When the Tesla 90D was announced in July – offered as the standard 90D or high-performance P90D – it was touted as the Model S with the longest range and quickest off-the-line speed.

The P90D is the only Model S capable of “Ludicrous” speed, making it the quickest-accelerating sedan at 0-60 mph in 2.8 seconds. With a 90 kilowatt-hour (kwh) battery, both variations of the 90D give owners a 6-percent boost in range (equaling about 15 more miles than the 85D or P85D).

This improved battery is perfectly in line with Tesla CEO Elon Musk’s target “to increase pack capacity by roughly 5 percent per year.”

So how did the 90D reach this level of performance, and how does Tesla expect to obtain such improvement in its batteries year over year?

The answer, in part, is silicon.

Extra Energy Storage

“It’s a race among the battery makers to get more and more silicon in,” said battery researcher Jeff Dahn.

Currently researching lithium-ion (li-ion) batteries at the Dalhousie University in Halifax, Nova Scotia, Dahn will join Tesla next year as part of an exclusive partnership.

Within the li-ion cell of the battery, Tesla has replaced a portion of the graphite with silicon, which increases the energy density of the battery. In a conference call, Musk directly attributed the gains of the 90D to this change.

“It is, actually, as a result of improved cell chemistry,” Musk said of the 90D’s boost in range and acceleration.

“We’re shifting the cell chemistry for the upgraded pack to partially use silicon in the anode,” he explained. “This is just sort of a baby step in the direction of using silicon in the anode. We’re still primarily using synthetic graphite, but over time we’ll be using increasing amounts of silicon in the anode.”

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Tesla isn’t the only one adding more silicon to its rechargeable batteries.

“The number of researchers around the world working on silicon for lithium-ion cells is mind-boggling,” Dahn said.

The list includes 3M, Penn State, University of Texas at Austin and Argonne National Laboratory, among others.

“Introducing silicon into automotive-grade lithium-ion cells represents a huge milestone for the EV industry,” writes electric vehicles magazine Charged. “Silicon is widely considered to be the next big thing in anode technology, because it has a theoretical charge capacity ten times higher than that of typical graphite anodes.”

Silicon’s Shortcoming

The challenge when working with silicon is balancing out its gains in energy density with its shortened life cycle. As a silicon particle absorbs the lithium, it swells drastically. The continuous series of growing and shrinking during the charge and discharge cycles diminishes silicon’s lifespan.

“The electrode is a whole bunch of particles glued together,” explained Dee Strand, chief scientific officer at Wildcat, an electrode material research firm.

“When you have particles that change dimensions so dramatically with every cycle, they tend to fall apart. The particles themselves pulverize. They crack. The glue comes undone. And your cycle life is very short,” Strand said.

“With silicon anodes, a nice passivation layer is formed on the particles. But as the silicon expands and contracts, it essentially cracks apart that layer and then makes more. Over time it ends up with a very thick resistive film on the anode, which causes it to lose both capacity and power. So that’s the other mechanism that causes the cell to fade very fast.”

Beyond The Silicon

In order to get the full potential from silicon, further developments are necessary for other components within the battery, such as the electrolyte formulas.

Adding more silicon to the anodes will “require better binders that hold the electrode material together, and better electrolytes that form more mechanically robust [solid electrolyte interface] layers on those particles,” said Strand.

As developers such as Wildcat work to fine-tune these chemistries, their research may further unlock the capabilities within Li-ion batteries. It still remains to be seen, though, how far Li-ion can go before the chemistry reaches its performance ceiling.