Block copolymer-derived nanoporous materials are featured with microstructures defined by the microphase separation of constituent blocks, enabling various applications in energy storage. Dictated by the molecular weights and volume fractions of constituent blocks, the microphase separation forms nanoscale microstructures of 1-100 nm. Selective removal of a sacrificial phase produces nanopores with tailored pore width, continuity, and tortuosity. The remaining phase customizes the properties of resulting nanoporous materials, including specific surface area, electrical conductivity/insulation, and mechanical performance. Therefore, block copolymer-derived porous materials are felicitous for use in high-performance energy storage. This dissertation presents the utilization of block copolymers to derive nanoporous materials: i) high-modulus polyimide separators for lithium-metal batteries, and ii) high-surface-area carbon electrodes for fast-charging zinc-ion batteries.
In lithium-metal batteries, the dendritic growth of lithium leads to deteriorating performance and severe safety concerns. Suppressing lithium dendrites is imperative to guarantee both high performance and safe cycling. Mesoporous polyimide separators are promising for dendrite suppression: i) the mesopores are smaller than the width of lithium dendrites, preventing lithium dendrites from penetrating the separator. ii) The high-modulus polyimide ceases the growth of lithium dendrites. Herein, this dissertation reports a mesoporous polyimide separator produced by thermalizing polylactide-b-polyimide-b-polylactide at 280 °C. The mesoporous polyimide separator exhibits a median pore width of 21 nm and a storage modulus of 1.8 GPa. When serving as a dendrite-suppressing separator in lithium-metal batteries, the mesoporous polyimide separator enables safe cycling for 500 hours at a current density of 4 mA/cm2.
In zinc-ion batteries, developing cathodes compatible with fast charging remains a challenge. Conventional MnO2 gravel cathodes suffer from low electrical conductivity and slow ion (de-)insertion, resulting in poor recharging performance. In this dissertation, porous carbon fiber (PCF) supported MnO2 (PCF@MnO2), comprising nanometer-thick MnO2 deposited on block copolymer-derived PCF, serves as a fast-charging cathode. The high electrical conductivity of PCF and fast ion (de-)insertion in nanometer-thick MnO2 both contribute to a high rate capability. The PCF@MnO2 cathode, with a MnO2 loading of 59.1 wt%, achieves a MnO2-based specific capacity of 326 and 184 mAh/g at a current density of 0.1 and 1.0 A/g, respectively.
This dissertation investigates approaches to utilizing block copolymers-derived nanoporous materials for high-performance energy storage. Those approaches are envisaged to inspire the design of block copolymer-derived nanoporous materials, and advance the development of "beyond Li-ion" energy storage. / Doctor of Philosophy / When we talk with friends on mobile phones, accomplish works on laptops, drive back home and see family's smiling faces under lamplights, we must have noticed that our daily life significantly relies on electrical energy. Although being predominantly employed in today's rechargeable energy storage, lithium-ion batteries using graphite anodes have approached their theoretical energy limits. We are expecting better-performance batteries for a more convenient life: to fully charge our phones faster, to use our laptops for a longer time, and to drive our electric cars for a further distance. Lithium-metal batteries and aqueous zinc-ion batteries stand out for "beyond lithium-ion" energy storage because they deliver more energy and charge faster. The commercialization of lithium-metal batteries and zinc-ion batteries may benefit from revolutionary porous materials derived from block copolymers.
On one hand, lithium-metal batteries employ metallic lithium anodes, storing about 10 times of energy compared to equal-weight graphite anodes and allowing faster charging rates. However, the lithium-metal anodes grow needle-shaped dendrites during cycling. Those lithium dendrites traverse the battery separator through its large pores, causing internal short circuits and even fire hazards. Suppressing lithium dendrites is imperative for safe lithium-metal batteries. Stiff separators with small pores can suppress lithium dendrites. The small pores prevent lithium dendrites from traversing, and the stiff separators cease the dendritic growth. This dissertation introduces a dendrite-suppressing separator derived from block copolymers comprising stiff polyimide blocks and vulnerable blocks. When those block copolymers form films, the vulnerable blocks spontaneously disperse as a network embedded in the polyimide. Then, the vulnerable blocks are removed at elevated temperatures to create interconnected small pores. This porous polyimide separator suppresses lithium dendrites to allow safe cycling for 500 hours, surpassing today's separators which encounter short circuits within 60 hours.
On the other hand, zinc-ion batteries require fast-charging cathodes for high charging rates. A fast-charging cathode demands both good electrical conductivity and fast ion insertion. Herein, this dissertation reports a porous carbon fiber supported MnO2 cathode. The block copolymers comprise a polyacrylonitrile block and a vulnerable block. The vulnerable blocks form a network dispersing in the polyacrylonitrile fibers. At elevated temperatures, polyacrylonitrile is converted to graphitic carbon fibers, and the vulnerable network decomposes to create interconnected pores. The porous carbon fibers afford a large surface area, allowing a high loading of MnO2 to deposit as nanometer-thick sheaths. The resulting cathode combines good electrical conductivity of porous carbon fibers and the fast ion insertion in thin MnO2 sheaths, therefore, exhibiting superior fast-charging performance.
This dissertation reports the methods of using block copolymers to produce porous materials for high-performance batteries. We envisage those methods to inspire the design of block copolymer-derived porous materials, and advance the development of high-performance energy storage for a more convenient life.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/110066 |
| Date | 12 May 2022 |
| Creators | Guo, Dong |
| Contributors | Chemistry, Liu, Guoliang, Madsen, Louis A., Long, Timothy E., Moore, Robert Bowen |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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