Quantifying the effect of conductive polymer binders on Li-O2 battery performance

Electric vehicles (EVs) have gained popularity in recent years for their ability to reduce carbon emissions. However, increased commercial adoption requires advancements in battery energy density, to enable longer drive times without significantly increasing vehicle weight. Li-O2 batteries are a promising new energy storage technology — with a theoretical energy density of 3500 Wh/kgbattery, Li-O2 batteries can enable EVs which can drive up to 1000 miles between charges. However, Li-O2 battery development is limited by a number of fundamental challenges, which result in poor rate capabilities due to transport and surface reactions, low energy efficiency, and rapid battery degradation due to material instability. This thesis project seeks to understand the effect of conductive polymer binders (such as Nafion) in Li-O2 battery cathodes on battery performance. Binders are a necessary component to battery cathodes—they literally bind cathode particles together, providing for electrical conductivity and a physically coherent structure—but add weight and do not contribute directly to electrochemical activity. Moreover, binders block access to reactive surfaces and impede transport of dissolved electrolyte species through the cathode thickness, reducing battery capacity, particularly under higher currents. Lastly, side reactions between binder materials and reactive intermediates in the electrolyte contribute to battery degradation. With the addition of conductive polymer binders such as Nafion in the cathode, we expect to see improved battery performance due to enhanced transport, reactive surface areas, and material stability. However, the direct role played by novel binder materials is poorly understood. Herein, we present combined Li-O2 battery experiments and numerical simulations to quantify the impact of novel conductive binders on battery performance. Carbon paper was used as a model Li-O2 cathode with limited microstructural variation, and battery cycling and electronic impedance spectroscopy data were collected for three types of cells: (i) no binder, (ii) a non-conductive PTFE binder, (iii) and a conductive Lithiated Nafion binder. The experimental results demonstrate that the battery performance is very sensitive to the cathode microstructure—results showed significant sample-to-sample variation, presumably due to variations in the binder deposition and dispersion. These variations prevent any definitive conclusions about the impact of the conductive binders, but guide future development of cathode fabrication processes. The experimental results also demonstrate the crucial impact of microstructure and surface area in Li-O2 battery operation. Even at very low currents, the battery charges and discharges at voltages well off from its thermodynamic equilibrium potential, demonstrating poor energy efficiency, and have capacities well below the theoretical limit. Validating a one-dimensional Li-O2 battery model against this battery data confirms the rate-limiting nature of the surface area for these cells.…