M. Stanley Whittingham a n d Russell R. Chianelli Exxon Research and Engineering Company Linden, N J 07036
I 1
I
Layered Compounds and Intercalation Chemistry An example of chemistry and diffusion in solids
The diffusion of ions in solids plavs a critical role in manv chemical reactions hoth in the laborHtory and in the marketplace. Thus the formation of many oxides and sulfides, hv the reaction of gaseous oxygen or sulfur with metals, occurs h i the diffusion of the metal ion through the chalcogenide layer. This reaction mechanism leads to some intriguing morphologies which will be illustrated later. It is critical in protective corrosion coatings. For example, if there were no thin coherent oxide film on aluminum, it would he destroyed rapidly in an oxidizing environment. One area where diffusion of atoms has been emphasized in the last decade is that of energy storaee reactions. Included among these reactions are the-formacon of "hydrides" and lithium compounds a t ambient temperature by the incorporation of hydrogen and lithium into the host crystalline lattice,
These reactions form the basis of safe hvdroeen . .. storaee devicrs, bntterieq, and electrochromic displ3ys. re,pectively, and a i l 1 bedessril~t:din d(,t:~ilin this I > ~ I I IInormnicc(,~nrx,~lnds T. are also of critical importance as Eaialysts &many important commercial reactions including petroleum refining and chemicals production. Zeolites and other shape-selective materials depend in large part for their selectivity on the different diffusion rates of the various reacting molecules. In this paper rather than cover a wide range of compounds we will consider in deoth . "iust one suhiect of materials., laver structures and their intercalation chemistry, and describe their synthesis and properties. The term intercalation literally refers to the act of inserting into a calendar some extra interval of time, such as February 29 in a leap year. Translated into chemical terms, it now describes the insertion of guest species into a lamellar host structure with substantial maintenance of the structural features of the host. Diffusion is important hoth on the microscopic and the macroscopic scale for some of these layer lattices. Thus, in order to intercalate potassium, for example, into a layer lattice the potassium ions must diffuse between the layers of the solids as shown schematically in Figure 1. These same layer lattices also show good luhricating properties in the solid state, because the individual layers, such as in graphite, can slide; that is, whole sheets diffuse relative to one another. Both graphite and molybdenum disulfide show excellent lubricating properties and are used commerciallv. In rhii paper we will first de;crihe th(, iyntht,sia of ium(:of these lover lnttiees and their interrnlates, then the i~roperties of the intercalates with particular emphasis on energy storage, and finally some catalytic properties of the layer lattices themselves.
-
Synthesis
The disulfides of the metals of group IVB, VB, and VIB all have layered structures. The layers comprise sulfur-metalPresented at the ACS National Meeting, March 25, 1980, as part of the State-of-the-ArtSymposium on Solid State Chemistry in the Undergraduate Curriculum sponsored by the Division of Chemical Education.
Figure 1. Microscopic and macroscopic diffusion in layered compounds
sulfur saodwicliei u,here t h t metal atoms reside in enher octahedral or rrironal pricmaric sites. These sandwiches are held together by weak van der Waals forces so that they can he fairly readily prized apart, thus allowing both other species to he inserted and the layers to slide relative to one another. Titanium and tantalum disulfide have been the most studied and will be the main focus of attention here. Titanium disulfide can be synthesized readily by a number of different routes. When a titanium wire is heated in sulfur vapor, crystals of titanium sulfide grow away from the wire to give wellformed hexaeonal crvstals as shown in Fieure 2a. It is believed that these crystals glow by the diffusionof titanium from the wire through the crvstal lattice to its surface where it can r e a d with the silfur vapor. Consistent with this mechanism a void is left in place of the wire. For such a mechanism to operate the crystal must show electrical conductivity, as well as high titanium diffnsivity, since electrons must diffuse along with the charged titanium ions to maintain electrical neutrality. (Protective films on metals must show either low metal diffusivitv or verv low electrical conductivitv to he effective.) In contrast, when titanium disulfide is prepared in the gaseous stare by reaction o i H.A with 'l'iC1 ,,mulriole nucleation uccurs and the flower shaped crystag shown in Figure 2h are found. A particularly interesting reaction is that which occurs between sulfur and the layer vanadium sulfur compound V L . ~ &to give vanadium tetrasulfide. This reaction exemplifies two points: the vast difference in reactivity between the basal plane and the edge sites of these layered sulfides and the role of anisotropic diffusion. As Figure 2c shows, the vanadium tetrasulfide has grown as hollow pipes with a cross-section essentially the same as that of the starting layer vanadium sulfide. This oarticular mornholoev .., results from the much higher rwctivitv oftht. tdgt. sites. so that gnwrh unlv occurs there The \,aoadiu111is h r l ~ e w dto d~ffusttfrom th(: in\.ered sulfide along the fiber axis of the VSa thus causing the one dimensional nature of the crystal. The reason for the difference in reactivity between edge and basal sites can be seen from Figure 3, which shows schematically the structure of a layered disulfide. Metal atoms are found between every other sulfur layer while alternate layers contain no metal and the only bonding is of a van der Waals nature. There are essentially no defects in the lattice on these planes so that it is difficult to nucleate a new ohase on the basal olane. In contrast the edges of these planar crystals can he t'hought of as containine only defects. because the bonding reauirements of the edge &miare not fulfilled. Thus, it is rathe; easy to nucleate Volume 57,Number 8, August 1980 / 569
Figure 4. Optical micrograph showing the intercalation of a single crystal of TiSl by lithium fromn-butyl lithium (4).
