Understanding and Mitigating Silicon Anode Swelling in Modern Batteries

By Ibtisam AbbasiReviewed by Lexie CornerDec 13 2024

The growing demand for energy has driven significant progress in energy storage systems, with a particular focus on improving the energy density of lithium-ion batteries (LIBs). In an effort to create more efficient LIBs, researchers have explored using silicon as an anode material to replace traditional electrodes made from materials like graphene.¹

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Despite its advantages, the use of lithium anodes presents challenges, particularly with volumetric changes that result in swelling. This swelling can lead to mechanical failures. Additionally, silicon anodes in LIBs have shown signs of electrical isolation after continuous charging and discharging, which reduces the battery's overall lifespan.²

What Causes Silicon Anode Swelling?

When a silicon anode reacts with lithium in a battery, it undergoes significant volumetric expansion. The electrochemical reaction between these two materials leads to the formation of several silicon-lithium phases, such as LiSi, Li₁₂Si₇, and Li₂₂Si₅.

Researchers have also observed the development of an amorphous Li_xSi compound-based shell during the lithiation process. As this shell forms, the expansion becomes more pronounced, with silicon nanowires—key components of the silicon anode—experiencing a volume increase of about 280 %.

This expansion disturbs the structural morphology of silicon, causing stress on the surface of the silicon particles. This process leads to harmful swelling of the anode and contributes to the pulverization of silicon.

Swelling is a significant issue as it causes peeling and cracking at the anode surface. The damage to the anode severely impacts the energy density of the battery, weakening the electrical connection between the anode and lithium. Additionally, swollen cells pose a major safety risk, which limits the potential for using silicon as an anode material in LIBs.³

Silicon Swelling: Effects and Challenges

The charging and discharging of LIBs involve lithiation and de-lithiation processes, which lead to volume expansion. This expansion significantly increases stress on the anode, leading to the release of silicon powder and the peeling of conductive material from the active material on the anode surface. As a result, the electrochemical performance of the battery declines.

During the first lithiation, a solid electrolyte interface (SEI) forms, allowing lithium-ion conduction while preventing electron flow. This helps improve the cyclic stability of LIBs. However, due to the volume expansion, the SEI becomes unstable. It decomposes and reforms on the silicon electrode surface, consuming a large amount of lithium ions in the process. The rapid depletion of lithium ions causes a significant drop in the initial Coulombic efficiency (ICE) and accelerates electrolyte depletion.⁴

In this way, swelling of the silicon anode contributes to a reduced battery lifespan. Increased mechanical stress on the anode surface, a decline in cyclic stability, and loss of electrical contact all hinder the long-term performance and commercialization potential of silicon-based anodes in LIBs.

Understanding the internal changes caused by swelling is essential to improving battery performance. Advanced imaging techniques are key to this process. As Tim Bradow, Senior Business Development Manager at Rigaku Americas, explains:

(CT imaging) allows us to examine the inner structure of a battery cell, detecting voids, swelling, cracks, and other defects in a non-destructive way. Once we finish evaluating the cell, it remains physically untouched and can be seamlessly returned to the production line, ready for sale or for further integration into a battery pack.

This capability enhances quality control and provides valuable insights into the structural issues caused by silicon swelling, ultimately helping to guide improvements in battery design.

Effective Strategies for Mitigating Silicon Anode Swelling

Material Design

Researchers have explored several strategies to address the issue of silicon anode swelling and volumetric changes. One of the most effective methods is the use of nanotechnology to reduce the impact of these changes. By shrinking silicon particles to sizes smaller than 150 nm, the contact area is increased, which helps improve the cycling performance of the anode.

Nanostructured silicon also helps minimize the mechanical stress caused by the abrupt volumetric changes that occur during the charging and discharging cycles. Zero-dimensional (0D) silicon nanoparticles, for example, show superior capacity retention and create a more stable material surface, reducing the likelihood of cracking.

Similarly, one-dimensional (1D) silicon nanoparticles have been shown to reduce fracture tendencies, while two-dimensional (2D) silicon nanosheets effectively minimize pulverization. This is because the volumetric change rate in 2D nanostructures is the lowest, making them especially effective in maintaining structural integrity.⁵

Industry experts highlight the crucial role that silicon anodes play in recent advancements in lithium-ion battery technology. As Francis Wang, CEO of NanoGraf Technologies, explains:

Anode and cathode technologies have historically been the performance drivers in lithium-ion technology. Silicon anodes have reached commercial adoption and are driving the performance improvements seen over the last few years.

This progress emphasizes the continued importance of innovation in anode materials to meet the increasing demand for higher performance and energy density.

Carbon-based materials are less susceptible to volumetric changes during the charging and discharging cycles. Silicon and carbon are highly compatible, and when combined in composite materials for the anode of LIBs, they help improve conductivity and optimize energy storage.

Currently, silicon-carbon composite materials often use silicon powder and SiO₂ as primary raw materials. Organic polymers, such as polyvinyl alcohol, are typically used as carbon sources. This combination of materials not only increases energy density but also enhances cyclic stability, making the batteries more reliable over time.⁶

Binder and Electrode Engineering

Binders, while not present in large quantities, are crucial for the performance of LIB anodes. Traditionally, polyvinylidene difluoride (PVdF) has been used as the binder, but new multifunctional binders are being developed to address key challenges, such as volume changes and SEI formation.

