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Mazzapioda et al. Energy Mater 2023;3:300019 https://dx.doi.org/10.20517/energymater.2023.03 Page 7 of 30

space group P4 /nmc. In this structure, Li-ion diffusion can occur along the two-dimensional diagonal
2
direction in the ab plane, with activation energies reduced to 0.30 eV and along the c-axis with activation
energies of 0.19 eV, both of which guarantee the superionic conductivity of LGPS [81,82] . Motivated by the cost
of Ge, there are strategies to replace Ge with Si and Sn or alternatively to form the LGPS phase exclusively
based on Li-P-S (specifically, Li P S ) .
[83]
9.6 3 12
Other types of sulphide-based fast ion conductors are glass-ceramics electrolytes, among which the binary
system (100-x)Li S-xP S (70 < x < 80) is of particular interest because of its low cost, high ionic conductivity
2 5
2
+
(10 -10 Scm ) and wide ESW to Li /Li . Additionally, glass-ceramic sulphides exhibit good plasticity to
[84]
-3
-4
-1
compensate for volumetric changes of Li metal and provide better stability towards the Li electrode
compared to crystalline electrolytes, in which grain boundaries facilitate dendrite formation and growth .
[85]
An effective method to yield high lithium-ion conductivity is based on the addition of lithium iodide (LiI)
in the Li S-P S electrolyte. In particular, enhanced mechanical stability and an optimised composition of
2 5
2
the SEI layer are achieved at the interface between the Li and the LiI-modified electrolyte, also preventing
the growth of Li dendrites [86,87] .
Lithium argyrodites (Li PS X with X = Cl, Br, or I) are also known as fast lithium-ion conductors with a
5
6
similar structure to Cu- and Ag-argyrodite compounds, which is based on tetrahedral close packing of
anions with cubic unit cell and F m space group. Within this structure, P atoms are coordinated with S to
form PS tetrahedra, while Li ions are distributed over the tetrahedral interstices (48 h and 24 g sites) [88,89] . Li
4
argyrodites, especially Li PS Cl and Li PS Br compounds, are able to approach higher ionic conductivities of
6
5
6
5
about 10 S cm -1[90,91] . In addition, their activation energy is low (0.2-0.3 eV), facilitating lithium diffusion
-3
into the structure.
Despite the advantageous features of sulphide-based ISEs, one of the main concerns with sulphide-
containing SEs is their poor chemical stability when they are accidentally exposed to atmospheric moisture.
All these materials undergo hydrolysis with moisture forming H S gas, leading to material degradation.
2
These sulphide-type electrolytes are not stable and also against the reduction of Li metal, increasing the
[92]
interfacial resistance, because insulating products such as Li P, Li S, LiX, and Li Ge can be formed .
3
15
2
4
Moreover, recent reports revealed that these electrolytes are unstable when in contact with delithiated, high-
voltage oxide cathode materials .
[93]
LI METAL SOLID ELECTROLYTE INTERFACE
Despite the significant advances of ISEs in terms of intrinsic ionic conductivity, designing Li/ISE interfaces
with favourable properties remains challenging. The term interfacial is defined as a combination of several
physical and chemical processes that occur at the Li/electrolyte interface during cycling, which define the
compatibility between the Li electrode and the SE, as well as the physical and mechanical stability of the
interface contact. These processes include SEI formation, Li structural and volumetric changes, formation of
[94]
Li-depleted space-charge layers, and Li dendrite growth . These complex processes affect and limit the
overall performance of SSBs.

The chemical-electrochemical process
To explain the fundamental interface reactions and mechanisms of SSBs, the open-circuit energy diagram of
[95]
a typical Li|SSE|Li M O SSLMB was proposed based on the band theory [Figure 2A] .
2
y
x