Battery Day: What is Tesla’s “lithium doping” patent all about?

With Tesla delaying its Battery Day several times now, there is opportunity to dig deeper into the various new methods and patents that Tesla CEO and c-founder Elon Musk may reveal at the much-anticipated investor event, which is now expected in two pieces, one online and one face-to-face.

We previously shared insight into Tesla’s “single crystal cathode” patent submitted by Dalhousie academic and head of the Tesla battery research team, Jeff Dahn on behalf of Tesla. It is thought to be one of the most significant pieces to the puzzle that will be Battery Day.

The single crystal cathode – for which Tesla’s patent discusses ways of achieving a cycle life of 4,000 or more – holds potential for the million-mile battery, as highlighted in a paper published by Dahn and his team in the Journal of The Electrochemical Society.

Currently, auto makers generally offer a warranty of 8 years or 160,000km for batteries in today’s electric vehicles, and an EV owner may have to shell out for another EV battery before the useful life of the vehicle itself over.

An EV battery with this kind of lifespan (of 4,000-5,000 cycles) would mean the buying proposition of electric cars would be  much more attractive and would accelerate the transition to clean transport.

Other research and patents assigned to Tesla have also attracted attention not because of a contribution to battery lifespan, but because of other factors such as energy density (and therefore increased driving range or decreased battery cost).

Drawing again on insight from Youtube channel “The Limiting Factor“, today we look at the “lithium doping” patent that Tesla filed in late April and which is significant for its method of addressing loss of energy density experienced during the production of the battery.

Titled “Active material for electrode and method of manufacturing thereof”, this patent discusses a method of adding lithium during the cathode creation (called pre-lithiation) such that energy losses are minimised.

Unlike the single crystal cathode patent which was submitted by Dahn’s lab in Canada, this patent was submitted on behalf of Tesla by its director of battery and powertrain technology Vineet Haresh Mehta, and staff engineer Sanketh R. Gowda.

The patent is important because, as The Limiting Factor host explains, rechargeable batteries lose around 7 to 10% of their capacity during their formation, when the solid electrolyte interphase (SEI) is formed within the battery cell.

“The SEI is a layer that forms over the cathode and anode as the cathode anode react with the electrolyte solution. After this initial reaction the SEI protects the cathode anode from reacting further with the electrolyte solution,” he explains.

“This allows the battery to last hundreds or thousands of cycles.”

But, because the SEI must come from within the battery, and much of the SEI is composed of lithium, once it is used to form the SEI it is no longer available to store energy – hence the loss in energy capacity.

For this reason, many researchers are looking to pre-lithiation as a means to increase the amount of lithium to make up for the lithium lost in the first cycle discharge and recharge when making the battery.

“There about a half a dozen different methods to pre-lithiated battery cell but to my knowledge none have been commercially successful yet,” says The Limiting Factor host.

In this patent, Tesla chooses a method referred to as positive electrode pre-lithiation , wherein the cathode material (made of nickel-cobalt-aluminum, or NCA, as per Tesla’s current cathodes) is made with extra lithium the extra lithium which is then dumped into the cells during the first cycle to create the SEI.

The patent discusses adding a lithium-nickel-copper oxide (technically known as LLNixCui-xCh – but we’ll stick with LNCO for short) as the pre-lithiation material, coated with 1% alumina presumably to stabilise the lithium in the LNCO material.

Tesla first tested the LNCO material in differing ratios – 70/30 copper-nickel, 50/50 and 30/70. According to the figures included in the patent, Tesla determines that as nickel increases, energy capacity decreases, but this is lost more slowly as the battery cycles over time.

We note that The Limiting Factor explains it is likely that Tesla has submitted the figures with a transposition error, as the capacity (shown in milliAmp hours per gram) for the 70/30 ratio in figure 5 should be 430mAh/g, and 30/70 should be 390mAh/gram – please let us know if you support this theory or not.

Tesla’s next step was to undergo a series of tests using the pre-lithiation material.

These were conducted using both a half cell (a battery cell that’s tested in a lab environment to verify that the material is meeting the theoretical expectations) and a full cell, using either 98% NCA and 2% inactive material including 1% carbon, or 96% NCA and 2% LNCO in addition to the same 2% inactive materials.

In the half cell test, Tesla shows that the measured energy capacity is very close to what the expected results were.

“Note the 4 milliAmp hour per gram increase when LNCO pre-lithiation material is added,” he says.

As The Limiting Factor host explains, the patent shows that, “the LNCO material is so potent that it only takes a small amount to increase the energy capacity.”

In the full cell test, the Limiting Factor host explains that, “Once again the battery cell comes out 4 milliAmp hours per gram better off with the LNCO pre-lithiation material added both before and after cycling.”

“For each weight percent of the LNCO material added it makes up for about 1% of the first cycle loss,” he says.

The outcome? For a more detailed explanation, you can listen to The Limiting Factor video at the bottom of this article, but put simply, Tesla has shown that by using the LNCO pre-lithiation method, it can drastically reduce first cycles losses.

As The Limiting Factor explains, “If Tesla’s high copper LNCO mixture reaches at least 430 milliAmp hours per gram it might be able to completely eliminate first cycle losses with less than 5% of the LNCO material added to their cathodes.

“This would mean a 7% greater improvement in the specific energy capacity of Tesla’s batteries. Bear in mind these samples only went up to 70% copper – it might be possible to incorporate even more.

“Regardless this would be enough to increase the specific energy of Tesla’s current batteries from 247 watt hours per kg to 270 watt hours per kg,” he says.

The host considers that the method is not ready to commercialise, but does show another facet of battery cell production that Tesla is working towards improving. He also notes that Maxwell Batteries, that Tesla acquired in 2019, also has a strategy in mind for dealing with first cycle losses, and has targetted a 2027 time frame in its own roadmap.

Whether or not this will be included as part of the picture at the Tesla Battery Day is as yet unknown, but if it is it will likely add to a number of technological developments that will all contribute to improved batteries for Tesla electric vehicles.

Citations:

Active material for electrode and method of manufacturing thereof
Publication Date: 30 April 2020
Inventor: Sanketh R. Gowda, Vineet Haresh Mehta
Assignee: Tesla
USPTO: US20200136142A1
https://patents.google.com/patent/US20200136142A1/en

A Wide Range of Testing Results on an Excellent Lithium-Ion Cell Chemistry to be used as Benchmarks for New Battery Technologies
Journal of The Electrochemical Society, Volume 166, Number 13
Publication Date: 6 September 2019
http://dx.doi.org/10.1149/2.0981913jes