Solid-state lithium battery research -Lithium - Ion Battery Equipment
Compared with traditional liquid lithium batteries, solid-state lithium batteries (SSLB) use solid electrolytes instead of organic electrolytes. Their safety and energy density are greatly improved, which can effectively reduce safety hazards of electric vehicles and relieve users' range anxiety. Solid electrolyte, as an electronic insulator and ionic conductor, is one of the core elements of SSLB. At the same time, it has problems such as low ionic conductivity, large interface impedance, and poor interface stability. By studying recent relevant literature, the ion conduction mechanism, research progress, existing important problems and solutions of sulfide solid electrolytes, oxide solid electrolytes, polymer solid electrolytes and composite solid electrolyte lithium batteries were reviewed and discussed. Regarding improving ionic conductivity, methods for adjusting the composition of solid electrolytes are highlighted. Regarding the issue of improving the interface, the ideas for improving the interface design and manufacturing process methods are mainly introduced. Comprehensive analysis shows that the comprehensive performance of solid electrolytes can be effectively improved by doping and coating modified solid electrolytes, exploring advanced interface research and diagnostic technologies, and guiding the design of interfaces with excellent lithium ion transport capabilities, as well as innovative and optimized processes. Finally, the solid-state lithium battery industrialization process of key domestic and foreign companies is listed, and the future application prospects of solid-state lithium batteries are analyzed and forecasted.(Lithium - Ion Battery Equipment)
Currently, safety hazards and safety accidents caused by battery thermal runaway are pain points in the development of new energy vehicles. Lithium precipitation, internal short circuit, single cell thermal runaway and battery system thermal diffusion during the charging process of lithium batteries are the root causes of safety problems. There are two problems with electrolytes currently used in commercial applications: the liquid electrolyte itself is easy to burn, and it has a tendency to have side reactions with positive and negative electrode materials. In a short period of time, active safety prevention and control of thermal runaway of liquid power lithium batteries can be achieved through means such as fast charging strategy control, battery system management optimization, thermal management design and thermal management strategy optimization. In the long run, the direction of industry technology development is to replace liquid electrolytes with solid electrolytes, develop solid lithium batteries with significantly improved energy density and safety, essentially solve battery thermal runaway, solve user pain points and achieve differentiated competitive leadership. .
The emergence of solid-state lithium battery technology is very promising to solve the current pain points of liquid lithium batteries. It has great potential in improving battery energy density, broadening the operating temperature range, and improving safety: ① Solid-state lithium batteries use solid electrolytes instead of liquid electrolytes , SSLB is expected to obtain high energy density (>500W·h/kg, >700W·h/ L) and power density (>10kW/kg); ② The solid-state lithium battery electrolyte is different from the electrolyte and separator of liquid lithium batteries, and its thermal stability is significantly improved, thus improving the high-temperature environment adaptability of solid-state lithium batteries and reducing auxiliary heat dissipation Mechanism redundancy simplifies system design while further improving energy density; ③ The solid electrolyte material used does not leak or burn, and can provide intrinsic safety in the triggering mechanism of thermal runaway, solving the safety problem of lithium batteries. Therefore, solid-state lithium batteries will be a better solution to balance specific energy, safety and performance.
1Research progress of solid-state lithium batteries
The structure of a solid-state lithium battery (positive electrode, electrolyte and negative electrode) is all composed of solid materials. The solid electrolyte conducts lithium ions but is electronically insulated, greatly simplifying the battery construction process. From the perspective of the reaction process, solid electrolytes are the basis of solid lithium batteries. Therefore, the development of solid electrolyte materials with excellent ionic conductivity is the key to commercializing solid lithium batteries. Polymers, oxides, sulfides and composite solid electrolytes are four important branches, each with advantages and disadvantages.
