In-Situ Neutron Diffraction Study of Lithium-Ion Battery Equalization Effects
Authors: Wilhelm Joern , Zinth Veronika , Erhard Simon Vincent , Buchberger Irmgard , Jossen Andreas 
Published: February 2020

Introduction

Graphite is the predominantly used anode material in commercial lithium-ion batteries. However, there are limitations if batteries are cycled at low temperature. Low temperature operation is linked to lithium plating on graphite electrodes during charging and decreased capacity after discharge1. The exponential deceleration of electrochemical processes leads to long equalization times of non-equilibrium states after discharge. In order to optimize battery electrodes detailed knowledge of lithium concentration gradients due to electrode morphology is helpful. For application, understanding of relaxation processes is a challenge in the design of battery control systems for low frequency operations. Modeling relaxation phenomena mostly relies on voltage and impedance data2. In-situ neutron diffraction allows for non-destructive observation of lithium intercalation processes adding information on active material phase composition during operation and relaxation.

Experimental

We performed neutron diffraction measurements on a commercial high power LiCoO2/ graphite pouch cell at STRESS-SPEC, MLZ3. Reflection data was collected in the 30-40° 2-theta range and the Li1-xC6 graphite reflection was continuously monitored during discharge and subsequent relaxation. In all tests constant current discharge was performed from 100% to 30% state of charge where LiC12 is delithiated through multiple stages. Tests were conducted at discharge current densities ranging from 1.25 mA cm-2 up to 7.5 mA cm-2 (with respect to the electrode surface area) at controlled temperatures of -10 °C, 0 °C and 25 °C. The relaxation after discharge was monitored for up to 6 hours in 3 min measurement intervals.

A p2D simulation model based on the porous electrode model by Doyle, Fuller and Newman4 was set up to model experimental relaxation timescales. The classical modeling approach is extended to include a simplified particle size distribution of the electrodes. Modeling parameters such as active materials open circuit voltage, layer thicknesses, and porosity are measured from the cell. The particle distribution is extracted from SEM-images and tortuosity properties are estimated using BrugemanEstimator5. Discharge and relaxation are modeled in low and high discharge rate scenarios under the variation of parameters such as solid and liquid diffusivities.

Results

We observe a lithiation gradient in the graphite anode during the discharge and subsequent relaxation process. Several Li1-xC6 phases with different lithium content coexist. The extent to which the phases are polarized is higher at low temperature (-10°C) compared to room temperature. After discharge, the inhomogeneity gradually disappears with time and the coexisting phases coalescent to a mean phase (fig.). This process is temperature dependent and takes ~30 min at room temperature and over 6 h at -10 °C. The timescale of this process can be reproduced by the extended p2D battery model and the lithiation gradient can be geometrically interpreted. Experimental data and modeling results will be presented and discussed in terms of electrode morphology effects.

Acknowledgments

This work was funded by the German Federal Ministry of Education and Research (BMBF), funding code 03x4633a (ExZellTUM).

Reference

T.M. Bandauer et al., J. Electrochem.
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D.M. Bernardi et al., J.
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M. Hofmann et al., Physica B, 385-86(2006), pp 1035-1037

M. Doyle, et al., J. Electrochem. Soc., 140(1993), pp 1526-1533

M. Ebner, et al., J. Electrochem. Soc., 162(2) (2015), A3064-A3070

Figure 1