Hierarchically Bicontinuous Porous Copper as Advanced 3D Skeleton

Mar 30, 2018 - ... large pore size and relatively low specific surface area. Herein, for the first time, we revisit CF and address these issues by rat...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 13552−13561

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Hierarchically Bicontinuous Porous Copper as Advanced 3D Skeleton for Stable Lithium Storage Xi Ke,†,∥ Yifeng Cheng,†,∥ Jun Liu,† Liying Liu,† Naiguang Wang,† Jianping Liu,† Chunyi Zhi,‡ Zhicong Shi,*,† and Zaiping Guo*,§ †

Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, Smart Energy Research Centre, School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China ‡ Department of Materials Science and Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong 999077, China § Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong, Innovation Campus, North Wollongong, NSW 2500, Australia S Supporting Information *

ABSTRACT: Rechargeable lithium metal anodes (LMAs) with long cycling life have been regarded as the “Holy Grail” for high-energy-density lithium metal secondary batteries. The skeleton plays an important role in determining the performance of LMAs. Commercially available copper foam (CF) is not normally regarded as a suitable skeleton for stable lithium storage owing to its relatively inappropriate large pore size and relatively low specific surface area. Herein, for the first time, we revisit CF and address these issues by rationally designing a highly porous copper (HPC) architecture grown on CF substrates (HPC/CF) as a three-dimensional (3D) hierarchically bicontinuous porous skeleton through a novel approach combining the self-assembly of polystyrene microspheres, electrodeposition of copper, and a thermal annealing treatment. Compared to the CF skeleton, the HPC/CF skeleton exhibits a significantly improved Li plating/stripping behavior with high Coulombic efficiency (CE) and superior Li dendrite growth suppression. The 3D HPC/CF-based LMAs can run for 620 h without short-circuiting in a symmetric Li/Li@Cu cell at 0.5 mA cm−2, and the Li@Cu/LiFePO4 full cell exhibits a high reversible capacity of 115 mAh g−1 with a high CE of 99.7% at 2 C for 500 cycles. These results demonstrate the effectiveness of the design strategy of 3D hierarchically bicontinuous porous skeletons for developing stable and safe LMAs. KEYWORDS: lithium metal anodes, 3D skeletons, hierarchically bicontinuous, polystyrene microspheres, electrodeposition

1. INTRODUCTION The development of power sources with high energy density has become more and more important in recent years with the exploding demands for portable electronics, electric vehicles, and grid-scale energy storage. Currently, rechargeable lithiumion batteries (LIBs) have been regarded as the state-of-the-art energy-storage devices, owing to their high energy density, high specific power, and long cycle life.1−3 Unfortunately, the energy density of LIBs is now approaching their theoretical limits owing to the relatively low capacity of the cathode and anode materials. In this context, lithium metal batteries (LMBs), such as Li−sulfur (Li−S)4,5 and Li−air (Li−O2)6,7 batteries, have attracted considerable attention because Li metal anodes (LMAs) possess a number of advantageous characteristics, including extremely high theoretical specific capacity (3860 mAh g−1), the lowest known negative electrochemical potential (−3.04 V vs the standard hydrogen electrode), and low density (0.534 g cm−3).8−11 Its widespread application has been severely impeded, however, by the problems of lithium dendrite growth,12,13 which may lead to serious safety concerns, and low © 2018 American Chemical Society

Coulombic efficiency (CE) during repeated Li plating/stripping processes, all of which stem from fundamental issues concerning the intrinsic reactivity of Li metal with the electrolyte at low potentials14 and nonuniform lithium plating,15 as well as the poor mechanical stability of the solid−electrolyte interphase (SEI).16,17 To address these issues for LMAs, a large number of strategies, such as adding electrolyte additives,18−21 construction of artificial SEI film on Li metal,22−25 inhibition of space charge layer formation,26,27 regulation of Li metal nucleation,28−30 reducing effective current density,31−33 and designing stable hosts,34−36 have been developed to stabilize lithium plating and suppress lithium dendrite growth. Alternatively, it is also very effective to employ large-surface-area conductive scaffolds with micro-/nanostructures as three-dimensional (3D) porous skeletons for LMA applications. For example, Received: February 2, 2018 Accepted: March 30, 2018 Published: March 30, 2018 13552

