Structural transformation of Li_2COPO_4F upon Li-deintercalation

摘要

Electrochemical performance and structural properties of the high-voltage cathode material Li_2COPO_4F have been investigated. The cyclic voltammetry and coulometry under potential step mode in the voltage range 3.0-5.1 V vs. Li revealed a structural transformation at potentials above 4.8 V. This transformation occurring upon Li-extraction appears to be irreversible: the subsequent Li-insertion does not result in restoration of the initial structure, but takes place within a new "modified" framework. According to the structure refinement this modification involves the mutual rotations of (CoO_4F_2) octahedra and (PO_4) tetrahedra accompanied by the considerable unit cell expansion which is expected to enhance the Li-transport upon subsequent cycling.The new framework demonstrates a reversible Li-insertion/extraction in a solid-solution regime with stabilized discharge capacity at around 60mAhg~(-1).