1.1 Research progress of sulfide solid-state lithium batteries
Sulfide solid electrolytes (such as thiophosphate electrolytes) have high room temperature ionic conductivity (about 10-2S/cm). From the perspective of crystal structure, sulfide solid electrolytes can be divided into crystalline, glassy and Glass-ceramic state. At the beginning of the 21st century, the research team of Professor Ryoji Kanno in Japan discovered through experiments the Li4-xGe1-xPxS4 solid electrolyte. Because it is very similar in structure to oxides, it was named thio-LISICON. It was the first crystalline sulfide solid to be discovered. Electrolytes. Crystalline sulfide solid electrolytes also include LGPS type and Argyrodite type. In addition, sulfide solid electrolytes such as layered structure and L-P-S system are also common. At present, glassy and glass-ceramic electrolytes are mainly produced under Li2S-P2S5 and similar systems. It has good stability, simple process flow, low cost, and has broad application prospects. Therefore, it is considered to be one of the very promising candidates for the next generation of new electrochemical energy storage systems. However, the inorganic sulfide electrolytes containing P element reported so far are unstable under air conditions. This is mainly because the chemical bond energy of P-S is much lower than the corresponding chemical bond energy of P-O. As a result, the P-S structure is prone to oxidation or desulfurization. Therefore, the interface between this type of sulfide electrolyte and the electrode causes irreversible chemical reactions with water vapor or oxygen in the air atmosphere at higher temperatures, resulting in structural changes and a reduction in ionic conductivity, which seriously restricts its use. Applications in solid-state lithium batteries.
Physical contact failure generally occurs between the electrode material and the sulfide interface. The important reasons for this problem are the volume expansion of the electrode material and the growth of lithium dendrites during the lithium deposition process. The Janek research team used β-Li3PS4 as the sulfide-based solid-state electrolyte and successfully applied the high-nickel (LiNi0.8Co0.1Mn0.1O2, NCM811) Li[Ni,Co,Mn]O2 material to bulk solid-state batteries, and demonstrated The interface between active materials and solid electrolytes plays an important role in battery performance. Through experimental tests, it was found that the formation of the interface phase mainly occurs in the first cycle. In addition to interfacial decomposition, the shrinkage of active materials after degradation can also lead to a decrease in the contact between the solid electrolyte and active material particles, thereby adding new interfacial resistance and capacity loss. In order to solve the interface problems discovered during the above experiments, Janek's research team put a certain pressure on the container during the battery cycle in a closed space, so that the internal microstructure of the battery was put under a certain pressure, thus improving the battery material interface problem to a certain extent. In addition to exerting a certain pressure during the operation of solid-state batteries, the contact between stable phase interfaces is stabilized. The research team also proposed "mechanical" hybrid electrode materials to enhance material mixing and achieve better cycle performance.
1.2 Research progress of oxide solid-state lithium batteries
Oxide solid electrolytes have a dense morphology, so compared with sulfides, they have higher mechanical strength and excellent stability in air environments. However, it is precisely because of its higher mechanical strength, poor deformability and softness, and the difficulty in improving the interface contact problem that the problems of oxide electrolytes are also more prominent. From a structural point of view, it can be classified into two types: crystalline and glassy. Perovskite type, NASICON type, anti-perovskite type and Garnet type are all crystalline forms. The ideal perovskite has a cubic face-centered close-packed structure, with lithium lanthanum titanate (Li1/2La1/2Ti3) as a typical representative. It has the advantages of stable structure, simple preparation process, and wide range of composition variability, but it is different from Li The chemical stability and ionic conductivity between negative electrode materials are poor. By introducing large ions into the perovskite solid electrolyte material, its conductivity and other related properties can be improved. LLZO (Li7La3Zr2O12) is a typical anti-perovskite material. Its conductivity reaches 5×10-4S/cm at room temperature and is relatively stable. However, lithium-rich LLZO will undergo proton exchange under certain conditions, causing the presence of The reduction of lithium causes its performance to decay rapidly, and the introduction of heteroatoms improves its conductivity and interface properties. The NASICON type molecular formula is M[A2B3O12], in which lithium ions are transferred through substitution between different points in the solid electrolyte. The size of [A2B3O12]- has a great impact on its conductivity. The method is to increase the skeleton ion gap doping. Miscellaneous as an important means. Garnet type materials are cubic crystals, which are stable and have low resistance. Li7La3Zr2O12 has high conductivity, but multiple sinterings result in the volatilization of a lot of lithium. Through unequal substitution, the vacancy concentration and carrier concentration of lithium ions can be optimized, and the room temperature ion conductivity of cubic garnet solid electrolyte materials can be improved.
1.3 Research progress of polymer solid-state lithium batteries
Solid polymer electrolyte (SPE) is composed of a polymer matrix and lithium salt. The SPE matrix includes polyethylene oxide, polysiloxane, and aliphatic polycarbonate. It has higher thermal stability than traditional liquid electrolytes. , and it is easier to achieve large-scale manufacturing than ceramic electrolytes. It has good elasticity and excellent machinability, and is a research hotspot for next-generation energy storage systems.
