P1-2865Pulsed-Field Gradient NMR Study of Pyrochlore-Type Oxyfluoride Li1.25La0.58Nb2O6FKey Words:An oxyfluoride compound Li1.25La0.58Nb2O6F (LLNOF) exhibits high ionic conductivity (7×10−3S cm−1). LLNOF has a pyrochlore-type crystal structureLi1.25La0.58□0.17Nb2O6F (LLNOF, x= 0.75)with three-dimensional channels occupied by Li, La, and vacancies. Here, we measured Li+ion diffusion in LLNOF using 7Li pulsed field gradient (PFG) NMR.⚫Fast Li+diffusion is observed in LLNOF.⚫High diffusivity and low Haven ratio is key factor.⚫Activation energy changes from 0.24 to 0.14.⚫19F NMR suggest disordering of Li+sites.ConclusionPFG-NMR of LLNOFT1relaxation and 19F MAS-NMR⚫Study the effect of Ta substitution. ⚫Improve electrochemical stability.⚫Clarify the grain-boundary diffusion mechanism.⚫Identify Li positions using neutron diffraction.Future PlanIntroductionThis study aims to clarify the lithium diffusion mechanism in LLNOF using PFG-NMR. Mobile Li carriers in pyrochlore-type LLNOF were identified by combining 7LiNMR and ionic conductivity measurements. Local and crystal structures were analyzed by 7Li /19F NMR and variable-temperature XRD to reveal the origin of its fast ionic conductivity.Theme underDiscussionSolid State Battery Ionics Group, GREEN Naoaki KuwataE-mail：： KUWATA.Naoaki@nims.go.jpLi1.25La0.58Nb2O6FNbO6FLi/La/vacancyCrystalstructureofpyrochlore-typeLLNOF.Li,Laandvacancy(□)occupytheA-site.[2] G. Hasegawa, N. Kuwata, et al., Chem. Mater. (2023) 35, 3815.•PFG-NMR [2]•JEOL ECA-400, 7Li frequency 155 MHz•JEOL ECA-500, 7Li frequency 194 MHz•Temperature•298 K ～700 K0.00.51.01.52.02.53.010-1100 g = 2 Tm−1 g = 4 Tm−1 g = 8 Tm−1 g = 12 Tm−1Intensity (a. u.)γ 2g 2δ 2(Δ - δ/3) (1011 s m-2) δ = 0.5 ms δ = 1.0 ms δ = 1.5 ms δ = 2.0 msδ-array0.05–3 msg-array0.1–12 Tm−1Δ = 100 ms393 K01234567891010-1100Intensity (a. u.)γ 2g 2δ 2(Δ - δ/3) (1011 s m-2) δ = 0.5 ms δ = 1.0 ms δ = 1.5 ms δ = 2.0 msδ-array0.05–3 msg-array0.1–12 Tm−1 g = 2 Tm−1 g = 4 Tm−1 g = 8 Tm−1 g = 12 Tm−1Δ = 100 ms303 KStimulated echo pulse sequence (g-array)Echo decay plot at 393 K and 303 K7Li PFG-NMR6040200−20−40−60Intensity (a. u.)Chemical shift (ppm)223 K393 K7Li static NMR spectra The7LiNMRofpolycrystallineLLNOFshowedamotional-narrowedsinglepeak.Thediffusioncoefficient(DNMR)wascalculatedfromtheechodecayplottobe1.0×10−7(303K)and3.8×10−8cm2s−1(393K).Stejskal-Tanner equationArrhenius plot of Dσ,bulk, DNMRand Dlocal1.52.02.53.03.54.04.55.010−910−810−710−6 Dσ, bulk DNMR Dlocal: BPP model Dlocal: Modified BPP modelDiffusion coefficient (cm2s-1)1000 / T (K-1)1.52.02.53.03.54.04.50.10.20.30.40.50.60.70.80.91T1 (s)1000/T (K−1) 155 MHz BPP model (155 MHz) 194 MHz BPP model (194 MHz)1.52.02.53.03.54.04.50.10.20.30.40.50.60.70.80.91 155 MHz Modified BPP-model (155 MHz) 194 MHz Modified BPP-model (194 MHz)T1 (s)1000/T (K−1)0.14 eV0.13 eVThe Dσ,bulk, DNMRand Dlocalare in good agreement with low activation energy (0.13-14 eV). Haven ratio (HR≡ DNMR/Dσ) is0.41~0.67. The Li+ions occupies the A site of the pyrochlore structure, causing strong Li+-Li+correlation to diffuse.Dlocal= l2/6τcModified Bloembergen–Purcell–Pound (BPP) modelNernst-Einstein equation19F MAS-NMRThe crystal structure is pyrochlore-type from 10 K to 300 K. At temperatures above 200 K, the lattice parameter increases.In 19F MAS-NMR, the peaks of FLi3La are averaged, suggesting disorder in the Li sites and metastable sites.123456710−1110−1010−910−810−710−610−5 DNMR LLNOF Dσ, bulk LLNOF Dcalc LLNOF DNMR LLTO Dσ, bulk LLTO DNMR sc-LLZTO Dσ, bulk sc-LLZTODiffusion coefficient (cm2s-1)1000 / T (K-1)Arrhenius plots of DNMRand Dσ. The magenta diamonds represent the results calculated by AIMD [3]. For comparison, the diffusion coefficients of perovskite-type LLTO and garnet-type LLZTO are also shown.Diffusion coefficient of oxide solid electrolytes
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