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.

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.

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%.

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.

Nanocomposites of and conductive carbon were prepared by two different methods which lead to enhanced electrochemical accessibility of the Fe redox centers in this insulating material. Method A employs a composite of the phosphate with a carbon xerogel formed from a resorcinol-formaldehyde precursor; method B uses surface-oxidized carbon particles to act as a nucleating agent for phosphate growth. Both particle size minimization and intimate carbon contact are necessary to optimize electrochemical performance. Although both methods succeed for the first criteria, the latter is best achieved with method A, affording excellent characteristics in room temperature, liquid electrolyte cells. The resultant composite achieves 90% theoretical capacity at C/2, with very good rate capability and excellent stability. © 2001 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.

Using ac impedance, we studied the formation process of solid electrolyte interface (SEI) film on graphite electrode during initial cycles. Results show that the SEI formation takes place through two major stages. The first stage takes place at voltages above 0.25 V (before lithiation of graphite), during which a loose and highly resistive film is formed. The second stage occurs at a narrow voltage range of 0.25-0.04 V, which proceeds simultaneously with lithiation of graphite electrode. In the second stage, a stable, compact, and highly conductive SEI film is produced. © 2001 The Electrochemical Society. All rights reserved.

The capacity and cyclability of solid‐state synthesized laminate cells of type have been studied at 23, 40, and . Larger capacities were obtained for cells cycled at the elevated temperatures. No evidence was found of potentially troublesome reactions between the different components of the cells under charged and discharged conditions up to . Remanant X‐ray diffraction peaks were observed from the delithiated phase for cells cycled at ambient temperature. Mössbauer spectroscopy showed different residual amounts of at the different temperatures: 22, 7, and 8% at 23, 40, and , respectively. The amount of residual was thus largest for cells cycled at room temperature, suggesting that slow kinetics hinder effective lithium insertion. ©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.

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.