Spherical Li Deposited inside 3D Cu Skeleton as Anode with

Jun 4, 2018 - Interestingly, the deposited Li in the 3D Cu skeleton was not tubular dendrite .... Both the lifespan of cells with 3D Cu or Cu foam cur...
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Letter Cite This: ACS Appl. Mater. Interfaces 2018, 10, 20244−20249

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Spherical Li Deposited inside 3D Cu Skeleton as Anode with Ultrastable Performance Yanyan Wang,† Zhijie Wang,† Danni Lei,†,‡ Wei Lv,† Qiang Zhao,†,‡ Bin Ni,†,‡ Yong Liu,§ Baohua Li,† Feiyu Kang,†,‡ and Yan-Bing He*,† †

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Engineering Laboratory for the Next Generation Power and Energy Storage Batteries, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, P. R. China ‡ Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China § School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471023, P. R. China S Supporting Information *

ABSTRACT: Porous current collectors are conducive to enhance the property of Li metal anode. Unfortunately, congestion in diffusion path during plating process damages the effects of current collectors. Herein, we developed a 3D Cu skeleton with open micrometer-sized pores by NaCl-assisted powder-sintering method. The unobstructed pores of 3D Cu skeleton help to reduce congestion during plating, thus most of Li deposited inside the current collector. Besides, the large smooth surface promotes the deposition of Li with smooth spherical shape, which mitigating Li dendrite growth. As a result, better safety and rechargeability of Li metal anode were achieved in this design. KEYWORDS: 3D Cu skeleton, current collector, Li metal anodes, high Coulombic efficiency, spherical Li, powder-sintering method indicates that the dendrite initiation time is proportional to J−2 (J is the effective current density).5,10 Moreover, according to Monroe and Newman’s work, the dendrite growth rate is also greatly affected by the effective current density.11 Therefore, fabricating conductive porous substrates with high surface area is useful to disperse local current density and thus mitigating Li dendrite growth.12,13 For example, various 3D porous Cu14−17 and porous carbon18−21 have been developed and applied as current collectors for Li metal anodes. However, the designed current collectors have relatively limited space and less unobstructed pore structure, which may affect the functions of 3D current collector. In a previous report, part of Li was preferentially deposited on the top of 3D current collector, the side closing to separator, due to the shorter diffusion pathway of Li+ and low porosity of current collector.17 The diffusion pathway of Li+ may be blocked by the previous deposited Li especially when the top surface of 3D current collector was covered. The subsequent deposition of Li+ can occur only on the top surface. Consequently, a large proportion of deposited Li was found outside the current collectors especially at high deposition capacity.14,16,22 This shortcoming may damage the function of 3D current collector in suppressing Li dendrite growth and mitigating volumetric change, which manifests as

L

i metal is widely regarded as the most promising anode material for next-generation batteries because of its extremely high specific capacity of 3860 mAh g−1 and low redox potential (−3.04 V vs the standard hydrogen electrode).1,2 However, long-standing challenges, especially the safety hazard and poor lifespan, have hindered the practical use of Li metal batteries.2,3 Different from traditional anodes like graphite and Li4Ti5O12 that host Li+ in their lattice, Li metal anode is “hostless” because it accommodates Li+ at the Li/ electrolyte interface. This storage mechanism of Li+ results in virtually infinite dimensional change during the plating/ stripping process.4 These characteristics of Li metal anode cause two serious consequences. One is the uncontrollable dendrite growth during cycling, which may pierce through the separator and provoke internal short circuit of batteries.5 Another one is the inevitable destruction of solid electrolyte interphase (SEI) layer on the surface of Li metal owning to volumetric changes.6,7 When the fragile SEI layer cracks, fresh Li is exposed and reacts quickly with electrolyte to form a new SEI layer.8 The excessive growth of SEI layer continuously consumes Li and electrolyte, and therefore damages the Coulombic efficiency of Li metal batteries.9 Extensive research has been conducted to address the abovementioned issues of Li metal anodes. Among the strategies, the designs of 3D porous host are demonstrated to be efficacious in suppressing dendrite growth and enhancing cycling efficiency of Li metal anodes. This strategy is based on the Sand’s law, which © 2018 American Chemical Society

