Observation of Elemental Inhomogeneity and Its Impact on Ionic Conductivity in Li‐Conducting Garnets Prepared with Different Synthesis Methods

Abstract

Garnet-type lithium lanthanum zirconate (Li 7 La 3 Zr 2 O 12 [LLZO]) and its doped derivatives continue to generate interest as solid electrolytes for future solid-state lithium batteries. Most Li-conducting garnets adopt the cubic garnet crystal structure (Ia3d). [1] However, without extrinsic dopants, LLZO adopts a tetragonal structure (I4 1 /acd) due to its uniquely high Li content, which causes a spontaneous ordering of the Li sublattice. [2][3][4] This thermodynamically favorable (at room temperature) tetragonal phase has lower ionic conductivity (%10 À6 S cm À1 at room temperature) compared with the cubic phase (%10 À4 -10 À3 S cm À1 ). [1,5,6] For this reason, an aliovalent dopant is needed to introduce Li vacancies to stabilize the highly conducting cubic phase. [1,3,[7][8][9][10] Many doping schemes have been explored, including Al 3þ [6,7,11] and Ga 3þ [7,12] to dope the Li sublattice, Nb 5þ [13,14] and Ta 5þ [7][8][9][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] to dope the Zr sublattice, Ca 2þ [22,29] and other alkaline earths such as Ba 2þ [30] to dope the La sublattice (often used in conjunction with another site dopant), and even recently F À[31-33] to dope the oxygen sublattice. Of the many possible compositions of LLZO, Ta-doped LLZO (LLZTO) with formula Li 7Àx La 3 Zr 2Àx Ta x O 12 (0.2 < x < 1) combines good electrochemical stability with lithium metal [14] and high ionic conductivity (>1 mS cm À1 ) [23,34,35] and, by virtue of doping the Zr sites in the garnet structure, does not block sites on the Li sublattice [7] unlike other dopants such as Al or Ga.Li-conducting garnets are generally synthesized via conventional solid-state reaction (SSR) methods that require high reaction temperatures (in excess of 900 C) and long reaction times (usually in excess of 8 h) (Scheme 1a). [10,36] Initial blending of oxide precursors is typically accomplished via ball milling for an extended time for mixing, particle size reduction, and to minimize the diffusion distance between individual atomic species. [10,36,37] After the synthesis, further high-energy ball milling is often required to reduce the particle size of the resultant coarse powder to confer greater sinterability. [20] In many cases, multiple milling and calcination steps are used to ensure full intermixing of atomic species. [10] Each of these steps has a large time cost in addition to requiring substantial thermal and

Lithium Dendrite Propagation in Ta-Doped Li7La3Zr2O12 (LLZTO): Comparison of Reactively Sintered Pyrochlore-to-Garnet vs LLZTO by Solid-State Reaction and Conventional Sintering

“…Second, the deleterious effects of compositional inhomogeneity may be mitigated via the P2G process. Our previous studies on LLZTO showed that inhomogeneous distribution of the Ta-dopant among various grains can lead to lower ionic conductivity and that the P2G method is one approach to mitigate composition inhomogeneity . Not only is compositional uniformity important for achieving high ionic conductivity and stabilization of the cubic phase of LLZTO, but since Zr has been demonstrated to be less stable to reduction by Li metal compared to Ta, , grains with local Zr enrichment could be susceptible to the formation of electronically conducting pathways.…”

Processing and Properties of Garnet‐Type Li7La3Zr2O12 Ceramic Electrolytes

“…Chan et al studied MSS systematically in terms of molten salt type, basicity, and washing effect. [35,[95][96][97]114] As shown in Figure 8b, during synthesis, LaOCl was first formed at ≈500 °C and it gradually transformed into La 2 Zr 2 O 7 nanocrystal as temperature increased. Finally, cubic LLZO powder with fine particle sizes of 0.3-3 µm was formed at 900 °C.…”

Synergized Tricomponent All‐Inorganics Solid Electrolyte for Highly Stable Solid‐State Li‐Ion Batteries

“…[ 4 ] In particular, Ta‐doped lithium lanthanum zirconate garnet, Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO) delivers a high ionic conductivity up to 10 −3 S cm −1 at room temperature, comparable to the values of organic liquid electrolytes used in commercial Li‐ion batteries. [ 5 , 6 , 7 ] With a high shear modulus (≈60 GPa), however, the garnet‐type oxides are typically stiff and very difficult to process. [ 8 ] It is necessary to densify LLZTO pellets to achieve high ionic conductivity and to function as an effective separator, which typically requires high‐temperature sintering treatment (≈1200 °C, 24 h).…”