Researchers at Chalmers University of Technology, Sweden, have developed a nanometric graphite-like anode for sodium ion (Na⁺ storage), formed by stacked graphene sheets functionalized only on one side, termed Janus graphene. The estimated sodium storage up to C_6.9Na is comparable to graphite for standard lithium ion batteries.

The approach, reported in an open-access paper in Science Advances, provides a way to design carbon-based materials for sodium-ion batteries.

Lithium ions can intercalate reversibly in graphite with high Li⁺ loadings, up to C₆Li, yielding a specific capacity of 372 mA h g⁻¹ with the formation of binary graphite intercalation compounds (binary GICs). This nanoscale and reversible process is now at the base of most batteries. Sodium would be a cheaper, more abundant alternative charge carrier compared with lithium for batteries, but unfortunately, it can reach only lower loadings in graphite, with a stoichiometry of C₆₄Na and a corresponding capacity of ~35 mA h g⁻¹.

The poor Na⁺ intercalation was initially attributed to the small interlayer distance of graphite, blocking intercalation of the large sodium ions. However, this cannot be the only reason, as experimental evidence shows that potassium ions or solvated Na⁺ (e.g., diethylene glycol dimethyl ether solvated Na⁺) that are larger than Na⁺ can reversibly intercalate in graphite to form binary or ternary GIC, respectively. In contrast with intercalation of bare sodium ions, the co-intercalation of sodium ions with solvent molecules would result, however, in huge volume changes of graphite electrodes and lower specific capacity (ca. 100 to 150 mA h g⁻¹) because the intercalated solvent molecules occupy the space between the graphene layers. The development of novel sodium ion batteries (SIBs) will thus require the intercalation of the bare Na⁺ into graphite, without any solvent, in analogy with the bare Li⁺ intercalation occurring in current commercial batteries.

… Here, we describe an artificial graphite nanostructure made of stacked graphene sheets, with the upper face of each sheet being functionalized with a molecule acting both as spacer and as an active site for Na⁺. Each molecule in between two stacked graphene sheets is connected by a covalent bond to the lower graphene sheet and interacts through electrostatic interactions with the upper graphene sheet, resulting in a unique structure. The use of asymmetric spacers allows control over noncovalent interactions between the head group of the spacer molecule (in this case, -NH₂) and the graphene surface. Nanosheets having asymmetric chemical functionalization on opposite faces are commonly referred to as “Janus” graphene, named after a two-faced ancient Roman god.

—Sun et al.

Schematic illustration of the preparation of the Janus graphene and the stacked Janus graphene thin film. The material has a unique artificial nanostructure. The upper face of each graphene sheet has a molecule that acts as both spacer and active interaction site for the sodium ions. Each molecule in between two stacked graphene sheets is connected by a covalent bond to the lower graphene sheet and interacts through electrostatic interactions with the upper graphene sheet. The graphene layers also have uniform pore size, controllable functionalization density, and few edges. Sun et al.

Sodium is an abundant low-cost metal, and a main ingredient in seawater. This makes sodium-ion batteries an interesting and sustainable alternative for reducing the need for critical raw materials. However, one major challenge is to increase the capacity of sodium-ion batteries, which, at the current level of performance, sodium-ion batteries cannot compete with lithium-ion cells.

One limiting factor is the graphite—composed of stacked layers of graphene—used as the anode in today’s lithium-ion batteries. Sodium ions are larger than lithium ions and interact differently, and cannot be efficiently stored in the graphite structure as Li ions are. The Chalmers researchers devised a novel way to solve this.

We have added a molecule spacer on one side of the graphene layer. When the layers are stacked together, the molecule creates larger space between graphene sheets and provides an interaction point, which leads to a significantly higher capacity.

—Jinhua Sun, first author

Typically, the capacity of sodium intercalation in standard graphite is about 35 milliampere hours per gram—less than one tenth of the capacity for lithium-ion intercalation in graphite. With the novel graphene the specific capacity for sodium ions is 332 milliampere hours per gram—approaching the value for lithium in graphite. The results also showed full reversibility and high cycling stability.

It was really exciting when we observed the sodium-ion intercalation with such high capacity. The research is still at an early stage, but the results are very promising. This shows that it’s possible to design graphene layers in an ordered structure that suits sodium ions, making it comparable to graphite.

—Professor Aleksandar Matic

The study was initiated by Vincenzo Palermo in his previous role as Vice-Director of the Graphene Flagship, a European Commission-funded project coordinated by Chalmers University of Technology.

Our Janus material is still far from industrial applications, but the new results show that we can engineer the ultrathin graphene sheets—and the tiny space in between them—for high-capacity energy storage. We are very happy to present a concept with cost-efficient, abundant and sustainable metals.

—Vincenzo Palermo

Resources

• Jinhua Sun, Matthew Sadd, Philip Edenborg, Henrik Grönbeck, Peter H. Thiesen, Zhenyuan Xia, Vanesa Quintano, Ren Qiu, Aleksandar Matic, Vincenzo Palermo (2021) “Real-time imaging of Na⁺ reversible intercalation in “Janus” graphene stacks for battery applications” Science Advances doi: 10.1126/sciadv.abf0812