- Email: [email protected]
Ambient lithium–air battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
Ambient lithiumeair battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium Yanli Ruan, Limei Yu, Shidong Song*, Xuhui Qin, Jian Sun, Wanjun Li, Butian Chen School of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin, 300387, China
highlights Non-carbon-based trode
shows
graphical abstract Ru/B4C
good
elec-
ORR/OER
activity. Ru/B4C
effectively
decompose
Li2CO3 on charge. High stable Ru/B4C can be oxidation-resistive. Li-air batteries exhibit comparable performance in air to that in O2. Stable and versatile Ru/B4C oxygen electrode is promising.
article info
abstract
Article history:
Li-air batteries (LABs) operated in ambient air containing moisture and CO2 highly desire
Received 14 August 2019
the oxygen electrodes to have capability of Li2CO3 and LiOH decomposition and electro-
Received in revised form
chemical stability. Here we report the application of a stable non-carbon based oxygen
29 September 2019
electrode based on boron carbide supported ruthenium (Ru/B4C) for ambient LABs. LABs
Accepted 8 October 2019
using Ru/B4C deliver a discharge capacity of 2689 mA h g1 and voltage plateaus of 2.7 V
Available online xxx
and 3.8 V for discharge and charge process, respectively at 0.1 mA cm2, which are comparable to those for Ru/B4C-based LieO2 battery (2796 mA h g1, 2.8 V and 3.7 V, respec-
Keywords:
tively). Under limited capacity of 300 mA h g1, LAB exhibits 45 stable cycles, close to the 50
Lieair batteries
cycles for its LieO2 battery counterpart. The typical product for the first discharge for LAB is
Li2CO3 decomposition
the mixture of Li2CO3 and Li2O2 with relative content ratio of 62:38, which cannot be
Non-carbon electrode
detected after the first charge. The non-carbon based Ru/B4C oxygen electrode provides a
Ruthenium
promising approach for the stable operation of LABs in ambient air.
Boron carbide
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (S. Song). https://doi.org/10.1016/j.ijhydene.2019.10.050 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Ruan Y et al., Ambient lithiumeair battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.050
2
international journal of hydrogen energy xxx (xxxx) xxx
Introduction Aprotic Li-air batteries (LABs) have attracted world-wide attention due to their potential to offer energy densities 3e5 times that of the state-of-the-art Li-ion batteries [1e4]. However, most of current LABs are studied in dry and pure O2 as moisture and CO2 from atmosphere can incur severe parasitic reactions and alter discharge product formation, even cause safety issues [5]. So far, the reversible operation of ambient LABs is still a considerable challenge [6]. The impact of water and CO2 contamination on the discharge and charge processes of LABs is intricate (see Eqns. S1-10) and inevitable [7,8]. Side products like Li2CO3 and LiOH may exist in the discharge product and should be efficiently cleaned up during cycling. Thus, it is highly desirable to have oxygen electrodes with special activity for the decomposition of Li2CO3 and LiOH for ambient LABs [9], as illustrated in Fig. 1a. Such oxygen electrodes possess versatile functions in contrast to the conventional bifunctional oxygen electrode for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Ru can effectively reduces the overpotential for Li-extraction from the Li2O2 [10,11], which is also recognized as the ratedetermining steps for Li2CO3 and LiOH decomposition [12]. Accordingly, Ru-based electro-catalysts can be potential oxygen electrode materials for LABs. However, most of the Ru-based oxygen catalysts are supported by thermodynamically unstable carbon materials and applied in LABs using pure O2, which are denominated as LieO2 batteries. In our previous work, B4C
exhibited good bifunctional activity and stability for ORR and OER in LieO2 batteries [13]. Nevertheless, it lacks sufficient catalytic capability towards decomposing side products like Li2CO3, LiOH and alkyl carbonates, which arise from corrosion of unstable electrolytes and carbon materials [9]. Subsequently, B4C was applied as non-carbon based substrate for Ir- and Rudeposited B4C (Ir(Ru)/B4C) for LieO2 batteries, which both exhibited excellent activities for side product decomposition and accordingly extended the cycle life of LieO2 batteries [9,14]. Considering the harsh environment for LABs in contrast to LieO2 batteries, even if corrosion problem can be overcome by developing stable electrodes and electrolytes, the formation of Li2CO3 and LiOH still cannot be avoided. The versatile oxygen electrodes would accelerate the technique transfer from LieO2 batteries to LABs. Here we report for the first time the application of Ru/B4C for ambient LABs. Ru/B4C oxygen electrodes exhibit significant activity and stability for ORR, OER and decomposition of side product. Accordingly, LABs using Ru/B4C electrodes deliver comparably high rate capacity and cycle performance in ambient air to those for LieO2 batteries in pure O2.
Experimental Materials and preparation B4C and RuCl3$xH2O were commercial products purchased from Macklin Biochemical (Shanghai) and Shanghai Civic Chemical Technology Co. Ltd, respectively. Ru/B4C was
Fig. 1 e Schematic of Ru/B4C with versatile functions (a); TEM images of Ru/B4C under low (b) and high (c) magnifications; CV curves of Lieair cells under various gas atmosphere. Please cite this article as: Ruan Y et al., Ambient lithiumeair battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.050
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synthesized by depositing Ru nanoparticles (NPs) on B4C via a modified hydrothermal synthesis method [15]. The oxygen electrodes for Lieair cells were prepared according to the method in [14]. PVDF and Ni foam was used as the binder and current collector, respectively. The typical loading of the oxygen electrode was 0.8e1.2 mg(Ru/B4C) cm2.
