Multishelled Si@Cu Microparticles Supported on 3D Cu Current

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Multishelled Si@Cu Microparticles Supported on 3D Cu Current Collectors for Stable and Binder-free Anodes of Lithium-ion Batteries Zailei Zhang, Zhong Lin Wang, and Xianmao Lu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00703 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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Multishelled Si@Cu Microparticles Supported on 3D Cu Current Collectors for Stable and Binder-free Anodes of Lithium-ion Batteries Zailei Zhang,†,‡,|| Zhong Lin Wang*,†,‡,||,§ and Xianmao Lu*,†,‡,|| †

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China. ‡ School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, China. || CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing, 100190, China. § School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332–0245, USA. *Correspondence authors. emails: [email protected] and [email protected].

ABSTRACT Silicon has proved to be a promising anode material of high specific capacity for the nextgeneration lithium ion batteries (LIBs). However, during repeated discharge/charge cycles, Sibased electrodes, especially those in microscale size, pulverize and lose electrical contact with the current collectors due to large volume expansion. Here, we introduce a general method to synthesize Cu@M (M = Si, Al, C, SiO2, Si3N4, Ag, Ti, Ta, SnIn2O5, Au, V, Nb, W, Mg, Fe, Ni, Sn, ZnO, TiN, Al2O3, HfO2, TiO2) core-shell nanowire arrays on Cu substrates. The resulting Cu@Si nanowire arrays were employed as LIB anodes that can be reused via HCl etching and H2-reduction. Multishelled Cu@Si@Cu microparticles supported on 3D Cu current collectors was further prepared as stable and binder-free LIB anodes. This 3D Cu@Si@Cu structure allows the interior conductive Cu network to effectively accommodate the volume expansion of the electrode and facilitates the contact between the Cu@Si@Cu particles and the current collectors during repeated insertion/extraction of lithium ions. As a result, the 3D Cu@Si@Cu microparticles at a high Si loading of 1.08 mg/cm2 showed a capacity retention of 81% after 200 cycles. In addition, charging tests of 3D Cu@Si@Cu-LiFePO4 full cells by a triboelectric nanogenerator with pulsed current demonstrated that LIBs with silicon anodes can effectively store energy delivered by mechanical energy harvesters. KEYWORDS: silicon, copper nanowires, binder-free, anode, lithium ion batteries 1 Environment ACS Paragon Plus

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Si as a potential anode material for next-generation lithium-ion batteries has attracted immense research interests in recent years due to its high gravimetric capacity (3579 mAh g-1 corresponding to Li15Si4).1-5 However, during repeated lithium insertion/extraction, Si-based electrodes are subject to drastic volume expansion (~300%) and prone to severe pulverization, loss of electrical contact, and detachment from the current collectors.6-8 Therefore, best performances of Si-based anodes have been mostly achieved with nanoscale structures such as nanoparticles, nanowires, nanotubes, and porous structures that may accommodate large volume changes.9-17 Most recently, some encouraging results have emerged from the study of using Si-based microparticles (MPs) as anode and improved cycling performance has been reported. For instance, Bao and co-workers coated a self-healing polymer on Si MPs with sizes of 3~8 μm and demonstrated a good cycling life of 80% capacity retention after 90 cycles in half-cell configuration.18 Cui et al. developed a method to encapsulate Si MPs (1~3 μm) with multilayered graphene cages and attained a capacity retention of 90% after 100 cycles of full-cell test.19 Choi and co-workers reported highly elastic binders integrating polyrotaxanes for Si MPs (~2.1 μm) that afforded 98% capacity retention after 50 cycles in full cells.20 Attempts to use layered Si composites as LIB anodes have also showed promising progress. Cho et al. fabricated silicon-nanolayer-embedded graphite/carbon and achieved a capacity retention of 96% after 100 cycles in full cells.21 Placke et al. also reported that 190-nm thick Si/C/Si film can retain 83% of its capacity after 150 cycles in half-cell test.15 In addition to carbon materials, copper has also been employed to form layered films with Si as anodes since Cu may enhance the overall electronic conductivity and accommodate the volume variation of Si. For example, Peng and coworkers prepared Si/Cu films (340 nm) via a cluster beam deposition method and a capacity retention of 95% was attained after 1000 cycles in half cells.14 Although most reported layered Si structures are based on thin films, their promising electrochemical performances highlight the potential of using layered Si-based microparticles as LIB anodes to afford long cycling life. Here we introduce a strategy of growing multishelled Si@Cu microparticles with onion-like structure on 3D Cu current collectors (denoted as 3D Cu@Si@Cu MPs) for stable LIB anodes. In our synthesized 3D Cu@Si@Cu MPs, the interior Cu network guarantees the contact between Si active materials and the 3D current collectors during battery cycling. Therefore, the electrode integrity can be maintained for repeated insertion/extraction of Li+ ions and transport of electrons even at high Si loading (Figure 1). Electrochemical tests of the 3D Cu@Si@Cu MPs with a Si

