Polymer Polymer Material Enables Better Solid-State Batteries

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Empa researchers have developed a solid electrolyte based on a stretchable polymer. The scalable material could enable better solid-state batteries and be used in flexible rechargeable batteries for medical applications, for example.

Two differently polarized electrodes and an electrolyte between them that enables the charged ions to be transported between the electrodes and blocks electron conductivity—thus enabling the battery to be charged and discharged: this is a battery. In most batteries, the electrolyte is a flammable liquid. So-called solid-state batteries use a solid as the electrolyte instead. This not only makes them safer: 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, but this technology still has some "teething troubles" that pose challenges for research and industry.

Silicone-Based Ionic Conductor

Empa researchers from the Laboratory for Functional Polymers are working on a new type of electrolyte that could provide a remedy in several respects. Whereas most electrolytes for solid-state batteries are made of rigid materials, their solid electrolyte is soft and ductile. The base polymer for the electrolyte is a polysiloxane, more commonly known as silicone. The elastic plastic has one major disadvantage for battery research: it is apolar. The charged particles, the ions, do not dissolve in it at all. The researchers working with Dorina Opris have succeeded in providing the "backbone" of the polymer with functional groups that make it a good ion conductor—while retaining its advantageous elastic properties, as elasticity is a major strength of the polymer electrolyte. 
Today's lithium-ion batteries use an anode based on lithium salts. Higher energy densities could be achieved with pure lithium metal as the anode material. When the battery is discharged, lithium ions "migrate" from the anode and return when the battery is charged. However, they are not deposited in an even layer on the surface of the anode, but form so-called dendrites: tree-like structures of lithium that "grow" to the cathode within a few charging cycles and thus cause a short circuit.the use of a solid electrolyte counteracts dendrite growth. However, if the ions "migrate" from the anode, cavities are created which can lead to the anode losing contact with the electrolyte and the capacity of the battery decreasing. This is where the elastic electrolyte developed by the Empa researchers kills two birds with one stone: it is firm enough to prevent dendrites, but elastic enough to fill the resulting cavities and thus compensate for the changes in volume of the anode during charging and discharging.

Cheaper than Conventional Solid Polymer Electrolytes

With the right materials for the electrodes, the electrolyte could also be used in 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 binding material for the cathode." Empa researcher Can Zimmerli adds: "The flexible polymer can be combined with different active materials for the cathode, which enables batteries for a wide range of applications. "Flexibility and safety are not the only advantages of the innovative electrolyte. "The material can be processed into thin layers of just a few micrometers and is scalable," says Opris. "If it is 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 electrolyteand—are also looking for a suitable industrial partner to start commercializing the technology.

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