Discussing the Homogeneity of Aluminum Distribution in LiCo1-­YAlyO2 for Low Doping Amounts (y ≤ 0.04)
Authors: Duffiet Marie , Blangero Maxime , Cabelguen Pierre Etienne , Delmas Claude , Carlier Dany 
Published: February 2020

Although LiCoO2 has been now used for decades as positive electrode material in Li-ion batteries, the maximum x(Li+) exchanged between the electrodes in a commercial cell has remained quite unchanged. Indeed, only approximately 0.62 Li+ ions are reversibly de-intercalated from the lamellar structure of LiCoO2 (analogue to NaFeO2 structure, crystallizing in the R-3m space group), giving a capacity of 174 mAh/g and a typical cycling electrochemical window of 3.0 – 4.4 V vs. graphite (3.0 – 4.3 V vs. Li). One possible strategy to try to access more capacity in a LiCoO2//C cell is then to succeed in reversibly extracting the remaining 0.38 Li+ ions, that could lead to a potential capacity gain of roughly 100 mAh/g (272 mAh/g being the theoretical capacity). Even though the full extraction seems rather unlikely, as it was previously shown that the final CoO2 suffers from critical oxygen losses1, a partial de-intercalation of the remaining 0.38 Li+ looks reasonable enough. The number of publications dealing with the performance of LiCoO2 cycled up to 4.5 V vs. Li or even 4.7 V vs. Li has then increased in the last years. However, bare LiCoO2 has always proven to show a very poor cycling performance in such conditions. It is believed that the main causes for this are i) structural instabilities1–3, ii) electrolyte degradation4,5 and iii) cobalt dissolution6. Two main strategies have been successfully investigated to overcome these issues: coatings and doping.

Aluminum ions Al3+ as dopants were among the first considered, by the means of theoretical calculations from Ceder’s group7, followed by experimental work from Jang et. al
8 and other groups9–11. The choice for aluminum was motivated by i) the low cost and non-toxicity of aluminum, ii) an increased overall voltage necessary to achieve higher power densities, iii) a similar radius for Al3+ compared to Co3+ (53.5 pm vs. 54.5 pm) facilitating the substitution of the latter and preserving the structure leading to the full solid solution LiCo1-yAlyO2. These phases were synthesized in many different ways from one article to another, but investigation on the aluminum distribution is often missing. Dahn’s group recently studied the interdiffusion of Co3+ and Al3+ in the case of coated LiCoO2
12, but what about their interdiffusion during the synthesis of Al-doped LiCo1-yAlyO2?

In this talk, we will discuss the aluminum distribution homogeneity in Al-doped LiCo1-yAlyO2 phases obtained from different solid state syntheses with well-controlled low doping amount (y = 0.01 or 0.04) and particle size. We will show that synchrotron X-ray diffraction data (SXRD) and 27Al & 59Co nuclear magnetic resonance (NMR) techniques are powerful tools in that matter. A special focus will also be given to the relation between the homogeneity of the Al-doping and the electrochemical performances of the materials used as positive electrode in Li cells.

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