Lithium Battery Cathode Materials, Li-Rich Li-Fe-M-O, Based on Tetrahedrally-Coordinated Structure
Authors: Taminato Sou , Goto Ryosuke , Sawada Aiyu , Mori Daisuke , Imanishi Nobuyuki 
Published: December 2024

The development of high-energy-density batteries with lithium (de-)intercalation reaction is one of the most promising solutions to realize electric vehicles and power storage system applications with long-term stability. The cathode materials exhibit smaller capacity than the anodes, which limits the energy density of the battery cells [1]. Consequently, the development of new cathode materials with higher capacity than previously reported is required. In the lithium 3d transition metal system, Li-rich materials based on tetrahedrally-coordinated structure, ex. Li5FeO4 and Li6CoO4, exhibited a reversible two lithium (de-)intercalation reaction over 350 mAh/g with mainly anionic redox [2,3]. The capacity was much higher than the practical cathode materials. Therefore, these Li-rich transition metal oxides with tetrahedrally coordinated structure have potential as high capacity cathode materials. In this study, Li-rich cathode materials based on tetrahedrally-coordinated structure were synthesized, and electrochemical properties in the lithium battery cell were characterized by electrochemical and structural investigations.
One of the Li-rich transition metal oxides; Li5+x
Fe1-x
Mn
x
O4 was synthesized by solid state reaction [4]. Phase identification by X-ray diffraction (XRD) measurement was conducted using a diffractometer with Cu Kα radiation. Observation of particle morphology and elemental distribution was performed using scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX). The prepared active material samples were mixed with Ketjen black as conductor and polytetrafluoroethylene as a binding agent using an agate mortar to prepare a composite electrode. A 2032-type coin cell was prepared using Li metal as the anode and 1 mol/dm3 LiPF6/ethylene carbonate:diethylene carbonate = 50:50 vol% as the electrolyte. Charge-discharge measurements were conducted in the voltage range of 1.7−3.9 V at 50 °C. The ball-milling process with acetylene black (AB) was applied before electrode fabrication to decrease the particle size of the prepared samples and achieve homogeneous electrical conduction in the composite electrode. The active material and AB were mixed with ZrO2 balls in a ZrO2 pot. A similar electrode fabrication process was conducted using a mixture of active material and AB.
An antifluorite structured Li6MnO4-type phase with the small starting materials was obtained in the range between 0.6 and 1.0 composition. The amount of residual reagents decreased with increasing excess lithium content in the starting mixture. A single phase with an Li6MnO4-type structure was obtained for an excess lithium composition of 10 mol%. The particle size of the synthesized samples was about 50 µm, which is quite large compared with conventional cathode materials for lithium batteries. The as-synthesized Li5.6Fe0.4Mn0.6O4 exhibited first charge and discharge capacities of 680 and 300 mAh/g, respectively, with a large irreversible capacity of 380 mAh/g (coulombic efficiency = 44%). This indicates that the new iron-manganese composition with ordered antifluorite-type structure is lithium insertion/extraction-active although a large irreversible electrode reaction was observed.
The ball milling process with AB provided that the particle size of the sample was decreased from ca. 50 µm to 1 μm, and a uniform distribution of Mn, Fe and C signals was realized for the mixture without any local signals. This suggests that a fine and homogeneous composite material composed of the active material and AB was obtained by the ball-milling process. The first discharge capacity of the Li5.6Fe0.4Mn0.6O4 electrode treated by ball-milling was 450 mAh/g. The coulombic efficiency was 60%. A reversible reaction continued to proceed with ca. 200 mAh/g after the following cycles. The differential capacity plots for the ball-milled Li5.6Fe0.4Mn0.6O4 (x = 0.6) at the second, third, and tenth cycles exhibited two pair of redox peaks at around 2.5 V and 3.2 V, which reveals that the lithium (de-)intercalation reaction successfully occurred after the 2nd cycle. A new iron-manganese based ordered antifluorite-type compound has potential as a high-capacity oxide cathode material and thus further investigation is required. Phase formation and electrochemical properties of the Li-rich materials based on tetrahedrally-coordinated structure including the other cation will also be discussed in the presentation.

Acknowledgement:

This work was financially supported by Tokuyama Science Foundation, Nippon Sheet Glass Foundation for Materials Science and Engineering, and JSPS KAKENHI Grant Number 24K01582.

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