Tailoring ceramics for specific applications: A case study of the development of all-solid-state lithium batteries
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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 Li2−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.
KeywordsCathode Material Negative Electrode Positive Electrode Bulk Conductivity Alkaline Earth Element
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