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Mazzapioda et al. Energy Mater 2023;3:300019 https://dx.doi.org/10.20517/energymater.2023.03 Page 19 of 30

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was improved to 0.4 × 10 S cm . Using this composite electrolyte, QSSLMB batteries with the
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configuration of Li/LiCoO were assembled, showing initial charge/discharge capacities of 140 mAh g at
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0.1C. The authors also prepared the mixture based on Al O and IL-LiTFSI. In this case, the cell capacity
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was 40/50 mAh g , confirming not only the ILs but also LLZO contribute to the Li-ion-conduction . In
[145]
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addition to the ILs with TFSI, ILs with FSI or DFOB are also known to be useful for the reduction/
[146]
elimination of the grain boundary resistance .
Generally, ILEs containing lithium salt are used to form QSSEs to enrich the concentration of Li . In
+
contrast, Zhang et al. reported that IL without salt addition is also useful for the formation of continuous
conduction pathways in ISEs. The addition of [Pyr ][TFSI] not only increased the density of SSEs,
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suppressing Li-dendrite growth, but also improved the interfacial wettability of QSSEs towards Li metal.
The Li|LiNi Co Mn O cell, employing LLZO-[Pyr ][TFSI] composites, showed a high reversible
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0.8
2
0.1
0.1
capacity of above 100 mAh g throughout 200 cycles, which was comparable to those of cells based on liquid
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electrolytes. The cell retains the capacity value above 100 mAh g even at 1C. Also, a LiFePO |LLZO-IL|Li
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cell exhibited a discharge specific capacity of 119 mAh g with minimal capacity loss during the first 60
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cycles .
[147]
Xiong et al. designed a composite-type QSSE consisting of LAGP and [Bmim][TFSI] and used this as the
interlayer. The Li|LAGP interface stability was investigated in Li|Li symmetrical cells with either the LAPG-
ILE composite or a conventional LE as the interlayer. Using the latter interlayer, the cell overpotential
increased on cycling, revealing the formation of an unstable SEI on Li, which grew continuously.
Additionally, the cell voltage dropped after 750 h, corresponding to 375 cycles, due to the penetration of
dendrites through the SSE. Conversely, the cell employing the LAGP-ILE interlayer showed a low and
steady over-voltage of 30 mV for 1,500 h, demonstrating the formation of a stable interphase, which
impeded the direct contact of bulk LAGP and Li that prevented the reduction of Ge in the SSE and
4+
suppressed the growth of Li dendrites. As a result, Li|LAGP-IL|LiFePO cells offered ultra-stable cycling
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with specific capacities higher than 110 mAh g at 2C [Figure 7A] . Information detailing the
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[148]
composition and ionic conductivity of the QSSE composite are reported in Table 2, while the performances
of QSSLMB, in which the QSSE is used as interlayer, are reported in Table 3 together with those of other
QSSEs.
It is also possible to use ILE as an interlayer on or in electrodes. Basile et al. reported a facile SEI formation
via a chemical interaction between Li metal anodes and [Py ][FSI] containing different lithium salts as the
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ILE for applications in LMBs. Symmetrical Li|ILE|Li cells cycled at 0.1 and 1.0 mA cm displayed stable Li
stripping/plating voltage profiles upon extended cycling (2,500 h) without any evidence of dendrite
formation. Full Li|ILE|LiFePO cells displayed safe cycling at 1C rate, achieving 1,000 cycles with a
4
Coulombic efficiency greater than 99.5% . Taking this into account, ILEs, like LE, are expected to
[149]
effectively improve the wettability between ISEs and Li and provide the building blocks to form a stable SEI
and suppress Li dendrite formation.

When ILE is used as the interlayer material, it is generally applied over ISE on Li anode, while on the
cathode side, it can be applied over or mixed in the cathode. Zheng et al. demonstrated an improved
stability of the Li|LSPS interface by using a small amount of 1.5 M LiTFSI in [Pyr ][TFSI], which led to the
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formation of a stable SEI layer rather than the MCI formed when LSPS is in direct contact with Li. The ILE
thin layers applied over the Li anode and also mixed in LiFePO provided a uniform ionic conductivity
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through the electrolyte/electrode interfaces, compensating for poor mechanical contact arising upon
cycling. As a result, the QSSE-based Li|LiFePO cell showed a higher initial discharge capacity
4