LATTICE MODIFICATIONS ON SOLID STATE ELECTROLYTES FOR THE OPTIMIZATION OF ION TRANSPORT
Jolley, Adam Garrett
Wachsman, Eric D
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A solid state electrolyte is one of the primary components of both a solid oxide fuel cells (SOFC) and an all-solid-state sodium battery. In both cases, the ionic conductivity of the electrolyte has a major impact on the performance of the electrochemical cell. For SOFCs, the conductivity of traditional electrolytes is not high enough for sufficient performance at intermediate and low temperature operation. Therefore, novel bismuth oxide compositions were developed to achieve higher conductivity. The conductivity of Bi2O3 was improved by reducing the total dopant concentration required to stabilize the highly conductive cubic phase. This strategy lead to the development of a Bi2O3 electrolyte (La7Zr3) with the highest oxygen ion conductivity to date. Unfortunately, at temperatures below 600°C the conductivity of the cubic phase was unstable. Therefore, rhombohedral bismuth oxide was investigated for low temperature SOFC operation due to its stability. For the first time, a dopant concentration less than 10% was used to stabilize the rhombohedral phase of Bi2O3. Furthermore, a novel phase diagram was constructed for the low dopant regime of the rhombohedral phase. Ultimately, the double doped bismuth oxide material (La5.1Y1.4) developed here was among the highest and most stable oxygen ion conductors below 600°C. Performance of an SOFC with a La5.1Y1.4 electrolyte verified that it is a promising material for low temperature SOFCs. A similar strategy of doping an electrolyte material to increase ionic conductivity was carried out on NASICON (Na3Zr2Si2PO12). NASICON is a promising electrolyte for room temperature sodium batteries, but traditionally it does not exhibit high enough conductivity to garner high performance. For the first time, the mechanism driving the phase transition in NASICON was determined and mapped out. Mitigation of the phase transition in the material was established to lower the activation energy barrier for sodium ion transport. Additionally, divalent cations were substituted into the NASICON lattice to generate an increase in sodium ion conductivity. Ultimately the phase and dopant concentration was optimized to deliver a material that is among the best sodium ion conducting ceramics to date (20% Zn-doped NASICON).