- Email: [email protected]
Discharging temperature dependence of Li2O2 formation and its effect on charging polarization for Li–O2 Battery
Accepted Manuscript Title: Discharging temperature dependence of Li2 O2 formation and its effect on charging polarization for Li-O2 Battery Author: Ming Song Xihua Du Yan Chen Lei Zhang Ding Zhu Yungui Chen PII: DOI: Reference:
S0025-5408(15)00179-8 http://dx.doi.org/doi:10.1016/j.materresbull.2015.03.023 MRB 8094
To appear in:
MRB
Received date: Revised date: Accepted date:
10-2-2015 8-3-2015 9-3-2015
Please cite this article as: Ming Song, Xihua Du, Yan Chen, Lei Zhang, Ding Zhu, Yungui Chen, Discharging temperature dependence of Li2O2 formation and its effect on charging polarization for Li-O2 Battery, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2015.03.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Discharging temperature dependence of Li2O2 formation and its effect on charging polarization for Li-O2 Battery
Ming Songa*[email protected], Xihua Dua, Yan Chena, Lei Zhangb, Ding Zhuc, Yungui Chenc
a
College of Chemistry and Chemical Engineering, Xuzhou Institute of Technology,
Xuzhou 221111 b
College of Materials Science and Engineering, Xi’an University of Science and
Technology, Xian 710054 c
College of Materials Science and Engineering, Sichuan University, Chengdu 610065
*
Corresponding author at: College of Chemistry and Chemical Engineering, Xuzhou
Institute of Technology, Li Shui Road, Xuzhou, PR China, Tel.: +86 516 83689723
Graphical abstract
Highlights 1. Varied morphologic Li2O2 were produced firstly by changing discharge temperature. 2. The particle size of Li2O2 obtained at higher temperature is finer. 3. Charging polarization of Li-O2 battery is lower with the finer Li2O2.
Abstract Among all the factors that restrict the application of Li-air battery, large charging polarization, due to the poor bulk charge transport ability of discharge product Li2O2, comes first. In this paper, we adopt the method of controlling discharging temperature to reduce the charging polarization during the first cycle of Li2O2 based on the fact that controlling discharging temperature can reduce Li2O2 bulk charge transport path since finer Li2O2 particles can be observed at higher temperature. The mechanism of reducing charging polarization simply through increasing the discharging test temperature has also been researched in this paper.
Keywords A. oxides, B. crystal growth, C. electrochemical measurements, D. electrochemical properties, D. energy storage
1. Introduction Li-air battery, generally referred to as lithium-oxygen (Li-O2) batteries, has the highest theoretical specific energy (3,505 Wh kg−1) which is about ten times that of current Li-ion batteries [1] although cathodes[2] and anodes[3] with high specific capacities are still being researched, and hence, this battery chemistry has attracted enormous research attention in the past couple of years. Currently, the high charging overpotential that limits discharge-charge cycle efficiency hinders the development of any practical non-aqueous Li-air battery and the poor charge transport ability (slow decomposition kinetics) of discharging product Li2O2 is an important factor. For Li2O2, the most stable surface, an oxygen-rich {0001} termination, is half-metallic [4], and hence it provides the conductive surface pathways in Li2 O2 [5, 6]. However, the bulk conductive ability is poor since the self-trapping of electrons or polaronic hole trapping in Li2O2[7,8] and hence when the Li2O2 thickness is greater than the critical thickness [9], the bulk charge transport can no longer support the electrochemical current. Therefore, increasing the bulk conductive ability is necessary to improve the decomposition kinetics of Li2O2 during charging process. Researchers have found that the low crystallinity of Li2O2 formed in discharging process has improved decomposition kinetics of charging process [10]. Moreover, the Li2O2 morphology on oxygen reduction reaction (ORR) can be influenced by discharging current density and then, oxygen evolution reaction (OER) kinetics will be changed [11-13].
In this paper, the particle size and morphology of Li2O2 are controlled to reduce the bulk charge transport distance by novel method of changing discharging temperature. Meanwhile, the relationship between discharging temperature and charging polarization characteristic has been researched in this manuscript. These results may provide a new direction for reducing the charging overpotential for the non-aqueous Li-O2 batteries.
2. Experimental Cathodes used in this paper were prepared by coating a Super P carbon/PTFE slurry onto a 304 SS mesh (Shenzhen Kejingstar, Ltd.) with a diameter of 1.5 cm and the electrolytes were prepared by mixing lithium trifluoromethane sulfonimide (LiTFSI) in tetraglyme (TEGDME) (Aladdin-Reagent, Inc.) with the molar ratio between LiTFSI and TEGDME is 1:5 (~0.89 M) in a glove box (Chengdu Dellix Industry Co., Ltd.) filled with argon ([H2O]<1 ppm). Although it has lower ionic conductivity [14], TEGDME is more stable than the common electrolyte of Li-ion battery. More details about the cathodes and electrolytes preparations can be found elsewhere [15].