However, studies have shown that interfacial instability between polymer solid electrolytes and other battery components hinders their practical applications. At present, the basic understanding of the polymer/electrode interface and polymer/ceramic electrolyte interface is still limited, and more research needs to be carried out on how lithium dendrite growth will alleviate the interface instability on the anode side. In order to achieve the above research purposes, the application of cryo-electron microscopy has played a good role in the research process. Furthermore, theoretical simulations are another effective way to circumvent the challenges in experiments and help better understand the instabilities of the aforementioned interfaces. Through a combination of advanced experimental studies and model studies, new fundamental understanding of the interface stability of polymer electrolyte-based solid-state lithium batteries will be gained, thereby accelerating their application in the commercial market.
1.4 Research progress of composite solid electrolyte lithium batteries
Composite solid electrolytes (CSSEs) are mainly a combination of inorganic solid electrolytes represented by oxides, sulfides, etc. and organic solid electrolytes represented by polymers such as polyethylene oxide to achieve "hardness and softness", using Lewis Acids and bases use each other to increase chain segment mobility and synergistically improve interfacial ion transport.
Organic/inorganic CSSEs can be roughly divided into three categories in terms of combination methods: filling organic solids with inorganic components, organic/inorganic double-layer or multi-layer structures, and filling three-dimensional continuous inorganic structures with organic components. Among them, composite solid electrolytes related to open skeleton structure materials such as MOFs, COFs and POCs mainly belong to the first category in terms of combination methods.
CSSEs have great development potential, and people have conducted a lot of research on them in recent years. Long et al. used a method that combines solution casting and hot pressing, using layered hectorite (LiMNT), polyethylene carbonate (PEC), lithium bisfluorosulfonimide (LiFSI), and high-voltage fluorocarbonate. Vinyl ester (FEC) additive and polytetrafluoroethylene (PTFE) binder were used to prepare intercalated CSSEs with high ion migration numbers. At 25°C, the electrolyte has high ionic conductivity, wide electrochemical window and high ion migration number. In addition, a three-dimensional lithium anode was obtained through a simple hot melt infusion strategy. The synergistic effect of the high ion migration number intercalation electrolyte and the 3D lithium anode is more conducive to inhibiting the growth of lithium dendrites. Solid-state batteries based on lithium iron phosphate (or NCM523), CSSEs and 3D lithium have excellent cycle and rate performance.
In summary, sulfide electrolytes have high conductivity, but poor chemical stability and poor processability. Oxide electrolytes have high conductivity, but have problems with rigid interface contact and serious side reactions, and are difficult to process. Polymer electrolytes have good interfacial compatibility and machinability, but their low room temperature ionic conductivity limits their application temperature range. In response to these problems, composite solid electrolytes are currently the material system with the most potential for development.
Currently, safety hazards and safety accidents caused by battery thermal runaway are pain points in the development of new energy vehicles. Lithium precipitation, internal short circuit, single cell thermal runaway and battery system thermal diffusion during the charging process of lithium batteries are the root causes of safety problems. There are two problems with electrolytes currently used in commercial applications: the liquid electrolyte itself is easy to burn, and it has a tendency to have side reactions with positive and negative electrode materials. In a short period of time, active safety prevention and control of thermal runaway of liquid power lithium batteries can be achieved through means such as fast charging strategy control, battery system management optimization, thermal management design and thermal management strategy optimization. In the long run, the direction of industry technology development is to replace liquid electrolytes with solid electrolytes, develop solid lithium batteries with significantly improved energy density and safety, essentially solve battery thermal runaway, solve user pain points and achieve differentiated competitive leadership. .
The emergence of solid-state lithium battery technology is very promising to solve the current pain points of liquid lithium batteries. It has great potential in improving battery energy density, broadening the operating temperature range, and improving safety: ① Solid-state lithium batteries use solid electrolytes instead of liquid electrolytes , SSLB is expected to obtain high energy density (>500W·h/kg, >700W·h/ L) and power density (>10kW/kg); ② The solid-state lithium battery electrolyte is different from the electrolyte and separator of liquid lithium batteries, and its thermal stability is significantly improved, thus improving the high-temperature environment adaptability of solid-state lithium batteries and reducing auxiliary heat dissipation Mechanism redundancy simplifies system design while further improving energy density; ③ The solid electrolyte material used does not leak or burn, and can provide intrinsic safety in the triggering mechanism of thermal runaway, solving the safety problem of lithium batteries. Therefore, solid-state lithium batteries will be a better solution to balance specific energy, safety and performance.
