Hosting Ultrahigh Areal Capacity and Dendrite-free Lithium via

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Hosting Ultrahigh Areal Capacity and Dendrite-free Lithium via Porous Scaffold Huibo Yan, Chao Shen, Kai Yuan, Kun Zhang, Xing-Rui Liu, Jian-Gan Wang, and Keyu Xie ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03910 • Publication Date (Web): 18 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018

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ACS Sustainable Chemistry & Engineering

Hosting Ultrahigh Areal Capacity and Dendrite-free Lithium via Porous Scaffold

Huibo Yan†, Chao Shen†,*, Kai Yuan†, Kun Zhang†, Xingrui Liu†, Jian-Gan Wang†, Keyu Xie†,*.

†

State Key Laboratory of Solidification Processing, Center for Nano Energy

Materials, School of Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Laboratory of Graphene (NPU), No.127 Youyi West Road, Xi’an 710072, China.

* Corresponding author: Chao Shen; Keyu Xie E-mail: [email protected]; [email protected]

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Abstract: Lithium (Li) metal is one of the most attractive anode materials for next-generation energy storage batteries. However, the undesirable Li dendrite growth usually leads to low Coulombic efficiency (CE) and safety hazards. Here, we employ reduced graphene oxide (rGO) infiltrated Ni foam (rGO@Ni) as a three-dimensional (3D) current collector to suppress Li dendrites. After Ni foam is soaked up with rGO, the enlarged working area conduces to a low local effective current density and a homogeneous Li deposition, therefore renders a dendrite-free morphology. Compared with the Cu foil and Ni foam electrodes, the rGO@Ni electrode achieves enhanced CE and longer cycle life. Even for an extreme areal capacity of 20 mAh cm-2, so far the highest areal capacity has been reported in the literatures, the rGO@Ni current collector not merely still functions after 50 cycles, also delivers an enhanced CE of above 95%. Moreover, the symmetrical cell runs for 1100 h without fluctuation. The exceptional electrochemical performance of rGO@Ni current collector indicates a rosy prospect for remedy of the dendrite problem. Hopefully, this study can shed light on further commercialization of Li metal-based batteries.

Keywords: rGO@Ni; current collector; Li metal anode; Li dendrites.

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INTRODUCTION Technological improvements in high-energy-density rechargeable batteries are being driven by an ever-increasing demand for portable electronic devices, electric car (bus), as well as smart grid storage system

1-3

. Due to its high theoretical specific

capacity (3860 mAh g−1 compared to 372 mAh g−1 for the widely used graphite anode in Li ion batteries), low density (0.59 g cm−3), and low negative electrochemical potential (−3.040 V vs standard hydrogen electrode) of Li metal anode, research has been focused on Li metal battery (LMB) since 1970s 4, 5. LMBs should have played a critical role in meeting the ever-growing demands for higher energy density rechargeable batteries (such as Li-sulfur and Li-oxygen batteries)

6-8

. Unfortunately,

there are some safety and efficiency issues emerging during charge/discharge cycles preventing practical use of Li metal anode in rechargeable LMBs 5, 9-13. Upon repeated Li stripping/plating processes, Li metal uncontrollably grows into dendrites that can penetrate the separator and cause an internal short with the cathode

14-16

. A natural

solid electrolyte-interphase (SEI) layer forms on Li metal surfaces and acts as a passivation layer when Li metal contacts with organic electrolytes

17, 18

. The large

surface area among dendrites and electrolytes facilitates the re-reaction and re-formation of SEI layer on the Li surface, and these in situ formed natural SEI layer usually does not have enough mechanical strength to withstand the large volume change during Li charge/discharge cycle processes

17, 19

. Consequently, corrosion and

isolation of Li (dead Li) is formed, leading to a low CE and short cycle life. It is well accepted that the unfavorable growth of Li dendrites is mainly attributed to two 3



factors

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20-22

. 1) The inhomogeneous and unstable SEI layer forms on the Li metal

anode. 2) Li metal itself is not a host material for Li atom accommodation. Scientists have been making attempts to address the inherent problems of Li metal anode, and have achieved solid progress on suppressing dendrite formation and growth. Various electrolyte additives have been employed to reinforce the SEI layer and suppress the formation and growth of Li dendrites. Additives currently proven effectual mainly include vinylene carbonate

