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TiS cells intercalation of tetrabutylammonium ions in TiS
Mat. Res. B u l l . , Vol. 16, p p . 919-922, 1981. P r i n t e d in the USA. 0025-5408/81/080919-04502.00/0 C o p y r i g h t (c) 1981 Pergamon P r e s s Ltd.
SUPPORTING ELECTROLYTES CONTAINING TETRABUTYLAMMONIUMIONS IN L i / T i S INTERCALATION OF TETRABUTYLAMMONIUM IONS IN TiS
CELLS
B. M. L. Rao and T. R. Halbert Corporate Research Laboratories Exxon Research and Engineering Company P.O. Box 45 Linden, New Jersey 07036
(Received October 15, 1980; R e f r e e d ) ABSTRACT In context with the development of L i / T i S 2 c e l l s , the role of tetrabutylammonium bromide (TBAB) was investigated as Supporting Salt in LiSCN-I:3 dioxolane organic e l e c t r o l y t e s . TBAB had l i t t l e e f f e c t on the o v e r a l l c o n d u c t i v i t y . A d e t e r i o r a t i o n of c e l l performance was noted upon a d d i t i o n of TBAB to the e l e c t r o l y t e in spite of an improvement in the l i t h i u m h a l f - c e l l . This behavior is a t t r i b u t e d to the i n t e r c a l a t i o n of TBA+ ions which have been shown in independent experiments to i n s e r t i n t o TiS 2 to form the new compound TBAx+ T IS2. The advantages of employing supporting e l e c t r o l y t e s to favorably influence e l e c t r o d e / s o l u t i o n i n t e r f a c e properties and mass t r a n s p o r t propert i e s in the bulk s o l u t i o n of electrochemical systems have been discussed in d e t a i l by Newman(1). S p e c i f i c a l l y , supporting e l e c t r o l y t e s are used to improve e l e c t r o l y t e c o n d u c t i v i t y , thereby p e r m i t t i n g higher and more uniform current d e n s i t i e s , minimizing migration e f f e c t s , and reducing ohmic drop in the double layer. I t i s , t h e r e f o r e , of i n t e r e s t to consider the use of supporting e l e c t r o l y t e s to improve the o v e r a l l performance c a p a b i l i t y of high energy density l i t h i u m organic e l e c t r o l y t e c e l l s , in which the low d i e l e c t r i c constant of the solvent leads to a high degree of i o n - a s s o c i a t i o n , low c o n d u c t i v i t y and migration e f f e c t s . However, the large negative p o t e n t i a l of the l i t h i u m electrode thermodynamically r e s t r i c t s the choice of supporting s a l t s , e f f e c t i v e l y r u l i n g out many such s a l t s with inorganic cations because of electrochemical displacement r e a c t i o n s . Supporting s a l t s with t e t r a a l k y l ammonium cations should not have t h i s problem in that such cations are expected to be stable to Li metal. In t h i s i n v e s t i g a t i o n , we have examined the r o l e of tetrabutylammonium bromide as a supporting s a l t in a l i t h i u m thiocyanate (LiSCN)/l,3-dioxolane e l e c t r o l y t e ( 2~. E l e c t r o l y t e c o n d u c t i v i t y , l i t h i u m p l a t i n g and s t r i p p i n g , and Li/TiS2 c e l l c y c l i n g studies are reported. An unexpected phenomenon encountered in the c e l l c y c l i n g study also led to the discovery of a novel i n t e r c a l a t i o n reaction of TBA+ and TiS2. 919
920
B . M . L . RAO, et al.
