Electrochemical Behavior of Li–Cu Composite Powder Electrodes in

Jun 13, 2017 - ... Behavior of Li–Cu Composite Powder Electrodes in Lithium Metal Secondary Batteries. Sun Woo ... Energy Storage Materials 2018 15,...
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Electrochemical Behavior of Li−Cu Composite Powder Electrodes in Lithium Metal Secondary Batteries Sun Woo Hwang, Jee Ho Yom, Sung Man Cho, and Woo Young Yoon* Department of Materials Science and Engineering, Korea University, 1, 5Ga, Anam-dong, Sungbuk-Gu, Seoul 136-701, Republic of Korea S Supporting Information *

ABSTRACT: A new type of Li−Cu composite powder electrode (Li−Cu CE) was fabricated via mechanical blending of Li and Cu powders. The new Li−Cu electrode is capable of replacing Li metal anodes in Li metal secondary battery (LMSB) systems without exhibiting typical intrinsic problems such as dendrite growth, volume change, and electrolyte depletion. Thus, Li−Cu CE cells can offer longer lives and very high capacities. The Li particles in Li−Cu CEs are surrounded by Cu particles and thus cannot form networks that extend throughout the electrode. Isolation of the Li powder enables the control of Li+ ion migration during deposition and dissolution. The Li−Cu CE can thus prevent problematic volume changes and dendrite growth on the anode during battery operation. Symmetric Li−Cu CE cells are stable for up to 200 cycles at a rate of 3 C, and the anode capacity is estimated to be 1158 mAh g−1 (Li+ ion usage of 30%). These results are thought to represent a largest anode capacity for Li-metal in LMSBs. KEYWORDS: lithium metal, electrodes, Li metal secondary batteries, composites, microstructures The most common Li metal anode (∼100 μm of Li foil) does not typically achieve its capacity which is known as 3860 mAh g−1. Several previous reports detailed attempts to restrict the capacity of Li metal to ∼193 mAh g−1 (convert Li to Li+ ions at ∼5%) and determine its electrochemical properties.17−19 An actual Li metal capacity greater than 965 mAh g−1 is needed for post-Li battery types such as Li−S and Li−air (this requires increasing Li+ ion usage to 25%) that meet the rational capacity ratio of negative-to-positive electrodes (N/P ratio) (i.e., the correct ratio of cathode and anode materials).20−23 Therefore, research has focused on the problematic Li metal volume changes that occur during battery cycling and adjusting the Li usage to improve the suitability of Li metal anodes, as well as the dendrite growth problem.12 Increasing the real active capacity values of the anodes has been linked to dendrite growth. Studies have been conducted in which Li+ ion usage was increased in order to better exploit the high capacity that Li offers.24,25 These studies show that Li powder is a better dendrite growth retardant than plain Li foil under high current density conditions.26,27 However, a bulk network, followed by dendrite growth, was seen on the Li powder surface when the Li-ion content was increased by more than 25%.28 This structural change, which occurs during charge−discharge cycles, was investigated to determine how Li metal stacking can be avoided. Previous research has shown that a modified Cu

1. INTRODUCTION The lithium metal secondary battery (LMSB) is a powerful energy storage system.1,2 Li is a promising anode material for future high-performance battery systems because it offers a large theoretical capacity (3860 mAh g−1), a low density in its elemental form (0.534 g cm−3), and a large, negative standard reduction potential [−3.045 V vs the standard hydrogen electrode (SHE)].3−5 Despite these advantages, the commercialization of Li metal anodes has been hampered by the tendency of lithium dendrites to grow from their surfaces, particularly during fast charging and cycling.6 Dendrite growth limits the use of Li metal anodes because of both reduced capacities and safety problems, such as short-circuits.7 Dendrite growth occurs because of the focalization of high current densities during the electrochemical reaction. 8 During delithiation at high current densities, focalized Li+ ions are deposited on the metal surface. Li deposition can produce bulk networks of Li dendrites, thus reducing the capacity of the cell. When dendrites bridge the space between electrodes, electrical shorts can occur, causing overheating and sometimes even fire.9,10 Formation of an unstable SEI (solid-electrolyte interface) layer can also lead to dendrite growth.11 Therefore, controlling the structure of the SEI layer to produce a stable interface is a promising method to suppress dendrites.12 Another solution to the dendrite problem is to enclose lithium metal inside a carefully structured electrode.13,14 Such structured electrodes limit Li+ ion deposition to external Li sites, thereby preventing the development of a network of Li dendrites upon Li+ deposition.15,16 © 2017 American Chemical Society

