This Battery Material Literally Breathes While Charging & Could Make Lithium-Ion Batteries Last Longer

Silicon has long been viewed as one of the most promising materials for building the next generation of lithium-ion batteries. It can store far more lithium than the graphite used in most commercial battery anodes, potentially allowing batteries to hold much more energy without dramatically increasing their size.

But silicon has a major weakness: it expands and contracts dramatically every time a battery is charged and discharged. This repeated movement can damage the material, break electrical connections and destabilize the protective layer around the electrode. Eventually, the battery loses capacity and performance.

Now, researchers led by Qiangfeng Xiao have developed a new type of silicon structure designed to solve this problem. Their hierarchically porous silicon nanospheres (hp-SiNSs) contain both a porous outer shell and a hollow interior. Instead of expanding outward like conventional silicon particles, these structures can accommodate lithium by expanding mainly inward.

The result is a silicon anode that combines high capacity, fast performance and improved cycling stability.

Why Silicon Is So Attractive for Batteries

Lithium-ion batteries power everything from smartphones and laptops to electric vehicles. As demand for longer-lasting electronics and electric transportation grows, researchers are looking for electrode materials capable of storing more energy.

Silicon stands out because its theoretical lithium-storage capacity is approximately 4,200 mAh per gram, far higher than that of conventional graphite anodes.

This enormous capacity makes silicon an attractive candidate for high-energy-density batteries.

However, there is a serious trade-off. When silicon absorbs lithium during charging, its volume can increase dramatically. During discharge, the material contracts again.

Repeated expansion and contraction can create mechanical stress inside the particles.

Over many cycles, this can cause silicon particles to:

• crack or pulverize,

• lose contact with the electrical network,

• damage the solid-electrolyte interphase (SEI),

• disrupt pathways for lithium and electrons, and

• rapidly lose their ability to store energy.

This has been one of the biggest barriers preventing silicon from completely replacing graphite in commercial batteries.

Researchers Have Tried Many Solutions

Scientists have already explored numerous ways to control silicon's mechanical instability.

One approach is to make silicon extremely small. Nanoparticles, nanowires and nanotubes can better tolerate mechanical stress than large solid particles.

Another strategy is to create hollow or porous silicon structures. Empty spaces inside the material can provide room for expansion.

Researchers have also combined silicon with carbon. In these silicon-carbon composites, carbon can provide electrical pathways while helping accommodate silicon's volume changes.

Special binders have also been investigated. Materials such as sodium alginate, poly(acrylic acid), conductive polymers and self-healing polymers can help maintain contact between particles as they expand and contract.

Despite these advances, a fundamental problem remains: much of the expansion still occurs outward. This can disturb the electrode surface and contribute to unwanted reactions with the electrolyte.

The new hp-SiNS design takes a different approach.

A Silicon Particle With a Porous Shell and Hollow Core

The researchers developed a chemical transformation technique to produce hierarchically porous silicon nanospheres.

The particles have two important structural features: a mesoporous shell and a hollow core.

The fabrication process begins with submicron silicon dioxide spheres containing a solid core and a mesoporous outer shell.

These spheres are then subjected to magnesiothermal reduction. Because reactions occur at different speeds in different parts of the structure, the porous shell reacts rapidly while the solid core reacts more slowly.

This produces a structure containing porous silicon around a remaining solid silicon dioxide core.

The researchers then remove the remaining silicon dioxide through acid etching.

The final product is the hierarchically porous silicon nanosphere, featuring an interconnected porous shell surrounding an internal empty space.

This size-dependent chemical reaction is important because it allows the researchers to preserve different structural features during the transformation rather than converting the entire particle uniformly.

The Key Discovery: Silicon Can "Breathe" Inward

The most interesting feature of these particles appears during battery operation.

When conventional solid silicon absorbs lithium, it expands outward. This can dramatically increase the particle's external dimensions.

The hp-SiNSs behave differently.