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loading of 1.08 mg/cm2 showed that capacities in the range of 900−2200 mAh g−1 can be attained from 2C to 0.2C, corresponding to areal capacities of 1.0−2.4 mAh cm−2. Meanwhile, full cells assembled from 3D Cu@Si@Cu MPs anode and LiFePO4 cathode demonstrated 80.2% capacity retention after 100 cycles. Encouragingly, the 3D Cu@Si@Cu-LiFePO4 batteries are capable of powering electronics by charging with a rotating triboelectric nanogenerator (TENG) with pulsed current. We further showed that the fabrication method demonstrated in this work can be extended to a range of anode materials including Al, Sn, Au and SnIn2O5.

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Figure 1. (a) Microscale Si film lose contact with Cu foil current collectors during lithiation/delithiation due to pulverization of the electrode and growth of thick SEI, the transports of Li+ and e– are largely blocked after cycling. (b) Multishelled Cu@Si@Cu microparticles coated on 3D Cu current collectors can maintain their structural integrity and contact with the current collectors to allow continuous transports of Li+ and e– during cycling.

RESULTS AND DISCUSSION Fabrication of Cu-based Core-shell Nanowires We developed a scalable approach for the fabrication of multishelled 3D Cu@M1@M2 (M1 and M2 represent Cu, Al, C, Si, Si3N4, SiO2, Ag, Ti, Ta, SnIn2O5, Au, V, Nb, W, Mg, Fe, Ni, Sn, ZnO, TiN, Al2O3, HfO2, or TiO2) structures on Cu substrates (Figure 2a). Firstly, Cu(OH)2 nanowire arrays were synthesized by the reaction of NaOH and (NH4)2S2O8 with Cu foils. Subsequently, Cu(OH)2@M1 core-shell structures were obtained by magnetron sputtering of M1 on the surface of Cu(OH)2 wires, which were then converted to 3D Cu@M1 nanowires via H2-reduction at an elevated temperature. Photographs of Cu(OH)2 nanowire arrays on Cu substrates (total area 1 m2) (Figure 2b), Cu(OH)2@Cu (Figure 2c), 3D Cu (Figure 2d) (~2000 cm2), 3D Cu(OH)2@Si, 3D


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Cu@Si (Figure 2e), as well as other Cu(OH)2@M1 and 3D Cu@M1 samples are shown in Figures 2f-j and Figure S1. Repeated deposition of M1 or M2 via magnetron sputtering, thermal evaporation or atomic layer deposition led to the formation of multishelled wires (denoted as 3D Cu@M1@M2). Following this preparation method, 3D Cu@Si@Cu nanowires and multishelled 3D Cu@Si@Cu microparticles with onion-like structure can be obtained. It is worth noting that this synthetic strategy offers great application potential not only on energy storage materials, but also in the fields of catalysis and electronics, because there are few examples of structurally stable metallic nanowires at high-temperature H2 atmosphere.22-25 b

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Figure 2. (a) Schematic illustration of the preparation process for Cu(OH)2 NWs, Cu(OH)2@M1 NWs, 3D Cu@M1 NWs, and 3D Cu@M1@M2 NWs. M1 & M2 refer to Cu, Al, C, Si, SiO2, Si3N4, Ag, Ti, Ta, SnIn2O5, Au, V, Nb, W, Mg, Fe, Ni, Sn, ZnO, or TiN. (b-j) Photographs of (b) Cu(OH)2 (100 squares), (c) Cu(OH)2@Cu (20 squares), (d) 3D Cu (20 squares), and (e-j) Cu(OH)2@M1 and 3D Cu@M1 samples. Note: each square in (b-j) is 10 cm x 10 cm.


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Figure 3. (a) SEM images of (a1) Cu(OH)2, (a2) Cu(OH)2@Cu, (a3) 3D Cu nanowires, and (a4) XRD pattern of 3D Cu nanowires. (b-u) SEM images of 3D Cu@M1 (M1 = Si, Al, Sn/SnO, Au, Ta, Nb, V, C, Ag, Ti, SnIn2O5, ZnO, W, Ni, TiN, MgO, Fe, TiO2), 3D Cu@Si@Cu, and 3D Cu@Si@C nanowires.

