Lithium–sulfur batteries (LSBs) represent a promising next-generation energy storage system, with advantages such as high specific capacity (1675 mAh g−1), abundant resources, low price, and ecological friendliness. During the application of liquid electrolytes, the flammability of organic electrolytes, and the dissolution/shuttle of polysulfide seriously damage the safety and the cycle life of lithium–sulfur batteries. Replacing a liquid electrolyte with a solid one is a good solution, while the higher mechanical strength of solid-state electrolytes (SSEs) has an inhibitory effect on the growth of lithium dendrites. However, the lower ionic conductivity, poor interfacial contact, and relatively narrow electrochemical window of solid-state electrolytes limit the commercialization of solid-state lithium–sulfur batteries (SSLSBs).
With the continuous development of science and technology, new energy vehicles, portable electronic devices, and energy storage systems have resulted in higher requirements for the energy density of secondary batteries 
. At present, the energy density of lithium–ion batteries is close to the limit of theoretical calculation. It is challenging for the energy density to break 400 Wh kg−1
, even with advanced material design 
. LSB is a prospective next-generation electrochemical energy storage system due to its high theoretical energy density of 2570 Wh kg−1
, low price, and environmental friendliness 
Although lithium–sulfur batteries have many advantages, there are still some problems that hinder their commercialization: (1) the volume effect of the positive sulfur electrode in the process of charge and discharge within a volume expansion about 80% 
; (2) the shuttle effect caused by the dissolution of the intermediate 
; (3) the low conductivity of sulfur (10−7
at room temperature) 
; and (4) the inhomogeneous lithium metal deposition caused by current density 
. These problems accelerate battery aging and capacity decay. To suppress these problems, researchers have adopted a variety of strategies, such as developing various materials as sulfur carriers 
, introducing an intermediate layer as a physical barrier for chemisorption 
, improving the conversion efficiency of polysulfide 
, and modifying the biphasic interface 
. Although these strategies have made some progress, they do not solve the root of the problem. Many researchers have turned their attention from flammable organic electrolytes to safer and more energy-dense solid electrolytes. Solid electrolytes can effectively avoid the shuttle effect induced by long-chain polysulfides. At the same time, the higher thermal stability of solid electrolytes avoids potential risks such as leakage or high-temperature flatulence, significantly improving the safety of batteries 
As the core part of a solid-state lithium–sulfur battery, the solid electrolyte dramatically affects battery performance. A good SSE must have the following characteristics: (1) A high ion mobility number is required, and when the ion mobility number is low, the cell will have severe local polarization, resulting in uneven Li+
deposition and lithium dendrite generation 
. (2) Higher Li-ion conductivity: Low ionic conductivity means that the battery has higher internal resistance, which also reduces the charge and discharge rate of the battery 
. (3) A stable electrochemical window: The ideal SSE electrochemical window should be stable between 0 and 5 V 
. (4) Good flexibility ensuring mechanical strength: The mechanical strength can inhibit the penetration of lithium dendrites, and the flexibility makes the interface between electrode and electrolyte more fit and stable 
. (5) Good electrical insulation 
: Until now, a single SSE has not been able to cover all of these features. Different types of SSEs are applied in different situations according to actual requirements and to face different challenges.
2. Principles and Challenges of Lithium–Sulfur Batteries
2.1. The Composition and Working Principle of Lithium–Sulfur Battery
A typical Li–sulfur battery system consists of a sulfur cathode, a lithium metal anode, and an electrolyte. Unlike the de-embedded lithium energy storage mechanism of traditional lithium-ion batteries, LSBs consist of a reversible redox reaction between lithium metal and S8
for the mutual conversion of chemical energy and electric energy. The entire reaction process can be expressed by the equation S8
+ 16Li → 8Li2
. During the discharge process, the solid ring S8
gradually becomes the long liquid chain polysulfide Li2
(4 < n ≤ 8) and the short-chain polysulfide Li2
(2 < n ≤ 4), which are finally converted to insoluble Li2
S. As shown in Figure 1
a, in a liquid electrolyte one, both long-chain polysulfides and short-chain polysulfides are soluble in the electrolyte, so the shuttle effect inevitably occurs. LiPS shuttles back and forth in the electrolyte and reacts directly with lithium metal in the negative electrode, causing the irreversible loss of active substances and capacity in the battery. The shuttle effect is a common problem in liquid Li–sulfur batteries and is the main reason for capacity decay. As shown in Figure 1
b, a solid electrolyte without liquid components can prevent LiPS from moving to the anode, thus realizing the enhancement of LSB cycle stability and Coulomb efficiency.
Figure 1. The schematic structure of (a) liquid-electrolyte and (b) solid-electrolyte lithium–sulfur battery.
2.2. The Challenges of Lithium–Sulfur Solid Electrolytes
The biggest problem facing conventional liquid-electrolyte lithium–sulfur batteries is the shuttle effect of lithium polysulfide. The shuttle effect is the phenomenon in which LiPS dissolves in the electrolyte and shuttles back and forth between the two electrodes when the battery is being used. LiPS reacts with lithium metal in the shuttle process, which dramatically reduces the capacity and cycle stability of Li–sulfur batteries, causing severe and damaging corrosion to the anode. There are two commonly used solutions. One is to introduce functional materials into the cathode or electrolyte and to fix polysulfide on the surfaces of the functional materials through the physical barrier or chemisorption 
. The other is to insert an interlayer to prevent polysulfide diffusion 
. Unfortunately, these measures only suppress but do not entirely solve the shuttle effect. A practical solution is to use solid electrolytes. The solid-phase reaction of a solid electrolyte can directly transform S into Li2
S, which effectively avoids LiPS dissolution and the shuttle effect, and its charge and discharge curve only shows a voltage plateau. Although the solid electrolyte effectively avoids the shuttle effect, the solid-phase reaction also faces the challenge of slow kinetics.
