Dendrite-Free Lithium Deposition for Lithium Metal ... - ACS Publications

†Samsung Advanced Institute of Technology (SAIT), Samsung Electronics Co., Ltd., 130, ... ble growth of Li dendrites, it still has safety and cyclab...
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Dendrite-Free Lithium Deposition for Lithium Metal Anodes with Interconnected Microsphere Protection Yong-Gun Lee,† Saebom Ryu,† Toshinori Sugimoto,† Taehwan Yu,† Won-seok Chang,† Yooseong Yang,† Changhoon Jung,† Jaesung Woo,‡ Sung Gyu Kang,‡ Heung Nam Han,‡ Seok-Gwang Doo,† Yunil Hwang,† Hyuk Chang,† Jae-Myung Lee,*,† and Jeong-Yun Sun*,‡ †

Samsung Advanced Institute of Technology (SAIT), Samsung Electronics Co., Ltd., 130, Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 443-803, South Korea ‡ Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, South Korea S Supporting Information *

ABSTRACT: A lithium (Li) metal anode is required to achieve a high-energy-density battery, but because of an undesirable growth of Li dendrites, it still has safety and cyclability issues. In this study, we have developed a microsphere-protected (MSP) Li metal anode to suppress the growth of Li dendrites. Microspheres could guide Li ions to selective areas and pressurize dendrites during their growth. Interconnections between microspheres improved the pressurization. By using an MSP Li metal anode in a 200 mAh pouch-type Li/NCA full cell at 4.2 V, dendrite-free Li deposits with a density of 0.4 g/cm3, which is 3 times greater than that in the case of bare Li metal, were obtained after charging at 2.9 mAh/cm2. The MSP Li metal enhanced the cyclability to 190 cycles with a criterion of 90% capacity retention of the initial discharge capacity at a current density of 1.45 mA/cm2.

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INTRODUCTION Lithium (Li) metal anodes are promising to achieve highenergy-density batteries owing to their high theoretical capacity (3860 mAh/g) and low redox potential (−3.04 V vs standard hydrogen electrode).1,2 In spite of the superior electrochemical performance, the use of a Li metal anode in a battery is limited by an undesirable growth of Li dendrites. The random growth of Li dendrites is initiated by nonuniform factors such as protrusions, impurities, and roughness at nano- or microscale on the surface of Li metal.3−6 The initiated Li dendrites grow rapidly because Li ions are concentrated at the tip of Li dendrites during charging. Therefore, the growth of Li dendrites should be prohibited from the early stage to prevent fatal failures such as large volumetric expansion, continuous loss of Li source, and short-circuiting in a battery. Researchers have attempted to use various strategies to suppress the growth of Li dendrites. Solid electrolytes based on inorganic solid conductors7,8 and polymer electrolytes9−11 have been applied to prevent the propagation of Li dendrites by utilizing their high mechanical rigidity. However, their ionic conductivity is low compared to that of liquid electrolytes, and their solid−solid interface is unstable; these issues must be solved to achieve a high-energy-density battery. Therefore, researchers have investigated the use of protective layers on the Li metal anode to use liquid electrolytes directly. To stabilize and reinforce a solid electrolyte interphase (SEI) layer on the Li metal anode, LiF12 and Li2S8-LiNO313 have been applied as electrolyte additives. However, the mechanical strength of the © 2017 American Chemical Society

