Suppressing O3−O’3 phase transition in NaCrO2 cathode enabling high rate capability for sodium-ion batteries by Sb substitution

Abstract

Poor rate capability of cathode materials for sodium-ion batteries (SIBs) is one of the major obstacles hindering practical application of SIBs due to the large ion radius of Na+ and its sluggish kinetics. Improving rate capability for SIBs is a critical issue. Herein, we reveal that introducing small amount of Sb into O3-NaCrO2 can improve the rate capability of NaCrO2 cathode significantly. The occupation of Sb in Cr sites can expand the c/a ratio and the unit cell volume, thus promoting sodium diffusion in the a-b channels. Under a high rate of 5C, the 5% Sb-substituted Na0.9Cr0.95Sb0.05O2 (NCS-0.05) shows a reversible capacity of 114 mAh g−1 with capacity retention of 70.9% after 1000 cycles. Under an ultrahigh current rate of 32C, a reversible capacity of 96.1 mAh g−1 can still be maintained. Synchrotron in situ X-ray diffraction results reveals highly reversible phase transition behavior of NCS-0.05 during cycling even under high rates, and the O3 → O’3 phase transition in NaCrO2 is suppressed effectively by Sb substitution. Density functional theory calculations indicate that Sb substitution can also lower desodiation energy and decrease the bandgap of NaCrO2, leading to improved reaction kinetics and electronic conductivity.