Investigation of energy density limitation for lithium sulfur batteries

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Lithium ion batteries have limited energy density, and an alternative battery chemistry is needed to continue to transform the current energy landscape. Rechargeable lithium-sulfur (Li-S) batteries are among the most promising high-energy-density electrochemical devices for future energy storage applications. However, there are intrinsic limitations that potentially lower the achievable energy density of Li-S batteries. To date, performance of this battery type has been limited by capacity loss and degradation due to permanent loss of active material (sulfur) from the electrode and from reactions of the electrode with the electrolyte. To date, developers have used excess electrolyte to dilute the side reactions that occur that result in lowered performance. This project will use combined experimental and theoretical approaches to study critical issues for high-loading and high-energy Li-S batteries. Specially designed cathode structures and electrolyte configurations will be built to analyze the effects of Li polysulfide species (LiPS) solubility on cell capacity and battery specific energy. The project will yield knowledge on the operating conditions that lead to capacity loss and degradation. The project will also integrate the research outcomes into publicly available electrochemical system simulator (ESS). The fundamental research project will focus on using combined experimental and theoretical approaches to study two critical issues for high loading and high energy Li-S batteries. The first theme includes studies of the effect of lithium polysulfide (LiPS) solubility on cell capacity. Experiments will address the creation of LiPS saturated conditions with different and well-defined initial states-of-discharge and then the characterization of Li-S cell performance under these LiPS saturated conditions. The experiments will also investigate the rate dependence of the potential reaction pathways for LiPS reduction. The second theme addresses experimental verification and theoretical modeling of solid product deposition on the cathode. Focus of this theme is on the study of the Li2S/Li2S2 deposition process using electrochemical measurement combined with material characterization methods, including transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The accompanying feasibility modeling of a Li-S battery will incorporate the experimental verified solid product deposition process, as well as the analysis of the LiPS solubility effect in the built model. The fundamental knowledge and outcomes of this project will also be incorporated into an Electrochemical Systems Simulator (ESS) which is a simulation package for electrochemical systems that incorporates the theoretical models into a user-friendly battery and circuit simulator. The ESS will be a unified framework, in which users can define their own electrochemical devices, discretize them on finite element grids, and perform one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) simulations to study the transport and predict charge and discharge curves and the electrochemical impedance spectra of the cells.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.