Detailed “mapping” ofinternal resistance in Li-S batteries
Matthew J. Lacey, Kristina Edström and Daniel Brandell
Department of Chemistry – Ångström Laboratory, Uppsala University. E-mail: matthew.lacey@kemi.uu.se

1. Introduction and principle

3. Results and discussion

Frequent and continuous determination of battery internal resistance by a simple
current-interrupt method through the creation of resistance ”maps”, showing changes inresistance as a function of both capacity and cycle number. This new approach isdemonstrated here with an investigation of cell failure in the lithium-sulfur system with Lielectrode excesses optimised towards practically relevant specifications.

Thick Li: 125 µm (25.8 mAh/cm2)
Thin Li: 30 µm (6.2 mAh/cm2)

Figure 4: Specific discharge capacity and coulombic efficiency vs cycle number for lithium-sulfur cells containing
thick (125 mm) or thin (30 mm) Li foil negative electrodes cycled until failure.

Fig. 4 shows the cycling behaviour of two lithium-cells using negative electrodes ofdifferent thicknesses. The positive electrodes are optimised towards realistic
specifications (65% S in electrode, ~2 mAh/cm2, water-based functional binder), and theFigure 1: Typical discharge voltage profile showing repeated current interruptions. Inset is a zoomed viewof a single current interruption, indicating the voltage difference over which the resistance is determined.

electrolyte/sulfur ratio is reduced to 6 µL/mgS-1.

In this work, internal resistance was followed by by modifying a standard C/10 constantcurrent cycling program to include regular current interruptions (0.5 seconds interruption
every 5 minutes). Internal resistance was estimated from each current interruption
assuming Ohmic behaviour. Analysis and plotting of the datasets is easily automatedwith a simple program written in the freely available programming language R.
In this case, measurements with this frequency gives approx. 100-150 individual
measurements of resistance per cycle but only increases the duration of the experiment
relative to the standard cycling program by less than 0.2%.
Compared with common techniques, particularly impedance spectroscopy as widelyused in academic research, the technique benefits from ease of implementation, lack ofrequirement for sophisticated instrumentation and simplicity of analysis.

2. Validation

Figure 5: Specific capacity vs cycle number plots for lithium-sulfur cells with different negative electrode thicknesses,coloured according to the calculated internal resistance at each given point.

The current-interrupt approach was validated by comparison with other conventional
measurements, specifically impedance spectroscopy and current pulse approaches. The
comparison was made by measuring the resistance at approximately the same point
every cycle (1 hour after the start of the charge cycle).

Figure 2: internal resistance for a Li-S cell measured at approximately the samestate of charge over 60 cycles by three different techniques.

Figure 6: Median internal resistance for each (dis)charge step vs cycle number, as calculated from the data given
in Fig. 2.

Figs. 5 and 6 show the “raw” results of a typical experiment and a simple statistical

The resistance as determined by the current-interrupt method is approximately the sum

analysis of the results respectively. In this case the effect of reducing the negativeof electronic and ionic resistances in the electrodes and electrolyte, plus charge transfer

electrode excess on the cycle life and the changes in cell resistance is clear.

(kinetic) resistances, as determined by comparison with impedance spectroscopy.

Supplementary experiments/analysis can reveal further information on the processes
governing cell failure, including:Figure 3: typical impedance spectrum for a Li-S cell.

•SoC-dependent variation in resistance, and changing distribution with increasing cycles•

Asymmetry in charge and discharge•

Evidence of electrolyte decomposition leading to cell failure

4. Summary
The behaviour of Li-S cells optimised towards realistic specifications deserves greater

References and acknowledgements

attention. Reduction of Li excess results in earlier onset of capacity fade and/or cell

The authors wish to acknowledge the Era Net Transport project “MaLiSu” and Vinnova in Sweden for funding.

failure. The resistance “mapping” approach is a simple method for extracting detailed

This work is published as M. J. Lacey et al, Chem. Commun. (2015), DOI: 10.1039/C5CC07167D

information on changes in cell resistance. Such information is highly valuable inevaluating stability of batteries, especially those where resistance varies with SoC.