Received: April 11, 2017 Accepted: June 13, 2017 Published: June 13, 2017 22530

DOI: 10.1021/acsami.7b04577 ACS Appl. Mater. Interfaces 2017, 9, 22530−22538

Research Article

ACS Applied Materials & Interfaces

Figure 1. SEM image of (a) Li and (b) Cu powders. (c) SEM image of a Li−Cu composite electrode with an overlaid linear EDX profile. (d) Linear Cu K emission EDX data.

Figure 2. SEM and EDX mapping images of (a, b, and c) pristine, (d, e, and f) singly discharged, and (g, h, and i) singly charged Li−Cu CEs.

collector.30−32 That means converted current collector is difficult to prevent the dendrite growth perfectly. In ths study, we developed a Li−Cu composite powder electrode (Li−Cu CE) as a new anode material and investigated its electrochemical properties. Li and Cu powders were mechanically mixed and pressed to be in electrical contact

current collector nano structure is more effective than plain Cu at reducing over potential when Li ions are deposited.29−32 However, this experiment restricted Li+ ion usage to less than ∼1% when long cycles were tested. Thus, the converted Li metal capacity was ∼38.6 mAh g−1. When the Li+ ion usage is increased, the Li+ ions deposition occurs on the Cu current 22531

DOI: 10.1021/acsami.7b04577 ACS Appl. Mater. Interfaces 2017, 9, 22530−22538

Research Article

ACS Applied Materials & Interfaces

Figure 3. Schematic illustration of the deposition and dissolution of Li+ ions in the Li−Cu CE.

with each other.31 In this system, Cu is inert and does not participate in the electrochemical reactions.33 Since Cu is electrically conductive, it produced increased stability during high current density battery cycling.34 The Cu structure prevented the growth of bulk networked dendrites from isolated Li particles during Li+ ion deposition. Isolation of the Li powder within the Cu structure controlled the migration of Li+ ions, leading to the generation of a stable SEI layer on the Li−Cu CE.20,35 Our new anode material allowed the Li metal capacity to reach 1158 mAh g−1 (Li+ ion usage increased to 30%) at high current densities (rates of up to 3 C) without dendrite growth or bulk anode volume changes. In addition, Li−Cu CE was used as an anode material in a Li metal/lithium vanadate (LVO) full cell battery. LVO is well-known as a nonlithiated cathode material, without Li+ ion sources. Therefore, a LVO cathode is acceptable in a LMSB system.36 We compared the real Li metal capacities of Li−Cu CE/LVO and Li foil/LVO full cell batteries in constant current density conditions (rates of up to 1 C). The Li−Cu CE full cell exhibited a Li metal capacity of ∼1200 mAh g−1, while the Li foil full cell offered only ∼26 mAh g−1.

the Li particles are well-dispersed in the Cu powder, ensuring that Li cannot form a bulk network that extends throughout the electrode. SEM and EDX mapping images show the Li−Cu CE symmetric cell morphology changes during a charge/discharge reaction.37 Figure 2a shows the morphology of a pristine Li− Cu CE before charging or discharging. The EDX map presented in Figure 2b distinguishes between the Li and Cu particles, and confirms Li particle isolation by revealing the particle type distributions. EDX maps are overlaid onto their respective SEM images in Figure 2c, which shows Li particles forming domains approximately 40−50 μm in size. However, each domain is clearly surrounded by Cu particles and thus isolated from other Li domains. Figure 2 (panels d−f) and 2 (panels g−i) shows SEM images of Li−Cu CEs after discharging and charging, respectively. Discharging and charging the Li−Cu CE symmetric cell at a rate of 3.0 C requires 30% usage of Li+ ions (the real capacity is ∼1158 mAh g−1). Overlaying the Cu EDX maps onto their respective SEM images reveals that the Li+ ions are dissolved on the Li particles. The distorted spherical Li particles shown in Figure 2f confirm that only the Li particles participate in dissolution/deposition during electrochemical charging/discharging, while Cu does not. After charging, the Li+ ions are redeposited on the Li particles rather than the Cu particles, as shown by the reformation of Li spheres (Figure 2i). Dendrite growth is not observed on the Li−Cu CE surface after the first charging step. We fabricated an optical cell (Figure S1) and filmed the transient Li+ ion deposition (Movie S1) to confirm that Li+ deposits only on Li particles and not on Cu.38−41 The video shows that Li+ ions are deposited only on the Li foil (forming dendrites) and not on the Cu foil (Figure S2) when a current of 0.1 mA is applied. This result indicates that the deposition of Li+ ions on the Li−Cu CE surface should not affect the Cu structure because the Li+ ions prefer to deposit on the kink site, which is formed by the Li powder dissolution.42 Therefore, the Li−Cu CE effectively prevents dendrite growth, and thus is capable of stable cycling when the Li+ ion usage is increased to 30%. Dendrite prevention occurs because the Li particles are isolated from each other during Li+ ion deposition and dissolution. A schematic illustration of the morphology change

