Today, with the wave of electric vehicle sweeping the world, the range and charging speed restrict the further leap of the industry and the user’s experience. While enjoying the convenience brought by green travel, we are still troubled by “mileage anxiety” and safety worries.
The energy density of traditional lithium-ion batteries seems to have reached the theoretical ceiling, and the flammability of liquid electrolytes is the sword of Damocles hanging over their heads. As a result, the industry and academia have turned their attention to the next generation of battery technology-solid-state batteries.
It is widely regarded as the key to breaking the existing bottleneck and opening a new era of energy storage.
Solid-state batteries refer to lithium batteries that use solid-state electrolytes instead of traditional electrolytes. According to the amount of solid-state electrolytes, they can be divided into semi-solid-state batteries and all-solid-state batteries. Usually, we regard 10% of the liquid content in the battery as the dividing line between semi-solid batteries and liquid batteries, while all-solid batteries will use solid electrolyte completely, and the liquid content will be reduced to 0%.
A solid-state lithium battery is mainly compose of a positive electrode, a negative electrode and a solid-state electrolyte, and that most essential difference is that the electrolyte and the separator of the liquid battery are replaced by the solid-state electrolyte, so that the separator and the electrolyte are not use or are used less.
How Solid State Batteries Work
- Lithium metal or similar materials are usually used for the positive electrode. When lithium ions move from the solid-state electrolyte to the positive electrode, the positive electrode material undergoes an oxidation reaction, releasing electrons.
- Lithium alloy or similar materials are generally used for the negative electrode. When lithium ions move from the solid electrolyte to the negative electrode, the negative electrode material will undergo a reduction reaction and receive electrons.
- Solid-state electrolytes are composed of conductive solid materials, such as lithium-containing inorganic salts, polymers or ceramic materials. The electrolyte has high ion mobility, low resistance and high chemical stability.
Classification of solid-state batteries
According to the classification of electrolytes, batteries can be subdivided into four categories: liquid (25wt%), semi-solid (5-10wt%), quasi-solid (0-5wt%) and all-solid (0wt%), of which semi-solid, quasi-solid and all-solid are collectively referred to as solid-state batteries. Automobile companies use solid-state batteries, safety is the short-term driving factor, and energy density is the long-term driving factor.
Semi-solid battery
Compared with liquid batteries, semi-solid batteries reduce the amount of liquid electrolytes and increase the composite electrolytes of oxides and polymers, in which oxides are mainly added in the form of diaphragm coating and positive and negative electrode coating, polymers are filled in the form of frame network, and the negative electrode is upgraded from graphite system to pre-lithiated silicon-based negative electrode and lithium metal negative electrode. The cathode has been upgraded from high nickel to high nickel + high voltage, lithium-rich manganese-based cathode, the diaphragm is still retained and coated with solid electrolyte coating, the lithium salt has been upgraded from LiPF6 to LiTFSI, and the energy density can reach more than 350 Wh/kg. Although semi-solid batteries reduce the amount of liquid electrolyte, they still have the risk of flammability.
All-solid-state battery
Compared with liquid batteries, all-solid-state batteries cancel the original liquid electrolyte, select oxides, sulfides, polymers and other solid electrolytes, and divide the positive and negative electrodes in the form of thin films, thus replacing the role of diaphragms, in which oxides are making rapid progress, sulfides have the greatest potential in the future, and polymers have a lower upper limit of performance. The negative electrode has been upgraded from graphite system to pre-lithiated silicon-based negative electrode and lithium metal negative electrode, and the positive electrode has been upgraded from high nickel to ultra-high nickel, nickel lithium manganate, lithium-rich manganese-based positive electrode, with an energy density of 500 Wh/kg.
Solid-state batteries can be divided into several main categories, including sulfide, oxide, and polymer solid-state batteries, based on the material and characteristics of the solid-state electrolyte.
Sulfide solid state batteries
Sulfide solid state batteries use inorganic sulfide materials as electrolytes, which usually have high lithium ion conductivity, approaching or exceeding the level of traditional liquid electrolytes.
Sulfide solid state electrolytes have attracted much attention because of their high ionic conductivity, for example, the conductivity of Li10GeP2S12 (LGPS) electrolyte can reach 1.2 X 10 ^ -2 S/cm. However, sulfide electrolyte is sensitive to water vapor and easy to react with water to produce toxic H2S gas, and irreversible chemical reactions with oxygen and water vapor in the air lead to the decrease of ionic conductivity and structural damage.
Therefore, the development of sulfide solid electrolyte is difficult, and the requirements for the production environment are stringent.
Oxide solid state battery
Oxide solid state batteries use oxide materials as electrolytes, which generally have low ionic conductivity but good mechanical properties and chemical stability.
The representative of oxide electrolyte is Li7La3Zr2O12 (LLZO) with garnet structure, which has high ionic conductivity up to 10 ^ -4 S/cm at room temperature. The dense morphology of oxide electrolyte makes it have higher mechanical strength, good stability in air and high voltage tolerance. However, due to its high mechanical strength, poor deformability and softness of oxide electrolyte, easy brittle fracture of electrolyte sheet and large contact loss of solid-solid interface, its application is limited.
Polymer solid state battery
Polymer solid state battery is composed of polymer matrix and lithium salt, which has low ionic conductivity at room temperature, but when heated to more than 60 C, the ionic conductivity is significantly improved.
The polymer electrolyte has the characteristics of light weight, good elasticity and excellent mechanical processing performance, and the process of the polymer electrolyte is close to that of the existing lithium battery, so that the polymer electrolyte is easy for mass production. However, polymer electrolytes have low ionic conductivity at room temperature, and there is a risk of short circuit caused by lithium dendrite penetration, and their thermal stability is limited.