Vo]. 16, No. 8
LiSCN (a pseudohalide lithium salt) is highly soluble in 1:3 dioxolane (>4.5 M), while the supporting s a l t , tetrabutylammoniumbromide (TBAB) has lower s o l u b i l i t y (<2.5 M) in the solvent at room temperature. Table 1 gives the specific c o n d u c t i v i t y data for 2.5 M LiSCN-dioxolane electrolytes at various concentrations of TBAB. The data i l l u s t r a t e that there is l i t t l e change in conductivity due to addition of TBAB. Apparently, any TABLE 1 improvements in conductivity that might result CONDUCTIVITY OF ELECTROLYTE from increased total salt concentration are almost exactly offset by other factors (e.g., Solute, Sp. C o n d . * increased v i s c o s i t y or ion aggragation). ( o h m . c m ) -1 An electrolyte containing 2.5 M LiSCN in USCN TBAB 1,3-dioxolane, supported by 2.0 M TBAB was 2.6 x 10 -3 2.5M 0 chosen for further work. 2.2 x 10 -3 2.5M 1.0 2.5M
2.0
2.7 x 10 .3
2.5 x 10 .3 2.5M 2.25 Preliminary experiments were carried 2.2 x 10 .3 0 2.0 out to assess the effect of supporting e l e c t r o l y t e on the effeciency of lithium "23oc electrodeposition and anodic s t r i p p i n g on copper and stainless steel substrates. The test cell was of the configuration previously described by Selim and Bro s . The results indicated 30-40% u t i l i z a t i o n e f f i c i e n c y for anodic s t r i p p i n g of electrodeposited lithium in 2.5 M LiSCN l:3-dioxolane and 70-80% u t i l i z a t i o n e f f i c i e n c y in the TBAB supported e l e c t r o l y t e at 0.5-2.5 mA/cm2 and I-5 mAh/cm2. Other work has suggested that the low u t i l i z a t i o n of e f f i c i e n c y in the unsupported e l e c t r o l y t e is due p r i m a r i l y to poor morphology of Li p l a t i n g , and resulting loss of electronic contact between plated Li dendrites and the bulk electrode. With t h i s in mind, we believe the role of TBA+ in improving u t i l i z a t i o n e f f i c i e n c y involves improved plating morphology. Whether t h i s is due to adsorption or some other effect is presently a matter of speculation.
Having examined the effects of TBAB on conductivity and lithium u t i l i z a t i o n efficiency, we turn to Li/TiS2 cell cycling experiments. Figure 1 gives the f i r s t dis10( charge rate capability of L i / TiS 2 cel~s assembled in parallel plate configurations and discharged to 1.4 V cut-off, as described in previous pub~E o lication (4). I t is noted i u_ 0 from this figure that the rate capabolity and the u t i l i z a t i o n efficiency are reduced in the g presence of TBAB. The change N in performance was further reflected in the rapid drop of ~2 the cell u t i l i z a t i o n efficiency on cycling between 2.7 V and 1.4 V limits. W~th the TBAB supporting electrolyte, e f f i 0.2 0.5 ) 2 5 10 ciency dropped to 30% of theoretical TiS 2 capacity observed DISCHARGE CURRENT DENSITY (mA/cm2) for cells containing 2.5 M FIGURE 1. FIRST DISCHARGEBEHAVIOROF LI/TIS2 CELLS WITH 2.5 M LISCN IN LiSCN in 1,2-dioxolane without 1,3-DIOXOLANE ELECTROLYTE. A: WITHOUT SUPPORTING SALT. B: WITH tab. i
