Electrochemical and Solid-State Letters

The structural changes in silicon electrochemically lithiated and delithiated at room temperature were studied by X-ray powder diffraction. Crystalline silicon becomes amorphous during lithium insertion, confirming previous studies. Highly lithiated amorphous silicon suddenly crystallizes at 50 mV to form a new lithium-silicon phase, identified as This phase is the fully lithiated phase for silicon at room temperature, not as is widely believed. Delithiation of the phase results in the formation of amorphous silicon. Cycling silicon anodes above 50 mV avoids the formation of crystallized phases completely and results in better cycling performance. © 2004 The Electrochemical Society. All rights reserved.

The most accurate values to date were determined for conductivity of water from 0-100°C, permitting new determination of high-temperature hydroxide ion equivalent conductance. These values were incorporated into a fundamental water coefficient table including hydroxide and hydrogen ion mobilities, water ionization constant, density, conductivity, and resistivity. The conductivity/resistivity values were measured with a multiple-pass, closed, recirculating flow conductivity system, with improved multiple resistance temperature device measurement, and improved analysis of temperature and impurity effects. An accurate conductivity knowledge is necessary to understand water-limiting processes and to facilitate the analysis of trace ionic impurities in water. © 2004 The Electrochemical Society. All rights reserved.

A mechanism that may cause accelerated performance decay of fuel cells is presented. The mechanism is explained using a one-dimensional model of the potential profile. The analysis indicates that the electrolyte potential drops from 0 to (vs. RHE) when the anode is partially exposed to hydrogen and partially exposed to oxygen. This causes flow of current opposite to normal fuel cell mode at the oxygen-exposed region and raises the cathode interfacial potential difference to 1.44 V, causing carbon corrosion, which decreases performance. The decay mechanism was validated using two different experimental setups which reproduced the carbon-corrosion phenomenon.

Materials with the olivine structure form an important class of rechargeable battery cathodes. Using first-principles methods, activation barriers to Li ion motion are calculated and an estimate for Li diffusion constants, in the absence of electrical conductivity constraints, is made. Materials with Fe, Co, Ni are considered. Li diffuses through one-dimensional channels with high energy barriers to cross between the channels. Without electrical conductivity limitations the intrinsic Li diffusivity is high. © 2003 The Electrochemical Society. All rights reserved.

The mechanism by which is transformed into isostructural has been elucidated using electron microscopy on large, hydrothermally grown crystals following chemical delithiation. Lithium is extracted at narrow, disordered transition zones on the crystal surface as the phase boundary progresses in the direction of the -axis. The substantial lattice mismatch along (ca. 5%) causes crack formation in the plane. Despite considerable disorder in the transition zone, the general structural arrangement is preserved, leading to good crystallinity in the newly created domains. Implications for improved electrode performance are discussed.

A quantitative approach is used to identify sources of contribution of capacity fade in commercial rechargeable lithium battery cells in laboratory evaluations. Our approach comprises measurements of close-to-equilibrium open-circuit voltage (cte-OCV) of the cell after relaxation at the end of the charging and discharging regimes and an incremental capacity analysis, in addition to conventional cycle-life test protocols using the dynamic stress test schedule. This approach allows us to separate attributes to capacity fade due to intrinsic and extrinsic origins.

The stability of in water was investigated. From high-resolution transmission electron microscopy observation, electron energy-loss spectroscopy analyses, Mössbauer experiments, and chemical analyses, we showed that a layer is present at the grains surface after immersion in water. This layer, which is a few nanometers thick, is accompanied by an increase of the percentage in the grains and a continuous dissolution of the layer. If experimental conditions such as immersion time, pH, and concentration are set to optimal values, the transition to an aqueous route for preparing -based positive electrodes could, in fact, be achieved.

The results of a first principles investigation of lithium diffusion within the layered form of are presented. Kinetic Monte Carlo simulations predict that lithium diffusion is mediated through a divacancy mechanism between and and with isolated vacancies at infinite vacancy dilution. The activation barrier for the divacancy migration mechanism depends strongly on lithium concentration resulting in a diffusion coefficient that varies within several orders of magnitude. We also argue that the thermodynamic factor in the expression of the chemical diffusion coefficient plays an important role at high lithium concentration. ©2000 The Electrochemical Society

A new lithium salt based on a chelated borate anion, BOB [bis(oxalato)borate] is evaluated as the electrolyte solute for lithium-ion cells by both electrochemical means and cell testing. Controlled potential coulometry study reveals that the anion can stabilize aluminum substrate to more positive potentials than the popular hexafluorophosphate anion does, while slow scan cyclic voltammograms show good compatibility of the salt with graphitizable carbonaceous anode as well as satisfactory stability against charged cathode surface. The lithium-ion cells containing this salt as electrolyte solute exhibit excellent capacity utilization, capacity retention as well as rate capability at room temperature. Probably due to the fact that the new anion contains no labile fluorine and is thermally stable, electrolyte solutions based on it demonstrate stable performance in cells even at 60°C, where -based electrolytes would usually fail. The preliminary results reported herein provide a possible solution to the instability of the Li-ion cell performance at the elevated temperatures anticipated for heavy duty applications such as electric or hybrid electric vehicle missions. © 2001 The Electrochemical Society. All rights reserved.

On-board hydrogen storage remains a big challenge for fuel cell powered electric vehicles. Ammonia contains 17.6 wt % hydrogen and has been recognized as a potential on-board vehicular hydrogen media. Direct ammonia fuel cells are interesting because they do not require an ammonia cracking process to produce hydrogen, whereas conventional proton exchange membrane fuel cells based on acidic membranes such as Nafion are not compatible with . Here we report the operation of direct ammonia alkaline anion-exchange fuel cells based on low cost membrane and non-noble catalysts with potential use in transportation and other applications.