UC San Diego

An intercalation reaction is the most commonly utilized redox reaction scheme for commercial Li-ion batteries (LIBs) due to minimized crystal deformation during the redox reaction. However, to meet the growing demands for large-scale energy storage criteria, low cost and high energy density, other reaction schemes need to be explored. In order to fully utilize the new schemes, mechanistic understanding of the respective materials is crucial. A relaxation phenomenon facilitates the mechanistic understanding because it inspects both kinetic and thermodynamic products of the reaction.

MIL-101(Fe), a MOF material is applied to study the relaxation phenomenon. MIL-101(Fe) demonstrates a unique rate dependent rechargeability. Through X-ray absorption spectroscopy and electronic state calculation, a kinetically stable product and thermodynamically stable product of MIL-101(Fe) are proposed. Kinetically, Fe3+ is reduced upon reduction, however, due to thermodynamic stability, the Fe2+ self oxidizes back to Fe3+ after relaxation. This work demonstrates relaxation phenomenon on the molecular level.

Zn electrode is one of the conversion materials with high energy density, yet Zn electrode lacks rechargeability due to the zincate ion formation during oxidation reaction. The zincate ions dissolve into electrolyte making the Zn electrode not rechargeable. One of the ways to overcome this issue is incorporating Bi2O3 as a composite additive. We have added the Bi2O3 additive and identified a comprehensive role of the Bi2O3. Through electron microscopy coupled with energy dispersed X-ray spectroscopy and X-ray photoelectron spectroscopy, we have discovered that the ZnO and Bi2O3 form an intermediate phase that allows zincate ions to deposit and retain ZnO on the electrode surface. This work demonstrates relaxation phenomenon on the particle level.

Understanding the relaxation phenomenon upon redox reactions distinguishes the kinetic and thermodynamic products. The findings in this dissertation provide more comprehensive methods to identify the energy storage mechanism. MnO2 is one of the low-cost, environmentally benign, and promising material for energy storage system. A preliminary Mn2+ deposition test suggested that Bi2O3 addition can adhere Mn2+. Based on the knowledge gained through relaxation characterization, a series of characterizations and methods to improve MnO2 aqueous energy storage system are proposed.