Understanding Solid-state Batteries and the Four Primary Materials
The main feature of solid-state batteries is their use of a solid electrolyte instead of the traditional liquid electrolyte, a change that brings several advantages, including higher energy density, enhanced safety and better low-temperature performance.
Solid State Battery Overview
1. What is a solid-state battery
❖Traditional lithium-ion batteries consist of four main components: positive electrode, negative electrode, electrolyte, and separator. Solid state batteries replace the electrolyte with a solid electrolyte. The key difference between solid-state batteries and traditional lithium-ion batteries is that the electrolyte changes from liquid to solid, balancing safety and high energy density.
❖Solid state electrolyte batteries are the ultimate form of lithium sodium batteries, which can completely solve safety issues and are the rightful protagonists of the second half of new energy. The industry chain of solid-state batteries is roughly similar to that of liquid lithium batteries, with upstream including raw materials and minerals, mechanical equipment, and basic materials. The main difference between the two lies in the types of negative electrode materials and electrolytes. In terms of positive electrode materials, they are almost identical. If fully developed to all solid-state batteries, the separator will also be completely replaced. The midstream of the industrial chain is the processing and preparation process of battery packs, while the downstream application areas of the industrial chain include new energy vehicles, energy storage systems, consumer electronics, etc.
2. The advantages of solid-state batteries
(1) Using solid electrolytes instead of liquid electrolytes and separators, solid electrolytes have a very high ignition point, improving the thermal stability of batteries;
(2) The voltage platform of solid-state batteries is 5V, which is higher than the 4.3V of liquid batteries. They can match high-voltage electrode materials and have better energy density and specific capacity than liquid batteries;
(3) Solid electrolytes do not have fluidity, so there is no leakage phenomenon. Simplifying battery grouping design, reducing battery weight and volume, and energy density is expected to exceed 300Wh/kg.
Solid state battery materials
Electrolyte
Solid state electrolyte is the core component of solid-state lithium-ion batteries, which can serve as both the separator and electrolyte of the battery. The core function of electrolytes is to transport Li+between the positive and negative electrodes. The ideal solid electrolyte should meet the characteristics of high ionic conductivity, low interface impedance, high structural stability and safety, high mechanical strength, and low cost. As of now, depending on the electrolyte, it can be mainly classified into polymer solid state electrolytes and inorganic solid state electrolytes. The representative system of the former is PEO polyethylene oxide; The latter refers to oxide, sulfide, and halide systems.
❖Polymer solid electrolyte: good flexibility, light weight, low potential, poor room temperature conductivity
Polymer solid electrolyte is a system composed of high molecular weight polymers and lithium salts (such as LiClO4, LiAsF6, LiPF6, etc.), which has ion transport ability and ion conductivity when coordinated with alkali metal salts. Typical polymer matrices include ether based polymers, nitrile based polymers, siloxane based polymers, carbonate based polymers, and vinylidene fluoride based polymers.
At present, the main material system suitable for the commercial field is PEO (polyethylene oxide). Under the action of an electric field, the oxygen atoms and lithium ions in the PEO chain segment can continuously coordinate and dissociate, achieving the migration of lithium ions. At the same time, PEO has a high solubility for lithium salts, and due to its light weight, good viscoelasticity, simple preparation process, low brittleness, and good interface stability with metal Li electrodes, it is one of the earliest researched and applied systems. But PEO is prone to crystallization at room temperature, resulting in a room temperature ionic conductivity of only 10-6-10-8 S/cm (generally practical requirements require>10-3 S/cm), and it needs to operate at high temperatures of 60 ° C-85 ° C; At the same time, the PEO withstand voltage platform is only 3.8V, which is relatively low and can only be adapted to iron lithium positive electrode materials, with limited energy density.
