Interface modification in solid-state lithium batteries based on garnet-type electrolytes with high ionic conductivity

Abstract The garnet-structure lithium-stuffed solid electrolyte Li7La3Zr2O12 is a promising candidate as lithium-ion conductors for next-generation lithium batteries. We present a comprehensive investigation on the effect of alkaline-earth-metal elements (Ca, Sr, Ba) doping on the structure, mechanical and electrochemical properties in the garnet-type solution Li6.6La3Zr1.6Sb0.4O12. The crystal structure, micromorphology, bending strength, fracture toughness and ionic conductivity of Li6.65La2.95A0.05Zr1.6Sb0.4O12 (A = Ca, Sr, Ba) samples are analyzed systematically by X-ray diffraction (XRD), scanning electron microscopy (SEM), three-point bending method, single edge notch beam and impedance analysis. The results show that alkaline-earth-metal ion substitution may increase the total Li-ion conductivity by promoting ceramic densification in combination with enhancing Li-ion concentration. Among the investigated compounds, Li6.65La2.95Ba0.05Zr1.6Sb0.4O12 samples show excellent performances with a relative density of 96.6%, bending strength of 148.2±2.1 MPa, fracture toughness of 0.94 ± 0.055 MPa•m1/2, Li-ion conductivity of 1.14 × 10-3 S•cm-1 as well as activation energy of 0.361 eV. Additionally, Li/garnet interfacial problems in the solid-state lithium batteries need to be addressed because the poor contact between the garnet electrolyte and Li electrode will cause large interfacial impedance and uneven Li-ion flux during cycling. A methodology is proposed to negate the Li/garnet interfacial impedance by screen-printing the Ag layer on the garnet. A decrease in the interfacial resistance from 986 to 136.7 Ω•cm2 and a stable lithium stripping/platting profile at different current densities are observed, indicating a homogeneous Li-ion flux and appreciable electrochemical performance at Li/garnet interface. The fabricated full batteries with LiFePO4 cathode, modified garnet electrolyte and lithium metal anode show excellent cycling performances under high current density at room temperature.