Bor Yann Liaw

Micro‐Macroscopic Coupled Modeling of Batteries and Fuel Cells I. Model Development

A micro-macroscopic coupled model, aimed at incorporating solid-state physics of electrode materials and interface morphology and chemistry, has been developed for advanced batteries and fuel cells. Electrochemical cells considered consist of three phases: a solid matrix (electrode material or separator), an electrolyte (liquid or solid), and a gas phase. Macroscopic conservation equations are derived separately for each phase using the volume averaging technique and are shown to contain interfacial terms which allow for the incorporation of microscopic physical phenomena such as solid-state diffusion and ohmic drop, as well as interfacial phenomena such as phase transformation, precipitation, and passivation. Constitutive relations for these interfacial terms are developed and linked to the macroscopic conservation equations for species and charge transfer. A number of nonequilibrium effects encountered in high-energy-density and high-power-density power sources are assessed. Finally, conditions for interfacial chemical and electrical equilibrium are explored and their practical implications are discussed. Simplifications of the present model to previous macrohomogeneous models are examined. In a companion paper, illustrative calculations for nickel-cadmium and nickel-metal hydride batteries are carried out. The micro-macroscopic model can be used to explore material and interfacial properties for desired cell performance.

Incremental Capacity Analysis and Close-to-Equilibrium OCV Measurements to Quantify Capacity Fade in Commercial Rechargeable Lithium Batteries

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.

Numerical Modeling of Coupled Electrochemical and Transport Processes in Lead‐Acid Batteries

A numerical model is developed to predict transient behaviors of electric vehicle lead-acid batteries during discharge and charge processes. The model not only accounts for coupled processes of electrochemical kinetics and mass transport occurring in a battery cell, but also considers free convection resulting from density variations due to acid stratification. A single set of conservation equations valid for both porous electrodes and the free electrolyte region is derived and numerically solved using a computational fluid dynamics technique. This numerical methodology is capable of simulating a two-dimensional cell with the fluid flow taken into consideration and requires only tens of minutes of central processing unit time on engineering workstations. Four sample calculations are presented in this work to provide rigorous validation of the developed simulator. The simulator is capable of predicting the transient behavior of the acid concentration, the porosity of the electrodes, and the state of charge of the battery during discharge, rest, and charge cycles. The model can also be used to investigate the effects of various system parameters, such as electrode dimensions, separator design, temperature, and electrolyte composition on the battery performance (voltage, power, cold cranking amperage, etc.).

Accelerated calendar and pulse life analysis of lithium-ion cells

Abstract Sandia National Laboratories has been studying calendar and pulse discharge life of prototype high-power lithium-ion cells as part of the Advanced Technology Development (ATD) Program. One of the goals of ATD is to establish validated accelerated life test protocols for lithium-ion cells in the hybrid electric vehicle application. In order to accomplish this, aging experiments have been conducted on 18650-size cells containing a chemistry representative of these high-power designs. Loss of power and capacity are accompanied by increasing interfacial impedance at the cathode. These relationships are consistent within a given state-of-charge (SOC) over the range of storage temperatures and times. Inductive models have been used to construct detailed descriptions of the relationships between power fade and aging time and to relate power fade, capacity loss and impedance rise. These models can interpolate among the different experimental conditions and can also describe the error surface when fitting life prediction models to the data.

Micro‐Macroscopic Coupled Modeling of Batteries and Fuel Cells II. Application to Nickel‐Cadmium and Nickel‐Metal Hydride Cells

The micro-macroscopic coupled model developed in a companion paper is applied to predict the discharge and charge behaviors of nickel-cadmium (Ni-Cd) and nickel-metal hydride (Ni-MH) cells. The model integrates important microscopic phenomena such as proton or hydrogen diffusion and conduction of electrons in active materials into the macroscopic calculations of species and charge transfer. Simulation results for a full Ni-Cd cell and single MH electrode are presented and validated against the pseudo two-dimensional numerical model in the literature. In good agreement with the previous results, the present family of models is computationally more efficient and is particularly suitable for simulations of complex test conditions, such as the dynamic stress test and pulse charging for electric vehicles. In addition, a mathematical model for full Ni-MH cells is presented and sample simulations are performed for discharge and recharge with oxygen generation and recombination taken into account. These gas reactions represent an important mechanism for battery overcharge in the electric vehicle application.

