From Pouch-Cell Swelling to Rock-Salt Reconstruction: A Multimodal Mechanistic Framework for 6C Fast-Charging Degradation of NCM

초록

Fast-charging capability is essential for lithium-ion batteries, but performance loss in practical pouch cells often arises from coupled electrochemical-mechanical processes that are hard to identify by conventional postmortem methods. Here, we establish a multiscale, workflow-driven diagnosis that connects nondestructive cell inspection to electrode- and material-level signatures using identical Li(Ni,Co,Mn)O-2 (NCM)-based pouch cells subjected to 90 cycles under 1C/0.2C and 6C/0.2C protocols. Under the aggressive 6C/0.2C regime, the cell exhibits rapid capacity decay, retaining only similar to 47% of its initial capacity at the 90th cycle, accompanied by severe macroscopic swelling and a disproportionate shift of charge capacity into the constant-voltage (CV) step, accounting for over 90% of the total charge capacity, which is attributable to markedly increased polarization. X-ray computed tomography (CT) reveals internal void development and stack disruption without disassembly. Cross-sectional SEM and transmission electron microscopy (TEM) further show that fast charging accelerates mechanical damage in polycrystalline (PC) NCM and promotes near-surface reconstruction toward rock-salt-like domains, while X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) capture concomitant loss of layered ordering and intensified cathode-electrolyte interphase (CEI) chemical evolution. Material-level degradation is quantitatively reflected in a reduction of the XRD I(003)/I(104) ratio from 1.1 (pristine) to 0.9 (6C/0.2C). This study offers a practical framework to identify failure modes and guide durable fast-charge pouch cell design.

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