A Formal Analysis of Internal Resistances of Li‐Ion Cells Using EIS and DRT to Simultaneously Probe Both Working Electrodes

ABSTRACT Lithium‐ion battery performance degrades over time due to complex aging mechanisms, including lithium inventory loss, structural changes in electrode materials, and increases in contact resistance. Understanding these processes is essential for improving battery performance and extending cycle life. This work presents a combined electrochemical impedance spectroscopy and distribution of relaxation times approach using a three‐electrode configuration to investigate aging and state‐of‐charge (SOC)‐dependent internal resistances in NMC811│graphite Li‐ion cells. By simultaneously probing the cathode, anode, and full‐cell responses and deconvoluting overlapping impedance features using DRT, we identify distinct contributions from particle‐particle resistance, SEI resistance, intercalation (charge‐transfer) kinetics, and solid‐state diffusion. Our results show that cathode processes dominate impedance growth at high SOC due to structural degradation and surface reconstruction, whereas anode contributions arise mainly at low SOC through increasing intercalation‐kinetic limitations. SEI‐related resistance stabilizes after the initial cycles, while contact resistance and diffusion impedances grow steadily with aging. These trends correlate closely with SEM, XRD, and Raman evidence of particle cracking, lattice strain, and structural disorder, establishing strong links between electrochemical signatures and physical degradation. Overall, this methodology provides a quantitative, component‐resolved framework for diagnosing electrode‐specific failure modes and guiding materials or interface‐engineering strategies to enhance the lifetime of lithium‐ion cells.