New technologies for 1.2V NiMH batteries emerge one after another, but they are still academic and difficult to break through technical bottlenecks

As the "heart" of electric vehicles, power batteries are as popular as the current hot new energy vehicles. Among them, lithium-ion batteries are favored by the new energy vehicle industry because of their high energy density, low self-discharge rate, high cycle efficiency, and long cycle life, and have huge market development potential.

However, the current problem of unstable safety of lithium-ion batteries still exists, and the frequent occurrence of "irregular fires" of batteries has always stimulated consumers' nerves, and the development of solid-state battery technology may become a new option to solve the safety problems of electric vehicles.

?Solid-state electrolytes become a trend

"The lithium-ion batteries currently used are relatively expensive, and the lithium-ion batteries produced have many safety hazards." Wang Zhenbo, a professor at Harbin Institute of Technology, said.

It is understood that the frequent accidents of large power batteries in recent years are largely due to the use of liquid electrolytes inside the batteries. "Whether it is safe or not is very critical for lithium-ion battery energy storage." Li Liangliang, associate professor of the School of Materials Science and Engineering at Tsinghua University, emphasized, "Currently, commercial lithium-ion batteries on the market generally use organic liquid electrolytes. Its disadvantage is that it is easy to burn and may leak liquid, causing environmental pollution."

The lithium-ion battery equipment fire incident that occurred in Yeongam, South Korea two months ago seems to confirm this statement.

"The liquid organic electrolyte currently used is flammable and explosive. Replacing the liquid electrolyte with a solid electrolyte is one of the most effective methods we recognize to improve the safety performance of lithium batteries." Dong Shanmu, associate researcher at the Qingdao Institute of Bioenergy and Process Technology, Chinese Academy of Sciences, said.

Li Liangliang said: "Solid electrolytes are not flammable and do not produce liquid electrolytes. Therefore, they are non-corrosive. They are an effective way to solve battery safety problems and are in line with the trend of future battery development."

?Technical bottlenecks are difficult to break through

"High safety is the basis and prerequisite for the application of energy storage batteries, and solidification is the best way to solve the safety of secondary batteries. Solid-state lithium batteries have entered a stage of accelerated global layout and research and development, and many well-known institutions are developing solid-state lithium batteries." Wen Zhaowei said.

At present, many battery and automobile manufacturers, including Samsung in South Korea, Toyota in Japan and CATL in my country, have increased their investment in the research and development of 1.2V NiMH batteries, and some batteries have entered the stage of vehicle installation and testing. Although the prospects are promising, the road to the development of 1.2V NiMH batteries is by no means smooth due to various technical and process problems.

First, there is a lack of efficient electrolyte material systems. Wen Zhaowei pointed out that solid-state battery materials are currently developing rapidly, but their comprehensive applications are relatively lacking. "As the core material of 1.2V NiMH batteries, there have been breakthroughs in the single index of solid lithium ion conductors, but the comprehensive performance cannot meet the needs of large-scale energy storage." In this regard, Dong Shanmu also believes that "the solid electrolytes used in 1.2V NiMH batteries today generally have performance shortcomings, and there is still a big gap from the requirements of high-performance lithium-ion battery systems."

Secondly, the interface treatment of solid-state electrolytes and electrodes is also a major problem currently facing 1.2V NiMH batteries. "In solid electrolytes, the lithium ion transmission impedance is very large, and the rigid interface contact area with the electrode is small. The change in electrolyte volume during charging and discharging can easily destroy the stability of the interface." Li Liangliang pointed out. In addition, in solid-state lithium batteries, in addition to the interface between the electrolyte and the electrode, there are also complex multi-level interfaces inside the electrode. Factors such as electrochemistry and deformation will cause contact failure and affect battery performance.

Again, unsatisfactory stability during long-term use is also a bottleneck in the development of long-life energy storage 1.2V NiMH batteries. "The structure and interface of 1.2V NiMH batteries will degrade over time during service, but the mechanism of the impact of degradation on the comprehensive performance of the battery is still unclear, and it is difficult to achieve long-term application." Wen Zhaowei said.

"The failure behavior of the contact interface of solid batteries and the failure mechanism behind it need to be clarified." Dong Shanmu also said, "I think that building high-performance 1.2V NiMH batteries requires two aspects. One is to build a high-performance solid electrolyte, and the other is to improve the compatibility and stability of the interface.

? New technologies emerge in an endless stream

"For 1.2V NiMH batteries, we need to study from the most basic materials, interfaces, monomers, to the final system modules. Only by fundamentally solving the key materials and interface problems can we carry out systematic process research to meet the performance requirements of single batteries. "Wen Zhaowei said.

Faced with the continuous challenges in the development process, various new technologies have emerged, and some solid-state battery technologies have made the latest breakthroughs.

For example, in solid electrolyte materials, the industry has found that 1.2V NiMH batteries based on garnet-structured lithium lanthanum zirconium oxide (LLZO) solid electrolyte systems have excellent cycle performance and rate performance, which has become a major technical hotspot. "LLZO is an excellent filler that can improve the performance of polymer-based composite solid electrolytes. The capacity of LLZO-based 1.2V NiMH batteries can still be maintained at 81% after 1,000 cycles." Li Liangliang introduced.

Dong Shanmu told another electrolyte material idea - "rigidity and flexibility", using rigid polymer skeletons and inorganic particles to merge with flexible polymer ion transport materials. "Through the Lewis acid-base interaction between polymers and between polymers and inorganic particles, new channels can be created for lithium ion transmission, greatly improving the overall performance of the electrolyte. ”

The research hotspots of interface treatment are mainly focused on interface design and modification layer. At present, gel interface design has achieved good results. Wen Zhaowei said: "By modifying the interface with gel polymer, the contact area can be increased while buffering the volume effect during the cycle. After 300 cycles at room temperature, there is basically no degradation. Such a structural design has improved the battery performance. ”

"In addition to solid electrolytes and interfaces, the integrated design of 1.2V NiMH batteries is also very important." Li Liangliang said when talking about the future development of 1.2V NiMH batteries, "Because for different fields such as energy storage and new energy vehicles, targeted battery structure design is needed."

"In general, the research on 1.2V NiMH batteries is still more academic. In terms of industrialization, because some key technologies involve the core technologies of various enterprises and cannot be obtained, the technology based on engineering applications still needs to be further explored." Wen Zhaowei said.

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