Ta Doping Improves the Cyclability and Rate Performance of a Nickel-Rich NCA Cathode via Promoted Electronic and Cationic Conductivity

Abstract

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.