The Li–O2 battery configuration used in this paper has been described elsewhere [16], including a lithium foil (1.6 cm in diameter), two pieces of Celgard2325 separator (1.9 cm in diameter), and a carbon cathode (1.5 cm in diameter). The cell was assembled in a glove box with water contents of <1 ppm, and about 400 μl prepared electrolyte was added in each cell. After standing for at least 4 h at each test temperature, cells were discharged and charged under O2 with a 1.1 atm pressure using a LAND tester (CT2001A, Wuhan LAND Electronic Co., Ltd.). The current density was 0.05 mA
cm−2 (~160 mA gc−1). An electrical thermostatic drying chest (DHG 9202, Shanghai Hongdu Electronic Technology Co., Ltd.) was used to control the test temperature.
Scanning electron microscopy (SEM) analysis was performed in order to investigate the particle size and morphology of the obtained Li2O2 during discharging process, using a JSM-7500F scanning microscope.
Electrochemical impedance spectroscopy (EIS) was measured by a PARSTAT 2273 (Princeton Applied Research) and the spectra were obtained in the frequency range from 1 MHz to 100 mHz with an AC amplitude of 5 mV at different temperatures.
Linear sweep voltammetry (LSV) was also measured by a PARSTAT 2273 with the scan speed of 0.2 mV s-1.
3. Results and discussion
3.1 Charging polarization characteristics of Li2O2 obtained at different discharging temperatures Galvanostatic charge method was used to reveal the charging polarization characteristics of Li2O2 (at 343 K) that have been obtained through discharging at 303 K, 323 K and 343 K, respectively, using Super P electrodes, and the cut-off capacities are consistent (1mAh) to ensure the same amount of discharging products Li2O2. For the discharging process, as shown in Figure 1, discharging polarization decreases with the increase of discharging temperature from 303 K to 343 K because of the improved ORR kinetics which may result from the lower electrolyte viscosity [17] and
higher O2 diffusion rate [18-21] at higher discharging temperature. Similar phenomenon has been reported in our previous work about the O2 transport limitation during discharging process [22]. For the charging process, the cut-off voltage is 3.8 V considering the electrolyte stability. The charging voltage plateaus of electrodes that have been discharged at 303 K, 323 K and 343 K are about 3.75 V, 3.6 V and 3.5 V, respectively. These results mean the charging overpotential at this test temperature can be partly reduced simply through increasing the discharging test temperature to 343 K. More importantly, this method is meaningful and practical since it will not lower the specific capacity or add the cost of batteries, comparing with other methods [23], and the high temperatures like 343 K can be easily obtained in practical applications such as electric vehicles. Obviously, these results may be related with the different morphologies of Li2O2 obtained at different discharging temperatures.
3.2 Morphology characteristics of Li2O2 obtained at different discharging temperatures To reveal the Li2O2 morphologies after discharging at different temperatures, the discharged-state electrodes have been observed using field emission scanning electron microscope (FSEM) at a magnification of 10000 (Fig. 2) and 50000 (Fig.3). The images of pristine electrodes have also been provided for comparison. Apparently, the images of pristine Super P carbon electrodes change after discharging (Fig. 2b-2d) since the crystallization, growth and aggregation of Li2O2 on
the surfaces of Super P carbon particles. The image of smaller particles of the discharged electrode at 323 K (Fig. 2c), compared with that of 303 K discharged electrode (Fig. 2b), reveals a thinner Li2O2 coating. For the image of the discharged electrode at 343 K (Fig. 2d), it seems that some preferred orientation growth occurs since a “flower-like” sharp has been observed. The electrodes have also been observed at higher magnification (50000×) and Super P particles in the pristine electrode are shown with diameter of several tens of nanometers (Fig. 3a) and these particles grow to about hundreds of nanometers after discharging at 303 K (Fig. 3b) because of the coating of discharging products Li2O2. However, small particles still exist in the electrodes that have been discharged at higher test temperatures (Fig. 3c and Fig. 3d), especially in the 343 K discharged electrode, which reveals the thickness of Li2O2 coating on the Super P particles is smaller. Meanwhile, the special aggregation of Li2O2 obtained during the discharging process at 343 K, forms the “flower-like” sharp in Fig. 2d. The results demonstrate that the discharging temperature dramatically affects the morphologies of Li2O2 and electrode because of the different conditions of Li2O2 crystal growth at three discharging temperatures. 3.3 Discharging temperature and charging polarization mechanism LSV and EIS methods have been used to study the mechanism of discharging temperature dependence of Li2O2 formation and its effect on charging polarization. For the ORR process (Fig. 4), all the LSVs show only one cathodic peak, which corresponds to the reduction of oxygen molecules to superoxide ions. Obviously, the current response of O2 cathode becomes stronger with the increased discharging
