A SnO2-Based Cathode Catalyst for Lithium-Air Batteries - ACS

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SnO2-based cathode catalyst for lithium air batteries Delong Mei, Xianxia Yuan, Zhong Ma, Ping Wei, Xuebin Yu, Jun Yang, and Zi-Feng Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02402 • Publication Date (Web): 06 May 2016 Downloaded from http://pubs.acs.org on May 11, 2016

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ACS Applied Materials & Interfaces

SnO2-based cathode catalyst for lithium air batteries

Delong Meia, Xianxia Yuana,*, Zhong Maa,d, Ping Weib, Xuebin Yuc, Jun Yanga, Zi-Feng Maa a

b

Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China

School of Mechanical-Electrical and Quality Technology Engineering, Nanning University, Nanning, Guangxi, 532999, China c d

Department of Materials Science, Fudan University, Shanghai, 200433, China

Energy & Photon Sciences department, Brookhaven National Laboratory, Upton, NY, 11973, USA

Abstract: SnO2 and SnO2@C have been successfully synthesized with a simple hydrothermal procedure combined with heat-treatment, their performance as cathode catalysts of Li-air batteries have been comparatively evaluated and discussed. The results show that both SnO2 and SnO2@C are capable of catalyzing ORR and OER at the cathode of Li-air batteries, but the battery with SnO2@C displays better performance owing to the unique higher conductivity, larger surface area, complex pore distribution and huge internal space.

Keywords: Tin dioxide; Carbon coated tin dioxide; Cathode catalyst; Microstructure; Li-air batteries

1

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1. Introduction With increasing attention to energy crisis and environmental pollution, lithium air (Li-air) batteries have become appealing in recent years owing to their distinctive advantages of extremely high energy density (which is comparable to that of gasoline and ten times higher than that of Li-ion batteries) and the active material of oxygen on cathode can be obtained from air instead of being provided internally.1-4 However, there are still many challenges,5,6 such as low discharge capacity, high overpotentials, poor rate capability and cycling performance, to overcome before practical applications of Li-air batteries. Among diverse factors affecting the performance of Li-air batteries, the cathode catalyst plays a crucial role, it should be bifunctionally active towards both oxygen reduction reaction (ORR) during discharge and oxygen evolution reaction (OER) during recharge of the batteries.7 At the current state of technology, the cathode catalysts that have been explored in Li-air batteries could be summarized mainly into carbon materials,8-13 metals and/or metal oxides14-19 and composite materials.20-25 As a typical member of metal oxides, tin dioxide (SnO2) has great applications in gas sensing26-28 as well as lithium-ion batteries29-35 exhibiting high lithium storage capacity and excellent cycling performance. However, it has never been incorporated into Li-air batteries to our best knowledge. Generally, SnO2 can be achieved with many methods, such as sol-gel,36 precipitation in solution,37 electrochemical deposition,38 hydrothermal synthesis39,40 and so on. With the consideration that hydrothermal synthesis is in favor of obtaining 2


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well dispersed porous SnO2 nanoparticles with large surface area39-41 which is preferred for cathode catalysts of Li-air batteries, it is employed in this study, combined with subsequent heat-treatment, to prepare SnO2 and SnO2@C (carbon coated SnO2) nanoparticles. Their performance as cathode catalysts in Li-air batteries have been explored and comparatively investigated. 2. Experimental 2.1 Material synthesis SnO2 and SnO2@C nanoparticles were synthesized with the glucose-mediated hydrothermal method ameliorated from that of Lou’s.31, 39 The specific procedure was as follows: 1.0 g of K2SnO3·3H2O was added to 40 ml of 0.4 M glucose aqueous solution. After vigorous stirring for 30 minutes, the obtained solution was transferred to a teflon lined stainless steel autoclave (50 ml in capability) and kept at 180 oC for 4 hours. After cooling down to room temperature, the sample was collected by centrifugation and washed thoroughly with distilled water and ethanol alternate for several times followed by a drying step in vacuum at room temperature overnight. The resulted black particles were then calcinated at 500 oC for 4 hours in an air and argon atmosphere, respectively, to obtain SnO2 and SnO2@C nanoparticles as schematically illustrated in Fig. 1. 2.2 Characterization Phase components in the as-prepared SnO2 and SnO2@C nanoparticles and the fresh, discharged and charged cathodes were characterized by powder X-ray diffraction (XRD) using a Bruker D8 advance diffractometer. Morphologies of the 3