At present, SSEs can be roughly divided into three types according to the different materials: polymer electrolyte (PE), inorganic solid electrolyte (ISE), and composite solid electrolyte (CSE). The problems and challenges faced by several types of solid-state lithium–sulfur batteries include the low ionic conductivity of the solid-state dielectric, interface incompatibility, poor chemical/electrochemical stability, and lithium dendrite growth.
2.2.1. Low Ionic Conductivity
Ion conductivity (σ) is a critical index that determines the internal resistance and multiplier performance of a battery. SSEs with excellent performance should have high ionic conductivity to realize the rapid transport of lithium ions. At the same time, it has electronic insulation to avoid the growth of lithium dendrites and the occurrence of self-discharge. The room-temperature ion conductivity of most SSEs is still low compared to liquid electrolytes, resulting in the poor electrochemical performance of the cells and limiting the development of SSLSB. The polymer electrolyte is easy to manufacture and is flexible enough to form good interfacial contact with the anode and cathode. The most common mechanism of lithium-ion transport in polymer electrolytes is segmental motion, which is characterized by the coordination and dissociation of migrating ions and polymer groups under the action of the electric field. Most ion transport occurs in the amorphous region, and the crystallization state at room temperature leads to low ionic conductivity. Generally, ionic conductivity is improved by reducing the crystallinity of polymers, manifested as polymer blending and copolymerization. Compared to solid-polymer electrolytes, inorganic solid electrolytes have higher ionic conductivity, but inorganic solid electrolytes are brittle, which leads to the SSLSB having high interface contact 
. To some extent, inorganic solid electrolytes and polymer electrolytes can complement and combine to form inorganic–organic composite electrolytes. Compared to the single electrolyte mentioned above, the inorganic–organic composite electrolyte has higher ionic conductivity and a lower glass transition temperature.
2.2.2. Interface Incompatibility
Improving interface compatibility has always been a research hotspot in the SSE field. Compared to flexible gel polymer electrolytes, the interface incompatibility between the solid electrolyte and electrode are more prominent. Among them, the oxide SSE is more rigid, and the interface contact area between the electrode and SSE is limited. The interface problem is severe and even causes a serious polarization phenomenon and slow kinetics. Improvement strategies are generally divided into two aspects: increasing the contact area using porous and multilayer structures designed by deposition technology or manufacturing by selecting an intermediate buffer layer comprising appropriate materials 
. Unlike oxide SSEs, sulfide SSEs are relatively soft and have good interface contact with the cathode and anode. The interface problem of sulfide SSE is mainly due to the space charge layer effect caused by the limited electrochemical window and the insulation of sulfur. To avoid the impact of the space charge layer, interface coating is a decent strategy.
2.2.3. Chemical/Electrochemical Stability
Chemical stability refers to the ability of the battery to resist chemical reactions in the idle state to maintain stable physical and chemical properties. Electrochemical stability refers to the ability of the battery to maintain physical and chemical properties under the action of an external electric field 
. LSBs often use lithium as the anode, which is relatively reactive and easy to react with SSE, resulting in chemical/electrochemical instability. Among the common SSEs, solid polymers are relatively stable with lithium metal, and sulfide-based SSEs are less electrochemically stable. Sulfide-based SSs have become the SSEs with the best potential due to its their ionic conductivity and low interfacial impedance. However, sulfide-based SSEs are extremely sensitive to water and unstable to lithium metal, which significantly limits the application of sulfide-based SSEs in batteries with a high-energy-density power batteries 
. Adding a coating material at the interface between the SSE and the electrode represents an excellent method for surface optimization. The coating material can provide surface passivation protection and prevent the diffusion of non-lithium elements at the interface. When the thickness of the coating material is lower than that of the decomposition product layer, the interface resistance can also be effectively reduced.
2.2.4. Lithium Dendrite
Lithium dendrites are mainly caused by inhomogeneous deposition at the Li/electrolyte interface 
. Uncontrolled Li dendrite growth can cause serious safety problems. Researchers hope to prevent the growth and penetration of lithium dendrites via inorganic solid electrolytes with high mechanical strength. With the development of characterization techniques, researchers have found that although the high mechanical strength of inorganic SSE could theoretically resist dendrite growth, Li dendrites still appear in the bulk phase of inorganic SSEs, eventually penetrating the SSE and causing short circuiting in the battery 
. On the one hand, the crystal structure of inorganic SSE electrolytes promotes the growth of Li dendrites. Once there is a gap, Li dendrites will penetrate the inorganic SSE, grow along the grain boundary, and gradually penetrate the positive pole 
. On the other hand, the high electronic conductivity of SSE promotes Li+
binding to electrons in the electrolyte and the gradual formation of Li dendrites. There are currently many strategies for inhibiting Li dendrites, but all of them have some defects. It is necessary to deepen our understanding of the mechanism of lithium dendrite growth to provide a possible way to inhibit Li dendrite effectively.