Thomas F. Fuller

Modeling of Galvanostatic Charge and Discharge of the Lithium/Polymer/Insertion Cell

The galvanostatic charge and discharge of a lithium anode/solid polymer separator/insertion cathode cell is modeled using concentrated solution theory. The model is general enough to include a wide range of polymeric separator materials, lithium salts, and composite insertion cathodes. Insertion of lithium into the active cathode material is simulated using superposition, thus greatly simplifying the numerical calculations. Variable physical properties are permitted in the model. The results of a simulation of the charge/discharge behavior of the system are presented. Criteria are established to assess the importance of diffusion in the solid matrix and transport in the electrolyte. Consideration is also given to various procedures for optimization of the utilization of active cathode material.

Simulation and Optimization of the Dual Lithium Ion Insertion Cell

The galvanostatic charge and discharge of a dual lithium ion insertion (rocking-chair) cell are modeled. Transport in the electrolyte is described with concentrated solution theory. Insertion of lithium into and out of the active electrode material is simulated using superposition, greatly simplifying the numerical calculations. Simulation results are presented for the Li=C~lpropylene carbonate +IM LiC1OJLiyMn204 cell, and these results are compared with experimental data from the literature. Criteria are established to assess the importance of diffusion in the solid matrix and of transport in the electrolytic solution. Various procedures to optimize the utilization of active material are considered. Simulation results for the dual lithium ion insertion cell are compared with those for a cell with a solid lithium negative electrode. The storage and conversion of energy continues to be important to society. Batteries, which interconvert chemical and electrical energy, are widely used in industry and for consumer applications (e.g., appliances and laptop computers). At the same time, environmental concerns are reshaping many industries. The ecological hazards of batteries through their operation and disposal is a primary consideration for battery manufacturers. In addition, stricter emission standards on automobiles are spurring interest in batteries for electric-vehicle applications. The energy and power requirements for vehicle propulsion are rigorous. 1 Consequently, research on rechargeable battery systems is receiving renewed attention. Lithium batteries are attractive for energy storage because of their high theoretical energy densities. Furthermore, they are less toxic than nickel cadmium or lead acid cells, and their disposal poses fewer environmental problems. Although primary lithium batteries have been massproduced for years, 2 the secondary (rechargeable) lithium cell has been commercialized only recently. 3-~ The typical lithium cell is made up of a lithium metal negative electrode, an electrolyte which serves as an ionic path between electrodes and separates the two materials, and a positive electrode, such as Mn204. In general, a highly reactive material is desired for the negative electrode to give a higher cell potential, and hence a higher theoretical energy density. Unfortunately, the more reactive the electrode material the more likely it reacts irreversibly with the electrolyte. The high reactivity of lithium metal is a significant problem for lithium batteries. Successful lithium battery systems operate due to a protective film that forms on the electrode surface. ~ This protective film retards further reaction with the electrolyte but impacts the capacity and cycle life of the cell through increased resistance and material isolation. The highly reactive lithium metal is a safety issue as well, becoming especially important in larger cells. One alternative scheme has been to replace the lithium metal negative electrode with a lithium alloy or compound, such as: LiA1, 7 LiFe203, s LiW02, 8 or LiC6. 9 Although these materials stabilize the lithium, this reduces the energy density of the cell, since the added material is not used in the

Water and Thermal Management in Solid‐Polymer‐Electrolyte Fuel Cells

A mathematical model of transport in a solid-polymer-electrolyte fuel cell is presented. A two-dimensional membrane-electrode assembly is considered. Water management, thermal management, and utilization of fuel are examined in detail. Because the equilibrium sorption of water between the gas phase and the polymer-electrolyte depends strongly on temperature, water and thermal management are interrelated. The rate of heat removal is shown to be a critical parameter in the operation of these fuel cells.

A Critical Review of Thermal Issues in Lithium-Ion Batteries

Lithium-ion batteries are well-suited for fully electric and hybrid electric vehicles due to their high specific energy and energy density relative to other rechargeable cell chemistries. However, these batteries have not been widely deployed commercially in these vehicles yet due to safety, cost, and poor low temperature performance, which are all challenges related to battery thermal management. In this paper, a critical review of the available literature on the major thermal issues for lithium-ion batteries is presented. Specific attention is paid to the effects of temperature and thermal management on capacity/power fade, thermal runaway, and pack electrical imbalance and to the performance of lithium-ion cells at cold temperatures. Furthermore, insights gained from previous experimental and modeling investigations are elucidated. These include the need for more accurate heat generation measurements, improved modeling of the heat generation rate, and clarity in the relative magnitudes of the various thermal effects observed at high charge and discharge rates seen in electric vehicle applications. From an analysis of the literature, the requirements for lithium-ion thermal management systems for optimal performance in these applications are suggested, and it is clear that no existing thermal management strategy or technology meets all these requirements.