i
i
1.0 M TBAB SUPPORTING SALT.
i
I
Vo], 16, No. 8
L i / T i S 2 CELLS
921
The e f f e c t s o f TBAB in Figure I are traced to TiS 2 cathode, since l i t h i u m p l a t i n g was shown to improve in the presence o f the supporting s a l t . One of the possible explanations might be the mass t r a n s p o r t related p r e c i p i t a t i o n o f s a l t in the f i n e pores of TiS 2 electrode, due to an accumulation o f the s a l t at a concentration exceeding the s o l u b i l i t y l i m i t during the i n i t i a l high rate i f discharge. A l t e r n a t e l y , the i n t e r c a l a t i o n of TBA+ ion might take place and i n t e r f e r e w i t h the i n t e r c a l a t i o n o f l i t h i u m ion in some manner. Calculat i o n s i n d i c a t e t h a t the coulombic charge passed during c e l l discharge at high currents amounts to <10% change in concentration at the electrode; t h e r e f o r e , the s a l t p r e c i p i t a t i o n mechanism appears u n l i k e l y . I n t e r c a l a t i o n of TBA+ ions i n t o TiS 2 i s p l a u s i b l e , as i n t e r c a l a t i o n of tetrabutylammonium ions i n t o l a y e r ed host l a t t i c e s , TaS z (5), MoS2 (6), and FeOC~ ( 6 , 7 ) , and i n t e r c a l a t i o n o f NHw+, RNH3+ and C6HsNH3+ i n t o TiS 2 have been reported p r e v i o u s l y (8,9). In order to determine i f TBA+ ion i n t e r c a l a t i o n i n t o TiS 2 indeed occurs, we catho d i c a l l y polarized a sample of TiS 2 g a l v a n o s t a t i c a l l y in a 0 . 1 M TBAB/CH3CN electrolyte. The TiS2 electrode, containing I00 mg of >I00 mesh TiS 2, served as the working electrode of a conventional t h r e e - e l e c t r o d e c e l l . The counter electrode was a Pt gauze, separated by a porous Vycor f r i t . The reference electrode was a Ag wire in 0 . I M AgNO3/CH3CN. The reduction was c a r r i e d out at i = 0.05 ma u n t i l I I coulombs had passed, corresponding to a nominal stoichiometry (TBA+)o 12TiS2. The i n i t i a l open c i r c u i t p o t e n t i a l was 0.735 V vs. Ag/Ag+; during the reduction, the p o t e n t i a l g r a d u a l l y dropped from -0.735 to -0.830 V. A f t e r reduction, the sample was removed from the e l e c t r o l y t e and washed with a c e t o n i t r i l e under i n e r t atmosphere. Data from the X-ray powder d i f f r a c t i o n pattern of the sample are reproduced in Table 2. These data can be i n t e r p r e t e d on the basis of a two-component mixture: a minor component of unreacted TiS2 and a major component of TBA+ i n t e r c a l a t e d TiS 2. Due to a preTABLE 2 X-RAY POWDER DIFFRACTION DATA FOR ferred o r i e n t a t i o n of the sample c r y s t a l TBA+ INTERCALATED TIS 2 l i t e s , only basal plane r e f l e c t i o n s (o,o,&) are observed f o r the i n t e r c a l a t e . These .,2e d. ~ Component give r i s e to a calculated layer spacing of 6.10 14.48 0,0,1 (TBA)xTIS 2 14.47A o, corresponding to an expansion of the vanderWaals gap by 8.76 Ao upon i n t e r 12.20 7.248 0,0,2 (TBA)xTIS 2 c a l a t i o n . The e f f e c t i v e diameter of TBA+ 15.50 5.712 0,0,t TiS 2 in e l e c t r o l y t e s o l u t i o n s has been reported 18.33 4.836 0,0,3 (TBA)xTIS 2 to be 9.88 AoI0. Therefore, we suggest 24.55 3.623 0,0,4 (TBA+)xTiS2 that the vanderWaals gap is occupied by 30.85 2.896 0,0,5 (TBA)xTJS 2 a s i n g l e layer of s l i g h t l y f l a t t e n e d TBA+ 31.31 2.854 0,0,2 T)S 2 ions, as i l l u s t r a t e d schematically in Figure 34.10 2.627 1,0,1 TiS 2 2. Space f i l l i n g models show that such 44.05 2.054 1,0,2 TiS 2 f l a t t e n i n g can r e a d i l y be accommodated 44.49 2.035 0,0,7 (TBA+)xTiS2 by adjusting the o r i e n t a t i o n of the butyl 47.85 1.899 0,0,3 TiS 2 groups on the TBA+ion. A l t e r n a t i v e l y , the TBA+ may "nest" p a r t l y i n t o the TiS 2 layers, 53.65 1.707 1,1,0 TiS 2 which are not ideal f l a t sheets. Futhermore, using the e f f e c t i v e diameter of TBA+ to estimate the area occupied by each cation (~73 A2/TBA+), and the area per Ti in TiS 2 (8.7 A L / T i ) , we can c a l c u l a t e the maximum stoichiometry for a s i n g l e layer of TBA+ in the vanderWaals gap of TiS 2 to be 0.12 TBA+/Ti. This is in good agreement with the nominal coulometric stoichiometry TBA+o.12TiS2 I I +
,
+
,
+
.
