Carbon Nanotube@RuO2 as a High Performance Catalyst for LiâCO2

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CNT@RuO as a high performance catalyst for Li-CO batteries Shiyu Bie, Meili Du, Wenxiang He, Huigang Zhang, Zhentao Yu, Jianguo Liu, Meng Liu, Wuwei Yan, Liang Zhou, and Zhigang Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20573 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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ACS Applied Materials & Interfaces

CNT@RuO2 as a high performance catalyst for LiCO2 batteries Shiyu Bie,† Meili Du,† Wenxiang He,† Huigang Zhang,† Zhentao Yu,† Jianguo Liu,*,†,‡ Meng Liu,† Wuwei Yan,‡ Liang Zhou,‡ Zhigang Zou†,‡ † Jiangsu Key Laboratory for Nano Technology, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, 22 Hankou Road, Nanjing 210093, China. ‡ Kunshan Sunlaite New Energy Co., Ltd., Kunshan, 1666# South Zuchongzhi Road, 215347, Jiangsu, China ABSTRACT: Efficient electrocatalysts for Li2CO3 decomposition play an important role in LiCO2 batteries. In this paper, CNT decorated with RuO2 is firstly introduced as cathode materials for Li-CO2 batteries. CNT@RuO2 composite can not only deliver high specific capacity, but also lower charge voltage. With the CNT@RuO2 cathodes, the coulombic efficiency still remains around 100 % until the 15th cycle. The charge voltage of early 30 cycles at current of 50 mA g-1 with capacity limit of 500 mA h g-1 can be fully lowered under 4.0 V. Particularly, CNT@RuO2 cathode can realize most decomposition of prefilled Li2CO3 and show a platform at around 3.9 V. This catalytic activity towards both in situ formed and preloaded Li2CO3 is more feasible for practical application in complex environment. ACS Paragon1/19 Plus Environment

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KEYWORDS: Li-CO2 batteries, decomposition of Li2CO3, CO2 capture and utilization, nanocomposites, electrochemical catalyst INTRODUCTION Li-CO2 battery has attracted increasing attentions because it can not only store energy but also capture CO2.1-5 Similar to that of Li-O2 batteries, Li-CO2 batteries mainly consist of lithium anodes, separator and porous cathodes that are immersed within CO2.6 During discharge, carbon and Li2CO3 are usually produced in the cathodes (Equation 1) 7 whereas the charge process may involve two different routes. One is the direct decompose of Li2CO3 (Equation 2). 3,8-10 The LiCO2 electrochemistry featuring the direct decomposition of Li2CO3 is irreversible but rechargeable with carbon accumulating on cathodes.3,10 The other is the electrochemical reaction between carbon and Li2CO3 (Equation3), realizing reversible energy storage.3,6 The two reaction routes are much different from a typical rechargeable Li-O2 battery, in which process the insoluble discharge products (Li2O2) can reversibly form and decompose during discharge and subsequent charge processes ( 2Li + + O2 +2e ― ↔Li2O2 ), but Li2CO3 can also be produced after long cycles due to some side effects on the O2 cathodes.11-12 Discharge: 3

1

2Li + + 2CO2 +2e ― →Li2CO3 + 2C

(Equation 1)

Charge: 1 Li2CO3→2Li + + 2e ― + CO2 + O2 (Equation 2) 2 1

3

Li2CO3 + 2C→2Li + + 2CO2 +2e ―

( Equation 3)

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It is well known that Li2CO3 is an insulator with low electron conductivity and inert electrochemical activity.13-15 The complete decomposition of Li2CO3 is difficult even at a voltage of over 4.0 V.14-17 The accumulation of Li2CO3 in air cathodes during cycling blocks gas diffusion way and covers the reactivity sites, finally leading to the cell death.

