Solid Electrolytes for Advanced Applications

The chapter systematically describes how the structural framework dictates the pathways for ion mobility (e.g., 1D, 2D and 3D) in solid-state electrolytes. In lithium-stuffed garnets, for example, Li⁺-ion shows three-dimensional nature of ion transport; whereas, the motion of same Li⁺-ion occurs in one- and two-dimensions in β-eucryptite (LiAlSiO₄) and Li₃N, respectively. In addition to Li⁺-ion, Na⁺, H⁺ and O^2- ion-conducting solid-state electrolytes are also introduced in the chapter recognizing their greater importance on developing novel materials for renewable energy applications.

The garnet-type Li⁺ ion conductor Li₇La₃Zr₂O₁₂ (LLZO) is a promising solid electrolyte for all-solid-state lithium batteries. Numerous synthesis methods have been developed as alternatives to the conventional solid-state reaction first used to prepare LLZO. This chapter provides a comprehensive review of the research progress that has been made on synthetic methods to obtain nanostructured LLZO, with a focus on the synthesis of materials prepared using sol–gel/combustion, electrospinning, cellulose templating, spray pyrolysis, co-precipitation, and molten salt methods, as well as to highlight some unique properties of fine-grained and nanostructured LLZO.

Garnet-type oxide, Li₇La₃Zr₂O₁₂ (LLZO), is a promising solid electrolyte material for all-solid-state lithium-ion batteries. This chapter reviews the latest research efforts on the air stability of lithium garnets, which includes the origin of the air stability and the reaction mechanisms of LLZO electrolytes in ambient air; the factors affecting the air stability such as synthesis conditions, materials properties, and storage conditions; and finally the improvement strategies such as mechanical polishing, using additives, synthesis optimization, and etching.

All-solid-state batteries (ASSBs) using inorganic solid electrolytes are one of the candidates of next-generation batteries. However, ASSBs suffer from various issues, most of which do not matter in conventional lithium-ion batteries with liquid electrolytes. In this chapter, mechanical stress in solid electrolytes are focused. First, we reveal that garnet-type solid electrolytes, Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZT), prepared by the spark plasma sintering (SPS) method, exhibit a residual tensile stress of more than 100 MPa in the direction of uni-axial pressure during the SPS process, which was revealed by XRD (side-inclination method). Then, the influence of the stress on ionic conduction is studied. Detailed analyses reveal that the stress mainly influences grain boundary resistance with little change in the bulk resistance of LLZT. The results suggest the importance of mechanically strong grain boundaries (including interfaces) for practical ASSBs.

Producing densified garnet-type solid electrolytes by lowering sintering temperature is an important target, which can prevent not only the lithium loss (controlling chemical stoichiometry) but also make it more compatible with cathode electrode materials. In this chapter, the use of sintering additives for enhancing the densification and microstructure of high conductive garnet-type solid electrolytes at low temperatures of ≤900 °C is reviewed. Sintering additives can modify the grain and grain boundary, both contributing to the optimization of the chemical and electrochemical properties of garnet-type solid electrolytes.

While the general working principle of all-solid-state batteries on a laboratory scale has nowadays already been frequently proven, one major improvement opportunity in optimizing the energy density still lies in the drastic reduction in the electrolyte thickness down to the range of a few micrometers. In this way, the overall inactive mass of the cell will be reduced and a potentially lower Li-ion conductivity compared to conventional liquid electrolytes can be counterbalanced. The focus of this chapter is on the thin film deposition of garnet structured Li-ion conductors. In the first part, different deposition approaches by wet chemical routes and gas-phase techniques are discussed. In the second part, an overview of the compositional analysis of Li-containing thin films, which is an elaborate and important part of the target-oriented development of garnet-structured thin films, is given.

Recent years, all solid-state battery (ASSB) has attracted much attention mainly due to the safety concerns. As the key factor of ASSB, the solid electrolyte determines the performance of the battery. Among the numerous electrolytes, the garnet type Li₇La₃Zr₂O₁₂ (LLZO) possesses the advantages of ionic conductivity and excellent chemical stability. Compared to the LLZO bulk ceramic electrolyte, LLZO thin films have advantages in better contact with electrodes, less lithium diffusion time in electrolytes and smaller resistance in ASSB. In this chapter, we will introduce LLZO films prepared by different thin-film preparation methods, including pulsed laser deposition (PLD), radio frequency (RF), sol–gel method, chemical vapor deposition (CVD), focused ion beam (FIB) milling, atomic layer deposition (ALD), and wet coating. Their methods, performance, and applications will be detailed expounded in each section.

All-solid-state batteries are known to be in great demand both on an industrial scale (electric vehicles, stand-alone devices) and for special population needs (pacemakers), and therefore attract scientific interest. The difficulty of high-capacity lithium-ion batteries creation consists in the development of suitable solid electrolytes that would have high lithium conductivity and would be chemically and thermodynamically resistant to metallic lithium (the most energy-intensive anode). Among the well-known oxide conductors, only the Li₇La₃Zr₂O₁₂ compound satisfies all the requirements. However, there is a problem of high porosity and low density of this electrolyte with tetragonal structure. The introduction of inorganic binders, in particular glasses, into the ceramic electrolyte can solve the indicated problem. The total conductivities of two Li₇La₃Zr₂O₁₂ modifications (tetragonal and cubic) are comparable in the region of medium temperatures (above 200 °C). However, the synthesis of tetragonal Li₇La₃Zr₂O₁₂ is realized without the dopants’ introduction. The 50Li₂O·50P₂O₅, 65Li₂O·27B₂O₃·8SiO₂, 40.2Li₂O·5.7Y₂O₃·54.1SiO₂ glasses were chosen and the effect of various additives on the functional properties of the tetragonal Li₇La₃Zr₂O₁₂ was studied. By a complex of modern techniques, it was established that the additives have a good effect on the electrical conductivity and the density of solid electrolytes. Thus, the proposed approach allows an increase electrical conductivity of the electrolytic membrane for all-solid-state batteries by several orders of magnitude.

