context: Long-term cyclic stability of symmetric cells. One of the most promising applications of metal anodes is to combine them with a sulfur or air cathode with a high energy density. It is thus important to find an efficient way of infusing the alkali metal inside the anode scaffold. To introduce Na in the network, the anode was pre-assembled by stacking the CNF film on top of a pure Na foil. The Na foil was manually pressed into a thickness ranging between 200 and 400 μm. The use of this stacked design saved materials and processing time better than the classical sacrificial-cell electrodeposition method. The thickness of Na foil could be further reduced using industrial pressing tools. Fig. 6a presents a schematic of the stacking scheme of the assembled electrode which allowed Na to be effectively stripped and plated by removal and re-infiltration into the fiber network. Fig. 6b presents a side view of the CNF film stacked on top of a Na layer after 6 mA h cm−2 of Na was stripped and re-deposited at a current density of 1 mA cm−2. The image signifies successful re-infiltration of Na in the fiber network after the two-step process. The fiber diameter increased to ∼600–800 nm from the initial ∼150 nm, proving a large load of deposited Na. The final fiber thickness obtained with the stacked design was compatible with the thickness obtained with regular electrodeposition, indicating that a majority of Na was infiltrated from the initial film into the network. In view of the large variation in the measured Na thicknesses, however, it may be possible that some part of Na had been redeposited back onto the original Na film, as shown in the schematics. The above fiber network was likely to hinder any dendritic growth resulting from this scenario. The overall ZNO@CNF film thickness remained at ∼100 μm after infiltration, suggesting that the network could withstand the mechanical pressure inside of the cell while retaining its original shape. The smooth top view of the same anode shown in Fig. S26† indicates that the ZnO@CNF film was able to hold and control the deposited Na even with a high areal loading of 6 mA h cm−2.
answer: sulfur 