Work

Why Electrochemical–Thermal–Mechanical Understanding is Needed to Explain Knees in Lithium-Ion Batteries Degradation

Year: 2026

Type: preprint

Abstract: Lithium-ion batteries are essential to modern life, spanning portable electronics, electric vehicles and grid storage. Yet, their degradation mechanisms remain incompletely understood, particularly the sudden capacity drops known as knee points. In this study, commercial Samsung 50S cells are degraded under various cooling setups and charge/discharge rates, revealing consistent knees in slow-discharge cells even above 80% of initial capacity, while fast-discharge cells degraded beyond 60% without knees. Using a multiphysics characterisation approach, we decipher the origins of these knees: attainment of lower voltages enhances silicon activity, triggering a chain of mechanisms that culminate in sudden failure. Specifically, recurring SEI reformation drives a dual pathway: gas release displaces electrolyte, inducing localised current concentration, overcharge and a first pattern of lithium plating, while simultaneous electrode thickening deforms the jelly roll. This mechanical effect, combined with geometrical heterogeneities from the welded tab design, produces a second pattern of lithium plating in later ageing states, identified as the primary cause of the knees. These findings elucidate the complex coupling of multiscale degradation mechanisms and demonstrate that robust state-of-health monitoring must be holistic, incorporating indicators beyond traditional temperature, capacity and resistance.

Source: ChemRxiv

DOI: 10.26434/chemrxiv.15009177/v1