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An Advanced Lithium Metal-Carbon Nanotube Composite Anode for High-Performance Lithium-Oxygen Batteries Feng Guo, Tuo Kang, Zhenjie Liu, Bo Tong, Limin Guo, Yalong Wang, Chenghao Liu, Xi Chen, Yanfei Zhao, Yanbin Shen, Wei Lu, Liwei Chen, and Zhangquan Peng Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b02560 • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 5, 2019

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An Advanced Lithium Metal-Carbon Nanotube Composite Anode for High-Performance Lithium-Oxygen Batteries Feng Guo,†,‡ Tuo Kang,‡,§ Zhenjie Liu,± Bo Tong,±,ǂ Limin Guo,± Yalong Wang,┴ Chenghao Liu,┴ Xi Chen,+ Yanfei Zhao,├ Yanbin Shen,*,†,‡ Wei Lu,†,‡ Liwei Chen,*,†,‡,║ and Zhangquan Peng*,± †School

of Nano Technology and Nano Bionics, University of Science and Technology of China, Hefei 230026, China. ‡i-Lab, CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China. §Shenzhen Engineering Lab of Flexible Transparent Conductive Films, Department of Materials Science and Engineering, Harbin Institute of Technology, Shenzhen 518055, China. ±State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China. ǂKey Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry of Education), School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China. ┴China Energy Lithium Co., No. 100, the 9th Avenue of Xinye, West TEDA, Tianjin 300465, China. +Division of Physics, Department of Mathematical Sciences, Xi'an Jiaotong-Liverpool University, 111 Ren'ai Road, Suzhou 215123, China. ├Vacuum Interconnected Nanotech Workstation (Nano-X), Suzhou Institute of NanoTech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China. ║In-Situ Center for Physical Sciences, School of Chemistry and Chemical Engineering, Shanghai Jiaotong University, Shanghai 200240, China. Keywords: Li-O2 batteries, lithium metal electrode, lithium-carbon composite, electrode-electrolyte interface, electrochemical reversibility

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Abstract: The low Coulombic efficiency and hazardous dendrite growth hinder the adoption of lithium anode in high energy density batteries. Herein, we report a lithium metal-carbon nanotube (Li-CNT) composite as an alternative to the long-term untamed lithium electrode to address the critical issues associated with the lithium anode in LiO2 batteries, where the lithium metal is impregnated in a porous carbon nanotube microsphere matrix (CNTm) and surface-passivated with a self-assembled monolayer of octadecylphosphonic acid as a tailor-designed solid electrolyte interphase (SEI). The high specific surface area of the Li-CNT composite reduces the local current density and thus suppresses the lithium dendrite formation upon cycling. Moreover, the tailordesigned SEI effectively seperates the Li-CNT composite from the electrolyte solution and prevents the latter’s further decomposition. When the Li-CNT composite anode is coupled with another CNTm-based O2 cathode, the reversibility and cycle life of the resultant Li-O2 batteries are drastically elevated.

To meet society’s ever-growing energy storage demands, a great deal of research effort has been devoted to exploring radically new battery chemistries, with the aim of developing energy storage devices having higher capacity and lower cost than today’s best Li-ion batteries.1-3 Among these new battery chemistries, the aprotic Li-O2 battery has attracted the most attention because of its unrivaled theoretical specific energy (3458 Wh kg-1, based on Li2O2 as the discharge product) compared to other rechargeable batteries.4,5 Typically, a Li-O2 battery consists of a Li metal anode separated from an O2 cathode by a Li+ conducting electrolyte, and its operation relies on the O2 reduction reaction producing solid Li2O2 on discharge and reverse oxidation upon recharge.6-9 Because the active material (i.e., O2) of the positive electrode can be accessed from the breathing air, the high specific energy of the Li–O2 battery could arguably be ascribed to the utilization of Li metal as the negative electrode that has an unbeatably high specific capacity (3860 mAh g-1) and low formal potential (-3.04 V vs. SHE).6,10-13

