First-principles study on LiMn0.5Fe0.5PO4 doping to decrease the Jahn-Teller effect

Transition metal Mn ions are highly promising cathode dopant materials. Due to the introduction of Mn ions, as lithium ions are deintercalated, Mn2+ will be transformed into Mn3+. This situation will lead to a severe Jahn-Teller effect, causing significant local lattice distortion and greatly reducing electrochemical stability. This article utilizes first-principles calculations to investigate the doping of Mg, Co, and V to weaken Jahn-Teller effect in LiMn0.5Fe0.5PO4 cathode. The oxidation-reduction processes of three doped models were analyzed, and the electronic structure and charge transfer amount between the Mn ion and the O ion were calculated for each. It was found that, when Mg ions are doped into the crystal, Mn ions will stabilize as Mn2+, thereby weakening the Jahn-Teller effect. However, the addition of V and Co will not alter the Jahn-Teller effect. The differential charge density and partial density of states (PDOS) were also calculated. It was found that only the doping of Mg ions can enable the material to achieve the lowest energy and the smallest volume change rate, which attribute to weaken the Jahn-Teller effect. Only doping with V and Co ions can achieve the highest lithium removal voltage, increasing the average lithium removal voltage from 4.22 to 4.42 V. Mechanical performance calculations show that the structures with two types of doped Mg ion are prone to shear deformation and cannot improve the ductility of the material. Additionally, it was found that the migration barrier of the three doped models was reduced to varying degrees, which is beneficial for the transition of lithium ions. Moreover, the diffusion coefficient of lithium ions also increased by 1-4 orders of magnitude.