Rechargeable energy storage systems with high energy density and round-trip efficiency are urgently needed to capture and deliver renewable energy for applications such as electric transportation. Lithium–air/lithium–oxygen (Li–O₂) batteries have received extraordinary research attention recently owing to their potential to provide positive electrode gravimetric energies considerably higher (∼3 to 5×) than Li-ion positive electrodes, although the packaged device energy density advantage will be lower (∼2×). In light of the major technological challenges of Li–O₂ batteries, we discuss current understanding developed in non-carbonate electrolytes of Li–O₂ redox chemistry upon discharge and charge, oxygen reduction reaction product characteristics upon discharge, and the chemical instability of electrolytes and carbon commonly used in the oxygen electrode. We show that the kinetics of oxygen reduction reaction are influenced by catalysts at small discharge capacities (Li₂O₂ thickness less than ∼1 nm), but not at large Li₂O₂ thicknesses, yielding insights into the governing processes during discharge. In addition, we discuss the characteristics of discharge products (mainly Li₂O₂) including morphological, electronic and surface features and parasitic reactivity with carbon. On charge, we examine the reaction mechanism of the oxygen evolution reaction from Li₂O₂ and the influence of catalysts on bulk Li₂O₂ decomposition. These analyses provide insights into major discrepancies regarding Li–O₂ charge kinetics and the role of catalyst. In light of these findings, we highlight open questions and challenges in the Li–O₂ field relevant to developing practical, reversible batteries that achieve the anticipated energy density advantage with a long cycle life.

The O₂electrode in Li–O₂cells was shown to exhibit gravimetric energy densities (considering the total weight of oxygen electrode in the discharged state) four times that of LiCoO₂ with comparable gravimetric power. The discharge rate capability of Au-catalyzed Vulcan carbon and pure Vulcan carbon (VC) as the O₂electrode was studied in the range of 100 to 2000 mA g_carbon ⁻¹. The discharge voltage and capacity of the Li−O₂ cells were shown to decrease with increasing rates. Unlike propylene carbonate based electrolytes, the rate capability of Li−O₂ cells tested with 1,2-dimethoxyethane was found not to be limited by oxygen transport in the electrolyte. X-Ray diffraction (XRD) showed lithium peroxide as the discharge product and no evidence of Li₂CO₃ and LiOH was found. It is hypothesized that higher discharge voltages of cells with Au/C than VC at low rates could have originated from higher oxygen reduction activity of Au/C. At high rates, higher discharge voltages with Au/C than VC could be attributed to faster lithium transport in nonstoichiometric and defective lithium peroxide formed upon discharge, which is supported by XRD and X-ray absorption near edge structure O and Li K edge data.

The lack of fundamental understanding of the oxygen reduction and oxygen evolution in nonaqueous electrolytes significantly hinders the development of rechargeable lithium-air batteries. Here we employ a solid-state Li_4+xTi₅O₁₂/LiPON/Li_xV₂O₅ cell and examine in situ the chemistry of Li-O₂ reaction products on Li_xV₂O₅ as a function of applied voltage under ultra high vacuum (UHV) and at 500 mtorr of oxygen pressure using ambient pressure X-ray photoelectron spectroscopy (APXPS). Under UHV, lithium intercalated into Li_xV₂O₅ while molecular oxygen was reduced to form lithium peroxide on Li_xV₂O₅ in the presence of oxygen upon discharge. Interestingly, the oxidation of Li₂O₂ began at much lower overpotentials (~240 mV) than the charge overpotentials of conventional Li-O₂ cells with aprotic electrolytes (~1000 mV). Our study provides the first evidence of reversible lithium peroxide formation and decomposition in situ on an oxide surface using a solid-state cell, and new insights into the reaction mechanism of Li-O₂ chemistry.

The oxidation kinetics of Li₂O₂ was studied in a carbonate-free electrolyte using electrodes consisting of non-catalyzed and catalyzed Vulcan carbon (VC) and chemically synthesized Li₂O₂ particles. VC and Au nanoparticles supported on VC (Au/C) were fairly inactive for catalyzing the oxidation of Li₂O₂, where oxidation currents greater than 10 mA g_carbon ⁻¹ were found only at voltages equal to and greater than 4.0 V vs. Li (V_Li). Pt and Ru nanoparticles supported on VC (Pt/C and Ru/C) could significantly increase the kinetics of Li₂O₂ oxidation, where Li₂O₂ could be removed largely at voltages below 4 V_Li. In addition, Pt/C and Ru/C showed quick initiation of Li₂O₂ oxidation in contrast to VC and Au/C.