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Reasons for the decline of lithium battery
Release time：
2022-11-17 15:15
According to the research survey, due to the high energy storage density, materials such as metal oxides, sulfides and fluorides are promising electrode materials for lithium-ion batteries for electric vehicles. However, their energy storage capacity decays quickly. A few days ago, scientists discovered by studying a lithium-ion battery with an iron oxide electrode that the loss of the battery after charging and discharging more than 100 times is caused by the accumulation of lithium oxide and the decomposition of the electrolyte.
The iron oxide electrodes used in the research are made of cheap and non-toxic magnetite. Conversion electrode materials, such as magnetite, which convert to a new product when reacting with lithium, can store more energy than current electrode materials 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 sharply within the first 10 high-speed charge-discharge cycles." Dong Su, who led the study and led the electron microscopy group at the Center for Functional Nanomaterials (CFN), said. CFN is the U.S. Department of Energy's Office of Science User Facility located at Brookhaven National Laboratory.
To find out why the cycling was unstable, the scientists tried to observe how the crystal structure and chemistry of magnetite changed after the battery completed 100 cycles. They conducted their research using a combination of transmission electron microscopy (TEM) and simultaneous X-ray absorption spectroscopy (XAS). While TEM transmits an electron beam through a sample, producing structural images or diffraction patterns that characterize the substance, XAS uses X-rays to probe the chemical properties of materials.
Using these techniques, the scientists found that on the first discharge, the magnetite completely decomposed into metallic iron nanoparticles and lithium oxide. But in the subsequent charging process, this conversion reaction is not completely reversible, and the residues of metallic iron and lithium oxide still exist. In addition, the original "spinel" structure of magnetite evolved into a "rock-salt" structure in the charged state (in the two structures, the positions of the iron atoms are not exactly the same). During subsequent charge-discharge cycles, rock salt iron oxide interacts with lithium to form a composite of lithium oxide and metallic iron nanoparticles. Because the conversion reaction is not fully reversible, these residual products gradually accumulate. The scientists also found that the electrolyte -- the chemical medium that enables lithium ions to flow between the two electrodes -- breaks down during subsequent cycles.
Based on the findings, the scientists propose an explanation for the decline in energy storage capacity. Co-lead author Sooyeon Hwang, a scientist in CFN's Electron Microscopy Group, said, "Because lithium oxide has low electronic conductivity, its accumulation creates a barrier for electrons shuttling between the positive and negative electrodes of the battery, which we call internal passivation. layer. Similarly, electrolyte decomposition can also form a surface passivation layer that hinders ion conduction. These barriers accumulate and prevent electrons and lithium ions from reaching the active electrode material where the electrochemical reaction takes place." Operating the battery at low currents, the scientists noted
, It is possible to restore some of the capacity by slowing down the charging rate to provide sufficient time for electron transport; 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 material and changing the electrolyte.
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