The power battery plays a decisive role in the performance of the vehicle. Compared solid state batteries vs lithium ion, the current lithium-ion battery system is still unable to fully meet the safety, energy density and cost requirements of power batteries. Solid state batteries have greater potential in terms of safety and energy density, and have received extensive attention from the industry in recent years. This paper discusses the characteristics and industrial chain of solid state batteries in combination with China's relevant industrial policies.
1. Power battery industry policy
In recent years, the subsidy policy for new energy vehicles has undergone several adjustments. Judging from the orientation of previous policy adjustments, the support for high cruising range and high energy density technical routes has not weakened or even increased. After the subsidy policy is completely withdrawn, the dual-point system may still achieve a guiding role in the technological upgrading of the industry by setting differentiated points for different energy densities and different cruising ranges.
According to relevant documents, the energy density target of power batteries is 400Wh/kg in 2025 and 500Wh/kg in 2030. The energy density of the liquid state is around 350Wh/kg. To achieve the goal of 2030, the existing liquid lithium battery technology route is difficult to achieve. From this perspective, the development of solid state batteries is a deterministic trend.
Experts pointed out that the goal of 2025 can be achieved by relying on the transformation of cathode materials from high-nickel ternary to high-capacity lithium-rich manganese-based materials, but it has basically reached the limit. To achieve the 2030 goal, a breakthrough at the solid electrolyte level is the only way.
2. The development history of solid state batteries
In the 1970s, there was an oil crisis in the United States. The U.S. government, realizing its over-reliance on oil imports, began to develop solar and wind power vigorously. Due to the intermittent nature of solar and wind power, rechargeable batteries will eventually be needed to store these renewable and clean sources of energy. At that time, non-rechargeable batteries mostly used lithium anodes and organic electrolytes. To realize rechargeable batteries, efforts have been made to reversibly intercalate lithium ions into layered transition metal sulfide cathodes.
In order to reduce costs and improve safety, all solid state rechargeable batteries using solids as electrolytes seem to be an important direction for future development. In recent years, some industry experts believe that in terms of performance, cost and safety, all solid state rechargeable batteries are the best choice to replace fossil energy and ultimately realize the road to new energy vehicles.
3. Characteristics of solid state batteries
The solid-state lithium battery refers to a lithium secondary battery in which the electrolyte part of the battery adopts a solid-state material. The difference between solid state batteries and traditional lithium-ion batteries is that solid state batteries replace the electrolyte, electrolyte salt, and separator of traditional lithium-ion batteries with solid-state electrolytes. Safety, energy density and cycle life are the three major requirements for power batteries. Since solid state batteries have the potential to outperform conventional lithium-ion batteries in all three aspects, they are also considered as the next-generation battery technology.
● High safety, no risk of spontaneous combustion and explosion
Traditional lithium-ion batteries using organic electrolytes can easily cause the electrolyte to heat up under abnormal conditions such as overcharge and internal short circuit, and there is a risk of spontaneous combustion or even explosion. All-solid state lithium batteries are based on solid-state materials that are non-flammable, non-corrosive, non-volatile, and have no leakage problems, and are expected to overcome the phenomenon of lithium dendrites. Semi-solid and quasi solid state batteries still have certain flammability risks, but they are also safer than liquid electrolyte batteries.
● High energy density
Lithium-ion batteries are approaching the performance limit under the current technical system. The energy density of Tesla 18650 cells reaches 250Wh/Kg, the energy density of 21700 cells used in Model 3 is about 300Wh/Kg, and the supported cruising range is about 400 to 500 kilometers, but it still cannot completely solve the mileage anxiety.
For solid state batteries, on the one hand, solid-state electrolytes do not require separators and electrolytes, which together account for nearly 40% of the volume and 25% of the mass in traditional lithium-ion batteries. On the other hand, since there are no problems such as liquid leakage and corrosion, the battery casing and cooling system modules can be simplified, and the weight of the battery system can be further reduced.
