Pitfalls of Critical Current Density Tests to Investigate Alkali Metal Electrodes

Using lithium metal as anode material promises a drastic increase in energy and power density in solid-state batteries. However, morphological challenges such as local contact loss of the metal electrode or dendrite growth occur during cell cycling,(1) limit the applicable current density. Designing reliable and reasonable testing protocols is of utmost importance to ensure an error-free analysis of the Li|Solid Electrolyte(SE) interface.<br/><br/>Testing the so-called “critical current density” (CCD) via a stepwise increase in applied current density in an alternating direction is often applied. These CCD tests should determine the maximum current density a cell can withstand before undergoing critical failure, which suggests that the CCD is a material property of the SE itself. However, our analysis shows that the CCD is neither an inherent cell nor an SE property. It highly depends on several external factors, such as the used current profile, exact interface conditions, applied pressure and temperature and even the cell geometry and size. This is evident from the spread of CCD values over orders of magnitude obtained on very similar materials in different studies.(2)<br/><br/>Therefore, to interpret the results obtained by CCD tests, it is necessary to know the microscopic mechanism that causes cell failure. This could be a direct failure of the SE, e.g. by cracking and dendrite formation, or it could also be a pore formation that precedes the growth of dendrites and acts as a precursor for short-circuiting by focusing the current at remaining contact spots. As all these effects overlap and gradually worsen and influence each other during a CCD test, data interpretation is quite challenging. We therefore suggest a differentiation between the different failure mechanisms by carefully tailoring the test protocol to induce the desired case, being either a direct short-circuiting or a short-circuiting as a result of pore formation.<br/><br/>For example, we suggest using an intermediate pressure and a waiting time between the current steps of a test to close potentially forming pores when investigating the SE. If instead the SE is a constant during the experiment and the interface conditions are changed, e.g. by interlayers or surface treatments, we suggest the omission of stack pressure and instead concentrate on longer current steps to not miss voiding at the interface. Generally, we hope our guidelines on how to investigate the Li|SE interface will help to streamline the efforts made by the community and ensure a better comparability between the results of different groups.<br/><br/>(1) Krauskopf, T.; Richter, F. H.; Zeier, W. G.; Janek, J. Physicochemical Concepts of the Lithium Metal Anode in Solid-State-Batteries. <i>Chem. Rev.</i> <b>2020</b>, <i>120 (15)</i>, 7745-7794.<br/>(2) Flatscher, F.; Philipp, M.; Ganschow, S.; Wilkening, H.M.R.; Rettenwander, D. The Natural critical current density limit for Li₇La₃Zr₂O₁₂ garnets. <i>J Mater. Chem. A. </i><b>2020</b>, <i>8</i>, 15782-15788