According to foreign media reports, due to their high energy storage density, materials such as metal oxides, sulfides and fluorides are promising electrode materials for electric vehicle lithium-ion batteries. However, their energy storage capacity declines quickly. Recently, scientists studied a lithium-ion battery with an iron oxide electrode and found that the loss of the battery after charging and discharging more than 100 times was caused by the accumulation of lithium oxide and the decomposition of the electrolyte. 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 new products when reacting with lithium) can store more energy because they can accommodate more lithium ions. "However, the energy storage capacity of these materials decays very quickly and is dependent on current density. For example, our electrochemical tests on magnetite show that the capacity of magnetite drops rapidly within the first 10 high-speed charge and discharge cycles." Dong Su, the person in charge of this research and leader of the electron microscopy group of the Center for Functional Nanomaterials (CFN), said. CFN is a U.S. Department of Energy Office of Science User Facility located at Brookhaven National Laboratory. In order to find out the cause of cycle instability, the scientists tried to observe the changes in the crystal structure and chemical properties of magnetite after the battery completed 100 cycles. They conducted the study using a combination of transmission electron microscopy (TEM) and synchrotron X-ray absorption spectroscopy (XAS). TEM's electron beam is transmitted through the sample to produce structural images or diffraction patterns of characteristic substances. XAS uses X-rays to detect the chemical properties of materials. Using these techniques, the scientists found that upon first discharge, magnetite completely decomposes into metallic iron nanoparticles and lithium oxide. But during the subsequent charging process, this conversion reaction is not completely reversible, and residues of metallic iron and lithium oxide still remain. In addition, the original "spinel" structure of magnetite evolves into a "rock salt" structure in the charged state (the positions of the iron atoms are not exactly the same in the two structures). In subsequent charge and discharge cycles, rock salt iron oxide interacts with lithium to form a composite material of lithium oxide and metallic iron nanoparticles. Because the conversion reaction is not completely reversible, these residual products gradually accumulate. The scientists also found that the electrolyte, the chemical medium that allows lithium ions to flow between the two electrodes, breaks down during subsequent cycles. On the basis of the findings, the scientists proposed an explanation for the decline in energy storage capacity. Sooyeon Hwang, a scientist in CFN's electron microscopy group and co-lead author, said, "Because lithium oxide has low electronic conductivity, its accumulation will form a barrier for electrons shuttling between the positive and negative electrodes of the battery. We call this an internal passivation layer. "Similarly, electrolyte decomposition can also form a surface passivation layer that blocks ion conduction. These obstacles accumulate, preventing electrons and lithium ions from reaching the active electrode materials where electrochemical reactions occur." The scientists point out that operating the battery at low currents can By slowing down charging, restoring some capacity provides enough time for electron transfer; however, other solutions are needed to completely solve this problem. They believe that capacity fading can be improved by adding other elements to the electrode materials and changing the electrolyte.

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