Electrochemical Kinetics of Graphite Degradation in Lithium-Ion Batteries
Authors: Das Supratim , Bazant Martin Z. 
Published: December 2020

Growth of the solid electrolyte interphase (SEI) is a major driver of capacity fade in LIBs. Despite its importance, the fundamental mechanisms remain unclear, primarily because of the complicated reaction pathways involved [1–3]. SEI growth can be both electrochemical and chemical in nature [4,5], and thus, it is a strong function of the potential and degree of lithiation of the electrode. We model the early-stage and long-term growth of SEI by accurately capturing the potential dependence of its formation kinetics as well as long term rate limiting steps. Battery degradation involves a complex interplay of multiple phenomena, most of which are unknown. Our model captures some of the essential trends that we see while cycling hundreds of commercial cells [6]. This is done using the Multiphase Porous Electrode Theory (MPET) framework [7] on graphite (phase separating) and carbon black (non phase separating) particles.
Lithium plating is another key degradation phenomenon that has been elusive, and it becomes important while trying to fast-charge batteries, i.e., 0% - 80% state-of-charge in 30 mins. We show that lithium plating is a key function of electrode morphology, phase-separation dynamics and potential. Phase-separation in graphite is modeled in the electrode using the Cahn-Hilliard Reaction framework described by Bazant [8]. We understand the electrochemistry of the onset of lithium plating with in-situ measurements connected to real time cell potential in a phase-separating electrode [9].
Results indicate that the peak SEI-forming currents are higher for higher driving currents and that SEI only grows during electrode lithiation, i.e. the battery only degrades while being charged. Additionally we capture a transition in the time-dependence of capacity fade from a steep initial drop to a more gradual ‘square-root-of-time’ trend by modeling the SEI as a bilayer with different rate-limiting steps for each type of SEI. We also find that onset of lithium plating is correctly captured only when phase separation in active material is accounted for. Further, the onset of plating is delayed on electrodes with a thick SEI layer – understanding SEI/plating coupling is integral to predicting fast charging manufacturing protocols for LIBs. This work holds promise for the predictive design of procedures [10] for manufacture and formation of LIBs.
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Letters 2019 19 (8), 5140-5148. DOI: 10.1021/acs.nanolett.9b01515