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Page 6 of 30 Mazzapioda et al. Energy Mater 2023;3:300019 https://dx.doi.org/10.20517/energymater.2023.03

[59]
LISICON: First described by Hong in 1978, LISICON [Li Zn(GeO ) ] is the typical representative of this
14
4 4
-1
-1
family of ISEs. This material offers high ionic conductivity at elevated temperatures (1.25 × 10 Scm at
300 °C), but not at room temperature. The crystal structures of these compounds are similar to γ-Li PO ,
4
3
[60]
with an orthorhombic unit cell (Pnma space group) where all cations are tetrahedrally coordinated . The
substitution of P by aliovalent cations (Si or Ge ) to balance a Li excess in the structure, resulting in the
5+
4+
4+
general structure of Li (P Si )O 4 [61,62] . The excess Li resides in interstitial sites that have a shorter distance
3+x
x
1-x
than in tetrahedral sites, resulting in a slightly higher Li ion conductivity (3 × 10 Scm ). Derivatives of
-6
-1
LISICON, such as thio-LISICON and LGPS, are explained in the section on sulphide-based ISEs.
LiPON: Lithium phosphorus oxynitrides (LiPON) first received significant attention in the 1970s as SE for
the development of thin-film solid-state macro batteries . LiPONs are commonly prepared via sputtering
[63]
of Li PO in N plasma, resulting in amorphous, glassy thin films with compositions of Li PO N
3
x
2
4
y
z
(2.6 ≤ x ≤ 3.5, 1.9 ≤ y ≤ 3.8, 0.1 ≤ z ≤ 1.3) [64,65] . One of the first LiPON glass SSEs was a nitrided Li O-P O glass
2
5
2
matrix prepared by ion beam sputtering, offering an ionic conductivity of around 2 × 10 S cm at room
-7
-1
temperature with a maximum conductance of 0.2 × 10 S cm due to their ultra-small thickness . Unlike
-2
-1
[66]
other SSE in powder form, sputter deposited LiPON possesses high mechanical and electrochemical
stability, and forms a favourable SEI on the Li metal surface [67,68] . However, this SSE possesses very limited
ionic conductivity, which only allows battery cycling at low current densities (on the order of tens of
μA cm ) unless the film thickness is reduced to a nanometric scale . Recently, some papers reported an
-2
[69]
improvement in ionic conductivity by incorporating silicon into the LiPON network (LiSiPON) [70,71] .
Su et al. reported LiSiPON thin film prepared by radio frequency magnetron sputtering (RFMS) showing
the highest Li ion conductivity (9.7 × 10 S cm at room temperature) and an activation energy of only
-1
-6
0.41 eV. The substitution of phosphorus with silicon in the film created Si-O-P cross-link structures and
promoted the mobility of lithium ions. These prepared LiSiPON films with higher ionic conductivity could
[72]
be an interesting alternative to LiPON for applications in high-energy-density lithium batteries .
NASICON: NASICON refers to a sodium super ion conductor with the chemical formula of NaM (PO ) ,
2
4 3
[76]
where M can be a transition metal (Zr , Ge [74,75] , Ti ). In 1976 Goodenough and Hong et al. identified the
[73]
first ISE Na Zr Si P O (0 ≤ x ≤3) with a rhombohedral crystal structure and a space group of R-3c. In the
12
x 3-x
1-x
2
range of 1.8 ≤ x ≤ 2.2, the structure undergoes a small distortion to monoclinic symmetry with space group
C2/c at ambient temperature . NASICON-type Li-ion ISE, LiM (PO ) , can be prepared by replacing Na-
[42]
2
4 3
ion with Li-ion. However, the ionic conductivities of LiTi (PO ) and LiGe (PO ) materials are lower than
2
2
4 3
4 3
that of the Na-ion analogues. To address this issue, a partial substitution of Ti and Ge with d-block elements
was investigated to promote Li ion migration . One of the most successful dopants is aluminium (Al)
[77]
which allows the presence of more lithium ions in the crystal structure, resulting in enhanced Li ion
conductivity. ISEs with compositions of Li Al Ti (PO ) (LATP) and Li Al Ge (PO ) (LAGP) show
x
4 3
2x
1+x
2x
4 3
1+x
x
room temperature ionic conductivity around 10 Scm -1[78,79] .
-3
Sulphide electrolytes: To improve the low room-temperature ionic conductivity of LISICON-type oxide
ISEs, a new class of materials was developed by the substitution of O with S in the framework (thio-
2-
2-
-1
LISICON). Kamaya et al. developed Li GeP S (LGPS) with an ionic conductivity of 1.2 × 10 S cm at
-2
10
2 12
room temperature, i.e., comparable to that of organic LEs . These SSEs are attracting increased interest
[80]
owing to their high Li-ion conductivity resulting from the larger size of S , which broadens ion conduction
2-
pathways in the electrolyte structure, and the higher polarisability of S , which weakens the attraction
2-
between lithium and sulphide ions. Several first principles modelling studies were carried out to interpret
the high ionic conductivity obtained for these compounds. These results suggested that the most
energetically favoured LGPS is a 3D superionic conductor with a tetragonal unit cell of PS and GeS and
4
4