The researchers, working at various U.S. Department of Energy light source facilities as well as at Cambridge and Stony Brook universities, chose ruthenium oxide (RuO2) as a model system to study these so-called ‘conversion materials, named because they undergo large structural changes when reacting with lithium ions, reversibly forming metal nanoparticles and salts (here Ru and Li2O). The reactions are different from those that occur in conventional electrodes, which store charge by allowing Li ions to nestle into spaces within the crystal lattice.
Our investigation identified the source of the additional capacity found for RuO2, and has also yielded a protocol for studying the ‘passivation layer’ that forms on battery electrodes, which protects the electrolyte from undergoing further decomposition reactions in subsequent charge-discharge cycles, said the study’s corresponding researcher, Clare Grey, a professor in the chemistry departments at Cambridge and Stony Brook universities. Understanding the structures of these passivation layers is key to making batteries that last long enough for use in applications such as transportation and power-grid storage.
At Brookhaven National Laboratorys National Synchrotron Light Source, the team studied their samples using x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS). At the Advanced Photon Source at Argonne National Laboratory, they used two additional techniques, high-resolution x-ray diffraction (XRD) and scattering pair distribution function (PDF) analysis, to extract information on the electronic and long/short-range structural changes of the RuO2 electrode in real time as the battery was discharged and charged. Using these methods, the team showed that RuO2 was reduced to Ru nanoparticles and Li2O via the formation of intermediate phases, LixRuO2.
Because the results did not explain the source of the additional charge-storage mechanism, the group used another technique, high-resolution solid-state nuclear magnetic resonance (NMR). The method involves subjecting a sample to a magnetic field and measuring the response of the nuclei within the sample. It can yield specific information on the chemical compositions and local structures, and is particularly useful for studying compounds that contain only ‘light’ elements, such as hydrogen (H), Li, and oxygen (O), which are difficult to detect using XRD. The NMR data showed that the major contributor to the capacity is the formation of LiOH, which reversibly converts to Li2O and LiH. Minor contributors to the capacity come from Li storage on the Ru nanoparticle surfaces, forming a LixRu alloy, and the decomposition of the electrolyte. The latter, however, ultimately causes the capacity to diminish and will result in the death of the battery following multiple charge cycles.
Scientists from the University of Cambridge, Brookhaven National Laboratory, Argonne National Laboratory, and Stony Brook University conducted the research.
Source: http://www.electronics-eetimes.com/en/study-pinpoints-how-to-make-li-ion-batteries-last-longer.html?cmp_id=7&news_id=222921889&page=1
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