Li rod

Triethylene glycol dimethyl ether (triglyme, 99%) and molecular sieves
(pore size of 3 Å, diameter of 1–2 mm) were purchased
from Alfa Aesar. Ethylene carbonate (EC, 98%) and dimethyl carbonate
(DMC, 98%) were purchased from Sigma-Aldrich. In order to remove the
residual moisture, molecular sieves were first activated by heating
to 180 °C under vacuum overnight and then added to the solvents.
LiTf (LiCF₃SO₃, 98%, Sigma-Aldrich) and NaTf
(NaCF₃SO₃, 99.5%, Solvionic) salts were dried
prior to use (at 120 °C under vacuum, overnight). The moisture
in the electrolytes was controlled to be under 20 ppm, as confirmed
by Karl Fischer titration performed in an Ar-filled glovebox (atmosphere:
O₂ < 0.1 ppm, H₂O < 0.1 ppm).
Due
to the high reactivity of lithium and sodium metals, surface degradation
is expected even in an Ar-filled glovebox. Thus, alkali metals used
(Li rod with 99.9% trace metal basis and Na cubes containing mineral
oil with 99.9% trace metals basis, both purchased from Sigma-Aldrich)
were cut freshly each time right before the electrodes were prepared.
Li and Na were subsequently sandwiched between two Celgard separators,
roll-pressed to approximately the same thickness (0.15 mm), and then
cut into disks with a diameter of 10 mm.

The Li rod (99.9%, trace
metals basis) and Na cubes stored in a mineral oil (99.9%, trace metals
basis) were purchased from Sigma-Aldrich and were stored in a glovebox
under an Ar atmosphere. S powder (99.998%, trace metals basis, Sigma-Aldrich)
was dried in a vacuum oven at 100 °C overnight before use. Li
and Na metals were wiped with a tissue paper to remove the residual
oil and freshly cut with stainless-steel blades. Subsequently, they
were sandwiched between two Celgard separators and roll-pressed to
approximately the same thickness (0.15 mm). The Celgard separator
was used during pressing since Li and Na adhere strongly to the stainless-steel
pressing plate. After the Li(Na) pressing procedure, discs with a
diameter of 10 mm were cut and attached to stainless steel discs with
a diameter of 18 mm. The stainless-steel discs act as a supporting
substrate since the alkali metals easily lose their form. The so-prepared
electrodes were placed in homemade glass ampoules with a length of
600 mm together with the S powder. The glass ampoule was placed in
an oven and heated at different temperatures (in the temperature range
from 298 to 433 K). All processes were performed in a glovebox (both
O₂ and H₂O levels were below 0.1 ppm). A schematic
diagram of the Li_xS_y(Na_xS_y) synthesis is shown in Figure 1.