+
.
922
B . M . L . RAO, et a].
Having established that TBA+ can intercalate TiS 2 in the absence of other cations, we examined the x-ray powder d i f fraction pattern of a TiS 2 electrode discharged to 0.7 e- in a TiS2/Li cell with TBAB supported LiSCN/l,3-dioxolane electrolyte. The only major phase apparent was LixTiS 2 (x~O.7), although several r e l a t i v e l y weak unindexed lines, not related to the above
Vol. 16, No. 8
Tis2
~i+~
~ N-M~
~'~.N+rJ ~ N-~ TiS2
FIGURE2. SCHEMATICREPRESENTATIONOF TBA+ INTERCALATED (d = 9.42, 4.89, 3.90 A°). The Ti~. absence of l a t t i c e parameters relating to (TBA)x TiS2 in this x-ray data is puzzling. I t is possible that the intercalated TBA+ ion may undergo ion exchange with Li + ions in the TBAB supported electrolyte. Conditions for ion exchange appear favorable, as the potential of TBA+ intercalated TiS 2 is noted to be more noble than LixTiS 2. Thus, the electro-intercalation of TBA+ might be occurring under cycling conditions, might interfere.with the rate capability and c y c l a b i l i t y of Li/TiS 2 cells, and yet not be observed in subsequent x-ray d i f f r a c t i o n studies of the electrode. TBA+ i n t e r c a l a t e , were noted
In summary, the addition of TBAB supporting salt to a LiSCN/dioxolane e l e c t r o l y t e does result in improved Li plating c h a r a c t e r i s t i c s . However, the supporting salt has l i t t l e effect on the overall c o n d u c t i v i t y of the e l e c t r o l y t e , and actually results in deterioration of cell c y c l a b i l i t y . This deterioration may be related to i n t e r c a l a t i o n of TBA+ ions, which have been shown in independent experiments to intercalate into TiS 2 to produce the new compound TBAx+TiS2 .
References I. Newman, J., Electrochemical Systems, Prentice Hall, 1973. 2. Rao, B. M. L., Eustace, D. J. and Shropshire, J. A., J. Applied Electrochemistry, 10,757 (1980). 3. Selim, and Bro, P., J. Electrochem. Soc., 121, 1457 (1972). 4. Gaines, L. H., Francis, R. W., Newman, G. H., and Rao, B. M. L., Proc. l l t h I.E.C.C., 418 (1976). 5. Kanazaki, Y., Konuma, M., and Matsumoto, D., J. Phys. Chem. Solids, 41,
525 (1980. 6. Schollhorn, R. and Weiss, A., J. Less Comm. Met, 36, 229 (1974). 7. Meyer, H., Weiss, A., and Besenhard, J. B., Mat. Res. B u l l . , I__33,913 (1978). 8. Schollhorn, R. and Meyer, H., Mat. Res Bull , 9, 1237 (1974). 9. Schollhorn, R., Sick, E., and Lerf, A., Mat. ~ s . B u l l . , I0, 1005 (1975). I0. Robinson, R. A. and Stokes, R. H., " E l e c t r o l y t e Solutions,--Ref. Ed.", Butterworth, London, 1968, p. 125. I I . This number is of course approximate, due to the f a c t that the sample is incompletely reacted.
