Predicting Electrolyte Performance in Lithium Metal Batteries at Low and High Current Densities

摘要

For lithium metal batteries to surpass graphite-based lithium ion batteries as the standard for advanced applications, electrolytes must be designed to 1) stabilize the lithium metal anode and high-voltage cathode and 2) facilitating ion transport at fast rates. Composite electrolyte systems (e.g. a glassy polymer or nanoparticle dispersed in an ion-conducting medium) have frequently been proposed in an effort to fulfill the first design principle; however, it is often not clear if the second design principle can be met. Ultimately, the performance of new electrolytes should be based on comprehensive cycling experiments in full-cell configurations. Practically, we would like to leverage simple characterization techniques in order to predict full-cell performance and allow design iteration more quickly. In this talk, we will discuss experiments utilizing symmetric cell configurations (i.e. lithium/electrolyte/lithium) which can be used to predict electrolyte performance in a battery. For applications requiring small applied currents, the product of the ionic conductivity, k, and the current fraction, p_+, can be used to evaluate electrolyte efficacy and can be measured in a simple polarization experiment in a symmetric cell. This method is well established, and we have compiled data from the literature which reveals a tradeoff between the two parameters. For applications requiring large current densities, the problem is more complicated, especially for composite electrolytes where many parameters (six or more) are required to fully define ion transport. We have applied an approximation to Newman's concentrated solution theory to define ion transport in a block copolymer electrolyte system (polystyrene-block-polyethylene oxide with LiTFSI) with only three transport parameters. We will discuss the results and predictions made by the theory and compare the predictions to experimental data. Our approach to characterize electrolyte performance at low and high current densities can be broadly applied to any electrolyte which can be studied in a symmetric cell configuration.