Design And Mechanism Study Of Novel High Capacity Metal Hydride Electrodes

Nowadays,efforts in electrode materials research are underway to improve existing battery technologies and develop new generation of electrochemical devices with high energy density.MH electrodes,which have been extensively investigated for decades,are the linchpin in the development of several practical electrochemical energy storage technologies.Much of current research in metal hydride(MH)electrodes follows the principle that the materials should be composed of hydrogen-absorbing elements(A)and non-hydrogen-absorbing elements(B).The non-hydrogen-absorbing elements,which allows the adjustment of the metal-hydrogen bond strength and provide catalytic properties in electrochemical process,are generally regarded as indispensable for MH electrode.However,the classical A-B type MH electrodes edge toward their theoretical capacity limits(typically<400 mAh g-1)due to the heavy masses of B elements.Herein,we demonstrate a fundamentally new strategy to design high capacity MH electrodes without using B elements.If an intermetallic is stable with respect to its constituent elements,the hydrides can cycle between the hydrides and the intermetallic instead of the elemental metals.The formation of intermetallic can be used for tailoring Gibbs free energy and inducing synergism in dehydrogenation process.Convincing experimental and theoretical analysis of hydrogenation(charge)and dehydrogenation(discharge)of Mg24Y5 electrodes was presented.The present work provides a unique example of high performance MH electrode against the classic design principle and opens novel opportunities for the revival of Ni-MH batteries,and the emergence of next generation MH-air batteries.Primary conclusions are listed as follow:(1)Combined studies by PCI measurements and CALPHAD technique are carried out to provide more quantitative understanding on the thermodynamics of the rare earth-hydrogen(RE-H)binary systems.The calculated phase equilibria and thermodynamic quantities agree rather well with experimental data.The thermodynamic database built by CALPHAD technique offer the possibility to design new hydrogen storage materials.(2)High reversible electrochemical hydrogen storage capacity of～1500 mAh g-1(equivalent to 57.7 H/Mg24Y5 or 5.6 wt%)can be obtained in Mg24Ys thin film electrodes through a fundamentally new strategy.All the absorbed hydrogen can be reversibly desorbed and restore the Mg24Y5 phase by altering the reaction pathways of dehydrogenation.Full hydrogenation of Mg24Y5 thin film electrodes leads to formation of 24MgH2·5YH3 composite and all the hydrogen can be reversibly desorbed to restore the Mg24Y5 phase through a stepwise dehydrogenation mechanism.(3)A single step dehydrogenation of all the 5 8H in the 24MgH2·5 YH2 composite is observed,which liberates the two extremely stable H in YH2 through thermodynamic synergism.The synergism in dehydrogenation process can overcome the kinetic and thermodynamic barriers,leading to exceptional capacity and fully reversible hydriding/dehydriding in Mg24Y5 thin film electrodes.Energy density calculations show that high capacity Mg24Y5 electrodes may open the door for future development of Ni-MH batteries and the emergence of next generation MH-air batteries.