13-14

For Li-air

batteries, the main discharge product, Li2O2, is able to react with CO2 in ambient air and forms the side product, Li2CO3.16-19 Therefore, studies on Li-CO2 batteries also help to overcome the hindrance from Li-O2 to Li-air batteries. The development of Li-CO2 battery is still in its infancy and faces many challenges such as low discharge capacity, weak rate capability, high charge overpotential, poor cyclability, and other problems.20

Many recent reports on Li-CO2 batteries used carbon materials such as

carbon nanotubes (CNT) and graphene as cathode materials of Li-CO2 batteries and demonstrated the formation/decomposition of Li2CO3.21-25 However, the charge voltage is higher than 4.0 V and even exceeds 4.5 V in the terminal charge state with a limited capacity.7,25 Previous reports revealed that at high charge potentials, superoxide radicals can be generated from preloaded Li2CO3 (main discharge product of Li-CO2 batteries) and further oxidize the electrolyte solvent, leading to electrolyte decomposition.10 That’s why pure carbon materials always show high specific capacity but poor cycle life. Some metal-based materials showed higher activity towards Li2CO3 decomposition than pure carbon materials for rechargeable LiCO2 batteries.26-27 Zhou’s group firstly used Ru-Super P as cathode materials of Li-CO2 batteries. They have confirmed that Ru nanoparticles can promote the reaction between Li2CO3 and carbon and decrease the charge voltage from 4.5 to 3.8 V.6 Ni nanoparticles dispersed on N-doped graphene (Ni-NG) and Ir nanoparticles embedded in carbon nanofiber networks have been explored and exhibited excellent performance.26-27 Composites with transition metal oxide such



as NiO-CNT, Metal-organic frameworks (MOFs) and TiO2-NP@CNT/CNF also demonstrated the capability to activate the inert Li2CO3 and improve both charge and discharge properties.8, 2829

Once Li2CO3 participates in the electrochemical process, it seems very difficult to keep the charge voltage under 4.0 V.9-10 Hence further work is still required to optimize the performance of Li-CO2 batteries. A new discharge-charge route without Li2CO3 has been proposed by Hou et al.21 To improve the performance of Li-CO2 batteries, Mo2C was used as a catalyst forming Li2C2O4-Mo2C during the discharging process instead of Li2CO3 and this new discharge product could be well decomposed below 3.5V upon charging. More recently, Zhou and his co-workers have developed a super-concentrated electrolyte to stabilize the peroxodicarbonate in the LiO2/CO2 batteries by preventing its further reduction into Li2CO3 during cycling.23 This new strategy successfully led to obvious decrease of charge potential (from 4.2 to 3.5 V) and remarkable increase of the energy efficiency. However, it can be seen that the discharge capacities are not high as compared to others featuring Li2CO3 as discharge product.23-24, 28 Since the environment of practical application is complex, it may be hard to control the reaction way and avoid the formation of Li2CO3, especially for Li-air batteries.30-32 Therefore, no matter if CO2 acts as contamination gas or energy source, decreasing the charge voltage for Li2CO3 is of great importance for practical application.33 In this study, CNT decorated with RuO2 nanoparticles (CNT@RuO2) is, for the first time, developed and used as the cathode material for Li-CO2 batteries and its electrochemical activity towards preloaded Li2CO3 was also tested. Although CNT with high specific surface area and good conductivity shows good CO2 reduction reaction activity 25 and RuO2 has been widely used as the cathode material for Li-O2 batteries，


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there is no studies on catalytic decomposition of Li2CO3 using a composite of CNT and RuO2.3435

Our results show that CNT@RuO2 is able to maintain the charge voltage platform around 3.9

V and decrease the charge voltage of Li-CO2 batteries below 4.0 V during the first 30 cycles at 50 mA g-1 with a limited capacity of 500 mA h g-1. This improvement is ascribed to the combined advantages of CNT and RuO2. Besides, the catalysis performance towards preloaded Li2CO3 was also studied. Our investigation on charging the Li2CO3 prefilled CNT@RuO2 electrode also demonstrates a platform at around 3.9 V and can decompose Li2CO3 with efficiency higher than 90%. This activity towards both in situ formed and preloaded Li2CO3 can be more feasible for complex practical environment. EXPERIMENT SECTION Materials. All reagents used here were analytical grade and used without further purification. CNT was purchased from XFNANO, Inc (Nanjing, China). HNO3, H2SO4, NaHCO3 and Nmethyl-2-pyrrolidone (NMP) were purchased from Guoyao Chemical Reagent Co., Ltd. RuCl3·xH2O and Li2CO3 were bought from Aladdin Ltd. (Shanghai, China). The water was purified through a Millipore system. Sample Preparation. Preparation of CNT@RuO2: CNT was pretreated in the mixture of nitric acid (60 wt%) and sulfuric acid (98 wt%) with a volume ratio of 1:3 at 60 ºC for 8 h. The pretreated CNT was then dispersed in a 30 mL solution containing about 0.1 g RuCl3·xH2O. NaHCO3 was added slowly into the suspension under stirring until the pH value reaches 7. After stirred for 15 h, the solution was filtered and washed with pure water. The precursor-coated CNT was dried at 50 ºC under vacuum for 8 h and finally treated at 200 ºC under vacuum for other 10