1Research progress of solid-state lithium batteries
The structure of a solid-state lithium battery (positive electrode, electrolyte and negative electrode) is all composed of solid materials. The solid electrolyte conducts lithium ions but is electronically insulated, greatly simplifying the battery construction process. From the perspective of the reaction process, solid electrolytes are the basis of solid lithium batteries. Therefore, the development of solid electrolyte materials with excellent ionic conductivity is the key to commercializing solid lithium batteries. Polymers, oxides, sulfides and composite solid electrolytes are four important branches, each with advantages and disadvantages.
1.1 Research progress of sulfide solid-state lithium batteries
Sulfide solid electrolytes (such as thiophosphate electrolytes) have high room temperature ionic conductivity (about 10-2S/cm). From the perspective of crystal structure, sulfide solid electrolytes can be divided into crystalline, glassy and Glass-ceramic state. At the beginning of the 21st century, the research team of Professor Ryoji Kanno in Japan discovered through experiments the Li4-xGe1-xPxS4 solid electrolyte. Because it is very similar in structure to oxides, it was named thio-LISICON. It was the first crystalline sulfide solid to be discovered. Electrolytes. Crystalline sulfide solid electrolytes also include LGPS type and Argyrodite type. In addition, sulfide solid electrolytes such as layered structure and L-P-S system are also common. At present, glassy and glass-ceramic electrolytes are mainly produced under Li2S-P2S5 and similar systems. It has good stability, simple process flow, low cost, and has broad application prospects. Therefore, it is considered to be one of the very promising candidates for the next generation of new electrochemical energy storage systems. However, the inorganic sulfide electrolytes containing P element reported so far are unstable under air conditions. This is mainly because the chemical bond energy of P-S is much lower than the corresponding chemical bond energy of P-O. As a result, the P-S structure is prone to oxidation or desulfurization. Therefore, the interface between this type of sulfide electrolyte and the electrode causes irreversible chemical reactions with water vapor or oxygen in the air atmosphere at higher temperatures, resulting in structural changes and a reduction in ionic conductivity, which seriously restricts its use. Applications in solid-state lithium batteries.
Physical contact failure generally occurs between the electrode material and the sulfide interface. The important reasons for this problem are the volume expansion of the electrode material and the growth of lithium dendrites during the lithium deposition process. The Janek research team used β-Li3PS4 as the sulfide-based solid-state electrolyte and successfully applied the high-nickel (LiNi0.8Co0.1Mn0.1O2, NCM811) Li[Ni,Co,Mn]O2 material to bulk solid-state batteries, and demonstrated The interface between active materials and solid electrolytes plays an important role in battery performance. Through experimental tests, it was found that the formation of the interface phase mainly occurs in the first cycle. In addition to interfacial decomposition, the shrinkage of active materials after degradation can also lead to a decrease in the contact between the solid electrolyte and active material particles, thereby adding new interfacial resistance and capacity loss. In order to solve the interface problems discovered during the above experiments, Janek's research team put a certain pressure on the container during the battery cycle in a closed space, so that the internal microstructure of the battery was put under a certain pressure, thus improving the battery material interface problem to a certain extent. In addition to exerting a certain pressure during the operation of solid-state batteries, the contact between stable phase interfaces is stabilized. The research team also proposed "mechanical" hybrid electrode materials to enhance material mixing and achieve better cycle performance.