SEM and XRD results in Figure 3a1 and Figure S2 confirm that uniform Cu(OH)2 nanowires


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(16~20 μm in length) were prepared. After magnetron sputtering of Cu, the Cu(OH)2 nanowires were converted to Cu(OH)2@Cu core-shell nanowires (Figure 3a2, Figures S3a,b). Depending on the duration of sputtering, the thickness of the Cu layer can be controlled between 20 to 500 nm. Subsequent reduction in H2 of the Cu(OH)2@Cu core-shells led to the formation of Cu nanowires (denoted as 3D Cu, Figures 3a3-a4, Figures S3c-f). Coating with a Cu layer is critical to preserving the structural integrity of the nanowires during high-temperature H2 reduction -- without the protection Cu layer, kinked and broken wires with rough surface were formed (Figure S3g, Schematic Illustration S1). The surface roughness of the Cu nanowires can be tuned by the power and duration of magnetron sputtering, as well as H2-reduction temperature and duration (Figure S4). In addition to Cu, the combination of magnetron sputtering of M1 (M1 = Si, C, Sn, Al, Fe, Au, Ta, Nb, V, Ag, Ti, SnIn2O5, ZnO, W, Ni, TiN, Mg, Si3N4, SiO2) and H2-reduction using Cu(OH)2 nanowire template led to a variety of 3D Cu@M1 core-shell wires, as confirmed from SEM and TEM images, XRD patterns, and EDS analyses (Figures 3b-t, Figures S5-24). This method can also be used to synthesize 3D Cu@Al2O3, 3D Cu@HfO2, and 3D Cu@TiO2 (Figure 3u, Figures S25-27) via atomic layer deposition of Al2O3, TiO2, HfO2 on Cu nanowires, as well as 3D Cu@Au by thermal evaporation Au on Cu(OH)2 nanowires followed by H2-reduction (Figure S28).

Characterizations and Electrochemical Tests of 3D Cu@Si@Cu NWs The electrochemical performances of 3D Cu@Si@Cu NWs were examined and compared with two more structures, namely 3D Cu@Si NWs and planar Cu@Si@Cu NWs. 3D Cu@Si NWs were prepared in the same way as 3D Cu@Si@Cu NWs, but without the deposition of the outer Cu layer. Planar Cu@Si@Cu NWs were prepared by scraping off 3D Cu@Si@Cu NWs from the Cu substrate followed by mixing with carbon black/PVDF and casting onto a Cu foil. Figure 4 shows the battery test results, in which all specific capacities were calculated based on the mass loading of Si. Cycling performances in Figure 4a indicate much faster capacity fading for both 3D Cu@Si NWs and planar Cu@Si@Cu NWs than 3D Cu@Si@Cu NWs. The second-cycle capacity of 3D Cu@Si@Cu NWs was 1292 mAh g-1 and 87% was retained after 1500 cycles, while for 3D Cu@Si and planar Cu@Si@Cu NWs, the corresponding capacity retentions after 1500 cycles were 31% and 35%, respectively. It should be noted that the measured capacity of the planar Cu@Si@Cu electrode may be slightly larger than the actual capacity of Si due to the presence of small amount of carbon, although the difference should be smaller than 37 mAh g-1 considering the theoretical capacity of 370 mAh g-1 for graphite. Electrochemical impedance measurements of the three


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electrodes after 1500 cycles revealed lower interfacial charge transfer resistance of 3D Cu@Si@Cu NWs compared to that of planar Cu@Si@Cu NWs or 3D Cu@Si NWs (Figure S29a). Cyclic voltammetry (CV) tests indicate that the lithiation process of 3D Cu@Si@Cu NWs occurred at 0– 0.18 V via the alloying reaction of Si with lithium, while the delithiation process was observed at 0.19–0.52 V (Figure 4b). Compared with the first three CV cycles, the CV at 1500th cycle shows reduced peak intensity during lithiation process (Figure 4b). In addition to the superior cycling stability, 3D Cu@Si@Cu NWs also exhibited excellent high rate capability. When cycled from 1C to 10C, the sample delivered capacities in the range of 1300 and 990 mAh g-1, with 76% capacity retention (Figure 4c and raw data in Figure S29b), corresponding to 76% capacity retention that is much higher than reported Si/C/Si films with ~40% capacity retention from 1C to 10C.15 The good rate performance of 3D Cu@Si@Cu NWs can be attributed to the presence of highly conductive Cu as the core and outer layers of the nanowires.14,15 In addition, the voltage profiles of 3D Cu@Si@Cu in Figure 4d reveal similar lithiation and delithiation potentials between 1C and 10C, indicating that even at very high rates lithium ions can rapidly pass through the SEI layer and electrons can be efficiently transferred between the 3D Cu collectors and Si. Notably, when 3D Cu@Si@Cu NW anode was subject to ultra-long cycling test for 10000 cycles at 10C followed by 2000 cycles at 5C, the final capacity was still maintained at 569 mAh g-1 (Figure 4e). It is worth noting that because the 3D Cu@Si NW anodes are free of binder and carbon black, it can serve as a good model system to examine the reuse of Si electrodes. As shown in Figure 4f, the capacity of 3D Cu@Si NWs decreased from 1054 to 379 mAh g-1 (36% capacity retention) after 2000 cycles. After etching the SEI with HCl followed by H2-reduction, the capacity of 3D Cu@Si NWs was recovered to 904 mAh g-1, and it was maintained at 613 mAh g-1 after another 1000 cycles due to partial removal of the SEI after etching (Figures S30-33).


