Silver Treatment Strengthens Solid-State Lithium Metal Batteries

Small imperfections concealed within batteries are silently restricting both their charging speed and safe operation. With rapid charging becoming standard for phones, laptops and electric vehicles, these minute faults are becoming a major concern.

The issue is particularly acute for solid-state lithium metal batteries, which use rigid ceramic layers instead of flammable liquids.

Although this change could deliver safer, higher-capacity cells, the solid layer may fracture under pressure while charging or during production. When a crack develops, lithium can penetrate it and cause abrupt failure.

Engineers at Stanford University have now described a subtle method of reinforcing this susceptible layer before damage can spread.

Their team investigated whether minor alterations to the surface could enable solid batteries to withstand the strain of fast charging while maintaining power flow.

The potential of solid batteries

Most lithium-ion batteries contain a liquid electrolyte-the ion-conducting fluid separating their two electrodes-and this solvent may catch fire when damaged.

Solid alternatives replace the liquid with a ceramic or glass-like layer that permits ion movement while preventing leaks.

A 2022 review found that hazardous heat and gas may still be produced when lithium grows uncontrollably within solid batteries.

This danger has kept researchers focused on the rigid central layer, which needs to resist both chemical reactions and mechanical stress.

Reinforcing LLZO with silver beneath the surface

The research focused on LLZO, a hard ceramic already used in experimental solid-state batteries to transport lithium without flammable liquids.

The researchers applied an exceptionally thin silver layer to its surface, then heated it gently so the metal could move just below the outermost layer.

At this limited depth, certain silver atoms replaced lithium atoms instead of creating an independent coating. This small substitution reinforced the surface from within, without increasing its bulk or disrupting lithium and electrical movement.

By slightly compressing the surface, the silver generated consistent internal pressure that makes it more difficult for cracks to begin or propagate.

Under battery-like conditions, the additional toughness also restricted lithium from entering minute defects during charging, reducing the likelihood of small flaws developing into short circuits.

Testing battery cracks under pressure

To assess the material's strength directly, the team pushed a tiny probe into it until it cracked.

The silver-treated surface tolerated almost five times as much force before breaking as the untreated material, demonstrating greater toughness before charging had even started.

This is important during manufacture and handling, when solid battery layers are regularly cut, stacked and compressed.

Greater strength also altered lithium's behaviour during rapid charging. As lithium accumulates rapidly on the anode, it can create needle-like dendrites that force stress into surface cracks and extend them further.

Cracks developed differently on the silver-treated surface, prompting lithium to distribute itself across the surface rather than penetrate inwards.

This broader plating behaviour might provide important extra time during high-current charging, although it requires validation in complete battery cells.

Looking for lower-cost solutions

Silver is useful, but it is not inexpensive, so the researchers examined other metals that could provide comparable surface reinforcement.

Initial laboratory tests indicated that copper might also strengthen LLZO, as its ions can occupy surface sites after heating.

“We decided a protective surface may be more realistic, and just a little bit of silver seems to do a pretty good job,” said study co-author X. Wendy Gu, a professor in the Department of Mechanical Engineering at Stanford University.

That adaptability is significant as demand for lithium batteries continues to grow and supply chains come under greater strain. Eventually, the same surface approach could also support sodium cells, potentially reducing pressure on lithium supply chains.

Moving coatings beyond the laboratory

To date, the silver treatment has been evaluated only on small electrolyte specimens rather than full battery cells containing both electrodes.

Complete cells bring additional potential failures at their interfaces, where inadequate contact can impede ion movement and increase heat.

Nevertheless, the coating did not noticeably alter the movement of either electricity or lithium at the surface, suggesting that improved mechanical strength is the principal reason for the benefit.

Throughout the study, this surface-based treatment transformed a known weak point into a more robust defence against cracking and lithium intrusion.

Should future testing show that the durability persists over numerous charging cycles, the method could be combined with other approaches intended to stabilise solid-state batteries.

The mass-production challenge

The transition from laboratory work to factory production presents a further challenge. During assembly, solid electrolyte sheets are cut, stacked and compressed thousands of times, making tiny imperfections nearly impossible to avoid.

As one researcher observed, manufacturing perfectly defect-free layers at scale would be exceptionally difficult and expensive.

Whether the silver-treated surface can endure these practical conditions will determine whether the technique makes its way into commercial batteries. Manufacturers must also address cost and recycling implications.

Image credit: Chaoyang Zhao