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SnO2 and SnO2@C nanoparticles and the discharged/charged cathodes were analyzed with the Philips-FEI Sirion scanning electron microscopy (SEM) and the JEOL JEM-2010 transmission electron microscopy (TEM). Because of the magnetic feature of nickel foam and its possible damage to the SEM instrument, the cathodes for morphology study were with carbon paper as the substrate. Carbon content in SnO2@C nanoparticles was evaluated by thermogravimetric (TG) analysis under air flow on Netzsch STA 449 F3 Jupiter instrument and elemental analysis (EA) on a Perkin Elmer PE 2400 II CHNS/O analyzer. BET specific surface areas and pore size distributions of SnO2 and SnO2@C nanoparticles were calculated with the N2 adsorption-desorption isotherms captured using Micromeritics ASAP 2010 M+C instrument at 77K. 2.3 Electrochemical measurements In order to evaluate the electrochemical catalytic performance of the SnO2 and SnO2@C nanoparticles towards ORR as well as OER, Li-air batteries with SnO2 and SnO2@C as the cathode catalyst, respectively, were fabricated with the following steps: SnO2 or SnO2@C catalyst, Super P carbon and polytetrafluoroethylene (PTFE) with a weight ratio of 3:6:1 were dispersed in ethanol to make a slurry which was sprayed evenly onto nickel foam (11mm in diameter) as the substrate with a paint spray gun, the loading of materials (catalyst, Super P carbon and PTFE) in each cathode was controlled to be 0.5 mg. For comparison, the pure carbon cathode without catalyst was also fabricated with the slurry made of 90 wt% Super P carbon and 10 wt% PTFE using the same procedure. After drying at 80 oC in vacuum 4


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overnight to remove the ethanol, an as-prepared cathode, a lithium metal anode, a glass

fiber

separator

and

an

electrolyte

of

1

M

LiTFSI

(lithium

bis-(trifluoromethanesulfonyl)-imide) in TEGDME (tetrathylene glycol dimethyl ether) were used together to assemble a Li-air battery with a self-modified Swagelok-type cell42 in an argon-filled glove box with oxygen and water contents less than 0.1 ppm. The discharge-charge performance of the batteries was measured in a 1.0 atm O2 atmosphere using a LAND CT2001A battery testing system at room temperature. The specific capacity and current density were calculated based on the amount of Super P carbon in the cathode. The cyclic voltammetry (CV) curves for pure Super P, SnO2 and SnO2@C based cathodes were recorded using a CHI 750a electrochemical potentiostat/galvanostat within the voltage range of 2.0-4.5 V (vs. Li metal) under O2 atmosphere with a potential scanning rate of 0.5 mV s-1 at room temperature. The electrochemical impedance spectroscopy (EIS) was measured in a frequency range from 1000 kHz to 100 mHz with an amplitude of 5 mV controlled by an Autolab PGSTAT302N. The rotating-disk-electrode (RDE) experiment was conducted in Ar- and O2- saturated electrolyte of 1 M LiTFSI in TEGDME, respectively, employing a glassy carbon disk (∅=3 mm) as working electrode at a rotating rate of 1600 rpm controlled with a RDE system from Autolab. The catalyst layer on the working electrode was prepared with the following procedure: 5 mg of catalyst was ultrasonically dispersed in a solution containing 30 µl PTFE aqueous solution (24wt%) and 970µl deionized water. Then, 8 µl of the obtained ink was pipetted onto the glassy carbon disk and dried at room 5



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temperature. For comparison, RDE experiment was also conducted on commercial Pt(20wt%)/C catalyst.

