- Email: [email protected]
Synergistic effect of bifunctional catalytic sites and defect engineering for high-performance Li–CO2 batteries
Journal Pre-proof Synergistic Effect of Bifunctional Catalytic Sites and Defect Engineering for Highperformance Li-CO2 Batteries Yun Qiao, Jiawei Wu, Jin Zhao, Qingling Li, Pengjian Zhang, Changshi Hao, Xili Liu, Shuting Yang, Yang Liu PII:
S2405-8297(20)30028-3
DOI:
https://doi.org/10.1016/j.ensm.2020.01.021
Reference:
ENSM 1071
To appear in:
Energy Storage Materials
Received Date: 27 November 2019 Revised Date:
14 January 2020
Accepted Date: 19 January 2020
Please cite this article as: Y. Qiao, J. Wu, J. Zhao, Q. Li, P. Zhang, C. Hao, X. Liu, S. Yang, Y. Liu, Synergistic Effect of Bifunctional Catalytic Sites and Defect Engineering for High-performance Li-CO2 Batteries, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2020.01.021. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
CRediT Author Statement
Yun Qiao: Conceptualization, Methodology, Funding acquisition, Formal analysis, WritingReview&Editing; Jiawei Wu: Investigation, Data Curation, Formal analysis, Writing-Original Draft; Jin Zhao: Formal analysis, Data Curation; Qingling Li: Investigation, Data Curation; Pengjian Zhang: Investigation, Formal analysis; Changshi Hao: Data Curation, Formal analysis; Xili Liu: Investigation, Formal analysis; Shuting Yang: Funding acquisition; Yang Liu: Conceptualization, Methodology, Funding acquisition, Supervision, Writing-Review&Editing.
Synergistic Effect of Bifunctional Catalytic Sites and Defect Engineering for High-performance Li-CO2 Batteries Yun Qiao a, b*, Jiawei Wu a, Jin Zhao a, Qingling Li a, Pengjian Zhang a, Changshi Hao a, Xili Liu a
a
, Shuting Yang a and Yang Liu a*
School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan,
453007, China. E-mail: [email protected]; [email protected] b
School of Environment and Chemical Engineering, Shanghai University, Shanghai, 200444,
China.
Synergistic Effect of Bifunctional Catalytic Sites and Defect Engineering for High-performance Li-CO2 Batteries Abstract Li-CO2 batteries are regarded as promising energy storage system due to their high energy density, CO2 fixation ability and environmental friendliness. However, Li-CO2 batteries are still suffering from large overpotential and poor cycling performance, which severely hinder their practical applications. Therefore, it is vital to design highly efficient cathode catalysts to improve the performance of Li-CO2 batteries. Here, we designed a highly efficient synergistic catalyst by incorporating ultrafine Ru nanoparticles on N, S co-doped graphene (Ru/NS-G) as cathode for Li-CO2 batteries. The ultrafine Ru catalyst can reduce the decomposition voltage of the discharge product
Li2CO3.
In addition, N, S co-doped graphene (NS-G) with a
three-dimensional structure provides excellent conductivity and adequate space for the reversible reactions in batteries. The Ru/NS-G catalyst achieves enhanced catalytic activities for the CO2 reduction/evolution reactions, assigned to the strong synergistic effect between the bifunctional catalyst Ru nanoparticles and the NS-G substrate with numerous defect structures and accessibly active sites. The batteries achieve an impressive discharge capacity (12448 mA h g−1 at a current density of 100 mA g−1), high Coulombic efficiency (94.6 %) and excellent cycling stability (over 100 cycles) with a low overpotential of 1.40 V. This work provides an engineering strategy on the cathode design for metal-gas batteries and other devices that are not limited to Li-CO2 batteries. Keywords: Li-CO2 batteries, composite cathodes, synergistic effect, electrocatalytic activity, cycling performance
1
1. Introduction In recent years, large amounts of carbon dioxide emissions have caused the “greenhouse effect” through the continuous consumption of fossil fuels, which has adversely given rise to the global environmental and energy crisis.[1-3] Therefore, it is more important than ever to develop a new type of sustainable energy system. Notably, the rechargeable Li-CO2 batteries have attracted worldwide attention owing to their ability to capture CO2 and convert CO2 to green energy.[4-8] Simultaneously, Li-CO2 battery possesses a high theoretical energy density (1876 Wh kg−1) and displays a significant effect on space exploration.[9-11] Typically, the electrochemical reaction in Li-CO2 batteries is confirmed to be 4Li + 3CO2 ↔ 2Li2CO3 + C.[12-14] Li2CO3 has been proved to be the main discharge product, which is a wide-bandgap insulator with pretty low electronic conductivity and
sluggish
decomposition
kinetics.[15-17]
Unfortunately,
the
incomplete
decomposition of Li2CO3 will gradually accumulate on the surface of the cathode, cover the active sites and hinder the gas diffusion way, eventually causing a “sudden death” for the Li-CO2 batteries.[18, 19] Besides, the Li-CO2 batteries exhibit large polarization, poor reversibility, limited cycling performance and low Coulombic efficiency.[20-22] Therefore, developing novel catalysts and designing promising structures are regarded as critical strategies to achieve superior battery performance. Carbonaceous materials have been used as cathodes in Li-CO2 batteries, owing to their large surface areas and high electrical conductivity.[23-25] However, pure carbonaceous materials are hardly to accomplish the decomposition of discharge product
Li2CO3.
Moreover,
the
superoxide
radicals
produced
by
the
self-decomposition of Li2CO3 will corrode the electrodes and cause electrolyte decomposition.[26, 27] Generally, the introduction of heteroatoms (such as N, S, B, P) in carbon materials is a feasible strategy to improve their chemical and electrical properties, which also have been used in Li-CO2 battery electrode.[28, 29] Representatively, the higher electronegativity of nitrogen (3.04) than that of carbon (2.55) can induce inhomogeneous distribution of electrons in carbon materials, and 2
thus provide more free electrons with strong electron affinity.[30] Additionally, the electronegativity of sulfur (2.58) is close to that of carbon (2.55), the charge distribution of carbon around S atom will be lightly changed. Meanwhile, the S dopants can provide a high spin density to further increase the catalytic activity of cathode.[31] Compared with single atom doping, strong synergistic interactions can be produced with different heteroatoms to further adjust the surface polarities and electronic properties of carbon material, and subsequent enhance the electrocatalytic activity.[32, 33] To date, the metal nanoparticles as catalysts can enhance the electrochemical reactions for Li-CO2 battery. Remarkably, among various metal nanoparticle catalysts, Ru-based catalysts possess a wide range of applications and excellent electrocatalytic activity not only in water reduction but also in the decomposition of Li2O2 or Li2CO3.[26, 34, 35] Zhou's group first synthesized Ru@Super P as anode for Li-CO2 battery.[26] Despite the cycling stability is unsatisfactory (less than 80 cycles), the reversibility is significantly improved. Therefore, it is desirable to design an effective electrode to combine metal nanoparticles and dual heteroatom-doped carbon materials for achieving high electrochemical behaviors in Li-CO2 batteries. In this study, we prepared the uniform dispersed Ru nanoparticles on N, S co-doped graphene (Ru/NS-G) as cathode for Li-CO2 batteries. The cathode displays a superb discharge capacity of 12448 mA h g−1 with a higher Coulombic efficiency of 94.6 %, and a low overpotential of 1.40 V at 100 mA g−1 after 100 cycles, which may be assigned to the synergistic effect of defect engineering and bifunctional catalytic sites on the Ru/NS-G cathode. Figure 1 illustrates the structure of a typical Li-CO2 battery, consisting of a Li metal anode, a glass fiber soaked with electrolyte as the separator and catalysts decorated porous cathode. During the discharge process, Li+ ions move towards the cathode and react with dissolved CO2 molecules to form Li2CO3 and C on the surface of the cathode. Upon the subsequent charge process, the reversible decomposition of Li2CO3 regenerate Li+ and CO2 molecules. Notably, there are three types of N atoms in the porous Ru/NS-G cathode, in which pyrrolic-N (N-5) 3
can effectively adsorb Li+, pyridinic-N (N-6) is favorable for adsorption of CO2 molecules, while graphitic-N (N-Q) is benefit to enhance the conductivity. Moreover, S dopant can endow a large spin density and thus enhance the catalytic activity. The synergistic effect between the bifunctional catalyst Ru nanoparticles and the N, S co-doped graphene (NS-G) significantly promotes the reversible formation and decomposition of the discharge products of Li2CO3 and C. Therefore, the Li-CO2 batteries based on the Ru/NS-G electrode will absolutely achieve low overpotential, excellent cyclability and enhanced discharge capacity. 2. Experimental Section Preparation of the Ru/NS-G: Graphene oxide aqueous suspension was purchased from Hangzhou Gaoxi Technology Co., Ltd. First, 400 mg thiourea was added to 5 mL graphene oxide aqueous suspension (10 mg/mL) under the strong ultrasonication and stirring for 2 h to form a uniform black solution. And then, the mixed solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave and treated at 180 °C for 12 hours. After the reaction was cooled to room temperature, the mixture was washed three times with deionized water and freeze-dried overnight. NS-G was obtained by annealing the above mixture at 700 °C for 2 h under an N2 atmosphere. In order to synthesize Ru/NS-G, the NS-G and RuCl3·3H2O (40% Ru content) were dispersed in deionized water under the strong ultrasonication and stirring (the mass ratio of Ru salt to NS-G was 1.5:1). The well-dispersed mixture was freeze-dried and then annealed at 300 °C for 3 h in 10% H2/Ar gas to obtain the final product. For comparison, reduced graphene oxide (r-GO) powder was obtained by using the same process without adding thiourea. Materials Characterization: Before the ex-situ measurements, the discharged and recharged cathodes were washed via tetraethylene glycol dimethyl ether (TEGDME) and dried overnight to remove the residual lithium Bis(trifluoromethane)sulfonimide (LiTFSI) salt. The morphology of the samples and discharge products were observed by field-emission scanning electronic microscopy (FE-SEM; SU8010). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were 4