23

, fluoroethylene carbonate

24

, LiNO3

and lithium polysulfide 25, and lithium bis (trifluoromethanesulfonyl) imide (LiFSI) 26. However, stable cycling cannot be guaranteed due to the consumption of additives in long-term cycle, and the SEI film can not maintain enough mechanical strength to withstand large volume change in extended period. Modifying the surface of the Li metal and forming an artificial protective film on Li metal are also considered. The concept of artificial protective film has been deeply explored in previous studies, and various artificial films have been applied on Li foil surfaces, such as polyacetylene 27, tetraethoxysilane

28

, lithium phosphorus oxynitride

compounded styrene butadiene rubber boron nitride layer

15

, a graphene layer

14

29

, Cu3N nanoparticles

, a hollow carbon nanospheres layer

16

, a

21

, and a Li3PO4 layer 30. Nonetheless, these

strategies have presented quite a few side-reactions that may render the electrolyte ineffectiveness. Some separators and intermediate barrier layers have also been used as chemically and physically stable barriers on Li metal anode. Cui et al. designed a novel silica nanoparticle sandwiched separator extending the life of Li metal batteries up to approximately five times

31

., Anodic aluminum oxide (AAO) membrane 4


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separator

32

, glass fiber cloths

22

, polyimide-coating layer with vertical nanoscale

channels 33, carbon nanotube film 34, and graphene

21

have also been employed to

inhibit the growth of Li dendrites. Although the protective method can enhance compatibility between the Li anode and electrolyte, the problems of low conductivity and high interfacial resistance are introduced. Compared with the above methods, the most effective way is the modified current collector. Cu foil as a widely accepted current collector for Li anode, with protuberances on the electrode surface, inevitably brings about the tip effect issue 20, 35. Scientists have gained ground on the conversion of Cu foil into a 3D host or large specific surface area (SSA) scaffold to address this issue. A 3D porous Cu current collector was obtained by dissolution of zinc out of commercial Cu−Zn binary alloy

36

, while a 3D porous submicrometer fibers Cu

current collector was obtained by reducing the Cu(OH)2 fibers on Cu foil to Cu fibers 37

. On top of that, Lu et al. designed a simple solvent evaporation assisted assembly

technique to prepare a Cu nanowires membrane which was synthesized by using hydrazine to reduce Cu(NO3)2 in the NaOH aqueous solution 38. These studies find that the 3D current collectors not only conduce a homogeneous electric field to eliminate the tip effect, but also increase the Li metal deposition capacity. An unstacked graphene nanostructured current collector proposed by Zhang et al. demonstrates that a large SSA current collector can guarantee an ultralow local current density in LMBs 39. Hence employing a 3D host scaffold, with extended SSA to reduce the effective current density, is a plausible approach to homogenate electric field and decrease local current density. 5



Based on our previous work 40, few-layer graphene compactly coated on the surface of Ni foam (graphene@Ni foam) prepared by chemical vapor deposition (CVD) is used as the artificial protective layer. Although graphene@Ni foam can suppress Li dendrites, the macropore of Ni foam is not fully utilized since there is no Li deposition site within it. Hence we propose a strategy of rGO infiltrated Ni foam as a current collector to acquire a 3D scaffold with a large SSA and a big host capacity. RGO is chosen due to its large SSA, high conductivity, and excellent electrochemical stability reactions

41-43

. Meanwhile, Ni foam is a stable substrate during electrochemical

10, 44-46

. We calculate via a quantitative model calculation that the working

area of the rGO@Ni current collector increased by 1118.25 cm-2 compared to Ni foam, which reduces the local current density and suppresses the Li dendrite growth. Even for an extreme areal capacity of 20 mAh cm-2, the rGO@Ni current collector not merely still functions after 50 cycles, also delivers an enhanced CE of above 95%. EXPERIMENTAL Preparation of rGO@Ni GO was prepared from purified natural graphite with a mean particle size of 325 mesh according to a modified Hummers’ method