SUPPORTING ELECTROLYTES CONTAINING TETRABUTYLAMMONIUMIONS IN L i / T i S INTERCALATION OF TETRABUTYLAMMONIUM IONS IN TiS
CELLS
B. M. L. Rao and T. R. Halbert Corporate Research Laboratories Exxon Research and Engineering Company P.O. Box 45 Linden, New Jersey 07036
(Received October 15, 1980; R e f r e e d ) ABSTRACT In context with the development of L i / T i S 2 c e l l s , the role of tetrabutylammonium bromide (TBAB) was investigated as Supporting Salt in LiSCN-I:3 dioxolane organic e l e c t r o l y t e s . TBAB had l i t t l e e f f e c t on the o v e r a l l c o n d u c t i v i t y . A d e t e r i o r a t i o n of c e l l performance was noted upon a d d i t i o n of TBAB to the e l e c t r o l y t e in spite of an improvement in the l i t h i u m h a l f - c e l l . This behavior is a t t r i b u t e d to the i n t e r c a l a t i o n of TBA+ ions which have been shown in independent experiments to i n s e r t i n t o TiS 2 to form the new compound TBAx+ T IS2. The advantages of employing supporting e l e c t r o l y t e s to favorably influence e l e c t r o d e / s o l u t i o n i n t e r f a c e properties and mass t r a n s p o r t propert i e s in the bulk s o l u t i o n of electrochemical systems have been discussed in d e t a i l by Newman(1). S p e c i f i c a l l y , supporting e l e c t r o l y t e s are used to improve e l e c t r o l y t e c o n d u c t i v i t y , thereby p e r m i t t i n g higher and more uniform current d e n s i t i e s , minimizing migration e f f e c t s , and reducing ohmic drop in the double layer. I t i s , t h e r e f o r e , of i n t e r e s t to consider the use of supporting e l e c t r o l y t e s to improve the o v e r a l l performance c a p a b i l i t y of high energy density l i t h i u m organic e l e c t r o l y t e c e l l s , in which the low d i e l e c t r i c constant of the solvent leads to a high degree of i o n - a s s o c i a t i o n , low c o n d u c t i v i t y and migration e f f e c t s . However, the large negative p o t e n t i a l of the l i t h i u m electrode thermodynamically r e s t r i c t s the choice of supporting s a l t s , e f f e c t i v e l y r u l i n g out many such s a l t s with inorganic cations because of electrochemical displacement r e a c t i o n s . Supporting s a l t s with t e t r a a l k y l ammonium cations should not have t h i s problem in that such cations are expected to be stable to Li metal. In t h i s i n v e s t i g a t i o n , we have examined the r o l e of tetrabutylammonium bromide as a supporting s a l t in a l i t h i u m thiocyanate (LiSCN)/l,3-dioxolane e l e c t r o l y t e ( 2~. E l e c t r o l y t e c o n d u c t i v i t y , l i t h i u m p l a t i n g and s t r i p p i n g , and Li/TiS2 c e l l c y c l i n g studies are reported. An unexpected phenomenon encountered in the c e l l c y c l i n g study also led to the discovery of a novel i n t e r c a l a t i o n reaction of TBA+ and TiS2. 919
920
B . M . L . RAO, et al.