2. RESULTS AND DISCUSSION 2.1. Li−Cu Composite Electrode (Li−Cu CE) Analysis. We investigated the morphologies of the Li and Cu powders via SEM. Li powder was produced using the droplet emulsion technique.25 The resulting Li particles were spherical, with estimated diameters of ∼10 μm, as shown in Figure 1a. SEM images of the pristine Cu powder in Figure 1b show that the Cu particles exhibit rough surfaces and diameters of approximately 10−20 μm.34 Full electrode fabrication details are given in the Experimental Section. Briefly, a composite of the Li and Cu powders was formed via mechanical mixing using a Wye-tube. A Li−Cu CE was fabricated from this composite via pressing. As shown in Figure 1c, Li and Cu powders are well mixed in the Li−Cu CE, with the two metals easily distinguished by their distinct shapes, which remained intact after mixing. We verified the compositions of the Li and Cu powders more clearly by measuring the linear EDX profile of a Li−Cu CE. Figure 1d shows the Cu K emission EDX data from the SEM image of the Li−Cu CE sample shown in Figure 1c. The data confirms that 22532

DOI: 10.1021/acsami.7b04577 ACS Appl. Mater. Interfaces 2017, 9, 22530−22538

Research Article

ACS Applied Materials & Interfaces

Figure 4. Scanning electron microscopy images of (a) pristine, (b) singly discharged, and (c) singly charged Li−Cu CEs recorded at a 30° tilt.

Figure 5. Voltage graphs from (a) Li foil, (b) LPE, and (c) Li−Cu CE symmetric cells, and (d) a comparison of the cycling properties of each cell. EIS analyses of (e) Li foil and (f) LPE and Li−Cu CE samples, respectively.

growth and enables stable electrochemical reactions that use a higher proportion of Li+ ions. We acquired SEM images of pristine, discharged, and charged Li−Cu CEs at an angle of 30° (Figure 4) to clearly show this redeposition behavior. The SEM images reveal that the dissolution of Li+ ions leads to particle shape distortions (Figure 4b). Figure 4c shows that the spherical Li particles reform after charging, suggesting that Li+ ions are preferentially redeposited on the distorted region. These results confirm that the Li−Cu CE structure leads to

during charging and discharging of our Li−Cu CE is shown in Figure 3. After discharging, Li+ ions are dissolved from the spherical Li particles, which become distorted (represented by the depressions in the Li particles in Figure 3).6 During charging, Li+ ions are fully redeposited on the distorted Li particles, which regain their original shape because the ions preferentially redeposit in the distorted regions.43,44 This, a new method of controlling the migration of Li+ ions, has been demonstrated. This redeposition behavior represses dendrite 22533

DOI: 10.1021/acsami.7b04577 ACS Appl. Mater. Interfaces 2017, 9, 22530−22538

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ACS Applied Materials & Interfaces

Figure 6. Scanning electron microscopy images of pristine (a) Li foil, (c) LPE, and (e) Li−Cu CE samples, before and after (b) 18, (d) 41, and (f) 200 cycles, respectively. The Cu peak is identified via (g) EDX mapping and (h) overlaid on the Li−Cu CE SEM image.

