Thread by @enn_nafnlaus

ht/@davidtayar5

Tesla's Roadruner cells are said to be using new additives (any bets that NaPO2F2 is among them) and "the amount of DMC in the solvent will be greatly increased". This means "greatly decreasing" the amount of EC - a chemical that provides for the formation of..

an excellent SEI (solid-electrolyte interphase, which protects the electrodes, particularly the anode), but has recently been indicted in various failure modes, including dendrite formation. EC also causes cell problems at low temperatures, as it has a high melting point.

EC freezing out of solution decreases the solvation ability of the remaining electrolyte, which causes lithium salts to precipitate out of solution - not only consuming them, but also covering the electrodes (increasing impedance) and potentially damaging the separator membrane.

DMC has a significantly lower melting point than EC, and is also not implicated in dendrite formation. However, it does not create and maintain the durable SEI that EC does. Hence, Tesla either needs additives to create and maintain the SEI, or to non-passively create a...

protective SEI during the manufacturing phase. Part of the chemicals list for the Kato Road facility is suggestive of the possibility of a titanate coating - but we won't know until battery day.

It should also be noted that if Tesla is increasing the amount of silicon in its...

anodes - as a reminder, silicon holds over 10x as much lithium as graphite, and nearly as much as lithium metal (albeit at a slightly lower voltage) - the problems they need to solve are not *just* the well-known cracking problem. Another issue is that traditional EC/DMC...

... mixtures tend to create a "bad" SEI on silicon... that is to say, one which does not protect the silicon against further degradation reactions due to the heavy volume change of silicon during cycling. Creating a better protective barrier has been a significant research topic.

Followup: the new electrolyte is reported to be up to 70% DMC.