Probing Al Distribution in LiCo_0.96Al_0.04O₂ Materials Using ⁷Li, ²⁷Al, and ⁵⁹Co MAS NMR Combined with Synchrotron X-ray Diffraction

Abstract

We prepared Al-doped LCO (LCA) powders with low Al content (4%) with a controlled Li/(Co + Al) stoichiometry by a solid-state reaction using Li₂CO₃ and two types of Co/Al precursors: simply mixed (Co₃O₄ and Al₂O₃) or heat-treated (Co₃O₄ and Al₂O₃). These samples were thereby used to propose a reliable protocol with the aim to discuss the homogeneity of the Al doping for LiCo_1-<i>y</i>Al_<i>y</i>O₂ (LCA) prepared with low Al content by evidencing the distribution of Al within the powders, which clearly affects the electrochemical profiles of associated LCA//Li cells. For all samples we initially also characterized the Li/(Co + Al) stoichiometry by ⁷Li MAS NMR, to discard the possible effect of excess Li in the samples. Synchrotron XRD combined with ²⁷Al and ⁵⁹Co MAS NMR then provided a deep understanding of the doping homogeneity at the powder or particle scale. We showed that doping the Co₃O₄ spinel precursor by reacting it with Al₂O₃ may be avoided, as it most likely leads to an inhomogeneous mixture of Co₃O₄ and Co_3-<i>z</i>Al_<i>z</i>O₄ as precursor, eventually reflecting in the final LiCo_0.96Al_0.04O₂ powder, which shows a nonhomogeneous Al distribution. We believe that such a detailed characterization should be the first step toward a deeper understanding of the real beneficial effect(s) of Al doping on the high voltage performance of LCO.