Structural transformation of Li2CoPO4F upon Li-deintercalation

Electrochemical performance and structural properties of the high-voltage cathode material Li2CoPO4F 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 (CoO4F2) octahedra and (PO4) 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 60 mAh g−1.