Zhang Yuegang, Professor of Tsinghua University: Solid-state batteries are stuck at the "two interfaces", while lithium metal batteries are trying to bypass this hurdle | Frontline

Solid-state batteries are not necessarily the only answer for next-generation lithium batteries.

Recently, at the 2026 Lithium Metal Battery Frontier Forum (LMBF 2026) held in Beijing, 36Kr had an exchange with Zhang Yuegang, the forum chairman and tenured professor at Tsinghua University.

In the past few years, solid-state batteries have almost become one of the most popular technical directions in the power battery industry. However, in Zhang Yuegang's view, the industry first needs to clarify a basic concept: solid-state batteries and lithium metal batteries are not two technical generations that can be directly compared in the first place.

The reason is that the two belong to different classification dimensions.

The energy density of batteries is mainly determined by cathode and anode materials. The cathode can be lithium iron phosphate, high-nickel ternary, lithium-rich manganese-based or even sulfur, while the anode has evolved from graphite to silicon-carbon and lithium metal; the so-called solid state and liquid state distinguish the form of electrolyte.

In other words, solid-state batteries change the electrolyte, while lithium metal batteries change the anode. It is still the cathode and anode that fundamentally determine how much energy a battery can store. Therefore, lithium metal anode batteries are also a key evolutionary branch of next-generation battery technology.

In Zhang Yuegang's view, the currently booming solid-state battery route in the industry still has a number of material technical difficulties to overcome.

As introduced, Zhang Yuegang is a tenured professor at Tsinghua University, an internationally renowned materials scientist with more than 30 years of experience in materials research at home and abroad. He has successively worked at the Nippon Electric Basic Research Institute, Stanford University in the United States, Intel, Lawrence Berkeley National Laboratory, Chinese Academy of Sciences and other institutions, and has achieved a number of important research results in the fields of nanomaterials, energy storage technology and other fields.

Solid-state batteries are stuck at two "interfaces"

The most attractive part of solid-state batteries for the industry is that they use solid electrolyte to replace flammable organic liquid electrolyte, so as to apply more cutting-edge cathode and anode materials and push the energy density up to the 500Wh/kg level. However, the interface problem of solid electrolyte still plagues the industry today, and Zhang Yuegang explained these difficulties in detail to 36Kr.

Zhang Yuegang believes that the core problems can actually be attributed to two "interfaces".

The first is the microscopic interface inside the cathode.

The battery cathode is not a whole dense material, but composed of a large number of particles, with a large number of nano-scale pores inside. Liquid electrolyte can penetrate into these tiny spaces like water and fully contact with the cathode material; but solid electrolyte can hardly do this.

If more solid electrolyte is added to improve ion transmission, it will take up the space of active materials that actually store energy, and eventually drag down the energy density of the whole battery.

The second problem is the interface between the anode and the solid electrolyte as well as the lithium dendrite problem.

A common judgment in the past was that solid electrolyte has higher mechanical strength and can physically block lithium dendrites. However, Zhang Yuegang believes that after more than ten years of research, various solid electrolytes still do not truly solve this problem at present.

Solid materials naturally have microscopic defects, along which lithium will continue to grow. Once a penetration channel is formed, it may even quickly lead to internal short circuit.

In addition, unlike liquids, solids cannot be fully wetted with each other, which is essentially closer to "point contact". In order to reduce the interface impedance, it is necessary to increase the proportion of electrolyte and the working pressure, but this contradicts the goal of improving energy density.

Therefore, another evolutionary idea for the next generation of battery technology is to temporarily retain the liquid electrolyte and replace the anode with lithium metal first.

The biggest obstacle for this route in the past was also lithium dendrites.

It is difficult for lithium to always remain absolutely flat during the repeated deposition process, and it will gradually grow a structure similar to branches. Once the dendrites continue to grow and pierce the separator, it may cause an internal short circuit.

Zhang Yuegang introduced that an important solution idea at present is to use additives in the liquid electrolyte to form a SEI protective film on the surface of lithium metal. When cracks appear in this film during the charging and discharging process, the additives can continue to react to dynamically repair the damaged area.

In this way, dendrites can be controlled at the micron scale, which greatly reduces the risk of short circuit caused by their continuous growth. But the cost is also obvious: the continuous damage and repair of the SEI film means that the electrolyte will be continuously consumed.

According to the progress given by Zhang Yuegang, relevant lithium metal batteries can now achieve hundreds of cycles, some high-specific-energy products at the 500Wh/kg level have exceeded 300 cycles, and products at the 400Wh/kg level can reach 600 cycles, but there is still a gap from the thousands of cycles generally required by new energy vehicles.

The first batch of lithium metal batteries may "fly to the sky" first

Zhang Yuegang judges that the more realistic commercialization path for lithium metal batteries is to enter drones and eVTOL first, then enter embodied intelligent robots, and finally enter new energy vehicles.

The reason is not complicated.

For automobiles, 300 or even 600 cycles are obviously not enough; but for aircraft, weight is directly related to range and payload, and every increase in energy density may generate higher commercial value. Therefore, drones and eVTOL are willing to pay higher costs for high-specific-energy batteries and are more motivated to adopt new technologies first.

The same logic applies to robots. As embodied intelligence is put into practical application, battery life, weight and instantaneous power will all become core indicators, and the value of high-energy-density batteries will be further amplified.

According to Zhang Yuegang's judgment, if the cycle life of 400Wh/kg-level lithium metal batteries can be increased to thousands of cycles in the next two or three years, they will have the opportunity to enter car enterprises for verification; if 500Wh/kg-level products reach 700-800 cycles, they may also start on-board tests.

In Zhang Yuegang's view, liquid electrolyte may help lithium metal anode achieve industrialization first at present; a few years later, when new demands for higher-specific-energy cathode materials, such as sulfur cathodes, emerge, solid electrolyte may become an important puzzle piece again.

The so-called "next-generation battery" may not eventually belong to a certain technical term that sounds sufficiently revolutionary, but to the material combination that is the first to simultaneously solve the problems of energy density, interface and cycle life.