High-T_c superconducting YBa₂Cu₃O₇ ceramic samples are lithiated by the reaction with n-butyllithium. For lithium nominal contents less than unity per formula the presence of '123' and '124' phases are deduced from X-ray diffraction and high-resolution electron microscopy experiments. NMR and impedance spectroscopy techniques have been conducted in a sample with a nominal Li content of 0.9 per formula. Spin-lattice relaxation times and electrical conductivity relaxation (ECR) are measured as a function of temperature. NMR and ECR data are interpreted in terms of a stretched exponential decay function in the time domain. Microscopic activation energies for lithium motion of 0.49 eV (NMR measurements) and of 0.41 eV (ECR) are deduced for short range motion. Activation energies for long range motion of 0.98 eV (NMR) and 1.03 eV (ECR) are also deduced.