Relaxation Structure Analysis of LixNiO2 (x=0.09) after Li-Extraction

Introduction

We have investigated the structural variation of γ-Fe₂O₃ after the termination of lithium insertion, and revealed the transition of electrode material from kinetic state to equilibrium state clear [1,2].  We named this technique as “Relaxation analysis” and conducted this technique to various types of cathode or anode materials, such as LiMn₂O₄[3], LiFePO₄[4], LiCoO₂[5], or graphite [6] to clarify the insertion/extraction process as well as relaxation mechanism . 

LiNiO₂ is one of the major cathode materials with low cost and low toxic nature in comparison with the widely used LiCoO₂.  LiNiO₂ shows the rock-salt type structure with the space group, .  When lithium is electrochemically extracted from the sample, two phases with similar structures (Li-rich and Li-lean phases) coexist below x = 0.32 of Li_xNiO₂ [7,8].  In the present study, relaxation analysis has been performed on Li_xNiO₂ (x = 0.09) after Li-extraction.  We have investigated the structural relaxation of Li_xNiO₂ (x = 0.09) by means of X-ray diffraction and the Rietveld analysis focusing on both the change of molar ratio and structure parameter in each phase.

Experiment

Working electrode was prepared by mixing LiNiO₂ powder (Sumitomo Metal Mining Co., Ltd.) with Acetylene Black (AB) as a supplemental conductor and PVdF powder as an adhesive agent with the weight ratio of 80:10:10.  Lithium foil was employed as the counter electrode and 1 mol∙dm^-3 LiPF₆ in EC/DMC (2:1 v/v%, Kishida chemical Co., Ltd) was used as the electrolyte. Lithium was electrochemically extracted from LiNiO₂ using two electrode cell (Hohsen Co.) at a constant current of 0.01C rate to obtain the sample of Li_0.09NiO₂.  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 15 ° to 75° in 2θ for CuKα radiaton (RINT-TTR, Rigaku Co., Japan) at various relaxation time and were served for Rietveld structure analysis using RIEVEC program [9].

Results and discussion

Fig.1 shows X-ray diffraction patterns of two phases of Li_0.09NiO₂ at various relaxation time after the termination of lithium extraction. Diffraction pattern gradually evolves with increasing relaxation time, i.e. the intensity of 003_rich peak decreases and 003_lean shifts toward the higher angle as observed in the inset. Each XRD profile can be well fitted with the Rietveld calculation.

Fig.2 shows mole fraction changes of Li-lean phase. The mole fraction of Li-lean phase decreases while that of Li-rich phase increases with the relaxation time after lithium extraction. Fig.3 shows the variation of lattice constant (c-axis) for both phases. The Li-rich phase retains the c-length almost constant during the relaxation time, while Li-lean phase shrinks the c-axis.

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 phase without excess lithium ion and Li-rich phase 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 with relaxation time.

References

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