High-Rate Cycling of Lithium-Metal Batteries ... - ACS Publications

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High-Rate Cycling of Lithium-Metal Batteries Enabled by Dual-Salt Electrolyte-Assisted Micro-Patterned Interfaces Byeolhee Yoon, Jinkyu Park, Jinhong Lee, Seokwoo Kim, Xiaodi Ren, Yong Min Lee, Hee-Tak Kim, Hongkyung Lee, and Myung-Hyun Ryou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05492 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 12, 2019

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ACS Applied Materials & Interfaces

High-Rate Cycling of Lithium-Metal Batteries Enabled by Dual-Salt Electrolyte-Assisted MicroPatterned Interfaces Byeolhee Yoon†, Jinkyu Park†, Jinhong Lee‡, Seokwoo Kim†, Xiaodi Ren⊥, Yong Min Lee*,§, Hee-Tak Kim*,‡, Hongkyung Lee*,⊥, and Myung-Hyun Ryou*,† † Department of Chemical and Biological Engineering, Hanbat National University, 125 Dongseo-daero, Yuseong-gu, Daejeon, 34158, Republic of Korea ‡ Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, 34141, South Korea § Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), 333 Techno Jungang-Daero, Daegu 42988, Republic of Korea ⊥ Energy and Environment Directorate, Pacific Northwest National Laboratory (PNNL), 902 Battelle Boulevard, Richland, Washington 99354 United States.

KEYWORDS: Dual-salt electrolyte, Fast-charging, Lithium dendrite, Lithium metal battery, Micro-patterning

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ABSTRACT: We present a synergistic strategy to boost the cycling performance of Li-metal batteries. The strategy is based on the combined use of a micro-pattern (MP) on the surface of the Li-metal electrode and an advanced dual-salt electrolyte (DSE) system to more efficiently control undesired Li-metal deposition at higher current density (~3 mA cm-2). The MP-Li electrode induces spatially uniform current distribution to achieve dendrite-free Li-metal deposition beneath the surface layer formed by the DSE. The MP-Li/DSE combination exhibited excellent synergistic rate capability improvements that were neither observed with the MP-Li system nor for the bare Li/DSE system. The combination also resulted in the Li||LiMn2O4 battery attaining over 1,000 cycles, which is twice as long at the same capacity retention (80 %) compared with the control cells (MP-Li without DSE). We further demonstrated extremely fast charging at a rate of 15 C (19.5 mA cm-2). INTRODUCTION Boosting the energy density of a battery beyond that of current lithium (Li)-ion batteries (LIBs) has been highly desired to achieve full electrical mobility for portable electronics, electric vehicles, and unmanned aircraft, amongst others.1-4 To overcome the theoretical limits of LIBs, next-generation Li secondary batteries, including Li-air (oxygen, O2), Li-sulfur (S), and Li-metal batteries (LMBs) have attracted great interest in recent decades. In terms of the selection of an anodic material for such a battery system, Li metal has been regarded as the ideal material for the anode owing to its superior properties: the lowest reduction potential (– 3.04 V vs. standard hydrogen), low density (0.534 g cc−1), and ultrahigh specific capacity (3860 mA h g−1 or 2060 mA h cm−3).5,6 Indeed, LMBs coupling with Li-rich or Ni-rich transient metal oxide cathodes can achieve high specific energies up to 500 W h kg−1, which far exceeds those of state-of-the-art LIBs.7,8


