## 1 Introduction  
Lithium-ion batteries (LIBs) are one of the great successes of electrochemical energy storage devices utilized in diverse applications such as portable electronics, hybrid automobiles and even large-scale electrical power storage systems [1; 2; 3; 4]. Since the first market emergence of LIBs in the 1990s, the performance of LIBs has been remarkably improved to meet the increasing demand for new energy storage systems with high energy density, high power density, long cycle life and a wide range of operating temperatures [5; 6]. Moreover, rechargeable batteries are rapidly expanding to drivetrains [7; 8], as can be seen from the quadrupled global sales of plug-in light vehicles from 0.55 to 2.21 million cars annually from 2015 to 2019 [9]. Thus, developing revolutionary energy storage systems is a critical task in today's energy-dependent society. LIBs are composed of a cathode and an anode separated by an electrolyte. During discharging, the lithium ions (Li\({}^{+}\)) migrate through the electrolyte from the anode to the cathode and a discharging current flows through the external circuit, whereas the use of electrical energy pushes the electrons and Li\({}^{+}\) back to the anode during the charging process. Most of the commercial LIBs employ liquid electrolytes owing to their large electrochemical voltage windows, high ionic conductivities and great wettability with the internal components of LIBs [10]. Generally, the liquid electrolyte is a mixture of linear and cyclic carbonate-based organic solvents such as diethyl carbonate (DEC) [11], ethyl methyl carbonate (EMC) [11; 12; 13], dimethyl carbonate (DMC) [11; 12], ethylene carbonate (EC) [12; 13; 14], propylene carbonate (PC) [11], and lithium salt such as lithium hexafluorophosphate (LiPF\({}_{6}\)), lithium hexafluoroarsenate monohydrate (LiASF\({}_{6}\)), lithium perchlorate (LiClO\({}_{4}\)) and lithium tetrafluoroborate (LiBF\({}_{4}\)) [13; 15; 16]. However, these liquid electrolytes have severe drawbacks [17], including high flammability, narrow electrochemical stability windows, limited operating temperatures and irreversible decomposition. Due to their high flammability, organic liquid electrolytes are believed to be the main reason for fires and explosions in LIBs [18]. In addition, the formation of lithium dendrites with organic liquid electrolytes [19] leads to internal short circuits causing catastrophic failure of lithium-based batteries [20]. Therefore, developing alternative battery systems to prevent such issues of the liquid electrolytes as well as to provide high energy density and power is indispensable. All-solid-state batteries (SSBs), which use non-volatile solid electrolytes, have emerged as an alternative battery system to replace the conventional LIBs with liquid electrolytes [21; 22; 23]. Not only are SSBs inherently safer owing to the lack of flammable organic components, SSBs also have a large electrochemical stability window, thus enabling a dramatic improvement in the energy density [24; 25; 26]. Furthermore, the SSBs have much higher power and energy characteristics compared with various batteries which are currently being developed as next-generation batteries [27; 28]. The electrodes are required to have the following features in order to achieve high energy density: (i) high gravimetric capacity (in Ah/kg) and volumetric capacity (in Ah/L), i.e., a high number of electrons transferred per unit of reaction; (ii) high (cathode) and low (anode) standard redox potential of the respective electrode redox reaction, leading to high cell voltage. Moreover, electrochemical reactions in rechargeable cells at both anode and cathode electrodes must be highly reversible to maintain the capacity for thousands of cycles. Recent studies, therefore, have focused on developing new electrode materials [29; 30] or engineering electrode architectures [31; 32] to increase the energy density of SSBs.