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

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.

SOURCE: THE LIMITING FACTOR
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.

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