DOI: 10.1021/acsami.8b01978 ACS Appl. Mater. Interfaces 2018, 10, 13552−13561

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ACS Applied Materials & Interfaces graphitized carbon fibers37 and copper skeletons38−40 with 3D porous structures have been demonstrated to effectively accommodate lithium and address the issue of Li dendrite growth. It is thus generally concluded that the porous conductive architecture can be employed as candidate 3D skeletons under conditions of large pore volume, proper pore size in the submicron range, and high electroactive surface area because a 3D skeleton with such a pore structure is beneficial for stable and reversible Li plating/stripping.38−40 From this point of view, commercially available metal foam skeletons, such as copper foam (CF) and nickel foam (NF), are not normally regarded as suitable 3D porous skeletons for LMAs owing to their inappropriately large pore size and relatively low surface area. Previous studies have shown that the CF or NF skeletons exhibit relatively low CE and short cycle life, with a high tendency toward lithium dendrite growth and dead lithium formation.38,39,41 Nevertheless, the metal foams still stand a chance of acting as skeletons in LMA configuration through rationally constructing their pore structures so as to effectively enlarge their electroactive surface areas to lower the local current density and delay the growth of lithium dendrites during Li plating/stripping. Colloidal crystals are one promising class of materials formed through self-assembly from monodisperse colloidal building blocks, such as polystyrene (PS) and silica microspheres, which have long been used as effective templates for the fabrication of a wide range of advanced porous materials at low cost and on a large scale.42 Normally, the 3D colloidal crystal template technique is employed in the fabrication of porous materials with long-range-ordered porosity and well-defined pore size, namely, an inverse opal structure in most cases, which generally involves a three-step procedure, including first the selfassembling of colloidal microspheres into a colloidal crystal, then infiltrating or depositing functional materials into the interstitial void spaces in the colloidal crystal template, and finally removing the colloidal crystal template.43,44 The fabrication of such porous materials is usually carried out on flat solid substrates, however, such as ordinary glass slides,43 indium-doped tin oxide glass,45 and gold-coated silicon wafers,46 and the self-assembly of colloidal crystal templates on porous curved substrates with irregular surfaces such as metal foams has rarely been reported. Herein, we extend the colloidal crystal template technique to assist in the fabrication of 3D highly porous copper (HPC) frameworks grown on commercially available CF substrates (HPC/CF) via a four-step procedure (Figure 1). Specifically, synthetic PS microspheres are first self-assembled on a CF substrate (PS/CF). Second, the copper electrodeposition process is carried out on the asprepared PS/CF substrate to yield a copper-deposited PS/CF substrate. Third, the copper-deposited PS/CF substrate is washed with chloroform to remove the colloidal crystal template to produce a relatively ordered porous copper structure on the CF substrate. Finally, the obtained samples are subjected to a thermal annealing treatment to simultaneously coarsen the copper ligaments and enlarge the pore size, to create an HPC film that seamlessly integrated with the CF substrate (HPC/CF). The as-fabricated HPC/CF composite fulfills the aforementioned criteria required for potential 3D porous skeletons for high-performance LMAs, considering that such a 3D hierarchically bicontinuous porous copper architecture may provide a proper pore structure as well as intrinsically superior electrical conductivity.

