The room-temperature CCDs for the Li/NaK/LLZTO/NaK/Li symmetric cells are higher on average than that for plain Li metal , reaching a maximum value around 2 mA cm -2 within the areal capacity limits of the data ranging from 3 to 30 mAh cm -2 (Fig. ). In contrast, the Li/LLZTO/Li symmetric cells exhibit critical current densities < 1 mA cm -2 with accessible areal capacities being limited to values below 1.5 mAh cm -2 . This limitation in capacity is due to the rapid increase in cell impedance that is observed beyond 1.5 mAh cm -2 of plated capacity, when stripping and plating at a current density of 0.5 mA cm -2 or higher (Fig. ) .