Communication Lithium-Metal Anode
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Bending-Tolerant Anodes for Lithium-Metal Batteries Aoxuan Wang, Shan Tang, Debin Kong, Shan Liu, Kevin Chiou, Linjie Zhi, Jiaxing Huang, Yong-Yao Xia, and Jiayan Luo* used directly when device deformation is relatively small. Batteries that can sustain deformation also reduce chances of device failure upon incidental damage. Of all materials that can be used for Li battery electrodes, Li metal itself is the ideal anode since it has the highest theore tical capacity of 3860 mA h g−1 and lowest electrochemical potentials (i.e., −3.04 V vs standard hydrogen electrode). Therefore, lithium metal batteries (LMBs) including lithium–oxygen (Li–O2) and lithium– sulfur (Li–S) cells can have much higher theoretical energy density than lithium ion batteries (Figure S1, Supporting Informa tion).[5] They are promising power sources for light weight, wearable, and flexible electronics. Typically, a full battery cell is made of stacked layers of cathode, anode, current collectors, separator, and cell package, all of which need to be bendable to make a flexible LMB. The separator is usually made of a polymer membrane. Flexible carbon thin films such as those made of carbon nanotubes (CNTs) and/or graphene based materials can be used as both the current collectors and the conductive support for the O2 and S cathodes (Table S1, Supporting Information).[6–11] Using Li metal anode in a bendable battery is especially challenging due to its tendency to grow dendrites or filaments during charging/discharging. Dendritic filament growth of Li lead to significant volume fluctuation of the anode layer during deposition/dissolution and hinders the formation of a stable solid electrolyte interphase (SEI) between Li and the electro lyte, resulting in limited cycling Coulombic efficiency and
Bendable energy-storage systems with high energy density are demanded for conformal electronics. Lithium-metal batteries including lithium–sulfur and lithium–oxygen cells have much higher theoretical energy density than lithium-ion batteries. Reckoned as the ideal anode, however, Li has many challenges when directly used, especially its tendency to form dendrite. Under bending conditions, the Li-dendrite growth can be further aggravated due to bending-induced local plastic deformation and Li-filaments pulverization. Here, the Li-metal anodes are made bending tolerant by integrating Li into bendable scaffolds such as reduced graphene oxide (r-GO) films. In the composites, the bending stress is largely dissipated by the scaffolds. The scaffolds have increased available surface for homogeneous Li plating and minimize volume fluctuation of Li electrodes during cycling. Significantly improved cycling performance under bending conditions is achieved. With the bending-tolerant r-GO/Li-metal anode, bendable lithium–sulfur and lithium– oxygen batteries with long cycling stability are realized. A bendable integrated solar cell–battery system charged by light with stable output and a series connected bendable battery pack with higher voltage is also demonstrated. It is anticipated that this bending-tolerant anode can be combined with further electrolytes and cathodes to develop new bendable energy systems.
Conformal electronics[1] require power sources (e.g., batteries) that are both high energy density and tolerant to mechanical deformation during device operations. One way to achieve bending-tolerant battery is to replace a bulk piece with an array of smaller ones linked by stretchable interconnects, so that significant macroscopic strains can be accommodated by deforming the interconnects instead of the individual bat teries.[2–4] On the other hand, making the bulk batteries themselves more bending tolerant would allow them to be A. Wang, S. Liu, Prof. J. Luo Key Laboratory for Green Chemical Technology of Ministry of Education School of Chemical Engineering and Technology Tianjin University and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin) Tianjin 300072, China E-mail:
[email protected] Prof. S. Tang State Key Laboratory of Structural Analysis for Industrial Equipment Department of Mechanics Dalian University of Technology Dalian 116024, China The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201703891.