Figure 1. Schematic illustration of the fabrication procedure for the 3D HPC/CF composite.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Polystyrene (PS) Microspheres. Briefly, 100 mL of water, 5 mg of sodium dodecyl sulfate, 32 g of styrene (previously extracted three times with 0.1 M NaOH and three times with distilled water), and 0.1 g of α-methylacrylic acid were added into a 500 mL three-necked round-bottom flask with a magnetic stir bar and a N2 inlet. The mixture was purged with N2 for 30 min with magnetic stirring. After the temperature was raised to 80 °C, 0.1 g of potassium persulfate dissolved in 3 mL of water was rapidly added to the mixture. The mixture was then kept stirring for 8 h under N2 atmosphere. Finally, the suspension was diluted to 3 wt % PS microspheres. The average diameter of the PS microspheres was about 500 nm (Figure S1). 2.2. Fabrication of 3D HPC/CF Skeletons. Copper foam (CF) substrates were cut into 1 × 5 cm2 pieces and washed successively in acetone, ethanol, and distilled water for 30 min. The clean CF substrates were then immersed in a dilute hydrochloric acid solution for 30 min to remove the native oxide on the copper surface. The pretreated CF substrates were then immersed in the monodispersed PS microsphere solution at 70 °C for 2 h. After the self-assembly process, the CF substrates modified with the PS colloidal crystal template were used as the working electrodes for copper electrodeposition in a three-electrode system, with two Pt mesh electrodes and a saturated calomel electrode as the counter and the reference electrode, respectively. The electrolyte was an aqueous solution of CuSO4 (16 g L−1) and H2SO4 (0.12 g L−1). The electrodeposition process was carried out in galvanostatic mode at a current density of 12 mA cm−2 for 60 min at room temperature. After copper electrodeposition, the electrodes were rinsed with chloroform to first remove the PS microspheres. Finally, the electrodes were annealed successively at 120 °C for 30 min, 300 °C for 1 h, and 450 °C for 2 h in a 5% H2/Ar mixture. The as-prepared 3D HPC/CF composites were cut into square disks (0.8 cm2) as skeletons for LMAs after vacuum drying. The areal weights of the as-obtained CF and HPC/CF skeletons were 63.1 and 75.9 mg cm−2, respectively. 2.3. Characterization and Electrochemical Measurements. The crystal structures of the samples were analyzed by X-ray diffraction (XRD, Rigaku D/max-2200/PC) using a Cu Kα radiation source at a wavelength of 0.1541 nm. The samples at different fabrication stages and the LMAs were characterized by field-emission scanning electron microscopy (SEM, JEOL, JSM-6700F). Specifically, for the ex situ characterization of the LMAs, the coin cell batteries containing them were first disassembled in a glovebox to obtain the LMAs, which were rinsed with 1,3-dioxolane/dimethoxyethane (DOL/DME) solvents to remove residual electrolyte and lithium salt and then dried in a glovebox. Finally, the LMAs were sealed in a plastic box in the glovebox to avoid air exposure during transfer to the vacuum chamber of the SEM. The electrochemical performances of the 3D HPC/CF and the CF skeletons for lithium storage were characterized in CR2032-type coin 13553

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ACS Applied Materials & Interfaces cells, which were assembled in an argon-filled glovebox. The coin cells were composed of an HPC/CF or a CF skeleton as the working electrode, a Celgard separator, and a Li foil as the counter/reference electrode. The electrolyte was 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in DOL/DME (1:1 by volume) with LiNO3 as additive. The Coulombic efficiency (CE) was measured at 1, 1.5, 2, and 3 mA cm−2 on a LAND electrochemical testing system under ambient conditions. The coin cells were initially cycled at 0−1 V (vs Li+/Li) at 50 μA for five cycles to remove the surface impurities and stabilize the solid−electrolyte interphase (SEI). Then, 1 mAh cm−2 of Li metal was plated onto the HPC/CF or the CF skeleton and then charged to 1.0 V (vs Li+/Li) to strip the Li metal at 1, 1.5, 2, and 3 mA cm−2 for each cycle. The CEs were calculated on the basis of the ratio of the stripped to plated amounts of Li. Symmetric cells were used to analyze the cycling stability and cycle life of the LMAs on 3D HPC/CF and CF skeletons. For long-term galvanostatic charge/discharge measurements, 3 mAh cm−2 of Li was initially plated on the skeletons at 0.5 mA cm−2, and the cells were then charged and discharged at 0.5, 1, and 1.5 mA cm−2 for 1 mAh cm−2 in each half cycle. Electrochemical impedance spectra (EIS) measurements were conducted using an electrochemical workstation (Solartron 1287/1260) in the frequency range of 100 kHz to 0.1 Hz after selected cycles. Furthermore, LiFePO4 was employed as the cathode material for full cell characterization. LiFePO4 powder was mixed with Super-P and poly(vinylidene fluoride) in N-methylpyrrolidone solvent to form a slurry, which was pasted onto carbon-coated aluminum foil to prepare the cathode. The 3D HPC/CF or the CF skeletons were initially assembled into a half cell with Li foil as the counter electrode. After plating 2 mAh cm−2 of Li metal into the skeletons, the LMAs in the copper skeletons were unloaded from the half cells and reassembled against the LiFePO4 cathode in a full cell with the same electrolyte composition as that in the half cells.