stable Li+ ion dissolution and deposition, without dendrite growth. 2.2. Electrochemical Properties. Symmetric cells were cycled to characterize their electrochemical properties and cycling stabilities, thus confirming that Li−Cu CE is an effective high current density Li metal electrode. Figure 5 depicts the varying electrochemical properties of symmetrical Li−Cu CE cells, as well as cells made with Li foil and powder electrodes (LPE). The Li+ ion usage and current density were 30% and 3.0 C, respectively. The battery voltages increased sharply to 5.0 V after 18 and 41 cycles for the Li foil and LPE cells, respectively. This suggests that dendrite formation occurred unhindered during cycling, caused by a reaction between the Li metal and electrolyte at the electrode surface.27 When a symmetric Li−Cu CE cell is cycled, a stable reaction with no sudden voltage increase is observed for up to 200 cycles (Figure 5, panels c and d). This phenomenon was confirmed by electrochemical impedance spectroscopy (EIS) data.45,46 After the first cycle, the charge transfer resistance (Rct) increases when Li foil or LPE is used in the symmetric battery. However, the Li−Cu CE symmetric battery exhibits no sharp resistance change after the first cycle (Figure 5, panels e and f). To confirm more detailed EIS, the resistance change is not observed until 50th in the Li− Cu CE battery (Figure S3). Which means that the Li−Cu CE forms a stable SEI layer without electrolyte depletion.47,48 These results show that the reaction that deposits Li+ ions on isolated Li sites in the Li−Cu CE remains stable as the battery cycling. Li particles isolated in the Cu structure may direct the flow of Li+ ions during dissolution and deposition and prevent Li particle volume changes and bulk network formation during cycling. This observation was confirmed using SEM images of the Li foil, LPE, and Li−Cu CE samples. We examined the surfaces of the pristine electrodes before (Figure 6, panels a, c, and e) and after cycling (Figure 6, panels b, d, and f) to compare their morphological changes. A large quantity of postbattery failure dendrites is visible on the clean Li foil surface after 18 cycles (Figure 6b). The dendrites interfere with the reactions in the battery, significantly reducing its cycle life and safety. A comparison of the LPE SEM images before (Figure 6c) and after 41 cycles (Figure 6d) reveals that some Li particles retained their spherical shape, although most exhibit modified morphologies. Furthermore, the dendrite growth that

produced an unexpected voltage increase after 41 cycles appears at random points on the Li particle surfaces. However, the Li−Cu CE exhibits only limited dendrite growth after 200 cycles (Figure 6f). The Li−Cu CE surface becomes roughened after 200 cycles. However, Li+ ions are typically deposited on the isolated Li particles, which regain their sphericities without volume changes. The resulting spherical Li particles were identified by overlaying Cu EDX maps onto their respective SEM images, as described previously (Figure 6h). The presence of spherical Li particles suggests that the Li−Cu CE cells exhibit stable cycling performance with increased Li+ ion usage. The SEM images also show that the Cu particle morphologies are unaffected by cycling, which indicates that they remain tightly bound to the Li particles after 200 cycles and therefore that no distinct volume change occurs at the Li−Cu CE electrode. These results demonstrate that the Li−Cu CE exhibits excellent electrochemical properties under conditions of high current density and increased Li+ ion usage because the Li+ deposits onto isolated Li particles.15 Deposition onto isolated Li sites impedes bulk networking of the Li powder. Thus, indiscriminate dendrite growth on the Li−Cu CE surface is prevented. Also to confirm the volume change of electrode, we analyze the cross section SEM images of pure, first delithiated, first lithiated, and 100th lithiated Li−Cu CE, respectively (Figure S4). It seems that the thickness of each Li− Cu CEs are estimated approximately similar. That means, the Li−Cu CE is able to suppress the volume change of the Li metal electrode. The stability of the Li−Cu CE during charging and discharging was also confirmed using rate capability tests (Figure 7). These tests show that a symmetric Li−Cu CE cell works well at all applied current densities (0.2, 0.5, 1.0, and 3.0 C). In these measurements, the battery capacity is 1158 mAh g−1 and the Li+ ion usage is 30%. Thus, we conclude that the Li−Cu CE exhibits stable cycling without dendrite growth at various current densities. To further test the electrochemical properties of Li−Cu CE as an anode material, a Li−Cu CE/LVO full cell was assembled, as shown in Figure 8a. The Columbic efficiency is slightly higher with a Li−Cu CE than with Li foil as the LVO full cell electrode. This shows that the Li−Cu-based battery has no anodic dendrite growth, thus reducing capacity loss caused by 22534

