Achieving high stability at high voltage remains a major challenge for Ni-rich NCM 811 cathodes. Herein, we report for the first time metal phosphides as surface decoration materials to mitigate capacity fading, structural degradation, and interfacial instability. Fe2P, Ni2P, and Ni12P5 were selected due to their sustainability, low cost, non-toxicity, and unique phase diagram properties. Fe2P was synthesized via an innovative low-temperature reduction of FePO4.2H2O using NaBH4. As a result, 2 wt% Fe2P-decorated NCM 811 exhibited enhanced electrochemical performance, delivering a capacity retention of 90.35% after 100 cycles at 0.1 C and 110 mAh.g-1 at 5 C, outperforming pristine NCM 811. Ex situ XRD and Raman, Operando EIS, and GITT analyses revealed reduced interfacial resistance, improved Li+ diffusion kinetics, and the formation of a stable cathode-electrolyte interphase. Moreover, NCM 811@Fe₂P delivered 220 mAh.g-1 at 0.1 C when charged to 4.5 V and maintained 118 and 100 mAh.g-1 at 40 °C and -15 °C, respectively. DFT reveals strong Fe-O and P-O interactions effectively suppressed oxygen release and transition-metal dissolution. The Fe2P coating also enhanced thermal and air stability, reducing heat generation during DSC analysis and preserving performance after prolonged air exposure. Full-cell tests (LTO||NCM 811@Fe2P) demonstrated excellent cycling stability with 88.78% capacity retention over 500 cycles, highlighting Fe2P as a scalable and multifunctional surface modification strategy for high-energy Ni-rich cathodes.