Optimization of electrode surface and electrolyte interface for efficient electrochemical energy storage and conversion devices

Abstract

Efficient energy storage and conversion of renewable energy is in great demand due to the population growth, increased energy consumption, global warming, and depletion of petroleum resources. However, the intermittence and unpredictability make the practical utilization of renewable energy such as wind and solar energy inefficient. Advanced electrochemical energy storage and conversion devices such as fuel cells, lithium-ion batteries, electrolyzers, and metal-air batteries are expected to help resolve these critical issues. In this thesis, the focus is on surface/interface of complex oxides for potential applications in electrochemical energy conversion (alkaline fuel cells) and storage (lithium-ion batteries) devices. In the first part, the surface of La_0.8Sr_0.2MnO₃(LSMO) is treated with diluted HNO₃. This process leads to the preferential formation of MnO_x/LSMO on the surface, leading to the exposure of Mn cations at the surface of LSMO. The electrocatalytic activity of the MnO_x/LSMO heterostructure towards oxygen reduction reaction (ORR) is shown to increase when compared to the untreated LSMO. Thanks to the formation of MnO_x at the surface, the resulting MnO_x/LSMO possesses i) a relatively high specific surface area and a mesoporous structure; ii) a higher coverage of Mn⁴⁺/Mn³⁺ cations at the surface; and iii) a high concentration of highly oxidative oxygen species. This work develops a facile strategy (i.e., HNO₃ treatment) for improving the ORR-activity of perovskite oxides in alkaline solution at room temperature. In the second part, we develop a LLZTO and PVDF solid-state composite membrane characterized by high conductivity, tensile strength, and flexibility as well as low impedance when interfacially modified by a minute amount of liquid electrolyte. A corresponding solid-state lithium-ion battery with LiFePO₄ and Li as electrodes delivers excellent rate capability and cycling stability at room temperature. In particular, the battery shows an initial discharge capacity of 155 mAh g^-1 and, after 100 cycles at 1C, of 145 mAh g^-1. Even at 4C, the discharge capacity is 96 mAh g^-1. Our study suggests that the interfacially modified LLZTO-PVDF membrane is a promising electrolyte for solid-state lithium-ion batteries.

Date of Award	2018
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology