<p>In this study, we prepared a carbon-coated prelithiation additive to replenish lithium ions lost to solid electrolyte interphase formation during the initial charge–discharge process. After mixing LiOH and FePO₄ in various weight ratios, Li₃PO₄–Li₅FeO₄ composites were synthesized through a two-step heat treatment involving precursor hydrolysis and a solid-state reaction. To improve the surface stability of the synthesized composites, a carbon coating was applied via chemical vapor deposition using acetylene as the carbon source. The physical properties of the mixed-phase composites were analyzed using scanning electron microscopy(SEM), energy-dispersive X-ray spectroscopy(EDS), X-ray diffraction(XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy(XPS). The electrochemical properties were analyzed by charge–discharge cycling, rate capability tests, differential capacity(d<i>Q</i>/d<i>V)</i> analysis, and electrochemical impedance spectroscopy(EIS). The NCM811 cathode containing the prelithiation additive prepared at a LiOH/FePO₄ weight ratio of 11:1 exhibited the highest initial capacity of 242.3 mAh/g, corresponding to a 7.7% increase compared to pristine NCM811. Compared with NCM811 containing uncoated Li₃PO₄–Li₅FeO₄, the carbon-coated composite exhibited 7% higher capacity retention after 100 cycles and 7.9% higher charge capacity. Additionally, it maintained a high capacity of 161.2 mAh/g even at a rate of 6&#xa0;C.</p>