Easily Decomposed Discharge Products Induced by Cathode

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Easily Decomposed Discharge Products Induced by Cathode Construction for High Energy Efficiency Lithium-Oxygen Batteries Jingming Fu, Xiangxin Guo, Hanyu Huo, Yue Chen, and Tao Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01673 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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ACS Applied Materials & Interfaces

Easily Decomposed Discharge Products Induced by Cathode Construction for High Energy Efficiency Lithium-Oxygen Batteries Jingming Fu,a,c Xiangxin Guo,*a,b Hanyu Huo,a Yue Chen,a and Tao Zhang*a

a

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai

Institute of Ceramics, Chinese Academy of Sciences, No. 1295 Dingxi Road, Shanghai, 200050, P.R. China b

College of Physics, Qingdao University, No. 308 Ningxia Road, Qingdao, 266071, P.R. China

c

University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing, 100049, P.R.

China

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ABSTRACT The lithium-oxygen (Li-O2) battery is deemed as a promising candidate for the next generation energy storage system due to its ultrahigh theoretical energy density. However, low energy efficiency and inferior cycle stability induced by the sluggish kinetics of charge transfer in discharge products limit its further development in practical application. In this work, tin dioxide (SnO2) nanoparticles decorated carbon nanotubes (SnO2/CNTs) have been constructed as composite cathodes to manipulate the morphology and component of discharge products in Li-O2 batteries. Owing to the strong oxygen adsorption of SnO2, oxygen reduction reactions tend to occur on composite cathode surfaces, resulting in formation of flake-like discharge products of Li2-xO2 below 10 nm in thickness rather than toroidal particles of several hundred nanometers. Such homogeneous nanosized discharge products with lithium vacancies markedly enhance the electrode kinetics and charge transfer in discharge products. Consequently, the Li-O2 batteries based on the SnO2/CNT cathodes show the small polarization voltage gap, which leads to the superior energy efficiency (80%) compared with that based on the pristine CNT cathodes. The results demonstrate that optimizing the discharge products with nanosized morphology and defective component by cathode construction is an effective strategy to realize the Li-O2 batteries with the increased energy efficiency and the improved cycle stability. KEYWORDS Li-O2 batteries, SnO2/CNTs, cathode construction, size effect, defects, energy efficiency

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1. INTRODUCTION With the rapidly increasing demands of electric vehicles and portable electronic products, the research and development of advanced energy storage devices with the increased energy density to satisfy the practical applications have attracted a growing number of attention in the past few decades.1-4 The lithium-oxygen (Li-O2) battery based on the electrochemical reaction: 2Li+ + O2 + 2e- ⇌ Li2O2 (E0 = 2.96 V vs Li/Li+), has been deemed as one of the great promising candidates for energy storage primarily due to its superhigh theoretical energy densities of 3505 W h kg-1.1,5 However, because the typical discharge product Li2O2 possesses the sluggish charge transfer kinetics, the high potential is inevitably required for decomposing the products, thereby resulting in poor energy efficiency and inferior rate capability.6,7 In addition, increased potential is needed in the case of increased product size.8 By-product lithium carbonates (Li2CO3) are also generated when the carbon-based cathodes are charged above 3.5 V, sharply degrading the battery cycle stability.9 Thus, achievement of easily decomposed discharge products through manipulation of composition and size is essential for promotion of charge transfer kinetics and realization of high-performance Li-O2 batteries.10-12 Transforming discharge product Li2O2 to other lithium compounds is useful to lower the overpotential, and thus enhance the battery performance.13,14 Gery et al. acquired crystalline discharge product LiOH through adding trace water in electrolyte, which led to improve specific capacity and energy efficiency (93.2%) with a narrow voltage gap of merely 0.2 V.15 But how to effectively suppress the side reactions induced by water becomes an additional huge challenge. 3