36, 44

(refer to the Supporting

Information for details). Ni foam (110 PPI,) was carefully cleaned treated with acetone to remove contaminants and hydrochloric acid to etch the surface, and then washed in sequence with deionized water and absolute ethanol. All reagents were purchased from national medicines Co. Ltd., China. Before experiments, cleaned Ni foam was fully wetted with deionized water. The preparation of rGO@Ni mainly

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consists of two steps. As illustrated in Fig. 1, the two steps involved in synthesizing rGO@Ni current collector. First, the Ni foam (Fig. 1a) was immersed into GO suspension and dried for several hours in air to remove the water at room temperature, repeated ten times until the surface of Ni foam skeleton was fully covered by GO (GO@Ni). Second, the GO@Ni was thermally reduced using quartz tube furnace at 950 °C inside a 2 cm internal diameter cylindrical quartz boat with open ends. The boat was introduced in a 1 in. internal diameter glass tube oven fitted with controlled vacuum and gas flows. The materials was annealed under a continuous flow of 70 standard cubic centimetres of ultrapure argon, at a rate of 20 °C min-1, and held the 950 °C for 1 h, and allowed it to cool to room temperature over 1 h. After annealing, the rGO@Ni current collector could be obtained (Fig. 1c). Characterizations Field emission scanning electron microscopy (FESEM) measurements were carried out with Nova NanoSEM 450. X-ray diffraction (XRD) patterns of the GO powder (produced by modified Hummers' method) and rGO powder (obtained from the as-prepared rGO@Ni acid-treated in HCl (1.5 M) solution for 30 minutes, washed with deionized water and dried in vacuum oven.) were from 5 to 60o on a Bruker D8 advance diffractometer with CuKα radiation (λ=1.5406 Ǻ). N2 adsorption–desorption isotherms of rGO powder were obtained at −196 °C (77 K) with an ASAP2020 system (Micromeritics, USA). The SSA was determined by the Brunauer–Emmett– Teller (BET) method. The pore size distribution (pore diameter and pore volume of the samples) was determined by the Barrett– Joyner–Halenda (BJH) method from the 7



adsorption branch of the isotherms. The contact angle was measured by an Optical Contact Angle & interface tension meter (SL200KB, Kino, USA) at room temperature in air, and a 3.0 µL droplet of the ether-based electrolyte was used in the experiment. Raman spectra were recorded on a Renishaw Raman microscope with a 532 nm wavelength laser. XPS analyses were carried out with a Scienta-ESCA300 spectrometer using a monochromatic Al Kα source (15 mA, 14 kV). Electrochemical measurements The Cu foil, Ni foam, or rGO@Ni was first pressed and punched out into circular disks with a diameter of 12 mm as differnt current collectors for Li metal anodes. For repeated Li deposition/stripping testing, CR2032-type coin cells were assembled to deposit Li on the current collectors to evaluate the CE, electrochemical impedance spectra and other properties. CR2032 coin cells were assembled using the current collectors mentioned above as the working electrode, a Li foil as the counter/reference electrode, and a Celgard-2325 microporous polypropylene film as the separator. The electrolyte was 1 M lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) in cosolvent of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1 in volume) with 2 % LiNO3. To standardize the measurement, a fixed amount (40 µL) of electrolyte was used in each coin cell. These batteries were assembled in an Ar-filled glove box and first cycled from 0 to 1 V at 50 µA for 5 cycles to remove surface contaminations and stabilize the interface. The Li deposition capacity is fixed at 1.0 mAh cm-2 and the cut-off potential for the stripping process is configured to be 1.0 V. EIS measurements were obtained over the frequency range of 100 KHz to 10 mHz 8