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Nonetheless, the meaningful realization of Li-metal-based batteries has long been hampered by the unstable nature of the Li-metal anode. This anode inherently creates a heterogeneous solidelectrolyte interphase (SEI) between the Li-metal surface and liquid electrolyte during operation. During repeated Li plating/stripping, uncontrolled and uneven Li is formed in dendrite-like shapes, such as granular Li and moss Li. This consumes a large amount of liquid electrolyte because these dendrites create newly exposed Li-metal surfaces on the electrolyte to form a new SEI. Finally, they lead to electrochemically isolated Li, i.e., “dead” Li, resulting in poor Coulombic efficiency (CE) and poor cycle performance of the Li-metal anode.4,6 Furthermore, uncontrolled nanostructured-Li, including fractal Li and “dead” Li, can cause a serious safety problem due to the explosive and flammable tendencies of nanostructured metal powder.9 In particular, the sharp protrusions of the grown Li dendrites may cause cell short-circuiting by piercing the separator, resulting in catastrophic battery explosion. Attempts to suppress the formation of Li dendrites have led to the intensive exploitation of numerous approaches such as electrolyte modifications,10-15 protective layers,16-21 threedimensional architectures for the Li-host,5,10,22 separator modifications,3,23,24 and current collector modifications,25,26 in recent years. Among them, Li-metal plating, which can be controlled with a highly stable SEI on the Li-metal surface, i.e., Li metal without dendrites, is a precondition because in all approaches SEI formation accompanies any form. Advanced electrolytes prepared by manipulating the electrolyte composition (by varying the solvent or type of Li salt) and/or adding small amounts of functional additives are a highly efficient and economical approach to improve the cycling stability of LMBs. The use of advanced electrolytes constitutes an easy way to manipulate the SEI characteristics of the Li metal, and the uniform and robust SEI formed thereby can improve the interfacial properties and prevent further


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decomposition of the electrolyte upon cycling. Recently, the dual-salt electrolyte (DSE) containing Li bis(oxalate)borate (LiBOB) and Li bis(fluorosulfonyl)imide (LiTFSI) salts was reported to significantly improve the cycling stability of LMBs over Li hexafluorophosphate (LiPF6), a Li salt commercialized for use in LIBs.1,11,27 The advantage of the DSE is that it leads to the formation of a polycarbonate-enriched SEI to protect the Li-metal surface from electrolyte attack during cycling.11 In recent years, to improve the cycle performance of Li metal, we developed a mechanical surface modification technique whereby a surface pattern is introduced on the Li-metal surface.2,28,29 The unique architecture of this micro-patterned Li-metal (MP-Li) electrode is beneficial to control the deposition of Li metal efficiently through spatially and evenly distributed pyramid-shaped holes arranged in a pattern, and which can induce uniform current distribution and Li+ ion fluxes. However, irrespective of the extent to which the surface structure is effectively manipulated to control the current distribution on the Li-metal surface, MP-Li continues to experience mossy-like Li granulation at high current density (>2.0 mA cm−2). This phenomenon adversely affects the performance of LMBs, which experience continuous electrolyte consumption and have a limited lifetime. In this study, we further verified the efficacy of the DSE by using a combination of mechanical and chemical modifications to enable Li-metal deposition to be more efficiently controlled to obtain MP-Li with excellent cycling stability. DSE-derived SEI formation at the MPLi electrode surface can greatly enhance its mechanical properties, thereby enabling the electrochemical deposition of Li metal to be more efficiently controlled with no dendritic morphology even at higher current density (~2.4 mA cm−2). As a result, MP-Li with DSE exhibited excellent cycling stability over 1,000 cycles under harsh operating conditions with high current (it


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maintained 77.5 mA h g−1, which is 74.2 % of the initial discharge capacity, at both charging/discharging rates at a 2 C rate of 2.6 mA cm−2), exceeding the cycle stability of the respective MP-Li and DSE systems. MP-Li with DSE demonstrated extremely fast charging at a rate of 15 C (19.5 mA cm-2) to retain 48.3 % of the initial capacity.