Modeling discharge and charge characteristics of nickel–metal hydride batteries

A combined numerical and experimental study of the discharge and charge of nickel–metal hydride (Ni–MH) batteries has been performed. Numerical simulations were based on a previously developed micro–macroscopic coupled model which includes both the proton diffusion in the nickel active material and the hydrogen diffusion in metal-hydride particles. Oxygen generation and recombination, a principal mechanism to ensure safe operation of Ni–MH cells during overcharge, is also accounted for. A sealed Ni–MH cell (AA size, 1 Ah capacity) was prepared and discharge experiments were conducted at various rates. These experimental data along with other discharge and charge data available in the literature were used to validate the present model. Numerical results were presented to show the effects of oxygen evolution on battery performance, particularly on the charge acceptance, cell pressure build-up and self-discharge. This combined experimental and numerical study yields a computer-aided tool for the design and optimization of Ni–MH batteries.

Following the Transient Reactions in Lithium–Sulfur Batteries Using an In Situ Nuclear Magnetic Resonance Technique

A fundamental understanding of electrochemical reaction pathways is critical to improving the performance of Li-S batteries, but few techniques can be used to directly identify and quantify the reaction species during disharge/charge cycling processes in real time. Here, an in situ (7)Li NMR technique employing a specially designed cylindrical microbattery was used to probe the transient electrochemical and chemical reactions occurring during the cycling of a Li-S system. In situ NMR provides real time, semiquantitative information related to the temporal evolution of lithium polysulfide allotropes during both discharge/charge processes. This technique uniquely reveals that the polysulfide redox reactions involve charged free radicals as intermediate species that are difficult to detect in ex situ NMR studies. Additionally, it also uncovers vital information about the (7)Li chemical environments during the electrochemical and parasitic reactions on the Li metal anode. These new molecular-level insights about transient species and the associated anode failure mechanism are crucial to delineating effective strategies to accelerate the development of Li-S battery technologies.

Low Temperature Limiting‐Current Oxygen Sensors Based on Tetragonal Zirconia Polycrystals

This paper reports that yttria doped tetragonal zirconia polycrystals can overcome the phase transition into the monoclinic phase at about 500{degrees} C and show higher ionic conductivities than cubic stabilized zirconia in spite of the lower defect concentration. This material is applied in oxygen gas sensors under limiting current conditions at intermediate and ambient temperatures. AC and dc conductivities, Tafel behavior, minority charge carrier diffusivity, and the i-V characteristics are reported. The detection limit of the oxygen partial pressure and the response time depend on the thickness of the electrolyte and are related to the oxygen ion conductance and the electronic mobilities, respectively. The sensors may be optimized by the application of thin film electrolytes and of modified configurations with solid oxide bulk conducting diffusion barriers.

In Situ Characterization of Electrochemical Polymerization of Methylene Green on Platinum Electrodes

A novel in situ characterization technique using imaging ellipsometry in conjunction with cyclic voltammetric (CV) deposition has been applied to correlate polymerized methylene green (poly-MG) film thickness and morphology, with charge transferred during CV cycles. We also report values for the refractive index (n) and extinction coefficient (k) of the monomer solution and the poly-MG film, for the first time. These values have been derived from measurements that employ multiple angles of incidence in the ellipsometry and analysis using an optical model based on Fresnel theory. The in situ characterization of film growth with thickness progression provided comprehensive information on the deposition process that could not be obtained using ex situ techniques such as scanning electron microscopy or optical microscopy. The results demonstrate the value of imaging ellipsometry as a quantitative characterization technique to monitor polymer film growth in situ and to characterize the quality of the film in terms of surface morphology and homogeneity as a function of film thickness. Comment is also provided on how the technique can be combined with other characterization techniques to deliver a more comprehensive analysis and control of the electrochemical deposition process.

Thermodynamic and structural considerations of insertion reactions in lithium vanadium bronze structures

Abstract Lithium vanadium bronze structures show promise for use as positive electrode materials in advanced lithium batteries. Many studies indicate that the insertion of lithium in these structures is reversible and at high rates. However, the electrochemical behaviors among these phases are quite different and also deviate from that predicted by the high temperature phase diagram. In this paper, a rationalization based on the thermodynamic and structural considerations is proposed. Strikingly, metastable reactions are found during the insertion and metastable allotropes are produced. We call these types of reactions “allotropic insertions” to distinguish from the conventional isomorphic insertion and displacement reactions.