(491b) Understanding Electrochemical Discharge and Degradation Mechanisms in Li-CFx Batteries from the Atomic to Macroscopic Scales

NASA’s Jet Propulsion Laboratory (JPL) is vetting ultra-high-energy-density Li-CF_x batteries for its mission concept to Europa, a moon of Jupiter which contains liquid water underneath its icy surface. The main challenges associated with this mission are battery aging during the long journey to Jupiter, very low surface temperatures, and high gamma-ray radiation. These factors can have a significant negative impact on battery materials and electrochemical performance. Furthermore, due to the low the solar flux at these distances, solar-powered recharging is ineffective, opening the possibility of using non-rechargeable battery chemistries. Traditional lithium-ion batteries cannot adequately meet the ultra-high-energy-density specifications required for the proposed mission—though non-rechargeable Li-CF_x batteries can. However, much remains to be understood about how Li-CF_x battery chemistry evolves at a molecular level, including the effects of radiation, aging, and temperature.

Here, we investigate molecular-level and interfacial changes in CF_x electrodes upon electrochemical discharge and exposure to radiation through correlated solid-state nuclear magnetic resonance (NMR) and electrochemical impedance spectroscopy (EIS) measurements. Quantitative solid-state ¹⁹F single-pulse magic-angle-spinning (MAS) NMR experiments were performed on pristine and irradiated CF_x electrodes, revealing that pristine CF_x electrodes and PVDF binder are remarkably stable to radiation (10 MRad). However, for Li-CF_x cells irradiated after formation or full discharge, ¹⁹F single-pulse NMR experiments establish differences in ¹⁹F signals associated with edge and surface sites, suggesting that radiation affects the electrochemical interfaces on the CF_x electrode surface. Furthermore, EIS measurements reveal an increase of the inner cell resistance after irradiation that can also be linked to possible decomposition of the electrolyte into other reactive species and/or the decomposition of the protective electrode passivation layer, ultimately destabilizing the electrolyte-electrode interface. Quantitative ¹⁹F and ⁷Li single-pulse MAS NMR experiments, as well as galvanostatic EIS measurements, were performed at different depths-of-discharge (DoD), enabling molecular and interfacial changes to be correlated. In particular, the solid-state NMR spectra enable the amount of electrochemically formed LiF to be quantified, while the EIS measurements reveal the charge transfer resistances associated with both the CF_x cathode and Li metal anode (which can be disentangled due to their frequency responses). In aggregate, the results reveal new insights into the molecular-level environments present in Li-CF_x battery chemistries, the solubility of the electrochemical discharge products, and how the electrochemically formed interfaces change with electrochemical cycling and radiation exposure. The results furthermore suggest design strategies towards realizing ultra-high-energy-density Li-CF_x batteries for space applications.