I just published a paper work entitled “‘Knees’ in lithium-ion battery aging trajectories”, along with a broad consortium of contributors with affiliations in five countries (and seven US states) across academia, national labs, and industry. This paper is a comprehensive review of the “knee” effect commonly observed as lithium-ion batteries age, where the energy and/or capacity suddenly drops (red curve below).
Many papers have proposed knee mechanisms, with various degrees of validation and supporting evidence. Our aim here was to create a unifying framework for current and future knee mechanisms.
Some of the key conclusions of this work include:
- We identified six “knee pathways” (physical routes by which knees can occur), but also three “internal state trajectories” (how the internal states vary and interact to produce an observable knee); see below figure
- Some of these pathways have readily-observable electrochemical signals, while others involve subtle changes that are challenging to measure nondestructively (e.g., local porosity, local additive concentrations)
- Knee pathways are subtle and complex; comprehesive knee prediction is not easy!
I hope this paper serves as a starting point for folks working on challenging battery lifetime problems across the battery industry. I’m quite proud of this work for its breadth and depth on an important but poorly understood topic. Kudos to my coauthors – I learned a lot working with such a wide range of contributors and time zones!
Lastly, a presentation I gave on this topic at AABC in December 2021 is present below.