Vo]. 16, No. 8
LiSCN (a pseudohalide lithium salt) is highly soluble in 1:3 dioxolane (>4.5 M), while the supporting s a l t , tetrabutylammoniumbromide (TBAB) has lower s o l u b i l i t y (<2.5 M) in the solvent at room temperature. Table 1 gives the specific c o n d u c t i v i t y data for 2.5 M LiSCN-dioxolane electrolytes at various concentrations of TBAB. The data i l l u s t r a t e that there is l i t t l e change in conductivity due to addition of TBAB. Apparently, any TABLE 1 improvements in conductivity that might result CONDUCTIVITY OF ELECTROLYTE from increased total salt concentration are almost exactly offset by other factors (e.g., Solute, Sp. C o n d . * increased v i s c o s i t y or ion aggragation). ( o h m . c m ) -1 An electrolyte containing 2.5 M LiSCN in USCN TBAB 1,3-dioxolane, supported by 2.0 M TBAB was 2.6 x 10 -3 2.5M 0 chosen for further work. 2.2 x 10 -3 2.5M 1.0 2.5M
2.0
2.7 x 10 .3
2.5 x 10 .3 2.5M 2.25 Preliminary experiments were carried 2.2 x 10 .3 0 2.0 out to assess the effect of supporting e l e c t r o l y t e on the effeciency of lithium "23oc electrodeposition and anodic s t r i p p i n g on copper and stainless steel substrates. The test cell was of the configuration previously described by Selim and Bro s . The results indicated 30-40% u t i l i z a t i o n e f f i c i e n c y for anodic s t r i p p i n g of electrodeposited lithium in 2.5 M LiSCN l:3-dioxolane and 70-80% u t i l i z a t i o n e f f i c i e n c y in the TBAB supported e l e c t r o l y t e at 0.5-2.5 mA/cm2 and I-5 mAh/cm2. Other work has suggested that the low u t i l i z a t i o n of e f f i c i e n c y in the unsupported e l e c t r o l y t e is due p r i m a r i l y to poor morphology of Li p l a t i n g , and resulting loss of electronic contact between plated Li dendrites and the bulk electrode. With t h i s in mind, we believe the role of TBA+ in improving u t i l i z a t i o n e f f i c i e n c y involves improved plating morphology. Whether t h i s is due to adsorption or some other effect is presently a matter of speculation.
Having examined the effects of TBAB on conductivity and lithium u t i l i z a t i o n efficiency, we turn to Li/TiS2 cell cycling experiments. Figure 1 gives the f i r s t dis10( charge rate capability of L i / TiS 2 cel~s assembled in parallel plate configurations and discharged to 1.4 V cut-off, as described in previous pub~E o lication (4). I t is noted i u_ 0 from this figure that the rate capabolity and the u t i l i z a t i o n efficiency are reduced in the g presence of TBAB. The change N in performance was further reflected in the rapid drop of ~2 the cell u t i l i z a t i o n efficiency on cycling between 2.7 V and 1.4 V limits. W~th the TBAB supporting electrolyte, e f f i 0.2 0.5 ) 2 5 10 ciency dropped to 30% of theoretical TiS 2 capacity observed DISCHARGE CURRENT DENSITY (mA/cm2) for cells containing 2.5 M FIGURE 1. FIRST DISCHARGEBEHAVIOROF LI/TIS2 CELLS WITH 2.5 M LISCN IN LiSCN in 1,2-dioxolane without 1,3-DIOXOLANE ELECTROLYTE. A: WITHOUT SUPPORTING SALT. B: WITH tab. i
