Free-Standing Copper Nanowire Network Current Collector for

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Free-Standing Copper Nanowire Network Current Collector for Improving Lithium Anode Performance Lei-Lei Lu, Jin Ge, Jun-Nan Yang, Si-Ming Chen, Hongbin Yao, Fei Zhou, and Shu-Hong Yu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b01581 • Publication Date (Web): 02 Jun 2016 Downloaded from http://pubs.acs.org on June 3, 2016

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Free-Standing Copper Nanowire Network Current Collector for Improving Lithium Anode Performance

Lei-Lei Lu, Jin Ge, Jun-Nan Yang, Si-Ming Chen, Hong-Bin Yao*, Fei Zhou, Shu-Hong Yu*

Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, CAS Center for Excellence in Nanoscience, Department of Chemistry, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, China.

* Corresponding author. [email protected], [email protected]

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Graphic Abstract:

Confining the lithium dendrites in the porous nanowire network: A rational design of copper nanowire (CuNW) network current collector to improve the safety and the stability of lithium anode is reported. As high as 7.5 mAh cm-2 of lithium can be plated into the free-standing CuNW current collector without any growth of dendritic lithium. The lithium metal anode based on the Cu NWs exhibited high Coulombic efficiency (average 98.6% during 200 cycles).

Keywords: Lithium metal; anode; current collector; copper nanowires; Coulombic efficiency.


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Abstract: Lithium metal is one of the most attractive anode materials for next-generation lithium batteries due to its high specific capacity and low electrochemical potential. However, the poor cycling performance and serious safety hazards, caused by the growth of dendritic and mossy lithium, has long hindered the application of lithium metal based batteries. Herein, we reported a rational design of free-standing Cu nanowire (CuNW) network to suppress the growth of dendritic lithium via accommodating the lithium metal in three dimensional (3D) nanostructures. We demonstrated that as high as 7.5 mAh cm-2 of lithium can be plated into the free-standing copper nanowire (CuNW) current collector without the growth of dendritic lithium. The lithium metal anode based on the CuNW exhibited high Coulombic efficiency (average 98.6% during 200 cycles) and outstanding rate performance owing to the suppression of lithium dendrite growth and high conductivity of CuNW network. Our results demonstrate that the rational nanostructural design of current collector could be a promising strategy to improve the performance of lithium metal anode enabling its application in next-generation lithium-metal based batteries.


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Lithium metal has long been considered as an ideal anode material in lithium batteries because of its low gravimetric density (0.53 g/cm3), low potential (−3.04 V vs. the standard hydrogen electrode), and high theoretical specific capacity (3860 mAh/g).1 However, the problems of dendritic and mossy Li formation hindered the practical application of Li metal anode in past four decades.2 As alternative, Sony and Asahi Kasei used graphite to replace Li as the anode to suppress the lithium dendrite growth, which gained great success in commercial lithium ion batteries.3, 4 However, the limited theoretical energy density of the traditional lithium ion batteries will no longer meet the requirement of advanced energy storage, especially the vehicle electrification.5, 6 To further improve the energy density of batteries requires exploiting new chemistry beyond lithium ion. Within this context, Li metal is revived to develop advanced Li metal based batteries with high energy densities.2, 7, 8 In particular, recent researches on lithium sulfur (Li-S) and lithium air (Li-air) batteries have shown potentials of increasing the energy density of lithium batteries by ~5-10 times.9 Intensive progresses have recently been gained on the sulfur and air cathodes to improve the electrochemical performance of Li-S and Li-air batteries.10-12 In contrast, adequate attention is lack to be paid to the equivalently problematic lithium anode in past several years. One distinct difference between Li metal and other anode materials is that the Li metal is a ‘host-less’ anode.13, 14 The stripping/plating of Li on the electrode surface leads to the most volume change comparing to the host anode materials. Associating with this uncommon volume change, the brittle solid-electrolyte interface (SEI) spontaneously formed on the Li metal surface cracks and the fresh Li metal underneath exposes to the electrolyte,


