Revealing the Equilibrium of Lithium Cations Across a Solid–Electrolyte Interface by T_1 NMR Relaxation

Battery fast charging is pivotal for broader acceptance of electric mobility. While demonstrated for lithium titanate (Li_4Ti_5O_12, LTO) anodes, the underlying mechanisms are still poorly understood. Recently, NMR T_1 relaxation time constants of ^7Li in the bulk of LTO were found to change if the surrounding electrolyte was altered. It was explained by interdiffusion of mobile lithium ions between the two phases, facilitated by unpinning of polarons from surface defects and leading to a pseudocapacitive effect that potentially influences fast charging. This effect is explored further by systematically varying the lithium salt concentration in an aprotic electrolyte in contact with LTO. Spectrally resolved ^7Li T_1 NMR relaxation times were used as a measure for bulk concentration changes of paramagnetic polaronic charges in LTO. Correlation of electrolyte concentration and ^7Li T_1 showed qualitatively different behavior above and below a salt concentration of about 5 mM, leading to a relaxation dispersion maximum in the LTO bulk. At intermediate concentrations, relaxation was consistent with a ^7Li exchange equilibrium between LTO and electrolyte. Upon contact of the two phases, yet without insertion into an electrochemical cell or applying an external potential, lithium ions redistributed between LTO bulk and liquid electrolyte. The results can be understood analogously to the distribution of mobile lithium ions between two phases separated by a Li^+ permeable membrane. This is the first demonstration of such an equilibrium for non-faradaic lithium exchange at an interface between a solid ceramic electrode and a liquid electrolyte outside an electrochemical cell, substantiating our previous hypothesis of a polaron-supported mechanism. This study provides a basis for more quantitative (surface)-defect engineering, which is key to optimize battery fast-charging properties.