First-Principles Study of the Structures and Redox Mechanisms of Ni-Rich Lithium Nickel Manganese Cobalt Oxides

To reduce the cobalt (Co) content in lithium-ion batteries, Ni-rich (high-Ni) lithium nickel manganese cobalt oxides (NMC) are pursued as one of the next-generation cathode materials. The critical challenge associated with Ni-rich lithium nickel manganese cobalt oxides (NMCs) is understanding the electronic structure changing during delithiation. The redox activity in Ni-rich NMC is a complicated phenomenon. First, the transition metal (TM) ions in high-Ni NMC cathode are multi-valent, suggesting they could have oxidation states other than 3+ at the fully discharged state. Then the sequence of losing electrons becomes a competition between the electron occupancy on different TM-d and O-2p orbitals. To theoretically explore the electronic structure evolutions, need a deep understanding of the representative structures in modeling. However, there is still debate on the crystal and electronic structures of the baseline, LiNiO₂. The widely accepted parent structure of NMCs, LiNiO₂ with <i>R-3</i><i>m</i> symmetry, conflicts with the renowned JT activity and the experiment-measured conducting behavior.<br/><br/>In this work, Density Functional Theory (DFT) calculations were performed to provide a theoretical understanding of Ni-rich NMC. First, it was found that the commonly used <i>R</i>-3<i>m</i> structure for LiNiO₂ is metallic, contrary to the experimentally reported mix-conducting behavior. By comparing the four different space groups, <i>R</i>-3<i>m</i>, <i>C2/m</i>, <i>P2₁/c</i>, and <i>P2/c</i>, <i>P2/c</i> with charge disproportionation of Ni²⁺ and Ni⁴⁺ is the most energetically stable and semiconducting structure of LiNiO₂, which can serve as a parent structure of Ni-rich NMCs, offering insights into the redox mechanisms. Therefore, the atomic structures of representative Ni-rich NMC were built by partially replacing Ni with Co or Mn in the <i>P2/c</i> LiNiO₂ to form Li_xNi_yMn_zCo_1-y-zO₂. Additionally, by comparing the lattice structure evolution during delithiation with experimental observations, the <i>P2/c</i>-based Ni-rich NMC agrees well with the characteristic anisotropic response in the in-plane and out-plane lattice parameters as a function of lithium concentration. During delithiation, lattice parameters a and b decrease while the c-axis first expanses in the early stage of delithiation and rapid shrink, characterized as the H2–H3 phase transformation with further Li-extraction.<br/><br/>In fully lithiated (x=1.0) <i>P2/c</i>-based high Ni content NMC (y&gt;0.5), the oxidation state of all Mn ions becomes 4+, while Co ions still maintain 3+, and part of the Ni⁴⁺ ions become Ni³⁺ to compensate for the charge. Upon delithiation, the local environment shows more variation of the charge states on the TM ions. The average oxidation on each TM follows a sequence of losing electrons that starts from Ni²⁺ to Ni³⁺, then oxidizing Ni³⁺ and Co³⁺, while Mn⁴⁺ remains electrochemically inactive till x=0. Additionally, by observing the trend of the changing in the oxidation state of TM ions in the experiment and DFT calculations, a general relationship for the oxidation state change in each TM as a function of x (the stage in delithiation) is derived and shows agreement with both modeling and experimental data.<br/><br/>This work confirmed the representing structure of Ni-rich NMC, elucidated the sequence of electron loss during charging, and provided a general relationship for the oxidation state change on each transition metal, offering a theoretical approach to get insights into the defects and oxidation in NMC cathodes. The procedure provides a systematical investigation of the electronic structures changing in NMCs during delithiation.