Access Type

Open Access Dissertation

Date of Award

January 2020

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical Engineering

First Advisor

Leela Mohana Reddy Arava

Abstract

ABSTRACT

ADVANCED ELECTRODES AND ELECTROLYTES FOR LONG-LIVED AND HIGH-PERFORMANCE LITHIUM-SULFUR BATTERIES

by

DEEPESH GOPALAKRISHNAN

August 2020

Advisor: Dr. Leela Mohana Reddy Arava

Major: Mechanical Engineering

Degree: Doctor of Philosophy

Lithium – Sulfur (Li-S) batteries have received much attention and considered as a promising candidate for next generation energy storage devices because of their high theoretical energy density (≈2600 Wh kg‒1) and environmental friendliness. However, the uncontrollable growth of lithium dendrites in the lithium metal anode and the fatal effect of polysulfide shuttle hinder their practical applications. The formation of dendrites during repeated Li plating/stripping processes results in: reduced Li availability for the electrochemical reactions, disruption in Li transport through the interface causing rapid capacity decay and increased safety concerns due to short circuiting. Polysulfide shuttle is a common phenomenon in Li-S batteries where the soluble intermediate polysulfide species (Li2Sx, 4 ≤ × ≤ 8) are inevitably produced and shuttled between cathode and anode, and react with the Li-metal to form insoluble Li2S and Li2S2 on the surface of anode, resulting in surface passivation of Li metal anode, fast self-discharge and rapid capacity fading in Li–S batteries. Thus, the major problems from both anode and cathode side are needed to be addressed, preferably by employing effective strategies. These issues can be addressed only when we have a better mechanistic understanding about chemical and electrochemical processes occurring in the Li-S battery. In the past decade, several strategies have been developed around the world and recently, our group demonstrated utilization of electrocatalyst to improve the polysulfides reaction and trap them inside the cathode of Li−S battery[28, 29, 77]. The electrocatalyst reduces the energy barrier of electrochemical reaction and also act as an anchor for polysulfides and confide them to the cathode reducing their shuttle effect. Herein, we carried out fundamental electrochemical studies on the sulfur -electrocatalyst interface to develop a suitable catalytic cathode. The potentiodynamic and potentiostatic methodologies are used to infer diffusional, adsorption and the kinetics behavior of polysulfides with respect to catalytic and non-catalytic interfaces. In this context, we evaluated the kinetics of sulfur redox chemistry on different electrocatalytic surfaces such as Pt, WS2 and NbS2 and their influences on reaction kinetics at different stages. Also, we have demonstrated the influence of catalyst on solid-to-liquid & liquid-to-solid polysulfides reaction kinetics and their effect on Li2S nucleation ending up in gaining of high capacity during the discharge process. In addition, we have explained in detail the impact of catalytic interface on cathode surfaces as well as on the reversibility of sulfur redox chemistry. We studied the synergistic effect of electrocatalyst NbS2 and conductive carbon substrate in Li-S battery performance. The other issue that we address in the thesis is lithium dendrite formation in Li-S batteries. Though the dendrite formation is one of the oldest issues, fundamental understanding about how the interfacial chemistry and Li deposition is correlated, how anode overpotential affect the cell characteristics etc. are still have no answers which are essential to address the dendrite formation. Here, we demonstrate a novel strategy using a special class of ionic liquids (ILs) with liquid crystal properties called Ionic Liquid Crystals (ILCs) as electrolyte cum pseudo-separator to detain the dendrite growth with their anisotropic nature controlling the Li-ion mass transport. The thermotropic ILC with two-dimensional Li-ion conducting pathways have been synthesized and well characterized. Detailed microscopic and spectroscopic analysis elucidates that the ILC is formed with Smectic A phase and can be utilized for wide temperature window operation. The electrochemical results corroborate the efficacy of ILC electrolytes in mitigating dendrites formation even after 800 hrs. and further substantiated by numerical simulation and deduced the mechanism involved in dendritic suppression. Thus, the research combines experimental development, characterization of the Ionic liquid crystals (ILCs) and analysis of their potential as electrolyte for improving Li battery performance supported by the numerical models.

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