A combined experimental and computational study has been performed in order to elucidate the effect of electrolyte salt concentration on the performance of Li-O2 batteries. Four electrolyte solutions with varying lithium triflimide (LiTFSI) content in 1,2-dimethoxyethane (DME) were studied to identify principal failure mechanisms in Li-O2 batteries for dilute and concentrated electrolytes (0.1 M to saturation) in cells cycled with high overpotentials and/or deep discharge. Quantitative 19F NMR was employed to determine that in 0.1 M electrolyte solutions salt decomposition can contribute to limitations in cell recycling arising from low ionic conductivity due to a decrease in available soluble Li+ over multiple cycles. In contrast, increased salt decomposition in high-concentration electrolytes can result in cathode passivation by insoluble Li salts that impact capacity by hindering Li2O2 production and further inhibiting electronic conductivity. By employing first-principles calculations, we modeled different pathways for the decomposition of the TFSI anion and found that it was particularly susceptible to decomposition in its neutral state, for example, if H+ is present and bound to the TFSI anion. The cumulative results suggest that employing low-concentration electrolytes with more stable lithium salts are ideal for better cell performance.