相组成对La_(0.75)Mg_(0.25)Ni_(3.5)储氢合金电化学性能的影响

Effects of Phase Composition on Electrochemical Properties of La_(0.75)Mg_(0.25)Ni_(3.5) Hydrogen Storage Alloy

La-Mg-Ni基超晶格储氢合金与传统AB 5型合金相比,具有优越的电化学容量,近年来受到研究者们的广泛关注,但是其合金相组成复杂,综合电化学性能尤其是循环稳定性有待提高。本工作通过将A 2B 7型La_(0.75)Mg_(0.25)Ni_(3.5)合金在1203 K下退火40 h,制备了只含有(La,Mg)2Ni_(7)和(La,Mg)5Ni 19超晶格结构的合金,并研究了相转变对合金电化学性能的影响。研究表明,铸态合金含有(La,Mg)Ni_(3)、(La,Mg)2Ni_(7)和(La,Mg)5Ni_(9)超晶格相以及LaNi_(5)相,在退火过程中,位于相图相对边缘位置的LaNi_(5)相和(La,Mg)Ni_(3)相消失,得到了只含有(La,Mg)2Ni_(7)和(La,Mg)5Ni_(9)超晶格的双相合金。超晶格相总量的增加伴随着元素组成的均匀化和应力的降低,这不但有效提高了合金的储氢容量,而且减轻合金在充/放电循环过程中的粉化和氧化,显著提高合金电极的循环稳定性。合金电极的最大放电容量由铸态的355 mAh·g^(-1)提高到退火态的367 mAh·g^(-1),100周的循环稳定性由57.97%显著提高到81.47%。但是由于合金中的缺陷和晶界的减少,合金的高倍率放电性能有所降低。

Compared with traditional AB 5-type alloys,La-Mg-Ni-based superlattice hydrogen storage alloys have superior electrochemical capacity,which has attracted extensive attention in recent years.In this work,an A 2B 7-type La_(0.75)Mg_(0.25)Ni_(3.5)alloy containing only(La,Mg)2N i7 and(La,Mg)5Ni 19 superlattice structure was prepared by annealing the as-cast alloy at 1203 K for 40 h,and the effects of phase transformation on the electrochemical properties of the alloy has been studied.The results showed that the as-cast alloy contained(La,Mg)Ni_(3),(La,Mg)2Ni_(7)and(La,Mg)5Ni_(9)superlattice phases and LaNi_(5)phase.During annealing process,LaNi_(5)and(La,Mg)Ni_(3)phases at the relative edge of the phase diagram disappeared,resulting in the alloy containing only(La,Mg)2Ni_(7)and(La,Mg)5Ni_(9)superlattice phases.The increase of the superlattice phase amount was accompanied by the homogenization of the element composition and the reduction of stress,which not only effectively improved the hydrogen storage capacity of the alloy,but also reduced the pulverization and oxidation during charge/discharge cycling,thus significantly improving the cycling stability of the alloy electrode.The maximum discharge capacity of the alloy electrode increased from 355 mAh·g^(-1)of the as-cast alloy to 367 mAh·g^(-1)of the annealed alloy,and the cyclic stability significantly increased from 57.97%to 81.47%.However,because of the reduction of the defects and grain boundaries in the alloy,the high-rate discharge capacity of the alloy was slightly reduced.