During the past decades, the development of alternative energy sources has become increasingly important as the growing consumption of non-regenerative fossil energy poses a threat to the environment. Hence, the development of batteries with performance beyond the intrinsic limits of lithium-ion batteries plays an important role. Especially lithium metal anodes show highest volumetric and gravimetric energy density of all anode materials, however, suffering from safety issues and capacity fading due to uncontrolled electrodeposition.[1] One major issue is short circuits, which refer to small local electrical contacts between the electrodes. These contacts are limiting the performance of the battery and can lead to hazardous situations. Although lithium plating has been studied widely, a better understanding of the short-circuiting mechanisms and metal battery failure is required

The Li//Li symmetric cell is one basic configuration to study the degradation mechanism correlated with electrodeposition and interface layers. The cycling time of Li symmetric cells has been regarded as a key metric indicating the metal anodes’ lifespan. However, there is a considerable performance gap between symmetric and realistic lithium metal cells. Developing a reliable testing procedure for lithium metal cells is critical for realising the emerging “anode-free’’ and “beyond lithium-ion” batteries, like Li-S and Li-O₂ batteries.

Therefore, the involved local structural changes that correlate with the electrochemical processes need to be unveiled during the operation of lithium metal batteries, suitably by in situ methods. We coupled in situ impedance spectroscopy and operando NMR for the first time to detect that transient “soft”-short circuits are formed under realistic cycling conditions. Especially this degradation mechanism is typically overseen as their electrochemical signatures are often not distinct.[2]

The detection of reversible soft shorts during a symmetrical cell polarization experiment suggests that the critical current density should be redefined to reflect the current density at which the degradation is not recoverable anymore. Furthermore, we showed that medium-frequency GEIS, as a readily available and low-cost technique, could be used to predict upcoming catastrophic battery failures. [3] Hence, this work will potentially contribute to the development of low-cost state-of-health battery analysis that has the potential to be implemented in electric vehicles and mobile electronics. If implemented, the customers will benefit from safer and higher-energy batteries.

Therefore, in situ NMR and impedance spectroscopy are a powerful and non-destructive method combinations to investigate a key problem that leads to the degradation of lithium metal batteries and potential safety issues. This understanding is crucial to improve the safety of next-generation batteries and enables faster commercialization of, e.g., Li-S, anode-free (lithium and sodium) and solid-state batteries.