Work is being done worldwide on numerous electrochemical systems that could achieve higher energy densities than lithium-ion batteries. These include solid-state batteries based on lithium and sulfur. Theoretically, lithium-ion batteries can achieve a gravimetric energy density of almost 2600 watt hours per kilogram (Wh/kg). So far, prototypes have reached around 350 Wh/kg in the laboratory, which is around twice as much as lithium-ion batteries. However, short loading times and reliable long-term stability are still lacking. With the help of neutrons, researchers led by Robert Bradbury from the Helmholtz-Zentrum Berlin (HBZ) have now found the cause of some weaknesses in lithium-sulfur batteries.
For their analysis, Bradbury and colleagues built a special cell to more closely observe the transport of lithium ions between the anode and cathode. They exposed their test cell to neutrons that were sensitive to lithium. Neutron radiography and neutron tomography revealed how the lithium ions propagated through the battery’s cathode. This process was surprisingly slow, thereby limiting battery performance. In addition, the lithium was concentrated near the pantograph during charging. “This results in reduced capacity because only some of the lithium is transported back into the battery when it’s charging,” says Bradbury.
Weaknesses of lithium-sulfur batteries
These measurements thus revealed previously overlooked weaknesses in lithium-sulfur batteries. The observed inhomogeneous distribution of lithium could also be understood with model calculations. On this basis, developers of lithium-sulfur batteries can now work specifically on improvements for faster ion transport within the battery.
The change in neutron attenuation in the cathode shows where lithium accumulates: at the top when discharging, at the bottom when charging. i.e0 is the limit to the solid electrolyte, imax is the boundary between cathode and current collector.
(Image: HZB)
Whether and when lithium-sulphur batteries will be good and cheap enough to give electric vehicles long ranges of 1000 kilometers and more cannot yet be said. After all, the Berlin start-up Theion has already announced the first samples of so-called “crystal batteries” based on lithium-sulphur for this year, which are to be manufactured in a small series. Representatives from several research groups will discuss further progress at the beginning of July at the tenth workshop on lithium-sulfur batteries in Dresden. It is not unlikely that the recently published neutron measurements will also be intensively discussed there.
(jl)
