J. MATER. CHEM., 1994, 4( lo), 1579-1580 Structure of LiN(CF3S0J2, a Novel Salt for Electrochemistry Jan L. Nowinski, Philip Lightfoot* and Peter G. Bruce Department of Chemistry, University of St Andrews, St Andrews, Fife, UK KY16 9ST The crystal structure of lithium bis(trifIuoromethanesu1fonimide) [LiN(CF,S02)J has been solved from X-ray powder diffraction data and- refined by the Rietveld method: The salt crystallises in the orthorhombic system, space group Pnaa, a =9.6351 (2) A, b =5.41 54(1) A, c =16.2639(3)A. The anion, previously uncharacterised crystallographically, has two-fold symmetry around the nitrogen atom, with short [1.56(1) A] S-N distances and a large S-N-S angle [1129(1)"]. This is consistent with other similar salts and suggests a high degree of double-bond character and delocalisation around N.Li + is tetrahedrally coordinated by four oxygens from four neighbouring anions. The development of rechargeable lithium batteries, smart windows and electrochromic display devices is critically dependent on finding suitable liquid or solid polymer electro- lytes. The electrolytes in turn consist of a lithium salt dissolved in a non-aqueous liquid solvent or a coordinating polymer such as a polyether. Recently a new inorganic lithium salt, LiN(CF3S02)2, lithium bis(trifluoromethanesulfonimide), LiTFSI, has been synthesized. It yields significantly higher conductivities in both liquid and solid-polymer solvents com- pared with the previous maximum values from salts such as LiC104, LiCF,S03 and LiPF,.Dissolved in poly(ethy1ene oxide) it is believed to be the best candidate for use in practical devices employing a solid polymer electrolyte.' One of our interests in this field is to determine the crystal structures of these complexes which, since single-crystal specimens are generally unavailable, has been achieved by X-ray powder It is of fundamental interest also to investigate the structural properties of the salts themselves in order to understand fully the solid-state properties of the complexes. To this end we have recently determined the structure of LiCF,SO,.' Given the very considerable interest in LiN(CF,S02)2 in both liquid and solid-polymer electro-chemistry, we have determined the crystal structure of this anhydrous salt which, again due to the lack of single crystals, has been solved by the newly emerging powder diffraction method.5.6 + Structure Determination The sample of anhydrous LiN(CF,S02)2 was obtained from 3M.X-Ray powder diffraction data were collected on a Stoe STADI/P diffractometer operating in transmission niode. A powdered sample was mounted in a 0.5 mm glass capillary and data were collected over the range 5"<28<85' using Cu-K,, radiation. The powder pattern was indexed uing the program TREOR.7 Owing to a chance relationship hetween the unit-cell parameters (cz 3b), unambiguous space group determination at this stage proved impossible due to system- atic overlap of non-equivalent reflections. In particular, absences in the reflection class hkO could not be detrtrmined with certainty and unsuccessful attempts to solve the si ructure were made in Pnam before Pnaa was tried successfully. Data suitable for structure solution were extracted from the powder pattern by a structure-independent profile fitting routine' using the program GSAS.' This procedure led to 189 indepen- dent reflections which were used as input to the direct methods program SIR88.I' The position of the S atom was dettrrmined from this.All remaining atoms were found by successive difference Fourier and Rietveld refinement cycles. Final con- vergence was achieved at R,, =6.7%, x2 = 1.50,for 46 variables and 309 reflections spread over 4000 data points Fig. 1). Isotropic thermal parameters were assigned to all atoms, with that for Li being fixed.