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which largely changes the Li ions flux and accelerates the growth of lithium dendrites. The formed sharp Li metal dendrites will pierce through the separator and trigger internal short circuits, bringing a serious safety hazard.15-17 Beside the safety concern, the high chemical reactivity and large surface area of Li dendrite will cause a continuous consumption of Li and electrolyte with the repeated formation/dissolution of SEI during cycling, giving rise to a low Coulombic efficiency (CE).7, 18 To address the safety concern and the low CE caused by Li dendrite growth, commonly adopted strategy is to reinforce the SEI layer by adding various electrolyte additives.19 In previous works, hydrogen fluoride (HF),20 lithium fluoride (LiF),21 copper acetate (Cu(CH3COO)2),22 vinylene carbonate (VC)-lithium nitrate (LiNO3),23 lithium polysulfide,24 Cs+ and Rb+ ions25 have been explored to generate a reinforced protective layer on the Li metal surface. However, the reinforced SEI is still not strong enough to suppress the Li dendrite growth and is far away from the goal we anticipated. Recently, instead of improving the stability of the intrinsic SEI layer, the ex situ coated artificial SEI layers such as interconnected hollow carbon spheres,14 boron nitride/graphene layers,26, 27 and a graphite layer28 have shown their potentials for improving Li metal anode performance. While numerous studies have focused on the SEI layer design to stabilize the interface between the Li metal and the electrolyte, the dendrite formation nature of Li metal plating on the current collector is not changed. Thus, new strategies to suppress the Li dendrite growth via tuning Li metal plating models are attractive. The origin of the growth of Li dendrites arises from the spatial inhomogeneity in charge distribution over the entire electrode surface.29 To prevent the growth of Li dendrites, the


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control of Li ion flux in the Li plating/stripping surface is vital.13 Due to the ‘host-less’ feature of Li metal anode, the plating of Li usually occurred on the planar copper or Li foils. However, the Li ion flux distribution on these traditional surfaces has been demonstrated to be inhomogeneous, which is the main reason causing the growth of Li dendrites.7, 14, 29 Very recently, the nanostructured modification of the surface of Li plating has exhibited the potential of changing the Li plating behavior and thus improving Li anode stability.30, 31 These processes have been demonstrate to be a promising route to inhibit the growth of Li dendrites via novel nanostructural designs of electrodes or current collectors.32-34 Herein, we report a free-standing Cu nanowire (CuNW) network current collector to accommodate the Li metal inside the porous nanostructure to limit the growth of Li dendrite and enhance the cycling stability of the Li metal anode. Owing to the opened porous structure of as-prepared current collector, Li metal can be plated on the entire surface of 3D network and fill the pores forming a CuNWs reinforced Li composite anode without the formation of the Li dendrite. Totally, 7.5 mAh of Li metal can be plated into the pores of CuNW current collector with the area of 1.1 cm2, which largely exceeds the areal capacity of the commercial battery electrode. More attractively, the Li metal can be also thermally infused into the free-standing CuNW network forming a 3D current collector localized Li metal anode. Using this Li-CuNWs nanocomposite anode to couple with the LiCoO2 cathode, we demonstrated a full cell with high rate capability and cycling stability. The essential principle that using 3D network of CuNWs to inhibit the growth of Li metal dendrite is illustrated in Figure 1. On the surface of traditional planar Cu current collector, the lithium ion flux is concentrated on those spots where the cracks of SEI appeared. In the


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following Li plating process, the growth of Li at the cracking spots is accelerated by the concentrated Li ion flux and the Li metal dendrite grows very quickly (Figure 1a). In contrast, the high surface area of interconnected CuNW network largely decreases the ion flux density and thus enhances the homogeneity of Li ion flux distribution (Figure 1b), which results in the homogeneous plating of Li in the porous current collector.35 In addition, even if Li dendrites grow in the CuNW network in some cases, these dendrites will be confined in the network structure and merge together with each other forming the bulk Li in the current collector before piercing the separator.