3
respectively corresponding to ORR and OER process. During cathodic scanning, the LieO2 cell exhibits a relatively bigger current density and an earlier onset potential (2.8 V) than the LAB (2.7 V), probably due to the higher oxygen concentration at electrolyte/electrode boundary.
Effect of various atmosphere on rate performance Electrochemical evaluation LABs were assembled as coin cells (CR2032) with holes on positive cases in an argon filled glovebox using Ru/B4C oxygen electrode, metallic lithium anode and 100 mL 1 M LiTFSI/tetraglyme electrolyte. No oxygen-selective membrane (OSM) or gas diffusion layer (GDL) was applied. The assembled LABs were tested using ambient air dried by molecular sieves. The discharge and charge tests of LABs were carried out on the Lanhe battery tester within 2.0e4.2 V at room temperature. The specific capacity was based on the total mass of cathode material (catalyst þ binder). Electrochemical impedance spectroscopy (EIS) was conducted by a CHI 660 electrochemical work station in the frequency range of 1 MHz to 0.1 Hz at open circuit voltage with a 5 mV amplitude. Cyclic voltammetric (CV) scanning at 0.5 mV s1 for LABs were respectively measured in Ar, air and pure O2 using CHI 660 as well.
Characterizations Electrode samples for XRD analysis were tested by a Rigaku D/ max-2500 X-ray diffractometer using Cu Ka radiation. TEM investigation was conducted using a Hitachi H7650. The morphologies of the electrodes were examined by fieldemission SEM (Hitachi S4800). FT-IR measurement of the electrodes was performed with a Nicolet 6700.
Results Characterizations of catalysts To investigate the structure and morphology of Ru/B4C catalyst, high resolution TEM was conducted. Fig. 1b shows that the ultrafine Ru nanoparticles (NPs) are uniformly dispersed on B4C without apparent agglomeration. B4C particles present flat surfaces and have an average size of 50 nm. Ru NPs have a narrow distribution (0.5e2.3 nm) and a mean size of 1.3 nm (Fig. S1). The contact between Ru and B4C looks tight (Fig. 1c). The intimate interaction between Ru catalyst and B4C support is conducive to exerting synergistic effect during the discharge and charge processes of LABs. Ru NPs present well-matched distances of Ru(002) and Ru(101) crystal planes and B4C particles show a B4C(021) crystal plane. Fig. 1d shows the cyclic voltammetric (CV) curves of the LABs scanning at 0.5 mV s1 within 2.0e4.4 V under various gas atmosphere. For the CV test in Ar, there is no obvious redox peak, indicating that Ru/ B4C electrode has a high electrochemical stability. Both the Liair and LieO2 cells exhibit significant redox peaks,
The rate performances of LABs and LieO2 batteries were measured at various current densities to evaluate their performance difference in ambient air and pure O2. Fig. 2aec exhibit the initial discharge and charge capacities of LABs and LieO2 batteries at various current density. The discharge voltage plateaus for LABs are slightly lower than those for their LieO2 counterparts, in accordance with the ORR behavior revealed by CV in Fig. 1d. In Fig. 2a, LABs deliver a discharge capacity of 2689 mA h g1 and discharge and charge voltage plateaus of 2.7 V and 3.8 V, respectively, which are comparable to those for LieO2 battery (2796 mA h g1, 2.8 V and 3.7 V, respectively), suggesting that Ru/B4C electrode achieves comparable ORR and OER activity in air to that in pure O2. It is also worth pointing out that the discharge curve for LieO2 battery exhibits a typical character of ’sudden death’, which may be ascribed to the surface passivation. In contrast, LAB presents an inclined discharge plateau, indicating a continuous passivation and impedance rise under ambient air. The poor reversibility of Li2CO3 and LiOH may degrade the cycle performance of LABs. However, the comparable capacity and overpotential for LAB to those for LieO2 batteries at 0.1 mA cm2 demonstrates that ambient air has less impact on battery performance at low rate with the aid of Ru/B4C versatile oxygen electrode. Fig. 2b and c shows that the performance difference between LABs and LieO2 batteries becomes significant at high current densities. It is speculated that high current speeds up the parasitic reactions and the side product formation. On one hand, a large number of highly þ 2 active superoxide/peroxide species (O 2 , O2 ) and Li may be produced rapidly at high rate during cell discharge and directly react with CO2 and H2O to form Li2CO3 and LiOH, respectively, as shown in Eqns. 1e5 [16,17]. O 2 þ CO2 / CO4
(1)
Liþ þ CO 4 / LiCO4
(2)
LiCO4 þ 3Liþ þ CO2 þ 3e / 2Li2CO3
(3)
þ O 2 þ H2O þ Li / HO2 þ LiOH
(4)
þ HO 2 þ H2O þ Li / H2O2 þ LiOH
(5)
On the other hand, high current requires fast oxygen transport, which may cause difficulty in mass transfer for LABs. The impedance behavior of LAB and LieO2 battery is shown in Fig. 2d. The high frequency intercept of the impedance data gives the total ohmic resistance of the cell (designated Rb). The distance between the intercepts at high and low
Please cite this article as: Ruan Y et al., Ambient lithiumeair battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.050
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Fig. 2 e Initial discharge and charge voltage profiles of LABs and LieO2 batteries within 2.0e4.2 V at 0.1 mA cm¡2 (a), 0.2 mA cm¡2 (b) and 0.3 mA cm¡2 (c), and the Nyquist plots (d).
