EFFECTS OF AIR ELECTRODE AND APROTIC SOLVENT ON LITHIUM-OXYGEN BATTERY PERFORMANCE

Transcription

1 EFFECTS OF AIR ELECTRODE AND APROTIC SOLVENT ON LITHIUM-OXYGEN BATTERY PERFORMANCE 1 MICHAELTANG, 2 CHUN-CHEN YANG, 3 SHINGJIANG JESSIE LUE 1 Department of Chemical and Materials Engineering, Chang Gung University, Taiwan 2 Department of Chemical Engineering, Ming-Chi University of Technology, Taiwan 3 Department of Radiation Oncology, Chang Gung Memorial Hospital, Taiwan 1,2,3 Department of Safety, Health and Environmental Engineering, Ming-Chi University of Technology, Taiwan 1 Mike @yahoo.com.tw, 2 ccyang@mail.mcut.edu.tw, 3 jessie@mail.cgu.edu.tw Abstract - Lithium-oxygen battery has a high power density and has good potential for commercial energy storage device. In the rechargeable lithium-air battery, electrolyte and electrode play important roles to supply ion transport paths, to maintain sufficient conductivity, and to enable electrochemical reactions. The objective of this research is to investigate various designs of lithium air battery: including the use of microporous hydrophobic layer and different aprotic solvent. The effects of these constituents on the capacity and cycle life of lithium-oxygen batteries using lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/tetraethylene glycol dimethyl ether (tetraglyme) or diethylene glycol dimethyl ether electrolyte are investigated. The aged electrodes after discharge/charge cycles were examined for morphology and elemental composition. The best performance was resulted from applying catalyst on carbon cloth containing microporous hydrophobic layer and employing tetraglyme solvent. A capacity of 2000 mah/g-pt for 15 cycles (300 h) at current density of 0.05 ma/cm 2 were obtained. Lithium carbonate deposits were observed after cycling test, especially on the surface toward oxygen inlet.these carbonate deposits may lead to battery failure. Therefore catalyst on the surface toward air inlet is recommend for future study Index Terms - Lithium-oxygen battery, Organic electrolyte, Electrode structure, Battery aging. I. INTRODUCTION Rechargeable battery with high capacity is the target for energy storage system and electrical vehicle industry. The primary advantage of lithium-air battery lies in its extremely high energy capacity. Using lithium metal as anode and oxygen (which can be obtained from ambient air, as shown in Fig. 1), the maximum energy density can reach up to 11,586 Wh/kg, one order of magnitude higher than that of lithium ion battery [1,2]. Therefore, lithium-air battery is considered as ultimate battery [3]. In this research, aprotic configuration [4] of lithium-oxygen battery is investigated. The effects of electrode design and choice of electrolyte solvent are studied and their impactson battery performance of resulting coin cells are evaluated. The aged batteries are examined in order to provide future approach for prolong lifetime. II. EXPERIMENTAL A. Air Electrode Preparation The air cathode was prepared onptfe-coated carbon cloth,as shown in Fig. 2. A catalyst ink was prepared by mixing Pt/C(Johnson Matthey, carbon support of Vulcan XC-72), Nafion binder solution (D520, Chemours Co.), in water/isopropyl alcohol (from Sigma-Aldrich) solution. This ink was sprayed evenly onto the polytetrafluoroethylene (PTFE)surface of the carbon cloth (Cetech Co., Ltd.) containing microporous PTFE layer. Another electrode of carbon cloth without PTFE coating was prepared for comparison. The catalyst-loaded carbon cloth was dried at 120 C for 2 hours, weighed, and cut into circular shape. The resulting catalyst loading was 0.5 mg/cm 2. Fig.2. Illustration of air electrode preparation. Fig. 1. Operation principle of non-aqueous (aprotic) lithiumoxygen battery at discharge state [1]. B. Electrolyte Preparation Electrolyte consisted of 1 M of lithium bis(trifluoromethane)sulfonamide (LiTFSI, from Sigma-Aldrich) in tetraethylene glycol dimethyl ether (tetraglyme) or diethylene glycol dimethyl ether (tetraglyme) (from Sigma-Aldrich) solvent. These 101

