Structural Changes in Li2CoPO4F during Lithium-Ion Battery Reactions

The cobalt-based fluorophosphate Li2CoPO4F positive electrode has the potential to obtain high energy density in a lithium ion battery since its theoretical capacity is 287 mAh·g–1 when two electrons can react reversibly. This material promises to charge/discharge with an extremely high redox-couple voltage of over 4.8 V vs Li/Li+. Bulk structural analyses including X-ray diffraction, Co K-edge X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) reveal that an orthorhombic LiβCoPO4F phase is produced from pristine Li2CoPO4F by a combination of solid-solution and two-phase reaction manners during the first charging process, and these phases reversibly transform during charge–discharge cycling. The results of 7Li MAS NMR and classical molecular dynamics simulations suggest that Li ions located at Li(1) sites intercalate/deintercalate through a 1D diffusion path along the b axis, whereas those located at Li(2) and Li(3) sites are fixed. The aforementioned analyses were successfully performed with the enhancement of electrochemical properties by use of a fluoroethylene carbonate-based electrolyte instead of an ethylene carbonate-based one and reducing its volume. Further enhancement was achieved by adding SiO2 nanoparticles into the electrode slurry. The electrochemical results encourage the possibility of the intercalation/deintercalation of more than one Li ion from/into Li2CoPO4F during electrochemical cycling.