Response to Tim Holme

Original post at Twitter here, in response to this exchange with QuantumScape CTO Tim Holme.

Many thanks and credit to @ironmantimholme for taking the time to reply and give us all a bit more insight into QuantumScape’s thinking! I do also agree with a lot of it and appreciate the value of proving capability at single layer - if it doesn’t work there it’ll never work.

I would also agree that Li plating is likely a major failure mode for the 2170 cell. In Sweden we have had a close academic-industrial collaboration on degradation in commercial cells - the last phase being the so-called “fast-charging project” where my colleagues looked at the degradation of cells charged at rates up to 4C, finding Li plating was significant from 3C and associated gas evolution at 4C giving a large impedance rise that led to similarly rapid cap. loss. This was published a few years ago (open access).

I think the point of single layer performance being something that can be approached asymptotically at multilayer with good engineering is a nice way to think about it. I had similar thoughts about Jeff Dahn’s widely shared ‘million mile battery’ paper… which, while multilayer, are pretty small and similar capacity in the end (0.24 Ah) - more I thought as a demonstration of potential performance and not necessarily what we would see imminently in large scale cells.

Photograph of small multi-layer pouch cell, from Harlow et al., 2019

Anyway, hopefully we all agree that it’s healthy to keep a critical (and self-critical) perspective on performance when we’re still at small scale - and we’re all keen to see this level of performance reproduced in the “real thing”!

P.S.: one thing I also wanted to mention from the Mussa et al paper linked before is the observation of different extents of degradation in different sample locations in the cell - what I refer to as ‘within-cell heterogeneity’. This I believe is a big aspect of degradation in large scale cells simply because the bigger you go, the more scope you have for uneven distributions of pressure, temperature, stress - leading to distributions in the reactions, and degradation mechanisms, themselves.

And once you have a severe ageing mechanism such as lithium plating, or gas evolution, or cathode breakdown arising because the distribution of the current and electrode potential in the cell is inefficient, it will just help accelerate the process. It is also something that (I believe) can be mitigated through good engineering and understanding at the materials level, and is increasingly a major research interest of mine. I’m also coordinating the successor of the “fast charging project”, so watch this space for updates…

comments powered by Disqus