i
i
1.0 M TBAB SUPPORTING SALT.
i
I
Vo], 16, No. 8
L i / T i S 2 CELLS
921
The e f f e c t s o f TBAB in Figure I are traced to TiS 2 cathode, since l i t h i u m p l a t i n g was shown to improve in the presence o f the supporting s a l t . One of the possible explanations might be the mass t r a n s p o r t related p r e c i p i t a t i o n o f s a l t in the f i n e pores of TiS 2 electrode, due to an accumulation o f the s a l t at a concentration exceeding the s o l u b i l i t y l i m i t during the i n i t i a l high rate i f discharge. A l t e r n a t e l y , the i n t e r c a l a t i o n of TBA+ ion might take place and i n t e r f e r e w i t h the i n t e r c a l a t i o n o f l i t h i u m ion in some manner. Calculat i o n s i n d i c a t e t h a t the coulombic charge passed during c e l l discharge at high currents amounts to <10% change in concentration at the electrode; t h e r e f o r e , the s a l t p r e c i p i t a t i o n mechanism appears u n l i k e l y . I n t e r c a l a t i o n of TBA+ ions i n t o TiS 2 i s p l a u s i b l e , as i n t e r c a l a t i o n of tetrabutylammonium ions i n t o l a y e r ed host l a t t i c e s , TaS z (5), MoS2 (6), and FeOC~ ( 6 , 7 ) , and i n t e r c a l a t i o n o f NHw+, RNH3+ and C6HsNH3+ i n t o TiS 2 have been reported p r e v i o u s l y (8,9). In order to determine i f TBA+ ion i n t e r c a l a t i o n i n t o TiS 2 indeed occurs, we catho d i c a l l y polarized a sample of TiS 2 g a l v a n o s t a t i c a l l y in a 0 . 1 M TBAB/CH3CN electrolyte. The TiS2 electrode, containing I00 mg of >I00 mesh TiS 2, served as the working electrode of a conventional t h r e e - e l e c t r o d e c e l l . The counter electrode was a Pt gauze, separated by a porous Vycor f r i t . The reference electrode was a Ag wire in 0 . I M AgNO3/CH3CN. The reduction was c a r r i e d out at i = 0.05 ma u n t i l I I coulombs had passed, corresponding to a nominal stoichiometry (TBA+)o 12TiS2. The i n i t i a l open c i r c u i t p o t e n t i a l was 0.735 V vs. Ag/Ag+; during the reduction, the p o t e n t i a l g r a d u a l l y dropped from -0.735 to -0.830 V. A f t e r reduction, the sample was removed from the e l e c t r o l y t e and washed with a c e t o n i t r i l e under i n e r t atmosphere. Data from the X-ray powder d i f f r a c t i o n pattern of the sample are reproduced in Table 2. These data can be i n t e r p r e t e d on the basis of a two-component mixture: a minor component of unreacted TiS2 and a major component of TBA+ i n t e r c a l a t e d TiS 2. Due to a preTABLE 2 X-RAY POWDER DIFFRACTION DATA FOR ferred o r i e n t a t i o n of the sample c r y s t a l TBA+ INTERCALATED TIS 2 l i t e s , only basal plane r e f l e c t i o n s (o,o,&) are observed f o r the i n t e r c a l a t e . These .,2e d. ~ Component give r i s e to a calculated layer spacing of 6.10 14.48 0,0,1 (TBA)xTIS 2 14.47A o, corresponding to an expansion of the vanderWaals gap by 8.76 Ao upon i n t e r 12.20 7.248 0,0,2 (TBA)xTIS 2 c a l a t i o n . The e f f e c t i v e diameter of TBA+ 15.50 5.712 0,0,t TiS 2 in e l e c t r o l y t e s o l u t i o n s has been reported 18.33 4.836 0,0,3 (TBA)xTIS 2 to be 9.88 AoI0. Therefore, we suggest 24.55 3.623 0,0,4 (TBA+)xTiS2 that the vanderWaals gap is occupied by 30.85 2.896 0,0,5 (TBA)xTJS 2 a s i n g l e layer of s l i g h t l y f l a t t e n e d TBA+ 31.31 2.854 0,0,2 T)S 2 ions, as i l l u s t r a t e d schematically in Figure 34.10 2.627 1,0,1 TiS 2 2. Space f i l l i n g models show that such 44.05 2.054 1,0,2 TiS 2 f l a t t e n i n g can r e a d i l y be accommodated 44.49 2.035 0,0,7 (TBA+)xTiS2 by adjusting the o r i e n t a t i o n of the butyl 47.85 1.899 0,0,3 TiS 2 groups on the TBA+ion. A l t e r n a t i v e l y , the TBA+ may "nest" p a r t l y i n t o the TiS 2 layers, 53.65 1.707 1,1,0 TiS 2 which are not ideal f l a t sheets. Futhermore, using the e f f e c t i v e diameter of TBA+ to estimate the area occupied by each cation (~73 A2/TBA+), and the area per Ti in TiS 2 (8.7 A L / T i ) , we can c a l c u l a t e the maximum stoichiometry for a s i n g l e layer of TBA+ in the vanderWaals gap of TiS 2 to be 0.12 TBA+/Ti. This is in good agreement with the nominal coulometric stoichiometry TBA+o.12TiS2 I I +
,
+
,
+
.
