A Decade of Work: Sila Charges to Market with Silicon in Li-ion Batteries

September 10, 2021 by Jake Hertz

Sila Nanotechnologies has spent the last ten years trying to push Li-ion batteries to the next level using silicon as an anode material and has finally made it to market within the Whoop 4.0.

One of the most critical moments in modern electronics was the commercial introduction of the lithium-ion (Li-ion) battery in 1991. Since that day, Li-ion batteries have improved dramatically and have served to power virtually every portable electronic device imaginable. 

Today, 30 years later, the Li-ion battery is finding even greater importance with the rise of the electric vehicle. However, recent years have seen a plateau in the improvement of cost per kWh, a trend which has turned researchers to search for alternative solutions to conventional Li-ion. 


Conventional Li-ion battery chemistries are reaching a cost-per-performance plateau.

Conventional Li-ion battery chemistries are reaching a cost-per-performance plateau. Image used courtesy of Sila Nanotechnologies


One company that has been leading the charge for the last decade is Sila Nanotechnologies. This week, Sila's battery technology has been brought to market for the first time as the power source for the Whoop 4.0 fitness tracker. 

This article will cover the challenges facing further development of Li-ion batteries and how Sila is tackling the problem. 


The Trouble with Li-ion 

The development of Li-ion batteries can be summed up as a tradeoff between five major considerations: cost, capacity, lifetime, degradation rate, and charge time. 

As it stands, engineers can design batteries that are good at one or more of these parameters; however, nothing has been able to achieve all of them simultaneously. Since it's difficult to have them all, the parameter that is often optimized for most is the cost per kWh since it defines the overall costs of a product (i.e., the battery is the most expensive part of an EV). 


Parts of a standard lithium-ion battery.

Parts of a standard lithium-ion battery. Image used courtesy of Let's Talk Science


The theoretical limits of a battery's performance are always a function of four critical components: the anode, cathode, electrolyte, and separator. 

Today's Li-ion batteries are relatively limited in material choice, where roughly 99% of Li-ion anode's consist of only a few types of graphite. Though the industry has scaled graphite anodes in terms of performance and cost for decades, the technology is finally reaching its theoretical limit.

With that in mind, how is Sila approaching Li-ion battery technology?


Sila’s Approach = Silicon

Recently, it has become clear to researchers that if lithium-ion batteries want to continue to be scalable, it's necessary to explore replacements for graphite anodes. 

To this end, Sila Nanotechnologies have turned to silicon (Si). Since Si has the theoretical ability to bond with 10x more lithium ions by weight than graphite can, it has the potential to be used as an anode. A single Si atom could store up to 4 lithium atoms, while a graphite anode would need 24 carbon atoms to do the same. 

The result is that Si anodes could offer up to 10 times higher capacity per gram (mAh/g) and up to 3x volumetric capacity measured as (mAh/cc). Sila believes that Si-dominant anodes could, theoretically, increase the overall energy density by up to 50%. This increase in energy density could thereby drop the dollar per kWh cost by 30-40% in less than a decade.


Silicon is a prime contender to replace graphite in Li-ion anodes.

Silicon is a prime contender to replace graphite in Li-ion anodes. Image used courtesy of Asenbauer et al


However, a significant challenge with developing silicon anodes is that silicon expands 300% during charging and contracts 300% during discharging. Graphite, on the other hand, only changes by about 7%. This swelling is dangerous and unstable and is the major roadblock stopping silicon from replacing graphite. 

To approach this, Sila has spent the last ten years developing a material that compensates for the swelling of silicon through the design of an engineered particle structure. Basically, the concept was to make a particle that could allow the swelling and contraction of silicon to happen inside the particle, which would occur while keeping the electrolyte outside of the particle. 

The past ten years of research have finally paid off with Sila's technology finally reaching the market.


The Whoop 

After ten years of development, Sila's product has finally made it to market as the power source of the new Whoop 4.0. 

This new device features a partial-silicon anode, where ~25% of the capacity in the battery's anode is silicon, the rest being graphite. Even though this is only a hybrid between Si and graphite, the results are impressive, which claim to make the Whoop 4.0 33% smaller than the 3.0 version while maintaining the same 5-day battery life. 

In the near term, Sila predicts to be able to integrate its current partial-silicon batteries into EVs by 2025. In the long term, Sila hopes to achieve even better by eventually developing a full silicon anode.


Featured image used courtesy of Sila Nanotechnologies