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In practice, Li metal anode rarely, if ever, meets the required reversibility and cyclability of Li-O2 batteries.14-19 This is because Li metal electrode can readily react with the electrolyte components (solvent, anion, additive, etc.) producing a solid electrolyte interphase (SEI).12 The obtained SEI is expected optimistically by many researchers in the Li-O2 field to be an electronically insulating and ionically conducting passivation layer that can effectively separate the Li metal electrode from the electrolyte solution and prevent the latter’s decomposition.20 However, in practice the SEI formed on the Li metal electrode is far from the ideal case. For instance, a recent mechanistic study by Zhou et al., who have been armed with a spectrum of advanced characterization techniques,21 indicated that the Li metal electrode in aprotic Li-O2 batteries is not reversible, and instead, it ceaselessly reacts with electrolyte solution, consumes O2 transpired from the cathode, and produces hazardous Li dendrites upon cycling. These problems would be unacceptable for a potentially disruptive Li-O2 technology and represent a significant barrier to progress. To address the critical issues associated with the Li metal electrode in Li-O2 batteries, sparse research effort has begun to appear in the past few years.20-29 For instance, Kim et al. coated the Li metal electrode surface with an artificial protective layer of two-dimensional layered material that can effectively suppress electrolyte decomposition and Li dendrite growth.24 Choudhury et al. tailor-designed an ionomer salt and used it as an electrolyte additive to promote the formation of a high quality SEI.20 Liu et al. demonstrated that the Li metal electrode can be stabilized by coating a thin film of solid-state or quasi-solid-state electrolyte.25 However, the severe volume expansion and contraction of the Li metal electrode, particularly upon deep discharge and charge, could induce stress and crack within the SEI and even delaminate the SEI from the Li metal electrode surface, as pointed out recently by Shen et al.30 As a result, new anode materials as alternatives to the conventional Li metal are urgently required for the realization of practical Li-O2 batteries. In this work, we devised an advanced lithium-carbon nanotube (Li-CNT) composite anode for aprotic Li-O2 batteries. The Li-CNT composite has a microsphere appearance, 3

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a robust internal electronic conducting network formed by the interwoven CNTs,31 and an artificial SEI layer formed via molecular self-assembly of octadecylphosphonic acid (OPA) on the Li-CNT surface.32 Because of its unique structural design and surface chemistry, the Li-CNT composite anode shows essentially no volume expansion and no Li dendrite growth upon discharge and recharge. Moreover, the parasitic side reactions between the Li-CNT composite and the air/electrolyte solution have been effectively inhibited. Specifically, 95.0% of the initial capacity of the Li-CNT can be retained even after exposure to dry air (dew point: −40 °C) for 5 days. Furthermore, a low polarization of < 10 mV has been observed for a symmetric Li-CNT || Li-CNT cell cycled at a current density of 0.5 mA cm-2 in an O2-saturated electrolyte, and even after 300 cycles the polarization is still < 30 mV, indicating a highly stable Li stripping/plating process. When the Li-CNT composite anode is coupled with another CNT microsphere-based O2 cathode, a high-performance Li-O2 battery with much enhanced reversibility (oxygen recovery efficiency ~ 95.0%) and cycle life (>1800 h) has been realized. The synthesis and surface modification procedure of the Li-CNT composite have been described in the Methods Section. The physicochemical properties of the composite have been measured by a broad range of complementary characterization techniques. Scanning electron microscopy (SEM) images show that the Li-CNT composites are spherically shaped and have a diameter of ~ 5.0 μm (Figure 1a). The specific capacity of the Li-CNT composite is quantified to be ~1850 mAh g-1 and 95.0% (~1750 mAh g-1) of the initial capacity can be retained even after exposure to dry air (dew point: −40 °C) for 5 days (Figure 1b). This observation indicates that the Li-CNT composite has a better stability towards oxygen and moisture than Li metal anode does. The small amount of capacity loss of the Li-CNT composite during storage in dry air is probably caused by imperfect coverage of the OPA SAM on the surface due to large curvature of the Li-CNT sphere shape, which leads to reaction between the unpassivated Li and the oxygen/moisture in the dry air. X-ray photoelectron spectroscopy (XPS) has also been employed to characterize the surface chemistry of the Li-CNT 4