EXPERIMENTAL SECTION Material. LiMn2O4 (LMO) was purchased from Iljin Materials (South Korea) and used as cathodic material. Li metal with a thickness of 200 µm, provided by Honjo (Japan), was used as the anodic material. The Reference electrolyte (REL) was composed of 1.15 M LiPF6 and a mixture of the solvents ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (30:70 by volume). The DSE was prepared by mixing 0.6 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF6 in a mixture of the solvents EC and EMC (40:60 by weight). These chemicals, including LiTFSI, LiPF6, EC, and EMC, were purchased from EnChem, South Korea. LMO electrode preparation. The cathode laminates were prepared by casting an NMP-based slurry composed of 90 wt.% LMO, 5 wt.% conductive carbon (Super P Li®, Imerys), and 5 wt.% PVDF binder (KF-1300, Kureha, Japan) on Al foil (15 µm, Sam-A Aluminum, South Korea) using a gap-controlled doctor blade. The cathode loading was ~13 mg cm-2. The electrode film was dried in the oven at 130 °C for 1 hour and then roll-pressed with a gap-control-type roll-pressing machine (CLP-2025, CIS, South Korea). The electrode density was controlled to be 1.7 g cm-3. Electrochemical measurements. The reductive decomposition of electrolytes was investigated by conducting linear sweep voltammetry (LSV) by using a cell containing a graphite electrode (diameter = 12 mm) as working electrode and a Li metal disc as the counter and reference electrodes at a scan rate of 0.05 mV s-1 negatively swept from the open circuit voltage (OCV) to 0


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V vs. Li/Li+. The electrochemical properties were measured using CR2032 coin cells composed of an LMO cathode and two different types of Li metal (bare Li and MP-Li), PE separators (ND 420, Asahi Kasei E-materials, Japan) and REL and DSE electrolytes. The as-prepared LMO cathode was cut into a disc 12 mm in diameter and further dried in a vacuum oven at 60 °C for 12 h before use. The cell was assembled in an argon-filled glove box with a dew point below – 80 °C. Electrochemical testing of the Li||LMO cells was carried out after the formation process in the voltage range 3.0–4.3 V using a battery cycler (PNE solution, Korea) at 25 °C. Prior to battery cycling, all cells were subjected to pre-cycling processes including one cycle of formation (both charge/discharge at C/10 rate under a constant current (CC) mode) and three cycles of stabilization (charging at C/5 rate in CC mode and constant voltage (CV) mode at 4.3 V, and subsequent discharging at C/5 rate in CC mode). Then, the cycle performance was evaluated at the rate 2 C. The fast charging capability test was performed by increasing the charging current every seven cycles at 0.5 C, 1 C, 2 C, 3 C, 5 C, 7 C, 10 C, and 15 C with a fixed discharging rate of 0.5 C. Then, the cells were further cycled at the rate 0.5 C as the capacity recovery step. EIS was conducted using a VSP impedance analyzer (Bio-Logic SAS) over the frequency range from 5 × 10-2 to 106 Hz. Characterizations. Prior to the SEM and XPS analyses, the cycled cells were disassembled to recover the Li metal and they were immersed in diethyl carbonate (DEC) for 12 h and then rinsed with fresh DEC for 1 h before drying under vacuum. SEM characterization was performed with field-emission SEM (FE-SEM, S-4800, HITACH, Japan) at an accelerating voltage of 10 kV. XPS (Sigma probe, Thermo VG Scientific, USA) was carried out using Al Kα radiation. To prevent corrosion during sample transfer, Li-metal samples were hermetically sealed in a polypropylene container (Nalgen, USA) and quickly transferred to the SEM and XPS analyzing chambers.


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RESULTS AND DISCUSSION Figure 1a, b show the optical microscopic (OM) image and three-dimensional (3D) microstructure of the surface of the stainless-steel stamp used in this study. The appearance of the stainless-steel stamp is also shown in Figure 1a (inset). The surface of the stamp consists of a highly aligned array of microneedle arrays of rectangular pyramid tips (width = 50 μm, depth = 48 μm), and the pyramids are separated by a tip distance of 100 μm. As illustrated in Figure 1c, we prepared an MP-Li electrode by using a simple stamp-imprinting method. Owing to the ductile properties of metallic Li, as shown in Figure 1d, e, the micropatterns with pyramid-shaped holes (50 μm width and 46 μm depth) were successfully transferred onto the Li-metal surface without any surface defects. Hereafter, for convenience, we denote this micro-patterned Li electrode as the MP-Li electrode. The morphological change of the Li metal was investigated by acquiring SEM images of the MP-Li electrodes obtained from Li||Li symmetric cells containing different electrolytes (Case 1 = Reference electrolyte (REL) = 1.15

