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Page 22 of 30 Mazzapioda et al. Energy Mater 2023;3:300019 https://dx.doi.org/10.20517/energymater.2023.03

Solvate ILs, which consist of a coordinating solvent (e.g., tetraglyme) and salt (e.g., LiTFSI), are also useful
additives for QSSLMBs. Cao et al. investigated the combination of the [Li(triglyme)][TFSI] (LiG ) complex
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with LGPS discovering that the stabilised Li|LGPS interface was achieved through the formation of a SEI
layer by the in situ electrochemical reduction of LiG complex on Li. The LiG complex incorporated at the
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3
Li|LGPS interface was fundamental in regulating Li transfer and suppressing the interfacial side reactions.
+
The Li-S battery with such a QSSE exhibited improved electrochemical performance at room temperature,
delivering 1,100 mAh g at 0.2C rate and superior cyclic stability even at 1C rate .
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[151]
Polymer-liquid-inorganic QSSEs
Sintered ISE pellets offer high ionic conductivities (10 to 10 Scm ) but are intrinsically brittle, entailing
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poor contact with the electrodes including Li. This problem occurs especially on cycling when the electrode
volume changes, leading to high interfacial impedance and eventual battery failure. Additionally, they
cannot be easily adapted to the roll-to-roll production processes needed for large-volume production (such
as for EVs).

An interesting approach to tackle these issues is to introduce Li-ion conducting polymer electrolytes (PEs),
including both SPE and gel polymer electrolytes, as a matrix for ISE to form QSSEs and/or as a layer at the
Li|SSE interface. Because of their flexible and cohesive nature, PEs can further improve the interfacial
contact and mechanical stability of QSSEs. Inspired by the soft nature, high ionic conductivity, and better
wettability offered by the polymer, numerous studies have reported on their use as QSSE in combination
with ISEs, electrode binders, and electrode/electrolyte interlayers, with and without the further
incorporation of LEs and ILEs.

Liu et al. used poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP)-LE composite as interlayers
between Li La Ca Zr Nb O (LLCZNO) and electrodes to decrease the interfacial resistances against
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0.25
7
2.75
1.75
0.25
both the cathode and Li. Because of the presence of the PE interlayer, the interfacial resistance of LLCZNO
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4
3
2
decreases from 6.5 × 10 to 248 Ω cm against the cathode and from 1.4 × 10 to 214 Ω cm against Li.
Furthermore, the full cell consisting of Li, the ISE with the interlayers, and LiFePO demonstrated a high
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capacity of around 140 mAh g at 1C rate and stable cycling performance over 70 cycles. The results
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indicate that the PE layer can protect Li metal from the formation of a highly resistive interface, thus
extending the cycle life of the QSSLMB .
[154]
Non-volatile, ILE-polymer-ISE QSSEs have also been explored, using either PEO or PVDF-HFP as matrices
[Figure 8A] [155,156] . Huo et al. proposed a composite electrolyte consisting of Li-salt-free PEO and
Li La Zr Ta O (LLZTO) wetted by [Bmim][TFSI] abbreviated as PEO/LLZTO@IL. It was demonstrated
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6.4
0.6
12
1.4
that the incorporation of a small amount (1.8 μL cm ) of IL increased the conductivity of the composite by
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one order of magnitude with respect to the IL-free PEO/LLZTO through the formation of highly ion-
conductive paths within the QSSE and decreased impedance at the electrode/electrolyte interfaces.
Consequently, the Li|LiFePO cells employing PEO/LLZTO@IL delivered the specific discharge capacity of
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133.2 mAh g at 0.1C rate with a capacity retention rate of 88% after 150 cycles at 25 °C. The PEO/
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LLZTO@IL QSSE was also studied for potential application in high energy density Li|LiFe Mn PO
0.15
0.85
4
(LFMP) cells. The resulting battery delivered relatively smooth charge/discharge curves and a high
Coulombic efficiency (91%) at the first cycle. After 100 cycles, the capacity retention was maintained at
84.1%, confirming the improved electrode/electrolyte interfaces .
[157]
Recently, Wu et al. have designed a bilayer SSE architecture implementing a novel ultrathin (≤ 20 μm) PE
film in combination with LAGP to improve the interfacial stability with Li. The PE film was composed of