High capacity double-layer surface modified Li[Li0.2Mn0.54Ni0.13Co0.13]O2 cathode with improved rate capability

Abstract

Layered Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂, which is a solid solution between layered Li[Li_{1 / 3}Mn_{2 /3}]O₂ and Li[Ni_{1 / 3}Mn_{1 / 3}Co_{1 /3}]O₂, has been surface modified by single-layer coating with 2–5 wt.% AlPO₄, CoPO₄, and Al₂O₃ and double-layer coating with 2 wt.% AlPO₄ or 2 wt.% CoPO₄ inner layer and 2–3.5 wt.% Al₂O₃ outer layer. The samples have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements in lithium cells. The double-layer coated samples are found to exhibit lower irreversible capacity loss (C_irr) and higher discharge capacity values than both the pristine and the single-layer coated samples, which are due to the retention of a higher number of oxide ion vacancies in the layered lattice after the first charge. The double-layer coated composite cathodes exhibit C_irr values as low as 26 mAh g⁻¹ with a high discharge capacity of ∼300 mAh g⁻¹, which is two times higher than that achieved with layered LiCoO₂. Moreover, the double-layer coated samples exhibit higher rate capabilities than the pristine and single-layer coated samples, which are attributed to the suppression of undesired SEI layers and the fast charge transfer reaction kinetics as indicated by the EIS data.