Relaxation Structure Analysis of LixNiO2 (x=0.09) after Li-Extraction
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, LiFePO4, LiCoO2, or graphite  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 LixNiO2 [7,8]. In the present study, relaxation analysis has been performed on LixNiO2 (x = 0.09) after Li-extraction. We have investigated the structural relaxation of LixNiO2 (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.
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 Li0.09NiO2. 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 .
Results and discussion
Fig.1 shows X-ray diffraction patterns of two phases of Li0.09NiO2 at various relaxation time after the termination of lithium extraction. Diffraction pattern gradually evolves with increasing relaxation time, i.e. the intensity of 003rich peak decreases and 003lean 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.
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