SEI layer was not sufficient to suppress the growth of Li dendrites. Furthermore, protective layers such as hexagonal boron nitride (h-BN)14 and multilayered graphene (MLG)15 with high mechanical strength have been studied. The protective layers supply robust mechanical confinements to prevent the penetration of Li dendrites. However, because the flow of Li ions was not controlled to produce a uniform distribution of Li deposits during charging, the mechanically enhanced protective layers had difficulties in efficiently suppressing the concentrated growth of Li dendrites. Thus, protective layers such as a glass fiber16 and a polyacrylonitrile (PAN) nanofiber,17 which can reduce the concentration of Li ions, have been studied. However, the layers could not pressurize Li dendrites owing to their coarse structures, and the layers may lose their control after a flooding of Li deposition. Zheng and co-workers reported a protective layer with hollow carbon nanospheres with the dual functions of robust mechanical strength and uniform distribution of Li ions.18,19 However, the flow of Li ions to Li metal was impeded by a rigid protective layer, resulting in a decrease of Coulombic efficiency. Efficient design on a Li metal anode structure is also reported to protect Li metal anode.20−24 Polystyrene (PS) microspheres are chemically stable to organic electrolytes and Li metal, resulting in stable operation Received: March 30, 2017 Revised: June 7, 2017 Published: July 3, 2017 5906

DOI: 10.1021/acs.chemmater.7b01304 Chem. Mater. 2017, 29, 5906−5914

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Chemistry of Materials

Figure 1. Suppressed growth of Li dendrites in a Li metal anode with microsphere protection (MSP). (a) An MSP Li metal anode was fabricated with interconnected poly(styrene-co-divinylbenzene) (P(S-DVB)) microspheres and an ion-conductive layer of diethylene glycoldiacrylate (DEGDA). P(S-DVB) microspheres were uniformly coated on the Li metal anode. Top view (b) exhibits interconnected microspheres, and crosssectional view (c) shows the interface between microspheres and Li metal with an increased contact area. (d) Cross-linkable oligomer (DEGDA) was filled between microspheres, and cured. DEGDA was uniformly wetted between microspheres. (e) Cross-sectional view shows micropores filled with DEGDA. (f) Low-magnification view reveals the uniformity of the MSP Li metal anode after the fabrication. The mechanism of microsphere protection is illustrated in (g−j). (g) Microspheres were partially embedded into a soft Li metal. The partial embedment induced an oligomer coating in a selective area. (h) Li ions were guided by nonconductive microspheres to result in the growth of a Li deposit under the oligomer coating. (i) The Li deposit was pressurized by microspheres during the growth. Interconnections between microspheres improved the pressurization. (j) The density of the Li deposit was greatly enhanced by MSP.

in a Li metal battery.9,10 The stability of microspheres can be enhanced by copolymerization with divinylbenzene (DVB).25 The copolymerized microspheres could be interconnected after polymerization, resulting in a network of microspheres. The network of microspheres could be used as a protective layer because it could supply sufficient mechanical rigidity. Furthermore, the microspheres could guide Li ions to form a uniform distribution, because the microspheres are non-

conductive.26 Therefore, the network of microspheres might be a promising solution to suppress the growth of Li dendrites.

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EXPERIMENTAL SECTION

Reagents and Materials. Interconnected poly(styrene-co-divinylbenzene) (P(S-DVB), EPR-PSD-3, EPRUI, China) microspheres with 3 μm diameters were used to protect the surface of a Li metal anode. The interconnected structures of P(S-DVB) were formed during the 5907

