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Currently, China's power battery technology continues to upgrade, most battery manufacturers choose oxide electrolyte as the core material, mainly perovskite, garnet and sodium superionic conductor three configurations. Oxide electrolyte shows great application potential by virtue of its advantages. However, some key challenges still hinder the development of solid-state batteries, such as how to solve their strong rigidity, poor interface contact and grain boundary impedance. With the promotion of policies and the growth of market demand, the oxide route is becoming an important direction in China's solid-state battery field.
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Why do most battery manufacturers in China choose oxide as electrolyte?
According to the crystal structure of the electrolyte, the oxide electrolyte can be divided into perovskite structural type (such as LLTO), garnet structural type (such as LLZO), fast ion conduction type (LISICON, NASICON), etc. The intrinsic ionic conductance of perovskite-type LLTO electrolyte material is higher, but its stability is relatively poor.
Garnet type LLZO electrolyte has high ionic conductivity and good stability, which has been widely concerned. LATP with sodium fast ion conductor structure has a high electrochemical window and is considered as an ideal electrolyte for high voltage solid state batteries. The conductivity of LGPS obtained by thiosubstitution of lithium fast ion conductor structure electrolyte is close to that of liquid electrolyte.
The oxide electrolyte can withstand high voltage, high decomposition temperature and good mechanical strength, but due to the material characteristics of the oxide itself, there are also shortcomings such as strong rigidity and fragility, especially the poor contact ability of the electrode and the electrolyte interface, resulting in poor interface stability during the battery cycle, resulting in a rapid increase in interface impedance during the cycle. Therefore, oxide solid electrolytes often need to add some polymer components and mix with trace ionic liquid/high-performance lithium salt-electrolyte to use.
According to public information, in the first half of 2024, China's new energy vehicle production and sales reached 4.929 million and 4.944 million, respectively, an increase of 30.1% and 32%, and the market share reached 35.2%. At the same time, under the promotion of superimposed policies and the stimulation of eVTOL demand, the solid-state battery industry is currently in a period of rapid development.
In terms of loading capacity, according to the data of the China Automotive Power Battery Industry Innovation Alliance, from January to May 2024, China's semi-solid state battery accumulated 1621.8MWh. In the Chinese market, the research and development of solid electrolytes by companies such as Weilan New Energy, Qingtao Energy, Ganfeng Lithium, Huineng Technology, Lishen Battery, and Shandong Jinqihang mainly focuses on the solid-liquid mixing technology route based on oxide materials.
The ionic conductivity of oxide electrolyte is generally 10-6~10-3S/cm, and the dense morphology makes it have the advantages of high mechanical strength, good chemical/electrochemical stability and air stability, electrochemical window width, and the cost is relatively moderate. Oxide solid electrolytes can be divided into crystalline and amorphous electrolytes according to their morphology. The crystalline oxide electrolyte has high air and thermal stability, so it is easy to achieve large-scale production. Specifically, the main configurations of oxides include:
Perovskite LLTO
At present, LLTO exhibits high crystal conductivity in crystalline solid electrolytes. Under certain conditions, the room temperature conductivity can reach a very high level, and the crystal conductivity is 10-3S/cm. The molecular formula for typical perovskite ceramics is ABO3, where the A site is usually a rare earth or alkaline earth element ion, and the B site is a transition metal ion. The A and B sites can be partially replaced by metal ions with similar radii, and this substitution will not have a great impact on the crystal structure, and the structural stability of the composite oxide provides conditions for the transmission of lithium cation.
However, the grain boundary conductivity of LLTO is two orders of magnitude lower than the volume conductivity, resulting in a low total conductivity of LLTO ceramics, limiting its practical application in lithium batteries. In order to overcome this problem, it is necessary to improve the total conductivity by doping modification.
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Garnet type LLZO
In garnet solid electrolyte, the conductivity of cubic phase is better than that of tetragonal phase. In the cubic phase structure, Li ions are distributed within the crystal grid, which makes the shortest distance between adjacent Li ions a key factor in promoting fast ion transport, thus achieving high ionic conductivity. In addition, the ionic conductivity of LLZO (lithium lanthanum zirconium oxide) is comparable to its intracrystal ionic conductivity, ensuring its high overall ionic conductivity as a solid electrolyte.
LLZO solid electrolyte has great application prospects in the field of all-solid-state batteries, because it does not react with lithium metal, has low interface and grain impedance, shows good stability in atmospheric environment, and sintered ceramics obtained by heat treatment are dense and have high strength and hardness.
However, while undoped LLZO shows advantages such as stable electrochemical performance and wide electrochemical window, it has shortcomings in terms of phase structure stability, vibration density and room temperature ionic conductivity, especially its large grain boundary resistance can cause serious interface resistance problems in all-solid-state battery applications. In addition, when LLZO is exposed to an environment containing water and carbon dioxide, the battery performance will gradually degrade. Therefore, improving the chemical stability of solid electrolytes in LLZO systems under environmental conditions is a key challenge.
Nasicon Sodium superionic conduction type LATP
The LATP of Nasicon oxide solid electrolytes is a typical lithium-ion conductive material with ionic conductivity up to 10-4S/cm at room temperature. In addition, LATP exhibits good chemical stability in humid air or carbon dioxide environments and is expected to have a high oxidation potential. These characteristics make LATP electrolytes not only have high ionic conductivity, but also have a wide electrochemical window, which is an ideal solid electrolyte material for high voltage solid state batteries.
At the same time, the Nasicon electrolyte is stable to air and water, which means that material preparation and battery assembly can be carried out on a large scale in an air environment, providing the possibility of industrial production. However, at present, LATP has problems such as low conductivity of high-temperature ions and easy reaction with metal ions, which hinder its large-scale application, which is the main challenge facing the development of the industry.
Conclusion
Oxide solid electrolytes play an important role in China's battery industry because of their excellent mechanical strength, high voltage resistance and good chemical stability. However, in practical applications, perovskite, garnet and Nasicon oxide electrolytes have their advantages and disadvantages, and still face challenges in practical applications such as interface stability, grain boundary impedance, and environmental sensitivity.
However, with the innovation of lithium battery electrolyte research and development technology and the help of new energy vehicles and eVTOL and other emerging fields, oxide solid electrolyte is expected to become an important technical pillar of future power batteries, providing strong support for global energy transformation.
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