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As a new generation of energy storage solutions, sulfide solid-state batteries have garnered significant attention in the rapid advancement of new energy technologies. With their high energy density and outstanding safety performance, sulfide solid-state batteries are gradually becoming the focus of the electric vehicle, consumer electronics, and aerospace industries. However, this technology still faces numerous challenges in research and commercialization, including the complexity of synthesis processes, material stability, and cost control.
This article discusses the current status of research and development of sulfide solid-state batteries, and the development trends, and market potential of sulfide electrolytes.
Current status of sulfide solid-state battery R&D
The lithium battery industry chain is gradually recovering, and new technologies are expected to strengthen the growth trend. Currently, mainstream solid-state electrolytes are categorized into three types: polymer, oxide, and sulfide. Polymer solid-state electrolytes have good film-forming properties but low ionic conductivity at room temperature, making them inadequate for standalone use.
Oxide solid-state electrolytes are insensitive to water and oxygen, with moderate conductivity. In the past, they were favored by domestic teams due to their high thermal stability, wide electrochemical window, and low material cost, and they were the first to be used in semi-solid-state batteries. However, issues with interfacial contact with electrode materials remain a significant drawback.
Compared to oxide solid-state electrolytes, sulfide solid-state electrolytes have a lower synthesis temperature, lower Young's modulus, and are easier to process and densify, with better interfacial contact with cathode and anode materials.
After cold pressing the powder into sheets, relatively high ionic conductivity can be achieved. However, sulfide electrolytes also face issues of instability in air and high synthesis costs. Since sulfide rapidly hydrolyzes in air to produce toxic gases, electrolyte synthesis must be conducted in an inert atmosphere, leading to high research, manufacturing, transportation, and storage costs.
Before this, the sulfide route was mainly favored by overseas companies, including Japanese and Korean companies like Toyota and Samsung, as well as some European and American companies such as Solid Power and Svolt Energy. These companies have successfully manufactured 20 Ah sulfide all-solid-state batteries, and Solid Power, Samsung, and Nissan have begun constructing sulfide all-solid-state battery pilot lines.
At the same time, the academic community is also paying attention to the sulfide route. The Sichuan New Energy Vehicle Innovation Center, backed by the team of Academician Ouyang Minggao, is at the forefront of all solid state battery research and has also chosen sulfide all-solid-state batteries as the technology direction for the next generation of batteries.
Classification of sulfide solid-state battery electrolytes
Sulfide solid-state electrolytes are typically classified into glassy, glass-ceramic, and crystalline based on their crystal structure. For example, Li3.25Ge0.25P0.7S4 belongs to the thio-LISICON type sulfide solid-state electrolyte, Li6PS5X (X=Cl, Br, I) belongs to the Li-argyrodite type solid-state electrolyte, and Li10GeP2S12 belongs to the LGPS type solid-state electrolyte.
Glassy sulfide solid-state electrolytes are obtained by mechanical ball milling or rapid cooling after high-temperature melting, with no distinct peaks under XRD characterization.
Glass-ceramic sulfide solid-state electrolytes are typically obtained after low-temperature sintering following ball milling, forming a metastable phase that is a mixture of glassy and crystalline states, showing some peaks under XRD characterization. Studies have shown that glassy solid-state electrolytes are mainly composed of four types of small crystals: orthophosphate, pyrophosphate, metaphosphate, and hypophosphate, but their ion conduction mechanism is not yet fully understood.
Crystalline sulfide solid-state electrolytes are typically obtained through high-energy ball milling followed by high-temperature sintering, with some studies using ball milling, grinding, and sintering, as well as liquid-phase methods. Crystalline sulfide solid-state electrolytes are mainly classified into thio-LISICON, Li-argyrodite, and LGPS types according to their crystal structure. These three types of lithium ion battery electrolyte have specific crystal structures and lithium-ion transport channels, and their structural composition and ion migration mechanisms are relatively well understood.
Preparation processes and challenges of sulfide solid-state battery electrolytes
Sulfide solid-state electrolytes can be synthesized using solid-phase or liquid-phase methods. The solid-phase method primarily involves high-energy ball milling followed by heat treatment, with the main process steps as follows:
(1)Ball milling: Lithium sulfide, phosphorus pentasulfide, and lithium chloride are mixed in a certain ratio with the ball milling medium, and the mixture is mechanically ground in a ball mill to obtain a slurry.
