Rietveld Relaxation Analysis on LixNi0.5Mn1.5O4 (x = 0.1, 0.2) 5V Cathode Material
Authors: Yamada Keisuke , Takai Shigeomi , Yabutsuka Takeshi , Yao Takeshi 
Published: February 2020

Introduction

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 LixNi0.5Mn1.5O4 (x = 0.1, 0.2) after Li-extraction.
LiNi0.5Mn1.5O4 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 LixNi0.5Mn1.5O4 exhibits two existing phases (Li-rich and Li-lean phases) below x = 0.5 [9]. We investigated the structural relaxation of LixNi0.5Mn1.5O4 (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 LiNi0.5Mn1.5O4 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 LiNi0.5Mn1.5O4 using two electrode cell (Hohsen Corp.) at a constant current of 0.3C rate to obtain the sample of LixNi0.5Mn1.5O4 (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 LixNi0.5Mn1.5O4 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

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Figure 1