Building Sustainable Batteries : Organic electrodes based on Li- and Na-benzenediacrylate
Abstract: As possible alternatives to the conventional inorganic Li- or Na-ion battery electrode materials, organic compounds have recently drawn considerable attention. However, major challenges such as poor electronic conductivity, solubility in battery electrolyte or fast capacity decay of the resulting electrochemical cells are some of the reasons that hold these compounds back from becoming commercial solutions in the energy system.The goal of this thesis work was to investigate the background to these phenomena and find strategies for improvements. Two different compounds were studied: dilithium and disodium benzenediacrylate, in their respective cells. First, improving the performance of the dilithium compound was performed by applying different electrode fabrication strategies. A freeze-drying technique was combined with carbon coating in the liquid state, which rendered an improved electrode morphology. Moreover, when using the compound in pouch cell format instead of Swagelok® cells, a different technique was applied: calendaring. Successful results were obtained both in half-cells and in full-cells when the compound was cycled versus LiFePO4-based cathodes. Second, the sodium analogue was investigated, and while the synthesis of this compound is straightforward, the electrochemical performance in Na-ion battery cells displays an unexpected degree of complexity. The compound displays a considerably faster capacity decrease in comparison to the Li compound, and generally a poor chemical stability in the applied system. When cycled at higher currents (C-rates of C/4 or C/10, in comparison to C/40), the compound presents an capacity increase while the Li decreases, likely due to a chemical process more dependent on time than on the number of cycles for the Na compound.The fast capacity decay in the first cycles of these types of compounds is often considered to be related to the Solid Electrolyte Interphase (SEI) layer formation. Its study was also performed and it was concluded that the Na compound has a thicker SEI layer in comparison to the Li counterpart, and mostly consisted of inorganic species such as the electrolyte salt and its decomposition products. Finally, a concept for a sustainable manufacturing and recycling process of a hybrid full cell is also performed with positive results.Although the organic compounds cannot yet outperform the inorganic compounds used commercially in Li-ion batteries, important steps towards their employment in the energy system have been taken in this thesis work.
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