We have investigated the structural variation of γ-Fe₂O₃ after the termination of lithium insertion, and revealed the relaxation process from the discharging state to thermodynamically stable state [1-3]. We named this technique as “Relaxation analysis” and conducted this technique to various types of cathode or anode materials, such as LiMn₂O₄[4], LiFePO₄[5], LiCoO₂[6], or graphite [7] to clarify the insertion/extraction process as well as relaxation mechanism. In the present study, relaxation analysis has been made on the Li_xNi_0.5Mn_1.5O₄ (x = 0.1, 0.2) after Li-extraction.

LiNi_0.5Mn_1.5O₄ with spinel-type structure is a promising cathode material because it operates at a higher voltage of ~4.7V [8] in comparison with the spinel cathode LiMn₂O₄. In the lithium extraction process, it has been reported that Li_xNi_0.5Mn_1.5O₄ exhibits two existing phases (Li-rich and Li-lean phases) below x = 0.5 [9]. We investigated the structural relaxation of Li_xNi_0.5Mn_1.5O₄ (x = 0.1, 0.2) by means of X-ray diffraction and the Rietveld analysis focusing on the change in molar ratio and structure parameter of two phases.

Experiment

 Working electrode was prepared by LiNi_0.5Mn_1.5O₄ powder (TOSHIMA Manufacturing Co.,Ltd) with Acetylene Black (AB) as a supplemental conductor and PVdF powder as an adhesive agent with ratio of 80 : 10 : 10 (weight ratio). Lithium foil was employed as the counter electrode and 1 mol·dm^-3 LiPF₆ in EC/EMC (3:7 v/v%, Kishida chemical Corp.,Ltd) was used as the electrolyte. Lithium was electrochemically extracted from LiNi_0.5Mn_1.5O₄ using two electrode cell (Hohsen Corp.) at a constant current of 0.3C rate to obtain the sample of Li_xNi_0.5Mn_1.5O₄ (x = 0.1,0.2). After the termination of Li extraction, we immediately removed the working electrode from the cell in a glove box to avoid the local cell reaction between the electrode material and the current collector.

 XRD data were collected from 10° to 120° in 2θ by using CuKα radiation (UltimaIV, Rigaku corp., Japan) at various relaxation time up to 70 hours. XRD data were analyzed by the Rietveld method using RIEVEC program [10]. Lattice constants and scale factors were obtained.

Results and discussion

 Fig.1(a) and (b) show the X-ray diffraction patterns of x = 0.1 and x = 0.2 for Li_xNi_0.5Mn_1.5O₄ measured after the termination of lithium extraction for 3h to 68h. Diffraction pattern gradually evolves with increasing relaxation time, i.e. the intensity of 400 peak of Li-rich phase develops while that of Li-lean phase does not change largely during relaxation. Each XRD profile can be well fitted with the Rietveld calculation.

 Fig.2 shows mole fraction changes of Li-rich phase. The mole fraction of Li-rich phase increases while that of Li-lean phase decreases with the relaxation time after lithium extraction. Fig.3 shows the variation of distance between oxygen and lithium for both phases. The O-Li distance of Li-lean phase is larger than that of Li-rich phase for both compositions. With the relaxation time up to 300-600 hours, Li-lean phase shrinks the O-Li distance, while Li-rich phase stretches it.

 It is found that, during the lithium extraction process, Li-lean phase is formed with excess amount of lithium ions, and it separates the Li-lean and Li-rich phases during the relaxation process. At the lithium extraction process, Li-lean phase with larger number of vacancy is favorable for lithium diffusion, while the ratio of Li-rich/Li-lean get into the equilibrium at the relaxation process. The decrease of the amount of lithium ion in Li-lean phase appears as the changes in the distance of oxygen and lithium. This phenomenon appears more clearly than when lithium extracted at a current of 0.1C rate.

References

[1] S. Park, M. Oda, and T. Yao, Solid State Ionics, 203, 29-32 (2011).

[2] S. Park, S. Ito, K. Takasu and T. Yao, Electrochemistry, 80 (10) 804-807 (2012).

[3] T. Yao, Energy Procedia, 34, 9-12 (2013).

[4] I. S. Seo, S. Park, and T. Yao, ECS Electrochem. Lett., 2 (1) A6-A9 (2013).

[5] S. Park, K. Kameyama, and T. Yao, Electrochemical and Solid-State Letters, 15 (4) A49-A52 (2012).

[6] I. Seo, S. Nagashima, S. Takai and T. Yao, ECS Electrochem. Lett., 2 (7) A72-A74 (2013).

[7] T.Kitamura, S.Park, S.Takai and T.Yao, 225th Meet. Electrochem. Soc., Abstract. (2014).

[8] Q. Zhong, A. Bonakdarpour, M. Zhang, Y. Gao, J.R. Dahn, J. Electrochem. Soc., 144, 205 (1997).

[9] W. K. Pang, N. Sharma, V. K. Peterson, J.-J. Shiu and S. H. Wu, J. Power Sources, 246, 464–472(2014).

[10] T.Yao, N.Ozawa, T.Aikawa, and S.Yoshinaga, Solid State Ionics, 175,199-202 (2004).