temperature, indicating a gradual increase of ORR kinetics. Unlike common air cathodes for aqueous cells, the gas-solid-liquid three-phase interface of the cathode in non-aqueous Li-O2 batteries [24] is hard to form and only dissolved oxygen is available for the ORR process, which occurs at the electrolyte/carbon interface. It is understandable that the diffusion of dissolved oxygen to the reaction interface becomes easier at higher temperature because of the lower electrolyte viscosity (2.99 m Pa s at 303 K and1.45 m Pa s at 343 K) [17], and hence more amount of crystal nucleus can be formed resulting in the thinner coating of Li2O2 on the Super P particles (Fig. 3). However, Li2O2 tends to grow on the formed crystal nucleus because of limited O2 transport at low temperature (303 K), which explains the image of larger particles (Fig. 3b). For the OER process (Fig. 5), only LSVs of the discharged-state electrodes show anodic peaks and no appreciable current is observed for the LSVs of the pristine electrode or current collector stainless steel (SS), which indicates that the anodic peaks are related with the decompositions of discharging products Li2O2 and the current collector SS is stable during the OER process. Apparently, the current responses of the discharged-state electrodes become stronger with the increased discharging temperature, indicating the gradual increase of OER kinetics. Furthermore, there are slight shifts in the anodic peak positions for the higher temperature discharged electrodes. These results prove that by controlling the discharging temperature and then controlling the morphology characteristics of Li2O2 (Fig. 2 and Fig. 3), the high charging overpotential that limits discharge-charge cycle efficiency of Li-O2 battery
can be lowed indeed, which is consistent with the galvanostatic charging results (Fig. 1). EIS is introduced to the study of the variations of kinetic properties of discharged electrodes at three temperatures (Fig. 6). Laoire et al. [25] interpreted the impedance spectra of Li-O2 batteries and proposed the equivalent circuit (Rs (C (RpW)), where Rs is the electronic resistance of the electrodes and their contacts to the current collectors, and electrolyte resistance, C is the capacitive contributions of the two electrodes, Rp is the charge transfer resistance at the two electrodes, W is the linear Warburg element that may be attributed to the diffusion of the electroactive species to the electrode. Obviously, when the discharging temperature elevates, the radius of the semicircle decreases, namely, the charge-transfer resistance Rp of the electrode decreases, indicating an enhanced kinetics property, which is fairly in agreement with the results of LSV (Fig. 5) and galvanostatic charging (Fig. 1).
4. Conclusions
The influences of discharging temperature on Li2O2 formation and then on charging polarization for Li-O2 battery have been researched in this work. The charging plateaus of the discharged electrodes are 3.75, 3.6 and 3.5 V at 303, 323 and 343 K, respectively, according to the galvanostatic charging results. SEM images of the discharged electrodes reveal different morphology characteristics and the Li2O2 particles obtained at higher temperature are finer because of the enhanced O2 transport ability of electrolyte and a larger amount of Li2O2 crystal nucleus. The discharged-state
electrodes after discharging at higher temperatures have better OER kinetics according to the LSV and EIS analyses. These results reveal that the charging polarization of the cathode can be controlled through changing the discharging temperature and may provide a new direction for reducing the charging overpotential for the non-aqueous Li-O2 batteries.
Acknowledgments This study was financially supported by the National Natural Science Foundation of China (Grant No. 21472071).
References [1] B. An, Q. Ru, S. Hu, X. Song, J. Li,;1; Facile synthesis and electrochemical performance of Co2SnO4/Co3O4 nanocomposite for lithium-ion batteries, Mater. Res. Bull, 60 (2014) 640-647. [2] X. Zhang, M. Wu, S. Gao, Y. Xu, X. Cheng, H. Zhao,;1; Li Huo, Facile synthesis of uniform flower-like V2O5 hierarchical architecture for high-performance Li-ion battery, Mater. Res. Bull, 60 (2014) 659-664. [3] X. Sun, X. Wang, L. Qiao, D. Hu, N. Feng, X. Li, Y. Liu, D. He,;1; Electrochemical behaviors of porous SnO2-Sn/C composites derived from pyrolysis of SnO2/poly(vinylidene fluoride), Electrochim. Acta, 66 (2012) 204-209. . [4] M.D. Radin, F. Tian, D.J. Siegel,;1; Electronic structure of Li2O2 {0001} surfaces, J. Mater. Sci., 47 (2012) 7564-7570. [5] M.D. Radin, J.F. Rodriguez, F. Tian, D.J. Siegel,;1; Lithium peroxide surfaces are metallic, while lithium oxide surfaces are not, J. Am. Chem. Soc., 134 (2012) 1093-1103.
