A step forward into understanding sodium (Na) ion full batteries was taken by researchers who identified nanoarchitectured electrode materials suitable for high performance batteries. The team of scientists, led by Guihua Yu in the University of Texas at Austin, designed a sodium-ion full battery with two different electrodes based on Na₂Ti₃O₇ nanotubes and VOPO₄ nanosheets. The measured battery characteristics make the proposed materials excellent candidates for future large-scale energy storage devices according to their report published in the journal Energy & Environmental Science.

Sodium-ion batteries (SIBs) are a potentially low-cost and safe alternative to the prevailing lithium-ion battery (LIB) energy storage systems. LIBs together with renewable resources currently offer a potential exodus to the dead ends of the fossil fuel dependent energy economy. But lithium is a relatively scarce element and its price is constantly rising. On the other hand, the abundance of sodium resources (practically inexhaustible and evenly distributed around the world), and the fact that the ion insertion chemistry of sodium is largely identical to that of lithium, has created great interest in SIBs.

When working with SIBs, the challenges according to Yu are mostly related to size. The sodium ion’s larger radius is related to shorter battery life cycles, because their larger size can cause instability to the guest-host intercalation compound that is formed once the sodium-ions (guests) are inserted into a host-layered material. Additionally, sodium atoms have a smaller reduction potential when compared to lithium, which combined with its larger atomic weight results in lower gravimetric and volumetric energy densities—that is lower battery capacity in weight (Watt-hours/kg) and volume (Wh/l), respectively. Furthermore, “[m]uch of the [current] research is mainly focused on one electrode, either the cathode or the anode. In this work, we investigate a full battery,” says Yu.

The researchers chose two-dimensional ultrathin VOPO₄ nanosheets as the cathode material and layered Na₂Ti₃O₇ nanotubes for the anode. The nanosheets were produced through the inexpensive and up-scalable technique of liquid phase exfoliation of VOPO₄ ·2H2O bulk chunks in 2-propanol. The nanotubes were prepared through a hydrothermal method under alkaline conditions, using a titanium powder as the precursor.

Yu says that Na₂Ti₃O₇ can be an alternative for hard carbon, which is now mainly used as the anode material “but suffers from inferior rate capability; and its voltage plateau, related to most capacities, is too close to the sodium plating voltage, causing a serious safety concern.”

The reported sodium-ion full batteries exhibited specific energy density of 220 Wh/kg, which is higher than other sodium-ion full cell configurations recently reported in the literature and do not exceed 180 Wh/kg. This value makes the battery comparable to Li-ion cells, with an energy density range between 100 and 265 Wh/kg. Another characteristic of the SIBS Yu’s team created was the low average voltage (ca. 0.55 V) of sodium ion insertion/extraction, which Yu describes as “safe,” whereas “the voltage plateau related to most capacities is too close to the sodium plating voltage, causing a serious safety concern,” he says.

The combination of VOPO₄ and Na₂Ti₃O₇ led to excellent rate capability (around 74 mA·h/g at 2C rate) and cycling performance (92.4% capacity retention after 100 cycles), with a large reversible capacity of 114 mA·h/g at a rate of 0.1. The battery also showed good performance over a broad temperature range.

David Mitlin of Clarkson University in New York, whose team also works on materials for sodium-ion batteries, says that Yu’s team has achieved the remarkable feat of creating a practical and low-cost SIB which stands to compete with commercial LIBs in a range of important performance characteristics.  

Liangbing Hu of the University of Maryland, whose team specializes in energy devices, including Li-ion and Na-ion batteries, says, “This new sodium ion battery technology is particularly promising for large-scale, low-cost grid-scale energy storage to enable a range of renewable energy systems such as solar and wind.”

Yu and his team are now focusing their efforts on new promising electrode materials with rationally designed structures. They are also looking for ways to replace the high volatile and flammable organic electrolytes with solid-state or quasi-solid-state electrolytes. “Our goal is to build safer SIBs with much longer cycle life—like the ones required for stationary and grid-scale storage systems—and even higher energy and power densities.”

According to Mitlin, “While the SIB materials per se have undergone a major renaissance in the last several years, integrated devices have received much less attention, so this work should inspire a new generation of sodium based energy storage technologies for stationary, municipal and perhaps even automotive applications.”

Read the abstract in Energy & Environmental Science.