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composite. As shown in Figure 1c, the XPS spectrum of Li 1s can be deconvoluted into two peaks centered at 54.5 and 55.3 eV, which could be ascribed to lithium and lithium phosphate, respectively.33,34 The P 2p spectrum consists of two Gaussian peaks centered at 133.5 and 131.5 eV, which could be assigned to P−O and P−C bonds, respectively (Figure 1d).34,35 The C 1s spectrum shown in Figure 1e is deconvoluted into two peaks centered at ∼285.0 and 284.8 eV corresponding to C in C−P and C−C bonds, respectively.36,37 For the O 1s spectrum (Figure 1f), the major peak located at 531.5 eV could be assigned to the O in phosphates,38 while a small shoulder at 534.0 eV might be ascribed to adsorbed oxygen species.39 The above XPS results have clearly proved the existence of the lithium alkyl phosphate coating layer on the Li-CNT surface.

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Figure 1. (a) SEM images of Li-CNT composites. (b) Electrochemical delithiation voltage profiles of the Li-CNT before and after exposed to dry air for 5 days. XPS spectra of the Li-CNT composite: (c) Li 1s spectrum, (d) P 2p spectrum, (e) C 1s spectrum, and (f) O 1s spectrum. To assess the electrochemical performance of the Li-CNT composite anode in LiO2 battery electrolyte, symmetric cells containing two Li-CNT composite electrodes and an electrolyte of O2-saturated 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in tetra ethylene glycol dimethyl ether (TEGDME), i.e., the benchmark electrolyte for current Li-O2 batteries, have been assembled and tested. For comparison, Li || Li symmetrical cells have also been prepared and examined. Figure 2a shows the voltage profiles of the symmetric cells operated at a current density of 0.5 mA cm-2. For Li-CNT || Li-CNT cell, a low polarization < 10 mV upon both discharge and charge has been observed at the initial stage of cycling, and the polarization is still < 30 mV even after 300 cycles, indicating a highly stable Li stripping/plating process associated with the Li-CNT composite electrode. In contrast, the Li || Li cell presents an unstable voltage profile with apparent random fluctuation during cycling, which could be ascribed to the non-uniform deposition of Li and the irregular growth of Li dendrites.4042

Specifically, after 150 cycles the discharge/charge overpotentials increase sharply,

indicating the failure of the cell.43,44 The stability of the Li-CNT | TEGDME interface has also been evaluated with timedependent electrochemical impedance spectra (EIS) and compared with that of the Li | TEGDME. As shown in Figure 2b, impedance of the Li || Li cell calculated from the semicircle is ~70 Ω and this value increases with dwell time. The increasing impedance of the Li || Li cell can be ascribed to the incessant growth of the SEI film caused by the parasitic side reaction between the Li metal and the electrolyte solution.23,40,41 While for the Li-CNT || Li-CNT cell the impedance spectra (Figure 2c) exhibit no obvious change even after 48 h dwell time, suggesting a more stable interface of Li-CNT | TEGDME. In situ differential electrochemical mass spectrometry (DEMS) study further provides compelling evidence for the enhanced stability of the Li-CNT | TEGDME interface. As shown in Figure 2d the Li || Li cell produces H2 and consumes

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consumption/evolution (Figure 2e). The surface chemistry evolution of the Li-CNT and Li metal electrodes cycled in the above symmetric cells has also been investigated using XPS (Figure S1). As shown in Figure S1a, electrolyte decomposition products of Li2O, LiOH, and ROCO2Li have been identified on the Li metal electrode after 20 h shelf-time; while

