In Situ XANES Analysis of Olivine-Type Cathode Materials to Study Kinetically Preferred State
Authors: Satou Yoshinori , Kimura Yasuharu , Komine Shigeki , Takai Shigeomi , Yao Takeshi 
Published: February 2020

Introduction

Yao et al. invented "relaxation analysis" to make transition of electrode materials from kinetically preferred state to equilibrium state clear. Olivine-type cathode materials are included in it1-3. These materials are promising, having both large theoretical capacity and fine stability4.
Previously, Park et al. found that the amount of LiFePO4 decreased and that of FePO4 increased at the relaxation process after the termination of lithium insertion by use of relaxation analysis2. They considered that the LiFePO4 including lithium defects preferable for Li diffusion formed during lithium insertion process and that the defective LiFePO4 separated to LiFePO4 without defects and FePO4 at the relaxation process.
Synchrotron X-ray absorption near edge structure (XANES) method is a powerful tool to investigate the structural/electronic properties. XANES enable to measure the kinetically-preferred state directly during a reaction. In this study, we measured the in situ Fe K-edge XANES of LiFePO4 cathode during lithium insertion for the purpose of investigating the kinetically-preferred structure and comparing the result with that of relaxation analysis obtained by using the XRD-Rietveld method2.

Experimental

Cathode was prepared by mixing LiFePO4, carbon black, and polyvinylidenefluoride with a weight ratio of 70:15:15 in N-methlpyrrolidone solution. The slurry was spread onto aluminum foil current collector and dried in a 120°C vacuum oven. The cathode and lithium metal anode were separated by a polypropylene membrane separator. LiPF6 (1 M) in ethylene carbonate/diethyl carbonate (EC/DEC) (3:7 v/v) was used as the electrolyte. The 2032-type coin cell with X-Ray window was assembled in an argon-filled glove box. We first charged the cell to 4.0 V at a rate of 0.19 C and took 12 minutes for rest when the voltage got to 4.0 V. We subsequently discharged the cells at a rate of 0.32 C to 2.65 V.

In situ Fe K-edge XANES measurement was performed in transmission mode at beamline 5S1 of Aichi Synchrotron Radiation Center. We took 3 minutes measurement and 1 minute interval per scan; the scan range was 150 eV before the Fe K absorption edge and 1000 eV after that. We merged three continuous patterns into one pattern to decrease noise and to improve the analysis precision. The composition x in Li(1-x)FePO4 of each merged pattern was calculated by integrating the current. We carried out a two-components analysis based on the linear combination of the last merged pattern, called Li-rich phase hereafter, and the merged pattern during the rest time before lithium insertion, called Li-lean phase hereafter, for either end by using Athena5program.

Results and Discussion

The composition of Li-lean phase and Li-rich phase were calculated as Li0.09FePO4 and Li0.85FePO4 respectively by using the integrating current. Figure 1 shows an example of two-components analysis for the composition of x = 0.28 in Li(1-x)FePO4 . The XANES pattern was well fitted with a low χ2value of 0.15%. It was found that the relative amount of Li-rich phase from two components analysis, 89.6%, was larger than the amount, 82.9%, calculated from the composition x in Li(1-x)FePO4. The large amount of Li-rich phase out of linear relationship may indicate that Li-rich phase with lithium defects formed in large amount during lithium insertion process. This is consistent with the result of the "Relaxation Analysis"2using the XRD-Rietveld method.
[1] S. Park, M. Oda and T. Yao, Solid State Ionics 203(2011) 29-32.
[2] S. Park, K. Kameyama and T. Yao, Electrochem. solid-state lett. 15(2012) A49-A52
[3] Y. Satou, S. Komine, S. Park and T. Yao, Solid State Ionics, In Press
[4] A. K. Padhi, K. S. Nanjundaswamy and J. B. Goodenough, J. Electrchem. Soc. 144(1997) 1188-1194
[5] B. Ravel and M. Newville, J. Synchrotron Rad. 12(2005) 537-541