Mn-Rich P′2-Na0.67[Ni0.1Fe0.1Mn0.8]O2 As High-Energy-Densityand Long-Life Cathode Material for Sodium-Ion Batteries

Abstract

P′2-type Na0.67[Ni0.1Fe0.1Mn0.8]O2 is introduced as a promising new cathode material for sodium-ion batteries (SIBs) that exhibits remarkable structural stability during repetitive Na+ de/intercalation. The O-Ni-O-Mn-O-Fe-O bond in the octahedra of transition-metal layers is used to suppress the elongation of the Mn-O bond and to improve the electrochemical activity, leading to the highly reversible Na storage mechanism. A high discharge capacity of ≈220 mAh g−1 (≈605 Wh kg−1) is delivered at 0.05 C (13 mAg−1) with a high reversible capacity of ≈140 mAh g−1 at 3 C and excellent capacity retention of 80% over 200 cycles. This performance is associated with the reversible P′2–OP4 phase transition and small volume change upon charge and discharge (≈3%). The nature of the sodium storage mechanism in a full cell paired with a hard carbon anode reveals an unexpectedly high energy density of ≈542 Wh kg−1 at 0.2 C and good capacity retention of ≈81% for 500 cycles at 1 C (260 mAg−1).