Solid-state batteries are the only way forward in the post-lithium era

In addition, due to its thorough safety features, temperature control components such as BMS can be eliminated, and the battery system can be further "reduced" through a diaphragm-free design.

Solid-state batteries are the next generation battery technology that is most likely to be industrialized first

Solid-state battery systems have a smaller revolution. Lithium-sulfur batteries, lithium-air and other systems require the replacement of the entire battery structure framework, which poses more and greater challenges, while solid-state batteries mainly rely on the innovation of the electrolyte, and the positive and negative electrodes can continue to use the current system, which is relatively easy to achieve.

Lithium metal anode is compatible and is achieved through solid electrolytes. Both lithium sulfur and lithium air require lithium metal anodes, and lithium metal anodes are easier to achieve on solid electrolyte platforms.

Solid-state batteries, as the next generation battery technology closest to us, have become a consensus in the scientific and industrial communities and are the only way forward in the post-lithium battery era.

How far are we from solid-state batteries?

The core problem of high impedance and low magnification

The current bulk ionic conductivity of solid 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. Regardless of which category, the problem of ion conduction cannot be avoided.

The function of the electrolyte is to build a lithium ion transmission channel for lithium ions between the positive and negative electrodes during the battery charging and discharging process to realize the conduction of current inside the battery. The indicator that determines the smoothness of lithium ion transport is called ionic conductivity. Low ionic conductivity means that the electrolyte has poor lithium conductivity, which prevents lithium ions from moving smoothly between the positive and negative electrodes of the battery.

The room temperature conductivity of the polymer system is about 10-7-10-5S/cm, the room temperature conductivity of the oxide system is 10-6-10-3S/cm, the sulfide system has the highest conductivity, about 10-3-10-2S/cm at room temperature, and the room temperature ionic conductivity of the traditional liquid electrolyte is about 10-2S/cm, which is higher than the ionic conductivity of any type of solid electrolyte.

In addition, solid electrolytes have high interface impedance. On the interface between the electrode and the electrolyte, the contact between the traditional liquid electrolyte and the positive and negative electrodes is liquid/solid contact, the interface wettability is good, and there is no large impedance between the interfaces. In contrast, the solid electrolyte contacts the positive and negative electrodes in a solid/solid interface, the contact area is small, the contact tightness with the pole piece is poor, the interface impedance is high, and the transmission of lithium ions between the interfaces is hindered.

Low ionic conductivity and high interface impedance lead to high internal resistance of solid-state batteries. The transmission efficiency of lithium ions inside the battery is low, and their mobility is even worse under high rates and large currents, which directly affects the energy density and power density of the battery.

Industrialization progress of three major technical routes

The three major systems of solid-state batteries each have their own advantages. Among them, polymer electrolytes are organic electrolytes, and oxides and sulfides are inorganic ceramic electrolytes.

Looking at the global solid-state battery companies, there are start-ups and international manufacturers. Each company has its own unique belief in different electrolyte systems, and there is no trend of technology flow or integration. European and American companies prefer oxide and polymer systems, while Japanese and Korean companies are more committed to solving the industrialization problems of sulfide systems, represented by giants such as Toyota and Samsung.

The polymer system technology is the most mature, and the first EV-level products were born. Its conceptual and forward-looking nature has caused latecomers to accelerate investment in research and development, but the performance ceiling restricts development. Composite with inorganic solid electrolytes will be a possible solution in the future.

In the oxide system, the focus of thin film type development is on capacity expansion and large-scale production, while the non-thin film type has better overall performance and is the current research and development focus; the sulfide system is the most promising solid-state battery system for application in the field of electric vehicles, but it is in a polarized situation with huge development space and immature technology level. Solving safety and interface problems is the focus in the future.

Industrialization is still in its early stages, but the future is secure

The energy density of marketed products is low. At present, there are few mass-produced solid-state battery products, and the industrialization process is still in its early stages. The energy density of the products of Bollore, the only company that has achieved mass production in the field of power batteries, is only 100Wh/kg, which does not yet have a competitive advantage over traditional lithium batteries.

High-performance laboratory products will lay the foundation for industrialization. Judging from the experimental and pilot products of overseas companies, the energy density advantage of solid-state batteries has begun to emerge, significantly exceeding the current level of lithium batteries.

In my country, basic research on solid-state lithium batteries started early. During the Sixth and Seventh Five-Year Plans, the Chinese Academy of Sciences listed solid-state lithium batteries and fast ion conductors as key topics. In addition, Peking University, China Electronics Technology Group Tianjin Institute 18 and other institutions also launched projects to conduct research on solid-state lithium battery electrolytes and have made good progress in this field.

In the future, as industry investment gradually increases, the pace of product performance improvement is also expected to accelerate.

The impact of solid-state batteries on the lithium battery industry chain

In addition to electrolytes, solid-state batteries also have certain differences from traditional lithium batteries in the selection of other battery components.

The electrode material uses a composite electrode mixed with a solid electrolyte. Structurally, the biggest difference between the positive and negative electrodes of solid-state batteries and traditional electrodes is that in order to increase the contact area between the electrode and the electrolyte, the positive and negative electrodes of solid-state batteries are generally mixed with a solid electrolyte.

For example, hot pressing or filling solid electrolyte between positive and negative electrode particles, or introducing liquid on the electrode side to form a solid-liquid composite system, which is different from the traditional lithium battery that separately mixes the electrode slurry and coats it on aluminum/copper foil.

In terms of material selection, due to the generally higher electrochemical window of solid-state electrolytes, high-nickel high-voltage positive electrode materials are easier to carry, and new positive electrode material systems will continue to be used in the future. For negative electrode materials, high-capacity negative electrodes such as silicon and metallic lithium will be mostly used to give full play to the advantages of solid-state batteries.

There is a buffer layer between the electrode and the electrolyte. The addition of the buffer layer can improve the interfacial properties between the electrode and the electrolyte. Its components can be gel compounds, Al2O3, etc.

The diaphragm still exists and disappears after the battery achieves full solid-state. At this stage, most solid-state battery companies' products still need to add a small amount of liquid electrolyte to alleviate electrode interface problems and increase conductivity. Therefore, the diaphragm still exists in the battery to block the positive and negative electrodes and prevent the battery from short-circuiting.

This compromise solution has the performance advantages of solid-state batteries and is easier to implement in terms of technical difficulty. As technology advances, the amount of electrolyte used will decrease in the future. When the transition is to a state where there is no liquid at all or the liquid content is small enough, the battery will eliminate the diaphragm design and the system will be able to meet safety requirements.

Soft pack packaging technology is mostly used. After removing the liquid electrolyte, the packaging and PACK of solid-state batteries are more flexible and lighter than traditional lithium batteries, so soft pack packaging will be used.

▌The path of stage development: step by step, gradual penetration

Looking into future development trends, with steady progress in technology and gradual penetration in applications, the development path of solid-state batteries has become clear.

Structurally, the current battery system contains some liquid electrolytes to complement each other. However, the use of liquids will be gradually reduced during the technological development process, from semi-solid batteries to quasi-solid batteries, and finally to liquid-free all-solid batteries.

In terms of application areas, it is expected to take the lead in leveraging its safety and flexibility advantages and be applied to micro-battery fields that are less sensitive to cost, such as RFID, implantable medical devices, wireless sensors, etc.; after technological advancement, it will gradually penetrate into high-end consumer batteries; as the product matures, it will eventually enter the electric vehicle and energy storage market on a large scale, penetrating from high-end brands downwards to achieve a comprehensive outbreak of downstream demand.