In-depth Notes on Lithium-Ion Battery Degradation Modes and their Analysis

Overview of Lithium-Ion Battery Degradation Modes

  • Importance of understanding degradation modes to reduce complexity in studying aging mechanisms.

Aging Mechanisms Grouped into Three Modes

  • Conductivity Loss (CL): Increase in ohmic resistance due to damage to current collectors and binder degradation.
  • Loss of Lithium Inventory (LLI): Caused by solid electrolyte interface (SEI) formation, electrolyte decomposition, and lithium plating.
  • Loss of Active Material (LAM): Results from physical damage and chemical reaction decompositions affecting active electrode material.

Electrochemical Impedance Spectroscopy (EIS)

  • EIS is used for non-destructive identification and quantification of degradation modes.
  • Method enhanced by the use of equivalent circuit models for fitting impedance spectra.

Battery Cycle Life Test Overview

  • Experimental Setup: Utilized commercial nickel-cobalt-aluminum (NCA) lithium-ion batteries.
  • Cycle Life Test Matrix: Conducted under various temperatures and charge/discharge rates to assess degradation under different conditions.
    • Groups:
    • Group 1#: 25°C, 1–1 C
    • Group 2#: 25°C, 0.5–1 C
    • Group 3#: 45°C, 1–1 C
    • Group 4#: 45°C, 0.5–1 C

Impedance Measurement Process

  • Measurements were taken after every 50 cycles at specific state of charge (SoC) levels: 20%, 50%, 80%.
  • STabilization for 3 hours before measuring to eliminate polarization effects.
  • The excitation current used was tested at 300 mA, 500 mA, and 800 mA.

Impedance Spectrum Characteristics

  • Observations from the impedance spectra showed:
    • Potential for two separate semi-circles in high-frequency and mid-frequency regions.
    • Transition in the shape indicative of aging progression in the battery.

Quantification Methodologies

  • Second-Order Equivalent Circuit Model: Used for impedance curves with two semi-circles; relates resistances to degradation modes.
    • Parameters correspond as:
    • Rohm (CL), Rsei + Rct (LLI), Rw (LAM)
  • First-Order Equivalent Circuit Model: Introduced for cases showing single semi-circle behavior, combining Rsei and Rct into one resistance, Rc.

Results Summary

  • Degradation modes quantified exhibited upward trends correlated with aging, particularly LLI and LAM.
  • Impact of external factors (SoC and excitation) analyzed:
    • Higher temperatures (45°C) showed greater LLI due to accelerated SEI growth.
    • Reduction in charge current affected LAM and CL, with high current fostering side reactions.

Statistical Analysis of Degradation Modes

  • Two-way ANOVA concluded:
    • SoC had significant impacts on LLI and LAM values.
    • No significant impact from varying excitation intensities.
    • Notably, significant differences found between low SoC (20%) and both medium-high SoCs (50% and 80%).