Figure 1. Schematic illustration of Li ion flux distribution and Li-metal plating models on different current collectors. (a) Planar Cu foil and (b) CuNW network. The distributions of Li ion flux between two electrodes is schematically presented by the dash lines; the grey parts on current collectors denote the plated Li. To fabricate the free-standing CuNW network current collector, we used a facile solvent evaporation assisted assembly technique to prepare a CuNW membrane under ambient conditions and then activated it by the H2/Ar (5%/95%) flow at 450 oC. The basic building blocks, CuNWs, were synthesized via a scalable and low cost solution method by using hydrazine to reduce copper nitrate in the NaOH aqueous solution.36 Schematic illustration of


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the formation of the CuNW membrane is shown in Figure S1 (See Supporting Information). To prepare a CuNW membrane, 0.5 ml of ethanol-hydrazine suspension of CuNWs was poured into a stainless steel mould and then let the solvents evaporate under ambient conditions. The self-evaporation of solvents brought the assembly of CuNWs forming a membrane (Figure 2a), which is composed of nanowires with tens of micrometers in length (Figure 2b). The phase of as-synthesized nanowires is confirmed as pure Cu by powder diffraction X-ray pattern (PXRD) (Figure S2a). However, the X-ray photoelectron spectroscopy (XPS) analysis (Figure 2 c) indicates that most of the surface of Cu NWs was oxidized (Cu2+) resulting in the high electronic resistance of as-fabricated CuNW membrane (Figure 2a). To reduce Cu2+ on the surface of Cu NWs and enable its high conductivity, CuNW membrane was then annealed under the H2/Ar (5%/95%) flow at 450 oC for 4 hours. After annealing, the phase of the membrane still maintained as pure Cu (Figure S2a). The XPS analysis (Figure 2c) confirmed the reduction of Cu2+ on the surface of CuNWs and the obtained CuNW membrane shows a good electrical conductivity with an areal resistance below 20 mΩ cm-2 (Figure S2b). The improvement of the electronic conductivity of the Cu NWs membrane was attributed to not only the reduction of Cu2+ on the surface of Cu NWs but also the junction connection of CuNWs (Figure 2e and 2f). Notably, the finally obtained CuNW membrane is flexible and robust, which can be bent to a certain degree (Figure S2c), indicating the integrity of the entire CuNW network. A lower annealing temperature (300 oC) was also tried to fabricate the CuNW network current collector. But only some parts of the CuNWs presented infused junctions (Figure S3a and b), which is not enough to form a robust CuNW network.


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Figure 2. (a) Photograph of the CuNW membrane before annealing and the multi-meter shows the ultrahigh resistance of the CuNW membrane. (b) SEM image of the CuNW membrane before annealing. (c) The comparison of the XPS spectra of the CuNW membrane before and after annealing. (d) Photograph of the CuNW membrane after annealing at 450 oC under H2/Ar (5%/95%) atmosphere and the multi-meter shows that the resistance of the CuNW membrane is only ~2.1 Ω. (e) SEM image of the CuNW membrane after annealing at 450 oC under H2/Ar (5%/95%). (f) The magnified SEM image of the CuNW membrane after annealing at 450 oC under H2/Ar (5%/95%) atmosphere showing the infused junction connection. The free-standing CuNW membrane annealed at 450 oC is chosen as the current collector for Li metal stripping/plating due to its robust network nanostructures and high electronic conductivity. Firstly, we theoretically calculated the porosity of the free-standing CuNW membrane assuming that the shape of the membrane is a well-defined cylinder shape (the detailed calculation process please see Supporting Information). The results (Table S1) show that the porosity of the CuNW network membrane is up to 83.9%. According to the void space volume in the CuNW membrane, we can calculate out the theoretical loading of Li metal in the porous structure, which can be as high as 9.06 mAh cm-2. The high theoretical loading amount of Li metal enables the CuNW membrane to be potentially applied in the


