Mitigating the Capacity Degradation by Ion-Electron-Conducting Dual-Layer Coating on a Layered Oxide Cathode Material for Sodium Ion Batteries

A high-capacity and long-life layered P2-Na0.7[Ni0.35Mn0.60Co0.05]O2 (NMC) cathode material, dually coated with Na-ion-conducting Na2SiO3 and electron-conducting RGO, has been successfully synthesized and tested for half-cell as well as full-cell applications. The first coating layer of Na2SiO3 provides a three-dimensional (3D) diffusion channel for Na-ion migration, while the second coating layer of RGO offers the electron-conducting pathways to enhance the charge transfer. Moreover, Si4+ migration in the NMC lattice during Na2SiO3 coating causes the enhancement in the interlayer spacing, which significantly increases the Na+-diffusion rate. The structural, morphological, electronic, and electrochemical analyses of the prepared cathode materials have been performed. The synergic effect of dual-layer modification and Si4+ doping not only protects the cathode particles but also improves the Na-ion kinetics as well as charge transfer rate, resulting in superior electrochemical performance. The dually surface-modified cathode shows a maximum discharge capacity of 171 mAh g–1 at ∼13 mA g–1 and 62 mAh g–1 at ∼1300 mA g–1 with 76% capacity retention and ∼98% coulombic efficiency over 500 cycles at 1C rate (260 mA g–1) for the half cell, while for the full cell, it delivers an initial discharge capacity of ∼91 mAh g–1 and 66% capacity retention over 1000 cycles at 1C rate.