h.34 RuO2 powder was prepared in the same way except that none pretreated CNT was used and pH was over 10. Electrochemical Measurements.CNT@RuO2 and PVDF were mixed with a mass ratio of 9:1 in N-methyl-2-pyrrolidone (NMP) solution. The mixture was then sprayed onto carbon paper and dried at 90 ºC under vacuum. The mass loading of CNT@RuO2 on each CO2 electrode was 0.30.5 mg cm-2. LiCF3SO3 dissolved in TEGDME at molar ratio of 1:4 was used as electrolyte, of which water content is around 16 ppm. Lithium foil was used as anode. The assembling of 2032 coin cell was conducted in a glove box filled with argon, where humidity and oxygen content were both kept around 0.1 ppm. The coin cell was then transferred into a sealed box full of CO2. Galvanostatic charge-discharge cycles were conducted by a LAND CT2001A multi-channel battery system. The CNT@RuO2 cathodes prefilled with Li2CO3 were prepared by grinding Li2CO3, CNT@RuO2 and PVDF (at mass ratio of 6:2:2) with mortar. Then the derived mixture was stirred in NMP solvent for 3 hours and pasted on carbon paper followed by drying under vacuum at 90 ºC overnight. The corresponding cells were assembled in the same way and charged under Ar. Cyclic voltammograms (CV) were conducted on an electrochemical workstation (CHI 760E, Chenghua Corp., Shanghai, China) at scanning rate of 0.2 mV s-1. Materials Characterization. For the ex situ characterization of the discharged and charged electrodes, the electrodes were washed with TEGDME and dried under argon to remove the residual solvent. The morphology of electrodes was observed on a scanning electron microscopy (SEM, Hitachi S-4800). Scanning transmission electron microscopy (STEM) and energydispersive X-ray (EDX) mapping were conducted on a Zesis Gemini SEM 500 microscopy. Xray powders diffraction (XRD) patterns were collected on a Philips X-ray diffractometer (APD 3520) equipped with Cu Kα radiation. Selected area electron diffraction (SAED), transmission ACS Paragon6/19 Plus Environment

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electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were obtained on a FEI Tecnai G2 F20 S-Twin microscope. The specific surface area was tested using a Brunauer-Emmett-Teller (BET) method by N2 adsorption/desorption (TriStar-3000, Micromeritics USA). The weight ratio of RuO2 was determined with a NETZSCH STA 449 F3 differential scanning calorimeter under air flow at a ramp rate of 5 °C min-1. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a VG Scientific ESCALAB 2201XL instrument at scanning speed of 1 ° min-1. DISCUSSION Figure 1a-d shows the TEM and EDX mapping images of CNT@RuO2. The EDX elemental mapping image of C element matches well with TEM result. The O and Ru elements are evenly distributed along CNT, indicating a uniform decoration. The XRD pattern of CNT@RuO2 exhibits two diffraction peaks at 25.9 and 42.8 ° assigned to the (002) and (100) planes of the CNT, respectively (Figure 2e). Two wide bumps of CNT@RuO2 at around 32 and 45 ° can be attributed to RuO2.34-36 XPS was performed to determine the oxidation state of CNT@RuO2. Figure S1a shows the XPS results of CNT, RuO2 and CNT@RuO2. The survey spectrum of CNT@RuO2 shows the typical XPS signals of Ru, C, and O. The Ru 3d5/2 peak appearing at 280.8 eV (Figure S1b) indicates the state of RuO2 in the component.34 The proportion of RuO2 was estimated by thermogravimetry (TG) (Figure S1c). Considering that the weight loss mainly results from CNT combustion under air flow, we can determine that the percentage of RuO2 is around 28.9 %.



