<p>The electrochemical efficiency of lithium-ion batteries (LIBs) is largely governed by the structural and compositional design of their cathode materials. This study presents a comprehensive multiphysics simulation of carbon–LiMnPO₄ nanocomposite cathodes to optimize their high-rate electrochemical performance. By integrating Fick’s second law, Ohm’s law, and the Butler–Volmer equation within COMSOL Multiphysics, the coupled effects of particle size, carbon content, and carbon distribution were systematically investigated. The results reveal that reducing LiMnPO₄ particle size from 100 to 20 nm enhances lithium-ion diffusion by 30%, increasing discharge capacity from 108 to 125 mAh/g at 1C. An optimal carbon content of 30 wt% yields maximum conductivity (4.3 × 10⁻¹ S/cm), minimum charge-transfer resistance (23 Ω), and peak capacity (130 mAh/g). Furthermore, uniform carbon distribution minimizes current inhomogeneity (standard deviation 0.015 mA/cm²), resulting in 92% capacity retention after 100 cycles at 2C, compared with 76% for non-uniform configurations. These findings demonstrate the synergistic roles of nanoscale engineering, compositional tuning, and morphological control in enhancing both rate capability and cycling stability. The developed multiphysics framework provides predictive insights into the electrochemical behavior of composite cathodes, guiding the rational design of next-generation high-performance LIB materials.</p> Graphical abstract <p></p>