Using in situ transmission electron microscopy (TEM), the researchers observed that the particles undergo reversible inward volume expansion and contraction during lithiation and delithiation.

In simple terms, the silicon is able to accommodate the lithium-induced volume change within its existing structure rather than pushing the outer surface dramatically outward.

This behavior can be thought of as a type of "inward lithium breathing."

During charging, lithium enters the silicon and the material expands toward the internal hollow space. During discharge, it contracts again.

This unusual mechanical response helps protect the particle's exterior.

Why the Structure Matters

The researchers' chemomechanical modelling suggests that both components—the porous shell and the hollow interior—are essential.

The porous shell provides space and flexibility for the silicon to accommodate lithium. Meanwhile, the hollow core provides an additional internal region into which the material can expand.

The mechanics of the structure are also important.

According to the researchers, the lithiated layer is significantly stiffer than the unlithiated porous layer. This difference in mechanical properties helps direct the expansion inward instead of causing substantial outward growth.

This is a crucial distinction from many earlier porous and hollow silicon designs.

Better Protection for the Battery Interface

One of the biggest benefits of minimizing outward expansion is better stability at the particle's surface.

During battery operation, silicon particles interact with the electrolyte and develop a protective layer known as the solid-electrolyte interphase, or SEI.

Large changes in the particle's external dimensions can repeatedly damage this layer. When the SEI breaks, fresh silicon can become exposed to the electrolyte, triggering additional reactions.

These reactions consume battery components and can reduce efficiency and capacity over time.

By keeping outward expansion extremely small, hp-SiNSs can provide a more stable surface environment.

The researchers found improved capacity retention and Coulombic efficiency compared with several other nanostructured silicon materials.

High Capacity, Power and Cycle Life

The structural advantages of hp-SiNSs translate into promising electrochemical performance.

Battery cells using these silicon nanospheres demonstrated high capacity, high power capability and long cycling life.

The porous architecture also supports rapid lithium transport. Lithium ions can move through the porous structure more efficiently, helping the electrode operate at higher rates.

This combination is particularly important for electric vehicles and other applications where batteries need both high energy storage and the ability to deliver power quickly.

A Potential Path Toward Better Silicon Anodes

The work demonstrates that solving silicon's expansion problem does not necessarily require preventing volume change altogether.

Instead, researchers can control where and how the volume change occurs.

That is the central idea behind the hp-SiNS design.

By engineering pores at multiple length scales and adding an internal hollow region, the researchers created a structure capable of accommodating silicon's natural expansion while limiting changes to its external dimensions.

The synthesis approach is also described as low-cost and potentially adaptable to other materials.

The researchers suggest that the same concept could eventually be applied to materials such as silicon carbide (SiC) and silicon nitride (Si₃N₄).

However, challenges remain before such structures can be widely used in commercial batteries. Increasing the electrode's areal loading—the amount of active material packed into a given area—will be important for practical applications. Researchers will also need to optimize electrode manufacturing and electrolytes and demonstrate reliable performance at commercially relevant scales.

Conclusion

Silicon offers one of the most promising routes toward higher-capacity lithium-ion batteries, but its enormous volume changes have prevented widespread adoption as a standalone anode.

The hierarchically porous silicon nanospheres developed by Qiangfeng Xiao and colleagues offer a new way to manage this problem. Their porous shell and hollow core allow silicon to accommodate lithium through reversible inward expansion and contraction, greatly reducing damaging outward movement.

The study shows that carefully controlling the internal architecture of an electrode can be just as important as choosing the material itself.

If these structural concepts can be successfully translated into high-loading, large-scale battery electrodes, they could contribute to the development of higher-energy, faster and longer-lasting lithium-ion batteries for future electronics and electric vehicles.

Reference: Xiao, Q., Gu, M., Yang, H. et al. Inward lithium-ion breathing of hierarchically porous silicon anodes. Nat Commun 6, 8844 (2015). https://doi.org/10.1038/ncomms9844