The iron oxide electrodes used in the research were made from inexpensive and non-toxic magnetite. Compared to current electrode materials, conversion electrode materials like magnetite (which react with lithium to form entirely new products) can store more energy because they can hold 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 showed that its capacity drops rapidly within the first 10 high-speed charge-discharge cycles, said Dong Su, the lead researcher and head of the electron microscopy group at the Center for Functional Nanomaterials (CFN), the U.S. Department of Energy's Office of Science User Facilities located within Brookhaven National Laboratory.
To find the cause of cycling instability, scientists attempted to observe changes in the crystal structure and chemical properties of magnetite after the battery completed 100 cycles. They combined transmission electron microscopy (TEM) and synchrotron X-ray absorption spectroscopy (XAS) for their study. In TEM, the electron beam is transmitted through the sample, producing structural images or diffraction patterns characteristic of the material, while XAS uses X-rays to probe the chemical properties of the material.
Scientists used these techniques to discover that during the first discharge, magnetite completely decomposes into metallic iron nanoparticles and lithium oxide. However, this conversion reaction is not entirely reversible during subsequent charging, leaving residues of metallic iron and lithium oxide. Furthermore, the original spinel structure of magnetite evolves into a rock salt structure under charged conditions (in which the positions of iron atoms are not exactly the same). In subsequent charge-discharge cycles, the rock salt iron oxide interacts with lithium, forming a composite of lithium oxide and metallic iron nanoparticles. Because the conversion reaction is not entirely reversible, these residual products gradually accumulate. Scientists also found that the electrolyte (the chemical medium that allows lithium ions to flow between the two electrodes) decomposes in subsequent cycles.
Based on the findings, scientists have proposed an explanation for the degradation of energy storage capacity. Sooyeon Hwang, a scientist in the CFN electron microscopy group and co-lead author, explained that because lithium oxide has low electronic conductivity, its accumulation forms a barrier to electrons traveling between the positive and negative electrodes of the battery, which we call an internal passivation layer. Similarly, electrolyte decomposition also forms a surface passivation layer, hindering ion conduction. These barriers accumulate, preventing electrons and lithium ions from reaching the active electrode materials where electrochemical reactions occur.
Scientists point out that operating a battery at low current can restore some capacity by slowing down the charging rate, allowing enough time for electron transport; however, other methods are needed to completely solve this problem. They believe that adding other elements to the electrode material and changing the electrolyte can improve capacity decay.
