Influence of precursor morphology and cathode processing on performance and cycle life of sodium-zinc chloride (Na-ZnCl2) battery cells

Replacing nickel by cheap and abundant zinc may enable high-temperature sodium-nickel chloride (Na-NiCl2) batteries to become an economically viable and environmentally sustainable option for large-scale energy storage for stationary applications. However, changing the active cathode metal significantly affects the cathode microstructure, the electrochemical reaction mechanisms, the stability of cell components, and the specific cell energy. In this study, we investigate the influence of cathode microstructure on energy efficiency and cycle life of sodium-zinc chloride (Na-ZnCl2) cells operated at 300 ◦C. We correlate the dis-/charge cycling performance of Na-ZnCl2 cells with the ternary ZnCl2-NaCl-AlCl3 phase diagram, and identify mass transport through the secondary NaAlCl4 electrolyte as an important contribution to the cell resistance. These insights enable the design of tailored cathode microstructures, which we apply to cells with cathode granules and cathode pellets at an areal capacity of 50 mAh/cm2. With cathode pellets, we demonstrate >200 cycles at C/5 (10 mA/cm2), transferring a total capacity of 9 Ah/cm2 at >83% energy efficiency. We identify coarsening of zinc particles in the cathode microstructure as a major cause of performance degradation in terms of a reduction in energy efficiency. Our results set a basis to further enhance Na-ZnCl2 cells, e.g., by the use of suitable additives or structural elements to stabilize the cathode microstructure

Solid-state electrochemistry of organic battery materials : addressing the redox potential and solubility of quinone derivatives

In the past decades of human development, the Li-ion battery has established itself as a key technology to manage the storage and consumption of electrical energy, and even more so in the present context of a clean energy transition. Organic electrode materials, which offer the possibility of reduced environmental costs when considering production and recycling of Li-ion cells, constitute an alternative to commercial inorganic compounds. Since the late 2000’s, the search for viable and practical organic Li-ion positive electrode materials has witnessed a particular increase in interest. This thesis tackles the complex challenge to design organic molecules displaying a high energy density while remaining insoluble in battery electrolytes. Benzoquinone was chosen as starting material, for it presents interesting redox properties – notably a reversible two-electron redox reaction and a particularly high gravimetric capacity – and can undergo facile molecular modifications. In the first part of this thesis, we developed a new design paradigm based on intermolecular H-bonding to achieve insolubility of organic redox molecules in aprotic electrolytes. In a second phase, we developed a novel organic electrode material meeting all the critical prerequisites of Li-ion battery positive electrodes, while giving insight on a complex electrostatic effect leading to an important gain in redox potential. Finally, we present the preliminary exploration of a new organic redox chemistry based on the N-Li bond, with the perspective of developing new high-energy organic electrode materials.(SC - Sciences) -- UCL, 202

On the improved electrochemistry of hybrid conducting-redox polymer electrodes

The electrochemistry of poly(2,5-dihydroxyaniline) (PDHA), a novel hybrid molecular configuration with redox active sites and electrical charge conduction along the polymer chain, has been recently reported. The theoretical capacity of this material is estimated at 443 mAh g−1, with high power performances being proposed given the intrinsic electrical conductivity. However, the initial results were below the expectations: only half the theoretical capacity attained, poor cycling stability and modest power behavior calling for further investigations on improving these performances. Herein we detail the optimized chemical synthesis and electrode formulation for poly(2,5-dihydroxyaniline) resulting in improved cycling stability, power performances and defined electrochemical response. We also detail the alternative electrochemical synthesis and activation route for PDHA and compare the results with the chemical approach

Negative Redox Potential Shift in Fire-Retardant Electrolytes and Consequences for High-Energy Hybrid Batteries

Fire-retardant electrolyte chemistries have attracted great attention given their potential to solve the grand challenges of alkali-ion batteries: safety, use of metallic anodes, and anodic stability. Whereas extensive analysis and correlations are drawn to explain their unusual electrochemical behavior, one essential property, their effects on redox potentials of battery components (redox potential shift) pervasively lack a strict description and quantification. Here we show that the strong solvation of lithium cations by organic phosphates, the widely used flame-retardant constituents, induces a negative redox potential shift by as much as 500 mV. We demonstrate that the redox potential shift is characteristic of Li-cation (de)solvation processes whereas it is negligible for other processes. This has important consequences for high energy hybrid battery concepts such as high voltage dual-ion graphite or organic batteries. These findings also shine a different light on the enhanced anodic stability of these nonconventional battery electrolyte formulations

Conjugated sulfonamides as a class of organic lithium-ion positive electrodes

The applicability of organic battery materials in conventional rocking-chair lithium (Li)-ion cells remains deeply challenged by the lack of Li-containing and air-stable organic positive electrode chemistries. Decades of experimental and theoretical research in the field has resulted in only a few recent examples of Li-reservoir materials, all of which rely on the archetypal conjugated carbonyl redox chemistry. Here we extend the chemical space of organic Li-ion positive electrode materials with a class of conjugated sulfonamides (CSAs) and show that the electron delocalization on the sulfonyl groups endows the resulting CSAs with intrinsic oxidation and hydrolysis resistance when handled in ambient air, and yet display reversible electrochemistry for charge storage. The formal redox potential of the uncovered CSA chemistries spans a wide range between 2.85 V and 3.45 V (versus Li+/Li0), finely tunable through electrostatic or inductive molecular design. This class of organic Li-ion positive electrode materials challenges the realm of the inorganic battery cathode, as this first generation of CSA chemistries already displays gravimetric energy storage metrics comparable to those of the stereotypical LiFePO4

Exploring the potential of polymer battery cathodes with electrically conductive molecular backbone

Organic redox materials have the potential to radically shift the battery technology landscape. Here, the chemical synthesis of poly(2,5-dihydroxyaniline) with intrinsic electrical conduction and a theoretical energy storage capacity of 443 mA h g−1 is detailed for the first time. The genuine intramolecular cross-hybridization of quinone redox and polyaniline conductor moieties leads to a subtle interplay between redox stability and the lithiation capacity with the underlying processes being discussed. Superior to the conventional electrode materials performances are expected upon further optimization of this novel class of organic redox materials with ion and electron conduction for energy storage

A H-bond stabilized quinone electrode material for Li–organic batteries: the strength of weak bonds

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