Beyond conventional Li-ion: Aqueous Li ion batteries and mixed metal fluoride cathodes for Li and Li-ion batteries
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This thesis explores the promises of and challenges posed by several alternative Li- and Li-ion battery chemistries: substituting the organic or polymer electrolyte with an aqueous electrolyte, and substituting intercalation cathode materials with metal fluoride conversion materials. Aqueous electrolytes attract interest due to their intrinsic safety (nonflammability), environmental friendliness, low cost and high ionic conductivity. A variety of electrode active materials found in commercial Li ion cells exhibit similar redox behavior with aqueous electrolytes, and thus hold promise for use in aqueous Li ion batteries. However, a variety of these compounds also exhibit inferior cycle stability with aqueous electrolytes due to the presence of unique side reactions. In addition, aqueous electrolytes are typically stable over a narrower potential window, limiting the cell voltage. In this thesis, systematic studies of the charge-discharge behavior of LiFePO4 with aqueous electrolytes of varying composition reveal that undesirable side reactions between water and LiFePO4 induce electrochemical separation of individual particles within the electrode, resulting in capacity fading. However, increasing the electrolyte salt concentration effectively reduces the activity of the water molecules and the extent of these side reactions. Increasing the electrolyte molarity also improves the stability of other intercalation compounds (such as LiNi0.33Mn0.33Co0.33O2), suggesting that these findings are sufficiently broad. Metal fluorides are promising high-capacity cathode active materials for rechargeable Li and Li-ion batteries. Their use may allow for higher cell-level energy densities, reducing overall energy storage system manufacturing costs. Higher thermal stability and lower electrode potentials may also improve safety. However, the low electrical conductivity, the dramatic structural and morphological changes that accompany the electrochemical reactions, and the ease with which metal fluorides give up transition metal ions to the electrolyte may explain why metal fluoride cathodes typically suffer from large voltage hysteresis and short cycle life. In this thesis, systematic studies were performed on the charge-discharge behavior of metal fluorides of a variety of single and mixed metal compositions. For mixed metal difluorides, it is discovered that reduction occurs in a single step with a reduction potential intermediate to those for the single metal difluorides. Also, for the first time, progressive formation of metal trifluorides from repeated cycling of metal difluorides is reported. Perhaps most importantly, electrochemical measurements in combination with post-mortem microscopy and spectroscopy reveal that the cathode stability depends on the ability to prevent the formation and growth of a solid electrolyte interphase layer. This depends strongly on the metal composition, with the incorporation of Ni leading to a particularly thick and insulative solid electrolyte interphase layer.