"People have been trying to reach an all-lithium anode because the battery capacity can be increased by a factor of 10," commented Professor Eric Wachsam. "That has not been possible with a typical organic electrolyte because they form dendrites.
"In our battery, the lithium metal cannot pierce through the hard ceramic electrolyte, which means we can use lithium metal anodes. This means the batteries are non-flammable, lighter and smaller for the same amount of energy and, as they use less material, they are also potentially less expensive."
According to Wachsam, sulphur as a cathode has been the ‘holy grail’ of batteries because it is low cost and can hold a lot of lithium.
“So lithium metal with a sulphur cathode has been the ultimate battery configuration,” he added.
The issue with organic electrolytes is said to be that the sulphur materials on the cathode side dissolve into the organic electrolyte to form a polysulphide which then diffuses through the battery. With ceramic garnet, this does not occur.
When asked why the team decided upon garnet as a ceramic material, Wachsam answered that it possesses high ionic conductivity.
“Garnet is the best material because of its chemical stability. It’s not as reactive as some of the other materials, like the sulphide electrolytes,” he explained.
There had been little success, however, in developing garnet-based solid state batteries because the high impedance between the garnet electrolyte and electrode materials limited the flow of current, which decreases the battery's ability to charge and discharge.
“The problem with the garnet was that the lithium metal did not wet the surface of the garnet and so you had a high interfacial impedance,” said Wachsam.
Looking to solve the problem, the team inserted a thin layer of aluminium oxide between the lithium electrodes and the garnet. In this experiment, the layer of aluminium oxide is said to decrease the impedance 300 fold.
“Right now we’re running those lithium sulphur cells around 2V with energy densities of 280W.h/kg. We believe we can get to 540W.h/kg.”
Wachsam claims solid state batteries could completely replace lithium-ion batteries.
“We’re working on developing scalable processes using conventional ceramic processing that will allow us to make very large batteries for everything from automotive to grid scale storage applications,” he concluded.
