Electric vehicles have come a long way in terms of going a long way with a toll. But everyone is still looking for the next big leap in battery technology – a battery with significantly higher power density would mean more range or lower cost to reach the current range. There is always room for incremental advancement in current lithium-ion battery technology, but there is a sacred lithium grail that has remained out of reach for decades: occupying its graphite anode to shrink the cell.
A lithium metal battery will simply use solid lithium as the anode instead of requiring a graphite frame for lithium atoms to be inserted after the battery charges. The problem is that lithium does not form an orderly surface during recharging, so battery capacity drops drastically – dropping to 80 percent within 20 charge cycles in some configurations. Rogue lithium also tends to build dangerous, branching structures, such as needles that can pierce the separator between the anode and cathode and circulate the cell.
Last year, a Tesla-related Dalhousie University lab team developed a somewhat better-performing lithium metal battery. The lithium atoms are electrified at a copper electrode as the battery charges and then back again to a conventional lithium-nickel-manganese-cobalt cathode as the charge decreases. Through a new electrolyte, they were able to get this battery to last about 90 cycles before hitting 80 percent capacity to control the bad short circuit problem.
In a new study, the team reports on a post-mortem of this model that identifies the causes of capacity loss. As a result, they find a tweak that takes them up to about 200 cycles.
This battery would be a major breakthrough if it achieved durability, holding about 60 percent more energy per unit volume than lithium-ion batteries in use today. This could increase the electric car from 400 kilometers to 680 kilometers (or 250 miles to 400), the researchers note. Improved stability is due to an electrolyte consisting of two lithium-boron-fluorine salts in an organic solvent. To see what was going on inside the battery, the team analyzed changes in the electrolyte over time and also tracked changes in the behavior of the solid lithium that forms at the anode.
The salts in the electrolyte, it turns out, were being consumed as the battery went through charge cycles. Reactions on the cathode side convert one of the salts to the other, but reactions on the anode side consume both salts without regenerating either. So as the battery went through more and more cycles, there was less electrolyte available to do its job.
Throwing the anode side under an electron microscope showed that cyclic plating and removal of solid lithium became less and less regular during repetitive cycles. It starts by forming a very smooth layer but develops topography up to the fiftieth cycle. Pockets form between the walls with rings, leading to the growth of lithium-ion parts that are electrically insulated – no longer playing along with dancing back and forth on the battery. (See image at the top of this page.) This also means that the surface area of the solid lithium increases, so more electrolyte would be required to maintain contact everywhere.
One possible solution would be to increase the electrolyte volume, so it takes longer for the salts to deplete, as well as to better maintain contact across the entire surface area. This would reduce the energy density of the battery, however, so the team decided to try increasing the concentration of dissolved salts in the electrolyte.
With elevated concentration, the battery was able to maintain its capacity for more charge cycles – hitting at least 150 cycles before dropping to 80 percent. Or to put it another way, it took about 200 cycles to drop to the equivalent capacity of a lithium-ion battery of that size. Another thing that helps is actually a simple tightening pressure, as this encourages strong lithium to be packed together. All of these numbers come from batteries housed in a small stack for change. Without that pressure, capacity drops significantly faster.
These results show significant progress in a stubborn problem, but this model has a way of meeting the longevity of current lithium-ion batteries, which can perform 1,000 cycles before dropping to that 80 percent mark. The researchers write, “However, an increase in proper life is required before such cells are applicable to electric vehicles or electrified city aviation. If further progress in life can be achieved, anode-free anode lithium metal cells with liquid electrolytes represent the most direct and low cost route to high energy density lithium batteries. ”
Nature Energy, 2020. DOI: 10.1038 / s41560-020-0668-8 (About DOI).