1.2 Research progress of oxide solid-state lithium batteries
Oxide solid electrolytes have a dense morphology, so compared with sulfides, they have higher mechanical strength and excellent stability in air environments. However, it is precisely because of its higher mechanical strength, poor deformability and softness, and the difficulty in improving the interface contact problem that the problems of oxide electrolytes are also more prominent. From a structural point of view, it can be classified into two types: crystalline and glassy. Perovskite type, NASICON type, anti-perovskite type and Garnet type are all crystalline forms. The ideal perovskite has a cubic face-centered close-packed structure, with lithium lanthanum titanate (Li1/2La1/2Ti3) as a typical representative. It has the advantages of stable structure, simple preparation process, and wide range of composition variability, but it is different from Li The chemical stability and ionic conductivity between negative electrode materials are poor. By introducing large ions into the perovskite solid electrolyte material, its conductivity and other related properties can be improved. LLZO (Li7La3Zr2O12) is a typical anti-perovskite material. Its conductivity reaches 5×10-4S/cm at room temperature and is relatively stable. However, lithium-rich LLZO will undergo proton exchange under certain conditions, causing the presence of The reduction of lithium causes its performance to decay rapidly, and the introduction of heteroatoms improves its conductivity and interface properties. The NASICON type molecular formula is M[A2B3O12], in which lithium ions are transferred through substitution between different points in the solid electrolyte. The size of [A2B3O12]- has a great impact on its conductivity. The method is to increase the skeleton ion gap doping. Miscellaneous as an important means. Garnet type materials are cubic crystals, which are stable and have low resistance. Li7La3Zr2O12 has high conductivity, but multiple sinterings result in the volatilization of a lot of lithium. Through unequal substitution, the vacancy concentration and carrier concentration of lithium ions can be optimized, and the room temperature ion conductivity of cubic garnet solid electrolyte materials can be improved.
1.3 Research progress of polymer solid-state lithium batteries
Solid polymer electrolyte (SPE) is composed of a polymer matrix and lithium salt. The SPE matrix includes polyethylene oxide, polysiloxane, and aliphatic polycarbonate. It has higher thermal stability than traditional liquid electrolytes. , and it is easier to achieve large-scale manufacturing than ceramic electrolytes. It has good elasticity and excellent machinability, and is a research hotspot for next-generation energy storage systems.
However, studies have shown that interfacial instability between polymer solid electrolytes and other battery components hinders their practical applications. At present, the basic understanding of the polymer/electrode interface and polymer/ceramic electrolyte interface is still limited, and more research needs to be carried out on how lithium dendrite growth will alleviate the interface instability on the anode side. In order to achieve the above research purposes, the application of cryo-electron microscopy has played a good role in the research process. Furthermore, theoretical simulations are another effective way to circumvent the challenges in experiments and help better understand the instabilities of the aforementioned interfaces. Through a combination of advanced experimental studies and model studies, new fundamental understanding of the interface stability of polymer electrolyte-based solid-state lithium batteries will be gained, thereby accelerating their application in the commercial market.
1.4 Research progress of composite solid electrolyte lithium batteries
Composite solid electrolytes (CSSEs) are mainly a combination of inorganic solid electrolytes represented by oxides, sulfides, etc. and organic solid electrolytes represented by polymers such as polyethylene oxide to achieve "hardness and softness", using Lewis Acids and bases use each other to increase chain segment mobility and synergistically improve interfacial ion transport.
Organic/inorganic CSSEs can be roughly divided into three categories in terms of combination methods: filling organic solids with inorganic components, organic/inorganic double-layer or multi-layer structures, and filling three-dimensional continuous inorganic structures with organic components. Among them, composite solid electrolytes related to open skeleton structure materials such as MOFs, COFs and POCs mainly belong to the first category in terms of combination methods.
CSSEs have great development potential, and people have conducted a lot of research on them in recent years. Long et al. used a method that combines solution casting and hot pressing, using layered hectorite (LiMNT), polyethylene carbonate (PEC), lithium bisfluorosulfonimide (LiFSI), and high-voltage fluorocarbonate. Vinyl ester (FEC) additive and polytetrafluoroethylene (PTFE) binder were used to prepare intercalated CSSEs with high ion migration numbers. At 25°C, the electrolyte has high ionic conductivity, wide electrochemical window and high ion migration number. In addition, a three-dimensional lithium anode was obtained through a simple hot melt infusion strategy. The synergistic effect of the high ion migration number intercalation electrolyte and the 3D lithium anode is more conducive to inhibiting the growth of lithium dendrites. Solid-state batteries based on lithium iron phosphate (or NCM523), CSSEs and 3D lithium have excellent cycle and rate performance.
In summary, sulfide electrolytes have high conductivity, but poor chemical stability and poor processability. Oxide electrolytes have high conductivity, but have problems with rigid interface contact and serious side reactions, and are difficult to process. Polymer electrolytes have good interfacial compatibility and machinability, but their low room temperature ionic conductivity limits their application temperature range. In response to these problems, composite solid electrolytes are currently the material system with the most potential for development.