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Figure 4. (a) Cycling performances of 3D Cu@Si, 3D Cu@Si@Cu and planar Cu@Si@Cu NWs at 1C between 1 to 0.01V for 1500 cycles (1C = 3600 mA g–1 of Si). (b) CV curves of 3D Cu@Si@Cu NWs for the 1st, 2nd, 3rd, and 1500th cycles at a scan rate of 0.2 mV s-1. (c) Rate performances of 3D Cu@Si and 3D Cu@Si@Cu NWs cycled at various rates from 1C to 10C. (d) Discharge/charge profiles of 3D Cu@Si@Cu NWs cycled at various rates from 1C to 10C. (e) Cycling performance and CEs of 3D Cu@Si@Cu NWs cycled at 10C for 10000 cycles and 5C for 2000 cycles. (f) Cycling performance and CEs of 3D Cu@Si NWs for 2000 cycles. The electrode was then etched with HCl and reduced with H2 to remove SEI before it was cycled for another 1000 cycles. The Si loading is ~0.02 mg/cm2.

The structural and morphological changes of 3D Cu@Si and 3D Cu@Si@Cu NWs before and after cycling test were carefully examined. TEM images before cycling test reveal that the Si coated on Cu NWs is amorphous (Figures 5a,b). After Cu sputtering, a uniform layer of crystalline Cu


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with a thickness of 80 nm was coated on the Si surface (Figures 5c-e). Raman spectra of both 3D Cu@Si and Cu@Si@Cu exhibited a broad peak between 465 and 505 cm-1 that can be ascribed to amorphous Si and SiO.26-28 For 3D Cu@Si NWs, after battery cycling test, an SEI layer was formed on the NWs and it can be etched with HCl to form a porous structure, indicating that the SEI on 3D Cu@Si NWs is not stable (Figures 5g1-g4). However, 3D Cu@Si@Cu NWs exhibited a stable structure after 1500 cycles of battery test followed by HCl etching (Figures 5h1-h4). Even after 12000 cycles at high rates, the core-shell structure 3D Cu@Si@Cu NWs with an SEI layer was still well maintained (Figure S34), indicating much improved cycling stability than that of 3D Cu@Si NWs.

Figure 5. (a) TEM and (b) HRTEM images of a Cu@Si NW. (c) HAADF image and the corresponding elemental mappings of a Cu@Si@Cu NW. (d) TEM image a Cu@Si@Cu NW and the schematic illustration of its cross section. (e) HRTEM image of the surface of a Cu@Si@Cu NW. (f) Raman spectra of 3D Cu@Si and 3D Cu@Si@Cu NWs. SEM images of 3D Cu@Si NWs (g1) before and (g2) after battery best. (g3) SEM and (g4) TEM images of 3D Cu@Si NWs after battery test and HCl etching. SEM images of 3D Cu@Si@Cu NWs (h1) before and (h2) after battery best. (h3) SEM and (h4) TEM images of 3D Cu@Si@Cu NWs after battery test and HCl etching.


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Characterizations and Electrochemical Tests of 3D Cu@Si@Cu Microparticles b

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3D Cu@Si@Cu NWs displayed a long cycling life as battery anode, but the low Si loading limited its areal capacity. By additional magnetron sputtering of Si and Cu alternatively on 3D Cu@Si@Cu NWs, multishelled Cu@Si@Cu microparticles (MPs) supported on 3D Cu current collectors can be obtained. Battery tests showed that when the Si loading reached 0.22 mg/cm2


