According to foreign media reports, due to the high energy storage density, materials such as metal oxides, sulfides and fluorides are extremely promising electrode materials for electric vehicle lithium-ion batteries. However, their energy storage capacity declines rapidly. A few days ago, scientists discovered that a lithium-ion battery with iron oxide electrodes found that the loss caused by battery charging and discharging more than 100 times was caused by lithium oxide accumulation and electrolyte decomposition.
The iron oxide electrode used in the research process is made of cheap and non-toxic magnetite. Compared with current electrode materials, conversion electrode materials such as magnetite (that is, converted into brand-new products when reacting with lithium) can store more energy because they can hold more lithium ions. "However, the energy storage capacity of these materials decays very quickly and depends on the current density. For example, our electrochemical tests on magnetite show that the capacity of magnetite drops rapidly during the first 10 high-speed charge and discharge cycles." Dong Su, the person in charge of this research and the leader of the Electron Microscope Group at the Functional Nanomaterials Center (CFN), said. CFN is the US Department of Energy's Office of Scientific User Facilities located at Brookhaven National Laboratory.
In order to find out the reason for the instability of the cycle, the scientists tried to observe the change of the crystal structure and chemical properties of the magnetite after the battery completed 100 cycles. They combined transmission electron microscopy (TEM) and simultaneous X-ray absorption spectroscopy (XAS) to conduct research. The electron beam of the TEM is transmitted through the sample to produce a structural image or diffraction pattern of the characteristic substance. XAS uses X-rays to detect the chemical properties of the material.
Scientists used these techniques to find that during the first discharge, magnetite completely decomposed into metallic iron nanoparticles and lithium oxide. However, in the subsequent charging process, this conversion reaction is not completely reversible, and metal iron and lithium oxide residues still exist. In addition, the original "spinel" structure of magnetite evolved into a "rock salt" structure in the charged state (in both structures, the positions of iron atoms are not exactly the same). In the subsequent charge-discharge cycle, the rock salt iron oxide interacts with lithium to form a composite of lithium oxide and metallic iron nanoparticles. Because the conversion reaction is not completely reversible, these residual products will gradually accumulate. Scientists have also discovered that the electrolyte (the chemical medium that causes lithium ions to flow between the two electrodes) decomposes in subsequent cycles.
On the basis of the research results, scientists have proposed an explanation for the decline in energy storage capacity. Sooyeon Hwang, a scientist and co-lead author of the CFN electron microscope group, said, "Since the electrical conductivity of lithium oxide is low, its accumulation will form a barrier for the electrons that shuttle between the positive and negative electrodes of the battery. We call it internal passivation. Layer. Similarly, the decomposition of the electrolyte will form a surface passivation layer, hindering ion conduction. These barriers accumulate, preventing electrons and lithium ions from reaching the active electrode material that undergoes an electrochemical reaction. "
Scientists pointed out that running the battery at low current can provide sufficient time for electron transmission by slowing down the charging speed and restoring part of the capacity; however, to completely solve this problem, other solutions are needed. They believe that adding other elements to the electrode material and changing the electrolyte can improve capacity attenuation.
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