Figure 1. a) TEM and b-d) EDX elemental mapping images of CNT@RuO2: b) C, c) O, d) Ru elements; e) XRD patterns of CNT and CNT@RuO2; f) HRTEM image of CNT@RuO2. The adsorption-desorption curves of CNT@RuO2 and CNT are shown in Figure S1d and the specific surface areas of them are 105.97 cm2 g-1 and 204.02 cm2 g-1, respectively. The lower surface area of CNT@RuO2 can be attributed to the decoration of RuO2 nanoparticles. Figure S2 shows the TEM images of CNT@RuO2. It can be seen that the diameters of carbon nanotubes range from 10 to 30 nm. In Figure 1d, RuO2 nanoparticles with diameters around 2 nm are evenly distributed on the carbon tubes. The inset in the bottom right of Figure 1d is an enlarged image of the selected area marked with red dashed frame. The interplanar distances are 0.22 and 0.32 nm, which are in agreement with those of RuO2 (111) and (110) planes, respectively.


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Figure 2. a) CV curves of coin cells with CNT or CNT@RuO2 cathodes at a scan rate of 0.2 mV s-1 ; b) Discharge-charge profiles of CNT and CNT@RuO2 cathodes at a current density of 50 mA g-1 in the first cycle; c) XRD patterns of the CNT@RuO2 cathodes in pristine state, after 1st discharge and 1st charge; SEM images of CNT@RuO2 cathodes d) in the pristine state, e) after the 1st discharge and f) after the 1st charge. To evaluate the electrochemical performances of CNT@RuO2, the 2032 coin cell was assembled using CNT@RuO2 as the cathode of Li-CO2 battery. All the potential window of CNT@RuO2 cathodes was set between 2.2 and 4.2V (versus Li/Li+) to avoid RuO2 oxidation and reduction.37 Both the applied current density and specific capacity were normalized to the weight of CNT@RuO2 composite. Firstly, the cyclic voltammograms (CV) were obtained to compare the electrochemical activity of CNT and CNT@RuO2 (Figure 2a). The CV curves of CNT@RuO2 show lower potential of the anodic peaks and higher anodic current than those of CNT. This anodic potential is in accordance with discharge-charge profiles shown in Figure 2b.



We can see that CNT@RuO2 cathodes demonstrate a charge platform occurring at around 3.9 V whereas the charge voltage of pure CNT cathodes sharply increases to around 4.4V. Considering that no cathodic/anodic peaks was shown and nearly zero capacity was delivered on CNT@RuO2 cathodes in argon (compared with that in CO2), the appearing of cathodic/anodic peaks and the high discharge capacity delivering in CO2 atmosphere can be attributed to the participation of CO2 in the electrochemical process. Besides, CNT@RuO2 cathode can almost realize full decomposition of the discharge product in the first cycle, exhibiting a high columbic efficiency up to 100%. In contrast, CNT cathodes deliver a discharge capacity of 3874 mA h g-1 and showed a charge capacity of 1047 mA h g-1, indicating a columbic efficiency around 27 %. Both cathodes show a discharge platform around 2.5 V and the lower discharge capacity of CNT@RuO2 (2187 m Ah g-1) can be attributed to its lower specific surface area than that of CNT. To gain insight into the discharge and charge process, the X-ray diffraction (XRD) was conducted to study the first discharge product of Li-CO2 cells with CNT@RuO2 cathodes (Figure 2c). At 2.2 V, the main discharge product shows peaks consistent with Li2CO3. At 4.2 V, the peaks related to Li2CO3 disappear. The corresponding morphology changes of air cathodes shown in Figure 2d-e also confirm the formation/decomposition of Li2CO3. The recovery of cathode morphology after the first charge indicates excellent catalytic activity of CNT@RuO2 composite towards the charging process of Li-CO2 batteries. It is worth mentioning that the discharge product on the surface of the cathodes is needle-like, differing from the product morphology of pure CNT cathode reported by others.23 The enlarged image of the area marked with red dashed frame is displayed in the top right of Figure 2e, from which we found that Li2CO3 seems not to totally grow along surface of CNT@RuO2 composite. To figure out if it is caused by different operating environments, the SEM image of CNT cathodes after discharging


