Unveiling the Full-cycle Degradation Mechanisms of Copper Impurities in Lithium-ion Batteries

Abstract

Preventing thermal runaway from manufacturing defects is critical as lithium-ion battery energy densities rise. Copper （Cu） impurity contamination is highly hazardous, yet its role in progressive, latent degradation remains poorly understood. Here, we establish a quantitative classification framework categorizing cells into high-, latent-, and low-risk groups based on Cu impurity size and concentration. Within the latent-risk regime （0.005– 0.1%）, Cu shifts from a physical obstacle to an active chemical reactant driving cell-wide cascading degradation. Dissolved Cu²⁺ ions act as catalyst to make strong Lewis acid, H+, that accelerate LiPF₆ dissociation, triggering autocatalytic HF generation. This excess HF drives interfacial inorganicization （forming LiF and CuF2）, clogs separator pores up to 9.85 μm deep, and triggers localized graphite exfoliation. Concurrently, HF crosstalk to the cathode weakens Ni–O bonds, causing irreversible bulk structural collapse with anomalous c-axis lattice expansion （0.077%）. This study provides a rigorous foundation for setting strict production impurity limits and developing early-diagnosis platforms for latent battery failures.