Title: Characterization of Positive Electrodes in Sodium-Metal Chloride Batteries
Description: The high-performance sodium metal chloride battery has garnered significant interest in the past decade due to its multiple advantages such as high energy density, deep discharge cycling ability, high safety level, 100% coulombic efficiency, and a broad ambient-temperature operating range. Current development of the sodium-metal chloride batteries is focused on improving its performance and cycling life. This work investigates micro-scale mass transfer and kinetic parameters, which is related to cell performance, for building a complete model. In a typical commercial sodium metal chloride cell, there is mass transfer and conduction throughout the thick positive electrode. The electrode materials participate in redox reactions neither homogeneously nor simultaneously. Therefore, a much thinner positive electrode is introduced in this work in order to remove added macro-scale effects in the electrode from the measurement. Therefore, the number of parameters needed to describe the data was reduced because the experimental design minimizes spatial variations within the cell.

Chapter 2 discusses the impact of iron addition to a sodium nickel-chloride cell by investigating ionic transport within the metal chloride phase. The electrochemical performance of a sodium mixed-metal (Ni, Fe) halide cell is characterized for different cathode compositions and at different rates. Charge/discharge data are characterized by a smaller nickel-voltage plateau during discharge than during charge, indicating that some of the NiCl₂ reduces at cell potentials nominally associated with the iron plateau. One means of describing the difference between charge and discharge is to consider transport processes within the mixed NiCl₂/FeCl₂ solid phase. A one-dimensional model has been used to simulate the ionic transport within the (Ni,Fe)Cl₂ phase; the transport model predicts the ratio of discharge to charge iron plateaus reasonably well for most rates and compositions. In order to further investigate complex dynamic behavior of the open-circuit potential (OCP) and galvanic interactions in an iron-doped sodium nickel-chloride cell, a GITT (Galvanostatic Intermittent Titration Technique) method is used in Chapter 3.

The response to open-circuit interrupts of porous mixed iron-nickel cathodes has been characterized as a function of state of charge (SOC) for different iron loadings and different charge and discharge rates. After discharge, OCP can evolve in time from the iron plateau to the nickel plateau, and this behavior can be explained by galvanic interactions between iron metal and Ni²⁺. Characteristic times of the OCP transients depend on SOC and can be large. When the OCP has converged on a steady state during discharge, its value may provide an estimate of the mole fraction of NiCl₂ at the interface of the triclinic (Ni,Fe)Cl₂ film that resulted from metal oxidation. Sulfur-containing additives were shown to have dramatic impact on cell resistance and performance. In Chapter 4, the electrochemistry of iron sulfide in nickel/iron porous electrodes in molten sodium tetrachloroaluminate electrolyte was investigated. With the addition of FeS to the electrolyte, results indicate the formation of nickel sulfide species on the metal electrode and an increasing discharge capacity with increasing amount of iron sulfide.

The cathode with highest sulfide content appears to be highly resistive. Galvanostatic interrupt experiments shows complex dynamic behavior of sulfide-iron-NiCl₂ galvanic interactions. With a goal of extending knowledge of kinetic and mass transfer parameters for understanding mass transfer, Chapter 5 discusses the performance of nickel/iron cells for a broader range of temperature, composition and current. The experiments were tested at different temperatures. Also, three granule compositions with different iron levels are tested at four different current rates. The data from this study can be for use in a complete model of the sodium-nickel/iron chloride cell and in the optimization of the electrode. In the previous chapters, a thinner positive electrode is used in order to remove the effects of macro-scale mass transfer. Chapter 6 discusses the impact of thickness of the cathode on the mass macro-scale transfer and conduction within the metal chloride and metal phase. The goal is to improve modeling of tortuosity as a function of state of charge because transport is important in real systems, and modeling ohmic resistance, for example, can be challenging.