collected on a TitanTM G2 60-300 electron microscope. X-ray photoelectron spectroscopy (XPS) measurements were performed on ESCALAB 250Xi X-ray Photoelectron Spectrometer. X-ray diffraction (XRD) was recorded using X' Pert3 Powder diffractometer with Cu Kα radiation (λ=1.54 Å). Thermogravimetric analysis (TGA) was carried out using the NETZSCH STA449C Jupiter analyzer from room temperature to 1000 °C at a continuous heating rate of 10 °C min−1 under air atmosphere. Raman spectroscopy was recorded on a Horiba LabRAM HR Evolution Raman spectrometer with a 532 nm laser. Fourier transform infrared (FTIR) spectra were collected on a Spectrum 400F. Battery Assembly and Electrochemical Tests: Li-CO2 batteries were assembled with CR2032-type coin cells and conducted in a glove box filled with high-purity argon (O2 < 0.1 ppm and H2O < 0.1 ppm). For the cathode preparation, Ru/NS-G and polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 9:1 with N-methyl-2-pyrrolidinone (NMP) as a dispersing solvent. Then, the obtained slurry was uniformly coated on carbon paper (8 mm in diameter) and dried in an oven at 80 °C for 12 hours. The mass loading was about 0.1–0.3 mg. Pure Li foil was applied as the anode and Whatman glass fiber impregnated with 50 µL electrolyte (1 M LiTFSI dissolved in TEGDME) as the separator. The batteries were remained in a glove box for 2 hours and then transferred to hermetic glass bottles filled with dry CO2 gas. Finally, the galvanostatic charge and discharge processes of cells were tested on LAND-CTA2001A testing system. The electrochemical impedance spectroscopy (EIS) analysis and Cyclic voltammetry (CV) curves were carried out on the CHI760E electrochemical workstation. 3. Results and Discussion The morphology of the Ru/NS-G electrode was characterized by SEM, TEM and HRTEM. The SEM image in Figure 2a demonstrates that the Ru/NS-G sample is consistent of wrinkled and folded nanosheets. The Ru/NS-G maintains the same morphology as r-GO and NS-G (Figure S1), after the hydrothermal reaction and even high temperature calcination processes. Moreover, the Brunauer-Emmett-Teller (BET) 5
surface area for Ru/NS-G is 130 m2 g−1 and its pore size distribution displays a broad distribution in the range of 20–250 nm (Figure S2). On one hand, this unique structure is beneficial for the electrolyte infiltration and CO2 diffusion. On the other hand, the hierarchical structure with plenty of defects can provide sufficient space for the deposition of discharge products and further promote the reversibility of Li-CO2 batteries. Notably, Ru nanoparticles are too small to be observed in the SEM image. The TEM image of Ru/NS-G clearly shows that Ru nanoparticles are uniformly dispersed on N, S co-doped graphene (Figure 2b). The HRTEM image of the Ru/NS-G in Figure 2c displays two obvious lattice plane spacings of 0.234 nm and 0.205 nm, corresponding to the (100) and (101) planes of hexagonal Ru (JCPDS: 06-0663). Moreover, the high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image intuitively confirms that the Ru nanoparticles with a mean size of 2.6±0.6 nm are homogeneously distributed on the NS-G substrate (Figure 2d). The correspond elemental mapping images further prove that the graphene was successfully doped with nitrogen and sulfur heteroatoms and the Ru nanoparticles are uniform distributed on the substrate (Figure 2e). The selected-area electron diffraction (SAED) pattern illustrates the polycrystalline structure of Ru, as displayed in Figure 2f. Five clear diffraction rings agree well with (100), (101), (102), (110) and (103) planes of Ru. The structure of the Ru/NS-G composite was further investigated by XRD. Figure S3 shows the XRD spectrum of Ru/NS-G composite, in which, the broad peak at 26.3° is ascribe to the typical (002) plane of graphite. Additionally, the diffraction peaks at 38.3°, 44.0°, 58.3°, 69.4°, and 78.3° are assigned to the (100), (101), (102), (110), and (103) planes of hexagonal ruthenium (JCPDS 06-0663), which are consistent with the SAED pattern. The Ru content of Ru/NS-G is estimated to be about 29.23% through TGA (Figure S4). The chemical composition of Ru/NS-G was analyzed by XPS. The Ru, C, O, N and S elements can be observed through the survey spectrum (Figure S5). In the high-resolution C 1s spectrum (Figure 2g), there are two well fitted peaks around 280.6 and 285.4 eV, which are ascribed to the Ru 3d5/2 and Ru 3d3/2 of Ru 3d.[36] The 6
remaining two peaks located at around 284.7 and 286.4 eV are attributed to C-C and C-O bonds.[11, 37] In addition, Ru 3p can be divided into two peaks at 462.1 and 484.5 eV for Ru 3p3/2 and Ru 3p1/2 (Figure S6). As shown in Figure 2h, the high-resolution N 1s peaks prove the presence of three types of N functionalities in the composite, corresponding to N-6 (398.5 eV), N-5 (399.6 eV) and N-Q (401.3 eV).[28, 38] N dopant can break the charge neutrality of carbon and influence the correspond charge distribution due to the different electronegativity between nitrogen and carbon.[39] It is worth to mention, N-6 can not only effectively adsorb CO2 molecules but also is considered to be a connecting bridge for Ru/NS-G and Li2CO3 to promote the electron transfer, which is conducive for the rapid stabilization or decomposition of discharge products.[40] N-5 can facilely adsorb Li+ due to its high average adsorption energy.[41, 42] Moreover, N-Q is advantageous to boost the conductivity of Ru/NS-G catalyst.[40] These results demonstrate that the N doping will endow the cathode with unexpected catalytic activity for the CO2-related catalytic process. For the high-resolution S 2p XPS spectrum, the as-fitted two major peaks at 164.0 and 165.3 eV are derived from the S 2p3/2 and S 2p1/2 of the -C-S-C-covalent bond, and another peak at 168.9 eV is attributed to the C-SOx-C bond formed by sulfur oxides (Figure 2i).[43] The presence of S heteroatom endows a large spin density and further advanced catalytic activity.[31]
7
Figure 1. Schematic illustration of the discharge and charge processes in Li-CO2 battery with Ru/NS-G cathode, and the specific roles of the Ru nanoparticles and the heteroatoms. Raman spectroscopy is a sensitive surface analytical technique for exploring carbonaceous materials. All the three materials, including r-GO, NS-G and Ru/NS-G composite, present the typical D and G band peaks at 1344 cm−1 and 1580 cm−1, which assigned as disordered carbon (D-band) and graphitic carbon (G-band), respectively (Figure S7).[44] Moreover, the ID/IG can be utilized to estimate the extent of defects in carbon materials. It is obvious that the intensity ratio (ID/IG) of Ru/NS-G (1.18) and NS-G (1.20) is much larger than that of r-GO (0.75), implying that Ru/NS-G and NS-G with nitrogen and sulfur heteroatom dopants possess more defective sites. From the above measurements, we conclude that the composite materials have been successfully prepared and can be used as an excellent cathode for Li-CO2 battery.
8
Figure 2. (a) SEM image, (b) TEM image, (c) high-resolution TEM image, (d) high-angle annular dark-field scanning TEM image (inset: the histograms of Ru nanoparticle size distribution), (e) the corresponding EDS elemental mapping images, and (f) SAED pattern of the Ru/NS-G. High-resolution XPS spectra of (g) C 1s and Ru 3d, (h) N 1s, and (i) S 2p for the Ru/NS-G sample. We systematically evaluated the electrochemical performance of Ru/NS-G composite as cathode for Li-CO2 batteries. The CV measurements were carried out at a scan rate of 0.2 mV s−1 between 2.0 and 4.5 V (Figure 3a). The Ru/NS-G cathode performs an obvious reduction peak at around 2.5 V, which is consistent with the CO2 reduction reaction. Moreover, the Ru/NS-G cathode displays a larger peak current than that of NS-G and r-GO cathodes, indicating its enhanced catalytic activity for CO2 reduction. For the anodic scan, the Ru/NS-G electrode also possesses a strong oxidation peak near 4.2 V and a positive onset potential (around 3.1 V), while the oxidation peaks of the NS-G and r-GO cathodes are unremarkable, indicating that the Ru/NS-G electrode presents a considerable effect on the formation and decomposition of Li2CO3 in Li-CO2 batteries. More importantly, the CV integral area of the Ru/NS-G cathode is significantly larger than NS-G and r-GO cathodes, which indicates that the 9
Ru/NS-G based cathode possesses a larger capacity.[45] The full discharge/charge measurements were also carried out to further clarify the catalytic activity of the three cathodes at 100 mA g−1 with a limited voltage range of 2.0-4.5 V. The Ru/NS-G cathode demonstrates the highest specific capacity of 12448 mA h g−1 in contrast to NS-G and r-GO cathode. Moreover, the Ru/NS-G cathode performs a stable discharge voltage platform around 2.68 V, while the discharge voltage of NS-G and r-GO cathodes are lower than that of Ru/NS-G cathode. Upon recharge, the Ru/NS-G cathode contributes a charge capacity of 10660 mA h g−1 with a favorable Coulombic efficiency of 94.6%. Whereas, the Coulombic efficiency for the NS-G is only 81.0%, and r-GO could be barely recharged. The excellent capacity performance and Coulombic efficiency of the Ru/NS-G cathode can be assigned to not only the numerous defect structures and ample active sites but also the synergistic effect between the bifunctional Ru nanoparticles catalyst and NS-G substrate. In order to distinctly comprehend the catalytic activity and advantages of Ru/NS-G, we further compared the overpotentials of the cathodes in Li-CO2 battery with a limited specific capacity of 1000 mA h g−1 at 100 mA g−1 (Figure 3c and Figure S8). It is worth to note that Ru/NS-G displays the highest discharge voltage of 2.91 V and the lowest charge voltage of 4.04 V, indicating its low overpotential of 1.13 V and promising catalytic ability for CO2 reduction and evolution reactions. In which, the overpotentials for Ru/G and NS-G cathodes are up to1.48 and 1.64 V, respectively, and r-GO cathode displays the largest overpotential. Figure 3d shows the cycling profiles of the Ru/NS-G cathode at a current density of 100 mA g−1. The battery based on the Ru/NS-G cathode not only exhibits an excellent cycling stability of 100 cycles, but also maintains a stable discharge and charge plateau. It is noteworthy that after 100 cycles, the corresponding voltage gap between the charge and discharge terminal potentials keeps at about 1.40 V. Figure S9 presents the discharge/charge curves of the NS-G cathode at 100 mA g−1. The battery can also stand for 100 cycles because of the strong synergistic interactions between the N and S heteroatoms. The introduction of N, S heteroatoms into graphene effectively alters the surface polarities and electronic 10
structure of the carbon material, and significantly boosts the electrocatalytic activity. However, it is worth noting that the NS-G cathode displays a high charge platform (>4.2 V) and a large overpotential throughout the cycles. The charge terminal voltage gradually raises from 4.2 V to 4.5 V, owing to the accumulation of insoluble Li2CO3.[46] Figure 3e shows the overpotential comparison of Ru/NS-G and NS-G cathodes at different cycles. It can be clearly observed that the overpotentials of Ru/NS-G cathode in Li-CO2 battery are considerably lower than that of the NS-G cathode even after 100 cycles, indicating its excellent cycle stability. Meanwhile, the Ru/NS-G cathode displays lower overpotentials at different current densities in contrast to NS-G cathode. Even at a high current density of 500 mA g−1, the overpotential of Ru/NS-G cathode is as low as 1.69 V (Figure 3f and Figure S10). The above results further confirm that the highly dispersed ultrafine Ru nanoparticles can significantly improve the catalytic activity and stability. At the same time, the modification of graphene by N and S heteroatoms provides abundant surface active sites and defects, further enhancing the catalytic activity. Hence, the Li-CO2 battery exhibits remarkable electrochemical performance.
