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Porous Al Current Collector for Dendrite-Free Na Metal Anodes liu shan, Shan Tang, Xinyue Zhang, Aoxuan Wang, Quan-Hong Yang, and Jiayan Luo Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03185 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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Porous Al Current Collector for Dendrite-Free Na Metal Anodes Shan Liu1, Shan Tang2, Xinyue Zhang1, Aoxuan Wang1, Quan-Hong Yang1, Jiayan Luo1* 1

Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University; Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China 2

Department of Mechanicals, Dalian University of Technology, Dalian 116024, China

Keywords: Porous Al; Na metal anodes; SEI; Battery; Microstructure *Corresponding authors: E-mail: [email protected]

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Abstract: Na based batteries are proposed as promising energy storage candidates for beyond Li-ion technology due to the higher natural earth of Na metal. For its high capacity and low potential, Na metal may carve itself a niche when directly used as anodes. Similar to or even more problematic than Li, however, uneven plating/stripping of Na leads to dendrite formation. As the plating substrates, current collectors have a paramount influence on the Na plating/stripping behaviors. Here we propose porous Al current collectors as the plating substrate to suppress Na dendrites. Al does not alloy with Na. It is advantageous over Cu current collectors in terms of cost and weight. The interconnected porous structure can increase available surface for Na to nucleate and decrease the Na+ flux distribution, leading to homogeneous plating. The Na metal anodes can run for over 1000 cycles on porous Al with a low and stable voltage hysteresis and their average plating/stripping Coulombic efficiency was above 99.9%, which is greatly improved compared to planar Al. We used the porous Al for Na-O2, Na-Na3V2(PO4)3 cells with low Na amount and anode free Na-TiS2 batteries, and anticipate that using this strategy can be combined with further electrolyte and cathodes to develop high performance Na based batteries.


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Introduction In the exploration of new energy storage technologies beyond Li-ion, room-temperature Na-ion batteries are proposed as promising candidates due to the several orders of magnitude higher natural abundance of Na metal on earth. Coined in 1980’s, the Na-ion cells operate in a manner similar to Li-ion batteries, with Na+ intercalation, alloying or conversion reaction with the intercalation compounds, metals or oxides/sulfides/phosphides.1-3 The past decade has witnessed the great progress in the development of cathode and anode materials.4-7 Prototype Na-ion batteries using layered oxide cathode and hard carbon anode has recently been demonstrated.8-11 Thermodynamically, the Na+/Na electrode potential is 0.3 V higher than Li counterpart, rendering lower energy density, let alone the higher mass of Na. The power density of Na-ion batteries is also inferior because of the larger Na+ size induced slower reaction kinetics.12 As for rechargeability, the Na+ compounds upon sodiation/desodiation are, in principle, less stable compared to Li+ reaction due to more pronounced lattice fluctuation.13-15 To make Na-ion batteries competitive in overall electrochemical performance, breakthrough in new mechanism and discovery of high energy electrode materials are required. They, after all, could hardly become main players by only adopting the Li-ion strategies until Li exhaustion or the price advantage of Na resource overwhelming the battery components (including active materials, separators, electrolyte, additives and etc.) production and cell fabrication cost.16

Na, however, may carve itself a niche when directly used as metal anodes.17,18 For example, Na-O2 batteries are proved to have better cycling stability and lower overpotential than Li-O2 cells for the higher reversibility of discharge product NaO2 and fewer side reactions during charge.19-22 Indeed, high temperature rechargeable Na metal batteries are commercially


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available, i.e., Na/S and Na/NiCl2 systems.23 These batteries operate at high temperature (~300 °C) with molten Na and solid electrolyte (β"-alumina) to ensure the reaction kinetics and prevent Na dendrite formation. Concerns are safety hazards related to the high temperature feature that molten Na is highly reactive and corrosive, imposing significant burden on cell manufacturing and operation.24,25 Solid electrolytes operating at lower (ideally ambient) temperature are desired. But increasing their conductivities orders of magnitude higher to be comparable to liquid electrolytes and addressing the more serious solid-solid interface challenge still require great efforts.26,27 In organic electrolytes, highly reactive Na can reduce most solvents to forming solid electrolyte interface (SEI) passivation layers.24,28,29 Similar to or even more problematic than Li, uneven plating/stripping of Na leads to dendrite formation, whose growth may puncture the SEI passivation layer and expose itself to electrolyte for mutually consumed new SEI formation, resulting in low Coulombic efficiency. Without stabilized SEI, the accumulated dendrites can pierce the separator causing cell shorting.

