Tailoring ceramics for specific applications: A case study of the development of all-solid-state lithium batteries

Metal oxides with the nominal chemical compositions Li₅La₃M₂O₁₂ (M=Nb, Ta), possessing a garnet-like structure, exhibit ionic bulk conductivities of the order of magnitude of ～10^?6 S/cm at 25 °C. Partial substitution of La by alkaline earth elements (Ca, Sr, Ba) in Li₅La₃M₂O₁₂ yields new members of compounds with garnet-like structure with the composition Li₆ALa₂M₂O₁₂ (A=Ca, Sr, Ba). Among the investigated compounds, so far, the Ba-compound Li₆BaLa₂Ta₂O₁₂ exhibits the highest bulk conductivity of 4.0×10^?5 S/cm at 22 °C with an activation energy of 0.40 eV. All Ta-members were found to be stable against chemical reaction with molten elemental lithium. Li₆ALa₂Ta₂O₁₂ (A=Sr, Ba) exhibit also high electrochemical stability of ～6 V vs. lithium and chemical stability against reaction with LiCoO₂ cathode material. A novel high voltage thin-film battery was constructed using spinel-type Li₂MMn₃O₈ (M=Co, Fe) as positive electrode, LiPON as electrolyte and Al as negative electrode material. Li₂MMn₃O₈ (M=Fe, Co) electrodes show two reversible plateaus during the charging and discharging cycle at ～4 and ～5 V vs. Li. The former plateau is due to the valence change of Mn³⁺ to Mn⁴⁺ and the latter one is due to the oxidation of M³⁺ to M⁴⁺. The chemical diffusion coefficient ( $\tilde D$ ) was found to be in the range 10^?13–10^?12 cm²/sec for any composition x of Li_2?xMMn₃O₈ (M=Fe, Co) in the range from 0.1 to 1.6 by employing the galvanostatic intermittent titration technique (GITT). AC impedance studies revealed an electrolyte-electrode charge transfer resistance of 260–290 Θ and an electrode double layer capacity of ～45–70 µF for an electrode area of 6.7 cm² at room temperature. The chemical diffusion coefficient of the Al,LiAl negative electrode is about three orders of magnitude higher than that of the positive electrode materials. Accordingly, we believe that the diffusion of Li into and out of the cathode material is the rate determining process.