[6] M.D. Radin, J.F. Rodriguez, D.J. Siegel,;1; Lithium peroxide surfaces and point defects: Relevance for Li-air batteries, Proceedings of the Battery Congress, 60 (2011) 6-16. . [7] J. Kang, Y.S. Jung, S.H. Wei, A.C. Dillon,;1; Implications of the formation of small polarons in Li2O2 for Li-air batteries, Phys. Rev. B, 85 (2012) 035210(1)-035210(5). [8] S.P. Ong, Y. Mo, G. Ceder,;1; Low hole polaron migration barrier in lithium peroxide, Phys. Rev. B, 85 (2012) 081105(R)(1)-081105(R)(4). [9] V. Viswanathan, K.S. Thygesen, J.S. Hummelshøj, J.K. Nørskov, G. Girishkumar,;1; Electrical conductivity in Li2O2 and its role in determining capacity limitations in non-aqueous Li-O2 batteries, J. Chem. Phys., 135 (2011) 214704(1)-214704(10). [10] J.B. Park, J. Lee, C.S. Yoon, Y.K. Sun,;1; Ordered mesoporous carbon electrodes for Li-O2 Batteries, ACS Appl. Mater. Interfaces, 5 (2013) 13426-13431. [11] B.D. Adams, C. Radtke, R. Black, M. Trudeau, K. Zaghib, L.F. Nazar,;1; Current density dependence of peroxide formation in the Li-O2 battery and its effect on charge, Energy Environ. Sci., 6 (2013) 1772-1778. [12] B.M. Gallant, D.G. Kwabi, R.R. Mitchell, J. Zhou, C.V. Thompson,;1; Shao-Horn Y, Influence of Li2 O2 morphology on oxygen reduction and evolution kinetics in Li-O2 batterie, Energy. Environ. Sci, 6 (2013) 2518-2528. [13] R.R. Mitchell, B.M. Gallant, Shao-Horn Y, C.V. Thompson,;1; Mechanisms of morphological evolution of Li2O2 particles during electrochemical growth. J. Phys. Chem. Lett. 4 (2013) 1060-1064. [14] I.H. Jo, H.S. Ryu, D.G. Gu, J.S. Park, I.S. Ahn, H.J. Ahn, T.H. Nam, K.W. Kim,;1; The effect of electrolyte on the electrochemical properties of Na/α-NaMnO2 batteries, Mater. Res. Bull., 58 (2014) 74-77. [15] M. Song, D. Zhu, L. Zhang, X. Wang, R. Mi, H. Liu, J. Mei, L.W.M. Lau, Y. Chen,;1; Temperature characteristics of nonaqueous Li-O2 batteries, J. Solid State Electrochem., 18 (2014) 739-745. [16] M. Song, D. Zhu, L. Zhang, X. Wang, L. Huang, Q. Shi, R. Mi, H. Liu, J. Mei, L.W.M. Lau, Y. Chen,;1; Temperature dependence of charging characteristic of C-free Li2O2 cathode in Li-O2 battery, J. Solid State Electrochem., 17 (2013) 2061-2069.
[17] D. Kodama, M. Kanakubo, M. Kokubo, S. Hashimoto, H. Nanjo, M. Katoa,;1; Density, viscosity, and solubility of carbon dioxide in glymes, Fluid Phase Equilibria., 302 (2011) 103-108. [18] J. Read, K. Mutolo, M. Ervin, W. Behl, J. Wolfenstine, A. Driedger, D. Foster,;1; oxygen transport properties of organic electrolytes and performance of Lithium oxygen battery, J. Electrochem. Soc., 150 (2003) A1351-A1356. [19] J. Read,;1; Ether-based electrolytes for the Lithium/Oxygen organic electrolyte battery, J. Electrochem. Soc., 153 (2006) A96-A100. [20] W. Xu, J. Xiao, K. Xu, D. Wang, J. Zhang, J.G. Zhang,;1; Crown ethers in nonaqueous electrolytes for Lithium/Air batteries, Electrochem. Solid-State Lett., 13 (2010) A48-A51. [21] W. Xu, J. Xiao, J. Zhang, D. Wang,;1; Zhang, J.G., Optimization of nonaqueous electrolytes for primary Lithium/Air Batteries operated in ambient environment, J. Electrochem. Soc., 156 (2009) A773-A779. [22] D. Zhu, L. Zhang, M. Song, X. Wang, R. Mi, H. Liu, J. Mei, L.W.M. Lau,Y. Chen,;1; Intermittent operation of the aprotic Li-O2 battery: the mass recovery process upon discharge interval, J. Solid State Electrochem., 17 (2013) 2539-2544. [23] J. Zhang, B. Sun, H.J. Ahn, C. Wang, G. Wang,;1; Conducting polymer-doped polyprrrole as an effective cathode catalyst for Li-O2 batteries, Mater. Res. Bull, 48 (2013) 4979-4983. [24] S.S. Zhang, D. Foster, J. Read,;1; Discharge characteristic of a non-aqueous electrolyte Li/O2 battery, J. Power Sources, 195 (2010) 1235-1240. [25] C.Ó. Laoire, S. Mukerjee, E.J. Plichta, M.A. Hendrickson, K.M. Abraham,;1; Rechargeable Lithium/TEGDME-LiPF6/O2 battery, J. Electrochem. Soc., 158 (2011) A302-A308.
