Conductivity and electrochemical stability of perovskite-structured lithium-strontium-niobium-hafnium-oxide solid Li-ion conductors

摘要

Perovskite-structured Li_(2x-y)Sr_(1-x)Hf_(1- y)Nb_yO_3 (x = 0.75y) solid state electrolytes with various Nb contents y = 0.25, 0.5, 0.75, 0.77 and 0.8 were prepared by conventional solid state reaction method at high temperature. Influence of compositions on structure and ionic conductivity of these perovskite-type ceramic electrolytes was studied. The crystalline structure, cross section micro-structure, ionic conductivity and electronic conductivity were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), AC-impedance spectra and potentiostatic polarization experiment, respectively. All samples present perovskite structure. But impurity phases such as Nb_2O_5 and SrNb_2O_6 were detected. In this solid electrolyte system, a change from tetragonal perovskite structure to cubic perovskite structure was observed as the content of Nb was increased. Among these compositions, Li_(0.37)5Sr_(0.4375)V_(0.1875)Hf_(0.25)Nb_(0.75)O_3 (V = vacancy) have the highest conductivity of σ = 2.91 × 10~(-5) S cm~(-1) at room temperature. Electronic conductivity for Li_(0.375)Sr_(0.4375)V_(0.1875)Hf_(0.25)Nb_(0.75)O_3 is negligible compare to its total conductivity. The interfacial electrochemical stability between Li_(0.375)Sr_(0.4375)V_(0.1875)Hf_(0.25)Nb_(0.75)O_3 solid electrolytes and electrodes was studied via cyclic voltammeter testing. Furthermore, all solid state Li battery Li/LSNH-3/LiNi_(0.5)Mn_(1. 5)O_4 were fabricated, but these batteries are unstable and could not operate. Li_(0.375)Sr_(0.4375)V_(0.1875)Hf_(0.25)Nb_(0.75)O_3 is unstable with metal Li, but it is stable when it is in contact with Li_4Ti_5O_(12) anode and high-voltage LiNi_(0.5)Mn_(1.5)O_4 cathode.