Lithium-ion batteries (LiBs) have remained the gold standard for the development of advanced storage devices despite the rapid expansion of innovative technologies. However, boosting their energy density is still a work in progress¹. The use of silicon (Si) instead of graphite as the anode material in LiBs has resulted in significant improvements², owing to Si's ten-fold larger mass capacity than graphite. However, Si-based anode LiBs suffer from limited durability due to volume expansion anodes during lithiation³. Of late, studies have revealed that nanostructuring Si can compensate for the volume expansion impact by allowing free spaces. Herein, we report the behavior of on-chip porous Si fabricated by electrochemical etching method. We highlight the well-controlled surface modification of porous Si by Rapid Thermal Annealing (RTA) treatment to relatively close surface pores to minimize detrimental SEI formation. Electrochemical behavior of the prepared Si samples as an anode for LiB was examined.

Figure 1A displays the scanning electron microscopy (SEM, Thermo Scientific™ Scios™ 2 DualBeam™) images (top and cross-section/inset) of the electrochemically etched porous Si samples (p-type, 500 µm thick, <100> orientation) using HF/H₂O/EtOH (electrolyte) bath with a current density of 100 mA cm^-2 for 12.5 min. A porous layer of 25 µm thickness was obtained. The heat treatment was conducted at two different temperatures: 800°C and 1000°C. Surface porosity determination has been conducted with both SEM characterizations (ImageJ for post analysis) and X-ray reflectivity (XRR, Rigaku Smartlab HRXRD system with Cu K-alpha X-ray source and HYPIX-3000 hybrid pixel array 2D detector) were employed as shown in Figure 1B. Figure 1C shows the differential capacity (dQ/dV vs V) plots in the potential range 0–2.5 V. Cathodic peak can be observed that corresponds to solid electrolyte interphase (SEI) formation.⁴ The calculated area under the curve links that 1000°C- 60s is the most favored structure. We attribute this result to the effective structural modification of a typical porous Si. Our structure provides spaces (pores) for electrolyte interaction but limits the growth of detrimental SEI. An areal capacity of 6.23 mAh cm^-2 (C/40) for 1000°C-60s has been achieved. Figure 1D presents the post-mortem SEM images of various Si samples tested. It directly confirms the observed good capacity for 1000°C-60s whereas severe fractures can be seen for room temperature (RT) and 800°C samples. This is caused by the repeated volume changes during cycling.

In summary, we successfully demonstrated a novel Si structure with a well-controlled surface porosity via thermal treatment after electrochemical etching process. Generally, our work sheds light on the rational design of Si anodes for practical high-energy lithium-ion batteries

REFERENCES

1. Jin, Y., Zhu, B., Lu, Z., Liu, N. & Zhu, J. Challenges and recent progress in the development of Si anodes for lithium-ion battery. Adv. Energy Mater. 7, 1–17 (2017).
2. Yang, Y. et al. New nanostructured Li2S/Silicon rechargeable battery with high specific energy. Nano Lett. 10, 1486–1491 (2010).
3. Ashuri, M., He, Q. & Shaw, L. L. Silicon as a potential anode material for Li-ion batteries: Where size, geometry and structure matter. Nanoscale 8, 74–103 (2016).
4. Xie, L, Liu, H., Lin, S., Yang, X., Qi, M., Zhu, L., Guo. Y. & Guo, G., Modified SiO hierarchical structure materials with improved initial coulombic efficiency for advanced lithium-ion battery anodes, RSC Adv. 9, 11369 (2019)