Electrochemical Degradations Caused By Mechanical Damages in Silicon Negative Electrode
Authors: Yoshida Naoki , Sakamoto Tadashi , Kuwata Naoaki , Kawamura Junichi , Sato Kazuhisa , Hashida Toshiyuki 
Published: February 2020

Introduction

Silicon is a promising anode material for lithium-ion battery application due to its high specific capacity. Unfortunately, silicon undergoes a volume expansion of 300~400% full lithiation and suffers from severe capacity fade, which limit its successful application in commercial cells. In situ analyses by scanning electron microscopy (SEM) transmission electron microscopy (TEM) do not allow a detailed analysis of the silicon cracking process, which can significantly depend on the charge/discharge cycle. To solve this problem, more thorough understanding of how the material degrades is necessary to help cycling techniques that may be capable of reducing the capacity fading. In this study, we used in situ acoustic emission method for detecting the volume change in silicon negative electrodes.

 Experimental

The silicon negative electrodes were deposited onto a copper substrate. That substrate was ultrasonically cleaned in ethanol. The sample was characterize using X lay diffraction. Half cell using silicon negative electrode were carried out in a grove box filled with argon gas.The battery cell for in situ acoustic emission was prepared. Stainless steel 306 hardware was used to assemble coin cells containing a lithium foil (0.5 mm thick),Cellgard PP2400 separator, and a silicon negative electrode. A 1M solution of LiPF6 in ethylene carbonate and dimethyl carbonate, mixed 1:1 by weight, served as an electrolyte. A stainless steel spring and compression was used in the battery cell on the lithium side.

 Electrochemical properties and mechanical damages measurements

Constant current-constant voltage (CCCV) tests were performed. The CCCV tests applied a constant current onto the lithium ion battery until a voltage threshold was reached. The battery cell was then subjected to constant current cycles against metallic Li between 2V and 0.001V. The voltage was then held constant for 3 hours, at which point, the next charge or discharge step was commenced.In this study, Acoustic emission is used to record mechanical damage from silicon negative electrodes. Acoustic Emission system with AE-900S-WB AE sensors (NF corporation), 9917 preamps (NF corporation), 9922 main amps (NF corporation) and WaveRunner604Zi oscilloscopes (LeCroy) was used for monitoring Acoustic emission activity. A preamp and main amp gain of 40dB, band pass filter of 20kHz ~ 500kHz were used. Any noise signals were removed by inspection of waveforms and their fast fourier transform (FFT).

 Result and discuss

The largest energy of emissions occurred on the first lithiation and corresponding to volume change of the silicon negative electrode. The greatest acoustic emission energy occurred during the first discharge steps. The subsequent charge and discharge steps showed a much less acoustic emission energy. After the second cycle, acoustic emission energy recorded during the discharge step. Two types of signal were identified by the FFT analysis. One type was emission from the silicon damages. The other was emission from the gas generation. Acoustic emission signal of the gas generator has a peak of less than 100kHz. In contrast, Acoustic emission signal of silicon damage has a peak of more than 100kHz. All type acoustic emission signals occurred on the first charge/discharge. At the first lithiation step, gas generation and many Peeling on the surface of the silicon negative electrode was observed.After the second cycle, AE of silicon damages recorded during the discharge step.The largest capacity fade occurred between the first delithiation steps and was accompanied by the largest energy of emissions from the first delithiation. In general, the capacity continued to decrease with each step as did the energy of emissions. This strongly indicates that the energy of emission, and hence the amount of damage , is related to capacity fade.

 Conclusion

In this study, we used in situ acoustic emission of volume change in silicon negative electrode. Acoustic emission corresponding to silicon negative electrode damage was detected and analyzed.The present study revealed the micromechanism of the deterioration in the silicon negative electrode of the lithium ion batteries. It was shown that the long cycle performance of the silicon negative electrode was dictated by its spallation during the discharge steps, while only the first lithiation caused its delamination and spoliation in the charge step. This study demonstrate that acoustic emission is a powerful tool to survey the real-time mechanical damage and electrochemical degradation in the electrode.