Garnet type electrolytes based on the sum formula Li₇La₃Zr₂O₁₂ (LLZO) have been identified as promising materials for solid state electrolytes with high ionic conductivity and electrochemical stability. Since the large-scale processing of thin and brittle ceramic sheets remains challenging and possible volume changes in a solid state battery require flexibility from the electrolyte/separator, the concept of composite electrolytes consisting of LLZO and polymer electrolyte based on poly(ethylene oxide) (PEO) has been explored by multiple research groups. The chapter is dedicated to introduce the concept of polymer and composite electrolytes and gives an overview of LLZO/PEO electrolytes investigated so far.

Lithium insertion materials for lithium-ion batteries generally alter their lattice dimensions during the electrochemical reactions. However, some lithium insertion materials, which are called as zero-strain insertion materials, maintain the lattice dimensions during the whole electrochemical reaction. This zero-strain characteristic is ideal for electrode materials for all-solid-state lithium-ion batteries. The next chapter overviews characteristic of zero-strain insertion materials regarding macro- and microscopic structural change during the electrochemical reaction, and thermal stability when applying all-solid-state lithium-ion batteries. Furthermore, pseudo-zero-strain insertion materials and zero-strain insertion materials for sodium-ion batteries are briefly discussed.

Dense garnet structured solid electrolytes exhibit great promise for lithium metal batteries owing to its high lithium-ion conductivity, good mechanical and electrochemical properties. However, the rigid interfacial contact between garnet structured electrolyte and electrode stymie their practical application. In this chapter, salient features of various strategies utilized to enhance the room temperature performance of lithium metal battery based on garnet structured solid electrolyte are discussed. In particular, a detailed electrochemical investigation on lithium | garnet interface and cathode | garnet interface with buffer layers for solid-state battery. The application of garnet structured solid electrolyte in suppressing polysulfide shuttling in lithium-sulfur (Li-S) battery is also discussed. It is shown that the cycling performance of lithium metal and Li-S batteries can be greatly improved with the incorporation of garnet structured solid electrolyte.

Dendritic lithium plating is one of the critical issues that hinder practical application of lithium metal anode on secondary batteries. Even dense ceramic solid electrolytes, which have been expected to show high resistance on lithium penetration, suffer from short-circuit. In this work, to suppress the lithium penetration through voids along grain boundaries, garnet-type solid electrolytes of Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZT) were sintered with grain boundaries modified with Li₂CO₃ or LiOH. Although all prepared solid electrolyte pellets showed relative density higher than 96% and high ionic conductivity of 7 × 10⁻⁴ S cm⁻¹, critical current densities (CCD) depended on grain boundary phases. While CCD of LLZT without modification was 0.2 mA cm⁻², Li₂CO₃ and LiOH modified LLZT pellets exhibited CCD of 0.4 and 0.6 mA cm⁻², respectively. This is possibly caused by the two effects of modification phases as supported by microstructure analyses using SEM, STEM, and EIS. First, Li₂CO₃ and LiOH remaining on the grain boundary plug voids and protect lithium growth. Secondly, Li₂CO₃ and LiOH played roles of sintering agent and facilitated densification of grain boundaries. The obtained results indicate a strategy toward practical application of lithium metal batteries.

In this chapter, challenges in the development of all-solid-state batteries designed to solve the problem of safety of the chemical power sources are discussed. Difficulties in the development of such batteries are poor adhesion and electrical conductivity of solids and change in the volume of materials during charge/discharge processes. To solve these problems, the use of glassy or glass-ceramic materials as the electrode is suggested. Based on vanadates glasses and glass-ceramics have attracted the most significant attention among glassy electrode materials since glassy vanadates have rather high electrical conductivity in comparison with other oxide glasses (about 10⁻⁵ S·cm⁻¹ at room temperature). In addition, the electrical conductivity of vanadate glasses can be significantly improved by obtaining of glass-ceramic based on them. Further, results of the test of all-solid-state battery with vanadate cathode are presented. It is shown that the voltage of Li–Ga | glassy vanadate single cell achieves the value of 3.3 V.

All-solid-state lithium batteries with LiCoO₂-Li₃BO₃ composite cathode layer was formed on Al-doped Li₇La₃Zr₂O₁₂ pellets by an aerosol deposition method. The effects of heat treatment after the aerosol process on the electrochemical performance of the all-solid-sate batteries were investigated. The overpotentials of charge-discharge cycles decrease after the heat treatment, and the cycle performance improved. The interfacial resistance of between LiCoO₂-Li₃BO₃ composite cathode layer and Al-doped Li₇La₃Zr₂O₁₂ electrolyte with the heat treatment was smaller than without the heat treatment.

Polymer electrolytes are of prime importance for advanced lithium-based batteries in terms of high-energy density, design flexibility and safety. In this chapter, we summarize the fundamental property requirements of materials for polymer electrolyte application. State of the art polymer hosts, salts and additives are reviewed along with the challenges faced in the current state of the art. Finally, the fabrication process (scaling-up) of Li metal polymer battery components, along with an overview of the current status of Li metal polymer batteries (Industrial), is presented. A brief conclusion and perspective for Li metal polymer batteries are also discussed.