Combined First‐Principle Calculations and Experimental Study on Multi‐Component Olivine Cathode for Lithium Rechargeable Batteries

The electrochemical properties and phase stability of the multi‐component olivine compound LiMn_1/3Fe_1/3Co_1/3PO₄ are studied experimentally and with first‐principles calculation. The formation of a solid solution between LiMnPO₄, LiFePO₄, and LiCoPO₄ 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 Fe³⁺/Fe²⁺, Mn³⁺/Mn²⁺, and Co³⁺/Co²⁺ redox couples, respectively. However, exceptionally large polarization is observed only for the region near 4.1 V of Mn³⁺/Mn²⁺ 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 Li_xMn_1/3Fe_1/3Co_1/3PO₄ (0 ≤ x ≤ 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 mode.