This is a battery: two electrodes with different polarities and between them, an electrolyte, which enables the transfer of ions (and disables the electronic conductivity) between the electrodes and thus the charging and discharging of the battery. In most batteries, the electrolyte is a flammable liquid. So-called solid-state batteries use a solid substance as an electrolyte instead. This not only makes them safer, but the solid electrolyte also allows the use of alternative materials for the electrodes, such as pure lithium metal for the anode. As a result, solid-state batteries can potentially achieve much higher energy densities, i.e., store more electricity per volume – an advantage for a wide range of applications, from electric cars to portable electronics.

However, as is often the case, this promising technology still has a few “teething problems” that pose challenges for research and industry. Empa researchers from the Laboratory for Functional Polymers are working on a novel electrolyte that could remedy several issues at once. Unlike most electrolytes for solid-state batteries, which are made of rigid materials, their solid electrolyte is soft and stretchable.

Silicone-based ion conductor

This innovation is the result of some clever chemistry. The starting polymer for the electrolyte is a polysiloxane, better known as silicone. This elastic compound has one major disadvantage for battery research: It is nonpolar and thus unable to dissolve the charged particles, the ions. The researchers led by Dorina Opris have succeeded in adding functional groups to the “backbone” of the polymer, making it a good ion conductor while retaining its advantageous elastic properties.

Elasticity is a key strength of the polymer electrolyte. Today's lithium-ion batteries use an anode based on lithium salts. Using pure lithium metal as the anode material instead could potentially achieve higher energy densities. When the battery is discharged, lithium ions “migrate” away from the metallic anode; when it is charged, they return. However, they do not deposit themselves in an even layer on the surface of the anode, but form so-called dendrites: tree-like lithium structures that can “grow” to the cathode within a few charging cycles and cause a short circuit.

The use of a solid electrolyte hinders dendrite growth. However, when ions move away from the anode, they leave behind empty spaces – voids – which can cause the anode to lose contact with the electrolyte and reduce the battery's capacity. This is where the elastic electrolyte developed by Empa researchers kills two birds with one stone: It is solid enough to prevent dendrites, but elastic enough to fill the voids and compensate for the volume changes in the anode during charging and discharging.

Towards flexible batteries

With the appropriate electrode materials, the electrolyte could also be used to manufacture flexible batteries. “Today's batteries for medical implants, such as pacemakers, are usually hard and uncomfortable for patients,” explains Dorina Opris. “Our polymer can serve not only as an electrolyte, but also as a binder material for the cathode.” Empa researcher Can Zimmerli adds: “The flexible polymer can be combined with different cathode active materials, enabling batteries for various applications.”

Flexibility and safety are not the only advantages of the innovative electrolyte. “The material can be processed into thin films of a few micrometers in thickness, and it is scalable”, says Opris. “If produced on an industrial scale, it is also cheaper than conventional solid polymer electrolytes.” The researchers are now working on further improving the ionic conductivity of the silicone electrolyte – and at the same time looking for a suitable industrial partner to begin commercializing the technology.