Due to the high energy storage density, metal oxides, sulfides and fluorides are promising electrode materials for lithium-ion batteries of electric vehicles. However, their energy storage capacity declines rapidly. Recently, scientists have found that the loss of lithium-ion battery with iron oxide electrode is caused by the accumulation of lithium oxide and the decomposition of electrolyte.

The iron oxide electrode used in the research process is made of cheap and nontoxic magnetite. Compared with current electrode materials, conversion electrode materials such as magnetite (i.e. converted to 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 test of magnetite shows that the capacity of magnetite decreases rapidly in the first 10 high-speed charge discharge cycles. " Dong Su, who led the electron microscopy team at the center for functional nanomaterials (CFN), said. CFN is the Department of energy's office of science user facilities, located at Brookhaven National Laboratory.

In order to find out the reason of unstable circulation, scientists tried to observe the change of crystal structure and chemical properties of magnetite when the battery completed 100 cycles. They combined transmission electron microscopy (TEM) and synchronous X-ray absorption spectroscopy (XAS). The electron beam of TEM is transmitted through the sample to produce the structure image or diffraction pattern of the characteristic material. XAS uses X-ray to detect the chemical properties of the material.

Using these techniques, scientists found that during the first discharge, magnetite was completely decomposed into metallic iron nanoparticles and lithium oxide. But in the next charging process, the conversion reaction is not completely reversible, and the residues of iron and lithium oxide still exist. In addition, the original "spinel" structure of magnetite evolves into "rock salt" structure in charged state (in the two 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 material of lithium oxide and metal iron nanoparticles. Because the conversion reaction is not completely reversible, these residual products will gradually accumulate. The scientists also found that the electrolyte, the chemical medium that causes lithium ions to flow between the two electrodes, breaks down in subsequent cycles.

On the basis of the research results, scientists put forward an explanation for the decline of energy storage capacity. Sooyeon Hwang, a scientist and co lead author of CFN electron microscope team, said, "due to the low electronic conductivity of lithium oxide, its accumulation will form a barrier for electrons shuttling between the positive and negative electrodes of the battery, which we call the internal passivation layer. Similarly, the decomposition of electrolyte will also form a surface passivation layer, which hinders ion conduction. These barriers build up and prevent electrons and lithium ions from reaching the active electrode materials where electrochemical reactions take place. "

The scientists point out that running the battery at low current can provide enough time for electronic transmission by slowing down the charging speed and restoring part of the capacity; however, other solutions are needed to completely solve this problem. They think that the capacity attenuation can be improved by adding other elements to the electrode material and changing the electrolyte.