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with particle sizes ranging from 0.8 to 4 μm, the resulting 3D Cu@Si@Cu MPs delivered a capacity of 807 mAh g-1 while 77% of the initial capacity was retained after 800 cycles (Figures 6a,b). With further increase of Si loading to 1.08 mg/cm2 (with particle sizes of 5~15 μm), 81% of the initial capacity (852 mAh g−1) can be maintained after 200 cycles with an average CE of 99% after the 1st cycle (Figures 6c,d, raw data in Figure S35). Rate cycling performance and CEs of 3D Cu@Si@Cu MPs at various rates from 2C to 0.2C are displayed in Figure 6e, which shows the capacities varied in the range of 900−2200 mAh g−1, corresponding to areal capacities of 1.0−2.4 mAh cm−2. 3D Cu@Si@Cu MPs delivered capacities of 1174 and 907 mAh g-1 at 1C and 2C respectively, corresponding to 77.3% capacity retention that is higher than amorphous silicon nanolayer implantative in activated graphite particles (~56% capacity retention from 1C to 2C).29 Electrochemical impedance measurements of 3D Cu@Si@Cu MPs indicate that no obvious impedance increase for the first 10 cycles (Figure 6f and Figure S36). Compared with Cu@Si@Cu nanowires (87% capacity retention after 1500 cycles), the cycling performance of Cu@Si@Cu microparticles (91% and 81% capacity retention after 150 and 200 cycles) decreased mainly because the large volume expansion of microstructured Si. Compared with 3D Cu@Si@Cu MPs that were tightly bound to 3D Cu current collectors, Cu@Si@Cu films prepared by alternating Si and Cu sputtering on Cu foil started to crack when the thickness reached 10 micrometers (Figure 7a), indicating that the 3D structure is necessary to obtain stable electrodes with high Si-loading. Cross-sectional SEM images of the 3D Cu@Si@Cu MPs indicate that the electrode integrity was retained after 200 cycles of battery test, although the length of the MPs increased by ~25%, corresponding ~95% volume expansion (Figures 7b,c). In addition, although the surface of the MPs started to show cracks and growth of SEI after the battery test, the morphology of 3D Cu@Si@Cu MPs was well maintained and severe pulverization was not observed after the SEI was etched (Figures 7d-i). The well-retained electrode integrity after cycling test indicate that 3D Cu@Si@Cu MPs were able to accommodate volume expansion of Si via the interior Cu network that connects Si to the 3D Cu current collectors.14


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Figure 7. (a) SEM image of Si@Cu microscale film by magnetron sputtering of Si on planar Cu foil. (b) Left: cross-sectional SEM image of 3D Cu@Si@Cu MPs before cycling test and the schematic illustration of the layered structure. (c) Cross-sectional SEM image of 3D Cu@Si@Cu MPs after cycling test. Top-view SEM images of 3D Cu@Si@Cu MPs (d,e) before and (f,g) after 200 cycles of battery test. (h,i) SEM images and the corresponding elemental mappings and EDS spectrum of 3D Cu@Si@Cu MPs with etched SEI after cycling test.

The structural advantages of 3D Cu@Si@Cu NWs and MPs as LIB anodes with various Siloadings were further examined in full-cell configuration by pairing with LiFePO4 cathodes. First, we prepared Li-LiFePO4 hall cells (LiFePO4:carbon black:PVDF = 80:10:10 by weight) and obtained long cycling life between 2.0 and 3.6 V (Figure S37). For full cells, the cathode capacity was about 3 times that of the anode. As shown in Figures 8a,b and raw data in Figure S38, the 3D Cu@Si@Cu-LiFePO4 full cells achieved high initial capacities above 1100 mAh g−1, and demonstrated capacity retentions of 98.6%, 74.6%, 72.4% after 150 cycles at Si loadings of ~0.02, 0.22, and 1.08 mg/cm2, respectively. The average CEs after first cycle were 98.6, 99.0, and 99.3%, respectively. The lithiation potential shows a sloping profile between 3.2 and 3.4 V, and the delithiation potential is from 3.1 to 2.6 V (Figure 8c). Impedance measurements of 3D Cu@Si@Cu-LiFePO4 with 1.08 mg/cm2 of Si loading showed little change before and after 150


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cycles, indicating limited increase of internal resistance in full-cell configuration (Figure 8d). The rate capability and discharge/charge profiles were tested at rates of 0.2C–4C (Figures 8e,f). The capacity decreased at 0.2C but maintained a good rate performance between 0.4–4C. Compared with Li-LiFePO4 hall cells, the 3D Cu@Si@Cu-LiFePO4 full cells showed different lithiated and delithiated processes (CV and discharge/charge curves in Figure S39) due to the higher voltage plateau (0.2∼0.5 V) of LixSi in full cell. It is worth noting that although the 3D Cu@Si@Cu anodes showed a long cycling life with high capacity, the deposition rate of such structures should be improved for viable applications, which may be achieved with new technologies such as rapid vapor deposition.30

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3D Cu@Si@Cu-LiFePO4 with 0.22 mg/cm2 of Si loading. (d) Impedance measurements of 3D Cu@Si@CuLiFePO4 battery with 1.08 mg/cm2 of Si loading before and after 150 cycles. (e) The rate cycling performance and (f) voltage profiles of 3D Cu@Si@Cu-LiFePO4 battery with 1.08 mg/cm2 of Si loading at various rates from 0.2 to 4.0C.

