Low temperature synthesis of γ-LixMnO2 powders in ethylene glycol

Abstract

The reaction of Electrolytic Manganese Dioxide (EMD γ-MnO2) with anhydrous LiOH in ethylene glycol (EG) was studied. It was found that this reaction proceeds in three steps while the reaction temperature increases: (1) chemical insertion of lithium in γ-MnO2 structure leading to γ-LixMnO2 phase, (2) progressive dissolution of this reduced phase and (3) precipitation of a Li-free organo-metallic compound Mn(OCH2CH2O) with lamellar structure (Mn-EG). If the reaction is performed at moderate temperature (T<120°C), we can limit the dissolution reaction and completely avoid the precipitation of the organo-metallic phase. The x value in the resulting LixMnO2 phases is found to be mainly linked to the temperature of the reaction medium. Although a maximum value of x=0.35 can be obtained at 30°C, Li0.84MnO2 can be prepared at 120°C. We showed that the LixMnO2 formation mechanism consists of a direct lithium insertion in the structure of MnO2 concomitant with Mn reduction by the organic medium. By changing the medium cationic content of the medium, we were able to prepare various reduced MnO2 compositions, such as HxMnO2, (Li,H)xMnO2, (Na,H)xMnO2, (K,H)xMnO2. These LixMnO2 phases, when tested in half Li-cells, show poor cyclability but large first charge capacity (260 mAh/g for x=0.84) that takes place at significantly higher voltage than observed for electrochemically inserted EMD. This finding was related to the presence of chemically inserted protons besides lithium ions.