3. Results and discussion During the hydrothermal reaction at 180 oC, K2SnO3·3H2O decomposed into SnO2 with equation 1. Meanwhile, glucose lost some water and the resulted C6HxOy adhered onto the surface of SnO2. Then, as illustrated in Fig.1, the surface C6HxOy was burnt during the heat-treatment in air resulting in SnO2 particles, while it decomposed into carbon coated on the surface of SnO2 leading to SnO2@C nanoparticles when the heat-treatment was proceeded in argon. It is worth to note that the glucose as raw material during the synthesis was excessive since it could not only form the carbon wrapping SnO2 but also facilitate rapid precipitation of polycrystalline SnO2 nanoparticles.39 Moreover, the hydrothermal reaction employed in this research could make SnO2 and SnO2@C nanoparticles loose and porous with large surface area,39-41 which make them possible for the cathode catalysts of Li-air batteries. K2SnO3 ·3H2O =SnO2+2KOH+2H2O

(1)

Fig. 2 shows XRD patterns of the as-prepared SnO2 and SnO2@C nanoparticles. For both samples, all the diffraction peaks centered at 26.61o (110), 33.89o (101), 37.95 o (200), 38.97o (111), 51.78o (211), 54.76o (220), 57.82o (202), 61.87o (310), 64.72o (112), 65.94o (301) and 71.28o (202) match well with that of tetragonal SnO2 (PDF#41-1445), indicating successful synthesis of SnO2 in them. For SnO2@C nanoparticles, XRD signal of the carbon coating on SnO2 surface might overlap with 6


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the diffraction peak of SnO2 (110), or another probable case is that the carbon exists in amorphous form. To verify the existence of carbon coating on the surface of SnO2 in SnO2@C nanoparticles, the thermogravimetric curve was captured under air flow and the result is presented in Fig. 3. There are two obvious weight loss steps in the TG curve, the first one with a loss of 0.69 wt% from room temperature to about 270 oC could be attributed to the removal of adsorbed water, the other one large step above 270 oC is assigned to oxidation of the surface carbon coating in air, it gives a carbon content of about 17.20wt%. The surface carbon content in SnO2@C nanoparticles was also evaluated with the technique of elemental analysis, which exhibited a result of 15.52wt%, agreeing well with the TG result of 17.20wt% within experimental error. Moreover, the carbon coating can also be clearly observed in the high-resolution TEM (HRTEM) image (Fig. 4d) with an amorphous lattice space of 0.374 nm. Fig. 4 a & b displays SEM images of the obtained SnO2 and SnO2@C nanoparticles. Sphere-like nanoparticles are uniformally dispersed in both samples. The particle size of SnO2 is about 27 nm, while it is much larger for the SnO2@C nanoparticles with a diameter of about 50 nm. The HRTEM images shown as Fig.4 c & d for SnO2 and SnO2@C, respectively, provide insights into the detailed structure of the samples, the sphere-like SnO2 nanoparticles are composed of aggregates of several granulas rather than single solid small particle, there is internal space between them which is beneficial for discharge product storage during the discharge of Li-air batteries when employed as the cathode catalyst, this will lead to improved discharge capacity. Similarly, each SnO2@C particle is consisted of several granulas that enwrapped in 7



