Comprehensive Study and Fundamental Understanding of Lithium Sulfur Batteries for eVTOL

Abstract

This dissertation is an attempt to bridge the traditional isolation of electrochemistry and aeronautics, particularly rotary-wing aeronautics, where integration of power and platform is vital for the development of a viable aircraft. The dissertation topic addresses the principal barriers of electric-VTOL aircraft: energy and power. It explores a promising alternative to traditional lithium-ion technology; advanced lithium sulfur batteries are projected to supply double the energy of lithium-ion. The principal conclusions are: 1. lithium sulfur can indeed achieve double the energy of lithium-ion but only at low power settings, 2. lithium sulfur exhibits high internal resistance compared to lithium-ion which explains the loss of energy at high power, and 3. discharging lithium sulfur only halfway stabilizes the batteries. A conceptual sizing analysis was developed and verified with NASA. Key battery performance targets were determined for electric VTOL aircraft. Lithium sulfur and lithium-ion coin cells were fabricated with identical overhead for a clear and consistent comparison of specific energy and power. The characteristics measured were discharge cycles, cycle life, impedance under conditions unique to electric vertical takeoff and landing aircraft namely high C-Rates, half cycles, and high transients. Equivalent circuit models were developed and validated to predict the steady-state and transient behavior of these cells. Results show that lithium sulfur provides more than twice the specific energy of lithium-ion up to currents of almost C/2. At 1C, it is comparable. Above 1C it drops drastically and by 4C the energy vanishes almost entirely. This is traced to an order of magnitude higher impedance of these cells. The price to pay for high energy is low cycle life. However, it appears this problem can be eliminated by half cycles. The dynamic behavior of lithium sulfur is richer in comparison to lithium-ion. The response is still capacitative, hence first order, but the complex Warburg and constant phase elements have far greater influence. The behavior is harder to model as it does not fit neatly into linear equivalent circuits. The key conclusion is that lithium sulfur appears to be an attractive alternative to lithium-ion with characteristics that have significant ramifications on future electric VTOL aircraft design and infrastructure.