Defense Date

2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Dr. Hani M. El–Kaderi

Second Advisor

Dr. Heather R. Lucas

Third Advisor

Dr. Soma Dhakal

Fourth Advisor

Dr. Wei–Ning Wang

Abstract

Energy demands are anticipated to increase in the next decades with the consumption of fossil fuels predicted to grow from the current levels. This will place an enormous burden on the non–renewable reserves of our energy sources and add to the preexisting environmental crisis with the emission of greenhouse gases like carbon dioxide (CO2). Electrical energy storage (EES) is considered one of the most crucial needs of modern society to circumvent this crisis and potential applications include electric vehicles and devices, as well as storage systems for solar and wind sources.Lithium–ion batteries (LIBs) have proven to be successful in the portable electronics industry. However, LIBs are approaching the theoretical limit of electrode materials (250 mAh g1). Alternative battery chemistries such as Na–ion batteries, Zn–air, Li–air, and Lithium–Sulfur (Li–S) batteriesare attracting unprecedented attention in the scientific community. Notably, Li–S batteries have shown promise to replace the conventional LIBs due to the high theoretical specific capacities of sulfur (1675 mAh g–1) and lithium (3860 mAh g–1). Their theoretical specific energy density is 2600 Wh Kg–1, which is 3–5 times higher than conventional Li–ion batteries. An additional benefit is that sulfur is a low cost, eco–friendly, and one of the most abundant elements in the earth’s crust. Also, lithium has a low redox potential (–3.04 VSHE), that maximizes the energy density of Li–S chemistry.

However, sulfur cathodes have shown poor long–term cycling stability resulting from 1) a low conductivity for sulfur and sulfur discharge products, 2) a substantial volume change during the charge–discharge process, and 3) a shuttling effect of lithium polysulfides (Li2SX, 4 ≤ x ≤ 8) between the cathode and anode. These lithium polysulfides (LPS) are soluble in the electrolyte and tend to shuttle from the cathode to the lithium anode, where they deposit as insoluble Li2S products. This process leads to low sulfur utilization and poor coulombic efficiency, which results in a rapid decay of specific capacity.

To address these limitations, we have designed novel nanocages containing cobalt phosphide (CoP) nanoparticles embedded in highly porous nitrogen–doped carbon (CoP–N–GC) by annealing ZIF–67 in a reductive atmosphere and then performing a phosphidation step. The large surface area and the uniform homogeneous distribution of CoP nanoparticles within the N–doped nanocages graphitic carbon act as electrocatalysts to suppress the shuttle of soluble polysulfides through strong chemical interactions, while catalyzing the sulfur redox. As a result, the S@CoP–N–GC electrode delivers an extremely high specific capacity of 1410 mA h g–1 at 0.1 C (1 C = 1675 mA g–1) with an excellent coulombic efficiency of 99.7 %. Moreover, capacity retention from 864 to 678 mA h g–1 was obtained after 460 cycles with a very low decay rate of 0.046 % per cycle at 0.5 C. Therefore, the Li–S battery assembled with a CoP catalyst and a polar conductive porous carbon structure effectively stabilize the sulfur cathode, enhancing the electrochemical performance and stability.

Additionally, we have designed, for the first time, ultra–small cobalt nanoparticles (Co NPs) embedded in nitrogen–doped porous carbon via strong electrostatic adsorption as a sulfur host material for Li–S batteries. The large surface area and the uniform homogeneous distribution of Co NPs within the N–doped carbon matrix significantly immobilize the soluble polysulfides migrating out of the framework through strong chemical interactions, which improves the electrochemical performance through the catalytic effect. As a result of these multi–functional arrangements of the cathode, the Co–BIDC/S cathode presents a high sulfur loading of 71 wt% exhibited a high initial specific capacity of 1219 mAh g–1 at 0.1 C and performs with an excellent coulombic efficiency (99.1 %). At 0.5 C, the battery initially starts at 968 mAh g–1, and, after 100 cycles, it only decreased to 858 mAh g–1. At 1 C, the battery delivered a specific capacity of 579 mAh g–1 after 300 cycles with an ultra–low capacity decay of 0.07 % per cycle. This work presents the first study and strategy to develop cobalt nanoparticles in nitrogen–doped porous carbon through strong electrostatic adsorption for Li–S batteries.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

5-11-2020

Available for download on Saturday, May 10, 2025

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