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surface carbon which will enhance conductivity of the SnO2 particles. Moreover, there is greatly larger internal space in SnO2@C nanoparticles than in SnO2, which contributes not only to more discharge product storage but also larger surface area. Besides, the surface of SnO2@C nanoparticles is obviously coarser than that of the SnO2 particles. This is beneficial for increase of surface area which will accelerate the catalytic performance. N2 adsorption-desorption isotherms of SnO2 and SnO2@C nanoparticles are presented in Fig. 5 a & b, respectively. With these data, the BET surface areas were calculated to be 45.35 m2 g-1 for SnO2 and 96.79 m2 g-1 for SnO2@C. This matches well with the above discussion with SEM and TEM images that SnO2@C has larger surface area than SnO2. The obtained pore radius distribution curves are displayed as Fig. 5 c & d. The pores in SnO2 are mainly in the range from 3 to 20 nm, there is only very small amount of pores larger than 20 nm. While the distribution of pores in SnO2@C is greatly wider, both micropores centered at about 1 nm and 2nm and mesopores in the range from 3 to 60 nm are observed, the mesopores larger than 20 nm contribute much more pore volume in SnO2@C than that in SnO2. This agrees well with the discussion above with TEM images that there is greatly larger internal space in SnO2@C nanoparticles than in SnO2. Electrochemical properties of both SnO2 and SnO2@C as bifunctional catalysts of Li-air batteries were manifested with CV curves as shown in Fig. 6a, that of the cathode with only pure Super P is also displayed there for comparison. In the ORR process, SnO2 exhibits the highest discharge onset potential, implying the highest 8


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initial discharge voltage, while the similar discharge onset potential of SnO2@C and Super P indicating close starting discharge voltage. The ORR peak potential of SnO2 is slightly higher than that of Super P, and that of SnO2@C is greatly larger than SnO2’s, a difference of about 250 mV can be clearly observed. This infers the fastest oxygen reduction kinetics catalyzed by SnO2@C and the highest discharge voltage platform of the SnO2@C based Li-air batteries. In the OER process, all three cathodes demonstrate almost the same onset potentials suggesting similar starting charge potential. The OER peak potentials of both SnO2 and SnO2@C are almost the same and slightly lower than the Super P’s, this implies the slowest OER kinetics and the highest charge voltage platform of Super P based Li-air battery and the closely similar voltage platform of the other two batteries. With these results, it could be expected that the battery with SnO2@C as the cathode catalyst will exhibit the best performance. Fig.6b reveals the discharge-charge profiles of Li-air batteries with pure Super P, SnO2 and SnO2@C based cathodes at a current density of 75 mA g-1. Though the SnO2 based battery displays the highest initial discharge voltage, its discharge voltage platform is lower than the other two batteries’. The starting charge voltages of all three batteries are almost the same, but the voltage platform of Super P based battery is obviously higher than the other two batteries whose voltage platforms are closely similar to each other. This agrees well with the discussion above with CV curves. Round-trip efficiency is an important characteristic of Li-air batteries, an efficiency close to 100% is expected for high performance Li-air batteries. However, low 9



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round-trip efficiency is still a main challenge for Li-air batteries,3 and worldwide scientists are making their efforts to improve it. In the present work, the batteries with SnO2@C and SnO2 demonstrate round-trip efficiency of 62.0% and 60.5%, respectively, both are higher than that of 56.5% for the Super P based battery. Moreover, the discharge capacities of the batteries with SnO2 and SnO2@C as cathode catalysts are twice higher than that of the Super P based battery, and the capacity of the SnO2@C based battery is evidently higher by 780 mA h g-1 (7559 vs. 8339 mA h g-1) than the SnO2 based battery. These results indicate that both SnO2 and SnO2@C are active bifunctional catalysts for cathode reactions in Li-air batteries, while SnO2@C is more efficient and has higher activity. When the current density is increased to as high as 200 mA g-1, the capacities of both batteries with SnO2 and SnO2@C maintain drastically higher than that of the Super P based battery as shown in Fig. 6c, and the difference between the capacities of SnO2 and SnO2@C based batteries raised to be 1458 mA h g-1, almost doubles that at 75 mA g-1. But interestingly, the capacity retentions of SnO2@C and SnO2 based batteries, 70.89% and 58.92%, respectively, are seriously lower than that of 88.5% for Super P based battery. The actual reason for the low capacity retention is unclear at present, more detailed research should be conducted in the future to understand and improve it. Cycling performance of the batteries with pure Super P, SnO2 and SnO2@C based cathodes were evaluated at current densities of 75 and 200 mA g-1 with a controlled capacity of 1000 mA h g-1, the results are demonstrated in Fig. 7 & 8. At 75 mA g-1, 10


