The expansion of the battery industry and the search for alternatives to lithium–ion batteries are driving research into high-performance cathode materials for sodium–ion batteries (SIBs). The O3-type Ni-rich cathodes with high capacity are promising for use in SIBs. However, the large size of Na+ ions and side reactions with electrolyte mean that they require assistance with sluggish kinetics.This work reveals a fruitful solution in the form of strengthening the grain boundaries of polycrystalline cathodes, which are vital sites of electrochemical reactions. The infusion of Na2WO4 during calcination enhances the performance of NaNi0.8Mn0.2O2, while also opening new possibilities for research into SIBs. This process facilitates Na+ ion diffusion during cycling and suppresses undesirable side reactions, thereby significantly increasing the reversible capacity of NaNi0.8Mn0.2O2. The exceptional rate capability of Na2WO4-infused NaNi0.8Mn0.2O2, even at high current densities and low operating temperatures, instills a sense of optimism regarding the potential of grain boundary modification to improve performance for SIBs.