DOI: 10.1021/acs.chemmater.7b01304 Chem. Mater. 2017, 29, 5906−5914

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Chemistry of Materials copolymerization of divinylbenzene with styrene, and the interconnection was controlled by a concentration of DVB. Tetrahydrofuran (THF, 401757, Sigma-Aldrich) was used as a solvent to disperse microspheres. Diethylene glycoldiacrylate (DEGDA, 437433, SigmaAldrich) and 2-hydroxy-2-methylpropiophenone (HMPP, 405655, Sigma-Aldrich) were employed as a UV-cross-linkable oligomer and an initiator for polymerization, respectively. Lithium bis(fluorosulfonyl)imide (LiFSI, PANAX ETEC, Korea) was used as a Li salt in the electrolyte. 1.0 M LiFSI in 1,2-dimethoxyethane (DME, 259527, Sigma-Aldrich)/1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropylether (TTE, 2107-3-56, Synquest Laboratories) (2:8 volume ratio) was used as an ether-based liquid electrolyte. A 20-μm-thick Li metal foil (Honjo Metal Co., Ltd.) was used as an anode. Aluminum-doped lithium nickel cobalt oxide (NCA, Li[Ni0.898Co0.087Al0.015]O2, Ecopro Co.) was used as a cathode. The areal capacity of the NCA electrode was 2.9 mAh/cm2, and its thickness was 50 μm. A 12-μm-thick separator (G1212A CCS, Asahi) was applied between the anode and cathode. Fabrication of a Microsphere-Protected (MSP) Li Metal Anode. P(S-DVB) microspheres were uniformly dispersed in THF, and the microsphere solution was cast on a Li metal foil by using the doctor-blading method. The THF was allowed to evaporate under ambient conditions after casting. The thickness of MSP Li metal was 3 μm. Because the microspheres were coated in a single layer, the total thickness of the hybrid layer is thin; thus it could help to be applied to a high-energy-density battery. The UV cross-linkable oligomer (DEGDA) was combined with a photoinitiator (HMPP) in THF. The oligomer solution was cast on the microsphere arrays, followed by evaporation of THF. UV cross-linking was conducted using a crosslinker (BIO-LINK BLX-365 Cross-linker, VILBER Lourmat) with 40 W power and 365 nm wavelength for an hour. Fabrication of a Pouch-Type Full Cell. A pouch-type full cell with a capacity of 200 mAh was fabricated with dimensions of x = 5.0 cm, y = 9.5 cm, and z = 1.0 mm. Electrodes and separators were cut using a punching machine. The dimensions of the Li metal anode and NCA cathode were 4.3 cm × 8.8 and 4.2 cm × 8.65 cm, respectively. The separators were cut with an extra areal margin of 15%. An Al tap and Ni taps were welded onto the cathode and the anode, respectively, by using an ultrasonic welder. A separator, a cathode (NCA), a separator, and a fabricated MSP Li metal anode were stacked on a layer of a MSP Li metal anode. The assembly was sandwiched between 2 PET films to give mechanical rigidity to the cell. An aluminum pouch was used to encapsulate the cell. The pouch was sealed with a hightemperature sealer, leaving three electrode terminals out and one side open for electrolyte injection. 1.0 M LiFSI in DME-TTE (2:8 volumetric ratio) liquid electrolyte was filled into the cell. DME is used as a stable solvent for Li metal, and TTE is added to improve the highvoltage stability of the electrolyte.27,28 Vacuum sealing was performed at 180 °C for 30 s. Prior to the cycling test, the pouch cells were stabilized under an open circuit condition for 12 h at room temperature. All fabrication processes were conducted in a dry room. SEM Observation. The morphologies of the MSP Li metal anodes were observed using a field-emission scanning electron microscope (FE-SEM, SU-8030, Hitachi). To calculate the density of Li deposits, the change in thickness of the Li metal anode was measured from cross-sectional SEM images after charging at 0.1 C (0.29 mA/cm2) for 10 h. Elemental mapping was conducted with an energy-dispersive Xray spectroscope (EDS, X-max 80, Oxford) for the Li metal anode. Electrochemical Characterization. Li/NCA full cells were cycled twice at a rate of 0.1 C (0.29 mA/cm2) in a voltage range of 3.0−4.2 V for initial formation. The cells were charged at 0.7 C (2.03 mA/cm2) and discharged at 0.5 C (1.45 mA/cm2) for the cycling test in a voltage window between 3.0 and 4.2 V by using a battery-testing system (TOSCAT-3100, Toyo System). For a rate capability test, cells were cycled between 3.0 and 4.2 V with various discharging rates ranging from 0.2 C (0.58 mA/cm2) to 1.5 C (4.35 mA/cm2). The charging rate was kept constant at 0.7 C (2.03 mA/cm2) during the rate capability tests. The impedance of the full cell was evaluated through electrochemical impedance spectroscopy (EIS) using a Solartron 1455A frequency response analyzer (FRA) together with a

