Sodium-ion batteries are considered a promising, sustainable alternative to lithium-ion batteries. However, high storage losses during the first charging cycle have slowed down their development so far. Researchers at the Federal Institute for Materials Research and Testing (BAM) have now developed a design for anodes that combines efficiency and high storage capacity.
An outer protective layer shields the porous core of the carbon anode from unwanted deposits, thereby increasing the battery's performance. Image Credit: BAM
The irreversible loss of storage capacity during the first charge - while the battery is still being manufactured - is caused by a chemical reaction between the anode and the electrolyte, the conductive liquid in the battery. During this process, electrolyte molecules decompose at 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 halt once a stable protective film has formed on the anode.
The film protects the anode from further decomposition by the electrolyte but consumes some 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.
New Anode Material Required
This problem hardly ever occurs in lithium-ion batteries because the protective layer forms more easily on their dense graphite anodes, meaning that the battery's efficiency is usually over 90 percent. However, sodium cannot be stored in graphite. This type of battery therefore generally requires a different anode material, and so-called hard carbons have proven to be the best choice here – except for the aforementioned disadvantages during the first charging process.
Innovative Core-Shell Design
To solve this problem, the BAM team developed an innovative core-shell design for the anode. “We realized that large storage capacities and efficient film formation cannot be achieved with sodium-ion batteries using a single material,” explains Tim-Patrick Fellinger, BAM expert for energy materials. “This is due to the fact that materials better suited for storage are more prone to losses during film formation.”
The researchers developed a process in which they coat a porous, sponge-like hard carbon as storage material in the core of the anode with a very thin layer that acts like a filter: it allows the desired sodium ions to pass through but keeps disruptive electrolyte molecules out. This preserves the storage capacity of the anode and allows the battery to maintain its performance over many charging cycles. The customized material is based on activated carbon, an inexpensive and environmentally friendly material, which also makes the technology economically attractive. The results have now been published in the journal Angewandte Chemie.
Simultaneous Improvement of Efficiency and Storage Capacity
The materials developed in the study already achieve an initial efficiency of 82 percent – without coating, it is 18 percent. The BAM team is considering further progress likely. "The separation of ‘formation’, the technical term for film formation, and storage allows for the simultaneous improvement of efficiency and storage capacity through separate material developments. Until now, advances in batteries have 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 innovations in material development – keyword: Advanced Materials – can be used to achieve further progress," says Paul Appel from the team.
Further Development at the Berlin Battery Lab
The anode material will be further developed at the Berlin Battery Lab (BBL), a collaboration between BAM, the Helmholtz Zentrum Berlin, and Humboldt Universität Berlin.
The Berlin Battery Lab brings together the cutting-edge expertise of all three research institutions in the field of sustainable battery technologies and offers industry a unique platform that accelerates the translation of innovations into market-ready products.
The “DialySorb” and “NMR-Batt” projects, in which the new anode was developed, are funded by the German Federal Ministry of Research, Technology, and Space.