Studies of Electrochemical Energy Storage Systems: Capacitor and Sodium-Ion Battery

Date

2018-12

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Abstract

The current technologies urgently require different kinds of efficient energy storage systems to match their practical applications. In this dissertation, I focus on two types of electrochemical energy storage systems: electrochemical capacitors (ECs) and sodium-ion batteries (SIBs). ECs display high power densities ca. 10 kW kg-1, thus they are widely applied in cases requiring fast charge/discharge response. SIBs exhibit promising potentials for next-generation batteries alternative to lithium-ion batteries for the abundance, low cost of sodium carbonates, and considerable energy densities. The operation window of aqueous ECs is limited because thermodynamically water decomposes at a potential of 1.23 V. Expanding the operation window is critical to achieve high cell voltage and improve the energy density of the ECs. In Chapter 3, a wide operation window from -0.4 V to 1.6 V vs. RHE was achieved by using high concentration LiTFSI as an electrolyte. In Chapter 4–6, I explore the strategies of developing suitable anode materials for SIBs and present some in-depth understandings about the electrochemical processes during cycling. Anode materials based on alloying and conversion mechanisms display high capacities, but vast volume expansion causes the fast decay of capacity. In Chapter 4, a new anode material Bi2Se3/C was synthesized by simple high-energy ball milling. The advantages of Bi and Se are combined in Bi2Se3. Bi displays a small volume expansion among alloying-type materials; Se displays much faster reaction kinetics than O and S in the same group. The carbon integration significantly improves the stability. The phase changes during cycling were carefully revealed by ex-situ XRD. A carbon network displaying high conductivity and firm bonding with active materials benefits the improvement of the reaction dynamics and stability. In Chapter 5, I report the synthesis of SnS2@N/S-C with the assistance of dopamine. It maintains 141 mAh g-1 over 950 cycles at 1 A g-1. Characterizing the changes of active materials in their operation conditions can provide us with more fundamental understanding. In Chapter 6, I study the morphology evolution of FeS nanorod@rGO through in-situ TEM. Finally, Chapter 7 reviews the thesis and discusses potential improvements.

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Keywords

Electrochemistry, Capacitor, Sodium-ion battery

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