Combined first-principle calculations and experimental study on multi-component olivine cathode material

Abstract

The electrochemical properties and phase stabilities of the multi-component olivine compound $LiMn_{1/3}Fe_{1/3}Co_{1/3}PO_4$ are studied experimentally and with first-principles calculation. The formation of the solid solution between $LiMnPO_4$, $LiFePO_4$, and $LiCoPO_4$ at this composition is confirmed by XRD patterns and the calculated energy. The experimental and first principle results indicate that there are three distinct regions in the electrochemical profile at quasi-open-circuit potentials of ~3.5 V, ~4.1 V, and ~4.7 V, which are attributed to $Fe^{3+}/Fe^{2+}$, $Mn^{3+}/Mn^{2+}$, and $Co^{3+}/Co^{2+}$ redox couples, respectively. However, exceptionally large polarization is observed only for the region near 4.1 V of $Mn^{3+}/Mn^{2+}$ redox couples, implying an intrinsic charge transfer problem. An ex situ XRD study reveals that the reversible one-phase reaction of Li extraction/insertion mechanism prevails, unexpectedly, for all lithium compositions of $LixMn_{1/3}Fe_{1/3}Co_{1/3}PO_4$ $(0 \le x \le 1)$ at room temperature. This is the first demonstration that the well-ordered, non-nanocrystalline (less than 1% Li - M disorder and a few hundred nanometer size particle) olivine electrode can be operated solely in a one-phase behavior.