Charging 3D Cu@Si@Cu-based batteries with triboelectric nanogenerators Triboelectric nanogenerators (TENG) are promising mechanical energy harvesting devices. The pulsed output current of a TENG may show different concentration polarization when it is used to charge LIBs compared to constant current charging.31-33 LiFePO4-Li4Ti5O12 and LiMn2O4-C batteries have shown potential applications in storing pulsed output current delivered by TENG.32,34 Since Si is the most promising next-generation anode for LIBs with the potential to reduce battery size, we tested 3D Cu@Si@Cu-Li (0.02 mg/cm2 Si loading) half cells and 3D Cu@Si@CuLiFePO4 (0.22 mg/cm2 Si loading) full cells charged by a rotating TENG. The TENG was connected to a rectifier and a Zener diode to obtain a pulsed current (Figures 9a,b, Figures S40,41). After 1200 s of TENG charging (peak current ~100 A), the voltage of 3D Cu@Si@Cu-Li increased to 1.0 V. The cell was then discharged at 20 μA for ~2300 s before the voltage dropped to 0.01 V. This charge/discharge process was repeated for 10 cycles and the corresponding times for charge/discharge are shown in Figure 9c and Figure S42. Comparison of the charge/discharge voltage profiles before, after, and during charging with TENG revealed similar discharging behaviors after the cell was charged with pulsed current of a TENG or at a constant current (Figures 9d-f), confirming the feasibility of using 3D Cu@Si@Cu anode for pulsed current charging. For 3D Cu@Si@Cu-LiFePO4 full cells, the TENG charging test was conducted at a peak current of 160 μA (Figure 9g, Figure S43). And the voltage of the full cell increased to 3.7 V after 4000 s. The cell was then discharged at 100 μA constant current for 2450 s (Figure 9h). This charge/discharge process was repeated for 5 cycles. The 3D Cu@Si@Cu-LiFePO4 full cell charged by TENG can power two electric watches connected in series (Figure 9i). It should be noted that when charged at a constant current of 100 μA, the 3D Cu@Si@Cu-LiFePO4 full cell exhibited a charging voltage plateau at ~3.31 V (Figure 9j). However, by charging the battery with TENG, the charging voltage plateau increased to ~3.48 V (Figures 9k,l, Figure S44). This increased voltage plateau during charging with TENG is consist with previous investigations.32,34 In addition, compared with charging at constant current, the cell voltage oscillated (with an amplitude of ~0.01 V) when it was charged by TENG pulsed current as shown in the enlarge voltage profiles (Figures


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9f,l). The above results demonstrate reversible Li-ion extraction (delithiation) and insertion (lithiation) of 3D Cu@Si@Cu anodes by TENG charging and constant-current discharging, confirming Si-based anodes are promising candidate for storage energy harvested by TENG with pulsed output current. b

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Figure 9. (a) The charging circuit. (b) Short-circuit current output by the TENG for charging 3D Cu@Si@CuLi cell. (c) Cycling performance of 3D Cu@Si@Cu-Li half-cell charged by TENG and discharged at a constant current. Voltage profiles of 3D Cu@Si@Cu-Li charged at (d) a constant current and (e) pulsed current by TENG. The cell was discharged at a constant current. (f) Enlarged charging profiles of 3D Cu@Si@Cu-Li, top: charging by TENG, bottom: constant current charging. (g) Short-circuit current output by the TENG for charging 3D Cu@Si@Cu-LiFePO4 full-cell. (h) Cycling performance of 3D Cu@Si@Cu-LiFePO4 full-cell charged by TENG pulsed current and discharged at a constant current. (i) Photograph of two electrical watches powered by a 3D Cu@Si@Cu-LiFePO4 full-cell charged by TENG. Voltage profiles of 3D Cu@Si@Cu-LiFePO4 full cell charged at (j) a constant current and (k) by a TENG with pulsed current. The cell was discharged at a constant current. (l) Enlarged charging profiles of 3D Cu@Si@Cu-LiFePO4, top: charging by TENG, bottom: constant current


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0

Figure 10. Discharge/charge profiles for the 2nd, 5th, and 10th cycles of (a) 3D Cu@Al, (c) 3D Cu@Sn/SnO, (e) 3D Cu@SnIn2O5, and (g) 3D Cu@Au. 1C = 2200 mA g–1 for Al, 990 mA g–1 for Sn/SnO, 900 mA g–1 for SnIn2O5, 400 mA g–1 for Au. The cycling performances and CEs of (b) 3D Cu@Al, (d) Cu@Sn/SnO, (f) Cu@SnIn2O5, and (h) Cu@Au.