Figure 2. (a) Voltage profiles of Li stripping/plating in the Li-CNT || Li-CNT (red line) and Li || Li (black line) cells at a current density of 0.5 mA cm-2. Time-dependent EIS of (b) Li || Li and (c) Li-CNT || Li-CNT cells. In situ DEMS study of (d) Li || Li and (e) Li-CNT || Li-CNT cells cycled at a current density of 0.5 mA cm-2. on Li-CNT anode the XPS spectra remain essentially unchanged compared to the fresh electrode shown in Figure 1, indicating the enhanced stability of the Li-CNT | 7

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TEGDME interface. After the first cycle, moieties of C−O, ROCO2Li and –CF2 appear on the Li electrode surface, which are originated from the decomposition of the LiTFSI salt and the TEGDME solvent (Figure S1b). On the Li-CNT electrode, only a limited amount of LiF and C−O has been detected at this stage. After 20 cycles (Figure S1c), the decomposition products on Li metal electrode significantly increase, indicating incessant parasitic side reactions on the Li electrode. In contrast, there is only a limited amount of electrolyte decomposition products and almost no Li2O signal forms on the Li-CNT electrode surface. These results suggest that the Li-CNT | TEGDME interface is electrochemically more stable than its Li | TEGDME counterpart. After recognizing that the Li-CNT composite electrode is more stable than the conventional Li metal anode when in contact with the electrolyte solution, Li-O2 cells with a configuration of Li-CNT | 1.0 M LiTFSI TEGDME | Ketjen black (KB, a benchmark cathode material for current Li-O2 batteries) have been assembled and tested. For comparison, conventional Li-O2 cells of Li | 1.0 M LiTFSI TEGDME | KB have also been prepared and examined. Figure 3a and b show the voltage profiles of galvanostatic discharge and charge for the Li || KB and Li-CNT || KB cells at a current density of 50 mA g-1carbon and a fixed capacity of 500 mAh g-1carbon within a voltage range of 2.0 - 4.5 V. The Li-O2 cell with Li metal anode can only cycle for < 20 cycles and in the following cycles the discharging/charging overpotential increases drastically and the cell fails quickly.22 On the contrary, the Li-CNT || KB cell exhibits almost reproducible voltage profiles over ~ 40 cycles with only a minor increase in polarization voltage. This enhanced cycling performance clearly indicates that the more stable LiCNT composite anode can improve the overall electrochemical performance of Li-O2 batteries. To further study the reversibility of the Li || KB and Li-CNT || KB cells, in situ DEMS has been used to monitor the gas consumption/evolution during cycling, see Figure 3c-f. It has been known that for an ideal Li–O2 battery without any parasitic side reactions the amount of O2 consumed during discharge shall be equal to that of O2 released during subsequent recharge, i.e., reversible formation and decomposition of 8

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Li2O2.21,22 By integrating the area defined by the O2 consumption/evolution curves in the DEMS results, the O2 recovery efficiency of Li-CNT || KB cell has been quantified to be 79.0%, which is higher than that of Li || KB cell (67.8%). The DEMS results provide direct evidence that the Li-CNT composite anode contributes greatly to the enhanced reversibility and cyclability of the Li-CNT || KB cell, thanks to the less parasitic side reactions of the electrolyte and oxygen on the negative electrode side. However, a small amount of CO2 can still be detected at the late stage of charging process, which could be due to the decomposition of the byproducts formed during the previous discharge, and/or the decomposition of the carbon material and electrolyte at high charging voltages.45

Figure 3. Cycling performances and DEMS results of (a,c,e) Li || KB and (b,d,f) LiCNT || KB cells.

