Solid films are usually metastable or unstable in the as-deposited state, and they will dewet or agglomerate to form islands when heated to sufficiently high temperatures. This process is driven by surface energy minimization and can occur via surface diffusion well below a film's melting temperature, especially when the film is very thin. Dewetting during processing of films for use in micro- and nanosystems is often undesirable, and means of avoiding dewetting are important in this context. However, dewetting can also be useful in making arrays of nanoscale particles for electronic and photonic devices and for catalyzing growth of nanotubes and nanowires. Templating of dewetting using patterned surface topography or prepatterning of films can be used to create ordered arrays of particles and complex patterns of partially dewetted structures. Studies of dewetting can also provide fundamental new insight into the effects of surface energy anisotropy and facets on shape evolution.

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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.

▪ Abstract  Polycrystalline films have wide variety of applications in which their grain structures affect their performance and reliability. Thin film growth techniques and growth conditions affect grain shapes, the distribution of grain sizes, and the distribution of the crystallographic orientations of grains. Variations in these structural properties are affected by the conditions under which grain nucleation, growth, coarsening, coalescence, and thickening occur. General trends in structural evolution in polycystalline films, as a function of processing conditions and materials class, are discussed in terms of these fundamental kinetic processes.