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 instabilities^1–3, ii) electrolyte degradation^4,5 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. al⁸ and other groups^9–11. 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 LiCo_1-yAl_yO₂. 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 LiCoO₂¹², but what about their interdiffusion during the synthesis of Al-doped LiCo_1-yAl_yO₂?

In this talk, we will discuss the aluminum distribution homogeneity in Al-doped LiCo_1-yAl_yO₂ 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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