Developing advanced cathode materials that provide high specific energy and power is essential for the commercialization of Li-ion batteries for electric vehicles. The layered LiNi_0.8Co_0.15Al_0.05O₂ (NCA) has received attentions as a promising candidate due to its cheaper material cost than LiCoO₂, competitive electrochemical performance, and improved structural stability than LiNiO₂. Thorough understanding of the origin of structural instability of Ni-based cathodes and the mechanism of Co-substitution and Al-doping will enable further improvement of Ni-based cathodes and other types of cathode materials as well. In this talk, we present our computational work on the structural stability of NCA, with emphasis on the effect of magnetic ions.

LiNi_1-yCo_yO₂ (NC) was studied first. Atom-spin coupled cluster expansion (CE) based on the density functional theory (DFT) calculations was performed to find stable atom-spin configurations of NC. The cationic mixing was accounted for by assuming that each cationic lattice site can be occupied by any of Li, Co, or Ni. The result predicted three stable configurations at zero temperature: LiNi_0.8Co_0.2O_2, LiNi_0.67Co_0.33O_2, and LiNi_0.5Co_0.5O₂, as well as several more configurations at finite temperature, especially at y < 1/3, which agreed to the experimental observation on the range of solid-solution phase NC. Interestingly, the configurations of solid-solution phase NC had Ni ions in Li-layer (Ni-defects). We also found that that the solid-solution phase NC became less dominant when the Co content increased, which suggested that Co substitution might have suppressed the formation of Ni-defects.

DFT calculations with HSE06 functional were additionally performed to investigate the mechanism of Co substitution associated with the suppression of Ni-defects formation. The oxidation state of each ion was obtained by analyzing the magnetic moment and agreed well with experimental observations. We found that a Ni-defect tended to be Ni²⁺ by oxidizing one Ni ion in TM layer to Ni⁴⁺ when they formed the nearest neighbor pair.  In this case, the antiferromagnetic spin distribution was possible for all pairs of paramagnetic Ni ions (Ni²⁺ and Ni³⁺), which implies the configurations with Ni-defects (Ni ions in Li layer) are thermodynamically more favorable than non-defective configurations where such the antiferromagnetic spin distribution is not possible due to magnetic frustration. When Co is substitued for Ni, the oxidation state of Co is Co³⁺, which is diamagnetic like Ni⁴⁺. Then the antiferromagnetic spin distribution for all paramagnetic ions is also possible even without creating Ni-defects and we will demonstrate that the most ideal case is when y=1/3.

We performed further calculations to find stable NCA configurations by replacing a portion of Co in NC with Al. By comparing NC and NCA, the effect of Al-doping to the structural stability will be also discussed.