Ionic conductivity and exchange current density of non-aqueous lithium polysulfide electrolyte

Lithium-polysulfide flow batteries, which utilize the high solubility of lithium polysulfide in non-aqueous electrolytes to enable flowable electrodes, have high theoretical energy density and low raw materials cost. To achieve greater electrode-level energy density, higher sulfur concentrations are needed. In a given electrolyte system, sulfur charge storage capacity (e.g. mAh/g sulfur) decreases dramatically with increasing sulfur concentration at a fixed C-rate, which corresponds to higher current output in higher concentration system. Understanding the limiting factors that undercut the rate capacity is crucial to enhancing the performance of high energy density systems. In particular, we systematically investigate the ionic conductivity and exchange current density at the electrode surface with lithium polysulfide solutions of varying concentration and in differing solvents which solvent molecules of different sizes. Ionic conductivities are measured using a commercially available conductivity probe, while exchange current densities are measured using both impedance spectroscopy and galvanostatic polarization using glassy carbon working electrodes. The electrolyte solvent is found to dramatically affect the solution ionic conductivity and exchange current density. In the concentration range of interest (1-8 M [S]), the ionic conductivity monotonically decreases with increasing sulfur concentration while exchange current density shows a more complicated response in a given solvent system. Between solvent systems, we observed a five-fold increase in ionic conductivity, and a more than 15-fold enhancement in exchange current density. The conductivity and current density results are used to interpret the rate capability of suspension-based cells using lithium-polysulfide electrolyte and carbon black as the cathode with different solvents. With the improvement in kinetics parameters, we also observed better rate capability in solvent. We also study non-carbonaceous electrode materials to understand how the electrode material can affect exchange current density and thus cell capacity. Indium tin oxide electrode shows lower exchange current density then glassy carbon electrode in preliminary results.

Department dc:contributor.department

Massachusetts Institute of Technology. Department of Materials Science and Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2015

Author dc:creator

• Pan, Menghsuan Sam

Advisor dc:contributor.advisor

• Yet-Ming Chiang.

• Materials Science and Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/98665

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/98665