Quasi-Rechargeability in Li-CFx Batteries

Abstract

Lithium–fluorinated graphite (Li-CFx) has long been prized as a primary battery chemistry for its outstanding specific capacity (860 mAh/kg for CF1.0) and low self-discharge rate (<0.5%/year). Enabling rechargeability of this chemistry would make it an appealing candidate for short-haul electric aviation. However, progress toward this goal has been impeded by the high driving force required to revert the final discharge products: LiF and graphite. In this work, we instead aim to recharge from the temporary intermediate LiyCFx phase and dub this quasi-recharging. Chemical lithiation followed by potentiometry and XRD is used to assess the presence of the intermediate as a function of time. Shallow galvanostatic cycling (<10% depth of discharge) is proposed as a way to enable some capacity to be recharged by minimizing the time available for the intermediate to decompose, compared to full depth of discharge cycling. A 4-electrode setup using separate counter electrodes for discharge and charge has been constructed to indirectly observe the delithiation of CFx during charge, thereby separating the desired Faradaic capacity from capacitive and parasitic capacities. Results obtained from these methods with varying discharge rate, temperature, and rest time between discharge and charge will be discussed.