Steven M. Davis

Silver vanadium oxides and related battery applications

Abstract This review contains references from journals, proceedings volumes, and patents involving the preparation, characterization, reactivity, and battery applications of materials containing silver, vanadium, and oxygen, hereafter referred to as silver vanadium oxide (SVO). SVO has been a subject of regular study for a number of years, with earlier reports involving the synthesis and characterization of the various phases of SVO. However, with the relatively recent discovery of SVO as an important electrode material in batteries, the number of publications and patents involving the preparation, structure, reactivity and related battery applications of SVO has increased markedly. In light of this recent increase in research activity involving SVO and its related battery applications, this review is a timely examination of this exciting and growing area of research.

Anode Passivation and Electrolyte Solvent Disproportionation: Mechanism of Ester Exchange Reaction in Lithium‐Ion Batteries

Carbonate-based electrolytes used in lithium-ion electrochemical cells were found to undergo ester exchange reactions, where the use of dimethyl carbonate and diethyl carbonate resulted in the in situ formation of ethyl methyl carbonate. The reaction was found to be reversible and occurs during the first charge cycle of the LiCoO 2 /petroleum coke lithium-ion system. Mechanistic studies were carried out and determined that the ester exchange reaction is reductively initiated at the carbon anode. A mechanism has been deduced, with an intermediate alkoxide species responsible for the ester exchange reaction. Electrode passivation was found to limit the extent of the reaction during subsequent cycles, with the choice of electrolyte solvent impacting the passivation of the electrode.

Simulation of the Li-CFx system

The Li-CF x system has been successfully used in implantable batteries to power medical devices for humans. Amassed data is analyzed to show that the behavior is dominated by the open-circuit potential and Tafel kinetics. Data from several discharge load resistances can be collapsed onto a single curve on this basis. At the low rates of interest, transport limitations can be neglected. Data for open-circuit relaxation show that the potential is a straight line when plotted against the logarithm of time. This is in harmony with the dominance of Tafel kinetics.

Simulation of pulse discharge of the Li-CFx system

A zero-dimensional model is developed for the CF x system to account for Butler-Volmer kinetics, internal cell resistance, and double-layer (or capacitive) charging. At the low rates involved, other transport processes can be neglected. Furthermore, for this system, Tafel kinetics is appropriate. This model is used to compare with discharge data for periodic pulses superposed on an otherwise low rate of discharge. The internal resistance and high double-layer capacity account for some aspects of the pulses, such as a long time relaxation when the load is changed, but do not account for other time constants of less than 1 s. For a long constant-load discharge without pulses, the model can be simplified further, with the capacitance being negligible and the internal resistance being important only at low load resistances. Development of a modified open-circuit potential function allows all discharge data for cells of different design and load resistance to be collapsed using a generalized design equation.

Novel Impact of Short Term Aging on the Electrochemistry of CO 2 Treated Synthetic Graphite

A study was conducted of graphite prepared from heat-treatment of synthetic graphite LK-702 in flowing CO 2 . Samples were characterized by percent mass loss, X-ray powder diffraction, scanning electron microscopy imaging, thermogravimetric analysis, Brunauer-Emmett-Teller surface area, and methylene blue adsorption surface area measurements. Treated graphites were studied as possible electrode materials for Li-ion cells. Cycle tests demonstrated that the reversible capacity and percent capacity retained were relatively unaffected by CO 2 treatment, while the irreversible capacity was significantly influenced by CO 2 treatment. The change in irreversible capacity was strongly dependent upon the age of the treated samples, where samples freshly treated in CO 2 showed a decrease in irreversible capacity with treatment time, and treated samples aged for at least fourteen days (in a dry environment) showed an increase in irreversible capacity with treatment time. To our knowledge, this is the first report of the impact of short term aging on the electrochemistry of CO 2 treated graphite.

Heat-Treatment of Synthetic Graphite under Argon and Effect on Li-Ion Electrochemistry

The novel use of heat-treatment in flowing Ar to systematically affect particle and crystallite dimensions of synthetic graphite LK-702 is reported. Samples were characterized by percent mass loss, X-ray powder diffraction, scanning electron microscopy, Brunauer-Emmett-Teller (BET) surface area, and methylene blue adsorption surface area measurements. The treated graphites were studied as possible electrode materials for Li-ion cells. The BET and methylene blue surface areas increased for treatment times up to 48 h, then remained constant. For times up to 48 h, Ar treatment was demonstrated to break apart larger graphite particles, narrowing the particle size distribution. The irreversible capacity of the graphite decreased substantially with increased Ar treatment time, while the reversible capacity remained unchanged.