Sodium-ion batteries are similar to their lithium-ion cousins since they store energy in the same way. They consist of two electrodes – 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 into the bulk of the anode material. Sodium-based devices are in principle more attractive though since sodium is highly abundant on Earth (its Clarke’s number is 2.64) and is therefore much cheaper than lithium. Sodium is also more environmentally friendly than lithium.

However, the radius of the sodium ion is significantly larger than that of the lithium ion. This makes it difficult to find a host electrolyte material that allows ions to be rapidly absorbed and removed. What is more, sodium-ion batteries made thus far suffer from a relatively low working potential, large capacity decay during cycling (which leads to a limited battery life) and poor safety.

Faster battery charge/discharge

Now, researchers led by Guihua Yu in Texas have designed a new sodium-ion battery electrode nanomaterial in which the diffusion distance of sodium ions is much shortened. This leads to improved rate performance (faster battery charge/discharge). The nanostructured electrode also better withstands structural damage cased by sodium insertion and extraction.

“For our designed sodium-ion batteries, both the Na2Ti2O7 anode and the VOPO4 cathode can store sodium ions,” explains Yu. “The layered crystalline structure of the electrodes also makes them promising materials for fast transport of Na ions. Indeed, such a battery has good electrochemical characteristics, comparable with those of lithium-ion batteries and, what is more, it operates between –20 and +55°C.”

High energy density

The researchers made their battery using layered Na2Ti2O7 and VOPO4 as anode and cathode, Whatman glassy fibre as the separator and 1 M NaClO4 dissolved in propylene carbonate with 2% fluoroethylene carbonate additive as the electrolyte. “During the charging process, sodium ions extract from the VOPO4 cathode move through the electrolyte and insert in the Na2Ti2O7 anode,” explains Yu. “These reactions reverse during the discharging process.”

The battery boasts one of the highest operating voltages (of 3V) and delivers a large reversible capacity of 114 mA h/g, he tells nanotechweb.org. It also retains 92.4% of its capacity after 100 cycles. A high energy density of 220 W h/kg makes it competitive with state-of-the-art lithium ion batteries.

The batteries are also flexible

The device could be used in most applications in which lithium-ion batteries are used today, and potentially extended to large-scale stationary storage systems, says Yu. And as we demonstrate in our paper, published in Energy & Environmental Science DOI: 10.1039/C6EE00794E, the batteries are flexible and perform well even when bent, folded or rolled. They could be used to power flexible/wearable electronics too.

The researchers are now working on increasing the life cycle of its battery as well as understanding more about capacity decay mechanism in the system. “We are also in process of designing even more promising electrode materials for higher energy/power densities,” says Yu.