Dr. D. Kong, Prof. L. Zhi CAS Key Laboratory of Nanosystem and Hierarchical Fabrication CAS Center for Excellence in Nanoscience National Central for Nanoscience and Technology Beijing 100190, China K. Chiou, Prof. J. Huang Department of Materials Science and Engineering Northwestern University Evanston, IL 60208, USA Prof. Y.-Y. Xia Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials Institute of New Energy iChEM (Collaborative Innovation Center of Chemistry for Energy Materials) Fudan University Shanghai 200433, China
DOI: 10.1002/adma.201703891
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Figure 1. Bending aggravates dendrite growth on Li-metal anodes. Schematic in (a) shows that bending of a Li-metal foil can induce crease/crack formation. The electric field around these crease/crack areas is stronger than on flat areas, leading to severe dendrites grown locally and unevenly on bended Li during plating. b) Dendrites can form during the Li plating process. Bending the loosely packed Li layer pulverizes the Li filaments, leading to partial loss of Li under bending. At the same time, new creases/cracks form under bending and accelerate new dendrite growth. c) Representative SEM images of Li surface at the initial stage, after cycling, after cycling followed by bending, after bending, after bending followed by cycling, and after cycling under bending conditions in symmetric Li–Li cells with 1 m LiTFSI in TEGDME electrolyte showing bending-aggravated dendrite growth.
much reduced lifetime of the battery. Severe filament growth can even penetrate the separator to short the batteries causing catastrophic, hazardous failure.[12–22] As disucssed below, all these dendrite-related problems for Li metal anode become more serious upon bending, which cannot be resolved even by reducing its thickness. Li is a very soft metal and can readily undergo plastic defor mation upon bending,[23] which leaves creases and cracks on the metal surface (Figure 1a,c) as nucleation sites promoting local growth of dendrites and filaments. Upon charging, the electric field around these features is stronger than the rest of the elec trode, which increases local Li+ concentration and accelerates Li deposition, leading to preferential and excessive filament growth at these sites. If bending occurs after filament growth, it will squeeze and even fracture the loosely packed dendrite layer, leading to partial loss of Li (Figure 1b,c). At the same
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time, bending will again create new creases/cracks and acceler ates new dendrite growth. Therefore, all the problems associ ated with dendrite Li growth are aggravated upon bending Li metal anodes. In Li–S (O2) batteries, polysulfides and O2 can cross over the separator and corrode the Li anode, further desta bilizing the Li metal anode. In fact, it has been reported that Li metal anodes need to be replaced periodically during long-run tests of Li–S (O2) batteries.[24–26] We discovered that loading Li metal into a graphene-based scaffold can drastically increase its bending tolerance. Here Li metal is finely distributed in between reduced graphene oxide (r-GO) sheets (Figure 2a,b), which act as both conductive path ways to allow electrical continuity in the anode, and barriers to retard crack propagation in Li metal during bending. This significantly reduces the formation of surface creases upon bending, thus preventing the local dendrite growth problem
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Figure 2. Bending-tolerant Li-metal anodes supported by r-GO scaffold. Schematic in (a) shows that the increased available surface area of the scaffolds leads to more homogeneous plating of Li. Schematic in (b) shows that bending stress will be largely dissipated by the bendable scaffold in the composite. Even small creases/cracks were generated, they are less prone to propagate as the rest Li is protected by the beneath scaffold layers. Plating/ stripping voltage profiles of pure Li metal and r-GO/Li in symmetric cells with c,d) 1 m LiTFSI in DME:DOL and e,f) 1 m LiTFSI in TEGDME electrolyte cycled c,e) at normal state and d,f) under bending to 180°. g) Representative SEM images of Li surface at initial stage, after cycling, after cycling followed by bending, after bending, after bending followed by cycling, and after cycling under bending conditions in symmetric Li–Li cells with 1 m LiTFSI in TEGDME electrolyte showing the r-GO/Li electrode surface is more homogenous without significant protuberance under various test conditions.
