Stress relaxation due to structural fluctuations of silicon (Si) is critical for its application in lithium-ion batteries (LIBs). In a paper published in the Journal of the Electrochemical Society, mesoporous silica (SiO2) spheres were reduced using magnesium silicide (Mg2Si) to achieve nanoporous Si particles. The fabricated Si particles were applied as anode material for solid-state LIBs.
Study: High cycle stability of nanoporous Si compounds in solid state lithium ion batteries. Image Credit: Smile Fight/Shutterstock.com
Compared to non-porous Si half-cells, the prepared nanoporous counterparts showed exceptional cyclability with approximately 90% retention capacity at 50 cycles in solid-state LIBs. Furthermore, analytical methods, including electrochemical impedance spectroscopy (EIS) and field emission scanning electron microscopy (FE-SEM) equipped with energy dispersive X-ray spectroscopy (EDX), helped to compare the characteristics of nonporous and nanoporous Si composite anodes.
The results confirmed that the contraction and expansion of the Si pore in the nanoporous Si anode material and the elastic deformation of lithium phosphorus sulfide (Li3PS4) relieved the structural stress caused by the volume changes of the aggregates of Si during the lithiation and delithiation process, which resulted in the high stability of the cycle. As a result, the potential of Si-based anodes in solid-state LIBs was established.
Replacement of conventional LIBs with solid state LIBs
LIBs are robust alternative energy storage systems used for electric vehicles. However, graphite is not a suitable anode material to meet the energy demand in advanced automotive technologies due to its limited specific capacity. Although Si is a suitable material for automotive applications, its anode form has poor cycleability.
The storage and release of lithium ions (Li+) leads to volume fluctuations in LIBs, causing electrochemical disadvantages. To this end, previous studies on LIBs based on liquid electrolytes mentioned two approaches to relieve the structural stress of the Si anode, namely nanosization and composition.
For example, previous studies mentioned that Si nanowires based on stainless steel substrates with sufficient gaps could withstand the volume changes of LIBs. In addition, mesoporous Si nanospheres with internal voids supported internal expansion and contraction during volume changes of Li + ions.
Solid-state LIBs comprise a solid electrolyte instead of a liquid one. One of the biggest advantages of solid state electrolytes is that they never carry the safety issues of liquid electrolytes. Solid-state LIBs are one of the most promising replacements for conventional LIBs due to their wide operating temperature range, high inherent safety, and high energy density.
Therefore, developing solid-state LIBs that are safe and flexible, have a stable output voltage, and have good air-atmosphere fabrication is highly desirable for powering automotive technology. However, studies on solid-state LIBs with Si anodes are more limited than those on LIBs using liquid electrolytes.
Nanoporous Si composites in solid state LIBs
In the present work, a simple and cost-effective nanosizing process was adopted to facilitate the volume expansion of the Si anode. Nanoporous Si particles were prepared by fume reduction of Mg2Si-based SiO2 or by atmospheric oxidation of Mg2Si. As-prepared nanoporous Si particles were used as the anode material, a novel approach for solid-state LIBs.
Furthermore, Li3PS4 solid electrolyte was used to fabricate composite anodes in solid-state LIBs. Optimization of the nanoporous Si particle size, pore distribution, and surface area was critical, as the electrode microstructure strongly affected the performance of solid-state LIBs.
Compared to the atmospheric oxidation of Mg2Si, the oxygen supply was restricted for SiO2 under vacuum conditions. Therefore, it is essential to obtain Si particles of higher purity with fewer surface oxides. In addition, the SiO2 spheres have well-distributed pore channels. Nanoporous Si, a reduction product of SiO2 spheres, was developed to investigate the role of porous structure in electrochemical properties.
Analytical methods such as EIS, FE-SEM-EDX were used to investigate microscopic structures and electrochemical behavior of nanoporous Si composite anodes for solid-state LIBs. The results indicated that the nanoporous structure of Si mitigated the volume change of Si aggregates in the composite anodes, where the volume expansion of Si was damped by the contraction of nanosized pores.
Furthermore, the expansion stress of Si was relieved by the elastic deformation of the solid electrolyte in solid-state LIBs. This deformation of the solid electrolyte returned to its native state and maintained contact with the Si aggregates.
conclusion
Overall, the improved cycling performance of solid-state LIBs with nanoporous Si particles provided insight into the importance of Si-based anodes in automotive technology. The present study represented a critical step in the development of high-performance solid-state LIBs.
Through the present work, improvements in efficiency and Coulombic capacity were achieved for the practical application of Si composite anodes in solid-state LIBs. The increased capacity helps build a new conduction path for dispersed Si aggregates in the composite anode.
Despite the limited supply of an oxygen source to SiO2, the X-ray photoelectron spectroscopy (XPS) profile indicated the presence of Si oxides on the inner and outer sides of the Si nanospheres. This was suppressed by improving the Coulombic efficiency through mechanical milling or vacuum photoetching.
reference
Okuno, R., Yamamoto, M., Kato, A., Takahashi, M. (2022). High cycling stability of nanoporous Si composites in solid-state lithium-ion batteries. Journal of the Electrochemical Society. https://iopscience.iop.org/article/10.1149/1945-7111/ac81f6
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