Decomposition Mechanism of Discharged Product in Lithium Air Battery Optimized By Dandelion-like NiCo2O4 Hollow Sphere
Authors: Chi Bo , Zou Lu , Jiang Yuexing , Cheng Junfang , Li Jin , Li Jun 
Published: February 2020

Rechargeable non-aqueous lithium–air batteries have received worldwide attention over the past several years, because of their extremely high theoretical energy density which is 5–10 times higher than that of lithium-ion batteries.[1-2] However, there are many challenges to be overcome before it turning into practical devices, especially the high overpotentials and short cycling life. A lot of researchers have demonstrated the great influence of discharged products especially lithium peroxide (Li2O2) formation/decomposition on both discharge and charge process, which play a critical role in the LOB’s performance, like discharge capacity, overpotentials and cycling.[3-6]
Understanding the formation/decomposition mechanism of the products can effectively reduce the overpotential through the origins of high overpotentials. Here, we show the film characteristic Li2O2 morphologies formed on the cathode surface, which can be smoothly decomposed at low potential using NiCo2O4 electro-catalyst of designed structure.
The dandelion-like NiCo2O4 hollow sphere presents hybrid structure of macrospores and mesopores. The formation of oxygen vacancies on the metal oxide surface would also improve the adsorption of O2 and strengthen the reduction of O2. We also find that the designed NiCo­2O4 hollow sphere would enhance the OER kinetics and reduce the charge potential.
The battery consisting of lithium metal anode, electrolyte comprising 1 mol·L-1 LiTFSI in DMSO and porous cathode, is discharged and charged in O2 atmosphere. The tested cathode is then removed, washed with CH3CN, and examined by X-ray diffraction, FTIR and 1H NMR spectroscopy. The existence of Li2O2 could be demonstrated from XRD pattern. As for the maybe discharge products like Li2CO3, HCO2Li, C2H4(OCO2Li)2 etc, it is difficult to identify all the specific species by FTIR alone. To analyze discharged products in details, the porous cathode is washed with D2O and the solution is subjected to 1H NMR spectroscopy. Besides that, the ﬁeld emission scanning electron microscope is employed to observe the morphologies with different DODs (depth of discharge) and discharge/charge stages. Combined with SEM figures, the tested products can be observed directly and visually, which help us understand better how the products formation/decomposition influences the performance of LOBs.
References
[1] Z.Q. Peng, S.A. Freunberger and Y.H. Chen, Science, 337, 563-565 (2012).
[2] G. Girishkumar, B.D. McCloskey, A.C. Luntz, S. Swanson and W. Wilcke, J. Phys.Chem.Lett, 1, 2193–2203 (2010).
[3] Y.-C. Lu, H.A. Gasteiger, M.C. Parent, V. Chiloyan and Y. Shao-Horn, Electrochem. Solid-State Lett, 13, A69 (2010).
[4] B.D. McCloskey, D.S. Bethune, R.M. Shelby, G. Girishkumar and A.C. Luntz, J. Phys.Chem.Lett, 2, 1161-1166 (2011).
[5] B.D. McCloskey, R. Scheffler, A. Speidel, D.S. Bethune, R.M. Shelby and A.C. Luntz, J. Am.Chem.Soc,133, 18038 (2011).
[6] Q. Wang, D. Zheng, M.E. McKinnon, X.-Q. Yang and D. Qu, J. Power Sources 274, 1005-1008 (2015).

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