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using an electric IM6ex impedance analyzer. For the symmetrical cell test, 2 mAh cm-2 of Li was first plated onto the current collectors at a current density of 0.2 mA cm-2, then the cells were cycled at a current density of 0.2 mA cm-2 for 4 h in each half cycle. The LiFePO4 full cells were assembled using Li anode with current collectors as anodes, LiFePO4 as cathode material. LiFePO4 electrode was prepared by by mixing LiFePO4, polyvinylidene fluoride, and conductive carbon additives (mass ratio: 8:1:1) in N-methyl-2-pyrrolidone (NMP) casted on an aluminum foil followed by drying in a vacuum oven for 12 h. The average mass loading of LiFePO4 in the electrode one is about 2.4 mg cm-2 and the other is 15.3 mg cm-2. The anode was first assembled into a half cell using a Li foil as counter electrode. After depositing 5 mAh cm-2 of Li metal onto the current collector, the cell was disassembled in an Ar-filled glove box and new anode was further reassembled into a full cell against LiFePO4 cathode. The electrolyte is consisted of 1.0 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 in volume). These LiFePO4 full cells were galvanostatically cycled between 2.4 and 4.2 V at 1C. All the cells were tested using a CT2001A cell test instrument (LAND Electronic Co, BT2013A, China) . RESULTS AND DISCUSSION There are numbers of protuberances on the Cu foil surface (Fig. S1, refer to supporting information), inducing nonuniform distribution of electric field and leading to the tip effect 22, 36. So called “tip effect” is that those protuberances serve as charge centres as the charges accumulating at the protuberances in the electric field 9



(Fig. 2a). The subsequent Li metal is then deposited on these sharp ends and the agminated Li ions finally develop into Li dendrites (Fig. 2a) 22. Vice versa, these dendrites exacerbate the tip effect, leading to continuous growth of large dendrites 38. We propose a hypothesis that a smaller surface area of the current collector will produce a small amount of dendrites, and containing a large surface area of the current collector will not produce dendrites. As the Fig 2b shown, the 3D structure of Ni foam homogenizes electric field and weakens the tip effect, which reduces some Li dendrites. Instead of permeating through the Ni foam, the Li metal deposits on the foam surface, leaving plenty of interspace among the skeletons unoccupied. As a consequence, Ni foam alone does not satisfy the purpose of dendrite inhibition. After soaked up with rGO, the working area is significantly increased, the internal pore space amidst the rGO@Ni provides open ion channels for electrolyte infiltration, and more importantly, the large surface area current collector reduce the local current density, facilitating an even Li metal depositing (Fig. 2c). In a nutshell, the dendrite-free deposits stems from the large surface area decreasing the electrode local current density and the 3D structure homogenizing the Li ions distribution. The top-view SEM images of the 3D rGO@Ni current collector are displayed in Fig. 3a and b. In contrast with Fig. S2 (refer to Supporting Information), there are some flaky materials adhering to the Ni foam skeleton. The cross-sectional SEM images of the rGO@Ni current collector clearly show that rGO has successfully permeated through the big pores of the Ni foam and covered the whole skeleton (Fig. 3c and d). The Raman spectroscopy and XRD are employed to identify the flaky 10


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materials. As shown in Fig. 3e, the Raman spectra of the corresponding centre point on the image has two prominent peaks at around 1355 and 1598 cm-1, which are assigned to the D and G bands of carbon, respectively. The G band is related to graphitic carbon and the D band is associated with the structural defects or partially disordered structures of graphitic domains

41, 43, 47, 48

. The Raman spectrum of the

GO@Ni also contains both G and D bands (Fig. S4, refer to Supporting Information). However, with an increased D/G intensity ratio compared to that in rGO@Ni strongly suggests the restoration of graphitic carbon in rGO@Ni 48. The intensity ratio of D/G further proves the presence of rGO

48, 49

. Fig. S3 (refer to Supporting Information)

depicts the differences in XRD patterns between GO powder and rGO powder obtained from the as-prepared rGO@Ni acid-treated in HCl solution for 30 minutes, washed with deionized water and dried in vacuum oven. It can be clearly seen that the XRD patterns of rGO are quite different from that of GO powder. The characteristic diffraction peak (001) of GO almost disappears, while a new gradual peak emerges at around 24°, which is one characteristic diffraction peak observed in general rGO 28, 49-50

. Therefore, the GO infiltrated the Ni foam can be completely reduced to rGO. In

order to evaluate the effect of high temperature process for the design current collector, the XPS spectrum is adopted for analysis. As shown in Fig.3f, the peaks centers at 852.4 and 869.5 eV correspond to the characteristic Ni2p3/2 and Ni2p1/2 peaks of Ni0, whereas the peaks located at 856.1 and 873.8 eV represent the characteristic Ni2p3/2 and Ni2p1/2 peaks of Ni2+. 51-53 The XPS spectrum indicates that the Ni skeleton surface of both pristine Ni foam and rGO@Ni is covered by NiO. 11



