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Tesla research collaboration progresses on new battery chemistry



This is what the lithium deposited at the anode looks like under a scanning electron microscope.  Fully charged in the top row, depleted in the bottom row.
Enlarge / This is what the lithium deposited at the anode looks like under a scanning electron microscope. Fully charged in the top row, depleted in the bottom row.

Electric cars have come a long way in terms of terms walking a long way for a fee. But people are still looking for the next big step in battery technology – a battery with a significantly higher energy density would mean more range or lower cost to reach current range. in. There is always some room for the increasing advancement of current lithium-ion battery technology, but having a lithium holy grail remained out of reach for decades: removing graphite anodes to shrink cells .

A lithium metal battery would simply use solid lithium as an anode instead of requiring a graphite frame for the lithium atoms to attach when charging. The problem is that lithium doesn’t form an orderly surface during recharging, so the battery capacity drops drastically – down to 80 percent within 20 charge cycles in some configurations. False lithium also tends to form dangerous, branched, needle-like structures that can penetrate the interface between the anode and cathode and short the cell circuit.

Last year, a team of Dalhousie University labs connected with Tesla developed a lithium metal battery with slightly better performance. The lithium atoms charge a copper electrode as the battery charges and then switch back to a regular lithium-nickel-manganese-cobalt cathode when the charge is depleted. Through a new electrolyte, they are able to make this battery last for about 90 cycles before reaching 80% capacity to control nasty short circuit breakdown.

In a new study, the team reports on that design autopsy to determine the causes of power loss. As a result, they found a tweak that helped them achieve around 200 cycles.

Lithium batteries without an anode are thinner than current lithium-ion designs.  The energy density is compared and illustrated for a cylindrical cell, lower left, with the EV / cost range compared, lower right.
Enlarge / Lithium batteries without an anode are thinner than current lithium-ion designs. The energy density is compared and illustrated for a cylindrical cell, lower left, with the EV / cost range compared, lower right.

This type of battery would be a big step up if it were to survive, containing about 60% more energy per unit volume than the lithium-ion batteries in use today. That could increase tram range from 400 km to 680 km (250 miles or to 400), the researchers noted. Improved stability is due to an electrolyte consisting of two lithium-boron-fluorine salts in an organic solvent. To see what’s going on inside the battery, the team analyzed the changes in the electrolyte over time and also monitored changes in the activity of the lithium solids formed on the anode.

It turns out, the salts in the electrolyte were consumed as the battery went through the charging 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. So as the battery goes through more and more cycles, the less electrolyte there is to do its job.

Throwing the anode face under an electron microscope showed that the cyclic plating and removal of solid lithium became less orderly over repeated cycles. It begins to form a very fine layer but develops the terrain by the fiftieth cycle or so. Pockets form between the walls like rings, leading to more and more electrically isolated lithium parts – no longer playing back and forth of the battery. (See the image at the top of this page.) This also means that the surface area of ​​solid lithium increases, so more electrolytes will be needed to maintain the contact everywhere.

One possible solution is to increase the electrolyte volume so that the depleted salts take longer, as well as maintain a better contact over the entire surface area. However, that would reduce the battery’s energy density, so the team decided to try increasing the concentration of dissolved salts in the electrolyte.

With increased concentration, the battery can maintain capacity for more charge cycles – reaching at least 150 cycles before dropping to 80%. In other words, it takes about 200 cycles to drop to the equivalent capacity of a lithium-ion battery of that size. Another thing that is really helpful is some simple clamping pressures, as this encourages solid lithium to pack together better. All of these numbers come from batteries housed in a small conversion clip. Without that pressure, the capacity drops significantly faster.

These results show significant progress towards a stubborn problem, but the design has a way of meeting the life of the current lithium-ion battery, which can reach 1,000 cycles before dropping to the mark. That 80%. “However, it is necessary to increase longevity before such cells can be used for electric vehicles or electrified urban aviation,” the researchers wrote. If further advances can be made in their lifetime, non-anodic lithium metal cells with liquid electrolyte are the simplest and lowest cost way towards high energy density lithium batteries. Feasibility “

Natural Energy, 2020. DOI: 10.1038 / s41560-020-0668-8 (About DOI).


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