M

LiPF6 salt in a mixture of carbonate solvents (ethylene

carbonate (EC)/ethyl methyl carbonate (EMC) = 30:70, by vol., Case 2 = Dual salt electrolyte (DSE) = 0.6 M LiTFSI + 0.4 M LiBOB + 0.05 M LiPF6 in carbonate solvents (EC/EMC = 40:60 by wt.)). Li plating/stripping processes were conducted at 2.4 mA cm−2 for 5 min. The unique structural advantage of the MP-Li electrode developed by our group is that it improves battery stability by controlling Li-metal deposition such that it occurs at the uniformly distributed pyramidal holes to suppress the growth of Li dendrites.2,28,29 Nonetheless, as shown in Figure 2a, b, the MP-Li is still adversely affected by granular Li and mossy/whisker-like Li formation under high current density conditions), which is in good agreement with our previous finding.28,29 Non-


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planar type Li-metal deposition is not favorable because structural changes in the Li metal during cycling implies that a large amount of liquid electrolyte is consumed to form the SEI on the newly exposed Li-metal surface, resulting in the degradation of the cycle performance of the metal. Furthermore, an excessive accumulation of “dead” Li causes the Li surface to deteriorate and impedes Li+ ion transport, resulting in a dramatic increase in the cell impedance and capacity fading of LMBs.30,31 In strong contrast, Li-metal deposition when using MP-Li with the DSE exhibits non-dendritic morphology (Figure 2c). Furthermore, MP-Li can be fully recovered to its original structure in the subsequent Li stripping process (Figure 2d). Two different electrolytes were then used to further investigate the structural reversibility of MP-Li by increasing the Li deposition capacity up to 2.4 mA h cm−2 (current density of 2.4 mA cm−2, an hour each for plating and stripping, Figure S1 in Supporting Information). Compared to the REL, DSE enables an even distribution of Li deposits as a result of the anchoring effect of MP-Li, even if the overflow of Li deposits occurs. This overflow is a consequence of the limited amount of Li deposits that are acceptable in the total void space of the pattern holes (~1 mA h cm−2 of areal capacity), which is insufficient to accommodate a larger amount of deposited Li (2.4 mA h cm−2). Nonetheless, it is noteworthy that the synergistic combination of MP-Li and DSE is still feasible to regulate the larger amount of Li deposits in a more uniform manner. The cross-sectional SEM images also support this claim (Figure S2 in Supporting Information). The use of REL with MP-Li caused the pyramid-shaped holes of the MP-Li to become filled with “dead” Li even as the cycle proceeded at high current operating conditions of 2.4 mA cm−2 (Figure 3a, b and Figure S1a, b in Supporting Information). Furthermore, the surface of MP-Li metal was severely deteriorated as a result of full coverage due to excessive accumulation of “dead” Li. On the other hand, in the case of MP-Li with DSE, the deposited Li was efficiently


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stripped out and the original surface was recovered although the patterned holes were filled with planar Li (Figure 3c, d and Figure S1c, d in Supporting Information). Noteworthy is that the chemical composition of the Li-metal surface inside and outside the patterned holes is significantly different after cycling. As shown in the EDX elemental mapping images (Figure 3e), carbon (C), oxygen (O), and fluorine (F) were observed inside the patterned hole, which implies that the SEI largely formed inside the hole. Considering these results, it can be deduced that the accumulation of "dead" Li was effectively reduced by DSE-derived SEI modification at the MP-Li electrode. It should be noted that, regardless of the extent to which the Li-metal surface is chemically decorated by the DSE, Li dendrites continue to form in an uncontrolled manner without structural modification of the surface. Without the surface patterns, needle-like Li dendrites were observed at the planar bare Li electrode and the deposited Li aggregates were randomly distributed regardless of the electrolyte (Figure S3 in Supporting Information). These observations are in good agreement with the results of the previous study 11. These results provided convincing evidence that the strategy of combining MP-Li with DSE is beneficial to minimize uncontrolled Li deposition. According to the deterministic role of SEI on Li morphology, we examined the mechanism of SEI formation associated with DSE by investigating the reduction decomposition of each chemical component of DSE using linear sweep voltammetry (LSV). As shown in Figure S4, DSE containing three types of Li salts (LiBOB, LiTFSI, and LiPF6) has a much higher onset decomposition potential (~1.8 V vs. Li/Li+) than REL (~0.75 V vs. Li/Li+) containing only LiPF6. The onset decomposition potential of LiTFSI is also close to that of the REL (~0.75 V vs. Li/Li+). In contrast, the electrolyte containing only LiBOB reveals almost the same onset decomposition potential (~1.8 V vs. Li/Li+) to that of DSE system. This implies that the reduction decomposition