According to the XPS spectra of rGO from rGO@Ni (Fig.3g), no Ni signal appears. Therefore, Ni does react with GO during the high temperature process. We proceed to use a quantitative model to prove that the rGO provides a high SSA for the rGO@Ni current collector. First, the mass of one Ni foam current collector and one rGO@Ni current collector are measured, equal to 32.215 mg and 32.845 mg respectively, so we can calculate the mass of rGO in one rGO@Ni current collector is 0.630 mg. Then the N2 adsorption-desorption isotherms of rGO powder is used to prove that rGO has a large SSA (Fig. 3h), pore distribution, and pore volume (Fig. S6, refer to Supporting Information). The SSA of rGO calculated by BET method is 177.5 m2 g−1, and pore volume calculated by the Barrett–Joyner–Halenda (BJH) method from the adsorption branch is 0.843 cm3 g-1, whereupon the working area of rGO on the rGO@Ni current collector, equal to the SSA of rGO powder multiplied by the quality of rGO, is an low local areal current density of 1118.25 cm-2 in LMBs. The quantitative model can further confirm the critical role of a low local current density in Li dendrite inhibition (refer to Supporting information for detailed calculation). Consequently, when the cell with Cu foil as electrode is cycled at a current density of 1 mA cm−2 , the local current density of Li depositing on rGO@Ni current collector is about 9.9310-4 mA cm−2, which is less than one thousandth of that of Cu foil electrode. Contact angles of LiTFSI-based electrolyte on the Cu foil and rGO@Ni are measured to determine the wettability (Fig. S7, refer to Supporting Information), it is 38°on the Cu foil current collector while it is nearly 0° on the rGO@Ni current collector, indicating a superior wettability between the rGO@Ni and the electrolyte. 12


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To demonstrate the morphology after Li metal deposits on these three types of current collectors, we further characterize after 100 cycles at a current density of 1.0 mA cm−2 for areal capacities of 1 mAh cm−2. The discrepancies in morphology are confirmed by our results (Fig. 4). Li ions are prone to concentrate near the surface of the protuberances on the uneven surface of Cu foils

20, 22

, which expedites the

nucleation and eventually evolves into Li dendrites at the local point. The needle-like Li dendrites (marked with arrows in Fig. 4b) grow epitaxially on the whole surface of Cu foil (Fig. 4a). It can be estimated from Fig. 4b-d that the dendrites have a diameter of around 2 µm, a length of more than 5 µm, and a thickness over 50 µm after 100 cycles. These dendrites are very likely to pierce the membrane and cause the battery to fire and explode. Meanwhile, most of the Li metal permeates into the large interspace among the skeletons of the Ni foam (Fig. 4e,g), so dendrites on the Ni foam current collector are significantly less than on the Cu foil current collector (Fig. 4f,h). It is generally accepted that large interface area of Ni foam not only induces uniform distribution of electric field, also increases heterogeneous nucleation and reduces the effective electrode current density. Last but not the least, no dendrite can be observed protruding upward on the surface of the 3D rGO@Ni (Fig. 4i,j), ascribed to the stable SEI layer, large SSA, and the evenly distributed Li ions induced by the strong interaction between Li ions and the 3D rGO@Ni. Li metal is embedded homogenously into the interspace of the 3D rGO@Ni current collector (Fig. 4k,l), rendering a dendrite-free morphology, indicating that the rGO@Ni can prevent the formation of Li dendrites and eliminatablete the potential LMBs hazards.It’s very 13



