Triclinic Na3.12Co2.44(P2O7)2 as a High Redox Potential Cathode Material for Na-Ion Batteries

Two types of sodium cobalt pyrophosphates, triclinic Na 3.12 Co 2.44 (P 2 O 7 ) 2 and orthorhombic Na 2 CoP 2 O 7 , are compared as high-voltage cathode materials for Na-ion batteries.Na 2 CoP 2 O 7 shows no electrochemical activity, delivering negligible capacity.In contrast, Na 3.12 Co 2.44 (P 2 O 7 ) 2 exhibits good electrochemical performance, such as high redox potential at ca. 4.3 V (vs.Na/Na + ) and stable capacity retention over 50 cycles, although Na 3.12 Co 2.44 (P 2 O 7 ) 2 delivered approximately 40 mA h g -1 .This is attributed to the fact that Na 2 CoP 2 O 7 (~3.1 Å) has smaller diffusion channel size than Na 3.12 Co 2.44 (P 2 O 7 ) 2 (~4.2 Å).Moreover, the electrochemical performance of Na 3.12 Co 2.44 (P 2 O 7 ) 2 is examined using Na cells and Li cells.The overpotential of Na cells is smaller than that of Li cells.This is due to the fact that Na 3.12 Co 2.44 (P 2 O 7 ) 2…