+
.
922
B . M . L . RAO, et a].
Having established that TBA+ can intercalate TiS 2 in the absence of other cations, we examined the x-ray powder d i f fraction pattern of a TiS 2 electrode discharged to 0.7 e- in a TiS2/Li cell with TBAB supported LiSCN/l,3-dioxolane electrolyte. The only major phase apparent was LixTiS 2 (x~O.7), although several r e l a t i v e l y weak unindexed lines, not related to the above
Vol. 16, No. 8
Tis2
~i+~
~ N-M~
~'~.N+rJ ~ N-~ TiS2
FIGURE2. SCHEMATICREPRESENTATIONOF TBA+ INTERCALATED (d = 9.42, 4.89, 3.90 A°). The Ti~. absence of l a t t i c e parameters relating to (TBA)x TiS2 in this x-ray data is puzzling. I t is possible that the intercalated TBA+ ion may undergo ion exchange with Li + ions in the TBAB supported electrolyte. Conditions for ion exchange appear favorable, as the potential of TBA+ intercalated TiS 2 is noted to be more noble than LixTiS 2. Thus, the electro-intercalation of TBA+ might be occurring under cycling conditions, might interfere.with the rate capability and c y c l a b i l i t y of Li/TiS 2 cells, and yet not be observed in subsequent x-ray d i f f r a c t i o n studies of the electrode. TBA+ i n t e r c a l a t e , were noted
In summary, the addition of TBAB supporting salt to a LiSCN/dioxolane e l e c t r o l y t e does result in improved Li plating c h a r a c t e r i s t i c s . However, the supporting salt has l i t t l e effect on the overall c o n d u c t i v i t y of the e l e c t r o l y t e , and actually results in deterioration of cell c y c l a b i l i t y . This deterioration may be related to i n t e r c a l a t i o n of TBA+ ions, which have been shown in independent experiments to intercalate into TiS 2 to produce the new compound TBAx+TiS2 .
References I. Newman, J., Electrochemical Systems, Prentice Hall, 1973. 2. Rao, B. M. L., Eustace, D. J. and Shropshire, J. A., J. Applied Electrochemistry, 10,757 (1980). 3. Selim, and Bro, P., J. Electrochem. Soc., 121, 1457 (1972). 4. Gaines, L. H., Francis, R. W., Newman, G. H., and Rao, B. M. L., Proc. l l t h I.E.C.C., 418 (1976). 5. Kanazaki, Y., Konuma, M., and Matsumoto, D., J. Phys. Chem. Solids, 41,
525 (1980. 6. Schollhorn, R. and Weiss, A., J. Less Comm. Met, 36, 229 (1974). 7. Meyer, H., Weiss, A., and Besenhard, J. B., Mat. Res. B u l l . , I__33,913 (1978). 8. Schollhorn, R. and Meyer, H., Mat. Res Bull , 9, 1237 (1974). 9. Schollhorn, R., Sick, E., and Lerf, A., Mat. ~ s . B u l l . , I0, 1005 (1975). I0. Robinson, R. A. and Stokes, R. H., " E l e c t r o l y t e Solutions,--Ref. Ed.", Butterworth, London, 1968, p. 125. I I . This number is of course approximate, due to the f a c t that the sample is incompletely reacted.