Li-Ion Conduction Mechanism and Relaxation Dynamics in Sorosilicate Compound Li2Cu5(Si2O7)2

Inorganic solid electrolytes are the building blocks for all-solid-state lithium-ion batteries and have substantial advantages in safety and electrochemical as well as mechanical stability compared to organic liquid electrolytes. We report here the Li-ion conduction mechanism and relaxation dynamics in a potential solid electrolyte material, viz. sorosilicate compound Li2Cu5(Si2O7)2, by impedance spectroscopy. The dc conductivity follows the Arrhenius behavior with an activation energy of 1.25(4) eV, and it reveals thermally activated lithium-ion conduction. We have shown the importance of a careful selection of an equivalent circuit model to determine the dc conductivity parameters from the impedance data involving contributions from the grain and grain boundaries. The frequency- and temperature-dependent ac conductivity and dielectric constant measurements reveal that the ionic conduction in the present compound occurs through the correlated barrier hopping (CBH) of charge carriers. The ionic conduction pathways within the unit cell have been mapped by the soft bond valence sum (BVS) analysis of the measured neutron diffraction pattern. The analysis indicates the presence of bottlenecks in the lithium-ion conduction pathways along the a axis and along the diagonal direction of the bc plane of the triclinic unit cell which are responsible for the observed high value of activation energy and, hence, the low value of ionic conductivity. Electric modulus studies have confirmed that the ionic conduction relaxation process is thermally activated and it has a spread of relaxation time. Therefore, the present study involving the dc conductivity, ac conductivity, electric modulus, and dielectric constant analyses sheds light on the microscopic mechanism of ionic conduction in Li2Cu5(Si2O7)2. The acquired knowledge on the ionic conduction mechanism will be useful for designing efficient ionic conductors for battery applications.