Figure 2. SEM images of the CF at low (a) and high (b) magnification, top-view SEM image of the PS colloidal crystal template-modified CF substrate (c), and SEM images at different magnifications (d−f) of the 3D HPC/CF composite.

3. RESULTS AND DISCUSSION Figures S2 and 2 display typical digital photographs and the corresponding SEM images of the samples at different stages during the fabrication procedure for the HPC/CF composite, respectively. It can be observed from Figure 2a that the pretreated CF substrate is composed of a network of 3D interconnected ligaments with diameters of 60−90 μm and highly porous structures, whereas the corresponding higher magnification SEM image (Figure 2b) shows that a ligament consists of a number of irregularly shaped microsized granules. After the self-assembly process for the PS microspheres on the CF, the copper ligament surface is covered by close-packed arrays of PS microspheres (Figure 2c), which are employed as the template for the subsequent copper electrodeposition carried out in galvanostatic mode. Figure S3 displays a typical chronopotentiometric polarization curve for the galvanostatic copper electrodeposition on the PS colloidal crystal-covered CF substrate at a current density of 12 mA cm−2. After a sequential process including copper electrodeposition, PS colloidal crystal leaching, and a thermal annealing treatment, the HPC structures were firmly grown on the CF substrate, which clearly results in an increase in the copper ligament size up to about 100−150 μm (Figure 2d). The higher magnification SEM image (Figure 2e) reveals that a large number of porous copper granules with various sizes ranging from 5 to 25 μm were grown on the original CF ligament. Furthermore, it can be observed from Figure 2f that a bicontinuous network of copper ligaments and void pores was formed in these granules through this fabrication route. The sizes of the ligaments and pores are in the submicron range, which may be determined by the size of the PS microspheres. The energy-dispersive X-ray spectroscopy (EDX) and the corresponding elemental mapping results for the HPC/CF composite confirm the presence of Cu

(Figure S4). X-ray diffraction (XRD) measurements were employed to further characterize the crystal phase of the HPC/ CF composite. For comparison, the XRD pattern of the CF substrate was also collected. As shown in Figure S5, the XRD pattern of the HPC/CF composite was consistent with that of the CF substrate. The peaks appearing at 2θ values of 43.7, 50.8, and 74.4° can be indexed to the diffraction from the (111), (200), and (220) planes of cubic copper (JCPDS card no. 04-0836), respectively. Such results confirm that the asprepared HPC structures were made of metallic copper. According to mercury porosimetry measurements, the CF and the HPC/CF have a pore area of 0.023 and 0.05 m2 g−1, respectively (Figure S6a,b). The increase in specific surface area of the HPC/CF compared to that of the CF can be attributed to the presence of the HPC structures with high submicron porosity. In addition, the cumulative pore volume of the HPC/ CF composite was determined to be 0.5216 cm3 g−1 (Figure S6c). As such, a 3D hierarchically bicontinuous porous copper architecture was successfully constructed through PS colloidal crystal template-assisted copper electrodeposition on the commercially available CF substrate. Ideally, such a facile and low-cost fabrication procedure could be extended to the preparation of other types of 3D hierarchically porous metallic structures, which may be examined in our further studies. It was found that the thermal annealing step in the fabrication procedure plays an important role in the formation of the bicontinuous porous structure of the HPC granules. To understand the effect of annealing treatment on the morphology and structural evolution of the HPC, we characterized the samples at different stages to observe the morphological evolution of the electrodeposited copper structures, as shown in Figure 3. It can be seen that after the 13554