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the batteries with SnO2 and SnO2@C can run successfully for 27 and 31 cycles, respectively, before the discharge terminal voltage falling down to below 2.0 V, while that with pure Super P cathode can only sustain 22 cycles with the highest charge terminal voltage. When the current density is increased to 200 mA g-1, the battery with pure Super P cathode can only survive 12 cycles with a discharge terminal voltage above 2.0 V, compared to that of 17 and 25 cycles, respectively, for the batteries with SnO2 and SnO2@C. The cycling ability of SnO2@C based battery decreased 6 cycles, which is only sixty percent of the SnO2 and pure Super P based battery with a 10-cycle-decay. The battery with SnO2@C not only exhibits the best cycling ability, but also keeps the lowest charge terminal voltage. To understand the superior performance of SnO2@C as cathode catalyst, EIS spectra of the batteries based on pure Super P, SnO2 and SnO2@C were captured and displayed as Nyquist plots in Fig. 6d, where the intercept on horizontal axis in high-frequency region represents the internal resistance of the batteries and the diameter

of

the

semicircle

indicates

the

polarization

resistance

of

the

charge-transfer-reactions. It is observed that the battery with highly conductive Super P exhibits the lowest internal resistance, and the one with semiconductive SnO2 demonstrates the highest resistance, while the resistance of the SnO2@C based battery is between them. This implies that the conductivity of SnO2@C is higher than SnO2 because of the surface carbon coating. Moreover, the diameter of the semicircles decrease in the sequence of Super P, SnO2 and SnO2@C, suggesting the fastest cathode reaction in the SnO2@C based battery. This is in line with the discussion 11



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above with CV curves. SEM images and XRD patterns of SnO2 and SnO2@C based cathodes at various states of fresh, discharged and charged were also captured to gain further insight into the outstanding performance of SnO2@C as the catalyst. Greatly different morphologies of discharge products could be seen in Fig. 9. Both flocculent and wool-ball like particles could be clearly observed in the discharged SnO2 based cathode with the flocculent ones on the surface, probably implying that the discharge product grows from flocculent into wool-ball like particles with increase in the depth-of-discharge. While only toroid like particles could be found in the discharged SnO2@C based cathode. This result agrees well with the opinions by Ma et al.,43 Oh et al.44 and Viswanathan et al.45 that category and morphology of the cathode products can generally be affected by the catalysts used in Li-air batteries. For the recharged cathodes, the morphology of the SnO2@C based one recovers to almost the same as the fresh electrode, while that of the SnO2 based one is largely different from the fresh electrode, many discharge products with different morphology from that in the discharged cathode could be clearly seen, maybe hinting that the morphology of discharge product changes during its decomposition in the charge process. The morphology transfer during discharge-charge processes has also been observed by Zhai et al.46 These results elucidate that the ORR/OER on SnO2@C based cathode proceeds more reversibly than that on SnO2 based cathode. However, none characteristic peaks corresponding to the discharge products, lithium peroxide Li2O2, could be observed in the XRD pattern of recharged SnO2 based cathode as shown in Fig. 10, even though Li2O2 is evidently demonstrated in both discharged SnO2 and 12