Extension to 3D Cu@Al, Cu@Sn/SnO, Cu@SnIn2O5, and Cu@Au anodes Finally, we would like to emphasize that the preparation process of 3D Cu@Si can be extended to a variety of anode materials. High-capacity anode materials, including Al (2235 mAh g−1),35-37


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Sn (990 mAh g−1), SnO (876 mAh g−1), SnO2 (783 mAh g−1),38-41 SnIn2O5 (~900 mAh g−1),42,43 and Au (~400 mAh g−1),44-46 were selected as examples to demonstrate their discharge/charge performance in 3D Cu@M structure (XRD patterns of the samples can be found in Figure S45). Half-cell tests showed that 3D Cu@Al NWs delivered a capacity of 1127 mAh g−1 (Figures 10a,b), 3D Cu@Sn/SnO NWs exhibited a capacity of 766 mAh g−1 (Figures 10c,d), 3D Cu@SnIn2O5 NWs showed good rate performance from 0.5C to 8C with capacities ranging from 705 mAh g−1 to 358 mAh g−1 (Figures 10e,f), and 3D Cu@Au NWs displayed a stable cycling performance at 1C for 36 cycles (Figures 10g,h). It is worth noting that the capacity/cycling stability of these materials may be further improved by tuning the deposition processes to obtain optimal thickness and loading.

CONCLUSIONS Our strategy to prepare thermally stable Cu nanowire array in large scale by depositing a protection layer before H2-reduction of Cu(OH)2 nanowires is different from previous reports in which Cu(OH)2 are reduced directly to form Cu particles47 or templates are used to obtain small amount of Cu nanowires.48 We demonstrated that based on our approach, Cu(OH)2 and Cu nanowire arrays with areas larger than 100 cm2 can be prepared. During the Cu sputtering process, the presence of –OH groups in large amount on the surface of Cu(OH)2 nanowires facilitated the adsorption of Cu atoms to form Cu(OH)2@Cu core-shell structure (FTIR spectrum in Figure S46). Typically, when Cu(OH)2 or CuOx nanowires are subject to H2-reduction, voids are created and enhanced diffusion of atoms occurs, leading to severe deformation and aggregation of the resulting nanowires.49-51 In our high-temperature H2-reduction processes (300–600 oC), the deposited outer protection layer can maintain the core-shell structure. In addition, we have developed a versatile method for the fabrication of 3D Cu@M1@M2 (M1 and M2 represent a variety of metals and metal oxides) structures. Among these structures, 3D Cu@Si@Cu nanowires and microparticles with tunable Si loading up to 1 mg cm-2 have proved to be stable anode materials for LIBs. Without using any conductive additives and binders, the 3D Cu@Si@Cu microparticles have achieved stable cycling in both hall-cell and full-cell configurations. Our structural design allows the Si-based microparticles to attain electrode integrity after cycling, thanks to the Cu network which prevents particle pulverization. Meanwhile, we have demonstrated that the Si-based anodes are promising candidates for storing energy generated by triboelectric nanogenerators with pulsed output current. We believe our strategy provides a route to a variety of composite nanowire materials and offers a promising approach for preparing binder17 Environment ACS Paragon Plus

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free anodes of LIBs with enhanced electrochemical performances.

METHODS Synthesis of Cu(OH)2 Nanowires on Cu Foil. Cu foil (99.99%) was washed by ethanol, diluted hydrochloric acid and deionized water to remove surface impurities. A 200 ml aqueous solution containing sodium hydroxide (NaOH, 3 mol/L) and ammonium persulfate ((NH4)2S2O8, 0.1 mol/L) was prepared. Then, the Cu foil was immersed in the solution at room temperature for 0.5–2 h. Afterwards, the Cu foil washed with distilled water and ethanol, and dried under vacuum at 80 oC for 48 h.52 Preparation of Cu(OH)2@M and 3D Cu@M (M = Cu, Al, C, Si, SiO2, Si3N4, Ag, Ti, Ta, SnIn2O5, Au, W, V, Nb, Mg, Fe, Ni, Sn, ZnO, TiN) Core-shell Nanowires. The prepared Cu(OH)2 nanowires on Cu foil were loaded into a multifunctional ion coating machine (Kurt J. Lesker PVD75 Proline) for magnetron sputtering of Cu, Al, C, Si, SiO2, Si3N4, Ag, Ti, Ta, SnIn2O5, Au, W, V, Nb, Mg, Fe, Ni, Sn, ZnO, TiN at a chamber pressure below 1×10-6 Torr. The deposition power was set between 50 and 300W and the deposition times varied from 0.5 to 1 h (Table S1). The resulting Cu(OH)2@M nanowires on Cu foil was then placed in a tube furnace and treated under 5% H2/Ar at 300 oC–600 oC for 1 h–2 h (ramping rate at 5 oC min-1) to obtain 3D Cu@M nanowires. Synthesis of 3D Cu@Si@Cu Microparticles. The as-prepared 3D Cu@Si nanowires were sputtered with Cu and Si alternatively for multiple cycles at deposition powers of 100–300W with durations of 0.5–50 h. Reuse of 3D Cu@Si Electrodes. The cells using 3D Cu@Si anodes after cycling were disassembled and washed with dimethyl carbonate in an argon-filled glove box. Then, the electrodes were rinsed with diluted hydrochloric acid, distilled water and ethanol, and finally dried under vacuum at 80 oC. At last, the electrodes were placed in a tube furnace and reduced in 5% H2/Ar at 400 oC for 1 h (ramping rate at 5 oC min-1). Materials Characterization. X-ray diffraction patterns (XRD) were recorded on a PANalytical X'Pert3 (Netherlands) using Cu Kα radiation (λ = 1.54056 Å). The microscopic feature of the samples was characterized by field-emission scanning electron microscopy (FESEM) (FEI/Nova Nano SEM 450) and transmission electron microscopy (TEM) (FEI/Tecnai G2 F20 S-TWIN TMP) with energy-dispersive X-ray spectrometer. Thermal gravimetric (TG) analysis was carried out on a Mettler Toledo TGA/DSC 1 STARe system at a heating rate of 10 oC min-1 in O2/Ar at 50–800 18 Environment ACS Paragon Plus