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Figure 4 displays a top view of the surface morphology evolution of the pristine skeletons and the plated Li metal after different cycles. The electrolyte was 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in DOL/DME (1:1 by volume) with 1 wt % LiNO3 as the additive. This DOL/ DME ether-based electrolyte is commonly used in lithium metal sulfur batteries, and LiNO3 is employed to form a stable SEI on the lithium metal surface. In addition, the morphology of lithium metal deposits in this electrolyte is inclined to be spherical, which is convenient for particle size analysis from SEM images.40 For the CF skeleton, the low-magnification SEM images (Figure 4a−d) show that the metallic Li rapidly filled the pores of the CF with increasing cycle numbers, whereas the magnified SEM images (Figure 4e−h) reveal that the Li metal deposits were composed of mossy and dendritic structures, which were mainly loosely stacked in the pores of the skeletons. The rapid growth of Li metal into the pore spaces of the CF (Figure S7b) was possibly caused by the faster deposition rate of Li metal on the curved ridge regions of the copper ligament (indicated by the blue arrows in Figure S7a) compared to that on the smooth surface regions of the copper ligament (indicated by the red arrows in Figure S7a), which can be attributed to the intensified current density distribution and the greater Li ion flux on the curved ridge area compared to those on the smooth surface.40 In contrast, Li plating on the 3D HPC/CF skeleton leads to a distinct change in surface morphology evolution. The low-magnification SEM images (Figure 4i−l) show that the original macroporosity of the skeleton is maintained after cycling, indicating that the Li metal was well accommodated in the pores of the 3D HPC structures, whereas the magnified SEM images (Figure 4m−p) show that the Cu skeleton was covered with a homogeneous, dense, and smooth Li layer without any Li dendrites, which was further verified by the cross-sectional SEM images of the fresh and cycled HPC/CF skeletons (Figure S8). A similar trend in surface morphology evolution was also observed for Li plating at the current densities of 1.0 and 2.0 mA cm−2 (Figures S9 and S10). The above results indicate that the 3D HPC/CF skeleton with hierarchically bicontinuous porosity can effectively suppress the growth of Li dendrites, which can be attributed to these two synergistic factors: first, its increased specific surface area can not only provide more sites, but also reduce the local current density for Li plating, significantly retarding the growth of Li dendrites; second, its higher curvature in the HPC structures can induce more Li nucleation and growth in the pores owing to the tip effect, greatly suppressing the growth of Li dendrites.40 On the basis of the above results and previous reports,9,48 the Li plating behavior on the CF and 3D HPC/CF skeletons can be schematically illustrated in Figure 5. Specifically, for the CF skeleton, Li metal first forms small Li islands on the copper skeleton in the nucleation step, with most of the islands distributed on the curved ridges due to the intensified current density distribution and greater Li ion flux. As the plating progresses, a Li metal layer grows on the copper skeleton surface, with massive Li dendrites growing from the ridge area into the pore space, caused by the above-mentioned tip effect on the curved ridge area. In contrast, for the 3D HPC/CF skeleton, owing to the existence of numerous highly curved submicron-sized copper ligaments in the HPC granules, Li metal preferentially nucleates and grows in the HPC structures due to the tip effect. As the plating progresses, Li metal was well accommodated into the HPC structures, thus effectively suppressing the growth of Li dendrites.

Figure 3. Low-magnification (a, c, e) and high-magnification (b, d, f) SEM images showing the morphological evolution of HPC structures on the CF substrate at the initial stage (a, b) and after annealing successively at 120 °C for 30 min (c, d) and 300 °C for 1 h (e, f).

