Zwitterionic polymer-based electrolyte engineering for alkali metal ion batteries

Tufts University, Medford MA

Investigators

Abstract

Lithium-ion batteries are ubiquitous in modern society, powering everything from wearable/handheld devices to electric vehicles. Securing the future of reliable and cost-effective electrochemical energy storage, however, will depend on using a variety of battery chemistries, including those that utilize more abundant alkali metal ions such as sodium. For every battery that is developed, maximizing the motion of certain target ions and ensuring a high degree of user safety are two critically important goals. In this project, the investigators will examine the ability of a class of highly stable polymers featuring zwitterionic side groups to promote increased selective transport of sodium and lithium cations. Zwitterions are a class of molecules which contain both a positive and a negative charge. Moderate amounts of a nonflammable, room temperature molten salt (an ionic liquid) will be co-formulated in these electrolytes to enhance overall ionic motion while maintaining safe operation. This study will produce important new insights into the roles of zwitterion chemistry, ionic liquid content, and alkali metal cation identity in determining ideal chemical compositions for future battery electrolytes. Through mentored undergraduate and graduate research experiences for underrepresented students and participation in K-12 outreach activities, this project will also benefit society by enabling transformative educational and training experiences for a diverse group of students at multiple experience levels. The main objective of this project is to examine the intermolecular interactions and selective alkali metal cation (lithium, sodium) transport within zwitterionic polymer/salt electrolytes that contain ionic liquid (IL) mass fractions of approximately 5-80 wt.%. This is an underexplored compositional space that exists between conventional polymer electrolytes and IL-rich ionogels. The research objectives will be accomplished by pursuing multiple synthetic strategies to prepare such electrolytes, probing interactions between the various charged species using several multinuclear NMR and FTIR/Raman spectroscopy techniques, measuring total ionic conductivities and alkali metal cation transference number values via AC impedance spectroscopy and DC polarization tests, and interrogating electrochemical stability upon repeated strip-plate cycling in symmetric alkali metal electrode cells. It is hypothesized that high-mobility pathways for alkali metal cation transport may be created along the pendant zwitterionic functional groups on the polymer, which provides a motivation for examining the effects of reducing the IL content among this novel class of electrolytes. This project will be among the first to directly compare sodium versus lithium cation transport using different zwitterionic polymer chemistries, which will provide useful information for the future design of polymer-based electrolytes for “beyond-Li ion” batteries. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.