Defining an Optimal Boron-Doping Window to Navigate the Trade-Off between Li-Ion Kinetics and Interfacial Stability in LiMn _0.6 Fe _0.4 PO ₄ @C Cathodes

(LMFP) cathodes possess high operating voltages, yet their application is severely hindered by sluggish electron/lithium-ion transport kinetics and interfacial instability during cycling. To address these challenges, this work proposes a surface engineering strategy utilizing a boron-doped carbon coating to modulate the interfacial kinetics of LMFP. The introduction of boron modifies the local electronic structure, inducing defects and holes that facilitate Li-ion migration, while simultaneously maintaining the layer's physical continuity and protective function against manganese dissolution. Within the investigated doping range, the 2% B-doped sample exhibits the best equilibrium between rate performance and cyclability, retaining 95.66% capacity after 100 cycles at 1 C and 93.20% after 300 cycles at 5 C, significantly outperforming the pristine sample (82% and 74%, respectively). In situ electrochemical impedance spectroscopy (EIS) combined with distribution of relaxation times (DRT) analysis reveals that the coating synergistically reduces interfacial charge-transfer resistance throughout the charge-discharge process. Consequently, this study defines an optimal boron-doping window to successfully navigate the trade-off between achieving rapid Li-ion kinetics and ensuring robust interfacial stability in olivine cathodes.