Study finds causes of battery aaa alkaline degradation: accumulation of lithium oxide and electrolyte decomposition

　　Materials such as metal oxides, sulfides, and fluorides are ideal electrode materials for lithium-ion batteries for electric vehicles and other technologies due to their high energy storage density. However, their energy storage capabilities degrade rapidly. Now, researchers have proposed an electrode made from the cheap, nontoxic iron oxide material magnetite — described online May 20 in Nature Communications — that could explain why lithium batteries will decline.

　　"Magnetite, as well as other conversion electrode materials (materials that convert into completely new products when reacting with lithium), can store more energy than current electrode materials because they can hold more lithium ions," said the study leader Leader of the Electron Microscopy Group (CFN) at the Center for Functional Nanomaterials - U.S. Department of Energy (DOE) Office of Science User Facility at Brookhaven National Laboratory. "However, the capacity of these materials decreases very quickly and is dependent on current density. For example, our electrochemical tests on magnetite showed that the capacity of magnetite decreases rapidly within the first 10 high-speed charge and discharge cycles."

　　To find out what's behind this poor cycling stability, the scientists described how the crystal structure and chemical properties of magnetite evolve as the battery completes 100 cycles. In these characterization studies, they combined transmission electron microscopy (TEM) of CFN with synchrotron X-ray absorption spectroscopy (XAS) at the Advanced Photon Source (APS) at the DOE Office of Science Users at Argonne National Laboratory. In a transmission electron microscope (TEM), an electron beam is transmitted through a sample to produce an image or diffraction pattern that is characteristic of the material's structure; XAS uses an X-ray beam to probe the chemical properties of the material.

　　Using these techniques, the scientists found that magnetite completely decomposed into metallic iron nanoparticles and lithium oxide during the first discharge. In the subsequent reaction, this conversion reaction is not completely reversible - residues of metallic iron and lithium oxide remain. Furthermore, the original "spinel" structure of magnetite evolves into a "rock salt" structure in the charged state (the positions of the iron atoms in the two structures are not exactly the same). With subsequent charge and discharge cycles, the rock salt iron oxide interacts with the lithium to form a complex of lithium oxide and metallic iron nanoparticles. Since the conversion reaction is not completely reversible, these residual products 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.

　　Su said: "Our real-time transmission electron microscopy studies in ultrahigh vacuum allowed us to understand how the structure of rock salt iron oxide changes with the introduction of lithium after the initial cycle. This study uniquely represents In situ lithification of pre-cycled samples. Previous in situ studies have only looked at initial charge-discharge cycles. However, we need to know what happens over many cycles to design longer-lasting batteries because the structure of the charging electrode is different from that of the original The status is different.”

　　Based on their findings, the scientists proposed an explanation for the decline in ability.

　　"Because lithium oxide has low electronic conductivity, its accumulation can create a barrier for electrons shuttling back and forth between the positive and negative electrodes of the battery," explained co-lead author Sooyeon Hwang, a scientist in CFN's electron microscopy group. "We call this an internal passivation layer. Likewise, electrolyte decomposition blocks ion conduction by forming a surface passivation layer. The accumulation of these obstacles prevents electrons and lithium ions from reaching the active electrode material where electrochemical reactions occur."

　　The scientists note that operating the battery at low currents can restore some capacity by slowing down charging, allowing enough time for electron transfer; however, other solutions will ultimately be needed to address this issue. They believe that capacity fading can be improved by adding other elements to the electrode materials and changing the electrolyte.

　　"The knowledge we gain can generally be applied to other transformation compounds that face the same internal and external passivation layer issues," said Zhongwei Chen, a professor at the University of Waterloo in Canada. "We hope this research will help guide the future." Fundamental research into these promising switching electrode materials. "

　　The team consists of scientists from Brookhaven Laboratory's CFN and Sustainable Energy Technologies Division, Argonne National Laboratory's APS, the University of Pennsylvania, and the University of Waterloo in Canada. This research was supported by the Office of the Department of Energy.

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