Metal hydrides for Li-ion battery anodes

The project LiMBAT has investigated the use of metal hydrides as negative electrode (anode) and solid-state electrolyte in Li-ion batteries. In contrast to presently used graphite electrodes, they undergo major chemical changes when the batteries are in use. The benefit is a much higher energy capacity, but the capacity decreases rapidly when the battery is charged and discharged. The capacity loss has been shown to be highly dependent on the microstructure of the electrode material, and one aim of the project was to understand the relationship between microstructure and performance of such anodes. Magnesium hydride, MgH2, was used as model systems to reveal correlations between the microstructure and the electrochemical properties. Materials with a wide range of morphologies were produced by mechanical milling at IFE and characterized by different techniques such as X-ray diffraction at the Swiss-Norwegian Beamlines at the synchrotron ESRF (France) and by electron microscopy at the NORTEM infrastructure (SINTEF, Trondheim). Electrochemical properties, especially the cycling capacity, were investigated at the University of Oslo (UiO) and IFE. The electrochemical capacities of the anodes were found to vary a lot with different microstructures. It was a general trend that longer milling times and thus smaller particles gave better capacity retention. There were additional factors at play which are not yet fully understood. Electrodes of composite materials were also investigated. Cobalt oxide, CoO, is an anode material with low capacity but very good capacity retention on cycling. A very interesting discovery was that electrodes consisting of 25% CoO and 75% MgH2 get the best properties from the two materials; high capacity from MgH2 and good capacity retention on cycling from CoO. It was found that the cobalt in the electrode doesn't revert to CoO after the first cycle, and it is likely that the cobalt is rather present as metallic nanoparticles that catalyze the electrochemical process in MgH2 with a dramatically increased performance. Many lithium-containing borohydrides, e.g. LiBH4, have good ionic conductivities and have therefore been considered as electrolytes in Li-ion batteries. Such solid-state electrolytes will lead to much safer batteries than the present ones based on flammable liquid electrolytes. However, the performance is inferior at room temperature due to significantly lower ionic conductivity compared to liquid electrolytes. As a part of LiMBAT, it was investigated how LiBH4-based solid-state electrolytes work in conjunction with MgH2 anodes. Mixtures of Li(BH4)0.75I0.25 (LI) and amorphous 0.75Li2S+0.25P2S5 (LPS) in different ratios were studied in detail. A mixture with 33 weight% LPS had a Li-ion conductivity of 1 mS/cm at room temperature. This is almost an order of magnitude higher than the other investigated mixtures and is within the usable range for solid-state electrolytes. Computer modeling (DFT) showed that the mobility of Li-ions in LiBH4 increases dramatically in the vicinity of e.g. PS4(3-)-ions from the LPS that are substituted into the crystal structure. A MgH2 anode that was cycled with this solid-state electrolyte showed less capacity loss compared to anodes cycled with liquid electrolyte. The anode material TiS2 was cycled 6 times in a half-cell with the LI-LSP electrolyte and kept 90% of its theoretical capacity which is a very good performance. The LiMBAT project was a joint effort between IFE, UiO and SINTEF. The project was supported by the international partners Hiroshima University (Japan), Sendai University (Japan) and ICMPE, CNRS (France) as well as the national SME Grenland Energy AS. The project has led to 5 papers in international journals and 3 additional papers are under preparation. The work has also been presented at 7 international conferences. Moreover, an international workshop with 24 participants from leading actors in the field from Norway, Japan, the U.S., France and Switzerland has been arranged within the LiMBAT project.

Se resultatrapport

The project concerns the use of metal hydrides in Li-ion batteries as anode materials in conversion type electrodes. Such electrodes have far greater capacity than classical insertion type electrodes, but are hampered by loss in capacity on cycling. The capacity loss has been shown to be highly dependent on the microstructure of the electrode material, but the structure-property relationship has not been systematically investigated. MgH2 and LiH/Mg composites will be used as model systems to understand the relationships between the morphologies and the electrochemical properties. Ball milling at various conditions, including cryogenic temperatures and under hydrogen pressure, will be employed to make materials with a wide range of microstructures. The microstructure will be characterized by various techniques, including powder X-ray diffraction, SEM and small-angle scattering. TEM will be performed using the national infrastructure NORTEM. Electrochemical properties, especially the cycling capacity, will be measured by both galvanostatic and potentionstatic methods using electrochemical half cells. Compositionally more complicated metal hydrides and composites will be prepared and investigated as soon as the microstructure-property relations in the MgH2-based systems are well understood. In-situ PXD, both at SNBL (ESRF) and at the national infrastructure RECX will be conducted to gain fundamental insight in all the steps of the electrochemical processes. Borohydride-based materials with high Li-ion conductivity will be particularly considered due to the possible beneficial effects on the reaction kinetics. The work will be a joint effort between Institute for Energy Technology (IFE), University of Oslo (UiO) and SINTEF, where IFE and UiO will employ a PostDoc fellow each. The project is supported by the international partners Hiroshima University (Japan) and ICMPE, CNRS (France) as well as the national SME Grenland Energy AS.