# Research on the Evolution Law of Electrochemical Impedance Spectral Characteristics of Lithium-Ion Batteries in Different States

## TL;DR

This paper studies how the electrochemical properties of lithium-ion batteries change under different conditions to improve their monitoring and reliability.

## Contribution

The study systematically reveals how impedance characteristics evolve with SOC, SOH, temperature, and mechanical compression in NCM batteries.

## Key findings

- Higher SOC and temperature reduce impedance, with Rct showing a U-shaped dependence on SOC.
- Battery degradation (lower SOH) increases RSEI and Rct, indicating SEI thickening and electrode damage.
- Mechanical compression increases resistances, especially Rct at high SOC, due to structural deformation.

## Abstract

Lithium-ion batteries (LIBs) are pivotal for energy storage in electric vehicles and renewable systems, but how to effectively monitor their conditions and ensure their operational reliability is still a concern today. This study employs electrochemical impedance spectroscopy (EIS) to systematically investigate the evolution of impedance characteristics in nickel–cobalt–manganese oxide (NCM) lithium-ion batteries (LIBs) under varying states of charge (SOCs), states of health (SOHs), temperatures, and mechanical compression displacements. Results reveal that higher SOC and temperature reduce impedance by enhancing ion kinetics and interfacial activity, with Rct (charge transfer resistance) exhibiting a U-shaped dependence on SOC, minimized at 40–60%. As SOH declines from 100% to 80%, RSEI (SEI film resistance) and Rct increase progressively, reflecting SEI thickening and electrode degradation. Mechanical compression (0–8 mm) elevates all resistances, particularly Rct at high SOC, due to structural deformation and hindered diffusion. DRT (distribution of relaxation times) spectra highlight amplified low-frequency peaks with aging and low SOC, underscoring diffusion limitations. These findings elucidate multi-scale failure mechanisms, from interfacial polarization to structural instability, providing a framework for non-invasive health monitoring and lifetime prediction.