Received: March 26, 2018 Accepted: June 4, 2018 Published: June 4, 2018 20244

DOI: 10.1021/acsami.8b04881 ACS Appl. Mater. Interfaces 2018, 10, 20244−20249

Letter

ACS Applied Materials & Interfaces Scheme 1. Illustration of the Fabrication Process of 3D Cu Skeleton

low and unstable Columbic efficiency of Li metal anode during long cycling. Therefore, optimizing the geometry of the porous Cu current collector is of great importance for exploiting the advantages of current collector and promoting the electrochemical performances of Li-metal anodes. Powder-sintering is a facile and low-cost method, which can be used in the fabrication of porous materials.23,24 To further increase the porosity, NaCl crystal, an environmentally friendly and easy removal salt, can be selected as template to create pores.25 In this work, we developed NaCl-assisted powdersintering method to prepare a 3D porous Cu skeleton with open micron-size pores as current collector for Li metal anode. The as-prepared 3D Cu skeleton possessed high surface area to disperse Li+ flux and open porous structure to provide sufficient space for Li deposition and mitigate volume change. Because of the open unobstructed pore structure, most of deposited Li was plated in the internal pores rather than outside of 3D current collector even at a high deposition capacity of 7 mAh cm−2. Interestingly, the deposited Li in the 3D Cu skeleton was not tubular dendrite but smooth spherical shape, which was crucial to suppress dendrite growth and thus lengthen the lifespan of Li metal anode. As a result, the 3D Cu skeleton showed great advantages in increasing the cycling efficiency of Li metal anode. The Coulombic efficiency of Li plating/stripping on 3D Cu skeleton was kept higher than 95% for 700 cycles at a cycle current density of 0.5 mA cm−2 and 400 cycles at 1 mA cm−2. Therefore, both dendrite-free Li deposition and excellent cycling efficiency were achieved simultaneously in this design. The fabrication process of 3D porous Cu skeleton was illustrated in Scheme 1. Commercial Cu powders and ball milled NaCl templates (morphology can be seen in Figure S1) were well mixed with binder poly(ether sulfone) (PES) and solvent N-methyl-2-pyrrolidone (NMP), and then the slurry was shaped into thin Cu-NaCl-binder membrane. Afterward, the thin membrane was treated with a two-step heat treatment. The first-step was conducted in air at 620 °C to completely remove the polymer binder and the second-step was conducted under reducing atmosphere at 580 °C to obtain well connected 3D Cu skeleton. Before the second-step heat treatment, the NaCl templates were removed by water. The XRD patterns as shown in Figure S2 demonstrating that the final sample is pure Cu. As-prepared 3D Cu skeleton can be utilized as current collector for lithium metal anode. The structure changes of samples during fabrication process were observed by scanning electron microscope (SEM). As presented in Figure 1a, no distinct pores were found on CuNaCl-binder membrane when the slurry solidified. However, abundant pores appeared after polymer binder being burned at 620 °C as well as NaCl templates being washed by water (Figure 1b). The overwhelming majority of pores derived from

Figure 1. Cross-sectional SEM images of (a) Cu-NaCl-binder membrane, (b) CuO intermediate after first-step heat treatment and removal of NaCl templates, and (c) 3D Cu skeleton after second-step heat treatment; the inset shows the high-magnification image. (d) Surface SEM image of 3D Cu skeleton. The inset shows the optical photograph.

