Insights into soft short circuit-based degradation of lithium metal batteries

The demand for electric vehicles with extended ranges has created a renaissance of interest in replacing the common metal-ion with higher energy-density metal-anode batteries. However, the potential battery safety issues associated with lithium metal must be addressed to enable lithium metal battery chemistries. A considerable performance gap between lithium (Li) symmetric cells and practical Li batteries motivated us to explore the correlation between the shape of voltage traces and degradation. We coupled impedance spectroscopy and operando NMR and used the new approach to show that transient (i.e., soft) shorts form in realistic conditions for battery applications; however, they are typically overlooked, as their electrochemical signatures are often not distinct. The typical rectangular-shaped voltage trace, widely considered ideal, was proven, under the conditions studied here, to be a result of soft shorts. Recoverable soft-shorted cells were demonstrated during a symmetric cell polarisation experiment, defining a new type of critical current density: the current density at which the soft shorts are not reversible. Moreover, we demonstrated that soft shorts, detected via electrochemical impedance spectroscopy (EIS) and validated via operando NMR, are predictive towards the formation of hard shorts, showing the potential use of EIS as a relatively low-cost and non-destructive method for early detection of catastrophic shorts and battery failure while demonstrating the strength of operando NMR as a research tool for metal plating in lithium batteries.


Galvanostatic Electrochemical impedance spectroscopy
Galvanostatic Electrochemical impedance spectroscopy (GEIS) is a simple and non-destructive technique which measures impedance during cycling.As such, GEIS has been used for in-situ impedance measurements previously for measuring the impedance of solid electrolyte-based cells. 1 The symmetric cells are sensitive to small voltage perambulations; thus, GEIS is expected to have a lesser impact on the cell.GEIS spectra were acquired in a narrow frequency range (500-0.8Hz), probing three points per decade.The narrower range minimises the effect on the cell since the spectrum acquisition lasts only nine seconds, and low frequencies are avoided.Indeed, the prolonged galvanostatic experiments resulted in typical voltage traces, despite the disruptions due to the GEIS (which resulted in the spikes seen on the voltage traces, see Figures 5a, 6, S4-7).
Plots of impedance intensity vs time show similar trends comparing the traces at the entire range of the measured frequencies (Figure S2).The measurements at 9 Hz were chosen to represent the GEIS measurements.

Impedance analysis
To demonstrate how the nature of charge transport affects the GEIS spectra, the Nyquist plots were fitted using EC-lab software V 11.23.
The medium frequency semi-circle attributed to the SEI shifted to higher frequencies; hence, since the bulk electrolyte resistance is not likely to change, we assigned this to introducing an electronic component to charge transport in the soft shorted cell.

Operando NMR
The stacked plot of the acquired 7 Li NMR spectra (Figure S14, right) of the Li metal plating in LP30 shows two different metal signals changing during the experiment.At the beginning of plating, only one metal signal is detectable with a chemical shift of 247 ppm.When the plating started, a second signal appeared with a chemical shift of 263 ppm, which was assigned, due to the BMS effect, to the formed microstructures on the surface of the electrode. 2This new metal peak increases until the cell is shorted after 17 hours of plating.This short was detected with the impedance and galvanostatic measurements (Figure S14, left).After the short circuit, the shape and chemical shift of the metal signals stays constant, which indicates a permanent short circuit in the cell.This behaviour can be explained by the fact that no further plating occurs after 15 hours due to a short circuit in the cell.The voltage profile shows a normal plating curve around 15 hours, indicating a soft short degradation mechanism in the cell that prevents further Li metal plating.However, after 25 hours, the voltage and the impedance drop to zero, indicating a hard short circuit.(Figure S15, left).

Figure S2
Figure S2 Comparison of impedance intensities |Z| against at different frequencies across the range being investigated, recorded during galvanostatic plating of Li in LP30 electrolyte 1 mAcm -2 in coin cell.The impedance intensity follows a similar trend in 500, 9 and 0.8 Hz.

Figure
Figure S5Voltage-time profile of plating of 16 mA h cm -2 at 3.2 mA cm -2 in lithium symmetric cell, LP30 +FEC electrolyte, GF separator.EIS was measured at 10°C.GEIS intensity at 9 Hz is presented.

Figure
Figure S6Voltage-time profile of galvanostatic plating of 16 mA h cm -2 at 3.2 mA cm -2 in lithium symmetric cell, LP30 electrolyte, GF separator.EIS was measured at 10°C.GEIS intensity at 9 Hz is presented.

Figure S11
Figure S11 GEIS Nyquist plots measured with 100 µA amplitude at RT during Li plating at 0.5 mA cm -2 in an NMR in-situ cylindrical symmetric cell, using LP30 + FEC electrolyte with GF separator.Magnification of the plot at 5-15 mA h cm -2 .

Figure S14
Figure S14Operando 7 NMR spectra and impedance (GEIS) measured at 8 Hz intensities measured during unidirectional Li plating at 1 mA cm -2 in an NMR in situ cylindrical symmetrical Li cell with LP30 electrolyte.Measuring a cell with LP30 + FEC shows a similar behaviour during the first 15 hours of the experiment.During steady plating, a new signal with a higher chemical shift (265 ppm) next to the bulk metal peak (246 ppm) appears.2The new signal increases in intensity until 15 hours, and after that, both metal peaks stay constant in intensity and shape until the end of the experiment (FigureS15, right).

Figure S15
Figure S15Operando 7 Li NMR spectra and impedance (GEIS) measured at 8 Hz intensities measured unidirectional Li plating at 1 mA cm -2 in an NMR in situ cylindrical symmetrical Li cell with LP30 + FEC electrolyte.