Since 1991, when Sony made history by placing its lithium-ion batteries on the market, these batteries have become ubiquitous – they are now found in mobile phones, tablets, laptops, cameras and increasingly in electric cars. If the market for such devices continues to grow as expected, experts fear that lithium, an alkali metal, will soon become a scarce resource. This is why researchers around the world are looking for alternatives. The economic prospects are highly attractive. It is expected that global annual battery sales will reach 80 billion euros by 2020.

Sodium-ion batteries are regarded as the most promising candidates: sodium is ubiquitous; for example in the form of sodium chloride in salt deserts, underground salt domes and the sea. It is much easier to extract sodium than lithium, and hence much cheaper,” says Prof. Dr. Stefano Passerini, who heads up the research group Electrochemistry for Batteries at the Helmholtz Institute Ulm at the Karlsruhe Institute of Technology. Lithium is finely dispersed in the Earth’s crust, which makes its extraction time consuming, expensive and environmentally harmful.

In chemical terms, sodium (Na) is the big brother of lithium (Li). However, the ions cannot just be exchanged. As Na ions are around 25 percent bigger than Li ions, they cannot intercalate into the graphite electrodes commonly used in batteries. New carbon-based materials for the electrodes therefore need to be found.

How apples can be turned into electrodes

Passerini’s team has made an important step towards the development of active materials for environmentally friendly sodium-ion-based energy storage systems. The Ulm scientists have developed two new materials that have the potential to be used as the negative and positive electrodes of batteries in the future.

Rechargeable batteries (also called accumulators) produce power when ions such as lithium or sodium move from one electrode to the other. If an electricity-consuming device (e.g. a mobile phone) is connected to the battery, the latter discharges and chemical energy is converted into electrical energy. The ions move from the negative to the positive electrode. When the battery is charging, the process is reversed and electrical energy is converted into chemical energy. The ions move back to the negative electrode.

The negative electrode consists of a carbon-based material that can be produced from unused and windfall apples. The apples are dried, treated with acid and then heated, resulting in amorphic carbon. The carbon-based material possesses excellent electrochemical properties. So far, the researchers have demonstrated that the bioelectrodes survive more than a thousand charge and discharge cycles and have a stable performance. Passerini points out that it was a Chinese student who first became aware of the potential of apples. “He wondered why so many apples were still on trees and the ground in autumn.” As all apples that do not comply with EU standards, e.g. if they are too small, are either turned into animal feed or rot on the ground, they are a readily available and inexpensive resource. “Many types of biological waste and renewable resources can be used as electrode material and are thus sustainable,” says Passerini going on to explain that banana peel and peanut shells have already been used for producing battery materials.

The researchers from Ulm have also come up with a new material for the positive electrode, consisting of several layers of sodium oxides. The great advantage of this new active material is that it does not require the expensive and environmentally hazardous element cobalt frequently used in the active materials in commercial lithium-ion batteries.

Research into sodium batteries is not new, but was discontinued in the late 1980s with the emergence of the more powerful Li-ion batteries. Li ions are smaller than Na ions, and thus enable the production of lighter batteries. This was a clear advantage in these times of electromobility. In addition, the specific energy of Li batteries, i.e. the amount of energy that can be stored per gram battery, is higher.

Due to their larger size, it is harder for Na ions to intercalate into the crystalline structure of the electrodes, resulting in slower charging and discharging processes than those achieved with Li ions. Na-ion batteries will therefore be bigger and not quite as powerful as modern Li-ion batteries. However, battery size is not an issue in stationary solar and wind power plants. In such cases, the batteries would be used for the mass storage of intermittent energy sources, energy produced during hours of sunlight and in wind-rich periods, and deliver it when needed. Experts expect that the demand for such batteries will grow as society continues to shift from fossil fuel to alternative energies.

“Sodium batteries will complement rather than replace lithium batteries,” says Passerini. However, it will be several years before the new sustainable batteries are placed on the market. The researchers are also currently testing other materials. For Passerini this presents no difficulties as a so-called “drop in” technology is used, i. e. a technology that can be used without any major investment/alteration.

The Ulm researchers are not the only ones working on the development of a new generation of batteries: in November 2015, French researchers from the CNRS (Centre National de la Recherche Scientifique) presented the first prototype of a sodium-ion battery, Toyota are working on a sodium battery for electric cars, and a British battery business has come up with the world’s first sodium-ion powered e-bike.

References:

Keller M., Buchholz D., Passerini S. (2016). Layered Na-Ion Cathodes with Outstanding Performance Resulting from the Synergetic Effect of Mixed P- and O-Type Phases. Adv. Energy Mater., 6: 1501555. doi: 10.1002/aenm.201501555

L. Wu, D. Buchholz, C. Vaalma, G. A. Giffin, S. Passerini, Apple Biowaste-Derived Hard Carbon as a Powerful Anode Material for Na-Ion Batterie, ChemElectroChem 2016, 3, 292