necessary to provide evidence for homogeneous Li deposition on the 3D rGO@Ni current collector. The cross-sectional SEM images for 1 mAh cm-2 Li plated on rGO@Ni current collector are regard as the proof for uniform Li deposition on rGO@Ni scaffold. Fig. S8a shows that cross-sectional morphologies for 1 mAh cm-2 Li plated on rGO@Ni current collector. As Fig. S8b shown, the rGO on the Ni skeleton is attached with lithium. Fig. S8c indicates that the interior of the hole is also filled with Li metal. Fig. S8d shows the adhesion of Li on internal graphene of the scaffold. The electrochemical performance of the cells, with Cu foil, Ni foam, or rGO@Ni as current collector respectively, further confirm the superiority of the 3D rGO@Ni current collector. All the cells were cycled 5 times from 0 to 1 V at 50 µA before further electrochemical procedure to remove surface contamination and stabilize the SEI film 36, 37. CE is a critical index to evaluate the availability of Li metal in cycling. As shown in Fig. S9 (refer to Supporting Information) and Fig. 5a, Cu foil current collector displays a fluctuant CE merely at a current density of 0.25 mA cm-2, on account of the dead Li caused by the unfavorable growth of Li dendrites. And the decay on CE grows worse when the current density increases to 0.5 and 1 mA cm-2. The CE of Ni foam current collector sustains above 97% after 100 cycles at current density of 0.25 mA cm-2, indicating that introduction of large surface area current collector does help to obtain lower effective electrode current density, consequently the cycle life is extented and the growth of Li dendrites is inhibited 40. Nevertheless, due to the unstable SEI layer, the CE of Ni foam deteriorates instantly after 80 cycles 14


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when the current density increases to 0.5 mA cm−2, and when the current density increases to 1 mA cm−2, a rapid CE deterioration occurs after 40 cycles. Meanwhile, the 3D rGO@Ni always delivers an enhanced CE of above 95% even after 100 cycles, attributed to a homogeneous Li deposition stems from the large working area and 3D host structure. As demonstrated in the Fig. S11 (refer to Supporting Information) and Fig. 5b, the Cu foil and Ni foam current collectors display fluctuant CE for different areal capacities at a same current density of 1 mA cm-2 only before 40 cycles. It is well accepted that high current density and huge areal capacity expedite the dendrite growth which forms the dead Li and damage the SEI layer. But even for areal capacity as huge as 20 mAh cm-2, so far the highest areal capacity has been reported in literatures (Table S2, refer to the Supporting Information), the rGO@Ni current collector not merely still functions after 50 cycles, also delivers an enhanced CE of above 95%.The cycling stabilities of these three kinds of current collectors have been further monitored by the symmetric cell tests, which is a common technique to evaluate the interface characteristics and mimic the Li stripping/plating process in electrochemical devices. The voltage-time profiles of Li stripping/plating in symmetric cells are shown in Fig. 5c. The current density is fixed. As the continuous formation and reaction of the SEI layer, a sudden voltage change emerges on the voltage-time curve of Cu foil current collector at the 140th hours and of Ni foam current collector at the 100th hours. In the case of Cu foil and Ni foam current collectors, the voltage changes during Li stripping/plating process lead to the formation of cracks on the SEI layers. However, the voltage stays unfluctuating after a 15



stable host 3D rGO@Ni current collector is successfully introduced. The 3D rGO@Ni current collector shows the most stable cycling performance, confirming that its extended SSA can reduce the local current density and inhibit the continuous growth of dendrites. The prominent electrochemical property of the rGO@Ni current collector can be further evidenced by the electrochemical impedance spectroscopy (EIS) analysis conducted on after initialization process (Fig. S10a, refer to Supporting Information) and the 100th cycles (Fig. S10b, refer to Supporting Information) at current density of 1mA cm-2 for the area capacities of 1 mAh cm−2. The resulting EIS plots feature includes a spike in the low-frequency region and a semicircle in the high-frequency region. The diameter of the semicircle at the high frequency range is an indicator of the charge transfer resistance. The charge transfer resistance of the 3D rGO@Ni current collector is always the smallest of the three, indicating that rGO on the surface of Ni foam is beneficial for stabilization and high ionic conductivity of the SEI layer and thus facilitates easy and uniform transportation of Li ions throughout the whole electrode surface. To demonstrate the possible practical application of the 3D rGO@Ni current collector, full cells are built with Li metal with current collectors as anode and LiFePO4 as cathode. As explicated in Fig. 6, sudden capacity attenuation and huge fluctuation of CE appear after 240 cycles in the case of Cu foil and after 265 cycles in the case of Ni foam as current collector. The electrode with 3D rGO@Ni as current collector delivers more stable electrochemical cycles and longer service life at 1C. After 350 cycles, the reversible capacity of the 3D current collector remains 115 mAh 16


