Abstract
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.
Highlights
Calculated DifferenceBragg Positions cObserved Background Calculated Difference Bragg PositionsIntensity (a.u.) Intensity (a.u.)2-Theta d 1μm (003)5 nm optimal species
The initial first GCD curve at 1C (250 mA g−1) shows that the full cell delivers a discharge capacity of 133.9 mA h g−1 based on the mass of cathode active material with the average voltage of 3.1 V (Fig. 5a), generating a specific energy of up to 224 Wh kg−1 based on the mass of NLNFMB and hard carbon (HC) active material[44,45]
At a specific current of 1C (250 mA g−1) (Fig. 5b), the full cell retains a high capacity of 80.1% after 100 cycles and a stable Coulombic efficiency (Supplementary Fig. 11)
Summary
Calculated DifferenceBragg Positions cObserved Background Calculated Difference Bragg PositionsIntensity (a.u.) Intensity (a.u.)2-Theta (degree) d 1μm (003)5 nm optimal species. The electrochemical performance of hard carbon (HC)||NLNFMB full cells was further evaluated. The initial first GCD curve at 1C (250 mA g−1) shows that the full cell delivers a discharge capacity of 133.9 mA h g−1 based on the mass of cathode active material with the average voltage of 3.1 V (Fig. 5a), generating a specific energy of up to 224 Wh kg−1 based on the mass of NLNFMB and HC active material[44,45].
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