Relaxation Analysis of Electrochemically Lithium Inserted γ-Fe2O3 By Using a Solid-State Li NMR

Abstract

Introduction Recently, we have conducted the relaxation analysis for various electrode materials such as γ-Fe2O3 [1-2], LiMn2O4 [3], LiFePO4 [4], LiCoO2 [5], and graphite [6] after termination of electrochemical Li insertion/extraction. The relaxation analysis makes transition of electrode material from kinetic state to equilibrium state clear [7]. For γ-Fe2O3 with spinel structure, we have reported that the iron occupancy of 8a site decreased and that of 16c site increased with lithium insertion process and the iron occupancy of 8a site increased and that of 16c site decreased gradually at the relaxation process after termination of lithium insertion by using the X-ray Rietveld analysis. We considered that this indicates that lithium prefer 8a site to occupy kinetically, on the other hand, prefer 16c site thermodynamically. In this study, we investigated the lithium behavior directly by using a solid-state Li nuclear magnetic resonance (NMR) of electrochemically lithium inserted γ-Fe2O3 at the relaxation process. Experiment Electrochemical lithium insertion into γ-Fe2O3 has been made by using an Ar-sealed glass beaker cell. The mixture of γ-Fe2O3 powder (Alfa Aesar, >99%), Acetylene Black (AB) and PTFE powder with the weight ratio of 70:30:5 was employed for cathode material, which was spread onto a Ni mesh as a current collector. Lithium foil was used as the reference and counter electrodes, and 1 M LiPF6 in EC/DMC (2:1 v/v%, Kishida Chemical Co., Ltd) was used as the electrolyte. Lithium ion has been electrochemically inserted into the sample at a constant current of 0.01 Ag-1 to obtain Li1.5Fe2O3. After the termination of lithium insertion, the sample was scraped immediately out of the Ni mesh and set into a quartz tube. We prepared three samples for analysis of relaxation behavior. The relaxation time after the termination of lithium insertion of each sample were 15 h, 38 h and 2967 h, respectively. 7Li NMR spectra of the samples were acquired at room temperature in an AVANCE400 (Bruker Biospin) instrument at a spinning frequency of 0 kHz and using the Hahn echo sequence, taking 1 M aqueous solution of LiCl, at 155.65 MHz of resonance frequency as reference. Results and discussion Figure 1 shows the measured NMR spectra (Hahn echo, static) of the three samples. We separated the spectrum into two waveforms. We named the waveform at lower chemical shift as W1 and that at higher chemical shift as W2. For either sample, the total waveform of W1 and W2 is well fitted to the measured waveform as shown in Fig. 1. It is considered that W1 and W2 represents the two different conditions of lithium in the samples. The peak intensity ratios of W1 to W2 for samples with the relaxation time of 15 h, 38 h, and 2967 h were 1.05, 1.21 and 1.56, respectively, which increased with the increase of the relaxation time. According to the relaxation analysis by using the X-ray Rietveld method [1], it is considered that lithium at 8a site increases and at 16c site decreases with the increase of the relaxation time. If we ascribe W1 to lithium at 16c and W2 to that at 8a, the change in the intensity ratio is very consistent with the above consideration from the X-ray Rietveld analysis. In conclusion, we succeeded to observe lithium relaxation behavior directly by the NMR spectra, which coincided with that presented by the X-ray Rietveld analysis.