Unlike the recent intense research on Li metal anodes, the study of the Na plating/stripping behaviors in organic electrolyte is relatively scarce,30-32 which is largely due to that the SEI on Na is poorly characterized and understood. The stable SEI on graphite enabled the success of Li-ion batteries,33,34 which also make the investigation of SEI possible and its mastering lays the foundation for Li metal study. It is found that stable and compact SEI layer can be formed in NaPF6 in glyme based electrolytes with demonstrated high average efficiency of 99.9% over 300 Na plating/stripping cycles on Cu foil.35 To effectively suppress the infinite volume fluctuation of Na plating/stripping, strategies can be learning from Li metal anodes.36-42 Lowering the effective current density with higher plating substrate surface can allow more


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homogeneous deposition.41 Conductive scaffolds can accommodate the deposits and at the same time increase the plating area.39 Carbon nanostructures have been demonstrated as effective hosts to depress Li dendrite

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and start to get attention for Na metal anodes.46

Artificial SEI layer either formed by sacrificial electrolyte additives or physically inserted can reduce direct metal-electrolyte contact and strengthen the interface.42

Figure 1. Controlled deposition of Na on porous Al. (a) Density of several current collector candidates for Na-ion battery anodes and Na metal anodes. (b) Phase diagram of Na-Al. Al doesn’t alloy with Na, which allows it to be used as a lightweight and inert current collector. (c) Schematic of Na deposition on planar and porous Al foils. Uneven electron distribution in planar Al can lead to dendritic and mossy Na formation during deposition. While on porous Al, the much higher geometric surface area can provide more nucleation sites, grounding for homogeneous distributed Na growth.

With no doubt, the plating substrates, serving as current collectors in Li/Na-ion batteries, have a paramount influence on the Na plating behaviors.47 Al foil, the typical Li-ion battery cathode current collector which is advantageous over Cu foil in terms of cost and weight, can serve as the current collector for both Na-ion battery cathodes and anodes as it does not alloy


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with Na (Figure 1a,b).48 Recently, a pioneering work by Pint et al. designed a nucleation layer to both assist Na seeding on Al foil with a lower nucleation energy barrier and improved structure for stable sodium plating. Significant improved performance such as over 1000 plating/stripping cycles with 99.8% average Coulombic efficiency and 14 mV low voltage hysteresis and smooth Na film formation were achieved.49 Here, a porous Al foil is proposed as the plating substrate to suppress Na dendrites, which is found to greatly improve the cycling stability of Na metal anodes compared to planar Al. Results and Discussion The principle that porous plating substrates can suppress the Na dendrite growth is illustrated in Figure 1c. On charged conventional planar Al current collector, electron is distributed conformably with the current collector surface geometry, which can hardly be uniform as bumps and dents inevitably exist at sub-micrometer scale. The electric field around these rough spots is stronger than on flat areas, which concentrates the Na+ flux to nucleate. The following Na plating process at the rough spots is also accelerated by the plenteous Na+ supply. The net effect is that Na grows unevenly and dendritic and mossy Na forms during plating. If the shear modulus of SEI layers is not high enough, Na dendrites can puncture the SEI and expose itself to electrolyte for new SEI formation, product of mutually consumption of Na and electrolytes. During stripping process, the erected dendrites may lose electric contact with current collectors, especially under high current densities, resulting in dead Na. Electrochemically, dendrite formation is accompanied by low Coulombic efficiency of Na plating/stripping cycle and depletion of electrolyte. What’s worse is that the dendrites, if heavily accumulated, can pierce the separator causing cell shorting. Instead, the interconnected porous Al has much higher geometric surface area, which can largely increase


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available surface for Na to nucleate. At the same time, the effective current density will be lowered and thus the Na+ flux distribution. The multitudinous nucleation sites and diminished growth rate collectively lead to the homogeneous plating of Na on the surface of the porous current collectors. Even after the pores of the current collectors are filled with deposits, Na dendrites are less likely to form as the Al is already ubiquitously covered by Na. Except for Al, other porous current collectors can also work with the same principle, such as Cu, stainless steel and Ti but with higher cost and weight (Figure 1a).