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properties of DSE are mainly governed by the decomposition of LiBOB. The BOB− anion is preferentially decomposed on the surface of Li metal, effectively preventing parasitic reactions from occurring on the Li surface.32 According to previous studies,33 LiBOB-containing electrolytes enrich the SEI with polycarbonate species, which promotes the flexibility of the SEI. The effect of DSE on the modification of SEI for MP-Li was studied by analyzing the chemical composition of the SEI formed on the MP-Li electrode using X-ray photoelectron spectroscopy (XPS). Figure 4 presents the high-resolution elemental XPS spectra of C 1s, O 1s, and F 1s on MPLi surfaces operated in combination with REL and DSE. The spectra were collected and compared after precycling and after 10 cycles. There are obvious spectral differences between MP-Li with REL and DSE. After 10 cycles, MP-Li with DSE reveals sharper deconvoluted peaks of 291.0−292.0 eV for C 1s and 531.0−532.0 eV for O 1s, compared to MP-Li with REL. Both of these peaks correspond to the decomposition of compounds of the BOB− that contain oxalate moieties.34-36 Notably, the strong peak at 287.5 eV can be assigned to the polycarbonate species,33 which indicates that the SEI formed at the MP-Li electrode in DSE contains a similar polycarbonate species. Consequently, the mechanical stability of the SEI can be reinforced by increasing the flexible polymeric components, resulting in smooth Li-metal plating without granule-like Li deposition. The F 1s spectra clearly show the existence of LiF species (684.5 eV) at the MP-Li surface with REL, which indicates that decomposition of the LiPF6 salt occurred intensively. Beneath the DSE-derived SEI, LiF and LixPOyFx (686 eV) continue to exist. This implies that the PF6 anion is consumed in a self-limiting manner as it not only passivates the Li surfaces immediately but also contributes to the polymerization of the polycarbonate. MP-Li with REL reveals a sharper deconvoluted peak of 684.5 eV for F 1s, corresponding to LiF, which is ascribed to the reduction products of LiPF6, compared to MP-Li with DSE.35 Instead, the O 1s


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spectra of MP-Li with DSE clearly show a sharper deconvoluted peak of 528.0−529.0 eV, corresponding to the compound Li2O.11 According to previous studies,37,38 the enrichment of the SEI with Li2O as a component is beneficial to enhance the SEI rigidity to prevent dendritic growth and further decomposition of the electrolyte. Although LiF-rich SEI is preferable because it enhances the diffusion of Li+ on the surface,15,39 a shortage of polymeric compounds in the SEI film may cause SEI breakdown as a result of mechanical deformation upon Li plating at the MPLi surface. As illustrated in Figure 5, DSE-derived SEI at MP-Li is more capable of enduring such deformational stress when it is composed of polycarbonate and Li2O species as flexible organic and rigid inorganic parts, respectively. Therefore, we believe that the abundance of polymeric compounds in harmonized balance with rigid inorganic counterparts is preferred as a key SEI design principle to enhance SEI flexibility with sufficient rigidity at 3D-structured Li surfaces. The specific amounts of each atomic content are listed in Table S1 (Supporting Information). The effect of the DSE on MP-Li was further investigated by evaluating the electrochemical performance, including the cycle performance, and rate capability of MP-Li with and without DSE. The electrochemical properties of LMBs with four combinations of electrode and electrolyte: bare Li without a surface pattern, MP-Li, the reference electrolyte, and DSE were compared. Thus, unit cells (Li||LiMn2O4 (LMO)) composed of bare Li with REL, bare Li with DSE, MP-Li with REL, and MP-Li with DSE were prepared, denoted as Li@REL, Li@DSE, MP-Li@REL, and MPLi@DSE, respectively. The unit cells were cycled at a high current rate of 2 C-rate (2.6 mA cm-2) for both plating/stripping (Figure 6a). The cycle performance of both types of unit cells containing the MP-Li electrodes was superior to those of the bare Li-containing unit cells of the same type. That is, the cycle performance of MP-Li@REL (80 % of initial discharge capacity was maintained after 91 cycles) and MP-Li@DSE (80 % of initial discharge capacity was maintained after 760