Solartron 1470E multichannel potentiostat electrochemical interface in a frequency range of 1 MHz to 1 Hz with an amplitude of 10 mV. All electrochemical characterizations were conducted at room temperature. In Situ Indentation Measurements for P(S-DVB) Microspheres. P(S-DVB) microspheres dispersed in ethanol were randomly distributed on a (100) silicon wafer by spraying. To avoid any thermal damage on P(S-DVB) microspheres, samples were dried at room temperature for 48 h. Selective indentation was conducted with an in situ indentation system installed in the SEM (FEI Quanta FEG-250 SEM, Hysitron PI 85 SEM Picoindenter). A cono-spherical diamond tip with a 1 μm radius was used through the indentations. The indentations were performed under displacement-control mode by applying a constant loading rate of 20 nm/s.

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RESULTS AND DISCUSSION Microsphere Protection. Chemically cross-linked P(SDVB) was used to form microspheres. The chemical crosslinking by DVB is important to improve the chemical stability to organic solvents, such as THF, because non-cross-linked PS spheres could be dissolved in THF. Moreover, P(S-DVB) did not show glass transition under 150 °C owing to its chemical cross-linking (Figure S1). Figure 1a schematically illustrates the fabrication process for the MSP Li metal anode. The MSP Li metal anode can be easily fabricated using the doctor-blading method directly on a sheet of Li metal without using a binder and further intricate processes. The fabrication of a closepacked MSP layer on the Li metal anode is essentially required to control the growth of Li dendrites effectively. Pores between the spheres were then filled with an ion-conductive oligomer, DEGDA. Ethylene oxide (EO)-based electrolytes, i.e., DEGDA, were quite stable to Li metal.29−32 Complete pore-filling enhanced the interconnection between microspheres, and improved the dendrite-free Li deposition further. Figure 1b shows a scanning electron microscopy (SEM) image for interconnected poly(styrene-co-divinylbenzene) (P(S-DVB)) microspheres coated on a Li metal anode. The spheres were partially embedded in the Li metal to coat the oligomer selectively, as shown in Figure 1c. The elemental mapping of carbon atoms verifies the embedment, as shown in the inset of Figure 1c. Figure 1d,e shows micropores filled with a solidified oligomer after UV-curing. The infiltration of the oligomer was controlled by the concentration of oligomer in the THF solution. A monodispersely coated MSP Li metal anode after the UV-curing is shown in Figure 1f. The surface morphology of the MSP Li metal anode was observed by an atomic force microscope (AFM) (Figure S2). Microspheres were uniformly coated on Li metal, and the surface roughness of Li metal was 227 nm after applying the protection with 3 μm diameter microspheres. Mechanisms to suppress the growth of Li dendrites by microsphere protection are illustrated in Figure 1g−j. Li metal is sufficiently soft that its morphology is changed on applying a small pressure.33 Thus, when the high-modulus microspheres are coated on soft Li metal (yield strength of 0.655 MPa)34 with external pressure, the spheres could be partially embedded on the Li metal surface, as shown in Figure 1g. The embedment decreased the size of micropores between microspheres and Li metal, resulting in an oligomer coating in very selected areas. Microspheres are nonconductive, but the oligomer filled in micropores is ion-conductive.26 Therefore, Li ions in the electrolyte are preferentially guided toward micropores, resulting in deposition in selective areas, as illustrated in Figure 1h. In addition, because the micropores are uniformly 5908

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Figure 2. Mechanical characterizations of interconnected P(S-DVB) microspheres. (a) In situ indentation of a single P(S-DVB) microsphere. The indentation was performed on a (100) silicon wafer with a cono-spherical diamond tip. (b) Load−depth curves of single P(S-DVB) microspheres. (c) Boundary conditions of a finite element method (FEM) simulation for a singlet indentation. (d) Loading curves of indentations were compared with the simulation. An elastic modulus of 3.91 GPa was evaluated for the P(S-DVB) microspheres by the comparison. (e) An interconnected triplet of P(S-DVB) microspheres was indented to measure the strength of an interconnection. (f) A load−depth curve of the triplet up to the failure of the interconnection. (g) Boundary conditions of a FEM simulation for a triplet indentation. (h) The loading curve was compared with the simulation, and the maximum stress developed in the interconnection was plotted together. The strength of the interconnection was calculated as 460 MPa.

