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Page 4 of 30 Mazzapioda et al. Energy Mater 2023;3:300019 https://dx.doi.org/10.20517/energymater.2023.03

As battery development transitions from liquids to solids, the LE or IL additives in QSSEs can also be
combined with polymers to ensure the safety property of the overall QSSLMBs and the mechanical stability
of ISEs. In the development of battery electrolytes, incorporation of a small amount of inorganic fillers in
the polymer matrix has already been demonstrated, the combination of which reduces the crystallinity of
polymers, i.e., inorganic filler serves as a “solid plasticiser” . Ternary composites, an inorganic filler,
[35]
[36]
polymer, and LE or IL, have also been reported . In contrast to these filler-in-polymer systems, there is a
new trend in the development of polymer-in-ISE composites designed to improve both the electrochemical
and mechanical properties of ISEs. Since the ISE is the main component that governs the ion conduction
along with ILs, the strategy for material design should differ from the well-developed filler-in-polymer
systems, and this is summarised in Section “Polymer-liquid-inorganic QSSEs”.

Prior to the discussion on the development of QSSEs, we present an overview of ISEs along with the
chemical-electrochemical and mechanical properties which control dendritic Li formation in SSLMBs.
Principally, we discuss the new emerging classes of QSSEs based on ISEs with a focus on the Li/electrolyte
interface and interphase. Attention is also devoted to the mechanism of dendritic Li formation in SSLMBs.
Lastly, the discussion is expanded to different types of QSSEs containing a IL, ILs and hybrid systems based
on SPE-IL-SSE as an outlook for the development of novel SSEs with reliable safety and high performance.

OVERVIEW OF INORGANIC SOLID-STATE ELECTROLYTES
The history of ISEs dates back to 1,838 , when Faraday discovered that Ag S and PbF become good ion
[18]
2
2
-1
conductors when in their heated states (e.g., ~1 S cm at 400 °C for PbF ). It was a century later that early
2
SSBs based on silver salts, such as Ag/AgI/I , were reported, but their performance was poor, including low
2
[37]
+
voltage (< 1 V) and low discharge current . In the same period, fast Li conduction in inorganic solid
materials, such as LiI and Li N, was discovered [38-40] . Then, the 1960s marked an important turning point
3
when the fast 2D Na ion transport in β-alumina (Na O•11Al O ) was discovered, leading to the birth of the
+
2
3
2
term “solid-state ionics”. Its use in high-temperature sodium-sulphur batteries has allowed for the
proliferation of practical applications of ISEs in energy storage systems. The first LIBs with ISEs were
reported in the 1990s, employing lithium phosphorous oxynitride (LiPON). The research on this topic has
flourished since then and accelerated the study of solid-state LIBs and led to the development of other types
of materials, as summarised below.
The historical development of ISEs is reported in Figure 1 , together with their performances [18,42] .
[41]
Garnet: Garnet-type Li ion conductors have A B (XO ) as the general chemical formula (A = Mg, Ca, Y, La
4 3
3 3
or rare earth; B = Fe, Al, Ge, Ga, Ni, Mn or V; X = Ge, Si, Al), containing three different types of cation sites,
in which A, B, and C are eight, six, and four oxygen coordinated respectively . Thangadurai et al.,
[43]
developed a family of garnet oxides Li La M O (M = Nb, Ta), among which Li La Ta O exhibited the
2
12
3
3
5
2
12
5
highest chemical and electrochemical stabilities and a Li-ionic conductivity of 1 × 10 S cm at room
-6
-1
[44]
temperature . To improve the ionic conductivity of these compounds, Li-rich garnet-type, such as
Li ALa M O (A = Mg, Ca, Sr, Ba) , Li La C O (C = Zr, Sn) [46,47] and Li La Ta O 13 [48,49] have been reported,
[45]
7
2
2
3
6
2
3
12
2
7
12
in which the content of lithium ions can be increased by doping low valent ions to balance the total charge.
These substitutions allow the reduction of the activation energies for Li-hopping (0.35-0.4 eV), leading to an
ionic conductivity of 10 Scm at room temperature [50,51] . In this family, Li La Zr O (LLZO) and its
-3
-1
2
12
3
7
-4
-1
derivatives are considered among the most suitable ISEs due to their high ion conductivity (10 -10 Scm at
-3
room temperature), wide ESW and higher stability in the presence of Li metal ; however, the following
[52]
points still need to be addressed: (i) stable crystalline phase with high ionic conductivity; (ii) low porosity
and stiffness to prevent lithium dendrite formation; and (iii) good wettability against lithium for lower