Unified Design Flow for Facilitating Fast Li-Kinetics in Layered Oxide Cathodes

Non-parallel flow of electric vehicles (EVs) and the rechargeable battery market motivates energy materials researchers to study advanced batteries. Although, it is possible to produce a 90 kWh battery pack with a 300-mile cruise range, unfortunately, their charging rate still lags significantly behind that of internal combustion engine vehicles (ICEVs). This shortfall in power density causes a ‘range anxiety’ for existing EV owners and poses a barrier for potential owners. Li-layered oxides (LLOs), typically with an O3-type stacking sequence, have been the most mainstream cathode material since their commercialization by Sony in 1991. For O3-type LLOs, Li-ion occupies thermodynamic stable octahedral sites, which are the phases that have edge sharing with neighboring transition metals at the initial state of the hopping mechanism. And the environment at the intermediate tetrahedral site significantly influences the Li-kinetics, affecting the power density of a battery. And the activation barrier for Li-kinetics is determined by the size of the intermediate site, and the valence state of the transition metal surrounding the Li-ion at the tetrahedral site. Despite attempts to induce fast mass transport using traces of various dopants, the fundamental limitations of the O3-type stacking sequence have prevented breakthroughs. Therefore, a study aiming to break the originated limitations of the O3-type LLOs and uncover novel Li-ion transport mechanisms is indispensable. By performing computational calculations, we propose a two-design perspective to enhance the rate of (de)lithiation by intentionally arranging Li-ion sites for O3-type LiCoO2 (O3-LCO), the most conventional cathode material. For the first step, we introduce a new framework to overcome the fundamental limitations of the existing O3-type LLOs. We manipulated the stacking sequence of O3-LCO from O3- to O2-type, which induces a stable intermediate tetrahedral site. The O2-type LCO has edge-sharing relationships between the LiO₄ tetrahedral site, serving as an intermediate step in the hopping mechanism, and the adjacent MO₆(M refers to transition metal) octahedron in the transition metal layer. This configuration makes the intermediate site energetically more stable than the O3-type and allows for fast Li hopping from the relatively unstable intermediate site to the final site in the O2-type framework due to the lower activation barrier than the O3-type. Additionally, the anionic electrostatic repulsion between facing O-O in the LiO₂ layer results in a larger Li slab thickness, creating wider pathways for Li-ion movement. The second step involves screening the optimal material candidates for fast Li-kinetics by inducing even more stable intermediate sites through a change in material content by trace amounts of substitutional dopants. We design a strategy to lower the activation barrier by substituting contents (y) in O2-type Li_1-x(Co_1-yM_y)O₂ with 3d transition metals that have a large ionic radius, aiming for volumetric expansion of the tetrahedral site through in-plane direction broadening. While the nine candidates showed similar values of Li-slab thickness, the ionic radius, and ab-plane area showed overwhelmingly high values when Ti was substituted. The larger ionic radius causes a wider ab-plane and thus increases the volume of the LiO₄ tetrahedral site. We demonstrated through density functional theory (DFT) calculations that an expansion in the volume of the tetrahedral site lowers the activation barrier for Li-ion, resulting in high-rate performance that can be implemented. Furthermore, we figure out that materials designed according to our proposed concept induce fast Li-kinetics. Our unified design flow for facilitating fast Li-kinetics can serve as a cornerstone for the design of fast-charging LLOs cathode materials.