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Mazzapioda et al. Energy Mater 2023;3:300019 https://dx.doi.org/10.20517/energymater.2023.03 Page 15 of 30

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(1.27 × 10 S cm ). This material also enables homogeneous Li flux and surface potential distribution
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between the grain boundaries in bulk, which significantly improve the interfacial stability between LAO-
LLZOF and the Li metal anode and significantly suppress the penetration of Li dendrites [128]
The quality of the Li|SE interface/interphase plays an important role in Li dendrite growth, i.e., the CCD
value above which Li penetration occurs. In LLZO, surface cleaning and/or the use of interfacial layers allow
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the reduction of the interfacial impedance enabling current densities up to 1-2 mAcm at room
temperature . Kazyak et al. studied the Li|Al-doped LLZO, combining operando optical and post-mortem
[129]
electron microscopy. They recognised four distinct morphologies of Li filaments (straight, spalling,
branching, and diffuse) in the LLZO structure [Figure 5D] when the cell is cycled above CCD. The fourth
morphology, however, was not observed in any of the optimised cells offering low Li interfacial impedance.
Moreover, the Li within these structures can be reversibly cycled, but at high current densities, the Li
filaments propagate via a mechanical crack-opening mechanism, of which the rate increases with increasing
[130]
current density. Similar Li dendritic growth was also observed in glassy LPS .

LiPON is considered a prospective ISE that can prevent the penetration of Li from the anode to the cathode
due to its homogeneity (no grain boundaries nor porous structure arising from the radio frequency
magnetron sputtering). Westover et al. artificially synthesised a LiPON|LiPON interface and demonstrated
that Li can deposit at such an interface [Figure 5E], but is confined to a 2D layer, confirming the ability of
LiPON to block Li dendritic growth .
[131]

Overall, Li dendrites formation in SSLMBs is driven by inhomogeneity at the Li|ISE interface resulting from
non-uniform interphase and the intrinsic porous microstructure of ISEs, especially oxide-based ones. The
standing challenges, including chemical and mechanical stability, high ionic/electronic conductivity ratio,
good contact with Li metal and low charge transfer impedance over many plating/stripping cycles, are yet to
be resolved, requiring further design and engineering efforts.

QUASI-SOLID-STATE ELECTROLYTES
Quasi-solid-state electrolytes (QSSEs) can be considered as an intermediate state material between LE and
ISEs, providing a good compromise in ionic conductivity, interfacial properties, and mechanical stability.
Generally, they are expected to be an efficient strategy towards the improvement of the above-mentioned
challenges of SSBs, such as (i) chemical and electrochemical stability; (ii) optimal Li|ISE interface and
interphase, and to a lesser extent; and (iii) safety.

Liquid electrolyte-containing QSSEs
The addition of a minimum amount of LE at the electrode|SSE interface or into SSE pores can provide
chemical building blocks enabling the formation of a stable interphase. Additionally, the LE may provide
paths for Li ion transport hence ensuring a chemically and physically stable Li|electrolyte interface .
[132]

Despite the presence of the liquid component, QSSEs are expected to be safer than conventional LEs
because of the lower amount of flammable liquid, which is additionally confined in the SE pores and/or at
the interface. Overall, cells employing QSSEs are expected to overcome the poor performances of both LE-
and ISE-based batteries [Figure 6A] .
[133]
However, the two components must be chemically stable on contact; thus, the chosen combination of liquid
and ISE is critical. With respect to lithium sulphur batteries, Judez et al. reported on the following suitable
combinations: LATP-type with 1 M LiClO -EC/DMC, 1 M LiPF -EC/DMC/DEC or EC/DME,
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