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o

C. Raman spectroscopy (HORIBA/LabRAM HR Evolution, France) with an excitation

wavelength of 532 nm and a beam spot size of 1–2 mm was used to characterize the samples. To observe the electrode thickness and surface morphology after cycling, the cells were disassembled and washed with dimethyl carbonate in an argon-filled glove box. To observe the variations after removing the SEI film on the surface of electrode, the electrodes were rinsed with diluted hydrochloric acid, distilled water and ethanol, and finally dried under vacuum at 80 oC for 24 h. FTIR-ATR measurements were performed on a Burker Vertex80v spectrometer from 600 to 4000 cm-1 under vacuum. Electrochemical Measurements. 3D Cu current collectors with deposited Al, Sn/SnO, SnIn2O5, Au, Si, Si@Cu were dried at 120 oC under vacuum for 24 h. The weight difference before and after the deposition of active materials is regarded as the mass of loading of Si, Al, Sn/SnO, SnIn2O5, or Au (0.02–1.08 mg/cm2). The weight ratio of Si:Cu for 3D Cu@Si@Cu samples was 81:19. For planar Cu@Si@Cu NWs samples, the Cu@Si@Cu NWs were scraped off the Cu substrate and mixed with carbon black, polyvinylidenefiuoride (PVDF) at a weight ratio of 80:10:10 with Nmethylpyrrolidone (NMP) as the solvent. The resulting slurry was cast onto a Cu foil, and then dried at 40 oC for 24 h and at 120 oC under vacuum for 24 h. The cathode electrodes were fabricated by casting slurries on Al current collectors with commercial lithium iron phosphate (LiFePO4), carbon black, and PVDF binder in a weight ratio of 80:10:10. All electrochemical measurements were carried out at room temperature in two-electrode 2032 coin-type cells. CR2032 coin-type cells were assembled in an Ar-filled glovebox with lithium foils as the counter electrode (half cells) or lithium iron phosphate (LiFePO4) as the cathode (full cells) and polypropylene microporous films (Celgard 2400) as separators. The liquid electrolyte was 80 μL LiPF6 (1 mol L-1) in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, v/v). N-methyl-2-pyrrolidone (NMP) was used as the solvent. Galvanostatic discharge/charge tests were carried out with a CT2001A LAND testing instrument in voltage ranges between 0.01–1.0 V, 0.01–2.0 V, 0.01–3.0 V and 2.0–3.6 V at current densities of 0.2C–10C. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were performed on Solartron potentiostat or CHI660E electrochemical workstation, and the frequencies of the EIS ranged from 100 kHz to 1 Hz. The output short current and open-circuit voltage of the TENG were measured by a Keithley electrometer (Keithley 6514).

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at

.

Additional methods, figures including photographs, XRD patterns, SEM/TEM images, EDS analysis, FTIR spectra, TG analysis, CV curves, discharge/charge profiles, impedance spectra, short-circuit current and open-circuit voltage of the TENG, and the table of the power and duration of magnetron sputtering for the preparation of various samples.

ACKNOWLEDGMENTS The authors gratefully acknowledge the supports from the Minister of Science and Technology (2016YFA0202702) and National Natural Science Foundation of China (Grant No. 51402299).

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Multishelled Si@Cu Microparticles Supported on 3D Cu Current

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