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Polycrystalline ion conductors are widely used as solid electrolytes in energy storage technologies. However, they often exhibit poor ion transport across grain boundaries and pores. This work demonstrates that strategically tuning the mesoscale microstructures, including pore size, pore distribution, and chemical compositions of grain boundaries, can improve ion transport. Using LiTa₂PO₈as a case study, we have shown that the combination of LiF as a sintering agent with Hf⁴⁺implantation improves grain-grain contact, resulting in smaller, evenly distributed pores, reduced chemical contrast, and minimized nonconductive impurities. A suite of techniques has been used to decouple the effects of LiF and Hf⁴⁺. Specifically, LiF modifies particle shape and breaks large pores into smaller ones, while Hf⁴⁺addresses the chemical mismatches between grains and grain boundaries. Consequently, this approach achieves nearly two orders of magnitude improvement in ion conduction. Tuning mesoscale structures offers a cost-effective method for enhancing ion transport in polycrystalline systems and has notable implications for synthesizing high-performance ionic materials.