Effect of Mn-substitution for Co on the crystal structure and acid delithiation of LiMn yCo 1− yO 2 solid solutions

Abstract

Lithium-manganese-cobalt oxides, LiMn y Co 1− y O 2, where Li/(Mn+Co)≈1and 0< y<1, have been obtained by a solid state reaction between lithium hydroxide and manganese-cobalt spinels in air (0< y<0.2) or under nitrogen (0.2< y<1). The crystal structure of the ternary oxides depends on the Mn/(Mn+Co) ratio: up to y<0.2, oxides with a trigonal structure (s.g. R 3 m) are formed; at 0.2<y<0.7, oxides with a rock-salt type structure (s.g. Fm 3 m) appear; finally, at y<0.7, the oxides obtained have a tetragonal spinel structure (I4 1/amd). Acid delithiation of the lithium-manganese-cobalt oxides proceeds differently depending on their composition and crystal structure. With cobalt-rich oxides, the acid extraction of lithium proceeds within the initial trigonal structure, while with rock-salt and tetragonal oxides acid delithiation yields Li x (Mn y Co 1− y )O 2 oxides with a cubic spinel structure (s.g. Fd3m). Among LiMn y Co 1− y O 2 oxides, lithium is removed more easily and to a higher extent from the tetragonal oxides. Due to the high oxidation degree of the manganese and cobalt ions in delithiated samples, the electrochemical cell consisting of Li/Li x (Mn y Co 1− y )O 2 displays a high discharge voltage (4.1–4.2 V), which allows its direct discharge. The highest reversibility and capacity per formula unit (about 0.75 Li) are observed on manganese-rich spinel oxides with y<0.7.