the occupied space of NaCl templates, PES and the interspace of particle packing during solidification process. As shown in Figure 1c, the porous structure was well preserved after powder sintering at 580 °C, and the particles grew into connected onepiece skeleton by forming sintering necks which ensure the 3D Cu skeleton with good mechanical strength. The smooth and rounded surface of 3D Cu skeleton was quite different from the surface with protuberant secondary structure of porous Cu in previous reports.14,17,26 This feature was ascribed to the grain growth process in the second-step heat treatment, where the high curvature area tended to disappear and flat area tended to preserve. The BET surface area of 3D Cu skeleton was 0.34 m2 g−1, which was much higher than that of commercial Cu foam (Figure S4). When the 3D Cu skeleton served as current collector, the large and smooth surface was beneficial for uniform electric field distribution and avoided charges concentrating at the tips. The homogeneous charge distribution was supposed to be significant for Li+ regular deposition. As for the pores, they ranged from ∼500 nm to ∼10 μm, which were able to provide adequate space for deposited Li even at high deposition capacity. In Figure 1d, the large open pores on the surface of 3D Cu skeleton, which have average size of few micrometers (Figure S5), can reduce congestion caused by previous deposited Li and facilitate Li depositing into the internal pores continuously. To investigate the deposition behaviors of Li on 3D Cu skeleton, different amounts of Li were deposited at a current 20245

DOI: 10.1021/acsami.8b04881 ACS Appl. Mater. Interfaces 2018, 10, 20244−20249

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ACS Applied Materials & Interfaces

Figure 2. SEM images of 3D Cu skeleton and Cu foam with different amount of Li plating. (a−c) the surface SEM of 3D Cu skeleton for the plating amount of (a) 3 mAh cm−2, (b) 5 mAh cm−2, (c) 7 mAh cm−2; (d−f) the cross-sectional SEM of 3D Cu skeleton for the plating amount of (d) 3 mAh cm−2, (e) 5 mAh cm−2, (f) 7 mAh cm−2; (g−i) the corresponding high magnification of d−f, respectively; (j−l) the surface SEM of Cu foam for the plating amount of (j) 3, (k) 5, and (l) 7 mAh cm−2.

Figure 3. (a) Coulombic efficiency comparison of Li plating/stripping on 3D Cu skeleton or Cu foam at different test conditions. The crosssectional SEM images of 3D Cu skeleton: (b) the 50th, (c) 100th, and (d) 200th Li deposition. These cells were tested at 1 mA cm−2 with a fixed plating capacity of 1 mAh cm−2. The location of line was the top surface of 3D Cu skeleton.

density of 0.5 mA cm−2. As shown in Figure 2a−c, although the Li+ unavoidably deposited on the top surface of 3D Cu skeleton, it did not cover the surface even at high capacity of 7 mAh cm−2, which meant that a large proportion of Li was deposited inside the internal pores. The cross-sectional SEM image revealed that most of the deposited Li was stored in

pores of 3D Cu skeleton, and the EDS mapping of Cu element (Figure S6) confirmed that the spherical particles were deposited Li unquestionably. Although plating capacity increased from 3 mAh cm−2 to 7 mAh cm−2, the deposited Li layer on the surface of 3D Cu skeleton collector did not thicken visibly, as shown in Figure 2d−f. From the comparison 20246

DOI: 10.1021/acsami.8b04881 ACS Appl. Mater. Interfaces 2018, 10, 20244−20249

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Figure 4. (a) Galvanostatic cycling performance of Li@3D Cu skeleton and Li@Cu foam electrodes at a current density of 1 mA cm−2 with a stripping/plating capacity of 1 mAh cm−2 or 5 mAh cm−2. (b) Rate performance and (c) cycle performance of full cells with LiFePO4 as cathode and Li@3D Cu skeleton or Li@Cu foam as anode.

deposition of Li was expected to improve the safety and lifespan of Li metal anodes. In sharp contrast, the morphology of deposited Li on Cu foam was tubular dendrite as shown in Figure 2j−l. From the comparison, appropriate pore size was very important. The space of too small pores was limited, which may cause congestion during plating. However, too large pores would reduce the inside surface area of current collector and the nucleation sites would decrease. Because cycling efficiency is one of the most important factors to assess the feasibility of anode materials, the Coulombic efficiency of Li plating/stripping on 3D Cu skeleton and Cu foam was further examined by calculating the ratio of stripping capacity versus the plating capacity in each cycle. To measure the lifespan of Li metal anode at different working current densities, we tested the cells at current density of 0.5 mA cm−2 and 1 mA cm−2, respectively, with a fixed plating capacity of 1 mAh cm−2. As shown in Figure 3a, the Coulombic efficiency values of cells were similar at the beginning. However, differences occurred during cycling, revealing that the Coulombic efficiency was greatly affected by the current collector and applied current density. Both the lifespan of cells with 3D Cu or Cu foam current collectors impaired when the

