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Mazzapioda et al. Energy Mater 2023;3:300019 https://dx.doi.org/10.20517/energymater.2023.03 Page 9 of 30

Based on the intrinsic properties of different kinds of SSEs, three different electrode/electrolyte interphases
can be identified in SSLMB [Figure 2C]. In the same figure, the first interphase (a), being the ideal case, is
thermodynamically stable, i.e., no electrolyte decomposition or chemical side reactions take place. In the
second type (b), the formed interphase is a mixed electronic and ionic conductor, known as “a mixed
conductive interphase” (MCI), resulting in the continuous degradation of the SSE because electrons can
reach it. Eventually, the process leads to cell failure because SSE and/or Li are consumed in this parasitic
reaction, which in the worst case, causes internal cell short circuits. The third type (c) is the case when the
interphase is electronically insulating, thus obstructing the SSE decomposition. However, the interphase
provides Li ion conductivity allowing cell operation .
[98]

Zhu et al. reported first-principle calculations to investigate the chemical and electrochemical stabilities of
the Li|SSE interfaces. Their calculations demonstrated that most SSEs have a limited ESW. Sulphide-based
ISEs have significantly narrower ESW than the oxide-based ones, being reduced already below 1.6 V vs.
+
Li /Li and oxidised at as low as 2.3 V vs. Li /Li. Consequently, sulphide-based electrolytes are prone to
+
generate a thick interphase layer, resulting in reduced Li ion transport across the interphase [81,99] .
+
Combining in-situ X-ray photoelectron spectroscopy (XPS) with impedance spectroscopy, Wenzel et al.
demonstrated the growth of passivating interphase between Li P S -based SSE and Li, which is composed of
7 3 11
Li S, Li P and LiX, explaining the high interfacial resistance, low Coulombic efficiency, and poor cycling
2
3
reversibility, all being limiting factors to the performance of SSLMBs . Similar passivating interphase
[100]
formation has been reported in Li-argyrodite Li PS Cl and Li PS Br. In contrast, thio-LISICON SSEs such as
6
5
6
5
LGPS and Li SnP S (LSPS) form a non-passivating interphase upon contact with Li metal. The reduction
2 12
10
of Ge and Sn to the metallic state results in the formation of electron-conducting pathways, which
4+
4+
[101]
unfavourably promote the degradation process until the electrolyte or Li is completely consumed .
Conversely, oxide-based ISEs (e.g., the perovskite Li La TiO , NASICON-type LiTi (PO ) , LISICON-type
3
3.3
0.56
4 3
2
Li Zn(GeO ) , and garnet-type Li La Zr O ) exhibit better chemical stability against Li metal and
7
12
3
2
14
4 4
suppressed oxidative decomposition above 3 V vs. Li /Li. In particular, the NASICON materials, LATP and
+
LAGP, are thermodynamically stable up to ~4.2 V vs. Li /Li. Among all known ISEs, Li La Zr O offers the
+
3
12
7
2
best stability with Li. The good stability of these SSEs is not thermodynamically intrinsic to the materials but
originates from the sluggish kinetics of their decomposition reactions, which result in the formation of a
passivating interphase at the Li/ISE interface, i.e., a layer with poor electronic conductivity and the ability to
inhibit further decomposition of the ISE [102,103] .
Ma et al. demonstrated that two reactions: the reduction of LLZO surface upon contact with lithium metal
and the simultaneous Li ion diffusion into the ISE, maintain the charge balance, leading to the formation of
+
a stable and ultrathin (6 nm) tetragonal-like LLZO interphase, which prevents further interfacial reactions
without affecting the bulk ionic conductivity of LLZO [Figure 3A] . The presence of dopant species in the
[104]
+
lattice of LZZO, improving Li conductivity, has a key role in controlling interfacial reactivity, thus affecting
the stability of LLZO in contact with Li metal. Zhu et al. reported a study on Ta, Nb, and Al-doped LLZO
samples demonstrating the formation of oxygen-deficient interphase (ODI) due to the reduction of Zr .
4+
The formation of an extensive ODI layer on Al-doped LLZO (due to a significant Zr reduction) stabilises
4+
the ISE in contact with Li, resulting in a low interfacial impedance. In contrast, Nb-doped LLZO, exhibiting
slightly less Zr reduction, showed the highest interfacial impedance with Li, which increased consistently
4+
with time due to the propagation of the reaction into the bulk .
[105]
Hartmann et al. observed the coupled diffusion of lithium ions and electrons into the bulk of a commercial
ISE containing Ge, Ti, and Si (LATGP) by means of SEM and XPS. When a thin lithium film (~200 nm
thickness) was formed on the SSE via vacuum deposition, the interface undergoes changes, accompanied by