Electrode thickness dependence of charge–discharge performance and reaction distribution of an in-situ-formed solid electrolyte for MgH2 anodes

MgH2 conversion anodes are considered promising anode active materials for use in lithium batteries because of their high theoretical lithium storage capacity and suitable redox potential. In a previous study, MgH2 was clarified to function as an anode material that forms a solid electrolyte in situ and to work without an electrolyte added to the composite electrode because of the in situ formation of LiH with an ionic conductivity of 10−7 S cm−1 at 120°C. In the present study, to improve the energy density, cells with thick MgH2 electrodes were prepared and the reaction mechanism was investigated. With increasing thickness of the MgH2 electrode from 84 to 420 μm, the capacity decreased from 1628 to 425 mAh g−1 and the initial lithiated areal capacity was constant among cells with electrodes thicker than 140 μm. In addition, the unreacted MgH2 was found to be mainly distributed near the current collector by ex situ XPS (X-Ray Photoelectron Spectroscopy) measurement. This result differs substantially from that observed for a thick Mg(BH4)2 anode, which can self-generate LiBH4 with a high ionic conductivity of 10−3 S cm−1. The results of the present study indicate that the ionic conductivity of the in-situ-formed electrolyte strongly influences the relationship between battery performance and electrode thickness of an anode that forms an electrolyte in situ.