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Page 2 of 12 Cui et al. Energy Mater 2023;3:300034 https://dx.doi.org/10.20517/energymater.2023.19

INTRODUCTION
Solid-state lithium (Li)-sulfur (S) batteries are considered to be the most promising secondary batteries
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[1-8]
because of their high energy density (2,550 Wh kg ) and high safety . However, solid-state Li-S batteries
often suffer from poor Li-ions (Li ) transport in batteries due to low ionic conduction of the electrolyte and
+
+
unstable electrode/electrolyte interface during the charge/discharge processes [Figure 1A] [9-15] . The poor Li
transport severely hinders the practical application of solid-state Li-S batteries [16-20] .
Recently, many groups have designed electrolytes with high ionic conductivity and high mechanical
strength to improve the ionic conduction of electrolytes and the stability of the electrode/electrolyte
interface . These electrolytes include ceramic electrolytes and solid polymer electrolytes (SPE). Although
[1-8]
ceramic electrolytes generally exhibit high ionic conductivity and high strength, they often show poor
contact with electrodes, which affects the Li transport in the electrode/electrolyte interface . In contrast,
+
[1,6]
flexible polymer electrolytes can well contact with electrodes, but they exhibit relatively low room-
temperature ionic conductivity and low mechanical strength [9-12] . A common method to improve the ionic
conductivity of polymer electrolytes is to introduce plasticizers (such as Pyr FSI, hydroxypropyl
13
trimethylammonium bis (trifluoromethane) sulfonimide chitosan salt (HACC-TFSI), etc.) or nanofillers
(such as Ai O , TiO -TiN, Li La Zr O , etc.) [11,13-17] . The plasticizers and nanofillers reduce the crystallinity of
2
2
12
2
3
3
7
the polymer electrolytes, thus resulting in high ionic conductivity [14-17] . An alternative way to realize high
ionic conductivity is to prepare polymer-in-salt electrolytes (the content of lithium salt exceeding 50 wt%)
[8-20] . For polymer-in-salt electrolytes, Li can transport in their amorphous polymer regions through the
+
movement of polymer segments and through the ionic channels constructed by aggregated cation/anion
clusters .
[18]
SPE usually composite inorganic materials to improve their mechanical strength [6,21,22] . The composite
p o l y m e r e l e c t r o l y t e s ( s u c h a s p o l y e t h y l e n e o x i d e (PEO)/Li A l 0 . 3 T i 1 . 7 ( P O 4 ) / 3
1.3
bis(trifluoromethane)sulfonimide lithium salt (LiTFSI), PEO/Li La Ca Zr Nb O /LiTFSI, PEO/
0.25
7
2.75
1.75
0.25
12
polyvinylidene difluoride (PVDF)/boron-nitride/LiTFSI, etc.) combine good flexibility of the polymer
components and high strength of the inorganic components. The high strength can inhibit the growth of
lithium dendrites, resulting in a stable electrode/electrolyte interface. The above studies solved the Li
+
transport problems of solid-state Li-S batteries to a certain extent, but it is still difficult for solid-state
batteries to maintain fast and stable Li transport during long-term cycles.
+
Herein, we demonstrate that fast and stable Li transport can be achieved using a polyurethane (PU)-based
+
electrolyte (polyolefin (PO)-PU-LiTFSI) with high ionic conduction and high adhesion. The polar urethane/
urea groups of the electrolyte reduce the hopping energy barrier of Li , which contribute to high ionic
+
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conductivity (1.8 × 10 S cm ), high ion transference number (0.54), and low activation energy (0.39 eV),
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thereby achieving fast Li transport. At the same time, the polar urethane/urea groups of PU endow the
+
electrolyte with high adhesion, ensuring tight interfacial contact and self-healing electrode/electrolyte
interface [Figure 1B], leading to stable Li transport. Benefiting from the fast and stable Li transport, a
+
+
symmetric Li||Li cell using the PO-PU-LiTFSI electrolyte exhibits excellent cycling stability up to 800 h and
a low overpotential of approximately 40 mV. A solid-state Li-S battery using the electrolyte displays a
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specific capacity of approximately 610 mAh g even after testing for 125 cycles (S loading = 4 mg cm ). The
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robust electrode/electrolyte interface during cycling was observed in situ using a laser confocal microscope.
Our study demonstrates the importance of polar groups in electrolytes in maintaining fast and stable Li
+
transport. This concept can also be used to solve similar problems of other solid-state batteries.