Hydrous ruthenium oxide (RuO2·xH2O or RuO x H y ) is a mixed proton−electron conductor which could be used in fuel cells and ultracapacitors. Its charge-storage (pseudocapacitance) and electrocatalytic properties vary with water content and are maximized near the composition RuO2·0.5 mol % H2O. We studied the atomic structure of RuO2·xH2O as a function of water content from x = 0.84 to 0.02 using X-ray diffraction and atomic pair density function (PDF). Even though the diffraction patterns of samples containing 0.84 to 0.35 mole of water are suggestive of “amorphous” structures, the PDF analysis clearly shows that up to 0.7 nm, the short-range atomic structure of all of these RuO2·xH2O samples resembles that of the anhydrous rutile RuO2 structure. We conclude that RuO2·xH2O is a composite of anhydrous rutile-like RuO2 nanocrystals dispersed by boundaries of structural water associated with Ru−O. Metallic conduction is supported by the rutile-like nanocrystals, while proton conduction is facilitated by the structural water along the grain boundaries. This structural picture explains the charge-storage and electrocatalytic properties of RuO2·xH2O in terms of competing percolation networks of metallic and protonic conduction pathways, that vary in volume as a function of the water content of the RuO2·xH2O. The control and optimization of electron and proton conducting volumes and pathways will lead to improved performance and guide the design of new materials.

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Lithium-ion batteries are prone to failure at low temperatures and dendrite growth during charging is one suspect. We attempt to understand lithium dendrite growth by observing their number, initiation time and growth rate at ambient and sub-ambient temperatures: −10 • C, 5 • C, and 20 • C using an in-situ optical microscopy cell (Li 0 |Li 0 ). We find that while dendrites initiate quickly at −10 • C, the cells at 5 • C short-circuit most rapidly due in part to a favorable morphology at this temperature. The experimental approach has broad applicability to other electrochemical energy storage technologies where mass transport limitations are present at low temperatures, particularly Li-air, Li-S, and Zn-air batteries. © The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. [DOI: 10.1149/2.0041502eel] All rights reserved.Manuscript submitted October 29, 2014; revised manuscript received November 24, 2014. Published December 11, 2014 Lithium dendrite formation at the anode during charging of lithium-ion batteries can initiate internal short circuits, a known failure mechanism. Dendrite-induced short circuits at low temperature have been identified as a causal factor in recent lithium-ion battery failures aboard commercial aircraft.1 Low temperature charging affects the kinetic processes occurring within the cell causing lithium ions to reduce and plate metallic lithium (Li 0 ) on the carbon anode rather than favoring ion intercalation into the carbon electrode, according to Equation 1. At best, this Li 0 deposition is manifested as uniform plating leading to a loss of useful capacity of the battery. In the worst case the Li 0 electrodeposits grow uncontrollably to form an internal short to the cathode capable of initiating the thermal runaway reaction. 2Factors which favor unstable Li + reduction to form dendrites rather than ion intercalation at low temperature include: lower solubility of Li + within the liquid electrolyte, greater ion pairing between Li + and the anion (PF 6 − ), and increased viscosity of electrolyte causing slowed ion diffusion.Akolkar recently developed a temperature-dependent diffusionreaction model to predict dendrite growth rate based upon the ratio of the dendrite tip current density to the current density on a flat lithium surface. 4 The model is built upon the principle that dendrite growth is more pronounced at low temperature due to increased mass transport resistance of lithium ions through a viscous electrolyte and reduced charge transfer resistance created by a thinner solid electrolyte interface (SEI) layer. The model predicts −10• C as the critical temperature below whic...

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The metastability of lithium electrodeposition continues to be a scientific mystery. Local ionic depletion has been conventionally argued to be a root cause for nonlinear morphological manifestations. Given the bulk nature of electrolyte transport limitation, it should be absent for very small interelectrode separations; however, even under such conditions, sustained electrodeposition is not observed. We find that the passivating film formed due to lithium's high reactivity alters the surface energies and in turn deposition preference for fresh lithium. This asymmetry in deposition preference leads to nonuniform surface structure growth and traps the electrolyte layer. Such electrolyte confinement causes polarization, even at subcritical currents. The existence of confined electrolyte and associated electrochemical complexations is proved through temperature-controlled electrodeposition experiments.Letter http://pubs.acs.org/journal/aelccp

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