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is provided in Figure S3. The discharge product on the surface of CNT cathodes features particles with diameters less than 0.5μm and also seems not to fully grow along the surface of CNT. This can be explained by an intermediate product Li2C2O4 proposed by Archer et al.5 Li2C2O4 is produced in at initial stage (2Li + +2CO2 +2e ― →Li2C2O4) and then it can decompose into C and Li2CO3, releasing CO2 at the same time (2Li2C2O4→2Li2CO3 +C + CO2). Our discharge platform at the initial stage is very close to theoretical equilibrium potential (at around 2.6 V) 5 of the electrochemical reaction. The mobility of Li2C2O4 in electrolyte could be attributed to the small content of water on the electrode (1.4 wt% of CNT@RuO2 powder, shown in Figure S1b) and electrolyte (16 ppm). The total reaction is the same as Equation 1. As shown in Figure 3a, the Li-CO2 battery with CNT@RuO2 composite was fully discharged and charged at 50 mA h g-1 for 17 cycles controlled by potential limits from 2.2 to 4.2 V. The discharge/charge capacity can be as high as 2187 mA h g-1. The average discharge/charge plateaus are located at 2.46 and 3.97 V, respectively. Although the discharge capacity gradually decreases in the following cycles, the coulombic efficiency still remains around 100% until the 15th cycle. The cycle life and coulombic efficiency of the full charge-discharge test are superior to that of many carbon materials such as graphene (at a current of 50mA g-1, with efficiency less than 50% in the 1st cycle) and Ketjen Black (at a current of 30 mA g-1, to the same discharge voltage, 7 cycles) of other studies.2,7 The SAED pattern of the CNT@RuO2 composite collected from air cathode after the 17th charge ( Figure S4) demonstrates two diffraction rings in agreement with the (-110) and (-202) planes of Li2CO3. It indicates the accumulation of Li2CO3 on cathodes which explains why the capacity decreases in the discharge-charge process.



Figure 3. a) Discharge-charge curves of CNT@RuO2 at 50 mA g-1 controlled by potential limits from 2.2 to 4.2 V; Cycling performance of Li-CO2 batteries with CNT@RuO2 cathodes at b) 50 mA g-1, c) 100 mA g-1 and d) 150 mA g-1. To evaluate the stability of the catalytic activity of CNT@RuO2, continuous dischargecharge cycling of the Li-CO2 cell was conducted at varied current densities of 50, 100 and 150 mA g-1 with a capacity limited at 500 mA h g-1 (Figure 3b-d). CNT@RuO2 composite was able to be discharged and charged for 55 cycles and the terminal charge voltage was below 4.0 V in the initial 30 cycles. Compared to Li-CO2 batteries using CNT cathodes in other reports, CNT@RuO2 composite can reduce the charge potential by ~0.4 V.23 The average charge voltage in the first cycle at 50, 100, and 150 mA g-1 are 3.86, 3.88, and 3.92 V, respectively. The charge potential increases with cycles and the cyclable number sharply decreases with the increase of current. This indicates the fast accumulation of undecomposable Li2CO3 when the Li-CO2 battery was cycled under a high current. Moreover, the XPS spectra of C1s of the CNT@RuO2 cathodes after the 1st and 10th discharge/charge at 50 mA g-1 were displayed in Figure 4a-d. The ACS Paragon12/19 Plus Environment

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peak around 289 eV are corresponding to the C=O bond of Li2CO3. After the 1st charge, the intensity of this peak becomes very low, implying the decomposition of most Li2CO3 in the charge process. This result agrees well with the XRD result. However, the peak of C=O bond remains a certain intensity after the 10th charge (Figure 4d), indicating that Li2CO3 could not be completely decomposed and accumulate on the cathodes little by little during cycling.

Figure 4. XPS profiles of the CNT@RuO2 cathodes at different discharge/charge states: a) the 1st discharge, b) the 1st charge, c) the 10th discharge, d) the 10th charge. Considering the complex environment of practical application, it is hard to control the discharge route in real atmosphere and the electrochemistry for Li2CO3 decomposition can be much different. CNT@RuO2 and CNT cathodes with prefilled Li2CO3 were charged at 10 mA g-1 (based on the mass of CNT@RuO2 or CNT) to test its catalytic performance towards preloaded Li2CO3, as shown in Figure 5a. The preloaded mass of Li2CO3 of CNT@RuO2 cathodes are 3.98



