NiCo2S4 Bi-metal Sulfide Coating on LiNi0.6Co0.2Mn0.2O2 Cathode for High-Performance All-Solid-State Lithium Batteries

NiCo₂S₄ nanoparticles (NPs) were dry coated on LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) cathode using a resonant acoustic coating technique to produce all-solid-state lithium batteries. The NiCo₂S₄ coating improved the electrochemical properties of the NCM622 cathode. In addition, NiCo₂S₄ eliminated the space-charge layer and the cathode showed an excellent affinity with the interface with a sulfide-based solid electrolyte as an inert material. X-ray diffraction patterns of NCM622 coated with NiCo₂S₄ showed the same peak separations and lattice parameters as those of bare NCM622. Field-emission scanning electron microscopy and electron dispersive spectroscopy mapping analyses showed that 0.3 wt% NiCo₂S₄-coated NCM622 had an evenly modified surface with NiCo₂S₄ NPs. X-ray photoelectron spectroscopy (XPS) revealed that the surface of 0.3 wt% NiCo₂S₄-coated NCM622 had two different S 2p peaks, a Co-S peak, and Ni and Co peaks, compared to those of bare NCM622. Electrochemical studies with electrochemical impedance spectroscopy and galvanostatic charge-discharge cycle performances showed that NiCo₂S₄-coated NCM622 retained a higher specific capacity over multiple cycles than bare NCM622. Especially, 0.3 wt% NiCo₂S₄-coated NCM622 exhibited a capacity retention of 60.6% at a current density of 15 mA/g for 20 cycles, compared to only 37.3% for bare NCM622. Finally, interfacial XPS and transmission electron microscopy-electron energy loss spectroscopy analyses confirmed the stable state of 0.3 wt% NiCo₂S₄-coated NCM622 with minimal side reactions.