Figure 2. Comparison of Na deposition morphology on planar and porous Al current collectors. Photographs of (a) planar Al and (b) porous Al foils before and after plating/stripping cycling. (c,d) Cross-section and (e,f) top-view SEM images of planar (left) and porous (right) Al foils before and after plating/stripping cycling. Mossy Na deposited on planar Al foil can be clearly observed visually and under microscope. In contrast, the deposited Na layer on porous Al is homogenous without any protuberance.

Porous Al can be prepared by electrochemical corrosion of planar Al foil with porosity controlled by applied voltage (or current) and corrosion time (Figure S1). In fact, porous Al foils have been used as current collectors for high power Li-ion batteries and capacitors for their better contact with electrode materials than planar foils.50,51 A commercially available


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porous Al foil ((Toyal Pass®, Toyo Aluminium, Japan) is used in this work to eliminate impurities introduced during corrosion process, as confirmed by its X-ray diffraction pattern (Figure S2). The porous Al has a square-wave texture surface with submicron roughness (Figure S3). Pores also at submicron scale penetrating across the films are observed with porosity of the films controlled to be around 16%. The pores provide much higher geometric surface area, which can largely increase available surface for Na to nucleate and lower the effective current density (Figure S4). Higher porosity could bring in better electrochemical performance but at the expense of film integrity and flexibility due to the relatively low mechanical strength of Al.51 Mossy Na deposited on planar Al foil can be clearly observed visually and under microscope. The optical images in Figure 2a show the planar Al foil became patchy after 1mAh cm-2 Na plating using 1 M NaPF6 in diglyme electrolyte. In contrast, the originally textured surface of the porous Al foil disappears after 1mAh cm-2 Na plating, indicating a more homogeneous Na deposition (Figure 2b). With Na plating amount from 0.5 to 2 mAh cm-2, the deposits on porous Al are uniform while patchy on planer Al (Figure S5). After stripping, the planar Al remains patchy due to incomplete dissolution of Na. But the surface of porous Al is cleaner. The difference in surface morphology can be further validated by scanning electron microscopy (SEM) images. Mossy Na can be clearly observed to be plated on planar Al foil (Figure 2c,e). The Na plated on porous Al homogenously duplicated the surface topology of porous Al without any protuberance (Figure 2d,f). Electrochemical impedance of the planar and porous Al foils in cells with Na counter electrode illustrated that the porous Al has much lower charge transfer resistance in the first cycle (Figure S6). At later cycles, the resistance of Na plating/striping on porous Al was stable. However, the resistance on planar Al increased due to dendrites induced unstable SEI layer.


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To shed light on how the porous structural design of the substrates influences the plating/stripping behavior, the surface morphology of planar and porous Al current collectors foils after different level of Na plating/stripping were investigated. From the SEM images of porous Al with increased Na plating amounts from 0.25 to 0.5 mAh cm-2 (Figure 3a-c) using 1 M NaPF6 in diglyme electrolyte, Na grown first in the Al skeleton and gradually filled the pores of the porous Al foils and then coated the square-wave texture surface (Figure 3a,b). Cross sectional SEM images and energy dispersive X-ray spectroscopy (EDX) confirm Na started deposition first in the pores (Figure S7). Given that the pores can at most accommodate 0.6 mAh cm-2 of Na deposits (see Figure S3 for detailed calculation), 1 mAh cm-2 of Na deposits on porous Al still displayed relatively homogeneous distribution (Figure 3c, Figure S7). Further increasing the amount of Na deposits to 2 mAh cm-2 had not produced any obvious dendrites (Figure S8). The square-wave texture surface of porous Al was recovered after the Na deposits were stripped from the current collectors, denoting that the plated Na can be stripped completely from porous Al current collectors (Figure 3d). After 20 plating/stripping cycles, the plated surface of porous Al was remained dendrite free and the stripped surface maintained the structurally stable texture (Figure 3e,f). On the contrary, the planar Al surface became initially patchy and then mossy with Na plating amount increased from 0.25 to 1 mAh cm-2 (Figure 3g-i). The mossy topology became more severe after cycling (Figure 3k). Moreover, the mossy Na couldn’t be stripped completely as the mossy structure is loosely connected, giving rise to dead Na (Figure 3j,l).