❖Oxide solid electrolyte: wide electrochemical window, good stability, strong hardness, but prone to brittle fracture
Oxide solid electrolytes are composed of inorganic salts of oxides, which can be divided into crystalline electrolytes and amorphous electrolytes. In addition to the lithium phosphorus oxygen nitrogen LiPON type amorphous electrolyte that can be used in thin-film batteries, the current commercialization mainly focuses on the research of crystalline electrolyte materials. The mainstream crystalline electrolyte material systems include garnet (LLZO) structure solid electrolyte, perovskite (LLTO) structure solid electrolyte, NASICON sodium super ion conductive solid electrolyte, and LISICON type solid electrolyte.
The general formula for garnet type electrolyte is Li3+xA3B2012, and the main material system is Li7La3Zr2O12, which is currently widely used; The general formula of perovskite electrolyte is Li3x La2/3-xTiO3, which has the advantages of stable structure, simple preparation process, and a wide range of variable components, but its ionic conductivity is slightly lower; The NaSICON type electrolyte utilizes the NASICON framework structure to prepare high-performance Li+solid electrolytes through lithium sodium substitution. Currently, the mainstream materials include the Li1+x Alx Ti2-x (PO4) 3 (lithium titanium aluminum phosphate LATP) system. Among the above materials, LLZO has high compatibility with lithium negative electrodes; The electrochemical stability of metal Li is poor for NaSICON and perovskite electrolytes. Overall, oxide solid electrolytes have high room temperature ionic conductivity, reaching 10-5-10-3 S/cm, and have a wide electrochemical window, high chemical stability, and high mechanical strength, making them an ideal solid electrolyte material system. However, there are also risks of high sintering temperature and easy mechanical processing leading to brittle fracture.
❖Sulfide solid electrolyte: high room temperature conductivity, poor air stability.
Sulfide electrolytes belong to inorganic solid electrolytes and are derived from oxide solid electrolytes, where oxygen elements in the oxide body of the electrolyte are replaced by sulfur elements. Compared to O2-, S2- has a larger radius, resulting in a larger ion conduction channel; The lower electronegativity and interaction with Li+greatly improve the room temperature ionic conductivity of the electrolyte. Electrolytes can be classified into crystalline state, glassy state, and glass ceramic electrolyte according to their crystalline morphology. The typical representatives of crystalline solid electrolytes are the Thio LISICON and Li2SiP2S12 systems.
The chemical formula of Thio LISICON is Li4-xA1-yByS4 (A=Ge, Si, etc., B=P, Al, Zn, etc.), with a room temperature ionic conductivity of up to 2.2 × 10-3S/cm; The Li2SiP2S12 system has good compatibility with both metallic Li and high-voltage cathodes. The Li2S-P2S5 system is the main representative of glassy and glass ceramic electrolytes, with a wide range of composition changes and an ion conductivity of up to 10-4-10-2 S/cm. However, sulfides quickly hydrolyze into H2S gas when exposed to air, so electrolyte synthesis needs to be carried out in an inert atmosphere, resulting in high research and development, manufacturing, transportation, and storage costs. Due to the easier oxidation of S2- compared to O2-, sulfide electrolytes are more prone to oxidation and decomposition at high voltages, resulting in a narrower electrochemical window.
❖Halide electrolyte: high voltage resistance, high conductivity, sensitive to humidity and temperature
The chemical formula of halide electrolytes is Lia-M-Xb, which originates from introducing high valence transition metal element M cations into lithium halide LiX (X=Br, Cl, F), adjusting the concentration of Li+and vacancies to form compounds similar to Lia-M-Xb. Compared to oxides and sulfides, the interaction between monovalent halide anions and Li+is weaker and has a larger radius than S2- or O2-, greatly improving the room temperature ionic conductivity of electrolytes. The theoretical ionic conductivity of electrolytes can reach the order of 10-2 S/cm. Meanwhile, halides generally have high oxidation-reduction potentials and better compatibility with high-voltage cathode materials, enabling stable cycling under high voltage windows. They are considered to be highly promising materials for the development of all solid state lithium-ion batteries.
There are currently three common types of halide electrolytes: Lia-M-Cl6, Lia-M-Cl4, and Lia-M-Cl8 halides, with the first two types having an ion conductivity of 10-3S/cm. However, halide electrolytes are prone to phase transition at different temperatures, which affects their conductivity, and are also prone to hydrolysis in air, resulting in high synthesis costs. In addition, the reaction between transition metals and lithium metal results in poor compatibility of the lithium negative electrode.