-0.2-I I I I I I I II Fig. 1 Final Rietveld fit for LiN(CF,SO,),. (+) Raw data; (-) calculated model. The difference curve is shown below Table 1 Final refined atomic parameters tor LiN(CF,S02)?, space group Pnaa, a=9.6351(2) A, b=5.4154(1) A, c=16.2639(3) A atom Y J' Z u(iso)/A2 S N C 0(2) F( 1) F(2) Li O(1) F(3) 0.0811(2) 0.01 lO(9) 0.0949( 9) 0.2209 (4) 0.1455(4) 0.1740(4) -0.1490(4) -0.0180(4) -0.0291(4) 0.0692( 4) 0.25 0.2501(14) -0.1240(9) -0.0032 (7) 0.1079(8) 0.4428( 6) 0.33 10( 6) -0.25 0.1883( 2) 0.25 0.0884(5) 0.1690(3) 0.2052(2) 0.0346(3) 0.1040( 3) 0.0707( 3) 0.25 0.008(1) 0.006( 3) 0.025(4) 0.01 3( 2) 0.017(2) 0.041(2) 0.038(2) 0.035(2) 0.025 Table 2 Selected bond distances (/A)and angles (/degrees) s-O( 1) 1.451 (4) C-F( 1) 1.264(8) s-O(2) 1.429 (4) C-F( 2) 1.316( 9) S-N 1.55 7 (4) C-F( 3) 1.305( 9) s-c 1.901 (7) Li-O( 1)x 2 1.95(1) Li-0 (2) x 2 1.97(1) O(1)-S-0(2) 118(1) O(1)-Li-O( 1) 99(1)0(1)-S-N 108( 1) O(1)-Li-0(2) x 2 114(2) O(1)-s-c 104(1) O( 1)-Li-O(2') x 2 115(2) 0(2)-S-N 117(1) O(1)-Li-O( 2) lOl(1) O(2)-s-c 104(1) S-N-S 129( 1) N-S-C 105( 1) F( 1)-C-F( 2) 113(1) F(2)-C-F( 3) 108(1) F(1 )-C-F( 3) 114( 1) F( 2)-C-S 107(1) F( 1)-C-S 108( 1) F(3)-C-S 107(1) n Fig.2 The (CF,S02)N- ion viewed approximately down the two- fold axis at N, showing a transoid conformation Results and Discussion Final refined atomic coordinates and thermal parameters are given in Table 1 and selected bond distances and angles in Table 2.The anion lies on a two-fold axis centred on N and parallel to the a axis, with a transoid conformation of the two CF, groups with respect to the S-N-S group when viewed down the two-fold axis (Fig. 2). This is the first crystallo- graphic determination of the structure of this anion. Th,e S-N-S angle of 129( 1)" and short S-N distance of 1.56 A (compare the average value of 1.64A in the RSO,NR', moeity'') reflect the significant double-bond character of the S-N bond. The negative charge of the anion is not located on the N alone but is delocalised via the double bonds onto the neighbouring SO, moeities. The structure of the (CF3S02)2N-anion may be compared with related anions (CF,SO,),CH-,12 (CF,SO,),C-,I3 (CH,SO,),N-l4 and RS02NC1-15.Bond lengths and angles are in excellent agree- ment with those reported in these studies; the corresponding S-No distance and S-N-S angle in (CH,SO,),N-are 1.59 A and 122", respectively. As observed in the case of RS0,NCl-,15 it is found that 0 rather than N is the coordinat- J. MATER. CHEM., 1994. VOL. 4 0 Fig.3 Unit-cell packing of LiN(CF3S02)2 viewed down the b axis. Each Li is coordinated tetrahedrally by four oxygens from four different neighbouring anions, two above and two below ing atom of the anion. Hence, Li is coordinated in a tetrahedral manner by four oxygens from four different anions. A view of the unit-cell packing is shown in Fig. 3. Isolated chains of anions held together by intervening cations run parallel to the b axis.The transoid conformation of the anion is in contrast to that found for the anion in Rb(C'F,S02),CH,12 which adopts a cisoid conformation in order to act as a bidentate ligand to Rb' . The authors are grateful to the 3M company for supplying the salt. P.G.B. thanks the Royal Society for the award of a Pickering Research Fellowship. References P. G. Bruce and C. A. Vincent, J. Chem. Soc., Faruduy Trans., 1993,89,3187. P. Lightfoot, M. A. Mehta and P. G. Bruce, J. Mdrr. Chern., 1992, 2, 379. P. Lightfoot, M. A. Mehta and P. G. Bruce, Science. 1993, 262, 883. P. Lightfoot, J. L. Nowinski and P. G. Bruce, J Am. Chetn. 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Power, Inorg. Chem., 1986,25.4057. Paper 4/03568B; Received 13th June, 1994