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The compositional and structural evolution of the Li and Li-CNT anodes upon cycling have also been examined by a combination of advanced characterization techniques including SEM, Fourier transform infrared spectra (FTIR) and X-ray diffraction (XRD). SEM results show that after 20 cycles mossy and dendritic Li deposits have formed on the Li metal anode that is smooth prior to cycling (Figure 4a and b), and severe pulverization has occurred as can be seen from an obvious volume expansion of ~40% (Figure 4c and d). While for the Li-CNT anode there is essentially no morphology change after 40 cycles from the top- and cross-sectional views (Figure 4e-h). FTIR studies show the signals at wavenumbers of 1500 and 3680 cm-1 from a small quantity of the lithium carbonate (Li2CO3) and lithium hydroxide (LiOH) on both Li and Li-CNT anodes before cycling (Figure 4i).46 Upon Li anode cycling, new features from decomposition of the electrolyte begin to appear at the wavenumbers of 865, 935, 1352 and 1616 cm-1, and the intensity of LiOH at 3680 cm-1 also increases.47,48 In contrast, FTIR spectra (Figure 4j) of the Li-CNT composite anode remain virtually unchanged after 20 cycles. XRD, a characterization technique sensitive to crystalline species, has identified only bulk lithium phase for both Li and Li-CNT anodes before cycling (Figure 4k and l). Upon cycling of the Li metal anode, LiOH formation and accumulation have been revealed by XRD (Figure 4k), while for the Li-CNT anode there is essentially no LiOH identified (Figure 4l).

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Figure 4. SEM images of the surface and cross-section of (a-d) Li and (e-h) Li-CNT composite anodes before and after cycling at a current density of 50 mA g-1carbon; FTIR and XRD patterns of the (i,k) Li and (j,l) Li-CNT composite anodes before and after different cycle numbers. The above complementary studies have indicated that the cyclability (> 40 cycles) of the Li-CNT || KB cell is superior to that of the Li || KB cell (< 20 cycles), and at the end of 40 cycles the Li-CNT composite anode shows minor parasitic reactions. Therefore, the limited cyclability (~ 40 cycles) of the Li-CNT || KB cell could be ascribed to the failure of the KB cathode. To get insight into the KB cathode of the LiCNT || KB cell, FTIR has been employed to examine the KB cathode at the end of discharge and charge of various cycles (Figure S2). Prior to cycling, the peaks at 1200 and 1770 cm-1 obtained on the pristine cathode are probably from the oxygen containing species (e.g., C-O and C=O) that are frequently observed on various carbon surfaces.49 The peaks at 845, 1395, and 1510 cm-1, which can be assigned to Li2CO3 and lithium carboxylates (Figure S2),45,47 have increased with the cycle numbers, probably due to the decomposition of the KB carbon and electrolyte particularly at high charging voltages. Besides the increase of Li2CO3 with cycle number, a small amount of incompletely decomposed Li2O2 has also been observed in the FTIR curves after 5 discharge/change cycles. The residual Li2O2 would accumulate on the KB cathode and clog pores, resulting in high charging voltage that will decompose the cathode/electrolyte and deteriorate the cycling stability of the cell. To verify this 11