among Figure 2g−i, it was obvious that the spherical Li particles became serried when the plating capacity increased, demonstrating that the diffusion paths were not blocked and Li+ was continuously deposited in the pores. Interestingly, the morphology of deposited Li presented a smooth spherical shape, which was quite different from the irregular shapes observed in previous reports.17 It was reported that the first stage of Li deposition was to form a spherical lithium nucleus, but in later growth stage, one direction (length) of lithium nucleus grew rapidly while another direction (width) grew very slowly.27 Theoretical research found that mass transfer and electric field distribution are two main factors that affect the morphology of deposited Li.11 The interlinked pore structure of 3D Cu current collector serves as unobstructed channels for electrolyte fluxion and Li+ diffusion, which help to facilitate mass transfer. Besides, the large and smooth surface of 3D Cu skeleton helps to reduce electric field intensity and avoids charges gathering. The uniformity of Li+ concentration and electric field distribution was improved, which promoted the lithium nucleus to grow equably in all direction and finally grow into spherical shape. The particle size of Li may increase with the increasing pore size of current collector.15 The dendrite-free 20247

DOI: 10.1021/acsami.8b04881 ACS Appl. Mater. Interfaces 2018, 10, 20244−20249

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ACS Applied Materials & Interfaces current density increased from 0.5 mA cm−2 to 1 mA cm−2. The Li plating/stripping on 3D Cu skeleton kept a Coulombic efficiency over 95% for 700 cycles at a current density of 0.5 mA cm−2, while over 95% for 400 cycles at a current density of 1 mA cm−2. In contrast, the Coulombic efficiency of Li plating/ stripping on Cu foam shew drastic fluctuation after 190 cycles at 0.5 mA cm−2 and after 60 cycles at 1 mA cm−2. When tested under higher capacity of 2 mAh cm−2, the cell with 3D Cu skeleton still kept stable for 200 cycles at a current density of 1 mA cm−2 (Figure S7). As summarized in Table S1, the Coulombic efficiency of Li plating/stripping on 3D Cu skeleton was much superior than most of other work that reported before. To investigate the functions of the 3D Cu skeleton in accommodating deposited Li and mitigating volumetric changes, we disassembled the cells, cycling at 1 mA cm−2 with a fixed plating capacity of 1 mAh cm−2, at the 50th, 100th, and 200th plating. As shown in high-magnification SEM images (Figure S8), the deposited Li metal inevitably became porous accompanied by volume expansion during repeated plating/ stripping process, and the pores of 3D Cu skeleton were stuffed with porous Li and side reaction product. However, they were well-restricted within the pore space of 3D Cu skeleton, as shown in low-magnification SEM images (Figure 3b−d). These images illustrated that the 3D Cu skeleton worked well in accommodating Li metal and reducing volume expansion of electrode. We further examined the cycling stability of Li metal anode with different current collectors (Li@3D Cu skeleton or Li@ Cu foam) in symmetric cells. The galvanostatic plating/ stripping voltage profiles of different cycling capacities were presented in Figure 4. The Li@3D Cu skeleton electrode displayed a stable voltage plateaus in each cycle at 1 mA cm−2 with a cycling capacity of 1 mAh cm−2. Besides, its voltage hysteresis (the gap between the voltage of Li stripping and plating) was maintained at around 40 mV for 500 h, which revealed stable interfacial properties of Li metal anode and electrolyte.17 In contrast, the Li@Cu foam electrode showed an obvious short circuit after 190 h at 1 mA cm−2 with a cycling capacity of 1 mAh cm−2. When the cycling capacity increased to 5 mAh cm−2, the Li@3D Cu skeleton electrode still maintained stable for 500 h, while the voltage hysteresis of Li@Cu foam electrode sharply increased after 250 h, which finally resulted in cell failure. This excellent cycling stability of 3D Cu skeleton is comparable to that of outstanding current collectors reported before (Table S2). To investigate the performance of Li@3D Cu skeleton or Li@Cu foam electrodes, we assembled full cells with LiFePO4 as cathode. Figure 4b showed that the Li@3D Cu skeleton| LiFePO4 cell exhibited overall advantage in rate performance. From the cycling performance presented in Figure 4c, the Li@ 3D Cu skeleton electrode showed much better cycling stability compared with Li@Cu foam electrode. After 200 cycles at 1 C, the specific capacity of Li@3D Cu skeleton|LiFePO4 cell was 139 mAh g−1, corresponding to a capacity retention of 92%. As for Li@Cu foam|LiFePO4 cell, its specific capacity dropped rapidly to almost zero after 70 cycles. The excellent electrochemical performance of Li metal anode can be attributed to the advanced structure of 3D Cu skeleton. The large open pores on the top surface were effective in reducing the congestion caused by the previous deposited Li and facilitating Li deposition in the internal pores. In addition, the large surface area of 3D Cu skeleton was able to disperse local current density and facilitate regular deposition of Li

