Boron-doped sodium layered oxide for reversible oxygen redox reaction in Na-ion battery cathodes.

Na-ion cathode materials operating at high voltage with a stable cycling behavior are needed to develop future high-energy Na-ion cells. However, the irreversible oxygen redox reaction at the high-voltage region in sodium layered cathode materials generates structural instability and poor capacity retention upon cycling. Here, we report a doping strategy by incorporating light-weight boron into the cathode active material lattice to decrease the irreversible oxygen oxidation at high voltages (i.e., >4.0 V vs. Na+/Na). The presence of covalent B–O bonds and the negative charges of the oxygen atoms ensures a robust ligand framework for the NaLi1/9Ni2/9Fe2/9Mn4/9O2 cathode material while mitigating the excessive oxidation of oxygen for charge compensation and avoiding irreversible structural changes during cell operation. The B-doped cathode material promotes reversible transition metal redox reaction enabling a room-temperature capacity of 160.5 mAh g−1 at 25 mA g−1 and capacity retention of 82.8% after 200 cycles at 250 mA g−1. A 71.28 mAh single-coated lab-scale Na-ion pouch cell comprising a pre-sodiated hard carbon-based anode and B-doped cathode material is also reported as proof of concept. The irreversible oxygen redox reaction during charging to the high-voltage region causes cathode structural degradation and Na-ion cell capacity fading. Here, the authors report a B-doped cathode active material to mitigate the irreversible oxygen oxidation and increase the cell capacity.