Supporting Information

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1 Supporting Information Spherical Li Deposited inside 3D Cu Skeleton as Anode with Ultra-Stable Performance Yanyan Wang, 1 Zhijie Wang, 1 Danni Lei, 1,2 Wei Lv, 1 Qiang Zhao, 1,2 Bin Ni, 1,2 Yong Liu, 3 Baohua Li, 1 Feiyu Kang, 1,2 Yan-Bing He 1, * 1 Engineering Laboratory for the Next Generation Power and Energy Storage Batteries, Graduate School at Shenzhen, Tsinghua University, Shenzhen , P. R. China 2 Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Beijing , P. R. China 3 School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang , P. R. China *Corresponding he.yanbing@sz.tsinghua.edu.cn (Yan-Bing He) S-1

2 Experimental Section Preparation of 3D Cu skeleton: The 3D Cu skeleton was prepared by powder-sintering method with NaCl crystals as template to further increase its porosity. Firstly, NaCl (99.8%, Macklin Co., Ltd) was intensively dried and ball-milled with ethanol as dispersing agent for 10 hours at 400 rpm. The as-obtained slurry was dried at 60 C to obtain submicron NaCl templates (Figure S1a). Then, the commercial Cu nanopowders (AR, Tianjiu Metal Material Co., Ltd) (Figure S1b), NaCl templates, binder polyethersulfone (PES) (Solvay Advanced Polymers Co., Ltd) and solvent N-Methyl-2-pyrrolidone (NMP) (99%, Macklin Co., Ltd) were blended together at a mass ratio of 6:3:1:5, and ball-milled for 10 hours at 400 rpm. The mixed slurry was degassed for 15 minutes and immediately casted on clean glass. To avoided Cu powders from sedimentation, the shaped slurry was immersed in ethanol to accelerate the process of PES solidification and then a Cu-NaCl-binder membrane was obtained. After that, the Cu-NaCl-binder membrane was punched into circular disks with a diameter of 14 mm. Afterwards, the disks were treated with two-step heat treatment[s1]. The first step was conducted in air at 620 C for 4 hours to remove binder while the second-step was conducted under the H 2 /Ar (5 %/95 %) atmosphere at 580 C for 2 hours to obtain well connected 3D Cu skeleton. Before the second-step heat treatment, the intermediate was washed with deionized water for 5 times to remove NaCl templates thoroughly. The final diameter of the 3D Cu current collector was about 12 mm (Figure S3). Structure characterization: The powder X-ray diffraction (XRD) patterns were measured by a Rigaku D/MAX 2500/PC diffractometer using Cu K α radiation (λ = nm). The morphology and structure observation were conducted by field emission scanning electron microscopy (FE-SEM, HITACH S4800, Japan) with energy dispersive X-ray spectroscopy (EDS). To study the deposition behavior of Li metal on different current collectors, the batteries were disassembled in an Ar filled glove box to get the electrodes, then the electrodes S-2

3 were immersed with DME to remove electrolyte. After drying in the glove box at ambient temperature, the electrodes can be used for characterization. Electrochemical measurements: CR2032-type coin cells were assembled to evaluate the electrochemical performances of Li metal anodes with 3D Cu skeleton. For comparison, commercial Cu foam was also used as current collector. The current collector was used as working electrode, Celgard 2400 polypropylene membrane as the separator and Li foil as the counter/reference electrode. The electrolyte was 1 M LiTFSI in mixed solvent of 1, 3- dioxolane (DOL) and DME (volume ratio: 1:1) with 1% LiNO3. The amount of electrolyte added to each cell was fixed at 120 µl. The battery assembly was performed in an argonfilled glove box. Before tests, all the batteries were cycled at 0-1 V (vs. Li + /Li) at 50 µa for 5 times on a battery tester (Land CT2001A) to eliminate surface contaminations[s2]. The Coulombic efficiency was tested via plating 1 mah cm -2 of Li on current collector at a current density of 0.5 ma cm -2 or 1 ma cm -2 and then stripping up to 0.5 V at 0.5 ma cm -2 or 1 ma cm -2 for each cycle. For long-term galvanostatic charge/discharge test, 2 mah cm -2 of extra Li was firstly deposited into current collectors to form the Li@3D Cu skeleton and Li@Cu foam electrodes, and then the symmetric cells were cycled at a fixed plating/stripping capacity of 1 mah cm -2 or 5 mah cm -2 at 1 ma cm -2. Full cells were assembled using Li anode with current collectors (Li@3D Cu skeleton or Li@Cu foam) as anode and LiFePO4 as cathode. The active materials loading of LiFePO4 electrode was 2.7 mg cm -2. S-3

4 Figure S1. (a) NaCl templates after ball-milling at 400 rpm for 10 hours; (b) Commercial Cu powders. Figure S2. XRD patterns of the pristine Cu powders, intermediate and 3D Cu skeleton. S-4

5 Figure S3. The optical photograph of 3D Cu skeleton. Figure S4. BET surface area plot of 3D Cu skeleton and Cu foam. S-5

6 Figure S5. The surface SEM image of 3D Cu skeleton. Figure S6. (a) The cross-sectional SEM image and (b) the corresponding EDS mapping of Cu element in the 3D Cu skeleton with Li deposition. Figure S7. The Coulombic efficiency of Li plating/stripping on 3D Cu skeleton at 1 ma cm -2 with a fixed plating capacity of 2 mah cm -2. S-6

7 Figure S8. The cross-sectional SEM images of 3D Cu skeleton: (a) the 50th, (b) 100th and (c) 200th Li deposition; (d) enlarged SEM image of the selected area in (c). Figure S9. The high magnification SEM images of Li deposition on Cu foam at a capacity of 3 mah cm-2. S-7

8 Figure S10. (a) The Coulombic efficiency of cell with Cu foil as current collector at 0.5 ma cm -2 with a stripping/plating capacity of 1 mah cm -2. (b) Galvanostatic cycling performance of symmetric cells with Cu foil as current collector at a current density of 1 ma cm -2 with a stripping/plating capacity of 1 mah cm -2. S-8

9 Table S1. Comparison of the Coulombic efficiency of Li metal anodes with different current collectors. Sample Preparation method Test condition (ma cm -2 / mah cm -2 ) Cycling number Reference 3D Cu Skeleton powder sintering 0.5/ this work 3D graphene@ni scaffold CVD 0.25/1 0.5/ [S3] ALD-CNTS CVD and ALD 1/2 80 [S4] 3D Cu current collector chemical dealloying 0.5/ [S5] Cu nanowire reduction by N 2 H 4 1/2 200 [S6] nitrogen-doped graphene Treating graphene in ammonia 1/ [S7] Cu-CuO-Ni hybrid structure magnetron sputtering and thermal-oxidizing 250 [S8] unstacked graphene framework CVD 0.5/ [S9] Cu mesh commercial product 0.5/1 100 [S10] S-9

10 Table S2. Comparison of galvanostatic cycling performance of symmetric cells with different current collectors. Sample Preparation method Test condition (ma cm -2 / mah cm -2 ) Cycling time (h) Reference 3D Cu Skeleton powder sintering 1/ this work 3D Cu foil Self-assembly 0.2/ [S2] Carbon modified Ni foam (CMN) CVD 550 [S11] Carbonized wood Carbonization 3/ [S12] ALD-CNTS CVD and ALD 3/ [S4] 3D Cu current collector chemical dealloying 0.2/ [S5] Cu nanowire reduction by N 2 H 4 1/ [S6] nitrogen-doped graphene Treating graphene in ammonia 1/ [S7] Cu mesh commercial product 0.5/1 2/ [S10] S-10

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