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cycles) improved compared to that of Li@REL (80 % of initial discharge capacity was maintained after 36 cycles) and Li@DSE (80 % of initial discharge capacity was maintained after 390 cycles), respectively. The improved cycle performance of the MP-Li electrode compared to bare Li is attributed to the structural advantages that were clearly demonstrated in our previous studies.2,28,29 More importantly, replacing REL with DSE can significantly improve the performance of unit cells containing a Li-metal electrode. For instance, the performance of the MP-Li@REL system did not exceed 100 cycles (76.4 % of initial discharge capacity after 100 cycles), whereas that of MP-Li@DSE remained stable over 1,000 cycles (74.2 % of initial discharge capacity after 1,000 cycles). At the same capacity retention of 80 %, the cycle life of MP-Li@DSE was extended to nearly eight times of MP-Li@REL (91 → 760 cycles). The Coulombic efficiency of MP-Li@DSE was 100 % over 1,000 cycles, but that of MP-Li@REL showed a tendency to fluctuate after 100 cycles. Taking these results into consideration, it can be deduced that the combined effect of controlling the surface structure of the Li-metal electrode and DSE is an effective method to improve the LMB cycle stability even at high current to increase the Coulombic efficiency (please see the comparison table of the current work with state-of-the-art works on Li metal, Supporting Information Table S2). The electrochemical impedance spectra (EIS) were recorded after measuring the capacity during cycling (shown in Figure 6a). The total resistance (Rtotal) of the unit cell is composed of the bulk resistance (Rb), charge transfer resistance (Rct), and the resistance of the solid−electrolyte interphase (RSEI); that is, Rtotal = Rb + Rct + RSEI. The filled resistance values of the Li||LMO batteries after the cycling test are listed in Table S3 (Supporting Information). As shown in Figure 6b, Li@REL shows a sharp increase in Rtotal after 200 cycles. In contrast, the unit cells containing DSE exhibit much lower Rtotal. This suggests that REL undergoes severe electrochemical decomposition


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at the Li/electrolyte interface and is depleted inside the unit cell.3,4,31,40 On the other hand, DSE suppressed electrolyte consumption during cycling. The value of Rtotal of MP-Li@DSE is much lower than Rtotal of the Li@ DSE cell throughout 1,000 cycles (Figure 6b). In addition, MPLi@DSE experiences a smaller decrease in IR during cycling. The above results from the EIS study inspired us to investigate the rate capability of the unit cells in detail. Uncontrolled formation/growth of Li at the Li-metal electrode and the resulting electrochemical performance are known to be highly sensitive to the charging (Li plating) current density.3,41 The rate capability test was carried out by changing the charging rate (from C/2 to 15 C) while maintaining the same discharge rate of C/2. After the test with 15 C charging, we further cycled the unit cells at C/2 to determine whether the rate capability difference was due to battery component degradation or kinetic factors. As can be seen in Figure 6c, the discharge capacity of all the unit cells (Li@REL, Li@DSE, MP-Li@REL, and MP-Li@DSE) was restored to the original initial discharge capacity level. If deterioration of the Li metal was due to uncontrolled Li formation, the discharge capacity of the unit cells would have decreased in the additional cycles after the 56th cycle. Considering this result, it can be inferred that the difference in rate capability between the different types of unit cells can be explained kinetically. The unit cells containing bare Li metal (Li@REL and Li@DSE) were not cycled at a high Crate (15 C), regardless of whether REL or DSE was used. In contrast, the unit cells containing surface-patterned Li metal (MP-Li@REL and MP-Li@DSE) exhibited excellent rate capabilities when charged at very challenging current (15 C, corresponding to 19.5 mA cm-2). This result is surprising because DSE was proposed as an effective functional electrolytic system that forms a stable and robust SEI on Li-metal surfaces.11 As shown in Figure 6a, the cycle performance of Li@DSE improved significantly compared to that of Li@REL. For example, Li@DSE and


