My work on the time dependence of SEI growth has been published in JES. This work reexamines a commonly stated “fact” about SEI growth, perhaps the most common battery degradation mode: its growth with the square root of time.

The SEI is said to be “the most complex and least understood component of lithium-ion batteries”. As a new graduate student working in batteries, I was reassured to learn that such a simple theoretical model could explain such a complex phenomenon. However, as my experiments and understanding progressed, this reassurance turned into confusion. First, many of the assumptions in the simple theory behind this model didn’t really apply to complex battery systems. Second, my own data fit poorly to t0.5. I began to suspect that this question was less settled than previously thought.

In this work, we go through both the theoretical and empirical evidence for the t0.5 model and find that in general, both are lacking. Not surprisingly, this complex phenomenon requires complex models to explain it. While we don’t propose an alternative model, I think the starting point towards something better is a recognition that the current approach could be improved. I hope this work both clarifies things that confused me as an early graduate student and encourages new graduate students to think critically about the literature.

Much credit for this work goes to Steve Harris, who first brought up this problem to me. I’ve learned a lot from his “outsider” approach to the battery field, which in my opinion makes him an exceptionally creative and outside-the-box thinker. Steve, this one’s for you.