Figure 3. (a) CV curves of Li-CO2 batteries based on Ru/NS-G, NS-G and r-GO electrodes. (b) Full discharge–charge curves (at a current density of 100 mA g−1). (c) The initial discharge-charge curves of Ru/NS-G and NS-G cathodes. (d) The selected 11
charge/discharge curves of the Ru/NS-G cathode at various cycles. (e) The overpotential comparison of Ru/NS-G and NS-G cathodes under different cycles. (f) The charge/discharge profiles of the Ru/NS-G and NS-G cathodes at different current densities. Ex-situ SEM, XPS, XRD, FTIR and Raman measurements were introduced to reveal the reversible deposition and decomposition of the discharge products and the reversibility of the electrochemical reaction. The morphologies and structures were observed via SEM characterization for the pristine, discharged and recharged Ru/NS-G cathodes (Figure 4a-c). Compared with the pristine Ru/NS-G in Figure 4a, some parts of the cathode surface are covered by the particle-shaped products after the first discharge process, while the folded features are still maintained (Figure 4b). After recharge, the particle-shaped products are completely vanished and the folded morphology are recovered, indicating the excellent reversibility of the cathode (Figure 4c). The XPS measurement was performed to gain insight into the reversibility of the Li-CO2 batteries (Figure 4d-g). Compared to the C 1s spectra of pristine Ru/NS-G cathode in Figure 4d, a new peak ascribing to the O-C=O bond at 289.4 eV appears in the discharged electrode and significantly declined after the subsequent recharge.[47] These results indicate the reversible formation and decomposition of Li2CO3 during the discharge and recharge processes, which is coincident with the SEM characterization. Additionally, we also analyzed the Li 1s XPS spectrum (Figure 4g). The distinct peak at 55.0 eV indicates the emergence of Li2CO3 during the discharge stage.[48] After recharge, no characteristic peak is observed, implying the decomposition of Li2CO3.. The FTIR and Raman spectra of the cycled cathodes were further provided. The FTIR spectrum of the discharged cathode in Figure 4h displays new peaks at 860 cm−1, 1412 cm−1 and 1473 cm−1, which are consist with the standard spectrum of commercial Li2CO3.[49, 50] Whereas, these peaks thoroughly disappear upon the subsequent recharge process, and the other small peaks in the cycled electrode are attributed to the residual electrolyte (Figure S11). Through observing the Raman spectra (Figure 4i), the appearance-disappearance of the peak at 1080 cm−1 12
further imply the formation-decomposition of Li2CO3 during the discharge and charge process.[26] Meanwhile, the XRD patterns of electrode at different stages further confirm this point (Figure S12). Moreover, the electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) curves were also carried out and further prove the reversibility of Li-CO2 battery with Ru/NS-G as cathode (Figure S13). These above results demonstrate that the as-prepared Ru/NS-G composite displays excellent electrocatalytic activity for promoting CO2 reduction and evolution reactions. The superior performance of the Li-CO2 battery based on the Ru/NS-G cathode can be attributed to the following three aspects. First, graphene possesses high theoretical surface areas, remarkable electrical conductivity and mechanical properties. This unique structure can facilitate the electrolyte infiltration and CO2 diffusion to ensure superb electron transport performances. Secondly, N, S heteroatoms are inserted into the sp2-hybridized carbon skeleton for alternative chemical doping to boost electrochemical properties. Particularly, nitrogen dopants play a vital role in increasing conductivity, CO2 and Li+ adsorption. S dopants can further enhance the catalytic activity. Finally, the synergistic effect between bifunctional catalyst Ru nanoparticles and NS-G can efficiently facilitate the reversible formation and decomposition of Li2CO3 and C, thus achieving the high performance of Li-CO2 batteries.
13
Figure 4. SEM images of the Ru/NS-G electrode at different stages: (a) pristine, (b) discharged, and (c) recharged. XPS spectra of C 1s and Ru 3d for the Ru/NS-G-based cathodes: (d) pristine, (e) discharged, and (f) recharged, respectively. (g) Li 1s XPS spectra after discharge-charge processes. (h) FTIR spectra. (i) Raman spectra. 4. Conclusion In summary, we have synthesized the Ru/NS-G composite as an excellent cathode in Li–CO2 batteries. The Ru/NS-G cathode delivers a superb discharge capacity of 12448 mA h g−1 with a higher Coulombic efficiency of 94.6%, due to the impactful synergistic effect between the bifunctional Ru nanoparticles catalyst and NS-G substrate. The battery also performs improved cycling stability, which could be stably operated for 100 cycles at a current density of 100 mA g−1. Besides, the polarization voltage of the Li-CO2 battery based on Ru/NS-G cathode is significantly reduced to be 1.40 V after 100 cycles. In particular, the reversible formation and decomposition of the discharge products of Li2CO3 and C were further confirmed by ex-situ SEM, XPS, FTIR and Raman techniques. This work affords a critical guideline for the 14
development of multifunctional synergistic catalysts for Li-CO2 battery and other energy storage devices.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. U1904187 and 21501049), the Fund of Key Scientific and Technological Project of Henan Province (No. 182102410081).
Conflict of Interest There are no conflicts to declare.
15
References [1] Y. Qiao, J. Yi, S. Wu, Y. Liu, S. Yang, P. He, H. Zhou, Li-CO2 Electrochemistry: A New Strategy for CO2 Fixation and Energy Storage, Joule 1 (2017) 359-370. https://doi.org/10.1016/j.joule.2017.07.001. [2] J. M. Matter, M. Stute, S. O. Snaebjornsdottir, E. H. Oelkers, S. R. Gislason, E. S. Aradottir, B. Sigfusson, I. Gunnarsson, H. Sigurdardottir, E. Gunnlaugsson, G. Axelsson, H. A. Alfredsson, D. Wolff-Boenisch, K. Mesfin, D. Fernandez de la Reguera Taya, J. Hall, K. Dideriksen, W. S. Broecker, Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions, Science 352 (2016) 1312-1314. https://doi.org/10.1126/science.aad8132. [3] J.-Y. Lee, H.-S. Kim, J.-S. Lee, C.-J. Park, W.-H. Ryu, Blood Protein as a Sustainable Bifunctional Catalyst for Reversible Li-CO2 Batteries, ACS Sustainable Chem. Eng. 7 (2019) 16151-16159. https://doi.org/10.1021/acssuschemeng.9b03079. [4] Y. Qiao, S. Xu, Y. Liu, J. Dai, H. Xie, Y. Yao, X. Mu, C. Chen, D. J. Kline, E. M. Hitz, B. Liu, J. Song, P. He, M. R. Zachariah, L. Hu, Transient, in situ synthesis of ultrafine ruthenium nanoparticles for a high-rate Li–CO2 battery, Energy Environ. Sci. 12 (2019) 1100-1107. https://doi.org/10.1039/c8ee03506g. [5] S. Bie, M. Du, W. He, H. Zhang, Z. Yu, J. Liu, M. Liu, W. Yan, L. Zhou, Z. Zou, Carbon Nanotube@RuO2 as a High Performance Catalyst for Li-CO2 Batteries, ACS Appl Mater Interfaces 11 (2019) 5146-5151. https://doi.org/10.1021/acsami.8b20573. [6] Y. Qiao, Y. Liu, C. Chen, H. Xie, Y. Yao, S. He, W. Ping, B. Liu, L. Hu, 3D-Printed Graphene Oxide Framework with Thermal Shock Synthesized Nanoparticles for Li-CO2 Batteries, Adv. Funct. Mater. 28 (2018) 1805899. https://doi.org/10.1002/adfm.201805899. [7] Z. Xie, X. Zhang, Z. Zhang, Z. Zhou, Metal–CO2 Batteries on the Road: CO2 from Contamination Gas to Energy Source, Adv. Mater. 29 (2017) 1605891. https://doi.org/10.1002/adma.201605891. [8] Y. Lu, X. Rong, Y.-S. Hu, L. Chen, H. Li, Research and development of advanced battery materials
in
China,
Energy Storage Mater. 16
23
(2019)
144-153.
https://doi.org/https://doi.org/10.1016/j.ensm.2019.05.019. [9] J. Chen, K. Zou, P. Ding, J. Deng, C. Zha, Y. Hu, X. Zhao, J. Wu, J. Fan, Y. Li, Conjugated Cobalt Polyphthalocyanine as the Elastic and Reprocessable Catalyst for Flexible
Li–CO2
Batteries,
Adv.
Mater.
31
(2019)
1805484.
https://doi.org/10.1002/adma.201805484. [10] F. Qiu, S. Ren, X. Mu, Y. Liu, X. Zhang, P. He, H. Zhou, Towards a stable Li-CO2 battery: The effects of CO2 to the Li metal anode, Energy Storage Mater. (2019). https://doi.org/10.1016/j.ensm.2019.11.017. [11] Z. Zhang, C. Yang, S. Wu, A. Wang, L. Zhao, D. Zhai, B. Ren, K. Cao, Z. Zhou, Exploiting Synergistic Effect by Integrating Ruthenium–Copper Nanoparticles Highly Co-Dispersed on Graphene as Efficient Air Cathodes for Li–CO2 Batteries, Adv. Energy Mater. 9 (2019) 1802805. https://doi.org/10.1002/aenm.201802805. [12] Z. Zhao, Y. Su, Z. Peng, Probing Lithium Carbonate Formation in Trace-O2-Assisted Aprotic Li-CO2 Batteries Using in Situ Surface-Enhanced Raman Spectroscopy,
J.
Phys.
Chem.
Lett.
10
(2019)
322-328.
https://doi.org/10.1021/acs.jpclett.8b03272. [13] R. Pipes, A. Bhargav, A. Manthiram, Phenyl Disulfide Additive for Solution‐ Mediated Carbon Dioxide Utilization in Li–CO2 Batteries, Adv. Energy Mater. 9 (2019) 1900453. https://doi.org/10.1002/aenm.201900453. [14] Q.-C. Zhu, S.-M. Xu, Z.-P. Cai, M. M. Harris, K.-X. Wang, J.-S. Chen, Towards real Li-air batteries: A binder-free cathode with high electrochemical performance in CO2
and
O2,
Energy
Storage
Mater.
7
(2017)
209-215.
https://doi.org/10.1016/j.ensm.2017.03.004. [15] C. Wang, Q. Zhang, X. Zhang, X. G. Wang, Z. Xie, Z. Zhou, Fabricating Ir/C Nanofiber Networks as Free-Standing Air Cathodes for Rechargeable Li-CO2 Batteries, Small 14 (2018) 1800641. https://doi.org/10.1002/smll.201800641. [16] Y. Mao, C. Tang, Z. Tang, J. Xie, Z. Chen, J. Tu, G. Cao, X. Zhao, Long-life Li– CO2 cells with ultrafine IrO2-decorated few-layered δ-MnO2 enabling amorphous Li2CO3
growth,
Energy
Storage 17
Mater.
18
(2019)
405-413.
https://doi.org/10.1016/j.ensm.2018.08.011. [17] B. Liu, Y. Sun, L. Liu, J. Chen, B. Yang, S. Xu, X. Yan, Recent advances in understanding Li–CO2 electrochemistry, Energy Environ. Sci. 12 (2019) 887-922. https://doi.org/10.1039/c8ee03417f. [18] Y. Qiao, J. Yi, S. H. Guo, Y. Sun, S. C. Wu, X. Z. Liu, S. X. Yang, P. He, H. S. Zhou, Li2CO3-free Li-O2/CO2 battery with peroxide discharge product, Energy Environ. Sci. 11 (2018) 1211-1217. https://doi.org/10.1039/c7ee03341a. [19] Z. Guo, C. Li, J. Liu, Y. Wang, Y. Xia, A Long-Life Lithium-Air Battery in Ambient Air with a Polymer Electrolyte Containing a Redox Mediator, Angew. Chem., Int. Ed. 56 (2017) 7505-7509. https://doi.org/10.1002/anie.201701290. [20] C. Ling, R. Zhang, K. Takechi, F. Mizuno, Intrinsic Barrier to Electrochemically Decompose Li2CO3 and LiOH, J. Phys. Chem. C 118 (2014) 26591-26598. https://doi.org/10.1021/jp5093306. [21] Z. Zhao, J. Huang, Z. Peng, Achilles' Heel of Lithium-Air Batteries: Lithium Carbonate,
Angew.
