Sodium-ion batteries store energy in very much the same way as their lithium-ion counterparts. They consist of two electrodes – an anode and cathode – separated by an electrolyte. When the battery is being charged with electrical energy, metal ions move from the cathode through the electrolyte to the anode, where they are absorbed. Sodium-ion batteries could be better than lithium-ion ones though because sodium is a common material in the Earth’s crust, which means that batteries made from sodium would be cheaper. Sodium also has a favourable redox potential (around just 0.3V above that of lithium).

For sodium-ion batteries to become truly competitive, however, they need to be able to store more than 200 Wh/kg and have a power density of greater than 2000 W/kg. They also need to be able to operate over more than 4000 cycles without any degradation in their performance. The problem is that the sodium-ion batteries made so far suffer from a relatively low working potential and large capacity decay during cycling, which ultimately leads to a limited battery life.

2D energy materials have already shown promise as cathodes for lithium-ion batteries thanks to their promising electrochemical characteristics, which include high redox activity and rate capability. However, many of these materials suffer from layer “self-restacking”, which reduces the number of active surfaces available. This leads to sluggish charge and mass transport – especially when it comes to ions like sodium, which have a considerably larger radius than lithium ions.

Much improved sodium-ion transport kinetics

A team led by Guihua Yu of the University of Texas at Austin has now shown that vanadyl phosphate (VOPO4) nanosheets intercalated with organic molecules, such as triethylene glycol (TEG) and tetrahydrofuran, could come into their own here. Thanks to X-ray diffraction (XRD) and cross-sectional high-resolution transmission electron microscopy (HR-TEM) measurements in collobration with researchers at Yale University, the researchers were able to calculate that the intercalants help increase the interlayer distances in the VOPO4 nanosheets by between 20 to 43%.

“Thanks to the expanded interlayer spacing, the interlayer-engineered VOPO4 nanosheets show much improved sodium-ion transport kinetics and much improved rate capability and cycling stability for sodium-ion storage, compared with the pure VOPO4 nanosheets without any organic molecule intercalation,” says Yu. “Indeed, our DFT calculations (performed with colleagues at Nanjing Normal University) show that the energy barrier of sodium-ion transport in TEG intercalated VOPO4 nanosheets, for example, is 0.22 eV, which is much lower than that of the pure VOPO4 nanosheets (0.72 eV).

General interlayer engineering strategy

“The expanded interlayer distance in fact activates more interlayer surfaces for sodium-ion transport and storage, and it is this that contributes to much decreased energy barriers for sodium-ion transport though the VOPO4 layers,” he explains.

The general interlayer engineering strategy reported in this work, which is detailed in Nano Letters DOI: 10.1021/acs.nanolett.7b02958, could be extended to ions other than sodium, such as potassium, magnesium and zinc, he tells And of course, different organic molecules, such as amines, might be used as the intercalants for the VOPO4.