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Figure 3. Comparison of the deposition morphologies with different Na amount on planer and porous Al current collectors. SEM images of porous and planar Al foils after plating (a,g) 0.2 mAh cm-2, (b,h) 0.5 mAh cm-2, (c,i) 1 mAh cm-2 of Na, (d,j) then stripping to 0.5 V, (e,k) plating 1 mAh cm-2 and (f,l) stripping to 0.5 V after 20 cycles. (m) Galvanostatic discharge/charge voltage profiles at a current density of 0.5 mA cm-2.

The performance of porous Al current collectors for Na metal anodes were tested compared to that of planar Al foil with several electrochemical methods. First, 2 mAh cm-2 of Na was first deposited onto the planar Al and porous Al foils at 1.0 mA cm-2 using 1 M NaPF6 in diglyme electrolyte. The symmetric cells were cycled at constant current of 0.5 mA cm-2 with each cycle set to 1 h. As shown in Figure 4a, the voltage hysteresis of the cell using planar Al foil electrode gradually increased and the cell was shorted after 100 h. While in the cell with porous Al foil electrode, the voltage hysteresis was smaller and maintained unchanged for more than 1000 h (1000 cycles), indicating improved plating/stripping stability of porous Al over planar Al. When less amount of Na was deposited, the improved stability of porous Al was more obvious. For example, in the case of 0.6 mAh cm-2 of Na deposited, the cell with planar Al was short after 150 h but the cell with porous Al could still maintain stable over 600


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h at 0.25 mA cm-2 with each cycle set to 30 minutes using 1 M NaPF6 in diglyme electrolyte. (Figure S9). Na is known to perform much less stable than in carbonate electrolyte than in glymes based electrolytes.51 It was found that porous Al could also improve the Na cycling stability in carbonate electrolyte (Figure S10).

Figure 4b,c shows the voltage profiles obtained by directly using porous Al and planar Al as electrodes with Na metal counter electrode at 0.5 mA cm-2. The plating capacity was 1 mAh cm-2 and the stripping upper voltage was 0.5 V. It can be noticed the cell with planar Al exhibited voltage oscillations during stripping process, which was presumably caused by the dendrites induced unstable SEI layer and electrical disconnection. After 220 hours’ cycling, the cell was short circuited by the unceasingly growing dendrites reaching the counter electrode. On the opposite, the potential fluctuation of Na plating/stripping on the porous Al foil was negligible. The voltage profiles retained the shape for more than 350 hours. At higher current density of 1 mA cm-2 and higher plating capacity of 2 mAh cm-2, the porous Al electrodes exhibited smoother and tidier voltage profiles than planar Al electrodes (Figure S11). Even with platting capacity up to 12 mAh cm-2, no potential fluctuation was found on porous Al foil (Figure S12). In 1 M NaClO4 in EC/DEC electrolyte, the porous Al electrodes still outperformed planar Al electrodes (Figure S13). The voltage hysteresis of the voltage profile using porous Al was lower than using planar Al (Figure S14). For example, 12 mV hysteresis at 0.5 mA cm-2 and only 53 mV at 5 mA cm-2 were found using porous Al, which are comparable with the carbon coated Al.49 Also similar to previous work,49 the planar Al electrode failed at current density higher than 4 mA cm-2.


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Figure 4. Electrochemical performance of porous Al current collector for Na metal anodes using 1 M NaPF6 in diglyme electrolyte. (a) Galvanostatic cycling of symmetric Na@planar Al/Na and Na@porous Al/Na cells. The current density was fixed at 0.5 mA cm-2 with each cycle set to 1 h. Voltage profiles of Na plating/stripping on (b) planar and (c) porous Al current collectors at 0.5 mA cm-2. The plating capacity was 1 mAh cm-2 and the stripping upper voltage was 0.5 V. The insets are enlarged voltage profile at different cycles. The first 1000 cycling Coulombic efficiency of planar and porous Al with Na deposited amount of (d) 0.25 mAh cm-2 at 0.5 mA cm-2 and (e) 0.5 mAh cm-2 at 1 mA cm-2.


