Crystal Structure, Energetics, And Electrochemistry of Li2FeSiO4 Polymorphs from First Principles Calculations

The influence of crystal structure and relative stability on the electrochemical properties of Li2FeSiO4 polymorphs as cathode materials for Li-ion batteries is investigated. Six Li2FeSiO4 forms related to the crystal structure of Li3PO4 have been considered: three known polymorphs crystallizing in layered structures (with space groups P2(1), Pmnb, and Pmn2(1)) and three forms reported for other LiMSiO4 materials crystallizing in three-dimensional (3D) frameworks (with space groups Pmn2(1), Pbn2(1), and P2(1)/n). While Li2FeSiO4 polymorphs are very close in energy, their energies begin to differ substantially upon removal of Li, rendering those with lower electrostatic energy more stable. The change in relative stability of polymorphs upon delithiation results in a driving force for a phase transformation of the exiting two-dimensional (2D)-Li2FeSiO4 polymorphs to a more stable structure having a 3D network of [SiO4] and [FeO4] tetrahedra. The resulting phase exhibits a voltage plateau that is predicted to be 0.3 V below that of the original phase for the first electron process (Li2FeSiO4/LiFeSiO4 couple). The calculated voltage capacity curves for the first and second cycles of Li2FeSiO4 at room temperature, assuming transformation to the new polymorph occurs after the first cycle, are in excellent agreement with experiments. Independently of the polymorphs, removal of the second lithium ion occurs at too high of a voltage (above 4.7 V) and is accompanied by major structural rearrangements, precluding the utilization of any of the unmodified Li2FeSiO4 polymorph derived from Li3PO4 as high specific capacity material.