A multifunctional coating with high ionic and electronic conductivity is constructed on the surface of LiNi0.8Co0.1Mn0.1O2 (NCM) to boost the battery stability upon cycling and during storage as well. Phosphoric acid reacts with residual lithium species on the pristine NCM to form a Li3PO4 coating with extra carbon nanotubes (CNTs) penetrating through, which shows high ionic and electronic conductivity. NCM, Li3PO4, CNTs, and the electrolyte jointly form a four-phase cathode electrolyte interface, which plays a key role in the great enhancement of capacity retention, from 50.3% for pristine NCM to 84.8% for the modified one after 500 cycles at 0.5C at room temperature. The modified NCM also delivers superior electrochemical performances at a high cut-off voltage (4.5 V), high temperature (55 °C), and high rate (10C). Furthermore, it can deliver 154.2 mA h g-1 at the 500th cycle after exposed to air with high humidity for 2 weeks. These results demonstrate that the well-constructed multifunctional coating can remarkably enhance the chemical and electrochemical performances of NCM. The improved cycling, storage, and rate performance are attributed to the four-phase cathode electrolyte interface delivering high electron and ionic conductivity and securing the cathode against attack. This work broadens the horizon for constructing effective electrode/electrolyte interfaces for electrochemical energy storage and conversion.