DOI: 10.1021/acsami.7b04577 ACS Appl. Mater. Interfaces 2017, 9, 22530−22538

Research Article

ACS Applied Materials & Interfaces

Table 1. Summary of the Densities of the Electrodes and Capacities of the Full Cell Batteries Li foil/LVO full cell battery

Li−Cu CE/LVO full cell battery

14Φ Li anode unit density (absolute capacity) 14Φ LVO cathode unit density (absolute capacity) N/P ratio 1st discharge unit capacity (Li+ ions unit capacity usage = theoretical capacity × 100 )

9.2 mg cm−2 0.21 mg cm−2 (54.7 mAh) (1.24 mAh) 1.4 mg cm−2 (0.60 mAh) 91:1 26.0 mAh g−1 (0.68%)

2.1:1 1244 mAh g−1 (32.2%)

50th discharge unit capacity (Li+ ions usage)

23.8 mAh g−1 (0.67%)

1158 mAh g−1 (30.0%)

mAh g−1 capacity), far exceeding the usual limit of 25%.24,25 Top-down and tilted SEM images and EDX maps were collected to identify the mechanisms of dendrite growth. The dissolution and deposition of Li+ ions did not cause dendrite growth in the symmetric Li−Cu CE, while the symmetric Li foil and LPE cells displayed severe dendrite growth. We suggest that this growth was caused by the random deposition of Li+ ions in the Li foil and LPE-based cells. Also, the Li+ ion usage in the Li−Cu CE/LVO system is 30%, while that of the Li foilbased full cell is only 0.6%. Thus, Li−Cu CE is an attractive replacement for Li metal anodes and a promising future LMSB anode system. However, improvements of specific structure that contain isolated Li particles are required in order to achieve the desired increases in battery performance and Li+ ion usage. We will report on our attempts to improve Li particle isolation in due course.

Figure 7. Rate capability profile of a symmetric Li−Cu CE cell.

side reactions with the electrolyte.7 However, the cathode capacity cycling profile is not enough to confirm Li+ ion usage. The Li−Cu CE/LVO full cell N/P ratio is 2.1:1, while that of the Li foil-based battery is 91:1. The values are listed in Table 1. Theoretically, only 1% of the Li ions in the LVO cathode Li foil were used. The real Li usage information is shown in Figure 8b. The graph shows that the real discharge capacity of the Li−Cu CE full cell is estimated to be 1244 mAh g−1, while that of the Li foil-based cell is approximately 26 mAh g−1. Thus, the Li− Cu CE can produce a specific capacity 50 times larger than that of ordinary Li foil, without dendrite growth.

3. CONCLUSION A new anodic Li−Cu CE was fabricated from a mixture of spherical Li and Cu particles using a mechanical pressing method. The Li−Cu CE was found to both reduce dendrite growth and increase Li+ ion usage during tests using symmetrical Li−Cu CE cells. Good contact between Li and Cu particles was confirmed via SEM imaging. The isolated Li particles limited Li+ ion deposition and allowed a stable surface reaction to occur during cycling. Isolation of Li particles also prevented the formation of a bulk Li network and thus inhibited dendrite growth on the Li−Cu CE surface. A symmetric Li−Cu CE cell was tested at 3.0 C and 30% Li+ ion usage. The Li−Cu CE exhibited a stable voltage profile over 200 cycles without distinct electrode volume changes, while the Li foil and LPE cells degraded after 18 and 41 cycles, respectively. These results show that the Li−Cu CE is capable of increasing the Li+ ion use to 30% (as indicated by its 1158

4. EXPERIMENTAL SECTION 4.1. Electrode Fabrication. Li powder made from spherical particles was synthesized using the droplet emulsion technique (DET), as schematically illustrated in Figure 9a.25 Spherical Li powder (∼10 μm in diameter) and amorphous Cu powder (Sigma-Aldrich, USA, ∼20 μm in diameter) were placed into the tips of each side of a wye tube (Y-Tube) and mechanically mixed (1:1 Li/Cu by volume). The tube was rotated at 1000 rpm for 24 h to produce a mechanically combined Li−Cu powder mixture. Li−Cu powder and polyvinylidene fluoride (Sigma-Aldrich) were suspended in triethyl phosphate (Alfa Aesar, 99%) at a 10:1 ratio (by weight) to make a Li−Cu slurry. The slurry was then mixed in a planetary centrifugal mixer (THINKY) at 700 rpm. The Li−Cu slurry was cast onto the Cu current collector (the Cu foil thickness was estimated to be 20 μm) and dried overnight in a vacuum oven at 70 °C (Figure 9b). The whole electrode was pressed at 980 kPa to improve the contact between the active materials and the current collector. The morphology of the Li−Cu CE was