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g−1 with a CE of 98.8%, corresponding to a capacity retention of 88.5%, while the capacity of the Cu foil and Ni foam electrodes are only 2.3 and 5.5 mAh g−1, respectively. To prove the validity of high areal capacity cells, a higher loading cathode be tested in the full cell. Commercial LiFePO4 cathode with a higher areal loading of 15.3 mg cm-2, paired with conversion-type anode, was used in this work (refer to Fig. S12, refer to Supporting Information). As shown in Fig. S12, visible capacity attenuation before the 150th cycle in the case of Cu foil and Ni foam as current collector, while rGO@Ni as current collector keeps a stable work state with a 122 mAh g-1 discharge capacity after 200 cycles. The electrode with 3D rGO@Ni as current collector delivers more stable electrochemical cycles and longer service life at 1C. In the full cell, a high capacity and long lifespan can maintain for the conversion-type rGO@Ni with Li metal anode. The extended surface area of 3D rGO@Ni current collector can reduce the local current density and suppress the lithium dendrite growth, therefore renders an improved utilization of lithium and a superior cycle stability. CONCLUSION In summary, we have introduced a simple but effective strategy to suppress Li dendrite growth by using rGO@Ni as current collector. After Ni foam is soaked up with rGO, the extended SSA can reduce the local current density and homogenize the Li metal deposition. Even for an ultrahigh areal capacity of 20 mAh cm-2, the rGO@Ni current collector not merely still works after 50 cycles, also delivers an enhanced CE of above 95%. And the rGO@Ni current collector lasts for 1100 hours 17



without fluctuation in symmetric cell test. The reversible capacity remains 115 mAh g-1 when LiFePO4 with a 2.4 g cm-2 as the cathode, after circulating for 350 cycles at 1C and when a higher loading LiFePO4 with a 15.3 mg cm-2 as cathode the reversible capacity still remains 122 mAh g-1 after 200 cycles at 1C. Compared with Cu foil and Ni foam, the rGO@Ni current collector displays superior electrochemical cycling performance with higher and more stable CE and longer service life, suffice it to say the rGO@Ni current collector is promising for next-generation Li metal batteries that require high safety, stability and reliability. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxx. Description the preparation of GO, specific working area calculation of rGO@Ni electrode, the comparison of the Li areal plating capacity in recent literatures, SEMs of Cu foil (Fig. S1) and Ni foam (Fig. S2), XPS spectra (Fig. S5), Raman spectra of Raman optical microscope image of GO@Ni (Fig. S4), XRD patterns of GO and rGO (Fig. S5), the pore size distribution of rGO powder by (BJH) method from the adsorption branches of the isotherms (Fig. S6), contact angles of LiTFSI-based electrolyte on the Cu foil (left) and rGO@Ni (Fig. S7), the cross-sectional morphologies for 1 mAh cm-2 Li plated on rGO@Ni current collector (Fig. S8), comparison of CE at various current rates (Fig. S9), EIS curves (Fig. S10), comparison of CE between for various areal capacities (Fig. S11), and the cycling

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performance in a full cell paired with a higher loading LiFePO4 cathode at 1C (Fig. S12). AUTHOR INFORMATION Corresponding Author *E-mail for Chao Shen: [email protected]; *E-mail for Keyu Xie: [email protected] NOTES The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (51674202 and 51521061), and the Fundamental Research Funds for the Central Universities (G2016KY0307), and the Key R&D Program of Shaanxi (2017ZDCXL-GY-08-03). REFERENCES (1) Grande, L.; Paillard, E.; Hassoun, J.; Park, J. B.; Lee, Y. J.; Sun, Y. K.; Passerini, S.; Scrosati, B., The lithium/air battery: still an emerging system or a practical reality?. Advanced Materials 2015, 27, 784-800. (2) Lu, Y.; Tikekar, M.; Mohanty, R.; Hendrickson, K.; Ma, L.; Archer, L. A., Stable cycling of lithium Metal batteries using high transference number electrolytes. Advanced Energy Materials 2015, 5, 1402073. (3) Tu, Z.; Nath, P.; Lu, Y.; Tikekar, M. D.; Archer, L. A., Nanostructured electrolytes for stable lithium electrodeposition in secondary batteries. Accounts of Chemical 19