frequency gives the electrode polarization resistance (designated Rp). LAB shows a similar Rb but a slightly higher Rp than LieO2 battery, indicating the relatively more sluggish ORR in ambient air, probably due to the low oxygen concentration in air and the intervention of side reactions.
Cycle performance The cycle performances of LABs and LieO2 batteries were tested under different rates and capacity limiting protocols to assess the versatile activity and stability of Ru/B4C. Fig. 3a and b shows the full discharge/charge voltage profiles for LABs and LieO2 batteries, respectively between 2.0 and 4.2 V at 0.1 mA cm2. Both LAB and LieO2 battery deliver similar discharge and charge capacities for each cycle. The similar discharge capacities and degree of attenuation for both batteries explicitly indicate that Ru/B4C can not only enable ORR and OER to reach a comparable performance in ambient air to that in pure O2, but also efficiently decompose side products during charge. Fig. 3cef shows the discharge/charge voltage profiles of LABs and LieO2 batteries cycled under a limited capacity of 300 mA h g1 at 0.1 mA cm2 within the voltage range of 2.0e4.2 V. The LAB stably delivers the controlled discharge and charge capacity for 45 cycles (Fig. 3c), close to LieO2 battery that exhibits 50 cycles (Fig. 3d). The discharge voltages for the initial several cycles are slightly lower than those for the following cycles for LAB (Fig. 3e), in contrast with those for LieO2 battery (Fig. 3f), implying an activation process may be required in ambient air. As a non-carbon based oxygen electrode material, Ru/B4C should be adequately stable during
battery cycling [13,14]. The performance decay is likely ascribed to the electrolyte decomposition triggered by the active oxygen species during charging, which may result in the concomitant formation of side products with their decomposition [18]. Under the limited capacity of 200 mA h g1, the LAB can be stably cycled for 70 cycles (Figs. S2a and b), far longer than the cell tested under 300 mA h g1. However, under 500 mA h g1, the LAB exhibits shorter stable cycles (Figs. S2c and d). At high current density of 0.2 mA cm2 (Figs. S3a-c) and 0.3 mA cm2 (Figs. S3d-f), the LABs deliver lower capacity and cycle performance than those at 0.1 mA cm2 (Fig. 3a and c). The results indicate that Ru/B4C versatile oxygen electrode can enable LABs to achieve a good reversibility under relatively low rate and capacity.
Characterizations of oxygen electrodes The morphology and structure of Ru/B4C oxygen electrode under different state of charge was characterized to reveal the formation and decomposition of dischage product during cycling. The surface of before-testing Ru/B4C electrode has a hierarchically porous structure (Fig. 4a). After the first discharge, the electrode surface is packed by a solid film which is composed of overlapped microsheets (Fig. 4b). In addition, the solid film appears to be poorly conducting under SEM observation as demonstrated by the white color (serious charging effect). After the first charge, the solid surface film disappears (Fig. 4c), manifesting the electrode surface has been cleaned up on charge. After 45 cycles, no surface film can
Please cite this article as: Ruan Y et al., Ambient lithiumeair battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.050
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Fig. 3 e Discharge/charge voltage profiles of LAB (a, c, e) and LieO2 batteries (b, d, f) in the full discharge/charge mode within the voltage range of 2.0e4.2 V.
be observed as well (Fig. 4d). Fig. 4e shows the XRD patterns corresponding to the electrodes in various state in Fig. 4aed. The XRD pattern of the Ru/B4C electrode after the first discharge identifies that the discharge product is mainly a mixture consisting of Li2CO3 and Li2O2 with relative content ratio of 62 : 38. LiOH is not detected as it may be converted to Li2CO3 in the presence of CO2 during discharge. After the first charge, the diffraction peaks of Li2O2 and Li2CO3 cannot be found, indicating the Ru/B4C oxygen electrode can efficiently decompose the discharge product and Li2CO3 side product. However, after 45 cycles the diffraction peaks of Li2CO3 residue are observed, suggesting that Li2CO3 cannot be fully eliminated after a long-term cycling. The performance decay may be caused by the Li2CO3 formation during charge due to the oxidation of unstable electrolyte [18]. The application of stable electrolytes, such as solid-state electrolytes, molten salts and ionic liquids may eliminate the formation of Li2CO3 during charging [6,20]. If so, the Ru/B4C versatile oxygen
electrode may enable LABs to achieve a long-term cycling under relatively high rate and capacity. FTeIR spectra in Fig. 4f verifies the analysis of XRD patterns in Fig. 4e. Li2CO3 can be effectively removed in the first charge but remains after 45 cycles, as revealed by the IR absorption peaks at 850 cm1 and 1440 cm1, which corresponds to the vibration strengths of CeO and C]O, respectively. The impedance behavior for the LAB in different state shows that Rp can basically return to the initial value after the first charge, while increases largely after more cycles due to the accumulation of side product (Fig. S4).