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high energy density battery. To demonstrate the efficiency of the porous nanostructural design of the current collector, the plating behavior of Li metal on the planar Cu foil and in the 3D CuNW membrane were investigated in the coin cell by using the electrolyte of 1M lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) in 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME) (1:1 by volume) with 1wt.% LiNO3 and 5 mM Li2S8 additives. The tested cells were first cycled at 0– 1 V (vs. Li+/Li) at 50 µA for five cycles for the contamination removal and interface stabilization (Figure S4).The results show that only plating the Li of 2.5 mAh cm-2 on the planar Cu foil a lot of Li metal dendrites were formed (Figure S5). In contrast, the top surface of the 3D CuNW membrane still retained the porous nanostructural morphology after plating the Li of 2.5 mAh cm-2 (Figure 3a). With increasing the plating capacity to 5 mAh cm-2, the porous structure of the top surface of the CuNW membrane can be still observed (Figure 3b) but the diameter of the NWs increased intensively indicating that the Li metal deposited along with the surface of CuNWs. After further increasing the plating capacity to 7.5 mAh cm-2 approaching to the theoretical capacity of Li (9.3 mAh cm-2) that the CuNW network can accommodate, the pores on the top surface of CuNWs were fully filled with Li metal as shown in Figure 3c. But, when the capacity of Li plating increased to 10 mAh cm-2, the dendrite of Li grew on the top surface of the CuNW network as shown in Figure 3d. The stripping process of Li metal from the 3D CuNW current collector was also investigated along with the variation of the stripping capacity from 2.5 mAh cm-2 to 7.5 mAh cm-2. As shown in Figure 3e-g, the Li metal was gradually stripped from the surface of 3D CuNW membrane. Meanwhile, the 3D CuNW network structure remained the integrity without any


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collapse after the Li stripping. To show the stability of the CuNW network as the current collector for cycling, the CuNW network after ten cycles of 10 mAh cm-2 of plating/stripping of Li was also characterized by SEM. As shown in Figure 3h, at the Li plating of 7.5 mAh cm-2, the top surface of the CuNW network was filled with Li metal but without the formation of Li dendrites.

Figure 3. Morphology variation of the porous CuNW current collector with different amount of Li plating/stripping. SEM images of the CuNW membrane surface after plating (a) 2.5 mAh cm-2, (b) 5 mAh cm-2, (c) 7.5 mAh cm-2 and (d) 10 mAh cm-2 of Li metal into 3D current collectors; then after stripping (e) 2.5 mAh cm-2, (f) 5 mAh cm-2 and (g) 7.5 mAh cm-2 of Li, respectively. (h) SEM image of the Li anode with a 7.5 mAh cm-2 of Li plating after 10 cycles. The Li plating/stripping states (a–h) are indicated in (i) galvanostatic discharge/charge voltage profile at the current density of 1 mA cm-2. To clearly show the spatial distribution of the Li metal in the porous CuNW network current collector, we further characterized the cross-section of the porous CuNW membrane at different capacities of Li plating by SEM. As shown in Figure 4a, the plating of Li metal


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initially occurred underneath the top surface of the CuNW membrane at the status of the 2.5 mAh cm-2 of Li plating. The magnified SEM in Figure 4b shows that most of Li metal (denoted by dark green color) preferred to deposit on the top part of the CuNW network due to the shorter Li ion diffusion pathway comparing to the bottom part. The deposition of Li on the surface of the CuNW was confirmed by the scanning TEM-electron energy loss spectroscopy (STEM-EEL) analysis as shown in Figure S6. The bottom part of the CuNW network still retained the hollow porous nanostructures as shown in Figure 4c. The detailed comparison of the morphologies of the CuNW before and after the Li plating (Figure S7) indicates that the Li plating occurred on the surface of the CuNW leading to the increase of the diameter of the CuNW and the plating of Li metal into the CuNW network gradually occurred from the top region to the bottom region. After increasing the plating capacity to 7.5 mAh cm-2, the cross-sectional SEM characterization (Figure 4d-f) shows that the micro-size particles of Li metal formed on the top part of the CuNW current collector, which means that the Li metal shells around the CuNWs further grew and emerged together to fill the pores inside the CuNW network. At the bottom part, the Li metal also deposited inside the pores of the CuNW network, which indicates that the plating of Li metal occurred in the entire CuNW network (Figure S8). When the amount of Li plating further increased to 10 mAh cm-2, the Li-metal plated in the 3D CuNWs is consisted of two parts with totally different morphologies (Figure 4g-i). One part is the Li metal dendrite (denoted by the bright green color) that grew on the top surface of the CuNW membrane (Figure 4h). The other part is the Li metal that is confined in the entire CuNW membrane filling the pores of network (Figure 4i). The analysis of the spatial distribution of the Li metal plated in the 3D CuNW network


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demonstrates that in the porous CuNWs current collector the deposition of Li metal gradually expand from the top of the CuNW network to the bottom and only after the pores of CuNW network were fully filled the excess Li metal would grow on the top surface forming the dendrite as shown in Figure 4j. The proposed mechanism for this top-to-bottom plating process of Li is schematically illustrated in Figure S9, which indicates that the position for the Li plating is determined by the competition between electron transfer and Li ion diffusion in the CuNWs membrane.

