In recent frequent occurrences of electric vehicle fires and explosions, the formation and growth of lithium dendrites in solid electrolytes are the potential "culprits," and they are also the core factors determining the critical current density of the battery.

Although scientists in this field have clearly identified that lithium dendrites are produced during the cycling process of solid-state batteries, there has been no clear conclusion on issues such as "where lithium dendrites originate and what factors they are influenced by."

Recently, the Beijing University of Technology, in collaboration with the Xi'an Jiaotong University and Zhejiang University team, has independently developed a new "face-to-face" in-situ battery observation equipment and corresponding analysis methods, achieving for the first time the real-time in-situ scanning electron microscopy monitoring of solid-state battery reactions under working conditions.

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Compared with traditional probe analysis methods [1], this method can collect comprehensively the evolution process of the structure and morphology of the electrode, electrode/electrolyte interface, and electrolyte interior under the external current, and can avoid interference caused by local pressure and current concentration, thereby revealing the failure mechanism of all-solid-state batteries that is closer to actual service.

At the same time, researchers have also constructed a "mechanical-electrochemical" coupling model for the growth of lithium dendrites in solid electrolytes, helping to understand the relationship between the evolution of lithium dendrite growth morphology and electrochemical performance in solid-state batteries. Therefore, this research has significant implications for the optimization and improvement of all-solid-state batteries.Recently, the related paper was published in Nano Letters under the title "Chemomechanical Origins of the Dynamic Evolution of Isolated Li Filaments in Inorganic Solid-State Electrolytes" [2].

Beijing University of Technology Ph.D. student Tianci Cao and Professor Rong Xu from Xi'an Jiaotong University are the co-first authors. Assistant Researcher Xiaopeng Cheng from Beijing University of Technology is the co-first author and co-corresponding author. Associate Researcher Xianqiang Liu from Beijing University of Technology and Professor Yuefei Zhang from Zhejiang University serve as co-corresponding authors.

In the field of electron microscopy, "seeing is believing" is an unassailable truth.

The construction of the in-situ solid-state battery model under working conditions determines whether it is possible to directly obtain the growth and change images of lithium dendrites and the direct connection with the variation of external current density.

Common button batteries have a diameter of about 20mm. How can we accommodate the observation of such large-sized samples while also achieving the visualization of the morphological evolution details of battery materials at the micro-nano scale?In-situ optical microscopy is simple in design but has low resolution, making it difficult to observe the nucleation and initial growth of lithium. In-situ transmission electron microscopy (TEM) observations have sufficient spatial resolution, but they all use nano-scale battery models, which differ significantly from actual battery structures, and usually require a meticulous preparation process to obtain nano-scale batteries, increasing the difficulty and cost of experiments.

Compared to TEM, the scanning electron microscope (SEM) has a larger internal chamber that can accommodate an in-situ battery system closer to the actual situation, while also having nano-scale spatial resolution.

Therefore, after comprehensive consideration, the research team designed a "face-to-face" structure compatible with SEM based on the actual battery structure, that is, the "cathode-solid-state electrolyte-anode" in-situ battery model.

And achieved continuous in-situ observation of lithium dendrite growth in the solid-state electrolyte under real battery cycling conditions using SEM. Most importantly, the corresponding relationship between the lithium deposition dissolution behavior of the solid-state battery and the electrochemical curve was obtained simultaneously.

On the other hand, the active metal lithium in the lithium metal solid-state battery is easily oxidized, and the assembly of the battery generally needs to be carried out first in an inert atmosphere (argon) glove box.So, how can we transfer the assembled solid-state batteries into the scanning electron microscope without causing damage?

In previous studies, researchers often had to "race against time" when dealing with similar samples, trying to minimize the exposure to air during the transfer to the electron microscope chamber. However, even with this approach, contamination of the sample was inevitable.

To address this issue, the research team proposed a solution—combining the in-situ battery module and the sample transfer box into one, by designing an independent in-situ electrochemical testing platform.

The pre-assembled battery is sealed and placed into the sample transfer box by a motor drive, and then the entire setup is transferred to the scanning electron microscope chamber. When everything is ready for the experiment, the motor is controlled to slowly open the transfer box, ensuring a damage-free and contamination-free process. Additionally, it can prevent oxidation and waste of the prepared experimental samples due to hand tremors or other reasons.

