Ceramic materials are a broad category that includes a variety of inorganic nonmetallic materials obtained through high-temperature processing (such as sintering), usually in powder or granular form. A representative electrolyte in solid electrolyte is ceramic-based solid electrolyte, ceramic-based solid electrolyte has some beneficial inherent characteristics, such as non-flammability, large mechanical strength, wide electrochemical stability window, etc., they have unparalleled advantages in safety and service life, is one of the important directions of solid electrolyte research at home and abroad. Ceramic based solid electrolyte mainly includes oxide series and sulfide series, of which the oxide series mainly includes perovskite type, garnet type, LISICON (lithium superionic conductor) type, NASICON (sodium superionic conductor) type, sulfide series is mainly thiophosphate system. The representative material of perovskite solid electrolyte is LLTO (lithium lanthanum titanum oxide/lithium lanthanum titanate, Li0.33La0.56TiO3), and the representative material of garnet solid electrolyte is LLZO (lithium zirconium oxide/lithium lanthanum zirconate, Li7La3Zr2O12). The representative material of NASICON solid electrolyte is LATP (lithium aluminum titanium phosphate, Li1.3Al0.3Ti1.7(PO4)3), these three are the material types of the production line layout at present, and there are few reports about LISICON solid electrolyte. Although the solid electrolyte of sulfide in the phosphate thioate system has a high lithium ion conductivity, its stability is relatively poor, it may also release hydrogen sulfide gas when in use, and its mechanical properties are also relatively poor, so it may be mainly used in lithium-sulfur batteries.
The introduction and application of Perovskite type LLTO solid electrolyte
The molecular formula of typical perovskite ceramics is ABO3, the A site is generally rare earth or alkaline earth element ion, the B site is A transition metal ion, and the A and B sites can be partially replaced by metal ions with similar radii to keep their crystal structure basically unchanged. Perovskite structure composite oxides are usually structurally stable, where Li+ is transported by vacancy transition mechanism, that is, Li+ directional transition to adjacent vacancy to generate lithium ion conductance.
Currently, perovskite-structured LLtos (lithium lanthanum titanum oxide/lithium lanthanum titanate, Li0.33La0.56TiO3) have high crystal conductivity in crystalline solid electrolytes. In 1993, Inaguma et al. found that the room temperature ionic conductivity of LLTO single crystal body could reach 10-3S·cm-1, but its polycrystalline conductivity was two orders of magnitude smaller than that of monomer. The short-plate effect made the total conductivity of LLTO ceramics low, thus failing to meet the requirements of practical application in lithium batteries. The total conductivity should be improved by doping modification. In view of the problems existing in LLTO, the study of regulating crystal structure and improving electrochemical performance through doping modification of LLTO has been widely reported. At present, LLTO doping is divided into A-site doping, B-site doping and AB site co-doping. The use of rare earth elements in the sintering process can improve the LLTO-induced density and reduce the porosity, thus reducing the grain boundary impedance. The three-dimensional skeleton constructed by rare earth ions with large radius is conducive to ion diffusion and migration, and its doping can increase the LLTO cell volume, thereby expanding the ion transport channel and stabilizing the highly conductive cubic phase of LLTO.
The introduction and application of Garnet type LLZO solid electrolyte
The general formula of garnet structure is A3B2(XO4)3, in which ABX coordinates with eight, six and four oxygen ions respectively to form dodecahedron, octahedron and tetrahedron, which together build a three-dimensional skeleton structure. At present, the most studied garnet solid electrolyte is LLZO (Lithium zirconium oxygen/lithium lanthanum zirconate, Li7La3Zr2O12), and its main raw materials are LiCO3, La(OH)3, ZrO2 and Al2O3.
LLZO has two crystal structures: cubic phase (c-LLZO) and tetragonal phase (t-LLZO), and the most significant difference between the two structures is the occupation of Li, in the cubic phase Li partially occupies the gap position, while in the tetragonal phase Li occupies the gap position. Among them, c-LLZO has higher conductivity than t-LLZO, and its crystal skeleton network is composed of La3+, Zr2+ and O2- ions, and Li ions are distributed in the crystal grid. The shortest distance between Li ions in adjacent positions is the main reason for fast ion transport, and thus provides high ionic conductivity. At 25℃, the ionic conductivity of LLZO is 3×10-4S/cm, and the proportion of its grain boundary resistance to the total resistance is less than 50%. Therefore, the overall ionic conductivity of LLZO is in the same order of magnitude as the intracrystalline ionic conductivity, ensuring its high overall ionic conductivity as a solid electrolyte. LLZO solid electrolyte has great application potential in all-solid-state batteries, LLZO does not react with lithium metal, has low interface and grain impedance, and is relatively stable in air, and the strength and hardness of the densified ceramics sintered by LLZO heat treatment are relatively high. However, usually undoped LLZO has the advantages of stable electrochemical performance and wide electrochemical window, but the phase structure stability is poor, the vibration density is low, has a large grain boundary resistance and low room temperature ion conductivity, and will have a large interface resistance in the application of all-solid-state batteries. In addition, LLZO is exposed to air with moisture and CO2, and Li2CO3 will be generated on the surface due to the role of H+/Li+ exchange, resulting in gradual deterioration of performance. In order to enhance the chemical stability of LLZO system solid electrolyte in atmospheric environment, LLZO is mainly modified by doping different metal elements, the purpose is to stabilize the cubic phase structure, optimize the preparation route, reduce its interface resistance and grain boundary resistance, and improve its room temperature ionic conductivity.
LLZO, as one of the most marketable solid electrolyte materials, has attracted the attention of many researchers, and through in-depth understanding of the crystal structure of LLZO and optimization of the structure of lithium-rich garnet by element doping, the lithium ion conductivity of LLZO has been improved by an order of magnitude. Although researchers have made a large number of attempts to build LLZ-based solid-state batteries and develop a series of effective strategies to solve the problem of positive /LLZO and negative /LLZO physical contact, up to now, there is indeed no mass-produced product that can significantly outperform traditional lithium-ion batteries in all aspects.
The introduction and application of NASICON LATP solid electrolyte
The general formula of NASICON type material is M1xM2(2-x)(PO4)3, M1 and M2 can be a variety of different metal elements, A13+ doped NASICON type solid electrolyte LATP (lithium aluminum titanium phosphate, lithium titanium), Li1.3Al0.3Ti1.7(PO4)3) is one of the hot research objects of solid electrolyte in recent years.
LATP has a complex crystal structure consisting of alternating layers of Li/Ti tetrahedrons and Al/PO4 octahedrons. The Li/Ti tetrahedrons form a three-dimensional frame by connecting shared vertices, while the Al/PO4 octahedrons fill the space between the tetrahedrons. This structure creates channels through which lithium ions can move. LATP can be synthesized by a variety of methods, including solid-state reactions, sol-gel methods, and hydrothermal methods. In one common method, lithium carbonate, alumina (Al2O3), titanium oxide (TiO2), and ammonium dihydrogen phosphate are mixed in stoichiometric proportions and heated in a reducing atmosphere at high temperatures (usually 900 to 1200 ° C) to form LATP ceramics. Some of the advantages of LATP include relatively high ionic conductivity, a wide electrochemical stability window, good mechanical stability, compatibility with lithium metal anodes, low reactivity with cathode materials, and a wide temperature range. In addition, LATP is not easy to form dendrites, which can improve the safety and cycle life of the battery. However, LATP is relatively expensive and has lower ionic conductivity than LLZO.