Tesla moves to cobalt-free silicon battery cell with a new form factor

Technology News |
By Nick Flaherty

Electric car maker Tesla has developed a cobalt-free, silicon lithium ion battery cell that is says will dramatically change the way cars are powered.

The cell is in a new, larger form factor measuring 46mm in diameter and 80 mm long. This compares to the previous 1865 and 2170 cells that are 18mm and 21mm in diameter.

“We have a plan to halve the cost per kWh with engineering and industrialisation,” said Drew Baglino, senior vice president of powertrain and energy engineering, speaking at Tesla’s Battery Day (above left).

This part of a plan to scale battery cell production to 20TWh per year. The current battery Gigafactory in Nevada built with Panasonic will produce 150GWh a year. “We would need 135 gigafactories like that,” said Baglino.

The new cell is key to the scaling up of production. It uses a tables construction with a dry electrode process acquired from ultracapacitor maker Maxwell Technologies, with a simple silicon anode and cobalt-free high nickel cathode. The electrolyte wasn’t mentioned though.

All of this is built on a high speed continuous production line similar to a bottling plant. The pilot plant in Fremont California has made tens of thousands of the new cells, but the yield is a problem, says Elon Mush, CEO of Tesla.  

“The dry coating they had was proof of concept and we have revised the machine four times since the acquisition,” said Musk. “There is still a lot of work to do to go to pilot to volume production, its insanely difficult to scale up, but we have made tens of thousands of cells. The yield is not good but there is a clear path to success,” he added. “We will probably be on machine revision 6 or 7 for volume production with a new rev every three or 4 months.2

The new line produces 20GWh of batteries, 7x the capacity of existing lines, through the high speed and the higher energy density of silicon.

The high speed production comes from the tabless cell construction. Rather than using tabs top and bottom, the substrate of the ‘jelly roll’ that holds the anode, electrolyte and cathode has copper edges that are laser patterned. These fold over to produce the connections top and bottom. This avoids having to stop and start the line to insert the tabs

Using silicon provides higher energy density but suffers from cracking. “Silicon stores 9x more lithium than graphite but expands 4x when fully charged,” said Baglino. “Current approaches use engineered silicon materials and don’t scale – what we are proposing is a step change in capability and cost. We use the base silicon, which costs $1.2/kWh, and stabilise the surface with an elastic ion-conducting polymer coating, then use a highly elastic binder.”

Tesla is also mining its own lithium in Nevada using a saline process, and will build a cathode plant nearby. This will build cathodes with no cobalt with a variety of other materials such as iron and manganese that it already uses. Cutting out cobalt avoids sustainability and ethical production issues.

“This uses metal powder directly, eliminates billions in battery grade nickel production, with simpler mining and simpler recycling,” said Baglino. “We want to make sure we are not constrained by nickel supply but we need a three tier approach with iron [for stationary storage], Nickel Manganese [for mid range vehicles], then high nickel for long range for the cybertruck and the semi,” said Musk.  

The answer to how Tesla addresses the swelling of the silicon may well come from the packaging of the cell which will be used as part of the structure of the vehicle, rather than in a separate battery pack.

“This has a dual use as energy and structure – this is quite profound,” said Musk .”This allows higher packing as there is no intermediate structure, so there’s more space, the pack itself is structural. Instead of a flame retardant filler in the pack, the filler is structural adhesive that is also flame retardant. This allows shear transfer between top and bottom sheets and this gives stiffness. We use the steel shell case of the battery to transfer the shear,” he said.

In this case the steel of the cell casing could be significantly thicker to be used as structure and to prevent the casing bursting when it swells.

It is this combination of new materials and new processes from the mining to the final testing that is the way to reduce the overall cost of the cell and the battery pack, say Baglino and Musk. This halves the pack cost per kWh (by 56 percent), although Tesla doesn’t give a cost. Current cell costs are already under $150/kWh, so the overall pack cost for Tesla would be well under $100/kWh. This allows a lower cost electric vehicle platform.

“It will take us a year to 18 months to start to realise the advantages and probably about 3 years overall but this bodes well for the future,” said Musk. “We are confident we can design and manufacture a compelling $25,000 vehicle three years from now that’s also fully autonomous with an EV powertrain that costs less than a combustion engine.”

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