Indexed on: 14 Jul '19Published on: 13 Jul '19Published in: ACS Applied Materials & Interfaces
In the loop of numerous challenges and ambiguities, Li-O2 batteries are crawling to reach their commercialization phase. To achieve the progressive milestones, along with developments in the architecture of cathode, anode and electrolytes, understanding its failure mode is equally important. Under unrestricted charge-discharge protocol, cycleability of non-aqueous Li-O2 batteries are limited to only few cycles. This report examines an additive-free ether based Li-O2 battery in perspective of identifying the origin of possible side reactions and their affiliations to integral components of the battery. Structural and compositional changes during every charge-discharge sequence are studied using bottom-up sequential tear down analysis. The substantial increase in impedance and corresponding decrease in capacities after every cycle are interrelated to the amount of electrode passivation resulting from the discharge products and electrolyte decomposition. From the tear down analysis, it is approximated that, among the total capacity loss, ≈ 55% is attributed to the cathode, ≈ 28% of the loss corresponds to the anode and ≈ 17 % is attributed to the electrolyte, given that battery failure instigates from the "reactive oxygen species". Electrochemically formed Li2O2 via superoxide pathway induces large decomposition overpotentials up to 4.6 V vs. Li/Li+ due to its overrated reactivity with electrolyte and carbon support. In contrary, efficient decomposition of chemically formed Li2O2 below 3.9 V proves that the extra charge potential observed for electrochemically formed Li2O2 is in fact consumed for the decomposition of irreversibly formed side products via superoxide pathway. Spontaneous reactivity of Li2O2 and trivial reactivity of Li2O highlights the need of advanced strategies to manoeuver oxygen red-ox in selective pathways that unaffect the electrolyte and electrodes, and necessity of their synchronized performance for the evolution of practical Li-O2 batteries.