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Page 24 of 30 Mazzapioda et al. Energy Mater 2023;3:300019 https://dx.doi.org/10.20517/energymater.2023.03

oxide-based ISEs following experiments using intentionally damaged (rolled or cut) pouch cells. This
highlights the strength of polymer matrices that can enhance the interfacial stability with the electrode and
outstanding mechanical stability that cannot be achieved by ILEs.

CONCLUSIONS AND OUTLOOK
SSLMBs based on ISE are promising candidates for the next-generation rechargeable lithium batteries by
virtue of their safety benefits, energy density, as well as unique mechanical and thermal stabilities.
Significant research efforts in terms of the ionic conductivity of ISE have been made to deliver this
technology commercially in the mass energy market. However, attaining operational SSEs remains a
standing challenge due to the difficulty in forming an optimised Li|ISE interface offering fast Li -ion
+
transport for efficient cell operation and in minimising electronic conductivity to avoid the continuous
degradation of the cell components and performance. Additionally, the Li dendrite growth occurring at Li|
ISE interface, which relates to the electronic conductivity and also crystalline nature of ISE, remains the
main limitation that has yet to be resolved.

To solve these issues and realise ideal interfaces, the main challenges include: (1) the improvement of
electrochemical stability of ISE; and (2) the formation of ISE with low boundary resistances not only at ISE
grains but also at electrode grains. Although oxide-based ISEs possess generally good electrochemical
stability with Li metal, the poor Li|ISE interface contact induces the formation of Li dendrites rapidly
propagating at the grain boundaries, surface defects and interconnected pores of ISEs. While improvement
of inherent properties of ISEs has been intensively studied, it is also possible to improve their properties by
adding secondary materials, such as LE, ILE, and SP, to form QSSE.

As summarised in this review, QSSEs synergistically combine the advantages of ISE and LEs, offering
improved safety, durability, and electrochemical performance. The addition of infiltration materials to ISEs
can improve the poor Li-ISE contact and sluggish interfacial kinetics by infiltrating voids among ISE
particles, thus enhancing the overall performance of SSLBs. Among the three additives to form QSSE, i.e.,
LE, ILE, and SP, LE possess the highest ionic conductivity, yet its use should be limited in terms of amount
to form QSSEs so as to not impede the safety advantage of ISEs. For advanced battery systems, QSSEs
containing ILEs or PE would be favourable. As of today, only a limited number of ILEs have been tested as
interlayer or infiltrating material for ISEs because high chemical and electrochemical stabilities are required
for battery application and also for compatibility with ISEs. However, taking the numerous structural
possibilities of ILEs into consideration, it should be possible to design task-specific ILEs for QSSEs. For
example, several ILs based on oxalatoborate are known to form a protective layer on the cathode side. For
Na-ion batteries, the use of sacrificial salts has been proposed to be favourable for stable battery
performance. Similarly, it should be possible to synthesise ILE with the ability to form a protective layer
during battery cycling. In addition, the optimised amount of ILEs added to ISEs has not yet been defined.
This is because the value should depend on various factors such as (1) compatibility between ILs and ISEs to
suppress the leakage of ILs; (2) particle size of ISEs and surface area to be treated (pellets or powders); and
(3) viscosity of ILs; hence, further systematic analyses must be carried out.

The use of polymer matrices in QSSEs is an effective means to enhance the flexibility of electrolytes, which
would be more advantageous for industrial purposes not only in terms of better scalability of their
production but also improved safety of final battery products, as confirmed by Cheng et al. [160,161] . Currently,
the production of functional SSBs requires a high-temperature sintering process of ISEs and specific cell
design allowing external pressure application, both of which lead to an increase in the cost of batteries. By
using PE based on flexible polymer matrices, the scalability of material production, especially with seamless