Li ion diffusion mechanism in the crystalline electrolyte γ-li 3 PO 4

Transcription

1 Li ion diffusion mechnism in the crystlline electrolyte γ-li 3 Yojun Du nd N. A. W. Holzwrth The structure of thin film bttery 3 Solid stte electrolyte could be mde very thin to overcome to the low ionconductivity. Such s LiPON (Li 3 ) LiPON electrolyte bsed on Li 3, tht is chemiclly nd physiclly stble. is developed by ORNL 1. Conductivities of vrious Li 3 - bsed mterils re mesured 2 mteril γ-li 3 Li 2.88 PO 3.73 N 0.14 Li 2.7 PO 3.9 Li 3.3 PO 3.9 N σ ( T ) = K T e E A / kt E A (ev) σ (S cm -1 ) B. Wng et l., J. of Solid Stte Chemistry 115, 313 (1995). 2. J. B. Btes et l., Solid Stte Ionics 53-56, 647 (1992).. Mesured t 25 o C 3. Supported by NSF grnts DMR nd DEAC cluster

2 Gol nd Outline For single crystl. Intrinsic crriers re creted s Li vcncy-interstitil pir (Frenkel pir), which yields 1 σ T ~ Method. ne E m / kt = e ( E m + E f / 2) / Vcncy mechnism of Li ion. Interstitil mechnism of Li ion. Formtion of vcncy-interstitil pir. Conclusion. kt For doped crystl. extrinsic crriers re creted s doped, which yields σ T ~ e E m / kt Li 2.88 PO 3.73 N 0.14 with 12%vcncy s doped. γ-li 3 (Pnm) 1. A. R. West, Bsic Solid stte Chemistry, 2 nd ed; John Wiley & Sons: Chichester, U.K., 1999, p

3 Methods Quntum ESPRESSO (PWscf ) 1 pckge nd ultr-soft pseudopotentil formlism of Vnderbilt using GGA nd LDA. Single L-point k-mesh smpling, cutoff of plnewve is 30 Ry. Nudged elstic bnd 1 method determines the miniml energy pth connecting two djcent locl minim H. Jónsson et l., in Clssicl nd Quntum Dynmics in Condensed Phse Simultions, edited by B. J. Berne, G. Ciccotti, nd D. F. Coker (World Scientific, Singpore, 1998), P G. Henkelmn et l, J. Chem. Phys. 113, 9901 (2000). E m

4 Vcncy diffusion mechnism γ-li 3 Volume optimized by Prrinello-Rhmn scheme Experiment 1 GGA LDA (Å) b (Å) c (Å) Two types of Li (d nd c) result in two types of Li ion vcncy: E ( v(c) Liv(d) ) E(Li ) = 0.22 (0.20) ev -xis 0.69 (0.66 ) ev X coordinte is defined s uii xii = ΔX if where u if u ii I J = i+ 1 d J, J 1 nd d, 1 ( R R J J J J-1 ) 2.93 Å 3.17 Å 1. O. V. Ykubovich nd V. S. Urusov, Cyrstllogrphy Reports 42, 261 (1997).

5 Vcncy diffusion mechnism γ-li 3 c-xis 0.63 (0.68)eV 3.51 Å 2.70 Å b-xis 0.67 (0.74)eV 0.56 (0.63) ev 3.06 Å 3.11 Å 3.09 Å 3.09 Å

6 The configurtion of Li ion interstitil The crystl cn be divided into two distinct voids chnnel long the c- xis, which, in turn, provides generl scn of possible interstitil sites. I 0 X 0.30 I II II * Energy y Results re computed in GGA z The II 0 interstitil induces biggest distortion of neighboring c-type Li ion

7 Interstitil diffusion mechnism long the b-c xis The II 0 kicks nd replce neighboring d-type Li-ion. The kicked-out d-type Li-ion becomes n II 0. The whole process tkes plce between two djcent I chnnel. I ( , 0.75, 1.00) 2c I ( , 0.25, 0.50) b-c xis 0.21 (0.29) ev 2b

8 Interstitil diffusion mechnism long -xis I 0 (0.30, II 0 (0.52, * II (0.50, 0.25, 1.00) 0.07, 0.57) 0.00, 0.50) The whole process hs n inversion symmetry centered t the sddle point configurtion II* t the site (0.5, 0.0, 0.5) 2c II 0 (0.48, I 0 (0.70, 0.07, 0.25, 0.43) 0.00) -xis 0.23 (0.30) ev 2b Diffusion occurs between two different void chnnels: I nd II.

9 Formtion of interstitil-vcncy pir The interstitil-vcncy pir is constructed s I 0 interstitil nd its next-neighbor c-type vcncy. Formtion energy: E f =1.6 (1.7) ev Conductivity of γ-li 3 σ T ~ e ( E m + E f / 2)/ kt GGA(eV) Interstitil diffusion of brrier of 0.2 ev b domintes vcncy diffusion of ev c Experiment 1 (ev) LDA (ev) A. K. Ivnov-Shitz, et l., Crystllogrphy Report 46, 864 (2001)

10 Future work The doped Li 2.88 PO 3.73 N 0.14 hs mesured diffusion brrier of 0.97 compred to our computed ev in GGA nd 0.7 ev in LDA. The oxygen vcncy might provide trps for migrting Li ion. Interfce between node nd electrolyte my lso hve significnt effect on the diffusion. The role of N dopnts hs yet to be investigted. The diffusion within β-li 3 is currently under study. Preliminry results shows it hs comprble brriers s γ-li 3.

11 Conclusion Li ion cn migrte in Li 3 vi both vcncy nd interstitil mechnisms. For the vcncy mechnism, Li ion diffuses long three crystllogrphic directions with slight nisotropy of ev. The interstitil mechnism involves kick-out process, nd provides the lowest migrtion brrier of 0.21 (0.29) ev long the b nd c xes nd 0.23 (0.30) ev long the xis. The formtion energy of interstitil-vcncy pir is 1.6 (1.7) ev. Hence the intrinsic defects cn diffuse long three crystllogrphic directions with slight nisotropy of ev consistent with experimentl results.