Enhancing the Mn Redox Kinetics of LiMnFePO 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 LiFePO (LFP) and LiMnFePO (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 LiNbO formed with the aliovalent Nb doping resides mainly on the surface, while the isovalent Mg 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 Mn 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.