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To test the Coulombic efficiency, Na was first plated onto the porous or planar Al anode electrode at a fixed amount and then stripped away to 0.5 V. The porous Al electrodes exhibited more stable electrochemical cycling and much longer cell life (Figure 4d,e). Comparable to the bared Al electrode in previous result,49 the Coulombic efficiency on planar Al became unstable after 100 cycles at current density of 0.5 and 1.0 mA cm-2. While average Coulombic efficiency of porous Al electrode was 99.9% and 99.8% over 1000 cycles at current density of 0.5 and 1.0 mA cm-2, respectively. Even at platting capacity up to 12 mAh cm-2, the Coulombic efficiency could maintain 99.9% for 50 cycles (1200 h) at current density of 1 mA cm-2 (Figure S15). At current density up to 4 mA cm-2, the Coulombic efficiency could still maintain over 99.3% (Figure S15, S16). The values are comparable to the carbon coated Al electrodes and among the best results reported for Na anodes (Table S1, S2).30,32,35,43-46,49 Under the same conditions, the performance of planar Al electrodes was poorly stable, which is largely owned to that the mossy Na deposits on planar Al couldn’t be fully stripped during charge with persisted dead Na (Figure S17 and 3).

As a proof of concept, Na-O2 batteries with carbon black O2 cathodes were assembled using planar Al and porous Al foils deposited with 1 mAh cm-2 Na as the anodes in 1 M NaPF6 in diglyme electrolyte. As shown in Figure 5, the cell with porous Al can maintain the capacity over 200 cycles, much better compared to the cell using planar Al which shown fast fading after 20 cycles. Note that the Na deposited was only 1 mAh cm-2, which is much lower than the amount used in those Na reference electrodes in conventional Na-ion batteries. Anode free Na-TiS2 full cells with planar and porous Al anodes and pre-sodiated TiS2 cathodes using 1 M NaPF6 in diglyme electrolyte were also assembled in configuration similar to the carbon


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coated Al paired with FeS2.49 The cycling stability of the cell with porous Al was also much better compared to the cell using planar Al (Figure S18). Full cells in carbonate electrolyte with Na3V2(PO4)3 cathodes were also assembled using 1 M NaClO4 in EC/DEC (1:1) electrolyte. The planar and porous Al foils were deposited with 1 mAh cm-2 Na as the anodes before full cell assembly. The cell with porous Al could maintain 50% of its initial capacity after 350 cycles but the cell with planar Al shown fast fading after 150 cycles (Figure S19). When deposited Na amount on the planar and porous Al foils were decreased to 0.3 mAh cm-2, twice the capacity of the cathode, the cell with planar Al could only sustain 10 cycles while the cell with porous Al could maintain stable after 50 cycles (Figure S20).

Figure 5. Na-O2 full cell performance of porous Al current collector for Na metal anodes. (a) Voltage profiles and (b) cycling performance of Na-O2 batteries with Na@planar Al and Na@porous Al anodes using 1 M NaPF6 in diglyme electrolyte. The O2 electrodes were 0.5 mg cm-2 Super P carbon black.

Conclusion In conclusion, we for the first time demonstrated porous Al foils can serve as the plating substrate to suppress Na dendrites. The interconnected porous structure can largely increase available surface for Na to nucleate and decrease the Na+ flux distribution, leading to the homogeneous plating of Na on the surface of the porous current collectors without any


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protuberance. The Na metal anodes can run for 1000 cycles with a low and stable voltage hysteresis without potential fluctuation and their average plating/stripping Coulombic efficiency was above 99.9% over 1000 cycles. Proof of concept full cell test shown that the porous Al foil can be employed in Na-O2 batteries and Na-Na3V2(PO4)3 cells with low Na usage. Anode free Na-TiS2 full cells with pre-sodiated TiS2 cathodes were also assembled. Optimizing the pore structure of the Al foils could definitely further improve the electrochemical performance, which is under investigation. This work paves the way for ambient temperature Na metal anodes and their application in Na metal batteries.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pub.acs.org.

AUTHOR INFORMATION Corresponding Author *Email:[email protected] Note The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors appreciate support from National Natural Science Foundation of China (Grant Nos. U1601206, 51502197) and Natural Science Foundation of Tianjin, China (Grant No. 15JCYBJC53100). This work was also supported by State Key Laboratory of Chemical Engineering (Grant No. SKL-ChE-15B02).


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Keywords: Porous Al; Na metal anodes; SEI; Battery; Microstructure

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Porous Al Current Collector for Dendrite-Free Na ... - ACS Publications

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