Figure 8. Capacity profiles of Li−Cu CE/LVO and Li foil/LVO full cell batteries. Converted (a) LVO and (b) Li unit capacities. 22535

DOI: 10.1021/acsami.7b04577 ACS Appl. Mater. Interfaces 2017, 9, 22530−22538

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Figure 9. (a) Schematic illustration of the droplet emulsion technique (DET). (b) Schematic illustration of the fabrication of a Li−Cu CE.

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confirmed via scanning electron microscopy (SEM), which revealed that the powders were both well mixed. All experiments and fabrication steps were conducted in a room devoid of moisture. 4.2. Test Cell Assembly. We fabricated pure Li metal (Li foil) and Li powder electrodes (LPE) to compare their electrochemical properties. The LPE was fabricated using a specially manufactured mold. First, Li powder (25 mg) was loaded onto the SUS-mesh of the current collector.49 The powder was packed into the mold and pressed at 1960 kPa to produce a coin-shaped electrode. Symmetric cells, containing pure Li foil, LPE, or Li−Cu CE, were assembled in cointype batteries (CR 2032) so that the electrochemical properties of each material could be compared.50 The diameters of the counter and working electrodes were 1.6 and 1.4 cm, respectively. LiPF6 (1 M) in an ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC) mixture (1:1:1 by volume, Solbrain, Republic of Korea) was used as an the electrolyte.51 An LVO cathode was prepared for the Li metal/LVO full cell battery. To make the LVO slurry, LVO powder (∼10 μm in diameter), Ketchen black (KB), and PVdF were dissolved in N-methyl 2pyrrolidone (NMP, Sigma-Aldrich) at a ratio of 80:15:5 by weight. This slurry was then mixed in a planetary centrifugal mixer at 2000 rpm. The LVO slurry was cast on the Al foil (thickness estimated to be 18 μm) and dried for 12 h in a vacuum oven at 100 °C. The Li metal/ LVO full cell batteries were assembled in CR 2032 coin-type cells. The diameters of the Li foil and Li−Cu CE anodes were 1.6 cm, while the LVO cathodes were 1.4 cm in diameter. The 1 M LiPF6 solution in EC/DMC/EMC (1:1:1 by volume) was used as the electrolyte. Electrochemical properties such as voltage profiles and cycling data were analyzed galvanostatically using a WBCS 3000 battery cycler (WonATech, Republic of Korea). The Li metal symmetric batteries were tested at a rate of 3 C, and the usage capacity was settled at 1158 mAh g−1 for each batteries. The voltage cutoff was 5 V.52 Li metal/ LVO full cell batteries were tested at 1 C, with a voltage range of 2.4 to 4.0 V.53 SEM and energy dispersive X-ray (EDX) mapping were performed to examine the surface morphologies of the electrodes. EIS analysis was checked for charge transfer resistance, with measurements taken at 10−2 to 105 Hz and an AC amplitude of 5 mV.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Woo Young Yoon: 0000-0002-8482-9769 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by a grant from the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (2016R1A2B3009481), and the Ministry of Knowledge Economy (MKE, Korea) (10045221) SEM microstructural examinations were performed at the Korea Basic Science Institute, Seoul Center.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04577. Detailed experimental layout of Li+ ion deposition (Figure S1) and its optical microscope observations (Figure S2). Nyquist plot of long cycled Li−Cu CE is analyzed (Figure S3). The cross section back scattered electron microscope images of Li−Cu CE (Figure S4) (PDF) Optical microscope observations of Li+ ion deposition (AVI) 22536

DOI: 10.1021/acsami.7b04577 ACS Appl. Mater. Interfaces 2017, 9, 22530−22538

Research Article

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DOI: 10.1021/acsami.7b04577 ACS Appl. Mater. Interfaces 2017, 9, 22530−22538