Non-uniform in-plane current density distribution within current collectors is an intrinsic consequence of finite sheet resistance in large-format lithium-ion cells. It plays a key role in driving localized overpotentials, uneven electrochemical activity, and accelerated aging. Analytical models such as those derived by Wang et al. [1] predict that current spreading within single-sided tab configurations leads to a hyperbolic-cosine spatial profile of the in-plane current density with increasing distance from the tab.

This work presents a non-invasive magnetic-field methodology to experimentally infer such current redistribution in large-format lithium-ion cells and to validate the analytical predictions. In cells with single-sided tabs, electronic currents form closed current paths between the positive and negative tabs. This distribution introduces a transverse in-plane current component which, according to Biot-Savart’s law, generates a measurable magnetic-field component aligned with the longitudinal axis. Due to this directional coupling, the in-plane current redistribution (Jy) creates a longitudinal magnetic field (Bx) which is dominated by electronic currents within the current collectors.

Experiments were conducted on two NMC pouch cells using an array of tunnel magnetoresistive (TMR) sensors connected to a signal conditioning and digital processing stages. Spatially resolved magnetic field measurements were acquired on both surfaces at discrete points along the longitudinal axis during charge and discharge at multiple current rates. The measured magnetic field profiles were well described by hyperbolic-cosine profile functions, consistent with the analytical current-spreading models. Two-dimensional surface reconstruction is a natural next step toward capturing full-field redistribution.

A key result is the robustness of the extracted spatial decay parameter, consistent across all tested current rates, and collectors. It indicates that the observed redistribution pattern is an intrinsic structural property governed primarily by current-collector resistivity and cell geometry, rather than a transient operational effect.

In conclusion, magnetic field sensing becomes a practical, contactless diagnostic tool for quantifying in-plane current inhomogeneity in large-format lithium-ion cells. The proposed approach enables rapid end-of-line quality control, continuous monitoring of current-collector integrity in advanced battery management systems, and non-destructive screening of cells for second-life applications. Potential downstream use of the inferred current distribution for enhanced state estimation is beyond the scope of this work.

Reference
[1] Z. Wang, D. L. Danilov, R.-A. Eichel, and P.H.L. Notten. About the in-plane distribution of the reaction rate in lithium-ion batteries, Electrochimica Acta, vol. 475, art. no. 143582, 2024. https://doi.org/10.1016/j.electacta.2023.143582.