Battery Technology Sodium-Ion Batteries: BAM Develops New Anode Design for More Storage Capacity

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Sodium instead of lithium: Sodium-ion batteries are considered an alternative to lithium-ion batteries thanks to their low-cost, sustainable materials, high level of safety, fast-charging capability and performance in cold conditions. However, high storage losses during the first charging cycle have so far slowed down their development. BAM has now developed a design for the anode that increases the storage capacity.

Sodium-ion batteries are considered the holy grail of battery technology, as they offer many advantages over their lithium-ion counterparts. However, they also have some disadvantages. These include their low energy density compared to lithium batteries. Sodium batteries currently operate in a range of 34–75 Wh/lb, while lithium batteries are 54–73 Wh/lb. Sodium-ion batteries therefore require more space and weigh more to store the same amount of energy. In other words, they have less storage capacity for the same installation space.

In addition, the irreversible loss of storage capacity during the first charge is a significant minus point: a chemical reaction between the anode and the electrolyte occurs during the production of the battery. Electrolyte molecules decompose on the hard carbon anode and penetrate its pores. They occupy "empty spaces" that are actually intended for the storage of sodium ions. This process only comes to a standstill once a stable protective film has formed on the anode.

The film protects the anode from further decomposition by the electrolyte, but consumes part of the storable energy because it itself consists partly of sodium ions. It therefore binds the charge carriers that are responsible for charge transport in the battery.

Which Anode Material is Suitable for Sodium-Ion Batteries?

This problem hardly ever occurs with lithium-ion batteries because the protective layer forms more easily on their dense graphite anodes, meaning that the efficiency of the battery is usually above 90 percent. However, sodium cannot be embedded in graphite. Therefore, a different anode material is generally required for this type of battery and so-called hard carbons have proven to be the best choice here—except for the disadvantages mentioned during the first charging process.

To find a solution, the Federal Institute for Materials Research and Testing (BAM) developed a new type of core-shell design for the anode. "We have recognized that large storage capacities and efficient film formation cannot be achieved with a single material in sodium-ion batteries," explains Tim-Patrick Fellinger, BAM expert for energy materials, and emphasizes: "The more suitable a material is for storage, the more lossy the film formation is."

Porous Hard Carbon

The researchers have developed a process in which they coat a porous, i.e. sponge-like hard carbon as a storage material in the core of the anode with a wafer-thin layer that acts like a filter. It allows the desired sodium ions to pass through, but keeps out interfering electrolyte molecules. This maintains the storage capacity of the anode and the battery can retain its performance over many charging cycles. The tailor-made material is based on activated carbon, an inexpensive and environmentally friendly material.

High Initial Efficiency, More Possible

The materials developed in the study already achieve an initial efficiency of 82 percent—without coating, it is 18 percent. Further progress is considered likely at BAM. "The separation of formation allows the simultaneous improvement of efficiency and storage capacity through separate material developments. So far, progress in batteries has mainly been achieved through material innovations on the cathode side. Here we are close to the theoretical limits. With anode materials, on the other hand, it is still completely uncertain where these limits lie and which innovative approaches in material development can be used to achieve further progress," says Paul Appel from the team.

Further Development Planned at the Berlin Battery Lab

The anode material is to be further developed at the Berlin Battery Lab (BBL), a cooperation between BAM, the Helmholtz-Zentrum Berlin and Humboldt-Universität zu Berlin (Germany). The Berlin Battery Lab pools the expertise of all three research institutions in the field of sustainable battery technologies and offers industry a platform to turn new developments into marketable products more quickly.(se)

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