After the construction of the in-situ solid-state battery is completed, how to stably apply external current and eliminate the impact of stray charges on the scanning electron microscope imaging has become a challenge for the team.Researchers, under the condition of preventing battery short circuits, made fine-tunings to the dimensions of the positive and negative electrode current collectors and their corresponding tabs. Although the final output was simply numerical information on dimensions, in reality, they went through multiple trials to obtain this information.

Next, they discovered a brand-new phenomenon—the dissolution of dendrites under lithium deposition conditions. This led them into deep "self-doubt," questioning whether there was an error in the experimental process or if extraneous factors were introduced.

After discussing with Professor Xu Rong from Xi'an Jiaotong University, the team constructed the rationality of this phenomenon and verified it through repeated experiments. "Fortunately, we did not deny the new phenomenon just because it did not conform to existing common sense, and we further clarified that in the field of electron microscopy, 'seeing is believing' is an unshakable truth," said Zhang Yuefei.

After multiple attempts, they finally constructed an in-situ solid-state battery observation system. After applying a stable current, they clearly observed the bulging of the solid electrolyte and the subsequent rapid growth of lithium dendrites.

Cheng Xiaopeng recalled, "The scene at that moment was thrilling. It was as if we were watching a seedling break through the soil, declaring its existence to the world. Our hearts were filled with a sense of novelty and pride. Everyone was silent, yet it seemed as if everything had been said."Committed to Solving Problems in Solid-State Batteries with Different Systems

The team, led by Academicians of the Chinese Academy of Sciences, Zhang Ze and Professor Zhang Yuefei, has been engaged in long-term in-situ research on the relationship between material microstructures and properties[4-6], and is committed to developing in-situ scanning electron microscopy methods.

In recent years, they have independently developed a variety of scientific instruments based on scanning electron microscopy, including in-situ tensile, heating, and electrochemical testing, and Zhejiang Qiyue Technology Co., Ltd. has transformed the scientific and technological achievements, breaking through the problem of in-situ high-temperature imaging above 1000°C in scanning electron microscopy.

By introducing the preparation, processing, and service conditions of materials into the electron microscope, they strive to achieve synchronous correlation research between material property testing and corresponding microstructures.Due to the in-situ testing of samples in the scanning electron microscope reaching centimeter-level sizes, the sample performance behavior and service conditions in in-situ testing analysis are more in line with actual situations, making the test results highly convincing and instructive for solving practical engineering problems.

The actual operating environment of batteries is complex and diverse, but ultimately it is concentrated in the three aspects of "mechanical, thermal, and electrical": the magnitude and distribution of current density, temperature distribution, and mechanical stress distribution.

On the premise of the in-situ electrochemical test platform designed by the team, which has well introduced the electric field, they plan to couple the thermal field and mechanical field in the next step to better solve the problems that arise in the operation of solid-state batteries in different systems.

At the same time, they will independently develop atomic layer deposition equipment and processes, and apply them to the surface modification of solid-state electrolytes, providing strategies to improve poor interfacial contact and suppress lithium dendrites in solid-state batteries.

Zhang Yuefei said, "We are based on the in-situ experimental method of the scanning electron microscope, facing the national demand, and have carried out systematic research on nickel-based high-temperature alloys used in aero-engines and lithium-ion battery materials."Ultimate Development Goal: All-Solid-State Batteries

At present, China's solid-state battery technology mainly focuses on the hybrid of solid and liquid states, with some companies already attempting to apply them in vehicles. However, there are still a series of issues to be resolved in this field, such as yield rate, battery cost, and cycle life.

The ultimate development goal for solid-state batteries is the all-solid-state battery, which is also the inevitable path to solving the inherent safety issues of lithium-ion batteries. The industry generally believes that all-solid-state batteries are expected to achieve industrialization by 2030.

Not long ago, academician Ouyang Ming and others took the lead in establishing the China All-Solid-State Battery Industry-Academia-Research Collaborative Innovation Platform, which will concentrate the efforts of all aspects of the industry to jointly break through the key technologies for the industrialization of all-solid-state batteries. Driven by new technologies such as artificial intelligence, it is expected to accelerate the breakthrough in the industrialization of all-solid-state batteries.

In the future, this research group will continue to build a good in-situ testing research platform and share resources in the collaborative research and development of all-solid-state batteries. It is hoped that fundamental scientific issues of all-solid-state batteries will be resolved from the aspects of key materials and electrodes, aiding the industrialization process."When solid-state batteries achieve mass production and are successfully integrated into vehicles, the current concerns in the electric vehicle field, such as insufficient range and the risk of battery combustion, will all dissipate," said Cheng Xiaopeng in conclusion. "I look forward to the early arrival of this day."