(2)Drying: The slurry obtained after ball milling is dried under a protective atmosphere to obtain a sulfide solid-state electrolyte precursor.
(3)Sintering: The sulfide solid-state electrolyte precursor is placed in a sintering furnace under an inert gas atmosphere and sintered at high temperature to obtain the sulfide solid-state electrolyte.
(4) Crushing: The sulfide solid-state electrolyte is added to a jet mill for crushing to obtain the desired particle size of the sulfide solid-state electrolyte. The jet mill is placed inside a glove box, and the entire process is conducted in a closed environment.
The liquid-phase method involves placing the materials into a polar organic solvent for stirring, then evaporating the solvent and heat-treating to obtain the solid-state electrolyte, which can reduce production costs. However, due to the difficulty of dissolving raw materials such as Li2S and P2S5, a long reaction time is often required to obtain a precipitate.
Additionally, the precipitate contains solvent molecules, and during heat treatment, the solvent volatilizes and the decomposition of crystalline solvent molecules creates a porous structure inside the electrolyte particles, reducing ionic conductivity.
Stability is a significant bottleneck in the large-scale application of sulfide solid-state electrolytes. Crystalline sulfide solid-state electrolytes have very high ionic conductivity, comparable to or exceeding that of liquid electrolytes.
Therefore, for sulfide electrolytes, ionic conductivity is no longer a key constraint on their application in solid-state batteries. Instead, air stability and electrochemical stability are the critical bottlenecks limiting their large-scale application.
For example, the raw materials used to synthesize sulfide electrolytes, such as Li2S and P2S5, have poor air stability and need to be produced in an environment filled with inert gas, free of water and oxygen, which greatly increases equipment requirements. Additionally, toxic hydrogen sulfide is produced during the production process, which must be recovered to prevent leakage and avoid safety risks.
The preparation methods for sulfide electrolyte membranes can be divided into two categories: dry and wet. The wet process uses a solvent to dissolve a binder, which is then mixed with solid-state electrolyte powder to form a slurry.
The slurry is coated and dried to form a membrane, with the thickness determined by the solid content of the slurry and the width of the coating gap. For wet coating, the solvent-binder pair and process parameters are crucial to the ionic conductivity and processability of the sulfide electrolyte membrane.
The dry process involves dry mixing and fiberizing the electrolyte powder and binder, followed by rolling or extruding the powder into a membrane. The membrane's thickness is determined by the gap width of the rolling or extrusion equipment. The dry process can solve the problem of solvent residue and eliminates the drying step required in the wet process.
It also offers dual advantages of increasing conductivity (since the binder exists in a fibrous state, facilitating the passage of electrons and ions) and reducing costs. However, compared to the wet process, the dry process places higher demands on equipment working pressure, precision, and uniformity, and further research is needed on process parameters and scaling-up equipment.
The future prospects of sulfide solid-state battery development
With the global pursuit of new energy technologies, the market prospects for sulfide electrolytes are vast. It is predicted that by 2030, the global shipments of solid-state batteries will reach hundreds of GWh, with sulfide electrolytes expected to dominate the lithium battery market share. The high energy density and safety of sulfide electrolytes make them highly promising for applications in electric vehicles, consumer electronics, and aerospace.
In the electric vehicle sector, the increasing demand for longer driving ranges and fast charging has positioned sulfide solid-state batteries as a key technology for enhancing electric vehicle performance. Additionally, the high safety of sulfide electrolytes gives them a competitive edge in the consumer electronics market, particularly in applications where battery safety is of paramount importance.
Despite the optimistic market outlook, the commercialization of sulfide electrolytes still faces challenges in terms of cost and technological maturity. Currently, the production cost of sulfide electrolytes is relatively high, and large-scale production technology is not yet fully developed. In the future, as production scales up and manufacturing technologies improve, the cost of sulfide electrolytes is expected to gradually decrease, thereby driving their widespread adoption in the market.
Conclusion
Sulfide electrolytes, as a core material for solid-state batteries, play a crucial role in enhancing battery performance and safety. Although there are challenges in synthesis processes, stability, and cost, ongoing technological advancements and a maturing market suggest that sulfide electrolytes are poised to play a key role in the future energy technology revolution. As global demand for new energy solutions continues to grow, the market prospects for sulfide electrolytes are promising.
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