modulus of ion-conductive oligomer (DEGDA) after curing was obtained as 0.2−0.5 GPa by using a dynamic mechanical analysis (DMA, DMAQ800, TA Instruments) (Figure S3). To evaluate the bonding strength between P(S-DVB) microspheres, in situ indentation tests were conducted with an interconnected triplet of microspheres, as shown in Figure 2e,f. The cono-spherical diamond tip was aligned to the center of the triplet, and it pushed the triplet until the interconnection between the microspheres was broken. As shown in Figure 2f, the load−depth curve shows a sudden load drop at an indentation depth of 312 nm after making contact, which corresponds to a fracture of the interconnections between the microspheres. FEM simulations were conducted with boundary conditions shown in Figure 2g to evaluate the bonding strength of P(S-DVB) microspheres. In order to prevent a rotation of the triplet, a condition of symmetry was applied through the center line of the indenter. The indenter was set as a rigid body, and the triplet has an elastic modulus and a Poisson’s ratio of 3.91 GPa and 0.34, respectively. The contact between the indenter and the triplet was assumed to be frictionless. Figure 2h compares a calculated load−depth curve with the measured values. The measured loads were fitted very well with the calculated values, except in the early region of indentation. The discrepancy in the early region of indentation may have originated from imperfect alignment between the indenter and the triplet. At the breakage point, a bonding strength of 460 MPa was evaluated for the interconnection between P(S-DVB) triplets. The high bonding strength of the interconnection could support the mechanism of pressurization. Densification of Li Deposits. The density of Li deposits after charging could be a key parameter that determines the performance of a battery. Inevitably, a deposit with lower density tends to result in a greater amount of dead Li, which does not participate in the deposition/stripping process any

distributed on the MSP Li metal anode, the growing of Li deposit could be allocated uniformly. Although a growing pressure of few GPa can be generated by a Li dendrite,35 the Li dendrite can be plastically deformed and densified by a stress less than the growing pressure because it has a very low yield strength of 0.655 MPa.34 Figure 1i presents the pressurized growth of a Li dendrite with an interconnected MSP layer. Even with a small inertia of individual microspheres, the MSP layer can provide sufficient stress to pressurize the Li dendrites because the microspheres are interconnected with each other. Mechanical Characterizations of Interconnected P(SDVB) Microspheres. The elastic modulus of P(S-DVB) microspheres was measured through in situ nanoindentations. As shown in Figure 2a, a P(S-DVB) singlet was aligned to a cono-spherical diamond tip with 1 μm radius before the indentation. In situ indentation tests were performed under the displacement-control mode to a maximum displacement of 100 nm. The corresponding load−depth curves are plotted in Figure 2b. To evaluate the elastic modulus of the P(S-DVB) microspheres, finite element method (FEM) simulations were conducted using a commercial software package, ABAQUS (ver. 6.10), with boundary conditions displayed in Figure 2c. The elastic modulus and Poisson’s ratio of the (100) silicon wafer were set as 165 GPa and 0.22, respectively.36 The indenter tip was assumed to be a rigid body. The elastic modulus of P(S-DVB) was set to be unknown, while the Poisson’s ratio was set as the value for polystyrene, 0.34.37 The surface of contact between the sphere and the rigid indenter tip was set to be frictionless. As shown in Figure 2d, the measured loading curves from the indentations were compared with simulated values, and the modulus of the microspheres was evaluated by minimizing the difference. The average value of the elastic modulus of P(S-DVB) microspheres was calculated as 3.91 GPa after the comparison. In addition, the elastic 5909

