Phantom LAM and LLI: Resistance and Hysteresis Bias in Voltage-Curve Degradation Mode Analysis

TL;DR

This paper investigates how voltage measurement artifacts, such as hysteresis and ohmic drops, can lead to false identification of material loss in battery degradation analysis, proposing correction methods for more accurate diagnostics.

Contribution

It introduces a correction protocol for voltage curves that accounts for ohmic drops and hysteresis, improving the accuracy of degradation mode attribution in lithium-ion batteries.

Findings

IR correction improves LAM and LLI estimation accuracy

Voltage window adjustments affect degradation component detection

Correcting ohmic drops reduces false attribution of material loss

Abstract

Degradation mode analysis (DMA) is widely used to decompose capacity fade into loss of lithium inventory (LLI) and loss of active material (LAM) from low-rate voltage-capacity data. Yet the measured trace is a pseudo-OCV (pOCV) that includes two non-degradation contributions: an SOC-dependent ohmic drop and intrinsic charge-discharge hysteresis, especially in graphite--silicon oxide (C/SiOx) negative electrodes. We show these can dominate attribution and generate Phantom LAM/LLI --apparent material loss created by curve registration, branch choice and voltage-windowing rather than true degradation. Using two commercial 21700 cells (LG M50T: higher resistance; Molicel P45B: lower resistance), we extract an SOC-dependent instantaneous resistance $R_\Omega(\mathrm{SOC})$ from the first $\sim$50,ms pulse step and apply an IR correction to pOCV before fitting. In LG M50T, IR correction lifts the low-rate discharge pOCV by $+13$--$27$,mV with ageing; without it, PE-LAM is increasingly under-diagnosed (to $-8.80%$ relative error at late life) and LLI is suppressed (median $-3.07%$), with compensating inflation of apparent graphite loss. In P45B, on a branch-fair $3.0$--$4.2$,V window, end-of-life charge-branch DMA reports higher PE-LAM ($+3.42$,pp) and LLI ($+5.36$,pp), while the discharge branch recovers larger Si-LAM (discharge--charge difference to $+14.38$,pp). Raising the lower cutoff ($2.5$--$4.2 \rightarrow 3.0$--$4.2$,V) further under-reports Si-LAM by $13.61$,pp by removing the Si-sensitive low-voltage tail. We propose a practical protocol: correct only the instantaneous ohmic term, harmonize the voltage window, and base quantitative attribution on the discharge branch, treating anomalous/negative component LAMs on charge as allocation artefacts rather than recovery.