Cathode and Anode Materials for Na-Ion Battery - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 397

Fig. 10 Cycling curves of
Na/Na x Co 2/3 Mn 1/3 O 2 cells
obtained with a C/100 current
rate, starting by a charge (red
curve) or a discharge (black
curve)[ 20 ]
Fig. 10 . Unlike the multiple phase transitions of P2-Na 0.67 CoO 2 , it only showed a
typical solid-solution insertion/extraction process. The P2-Na 0.67 Co 0.67 Mn 0.33 O 2
electrode delivered a high reversible capacity of ~130 mAh g -1 (more than 0.5 Na
per formula unit). However, the prolonged cycling performance of this electrode is
not shown in the literature.
It is believed that in NaNi x M 1-x O 2 , the displacement of Na + by Ni 2+ can hardly
happen due to their large difference in ion radius (Na + : 1.02 Å, Ni 2+ : 0.69 Å).
Thus, for NaMnO 2 , the part replacement of Mn by Ni could possibly result in an
ordered structure. Komaba et al. prepared NaNi 0.5 Mn 0.5 O 2 via a co-precipitation
method [ 21 ]. XRD results showed that only 0.4 % of site exchange of Na with Ni
was detected, suggesting a stable layered structure and fast Na-ion diffusion
pathway. The NaNi 0.5 Mn 0.5 O 2 electrode demonstrated an initial discharge capacity
of 125 mAh g -1 in the potential range of 2.2-3.8 V and high cycling capacity of
[100 mAh g -1 over 20 cycles (Fig. 11 ), showing better performance than
NaMnO 2 [ 22 ]. However, the successive phase transition still occurred during
charge/discharge, capacity loss with cycling. Thus, stoichiometric Li was intro-
duced into the transition metal layer, to form a stabilizing charge ordering state
between Ni 2+ and Mn 4+ , for example, Na 1.0 Li 0.2 Ni 0.25 Mn 0.75 O d [ 23 ]. Normalized
Mn and Ni K-edge x-ray absorption near edge structure (XANES) spectra showed
that the presence of Mn and Ni in the Na 1.0 Li 0.2 Ni 0.25 Mn 0.75 O d are predominantly
Mn 4+ and Ni 2+ , respectively, resulting in no Jahn-Teller distortion in the structure.
The Na 1.0 Li 0.2 Ni 0.25 Mn 0.75 O d electrode lost only 2 % of its initial capacity over 50
cycles (Fig. 12 ), exhibiting excellent cycling stability. The shapes of charge/dis-
charge curves suggest a solid-solution insertion/extraction process, different from
the multiple phase transitions of Na 0.67 Ni 0.5 Mn 0.5 O 2 (Fig. 11 ). ICP-OES analysis
found that after being charged to 4.2 V, less than 5 % of the total Li in
Na 1.0 Li 0.2 Ni 0.25 Mn 0.75 O d was removed from the lattices. These results demon-
strated that Li can stabilize the transition metal layer and restrict the phase tran-
sition during Na intercalation/deintercalation.