Batteries: modelling tomorrow's materials today

Researchers from the Karlsruhe Institute of Technology (KIT) have used computer-based simulations to study the factors that determine how quickly a battery can be charged. The researchers used microstructural models to discover and investigate new electrode materials and found that when sodium-nickel-manganese oxide is used as a cathode material in sodium-ion batteries, modifications of the crystal structure can occur during charging. These modifications lead to an elastic deformation, as a result of which capacity decreases.

Research into new battery materials is aimed at optimising their performance and lifetime. Work is also underway to reduce the consumption of rare elements, such as lithium and cobalt, as well as toxic constituents. Sodium-ion batteries are considered very promising in this respect. They are based on principles similar to those of lithium-ion batteries, but can be produced from raw materials that are widely accessible. They are also suitable for both stationary and mobile applications.

Dr Simon Daubner, Group Leader at the Institute for Applied Materials – Microstructure Modelling and Simulation – in KIT said layered oxides such as sodium-nickel-manganese oxides are highly promising cathode materials. However, sodium-nickel-manganese oxides can change their crystal structure depending on how much sodium is stored; if the material is charged slowly, everything proceeds in a well-ordered way.

“Sodium leaves the material layer by layer, just like cars leaving a carpark storey by storey. But when charging is quick, sodium is extracted from all sides. This results in mechanical stress that may damage the material permanently,” Daubner said.

Researchers from the Institute of Nanotechnology (INT) and IAM-MMS of KIT, together with scientists from Ulm University and the Center for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW), recently carried out simulations to clarify the situation. Their research findings were reported in npj Computational Materials.

Experiments confirm simulation results

“Computer models can describe various length scales, from the arrangement of atoms in electrode materials to their microstructure to the cell as the functional unit of any battery,” Daubner said. To study the NaXNi1/3Mn2/3O2 layered oxide, microstructured models were combined with slow charge and discharge experiments. The material was found to exhibit several degradation mechanisms causing a loss of capacity. For this reason, it is not yet suited for commercial applications.

A change in the crystal structure results in an elastic deformation. The crystal shrinks, which may cause cracking and capacity reduction. INT and IAM-MMS simulations show that this mechanical influence decisively determines the time needed for charging the material. Experimental studies at ZSW confirmed these results.

The findings of the study can be transferred partly to other layered oxides. “Now, we understand basic processes and can work on the development of battery materials that are long-lasting and can be charged as quickly as possible,” Daubner said. This could lead to the widespread use of sodium-ion batteries in five to 10 years’ time.

Image caption: Section of a cathode layer (about 100 micrometres, left) consisting of spherical particles (of about 10 micrometres in diameter, centre) and simulation (right) of the sodium fraction in a sodium-nickel-manganese oxide crystal. Image credit: Simon Daubner, KIT.