Discussion So far, LABs are still in their infancy, mainly due to the lack of stable versatile oxygen electrodes and electrolytes, which incurs detrimental side reactions. Non-carbon based oxygen electro-catalysts can not only avoid carbon corrosion and
Please cite this article as: Ruan Y et al., Ambient lithiumeair battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.050
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Fig. 4 e SEM images of Ru/B4C oxygen electrodes (a) before testing, (b) after the first discharge, (c) after the first charge, and (d) after 45 cycles under 300 mA h g-1 at 0.1 mA cm-2, and the corresponding (e) XRD patterns and (f) FT-IR spectra.
serve reliable electrode performance, but also be stable reference materials for screening usable electrolytes [19]. Recently, a non-carbon based LixNiO2 composite cathode along with stable inorganic electrolyte of LiNO3/KNO3 eutectic molten salt enabled LieO2 batteries to achieve 150 highly efficient cycles via a reversible four-electron pathway [20]. Based on a non-carbon based molybdenum disulfide cathode and an ionic liquid/dimethyl sulfoxide electrolyte, a LAB could be stably operated for up to 700 cycles in a simulated air atmosphere [6]. It is undoubted that non-carbon based oxygen electrodes play significant roles for achieving durable LABs. In terms of ORR/OER activity and chemical stability, Ru/B4C demonstrates comparable performance to those above mentioned catalysts. Furthermore, B4C is a lightweight material (~2.5 g cm3 in density), which can provide a high mass specific energy. The high ORR/OER activity for Ru/B4C is probably due to the synergic effect between Ru catalyst and B4C substrate. Most recently, a similar Pd/B4C oxygen electrocatalyst was reported with a highly active and durable ORR performance for Zn-air batteries (ZABs) [21]. A supporting first-principle computation was conducted and revealed the
synergistic effect between B4C and Pd could significantly promote O2 adsorption and splitting, as well as the transformation of O* to OH [21]. With a view to the practical application of LABs for transportation and stationary applications [22e24], LABs are strongly impacted by environment, such as weather (temperature), climate (humidity), and oxygen partial pressure [25], which may deteriorate the oxygen electrode structure and further destroy electrolyte. B4C is refractory and super hard (comparable to diamond and boron nitride), demonstrating robust mechanical property and high environment tolerance, which can ensure a consistent structure to confine catalysts and exert the synergetic effect.
Conclusion In summary, Ru/B4C exhibits comparably high activity and stability for ORR and OER in ambient air to those in pure O2, which is conducive to the transfer from LieO2 batteries to LABs. The features of excellent catalytic activity, chemical and
Please cite this article as: Ruan Y et al., Ambient lithiumeair battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.050
international journal of hydrogen energy xxx (xxxx) xxx
electrochemical stability, and environment tolerance make Ru/ B4C suitable for practical application of LABs in ambient air.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21878232), Natural Science Foundation of Tianjin (No. 19JCYBJC21500), and the Program for Tianjin Distinguished Professor.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.050.
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[11] Wang Y, Liang Z, Zou Q, Cong G, Lu Y. Mechanistic insights into catalyst-assisted nonaqueous oxygen evolution reaction in lithium-oxygen batteries. J Phys Chem C 2016;120:6459e66. [12] Ling C, Zhang R, Takechi K, Mizuno F. Intrinsic barrier to electrochemically decompose Li2CO3 and LiOH. J Phys Chem C 2014;118:26591e8. [13] Song S, Xu W, Cao R, Luo L, Mark HE, Mark EB, Liu B, Esteveza L, Wang CM, Zhang JG. B4C as a stable non-carbonbased oxygen electrode material for lithium-oxygen batteries. Nano Energy 2017;33:195e204. [14] Song S, Yu L, Ruan Y, Sun J, Chen B, Xu W, Zhang J-G. Highly efficient Ru/B4C multifunctional oxygen electrode for rechargeable Li-O2 batteries. J Power Sources 2019;413:11e9. [15] Liao S, Holmes K-A, Tsaprailis H, Birss VI. High performance PtRuIr catalysts supported on carbon nanotubes for the anodic oxidation of methanol. J Am Chem Soc 2006;128:3504e5. [16] Yin W, Grimaud A, Lepoivre F, Yang C, Tarascon JM. Chemical vs electrochemical formation of Li2CO3 as a discharge product in Li-O2/CO2 batteries by controlling the superoxide intermediate. J Phys Chem Lett 2017;8:214e22. [17] Wu S, Tang J, Li F, Liu X, Zhou H. Low charge overpotentials in lithium-oxygen batteries based on tetraglyme electrolytes with a limited amount of water. Chem Commun 2015;51:16860e3. [18] Mahne N, Schafzahl B, Leypold C, Leypold M, Grumm S, Leitgeb A, Strohmeier GA, Wilkening M, Fontaine O, Kramer D, Slugovc C, Borisov SM, Freunberger SA. Singlet oxygen generation as a major cause for parasitic reactions during cycling of aprotic lithium-oxygen batteries. Nat Energy 2017;2:17036. [19] Abraham KM. Electrolyte-directed reactions of the oxygen electrode in lithium-air batteries. J Electrochem Soc 2015;162(2):A3021e31. [20] Xia C, Kwok CY, Nazar LF. A high-energy-density lithiumoxygen battery based on a reversible four-electron conversion to lithium oxide. Science 2018;361:777e81. [21] Chen Y-N, Zhang X, Cui H, Zhang X, Xie Z, Wang X-G, Jiao M, Zhou Z. Synergistic electrocatalytic oxygen reduction reactions of Pd/B4C for ultrastable Zn-air batteries. Energy Storage Materials 2018;15:226e33. [22] Kyriakopoulos GL, Arabatzis G, Tsialis P, Ioannou K. Electricity consumption and RES plants in Greece: typologies of regional units. Renew Energy 2018;127:134e44. [23] Arabatzis G, Kyriakopoulos G, Tsialis P. Typology of regional units based on RES plants: the case of Greece. Renew Sustain Energy Rev 2017;78:1424e34. [24] Kyriakopoulos GL, Arabatzis G. Electrical energy storage systems in electricity generation: energy policies, innovative technologies, and regulatory regimes. Renew Sustain Energy Rev 2016;56:1044e67. [25] Yang W, Kim DY, Yang L, Li N, Tang L, Amine K, Mao H-K. Oxygen-rich lithium oxide phases formed at high pressure for potential lithiumeair battery electrode. Advanced Science 2017;4(9):1600453.