copper electrodeposition and the following rinsing with chloroform, a large number of honeycomb-like copper microspheres with various sizes are distributed on the surfaces of the pristine copper ligaments (Figure 3a), and the uniformly distributed pores in these microspheres with a size of about 500 nm are apparently produced by the leaching of the PS microspheres with chloroform after copper electrodeposition (Figure 3b). After heating to 120 °C for 30 min, the sizes of the copper ligaments and pores start to increase (Figure 3c,d). The quasi-ordered porous structures on the copper microsphere surface began to become randomly bicontinuous porous structures. After further annealing at 300 °C for 1 h, the ligament coarsening becomes more apparent and the ligament surfaces become very smooth, as shown in Figure 3e,f. The final annealing step at 450 °C for 2 h employed to remove the possible residual PS has little effect on the morphology of the copper structures (data not shown). It is revealed that the surface morphology of the electrodeposited copper structures is transformed from the quasi-ordered honeycomb-like morphology to the bicontinuous spongelike morphology after this annealing stage. Therefore, it is demonstrated from the above results that the thermal annealing treatment, which coarsens the copper ligaments and enlarges the pore size, is a very essential step for the formation of HPC structures. It has been suggested in a previous report that the coarsening of the porous metals, which involves ligament pinch-off and the formation of void bubbles, results from a surface-diffusion-controlled solid-state Rayleigh instability, which controls the change in topology of the porous material.47 In this study, the surface diffusion of copper atoms may be activated during the thermal annealing process. The protocol presented here would provide a new avenue for the rational design of other porous metallic structures for many applications. 13555

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Figure 4. SEM images of the surface morphology of Li metal electrodeposited on the CF and 3D HPC/CF skeletons with a current density of 1.5 mA cm−2 for a capacity of 1 mAh cm−2 of Li. SEM images under different magnifications of the pristine CF skeleton (a, e), and the 10th (b, f), the 80th (c, g), and the 180th (d, h) Li plating. SEM images under different magnifications of the pristine 3D HPC/CF skeleton (i, m), and the 10th (j, n), the 80th (k, o), and the 180th (l, p) Li plating.

The electrochemical performance in an actual LMB is an important index for the application prospects of the as-prepared 3D HPC/CF skeletons. CEs at different current densities (1, 1.5, 2, and 3 mA cm−2) were measured, and the results are shown in Figure 6. At a current density of 1.0 mA cm−2, the HPC/CF-based electrode maintained a high CE of 98% over 270 cycles, whereas that of the CF-based electrode decreased markedly to below 70% after 160 cycles (Figure 6a). As the current density was increased to 1.5, 2, and 3 mA cm−2, the HPC/CF-based Cu/Li cell exhibited highly stable CEs at 96% over 200 cycles, at 95% over 150 cycles, and at 94% over 100 cycles, whereas the CF-based Cu/Li cell showed a rapid decrease in its CEs after 120 cycles, 60 cycles, and 40 cycles, respectively (Figure 6b−d). The rapid decline in the CEs, accompanied by large oscillations, can be ascribed to the uncontrollable growth of dendritic and mossy Li, which results in the continuous growth of the SEI layer, consuming much of the Li. Such results reveal that the hybridization of the 3D HPC architecture on the CF substrate effectively improves the cycling performance of the Li metal plating/stripping. It is worth noting that the cycling performance of the as-prepared 3D HPC/CF skeletons in LMAs also significantly outperforms recently reported 3D copper skeletons, for example, a 3D skeleton with a junglelike porous copper layer composed of Cu

Figure 5. Schematic diagram of the Li plating stages on the CF (top) and 3D HPC/CF (bottom) skeletons.

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Figure 6. CEs of LMAs on the 3D HPC/CF and the CF skeletons at different current densities of 1 mA cm−2 (a), 1.5 mA cm−2 (b), 2 mA cm−2 (c), and 3 mA cm−2 (d). The amount of Li plating in each cycle was fixed at 1.0 mAh cm−2.

