Innovative battery electrode made of tin foam

Metal-based electrodes in lithium-ion batteries promise significantly higher capacities than conventional graphite electrodes. Unfortunately, they degrade due to mechanical stress during charging and discharging cycles. Now a team at the HZB has shown that a highly porous foam made of tin can absorb the mechanical stress during charging cycles much better. This makes tin foams interesting as a potential material for lithium batteries.

Modern lithium-ion batteries generally use a multilayer graphite electrode, while the counter electrode is often made of cobalt oxide. When charging and discharging, lithium ions migrate into the graphite without causing significant volume changes in the material. However, the capacity of graphite is limited, which makes the search for alternative materials an exciting area of ​​research. Metal-based electrodes, for example made of aluminum or tin, potentially offer a higher capacity. However, they tend to expand significantly in volume when lithium is absorbed, which is associated with structural changes and material fatigue.

One option for creating metal electrodes that "fatigue" less quickly is to nanostructure the thin metal foils. Another option is to use porous metal foams. Tin is a particularly attractive metal because it has a capacity per kilogram that is almost three times higher than graphite and is not a rare raw material but is available in abundance.

A research team from the Helmholtz-Zentrum Berlin (HZB) has now investigated various types of tin electrodes during the discharge and charging process using operando X-ray imaging and developed an innovative approach to address this problem. Some of these investigations took place at the BAMline at BESSY II. In addition, high-resolution radioscopic X-ray images were created in collaboration with the imaging experts Dr. Nikolai Kardjilov and Dr. André Hilger at the HZB. "In this way, we were able to follow the structural changes in the Sn metal-based electrodes examined during the charging/discharging processes," says Dr. Bouchra Bouabadi, who carried out the experimental study. In collaboration with battery expert Dr. Sebastian Risse, she shows how the morphology of the tin electrodes changes during operation due to the inhomogeneous uptake of lithium ions.

The best version of the tin electrode was made by Dr. Francisco Garcia-Moreno: a foam made of tin with countless micrometer-sized pores. "We were able to show that in such a tin foam, significantly less mechanical stress occurs during volume expansion," says Dr. Risse. This makes tin foams interesting as a potential material for lithium batteries.

Garcia-Moreno has already investigated numerous metal foams, including those for components in the automotive industry and aluminum foams for battery electrodes. "The tin foams we developed at TU Berlin are highly porous and an interesting alternative to traditional electrode materials," he says. The structuring of tin foams is crucial to reduce mechanical stress as much as possible. Tin foam technology could also be interesting from an economic point of view: "Although tin foam is more expensive than conventional tin foils, it offers a more cost-effective alternative to expensive nanostructuring, while at the same time being able to store significantly more lithium ions, thus enabling an increase in capacity."