This is the second in a series of posts from the article, “How to Build a Safer, More Energy-Dense Lithium-ion Battery” authored by Ashok Lahiri, Nirav Shah, and Cam Dales of Enovix. Following is an excerpt from the article regarding how our 3D cell architecture enables us to incorporate a 100% silicon anode.
Our flat-cell architecture can take full advantage of a number of advances in electrode chemistry. To understand why that’s so, you need to know a little more about how a conventional Li-ion battery works, in particular about how the graphite anode absorbs lithium ions when the battery is charging and emits them back into the electrolyte when the battery is discharging. At the anode, one atom of lithium combines with six atoms of carbon in the graphite to form LiC6. This gives graphite a theoretical specific capacity of about 372 milliampere-hours per gram. Because the ratio of lithium-to-carbon atoms is 1:6, only modest swelling occurs.
Instead of graphite, we use silicon for the anode material. Silicon is attractive because it forms a Li22Si5 alloy. That very high ratio of lithium-to-silicon bonding allows silicon to store about 4,200 mAh/g, an extraordinary amount. But silicon’s increased absorption of lithium ions can cause it to swell by up to 400 percent.
Of course, any design that exploits the increased capacity of a silicon anode would have to match it on the other end by adding to the thickness of the cathode or using a better material. Commonly used cathodes such as lithium cobalt oxide (LCO), lithium nickel manganese cobalt oxide (NMC), and lithium nickel cobalt aluminum oxide (NCA) have usable capacities of 140 mAh/g, 170 mAh/g, and 185 mAh/g, respectively. Right now, we are using an NCA cathode, sized to match the capacity of the silicon anode. However, we can use any of the conventional Li-ion cathode materials, and that flexibility should allow us to meet the requirements of specific applications.
Although it’s possible to add silicon to the anodes of conventionally produced batteries, you can’t add too much. That’s because as silicon absorbs lithium and expands, it eventually pulls the anode apart from the metal-foil current collector. This explains why commercial Li-ion batteries have so far been limited to about a 5 to 10 percent silicon-to-graphite blend.
Enovix gets around this problem by making its silicon porous so that expansion pushes its tiny internal cavities together rather than swelling the entire anode. This feature maintains the structural integrity of the connection between the anode and its current collector during repeated charge-discharge cycles. This ability to control anode expansion is one of the key advantages of our system over the conventional Li-ion battery architecture that Sony pioneered.
Depending on size and thickness, our cells pack into a given volume from 1.5 to 3 times as much energy as conventional Li-ion cells do. Because our battery architecture makes it possible to exploit a wider variety of electrode materials, we expect to take advantage of ongoing research in materials, which so far has improved the performance of conventional batteries by roughly 5 percent per year. But because we can also exploit future efficiency gains within our structural design, we expect the energy density of our batteries to improve two to three times faster than that of conventional batteries.