Combined solid state battery
In addition to the above three main types of solid-state batteries, there are also combined solid-state batteries, such as composite solid-state electrolytes, which are electrolytes obtained by combining sulfide/oxide and polymer electrolytes. The composite electrolyte combines the advantages of inorganic and organic solid electrolytes, and has high lithium ion conductivity and electrochemical stability.
In addition, there are chloride solid electrolytes, which have the high ionic conductivity of sulfides, deformability and the stability of oxides to high-voltage cathode materials, but are not yet feasible for large-scale commercialization.
Advantages of solid-state batteries
Solid-state electrolytes are less fluid than electrolytes, so poor direct contact between solids and solid particles, coupled with electrochemical instability, leads to many interfacial problems. However, compared with liquid batteries, the potential advantages of solid-state batteries lie in:
High safety: Non-volatile and non-flammable solid electrolyte has higher safety than organic electrolyte.
Good temperature adaptability: All-solid-state batteries can operate in a wider temperature range, especially at higher temperatures.
High energy density: All-solid-state batteries are expected to solve the safety problem of lithium metal anode (lithium dendrite). Thereby improving the energy density of the lithium ion battery on the basis of graphite and silicon-carbon negative electrodes of the current commercial lithium battery.
Simplify cell, module, and system design: since the solid electrolyte is not fluid, an internal string can be used.
Solid-state battery production proces
Solid-state batteries and liquid batteries have many similarities in the manufacturing process. For example, the manufacturing process of electrode sheets is based on slurry mixing, coating and calendering. After cutting, the tab welding and PACK (battery pack processing into groups) are carried out, but there are also some differences.
There are three core differences:
Solid battery anode material compounding, namely, the mixture of solid electrolyte and anode active material is used as the composite anode;
Electrolyte is added in different ways. For liquid batteries, the electrolyte is injected into the battery and packaged after the tab is welded, while the solid electrolyte needs to be coated again on the calendered composite positive electrode in addition to forming a composite positive electrode with the positive active material.
Liquid lithium-ion battery pole pieces can be wound or laminated, while solid-state batteries are usually packaged in the form of laminations due to the poor toughness of solid-state electrolytes such as oxides and sulfides.
The core process of solid electrolyte is film formation, which can be divided into dry process, wet process and other processes.
The core process of solid-state battery manufacturing lies in the solid electrolyte film-forming link. The film-forming process of electrolyte will affect the thickness and related performance of electrolyte. If the thickness is too thin, it will lead to relatively poor mechanical properties, which will easily lead to damage and internal short circuit. If the thickness is too thick, the internal resistance will increase, and because the electrolyte itself does not contain active substances, the energy density of battery monomer and system will be reduced.
Wet film-forming process:
The mold support film forming is suitable for polymer and composite electrolyte, the solid electrolyte solution is poured into a mold, and the solid electrolyte film is obtained after the solvent is evaporated; the anode support film forming is suitable for inorganic and composite electrolyte films, namely, the solid electrolyte solution is directly poured on the surface of the anode, and the solid electrolyte film is formed on the surface of the anode after the solvent is evaporated; The framework supporting film is suitable for the composite electrolyte film, the electrolyte solution is injected into the framework, after the solvent is evaporated, the solid electrolyte film with the framework supporting is formed, and the mechanical strength of the electrolyte film can be improved.
Dry film-forming process:
The electrolyte and the binder are mixed, ground and dispersed, and the dispersed mixture is pressurized (heated) to prepare the solid electrolyte membrane. The method does not use a solvent, and there is no solvent residue. The disadvantage of the dry method is that the electrolyte membrane is relatively thick, and because it does not contain active substances, the energy density of the solid battery will be reduced.
Other film-forming processes:
Including chemical, physical, electrochemical vapor deposition, and the like. This kind of process has high cost and is suitable for thin film all-solid-state batteries.
The semi-solid battery can be compatible with the traditional lithium battery production process, the production equipment can be basically compatible with the lithium battery, only a new production line specializing in semi-solid diaphragm is needed, and the production equipment is compatible with the liquid battery diaphragm equipment.
Semi-solid batteries require larger pore size and higher strength of the separator, and adopt wet + coating process.
Compared with the traditional battery, the separator of the semi-solid battery has no obvious process change, and the parameters can be adjusted. However, because the semi-solid battery needs to improve the ionic conductivity, the aperture of the separator is required to be larger and the strength is higher, so the wet stretching + coating process is needed.
Solid state battery industry chain
The upstream of the solid-state battery industry chain is basic materials and equipment, including raw minerals, positive and negative materials, electrolytes and other battery core raw materials, as well as battery production equipment; the midstream is battery pack processing and preparation, including battery packaging integration, power management system, energy management system and other program design; The downstream is the application field, including EV, consumer electronics, energy storage, power tools and so on.
Throughout the technical map of solid-state batteries, from the innovation of material system to the exploration of process routes, we can see a deep reconstruction of the nature of batteries.
It is not a simple “repair”, but aims to eradicate the “chronic illness” of traditional batteries in principle-replacing liquid activity with solid stability, and expanding the application boundary with higher energy density.
Although the road ahead is still full of challenges, such as interface impedance, cost control and large-scale production process problems, the huge potential of high safety and high energy density is exciting enough.
From the gradual improvement of semi-solid state to the subversive revolution of all-solid state, this technological path is steadily advancing. It can be predicted that the maturity and popularization of solid-state batteries will not only drive EV into the next fast lane, but also profoundly change the way consumer electronics, large-scale energy storage and even the whole society use energy.
This silent energy revolution deserves our continuous attention and expectation.
