The Lithiation Properties Of Magnesium Hydride Anode Materials

High-capacity anode materials are very important for the development of next generation lithium ion batteries with high energy density. As a new type of conversion reaction anode material for lithium ion battery (MgH2+Li++e-(?)LiH+Mg), magnesium hydride (MgH2) has the advantages of low reaction potential (0.52 V vs. Li/Li+), high theoretical specific capacity (2038 mAh/g), and low voltage hysteresis between discharge and charge plateau. However, magnesium hydride is sensitive to the environment, making it difficult to the electrode preparation, and the reversibility and cycle stability are poor. Aiming to circumvent these problems, we investigated the electrode preparing process, and the effect of milling time, adding conductive agent or graphite, doping with metallic powders, and the thickness of thin films on the electrochemical performances of MgH2 powder or film. The main conclusions are drawn as follows:The powder compaction method was used to compact the MgH2 powder into the nickel foam, the result indicated that increasing the milling time and adding with conductive agent effectively improve the activity of MgH2. The discharge plateau of the 20h-milled MgH2 was obviously observed at 0.3 V (vs. Li/Li+), and the first discharge capacity was 762.5 mAh/g, however, it maintained only ca.200 mAh/g after 5 cycles, which mostly comes from nickel foam.We milled the MgH2 with Ni, Cu, Mn and conductive carbon black (SP) or graphite (C) with the mass ratio of 50:40:10 for different times to obtain MgH2/Ni/SP, MgH2/Mn/C, MgH2/Cu/C composites. The results indicates that the first discharge capacity of MgH2/Mn/C milled for 30h was 968.5mAh/g at 50mA/g and the plateau capacity was higher than 700 mAh/g, while the charge capacity was 596.3 mAh/g and the plateau capacity was ca.300 mAh/g, the coulomb efficiency was 58.8%. The results show that MgH2/Mn/C containing 30% graphite (50:20:30) has a first discharge capacity of 959.3 mAh/g, maintaining ca.200 mAh/g after 10 cycles.Mg film was also prepared on the copper foil by magnetron sputtering, which was coated with Pd film to enhance its hydrogen absorption properties. The film was directly used for the preparation of electrode after hydrogenation. It was found that the first discharge of MgH2 film appeared serious polarization phenomenon, which was improved in the subsequent discharge cycles. The plateau capacity of discharge process of MgH2 and Li reaction was ca. 0.3 mAh/cm2 at 25μA/cm2, while the plateau capacity of charge process was about 0.15 mAh/cm2, with the coulombic efficiency of 50%. The capacity and coulomb efficiency of MgH2 film deposited for 120min was better than that deposited for 60 min.To improve the reversibility of MgH2 in the charging process (Mg+LiH→MgH2+Li+) is the key to enhance the performance of MgH2 anode. It is an important way to improve the reversibility of MgH2 by increasing the phase boundaries of LiH/Mg with nano-technology and surface treatment to avoid the formation of passivation layer on the surface of magnesium in the electrolyte.