Low Temperature Synthesis for Mn Series Positive Electrode Active Material
Authors: Ishido Madoka , Nasu Akira , Kobayashi Hiroaki , Matsui Masaki 
Published: December 2024

Introduction

Conducted a series of synthesis of various manganese-based cathode active materials for lithium-ion batteries via hydroflux process. We obtained poorly crystalline ortho-LiMnO2 and Li2MnO3 mixture using MnO as the starting materials. ortho-LiMnO2 single phase was successfully synthesized using Mn2O3 and calcined under inert atmosphere. The control of oxidation state of Mn is critical to obtain the target material by hydroflux process.

Experimental methods

The starting materials were LiOH-H2O and NaOH and various Mn oxides ground by a ball mill. The precursors were mixed in a molar ratio of 1.5:0.5:1 for each metal elements and then watered to obtain a slurry or pellet-like precursor. The precursor was calcined in a tube furnace at 300 °C for 12 h to obtain samples. Excess alkali metal hydroxide in the obtained samples was washed and dried using ethanol and water, and then the samples were identified by powder X-ray diffraction measurement and their surface morphology was observed by SEM observation. A positive electrode was prepared by adding acetylene black and PVdF to the obtained positive electrode active material, preparing a slurry, and coating it on an Al foil. Half-cells were prepared using metallic lithium as the counter electrode and 1 molL-1 LiPF6 EC/DEC (1:1 vol) solution as the electrolyte, and charge-discharge measurements were performed at C/10 at 25 °C.

Results and Discussion

At first, we investigated the conditions for synthesis from divalent starting materials as in the synthesis of LiCoO2. When MnO was used as the starting material, a mixed phase of ortho-LiMnO2 and Li2MnO3 with a zigzag layered structure was obtained. Despite the addition of water, the peaks attributed to both phases were broad, indicating low crystallinity. SEM observations showed that the grain size remained similar to that of the starting material MnO, and no traces of crystal growth during synthesis were observed. Referring to the Pourbaix Diagram of Mn, the oxidation potential of Mn2+ is lower than that of Co2+, suggesting that the rapid oxidation reactions to Mn3+ and Mn4+ inhibited the crystal growth.
Therefore, we investigated synthesis in an inert atmosphere to prevent oxidation reactions of the starting materials. In order to obtain a single phase of ortho-LiMnO2, Mn2O3 was used as the starting material and synthesized in an inert atmosphere, and observed patterns attributed to ortho-LiMnO2. The crystallinity was also improved.
Charge-discharge measurement of the obtained ortho-LiMnO2 as positive electrode active material showed typical charge-discharge behavior of ortho-LiMnO2, such as phase transition to spinel phase in the first cycle.
These results indicate that the control of the oxidation state of Mn is an important parameter in obtaining the desired compound. In the presentation, we will also report the results of our investigation of other synthesis conditions.

References

Maeda, R. Nakanishi, M. Mizuhata, and M. Matsui, Inorg. Chem.,
in press

Li, Z. Su, and Y. Wang, Journal of Alloys and Compounds, 735, (2018) 2182–2189.