metal. The deposited Li was smooth spherical shape, which is the minimum specific surface area compared with other geometrical shapes. The smaller specific surface area resulted in less side reaction between Li metal and electrolyte, and thus led to less SEI film formation and higher utilization of Li metal. The smooth spherical shape also benefitted uniform electric field distribution on Li particle, because the same local curvature resulted in equal surface potential for sphere. In consideration of needle-like dendrites, the potential at the root (Φr) of dendrite is different from that of the tip (Φt) owing to the curvature difference. As a result, the potential difference (ΔΦ = Φt − Φr) becomes the driving force for the dendrite growth.11 However, for spherical deposited Li, the driving force of dendrite growth was greatly weakened or even did not exist, so the deposited Li is prone to grow into larger sphere continually instead of tubular dendrite. Less dendrite formation helped to reduce the possibility of short circuit during cycling and lengthen the lifespan of cells. As for the Cu foam, Li+ preferentially deposited on its skeleton edges on the top surface as tubular dendrites (Figure S9). Cu foam did not work as a host of the deposited Li although it possessed up to 98% porosity. The unchecked growth of Li dendrites led to excessive growth of SEI film, which accounted for the short circuit and poor Coulombic efficiency of cells. In conclusion, we developed a 3D Cu skeleton prepared by NaCl-assisted powder-sintering method, and used it as current collector for long-life Li metal anodes. The porous structure of 3D Cu skeleton served as a host for Li deposition and its high surface area helped to disperse Li+ flux, which jointly facilitated Li homogeneous plating. Most of the deposited Li was plated in the internal pores of 3D Cu skeleton. The deposited Li was smooth spherical shape, which was effective in mitigating Li dendrite growth and reducing side reaction between Li metal and electrolyte. Consequently, Coulombic efficiency of Li deposition on 3D Cu current collector maintained higher than 95% for 700 cycles at 0.5 mA cm−2 and 400 cycles at 1 mA cm−2 respectively. The 3D Cu skeleton was demonstrated to be effective in suppressing dendrite growth, improving Coulombic efficiency and mitigating volumetric change simultaneously.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b04881. Experimental section, additional SEM image of 3D Cu skeleton, XRD patterns, SEM images of deposited Li, BET results, and additional electrochemical results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.-B.H.). ORCID

Yan-Bing He: 0000-0001-5787-5498 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Basic Research Program of China (2014CB932400), the National Natural Science Foundation of China (51672156 and 51232005), the 20248

DOI: 10.1021/acsami.8b04881 ACS Appl. Mater. Interfaces 2018, 10, 20244−20249

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Guangdong special support program (2015TQ01N401), Guangdong Province Technical Plan Project (2017B010119001 and 2017B090907005), Dongguan City (2015509119213), Shenzhen Technical Plan Project (KQJSCX20160226191136, JCYJ20170412170706047, JCYJ20170307153806471 JCYJ20150529164918734 and GJHS20170314165324888).

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DOI: 10.1021/acsami.8b04881 ACS Appl. Mater. Interfaces 2018, 10, 20244−20249