Venkat Srinivasan

Discharge model for the lithium iron-phosphate electrode

This paper develops a mathematical model for lithium intercalation and phase change in an iron phosphate-based lithium-ion cell in order to understand the cause for the low power capability of the material. The juxtaposition of the two phases is assumed to be in the form of a shrinking core, where a shell of one phase covers a core of the second phase. Diffusion of lithium through the shell and the movement of the phase interface are described and incorporated into a porous electrode model consisting of two different particle sizes. Open-circuit measurements are used to estimate the composition ranges of the single-phase region. Model-experimental comparisons under constant current show that ohmic drops in the matrix phase, contact resistances between the current collector and the porous matrix, and transport limitations in the iron phosphate particle limit the power capability of the cells. Various design options, consisting of decreasing the ohmic drops, using smaller particles, and substituting the liquid electrolyte by a gel are explored, and their relative importance discussed. The model developed in this paper can be used as a means of optimizing the cell design to suit a particular application.

An Electrochemical Route for Making Porous Nickel Oxide Electrochemical Capacitors

Porous nickel oxide films were prepared by electrochemically precipitating nickel hydroxide and heating the hydroxide in air at 300 C. The resulting nickel oxide films behave as an electrochemical capacitor with a specific capacitance of 59 F/g electrode material. These nickel oxide films maintain high utilization at high rates of discharge (i.e., high power density) and have excellent cycle life. Porous cobalt oxide films were also synthesized. Although the specific capacitances of these films are approximately one-fifth that of the nickel oxide films, the results demonstrate the versatility of fabricating a wide range of porous metal oxide films using this electrochemical route for use in capacitor applications. Electrochemical capacitors have generated wide interest in recent years for use in high power applications (e.g., in a hybrid electric vehicle, where they are expected to work in conjunction with a conventional battery).

Capacitance studies of cobalt oxide films formed via electrochemical precipitation

Abstract Cobalt oxide films were prepared by electrochemically precipitating the hydroxide and heating it in air to form Co3O4. The resulting oxide films behave as a capacitor. The materials were studied emphasizing their use as either positive or negative electrodes in devices. The capacitance of the material was estimated at different heating temperatures and insight was gained into the charge storage mechanism. It was seen that the charge storage in cobalt oxide was similar to that of nickel oxide, although their capacitances were different. While, the material was shown to be inferior to nickel hydroxide/oxide when used as a positive electrode, it was seen to be superior as a negative. An asymmetric capacitor with NiO as a positive electrode and Co3O4 as a negative promises adequate capacitance over a large voltage window.

Studies on the Capacitance of Nickel Oxide Films: Effect of Heating Temperature and Electrolyte Concentration

Nickel oxide films were prepared by electrochemically precipitating the hydroxide and heating it in air to form the oxide. The resulting oxide films behave as a capacitor. The capacitance of the oxide depends on the heating temperature, showing a maximum at 300°C. The mechanism of charge storage was studied by measuring the capacitance and surface area as a function of heating temperature, and the capacitance in different electrolytes and potential windows. The charge‐storage mechanism is believed to be a surface redox reaction involving adsorbed hydroxyl ions.

In Situ Measurements of Stress-Potential Coupling in Lithiated Silicon

An analysis of the dependence of electric potential on the state of stress of a lithiated-silicon electrode is presented. Based on the Larche and Cahn chemical potential for a solid solution, a thermodynamic argument is made for the existence of the stresspotential coupling in lithiated silicon; based on the known properties of the material, the magnitude of the coupling is estimated to be 60 mV/GPa in thin-film geometry. An experimental investigation is carried out on silicon thin-film electrodes in which the stress is measured in situ during electrochemical lithiation and delithiation. By progressively varying the stress through incremental delithiation, the relation between stress change and electric-potential change is measured to be 100–120 mV/GPa, which is of the same order of magnitude as the prediction of the analysis. The importance of the coupling is discussed in interpreting the hysteresis observed in the potential vs state-of-charge plots and the role of stress in modifying the maximum charge capacity of a silicon electrode under stress.