Toward Efficient Binders for Li-Ion Battery Si-Based Anodes: Polyacrylic Acid

Si-based Li-ion battery anodes offer specific capacity an order of magnitude beyond that of conventional graphite. However, the formation of stable Si anodes is a challenge because of significant volume changes occurring during their electrochemical alloying and dealloying with Li. Binder selection and optimization may allow significant improvements in the stability of Si-based anodes. Most studies of Si anodes have involved the use of carboxymethylcellulose (CMC) and poly(vinylidene fluoride) (PVDF) binders. Herein, we show for the first time that pure poly(acrylic acid) (PAA), possessing certain mechanical properties comparable to those of CMC but containing a higher concentration of carboxylic functional groups, may offer superior performance as a binder for Si anodes. We further show the positive impact of carbon coating on the stability of the anode. The carbon-coated Si nanopowder anodes, tested between 0.01 and 1 V vs Li/Li+ and containing as little as 15 wt % of PAA, showed excellent stability during the first hundred cycles. The results obtained open new avenues to explore a novel series of binders from the polyvinyl acids (PVA) family.

Relaxation Phenomena in Lithium‐Ion‐Insertion Cells

Relaxation phenomena in lithium-ion-insertion cells are modeled. Simulation results are presented for a dual lithium-ion-insertion cell and for a cell using a lithium-foil negative electrode. A period of relaxation after a charge or discharge can cause appreciable changes in the distribution of material in the insertion electrodes. Local concentration cells in the solution phase and an open-circuit potential that depends on state of charge for the solid phase drive the redistribution of material. Concentration profiles in solid and solution phases during relaxation are analyzed, and the consequences for cell performance are discussed. The model predicts the effects of relaxation time on multiple charge-discharge cycles and on peak power. Galvanostatic and potentiostatic charging are simulated; the results are compared to experimental data for a commercial battery.

Experimental Determination of the Transport Number of Water in Nafion 117 Membrane

The transport number of water in Nafion 117 membrane over a wide range of water contents is determined experimentally using a concentration cell. The transport number of water, the ratio f[sup m][sub o]/Z[sub o], is about 1.4 for a membrane equilibrated with saturated water vapor at 25[degrees]C, decreases slowly as the membrane is dehydrated, and falls sharply toward zero as the concentration of water approaches zero. In this paper, the relationship between the transference number, the transport number, and the electro-osmotic drag coefficient is presented, and their relevance to water management is solid-polymer-electrolyte fuel cells is discussed. Results are compared with other data available in the literature and with the theoretical maximum.

Towards Ultrathick Battery Electrodes: Aligned Carbon Nanotube – Enabled Architecture

Vapor deposition techniques were utilized to synthesize very thick (∼1 mm) Li-ion battery anodes consisting of vertically aligned carbon nanotubes coated with silicon and carbon. The produced anode demonstrated ultrahigh thermal (>400 W·m(-1) ·K(-1)) and high electrical (>20 S·m(-1)) conductivities, high cycle stability, and high average capacity (>3000 mAh·g(Si) (-1)). The processes utilized allow for the conformal deposition of other materials, thus making it a promising architecture for the development of Li-ion anodes and cathodes with greatly enhanced electrical and thermal conductivities.

PEM Fuel Cell Pt ∕ C Dissolution and Deposition in Nafion Electrolyte

Dissolution at the cathode and subsequent transport of platinum to the other cell components causes catalyst degradation in proton exchange membrane (PEM) fuel cells. Deposition of platinum in Nafion membrane was observed after potential cycling under hydrogen/air conditions. The deposited Pt formed a band in the ionomer, and a straightforward model was proposed to describe its location. The predicted position of the Pt band agreed with the experimental data. A simple scanning electron microscopy-energy dispersive spectroscopy analysis was used to estimate that ∼ 13% of the platinum initially in the cathode was transported into the membrane following 3000 potential cycles.

Influence of rib spacing in proton-exchange membrane electrode assemblies

A two-dimensional design analysis of a membrane-electrode assembly for a proton-exchange membrane fuel cell is presented. Specifically, the ribs of the bipolar plates restrict the access of fuel and oxidant gases to the catalyst layer. The expected change in cell performance that results from the partial blocking of the substrate layer is studied by numerical simulation of the oxygen electrode and the membrane separator. The effects of rib sizing and the thickness of the gas-diffusion electrode on the current and water distributions within the cell are presented. For all of the cases considered, the two-dimensional effect only slightly alters the half-cell potential for a given applied current but has a significant influence on water management.Concentrated solution theory with variable transport properties is used in the membrane electrolyte to solve for the electrical potential and local water content. The Stefan-Maxwell equations are used in the gas-diffusion electrode to determine the local mole fractions of nitrogen, oxygen and water vapour.A control-volume formulation is used for the resolution of the coupled nonlinear differential equations. One advantage of the control-volume approach over finite-difference methods is the relative ease in which internal boundary points in fuel-cell and battery models are handled. This and other advantages are briefly discussed.