The all-solid-state lithium battery (ASSLIB) is one of the key points of future lithium battery technology development. Because solid-state electrolytes (SSEs) have higher safety performance than liquid electrolytes, and they can promote the application of Li-metal anodes to endow batteries with higher energy density. Glass-ceramic SSEs with excellent ionic conductivity and mechanical strength are one of the main focuses of SSE research.
1. Introduction
Since Sony first commercialized lithium-ion batteries (LIBs) in 1991, LIBs have been widely used in electronics, power and energy storage applications due to their high working voltage, high energy density, long cycle life and no memory characteristics
[1][2][3][1,2,3]. With the rapid development of electric vehicles (EVs), traditional LIBs have been insufficient to meet the range of EVs. The energy density of traditional LIBs has achieved 260 Wh·kg
−1, which is approaching the limitations of traditional LIBs
[4]. Metal lithium has a high theoretical specific capacity (3860 mAh·g
−1) and the lowest redox potential (−3.04 V vs. SHE) and can effectively increase the energy density of the battery when used as the anode
[5]. However, traditional liquid electrolytes restrict the application of the lithium-metal anode because they contain flammable organic solvents that cause some safety problems
[6][7][6,7]. All-solid-state lithium-metal batteries (ASSLMBs) with higher safety and higher energy density composed of lithium-metal anodes and solid-state electrolytes (SSEs) instead of traditional liquid electrolytes are expected to become the next generation of lithium battery.
In 1833, Faraday first discovered the ionic conductivity of solid Ag
2S and PbF
2, and research on the ionic conductivity of solids has been conducted since that time
[8]. In the 1960s, Na
2O·11Al
2O
3 with Na
+ ion conductivity was discovered, and researchers discovered that this type of material possessed the property of high ionic conductivity and had the potential to be used as SSEs
[9]. Therefore, using solids with satisfactory ionic conductivity to form ASSLIBs became possible. SSEs, the most important component of ASSLIBs, have many advantages over liquid electrolytes.
-
The non-flammable characteristics of SSEs make ASSLIBs have higher safety performance than LIBs
[10].
-
Compared to traditional LIBs, SSEs are able to replace the liquid electrolyte and separator to effectively reduce battery weight. Meanwhile, the energy density of the battery is increased by combining the application of a lithium-metal anode
[11].
-
Compared to conventional LIBs, ASSLIBs have greater structural design advantages because they can be connected in series internally to achieve higher voltages. Chen et al.
[12] stacked one, two and three solid-state cells in a button battery to obtain open-circuit voltages of 3.08, 6.51 and 9.12 V, respectively.
Although ASSLIBs have certain advantages, their process of industrialization is still limited by technological, marketing and financial factors. On the technological side, the research of SSE synthesis method, stability, conductivity and interfacial properties is the key to practical application. After years of development, SSEs can be divided into three categories: inorganic solid electrolytes (ISEs), polymer solid electrolytes (PSEs) and composite solid electrolytes (CSEs). Among them, ISEs can be divided into amorphous glass, glass-ceramic and polycrystalline ceramic. Glass is an amorphous supercooled liquid, while glass-ceramics are partially crystalline glasses, consisting of a mixture of crystalline and amorphous glass phases
[13][14][13,14]. The definition of glass-ceramic materials is an inorganic non-metal material prepared by controlling the crystallization of glass through different processing methods
[15]. They consist of at least one functional crystalline phase and residual glass. The volume fraction of the crystalline part in glass-ceramic materials is typically in the range of 10–90%
[14]. The main advantages of glass-ceramic materials are their dense, non-porous microstructure, and good mechanical, electrical and thermal properties. Glass-ceramic SSEs have become one of the hot research directions for SSEs due to their excellent ionic conductivity, electrochemical properties and better compatibility with electrodes.
Glass-ceramic SSEs are divided into two main categories, oxide glass-ceramic SSE systems and sulfide glass-ceramic SSE systems. Oxide glass-ceramic SSEs include NASICON-type electrolytes and some other oxides. They are mainly prepared by the melt-quenching method with subsequent heat treatment, and their main advantages are high ionic conductivity (10
−4~10
−3 S·cm
−1), large Li
+ transference number and high mechanical strength
[16][17][16,17]. The sulfide glass-ceramic SSEs are mainly Li
2S-P
2S
5 binary systems, which are prepared by mechanical ball milling and subsequent heat treatment, and their main advantages are high ionic conductivity (10
−3~10
−2 S·cm
−1)
[18][19][20][21][18,19,20,21]. Although glass-ceramic SSEs generally have high ionic conductivity, the stability of the SSE itself and the interface problems between the electrode/electrolyte are major impediments to the practical application of ASSLIBs
[22][23][22,23]. Improving the properties including ionic conductivity and chemical stability has become one of the main focuses of current research on glass-ceramic SSEs.
2. Ionic Conduction Mechanism
For designing high-performance SSEs, an understanding of their ion conduction mechanisms is necessary. Li
+ ion migration in ceramics relies on different types of defects, including point defects, line defects, planar defects, volume defects and electron defects. Compared to other defects, point defects have a greater impact on cation transport in crystals
[24]. In a perfectly ordered crystal, ions cannot leave their host position
[8]. The migration of ions in SSEs is accomplished by moving point defects in the crystal.