Figure caption Fig. 1 Galvanostatic discharge and charge profiles of Super P electrodes at different discharging and same charging temperatures
Fig. 2 SEM of discharged Super P electrodes (medium magnification). a) pristine electrode, b)303 K discharged electrode, c) 323 K discharged electrode, d) 343 K discharged electrode Fig. 3 SEM of discharged Super P electrodes (high magnification). a) pristine electrode, b)303 K discharged electrode, c) 323 K discharged electrode, d) 343 K discharged electrode Fig. 4 Cathodic LSV curves of Super P electrodes at different test temperatures Fig. 5 Anodic LSV curves（ 343K） of Super P electrodes that have discharged at different temperatures Fig. 6 Nyquist plots of Super P electrodes at 343K that have discharged at different temperatures FIGURES
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
S0025-5408(15)00179-8 http://dx.doi.org/doi:10.1016/j.materresbull.2015.03.023 MRB 8094
To appear in:
MRB
Received date: Revised date: Accepted date:
10-2-2015 8-3-2015 9-3-2015
Please cite this article as: Ming Song, Xihua Du, Yan Chen, Lei Zhang, Ding Zhu, Yungui Chen, Discharging temperature dependence of Li2O2 formation and its effect on charging polarization for Li-O2 Battery, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2015.03.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Discharging temperature dependence of Li2O2 formation and its effect on charging polarization for Li-O2 Battery
Ming Songa*[email protected], Xihua Dua, Yan Chena, Lei Zhangb, Ding Zhuc, Yungui Chenc
a
College of Chemistry and Chemical Engineering, Xuzhou Institute of Technology,
Xuzhou 221111 b
College of Materials Science and Engineering, Xi’an University of Science and
Technology, Xian 710054 c
College of Materials Science and Engineering, Sichuan University, Chengdu 610065
*
Corresponding author at: College of Chemistry and Chemical Engineering, Xuzhou
Institute of Technology, Li Shui Road, Xuzhou, PR China, Tel.: +86 516 83689723
Graphical abstract
Highlights 1. Varied morphologic Li2O2 were produced firstly by changing discharge temperature. 2. The particle size of Li2O2 obtained at higher temperature is finer. 3. Charging polarization of Li-O2 battery is lower with the finer Li2O2.
Abstract Among all the factors that restrict the application of Li-air battery, large charging polarization, due to the poor bulk charge transport ability of discharge product Li2O2, comes first. In this paper, we adopt the method of controlling discharging temperature to reduce the charging polarization during the first cycle of Li2O2 based on the fact that controlling discharging temperature can reduce Li2O2 bulk charge transport path since finer Li2O2 particles can be observed at higher temperature. The mechanism of reducing charging polarization simply through increasing the discharging test temperature has also been researched in this paper.
Keywords A. oxides, B. crystal growth, C. electrochemical measurements, D. electrochemical properties, D. energy storage
1. Introduction Li-air battery, generally referred to as lithium-oxygen (Li-O2) batteries, has the highest theoretical specific energy (3,505 Wh kg−1) which is about ten times that of current Li-ion batteries [1] although cathodes[2] and anodes[3] with high specific capacities are still being researched, and hence, this battery chemistry has attracted enormous research attention in the past couple of years. Currently, the high charging overpotential that limits discharge-charge cycle efficiency hinders the development of any practical non-aqueous Li-air battery and the poor charge transport ability (slow decomposition kinetics) of discharging product Li2O2 is an important factor. For Li2O2, the most stable surface, an oxygen-rich {0001} termination, is half-metallic [4], and hence it provides the conductive surface pathways in Li2 O2 [5, 6]. However, the bulk conductive ability is poor since the self-trapping of electrons or polaronic hole trapping in Li2O2[7,8] and hence when the Li2O2 thickness is greater than the critical thickness [9], the bulk charge transport can no longer support the electrochemical current. Therefore, increasing the bulk conductive ability is necessary to improve the decomposition kinetics of Li2O2 during charging process. Researchers have found that the low crystallinity of Li2O2 formed in discharging process has improved decomposition kinetics of charging process [10]. Moreover, the Li2O2 morphology on oxygen reduction reaction (ORR) can be influenced by discharging current density and then, oxygen evolution reaction (OER) kinetics will be changed [11-13].
In this paper, the particle size and morphology of Li2O2 are controlled to reduce the bulk charge transport distance by novel method of changing discharging temperature. Meanwhile, the relationship between discharging temperature and charging polarization characteristic has been researched in this manuscript. These results may provide a new direction for reducing the charging overpotential for the non-aqueous Li-O2 batteries.
2. Experimental Cathodes used in this paper were prepared by coating a Super P carbon/PTFE slurry onto a 304 SS mesh (Shenzhen Kejingstar, Ltd.) with a diameter of 1.5 cm and the electrolytes were prepared by mixing lithium trifluoromethane sulfonimide (LiTFSI) in tetraglyme (TEGDME) (Aladdin-Reagent, Inc.) with the molar ratio between LiTFSI and TEGDME is 1:5 (~0.89 M) in a glove box (Chengdu Dellix Industry Co., Ltd.) filled with argon ([H2O]<1 ppm). Although it has lower ionic conductivity [14], TEGDME is more stable than the common electrolyte of Li-ion battery. More details about the cathodes and electrolytes preparations can be found elsewhere [15].
