Effect of sodium and yttrium co-doping on electrochemical performance of LiNi0.8Co0.15Al0.05O2 cathode material for Li-ion batteries: Computational and experimental investigation

Abstract

LiNi0.8Co0.15Al0.05O2 (NCA) has attracted a lot of attention owing to its high voltage, specific energy density, and specific capacity. However, the cycle durability of the NCA material is still a challenge. Oxygen evolution, migration of nickel cations to the lithium layers (cation mixing), and the destruction of the layered structure of NCA are the most important reasons for thermal and structural instability, reducing capacity, and increasing impedance. Doping is one of the most effective ways to improve the thermal and structural stability of the NCA. In this study, the addition of sodium (Na) on Li-site and yttrium (Y) on metal-site, as well as the simultaneous addition of these elements to NCA material, were investigated experimentally and theoretically. For the first time, we proposed a framework for parameterization of the structural and thermal stability of NCA during the lithiation/delithiation process in terms of first-principle density functional theory (DFT) calculations. It is shown that variation in the Bulk modulus can reflect structural responses, and the calculated Bader charge on oxygen atoms is directly connected to thermal stability concerns. The computational results confirmed the positive effect of Na+ and Y3+ on the structural and thermal stability of the NCA cathode systematically. Following those outcomes, different percentages of the dopants were added to the cathode material experimentally, and electrochemical tests such as cyclability, electrochemical impedance spectroscopy, cyclic voltammetry, and rate capability were performed. The electrochemical tests revealed that in single-doped samples, the optimal amount of Na+ and Y3+ was 0.5 % (mol%), and in co-doped samples, 0.5 % of each dopant (0.5Na-0.5Y-NCA) produced the best results. The optimal 0.5Na-0.5Y-NCA sample retained 92.34 % of its capacity after 100 charge and discharge cycles at a rate of 0.5C, compared to 67.11 % for the pristine NCA. The cooperative addition of sodium and yttrium with different mechanisms such as reducing released oxygen, reducing the formation of insulating compounds, increasing the diffusion coefficient, reducing charge transfer resistance, and improving structural and thermal stability demonstrated synergistic effects on stabilizing the layered structure and improving cycling performance. The underlying mechanisms of observed experimental improvements were interpreted and discussed according to computational outcomes.