Chem.,
Int.
Ed.
57
(2018)
3874-3886.
https://doi.org/10.1002/anie.201710156. [22] X. Mu, H. Pan, P. He, H. Zhou, Li-CO2 and Na-CO2 Batteries: Toward Greener and Sustainable Electrical Energy Storage, Adv. Mater. (2019) e1903790. https://doi.org/10.1002/adma.201903790. [23] Z. Zhang, Q. Zhang, Y. Chen, J. Bao, X. Zhou, Z. Xie, J. Wei, Z. Zhou, The First Introduction of Graphene to Rechargeable Li–CO2 Batteries, Angew. Chem., Int. Ed. 54 (2015) 6550-6553. https://doi.org/10.1002/anie.201501214. [24] X. Zhang, Q. Zhang, Z. Zhang, Y. Chen, Z. Xie, J. Wei, Z. Zhou, Rechargeable Li–CO2 batteries with carbon nanotubes as air cathodes, Chem. Commun. 51 (2015) 14636-14639. https://doi.org/10.1039/C5CC05767A. [25] A. J. Hu, C. Shu, C. Xu, R. Liang, J. Li, R. Zheng, M. Li, J. Long, Design strategies toward catalytic materials and cathode structure for emerging Li−CO2 batteries,
J.
Mater.
Chem.
A
https://doi.org/10.1039/C9TA06506G. 18
7
(2019)
21605–21633.
[26] S. X. Yang, Y. Qiao, P. He, Y. J. Liu, Z. Cheng, J. J. Zhu, H. S. Zhou, A reversible lithium-CO2 battery with Ru nanoparticles as a cathode catalyst, Energy Environ. Sci. 10 (2017) 972-978. https://doi.org/10.1039/c6ee03770d. [27] S. Yang, P. He, H. Zhou, Exploring the electrochemical reaction mechanism of carbonate oxidation in Li–air/CO2 battery through tracing missing oxygen, Energy Environ. Sci. 9 (2016) 1650-1654. https://doi.org/10.1039/c6ee00004e. [28] L. Qie, Y. Lin, J. W. Connell, J. Xu, L. Dai, Highly Rechargeable Lithium-CO2 Batteries with a Boron- and Nitrogen-Codoped Holey-Graphene Cathode, Angew. Chem., Int. Ed. 56 (2017) 6970-6974. https://doi.org/10.1002/anie.201701826. [29] L. Zhao, J. Yu, C. Xing, Z. Ullah, C. Yu, S. Zhu, M. Chen, W. Li, Q. Li, L. Liu, Nanopore confined anthraquinone in MOF-derived N-doped microporous carbon as stable organic cathode for lithium-ion battery, Energy Storage Mater. 22 (2019) 433-440. https://doi.org/10.1016/j.ensm.2019.02.003. [30] J. Zhu, W. Li, S. Li, J. Zhang, H. Zhou, C. Zhang, J. Zhang, S. Mu, Defective N/S-Codoped 3D Cheese-Like Porous Carbon Nanomaterial toward Efficient Oxygen Reduction
and
Zn–Air
Batteries,
Small
14
(2018)
1800563.
https://doi.org/10.1002/smll.201800563. [31] X. Yu, M. Zhang, J. Chen, Y. Li, G. Shi, Nitrogen and Sulfur Codoped Graphite Foam as a Self-Supported Metal-Free Electrocatalytic Electrode for Water Oxidation, Adv. Energy Mater. 6 (2016) 1501492. https://doi.org/10.1002/aenm.201501492. [32] Y. Song, S. Bai, L. Zhu, M. Zhao, D. Han, S. Jiang, Y.-N. Zhou, Tuning Pseudocapacitance via C-S Bonding in WS2 Nanorods Anchored on N,S Codoped Graphene for High-Power Lithium Batteries, ACS Appl. Mater. Interfaces 10 (2018) 13606-13613. https://doi.org/10.1021/acsami.8b02506. [33] Y. Wu, Y. Yu, 2D material as anode for sodium ion batteries: Recent progress and perspectives,
Energy
Storage
Mater.
16
(2019)
323-343.
https://doi.org/10.1016/j.ensm.2018.05.026. [34] J. Wang, Z. Z. Wei, S. J. Mao, H. R. Li, Y. Wang, Highly uniform Ru nanoparticles over N-doped carbon: pH and temperature-universal hydrogen release 19
from
water
reduction,
Energy
Environ.
Sci.
11
(2018)
800-806.
https://doi.org/10.1039/c7ee03345a. [35] M. Nazarian-Samani, H.-D. Lim, S. Haghighat-Shishavan, H.-K. Kim, Y. Ko, M.-S. Kim, S.-W. Lee, S. F. Kashani-Bozorg, M. Abbasi, H.-U. Guim, D.-I. Kim, K.-C. Roh, K. Kang, K.-B. Kim, A robust design of Ru quantum dot/N-doped holey graphene for efficient Li-O2 batteries, J. Mater. Chem. A 5 (2017) 619-631. https://doi.org/10.1039/c6ta08427c. [36] H. Hwang, T. Kwon, H. Y. Kim, J. Park, A. Oh, B. Kim, H. Baik, S. H. Joo, K. Lee, Ni@Ru and NiCo@Ru Core–Shell Hexagonal Nanosandwiches with a Compositionally Tunable Core and a Regioselectively Grown Shell, Small 14 (2018) 1702353. https://doi.org/10.1002/smll.201702353. [37] X. Zhang, C. Y. Wang, H. H. Li, X. G. Wang, Y. N. Chen, Z. J. Xie, Z. Zhou, High performance Li-CO2 batteries with NiO-CNT cathodes, J. Mater. Chem. A 6 (2018) 2792-2796. https://doi.org/10.1039/c7ta11015d. [38] Q. Wang, Y. Ji, Y. Lei, Y. Wang, Y. Wang, Y. Li, S. Wang, Pyridinic-N-Dominated Doped
Defective
Graphene
as
a
Superior
Oxygen
Electrocatalyst
for
Ultrahigh-Energy-Density Zn–Air Batteries, ACS Energy Lett. 3 (2018) 1183-1191. https://doi.org/10.1021/acsenergylett.8b00303. [39] T. Akhter, M. M. Islam, S. N. Faisal, E. Hague, A. I. Minett, H. K. Liu, K. Konstantinov, S. X. Dou, Self-Assembled N/S Codoped Flexible Graphene Paper for High Performance Energy Storage and Oxygen Reduction Reaction, ACS Appl. Mater. Interfaces 8 (2016) 2078-2087. https://doi.org/10.1021/acsami.5b10545. [40] Y. Li, J. Zhou, T. Zhang, T. Wang, X. Li, Y. Jia, J. Cheng, Q. Guan, E. Liu, H. Peng, B. Wang, Highly Surface-Wrinkled and N-Doped CNTs Anchored on Metal Wire: A Novel Fiber-Shaped Cathode toward High-Performance Flexible Li–CO2 Batteries,
Adv.
Funct.
Mater.
29
(2019)
1808117.
https://doi.org/10.1002/adfm.201808117. [41] X. Wang, Q. Weng, X. Liu, X. Wang, D.-M. Tang, W. Tian, C. Zhang, W. Yi, D. Liu, Y. Bando, D. Golberg, Atomistic Origins of High Rate Capability and Capacity of 20
N-Doped Graphene for Lithium Storage, Nano Lett. 14 (2014) 1164-1171. https://doi.org/10.1021/nl4038592. [42] M. Wang, Y. Yao, Z. Tang, T. Zhao, F. Wu, Y. Yang, Q. Huang, Self-Nitrogen-Doped Carbon from Plant Waste as an Oxygen Electrode Material with Exceptional Capacity and Cycling Stability for Lithium-Oxygen Batteries, ACS Appl. Mater. Interfaces 10 (2018) 32212-32219. https://doi.org/10.1021/acsami.8b11282. [43] Y. Liu, X. Qiu, X. Liu, Y. Liu, L.-Z. Fan, 3D porous binary-heteroatom doped carbon nanosheet/electrochemically exfoliated graphene hybrids for high performance flexible solid-state supercapacitors, J. Mater. Chem. A 6 (2018) 8750-8756. https://doi.org/10.1039/c8ta01148f. [44] Y. Jin, C. Hu, Q. Dai, Y. Xiao, Y. Lin, J. W. Connell, F. Chen, L. Dai, High-Performance Li-CO2 Batteries Based on Metal-Free Carbon Quantum Dot/Holey Graphene Composite Catalysts, Adv. Funct. Mater. 28 (2018) 1804630. https://doi.org/10.1002/adfm.201804630. [45] T. Liu, X. Zhang, T. Huang, A. Yu, Pyridinic-N-dominated carbon frameworks with porous tungsten trioxide nano-lamellae as a promising bi-functional catalyst for Li-oxygen
batteries,
Nanoscale
10
(2018)
15763-15770.
https://doi.org/10.1039/c8nr04026e. [46] P. Tan, Z. H. Wei, W. Shyy, T. S. Zhao, X. B. Zhu, A nano-structured RuO2/NiO cathode enables the operation of non-aqueous lithium-air batteries in ambient air, Energy Environ. Sci. 9 (2016) 1783-1793. https://doi.org/10.1039/c6ee00550k. [47]Y. Xing, Y. Yang, D. Li, M. Luo, N. Chen, Y. Ye, J. Qian, L. Li, D. Yang, F. Wu, R. Chen, S. Guo, Crumpled Ir Nanosheets Fully Covered on Porous Carbon Nanofibers for Long-Life Rechargeable Lithium–CO2 Batteries, Adv. Mater. 30 (2018) 1803124. https://doi.org/10.1002/adma.201803124. [48] Z. Zhang, X. G. Wang, X. Zhang, Z. Xie, Y. N. Chen, L. Ma, Z. Peng, Z. Zhou, Verifying the Rechargeability of Li-CO2 Batteries on Working Cathodes of Ni Nanoparticles Highly Dispersed on N-Doped Graphene, Adv. Sci. 5 (2018) 1700567. https://doi.org/10.1002/advs.201700567. 21
[49] Z. Guo, J. Li, H. Qi, X. Sun, H. Li, A. G. Tamirat, J. Liu, Y. Wang, L. Wang, A Highly Reversible Long-Life Li-CO2 Battery with a RuP2-Based Catalytic Cathode, Small 15 (2019) e1803246. https://doi.org/10.1002/smll.201803246. [50] S.-M. Xu, Z.-C. Ren, X. Liu, X. Liang, K.-X. Wang, J.-S. Chen, Carbonate decomposition: Low-overpotential Li-CO2 battery based on interlayer-confined monodisperse
catalyst,
Energy
Storage
https://doi.org/10.1016/j.ensm.2018.05.015.