The Li–O2 battery configuration used in this paper has been described elsewhere [16], including a lithium foil (1.6 cm in diameter), two pieces of Celgard2325 separator (1.9 cm in diameter), and a carbon cathode (1.5 cm in diameter). The cell was assembled in a glove box with water contents of <1 ppm, and about 400 μl prepared electrolyte was added in each cell. After standing for at least 4 h at each test temperature, cells were discharged and charged under O2 with a 1.1 atm pressure using a LAND tester (CT2001A, Wuhan LAND Electronic Co., Ltd.). The current density was 0.05 mA
cm−2 (~160 mA gc−1). An electrical thermostatic drying chest (DHG 9202, Shanghai Hongdu Electronic Technology Co., Ltd.) was used to control the test temperature.
Scanning electron microscopy (SEM) analysis was performed in order to investigate the particle size and morphology of the obtained Li2O2 during discharging process, using a JSM-7500F scanning microscope.
Electrochemical impedance spectroscopy (EIS) was measured by a PARSTAT 2273 (Princeton Applied Research) and the spectra were obtained in the frequency range from 1 MHz to 100 mHz with an AC amplitude of 5 mV at different temperatures.
Linear sweep voltammetry (LSV) was also measured by a PARSTAT 2273 with the scan speed of 0.2 mV s-1.
3. Results and discussion
3.1 Charging polarization characteristics of Li2O2 obtained at different discharging temperatures Galvanostatic charge method was used to reveal the charging polarization characteristics of Li2O2 (at 343 K) that have been obtained through discharging at 303 K, 323 K and 343 K, respectively, using Super P electrodes, and the cut-off capacities are consistent (1mAh) to ensure the same amount of discharging products Li2O2. For the discharging process, as shown in Figure 1, discharging polarization decreases with the increase of discharging temperature from 303 K to 343 K because of the improved ORR kinetics which may result from the lower electrolyte viscosity [17] and
higher O2 diffusion rate [18-21] at higher discharging temperature. Similar phenomenon has been reported in our previous work about the O2 transport limitation during discharging process [22]. For the charging process, the cut-off voltage is 3.8 V considering the electrolyte stability. The charging voltage plateaus of electrodes that have been discharged at 303 K, 323 K and 343 K are about 3.75 V, 3.6 V and 3.5 V, respectively. These results mean the charging overpotential at this test temperature can be partly reduced simply through increasing the discharging test temperature to 343 K. More importantly, this method is meaningful and practical since it will not lower the specific capacity or add the cost of batteries, comparing with other methods [23], and the high temperatures like 343 K can be easily obtained in practical applications such as electric vehicles. Obviously, these results may be related with the different morphologies of Li2O2 obtained at different discharging temperatures.
3.2 Morphology characteristics of Li2O2 obtained at different discharging temperatures To reveal the Li2O2 morphologies after discharging at different temperatures, the discharged-state electrodes have been observed using field emission scanning electron microscope (FSEM) at a magnification of 10000 (Fig. 2) and 50000 (Fig.3). The images of pristine electrodes have also been provided for comparison. Apparently, the images of pristine Super P carbon electrodes change after discharging (Fig. 2b-2d) since the crystallization, growth and aggregation of Li2O2 on
the surfaces of Super P carbon particles. The image of smaller particles of the discharged electrode at 323 K (Fig. 2c), compared with that of 303 K discharged electrode (Fig. 2b), reveals a thinner Li2O2 coating. For the image of the discharged electrode at 343 K (Fig. 2d), it seems that some preferred orientation growth occurs since a “flower-like” sharp has been observed. The electrodes have also been observed at higher magnification (50000×) and Super P particles in the pristine electrode are shown with diameter of several tens of nanometers (Fig. 3a) and these particles grow to about hundreds of nanometers after discharging at 303 K (Fig. 3b) because of the coating of discharging products Li2O2. However, small particles still exist in the electrodes that have been discharged at higher test temperatures (Fig. 3c and Fig. 3d), especially in the 343 K discharged electrode, which reveals the thickness of Li2O2 coating on the Super P particles is smaller. Meanwhile, the special aggregation of Li2O2 obtained during the discharging process at 343 K, forms the “flower-like” sharp in Fig. 2d. The results demonstrate that the discharging temperature dramatically affects the morphologies of Li2O2 and electrode because of the different conditions of Li2O2 crystal growth at three discharging temperatures. 3.3 Discharging temperature and charging polarization mechanism LSV and EIS methods have been used to study the mechanism of discharging temperature dependence of Li2O2 formation and its effect on charging polarization. For the ORR process (Fig. 4), all the LSVs show only one cathodic peak, which corresponds to the reduction of oxygen molecules to superoxide ions. Obviously, the current response of O2 cathode becomes stronger with the increased discharging