shown in Figure 1a. On the other hand, using the nonreac tive, high surface area, conducting r-GO scaffold also leads to more evenly distributed effective current density, which helps to prevent Li dendritic growth at all stages of bending or battery operation (Figure 1b). As we illustrate below, the resulting Li metal anodes indeed show drastically improved plating/stripping cycling performance under various bending operations. The r-GO/Li composite films are made by capillary loading of molten Li in between r-GO sheets (Figure S2, Supporting Information).[21] Several properties of r-GO/Li composite films
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make them advantageous for constructing bending-tolerant Li metal anodes. First, the r-GO films themselves are bendable, which can dissipate the bending stress in the composite. Fur thermore, r-GO has been found to be lithiophilic, thus elimi nating the need for additional lithiophilic coating or Li grow seeds that are typically needed for other carbon materials.[22,27] In addition, the r-GO films also have high surface area, which can lower the electrical current density, allowing uniform and stable Li deposition/dissolution (Figures S3 and S4, Supporting Information). Besides, r-GO is lightweight (5 wt% in the com posite) and thus the maximum capacity that can be extracted
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from the composite anode was only sacrificed by 5.98% com pared to pure Li metal (Figure S5, Supporting Information). Upon contact with molten Li, the lamellar GO film under goes a self-propagating reduction reaction initiated by hot molten Li. Gas evolved during reduction expanded the GO film, and molten Li absorbed into the intersheet space.[21] Note that GO reduction can be rapidly initiated thermally and selfpropagates,[28,29] and does not need Li metal as the reducing agent. Control experiments showed that putting GO films in heated furnace also immediately induced the self-propagating exothermal deoxygenating reactions (Figure S6, Supporting Information),[28] and Li can then be absorbed into the thermally reduced r-GO films. This was also confirmed by X-ray diffrac tion studies showing that the amount of Li oxide was negligible in the r-GO/Li composites (Figure S7, Supporting Information). In order to have a fair comparison between r-GO/Li and Li metal itself, the areal mass and thickness of the corresponding electrodes were made the same. We heated molten Li to 400 °C to lower its viscosity, so that it can readily infiltrate into the r-GO film to achieve a highly compact composite with den sity similar to that of pure Li (Figures S2 and S8, Supporting Information). We chose to use 50 µm thick Li in this work as it can already deliver an area capacity of 10 mA h cm−2, which is sufficient for most applications (note: the area capacity for current LIB electrode is typically 3–4 mA h cm−2). The anodic performances of r-GO/Li composite and pure Li metal were first tested under a symmetric battery cell configuration. We chose 1 m LiTFSI in DME:DOL as the electrolyte, which is the common choice for Li–S batteries. The current density is set to be 200 mA g−1, and each plating/stripping cycles takes 3 h. As shown in Figure 2c, the Li metal cell already exhibited large voltage hysteresis at early stage and was short circuited after only 200 h cycling. In contrast, the r-GO/Li cell has much smaller initial voltage hysteresis and maintained stable per formance for more than 1000 h, indicating improved plating/ stripping stability of r-GO/Li composite (Figure S9a, Sup porting Information). When cycled under bending condition in pouch cell, shorting was observed after the first few cycles for Li–Li cell, presumably due to the dendrite problems discussed in Figure 1 (Figure 2d; Figure S9c, Supporting Information). However, the similarly bent r-GO/Li anode has much higher plating/stripping stability and remains stable over 700 h. Next, the anodes were evalu ated in 1 m LiTFSI in TEGDME electrolyte, the commonly used electrolyte for Li–O2 batteries. The r-GO/Li anode also outper formed pure Li metal electrode, under both normal and bending conditions (Figure 2e,f; Figure S9b and S9d, Supporting Infor mation). Scanning electron microscopy (SEM) studies show dendrites covering the surface of Li metal anode after cycling (Figure 1c). In contrast, the r-GO/Li surface is much smoother and absent of any significant protuberance even after various plating/stripping tests (Figure 2g; Figures S10 and S11, Sup porting Information), which explains its high bending toler ance. Furthermore, the r-GO/Li composite and pure Li anodes were tested after bending to 90° for 10 times or 30 times. The voltage hysteresis and cycling performance of r-GO/Li com posite electrode outperformed pure Li metal in both 1 m LiTFSI in DME:DOL and 1 m LiTFSI in TEGDME (Figure S12, Sup porting Information). SEM observation showed that more
