Construction of high performance N-doped carbon coated LiMn_0.8Fe_0.2PO₄ cathode for lithium-ion batteries

Highlights: ●LiMn_0.8Fe_0.2PO₄/C nanocrystal is synthesized by a facile solvothermal reaction. ●Melamine is used as a nitrogen source of N-doped carbon coated LiMn_0.8Fe_0.2PO₄/C. ●The transformation law of morphology from nanosheet to nanoparticle is studied. ●The positive effect of N atoms on the electrochemical performance is illustrated. ●Particle-like LMFP-7 delivers excellent cycling performance and chemical stability. -- Abstract: N-doped carbon coated LiMn_0.8Fe_0.2PO₄ nanocrystal for lithium-ion batteries was prepared by a facile solvothermal method. The doping effect of nitrogen is regulated by altering the addition amount of nitrogen source melamine. Result shows that the introduction of N atoms does not change the crystal structure of LiMn_0.8Fe_0.2PO₄/C. The ‒NH₂ functional groups in melamine can react with the ‒OH groups in pyrolytic carbon generated by the pre-sintering of sucrose. Then, multiple nanosheets in LMFP-0 are combined together and the morphology of LiMn_0.8Fe_0.2PO₄/C is transformed to nano particle-like in LMFP-7. The doped nitrogen in the forms of pyridinic, pyrrolic and graphitic N are derived from the combination of pyrolytic carbon and melamine, which can generate active defective sites and improve the electronic conductivity and diffusion rate of lithium ions. Sample LMFP-7 delivers the best electrochemical performance with a capacity of 154.7, 144.2 and 110.0 mAhg⁻¹ at 0.1, 1 and 5 C, respectively. The LiMn_0.8Fe_0.2PO₄/C cathode exhibits good electrochemical reversibility, low charge transfer resistance (46.9 Ω) and high diffusion coefficient (1.35 × 10⁻¹³ cm² s⁻¹). It also delivers excellent cyclic performance, structural stability and chemical stability.