Tesla’s battery research group has created a hybrid lithium-ion/lithium metal cell that could act as an all-electric range extender when required.
EVs and Range Anxiety
Range anxiety continues to slow the uptake of electric vehicles (EVs) around the world. In the U.S., 58% of drivers are afraid they will be left stranded by an exhausted EV battery, while 49% cite a lack of charging stations as a concern.
It’s an enduring perception, with many potential buyers eschewing EVs due to range concerns despite most modern EVs being capable of 200 to 300 miles of range, while high-end Teslas can have 300 to 400 miles. Tesla’s 2020 Roadster will reportedly travel an incredible 620 miles on a single charge.
Range anxiety tends to disappear after purchase. 65% of EV owners reported their range anxiety went away after a few months. This is because most people — with some notable exceptions — do not use their batteries to anywhere near their full capacity during their daily commute, and will only use the full battery range for a road trip once or twice a year. The average car owner in the U.S. drives only 29 miles per day.
To dispel range anxiety and provide peace-of-mind, EV makers including GM and BMW have included range extenders in vehicles such as the Chevrolet Volt and BMW i3: small gasoline combustion engines that can be used as generators for recharging the battery when the driver needs more range.
This means that EVs with gasoline-powered range extenders are technically not fully electric. However, a new paper published in Joule by Tesla’s Canada-based battery research group has found a way to create a truly all-electric EV range extender.
Hybrid Lithium-Ion/Lithium Metal Cells
Led by Dalhousie University’s Jeff Dahn, the research team has discovered a way to cycle lithium metal on graphite to form hybrid lithium-ion/lithium metal cells. This could lead to an energy density that is 20% higher than traditional lithium-ion cells.
The paper states: “Replacing the graphite anode in conventional lithium-ion cells with lithium metal significantly increases energy density. However, lithium metal anodes are plagued with rapid capacity loss and short cell lifetime. To develop a high energy density cell with longer lifetime, we propose a hybrid lithium-ion/lithium metal cell that is achieved by purposefully plating lithium metal on graphite. Although unwanted lithium plating is normally a degradation mechanism in conventional lithium-ion cells, we achieve reversible lithium plating on graphite with an optimized dual-salt electrolyte.”
Driving in Lithium-ion Mode, Extending in Lithium Metal Mode
How does this lead to an all-electric range extender? The paper explains, batteries are rarely cycled to full capacity: “These hybrid cells can [therefore] be operated in lithium-ion mode with little degradation for the majority of their life with periodic fully charged lithium metal cycles for extended capacity.”
A hybrid cell operating in lithium-ion mode would have an energy density of 530Wh/L, or 25% less than a conventional lithium-ion cell, resulting in a range of 186 miles.
Referring to a 2012 study on EV driving and charging behavior, the research team noted that typically only 1% of day-to-day drives are lengthier than 202 miles. EV owners would therefore be able to operate their hybrid cells for 99% of the time in lithium-ion mode (up to 186 miles), and would only rarely need to access the lithium metal part to extend the range up to 248 miles for longer trips.
What Does This Hybrid Approach Mean for Drivers?
Electrek notes that the majority of Tesla drivers as well as other long-range EV owners “are dragging hundreds of pounds of batteries every day that they […] only use a few times a year.”
The hybrid approach would enable greater efficiency and help maintain the longevity of the battery pack. Long-range electric cars would still be available for those who want them, but Dahn’s approach is based on real-world usage rather than catering to range anxiety.
Finally, removing gasoline-powered range extenders from EVs would make them truly all-electric vehicles.
The study is in its initial stages and hybrid cells may be a long way from commercialization.
