Development of Novel Fe2O3-Al2O3 Solid Solution Electrode by Using Mechanochemical Synthesis

Introduction

  α-Fe₂O₃ is expected to perform as a favorable electrode material for lithium ion battery due to low cost and low toxicity. The cathode reaction is Fe₂O₃+6Li⁺+6e^-→2Fe+3Li₂O. It is considered that Fe site in crystal structure of α-Fe₂O₃ is lost during cathode reaction, and then the specific capacity declines drastically.

When crystal structure of α-Fe₂O₃ is held, cyclic performance will be enhanced. In this study, we synthesized Fe₂O₃-Al₂O₃ solid solution in various molar ratios by mechanochemical synthesis. We investigated cyclic performance of the solid solution for the electrode of lithium ion battery.

Experiment

γ-Fe₂O₃ and γ-Al₂O₃ were mixed in molar ratios of Fe:Al=(1-x):x (x=0.05, 0.33, 0.40, 0.45, 0.50, 0.60, 0.65, 0.67, 0.95) by theta composer (Tokuju corp.), then synthesized Fe₂O₃-Al₂O₃ solid solution by mechanochemical synthesis with ball mill (Premium line, Fritsch GmbH). X-ray diffraction analysis of the obtained sample was conducted using Ultima IV (Rigaku co.) (CuKα, 40kV, 40mA).

 The electrode was fabricated by mixing powder of the solid solution as the active material, acetylene black as a conducting additive and PTFE as a binder at the ratio of 60:40:3 by weight. C。opper mesh was used as current collector. Metal lithium was used as counter electrode and reference. The electrolyte was an 1M LiPF₆ EC:EMC (3:7 v/v%).

 We discharged the electrode material to the cutoff voltage of 0.1V (vs.Li/Li⁺) at 0.1A/g and charged to the cutoff voltage of 3.5V (vs.Li/Li⁺) at 0.1A/g. We measured cyclic performance of the solid solution.

Result and Discussion

We measured X-ray diffraction (XRD) of the samples synthesized with ball mill in various molar ratios from 20 degrees to 80 degrees in 2θ. We obtained lattice constants precisely by fitting the XRD peaks with Lorentz function. Unit cell volume is calculated from the lattice constants. For every molar ratio, the lattice constants and the unit cell volume are almost equal to those calculated from the lattice constants of α-Al₂O₃ and α-Fe₂O₃ (JCPDS database) by using Vegard’s law. It is confirmed that the all samples obtained by this mechanochemical synthesis were solid solutions. It is considered that this is the first report of the synthesis of complete solid solutions of α-Fe₂O₃ and α-Al₂O₃.

Fig.1 shows an example of charge-discharge curves of the solid solution (Fe_0.6Al_0.4)₂O₃. It shows that 1.4V plateau of the discharge curve is getting to ambiguous as number of cycles increases. Fig.2 shows cyclic performance of the solid solution (Fe_0.6Al_0.4)₂O₃. It shows that the specific capacity did not decline even after 300 cycles. It is indicated that the obtained solid solution (Fe_0.6Al_0.4)₂O₃ has excellent cyclic performance. It seems that α-Al₂O₃ solved in α-Fe₂O₃ keeps crystal structure of the active material.

References

T. Yao, S. Esaki, M. Nishijima, T. Yoshie, S. Park, M. Oda and J. Harada, PCT/JP2012/ 83694