As electric cars grow in popularity, automobile makers are looking to pack more energy per volume into the cars’ batteries. One possible approach is to use batteries with lithium-metal anodes and solid electrolytes, but these batteries don’t yet match the performance of standard lithium-ion batteries because ions don’t move efficiently enough through the electrolytes. Now, one group of researchers has found that tweaking the chemistry of a promising solid electrolyte material increases its ionic conductivity by nearly four orders of magnitude (J. Am. Chem. Soc. 2018, DOI: 10.1021/jacs.8b10282). This improvement could lead to a lithium-metal battery powerful enough for practical use.
The material in question is an agyrodite, a class of lithium thiophosphates whose crystalline structure seems well suited to the movement of lithium ions. The researchers knew that the structure could be tweaked to make the ions flow even more easily, and they believe the material could be adapted to large-scale production. The researchers wanted to see if they could increase the ionic conductivity of one agyrodite, lithium phosphorus germanium sulfur iodide, which has the worst conductivity in its class. They replaced some of the phosphorus atoms with additional germanium which has a +4 charge compared with phosphorus’s +5 charge. That allowed the researchers to add more positively charged lithium to the material to balance its charge, increasing the number of charge-carrying lithium ions in the battery.
Researchers predicted that the substitution would increase the conductivity of the material by less than an order of magnitude, so they were surprised to find it went substantially higher—to 5.4 mS/cm when squeezed into pellets at room temperature, and to 18.4 mS/cm after heating and sintering the pellets together. “Suddenly the worst ionic conductor we have in this class of materials becomes the best one,” Wolfgang Zeier, a materials scientist at Justus Liebig University Giessen who led the work, says.
Zeier thinks the jump in conductivity has to do with the way the germanium atoms, which are larger than phosphorus atoms, increased the size of the crystal’s unit cells. Changing the structure, he says, may have opened up more diffusion pathways for the ions to traverse.
The higher conductivity allows the use of a thicker cathode, which increases the battery’s energy density. The electrode Zeier’s team designed was approximately 160 µm thick, compared with about 50 µm in previous agyrodite systems. The team built a battery out of their material and put it through 150 charge-discharge cycles with little drop in capacity, which is important for the lifetime of a rechargeable battery.
One concern is the expense of germanium. Zeier says he’s tried out other, cheaper elements and gotten similar results, though he doesn’t want to identify them because of the battery’s commercial potential. But he points out that one of the material’s precursors, lithium sulfide, accounts for most of the cost of the material.
John Goodenough, a materials scientist at the University of Texas at Austin, whose work led to the invention of the lithium-ion battery, says agyrodites are interesting but expensive and unstable at high voltages, meaning they probably can’t compete commercially with other solid-state electrolytes.
But Zeier says stability is not much of an issue as long as there’s a protective coating on the cathode, which is necessary in many other types of batteries. “Once you have a good protective coating, it prevents electrolyte decomposition, and the solid-state batteries run really well with the argyrodites due to their good conductivity,” he says.
CORRECTION: On Dec. 21, 2018, the illustration was updated to correct the label for the sulfur ion.