22
Mater.
15
(2018)
291-298.
Conflict of Interest There are no conflicts to declare.
S2405-8297(20)30028-3
DOI:
https://doi.org/10.1016/j.ensm.2020.01.021
Reference:
ENSM 1071
To appear in:
Energy Storage Materials
Received Date: 27 November 2019 Revised Date:
14 January 2020
Accepted Date: 19 January 2020
Please cite this article as: Y. Qiao, J. Wu, J. Zhao, Q. Li, P. Zhang, C. Hao, X. Liu, S. Yang, Y. Liu, Synergistic Effect of Bifunctional Catalytic Sites and Defect Engineering for High-performance Li-CO2 Batteries, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2020.01.021. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
CRediT Author Statement
Yun Qiao: Conceptualization, Methodology, Funding acquisition, Formal analysis, WritingReview&Editing; Jiawei Wu: Investigation, Data Curation, Formal analysis, Writing-Original Draft; Jin Zhao: Formal analysis, Data Curation; Qingling Li: Investigation, Data Curation; Pengjian Zhang: Investigation, Formal analysis; Changshi Hao: Data Curation, Formal analysis; Xili Liu: Investigation, Formal analysis; Shuting Yang: Funding acquisition; Yang Liu: Conceptualization, Methodology, Funding acquisition, Supervision, Writing-Review&Editing.
Synergistic Effect of Bifunctional Catalytic Sites and Defect Engineering for High-performance Li-CO2 Batteries Yun Qiao a, b*, Jiawei Wu a, Jin Zhao a, Qingling Li a, Pengjian Zhang a, Changshi Hao a, Xili Liu a
a
, Shuting Yang a and Yang Liu a*
School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan,
453007, China. E-mail: [email protected]; [email protected] b
School of Environment and Chemical Engineering, Shanghai University, Shanghai, 200444,
China.
Synergistic Effect of Bifunctional Catalytic Sites and Defect Engineering for High-performance Li-CO2 Batteries Abstract Li-CO2 batteries are regarded as promising energy storage system due to their high energy density, CO2 fixation ability and environmental friendliness. However, Li-CO2 batteries are still suffering from large overpotential and poor cycling performance, which severely hinder their practical applications. Therefore, it is vital to design highly efficient cathode catalysts to improve the performance of Li-CO2 batteries. Here, we designed a highly efficient synergistic catalyst by incorporating ultrafine Ru nanoparticles on N, S co-doped graphene (Ru/NS-G) as cathode for Li-CO2 batteries. The ultrafine Ru catalyst can reduce the decomposition voltage of the discharge product
Li2CO3.
In addition, N, S co-doped graphene (NS-G) with a
three-dimensional structure provides excellent conductivity and adequate space for the reversible reactions in batteries. The Ru/NS-G catalyst achieves enhanced catalytic activities for the CO2 reduction/evolution reactions, assigned to the strong synergistic effect between the bifunctional catalyst Ru nanoparticles and the NS-G substrate with numerous defect structures and accessibly active sites. The batteries achieve an impressive discharge capacity (12448 mA h g−1 at a current density of 100 mA g−1), high Coulombic efficiency (94.6 %) and excellent cycling stability (over 100 cycles) with a low overpotential of 1.40 V. This work provides an engineering strategy on the cathode design for metal-gas batteries and other devices that are not limited to Li-CO2 batteries. Keywords: Li-CO2 batteries, composite cathodes, synergistic effect, electrocatalytic activity, cycling performance
1
1. Introduction In recent years, large amounts of carbon dioxide emissions have caused the “greenhouse effect” through the continuous consumption of fossil fuels, which has adversely given rise to the global environmental and energy crisis.[1-3] Therefore, it is more important than ever to develop a new type of sustainable energy system. Notably, the rechargeable Li-CO2 batteries have attracted worldwide attention owing to their ability to capture CO2 and convert CO2 to green energy.[4-8] Simultaneously, Li-CO2 battery possesses a high theoretical energy density (1876 Wh kg−1) and displays a significant effect on space exploration.[9-11] Typically, the electrochemical reaction in Li-CO2 batteries is confirmed to be 4Li + 3CO2 ↔ 2Li2CO3 + C.[12-14] Li2CO3 has been proved to be the main discharge product, which is a wide-bandgap insulator with pretty low electronic conductivity and
sluggish
decomposition
kinetics.[15-17]
Unfortunately,
the
incomplete
decomposition of Li2CO3 will gradually accumulate on the surface of the cathode, cover the active sites and hinder the gas diffusion way, eventually causing a “sudden death” for the Li-CO2 batteries.[18, 19] Besides, the Li-CO2 batteries exhibit large polarization, poor reversibility, limited cycling performance and low Coulombic efficiency.[20-22] Therefore, developing novel catalysts and designing promising structures are regarded as critical strategies to achieve superior battery performance. Carbonaceous materials have been used as cathodes in Li-CO2 batteries, owing to their large surface areas and high electrical conductivity.[23-25] However, pure carbonaceous materials are hardly to accomplish the decomposition of discharge product
Li2CO3.
Moreover,
the
superoxide
radicals
produced
by
the
self-decomposition of Li2CO3 will corrode the electrodes and cause electrolyte decomposition.[26, 27] Generally, the introduction of heteroatoms (such as N, S, B, P) in carbon materials is a feasible strategy to improve their chemical and electrical properties, which also have been used in Li-CO2 battery electrode.[28, 29] Representatively, the higher electronegativity of nitrogen (3.04) than that of carbon (2.55) can induce inhomogeneous distribution of electrons in carbon materials, and 2
thus provide more free electrons with strong electron affinity.[30] Additionally, the electronegativity of sulfur (2.58) is close to that of carbon (2.55), the charge distribution of carbon around S atom will be lightly changed. Meanwhile, the S dopants can provide a high spin density to further increase the catalytic activity of cathode.[31] Compared with single atom doping, strong synergistic interactions can be produced with different heteroatoms to further adjust the surface polarities and electronic properties of carbon material, and subsequent enhance the electrocatalytic activity.[32, 33] To date, the metal nanoparticles as catalysts can enhance the electrochemical reactions for Li-CO2 battery. Remarkably, among various metal nanoparticle catalysts, Ru-based catalysts possess a wide range of applications and excellent electrocatalytic activity not only in water reduction but also in the decomposition of Li2O2 or Li2CO3.[26, 34, 35] Zhou's group first synthesized Ru@Super P as anode for Li-CO2 battery.[26] Despite the cycling stability is unsatisfactory (less than 80 cycles), the reversibility is significantly improved. Therefore, it is desirable to design an effective electrode to combine metal nanoparticles and dual heteroatom-doped carbon materials for achieving high electrochemical behaviors in Li-CO2 batteries. In this study, we prepared the uniform dispersed Ru nanoparticles on N, S co-doped graphene (Ru/NS-G) as cathode for Li-CO2 batteries. The cathode displays a superb discharge capacity of 12448 mA h g−1 with a higher Coulombic efficiency of 94.6 %, and a low overpotential of 1.40 V at 100 mA g−1 after 100 cycles, which may be assigned to the synergistic effect of defect engineering and bifunctional catalytic sites on the Ru/NS-G cathode. Figure 1 illustrates the structure of a typical Li-CO2 battery, consisting of a Li metal anode, a glass fiber soaked with electrolyte as the separator and catalysts decorated porous cathode. During the discharge process, Li+ ions move towards the cathode and react with dissolved CO2 molecules to form Li2CO3 and C on the surface of the cathode. Upon the subsequent charge process, the reversible decomposition of Li2CO3 regenerate Li+ and CO2 molecules. Notably, there are three types of N atoms in the porous Ru/NS-G cathode, in which pyrrolic-N (N-5) 3
can effectively adsorb Li+, pyridinic-N (N-6) is favorable for adsorption of CO2 molecules, while graphitic-N (N-Q) is benefit to enhance the conductivity. Moreover, S dopant can endow a large spin density and thus enhance the catalytic activity. The synergistic effect between the bifunctional catalyst Ru nanoparticles and the N, S co-doped graphene (NS-G) significantly promotes the reversible formation and decomposition of the discharge products of Li2CO3 and C. Therefore, the Li-CO2 batteries based on the Ru/NS-G electrode will absolutely achieve low overpotential, excellent cyclability and enhanced discharge capacity. 2. Experimental Section Preparation of the Ru/NS-G: Graphene oxide aqueous suspension was purchased from Hangzhou Gaoxi Technology Co., Ltd. First, 400 mg thiourea was added to 5 mL graphene oxide aqueous suspension (10 mg/mL) under the strong ultrasonication and stirring for 2 h to form a uniform black solution. And then, the mixed solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave and treated at 180 °C for 12 hours. After the reaction was cooled to room temperature, the mixture was washed three times with deionized water and freeze-dried overnight. NS-G was obtained by annealing the above mixture at 700 °C for 2 h under an N2 atmosphere. In order to synthesize Ru/NS-G, the NS-G and RuCl3·3H2O (40% Ru content) were dispersed in deionized water under the strong ultrasonication and stirring (the mass ratio of Ru salt to NS-G was 1.5:1). The well-dispersed mixture was freeze-dried and then annealed at 300 °C for 3 h in 10% H2/Ar gas to obtain the final product. For comparison, reduced graphene oxide (r-GO) powder was obtained by using the same process without adding thiourea. Materials Characterization: Before the ex-situ measurements, the discharged and recharged cathodes were washed via tetraethylene glycol dimethyl ether (TEGDME) and dried overnight to remove the residual lithium Bis(trifluoromethane)sulfonimide (LiTFSI) salt. The morphology of the samples and discharge products were observed by field-emission scanning electronic microscopy (FE-SEM; SU8010). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were 4
