Hunter Ford

Chemical and Biomolecular Engineering

Faculty Advisor: Jennifer Schaefer

Electrochemically Stable Ionomers with Minimal Sulfur Affinity for Mitigating the Polysulfide Shuttle in Metal-sulfur Batteries

The mitigation of fossil fuel driven climate change is a major global challenge, one that can be addressed only by a large-scale shift towards sustainable energy. For this large-scale transition to be realized, the intermittency associated with solar and wind based energy generation must be managed. A logical approach to overcome this barrier is to store the energy produced when the sun is shining and wind is blowing so the energy can be used later when needed. My Ph.D. research is concerned with developing better energy storage technologies that are designed to meet the energy needs of the future. Ranging in potential application from grid-scale storage to the electrification of transportation, my research seeks to contribute to the pursuit of a future without a reliance on fossil fuels. The standard lithium-ion battery delivers insufficient storage capacity for next generation technologies like long-range electric vehicles. Furthermore, the electrode materials consist of transition metals such as cobalt, the mining of which poses health hazards and raises numerous ethical concerns.1 New battery chemistries that address these shortcomings are highly desirable. Metal-sulfur batteries are potentially one such set of chemistries, coupling an alkali or alkaline earth metal anode with a sulfur based cathode. Metal-sulfur batteries can have theoretical energy capacities at least six times higher than commercially available lithium-ion, and can address both grid-scale and transportation storage needs.2 The versatility of the metal-sulfur chemistry, as well as the widespread abundance of sulfur and relevant metals such as magnesium, makes this an attractive and sustainable technology. Metal-sulfur batteries are not without their own set of unique challenges. Intermediate species called polysulfides are formed during the battery operation. These species are necessary for the storage and release of energy. The challenge is that polysulfides dissolve in the liquid electrolyte of the battery, where they can diffuse out of the cathode and reach the anode. Polysulfides can passivate the surface of the metal anode, and/or get trapped in a process of constant redox shuttling between the electrodes, both of which result in short lifetimes of metal-sulfur batteries and inefficient energy usage.3 Addressing this phenomenon, known as the polysulfide shuttle, is paramount for enabling the practical use of metal-sulfur batteries and is the subject of my Ph.D. research.

My research has previously shown that certain ionomers (polymers that contain covalently tethered charges) are effective at mitigating the polysulfide shuttle in metal-sulfur batteries. When employed as a separator, a component that is sandwiched between the cathode and anode in a cell, I have demonstrated that certain ionomer chemistries can successfully diminish the polysulfide shuttle in magnesium sulfur batteries. Although the performance of the cells with ionomer separators was improved, unfortunately there was still a large degree of capacity fading.4 The conclusions of that study as well as preliminary unpublished results have identified that the capacity fading was due in part to the absorption of the sulfur into the ionomer. The ionomer successfully prevented the sulfur from contacting the magnesium anode, thereby interrupting the polysulfide shuttle, but because the sulfur was absorbed into the ionomer the cell still suffered capacity fading. I have recently identified a series of ionomer chemistries that have low sulfur affinity, meaning that these materials should not absorb the active material and therefore lead to higher capacity retention. However, while these materials do not absorb sulfur under passive diffusion conditions, they appear to react with sulfur when in a full metal-sulfur cell. This electrochemical instability seems to result in the sulfur becoming covalently bound to the ionomer, once again resulting in a loss of capacity upon cycling. The identified materials that passively reject sulfur will be modified to no longer contain the electrochemically unstable components, resulting in an ionomer that will not absorb nor react with sulfur. In developing an electrochemically stable ionomer with low sulfur affinity, the performance of metal-sulfur batteries is expected to be dramatically improved.

(1) Banza Lubaba Nkulu, C.; et al. Sustainability of Artisanal Mining of Cobalt in DR Congo. Nat. Sustain. 2018.
(2) Manthiram, A.; et al. Rechargeable Lithium-Sulfur Batteries. Chemical Reviews. 2014, pp 11751–11787.
(3) Kim, H. S.; et al. Structure and Compatibility of a Magnesium Electrolyte with a Sulphur Cathode. Nat. Commun. 2011, 2 (1).
(4) Ford, H. O.; et al Cross-Linked Ionomer Gel Separators for Polysulfide Shuttle Mitigation in Magnesium-Sulfur Batteries: Elucidation of Structure-Property
Relationships. Macromolecules, Front Cover Featured (2018), 51 (21), 8629-8636.