In search of the super battery

Batteries are the key technology for the energy turnaround. The demand for batteries to store  renewable energy will grow strongly in coming years. Lithium-ion batteries are currently the uncontested market leader. However, research for more environmentally friendly materials continues on a global scale. Are there any suitable alternatives? Empa and the ETH Zurich researchers took a closer look – and point out the weaknesses of the new technologies.

The Energy Strategy 2050 foresees a nuclear energy phase out and the expansion of renewable energies from wind and sun. Because these energies are volatile, the need for inexpensive, stationary battery storage will increase strongly in the future. Today, the industry relies primarily on lithium-ion batteries (LIB). However, researchers worldwide are looking for more environmentally friendly alternatives.

The Empa and ETH Zurich researchers Kostiantyn Kravchyk and Maksym Kovalenko took a closer look at possible alternatives to lithium-ion batteries. They analysed dozens of scientific publications by research groups around the world and systematically conducted their own experiments. Their assessments were published in the "New Journal of Chemistry", as stated by Empa on its website.

Sodium as an alternative?

A battery in which lithium is replaced by sodium and that basically functions like LIB would be an obvious alternative. The advantage: Sodium is a sustainable raw material and available all over the world. It is found in seawater and can also be extracted from underground salt domes. Since a sodium ion is about 50 percent larger than a lithium ion, the sodium cathode materials show poorer electrochemical cycling performance and can sustain much fewer charging cycles in comparison to LIB. This basically eliminates the cost advantage achieved in the extraction of the raw material.

According to the researchers, problems also exist with anode material. Similar to LIB, graphite is not suitable for sodium batteries. Tests with inexpensive tin, antimony or phosphorus showed good charge storage properties. However, during charging the volume of the anode increases threefold, which impairs battery stability. And: When subjected to shocks, the material can easily disintegrate, and the battery is damaged.

Weaknesses in aluminium and magnesium

Researchers are also working with magnesium as an alternative to LIB, since it is inexpensive to extract and non-toxic. On the anode side of the battery, metallic magnesium can be inserted directly and no graphite is needed. However, the high electrical charge of the magnesium ion creates drawbacks on the cathode side. As a result, these types of batteries can only be used in a small voltage range if they are to last a long time. And: They cannot be charged quickly and are very inefficient.

Available in large quantities and non-toxic: This also applies to aluminium. Aluminium graphite batteries are about five times heavier than comparable LIB due to their chemical operating principle.  Because the graphite cathode expands to more than twice its original volume during each charging process, these batteries need flexible outer casings. The expansion and shrinkage also has a negative effect on long-term stability.

Tops in energy density

It is still uncertain whether one of these alternative battery technologies will prevail and one day replace lithium- ion batteries in some areas. According to the Empa and ETH researchers, primarily systems with graphite cathodes could have potential. However, it is clear: None of these technologies can compete with lithium-ion batteries in terms of energy density.

These types of alternative batteries are therefore only suitable for applications, in which electricity is to be stored as cheaply as possible and the focus is on sustainable production of the batteries. Furthermore, these alternative batteries require a new type of battery management – for example, the performance of an aluminium-graphite electrode increases significantly when it is cooled to 10 degrees centigrade and by up to 25 percent through skilful, step-by-step charging.

Rethinking required

Therefore, a lot of work remains to be done by research groups the world over if alternative battery technologies are to achieve a breakthrough. The Empa and ETH researchers call for a more holistic approach.  “With their lab experiments, researchers often only prove the feasibility of an idea – the cost of all necessary components and the estimated total weight of the entire battery system are mostly neglected,” says Kravchyk. However, precisely these parameters are crucial for a possible commercialisation.

More information on this topic is available in this scientific article.