Improvement of Charge Transfer Resistitance of Cathode in Ni-MH Batteries Using Ni/Al-LDH Prepared By Liquid Phase Deposition Method

Much attention has been paid to the layered double hydroxides containing transition-metal elements for various electrochemical devices. Nickel-aluminum layered double hydroxide (Ni-Al LDH) is the promising materials for the positive electrode of Ni-MH battery because of its good electrochemical properties and high stability in strong alkaline keeping the structure of α-Ni(OH)₂. Furthermore, α-Ni(OH)₂ have higher capacity than β-Ni(OH)₂ which is commonly used for the electrode materials of Ni-MH battery, but less stability in strong alkaline. Generally, electrode is prepared by painting colloidal active materials, however, the internal resistance in the electrode is high. Improvement of the adhesion between current collector and active materials contribute to reduce the internal resistance, especially charge transfer resistance (R_ct) of an electrode reaction. Previously, we reported the precipitation of Ni-Al LDH with high crystallinity and high purity by the liquid phase deposition (LPD) method. The LPD method is the syntheses method of metal oxides and hydroxides using the hydrolysis reaction of metal fluorine complexes. The merits of LPD method are easy operation and following capability for substrate surface shape. In this study, the Ni-Al LDHs were directly prepared on the surface of Ni foam as a current collector of Ni-MH battery, and investigated the reduction effect of the R_ct. From the cross-sectional SEM images of the sample, dense Ni-Al LDH was deposited on the Ni foam surface, and the thickness was up to 950 nm. The resistance of charge transfer, R_ct was measured by an ac impedance spectroscopy at 0.5 V vs Hg/HgO.  The apparent activation energy of chage transfer is calculated from Arrhenius plots of the temperature dependence of R_ct values.  The composition of LDH was determinded by ICP-AES spectroscopy for Ni and Al, and neutralization titration for OH, as Ni_0.95Al_0.05(OH)₂·A_0.05 ,where A is exchangable anionic species.  Deposition of Ni-Al LDH on the surface of Ni foam directly reduced the R_ct because of the improving the adhesion between current collectors and active materials as shown in Figure 1. In this case of using the sample where the Ni/Al LDH layer is deposited, the value of R_ct decreased with the thickness of LDH layer on the Ni metal form from ca. 0.8 to 0.2 Ω cm^-2, where those values for the sample using LDH power increase over 0.8 Ω cm^-2.  Furthermore, the activation energy for the electrode reaction decreased from 22.3 to 17.9 kJ mol^-1 . It is suggested that the ionic transport becomes easily in the LDH layer and are coupled electronic conduction from Ni form using deposited Ni/Al LDH on Ni form.  This Ni-Al LDH/ Ni foam composite will be able to provide a secondary battery electrode having high charge transfer efficiency.