I'm doing a PhD in batteries. Not this issue specifically, but I hear a lot about different battery fields so I think I can speak on it.

The drawback is that the anode expands and contracts a lot during a cycle. This puts a lot of strain on the binder holding the film together, and on the contact between the film and the aluminum foil. This makes the battery degrade and fail after fewer cycles.

Below is an article in nature from 2020 where a group is trying to solve this issue by coating the Si platelet particles with carbon (adding complexity and mass). You can read about this issue on greater detail in the abstract and introduction. There are many articles tackling the same issue (many cited in this article), I just picked this one because it had info in the intro/abstract. Stable high-capacity and high-rate silicon-based lithium battery anodes upon two-dimensional covalent encapsulation

In addition, the expansion/contraction cycles causes the electrolyte to dry up. During the first few cycles of any battery, the electrolyte reacts with the electrodes to form passivating layers on the electrodes. When the particle contracts/expands excessively, the particle breaks apart and the passivating layer is ripped up. The passivating layer is then reformed, now on a larger area, which consumes more electrolyte. Eventually the cell fails from the lack of electrolyte.

Below is an article in nature from 2024 where a group tries to solve this issue by designing an electrolyte that creates a passivating layer that keeps its shape when the particle contracts, creating a shell. You can read more about the issue In the abstract, intro, and figures. High voltage electrolytes for lithium-ion batteries with micro-sized silicon anodes

These are solvable issues, but a lot of the solutions are either too complicated to scale up, or add too much mass/volume to make it worth it, or slow down the discharge rate. And any change anywhere, needs to be taken into account on the rest of the parts of the battery.

I don't know what Sienza Energy did. It's an MIT spin-off, so they probably know their stuff. All issues don't need to be solved for a battery to be functional, it just needs to be good enough. Any new battery factory "just" needs to find and scale-up the state-of-the-art components in the right combination. There will be a ton of drawbacks, but it will be better than the last battery factory.