Nanoporous materials for energy applications
Date
2015
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Batteries have become ubiquitous in modern society by powering small,
consumer electronic devices such as flashlights, cell phones, and laptops. Increasingly,
batteries are also being examined as a method to improve energy efficiency (and
reduce greenhouse gas emissions) for vehicles and power transmission/distribution
applications. For lithium-ion based batteries to meet the demands of these new
applications, new electrode materials and morphologies are the key to access high
energy and/or power density. In this work, the research efforts include two major
thrusts, concentrating on the synthesis and understanding of novel porous materials as
potential electrodes for rechargeable lithium-ion batteries. The nano-sized walls and
multidimensional pore structures allow fast solid state and electrolytic transport, while
micron-sized particle ensure better interparticulate contact.
The first thrust of research focused on the development of new synthetic
approaches for porous material fabrication. A novel ionothermal synthetic method has
been developed using deep-eutectic solvents, such as choline chloride and N,Ndimethylurea,
to form iron, manganese and cobalt phosphates with a zeotype
framework. Through this advanced method the successful synthesis of 4 previously
undiscovered metal phosphate zeotypes was achieved. A careful control of water
content during the ionothermal synthesis elucidated the multistep decomposition of
our framework template and its impacts in the resulting zeotype structures. Upon
conclusion of the ionothermal work, the focus shifted to the methodology
development for mesoporous metal sulfides. An “oxide-to-sulfide” synthetic strategy
was developed for the first time, resulting in the first synthesis of ordered porous iron,
cobalt and nickel sulfides. More importantly, this is a general synthetic method,
relying primarily on volumetric calculations per metal atom, which could be further
extend to other metal-containing compounds, such as metal chalcogenides,
phosphides, and nitrides.
The second thrust was on the fundamental understanding of structural and
electrochemical properties of nanocast mesoporous metal oxides. A thorough
examination of three-dimensional porous TiO2 particles (solvothermal grown and
micron in particle size) has been performed in order to achieve a better understanding
of structure-property relationship. While ex-situ SEM analysis didn’t show any
notable change after the 1st discharge, significant structural alterations have occurred
during the 2nd discharge. This formation coincides with a significant drop in
performance, and it appears through ex-situ PXRD that the phase conversion of
anatase to Li-titanate, the essential mechanism for energy storage in this material, is no
longer occurring.
A detailed structure-property investigation of mesoporous β-MnO2 was also
performed. Previously, it has been reported that the material experiences a 7% volume
change during lithium intercalation/de-intercalation while still retaining its mesoscopic
order. The large volume change and new assertions about mesoscale β-MnO2 from
atomistic models makes determining additional structure-property insight of great
importance. The analysis, through ex-situ PXRD measurements and dQ/dV analysis of
the battery cycles indicate that a complex structural rearrangement occurs during
cycling. A rearrangement irreversibly occurred in the initial cycles of mesoporous β-
MnO2, and a complex structural dynamics has been observed. The knowledge
generated from this work may help the battery community better understand the
irreversible structural evolution occurring in commercialized lithium manganese oxide
electrode materials.