At the same time, the new positive and negative materials can make the electrochemical window reach more than 5V, which can fundamentally improve the energy density, which is expected to reach 500Wh/Kg, and the cruising range is expected to be increased to 600 to 700 kilometers under the same battery capacity. In addition, solid-state batteries have the advantages of long cycle life, wide operating temperature range, and fast charging.
4. Classification of solid state batteries
According to different electrolyte materials, solid state batteries can be divided into three systems: polymers, oxides and sulfides. Among them, polymer electrolytes belong to organic electrolytes, and oxides and sulfides belong to inorganic ceramic electrolytes. In general, polymer electrolyte technology is the most mature and has taken the lead in realizing small-scale mass production, but its theoretical energy density is not as good as that of the other two types of electrolytes.
The performance of oxide electrolytes is better than that of polymer electrolytes, but the capacity of thin film oxide batteries is small and can only be used in the field of consumer electronics, and the technology of non-thin film oxide batteries is relatively immature. Sulfide electrolytes are theoretically the most suitable for electric vehicles, but they are the most difficult to develop. According to the difference of cathode and anode materials, solid state batteries can be further divided into solid state li ion battery (using the current lithium-ion battery material system) and solid state lithium metal batteries (using metal lithium as the anode).
5. Technical difficulties of solid state batteries: high impedance, low rate
The bulk ionic conductivity of current solid-state electrolytes is much lower than that of liquid electrolytes, often by several orders of magnitude. According to the choice of materials, solid electrolytes can be divided into three systems: polymers, oxides, and sulfides, and no matter which category, the problem of ion conduction cannot be avoided. In addition, solid electrolytes possess high interfacial impedance. On the electrode and electrolyte interface, the traditional liquid electrolyte is in liquid/solid contact with the cathode and anode. The interface has good wettability and no large impedance is generated between the interfaces.
In contrast, the solid electrolyte and the cathode and anode are in solid/solid interface contact, the contact area is small, the contact tightness with the pole piece is poor, the interface impedance is high, and the transport of lithium ions between the interfaces is hindered. Low ionic conductivity and high interfacial impedance lead to high internal resistance of solid state batteries, low transfer efficiency of lithium ions inside the battery, and poor mobility at high rates and currents, which directly affect the energy density and power density of the battery.
6. Solid state batteries industry chain
The industrial chain of solid state batteries is roughly similar to that of liquid lithium batteries. The main difference between the two is that the anode materials and electrolytes in the middle and upstream are different, but the cathodes are almost the same. The semi-solid battery separator still exists, and the separator disappears after the battery is fully solid-state. Solid state batteries electrode materials use composite electrodes mixed with solid state electrolytes. Structurally, the biggest difference between the cathode and anode of solid state batteries and traditional electrodes is that in order to increase the contact area with the electrolyte, the cathode and anode of solid state batteries are generally mixed with solid state electrolytes.
For example, hot pressing, filling or coating solid electrolyte between cathode and anode particles, or coating solid electrolyte on the separator. In terms of material selection, due to the generally higher electrochemical window of solid-state electrolytes, high-nickel high-voltage cathode materials are easier to carry, and new cathode material systems will continue to be used in the future. High-capacity anodes such as silicon and metallic lithium are mostly used as anode materials to give full play to the advantages of solid state batteries.
At this stage, most solid state batteries companies still need to add a small amount of liquid electrolyte to their products to alleviate electrode interface problems and increase conductivity. Therefore, the separator still exists in the battery to block the positive and negative electrodes and avoid short circuit of the battery. With the advancement of technology, the amount of electrolyte will be less and less in the future. When it transitions to completely free of liquid or the liquid content is small enough, the battery will cancel the separator design, and the system can already meet the safety requirements.
Based on various plans in China, the development history of lithium battery technology, the direction of the path, and the progress of the solid state batteries industry of various companies, the development of the solid state batteries industry will become a definite trend. In the future, related technologies in China's solid state batteries industry will continue to improve, and solid state batteries will also show higher energy density, better safety and lower costs.
The time for large-scale production and commercial development is not far away. With the improvement of China's localization of the soft pack industry chain and the increase in the penetration rate of solid state batteries, the packaging form of soft packs will gradually become the mainstream.