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Li dendrite formed in the bent region than in the flat area (Figure S13, Supporting Information). With bendable Li anodes in hand, we are now able to make an entire Li–S battery bendable (Figure 3a). The cathode was com posed of S particles (1 mg cm−2) embedded in a flexible CNT thin film (Figure 3b,c; Figure S14, Supporting Information). Energy dispersive X-ray spectroscopy (EDX) mapping shows S particles are well distributed in the CNT matrix (Figure 3d). The S-CNT cathode was first tested in coin cell configuration with pure Li or r-GO/Li as anode (Figure 3e). At low current density of 0.1 A g−1, cells with both types of anodes exhibited comparable capacities and charge–discharge curves (Figure 3e; Figure S15, Supporting Information). But as the current density increases, the battery with r-GO/Li anode significantly outper formed the one with pure Li metal anode, showing both higher capacity and lower voltage hysteresis, which is consistent with the results obtained from symmetric cells (Figure 2c). To test the batteries under bending condition, pouch cells were assembled and then bent to 180° before testing (Figure 3f). The pouch cell with r-GO/Li anode could run for 100 cycles with stability com parable to the corresponding coin cells. However, the pouch cell with pure Li anode failed just after 35 cycles. To investigate the failure mechanism, both anodes were extracted from the pouch cells and washed before characterization by SEM and Fourier transform infrared spectroscopy (FT-IR). As expected, severe dendrites growth was observed on pure Li anode after cycling under bending condition. In addition, intermediate products of the S cathode–electrolyte soluble polysulfides crossed over the porous separator membrane and corroded the Li to form inert insulating Li2S (Figure 3g). Since corrosion took place at the interface between Li and polysulfides containing electro lyte, the erected dendrites increased the exposed area of Li and thus the corrosion degree, resulting in serious Li2S precipitated on the Li anode. As discussed previously, dendrite growth is aggravated by bending, which prevented the formation of stable SEI and continuously reduces the electrolyte to O, C, and F containing precipitates, as observed in FT-IR (Figure 3i). The combined effects of unstable SEI and polysulfide corrosion quickly deactivated the Li metal, especially when it was bent. In a few reports of long term cycling of S cathodes, the Li metal anodes had to be replaced periodically,[24] due to the deactiva tion of Li metal anode by polysulfides. With the r-GO support to suppress dendrite formation, the SEI was much stabilized even under bending condition. As a result, polysulfides corro sion and electrolyte decomposition on the surface of r-GO/Li electrode were suppressed, as reflected in the relatively uniform surface morphology (Figure 3h) and much reduced deposition of S, C or F containing precipitates (Figure 3h,i) after cycling. X-ray photoelectron spectroscopy confirmed the above men tioned difference in the elemental compositions between the deposits on pure Li and r-GO/Li electrodes (Figure S16, Sup porting Information). In addition to Li–S batteries, Li metal anodes are also very useful for making high energy density Li–O2 batteries. To make Li–O2 batteries bendable (Figure S17a, Supporting Informa tion), flexible cellulose paper (CP) dip coated with CNT and decorated with RuO2 nanoparticles was used as O2 cathode (Figures S17b–d,S18, and S19, Supporting Information). The RuO2–CNT–CP cathode leads to very high capacity Li–O2
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Figure 3. Bendable Li–S batteries. a) Schematic of a bendable Li–S battery. b) Photo, c) SEM image, and d) EDX mapping of the S–CNT cathode. e) Photo and cycling performance of the Li–S battery in CR2032-type coin cells with pure Li and r-GO/Li as anode. f) Photo and cycling performance of the Li–S battery in pouch cells (7 cm × 5 cm) with pure Li and r-GO/Li as anode cycled when bent to 180°. g) SEM images showing severe dendrites grown on pure Li anode after cycling under bending condition and were heavily contaminated by polysulfides, h) while the surface of r-GO/Li electrode after cycling was more uniform with much less SEI and Li2Sn. i) FT-IR absorption of the anodes after cycling under bending showing r-GO/Li anode was less contaminated than pure Li.
