On the discharge mechanism in Li-CFx batteries

Electrification of aviation presents the next frontier in the decarbonization of the transportation sector. While current and next-generation battery technologies such as lithium metal anodes and solid state electrolytes enable cell-level specific energy expected to be adequate for the electrification of ground-based transportation, the electrification of aviation requires batteries with over 800 Wh/kg and therefore require the development of rechargeable batteries with ultra-high specific energy. Li-CF_x is one such battery chemistry which possesses the specific energy required for the electrification of short and medium-range aircraft; however, rechargeability has not yet been achieved. Additionally, the discharge voltage is low relative to the open-circuit voltage by nearly 1 V even when discharged slowly. Understanding the discharge mechanism is therefore crucial to improve the performance of Li-CF_x batteries. 

In this work, density functional theory calculations and electrochemical measurements are employed to build mechanistic insights into the discharge mechanism. Due to sluggish fluorine diffusion kinetics, the defluorination of CF_x to yield carbon and LiF electrochemically is implausible; rather, lithium intercalates into CF_x during discharge followed by a chemical decomposition into LiF and carbon, the thermodynamically stable products. A possible crystal structure for the lithium-intercalated CF_x phase is proposed and mechanisms by which the phase transforms into LiF and carbon are investigated. Galvanostatic cycling in shallow DoD window showed some charge capacity, believed to be originating from the lithium-intercalated CF_x. The term quasi-rechargeability is used to distinguish this from true rechargeability arising from the electrochemical conversion of LiF and carbon, and the challenges to be solved for enabling true rechargeability is outlined. A modification to the current hypothesis on the discharge mechanism is proposed, and the insights from this investigation is expected to yield design rules for enhanced discharge performance of a primary Li-CF_x battery.