Confined-Space-Induced Side Reactions in Li-O2 Electrochemistry

Abstract

Rechargeable lithium-oxygen battery (LOB) is one of potentially promising candidates for next-generation batteries of which criteria typically set as > 500 Wh kg-1 in a full-cell configuration [1]. However this rechargeable battery system suffers from a wide spectrum of side reactions at the all main components, i.e. anode, electrolyte, and cathode, and also at their interfaces [2-5]. Therefore, the suppression of side reactions is a key challenge in LOB. Especially, understanding mechanism of side reactions occurring at the interface of porous cathode/electrolyte is important to improve the basic electrochemical properties of LOB, therefore, is also essential to enhance the battery performances of LOB. In our previous works, we prepared a variety of porous-carbon-based cathodes to investigate the effects of heteroatom doping or hierarchical porestructures in Li-O2 electrochemistry [6-9], and found that a porestructure gives considerably strong influences to electrochemical growth/decomposition of Li2O2 at a solid-liquid interface in porous electrodes. Driven by these previous findings, in this work, we aimed to understand further details of the Li-O2 electrode process in porous solids by, and show here that unfavorable side reactions in porous electrodes are induced if the reaction takes place at confined-spaces.In our study, we controlled the electrochemical growth of Li2O2 in a porous electrode by changing current densities, and therefore can selectively form Li2O2 at macropores or mesopores/macropores confirmed by nitrogen adsorption/desorption experiments and X-ray diffractometry. Furthermore, operando gas analysis, combining electrochemical experiments and on-line mass spectrometry, revealed that the Li2O2 formed in macropores can almost solely proceed the electrochemical decomposition reaction for O2 generation, on the other hand, Li2O2 in mesopores generate a considerable amount of CO2, even around 3.5 V vs Li/Li+. These results indicated that a confinement space could trigger Li2O2 to unfavorable side reactions to form CO2. Therefore, a fine control of porestructure in cathodes is important for LOB not only to improve round trip energy efficient but also to obtain reversible Li2O2 formation, i.e. O2 + 2Li+ + 2e − ↔ Li2O2 [10].In the presentation, a detailed mechanism will be explained how confined-space-induced side reactions are proceeded.[1] M. Ue, K. Sakaushi, K. Uosaki, Materials Horizons, DOI: 10.1039/D0MH00067A (2020).[2] B. D. McCloskey, D. S. Bethune, R. M. Shelby, G. Girishkumar, A. C. Luntz, J. Phys. Chem. Lett., 2, 1161 (2011).[3] B. D. McCloskey, A. Speidel, R. Scheffler, D. C. Miller, V. Viswanathan, J. S. Hummelshøj, J. K. Nørskov, A. C. Luntz, J. Phys. Chem. Lett., 3, 997 (2012).[4] W.-J. Kwak, Rosy, D. Sharon, C. Xia, H. Kim, L. R. Johnson, P. G. Bruce, L. F. Nazar, Y.-K. Sun, A. A. Frimer, M. Noked, S. A. Freunberger, D. Aurbach. Chem. Rev., DOI: 10.1021/acs.chemrev.9b00609 (2020).[5] T. Liu, J. P. Vivek, E. W. Zhao, J. Lei, N. Garcia-Araez, C. P. Grey, Chem. Rev., DOI: 10.1021/acs.chemrev.9b00545 (2020).[6] K. Sakaushi, T.-P. Fellinger, M. Antonietti, ChemSusChem, 8, 1156 (2015).[7] K. Sakaushi, S.-J. Yang, T.-P. Fellinger, M. Antonietti, J. Mater. Chem. A, 3, 11720 (2015).[8] K. Sakaushi, M. Antonietti, Bull. Chem. Soc. Jpn., 88, 386 (2015).[9] K. Sakaushi, M. Antonietti, Acc. Chem. Res., 48, 1591 (2015).[10] K. Sakaushi, H. Asahina, K. Uosaki, in preparation.