Hierarchical LiMn0.6Fe0.4PO4 microspheres with atomic mixture of Fe/Mn derived from (Mn0.6Fe0.4)3(PO4)2·xH2O precursors for high stability lithium ion batteries

Abstract

LiMn_xFe_1-xPO₄/C (LMFP) is one of the most promising alternatives of LiFePO₄ (LFP) for next generation high energy lithium-ion batteries (LIBs) due to its higher working potential. However, when the substitution level of Fe with Mn exceeds half, the resulting inhomogeneous distribution of Fe/Mn may subsequently impede the performance of lithium manganese iron phosphate (LMFP) in terms of its rate capability, long-term cyclability. In this study, a multi-strategy synergistic modification approach was employed to address the aforementioned issues. First of all, the atomic-level mixing of iron and manganese was achieved by synthesizing the microsphere (Mn_0.6Fe_0.4)₃(PO₄)₃·xH₂O precursor while nanoscale primary particles coated with uniform carbon layer were agglomerated into hierarchical LiMn_0.6Fe_0.4PO₄ microspheres through spray-drying to enhance the tap density and electrical conductivity. XAFS measurements reveal a shorter Fe-O bond length, which is beneficial for maintaining the structural stability of the LMFP. Moreover, in situ XRD analysis confirms the occurrence of complete solid-solution behavior during cycling, which minimizes the transport energy barrier at the interface of the two phases and enhances the kinetic properties. Furthermore, in situ XAFS verifies the redox reactions of transition metals in LiMn_0.6Fe_0.4PO₄ occur with a high degree of reversibility during electrochemical cycling. Consequently, as prepared microspherical LiMn_0.6Fe_0.4PO₄ cathode material demonstrates high tap density (1.28 g ml⁻¹), excellent rate performance with a capacity of 137 mAh g⁻¹ at a high rate of 3C and long cycling stability with capacity retention of 88.5 % after 800 cycles as well as minimal voltage decay of 0.21 mV per cycle.