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.

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.

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.

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.

We report live observations of lithium polymer batteries upon cycling within the microscope antechamber by means of a scanning electron microscope, equipped with a transfer system that avoids sample air exposure. The well-established direct correlation between current density and dendrite formation is confirmed, and the formation of a mossy or dendritic interface was evidenced to be at the origin of the delaminating between the substrate, where the lithium is plated, and the polymer. These experiments enable a better understanding of the dendrite formation mechanism, like the important finding that they grow tipwise as well as sidewise. © 2002 The Electrochemical Society. All rights reserved.

The structure, synthesis, and electrochemical behavior of layered for 5/12, and 1/2 are reported for the first time. is derived from or by substitution of and by while maintaining all the remaining Mn atoms in the 4+ oxidation state. with can deliver steady capacities of 150 and 160 mAh/g at 30 and 55°C, respectively, between 3.0 and 4.4 V using a current density of 30 mA/g. Differential scanning calorimetry experiments on charged electrodes of for indicate that this material should be safer than with 5/12, and 1/2 can be cycled between 2.0 and 4.6 V to give capacities of about 200, 180, and 160 mAh/g, respectively, at 30°C. with gives a capacity of 220 mAh/g at 55°C between 2.0 and 4.6 V using a current density of 30 mA/g. © 2001 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 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 electrochemical performance of negative electrodes based on commercially available crystalline Si powder and sodium carboxymethyl cellulose (CMC) binder was investigated. Compared to the conventional binder, polyvinylidene fluoride, Si electrodes using CMC binder show vastly improved cycling performance. A high specific capacity of about for has been achieved with a lower cutoff potential of vs . Si electrodes made using CMC binder have better capacity retention than those using a binder consisting of CMC and styrene butadiene rubber. CMC is an extremely stiff and brittle polymer, so it is surprising that it functions well as a binder in electrodes where the volume change of the active material particles is about 100%.

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