Multi-Temperature in Situ Magnetic Resonance Imaging of Polarization and Salt Precipitation in Lithium-Ion Battery Electrolytes

Accurate electrochemical modeling of lithium-ion batteries is an important direction in the development of battery management systems in automotive applications, for both real-time performance control and long-term state-of-health monitoring. Measurements of electrolyte-domain transport parameters, under a wide regime of temperature and current conditions, are a crucial aspect of the parametrization and validation of these models. This study constitutes the first exploration of the temperature dependence for the steady-state electrolyte concentration gradient under applied current with spatial resolution via the in situ magnetic resonance imaging (MRI) technique. The use of complementary pure phase-encoding MRI methods was found to provide quantitatively accurate measurements of the concentration gradient, in strong agreement with predictions based on ex situ NMR and electrochemical techniques. Temperature is demonstrated to have a marked influence on the steady-state concentration gradient as well as the rate of its buildup. This finding underlines the importance of utilizing spatially varying electrolyte transport parameters in modeling approaches. Additionally, a surprising outcome of this investigation was that a conventional 1.00 M LiPF6 electrolyte in an equal-parts ethylene carbonate/diethylene carbonate solvent mixture generated salt precipitation under polarization at 10 °C. The loss of salt under strong polarization and at low temperature is a previously unaddressed potential source of long-term capacity fade in lithium-ion batteries, and on the basis of this result, warrants further investigation.