Coating the surface of cathode materials with Li3PO4 to improve the cycling performance is commonly practiced; however, obtaining an effective and uniform coating of Li3PO4 on cathode materials is extremely difficult due to the lack of strong bonds between them. Herein, this issue is solved by coating the surface of LiNi0.5Mn1.5O4 with Li3PO4 via a novel hydrothermal method. It is shown that a uniform Li3PO4 surface coating can only be obtained by introducing a small amount of additional metal ions, such as Co, in the starting solution. X‐ray photoelectron microscopy combined with scanning transmission electron microscopy reveals that the Co ions diffuse into the 16c sites of the surface of the spinel LiNi0.5Mn1.5O4 structure, making the surface layer act as a bridging layer that connects olivine Li3PO4 with bulk spinel LiNi0.5Mn1.5O4. The high‐voltage LiNi0.5Mn1.5O4 half‐cell with the optimal Li3PO4 coating shows a discharge capacity retention of 95.8% after 50 cycles at 55 °C, compared with the only 81.1% discharge capacity retention of the bare counterpart.