Investigation of Structure and Electrochemical Properties of A5B19-Type La-Mg-Ni System Layered Superlattice Anode Materials in La0.8-xNdxMg0.2Ni3.8
In this paper,the structure and electrochemical characteristics of the annealed La0.8-xNd,Mg0.2Ni3.5(x=0,0.1,0.2,0.3)layered super lattice negative electrodes were investigated.The samples was prepared via vacuum induction melting followed by annealing treatment at 1223 K for 12 h.The crystal structure analysis showed that all alloys were multiphase structures,x=0 alloy contained CaCu5-type structures of LaNi5 phase,rhombohedral Ce5Co19-type(3R)structures of(La,Mg)5Ni19 phase(A5B19 phase).After partial substitution of Nd for La,the hexagonal Pr5Co19-type(2H)structures of(La,Mg)5 Ni19 phase appears,and the abundance of Pr5Co19-type phase increased from 10.76％(x=0.1)to 40.80％(x=0.3)while the abundance of CaCu5-type phase and Ce5Co19-type phase decreased directly from 62.45％(x=0)to 35.69％(x=0.3)and from 37.55％(x=0)to 23.81％(x=0.3)as the Nd content increasing.The content of the A5B19-type phase(3R+2H)was the highest for x=0.3 and more than 50％.The results demonstrate that substituting La for Nd,that had a smaller atomic radius,improved the stability of the A5B19-type layer stacking structure phase and helped to generate A5B19-type phase.Furthermore,the unit cell volumes of various phases fall linearly with increasing x,as predicted by Vegard's equation,owing to the fact that Nd's atomic radius is less than that of La element.At a temperature of 298 K,the electrochemical P-C isotherms of La0.8-xNdxMg0.2Ni3.5(x=0,0.1,0.2,0.3)alloy electrodes were measured.Because the equilibrium pressures of LaNi5 phase and(La,Mg)5Ni19 phase were comparable,the sample electrodes display one plateau.The plateau pressure improved as the x increased for this collection of alloys,having the highest pressure in x=0.3.This was mostly due to the partial substitution of La by the smaller atomic radius Nd,which reduced the lattice properties of various phases,resulting in a larger pressure for the H atom to escape from the alloy.A higher plateau pressure also suggested that the alloy hydrides were less stable.This suggested that adding Nd to the hydride decreased its stability but improved its kinetic performance as an electrode material.Furthermore,when the substitution element Nd increased,the maximum hydrogen storage capacity of alloy electrodes increased from 0.908 H/M(x=0)to 0.953 H/M(x=0.2),which was mainly due to the abundance of A5B19-type phase with higher hydrogen storage in the structure.However,when x increased further,the maximum hydrogen storage capacity decreased to 0.928 H/M(x=0.3),which could be attributed to the decrease in the unit cell volumes of various phases,resulting in the reduction of some interstitial positions in the cell volume and the inability to store hydrogen,thus causing the reduction of the intrinsic hydrogen absorption capacity of the alloy.x=0.2 alloy exhibited a maximum hydrogen storage capacity,higher than the current commercial AB5 type alloy(0.8 H/M).It could be seen from the curve for relationship of the discharge capacity and cycle number of the alloys electrodes measured that the partial substitution Nd element for La element had a little effect on the activation property.All alloys reached the maximum discharge capacity after three or four charging-discharging cycles.The maximum discharge capacities increased from 352.97 mAh·g-1(x=0)to 362.51 mAh·g-1(x=0.1),371.22 mAh·g-1(x=0.2)and then decreased to 360.91 mAh·g-1(x=0.3)as Nd element content increased,which was in well agreement with the hydrogen storage capacity in P-C isotherm.The capacity retention of the sample electrodes from 70.70％(x=0)to 80.52％(x=0.2),then decreased to 72.61％(x=0.3)after one hundred charge/discharge cycles.The high rate dischargeability of the sample electrodes obviously improved when La was substituted by Nd and the sample electrode in x=0.3 showed the best the high rate dischargeability.For example,at the discharge current density of 900 mA·g-1,the high rate dischargeability of La0.8-xNdxMg0.2Ni3.5 sample electrodes was 63.27％(x=0),73.91％(x=0.1),79.21％(x=0.2)and 84.3％(x=0.3).The high rate dischargeability of the hydrogen storage electrode mainly depends on two kinetic factors:the charge transfer reaction at the electrode-electrolyte interface and the hydrogen diffusion reaction in the alloy block,which were closely related to the exchange current density I0 and hydrogen diffusion coefficient D.I0 values obtained from the linear polarization curves of La0.8-x NdxMg0.2Ni3.5 sample electrodes continuously increases from 121.71 mA·g-1(x=0)to 217.92 mA·g-1(x=0.3)with increasing Nd element content.The diffusion coefficient D of the sample electrodes increased from 2.81 cm2·s-1(x=0)to8.72cm2 s-1(x=0.3).This trend of the exchange current density I0 and hydrogen diffusion coefficient D was consistent with the variation of the high rate dischargeability of the sample electrodes,owing to the synergistic effect of the charge-transfer on the sample surface and hydrogen diffusion rates in bulk of sample.
La-Mg-Ni-based A5B19 type super lattice negative materialsNd substitutionelectrochemical properties
NETL
NSTL
万方数据