collected on a TitanTM G2 60-300 electron microscope. X-ray photoelectron spectroscopy (XPS) measurements were performed on ESCALAB 250Xi X-ray Photoelectron Spectrometer. X-ray diffraction (XRD) was recorded using X' Pert3 Powder diffractometer with Cu Kα radiation (λ=1.54 Å). Thermogravimetric analysis (TGA) was carried out using the NETZSCH STA449C Jupiter analyzer from room temperature to 1000 °C at a continuous heating rate of 10 °C min−1 under air atmosphere. Raman spectroscopy was recorded on a Horiba LabRAM HR Evolution Raman spectrometer with a 532 nm laser. Fourier transform infrared (FTIR) spectra were collected on a Spectrum 400F. Battery Assembly and Electrochemical Tests: Li-CO2 batteries were assembled with CR2032-type coin cells and conducted in a glove box filled with high-purity argon (O2 < 0.1 ppm and H2O < 0.1 ppm). For the cathode preparation, Ru/NS-G and polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 9:1 with N-methyl-2-pyrrolidinone (NMP) as a dispersing solvent. Then, the obtained slurry was uniformly coated on carbon paper (8 mm in diameter) and dried in an oven at 80 °C for 12 hours. The mass loading was about 0.1–0.3 mg. Pure Li foil was applied as the anode and Whatman glass fiber impregnated with 50 µL electrolyte (1 M LiTFSI dissolved in TEGDME) as the separator. The batteries were remained in a glove box for 2 hours and then transferred to hermetic glass bottles filled with dry CO2 gas. Finally, the galvanostatic charge and discharge processes of cells were tested on LAND-CTA2001A testing system. The electrochemical impedance spectroscopy (EIS) analysis and Cyclic voltammetry (CV) curves were carried out on the CHI760E electrochemical workstation. 3. Results and Discussion The morphology of the Ru/NS-G electrode was characterized by SEM, TEM and HRTEM. The SEM image in Figure 2a demonstrates that the Ru/NS-G sample is consistent of wrinkled and folded nanosheets. The Ru/NS-G maintains the same morphology as r-GO and NS-G (Figure S1), after the hydrothermal reaction and even high temperature calcination processes. Moreover, the Brunauer-Emmett-Teller (BET) 5
surface area for Ru/NS-G is 130 m2 g−1 and its pore size distribution displays a broad distribution in the range of 20–250 nm (Figure S2). On one hand, this unique structure is beneficial for the electrolyte infiltration and CO2 diffusion. On the other hand, the hierarchical structure with plenty of defects can provide sufficient space for the deposition of discharge products and further promote the reversibility of Li-CO2 batteries. Notably, Ru nanoparticles are too small to be observed in the SEM image. The TEM image of Ru/NS-G clearly shows that Ru nanoparticles are uniformly dispersed on N, S co-doped graphene (Figure 2b). The HRTEM image of the Ru/NS-G in Figure 2c displays two obvious lattice plane spacings of 0.234 nm and 0.205 nm, corresponding to the (100) and (101) planes of hexagonal Ru (JCPDS: 06-0663). Moreover, the high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image intuitively confirms that the Ru nanoparticles with a mean size of 2.6±0.6 nm are homogeneously distributed on the NS-G substrate (Figure 2d). The correspond elemental mapping images further prove that the graphene was successfully doped with nitrogen and sulfur heteroatoms and the Ru nanoparticles are uniform distributed on the substrate (Figure 2e). The selected-area electron diffraction (SAED) pattern illustrates the polycrystalline structure of Ru, as displayed in Figure 2f. Five clear diffraction rings agree well with (100), (101), (102), (110) and (103) planes of Ru. The structure of the Ru/NS-G composite was further investigated by XRD. Figure S3 shows the XRD spectrum of Ru/NS-G composite, in which, the broad peak at 26.3° is ascribe to the typical (002) plane of graphite. Additionally, the diffraction peaks at 38.3°, 44.0°, 58.3°, 69.4°, and 78.3° are assigned to the (100), (101), (102), (110), and (103) planes of hexagonal ruthenium (JCPDS 06-0663), which are consistent with the SAED pattern. The Ru content of Ru/NS-G is estimated to be about 29.23% through TGA (Figure S4). The chemical composition of Ru/NS-G was analyzed by XPS. The Ru, C, O, N and S elements can be observed through the survey spectrum (Figure S5). In the high-resolution C 1s spectrum (Figure 2g), there are two well fitted peaks around 280.6 and 285.4 eV, which are ascribed to the Ru 3d5/2 and Ru 3d3/2 of Ru 3d.[36] The 6
remaining two peaks located at around 284.7 and 286.4 eV are attributed to C-C and C-O bonds.[11, 37] In addition, Ru 3p can be divided into two peaks at 462.1 and 484.5 eV for Ru 3p3/2 and Ru 3p1/2 (Figure S6). As shown in Figure 2h, the high-resolution N 1s peaks prove the presence of three types of N functionalities in the composite, corresponding to N-6 (398.5 eV), N-5 (399.6 eV) and N-Q (401.3 eV).[28, 38] N dopant can break the charge neutrality of carbon and influence the correspond charge distribution due to the different electronegativity between nitrogen and carbon.[39] It is worth to mention, N-6 can not only effectively adsorb CO2 molecules but also is considered to be a connecting bridge for Ru/NS-G and Li2CO3 to promote the electron transfer, which is conducive for the rapid stabilization or decomposition of discharge products.[40] N-5 can facilely adsorb Li+ due to its high average adsorption energy.[41, 42] Moreover, N-Q is advantageous to boost the conductivity of Ru/NS-G catalyst.[40] These results demonstrate that the N doping will endow the cathode with unexpected catalytic activity for the CO2-related catalytic process. For the high-resolution S 2p XPS spectrum, the as-fitted two major peaks at 164.0 and 165.3 eV are derived from the S 2p3/2 and S 2p1/2 of the -C-S-C-covalent bond, and another peak at 168.9 eV is attributed to the C-SOx-C bond formed by sulfur oxides (Figure 2i).[43] The presence of S heteroatom endows a large spin density and further advanced catalytic activity.[31]
7
Figure 1. Schematic illustration of the discharge and charge processes in Li-CO2 battery with Ru/NS-G cathode, and the specific roles of the Ru nanoparticles and the heteroatoms. Raman spectroscopy is a sensitive surface analytical technique for exploring carbonaceous materials. All the three materials, including r-GO, NS-G and Ru/NS-G composite, present the typical D and G band peaks at 1344 cm−1 and 1580 cm−1, which assigned as disordered carbon (D-band) and graphitic carbon (G-band), respectively (Figure S7).[44] Moreover, the ID/IG can be utilized to estimate the extent of defects in carbon materials. It is obvious that the intensity ratio (ID/IG) of Ru/NS-G (1.18) and NS-G (1.20) is much larger than that of r-GO (0.75), implying that Ru/NS-G and NS-G with nitrogen and sulfur heteroatom dopants possess more defective sites. From the above measurements, we conclude that the composite materials have been successfully prepared and can be used as an excellent cathode for Li-CO2 battery.
8
Figure 2. (a) SEM image, (b) TEM image, (c) high-resolution TEM image, (d) high-angle annular dark-field scanning TEM image (inset: the histograms of Ru nanoparticle size distribution), (e) the corresponding EDS elemental mapping images, and (f) SAED pattern of the Ru/NS-G. High-resolution XPS spectra of (g) C 1s and Ru 3d, (h) N 1s, and (i) S 2p for the Ru/NS-G sample. We systematically evaluated the electrochemical performance of Ru/NS-G composite as cathode for Li-CO2 batteries. The CV measurements were carried out at a scan rate of 0.2 mV s−1 between 2.0 and 4.5 V (Figure 3a). The Ru/NS-G cathode performs an obvious reduction peak at around 2.5 V, which is consistent with the CO2 reduction reaction. Moreover, the Ru/NS-G cathode displays a larger peak current than that of NS-G and r-GO cathodes, indicating its enhanced catalytic activity for CO2 reduction. For the anodic scan, the Ru/NS-G electrode also possesses a strong oxidation peak near 4.2 V and a positive onset potential (around 3.1 V), while the oxidation peaks of the NS-G and r-GO cathodes are unremarkable, indicating that the Ru/NS-G electrode presents a considerable effect on the formation and decomposition of Li2CO3 in Li-CO2 batteries. More importantly, the CV integral area of the Ru/NS-G cathode is significantly larger than NS-G and r-GO cathodes, which indicates that the 9
Ru/NS-G based cathode possesses a larger capacity.[45] The full discharge/charge measurements were also carried out to further clarify the catalytic activity of the three cathodes at 100 mA g−1 with a limited voltage range of 2.0-4.5 V. The Ru/NS-G cathode demonstrates the highest specific capacity of 12448 mA h g−1 in contrast to NS-G and r-GO cathode. Moreover, the Ru/NS-G cathode performs a stable discharge voltage platform around 2.68 V, while the discharge voltage of NS-G and r-GO cathodes are lower than that of Ru/NS-G cathode. Upon recharge, the Ru/NS-G cathode contributes a charge capacity of 10660 mA h g−1 with a favorable Coulombic efficiency of 94.6%. Whereas, the Coulombic efficiency for the NS-G is only 81.0%, and r-GO could be barely recharged. The excellent capacity performance and Coulombic efficiency of the Ru/NS-G cathode can be assigned to not only the numerous defect structures and ample active sites but also the synergistic effect between the bifunctional Ru nanoparticles catalyst and NS-G substrate. In order to distinctly comprehend the catalytic activity and advantages of Ru/NS-G, we further compared the overpotentials of the cathodes in Li-CO2 battery with a limited specific capacity of 1000 mA h g−1 at 100 mA g−1 (Figure 3c and Figure S8). It is worth to note that Ru/NS-G displays the highest discharge voltage of 2.91 V and the lowest charge voltage of 4.04 V, indicating its low overpotential of 1.13 V and promising catalytic ability for CO2 reduction and evolution reactions. In which, the overpotentials for Ru/G and NS-G cathodes are up to1.48 and 1.64 V, respectively, and r-GO cathode displays the largest overpotential. Figure 3d shows the cycling profiles of the Ru/NS-G cathode at a current density of 100 mA g−1. The battery based on the Ru/NS-G cathode not only exhibits an excellent cycling stability of 100 cycles, but also maintains a stable discharge and charge plateau. It is noteworthy that after 100 cycles, the corresponding voltage gap between the charge and discharge terminal potentials keeps at about 1.40 V. Figure S9 presents the discharge/charge curves of the NS-G cathode at 100 mA g−1. The battery can also stand for 100 cycles because of the strong synergistic interactions between the N and S heteroatoms. The introduction of N, S heteroatoms into graphene effectively alters the surface polarities and electronic 10
structure of the carbon material, and significantly boosts the electrocatalytic activity. However, it is worth noting that the NS-G cathode displays a high charge platform (>4.2 V) and a large overpotential throughout the cycles. The charge terminal voltage gradually raises from 4.2 V to 4.5 V, owing to the accumulation of insoluble Li2CO3.[46] Figure 3e shows the overpotential comparison of Ru/NS-G and NS-G cathodes at different cycles. It can be clearly observed that the overpotentials of Ru/NS-G cathode in Li-CO2 battery are considerably lower than that of the NS-G cathode even after 100 cycles, indicating its excellent cycle stability. Meanwhile, the Ru/NS-G cathode displays lower overpotentials at different current densities in contrast to NS-G cathode. Even at a high current density of 500 mA g−1, the overpotential of Ru/NS-G cathode is as low as 1.69 V (Figure 3f and Figure S10). The above results further confirm that the highly dispersed ultrafine Ru nanoparticles can significantly improve the catalytic activity and stability. At the same time, the modification of graphene by N and S heteroatoms provides abundant surface active sites and defects, further enhancing the catalytic activity. Hence, the Li-CO2 battery exhibits remarkable electrochemical performance.
