Lithium metal cells incorporating the eutectic Na-K alloy layer were assembled in a similar manner to those of the plain lithium cells except that ~5 μL of eutectic Na-K alloy (68 mol% K) was spread on one face of the punched lithium disc. The side of the lithium disc with the added Na-K alloy was placed in contact with the LLZTO solid electrolyte disc. Before electrochemical cycling, certain cells incorporating the Na-K alloy were subject to the same heat treatment as the plain lithium cells, whilst the remainder were not subject to any heat treatment. During electrochemical cycling, some cells had an applied stack pressure of 1.5 MPa (calibrated independently with a compression force gauge). For the cells cycled under low pressure (~75 kPa), the springs in the fixture were compressed to a few percent strain based on visual inspection.