temperature, indicating a gradual increase of ORR kinetics. Unlike common air cathodes for aqueous cells, the gas-solid-liquid three-phase interface of the cathode in non-aqueous Li-O2 batteries [24] is hard to form and only dissolved oxygen is available for the ORR process, which occurs at the electrolyte/carbon interface. It is understandable that the diffusion of dissolved oxygen to the reaction interface becomes easier at higher temperature because of the lower electrolyte viscosity (2.99 m Pa s at 303 K and1.45 m Pa s at 343 K) [17], and hence more amount of crystal nucleus can be formed resulting in the thinner coating of Li2O2 on the Super P particles (Fig. 3). However, Li2O2 tends to grow on the formed crystal nucleus because of limited O2 transport at low temperature (303 K), which explains the image of larger particles (Fig. 3b). For the OER process (Fig. 5), only LSVs of the discharged-state electrodes show anodic peaks and no appreciable current is observed for the LSVs of the pristine electrode or current collector stainless steel (SS), which indicates that the anodic peaks are related with the decompositions of discharging products Li2O2 and the current collector SS is stable during the OER process. Apparently, the current responses of the discharged-state electrodes become stronger with the increased discharging temperature, indicating the gradual increase of OER kinetics. Furthermore, there are slight shifts in the anodic peak positions for the higher temperature discharged electrodes. These results prove that by controlling the discharging temperature and then controlling the morphology characteristics of Li2O2 (Fig. 2 and Fig. 3), the high charging overpotential that limits discharge-charge cycle efficiency of Li-O2 battery
can be lowed indeed, which is consistent with the galvanostatic charging results (Fig. 1). EIS is introduced to the study of the variations of kinetic properties of discharged electrodes at three temperatures (Fig. 6). Laoire et al. [25] interpreted the impedance spectra of Li-O2 batteries and proposed the equivalent circuit (Rs (C (RpW)), where Rs is the electronic resistance of the electrodes and their contacts to the current collectors, and electrolyte resistance, C is the capacitive contributions of the two electrodes, Rp is the charge transfer resistance at the two electrodes, W is the linear Warburg element that may be attributed to the diffusion of the electroactive species to the electrode. Obviously, when the discharging temperature elevates, the radius of the semicircle decreases, namely, the charge-transfer resistance Rp of the electrode decreases, indicating an enhanced kinetics property, which is fairly in agreement with the results of LSV (Fig. 5) and galvanostatic charging (Fig. 1).
4. Conclusions
The influences of discharging temperature on Li2O2 formation and then on charging polarization for Li-O2 battery have been researched in this work. The charging plateaus of the discharged electrodes are 3.75, 3.6 and 3.5 V at 303, 323 and 343 K, respectively, according to the galvanostatic charging results. SEM images of the discharged electrodes reveal different morphology characteristics and the Li2O2 particles obtained at higher temperature are finer because of the enhanced O2 transport ability of electrolyte and a larger amount of Li2O2 crystal nucleus. The discharged-state
electrodes after discharging at higher temperatures have better OER kinetics according to the LSV and EIS analyses. These results reveal that the charging polarization of the cathode can be controlled through changing the discharging temperature and may provide a new direction for reducing the charging overpotential for the non-aqueous Li-O2 batteries.
Acknowledgments This study was financially supported by the National Natural Science Foundation of China (Grant No. 21472071).
References [1] B. An, Q. Ru, S. Hu, X. Song, J. Li,;1; Facile synthesis and electrochemical performance of Co2SnO4/Co3O4 nanocomposite for lithium-ion batteries, Mater. Res. Bull, 60 (2014) 640-647. [2] X. Zhang, M. Wu, S. Gao, Y. Xu, X. Cheng, H. Zhao,;1; Li Huo, Facile synthesis of uniform flower-like V2O5 hierarchical architecture for high-performance Li-ion battery, Mater. Res. Bull, 60 (2014) 659-664. [3] X. Sun, X. Wang, L. Qiao, D. Hu, N. Feng, X. Li, Y. Liu, D. He,;1; Electrochemical behaviors of porous SnO2-Sn/C composites derived from pyrolysis of SnO2/poly(vinylidene fluoride), Electrochim. Acta, 66 (2012) 204-209. . [4] M.D. Radin, F. Tian, D.J. Siegel,;1; Electronic structure of Li2O2 {0001} surfaces, J. Mater. Sci., 47 (2012) 7564-7570. [5] M.D. Radin, J.F. Rodriguez, F. Tian, D.J. Siegel,;1; Lithium peroxide surfaces are metallic, while lithium oxide surfaces are not, J. Am. Chem. Soc., 134 (2012) 1093-1103.
