Battery Technology New Anode Design for Sodium-Ion Batteries Reduces Storage Losses

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Sodium-ion batteries have several advantages—but high storage losses during the first charge cycle. A new anode design aims to change that.

Sodium-ion batteries are considered the holy grail of battery technology, as they offer many advantages compared to their lithium-ion counterparts. However, they are also associated with some disadvantages. One of these is the lower energy density compared to lithium batteries. Sodium batteries currently operate in a range of 34–75 watt-hours per pound, while lithium batteries range from 54–73 Wh per pound. This means sodium-ion batteries require more space and weigh more to store the same amount of energy.

In addition, the irreversible loss of storage capacity during the first charge is a significant drawback: This occurs during the manufacturing of the battery through a chemical reaction between the anode and the electrolyte. Electrolyte molecules decompose at the anode made of hard carbon and penetrate its pores.

They occupy "voids" that are actually intended for the storage of sodium ions. Only once a stable protective film has formed on the anode does this process come to a halt. The film protects the anode from further decomposition by the electrolyte but consumes part of the storable energy because it partly consists of sodium ions itself. In doing so, it binds those charge carriers responsible for charge transport within the battery.

Which Anode Material is Suitable for Sodium-Ion Batteries?

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

To address this, the Federal Institute for Materials Research and Testing (BAM) developed an innovative core-shell design for the anode. "We recognized that large storage capacities and efficient film formation in sodium-ion batteries cannot be achieved with a single material," explains Tim-Patrick Fellinger, BAM expert for energy materials, emphasizing: "The better a material is suited for storage, the more loss-prone the film formation process is."

Porous Hard Carbon As Storage Material

The researchers developed a process in which a porous, sponge-like hard carbon storage material at the core of the anode is coated with an ultrathin layer that acts as a filter. This layer allows the desired sodium ions to pass through while blocking disruptive electrolyte molecules. This preserves the anode's storage capacity and enables the battery to maintain its performance over many charging cycles. The layer is based on activated carbon, an affordable and environmentally friendly material.

Significantly Higher Initial Efficiency

The materials developed in the study already achieve an initial efficiency of 82 percent—compared to just 18 percent without the coating. Further progress is considered likely by researchers at BAM. "Separating the formation process allows for simultaneous improvement of efficiency and storage capacity through separate material developments. Until now, advances in batteries have primarily been achieved through material innovations on the cathode side."

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Here we are close to theoretical limits. For anode materials, however, it is still completely uncertain where these limits lie and what innovative approaches in material development can achieve further progress," says Paul Appel from the team.

Further Development Planned at the Berlin Battery Lab

The further development of the anode material is set to take place at the Berlin Battery Lab, a collaboration between BAM, the Helmholtz Center Berlin, and Humboldt University of Berlin, Germany. The Berlin Battery Lab combines the expertise of all three research institutions on sustainable battery technologies—and provides industry with a platform to accelerate the transition of new developments into market-ready products. (se)