High-energy Ni-rich lithium transition metal oxides such as Li[Ni$_{0.8}$Co$_{0.1}$Mn$_{0.1}$]O$_{2}$ (NCM$_{811}$) are appealing positive electrode materials for next-generation lithium batteries. However, the high sensitivity toward moist air during storage and the high reactivity with common organic electrolytes, especially at elevated temperatures, are hindering their commercial use. Herein, an effective strategy is reported to overcome these issues by coating the NCM$_{811}$ particles with a lithium phosphonate functionalized poly(aryl ether sulfone). The application of this coating allows for a substantial reduction of lithium-based surface impurities (e.g., LiOH, Li$_{2}$CO$_{3}$) and, generally, the suppression of detrimental side reactions upon both storage and cycling. As a result, the coated NCM811-based cathodes reveal superior Coulombic efficiency and cycling stability at ambient and, particularly, at elevated temperatures up to 60°C (a temperature at which the non-coated NCM$_{811}$ electrodes rapidly fail) owing to the formation of a stable cathode electrolyte interphase with enhanced Li$^{+}$ transport kinetics and the well-retained layered crystal structure. These results render the herein presented coating strategy generally applicable for high-performance lithium battery cathodes.