Battery research led by a team from Tesla that could produce a significant increase in driving range, and the new paper on hybrid batteries published recently points to a dramatic 20% jump in the capacity of electric car batteries.
In the battery world, hybrid does not refer to a combination of an electric/internal combustion engine drivetrain. Instead, it refers to the combination of two types of batteries or materials with differing characteristics that together can offer more than one type of battery on its own.
The research in question was published on April 30 in energy journal Joule by a research team that includes Tesla battery researcher Jeff Dahn, Tesla battery cell engineer Rochelle Weber, as well as Dalhousie University battery postdocs Matthew Genovese, Alexander Louli, and Tesla cell engineering intern Cameron Martin.
According to the researchers, hybrid cells can deliver 20% higher energy density than conventional lithium-ion cells.
This can mean one of two things for those wanting to buy an electric vehicle: it is either cheaper, because a smaller battery can be used in the vehicle, or it can drive further. In the case a vehicle with 500km driving range, if a battery the same size were used, as much as 600km driving range could instead be on offer.
The new paper discusses a hybrid battery that combines both lithium-ion and lithium metal. The researchers know that replacing the typical graphite anode present in li-ion batteries with lithium metal can greatly increase energy density, but as they point out, “lithium metal anodes are plagued with rapid capacity loss and short cell lifetime.”
The research team proposes that by instead plating the graphite anode with lithium metal, this can be avoided. But there is another problem: lithium plating can further degrade lithium-ion cells. To avoid this problem, the researchers propose implementing “reversible lithium plating on graphite with an optimized dual-salt electrolyte.”
As described by the research team’s summary of the paper:
A hybrid anode cell design is proposed involving lithium metal plating on top of graphite that provides a 20% increase in energy density over conventional lithium-ion cells.
Pouch cells with hybrid graphite-lithium metal anodes cycled with conventional electrolytes fell below 80% capacity in under 15 cycles. However, with a dual-salt electrolyte and applied mechanical pressure optimized for lithium metal cycling, hybrid cells achieved over 150 full (100% utilization) cycles before falling below 80% capacity with a CE of 99.6% for lithium metal plating on graphite.
We also found that intermittent high energy (100% utilization) cycles utilizing lithium metal can be dispersed among hundreds of conventional lithium-ion cycles where only the graphite is utilized. Operating the cell with this intermittent protocol shows minimal impact to the underlying graphite capacity. Therefore, these hybrid cells can operate well in “lithium-ion mode” with periodic high energy full cycles accessing the lithium metal capacity.
It sounds very promising. To get a better picture of the significance of the research, we reached out to Jacek Jasienak, who is a professor at Monash University in Melbourne, director of the Monash energy institute and chief investigator at the ARC Centre of Excellence in Exciton Science (ACEx).
“Transitioning to metallic lithium would have a huge impact on the overall capacity,” says Jasienak.
“Unfortunately, as has been well known for over three decades, plating of lithium causes dendrites, which causes shorting of the batteries and explosions.
“A question is how does one control the plating process to avoid this dendride formation and any adverse solid electrolyte layers forming. This paper plates the lithium onto graphite to provide a hybrid battery approach.”
As the paper in question is still “in press” mode, Jasienak could not comment further, except to say that he had concerns about the suitability of this approach for safe and long running operation versus a plain graphite anode.
With Tesla’s Battery Day slated for the third week of May – although a firm date still has not been confirmed – it raises again the question of what will learned about Tesla’s battery research, which CEO and co-founder Elon Musk said at the company’s first quarter earnings call will be “one of the most exciting days in Tesla’s history.”
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Bridie Schmidt is lead reporter for The Driven, sister site of Renew Economy. She specialises in writing about new technology and has been writing about electric vehicles for two years. She has a keen interest in the role that zero emissions transport has to play in sustainability and is co-organiser of the Northern Rivers Electric Vehicle Forum.