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hypothesis, a Li-O2 cell was built by using a new KB cathode, a fresh electrolyte and the used Li-CNT composite anode that has already been cycled for 40 cycles (see Figure S3a). The rebuilt cell can deliver another 40 stable cycles (see Figure S3b), and this observation further proves the highly stable interface of Li-CNT | electrolyte. Meanwhile, these results also indicate that the KB cathode with rich surface chemistry (e.g., C-O and C=O) is prone to decompose and limits the cycle life of the Li-CNT || KB cell.50 To obtain a more stable cathode, CNTs, whose graphitic sidewalls have less surface defects than the KB carbon, have been employed to prepare a CNT-based microsphere (CNTm). The CNTm has a porosity of ~ 94% (see porosity calculation in the Method section) and a pore volume of 2.2 cm3 g-1 (see Figure S4) that can provide enough space for the Li2O2 growth. Moreover, an interwoven electron-conducting network of the CNTs is very beneficial to the O2 electrochemistry (i.e., formation and decomposition of Li2O2). The obtained CNTm was then used to replace the KB carbon to build a LiCNT || CNTm cell (the areal loadings of the Li-CNT and the CNTm electrodes are 10 mg cm-2 and 1 mg cm-2, respectively). As shown in Figure 5a and b, lower discharge/charge polarization has been achieved and the cycle life has been enhanced to > 90 cycles (the areal cycling amount of the Li-CNT || CNTm cell is 1 A g-1 or 1 mAh cm-2). Additionally, the O2 recovery efficiency of the Li-CNT || CNTm cell has been quantified to be ~ 95.0% (Figure 5c and d), suggesting a much-enhanced electrochemical reversibility. Moreover, the formation and decomposition of toroidalshaped Li2O2 on the CNTm cathode have also been confirmed with combined SEM and XRD studies (Figure S5). The significantly enhanced electrochemical reversibility and cycle life of the Li-CNT || CNTm cell indicates that applying more stable cathode materials is of crucial importance for next-generation Li-O2 batteries.

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Figure 5. (a) Voltage profiles of the first discharge-charge cycles of Li || KB, Li-CNT || KB and Li-CNT || CNTm cells, (b) cycling performance and (c-d) DEMS results of the Li-CNT || CNTm cell. In summary, an advanced Li-CNT composite anode has been devised to replace the conventional Li metal electrode towards better Li-O2 batteries. The Li-CNT composite has a microsphere appearance and an internal interwoven CNT network that can promote the lithium deposition within the microsphere without Li dendrite formation and volume expansion. In addition, an artificial SEI layer coated on the Li-CNT composite surface has the ability to restrain the parasitic side reactions between the lithium metal component of the Li-CNT composite and the electrolyte components. When the Li-CNT composite anode is paired with another CNTm cathode, the resultant Li-CNT || CNTm cell exhibits much enhanced electrochemical reversibility (O2 recovery efficiency 95.0 %) and cycle life (> 90 cycles). Besides to being a highperformance alternative anode for aprotic Li-O2 batteries, the Li-CNT composite can also be used as the negative electrodes in other high energy density batteries such as all-solid-state, lithium-sulfur, etc. 13

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental section, calculation of the porosity, XPS spectra, FTIR patterns, cycling performance, N2 adsorption-desorption isotherms and the pore-size distribution, SEM images and corresponding XRD patterns

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The project was supported by the National Key Research and Development Program, the Ministry of Science and Technology of China (Grants No. 2016YFA0200703, 2016YFB0100100 and 2016YFB0100102), the “Strategic Priority Research Program” of Chinese Academy of Sciences (Grant no. XDA09010600), and the National Natural Science Foundation of China (Grant nos. 21625304, 21733012, 21825202, 21605136, 91545129, and 21575135).

REFERENCES (1) Tarascon, J. M.; Armand, M. Issues and challenges facing rechargeable lithium batteries. Nature 2001, 414, 359-367. (2) Li, M.; Lu, J.; Chen, Z.; Amine, K. 30 years of lithium-ion batteries. Adv. Mater. 2018, 30, 1800561. (3) Yang, X.; Li, X.; Adair, K.; Zhang, H., Sun X. Structural design of lithium-sulfur batteries: from fundamental research to practical application. Electrochem. Energy Rev. 2018, 1, 239-293. (4) Mahne, N.; Fontaine, O.; Thotiyl, M. O.; Wilkening, M.; Freunberger, S. A. Mechanism and performance of lithium-oxygen batteries - a perspective. Chem. Sci. 2017, 8, 6716-6729. (5) Zhang, P.; Zhao, Y.; Zhang, X. B. Functional and stability orientation synthesis of materials and structures in aprotic Li-O2 batteries. Chem. Soc. Rev. 2018, 47, 2921-3004. 14

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