Solid-state batteries do not contain liquid solvents (usually flammable organic solvents), electrolytes and additives, and there is no danger of explosion or fire. Therefore, the structure of the battery pack is simpler to save more space, so we have more space to place more active materials to increase the capacity of the battery. Compared with liquid batteries using organic solvent electrolytes, the most obvious advantage of solid-state batteries is that the energy density is greatly improved, which can make the car more durable in a limited volume.
Generally speaking, solid state electrolyte is generally divided into polymers, inorganic ceramics (inorganic electrolytes are usually based on oxides and sulfides) and composite systems composed of them. The solid state electrolyte film-forming process is a key process in the manufacture of solid-state batteries. Through decades of research, in terms of material development, different types of solid-state electrolyte (polymers, oxides, sulfides, etc.) have been successfully synthesized and prepared.
Solid-state batteries do not contain liquid solvents (generally flammable organic solvents), electrolytes, additives and other flammable substances. There is no danger of explosion or fire, so the structure of the battery pack is simpler and saves more space. Larger space to place more active materials to increase the capacity of the battery. Compared with liquid batteries using organic solvent electrolytes, the most obvious advantage of solid-state batteries is that the energy density is greatly improved, which can make the car more durable in a limited volume.
Sulfide solid electrolyte
Compared with oxygen ions, sulfur ions are less electronegative and less bound to lithium ions. At the same time, the radius of sulfur ions is large, which makes the transmission channel of lithium ions wider in the crystal structure, which is beneficial to the movement of lithium ions. Therefore, the sulfide solid state electrolyte has the highest ionic conductivity among the three types of electrolytes. Sulfide electrolytes mainly include vitreous, glass-ceramic, and crystalline sulfides.
Oxide solid electrolyte
Oxide solid state electrolyte mainly include garnet Li7La3Zr2O12 (LLZO), perovskite Li3.3La0.56TiO3 (LLTO), anti-perovskite structure, sodium superionic conductor (NASICON) and lithium superionic conductor (LISICON), etc., which have Excellent stability. Among the many oxide structures, LATP (LiAlxTi2xPO)4 is a kind of NASICON structure oxide solid state electrolyte material, which has become the most studied by virtue of its high ionic conductivity as high as 0.7mS/cm.
At present, analyzing the published data, solid-state batteries based on oxide electrolytes can generally be divided into three categories, including oxide thin-film all-solid-state batteries, oxide membrane batteries, and organic-oxide composite electrolyte batteries.
Polymer solid electrolyte
Compared with inorganic solid state electrolyte, solid polymer electrolytes with dissolved lithium salts have the advantages of good flexibility, light weight, low cost, and easy processing. Polymer solid state electrolyte usually consist of a polymer matrix, such as polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), and polymethyl methacrylate (PMMA), and some lithium salts, such as Composition of LiClO4, LiTFSI[LiN(CF3SO2)2], LiAsF6 and LiPF6.
Among them, PEO contains ethoxy chain segments, has high solubility for lithium salts, and is the most widely studied polymer matrix material. In addition to these widely studied polymer matrices, there are some acrylate monomers of different molecular weights, such as poly(ethylene glycol) methyl ether methacrylate (PEGMEA) and polyethylene glycol acrylic acid (PEGDA). The solid polymer electrolyte is prepared by in-situ polymerization in the form of photocuring or thermal curing, and the ionic conductivity can reach the level of 10-4S·cm-1.
In addition, aliphatic polycarbonate-based solid polymer electrolytes are considered to be a very promising class of polymer electrolyte systems due to their special molecular structures and high dielectric constants.
Basic electrochemical performance of lithium-ion solid state electrolyte
- Ionic conductivity
The role of the lithium ion battery electrolyte is to conduct lithium ions to the surface of the positive and negative active material particles in the battery. Therefore, the ion conductivity is the most important property of the electrolyte. Referring to the liquid electrolyte, its room temperature ionic conductivity reaches above 5×10-3 S/cm; In addition, the liquid electrolyte can well infiltrate between the positive and negative active material particles to build a three-dimensional ion-conducting network at the positive and negative electrodes.
In an all-solid-state battery, there is a solid-solid contact between the positive electrode active material and the solid state electrolyte. To achieve a contact effect with electrode materials similar to that of liquid electrolytes at room temperature, on the one hand, it is necessary to increase the mass/volume ratio of the solid state electrolyte to the positive electrode material, and on the other hand, the solid state electrolyte itself is required to have ions similar to or even higher than that of the liquid electrolyte. conductivity.
In semi-solid batteries, the solid state electrolyte is only used as a separator, and the internal resistance contributed by it is required to be low. The impedance value contributed by the solid state electrolyte separator with a thickness of less than 100 μm and an ionic conductivity greater than 1×10-3 S/cm is less than 10 Ω·cm2, which meets the use standard.
- Electronic conductivity
Another function of the lithium-ion solid state electrolyte is to separate the positive and negative electrodes, and its electronic conductivity should be extremely low to reduce the self-discharge of the battery.
- Electrochemical window
The lithium-ion solid state electrolyte should not be electrochemically decomposed during the charging and discharging process of the battery.
- Lithium/solid electrolyte interface stability
Metal lithium anodes have serious side reactions and lithium dendrite short circuit problems in liquid electrolytes, which may be solved by replacing liquid electrolytes with solid state electrolyte. In addition, since metal lithium anode is the only way to achieve high specific energy lithium batteries exceeding 350 W h/kg, the contact wettability, stability and interface stability of solid state electrolyte and metal lithium during lithium intercalation/extraction is very important.
Solid-state lithium-ion battery that uses solid state electrolyte instead of liquid organic electrolytes are expected to use positive and negative electrode materials with higher specific capacities, so as to realize a battery system with higher specific energy, and at the same time completely solve the safety problem of batteries, which is in line with future secondary batteries. The direction of development is an ideal power source for electric vehicles and large-scale energy storage.
As the core component of solid-state lithium batteries, solid state electrolyte are the key to achieving battery high energy density, high cycle stability and high safety performance of solid-state lithium batteries. At the current level of technology, Tesla's 4860 battery and the increase in energy density to 300Wh/kg already belong to the highest value of liquid lithium batteries. Therefore, solid-state batteries have unlimited potential in making electric vehicle battery systems that require large-capacity modules and battery packs.
In terms of the two major pain points of lithium batteries - energy density and safety, solid-state batteries have high hopes for their subversive characteristics, although the difficulty of manufacturing and high cost of all-solid-state batteries make their current commercialization difficult.