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.

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.

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.

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%.

A composite material (NSCM) has been prepared by mixing nanometer‐scale pure powder and carbon black. The electrochemical performance of NSCM anodes for lithium rechargeable batteries exhibited an extremely high reversible capacity, over , at the tenth cycle. The voltage profile is flat and smooth ranged from 0.4 to vs. . In addition, the cyclic performance is good even at larger current densities. Consequently, NSCM can be used as a novel high capacity anode material for lithium‐ion batteries. ©1999 The Electrochemical Society

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.

Li insertion electrodes are made by pressing a mixture of active material and additives on a metallic substrate. Here we estimate how various interphase contacts affect the electrode kinetics. We apply variable external mechanical pressure onto different cathodes and measure their impedance response. Similar experiments are performed on dry composites in contact with: Al or Cu foil, or Ag paste. Most surprisingly, we find that the high-frequency impedance arc is due to the contact impedance between the metal and the electrode material. This is in fundamental contradiction with previous interpretations. We propose an equivalent circuit explaining the observed phenomena.

The binder's influence on the cycling stability of high-energy anodes for lithium-ion batteries is demonstrated. Varying the binder's nature strongly influences the composite electrode's performance on deep charging/discharging. If sodium-carboxymethylcellulose is used as binding agent, then a chemical bond between binder and silicon particles can be detected (attenuated total reflection–Fourier transform infrared spectroscopy). Consequently, a reaction mechanism that describes a condensation reaction between the binder and the silicon particles is verbalized. It is shown that, not necessarily the binder's physical flexibility, but its chemical interaction with the active masses is the major claim leading to long-lasting reversible lithium uptake/release.