batteries when fully discharged (Figure S20, Supporting Infor mation), and the discharge product on the cathode was con firmed to be Li2O2 (Figures S21 and S22, Supporting Informa tion) which is consistent with previous reports.[30] When cycled at limited capacity of 1000 mA h g−1 in coin cells, the cut-off discharge voltage of RuO2–CNT–CP cathode can maintain at 2.6 V in the cell with r-GO/Li anode (Figure S17e, Supporting Information). In contrast, the cut-off discharge voltage in the pure Li anode cell starts to decay after the first few cycles and quickly drops after 35 cycles. In pouch cells bent to 180° and cycled with limited capacity of 300 mA h g−1, the cut-off discharge voltage drop in the pure Li anode cell occurs ahead after 25 cycles (Figure 3f). How ever, the cut-off discharge voltage in the r-GO/Li anode cell can still sustain for 50 cycles. Similar to or even more fatal than
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polysulfides, O2 on the cathode side in Li–O2 batteries can dif fuse to the anode side and corroded the Li to Li2O. So there are also reports in which the Li metal anodes in the Li–O2 bat teries had to be replaced intermittently.[25] In fact, Li has not been considered suitable as the counter electrode for Li–O2 bat teries.[26,31] With the severe dendrites grown on pure Li anode under bending condition, it is thus not surprising that the Li anode was heavily corroded and quickly deactivated after cycling (Figures S17g,i and S23). In this case, r-GO again stabilizes the SEI and prevents O2 corrosion. More uniform surface mor phology (Figure S17h, Supporting Information) and much less SEI and Li2O on the r-GO/Li anode were found (Figures S17i and S24, Supporting Information). Li–S and Li–O2 batteries that can tolerate bending or defor mation, as demonstrated in Figure 4a,b, could find widespread
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Figure 4. Bendable integrated solar cell–battery systems and series stacked batteries. Photos of bent a) Li–S and b) Li–O2 battery-powered strings of light-emitting diodes. c) Charge–discharge curves of a bendable integrated solar cell–Li–S battery charged by light and discharged at different current densities. The inset is a photo of the integrated device. d) Charge–discharge curves of series connected Li–S batteries. The insets are schemes and photos of the series stacked batteries.
applications in many areas. Due to the air and humidity sensi tivity of Li–O2 batteries, we only used Li–S pouch cells for the following proof-of-concept demonstrations. When a flexible thin film solar cell is combined with a bendable Li–S battery, a bend able integrated energy storage system is created (Figure 4c). An obstacle for direct use solar energy is the instability of the output voltage. Now the bendable system powered by sun can have more stable output. To increase the working voltage or cur rent, a number of bendable Li–S batteries can be connected in series or parallel, respectively. When three batteries are stacked and connected in series, they can deliver a discharge voltage up to 6 V (Figure 4d), without noticeable polarization. In summary, we demonstrated bending-tolerant r-GO sup ported Li metal anodes that can lead to flexible Li metal batteries. In r-GO/Li anodes, Li dendrites are significantly suppressed, even under bending conditions. The r-GO sheets can also reduce the loss of Li by promoting homogeneous plating of Li and con fining Li in the scaffolds. The flexible r-GO sheets embedded in Li metal can help to dissipate the bending stress and retard the propagation of any defects or even cracks generated by bending. The composite anode has significantly extended cycling lifetime during electrochemical plating/striping and much improved bending tolerance. With bendable Li metal anodes, it is now possible to construct high performance Li–S and Li–O2 batteries that are readily compatible with flexible solar cells to achieve bendable integrated solar cell–battery systems.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgement A.W., S.T., and D.K. contributed equally to this work. The authors appreciate support from the National Natural Science Foundation
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of China (Grant Nos. U1601206, 51502197) and Natural Science Foundation of Tianjin, China (Grant No. 15JCYBJC53100). K.C. is a National Science Foundation Graduate Research Fellow. J.H. also thanks the support from the Office of Naval Research in the US (ONR N000141612838).
Conflict of Interest The authors declare no conflict of interest.
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