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Li@REL maintained 80 % of their initial discharge capacity after 390 and 36 cycles, respectively, which is a difference of approximately ten times. These results indicate that the SEI derived from DSE is advantageous for improving the cyclic performance of Li metal, whereas the surface patterning process contributes the strength to improve the rate capability of Li metal. This strongly suggests that the performance improvement is attributable to the synergistic effect between DSE and the surface pattern. For a better understanding of the synergistic effect of the dual salt on surface-patterned Li metal, we observed the potential profiles of the Li||Li symmetric cells during galvanostatic cycling (Figure S5 in Supporting Information). Galvanostatic cycling results of Li||Li symmetric cells showed almost the same tendency to the rate capability results. Li@REL revealed the highest value of overpotential with the most irregular potential spikes during cycling compared to the others. MP-Li@DSE showed the smallest value of overpotential, followed by Li@DSE and MPLi@REL.


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CONCLUSIONS In this study, we present the combining strategy of a highly stable electrolyte and a Li-metal electrode with a three-dimensionally structured surface to prevent the formation of Li dendrites upon high-rate cycling of LMB. The DSE plays a critical role in the anode passivation by simultaneous formation of the flexible polycarbonate and rigid Li2O components upon SEI development. Building a uniform and flexible SEI onto MP-Li assisted by DSE is essential to allow smooth Li deposition even at high charging current (~2.6 mA cm–2) and suppress the “dead” Li accumulation upon subsequent cycling. Li||LMO cell integrated with MP-Li and DSE (MPLi@DSE) exhibits superior cycling stability for 1,000 cycles (74.2 % capacity retention) at the 2 C rate, a two- and eight-fold enhancement compared with the MP-Li@REL and Li@DSE cells, respectively. While extremely fast charging of Li metal anode is still challenging when using only DSE, MP-Li@DSE cell demonstrates excellent high-rate capability up to 15 C-rate (19.5 mA cm– 2).

Therefore, the synergistic combination of MP-Li with advanced electrolytes that exhibit high

Li Coulombic efficiency is a simple and practical approach to enhance the high-rate and long-term cycling performance of LMBs, and it holds promise for direct practical implementation.


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FIGURES

Figure 1. (a) Optical surface image and (b) software-assisted height-contour image of the stainless-steel stamp shown in the inset of Figure 1a. (c) Schematic illustration of stamp imprinting method for MP-Li electrode fabrication. SEM images for (d) top surface and (e) cross-section of MP-Li electrodes.


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Figure 2. Morphological investigation of Li deposition at the MP-Li electrode. The MP-Li electrode was obtained from Li||Li symmetric cells after Li deposition (2.4 mA cm−2 for 5 min) and subsequent Li stripping (−2.4 mA cm−2 for 5 min) containing (a, b) reference electrolyte and (c, d) DSE, respectively. (a’−d’) show the magnified SEM images of (a−d), respectively.


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Figure 3 Postmortem SEM investigation of MP-Li with various electrolytes. The Li plated surface during the last half-cycle was observed after 10 cycles (for each cycle, Li||Li symmetric cells were operated in the following sequence: 2.4 mA cm−2 for 5 min → Rest for 10 min → −2.4 mA cm−2 for 5 min → Rest for 10 min). Top images: low and high magnification of the MP-Li electrode with (a, b) REL and (c, d) DSE after 10 cycles, respectively. (e) Magnified SEM and elemental mapping EDX images of dashed-box shown in c.


