Limited understanding of the high ionic conductivity of solid electrolytes is one of the major hurdles preventing the development of all-solid-state batteries for future electric vehicles. This is particularly observed in recently discovered sulfide-based solid electrolytes such as Li10GeP2S12 and Li9.54Si1.74P1.44S11.7Cl0.3, which exhibit unprecedented ionic conductivity close to or even higher than that of their liquid electrolyte counterparts. Despite recently reported experiments and simulations on their topological structures and associated ionic conductivity, the mechanisms underlying the superionic transport rate observed for these solid electrolytes are still poorly understood. Herein, we report the first results of the effect of applied electric potential on the changes in the electronic structures associated with the addition of dopant materials to solid electrolytes. Atomic simulations confirm that both Si and Cl dopants promote the polarization of Si- and Cl-bearing ionic clusters of solid electrolytes. This renders the ionic clusters mechanically less stable and thus opens up the diffusion pathway for Li+ under the presence of an electric field, facilitating the fast transport of Li+. The present work offers some design criteria that can be used to develop high-rate performance solid electrolytes.