Investigating the Local Environment of Li in the Fast-Ion Conductor LLZ Using Molecular Dynamics to Understand Diffusion Mechanics and Phase Transition Phenomena Authors: Matthew Klenk; Wei Lai Introduction • • • • Garnet series Li7-xLa3Zr2-x,TaxO12 LLZ (x=0) composition Fast-ion conducting solid Promising material to replace volatile liquids in secondary Li ion batteries • Mechanics of Li diffusion and origin of phase transition are still debated Tetrahedral (Td) Observing Diffusion events Li Probability Density Function Tetragonal (300 K) Td-16e Oh-16f Tetragonal (500 K) Td-16e Oh-32g Oh-32g Td-8a Tetragonal (800 K) Octahedral (Oh) Bottleneck Cubic (1100 K) • At low Temps Lithium almost exclusively at Td-8a, Oh-32g and Oh-16f • As temp increases density starts to show on Td-16e sites • Li density is not complete until phase change to cubic about 900 K Origins of Phase Transformation • At low temperatures diffusion events are rare and rattling is observed • In the two clusters shown Oh lithium minimize their interactions by avoiding occupied Td sites or surrounding an empty Td • At high temperatures diffusion is observed • Td-8a Li push Oh-32 lithium to Td-16e site • Diffusion is highly coordinated and Li typically move in pairs Li Diffusion Pathway Through Bottleneck Oh Oh Td Oh Cage Occupancy 0.36 1.0 Oh-32g (500 K) 0.32 Td-8a (500 K) 0.8 Oh-16f (500 K) 0.24 0.6 2 MSD (Å ) Lattice Parameter and Phase Transition 0.20 0.16 Oh-32g (300 K) 0.12 13.2 Cubic Td-8a (300 K) 0.04 0 100 13.1 200 300 Time (ps) 400 0.0 200 400 600 800 1000 1200 1400 Temperature (K) 500 Oh Clusters Td Clusters 13.0 T10 T13 T00 T03 160 12.9 140 T11 T14 T01 T04 300 T12 T02 12.6 0 300 600 900 1200 1500 Temperature (K) 150 80 60 100 40 50 20 0 300 Conductivity MD AIMD (Miara) AIMD (Jalem) MD (Adams) 0 10 T12 T13 T14 600 900 1200 1500 Temperature (K) O10 O11 O12 0.10 -2 0.08 T00 T01 T02 T03 T04 O00 O01 O02  -1 10 0.06 -3 10 -4 0.5 Expt (Matsui) MD-corrected 1.0   1  zie 2 Li7La3Zr2O12 0.04 1.5 2.0 -1 1000/T (K ) Dc / (kBT ) 2.5 0.025 0.000 0.0 0.4 0.8 1.2 1.6 On-face-Li-to-edge Distance (Å) • Using molecular dynamics to probe to the local environment of Li we were able to predict phase transformation phenomena and conductivity values close to those reported in literature using diffraction and impedance spectroscopy • The phase change from tetragonal to cubic of LLZ is initiated by an order-disorder transition on lithium sites • We are continuing our work to investigate intermediary compositions on Li garnet. References Li5La3Ta2O12 10  (S/cm) T11 300 0.12 -1 10 T10 0 600 900 1200 1500 Temperature (K) 0.050 • Histograms of distance to nearest oxygen (top) and edge of tetrahedral (bot) • Normal distribution suggests that Li diffuses through the center of the Bottleneck • This counters claims of an edge pass mechanism suggested by DFT simulations Summary Number of Clusters Number of Clusters MD MD (psedocubic) ND (Wang) ND (Awaka) O12 O02 200 100 12.7 O11 O01 250 120 12.8 O10 O00 1.2 1.5 1.8 2.1 2.4 2.7 On-face-Li-to-oxygen Distance (Å) 0.075 0.2 0.08 Expt (Matsui) Lattice Parameters (Å) • Simulations predict a phase transition at 900 K • Lattice parameters and transition temp are in agreement with experimental results • First simulations to predict this transition using MD Tetragonal Td-8a Td-16e Oh-16f Oh-32g 0.4 Oh-16f (300 K) 0.05 0.00 Cage occupancy 0.28 Fraction Td 1100 K (MD) Fraction Atomic MSD Oh 0.10 200 400 600 800 1000 1200 1400 Temperature (K)     /  ln N T ,V / (kBT )  N /  • Conductivity (left) was calculated by applying thermodynamic factor (right) to correct for not being Ideal solution. Can be used as a proxy for coordinated diffusion. 2 N • Phase transformation is coupled with order-disorder transition on Li Td and Oh sites. • Order-disorder transformation is initiated by Td-8a lithium and Oh-32g lithium simultaneously diffusing to occupy Td-16e • The degree of Oh only diffusion is low at lower temperatures suggesting that Td-8a lithium are the most unstable and initiate diffusion 1. Matsui, M.; Sakamoto, K.; Takahashi, K.; Hirano, A.; Takeda, Y.; Yamamoto, O.; Imanishi, N., Phase transformation of the garnet structured lithium ion conductor: Li7La3Zr2O12. Solid State Ionics 2014, 262, 155-159. 2. Wang, Y.; Huq, A.; Lai, W., Insight into lithium distribution in lithium-stuffed garnet oxides through neutron diffraction and atomistic simulation: Li7-xLa3Zr2-xTaxO12 (x=0-2) series. Solid State Ionics 2014, 3. Miara, L. J.; Ong, S. P.; Mo, Y.; Richards, W. D.; Park, Y.; Lee, J.-M.; Lee, H. S.; Ceder, G., Effect of Rb and Ta Doping on the Ionic Conductivity and Stability of the Garnet Li7+2x-y(La3-xRbx)(Zr2-yTay)O-12 (0 <= x <= 0.375, 0 <= y <= 1) Superionic Conductor: A First Principles Investigation. Chemistry of Materials 2013, 25 (15), 3048-3055. 4. Adams, S.; Rao, R. P., Ion transport and phase transition in Li7-xLa3(Zr2-xMx)O-12 (M = Ta5+, Nb5+, x=0, 0.25). Journal of Materials Chemistry 2012, 22 (4), 1426-1434. 5. Jalem, R.; Yamamoto, Y.; Shiiba, H.; Nakayama, M.; Munakata, H.; Kasuga, T.; Kanamura, K., Concerted Migration Mechanism in the Li Ion Dynamics of Garnet-Type Li7La3Zr2O12. Chemistry of Materials 2013, 25 (3), 425-430. Acknowledgements This work is supported by the Ceramics Program of National Science Foundation (DMR-1206356). All simulations were performed on HPCC resources