DOI: 10.1021/acs.chemmater.7b01304 Chem. Mater. 2017, 29, 5906−5914

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Figure 3. Density of Li deposit in a 200 mAh pouch-type Li/LiNiCoAlO2 (Li/NCA) full cell after charging at a current density of 0.29 mA/cm2 for 10 h (voltage window: 3.0−4.2 V vs Li+/Li, at room temperature). (a) Pouch cells with dimensions of x = 5.0 cm, y = 9.5 cm, and z = 1.0 mm were investigated to measure the densities of Li deposits. Changes in thickness (Δz) of the pouch cells with a bare Li metal (b) and with a MSP Li metal (e) were examined after charging. (c and d) Morphology of a Li deposit on a bare Li metal was observed. Top view (c) and cross-sectional view (d) of the deposit showed porous structures. (f and g) Morphology of a Li deposit on a MSP Li metal. Highly densified structures were observed in the top (f) and cross-sectional images (g). (h) The densities of Li deposits were calculated based on the thicknesses measured from the cross-sectional images.

Figure 4. Effect of sphere diameter on density of Li deposit. (a−e) Change in thickness (Δz) of a 200 mAh pouch-type Li/LiNiCoAlO2 (Li/NCA) full cell after charging at a current density of 0.29 mA/cm2 for 10 h (voltage window: 3.0−4.2 V vs Li+/Li, at room temperature). Li metal anodes were protected with various diameters (0.01−10 μm) of spheres. (f) The densities of Li deposits were calculated for various sphere diameters after charging.

the growth direction of Li deposits. The changes in thickness of pouch cells with bare Li and MSP Li metal anodes are plotted in Figure 3b,e, respectively, after charging at 0.1 C (0.29 mA/ cm2). As shown in Figure 3b, the pouch cell with a bare Li

more. As shown in Figure 3a, the changes in external thickness after charging at 0.1 C (0.29 mA/cm2) for 10 h are measured at 9 investigated points using a 200 mAh pouch-type full cell with 1.0 M LiFSI in DME-TTE electrolyte. The Z-axis represents 5910

