Capturing the Current-Overpotential Nonlinearity of Lithium-Ion Batteries by Nonlinear Electrochemical Impedance Spectroscopy (NLEIS) in Charge and Discharge Direction

Sabine Ernst,Nicolas Schlüter,Uwe Schröder,Tom Patrick Heins

doi:10.3389/fenrg.2019.00151

Sabine Ernst, Nicolas Schlüter + Show 2 more

Open Access

https://doi.org/10.3389/fenrg.2019.00151

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Abstract

In this paper, a Nonlinear Electrochemical Impedance Spectroscopy (NLEIS) method is presented that allows capturing the nonlinearity of current and overpotential of a lithium-ion battery individually in charge and discharge direction. A DC bias is applied to the battery to shift its operating point to the nonlinear region of current and overpotential. An alternating current of a low amplitude (AC) is simultaneously superimposed in order to investigate the system additionally in the time domain. NLEIS cell spectra and selected electrode-resolved NLEIS spectra are recorded as a function of state of charge and temperature. Furthermore, Distribution of Relaxation Times (DRT) plots obtained from EIS measurements provide information, which electrochemical processes correlate with the occurrence of nonlinear distortions of current and overpotential. Moreover, the occurrence of overpotentials and their degree of nonlinearity at cathode and anode as a function of the state of charge are determined by current pulse measurements.

Highlights

Lithium-ion batteries in their entirety represent complex electrochemical systems
We present an approach to Nonlinear Electrochemical Impedance Spectroscopy (NLEIS) where the operating point of the lithium-ion battery on the Iη-curve is shifted by a direct current (DC) bias to the nonlinear region of current and overpotential
In order to gain a deeper understanding of when nonlinearity in the studied lithium-ion batteries becomes relevant and whether the electrode reactions are asymmetric, we initially recorded Iη-curves by current-pulse measurements (I-Pulse)

Summary

INTRODUCTION

Lithium-ion batteries in their entirety represent complex electrochemical systems. Their complexity is mainly reflected in the multitude of characteristically different electrochemical processes taking place within the battery. In this work, an approach to Nonlinear Electrochemical Impedance Spectroscopy (NLEIS) will be presented to investigate the nonlinear current-voltage ratio of lithium-ion batteries in charge and discharge direction individually. After intercalating into the active material, the lithium ions diffuse toward free lattice sites in the cathode material (in case of charging the same process takes place at the anode). This represents the solid-state diffusion process (ηdiff,solid) (Jossen, 2006). We focused on the investigation of electrochemical processes causing nonlinear distortion of current and overpotential such as charge transfer and diffusion processes (Figure 1). The result gives the magnitude of the corresponding higher harmonic (Hx)

MEASUREMENT METHODS

RESULTS

Current Pulse Results

CONCLUSION