submicron fibers grown on copper foil exhibits a stable CE of about 97% at 0.5 mA cm−2 over 50 cycles38 and another highly porous 3D skeleton, which was derived from chemical dealloying of commercially available Cu−Zn alloy foil, shows a stable CE of about 97% at 1.0 mA cm−2 for 140 cycles.39 Figure 7a,b shows the voltage profiles of Li plating/stripping on the CF and 3D HPC/CF skeletons at a current density of 1.5 mA cm−2 for a total of 1 mAh cm−2 of Li metal. It is shown that the charge/discharge curves of the 3D HPC/CF skeletons are nearly overlapping, except for the first cycle, whereas the amount of stripped Li was significantly reduced after cycling for the CF skeleton, which is in line with the results shown in Figure 6b. Moreover, as displayed in the insets of Figure 7a,b, the 3D HPC/CF skeleton exhibits smaller voltage hysteresis, which is defined as the voltage difference between the discharge and charge processes, than the CF skeleton, indicating reduced interfacial resistance in the 3D HPC/CF skeleton. Furthermore, the long-term cycling stabilities of the 3D HPC/CF and CF skeletons were characterized by testing a symmetric Li/Li cell containing a Li foil counter/reference electrode and a Li@ Cu working electrode. The 3D HPC/CF and CF skeletons were initially plated with 3 mAh cm−2 of Li metal and then cycled at a current density of 0.5 mA cm−2 and a cycling capacity of 1 mAh cm−2. As can be observed from the voltage− time profiles of the Li plating/stripping (Figure 7c), the LMA on the CF skeleton shows random voltage oscillations after cycling for 160 h, which could be attributed to the polarization resulting from the continuous formation of an SEI layer, and then a sudden voltage drop after cycling for 340 h, which could possibly be caused by a dendrite-induced short circuit of the cell. In comparison, the Li plating/stripping for the LMA on the 3D HPC/CF skeleton exhibits greatly improved cycling stability without notable voltage fluctuation. There appears to be no sign of a short circuit after cycling for 620 h, suggesting that the growth of Li dendrites has been effectively suppressed. The corresponding voltage hysteresis curves (Figure S11) also

clearly show the voltage fluctuation phenomenon and the short circuit event during the cycling. Specifically, at a current density of 0.5 mA cm−2, Li plating/stripping on the CF-based LMA leads to a continuously oscillating voltage hysteresis above 50 mV, and then, there is a sharp drop to about 17 mV after 80 cycles, indicating a short circuit, whereas the Li plating/ stripping on the 3D HPC/CF-based LMA leads to a very stable voltage hysteresis below 27 mV during the whole of the cycling. As is well known, the voltage hysteresis is mainly affected by the charge transfer resistance, interfacial properties, and current density.22,23 The decrease in voltage hysteresis on the 3D HPC/CF-based LMA may result from the increased specific surface area of the 3D HPC/CF skeleton, which can reduce the charge transfer resistance, offer a larger interfacial area, and lower the local current density. Electrochemical impedance spectroscopy (EIS) measurements of the two types of skeletons were further performed to investigate the interfacial properties. It is shown in Figure 8 that the values of the SEI resistance (RSEI) and the charge transfer resistance (Rct) (Figure 8d) of the 3D HPC/CF skeleton are always smaller than those for the CF skeleton, and these resistances decrease with cycling, which reveals that the HPC structure can stabilize the electrode/ electrolyte interface. Additionally, the long-term cycling at larger current densities of 1.0 and 1.5 mA cm−2 of the 3D HPC/CF skeleton also results in a smaller and much more stable voltage hysteresis than that for the CF skeleton (Figure S12). Furthermore, to evaluate the potential practical application of the as-prepared 3D HPC/CF skeleton, full cells were assembled with the 3D HPC/CF or CF skeletons, on which 2 mAh cm−2 of Li metal was preplated as the anode and LiFePO4 as the cathode. Figure 7d shows the cycling performance of the LMA with the 3D HPC/CF and the CF as the skeletons at 2 C for 500 cycles; it can be observed that the LMA based on the 3D HPC/CF skeleton exhibits greatly enhanced cycling performance compared to the LMA using the CF skeleton. The reversible capacity of the 3D HPC/CF 13557

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Figure 7. Voltage profiles of Li plating/stripping on CF (a) and 3D HPC/CF (b) skeletons at a current density of 1.5 mA cm−2 for a total of 1 mAh cm−2 of Li, with the insets showing enlargements of the low voltage range. Voltage−time profiles of the Li plating/stripping with a cycling capacity of 1 mAh cm−2 at 0.5 mA cm−2 in symmetric Li/Li@Cu cells with 3D HPC/CF and CF skeletons (c). Cycling performances of the LMAs with 3D HPC/CF and CF skeletons in a full cell with a LiFePO4 cathode at 2 C (d).