Larry et al. obtained stable discharge product of LiO2 in the reduced graphene oxide (rGO)-based Li-O2 batteries, significantly reducing the charge voltage to 3.2 V.16 This was explained by the higher electrical conductivity of LiO2 than that of the Li2O2. However, as superoxide radicals were directly exposed to electrolytes, the unavoidable by-products such as carbonates would simultaneously deteriorate the cycle performance.17 Previously, our group obtained the discharge products of Li-deficient Li2-xO2 by constructing Si and ZnO-anchored carbon nanotube (CNT) cathodes, achieving the decreased charge overpotential and the improved cycle stability.18,19 Along with the composition of discharge products, the product size is another critical factor for the charging potential and cell performance. Chen et al. systematically investigated the remarkable effect of Li2O2 particle size on the charging performance, revealing that the average potential of charge plateaus was lowered with decreasing particle size, attributing to the enhanced reactivity and electrode kinetics of Li2O2.20 Guo et al. reduced the size of typical toroidal Li2O2 particles to as low as 100 nm via adopting Au-anchored CNT cathodes, promoting the Li2O2 decomposition at the potential of 0.2 V lower than that of the Li-O2 cells with the bare CNTs.21 Chen et al. reported the similarly structured Li2O2 with the size smaller than 25 nm by utilizing Pd-functioned graphene nanosheets, which further lowered the charge potential and improved the energy efficiency.22 Since graphene cathodes tend to suffer from severe structural changes during cycles, Byon et al. synthesized RuO2 modified CNT cathodes to form the filmlike Li2O2 below 20 nm in thickness with the aid of increasing oxygen adsorption, achieving the charge-voltage reduction and cycle-life prolongation.23-25 4


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The formation of discharge products is determined by the competition between the solubility of LiO2 in the electrolytes and the adsorption free energy of LiO2* on the cathodes.26 Hence, it is of great significance to construct suitable carbon-based cathodes with surface modification in order to realize the component and size adjustment of discharge products. Tin dioxide is one of the most commonly used materials for commercial gas sensors due to its strong interplay with oxygen.27-29 The adsorption energy of oxygen for SnO2 is at least -2.74 eV, while that for bare CNT is merely -0.22 eV.30,31 After the CNT cathodes are decorated with SnO2 nanoparticles, O2 is easily adsorbed on the cathodes, followed by accepting an electron to generate the surface adsorbed LiO2*. Afterwards, the LiO2* probably drives the nanosized Li2O2 growth on cathode surface via surface model, thus contributing to a relatively low charge overpetential.26,32 Ohno et al. investigated the initial deposition of Li2O2 on the surface of SnO2 by virtue of theoretical calculations, which revealed that Li2O2 could wet the active surface of SnO2 and thus tend to generate the film-like morphology.33,34 Metal oxides have been employed as cathode catalysts in Li-O2 batteries.35 Chen et al. utilized δ-MnO2 nanoboxes and cobalt-manganese oxide nanocubes respectively as cathode catalysts for rechargeable Li-O2 batteries to reduce the overpotential and prolong the cycle life up to 100 cycles due to the large specific surface area and the intrinsic catalytic activity.36,37 In addition, Ma et.al synthesized SnO2/C nano-composites as cathode catalysts of Li-air batteries, which displayed expanded discharge capacity and enhanced cycle performance because of the large surface area and the complex pore structure of SnO2/C.38 The above-mentioned papers focused much on the catalytic functions of metal oxides on OER/ORR in Li-O2 batteries. In comparison, we utilized the strong interaction of SnO2 with oxygen to 5



manipulate the discharge products to improve the battery energy efficiency. To the best of our knowledge, there have been no experimental researches on optimizing discharge products via constructing SnO2-decorated CNT composite cathodes for Li-O2 batteries to date. Herein, SnO2 nanoparticles decorated carbon nanotubes (SnO2/CNTs) as self-standing cathodes prepared by magnetron sputtering are investigated for nonaqueous Li-O2 batteries. Compared with the non-modification CNT cathodes, the SnO2 decoration leads to formation of nanosized flake-like defective discharge products of Li2-xO2. Consequently, the Li-O2 batteries based on SnO2/CNT composite cathodes show the reduced voltage gap decreasing of 0.72 V, corresponding to the superior energy efficiency of 80%. These results demonstrate that modification of discharge product morphology and component by the SnO2/CNTs architecture is a powerful means to improve the energy efficiency of the Li-O2 batteries. 2. EXPERIMENTAL SECTION 2.1. Synthesis of SnO2/CNT cathodes Self-standing CNT (Microphase) was dripped with ethyl alcohol and dried in vacuum at 60 °C for 12 h. Afterwards, SnO2 nanoparticles were prepared on the surface of CNT by direct current (DC) magnetron sputtering with the sputtering pressure of 0.2 Pa, sputtering time of 300s and sputtering power of 30 W. 2.2. Lithium-oxygen batteries assembly and testing