Figure 3. (a) CV curves of Li-CO2 batteries based on Ru/NS-G, NS-G and r-GO electrodes. (b) Full discharge–charge curves (at a current density of 100 mA g−1). (c) The initial discharge-charge curves of Ru/NS-G and NS-G cathodes. (d) The selected 11
charge/discharge curves of the Ru/NS-G cathode at various cycles. (e) The overpotential comparison of Ru/NS-G and NS-G cathodes under different cycles. (f) The charge/discharge profiles of the Ru/NS-G and NS-G cathodes at different current densities. Ex-situ SEM, XPS, XRD, FTIR and Raman measurements were introduced to reveal the reversible deposition and decomposition of the discharge products and the reversibility of the electrochemical reaction. The morphologies and structures were observed via SEM characterization for the pristine, discharged and recharged Ru/NS-G cathodes (Figure 4a-c). Compared with the pristine Ru/NS-G in Figure 4a, some parts of the cathode surface are covered by the particle-shaped products after the first discharge process, while the folded features are still maintained (Figure 4b). After recharge, the particle-shaped products are completely vanished and the folded morphology are recovered, indicating the excellent reversibility of the cathode (Figure 4c). The XPS measurement was performed to gain insight into the reversibility of the Li-CO2 batteries (Figure 4d-g). Compared to the C 1s spectra of pristine Ru/NS-G cathode in Figure 4d, a new peak ascribing to the O-C=O bond at 289.4 eV appears in the discharged electrode and significantly declined after the subsequent recharge.[47] These results indicate the reversible formation and decomposition of Li2CO3 during the discharge and recharge processes, which is coincident with the SEM characterization. Additionally, we also analyzed the Li 1s XPS spectrum (Figure 4g). The distinct peak at 55.0 eV indicates the emergence of Li2CO3 during the discharge stage.[48] After recharge, no characteristic peak is observed, implying the decomposition of Li2CO3.. The FTIR and Raman spectra of the cycled cathodes were further provided. The FTIR spectrum of the discharged cathode in Figure 4h displays new peaks at 860 cm−1, 1412 cm−1 and 1473 cm−1, which are consist with the standard spectrum of commercial Li2CO3.[49, 50] Whereas, these peaks thoroughly disappear upon the subsequent recharge process, and the other small peaks in the cycled electrode are attributed to the residual electrolyte (Figure S11). Through observing the Raman spectra (Figure 4i), the appearance-disappearance of the peak at 1080 cm−1 12
further imply the formation-decomposition of Li2CO3 during the discharge and charge process.[26] Meanwhile, the XRD patterns of electrode at different stages further confirm this point (Figure S12). Moreover, the electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) curves were also carried out and further prove the reversibility of Li-CO2 battery with Ru/NS-G as cathode (Figure S13). These above results demonstrate that the as-prepared Ru/NS-G composite displays excellent electrocatalytic activity for promoting CO2 reduction and evolution reactions. The superior performance of the Li-CO2 battery based on the Ru/NS-G cathode can be attributed to the following three aspects. First, graphene possesses high theoretical surface areas, remarkable electrical conductivity and mechanical properties. This unique structure can facilitate the electrolyte infiltration and CO2 diffusion to ensure superb electron transport performances. Secondly, N, S heteroatoms are inserted into the sp2-hybridized carbon skeleton for alternative chemical doping to boost electrochemical properties. Particularly, nitrogen dopants play a vital role in increasing conductivity, CO2 and Li+ adsorption. S dopants can further enhance the catalytic activity. Finally, the synergistic effect between bifunctional catalyst Ru nanoparticles and NS-G can efficiently facilitate the reversible formation and decomposition of Li2CO3 and C, thus achieving the high performance of Li-CO2 batteries.
13
Figure 4. SEM images of the Ru/NS-G electrode at different stages: (a) pristine, (b) discharged, and (c) recharged. XPS spectra of C 1s and Ru 3d for the Ru/NS-G-based cathodes: (d) pristine, (e) discharged, and (f) recharged, respectively. (g) Li 1s XPS spectra after discharge-charge processes. (h) FTIR spectra. (i) Raman spectra. 4. Conclusion In summary, we have synthesized the Ru/NS-G composite as an excellent cathode in Li–CO2 batteries. The Ru/NS-G cathode delivers a superb discharge capacity of 12448 mA h g−1 with a higher Coulombic efficiency of 94.6%, due to the impactful synergistic effect between the bifunctional Ru nanoparticles catalyst and NS-G substrate. The battery also performs improved cycling stability, which could be stably operated for 100 cycles at a current density of 100 mA g−1. Besides, the polarization voltage of the Li-CO2 battery based on Ru/NS-G cathode is significantly reduced to be 1.40 V after 100 cycles. In particular, the reversible formation and decomposition of the discharge products of Li2CO3 and C were further confirmed by ex-situ SEM, XPS, FTIR and Raman techniques. This work affords a critical guideline for the 14
development of multifunctional synergistic catalysts for Li-CO2 battery and other energy storage devices.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. U1904187 and 21501049), the Fund of Key Scientific and Technological Project of Henan Province (No. 182102410081).
Conflict of Interest There are no conflicts to declare.
15
References [1] Y. Qiao, J. Yi, S. Wu, Y. Liu, S. Yang, P. He, H. Zhou, Li-CO2 Electrochemistry: A New Strategy for CO2 Fixation and Energy Storage, Joule 1 (2017) 359-370. https://doi.org/10.1016/j.joule.2017.07.001. [2] J. M. Matter, M. Stute, S. O. Snaebjornsdottir, E. H. Oelkers, S. R. Gislason, E. S. Aradottir, B. Sigfusson, I. Gunnarsson, H. Sigurdardottir, E. Gunnlaugsson, G. Axelsson, H. A. Alfredsson, D. Wolff-Boenisch, K. Mesfin, D. Fernandez de la Reguera Taya, J. Hall, K. Dideriksen, W. S. Broecker, Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions, Science 352 (2016) 1312-1314. https://doi.org/10.1126/science.aad8132. [3] J.-Y. Lee, H.-S. Kim, J.-S. Lee, C.-J. Park, W.-H. Ryu, Blood Protein as a Sustainable Bifunctional Catalyst for Reversible Li-CO2 Batteries, ACS Sustainable Chem. Eng. 7 (2019) 16151-16159. https://doi.org/10.1021/acssuschemeng.9b03079. [4] Y. Qiao, S. Xu, Y. Liu, J. Dai, H. Xie, Y. Yao, X. Mu, C. Chen, D. J. Kline, E. M. Hitz, B. Liu, J. Song, P. He, M. R. Zachariah, L. Hu, Transient, in situ synthesis of ultrafine ruthenium nanoparticles for a high-rate Li–CO2 battery, Energy Environ. Sci. 12 (2019) 1100-1107. https://doi.org/10.1039/c8ee03506g. [5] S. Bie, M. Du, W. He, H. Zhang, Z. Yu, J. Liu, M. Liu, W. Yan, L. Zhou, Z. Zou, Carbon Nanotube@RuO2 as a High Performance Catalyst for Li-CO2 Batteries, ACS Appl Mater Interfaces 11 (2019) 5146-5151. https://doi.org/10.1021/acsami.8b20573. [6] Y. Qiao, Y. Liu, C. Chen, H. Xie, Y. Yao, S. He, W. Ping, B. Liu, L. Hu, 3D-Printed Graphene Oxide Framework with Thermal Shock Synthesized Nanoparticles for Li-CO2 Batteries, Adv. Funct. Mater. 28 (2018) 1805899. https://doi.org/10.1002/adfm.201805899. [7] Z. Xie, X. Zhang, Z. Zhang, Z. Zhou, Metal–CO2 Batteries on the Road: CO2 from Contamination Gas to Energy Source, Adv. Mater. 29 (2017) 1605891. https://doi.org/10.1002/adma.201605891. [8] Y. Lu, X. Rong, Y.-S. Hu, L. Chen, H. Li, Research and development of advanced battery materials
in
China,
Energy Storage Mater. 16
23
(2019)
144-153.
https://doi.org/https://doi.org/10.1016/j.ensm.2019.05.019. [9] J. Chen, K. Zou, P. Ding, J. Deng, C. Zha, Y. Hu, X. Zhao, J. Wu, J. Fan, Y. Li, Conjugated Cobalt Polyphthalocyanine as the Elastic and Reprocessable Catalyst for Flexible
Li–CO2
Batteries,
Adv.
Mater.
31
(2019)
1805484.
https://doi.org/10.1002/adma.201805484. [10] F. Qiu, S. Ren, X. Mu, Y. Liu, X. Zhang, P. He, H. Zhou, Towards a stable Li-CO2 battery: The effects of CO2 to the Li metal anode, Energy Storage Mater. (2019). https://doi.org/10.1016/j.ensm.2019.11.017. [11] Z. Zhang, C. Yang, S. Wu, A. Wang, L. Zhao, D. Zhai, B. Ren, K. Cao, Z. Zhou, Exploiting Synergistic Effect by Integrating Ruthenium–Copper Nanoparticles Highly Co-Dispersed on Graphene as Efficient Air Cathodes for Li–CO2 Batteries, Adv. Energy Mater. 9 (2019) 1802805. https://doi.org/10.1002/aenm.201802805. [12] Z. Zhao, Y. Su, Z. Peng, Probing Lithium Carbonate Formation in Trace-O2-Assisted Aprotic Li-CO2 Batteries Using in Situ Surface-Enhanced Raman Spectroscopy,
J.
Phys.
Chem.
Lett.
10
(2019)
322-328.
https://doi.org/10.1021/acs.jpclett.8b03272. [13] R. Pipes, A. Bhargav, A. Manthiram, Phenyl Disulfide Additive for Solution‐ Mediated Carbon Dioxide Utilization in Li–CO2 Batteries, Adv. Energy Mater. 9 (2019) 1900453. https://doi.org/10.1002/aenm.201900453. [14] Q.-C. Zhu, S.-M. Xu, Z.-P. Cai, M. M. Harris, K.-X. Wang, J.-S. Chen, Towards real Li-air batteries: A binder-free cathode with high electrochemical performance in CO2
and
O2,
Energy
Storage
Mater.
7
(2017)
209-215.
https://doi.org/10.1016/j.ensm.2017.03.004. [15] C. Wang, Q. Zhang, X. Zhang, X. G. Wang, Z. Xie, Z. Zhou, Fabricating Ir/C Nanofiber Networks as Free-Standing Air Cathodes for Rechargeable Li-CO2 Batteries, Small 14 (2018) 1800641. https://doi.org/10.1002/smll.201800641. [16] Y. Mao, C. Tang, Z. Tang, J. Xie, Z. Chen, J. Tu, G. Cao, X. Zhao, Long-life Li– CO2 cells with ultrafine IrO2-decorated few-layered δ-MnO2 enabling amorphous Li2CO3
growth,
Energy
Storage 17
Mater.
18
(2019)
405-413.
https://doi.org/10.1016/j.ensm.2018.08.011. [17] B. Liu, Y. Sun, L. Liu, J. Chen, B. Yang, S. Xu, X. Yan, Recent advances in understanding Li–CO2 electrochemistry, Energy Environ. Sci. 12 (2019) 887-922. https://doi.org/10.1039/c8ee03417f. [18] Y. Qiao, J. Yi, S. H. Guo, Y. Sun, S. C. Wu, X. Z. Liu, S. X. Yang, P. He, H. S. Zhou, Li2CO3-free Li-O2/CO2 battery with peroxide discharge product, Energy Environ. Sci. 11 (2018) 1211-1217. https://doi.org/10.1039/c7ee03341a. [19] Z. Guo, C. Li, J. Liu, Y. Wang, Y. Xia, A Long-Life Lithium-Air Battery in Ambient Air with a Polymer Electrolyte Containing a Redox Mediator, Angew. Chem., Int. Ed. 56 (2017) 7505-7509. https://doi.org/10.1002/anie.201701290. [20] C. Ling, R. Zhang, K. Takechi, F. Mizuno, Intrinsic Barrier to Electrochemically Decompose Li2CO3 and LiOH, J. Phys. Chem. C 118 (2014) 26591-26598. https://doi.org/10.1021/jp5093306. [21] Z. Zhao, J. Huang, Z. Peng, Achilles' Heel of Lithium-Air Batteries: Lithium Carbonate,
Angew.
