Fabrication, Testing, and Comparative Analysis of Lithium Sulfur and Lithium-Ion Electrochemistries

Abstract:
This paper addresses the fundamental barriers of eVTOL aircraft- energy and power. 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. The key conclusions are 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 appear 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 eVTOL design and infrastructure.

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