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Each as-prepared SnO2/CNT composite cathode was punched into discs of 5 mm in diameter. Bis(trifluoromethysulfonyl) amine lithium (LiTFSI, Aldrich) salt was baked at 80 °C under vacuum for 24 h. 0.9 M LiTFSI dissolved in dimethyl sulfoxide (DMSO) with water content below 5 ppm, determined by Metrohm 831 KF Coulometer, was used as the electrolyte. The Swagelok-type Li-O2 battery, composed of a lithium foil anode, SnO2/CNT or pristine CNT cathode and a glass fiber (Whatman) soaked with the electrolyte, was assembled in an Ar-filled glove box (Vigor) with O2 and H2O contents lower than 0.1 ppm. After the assembly, the cells were sealed in homemade containers with constant high-purity O2 flow. After the cells were rested for 6 h, the galvanostatic discharge/charge measurements were conducted by an Arbin BT 2000 cycler. The discharge capacity was normalized by the cathode mass loading of 2 mg cm-2. 2.3. Characterizations X-ray Diffraction (XRD) measurement was carried out using a Bruker D8 discover diffractometer with Cu Kα radiation in a reflection mode. After the discharge/charge measurements, the cathodes were rinsed with acetonitrile solvent in the Ar-filled glovebox and then dried on a filter paper. The samples were sealed in a homemade quartz box for further analysis through Thermo DXR Raman spectroscopy. Scanning electron microscopy (SEM, FEI Magellan 400) attached with an energy dispersive spectrometer (EDS) and transmission electron microscopy (TEM, JEM-ARM200F) were selected to observe the morphology, component and element distribution of the discharge products. X-ray photoelectron spectroscopy (XPS) was conducted for the element valence analysis and discharge-product identification. 7



3. RESULTS AND DISCUSSION 3.1. Characterizations of the SnO2/CNT Cathodes Figure 1a shows the XRD patterns of the as-prepared SnO2/CNT and pristine CNT cathodes, respectively. The diffraction peaks located at 33.89°, 37.95° and 51.78° are indexed to (101), (200) and (211) of the tetragonal rutile SnO2 respectively, indicating the deposition of SnO2 on the CNT cathodes. The XPS spectrum of Sn 3d in Figure 1b also indicates existence of Sn4+ ions on the cathode surfaces, further confirming the successful SnO2 decoration on CNTs. Figure 1c and 1d display the SEM images of SnO2/CNT and pristine CNT cathodes, respectively. It can be clearly seen that the CNT surfaces of composite cathodes are coated with SnO2 nanoparticles in comparison with the smooth surfaces of pristine CNTs. This is further confirmed by the corresponding TEM images of SnO2/CNT cathodes, as shown in the inset of Figure 1c and Figure S1a. The EDS mapping in Figure S1b-d reveals the homogeneous and conformal distribution of the tin and oxygen elements, indicating that SnO2 nanoparticles are well dispersed across the CNT surfaces.

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Figure 1. Structure and morphology of SnO2/CNT composite cathodes: (a) XRD patterns of SnO2/CNTs and pristine CNTs; (b) XPS spectrum of Sn 3d in SnO2/CNTs; (c) SEM image of SnO2/CNTs, the inset gives the TEM image of SnO2/CNTs; (d) SEM image of the pristine CNTs. 3.2. Improved performance of the Li-O2 batteries with the SnO2/CNT cathodes Figure 2a shows the first-discharge/charge cycle of the Li-O2 cells based on the SnO2/CNT and pristine CNT cathodes under the discharge cutoff capacity of 1000 mA h g-1 at 0.1 mA cm-1. It can be seen that the discharge plateau for SnO2/CNTs is 2.80 V, which is 0.13 V greater than 9