[6] M.D. Radin, J.F. Rodriguez, D.J. Siegel,;1; Lithium peroxide surfaces and point defects: Relevance for Li-air batteries, Proceedings of the Battery Congress, 60 (2011) 6-16. . [7] J. Kang, Y.S. Jung, S.H. Wei, A.C. Dillon,;1; Implications of the formation of small polarons in Li2O2 for Li-air batteries, Phys. Rev. B, 85 (2012) 035210(1)-035210(5). [8] S.P. Ong, Y. Mo, G. Ceder,;1; Low hole polaron migration barrier in lithium peroxide, Phys. Rev. B, 85 (2012) 081105(R)(1)-081105(R)(4). [9] V. Viswanathan, K.S. Thygesen, J.S. Hummelshøj, J.K. Nørskov, G. Girishkumar,;1; Electrical conductivity in Li2O2 and its role in determining capacity limitations in non-aqueous Li-O2 batteries, J. Chem. Phys., 135 (2011) 214704(1)-214704(10). [10] J.B. Park, J. Lee, C.S. Yoon, Y.K. Sun,;1; Ordered mesoporous carbon electrodes for Li-O2 Batteries, ACS Appl. Mater. Interfaces, 5 (2013) 13426-13431. [11] B.D. Adams, C. Radtke, R. Black, M. Trudeau, K. Zaghib, L.F. Nazar,;1; Current density dependence of peroxide formation in the Li-O2 battery and its effect on charge, Energy Environ. Sci., 6 (2013) 1772-1778. [12] B.M. Gallant, D.G. Kwabi, R.R. Mitchell, J. Zhou, C.V. Thompson,;1; Shao-Horn Y, Influence of Li2 O2 morphology on oxygen reduction and evolution kinetics in Li-O2 batterie, Energy. Environ. Sci, 6 (2013) 2518-2528. [13] R.R. Mitchell, B.M. Gallant, Shao-Horn Y, C.V. Thompson,;1; Mechanisms of morphological evolution of Li2O2 particles during electrochemical growth. J. Phys. Chem. Lett. 4 (2013) 1060-1064. [14] I.H. Jo, H.S. Ryu, D.G. Gu, J.S. Park, I.S. Ahn, H.J. Ahn, T.H. Nam, K.W. Kim,;1; The effect of electrolyte on the electrochemical properties of Na/α-NaMnO2 batteries, Mater. Res. Bull., 58 (2014) 74-77. [15] M. Song, D. Zhu, L. Zhang, X. Wang, R. Mi, H. Liu, J. Mei, L.W.M. Lau, Y. Chen,;1; Temperature characteristics of nonaqueous Li-O2 batteries, J. Solid State Electrochem., 18 (2014) 739-745. [16] M. Song, D. Zhu, L. Zhang, X. Wang, L. Huang, Q. Shi, R. Mi, H. Liu, J. Mei, L.W.M. Lau, Y. Chen,;1; Temperature dependence of charging characteristic of C-free Li2O2 cathode in Li-O2 battery, J. Solid State Electrochem., 17 (2013) 2061-2069.
[17] D. Kodama, M. Kanakubo, M. Kokubo, S. Hashimoto, H. Nanjo, M. Katoa,;1; Density, viscosity, and solubility of carbon dioxide in glymes, Fluid Phase Equilibria., 302 (2011) 103-108. [18] J. Read, K. Mutolo, M. Ervin, W. Behl, J. Wolfenstine, A. Driedger, D. Foster,;1; oxygen transport properties of organic electrolytes and performance of Lithium oxygen battery, J. Electrochem. Soc., 150 (2003) A1351-A1356. [19] J. Read,;1; Ether-based electrolytes for the Lithium/Oxygen organic electrolyte battery, J. Electrochem. Soc., 153 (2006) A96-A100. [20] W. Xu, J. Xiao, K. Xu, D. Wang, J. Zhang, J.G. Zhang,;1; Crown ethers in nonaqueous electrolytes for Lithium/Air batteries, Electrochem. Solid-State Lett., 13 (2010) A48-A51. [21] W. Xu, J. Xiao, J. Zhang, D. Wang,;1; Zhang, J.G., Optimization of nonaqueous electrolytes for primary Lithium/Air Batteries operated in ambient environment, J. Electrochem. Soc., 156 (2009) A773-A779. [22] D. Zhu, L. Zhang, M. Song, X. Wang, R. Mi, H. Liu, J. Mei, L.W.M. Lau,Y. Chen,;1; Intermittent operation of the aprotic Li-O2 battery: the mass recovery process upon discharge interval, J. Solid State Electrochem., 17 (2013) 2539-2544. [23] J. Zhang, B. Sun, H.J. Ahn, C. Wang, G. Wang,;1; Conducting polymer-doped polyprrrole as an effective cathode catalyst for Li-O2 batteries, Mater. Res. Bull, 48 (2013) 4979-4983. [24] S.S. Zhang, D. Foster, J. Read,;1; Discharge characteristic of a non-aqueous electrolyte Li/O2 battery, J. Power Sources, 195 (2010) 1235-1240. [25] C.Ó. Laoire, S. Mukerjee, E.J. Plichta, M.A. Hendrickson, K.M. Abraham,;1; Rechargeable Lithium/TEGDME-LiPF6/O2 battery, J. Electrochem. Soc., 158 (2011) A302-A308.
Figure caption Fig. 1 Galvanostatic discharge and charge profiles of Super P electrodes at different discharging and same charging temperatures
Fig. 2 SEM of discharged Super P electrodes (medium magnification). a) pristine electrode, b)303 K discharged electrode, c) 323 K discharged electrode, d) 343 K discharged electrode Fig. 3 SEM of discharged Super P electrodes (high magnification). a) pristine electrode, b)303 K discharged electrode, c) 323 K discharged electrode, d) 343 K discharged electrode Fig. 4 Cathodic LSV curves of Super P electrodes at different test temperatures Fig. 5 Anodic LSV curves（ 343K） of Super P electrodes that have discharged at different temperatures Fig. 6 Nyquist plots of Super P electrodes at 343K that have discharged at different temperatures FIGURES
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6