The basic assumption about the ionic conduction mechanism in polycrystalline (ceramic) is that vacancies in the lattice and interstitial spaces in the cationic sublattice are considered as charged movable species
[25][26][25,26]. It is noteworthy that only a fraction of cations in a lattice has an ability to move having vacant stable or meta-stable lattice nodes within reach
[9]. Currently, there are three main types of cation migration.
-
Cation vacancy diffusion, cation migration from the initial position to its adjacent vacancy lattice position.
-
The cation occupying the interstitial migrates directly to the adjacent vacant interstitial.
-
Interstitialcy mechanism, cation occupying a lattice interstitial migrates to an adjacent lattice node, migrating the cation occupying that lattice to the next site.
For polycrystalline ceramic SSEs, the Li
+ transport mechanism depends on three factors: carrier type, diffusion pathways and diffusion type. The carrier type and concentration are determined by the point defects in the polycrystalline ceramic structure, which directly affect the ionic conductivity. The interactions between Li ions during migration in the crystal and between ones and the surrounding environment will significantly affect the ionic conductivity
[24][27][28][29][24,27,28,29].
Compared to ceramic SSEs, amorphous (glass) SSEs have better flexibility, uniformity and density. Meanwhile, the glass SSEs show no grain boundary resistance and isotropic Li
+ mobility. These properties of glass SSEs have prompted attempts to find its ionic conduction mechanism. At present, although many experimental data on Li
+ conduction in glass SSEs are available, the Li
+ conduction mechanism in glass SSEs is still not well explained, and no relevant general theory has been established. The main challenge is that glass SSEs are a short-range ordered, long-range disordered amorphous material. It means that the glass SSEs have no long-range crystalline order, no regular symmetric long-range ion migration pathways and no regular symmetric short-range coordination order
[9]. In glass SSEs with disordered structure, the migration of cations in SSEs cannot be explained by a single factor. During the migration of cations in glass SSEs, charge carrier interactions and even interactions with the transport matrix can have an effect on the migration of ions. This makes the theoretical development of the conduction mechanism of cations in glass SSEs difficult. However, hypotheses have been offered to explain how the cations migrate in the amorphous SSE
[30].
Funke et al.
[31] suggested that structure and kinetic disorder are major factors in the high ionic conductivity of amorphous materials. They defined the movement of ions in a completely ordered crystal structure as level 1. This material is regarded as an insulator without ion movement because of the absence of defects in the perfectly ordered crystal structure. Crystal structures with few defects are defined as level 2, and a single point defect can only move randomly to another location. Materials with disordered structure are defined as level 3, and ion movement cannot be described by defect theory but is related to multiple interactions with the surrounding environment. They suggested that the mismatch caused by the hopping of ions resulted in rearrangement of the particles of the neighborhood. The hopping ion is accommodated by the new site created by its neighborhood relaxation.
3. Synthesis and Characterization of Glass-Ceramic Solid-State Electrolytes
Currently, there are two main types of glass-ceramic SSEs, oxide glass-ceramic SSE systems and sulfide glass-ceramic SSE systems. Glass-ceramic SSEs are mainly prepared in two steps by the melt-quenching method and mechanical ball-milling method. In the first step, the required parent glass is prepared at a given ratio of raw materials by high temperature melting or mechanical ball milling. In the second step, the parent glass is heat-treated between the glass transition temperature T
g and the crystallization temperature T
c. T
g and T
c are determined by differential thermal analysis (DTA) and differential scanning calorimetry (DSC). Currently, glass-ceramic materials with high ionic conductivity are mainly obtained by changing the optimized raw material ratio, heat treatment temperature and time. Glass-ceramic SSEs prepared by wet chemical methods have also been reported recently
[32][33][34][35][32,33,34,35]. For the prepared glass-ceramic SSEs, the properties were investigated mainly by characterization means such as impedance spectroscopy (IS), X-ray diffraction (XRD), DTA, DSC and electron microscopy. In some cases, short-range ordering in glass-ceramic SSEs has also been investigated by nuclear magnetic resonance (NMR). In this chapter, the preparation and characterization of oxide glass-ceramic SSE systems and sulfide glass-ceramic SSE systems are highlighted in the following sections. Additionally, possible ways to improve their ionic conductivity will be discussed.
3.1. Oxide Glass-Ceramic SSE Systems
Most of the oxide SSEs are polycrystalline ceramic SSEs whose advantages are high ionic conductivity, high mechanical strength and a wide electrochemical stability window. However, the interface problem between this type of SSEs and electrodes is more prominent. Compared to polycrystalline ceramics, glass has certain advantages in terms of flexibility, homogeneity and density. Therefore, glass-ceramic SSEs are prepared by fusion glass and partial crystallization of glass, which not only improve the ionic conductivity but also optimize the interface between SSEs and electrodes to some extent. The current research on oxide glass-ceramic SSE systems is mainly focused on Na
+ superionic conductor (NASICON)-type SSEs and some other types of oxides.
3.2. Sulfide Glass-Ceramic SSE Systems
Compared to oxide SSEs, sulfide SSEs have been intensively studied in recent years due to their advantages such as higher ionic conductivity at room temperature and cheaper raw material. Sulfides can be processed into three forms: glass, glass-ceramic and crystalline. Glass-ceramic SSEs generally have better performance than the other two forms. Therefore, the sulfide glass-ceramic SSE system, represented by the glass-ceramic SSEs in the Li
2S-P
2S
5 binary system (LPS glass-ceramic SSEs), has been studied extensively in recent years.