Figure 4. (a) Cross-sectional SEM image of overall 3D CuNW membrane with the Li plating of 2.5 mAh cm-2. (b) and (c) Corresponding magnified SEM images of top region and bottom region of (a), respectively. (d) Cross-sectional SEM image of overall 3D CuNW membrane with the Li plating of 7.5 mAh cm-2. (e) and (f) Corresponding magnified SEM images of top


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region and bottom region of (d), respectively. (g) Cross-sectional SEM image of overall 3D CuNW membrane with the Li plating of 10 mAh cm-2. (h) and (i) Corresponding magnified SEM images of top region and bottom region of (g), respectively. The dark green colored part represents the region of lithium plating inside the 3D porous structure and the bright green colored part represents the region of lithium dendrites. (j) The schematic illustration of the Li plating in the 3D CuNWs network current collector at different status. To further show the advantages of the CuNW network membrane as the current collector in the Li anode, the efficiency and cycling stability of Li plating/stripping in the CuNW membrane were tested and compared to that of planar Cu foil. As shown in Figure 5a, the voltage plateau of Li plating/stripping on the planar Cu foil show a large increase from ~0.1 V at 2nd cycle to ~0.4 V at 50th cycle. This plating/stripping voltage increase was caused by the unstable Li/electrolyte interface on the planar Cu foil. In contrast, the voltage plateau of Li plating/stripping in the CuNW membrane stabilized at ~0.15 V during the initial 10 cycles and even decreased to ~0.02 V after 50th cycle (Figure 5b), which indicates that the stable Li/electrolyte interface was formed in the CuNWs network. Voltage profiles of long time cycling of Li plating/stripping with a constant areal capacity of 2 mAh cm-2 on the planar Cu foil and in the CuNW membrane are shown in Figure 5 c and d, respectively. The voltage plateaus of Li plating/stripping on the planar Cu foil displayed random voltage oscillations indicating the instability of Li/electrolyte interface. But the voltage plateaus of Li plating/stripping in the 3D NWs networks were very consistent with each cycle. Furthermore, the voltage hysteresis variation of Li plating/stripping can be derived from the long time cycling. The voltage hysteresis is the difference between the voltages of Li stripping and plating, which is an important parameter to reveal the stability of the Li anode because it is determined by the interfacial properties and the charge transfer.14, 31 As shown in Figure 5e, at a current density of 1 mAh cm-2, the Li plating/stripping on the planar Cu foil shows an


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increase of voltage hysteresis from 0.25 V to 1 V during the cycling with the increase of charge transfer caused by the accumulation of unstable SEI. In contrast, the voltage hysteresis of Li plating/stripping in the 3D Cu NWs network is always stable and maintained at ~0.04 V, which is lower than previously reported data (~0.05 V) collected on 3D Cu foil at a low current density (0.2 mA cm-2).31 This result of stable and low overpotential/polarization of Li plating/stripping is attributed to the large surface area of 3D CuNW membrane, which can provide a large Li/electrolyte interface to reduce the charge transfer and internal resistance during cycling. Moreover, electrochemical impedance spectroscopy (EIS) in Figure S10 also demonstrated the lower interfacial charge transfer resistance of 3D Cu NWs network comparing to that of planar Cu foil. In addition, the rate performance of Li plating/stripping in 3D CuNW membrane was tested as well. As shown in Figure S11, the voltage hysteresis of Li plating/stripping steadily increased with the increase of current density and when the current density returned back to 1 mA cm-2, the voltage hysteresis dropped to 0.02 V even lower than the initial value, indicating the excellent rate performance of the 3D CuNWs network for Li plating/stripping. The SEM images of the top surfaces of the CuNW membranes after the Li plating at high current densities (Figure S12) show the hollow porous nanostructures indicating that the Li metal plating occurred on the surface of CuNWs without the formation of Li dendrites. The CE of the Li anode on the Cu foil and CuNW membrane were also tested because it is a significant parameter to evaluate the life-span of Li batteries. In our cases, the CE is collected by plating constant 2 mA cm-2 of Li and then stripping out as much as possible via the voltage control. As shown in Figure 5f, on the planar Cu foil, the plating/stripping efficiency of Li declined rapidly to less than 50% with an initial value of


