Nanoscience and Microsystems ETDs

Publication Date

Summer 7-6-2017

Abstract

Secondary lithium sulfur (Li-S) batteries have been heavily studied in the battery community since the start of the 21st century due to their high theoretical capacity and specific energy density. The ground-breaking lithium ion battery has revolutionized society and its ability to store energy, however the lithium ion battery is reaching its limit with a theoretical capacity almost five times lower than that of lithium sulfur. With the improvement and commercialization of Li-S batteries, electric vehicles and personal electronics will have extended performance on a single charge. The viability of this promising chemistry relies on overcoming several key difficulties, one of the most imperative being high solubility, mobility, and reactivity of polysulfide reaction intermediates in liquid electrolytes. In situ Raman microscopy is used to study polysulfide speciation in the bulk electrolyte (1:1 1,2-dimethoxyethane (DME): 1,3-dioxolane (DOL) with 1 M lithium bis(trifluoromethane)sulfonimide (LiTFSI) and 0.25 M LiNO3) during the discharge and charge of a Li-S T-cell to better understand the complex interplay between chemical and electrochemical reactions in solution. Long chain polysulfides and the S3- radical appear in the electrolyte at 2.4 V indicating a rapid equilibrium of the dissociation reaction to form S3-. Interestingly, an increase in the concentration of all polysulfides was observed during charge indicating the importance of the E/S ratio and potentially a loss in the useful sulfur inventory from the cathode to the electrolyte. For peak comparisons, nominal Li2S8, Li2S6, and Li2S4 standards were chemically synthesized under argon and analyzed for stability, using Raman, for one week. All three standards equilibrated to contain the same polysulfide species, however the relative distributions of the species distinguished the standards from one another. Not all peaks observed in the in situ cell were seen in the standards, indicating that the electrochemical and chemical equilibria do not yield the same speciation.

Keywords

Raman microscopy, battery, polysulfides, standards, in situ characterization, lithium sulfur

Sponsors

Sandia National Laboratories

Document Type

Thesis

Language

English

Degree Name

Nanoscience and Microsystems

Level of Degree

Masters

Department Name

Nanoscience and Microsystems

First Committee Member (Chair)

Fernando Garzón

Second Committee Member

Christopher Apblett

Third Committee Member

Andrew Shreve

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