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List of Figures Fig. 1. Schematic illustration of the rGO@Ni formation process. (a) Ni foam, (b) (GO@Ni) and (c) rGO@Ni. Fig. 2. Schematic illustration of the Li deposition on (a) Cu foil, (b) Ni foam, and (c) rGO@Ni. Fig. 3. (a,b) Top-view and (c,d) cross-sectional view SEM images of the morphologies of rGO@Ni. (e) Raman spectra of the corresponding centre point on the Raman optical microscope image (shown insert e). (f) Ni2p XPS spectra of rGO@Ni and Ni foam. (g) XPS spectra of rGO from rGO@Ni. (h) N2 adsorption–desorption isotherms of rGO powder. Fig. 4. SEM images of the morphologies after Li metal deposits on (a-d) Cu foil, (e-h) Ni foam, and (i-l) rGO@Ni current collectors at a current density of 1 mA cm−2 for 1 mAh cm−2 after 100 cycles. (a,b) Top view and (c,d) cross-sectional view on Cu foil, (e,f) Top view and (g,h) cross-sectional view on Ni foam, (i,j) Top view, and (k,l) cross-sectional view on the rGO@Ni. Fig.

5.

Electrochemical

characterization

of

the

electrodes

during

Li

deposition/dissolution process. CE of Cu foil, Ni foam and rGO@Ni, (a) at various current densities of 0.25, 0.5, and 1 mA cm−2 respectively with same areal capacity of 1 mAh cm−2, (b) at same current density of 1 mA cm−2 for various areal capacities of 5, 10 and 20 mAh cm−2, respectively. (c) Voltage–time profiles with a cycling capacity of 0.8 mAh cm−2 for 0.2 mA cm−2 in symmetric cells.

28


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Fig. 6. Cycling performance of Li metal anode with (a) Cu foil, (b) Ni foam, and (c) rGO@Ni current collectors in a full cell with a 2.4mg cm-2 LiFePO4 cathode at 1C.

29



Fig. 1. Schematic illustration of the rGO@Ni formation process. (a) Ni foam, (b) (GO@Ni) and (c) rGO@Ni.

30


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Fig. 2. Schematic illustration of the Li deposition on (a) Cu foil, (b) Ni foam, and (c) rGO@Ni.

31



Fig. 3. (a,b) Top-view and (c,d) cross-sectional view SEM images of the morphologies of rGO@Ni. (e) Raman spectra of the corresponding centre point on the Raman optical microscope image (shown insert e). (f) Ni2p XPS spectra of rGO@Ni and Ni foam. (g) XPS spectra of rGO from rGO@Ni. (h) N2 adsorption–desorption isotherms of rGO powder. 32


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Fig. 4. SEM images of the morphologies after Li metal deposits on (a-d) Cu foil, (e-h) Ni foam, and (i-l) rGO@Ni current collectors at a current density of 1 mA cm−2 for 1 mAh cm−2 after 100 cycles. (a,b) Top view and (c,d) cross-sectional view on Cu foil, (e,f) Top view and (g,h) cross-sectional view on Ni foam, (i,j) Top view, and (k,l) cross-sectional view on the rGO@Ni.

33



Fig.

5.

Electrochemical

characterization

of

the

electrodes

Page 34 of 36

during

Li

deposition/dissolution process. CE of Cu foil, Ni foam and rGO@Ni, (a) at various current densities of 0.25, 0.5, and 1 mA cm−2 respectively with same areal capacity of 1 mAh cm−2, (b) at same current density of 1 mA cm−2 for various areal capacities of 5, 10 and 20 mAh cm−2, respectively. (c) Voltage–time profiles with a cycling capacity of 0.8 mAh cm−2 for 0.2 mA cm−2 in symmetric cells.

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Fig. 6. Cycling performance of Li metal anode with (a) Cu foil, (b) Ni foam, and (c) rGO@Ni current collectors in a full cell with a 2.4mg cm-2 LiFePO4 cathode at 1C.

35



TOC: The rGO@Ni scaffold renders ultrahigh Li areal capacity and stable cycling performance.

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Hosting Ultrahigh Areal Capacity and Dendrite-free Lithium via

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