mg (2.88 mA h) and the charge capacity is 2.69 mA h. For CNT cathodes with prefilled Li2CO3, the mass of Li2CO3 are around 2.4 mg (1.74mA h).The charge curve of CNT@RuO2 cathodes demonstrates a charge plateaus at ~3.9 V and can realize decomposition of Li2CO3 with a coulombic efficiency higher than 90%. As the charge capacity increases, the contact between Li2CO3 and catalysts could become worse and this can be the reason why the charge voltage increases gradually. The XRD pattern of CNT@RuO2 cathode (Figure 5b) shows the diffraction peaks in good agreement with Li2CO3 (JCPDS No.22-114). After charge, most of the peaks corresponding to Li2CO3 disappeared, but the peak at around 32 º still remains ， implying the incomplete decomposition of preloaded Li2CO3. SEM images of CNT@RuO2 cathodes before and after charge displayed in Figure 5c-d directly reveal the morphology change. Li2CO3 bulks with diameters around 1 μm preloaded on the cathodes could not be observed after charge. Therefore, it shows that CNT@RuO2 can lower the charge voltage of both in situ formed and preloaded Li2CO3, but the reaction routes of both are still uncertain. If CNT@RuO2 can promote the charge route of Equation 3, the carbon reacting with Li2CO3 is probably from the electrolyte or carbon materials, leading to an undesired result of electrolyte decomposition or cathode corrosion when Li2CO3 is produced without carbon (especially for Li-Air batteries) . We hope that more in situ work could be done to discuss these kinds of problems.


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Figure 5. a) Charge curve of CNT@RuO2 and CNT cathodes with preloaded Li2CO3; b) XRD patterns of CNT@RuO2 before and after charge; SEM images of CNT@RuO2 cathodes with preloaded Li2CO3 at different states: c) before and d) after charge. CONCLUSIONS In summary, CNT@RuO2 composite was prepared as a catalyst for the decomposition of both in situ formed and preloaded Li2CO3. Due to its superior catalytic activity, CNT@RuO2 cathodes can deliver a discharge capacity of 2187 mA h g-1 and lower the charge voltage around 3.97 V with a high Coulombic efficiency up to 100 % in the first cycle. The formation/decomposition of Li2CO3 was confirmed by XRD and SEM. The morphology of the discharge product is needlelike and Li2CO3 seems not to grow along the surface of CNT@RuO2 composite. The Li-CO2 batteries with CNT@RuO2 cathodes were able to be discharged and charged for 55 cycles and the charge voltage of the first 30 cycles can be totally controlled below 4.0 V. XPS analysis revealed the incomplete decomposition of Li2CO3 during cycling. Particularly, Li2CO3 was preloaded on CNT@RuO2 cathodes and charged at 10 mA g-1. CNT@RuO2 composite could ACS Paragon15/19 Plus Environment


realize the decomposition of prefilled Li2CO3 with an efficiency of 93%. The decomposition of Li2CO3 was proved by XRD and Li2CO3 particles disappeared after charge in SEM images. This catalytic activity towards both in situ formed and preloaded Li2CO3 is more feasible for practical application in complex environment. Although further in situ characterizations are still needed to understand the mechanism of the discharge/charge reactions on CNT@RuO2 cathodes, we hope this work can provide a new strategy to design catalysts of Li-CO2 or other metal-air batteries. ASSOCIATED CONTENT Supporting Information. Figures with XPS spectra of CNT@RuO2, CNT, RuO2, TG curve of CNT@RuO2, N2 adsorption-desorption curve of CNT@RuO2,TEM image of CNT@RuO2, diffraction pattern of CNT@RuO2 collected from air cathode after 10th recharge. The Supporting Information is available free of charge on the ACS Publications website at DOI: AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge financial support from Jiangsu Province Natural Science Foundation (BK20171247 and BK20171245), Scientific Instrument Develop Major Project of National Natural Science Foundation of China (51627810), the Graduate innovation Foundation of Nanjing University (2017ZDL05), Key Plan of NJU national demonstration base for


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innovation& Entrepreneurship (SCJD020901), Joint Funds of the National Natural Science Foundation and Liaoning of China (U1508202). Jianguo Liu also thanks the support of PAPD of Jiangsu Higher Education Institutions, “Six Talent Peaks Program” of Jiangsu Province, and Fundamental Research Funds for the Central Universities, China. REFERENCES (1)

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