Enhancing the Mn Redox Kinetics of LiMn0.5Fe0.5PO4 Cathodes Through a Synergistic Co-Doping with Niobium and Magnesium for Lithium-Ion Batteries

The concerns on the cost of lithium-ion batteries have created enormous interest on LiFePO4 (LFP) and LiMn1-xFexPO4 (LMFP) cathodes However, the inclusion of Mn into the olivine structure causes a non-uniform atomic distribution of Fe and Mn, resulting in a lowering of reversible capacity and hindering their practical application. Herein, a co-doping of LMFP with Nb and Mg is presented through a co-precipitation reaction, followed by a spray-drying process and calcination. It is found that LiNbO3 formed with the aliovalent Nb doping resides mainly on the surface, while the isovalent Mg2+ doping occurs into the bulk of the particle. Full cells assembled with the co-doped LMFP cathode and graphite anode demonstrate superior cycling stability and specific capacity, while maintaining good tap density, compared to the undoped or mono-doped (only with Nb or Mg). The co-doped sample exhibits a capacity retention of 99% over 300 cycles at a C/2 rate. The superior performance stems from the enhanced ionic/electronic transport facilitated by Nb coating and the enhanced Mn2+/3+ redox kinetics resulting from bulk Mg doping. Altogether, this work reveals the importance of the synergistic effect of different dopants in enhancing the capacity and cycle stability of LMFP.