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Cui et al. Energy Mater 2023;3:300034 https://dx.doi.org/10.20517/energymater.2023.19 Page 7 of 12

+
In addition, there are multiple Li transport pathways in the PO-PU-LiTFSI electrolyte [Figure 3A]. In the
+
case of a high degree of LiTFSI dissociation and high LiTFSI content, Li can transport not only through the
movement of the PU segments in the amorphous regions but also through the ionic channels constructed
-
by aggregated cation/anion clusters [18-20] . Meanwhile, the migration of TFSI is limited by the aggregated ion
clusters and the interaction between the urethane/urea group with TFSI . The dissociated lithium salts also
-
act as plasticizers to increase the amorphous regions of the polymer electrolyte [Figure 2C and
Supplementary Figure 5]. Therefore, the PO-PU-LiTFSI electrolyte exhibits high ionic conduction.

The PO-PU-LiTFSI electrolyte shows high electrochemical stability. The electrochemical stability window of
the PO-PU-LiTFSI electrolyte is 4.57 V, which is higher than that of PEO-based SPE (about 4 V)
[Figure 2F] [30] .

It is believed that the low electrochemical stability of PEO-based electrolytes is due to the high reactivity of
their terminal hydroxyl groups In this study, the terminal hydroxyl groups of PEG were consumed by
[30]
.
polyaddition reaction, so the PO-PU-LiTFSI electrolyte exhibits high electrochemical stability.
To further evaluate electrochemical performance of the PO-PU-LiTFSI electrolyte, symmetric Li||Li cells
and solid-state Li-S batteries were assembled. As shown in [Supplementary Figure 8] the Li||Li cell using the
PO-PU-LiTFSI electrolyte can cycle for about 800 h with a stable overpotential (about 40 mV). The results
show that the electrode/electrolyte interfaces of the battery are stable, and the PO-PU-LiTFSI electrolyte
exhibits high stability during cycling. At a high S loading of approximately 4 mg cm , the solid-state Li-S
-1
-1
battery using the PO-PU-LiTFSI electrolyte delivers a specific capacity of ~610 mAh g after testing for 125
cycles, which is one of the best performances among related solid-state Li-S batteries [Figure 5A and
Supplementary Table 1]. The solid-state Li-S battery using the electrolyte also exhibits a good rate capability
[Figure 5B]. At a S loading of approximately 3 mg cm , the solid-state Li-S battery delivers specific capacity
-1
values of 1,297, 994, and 822 mAh g at rates of 0.05 C, 0.1 C, and 0.2 C, respectively. The cycling
-1
performance and rate capability of the solid-state battery deteriorate when the LiTFSI content in the
electrolyte decreases, which is due to the poor contact between the electrolyte and the electrodes [Figure 5B
and Supplementary Figure 9].

The solid-state Li-S batteries exhibit low interfacial impedance and stable electrode/electrolyte interfaces
during cycling. Figure 5C records the voltage profiles of the batteries using PO-PU-LiTFSI electrolytes with
different LiTFSI contents. The discharge curve of solid-state batteries displays two plateaus at 2.3 and 2.1 V,
corresponding to two stages of lithiation of S , showing a similar discharge curve to liquid Li-S batteries. The
8
discharge plateau at around 2.3 V is associated with the ring opening of crown S and conversion into long-
8
chain polysulfides (Li S ; 3 ≤ n ≤ 8). The second plateau, corresponding to about 2.1 V, is attributed to the
2 n
further conversion of long-chain Li S to Li S. The battery using the electrolyte with 80 wt% LiTFSI content
2
2 n
exhibits small polarization and more stable voltage profiles during cycling, indicating low interfacial
impedance and excellent electrode/electrolyte interface stability. On the contrary, the battery using low-
LiTFSI-content (20 wt%) electrolyte exhibits severe polarization and unstable voltage profiles, showing high
interfacial impedance and unstable electrode/electrolyte interfaces.

Differences in interfacial impedance were also reflected by Nyquist plots of solid-state Li-S batteries. The
solid-state battery using a high-LiTFSI-content electrolyte exhibits nearly coincident Nyquist plots at
different cycles [Figure 5D]. The electrochemical impedance spectroscopy (EIS) fitting result shows small
changes in ohmic resistance (R ), interfacial charge transfer resistance (R ), and SEI layer resistance (R )
SEI
o
ct
during cycling [Supplementary Table 2]. The battery using a high-LiTFSI-content electrolyte shows a R of
ct