(435d) Understanding the Effects of Preconditioning Treatments and Discharge on the Electrochemical Interphases in Li-CFx Batteries

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 extreme conditions can have a significant negative impact on battery materials and electrochemical performance. Furthermore, due to the low solar flux at these distances, solar-powered recharging is ineffective, restricting the advantage of 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_xbatteries can. However, much remains to be understood regarding how the Li-CF_x battery electrodes and their electrochemical interphases evolve at a molecular level, both during preconditioning steps and during discharge under different conditions.

The solid electrolyte interphase (SEI) formed on the surface of the lithium metal anode upon electrochemical reduction of the electrolyte plays a key role in the stability and discharge performance of Li-CF_x batteries. To form a stable SEI, Li-CF_x batteries are subjected to a preconditioning treatment, which consists of an initial discharge at low rates, followed by a heat treatment. Here, we investigate the influence of temperature, aging, discharge rate, and electrochemical discharge on the chemical composition and stability of the electrochemical interphases formed on Li-CF_x battery electrodes. Coupled electrochemical impedance spectroscopy (EIS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), and solid-state nuclear magnetic resonance (NMR) measurements enabled molecular-level and interfacial changes in lithium metal and CF_x electrodes subjected to different preconditioning steps to be correlated to the chemical composition and stability of the passivation layers formed.

Potentiostatic EIS measurements revealed an increase of the inner cell resistance linked to both cell aging and increased temperatures during heat treatment, suggesting the degradation of the electrode passivation layer. ToF-SIMS measurements of the lithium metal electrode revealed the compositions and thicknesses of the Li-CF_x SEI. Quantitative solid-state ¹⁹F and ⁷Li single-pulse magic-angle-spinning (MAS) NMR experiments were performed on CF_x electrodes subjected to different preconditioning parameters, as well as those electrochemically discharged, enabling molecular and interfacial changes to be correlated. The experiments reveal insights into the molecular-level compositions and stability of the electrochemical interphases in Li-CF_x batteries and how they change with preconditioning heat treatment and electrochemical discharge. Overall, the results suggest design strategies for improving the stability of electrochemical interphases in ultra-high-energy-density Li-CF_x batteries for space applications.