that for the pristine CNTs. The increased discharge potential mainly results from the catalytic effect of SnO2 on oxygen reduction reaction, which is consistent with the previous reports.39 After discharge, the batteries with SnO2/CNT cathodes are recharged under the potential of 3.52 V, while the charge profile for the batteries with the CNTs show the two distinct stages, including a sharp slope under 4.0 V and a gradually increased plateau of 4.2 V to 4.39 V. Correspondingly, the energy efficiency for CNTs is 71%, calculated by the charge capacity in the first stage accounting for 40%, while that for SnO2/CNTs is 80%, calculated by the charge plateau of 3.52 V. Thus, the SnO2 decoration dramatically reduces the voltage gap between the discharge and charge plateau from 1.45 V to 0.72 V, and increases the energy efficiency from 71% to 80%. Similar discharge/charge behaviors of SnO2/CNT and CNT cathodes can also be obtained in deep discharge-charge processes. Figure 2b illustrates the first cycle profiles for the batteries based on the SnO2/CNT and CNT cathodes under the cutoff capacity of 3000 mA h g-1 at the current density of 0.4 mA cm-1. During the discharge-charge processes, the cells with the SnO2/CNTs exhibit much lower discharge-charge voltage gap than that of the cells with the CNTs. The first full discharge/charge curves in Figure S2 show a discharge capacity of 6277 mAh g-1 for the SnO2/CNT-based batteries, much higher than that (3022 mAh g-1) for the CNTbased batteries. Overall, the SnO2 modified CNTs benefit the voltage-gap decrease, energyefficiency promotion and discharge-capacity enlargement.

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Figure 2. The first-cycle discharge/charge curves of the Li-O2 cells with pristine CNT and SnO2/CNT cathodes under different discharge cutoff capacity of (a) 1000 mA h g-1 and (b) 3000 mA h g-1, respectively. 3.3. Analysis of discharge products in the Li-O2 batteries with the SnO2/CNT cathodes The analyses of discharge products in Li-O2 batteries with the SnO2/CNT and CNT cathodes are conducted to explain the different discharge-charge behaviors. The morphologies of discharge products in Li-O2 batteries based on the SnO2/CNT and CNT cathodes after firstdischarge to 3000 mA h g-1 were observed using SEM and TEM measurements in Figure 3. The SEM image of discharge products for the SnO2/CNTs is presented in Figure 3a, where the products are conformally attached on the surfaces of the composite cathodes. The energy dispersive spectrum (EDS) in Figure S3 shows the detailed components of the discharge products, where the C element is derived from the CNTs, while S and F elements come from the electrolyte decomposition. Noticeably, the ratio of oxygen atoms is 59.01%, much higher than that of pure SnO2 corresponding to the stoichiometric ratio of 13.38%, which is most probably 11



attributed to contribution of the discharge products. Figure 3b shows the TEM image of discharge products for the SnO2/CNT cathodes. It is apparent that the flake-like products with thickness under 10 nm are uniformly generated on the cathode surfaces. However, the toroidal discharge products with approximate several hundred nanometers are stacked on the CNT surfaces for the pristine CNT cathodes, as shown in Figure 3c and d. Apparently, the products are selectively grown on defective active sites of the CNTs, which is consistent with our group’s previous work.40

Figure 3. (a) SEM and (b) TEM images of SnO2/CNT cathodes after discharge to 3000 mA h g1,

respectively; (c) SEM and (d) TEM images of CNT cathodes after discharge to 3000 mA h g-1, 12


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respectively. The TEM results also provide componential information of the discharge products in the Li-O2 cells with the SnO2/CNT cathodes after the first discharge to 3000 mA h g-1, as shown in Figure 4a-c. High-resolution transmission electron microscopy (HRTEM) is applied to identify the flake-like discharge products observed in the TEM image of Figure 4a. Figure 4b and 4c are the corresponding zoom-in views for the marked areas in Figure 4a, where the Li2O2 in form of nanosized flake-like products (

Easily Decomposed Discharge Products Induced by Cathode

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