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.

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The oxidation kinetics of Li(2)O(2) was studied in a carbonate-free electrolyte using electrodes consisting of non-catalyzed and catalyzed Vulcan carbon (VC) and chemically synthesized Li(2)O(2) particles. VC and Au nanoparticles supported on VC (Au/C) were fairly inactive for catalyzing the oxidation of Li(2)O(2), 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 Li(2)O(2) oxidation, where Li(2)O(2) could be removed largely at voltages below 4 V(Li). In addition, Pt/C and Ru/C showed quick initiation of Li(2)O(2) oxidation in contrast to VC and Au/C.

Recent studies have shown that many aprotic electrolytes used in lithium-air batteries are not stable against superoxide and peroxide species formed upon discharge and charge. However, the stability of polymers often used as binders and as electrolytes is poorly understood. In this work, we select a number of polymers heavily used in the Li-air/Li-ion battery literature, and examine their stability, and the changes in molecular structure in the presence of commercial Li 2 O 2 . Of the polymers studied, poly (acrylonitrile) (PAN), poly(vinyl chloride) (PVC), poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-cohexafluoropropylene) (PVDF-HFP), and poly(vinyl pyrrolidone) (PVP) are reactive and unstable in the presence of Li 2 O 2 . The presence of the electrophilic nitrile group in PAN allows for nucleophilic attack by Li 2 O 2 at the nitrile carbon, before further degradation of the polymer backbone. For the halogenated polymers, the presence of the electron-withdrawing halogens, and adjacent alpha and beta hydrogen atoms that become electron-deficient due to hyperconjugation makes PVC, PVDF, and PVDF-HFP undergo dehydrohalogenation reactions with Li 2 O 2 . PVP is also reactive, but with much slower kinetics. On the other hand, the polymers poly(tetrafluoroethylene) (PTFE), Nafion ® , and poly(methyl methacrylate) (PMMA) appear stable against nucleophilic Li 2 O 2 attack. The lack of labile hydrogen atoms and the poor leaving nature of the fluoride group allows for the stability of PTFE and Nafion ® , while the methyl and methoxy functionalities in PMMA reduce the number of potential reaction pathways for Li 2 O 2 attack in PMMA. Polyethylene oxide (PEO) appears relatively stable, but may undergo some crosslinking in the presence of Li 2 O 2 . Knowledge gained from this work will be essential in selecting and developing new polymers as stable binders and solid or gel electrolytes for lithiumair batteries.

We examine single step reactive electrospinning of poly(vinyl alcohol) (PVA) and a chemical cross-linking agent, glutaraldehyde (GA), with hydrochloric acid (HCl) as a catalyst to generate water insoluble PVA nanofibers. Such an approach using a conventional setup with no modification enables the fibers to cross-link during the electrospinning process, thereby eliminating the need for post-treatment. Significant changes in the rheological properties occur during in situ cross-linking, which we correlate with electrospinnability. In particular, we associate changes in dynamic rheological properties to changes in fiber morphology for two regions: (1) below the critical concentration to electrospin PVA only and (2) above the critical concentration to electrospin PVA only. In region 1 fiber morphology changes from beaded fibers to uniform fibers to flat fibers, and in region 2 fiber morphology changes from uniform fibers to flat fibers. Electrospinning windows to generate uniform fibers for both regions are determined and can be manipulated by changing the molar ratio of GA to PVA and the volume ratio of HCl to GA. The electrospun fibrous material generated can be rendered insoluble in water, and the uniform fiber morphology can be maintained after soaking in water overnight. The reactive electrospinning process also lowers the critical PVA concentration required for successful electrospinning of the system.