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 2 O.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/110,834, filed Feb. 2, 2015, which is incorporated herein by reference. 
     
    
     STATEMENT OF GOVERNMENT INTEREST 
       [0002]    This invention was made with Government support under contract no. DE-AC04-94AL85000 awarded by the U. S. Department of Energy to Sandia Corporation. The Government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION 
       [0003]    The present invention relates to lithium ion conducting ceramics and, in particular, to an improved method for producing dense lithium lanthanum tantalite lithium ion conducting ceramics. 
       BACKGROUND OF THE INVENTION 
       [0004]    Lithium lanthanum tantalate (LLTO) ceramics exhibit broad technological significance as lithium-ion conductors, with particular relevance for applications such as lithium-ion battery systems. More recently, LLTO ceramics have been identified as useful materials for nuclear waste separation and fuel reprocessing due to a difference in Li and Cs diffusivity. See U.S. Provisional application Ser. No. 14/660,696, filed Mar. 17, 2015, which is incorporated herein by reference. For optimal ion conductivity and robust structural and chemical stability, however, lithium-ion conducting ceramics must be highly dense with no interconnected porosity. High density ceramics can also exhibit higher total conductivity due to their apparent lack of volumetric pore dilution. 
         [0005]    Pressureless sintering is an important processing requirement for production of low cost ion conductors. Pressure assisted densification (e.g., hot pressing), although often an effective means of achieving high density ceramics, is generally considered a batch-type process that requires significant capital investment and recurring tooling costs. Moreover, it can be difficult or impossible to create contoured ceramic geometries (e.g., tubes) using hot pressing. As a result, processing methods that enable pressureless sintering are often favored over hot isostatic pressing or hot pressing. 
         [0006]    LLTO is an alkali containing ceramic, and alkali ceramics are particularly difficult to sinter to full density for several reasons. The Li 2 O—La 2 O 3 —Ta 2 O 5  phase diagram is very complex, making LLTO processing difficult. See K. Hayashi et al.,  Mat. Res. Bull.  21(3), (1986). Further, alkali oxides (R 2 O, e.g., Li 2 O) exhibit significant vapor pressures at common sintering temperatures, leading to significant reagent loss to volatility. See R. H. Lamoreaux and D. L. Hildenbrand,  J. Phys. Chem. Ref. Data  13(1), (1984). Common sintering temperatures for alkali-based ceramics are frequently above 1000° C. which exacerbates the effects of R 2 O volatility. Alkali-based ceramic particles are also prone to coarsening due in part to R 2 O volatility as well as the propensity to form liquid phases during calcination. If the alkali is allowed to escape from the atmosphere surrounding the ceramic, a typical microstructure will exhibit a porous outer rim and semi-dense interior. Therefore, the composition of the sintered ceramic depends on location within the specimen and will be alkali deficient near the outermost regions of the sintered ceramic. Finally, Li 2 O(g) is reactive with most oxide kiln hardware in which many sintering studies are conducted. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention is directed to a new method to produce Li 5 La 3 Ta 2 O 12  (LLTO) lithium-ion conducting ceramics. To achieve high density, uniform LLTO ceramics with pressureless sintering, powders with small particle sizes are first produced and then sintered in an environment that limits loss of R 2 O. Using the hybrid solid-state synthesis method, near full density and phase pure LLTO ceramics can be produced. 
         [0008]    As an example of the present invention, LLTO powders were first synthesized from tantalum oxide (Ta 2 O 5 ), lanthanum acetate (La(OAc)), and lithium nitrate (LiNO 3 ). LiNO 3  was first dissolved in ethanol while La(OAc) was dissolved in propionic acid under mild heating. A ceramic suspension of Ta 2 O 5  in ethanol was then blended with the LiNO 3  and La(OAc) solutions and reduced in volume by evaporation of the solvents. The resulting precursor was combusted at ˜500° C. to remove the acetate ion and remaining propionic acid and then calcined above 800° C. to remove carbonates. The calcination step resulted in a mixed phase powder that can then be used to reactively sinter LLTO ceramics to &gt;95% of the theoretical density (6.334 g/cm 3 ). These ceramics exhibit room temperature conductivities of 2×10 −5  S/cm which is about 10× higher than previously reported values. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The detailed description will refer to the following drawings, wherein like elements are referred to by like numbers. 
           [0010]      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. 
           [0011]      FIG. 2  is a scanning electron micrograph of a calcined powder. 
           [0012]      FIG. 3  is a schematic illustration of a Li 2 O vapor containment crucible for LLTO sintering. 
           [0013]      FIG. 4  is a graph of the room temperature impedance behavior of a sintered LLTO ceramic. 
           [0014]      FIGS. 5( a ) and 5( b )  are graphs of the temperature-dependent ionic conductivity of a sintered LLTO ceramic. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    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 3 , La(CH 3 CO 2 ) 3 -xH 2 O(La(OAc)), and Ta 2 O 5  precursors. As an example of the invention, LiNO 3  and La(OAc) were dissolved in alcohol and propionic acid, respectively, to produce ˜0.4M LiNO 3  and 1.9M La(OAc) solutions. These solutions were then blended with an alcohol-Ta 2 O 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. 
         [0016]    After calcination, the particle size of the synthesized and calcined powder is less than 2 μm, 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 2 O diffusion and reaction with R 2 O. 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. 
         [0017]    A diagram of an exemplary R 2 O 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 Pt—Pt interfaces prevent Li 2 O from leaving the crucible, i.e., Li 2 O permeability in Pt is sufficiently low to prevent diffusion out of the crucible. Al 2 O 3  crucibles alone are not sufficient to prevent Li 2 O volatilization because Li 2 O vapor can readily react with Al 2 O 3  to form Li 2 O—Al 2 O 3  and Li 2 O-5Al 2 O 3  intermediate compounds. These intermediate compounds consume volatile Li 2 O 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. 
         [0018]      FIG. 4  is a plot of the impedance of the sintered ceramic measured at room temperature. The ionic conductivity values (˜2×10 −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. 
         [0019]      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 5 La 3 Ta 2 O 12 ”) surpasses literature reports of LLTO (labeled “Thangadurai Li 5 La 3 Ta 2 O 12 ) and LBLTO (labeled “Thangadurai Li 6 BaLa 2 Ta 2 O 12 ) and exceeds LLZO (labeled “Murugan Li 7 La 3 Zr 2 O 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 a =0.46 eV) and compares well with literature LLTO values (E a =0.56 eV). 
         [0020]    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.