System Design and Electrochemical Characterization of Lithium-Ion Active Material Suspensions

To further reduce the usage of fossil fuels, large scale energy storage technologies are needed to store electrical energy and improve the energy quality from the renewable energies effectively at a low cost. Redox flow batteries are ideal for large scale energy storage because of the decoupling of the power and the energy in the system, which provides the flexibility to independently adjust and design the power and energy requirements for an application. The energy density in conventional flow batteries, however, is highly limited by the solubility of the active species. In this dissertation, a new type of flow battery with lithium-ion active materials is first demonstrated to address this limitation. The energy density is increased significantly comparing with conventional redox flow battery while maintaining the benefits of design flexibility.

This new type of flow battery incorporates solid electroactive materials dispersed in lithium-ion battery electrolyte as flowing suspensions. Such type of battery has never been investigated before. To fill this missing knowledge, challenges including active material selection, flow cell architecture design, rheological characterization, and electrochemical performance measurements were addressed in this dissertation. A half-cell design was demonstrated with Li4Ti5O12 suspension, an anode active material. This was the first demonstration on this type of flow battery based on lithium-ion active materials in literature and discussed in Chapter II. Moving forward to the cathode material suspension demonstration, a sub-micrometer sized LiCoO2 material was synthesized and characterized because of the need of a small sized cathode active material with high conductivity. This synthesis method was expected to be a scalable synthesis approach and included in Chapter III. Based on this, another half-cell demonstration was further conducted with LiCoO2 suspension as the first report on lithium-ion cathode material suspension as the energy storage media. Combining Li4Ti5O12 suspension and LiCoO2 suspension in the same system, a full cell demonstration was studied as well. Both pieces of work were discussed in Chapter IV.

In this route, the high resistance of the electrochemical reaction was found to be the key barrier limiting the cycling performance. Therefore, a technique to characterize the resistance and identify materials for best performance was developed and named “Dispersed Particle Resistance”. Chapter V first introduced this concept with Li4Ti5O12 anode material as the model material and characterized in organic lithium-ion electrolyte. This technique was also found to be effective to characterize the performance of active materials in conventional coin cells as the measured resistance parameter was inversely related with the rate capability of active materials in conventional cells. To further demonstrate the applicability of this technique, a new class of lithium-ion active materials – cathode materials was demonstrated using six LiFePO4 cathode materials in aqueous electrolyte. This demonstration also introduced a new design using aqueous electrolyte suspensions for improved performance and was reported in Chapter VI.