One promising development is the use of partially lithiated Nafion (P-LiNF) binders in electrodes. These binders have shown stable performance over more than 100 cycles. They offer excellent mechanical properties, strong adhesion, and the ability to accommodate significant volume changes during cycling.

Imaging studies of the anodes have revealed crack-free surfaces even after 50 or more charge-discharge cycles.⁷ In fact, these novel conductive binders have proven effective in addressing swelling issues associated with silicon anodes.

Electrolyte Optimization

Electrolytes heavily influence the stability, safety, electrochemical performance, and volumetric alterations of LIBs. To improve efficiency and address the large volume changes of silicon anodes, researchers have developed new electrolytes and additives.

One such innovation is a non-flammable, ether-based electrolyte that includes a fluoroethylene carbonate additive. This electrolyte promotes the formation of a high-modulus SEI and achieves a coulombic efficiency of 91 %. It also helps manage swelling and volumetric changes, with a degradation rate of only about 0.06 % per cycle. This allows the electrolyte to support significant volumetric changes while extending the battery’s lifespan by 150 to 200 cycles.⁸

Overall, developing efficient electrolytes with the right additives offers a scalable solution to the challenges associated with silicon anodes in modern batteries and energy storage systems.

Advanced Coatings

Coatings are an effective solution for addressing issues related to electrochemical stability and significant volume expansion. Researchers have been experimenting with various coating techniques, focusing on optimizing the thickness of the layers to improve performance while minimizing swelling in the anodes.

One such innovation is the use of layered conductive polyaniline (LCP) coatings on silicon nanoparticles in LIBs. This coating helps achieve higher electrochemical performance and reduces the rapid volume changes that can shorten the lifespan of modern batteries. The LCP coating is particularly effective in enhancing the silicon anode’s performance by storing electrolytes within its interlaminated spaces.

This design increases the flexibility of the anode, helping to manage volume changes during the charge-discharge cycles while ensuring consistent ion transport. Experimental results showed that the silicon anode maintained excellent cycling stability, even under high areal capacity conditions, with good performance sustained over 150 cycles.⁹

Carbon-based coatings have been widely used in recent years to improve silicon anode performance and tensile properties. Studies have explored the impact of varying the number of coating layers, and findings suggest that 2 to 4 layers of carbon coating provide an effective, cost-efficient way to manage the dramatic volumetric expansion of silicon anodes.¹⁰

Ensuring the Future of Silicon-Based Energy Storage

The growing demand for efficient portable electronics and electric vehicles has made reliable energy systems more essential than ever. Silicon-based LIBs play a central role in modern energy storage, and significant progress has been made to address the challenges of swelling and volume changes, which could hinder their commercialization.

Many of the issues with modern battery electrodes have been largely resolved through the innovative use of nanotechnology, protective coatings, and carefully designed electrolytes. These advancements are key to ensuring the continued growth and success of this multi-billion-dollar industry in the future.

Decoding the Electrode Swelling for Advanced Battery Diagnostics and ManagementPlay

Further Reading and Further Reading

1. Sung, J., et al. (2021) Subnano-sized silicon anode via crystal growth inhibition mechanism and its application in a prototype battery pack. Nat Energy. https://doi.org/10.1038/s41560-021-00945-z
2. Sung, J., et al. (2022). Highly densified fracture‐free silicon‐based electrode for high energy lithium‐ion batteries. Batteries & Supercaps. Available at: https://doi.org/10.1002/batt.202200136
3. Zhang, C., et al. (2021). Challenges and recent progress on silicon‐based anode materials for next‐generation lithium‐ion batteries. Small Structures. https://doi.org/10.1002/sstr.202100009
4. Kong, X., et al. (2023). Recent Progress in Silicon−Based Materials for Performance−Enhanced Lithium−Ion Batteries. Molecules. Available at: https://doi.org/10.3390/molecules28052079
5. Xu, L. (2023). Analysis of Research Progress in Improving the Performance of Silicon-based Anode Materials for Lithium-ion Batteries: A Combination of Nanostructured Silicon and Silicon-Based Composites. Journal of Physics: Conference Series. IOP Publishing. Available at: https://doi.org/10.1088/1742-6596/2608/1/012001
6. Zhang, X., et al. (2019). Dimensionally designed carbon–silicon hybrids for lithium storage. Advanced Functional Materials. Available at: https://doi.org/10.1002/adfm.201806061
7. Li, Z., et. al. (2020). Silicon anode with high initial coulombic efficiency by modulated trifunctional binder for high‐areal‐capacity lithium‐ion batteries. Advanced Energy Materials. 10 (20). 1903110. Available at: https://doi.org/10.1002/aenm.201903110
8. Cao, Z., et al. (2021). Electrolyte design enabling a high‐safety and high‐performance si anode with a tailored electrode–electrolyte interphase. Advanced Materials. Available at: https://doi.org/10.1002/adma.202103178
9. Pan, S., et al.  (2022). Integrating SEI into layered conductive polymer coatings for ultrastable silicon anodes. Advanced Materials. Available at: https://doi.org/10.1002/adma.202203617
10. Qi, C., et al. (2022). Suitable thickness of carbon coating layers for silicon anode. Carbon. https://doi.org/10.1016/j.carbon.2021.10.062

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