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Page 2 of 30 Mazzapioda et al. Energy Mater 2023;3:300019 https://dx.doi.org/10.20517/energymater.2023.03

Keywords: All solid-state batteries, lithium metal anode, inorganic solid-state electrolytes, interfacial issues, quasi-
solid-state batteries

INTRODUCTION
The accelerated demand for high energy density and long cycling batteries is expediting the development of
next-generation energy storage technologies. Rechargeable lithium-ion batteries (LIBs) account for a wide
range of applications, such as electric vehicles, portable devices, and stationary energy storage, due to their
long cycle lives, high charge-discharge rates, high specific capacity, and voltage, as well as reasonable
temperature range of operation . However, commercial LIBs are associated with limited energy density
[1-3]
and safety-related concerns linked to poor abuse tolerance. The internal failure of a LIB is mainly caused by
the intrinsic flammability of organic carbonate-based liquid electrolytes (LEs), such as ethylene carbonate
(EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC), volume expansion of electrodes during
[4,5]
cycling, and uncontrollable lithium dendrite formation, affecting their broad applicability .
To achieve higher energy density, lithium metal is considered the most promising anode material due to its
-1
high theoretical capacity (3,860 mAh g ) and low electrode potential (-3.04 V vs. standard hydrogen
electrode) . However, its use in combination with conventional LE is prevented by the thermodynamic
[6,7]
instability of the carbonate-based organic solvent and the inability of lithium salt anion which cannot form
a stable solid electrolyte interphase (SEI) on the Li surface . During the charge and discharge processes, the
[8]
SEI layer is partially fractured, which causes inhomogeneous Li stripping/plating and the formation of Li
dendrites with small needle-shaped structures on the anode surface. These phenomena occur continuously
during cycling and cause consumption of both Li metal and electrolyte, reducing the Coulombic efficiency
of the batteries. In the worst case, Li dendrites grow across the electrolyte towards the cathode and cause
internal short circuits which may eventually lead to fires and/or explosions .
[8,9]

Considering the aforementioned limitations of LIBs, solid-state Li metal batteries (SSLMBs) have been
extensively studied as prospective candidates for enhancing the energy density of next-generation
rechargeable batteries. A replacement of conventional LEs with solid-state electrolytes (SSEs) may offer
improved performance and safety of the battery cell. SSEs can provide higher safety, acting as a physical
barrier to separate negative and positive electrodes, and prevent thermal runaway phenomena under high
temperatures. In addition, the excellent mechanical properties of SSEs are an advantage for the use of
lithium metal as the anode material with effective suppression of Li dendrite formation [10,11] .

Currently, the most common SSEs are classified into two main groups, namely solid polymer electrolytes
(SPEs) and inorganic solid electrolytes (ISEs) . SPEs are composed of a polymer matrix and a lithium salt
[12]
which provide the lithium ions for conduction. Many polymers have been investigated, including
poly(ethylene oxide) (PEO), polycarbonate (PC), poly(methylmethacrylate) (PMMA), poly(vinylidene di-
fluoride) (PVDF) and poly(acrylonitrile) (PAN). Among them, PEO-based electrolytes are the most widely
studied for solid-state batteries due to their good electrochemical stability with Li anode and excellent
compatibility with Li salts [13,14] . SPEs offer advantages over ISE, such as good processability and outstanding
flexibility, but their applicability is limited by low ionic conductivity at room temperature and poor anodic
electrochemical stability .
[15]
ISEs, which can be divided into three main groups, oxide-, phosphate- and sulphide-based electrolytes,
exhibit high ionic conductivity (> 0.1 mScm at room temperature) and excellent thermal stability (thermal
-1
runaway temperature when in contact with Li >300 °C) . Some ISEs also offer high electrochemical
[16]