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The rapid development of battery technology has changed the lives of many people, the most obvious may be new energy vehicles, a fully charged new energy vehicle, can run four or five hundred kilometers, the cost of charging, compared with the fuel cost of traditional fuel vehicles, is more than a little lower. What's more, the energy of new energy vehicles** is cleaner and there is less direct or indirect pollution to the environment.
However, the batteries used in new energy vehicles still have not got rid of two key problems, one is the life attenuation of the battery, and the other is the safety of the battery.
When current is passed through the battery, the material inside the battery is more or less lost, and in the long run, the life of the battery gradually decays. Scientists have found that the physical quantities of stress and strain also play a role in this process, but their exact impact on battery performance and longevity is not fully understood.
At present, most of the batteries used in automobiles are lithium batteries, which are inseparable from electrolytes. Electrolyte is one of the four main materials of lithium batteries (the other three are the positive electrode, negative electrode and separator), and its main role is to conduct lithium ions between the positive and negative electrodes, and then generate electricity. The electrolyte has a fatal disadvantage: it is flammable, and if there is a fault such as a short circuit inside the battery, the lithium battery is likely to ignite a fire within a few seconds, engulfing the entire car, and posing a great threat to people's safety.
To address these pain points, a research team led by Oak Ridge National Laboratory (ORNL) in the United States developed a framework for designing solid-state batteries (SSBs) with the mechanics in mind. A recent article** published in the journal Science reviewed how mechanical factors change during the SSB cycle.
Scientists from the ORNL Multiphysics Modeling & Flow Group said: "Our research focuses on emphasizing the importance of mechanics in battery performance. Many studies have focused on chemical or electrical properties, but neglected to show the underlying mechanical properties. ”
The team spans several research areas at ORNL, including computational, chemical, and materials science. Their review paints a more cohesive picture of the conditions that affect SSB by using perspectives from the scientific field.
In batteries, charged particles flow through materials called electrolytes, which are mostly liquids, such as lithium-ion batteries in electric vehicles, but solid electrolytes are also being developed. These conductors are typically made of glass or ceramic and can offer benefits such as enhanced safety and strength. "True solid-state batteries have no flammable liquids inside, which means they will be less dangerous than batteries commonly used today," the researchers said. ”
However, solid electrolytes are still in the early stages of development due to the challenges associated with these novel materials. SSB components expand and contract during charge and mass transport, altering the overall system. The electrodes are constantly deformed during battery operation, creating stratification and voids at the interface with the solid electrolyte, and in today's systems, the best solution is to apply a lot of pressure to keep everything together.
These dimensional changes can damage solid electrolytes made of brittle materials, which often break due to stress and pressure. Making these materials more malleable will allow them to withstand stress by flowing rather than cracking. This behavior can be achieved through some techniques that introduce small crystal defects into ceramic electrolytes.
The electrons leave the system through the anode, and in SSB this component can be made of pure lithium, the metal with the highest energy density, and although this material offers an advantage to the power of the battery, it can also generate pressure that can damage the electrolyte.
Scientists from the ORNL Mechanical Properties and Mechanics Group said: "During charging, uneven plating and the lack of a stress relief mechanism can create stress concentrations. These can support a large amount of pressure to allow lithium metal to flow, and to optimize the performance and lifetime of SSBs, we need to design next-generation anodes and solid electrolytes that can maintain mechanically stable interfaces without damaging the solid electrolyte diaphragm. ”
The team's work is part of ORNL's long history of studying SSB materials. In the early 90s of the 20th century, the laboratory developed a glass-like electrolyte called lithium phosphate oxide or lipon. Lipon has been widely used as an electrolyte in thin-film batteries with metallic lithium anodes. The part can withstand multiple charge-discharge cycles without failing, mainly thanks to the ductility of Lipon. When it encounters mechanical stress, it flows instead of breaking.
The team's efforts highlight one aspect of SSB's under-researched – understanding the factors that influence its longevity and efficacy. "The research community needs a roadmap," the researchers said, "In our **, we outline the mechanics of solid-state electrolyte materials, encouraging scientists to consider these issues when designing new batteries." ”