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SnO2@C based electrodes. This is probably because of low crystallinity of Li2O2 during its decomposition in the charge process or its low content in the whole recharged SnO2 based electrode. Therefore, the outstanding performance of SnO2@C as cathode catalyst of Li-air batteries can mainly be ascribed to the improved conductivity, the enhanced catalytic kinetics and its reversibility, and the enlarged surface area and internal space resulted from the carbon coating. It drastically enhanced the conductivity of the semiconductive SnO2 which is essential for electro-catalysts,43 although this carbon coating strategy generally leads to a concession between capacity and cycle property in lithium ion batteries,47-48 it performs well in Li-air batteries owing to different mechanisms. Moreover, during the heat-treatment in argon to prepare SnO2@C nanoparticles, the carbonization of glucose on the surface of SnO2 accelerated the porous surface, complex pore distribution, large internal space and huge surface area, leading to the result that: (1) more discharge product could be stored in the cathode resulting in improved discharge capacity; (2) more active sites could be exposed on the surface leading to enhanced ORR/OER kinetics and reversibility; (3) the complex pore structure is beneficial for facilitating oxygen diffusion and improving electrolyte wettability leading to fast accumulation and decomposition of discharge products in the cathode and improved cycling performance. However, when compared with the commercial Pt/C catalyst, which is active to many chemical reactions, the performance of SnO2@C is still a little lower as shown in Fig. 11, and much more detailed research should be conducted in the future to further improve the catalytic 13



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performance and accelerate its practical applications in Li-air batteries. 4. Conclusions SnO2 and SnO2@C that widely used in lithium ion batteries were explored for the first time as cathode catalysts of Li-air batteries. Both of them have been found to be efficient towards the oxygen reduction and evolution reactions, but the carbon coated material of SnO2@C demonstrated evidently better performance, including lower overpotential, larger discharge capacity and enhanced cycling performance. This could be ascribed to the peculiar characteristics of higher conductivity, larger surface area and internal space, and the complex pore structure/distribution in SnO2@C. This result hints a new direction for the development of cathode catalyst of Li-air batteries.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest.

Acknowledgments The authors are grateful for the financial support of this work by National Key Basic Research Program of China (No. 2014CB932303) and the National Natural Science Foundation of China (21176155 & 21476138).

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18


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19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure Captions Fig. 1

Schematic illustration of the calcination during the synthesis of SnO2 and SnO2@C

Fig. 2

XRD patterns of as-prepared SnO2 and SnO2@C

Fig. 3

TG curve of SnO2@C in air flow with a heating rate of 5 oC min-1

Fig. 4

SEM images of SnO2 (a) and SnO2@C (b); HRTEM images of SnO2 (c) and SnO2@C (d)

Fig. 5

N2 adsorption-desorption isotherms and the calculated pore size distributions of SnO2 (a, c) and SnO2@C (b, d)

Fig. 6

CV curves of pure Super P, SnO2 and SnO2@C based cathodes in oxygen saturated 1 M LITFSI/TEGDME electrolyte at a potential scanning rate of 0.5 mV s-1 (a); Discharge-charge profiles of Li-air batteries with pure Super P, SnO2 and SnO2@C based cathodes at a current density of 75 mA g-1 (b); Capacities of Li-air batteries with pure Super P, SnO2 and SnO2@C based cathodes at a high current density of 200 mA g-1 (c); Nyquist plots of Li-air batteries with pure Super P, SnO2 and SnO2@C based cathodes (d)

Fig. 7

Cycling performance of Li-air batteries with pure Super P (a, d), SnO2 (b, e) and SnO2@C (c, f) based cathodes at a current density of 75 mA g-1 with a controlled capacity of 1000 mAh g-1

Fig. 8

Cycling performance of Li-air batteries with pure Super P (a, d), SnO2 (b, e) and SnO2@C (c, f) based cathodes at a current density of 200 mA g-1 with a controlled capacity of 1000 mAh g-1 20


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Fig. 9

SEM images of SnO2 (a, b, c) and SnO2@C (d, e, f) based cathodes at various states

Fig. 10

XRD patterns of SnO2 and SnO2@C based cathodes at various states

Fig. 11

RDE polarization curves of SnO2@C and Pt/C in 1 M LITFSI/TEGDME electrolyte at a disk rotating rate of 1600 rpm

21



TOC

5.0 4.5

Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

4.0 3.5 3.0

500¡æ

500¡æ

Air

Ar

SnO2

C6HxOy

C

2.5 2.0 0

3000

6000

9000 -1

Specific capacity / mA h g

22


Page 23 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


500℃×4h

500℃×4h

Air

SnO2

Fig. 1

Ar

C6HxOy


C


SnO2@C

Intensity / a.u

SnO2

20

40

50

2

Fig. 2

60 


(202)

