Discovering and engineering solid-state ionic conductors is central to the advancement of electrochemical energy storage. Crystal structures with an array of diffusion mechanisms and pathways have been developed to facilitate rapid ionic transport for electrode (mixed ionicelectronic conductors) and solid-electrolyte (pure ionic conductors, ideally) applications. As the most basic design criterion, low hopping barriers with a relatively flat energy landscape for the mobile ion are a prerequisite for fast transport. In real, defective, materials, there is another key consideration which is the dimensionality of diffusion. Two-dimensional (2D) or threedimensional (3D) conduction is generally favored relative to a one-dimensional (1D) tunnel diffusion mechanism because the latter is susceptible to blocking defects. Perhaps the best example of this is olivine LiFePO4 (LFP), which is a 1D Li + conductor along channels in the [010] direction. Lithium diffusivity at 300 K in pristine LFP is ~10 -12 m 2 s -1 ; this extraordinarily high value should translate to rapid, long-range conduction in bulk solid particles. However, upon the introduction of just 1% of Li/Fe antisite defects, the intrinsic conductivity drops by two orders of magnitude and 90% of the accessible capacity of an equant 1 µm 3 particle is lost. Owing to this defect intolerance, LFP particles must be nanosized along the b-axis to retain good kinetics and site accessibility. The design of not just highrate but defect-tolerant battery materials is an outstanding challenge for the field.