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Figure 4. High-resolution XPS results for MP-Li surfaces with REL and DSE. Each MP-Li sample was obtained from the Li||LiMn2O4 cells (3.0−4.3V) after formation and 10 subsequent cycles, respectively (2.6 mA cm-2). The upper, middle, and lower rows correspond to the C 1s, F 1s, and O 1s spectra, respectively.


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Figure 5. Schemes for electrodeposition of Li metal at (a) bare Li surfaces decorated with RELderived SEI and at MP-Li surfaces decorated with (b) REL-derived and (c) DSE-derived SEI, respectively.


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Figure 6. Electrochemical cycling of Li||LiMn2O4 batteries with MP-Li and DSE. (a) Capacity retention and CE evolution at 2 C-rate (= 2.6 mA cm-2) for the four different cells, bare Li with REL and DSE, respectively, and MP-Li with REL and DSE, respectively. (b) Nyquist plots of electrochemical impedance spectra (EIS) obtained after 200 and 1,000 cycles. (c) Rate capability results on varying the charging range (from C/2 to 15 C, while maintaining the same discharging current of C/2).


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. SEM images of Li metal (bare Li and MP-Li) after Li plating and stripping processes in different electrolyte systems (REL and DSE), LSV of different electrolytes (REL and DSE), potential profiles of Li/Li symmetric cells, XPS analysis results of Li metal obtained from the Li||LiMn2O4 cells after cycling, and fitted resistance values of the Li||LiMn2O4 cells.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. (M.-H.R.).

ORCID Yong Min Lee: 0000-0002-7732-2089 Hee-Tak Kim: 0000-0003-4578-5422 Hongkyung Lee: 0000-0003-2002-2218 Myung-Hyun Ryou: 0000-0001-8899-019X

Author Contributions B.Y. and J.P. contributed equally to this work.


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT B. Y and J. P equally contributed to this work. This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1D1A3B03933293). This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No.2018M3A7B4071066).

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Figure 1. (a) Optical surface image and (b) software-assisted height-contour image of the stainless-steel stamp shown in the inset of Figure 1a. (c) Schematic illustration of stamp imprinting method for MP-Li electrode fabrication. SEM images for (d) top surface and (e) cross-section of MP-Li electrodes.


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Morphological investigation of Li deposition at the MP-Li electrode. The MP-Li electrode was obtained from Li||Li symmetric cells after Li deposition (2.4 mA cm−2 for 5 min) and subsequent Li stripping (−2.4 mA cm−2 for 5 min) containing (a, b) reference electrolyte and (c, d) DSE, respectively. (a’−d’) show the magnified SEM images of (a−d), respectively.



Postmortem SEM investigation of MP-Li with various electrolytes. Top images: low and high magnification of the MP-Li electrode with (a, b) REL and (c, d) DSE after 10 cycles, respectively (for each cycle, Li||Li symmetric cells were operated in the following sequence: 2.4 mA cm−2 for 5 min → Rest for 10 min → −2.4 mA cm−2 for 5 min → Rest for 10 min). (e) Magnified SEM and elemental mapping EDX images of dashedbox shown in c.


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High-resolution XPS results for MP-Li surfaces with REL and DSE. Each MP-Li sample was obtained from the Li||LiMn2O4 cells (3.0−4.3V) after formation and 10 subsequent cycles, respectively (2.6 mA cm−2). The upper, middle, and lower rows correspond to the C 1s, F 1s, and O 1s spectra, respectively.



Schemes for electrodeposition of Li metal at (a) bare Li surfaces decorated with REL-derived SEI and at MPLi surfaces decorated with (b) REL-derived and (c) DSE-derived SEI, respectively.


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Electrochemical cycling of Li||LiMn2O4 batteries with MP-Li and DSE. (a) Capacity retention and CE evolution at 2 C-rate (= 2.6 mA cm−2) for the four different cells, bare Li with REL and DSE, respectively, and MP-Li with REL and DSE, respectively. (b) Nyquist plots of electrochemical impedance spectra (EIS) obtained after 200 and 1,000 cycles. (c) Rate capability results on varying the charging range (from C/2 to 15 C, while maintaining the same discharging current of C/2).



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High-Rate Cycling of Lithium-Metal Batteries ... - ACS Publications

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