Quantifying the promise of lithium–air batteries for electric vehicles

Researchers worldwide view the high theoretical specific energy of the lithium–air or lithium–oxygen battery as a promising path to a transformational energy-storage system for electric vehicles. Here, we present a self-consistent material-to-system analysis of the best-case mass, volume, and cost values for the nonaqueous lithium–oxygen battery and compare them with current and advanced lithium-based batteries using metal-oxide positive electrodes. Surprisingly, despite their high theoretical specific energy, lithium–oxygen systems were projected to achieve parity with other candidate chemistries as a result of the requirement to deliver and purify or to enclose the gaseous oxygen reactant. The theoretical specific energy, which leads to predictions of an order of magnitude improvement over a traditional lithium-ion battery, is shown to be an inadequate predictor of systems-level cost, volume, and mass. This analysis reveals the importance of system-level considerations and identifies the reversible lithium-metal negative electrode as a common, critical high-risk technology needed for batteries to reach long-term automotive objectives. Additionally, advanced lithium-ion technology was found to be a moderate risk pathway to achieve the majority of volume and cost reductions.

Existence of path-dependence in the LiFePO4 electrode

This paper explores two unique features of the lithium iron phosphate (LiFePO 4 ) electrode that provide insight into the electrochemical behavior of this system. First, we show the existence of an asymmetric behavior between charge and discharge, whereby the utilization on charge is considerably larger than that on discharge under current densities where transport limitations are important. Second, we show the existence of a path-dependence in this system whereby the high-rate electrochemical behavior of the electrode at a particular state of charge (SOC) depends on the path by which the electrode was brought to that SOC. We qualitatively explain both these features using a shrinking-core model to account for the juxtaposition of the two phases. The path-dependence reported in this paper could have implications in batteries used in hybrid-electric-vehicles as the power capability of this chemistry will depend on its cycling history, thereby complicating predictions of power. In addition, the data reported here emphasizes the importance of ensuring consistency in defining the SOC in experiments on this electrode.

Cyclic voltammetric studies of the effects of time and temperature on the capacitance of electrochemically deposited nickel hydroxide

A cyclic voltammetric (CV) technique was used to study the combined effects of annealing temperature and time on the pseudocapacitance of thermally treated electroprecipitated nickel hydroxide thin films. Through the analysis of the areas of the CVs cycled between 0 and 0.35 V (versus Ag/AgCl) it is shown that the optimal treatment condition for maximum film capacitance occurs at 300°C for 3 h. On the other hand, using the anodic and cathodic peak currents of the CVs cycled between 0 and 0.5 V (versus Ag/AgCl), the maximum film capacitance is also shown to occur at a thermal treatment condition of 300°C and 3.5 h (or 320°C and 3.2 h for linear approximations). The two methods demonstrate simple ways of extracting useful information on the electrochemical performance properties of thin films.

Design and Optimization of a Natural Graphite/Iron Phosphate Lithium-Ion Cell

This paper uses a model for a natural graphite/lithium hexafluoro phosphate (ethylene carbonate:diethyl carbonate)/iron phosphate lithium-ion cell in order to study its performance and aid in its optimization. The model is used to generate Ragone plots for various designs, where both the average power of the cell and the peak power, defined at 80% depth-of-discharge for a 30 s pulse, are evaluated. This allows us to assess the ability of this chemistry to achieve the U.S. Department of Energy goals. The model is then used to maximize the specific energy of the cell by optimizing the design for a fixed time of discharge. The cell was optimized for the porosity and thickness of the positive electrode, while holding constant the capacity ratio of the two electrodes, the thickness and porosity of the separator, the electrolyte concentration, and the porosity of the negative electrode. The effect of the capacity ratio was qualitatively examined. The optimization was performed for discharge times ranging from 10 h to 2 min in order to map the maximum performance of this chemistry under a wide operating range. The study allows us to gauge the ability of this chemistry to be used in a particular application. The optimized designs derived in this paper are expected to be a starting point for battery manufacturers and to help decrease the time to commercialization.

Mathematical Modeling of Electrochemical Capacitors

Analytic solutions to the mathematical model of an electrochemical capacitor (EC) are used to study cell performance under two types of operating conditions: (i) constant current and (ii) electrochemical impedance spectroscopy. The analytic solution under constant-current operation is used to investigate the relative importance of ionic resistance in the separator, and ionic and electronic resistances in the porous electrode in the design and operation of an EC. Model results are presented that show the trade-off between energy and power density, as the physical properties of the cell components are varied (e.g., electrode thickness). The analytic solution is also used to study the effect of cell design and operation on the heat generation during constant-current cycling. The impedance model is presented as an alternative to equivalent-circuit models for data analysis. The analytic solution can be used to gain a physical understanding of the various processes that occur in an EC.