2 electrolyte solutions were prepared in an Mbraun glove box to avoid contact with moisture and oxygen. the PTFE layer helped stabilized discharge voltage over the running cycle. C. Battery Assembly and Test A glass fiber filter (with thickness of 0.42 mm, pore size of 290 m) was used as the separator and soaked in an electrolyte solution for 24 h. Theair cathode, electrolyte (3 drops), glass fiber filter, lithium foil (0.2 mm thick),andspring leaf were loaded in order into a CR2032 coin cell compartment. After the battery was sealed, pure oxygen at a flow rate of 50 ml/min was circulated to the coin cell to activate the battery for 8 h before further analyses.the battery was tested for cycling charge/discharge condition between 2 and 4.5 V at 0.1 ma current. The charge or discharge period lasted for 10 h with 5 minutes of intermission, unless the voltage limit was reached. The discharge voltage and specific energy capacity were calculated to demonstrate the electrochemical performance of the battery. III. RESULTS AND DISCUSSION A. Effect of PTFE microporous layer The pristine carbon cloth had pore sizes of m, and the pore sizes shrank to m after the PTFE layer coating (Fig. 3). The microstructure of the catalyst-loaded air electrodes with and without PTFE layer is shown in Fig. 3. The carbon fibers (with diameter of in m) were woven into cloth. After the catalyst was sprayed onto the carbon fiber mat, the surface became rough, with nano-sized Ptloaded carbon spheres. The carbon spheres were about nm in diameter (Fig. 4 top). When the catalyst was sprayed on the PTFE coating, this surface exhibited cracks and nano-sized Pt/C (Fig. 4 bottom). The batteries with these electrodes were cycled on discharge and charge runs. The battery without PTFE had a life time of 170 h, whereas the PTFEcontaining battery sustained longer at 300 h (Fig. 5). Their capacity values were at 2000 mah/g-pt. It is clear that PTFE-free sample needed higher charge voltage than the PTFE-containing one. In addition, B. Electrolyte Effect Two kinds of electrolytes were used in battery and tested in this experiment: 1 M LiTFSI in tetraglyme and 1 M LiTFSI in diglyme, using PTFE-containing air electrode. The battery of tetraglyme had larger bulk resistance than diglyme sample (14.5 vs ), probably due to its slight lower conductivity (2.25 vs ms/cm). The charge-transfer resistance was smaller for tetraglyme (18.9 vs ) than the diglyme battery, as shown in Fig. 6 top graphs. This may be associated with the solvents contact angles (70 vs. 59 ). The battery performance using these electrolytes is shown in Fig. 6 bottom graphs. The tetraglyme battery discharged stably at 2.7 V, whereas the diglyme battery exhibited severe voltage decay. In addition, tetraglyme battery had a longer lifetime than the diglyme one (300 vs. 220 h). The better performance of the tetraglyme solvent than the diglyme may be associated with the chemical inertia of the ether groups [5], which is resistant to decomposition by the oxygen radical anion (O 2 ) [6,7] resulted from the oxygen reduction process to the formation of lithium peroxide (Li 2 O 2 ). The tetraglyme is more viscous than the diglyme solution (15.9 vs mpa s), which may maintain wettability of the electrode and prevent solvent from evaporation. Other potential solvents and lithium salts are tested and reported in our recent publication [8]. C. Diagnosis on aged batteries These two batteries in Fig. 6 were dissembled after cycling test and the air electrodes were examined, as shown in Figs The batteries containing tetraglyme and diglyme showed deposits on the air electrodes. These deposits were more abundant on the carbon fibers toward the air side, rather than the PTFE side. The catalyst/ptfe surface was facing lithium anode and the migrated lithium ion flux might have been distributed evenly due to the PTFE layer and maintained soluble in the electrolyte. Fig. 3. Pore size distribution of pristine carbon cloth (left) and carbon cloth with PTFE microporous layer (right). 102