DOI: 10.1021/acs.chemmater.7b01304 Chem. Mater. 2017, 29, 5906−5914

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Chemistry of Materials metal anode showed a large increase (67.8 μm in average) in thickness after charging (2.9 mAh/cm2). Moreover, the bare Li metal showed a nonuniform growth of Li deposit with a standard deviation of 8.4 μm in thickness. In contrast, the MSP Li metal showed a small increase in thickness (21.8 μm on average) with a uniform distribution (standard deviation of 1.1 μm) after the same amount of charging (2.9 mAh/cm2) (Figure 3e). After the Li metal anodes were disassembled from pouch cells, the thicknesses of Li deposits were measured under SEM observation. A thick and irregular Li deposit with micrometersized pores was observed on the bare Li metal anode, as shown in Figure 3c,d. In contrast, a uniform and dense Li deposit was acquired from the MSP Li metal anode, as shown in Figure 3f,g. Thus, Li dendrites were not observed in a Li deposit after charging when the Li metal anode was protected by microspheres. In addition, the layer of microspheres was kept intact after charging without the penetration of Li dendrites (Figure 3f). Li ions were uniformly and densely deposited beneath the MSP layer, and the MSP layer was lifted by the Li deposit. We have observed the morphology of cycled Li metal anodes after 10 cycles in Figure S4. As shown in Figure 3h, the densities of Li deposits were calculated using the measured thicknesses from the cross-sectional images. When microsphere protection was applied, the density of the Li deposit was greatly improved to 0.4 g/cm3, which was 3 times higher than that in the case of bare Li metal (0.12 g/cm3). The density value was close to the theoretical density of pure Li metal (0.534 g/cm3). The effect of individual components of MSP Li metal on density of Li deposit was explored in Figure S5. Synergies between microspheres and ion-conductive oligomers improved the density of Li deposits. Effect of sphere diameter on density of Li deposits was tested in Figure 4. The densities of Li deposits were calculated by measuring the change in external thickness of the pouch cell after charging as shown in Figure 4a−e. Interconnected P(SDVB) spheres with diameters of 1, 3, 5, and 9 μm were used as microspheres. Because nanosized P(S-DVB) was not available, Al2O3 nanospheres which have average diameters of 10 and 50 nm were used with poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) binder to examine nanometer ranges. The effect of sphere diameters is summarized in Figure 4f. The Li metal anode protected by microsize spheres showed higher density of Li deposit (>0.32 g/cm3) than that of Li deposit in nanosized sphere protected Li metal (>0.25 g/cm3) after the same amount of charging (2.9 mAh/cm2). The superior effect of microspheres on the density of Li deposit could be related to the diameter of the Li dendrite, which is in a range of 1−2 μm.3 Micropores created by microsized spheres, which are larger than the initial diameter of a Li dendrite (1−2 μm), made a space for Li dendrite growing. Also, the interconnected microspheres might pressurize the Li dendrite. Coulombic Efficiency in Cu/Li Cells. Cu/Li cells with microsphere protections were employed to investigate the cycling performance of Li depositing/stripping in Figure 5. Coulombic efficiencies were monitored at different current densities (1.0, 2.0, and 5.0 mA/cm2). At a current density of 1.0 mA/cm2, both MSP Cu and bare Cu retained high Coulombic efficiency (>99.8%) over 100 cycles. However, the Coulombic efficiency of bare Cu started to decrease after the 55th cycle as the current density increased over 2.0 mA/cm2. Surprisingly MSP Cu/Li cells showed very high stability of Coulombic efficiency at a high current density of 5.0 mA/cm2 over 200 cycles, whereas bare Cu/Li cells showed a rapid decrease of

Figure 5. Cycling performance of Cu/Li cells with microsphere protection. Coulombic efficiencies are monitored at different current densities of (a) 1.0 mA/cm2, (b) 2.0 mA/cm2, and (c) 5.0 mA/cm2. The amount of Li deposition in each cycle was fixed at 1.0 mAh/cm2. Voltage profiles at the 50th cycle for cells are plotted as insets.

Coulombic efficiency after the 50th cycle. Voltage profiles at the 50th cycle for cells were plotted as insets. Electrochemical Characterizations. A 20-μm-thick Li metal anode was adopted with a high areal capacity NCA cathode (2.9 mAh/cm2) into a 200 mAh pouch-type full cell (active area: 4.2 × 8.65 cm) to make practical Li metal batteries. The charge−discharge curves of the 200 mAh pouch-type full cell with a MSP Li metal anode are plotted for different cycle numbers in Figure 6a. The cells were operated in a voltage range of 3.0−4.2 V at a rate of 0.7 C (2.03 mA/cm2)/0.5 C (1.45 mA/cm2) (charge/discharge rate). The average voltage of the cell was approximately 3.65 V, and the discharge capacity after 150 cycles was greater than 175 mAh/g. Figure 6b compares the cyclability of a full cell with a MSP Li metal anode to that of a cell with a bare Li metal anode. The end condition for the cycling test was set to 90% of the initial discharge capacity for safety. The cell with a bare Li metal anode reached the end condition at 140 cycles, but the cell with an MSP Li metal anode retained its discharge capacity over the end condition even after 190 cycles. The cyclability of the cell was enhanced by 36% when the MSP Li metal anode was applied. The Coulombic efficiencies of the cells are shown in the inset of Figure 6b. Both cells with the MSP and bare Li metal anode 5911

