Excellent rate performance of Li4+xTi5O12 (0 < x < 3, LTO) electrodes results from mixed Li site occupancy and facile Li ion exchange at 8a and 16c sites. In this paper, we reveal that inter-particle connectivity within LTO electrodes affects 8a and 16c site occupancies upon discharge and impacts Li ion diffusion. LTO electrodes of the same primary crystal structure but of different grain boundary structures were prepared and they showed significantly different electrochemical performance. LTO electrodes with a percolated 3D structural network and bridged grain boundaries offered balanced 8a-16c occupancy, Li ion exchange at 8a and 16c sites upon discharge, high ionic conductivities, and good rate performance. While LTO electrodes with isolated clusters of particles showed strong rate dependence of 8a-16c occupancy, a lack of Li ion exchange at 8a and 16c sites, large over-potential, and substantial capacity decay upon fast charging. Bridged grain boundaries in LTO secondary particles facilitate apparent solid-solution process during electrochemical cycling by maintaining Li site exchange and thus enhance the rate performance of LTO electrodes.