Please cite this article as: Ruan Y et al., Ambient lithiumeair battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.050
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
Ambient lithiumeair battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium Yanli Ruan, Limei Yu, Shidong Song*, Xuhui Qin, Jian Sun, Wanjun Li, Butian Chen School of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin, 300387, China
highlights Non-carbon-based trode
shows
graphical abstract Ru/B4C
good
elec-
ORR/OER
activity. Ru/B4C
effectively
decompose
Li2CO3 on charge. High stable Ru/B4C can be oxidation-resistive. Li-air batteries exhibit comparable performance in air to that in O2. Stable and versatile Ru/B4C oxygen electrode is promising.
article info
abstract
Article history:
Li-air batteries (LABs) operated in ambient air containing moisture and CO2 highly desire
Received 14 August 2019
the oxygen electrodes to have capability of Li2CO3 and LiOH decomposition and electro-
Received in revised form
chemical stability. Here we report the application of a stable non-carbon based oxygen
29 September 2019
electrode based on boron carbide supported ruthenium (Ru/B4C) for ambient LABs. LABs
Accepted 8 October 2019
using Ru/B4C deliver a discharge capacity of 2689 mA h g1 and voltage plateaus of 2.7 V
Available online xxx
and 3.8 V for discharge and charge process, respectively at 0.1 mA cm2, which are comparable to those for Ru/B4C-based LieO2 battery (2796 mA h g1, 2.8 V and 3.7 V, respec-
Keywords:
tively). Under limited capacity of 300 mA h g1, LAB exhibits 45 stable cycles, close to the 50
Lieair batteries
cycles for its LieO2 battery counterpart. The typical product for the first discharge for LAB is
Li2CO3 decomposition
the mixture of Li2CO3 and Li2O2 with relative content ratio of 62:38, which cannot be
Non-carbon electrode
detected after the first charge. The non-carbon based Ru/B4C oxygen electrode provides a
Ruthenium
promising approach for the stable operation of LABs in ambient air.
Boron carbide
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (S. Song). https://doi.org/10.1016/j.ijhydene.2019.10.050 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Ruan Y et al., Ambient lithiumeair battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.050
2
international journal of hydrogen energy xxx (xxxx) xxx
Introduction Aprotic Li-air batteries (LABs) have attracted world-wide attention due to their potential to offer energy densities 3e5 times that of the state-of-the-art Li-ion batteries [1e4]. However, most of current LABs are studied in dry and pure O2 as moisture and CO2 from atmosphere can incur severe parasitic reactions and alter discharge product formation, even cause safety issues [5]. So far, the reversible operation of ambient LABs is still a considerable challenge [6]. The impact of water and CO2 contamination on the discharge and charge processes of LABs is intricate (see Eqns. S1-10) and inevitable [7,8]. Side products like Li2CO3 and LiOH may exist in the discharge product and should be efficiently cleaned up during cycling. Thus, it is highly desirable to have oxygen electrodes with special activity for the decomposition of Li2CO3 and LiOH for ambient LABs [9], as illustrated in Fig. 1a. Such oxygen electrodes possess versatile functions in contrast to the conventional bifunctional oxygen electrode for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Ru can effectively reduces the overpotential for Li-extraction from the Li2O2 [10,11], which is also recognized as the ratedetermining steps for Li2CO3 and LiOH decomposition [12]. Accordingly, Ru-based electro-catalysts can be potential oxygen electrode materials for LABs. However, most of the Ru-based oxygen catalysts are supported by thermodynamically unstable carbon materials and applied in LABs using pure O2, which are denominated as LieO2 batteries. In our previous work, B4C
exhibited good bifunctional activity and stability for ORR and OER in LieO2 batteries [13]. Nevertheless, it lacks sufficient catalytic capability towards decomposing side products like Li2CO3, LiOH and alkyl carbonates, which arise from corrosion of unstable electrolytes and carbon materials [9]. Subsequently, B4C was applied as non-carbon based substrate for Ir- and Rudeposited B4C (Ir(Ru)/B4C) for LieO2 batteries, which both exhibited excellent activities for side product decomposition and accordingly extended the cycle life of LieO2 batteries [9,14]. Considering the harsh environment for LABs in contrast to LieO2 batteries, even if corrosion problem can be overcome by developing stable electrodes and electrolytes, the formation of Li2CO3 and LiOH still cannot be avoided. The versatile oxygen electrodes would accelerate the technique transfer from LieO2 batteries to LABs. Here we report for the first time the application of Ru/B4C for ambient LABs. Ru/B4C oxygen electrodes exhibit significant activity and stability for ORR, OER and decomposition of side product. Accordingly, LABs using Ru/B4C electrodes deliver comparably high rate capacity and cycle performance in ambient air to those for LieO2 batteries in pure O2.
