Strategic Design of High-Capacity P2-Type Sodium Layered Oxides Enabled by Multifunctional Li/F/Cu Co-Doping
P2-type layered cathode materials have low energy density due to low initial Na content. In this research, high sodium content P2-type cathode material Na0.8Li0.08Ni0.23Mn0.67Cu0.1O1.92F0.08 (NMC-LF), was synthesized by a simple high-temperature solid-phase method. The strong electronegativity of F allows the local TM-O bond to be transformed into a TM-F bond, and the TMO6 octahedron becomes more stable. The doping of Li+ not only maintains structural stability but also transforms the two-phase reaction in the high-voltage range into a solid-solution reaction. This leads to an effective suppression of the irreversible P2–O2 phase transition in the high-voltage region. At 20 mA·g–1, the initial discharge capacity of NMC-LF reaches 131.05 mAh·g–1. At a high current density of 200 mA·g–1, the discharge capacity reaches 101.9 mAh·g–1 and remains at 99.31% after 150 cycles. The diffusion coefficient of NMC-LF is as high as 8.67 × 10–13 cm2·s–1. Furthermore, the structural evolution during material formation is elaborated by high-temperature XRD, and it is determined that there are no impurities at 900 °C. Under the joint synergy of Li/F/Cu and the design of high sodium content, the long-cyclic and high-rate performance has been significantly improved. In-situ XRD tests show that Li/F/Cu codoping inhibits the P2–O2 phase transition and enhances structural reversibility. This provides a new idea for the research of layered cathode materials.
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