High performance LiNi_0.5Mn_1.5O₄ cathode by Al-coating and Al³⁺-doping through a physical vapor deposition method

Highlights: • Metal Al was used as an electrical conductive coating material for LiNi_0.5Mn_1.5O₄. • The uniform surface coating layer of metal Al was successfully achieved with adjusted thickness through a physical vapor deposition technology. • Al³⁺-doped LiNi_0.5Mn_1.5O₄ can be easily obtained by further directly annealing of Al-coated LiNi_0.5Mn_1.5O₄ in air. • The conductive Al-coating layer can greatly improve the rate performance and cycle stability of LiNi_0.5Mn_1.5O₄. - Abstract: In this work, spinel LiNi_0.5Mn_1.5O₄ (LNMO) hollow microspheres are synthesized by an impregnation method using microsphere MnO₂ as both the precursor and template. To enhance the electrical conductivity of LNMO, metal Al was employed for the first time as a coating material for LNMO. Though an Electron-beam Vapor Deposition approach, the surface of LNMO can be easily coated by a tight layer of Al nanoparticles with adjusted thickness. Further annealing the Al-coated sample at 800 °C in air, the Al³⁺-doped LNMO can be obtained. The effects of Al-coating and Al³⁺-doping on the sample morphology and structure are investigated by SEM, TEM, XRD and FT-IR. The electrochemical properties of Al-coated LNMO and Al³⁺-doped LNMO are measured with comparison of bare LNMO by charge/discharge tests and electrochemical impedance spectroscopy (EIS). The results show that both Al-coating and Al³⁺-doping can greatly enhance the cycle performance and rate capability of LNMO. Especially for Al-coated LNMO, it shows the lowest battery impedance due to the existence of conductive Al coating layer, thus delivers the best rate performance among the three. The physical coating procedure used in this work may provide a new facile modification approach for other cathode materials.