Experimental Materials and preparation B4C and RuCl3$xH2O were commercial products purchased from Macklin Biochemical (Shanghai) and Shanghai Civic Chemical Technology Co. Ltd, respectively. Ru/B4C was
Fig. 1 e Schematic of Ru/B4C with versatile functions (a); TEM images of Ru/B4C under low (b) and high (c) magnifications; CV curves of Lieair cells under various gas atmosphere. Please cite this article as: Ruan Y et al., Ambient lithiumeair battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.050
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synthesized by depositing Ru nanoparticles (NPs) on B4C via a modified hydrothermal synthesis method [15]. The oxygen electrodes for Lieair cells were prepared according to the method in [14]. PVDF and Ni foam was used as the binder and current collector, respectively. The typical loading of the oxygen electrode was 0.8e1.2 mg(Ru/B4C) cm2.
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respectively corresponding to ORR and OER process. During cathodic scanning, the LieO2 cell exhibits a relatively bigger current density and an earlier onset potential (2.8 V) than the LAB (2.7 V), probably due to the higher oxygen concentration at electrolyte/electrode boundary.
Effect of various atmosphere on rate performance Electrochemical evaluation LABs were assembled as coin cells (CR2032) with holes on positive cases in an argon filled glovebox using Ru/B4C oxygen electrode, metallic lithium anode and 100 mL 1 M LiTFSI/tetraglyme electrolyte. No oxygen-selective membrane (OSM) or gas diffusion layer (GDL) was applied. The assembled LABs were tested using ambient air dried by molecular sieves. The discharge and charge tests of LABs were carried out on the Lanhe battery tester within 2.0e4.2 V at room temperature. The specific capacity was based on the total mass of cathode material (catalyst þ binder). Electrochemical impedance spectroscopy (EIS) was conducted by a CHI 660 electrochemical work station in the frequency range of 1 MHz to 0.1 Hz at open circuit voltage with a 5 mV amplitude. Cyclic voltammetric (CV) scanning at 0.5 mV s1 for LABs were respectively measured in Ar, air and pure O2 using CHI 660 as well.
Characterizations Electrode samples for XRD analysis were tested by a Rigaku D/ max-2500 X-ray diffractometer using Cu Ka radiation. TEM investigation was conducted using a Hitachi H7650. The morphologies of the electrodes were examined by fieldemission SEM (Hitachi S4800). FT-IR measurement of the electrodes was performed with a Nicolet 6700.
