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

[17]
stability (> 4.0 V as measured by linear sweep voltammetry and also supported by DFT calculation ).
Several types of ISEs have been studied, including Li superionic conductors (LISICONs), Na superionic
conductors (NASICONs), garnets, and perovskites as oxide/phosphate-based ISEs and thio-LISICON,
Li GeP S (LGPS), glassy-type Li S-P S , and argyrodites with the chemical composition of Li PS X (X = Cl,
5
2 12
2
10
2 5
6
Br, I) as sulphide-based ISEs [17,18] .
Although practical applications of ISEs are still limited by numerous concerns including high cost, difficult
manufacturing, and high interfacial charge transfer resistances arising from rough contact with the
electrodes [19-21] , ISEs are considered the most promising materials to replace the current LEs. For practical
applications, the SSEs must satisfy several prerequisites, namely (i) high ionic conductivity, more than
-1
10 S cm at room temperature with a Li-ion transference number (t ) close to 1; (ii) good chemical
-4
Li+
stability at the interface preventing any side reactions during SSLMB operation; (iii) good electrochemical
stability in a wide electrochemical operating window in order to obtain high energy density; and (iv) good
mechanical stability to prevent or suppress lithium dendrites growth [22-24] . While these main concerns have
been addressed, some hurdles still remain, such as (i) poor wettability to Li surface; (ii) electrolyte/electrode
microstructures and stress cracking, as well as volume expansion of lithium metal caused during cycling;
and (iii) interfacial side reactions. These drawbacks must be resolved since they generate defective Li anode/
electrolyte interfacial contact, i.e., high electrode/electrolyte resistance, which is one of the most detrimental
issues that limit the overall performance of SSLMBs [25,26] .

Recently, a new concept was proposed for the design of novel SSEs with improved safety, durability, and
electrochemical performance, involving the addition of a small amount of liquid material (usually a LE) to
ISEs, yielding quasi-solid-state electrolytes (QSSEs). The resulting composites based on LE and ISEs retain
the solid-state nature, i.e., no fluidity, self-standing, and no liquid leakage, which are different from liquid or
gel-type electrolytes. In such systems, ISEs function as a separator while at the same time granting ionic
conductivity, whereas LEs solve the problems of poor Li-ISE contact and sluggish interfacial kinetics, which
cause the decline of SSLMB performances. In this context, ionic liquids (ILs) have attracted significant
attention in addition to conventional LE for the development of QSSEs because of their remarkable
properties, such as thermal and chemical stability, wide electrochemical stability window (ESW) up to 5 or
-1
6 V vs. Li /Li, high ionic conductivity (10 -10 S cm at room temperature) as well as low volatility and
+
-4
-2
flammability [27,28] .
In the past decades, ILs have been studied as a potential electrolyte additive in advanced electrochemical
devices. For instance, in polymer-IL composite electrolytes, i.e., organic-organic system, the incorporation
of IL in polymers as plasticisers and as ion sources results in fast ion conduction and improved
electrochemical, thermal, and interfacial properties [29-32] . This is also demonstrated in an organic-inorganic
hybrid system, whereby ILs are combined with either Li conductive filler (i.e., ISE) or non-conductive
+
inorganic fillers (the former is of interest and summarised in Section “Ionic liquid electrolyte (ILE)-
containing QSSEs”). As one example of the latter combination, Ito et al. investigated silica nanoparticle
composites with 75 vol% IL-Li salt, which exhibited desirable ion conductivity (> 10 S cm at room
-4
-1
temperature) and reported a quasi-solid-state lithium metal battery (QSSLMB), Li|QSSE|LiCoO , exhibiting
2
high capacity of 126 mAh g -1[33] . Wen et al. reported a QSSE based on a biomimetic leaf-like Al O and ILs.
3
2
This system promoted the migration of Li both in bulk and at the interface, improving the interfacial
+
stability and restraining Li dendrite formation during long-term cycling. Symmetric Li cells assembled with
this QSSE exhibited a long cycle lifetime of 1,100 h at a high constant current density of 0.5 mA cm -2[34] .