In-Situ Acoustic Emission Study of Sn Anode in Li Ion Battery

1. Introduction

Acoustic emission (AE) technique has been developed to evaluate the degradation of lithium ion battery during charge/discharge process by detecting supersonic elastic waves. In this research, the AE technique was applied to detect the SEI formation and the crack formation, respectively. The tin (Sn) thin film anode shows catalytic influence which causes the decomposition of ethylene carbonate (EC) in electrolyte from 2.4 to 0.7 V^{1), 2)}. On the other hand, it is reported that the AE signals from the tin oxide (SnO) thin film anodes are mainly detected in the Li extraction process by the crack formation due to the volume contraction³⁾. The AE results obtained from the Sn and the SnO are compared to characterize the AE signals.

2. Experimental

 Sn and SnO thin films were prepared on cupper substrates by pulsed laser deposition (PLD) method using 4th harmonics of Nd:YAG laser, where the substrate was kept at room temperature under vacuum condition. Two-electrode stainless-steel cell attached with AE sensor (NF corp., AE-900S-WB) was used for electrochemical measurements. Li metal foil was used as counter electrode. The electrolyte solutions consisted of 1M LiPF₆ and 1:1 volume ratio of EC and dimethyl carbonate. The AE sensor was attached to outside of the stainless-steel cell with grease to enhance contact. The AE signals were collected during constant current charge and discharge measurements (C rate: 0.1). The AE signals were amplified with two amplifiers (40 dB) and recorded by an oscilloscope (LeCroy, WaveSurfer104MXs-B).

3. Results and discussion

 Figure 1 and 2 show the results of the AE measurements for Sn and SnO electrodes, respectively. The solid curves represent charge/discharge voltage profiles and the dashed curves represent cumulated counts of AE events, plotted as a function of time. In figure 2, large amount of low frequency AE signals (20~50 kHz) were detected at 1.5~1.2 V plateau (region (A)), which corresponds to electrolyte decomposition the catalytic effect of the Sn^{1), 2)}. The EC shows the formation of a high weight polymeric species and alkyl carbonates at the first cycle with gas generation^{1), 4)}. These AE signals can be attributed to the SEI formation by EC polymerization or the gas generation. The gas generation was also confirmed at the voltage range (2.4~0.7 V) by in-situ microscope observation. In the case of SnO, low frequency AE signals (20~50 kHz, at region (B)) and high frequency AE signals (200~400 kHz, at region (C)) are detected. At region (C), the high frequency signals are due to crack formation of SnO electrode in large volume contraction process (SnLi_x → Sn + xLi)³⁾. Figure 3 shows fast Foulier transform (FFT) spectra of average AE signals at regions (A), (B) and (C). At region (C), there are many peaks at higher frequency than 80 kHz. The high frequency signals were caused by the crack formation on the SnO by lithium extraction. At both region (A) and (B), there are two main peaks at 20 kHz and 50 kHz. From the similarity of the FFT, the electrolyte decomposition might take place also in the case of SnO at region (B). In conclusion, the gas generation during the electrolyte decomposition or the SEI formation can be detected by using the AE technique.

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2)    I. T. Lucas, et al., Electrochem. Commun., 11 (2009) 2157

3)    Syutaro Kato, The 54^th Battery Symposium In Japan abstract, 82 (2011)

4)    Rotem Marom, et al., J. Mater. Chem., 2011 21 9938-9954