Method for producing dense lithium lanthanum tantalate lithium-ion conducting ceramics

Abstract

A method to produce high density, uniform lithium lanthanum tantalate lithium-ion conducting ceramics uses small particles that are sintered in a pressureless crucible that limits loss of Li.sub.2O.

Claims

1. A method for producing a lithium lanthanum tantalate ceramic, comprising: dissolving lithium nitrate in an alcohol solvent; dissolving lanthanum acetate in an acid solvent; suspending tantalum oxide in an alcohol; blending the lithium nitrate solution, the lanthanum acetate solution, and the tantalum oxide suspension and evaporating the solvents to provide a stoichiometric mixture; combusting the stoichiometric mixture at a sufficiently high temperature to remove organics, thereby providing an inorganic mixture; calcining the inorganic mixture at a sufficiently high temperature to remove carbonates, thereby providing a mixed oxide powder; and sintering the mixed oxide powder in a closed and non-reactive crucible at a sufficiently high temperature and pressure to provide a dense lithium lanthanum tantalate ceramic.

2. The method of claim 1, wherein the alcohol solvent for dissolving lithium nitrate comprises ethanol.

3. The method of claim 1, wherein the acid solvent for dissolving lanthanum acetate comprises propionic acid.

4. The method of claim 1, wherein the alcohol for suspending tantalum oxide comprises ethanol.

5. The method of claim 1, wherein the sufficiently high temperature for combusting is greater than 500 C.

6. The method of claim 1, wherein the sufficiently high temperature for calcining is greater than 800 C.

7. The method of claim 1, wherein the sufficiently high temperature for sintering is greater than 1000 C.

8. The method of claim 1, wherein the sufficiently high pressure for sintering is ambient pressure.

9. The method of claim 1, wherein the closed and non-reactive crucible comprises a platinum crucible.

10. The method of claim 1, wherein the closed and non-reactive crucible comprises a transition metal crucible.

11. The method of claim 10, wherein the transition metal comprises nickel, platinum, palladium, iridium, or alloys thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The detailed description will refer to the following drawings, wherein like elements are referred to by like numbers.

(2) FIG. 1 is an illustration of a synthesis method for reducing the particle size of ceramic powders to enable the production of dense LLTO ceramics.

(3) FIG. 2 is a scanning electron micrograph of a calcined powder.

(4) FIG. 3 is a schematic illustration of a Li.sub.2O vapor containment crucible for LLTO sintering.

(5) FIG. 4 is a graph of the room temperature impedance behavior of a sintered LLTO ceramic.

(6) FIGS. 5(a) and 5(b) are graphs of the temperature-dependent ionic conductivity of a sintered LLTO ceramic.

DETAILED DESCRIPTION OF THE INVENTION

(7) The invention is directed to a method to produce near full density LLTO ceramics without phase decomposition by pressureless sintering. As shown in FIG. 1, LLTO powders were synthesized from LiNO.sub.3, La(CH.sub.3CO.sub.2).sub.3-xH.sub.2O(La(OAc)), and Ta.sub.2O.sub.5 precursors. As an example of the invention, LiNO.sub.3 and La(OAc) were dissolved in alcohol and propionic acid, respectively, to produce 0.4M LiNO.sub.3 and 1.9M La(OAc) solutions. These solutions were then blended with an alcohol-Ta.sub.2O.sub.5 ceramic suspension in stoichiometric quantities to achieve the target cation ratio of 5Li:3La:2Ta. After the alcohol and propionic were removed via evaporation, the mixture was then heated to achieve slow combustion within a furnace thus removing organics to produce a homogenous mixture of carbonates and oxides. This mixture was then heated in excess of 800 C. to remove carbonates of lanthanum and form the mixed oxides that will eventually form LLTO.

(8) After calcination, the particle size of the synthesized and calcined powder is less than 2 m, as shown in FIG. 2. The particle size can then be further reduced by low energy milling (e.g., ball milling). Prepared powder compacts or green ceramic bodies can then be placed in a closed crucible whose composition is appropriate to resist R.sub.2O diffusion and reaction with R.sub.2O. The powder compacts can then be sintered in this crucible at 1300 C. for 12 hours. With this method, the sealed crucible can produce dense (98.4% relative density) ceramic specimens with only trace minor phases.

(9) A diagram of an exemplary R.sub.2O vapor containment crucible is shown in FIG. 3. This system contains a fully closed (not hermetically sealed) Pt crucible with Pt foil. Other crucibles that are non-reactive with LLTO ceramics and can be used include Ni, Pt, Pd, Ir, other transition group metals, and alloys thereof. LLTO powders around the exterior of the PtPt interfaces prevent Li.sub.2O from leaving the crucible, i.e., Li.sub.2O permeability in Pt is sufficiently low to prevent diffusion out of the crucible. Al.sub.2O.sub.3 crucibles alone are not sufficient to prevent Li.sub.2O volatilization because Li.sub.2O vapor can readily react with Al.sub.2O.sub.3 to form Li.sub.2OAl.sub.2O.sub.3 and Li.sub.2O-5Al.sub.2O.sub.3 intermediate compounds. These intermediate compounds consume volatile Li.sub.2O and the LLTO stoichiometry shifts to Li-deficient compounds. Conversely, oxidation of Pt crucible materials can be controlled by appropriate control of oxygen partial pressure. Any volatile alkali oxides are sufficiently maintained within the closed Pt crucible to maintain stoichiometry. If the volatile oxides are maintained within the closed and non-reactive crucible, the composition and density will be uniform throughout the ceramic body.

(10) FIG. 4 is a plot of the impedance of the sintered ceramic measured at room temperature. The ionic conductivity values (210.sup.5 S/cm) are significantly higher than any LLTO reported in the literature (by 10) and are comparable to Ba-substituted compounds and other fast ion conductors. The conductivity improvement is likely due to the improved density.

(11) FIGS. 5(a) and 5(b) are graphs of the ionic conductivity as a function of temperature. As shown in FIG. 5(a), the ionic conductivity of the ceramic of the present invention (labeled Sandia Li.sub.5La.sub.3Ta.sub.2O.sub.12) surpasses literature reports of LLTO (labeled Thangadurai Li.sub.5La.sub.3Ta.sub.2O.sub.12) and LBLTO (labeled Thangadurai Li.sub.6BaLa.sub.2Ta.sub.2O.sub.12) and exceeds LLZO (labeled Murugan Li.sub.7La.sub.3Zr.sub.2O.sub.12) at 125 C. As shown in FIG. 5(b), the activation energy for Li-conduction is 0.53 eV. This activation energy is slightly higher than that determined for LLZO (E.sub.a=0.46 eV) and compares well with literature LLTO values (E.sub.a=0.56 eV).

(12) The present invention has been described as a method for producing dense lithium lanthanum tantalate lithium-ion conducting ceramics. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.