Chem.,
Int.
Ed.
57
(2018)
3874-3886.
https://doi.org/10.1002/anie.201710156. [22] X. Mu, H. Pan, P. He, H. Zhou, Li-CO2 and Na-CO2 Batteries: Toward Greener and Sustainable Electrical Energy Storage, Adv. Mater. (2019) e1903790. https://doi.org/10.1002/adma.201903790. [23] Z. Zhang, Q. Zhang, Y. Chen, J. Bao, X. Zhou, Z. Xie, J. Wei, Z. Zhou, The First Introduction of Graphene to Rechargeable Li–CO2 Batteries, Angew. Chem., Int. Ed. 54 (2015) 6550-6553. https://doi.org/10.1002/anie.201501214. [24] X. Zhang, Q. Zhang, Z. Zhang, Y. Chen, Z. Xie, J. Wei, Z. Zhou, Rechargeable Li–CO2 batteries with carbon nanotubes as air cathodes, Chem. Commun. 51 (2015) 14636-14639. https://doi.org/10.1039/C5CC05767A. [25] A. J. Hu, C. Shu, C. Xu, R. Liang, J. Li, R. Zheng, M. Li, J. Long, Design strategies toward catalytic materials and cathode structure for emerging Li−CO2 batteries,
J.
Mater.
Chem.
A
https://doi.org/10.1039/C9TA06506G. 18
7
(2019)
21605–21633.
[26] S. X. Yang, Y. Qiao, P. He, Y. J. Liu, Z. Cheng, J. J. Zhu, H. S. Zhou, A reversible lithium-CO2 battery with Ru nanoparticles as a cathode catalyst, Energy Environ. Sci. 10 (2017) 972-978. https://doi.org/10.1039/c6ee03770d. [27] S. Yang, P. He, H. Zhou, Exploring the electrochemical reaction mechanism of carbonate oxidation in Li–air/CO2 battery through tracing missing oxygen, Energy Environ. Sci. 9 (2016) 1650-1654. https://doi.org/10.1039/c6ee00004e. [28] L. Qie, Y. Lin, J. W. Connell, J. Xu, L. Dai, Highly Rechargeable Lithium-CO2 Batteries with a Boron- and Nitrogen-Codoped Holey-Graphene Cathode, Angew. Chem., Int. Ed. 56 (2017) 6970-6974. https://doi.org/10.1002/anie.201701826. [29] L. Zhao, J. Yu, C. Xing, Z. Ullah, C. Yu, S. Zhu, M. Chen, W. Li, Q. Li, L. Liu, Nanopore confined anthraquinone in MOF-derived N-doped microporous carbon as stable organic cathode for lithium-ion battery, Energy Storage Mater. 22 (2019) 433-440. https://doi.org/10.1016/j.ensm.2019.02.003. [30] J. Zhu, W. Li, S. Li, J. Zhang, H. Zhou, C. Zhang, J. Zhang, S. Mu, Defective N/S-Codoped 3D Cheese-Like Porous Carbon Nanomaterial toward Efficient Oxygen Reduction
and
Zn–Air
Batteries,
Small
14
(2018)
1800563.
https://doi.org/10.1002/smll.201800563. [31] X. Yu, M. Zhang, J. Chen, Y. Li, G. Shi, Nitrogen and Sulfur Codoped Graphite Foam as a Self-Supported Metal-Free Electrocatalytic Electrode for Water Oxidation, Adv. Energy Mater. 6 (2016) 1501492. https://doi.org/10.1002/aenm.201501492. [32] Y. Song, S. Bai, L. Zhu, M. Zhao, D. Han, S. Jiang, Y.-N. Zhou, Tuning Pseudocapacitance via C-S Bonding in WS2 Nanorods Anchored on N,S Codoped Graphene for High-Power Lithium Batteries, ACS Appl. Mater. Interfaces 10 (2018) 13606-13613. https://doi.org/10.1021/acsami.8b02506. [33] Y. Wu, Y. Yu, 2D material as anode for sodium ion batteries: Recent progress and perspectives,
Energy
Storage
Mater.
16
(2019)
323-343.
https://doi.org/10.1016/j.ensm.2018.05.026. [34] J. Wang, Z. Z. Wei, S. J. Mao, H. R. Li, Y. Wang, Highly uniform Ru nanoparticles over N-doped carbon: pH and temperature-universal hydrogen release 19
from
water
reduction,
Energy
Environ.
Sci.
11
(2018)
800-806.
https://doi.org/10.1039/c7ee03345a. [35] M. Nazarian-Samani, H.-D. Lim, S. Haghighat-Shishavan, H.-K. Kim, Y. Ko, M.-S. Kim, S.-W. Lee, S. F. Kashani-Bozorg, M. Abbasi, H.-U. Guim, D.-I. Kim, K.-C. Roh, K. Kang, K.-B. Kim, A robust design of Ru quantum dot/N-doped holey graphene for efficient Li-O2 batteries, J. Mater. Chem. A 5 (2017) 619-631. https://doi.org/10.1039/c6ta08427c. [36] H. Hwang, T. Kwon, H. Y. Kim, J. Park, A. Oh, B. Kim, H. Baik, S. H. Joo, K. Lee, Ni@Ru and NiCo@Ru Core–Shell Hexagonal Nanosandwiches with a Compositionally Tunable Core and a Regioselectively Grown Shell, Small 14 (2018) 1702353. https://doi.org/10.1002/smll.201702353. [37] X. Zhang, C. Y. Wang, H. H. Li, X. G. Wang, Y. N. Chen, Z. J. Xie, Z. Zhou, High performance Li-CO2 batteries with NiO-CNT cathodes, J. Mater. Chem. A 6 (2018) 2792-2796. https://doi.org/10.1039/c7ta11015d. [38] Q. Wang, Y. Ji, Y. Lei, Y. Wang, Y. Wang, Y. Li, S. Wang, Pyridinic-N-Dominated Doped
Defective
Graphene
as
a
Superior
Oxygen
Electrocatalyst
for
Ultrahigh-Energy-Density Zn–Air Batteries, ACS Energy Lett. 3 (2018) 1183-1191. https://doi.org/10.1021/acsenergylett.8b00303. [39] T. Akhter, M. M. Islam, S. N. Faisal, E. Hague, A. I. Minett, H. K. Liu, K. Konstantinov, S. X. Dou, Self-Assembled N/S Codoped Flexible Graphene Paper for High Performance Energy Storage and Oxygen Reduction Reaction, ACS Appl. Mater. Interfaces 8 (2016) 2078-2087. https://doi.org/10.1021/acsami.5b10545. [40] Y. Li, J. Zhou, T. Zhang, T. Wang, X. Li, Y. Jia, J. Cheng, Q. Guan, E. Liu, H. Peng, B. Wang, Highly Surface-Wrinkled and N-Doped CNTs Anchored on Metal Wire: A Novel Fiber-Shaped Cathode toward High-Performance Flexible Li–CO2 Batteries,
Adv.
Funct.
Mater.
29
(2019)
1808117.
https://doi.org/10.1002/adfm.201808117. [41] X. Wang, Q. Weng, X. Liu, X. Wang, D.-M. Tang, W. Tian, C. Zhang, W. Yi, D. Liu, Y. Bando, D. Golberg, Atomistic Origins of High Rate Capability and Capacity of 20
N-Doped Graphene for Lithium Storage, Nano Lett. 14 (2014) 1164-1171. https://doi.org/10.1021/nl4038592. [42] M. Wang, Y. Yao, Z. Tang, T. Zhao, F. Wu, Y. Yang, Q. Huang, Self-Nitrogen-Doped Carbon from Plant Waste as an Oxygen Electrode Material with Exceptional Capacity and Cycling Stability for Lithium-Oxygen Batteries, ACS Appl. Mater. Interfaces 10 (2018) 32212-32219. https://doi.org/10.1021/acsami.8b11282. [43] Y. Liu, X. Qiu, X. Liu, Y. Liu, L.-Z. Fan, 3D porous binary-heteroatom doped carbon nanosheet/electrochemically exfoliated graphene hybrids for high performance flexible solid-state supercapacitors, J. Mater. Chem. A 6 (2018) 8750-8756. https://doi.org/10.1039/c8ta01148f. [44] Y. Jin, C. Hu, Q. Dai, Y. Xiao, Y. Lin, J. W. Connell, F. Chen, L. Dai, High-Performance Li-CO2 Batteries Based on Metal-Free Carbon Quantum Dot/Holey Graphene Composite Catalysts, Adv. Funct. Mater. 28 (2018) 1804630. https://doi.org/10.1002/adfm.201804630. [45] T. Liu, X. Zhang, T. Huang, A. Yu, Pyridinic-N-dominated carbon frameworks with porous tungsten trioxide nano-lamellae as a promising bi-functional catalyst for Li-oxygen
batteries,
Nanoscale
10
(2018)
15763-15770.
https://doi.org/10.1039/c8nr04026e. [46] P. Tan, Z. H. Wei, W. Shyy, T. S. Zhao, X. B. Zhu, A nano-structured RuO2/NiO cathode enables the operation of non-aqueous lithium-air batteries in ambient air, Energy Environ. Sci. 9 (2016) 1783-1793. https://doi.org/10.1039/c6ee00550k. [47]Y. Xing, Y. Yang, D. Li, M. Luo, N. Chen, Y. Ye, J. Qian, L. Li, D. Yang, F. Wu, R. Chen, S. Guo, Crumpled Ir Nanosheets Fully Covered on Porous Carbon Nanofibers for Long-Life Rechargeable Lithium–CO2 Batteries, Adv. Mater. 30 (2018) 1803124. https://doi.org/10.1002/adma.201803124. [48] Z. Zhang, X. G. Wang, X. Zhang, Z. Xie, Y. N. Chen, L. Ma, Z. Peng, Z. Zhou, Verifying the Rechargeability of Li-CO2 Batteries on Working Cathodes of Ni Nanoparticles Highly Dispersed on N-Doped Graphene, Adv. Sci. 5 (2018) 1700567. https://doi.org/10.1002/advs.201700567. 21
[49] Z. Guo, J. Li, H. Qi, X. Sun, H. Li, A. G. Tamirat, J. Liu, Y. Wang, L. Wang, A Highly Reversible Long-Life Li-CO2 Battery with a RuP2-Based Catalytic Cathode, Small 15 (2019) e1803246. https://doi.org/10.1002/smll.201803246. [50] S.-M. Xu, Z.-C. Ren, X. Liu, X. Liang, K.-X. Wang, J.-S. Chen, Carbonate decomposition: Low-overpotential Li-CO2 battery based on interlayer-confined monodisperse
catalyst,
Energy
Storage
https://doi.org/10.1016/j.ensm.2018.05.015.
22
Mater.
15
(2018)
291-298.
Conflict of Interest There are no conflicts to declare.