Status and Targets for Polymer-Based Solid-State Batteries for Electric Vehicle Applications

Abstract

As the market of electric vehicles (EVs) expands, numerous researchers and companies are focusing on the use of lithium metal anodes, which can enable higher energy densities required to extend the driving range of EVs and allow for lower cost [1]. However, the implementation of these metal anode batteries has several safety issues that emerge from dendrite growth in flammable liquid electrolytes. Solid polymer electrolyte has been studied as one of the promising candidates for lithium metal batteries because of the advantages of the non-flammability and relatively higher mechanical properties than liquid electrolytes [2, 3]. Compared to existing batteries based on liquid electrolytes, solid polymer electrolytes still have limitations, such as low ionic conductivity under room temperature conditions, which can significantly hinder its application in EVs. Hence, research on next-generation solid polymer electrolyte is being conducted to improve its transport properties. In this work, by using a model-based approach, the status of the present-day solid polymer electrolyte battery is examined, and compared with the traditional batteries with liquid electrolyte, with a focus on EVs. Using a mathematical model developed based on the macro-homogeneous approach pioneered by Newman and coworkers [4], we investigated Li-metal/LiFePO4 (LFP) batteries containing three different electrolytes, namely (i) liquid electrolyte (ii) polystyrene-b-poly(ethylene oxide) (SEO) block copolymer electrolyte and (iii) a single-ion conducting block copolymer electrolyte. Concentrated solution theory and modified Ohm’s law are used to calculate the mass transport and potential gradient inside the electrolyte. In addition, the mass balance of lithium, potential drop in the active material, and the charge-transfer reactions are considered for a composite positive electrode that consists of LFP active particles. More detailed information about the continuum model is available in the literatures [4, 5]. All the transport parameters for the solid polymers, which include SEO and single-ion block copolymer, are adopted from the previously-published experimental results [3, 6-8]. For validation, model predicted results of the three Li/LFP batteries are compared with experimental data. Next, we compared the performance of the three different electrolytes operating at the same specified electrode thickness and weight percent of electrolyte. Optimal design points of each cell that provides the highest specific energy and power are determined. Finally, the results demonstrate how the specific energy and power of a solid polymer cell should be improved to achieve similar performance as that of a liquid electrolyte cell. This work provides directions for improvements in present-day solid polymer electrolytes for usage in electric vehicle. References Gallagher, K.G., et al., Quantifying the promise of lithium-air batteries for electric vehicles. Energy & Environmental Science, 2014. 7(5): p. 1555-1563.Armand, M. and J.M. Tarascon, Building better batteries. Nature, 2008. 451(7179): p. 652-657.Bouchet, R., et al., Single-ion BAB triblock copolymers as highly efficient electrolytes for lithium-metal batteries. Nature Materials, 2013. 12(5): p. 452-457.Doyle, M., T.F. Fuller, and J. Newman, Modeling of Galvanostatic Charge and Discharge of the Lithium Polymer Insertion Cell. Journal of the Electrochemical Society, 1993. 140(6): p. 1526-1533.Fuller, T.F., M. Doyle, and J. Newman, Simulation and Optimization of the Dual Lithium Ion Insertion Cell. Journal of the Electrochemical Society, 1994. 141(1): p. 1-10.Bruce, P.G., et al., Li-O-2 and Li-S batteries with high energy storage. Nature Materials, 2012. 11(1): p. 19-29.Devaux, D., et al., Failure Mode of Lithium Metal Batteries with a Block Copolymer Electrolyte Analyzed by X-Ray Microtomography. Journal of the Electrochemical Society, 2015. 162(7): p. A1301-A1309.Pesko, D.M., et al., Comparing Two Electrochemical Approaches for Measuring Transference Numbers in Concentrated Electrolytes. Journal of the Electrochemical Society, 2018. 165(13): p. A3014-A3021.