contained numerous physically activated sites, do not exhibit this unique voltage plateau (∼3.7 V) because of electrochemical activation during precycling (Figure S2, Supporting Information). The exposed Li metal sites formed via physical activation might be preferentially consumed rather than forming new Li metal sites via electrochemical activation requiring high polarization. Although the 40% compressed LiMP electrodes had large numbers of physically activated sites, they could be consumed during the long cycle performance, consequently requiring additional activated Li metal sources. After electrochemical activation of the nonphysically activated LiMPs, the 40% compressed LiMP electrodes were self-healed and their initial discharge capacities were recovered. The true average CE of a system with an excess of cyclable Li can change because of the lack of information about the actual Li loss per cycle, which varies with the thickness of the Li metal electrode. In contrast, an LIB system has only one source of Li; thus, the capacity will decrease if an irreversible Li loss occurs. Therefore, we propose a new method of calculating the average CEs of Li battery systems employing Li metal electrodes (please see the Supporting Information for details). An advantage of the average CE method is that it enables comparison of systems with different solutions applied to the Li metal electrodes, revealing the CE of the investigated system. The typical cycling behavior of a system with excess Li shows two main stages (Figure S3, Supporting Information): a pronounced cycling period with high CE, where the total available amount of Li is greater than the total amount of Li from the cathode (region A, Qavailable > Qcathode), followed by a sudden drop in the capacity (region B, Qavailable < Qcathode). The intersection point can be identified as the point at which the available amount of Li is similar to the total amount of Li from the cathode (Qavailable ≈ Qcathode). At this point, the Li reservoir is completely depleted. Considering the Li excess in the anode, which can be calculated or estimated based on the geometry, measured electrochemically or by weight, the total amount of Li in the system can be determined. The measured data and the total amount of Li in the system can then be used to fit the intersection point B with an average CE. By fitting the cycling data, the 40% compressed LiMP electrode was observed to have a higher average CE (>97.1%) than the bare Li metal foil electrode (>91.6%) (Table S2, Supporting Information). Thus, we can infer that application of compressed LiMP electrodes provides a means of increasing the CE.

stable CE up to the 350th cycle compared to other cases (Figure 6b). Furthermore, the rate capabilities of the LiMP electrodes were measured in the Li/LMO cells as a function of current density while maintaining the charge current densities at a rate of C/2 (0.435 mA cm−2). Although the rate capability of the 40% compressed LiMP electrode is slightly lower than that of the bare Li metal foil electrode, it is improved compared to those of the 0 and 20% compressed LiMP electrodes (Figure 6c). The improved cycling and rate capability of the 40% compressed LiMP electrode can be explained by the voltage profiles of the selected cycles that are presented in Figure 7.

Figure 7. Voltage profiles of selected cycles for (a) the noncompressed LiMP electrode and (b) the 40% compressed LiMP electrodes.

The 40% compressed LiMP electrode shows highly improved and stable electrochemical performance up to the 150th cycle. During this cycle, the performance of the 40% compressed LiMP electrode fluctuates after prolonged cycling (Figure 6a). To understand the mechanism of this fluctuation during cycling, we selected the cycles showing discharge capacity “valleys” (the 289th and 340th cycles of the 40% compressed LiMP electrode relevant to Figure 6a) and redrew the voltage profile (Figure 7a). For comparison, we also redrew the voltage profile of the cycle (Figure 7b) in which the discharge capacity recovered (the 300th cycle of the 40% compressed LiMP electrode relevant to Figure 6a). The former two cycles (289th and 340th cycles) show unique voltage plateaus at ∼3.4 V, but the discharge-recovered cycle (300th cycle) does not. Interestingly, the 0% compressed LiMP electrode shows an unusual voltage plateau at the end of the discharging step at ∼3.7 V (marked with *). The voltage gap between the two cases might be attributable to the increased polarization of the unit cell (Li metal/LMO) after the long cycle performance.

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CONCLUSIONS In this report, we proposed the application of compressed SLMP electrodes as anodes. The investigated LiMP electrodes exhibited several advantages in terms of handling, performance, and safety, in comparison to the Li metal foil electrodes. Moreover, the LiMP electrode preparation enables the use of regular battery manufacturing equipment. Besides the practical advantages, the Li plating and stripping behavior varied with the compression level in the electrochemical investigations. The compressed LiMP electrodes showed promising properties in combination with carbonate-based electrolytes and with cathodes as their Li sources. The increase in the energy G


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density on the stack level compared with that obtained by directly replacing graphite with a 40 μm thick LiMP electrode was calculated to be 124% (Table S3, Supporting Information). Moreover, fitting the cycling data revealed average CEs of >97.1 and >91.6% for the LiMP and Li metal foil electrodes, respectively. Nevertheless, these CEs remain insufficient for the successful commercialization of Li metal anodes. However, the results elucidate a promising pathway for future work, and it may be possible to combine these methods with others to increase the CE further, for example, by coating of the Li metal surface,14 electrolyte engineering,14 or using solid-state electrolytes.16 We believe that the only method of commercializing Li metal electrodes is to combine several dendrite-suppressing methods, which we will pursue in our future work.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02740. Comparison of the current work with the state-of-the-art work on Li metal anodes and the true average CE (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone.: +53-785-6425. Fax: +82-53-785-6409 (Y.M.L.). *E-mail: [email protected]. Phone: +82-42-821-1534. Fax: +82-42-821-1534 (M.-H.R.). ORCID

Myung-Hyun Ryou: 0000-0001-8899-019X Author Contributions ⊥

D.J. and J.O. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Commercializations Promotion Agency for R&D Outcomes (COMPA) funded by the Ministry of Science, ICT, and Future Planning (MISP) (2017K000216). This work was also supported by the international Collaborative Energy Technology R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (no. 20158510050020).

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