Investigating the Degradation Behavior of Silicon Anodes for Lithium-Ion Batteries
Authors: Jeon Jiyun , Lee Hochun 
Published: December 2024

Silicon (Si), which delivers higher capacity than that of graphite, is an adequate material for enhancing the energy densities of lithium-ion batteries (LIBs). However, rapid capacity deterioration has made it difficult for practical use and commercialization. Furthermore, there is limited understanding regarding the degradation conditions and mechanisms of Si anode. Thus, our study aims to elucidate the degradation behavior and conditions of Si anodes while exploring the underlying degradation mechanisms.

To investigate the degradation conditions of Si anodes, we conducted electrochemical experiments on NCM||Si full cells under various conditions. Cycling under sparse and ample electrolyte conditions was compared. It revealed stable cycling up to 400 cycles under sufficient conditions, while a rapid deterioration was observed after 200 cycles under sparse conditions. Additionally, we compared cycling performance under room temperature and elevated temperature conditions. Unlike conventional graphite-based LIBs, which typically exhibit more unstable cycling at high temperatures than room temperatures, NCM||Si full cells demonstrated more stable cycling behavior at elevated temperatures. While a sharp degradation occurred after approximately 200 cycles at room temperature, high temperatures yielded a remarkable 99.8% Coulombic efficiency and stable cycling beyond 300 cycles. Moreover, cycling performance was compared at low and high C-rates. At low C-rates, lifetimes exceeding 300 cycles were observed, whereas at high C-rates, rapid degradation commenced around 250 cycles. Through degradation condition experiments, we confirmed that Si anodes experience accelerated degradation with sparser electrolytes, lower temperatures, and higher C rates.

Conditions of low temperature and high C-rate kinetically limit the movement of electrons and Li ions. Therefore, we hypothesized that the observed degradation behavior of Si may be attributed to restricted kinetics. To investigate this, we conducted Electrochemical impedance spectroscopy (EIS) measurements on Si, Si/Gr composite, and graphite (Gr) anodes across various states of charge (SOC). Our findings revealed pronounced differences in resistance behavior among these materials throughout the lithiation process. While graphite exhibited a linear resistance increase with lithiation, the Si/Gr composite maintained low resistance. However, pure Si anodes exhibited low resistance up to SOC60 but sharply increased resistance, quadrupling at SOC80. This observation suggests that Si encounters a significant kinetic barrier at high SOC ranges, which likely serves as a primary trigger for degradation.

To enhance our comprehension of the degradation mechanism, we examined electrode surfaces post-cycling through scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). In the degraded Si electrode after 200 cycles, we observed pits scattered throughout the electrode surface which appeared to result from repeated expansion and contraction cycles. Furthermore, through EDS image analysis, we noted a significant increase in F peak content in areas where the Si peak had decreased. These observations suggest a correlation between the degradation of Si electrodes and an increase in side-reaction with electrolytes.

It is known that Si undergoes a structural transformation from amorphous to crystalline, forming an over-lithiated structure, within a voltage range of less than 50 mV. Unlike graphite, which undergoes lithiation by intercalation, Si undergoes lithiation by chemical transformation, resulting in different products depending on the operating voltage. Therefore, understanding the actual driving voltage range of Si anodes is crucial for deducing the products of Si during charging and discharging. Thus, we conducted a three-electrode experiment in NCM||Si full cells. During the charging and discharging, the actual operating voltage of the Si electrode was observed to range from 0.031V to 2V. Similarly, in NCM||SiGr full cells, the actual operating voltage of the SiGr electrode ranged from 0.073V to 0.85V. Given that charging and discharging of the Si electrode occur even below 50mV, it suggests the presence of overcharged crystalline Si.

To confirm the existence of this crystalline Si, we additionally conducted X-ray diffraction (XRD) and dQ/dV analysis. XRD analysis of degraded electrodes exhibited that as the cycle increased, indicating accelerated degradation, the peak of crystalline Si increased. The trend of increasing crystalline peak with progressing degradation was also confirmed through dQ/dV analysis.

In conclusion, We observed the degradation behavior on Si electrodes and confirmed that the generation of over-lithiated crystalline Si is the main degradation cause. This discovery is expected to contribute to the future development of Si-based LIBs by providing valuable insights into achieving long cycles of Si anodes.