NCA
layered cathode materials are characterized by low cost and
high reversible specific capacity compared with olivine and spinel
cathodes. However, the high Ni content also leads to poor cyclability.
In this paper, other than the mechanism reported by another group,
we proposed the increase in electronic and cationic conductivity of
Ta-doped LiNi0.94Co0.04Al0.02O2 cathode material could be the other reason for the promoted
electrochemical behavior. DFT calculation revealed that the increased
amount of electrons near Fermi energy which reduces the bandgap (from
1.2953 eV to 1.2171 eV) of Ta-NCA can be ascribed to the enhanced
electronic conductivity. Such improvement later leads to reduced polarization
and therefore the cyclability of the half cell is increased. Meanwhile,
the suppressed H2–H3 phase transition, is also found to be
responsible for the promoted cyclic performance. For the Ta-NCA cathode,
the capacity retention after 100 cycles is promoted from 42.11% to
93.62% (from 81.16 mAh·g–1 to 181.54 mAh·g–1). Ta doping also increases the Li diffusion coefficient
due to the expanded interplanar spacing of the Li slab. For Ta-NCA
cathode with a promoted diffusion coefficient of 3.11 × 10–11 cm2·s–1, the specific
capacity at 5C is increased from 118.51 mAh·g–1 to 158.47 mAh·g–1. Both the increase in cyclability
and rate behavior indicate the high potential of the Ta-NCA cathode
material in high-energy lithium-ion batteries.
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