Cathode material
The positive electrode materials of solid-state batteries mainly include lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt oxide, and lithium aluminum cobalt oxide.
1. Lithium cobalt oxide: a commonly used positive electrode material in lithium-ion batteries, which can provide high energy density and long cycle life, but there are safety issues.
2. Lithium iron phosphate: Compared to lithium cobalt oxide, lithium iron phosphate has better safety and longer lifespan, but lower energy density.
3. Lithium nickel cobalt oxide: high energy density, long cycle life, but high material cost and safety issues.
4. Lithium aluminum cobalt oxide: High energy density, but slightly lower cycle life than lithium nickel cobalt oxide.
5. Multiple material combinations in solid electrolytes, such as lithium permanganate (LiMn204) and lithium titanate (Li4Ti5012), can provide higher safety and longer lifespan, but with relatively lower energy density.
Anode material
There are three main types of negative electrode materials for solid-state batteries: metallic lithium, carbon materials, and silicon materials.
1. Lithium metal is mainly used in solid-state lithium-ion batteries and solid-state lithium sulfur batteries. Among them, solid-state lithium-ion batteries are high-energy density batteries that can be applied in fields such as electric vehicles and drones; Solid state lithium sulfur batteries are a type of battery with high energy density and high safety, which can be applied in fields such as aerospace and military.
2. Carbon materials are mainly used in solid-state lithium-ion batteries. Among them, carbon nanotubes are a common carbon material with high specific surface area and excellent electrochemical performance, which can be applied in high-performance solid-state lithium-ion batteries.
3. Silicon material is a new type of negative electrode material with high specific capacity and low cost. In solid-state batteries, silicon materials can react with solid-state electrolytes to form lithium ions, thereby achieving charging and discharging of the battery. Compared with metallic lithium and carbon materials, silicon materials have a higher specific capacity, but their cycling stability is poor, and they are prone to volume expansion and structural damage. Silicon materials are mainly used in solid-state lithium-ion batteries. Among them, silicon nanowires are a common silicon material with high specific surface area and excellent electrochemical performance, which can be applied in high-performance solid-state lithium-ion batteries.
Separator
Separator material is an important component of solid-state batteries, mainly used to isolate positive and negative electrodes and prevent electronic conduction. The components of separator materials mainly include polymers, nanoscale powders, etc.
Research suggests that double-layer coating can replace separators. Inorganic solid electrolyte layers are coated on both sides of the negative electrode, and organic polymer layers are coated on the surface of the inorganic solid electrolyte layer. Currently, there is a view that sulfide and oxide all solid state batteries do not require separators. In various publicly available patents for solid-state batteries, the concept of composite separators has also been proposed, such as inorganic organic composite separators.
Conclusion
From market information, the products released by various enterprises are still mainly semi-solid state batteries. The manufacturing process and equipment of semi-solid state batteries are highly compatible with current liquid lithium batteries, and are expected to achieve commercial mass production in a short period of time. However, as semi-solid state batteries are only a transitional technology route, market recognition and technological sustainability need to be verified. If solid-state batteries are to be industrialized in the future, cost reduction is a long and arduous task. At the same time, the existing industrial chain of liquid lithium batteries will undergo significant changes, and the application of solid electrolytes will replace the separator and electrolyte links. The positive electrode industry will be less affected, and technological and product iteration will continue. If solid-state batteries can be mass-produced, relevant companies are exploring technological compatibility through production line transformation due to different processes in the middle and later stages. From a competitive perspective, in addition to traditional battery manufacturers conducting research and development on solid-state batteries, there are also start-up companies led by automotive companies and researchers, as well as upstream material manufacturers entering the field of solid-state battery research and development. It cannot be ruled out that some companies may break through in the field of solid-state battery research and development, achieve curve overtaking, and affect the global competition pattern of the battery industry.