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Figure 6. Electrochemical characterizations of 200 mAh pouch-type Li/LiNiCoAlO2 (Li/NCA) full cells with an MSP Li metal anode. (a) Charge− discharge curves of the full cell with a MSP Li metal anode. (b) Cyclability of the full cell. A charge/discharge rate of 0.7 C (2.03 mA/cm2)/0.5 C (1.45 mA/cm2) was used with a voltage window of 3.0−4.2 V vs Li+/Li. The tests were terminated at a capacity retention of 90% compared to the initial discharge capacity for safety. The Coulombic efficiency of the full cell is plotted in the inset. (c) Discharge capacity of the cell with various charge/discharge rates. (1 C = 2.9 mA/cm2). (d) Impedance spectrum of a Li/NCA full cell with an MSP Li metal anode.

more than 3 times that in the case of bare Li metal, were obtained after charging in a 200 mAh pouch-type Li/NCA full cell at 4.2 V. The cyclability of the cell was enhanced by 36% when an MSP Li metal anode was applied. We also suggested related mechanisms to suppress the growth of Li dendrites with microsphere protection. Selective oligomer coating, Li ion guiding, and dendrite pressurization are the key factors to obtain a high-density Li deposit. Our research suggests the possibility of realization of high-energy-density Li metal batteries with long-term cyclability and safety.

exhibit a high Coulombic efficiency greater than 99.7%. The rate capabilities of full cells with different discharging rates ranging from 0.2 to 1.5 C (0.58 mA/cm2 to 4.35 mA/cm2) at a fixed charging rate of 0.7 C (2.03 mA/cm2) are monitored to investigate the performance of cells at a high current density, as shown in Figure 6c. The discharge capacity of a cell based on the MSP Li metal anode was identical to that of a cell with a bare Li metal anode up to 1.5 C (4.35 mA/cm2). Electrochemical impedance spectroscopy was performed before a cycle and after 100 cycles with a pouch-type full cell. Pouch cells with bare Li anode and MSP Li metal anode are compared in Figure 6d. The cell with the MSP Li metal anode shows nearly the same bulk resistance as that with the bare Li metal and slightly higher interfacial resistance (0.5 Ω) compared to that with bare Li metal (0.3 Ω) before a cycle. After 100 cycles, the bare Li metal and MSP Li metal showed similar bulk resistance, but the interfacial resistance of bare Li metal was increased to 0.8 Ω, which is greater than that of MSP Li metal (0.6 Ω). Because the increase in interfacial resistance implies an expansion of the unstable interface between the electrolyte and Li metal, bare Li metal could reach failure earlier. In order to verify the effect of the Li metal anode separately, without considering the effect of the cathode, interfacial resistance was measured in a Li/Li symmetric cell (Figure S6).

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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/acs.chemmater.7b01304. Thermal characterization of interconnected P(S-DVB) microspheres, surface morphology of MSP Li metal anode by AFM, mechanical characterizations of ionconductive oligomer, density of Li deposits after charging with individual components of MSP Li metal, and impedance spectrum of Li/Li symmetric cell with MSP Li metal anode (PDF)

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CONCLUSIONS We achieved dendrite-free Li deposition in a Li metal battery by applying microsphere protection. With microsphere protection, dendrite-free Li deposits having a density of 0.4 g/cm3, which is

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.-M.L.). *E-mail: [email protected] (J.-Y.S.). 5912

DOI: 10.1021/acs.chemmater.7b01304 Chem. Mater. 2017, 29, 5906−5914

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Chemistry of Materials ORCID

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Jeong-Yun Sun: 0000-0002-7276-1947 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by funds from Samsung Electronics Co. Ltd. J.-Y.S. and J.W. acknowledge support from a National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (No. 2016R1C1B2007569). J.Y.S. acknowledges support from the Creative-Pioneering Researchers Program through Seoul National University (SNU). H.N.H. was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT & Future Planning (MSIP) (No. NRF2015R1A5A1037627).

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DOI: 10.1021/acs.chemmater.7b01304 Chem. Mater. 2017, 29, 5906−5914

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DOI: 10.1021/acs.chemmater.7b01304 Chem. Mater. 2017, 29, 5906−5914