Results Characterizations of catalysts To investigate the structure and morphology of Ru/B4C catalyst, high resolution TEM was conducted. Fig. 1b shows that the ultrafine Ru nanoparticles (NPs) are uniformly dispersed on B4C without apparent agglomeration. B4C particles present flat surfaces and have an average size of 50 nm. Ru NPs have a narrow distribution (0.5e2.3 nm) and a mean size of 1.3 nm (Fig. S1). The contact between Ru and B4C looks tight (Fig. 1c). The intimate interaction between Ru catalyst and B4C support is conducive to exerting synergistic effect during the discharge and charge processes of LABs. Ru NPs present well-matched distances of Ru(002) and Ru(101) crystal planes and B4C particles show a B4C(021) crystal plane. Fig. 1d shows the cyclic voltammetric (CV) curves of the LABs scanning at 0.5 mV s1 within 2.0e4.4 V under various gas atmosphere. For the CV test in Ar, there is no obvious redox peak, indicating that Ru/ B4C electrode has a high electrochemical stability. Both the Liair and LieO2 cells exhibit significant redox peaks,
The rate performances of LABs and LieO2 batteries were measured at various current densities to evaluate their performance difference in ambient air and pure O2. Fig. 2aec exhibit the initial discharge and charge capacities of LABs and LieO2 batteries at various current density. The discharge voltage plateaus for LABs are slightly lower than those for their LieO2 counterparts, in accordance with the ORR behavior revealed by CV in Fig. 1d. In Fig. 2a, LABs deliver a discharge capacity of 2689 mA h g1 and discharge and charge voltage plateaus of 2.7 V and 3.8 V, respectively, which are comparable to those for LieO2 battery (2796 mA h g1, 2.8 V and 3.7 V, respectively), suggesting that Ru/B4C electrode achieves comparable ORR and OER activity in air to that in pure O2. It is also worth pointing out that the discharge curve for LieO2 battery exhibits a typical character of ’sudden death’, which may be ascribed to the surface passivation. In contrast, LAB presents an inclined discharge plateau, indicating a continuous passivation and impedance rise under ambient air. The poor reversibility of Li2CO3 and LiOH may degrade the cycle performance of LABs. However, the comparable capacity and overpotential for LAB to those for LieO2 batteries at 0.1 mA cm2 demonstrates that ambient air has less impact on battery performance at low rate with the aid of Ru/B4C versatile oxygen electrode. Fig. 2b and c shows that the performance difference between LABs and LieO2 batteries becomes significant at high current densities. It is speculated that high current speeds up the parasitic reactions and the side product formation. On one hand, a large number of highly þ 2 active superoxide/peroxide species (O 2 , O2 ) and Li may be produced rapidly at high rate during cell discharge and directly react with CO2 and H2O to form Li2CO3 and LiOH, respectively, as shown in Eqns. 1e5 [16,17]. O 2 þ CO2 / CO4
(1)
Liþ þ CO 4 / LiCO4
(2)
LiCO4 þ 3Liþ þ CO2 þ 3e / 2Li2CO3
(3)
þ O 2 þ H2O þ Li / HO2 þ LiOH
(4)
þ HO 2 þ H2O þ Li / H2O2 þ LiOH
(5)
On the other hand, high current requires fast oxygen transport, which may cause difficulty in mass transfer for LABs. The impedance behavior of LAB and LieO2 battery is shown in Fig. 2d. The high frequency intercept of the impedance data gives the total ohmic resistance of the cell (designated Rb). The distance between the intercepts at high and low
Please cite this article as: Ruan Y et al., Ambient lithiumeair battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.050
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Fig. 2 e Initial discharge and charge voltage profiles of LABs and LieO2 batteries within 2.0e4.2 V at 0.1 mA cm¡2 (a), 0.2 mA cm¡2 (b) and 0.3 mA cm¡2 (c), and the Nyquist plots (d).
frequency gives the electrode polarization resistance (designated Rp). LAB shows a similar Rb but a slightly higher Rp than LieO2 battery, indicating the relatively more sluggish ORR in ambient air, probably due to the low oxygen concentration in air and the intervention of side reactions.
Cycle performance The cycle performances of LABs and LieO2 batteries were tested under different rates and capacity limiting protocols to assess the versatile activity and stability of Ru/B4C. Fig. 3a and b shows the full discharge/charge voltage profiles for LABs and LieO2 batteries, respectively between 2.0 and 4.2 V at 0.1 mA cm2. Both LAB and LieO2 battery deliver similar discharge and charge capacities for each cycle. The similar discharge capacities and degree of attenuation for both batteries explicitly indicate that Ru/B4C can not only enable ORR and OER to reach a comparable performance in ambient air to that in pure O2, but also efficiently decompose side products during charge. Fig. 3cef shows the discharge/charge voltage profiles of LABs and LieO2 batteries cycled under a limited capacity of 300 mA h g1 at 0.1 mA cm2 within the voltage range of 2.0e4.2 V. The LAB stably delivers the controlled discharge and charge capacity for 45 cycles (Fig. 3c), close to LieO2 battery that exhibits 50 cycles (Fig. 3d). The discharge voltages for the initial several cycles are slightly lower than those for the following cycles for LAB (Fig. 3e), in contrast with those for LieO2 battery (Fig. 3f), implying an activation process may be required in ambient air. As a non-carbon based oxygen electrode material, Ru/B4C should be adequately stable during
battery cycling [13,14]. The performance decay is likely ascribed to the electrolyte decomposition triggered by the active oxygen species during charging, which may result in the concomitant formation of side products with their decomposition [18]. Under the limited capacity of 200 mA h g1, the LAB can be stably cycled for 70 cycles (Figs. S2a and b), far longer than the cell tested under 300 mA h g1. However, under 500 mA h g1, the LAB exhibits shorter stable cycles (Figs. S2c and d). At high current density of 0.2 mA cm2 (Figs. S3a-c) and 0.3 mA cm2 (Figs. S3d-f), the LABs deliver lower capacity and cycle performance than those at 0.1 mA cm2 (Fig. 3a and c). The results indicate that Ru/B4C versatile oxygen electrode can enable LABs to achieve a good reversibility under relatively low rate and capacity.
