In situ neutron radiography of lithium-ion batteries: the gas evolution on graphite electrodes during the charging

doi:10.1016/j.jpowsour.2003.11.065

Journal of Power Sources

Volume 130, Issues 1–2, 3 May 2004, Pages 221-226

https://doi.org/10.1016/j.jpowsour.2003.11.065 Get rights and content

Abstract

In situ neutron radiography (NR) was used to study the gas evolution on graphite electrodes in lithium-ion cells containing different PVDF-based gel-type electrolytes. The amount of gas bubbles and channels was calculated by image analysis. Gas production was extremely high in the case of the electrolyte containing ethylene carbonate (EC) and propylene carbonate (PC) (2:3, w/w), 1 M LiClO₄. About 60% of the electrode surface consisted of the gas phase which resulted in an inhomogeneous local current distribution. In contrast, the electrolyte containing EC and γ-butyrolactone (GBL) (1:1, w/w), 1 M LiBF₄ only showed a small increase of the gas volume between the electrodes of about 3%. In situ NR also revealed the displacement of the electrolyte due to gas evolution and volume changes of the electrodes.

Introduction

In situ techniques are useful to study the charge/discharge processes occurring in lithium-ion batteries [1], [2], [3]. The formation of gas during the first charge of these batteries is an important criterion for the choice of electrode materials and the electrolyte solution. High amounts of gas evolved would cause severe leakage and safety problems. We reported that the kind of the evolved gases can be determined using differential electrochemical mass spectrometry (DEMS) [4], [5], [6]. However, using DEMS it is difficult to determine the total gas volume. Furthermore, the interest in both, the spatial distribution and kinetic behavior of the gas evolution as well as the sensitivity of a lithium-ion battery towards humidity necessitate a rapid and non-destructive inspection method to observe the battery during electrochemical cycling.

Neutron radiography (NR) has been developed to study various technical devices and archaeological objects [7], [8]. The application of NR to lithium-ion batteries is very interesting due to the favorable properties with respect to mass attenuation on materials commonly used in these batteries, above all the strong attenuation by hydrogen-containing materials. We applied this technique to commercial lithium-ion batteries in order to study both, the production of gas and the level of the electrolyte solution, under practical conditions [9]. However, the use of arbitrary combinations of electrode materials and electrolytes, in particular those producing a larger gas volume, requires specially designed laboratory test cells.

We developed such a cell, which allows the study of both, the gas evolution rate and the gas spatial distribution. The goal was to compare different electrolyte solutions and to show the consequences of their choice for the electrochemical behavior of the lithium-ion cell. Two extreme cases were chosen for demonstration in this publication: (i) a propylene carbonate (PC) based electrolyte known to cause important graphite exfoliation under production of propylene gas, and (ii) an ethylene carbonate (EC)/γ-butyrolactone (GBL) based electrolyte which shows only minor emission of gas [5], [10], [11], [12].

Section snippets

Lithium-ion cells and electrochemical cycling

A specially designed laboratory test cell was developed. The schematic drawing of the cell is shown in Fig. 1. Both, the cell housing and the current collector plates were made of aluminum, known to have only a small cross section for the neutron beam [9]. To avoid alloying of lithium with the aluminum current collector plate of the negative electrode, the aluminum was electroplated with copper (20 μm). The negative electrodes were made of 90% TIMREX^® SLP30 graphite (TIMCAL SA) and 10% PVDF

Results and discussion

Fig. 2 shows the NR image of the in situ cell. The area of interest, representing the properties of the two electrodes and the gel-type electrolyte, can be found as the lighter circle in the middle of the image. Gas bubbles coming from the argon imprisoned in the electrolyte upon the cell assembly are seen as inhomogeneities and do not disturb the measurement. The varying intensity outside of the central circle is due to the gel electrolyte that flew besides the electrode area upon the cell

Conclusions

Neutron radiography was demonstrated to be a very useful tool to investigate in situ the gas evolution and the level of electrolyte solution in gel-type lithium-ion cells during cycling. The determination of the lateral gas distribution is possible and reveals the formation of growing gas channels and bubbles. PC-containing electrolytes cause the production of large amounts of gas, which results in an unfavorable distribution of the local current density and reduction of the cell charge

Acknowledgements

We thank Mr. Gabriel Frei for the assistance, permitting easy operation at the NEUTRA beam line at PSI.

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In situ neutron radiography of lithium-ion batteries: the gas evolution on graphite electrodes during the charging

Abstract

Introduction

Section snippets

Lithium-ion cells and electrochemical cycling

Results and discussion

Conclusions

Acknowledgements

J. Power Sources

J. Power Sources

J. Power Sources

J. Electroanal. Chem.

J. Power Sources

Nucl. Instrum. Methods B

Appl. Spectrosc.