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88.8% because of the large morphology variation of Li dendrites and the unstable SEI during the cycling. Due to the suppression of Li dendrite growth and relatively stable SEI, the CE of Li plating/stripping in the 3D CuNW membrane raised to ~99.2% after 50 cycles with an initial value of 94.3% and maintained at an average value of 98.6% during 200 cycles at the current density of 1 mA cm-2. The CE of Li plating/stripping in the 3D CuNW membrane also maintained at good values at high current densities (97.6% at 2 mAh cm-2 and 97.1% at 5 mAh cm-2) as shown in Figure S13. This result is even better than previously reported CE obtained on the 3D Cu foil23 indicating that our developed free-standing 3D CuNW membrane is effective to suppress the Li dendrite growth and stabilize the SEI.


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Figure 5. (a) and (b) voltage profiles of Li plating/stripping at 1 mA cm-2 on planar Cu foil and 3D CuNW membrane, respectively. The plating/stripping of Li was controlled by the voltage. (c) and (d) voltage profiles of Li plating/stripping at 1 mA cm-2 on the Cu foil and in the 3D CuNW membrane, respectively. The plating/stripping of Li was controlled by the capacity. (e) The plot of voltage hysteresis with cycling number. (f) Coulombic efficiencies comparison of Li anode on planar Cu foil and 3D CuNW membrane with the cycling number. In addition, we examined the CE of the Li plating/stripping in 3D CuNW membrane by using commercial carbonate electrolyte (Figure S14). The CE is 92~95% in the 50 cycles because of the decomposition of electrolyte and the consumption of lithium. The morphology characterization of Li plating with carbonate electrolyte (Figure S15) shows that the Li metal could also be accommodated in the CuNW network without the formation of Li dendrites.


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Furthermore, a nano-composite Li-CuNWs anode was prepared via the thermal infusion of Li into ZnO nanoparticles decorated CuNWs membrane and assembled into a full cell against a LiCoO2 cathode (experimental details please see Supporting Information). As shown in Figure S16, there is a remarkable increase of ~30% in the specific capacity of the cell run at 5C based on the Li-CuNWs anode in comparison with Li foil anode after 100 cycles indicating the advantage of the Li-CuNWs nanocomposite anode for high rate Li metal batteries. In conclusion, we have demonstrated that a free-standing CuNW network current collector can accommodate the lithium deposition inside the porous nanostructure suppressing the growth of Li dendrites thus improving the performance of the Li anode. In our case, the Li metal anode can run for 200 cycles with a low and stable voltage hysteresis of ~0.04 V and the average CE of Li plating/stripping can be up to ~98.6%. The free-standing 3D CuNW membrane is expected to present a synergistic effect with other rational design to realize its practical application in Li metal batteries such as all-solid-state Li metal battery or flexible lithium battery.

Acknowledgments We acknowledge the funding support from the National Natural Science Foundation of China (Grant 51571184, 21431006), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant 21521001), the National Basic Research Program of China (Grants 2014CB931800, 2013CB931800), and the Users with Excellence and Scientific Research Grant of Hefei Science Center of CAS (2015HSC-UE007, 2015SRG-HSC038). H. B. Yao thanks the support by “the Recruitment Program of Global


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Experts”.

Supporting Information experimental details, schematic illustration of CuNWs membrane fabrication, XRD, sheet resistance, and photograph of as-fabricated CuNWs membrane, SEM images of lithium deposited on Cu foil, EIS spectra, Rate performance of 3D Cu NWs-Li metal anodes, Coulombic efficiencies of Li metal anode with carbonate electrolyte, cycling performance of Full-cells using 3D Cu-Li nanocomposite anode and LiCoO2 cathode, respectively. This material is available free of charge via the Internet at http://pubs.acs.org. Reference 1.

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