Role of Discharge-Product Morphology and Formation Rates in Li–O2 Battery Performance

Abstract

Lithium–oxygen batteries have unprecedented theoretical capacity. Achieving the theoretical capacity in practice remains a challenge – even during low-power operation – because of higher-order phenomena that accompany the formation of discharge products, such as surface passivation and clogging of the cathode’s pores. In the last few years, Li–O2 battery research has moved towards systems that preferentially form large discharge-product particles, which can be achieved by using high-donicity electrolytes and including mediators as additives.1 In this study we examine the implications of discharge-product growth and morphology on performance using numerical simulations.A continuum model is developed which incorporates multicomponent mass transfer2 described by Onsager-Stefan-Maxwell laws. Including more details about liquid-phase transport is particularly important when modelling solution-mediated discharge processes, because the diffusion of soluble reaction intermediates can control the battery’s apparent discharge capacity. Detailed reaction models, which include precipitation and film-growth kinetics are also necessary, to elucidate the competition between discharge-product formation pathways3: thin-film formation can slow interfacial reaction kinetics, whereas pore clogging raises overpotentials by hindering the transport of reactive species.Large discharge products form through a solution-phase mechanism, which is favoured by electrolytes that sustain the presence of an LiO2 intermediate for a relatively long time. These intermediate forms the final Li2O2 discharge product either by electrochemically reacting with a lithium ion or by chemically disproportionating, thereby contributing to discharge-product-particle growth. When LiO2 reaction intermediates are long-lived, they can build up in the electrolyte, particularly near the electrode/electrolyte interface. Thus, the system is locally pushed from the dilute into the concentrated regime, and pairwise interactions between species can become significant to device performance.(1) Torayev, A.; Magusin, P. C. M. M.; Grey, C. P.; Merlet, C.; Franco, A. A. Text Mining Assisted Review of the Literature on Li-O2 Batteries . J. Phys. Mater. 2019, 2, 044004.(2) Liu, J.; Khaleghi Rahimian, S.; Monroe, C. W. Capacity-Limiting Mechanisms in Li/O2 batteries. Phys. Chem. Chem. Phys. 2016, 18, 22840–22851.(3) Yin, Y.; Torayev, A.; Gaya, C.; Mammeri, Y.; Franco, A. A. Linking the Performances of Li–O2 Batteries to Discharge Rate and Electrode and Electrolyte Properties through the Nucleation Mechanism of Li 2 O 2. J. Phys. Chem. C 2017, 121, 19577–19585.