skeleton-based LMA could be retained at 115 mAh g−1 with a high CE of 99.7%, revealing a capacity retention of 71.1%, whereas that of the CF skeleton-based LMA remained at 102.5 mAh g−1 after 300 cycles with a CE of 99.4%, corresponding to a capacity retention of 67.7% and then abruptly decreased to only 7.6 mAh g−1 after 500 cycles with a CE of 92.8%. The significantly enhanced electrochemical performance of the 3D HPC/CF, in comparison to that of the CF skeleton, indicates good application prospects for 3D HPC/CF skeletons in practical LMBs.

deposition, and thermal annealing. The thermal annealing treatment plays an essential role in the surface morphology and structural evolution of the HPC structures. The engineering of the structure of the CF skeleton by integrating the HPC frameworks provides a 3D hierarchically bicontinuous porous copper skeleton with an increased specific surface area, which can not only provide a larger interfacial area, but also reduce the charge transfer resistance and reduce the local current density during Li plating/stripping. Additionally, Li metal preferentially nucleates and grows in the HPC frameworks due to the tip effect, which effectively suppresses the growth of Li dendrites, leading to high electrochemical performance of the LMAs. The cells with the 3D HPC/CF skeletons delivered enhanced CEs and thus presented superior Li plating/stripping cycling performance. The 3D HPC/CF-based LMAs can run for 620 h without short-circuiting in a symmetric Li/Li@Cu cell at a current density of 0.5 mA cm−2. Furthermore, the Li@Cu/ LiFePO4 full cell exhibits superior capacity retention with a

4. CONCLUSIONS In summary, we have demonstrated a novel fabrication approach for constructing HPC frameworks on the commercially available CF substrate to produce a 3D hierarchically bicontinuous porous copper skeleton (HPC/CF) by a protocol combining self-assembly of PS microspheres, Cu electro13558

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Figure 8. EIS curves for selected cycles of CF (a) and 3D HPC/CF (b) skeletons at a current density of 1.5 mA cm−2 for a total of 1 mAh cm−2 of Li; the corresponding equivalent circuit model used to fit the experimental results (c) and the fitting results for the circuit elements (d).

high CE of 99.7% at 2 C for 500 cycles, revealing the potential practical applications of the 3D HPC/CF skeletons in LMBs. This work paves the way for engineering the structure of the commercially available CF to significantly improve its electrochemical performance for application in LMAs and provides insight into the rational design of 3D hierarchically bicontinuous porous skeletons for highly efficient and safe LMAs for next-generation high-energy-density LMBs.

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Additional optical, SEM, EDX, and XRD characterizations; mercury porosimetry measurement; and additional electrochemical tests (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.S.). *E-mail: [email protected] (Z.G.). ORCID

Chunyi Zhi: 0000-0001-6766-5953 Zhicong Shi: 0000-0003-2360-7668 Zaiping Guo: 0000-0003-3464-5301

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b01978.

Author Contributions ∥

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X.K. and Y.C. contributed equally. DOI: 10.1021/acsami.8b01978 ACS Appl. Mater. Interfaces 2018, 10, 13552−13561

Research Article

ACS Applied Materials & Interfaces Author Contributions

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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 the National Natural Science Foundation of China (Nos. 21673051 and 51604086), the Guangdong Science and Technology Department (Nos. 2016A010104015 and 2017B010119003), the Guangzhou Science & Innovative Committee (Nos. 201604030037 and 201704030011), the “One-hundred Talents plan” (No. 220418056), the “One-hundred Young Talents plan” (No. 220413126), and the Youth Foundation of Guangdong University of Technology (No. 252151038). The authors also appreciate Dan Liu and Qinghai Li from Dynavolt Renewable Energy Technology Co., Ltd. for helpful discussion.

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DOI: 10.1021/acsami.8b01978 ACS Appl. Mater. Interfaces 2018, 10, 13552−13561