Revealing the interdiffusion equilibrium of lithium cations across a solid--electrolyte interface by T1 NMR relaxation

Fast charging of batteries is pivotal for a broader acceptance of electric mobility. While fast charging capabilities have been demonstrated for lithium titanate (Li4Ti5O12, LTO) anode materials, the underlying mechanisms are still poorly understood. Recently the interdiffusion of mobile lithium ions between a liquid electrolyte and LTO, facilitated by unpinning of polarons from surface defects, has been suggested as a potential factor influencing fast charging. This effect is explored further by systematically varying the concentration of the electrolyte salt in an aprotic electrolyte in contact with LTO, and acquiring Li-7 T1 NMR relaxation time constants as a measure for the bulk concentration of paramagnetic polarons. A systematic relation between electrolyte concentration and Li-7 T1 was found, with a qualitatively different behavior above and below about 5\;mM. In the high concentration region, the observed relation was consistent with the law of mass action for a biphasic equilibrium exchange of lithium ions between LTO and electrolyte. Upon contact of the two phases, there was a redistribution of lithium ions between anode and electrolyte, which can be understood analogously to osmotic pressure of mobile lithium ions. This is the first demonstration of such an equilibrium for a non-faradaic lithium exchange at a solid–-electrolyte interface, substantiating our previous hypothesis of a polaron-supported fast-charging mechanism. This study provides a basis for more quantitative (surface)-defect engineering, which is key to optimize battery fast-charging properties.