(310) (112) (301)

(202)

(220)

(211)

(101)

30

(200) (111)

SnO2-PDF#41-1445 (110)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 24 of 33

70

80

Page 25 of 33

105

Weight percent / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


0.69%

100 95

17.20%

90 85 SnO2@C

80 75

0

200

400

600

Temperature / °C

Fig. 3


800

1000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

a

b

c

d

Fig. 4


Page 26 of 33

Page 27 of 33

200

a

b

Adsorption Desorption

150

Adsorption Desorption

150

3

-1

Pore volume / cm g STP

200

100

100

50

50

0

0.0

0.025

0.4

0.6

0.8

0

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure / P/P0

0.010

d

c 0.008

0.020 0.006

3

-1

0.2

Relative pressure / P/P0

0.030

dV/dr / m g -nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


0.015 0.004

0.010 0.002

0.005 0.000

1

10

100

0.000

1

Pore radius / nm

Fig. 5

10

Pore radius / nm ACS Paragon Plus Environment

100


5.0

a

b

-0.4

4.5

-0.2

4.0

Voltage / V

Current density / mA cmgeo

-2

-0.6

0.0 0.2 0.4

Super P SnO2

0.6

SnO2@C

0.8

3.5

Super P SnO2

3.0

SnO2@C

2.5 2.0

2.0

2.5

3.0

3.5

4.0

0

4.5

3000

6000

9000 -1

Specific capacity / mA h g

Potential / V (vs. Li metal) 350

10000

c 8000

Super P SnO2

300

SnO2@C

250

6000

Z'' / 

Specific capacity / mA h g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 28 of 33

4000

SnO2 Super P

200 150 100

2000 0

50

75

200

0

0

50

-1

Current density / mA g

Fig. 6

d

SnO2@C


100

150

200

Z' / 

250

300

350

400

Page 29 of 33

5.0

Voltage / V

b

a

4.5

c

4.0 1st 15th 23th

3.5 3.0

5th 20th

1st 15th 27th

10th 22rd

5th 20th 28th

10th 25th

1st 15th 30th

5th 20th 31st

10th 25th 32nd

2.5 2.0 1.5

0

5

200

400 600 800 -1 Capacity / mA h g

1000

d

200

400 600 800 -1 Capacity / mAh g

1000

200

400 600 800 -1 Capacity / mA h g

1100

f

-1

e

1000

4 1000

3

2 0

5

Fig. 7

10 15 Cycle number

20

25

5

10 15 20 Cycle number

25


30

5

10


30

900 35

Capacity / mAh g

Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47



5.0

b

a

4.5

c

Voltage / V

4.0 1st 12nd

3.5 3.0

5th 13rd

1st 15th

10th

5th 17th

10th 18th

1st 15th 26th

5th 20th

10th 25th

2.5 2.0 1.5

0

200

400 600 800 -1 Capacity / mAh g

1000

d

200

400 600 800 -1 Capacity/mAh g

1000

200

400 600 800 -1 Capacity/mAh g

1100

f

e

1000

4 1000

3

2 0

Fig. 8

5 10 Cycle number

15

5

10 15 Cycle number


20

5


25

900 30

Capacity / mAh g

-1

5 Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


a

b

d

e

fresh

Fig. 9

c

f

discharged


recharged


 





 SnO2



Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 32 of 33

 Li2O2 SnO2@C recharged SnO2@C discharged SnO2@C fresh SnO2 recharged SnO2 discharged SnO2 fresh

30

40

50

60

2

Fig. 10




70

80

Current density / mA cmgeo

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


-2

Page 33 of 33

0.0

-0.1

-0.2

SnO2@C Pt/C

-0.3

-0.4

1.8

2.1

2.4

2.7

Potential / V (vs. Li metal)

Fig. 11


3.0

A SnO2-Based Cathode Catalyst for Lithium-Air Batteries - ACS

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