Dead Li Formation in Low-Pressure Anode-Free Polymer Electrolyte Battery Configuration: An Operando and Ex Situ SEM Study at High and Low Current Densities

Lithium loss during cycling occurs in low-pressure anode-free Li metal-solid polymer electrolyte cells due to the irreversible formation of dead lithium. Here, operando and ex situ scanning electron microscopy (SEM) were used to observe lithium plating morphology in cross sections of an anode free Li|TPV|Cu cell with a stiff thermoplastic vulcanizate electrolyte at low and high current densities. Large, dense irregular morphology was observed by ex situ methods at a low current density of 0.1 mA cm −2 without apparent failure. Post-mortem analysis of the plated lithium by 7 Li nuclear magnetic resonance spectroscopy showed that dead lithium formation was more prevalent than dendrite growth. The origin of this irregular plating pattern is suspected to come from the initial solid electrolyte interphase formed between the lithium and the current collector, as observed by X-ray photoelectron spectroscopy. Increasing current density to 0.3 mA cm −2 resulted in the morphology of the plated lithium becoming more uneven. Dead Li formation was triggered in SEM operando mode with a staircase polarization between 0.3 and 1.2 mA cm −2 until 3.7 mAh cm −2 of charge was transmitted. Irregular dead lithium is likely formed in both current regimes due to difficulties in homogeneous lithium plating in the anode-free configuration using a single polymer electrolyte.