3 Fig. 4. Scanning electron microscopy images of carbon cloth (top), with PTFE coated layer (middle), and loaded with catalyst (bottom). The EDX analysis on the air electrodes revealed that oxygen content was higher in the diglyme battery than the tetraglyme counterpart (60% vs. 54%, Fig. 9), although both oxygen contents of the aged batteries were significantly higher than the fresh electrode (13.7%). The XRD spectra confirmed further that the deposits were lithium carbonate (fig. 10). The source of the carbonate may come from the deposition of electrolyte, carbon support, and ambient carbon dioxide gas in the air.the abundant carbonate deposits on the air electrode toward oxygen inlet may lead to battery failure. Further investigation is needed to minimize the carbonate occurrence in order to prolong the battery life. Fig. 5. Lithium-oxygen battery discharge/charge voltage and capacity for air electrodes with and without PTFE coating. 103

4 Fig. 6. Battery impedance (top) and cycling voltage (bottom) of lithium-oxygen batteries containing tetraglyme (left) or diglyme (right) solvent. Fig. 7. Morphology of air electrode for tetraglyme-containing lithium-oxygen battery: fresh (far left) and aged (300 h) samples. Fig. 8. Morphology of air electrode for diglyme-containing lithium-oxygen battery: fresh (far left) and aged (220 h) samples. 104

5 Fig. 9. Morphology (top) and EDX spectra (bottom) of deposits of air electrodeof aged lithium-oxygen battery containing tetraglyme (left) or diglyme (right)solvent. carbonate deposits on the air electrode toward oxygen inlet may lead to battery failure. Therefore catalyst on the air side is recommend for future study. REFERENCES Fig. 10. XRD spectra of fresh air electrode and aged electrode from tetraglyme battery. Li side refers to the catalyst/ptfe surface and air side to the carbon fiber surface (as shown in Figs. 7-8). CONCLUSION In this work, we confirm the beneficial effect of PTFE coating on carbon cloth for battery performance. The battery with tetraglyme solvent outperformed that with the diglyme one. The best performance was obtained whenlitfsi(tetraglyme) was used on PTFE-containing carbon cloth as air electrode, which resulting in a capacity of 2000 mah/g-catalyst and 15 cycles (300 h). The abundant [1] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, and J.M. Tarascon, Li-O 2 and Li-S batteries with high energy storage, Nature Mater. 11 (2012) [2] G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson, and W. Wilcke, Lithium-air battery: promise and challenges, J. Phys. Chem. Lett. 1 (2010) [3] [4] R. Younesi, G.M. Veith, P. Johansson, K. Edström, and T. Vegge, Lithium salts for advanced lithium batteries: Li metal, Li O 2, and Li S, Energy Environ. Sci. 8 (2015) [5] H.G. Jung, J. Hassoun, J.B. Park, Y.K. Sun, and B. Scrosati, An improved high-performance lithium air battery, Nature Chem. 4 (2012) [6] Y.C. Lu, H.A. Gasteiger, and Y. Shao-Horn, Method development to evaluate the oxygen reduction activity of high-surface-area catalysts for Li air batteries, Electrochem. Solid State Lett. 14 (2011) A70 A74. [7] M. Balaish, A. Kraytsberg, Y. Ein-Eli, A critical review on lithium-air battery electrolytes, Phys.Chem. Chem. Phys. 16 (2014) [8] M. Tang, K.L. Lin, C.C. Yang, and S.J. Lue, Preliminary studies on electrolyte and electrode designs for lithium-oxygen batteries and their electrochemical performance, Inter. J. Nanopart. Nanotechnol., in press. 105