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-O 2 ) batteries have received extraordinary research attention recently owing to their potential to provide positive electrode gravimetric energies considerably higher ($3 to 5Â) than Li-ion positive electrodes, although the packaged device energy density advantage will be lower ($2Â). In light of the major technological challenges of Li-O 2 batteries, we discuss current understanding developed in non-carbonate electrolytes of Li-O 2 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 (Li 2 O 2 thickness less than $1 nm), but not at large Li 2 O 2 thicknesses, yielding insights into the governing processes during discharge. In addition, we discuss the characteristics of discharge products (mainly Li 2 O 2 ) including morphological, electronic and surface features and parasitic reactivity with carbon. On charge, we examine the reaction mechanism of the oxygen evolution reaction from Li 2 O 2 and the influence of catalysts on bulk Li 2 O 2 decomposition. These analyses provide insights into major discrepancies regarding Li-O 2 charge kinetics and the role of catalyst. In light of these findings, we highlight open questions and challenges in the Li-O 2 field relevant to developing practical, reversible batteries that achieve the anticipated energy density advantage with a long cycle life. Broader contextLithium-O 2 batteries have received heightened attention in the last ve years owing to an increasing need for high-density energy storage for electric vehicles. Among the available battery chemistries, the Li-O 2 system is, in some regards, one of the most promising. This is largely attributed to a signicant gravimetric energy enhancement compared to Li-ion, with Li-O 2 projected to have at least a factor of two enhancement for a fully packaged battery. However, practical Li-O 2 batteries will only be successfully developed once current battery performance challenges are adequately addressed. Critical challenges include low round-trip efficiency resulting from high charging overpotentials, poor cycle life, and low power. These challenges present exciting opportunities for continued fundamental studies that can pave the way for improving electrode performance. Developing deeper mechanistic understanding of oxygen redox reactions in organic electrolytes, morphological and electronic features of reaction products, and improving the chemical stability of electrode and electrolyte would enable more effective rational design of electrodes and Li-O 2 batteries to meet high expectations for improved performance.

Understanding the origins of high overpotentials required for Li 2 O 2 oxidation in Li-O 2 batteries is critical for developing practical devices with improved round-trip efficiency. While a number of studies have reported different Li 2 O 2 morphologies formed during discharge, the influence of the morphology and structure of Li 2 O 2 on the oxygen evolution reaction (OER) kinetics and pathways is not known. Here, we show that two characteristic Li 2 O 2 morphologies are formed in carbon nanotube (CNT) electrodes in a 1,2-dimethoxyethane (DME) electrolyte: discs/toroids (50-200 nm) at low rates/overpotentials (10 mA g CÀ1 or E > 2.7 V vs. Li), or small particles (<20 nm) at higher rates/overpotentials. Upon galvanostatic charging, small particles exhibit a sloping profile with low overpotential (<4 V) while discs exhibit a twostage process involving an initially sloping region followed by a voltage plateau. Potentiostatic intermittent titration technique (PITT) measurements reveal that charging in the sloping region corresponds to solid solution-like delithiation, whereas the voltage plateau (E ¼ 3.4 V vs. Li) corresponds to two-phase oxidation. The marked differences in charging profiles are attributed to differences in surface structure, as supported by X-ray absorption near edge structure (XANES) data showing that oxygen anions on disc surfaces have LiO 2 -like electronic features while those on the particle surfaces are more bulk Li 2 O 2 -like with modified electronic structure compared to commercial Li 2 O 2 . Such an integrated structural, chemical, and morphological approach to understanding the OER kinetics provides new insights into the desirable discharge product structure for charging at lower overpotentials. Broader contextLi-O 2 batteries are promising as a next-generation electrochemical energy storage technology due to the potentially several-fold improvement in gravimetric energy compared to today's Li-ion batteries. However, Li-O 2 batteries face substantial challenges that currently limit their practical use such as low rate capability, limited cycle life (typically below 100 cycles) resulting largely from the poor chemical and electrochemical stability of the electrode and electrolyte, and the large voltage polarization ($0.6 to 1 V above thermodynamic) required on charge due to the slow kinetics of oxygen evolution from Li 2 O 2 . While select surfaces or catalysts have been demonstrated to lower the charging voltage, the processes occurring in electrochemically formed Li 2 O 2 during charge are not well understood. Further, the inuence of Li 2 O 2 morphologies (e.g. particle shape and structure) and corresponding surface properties on the charging voltages are not known. Characterization and control over products formed upon discharge, as reported in this study, enable new insights into the governing physical parameters of Li 2 O 2 that inuence the charging behaviour. Combined chemical, electrochemical, morphological and electronic understanding is increasingly important as researchers se...

We report considerable chemical and morphological changes of reaction products in binder-free, vertically-aligned carbon nanotube (VACNT) electrodes during Li-O 2 battery cycling with a 1,2-dimethoxyethane (DME)-based electrolyte. X-ray absorption near edge structure (XANES) of discharged oxygen electrodes shows direct evidence for the formation of Li 2 CO 3 -like species at the interface between VACNTs and Li 2 O 2 , but not significantly on the Li 2 O 2 surfaces exposed to the electrolyte. Although Li 2 O 2 and Li 2 CO 3 -like species were largely removed upon first charge, the oxidation kinetics became increasingly difficult during Li-O 2 cycling, which is accompanied by the accumulation of Li 2 CO 3 in the discharged and charged electrodes as evidenced by selected area electron diffraction (SAED) and transmission electron microscopy (TEM). Together, these results indicate that the irreversibility during Li-O 2 cycling in DME can be attributed largely to the growth of Li 2 CO 3 -like species associated with the reactivity between carbon and Li 2 O 2 or other reaction intermediates.

Aqueous organic redox flow batteries (RFBs) could enable widespread integration of renewable energy, but only if costs are sufficiently low. Because the levelized cost of storage for an RFB is a function of electrolyte lifetime, understanding and improving the chemical stability of active reactants in RFBs is a critical research challenge. We review known or hypothesized molecular decomposition mechanisms for all five classes of aqueous redox-active organics and organometallics for which cycling lifetime results have been reported: quinones, viologens, aza-aromatics, iron coordination complexes, and nitroxide radicals. We collect, analyze, and compare capacity fade rates from all aqueous organic electrolytes that have been utilized in the capacity-limiting side of flow or hybrid flow/nonflow cells, noting also their redox potentials and demonstrated concentrations of transferrable electrons. We categorize capacity fade rates as being “high” (>1%/day), “moderate” (0.1–1%/day), “low” (0.02–0.1%/day), and “extremely low” (≤0.02%/day) and discuss the degree to which the fade rates have been linked to decomposition mechanisms. Capacity fade is observed to be time-denominated rather than cycle-denominated, with a temporal rate that can depend on molecular concentrations and electrolyte state of charge through, e.g., bimolecular decomposition mechanisms. We then review measurement methods for capacity fade rate and find that simple galvanostatic charge–discharge cycling is inadequate for assessing capacity fade when fade rates are low or extremely low and recommend refining methods to include potential holds for accurately assessing molecular lifetimes under such circumstances. We consider separately symmetric cell cycling results, the interpretation of which is simplified by the absence of a different counter-electrolyte. We point out the chemistries with low or extremely low established fade rates that also exhibit open circuit potentials of 1.0 V or higher and transferrable electron concentrations of 1.0 M or higher, which are promising performance characteristics for RFB commercialization. We point out important directions for future research.

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