Confined-Space-Induced Side Reactions in Li-O2 Electrochemistry
Authors: Sakaushi Ken , Asahina Hitoshi , Matsuda Shoichi , Uosaki Kohei 
Published: December 2020

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.
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