Characterizations of oxygen electrodes The morphology and structure of Ru/B4C oxygen electrode under different state of charge was characterized to reveal the formation and decomposition of dischage product during cycling. The surface of before-testing Ru/B4C electrode has a hierarchically porous structure (Fig. 4a). After the first discharge, the electrode surface is packed by a solid film which is composed of overlapped microsheets (Fig. 4b). In addition, the solid film appears to be poorly conducting under SEM observation as demonstrated by the white color (serious charging effect). After the first charge, the solid surface film disappears (Fig. 4c), manifesting the electrode surface has been cleaned up on charge. After 45 cycles, no surface film can
Please cite this article as: Ruan Y et al., Ambient lithiumeair battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.050
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Fig. 3 e Discharge/charge voltage profiles of LAB (a, c, e) and LieO2 batteries (b, d, f) in the full discharge/charge mode within the voltage range of 2.0e4.2 V.
be observed as well (Fig. 4d). Fig. 4e shows the XRD patterns corresponding to the electrodes in various state in Fig. 4aed. The XRD pattern of the Ru/B4C electrode after the first discharge identifies that the discharge product is mainly a mixture consisting of Li2CO3 and Li2O2 with relative content ratio of 62 : 38. LiOH is not detected as it may be converted to Li2CO3 in the presence of CO2 during discharge. After the first charge, the diffraction peaks of Li2O2 and Li2CO3 cannot be found, indicating the Ru/B4C oxygen electrode can efficiently decompose the discharge product and Li2CO3 side product. However, after 45 cycles the diffraction peaks of Li2CO3 residue are observed, suggesting that Li2CO3 cannot be fully eliminated after a long-term cycling. The performance decay may be caused by the Li2CO3 formation during charge due to the oxidation of unstable electrolyte [18]. The application of stable electrolytes, such as solid-state electrolytes, molten salts and ionic liquids may eliminate the formation of Li2CO3 during charging [6,20]. If so, the Ru/B4C versatile oxygen
electrode may enable LABs to achieve a long-term cycling under relatively high rate and capacity. FTeIR spectra in Fig. 4f verifies the analysis of XRD patterns in Fig. 4e. Li2CO3 can be effectively removed in the first charge but remains after 45 cycles, as revealed by the IR absorption peaks at 850 cm1 and 1440 cm1, which corresponds to the vibration strengths of CeO and C]O, respectively. The impedance behavior for the LAB in different state shows that Rp can basically return to the initial value after the first charge, while increases largely after more cycles due to the accumulation of side product (Fig. S4).
Discussion So far, LABs are still in their infancy, mainly due to the lack of stable versatile oxygen electrodes and electrolytes, which incurs detrimental side reactions. Non-carbon based oxygen electro-catalysts can not only avoid carbon corrosion and
Please cite this article as: Ruan Y et al., Ambient lithiumeair battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.050
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Fig. 4 e SEM images of Ru/B4C oxygen electrodes (a) before testing, (b) after the first discharge, (c) after the first charge, and (d) after 45 cycles under 300 mA h g-1 at 0.1 mA cm-2, and the corresponding (e) XRD patterns and (f) FT-IR spectra.
serve reliable electrode performance, but also be stable reference materials for screening usable electrolytes [19]. Recently, a non-carbon based LixNiO2 composite cathode along with stable inorganic electrolyte of LiNO3/KNO3 eutectic molten salt enabled LieO2 batteries to achieve 150 highly efficient cycles via a reversible four-electron pathway [20]. Based on a non-carbon based molybdenum disulfide cathode and an ionic liquid/dimethyl sulfoxide electrolyte, a LAB could be stably operated for up to 700 cycles in a simulated air atmosphere [6]. It is undoubted that non-carbon based oxygen electrodes play significant roles for achieving durable LABs. In terms of ORR/OER activity and chemical stability, Ru/B4C demonstrates comparable performance to those above mentioned catalysts. Furthermore, B4C is a lightweight material (~2.5 g cm3 in density), which can provide a high mass specific energy. The high ORR/OER activity for Ru/B4C is probably due to the synergic effect between Ru catalyst and B4C substrate. Most recently, a similar Pd/B4C oxygen electrocatalyst was reported with a highly active and durable ORR performance for Zn-air batteries (ZABs) [21]. A supporting first-principle computation was conducted and revealed the
synergistic effect between B4C and Pd could significantly promote O2 adsorption and splitting, as well as the transformation of O* to OH [21]. With a view to the practical application of LABs for transportation and stationary applications [22e24], LABs are strongly impacted by environment, such as weather (temperature), climate (humidity), and oxygen partial pressure [25], which may deteriorate the oxygen electrode structure and further destroy electrolyte. B4C is refractory and super hard (comparable to diamond and boron nitride), demonstrating robust mechanical property and high environment tolerance, which can ensure a consistent structure to confine catalysts and exert the synergetic effect.
Conclusion In summary, Ru/B4C exhibits comparably high activity and stability for ORR and OER in ambient air to those in pure O2, which is conducive to the transfer from LieO2 batteries to LABs. The features of excellent catalytic activity, chemical and
Please cite this article as: Ruan Y et al., Ambient lithiumeair battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.050
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electrochemical stability, and environment tolerance make Ru/ B4C suitable for practical application of LABs in ambient air.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21878232), Natural Science Foundation of Tianjin (No. 19JCYBJC21500), and the Program for Tianjin Distinguished Professor.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.050.
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Please cite this article as: Ruan Y et al., Ambient lithiumeair battery enabled by a versatile oxygen electrode based on boron carbide supported ruthenium, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.050