Structural Changes in Li₂CoPO₄F during Lithium-Ion Battery Reactions

The cobalt-based fluorophosphate Li 2 CoPO 4 F 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 Li β CoPO 4 F phase is produced from pristine Li 2 CoPO 4 F 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 7 Li 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 SiO 2 nanoparticles into the electrode slurry. The electrochemical results encourage the possibility of the intercalation/deintercalation of more than one Li ion from/into Li 2 CoPO 4 F during electrochemical cycling.