EFFECT OF LITHIUM CONTENT SINGLE CONTINUOUS PHASE FOUND FOR LixTiS2 F O R O Q x S 1.
Figure 2. Electron micrographs of TiS2 crystals grown (a)inthe solid state( 1). (b) from the gas phase, and (clV S I crystals
(a.
a new phase or to continue the growth of the crystal at these sites. Lithium Intercalation (3) The simplest of all the intercalation reactions is that between lithium and titanium disulfide. Among many possible synthetic techniques, just two will be described here. In the first, the lithiating agent used is n-butyl lithium dissolved in a hydrocarbon solvent such as hexane:
-
C4H9Li+ TiSP LiTiS? + lIZC8H18 The major by-product is octane so that the product can be obtained readily by filtration. Naturally this reaction must he carried out in an inert atmosphere. Because the liquid reactants are all colorless, the reaction can be readily followed optically. Figure 4 shows a hexagonal crystal of titanium di570 1 Journal of Chemical Education
Lattice parameter perpendicularto the layers of Li.TiS2 (5).
sulfide immersed in n-hutyl lithium. This crystal is partially reacted and the reaction front can he seen moving in from the edges; there is about a 10% expansion perpendicular to the basal planes during the reaction. This lithium diffusion is rapid so that in crystals of this size reaction is complete within about an hour. In powders the reaction is complete within minutes. and in lareer cwstals the strains induced during the expansidn of the lattice ciack the crystal (as can he seen i n ~ i g . 4 in the background) so that the reaction rate tends to increase " with time. Just as this reaction can be followed optically, so i t can be carried out in an X-ray diffractometer and thecrystal structure changes monitored. Figure 5 shows the change in the lattice size between the titanium containing layers as the reaction proceeds. This change indicates that the reaction occurs contin"ously with all compositions between x = 0 and x = 1 in
Li,TiSn existing. This is an ideal example of a nonstoichiometric reaction:
-
xLi + Ti& LirTiS2 for 0 < x > 1 There is no change in the structure of the titanium disulfide lattice during the reaction except for a 10%expansion perpendicular to the planes to accommodate the lithium. The kxpansion parallelto the layers, -1.470, is much smaller and is associated with the increased electron densitv on the sulfur ions which effectively increases their size. The second procedure for synthesizing the lithium intercalate involves the reduction of the titanium disulfide in an electrochemical cell, such as that shown schematically in Figure 6. A single crystal of the sulfide or powder bonded with, say, Telfonm into a metal grid is simply immersed into a polar oreanic solvent (e.e.. dioxolane) in which a lithium salt~. (e.e.. -, litrhium perchloiatej is dissolvd; a sheet of lithium metal serves as the anode. On simply electrically shorting the two electrodes, the lithium ions intercalate the titanium disulfide lattice; and the charge compensating electrons pass through the external circuit. The reactions occurring a t the two electrodes are
makes the LitTiSz couple ideal for energy storage. Such a battery is constructed as shown in Figure 6, hut with thin porous polypropylene sheets separating the electrodes thus minimizing the distance hetween them. As the battery is discharged, the lithium intercalates the Ti&, and the cell voltage drops steadily until reaction is complete. Revenihility of the reaction is tested by reversing the current flow, and as can he seen in Figure 8 the voltage increases in a continuous manner. By folding this curve over the discharge curve, Figure 9, it can he seen that the two curves are almost identical except for a slight shift in voltage which is due to resistance losses in the electrolyte. This curve indicates that the reaction is ideally B; O F FORMATION OFUnTlS2
I
Li r Lit + e- at anode Ti& + e- + Ti% at cathode Li + Ti& + Li+TiS*- overall reaction The rate of intercalation can he controlled bv- i m .~ o s i n ean external voltnge across the cell, and whcn this voltage equals the f r ~ e e n e r e v othe f intercalation rtwction the reaction will stop. When ;he voltage exceeds the free energy, the lithium ions will be deintercalated. Thkquilibrium voltage, E, and the free energy of reaction, AG, are related through the equation.
-
XG = -EF where F is Faraday's constant. Thus by measuring E as a function of x , the free energy of formation of Li,TiSz can he 7. and aeain a comouted. These results are shown in Fieure " continuous variation of the free energy with composition is observed indicative of a nonstoichiometric reaction. The drop in the free energy with increasing lithium content is expected both because the repulsive interactions between the positively charged lithium ions must increase as the lithium content increases and because the electrons which are donated to the TiS2must occupy continuously increasing energy levels. There is very little change in the free energy . . with temDerature. A S = 25 e.u. at x = I , just as expected fin a solid state reaction. This high free energy of reaction, coupled with the high lithium ion ditfusivity and the electrunic conductivity of TiS2
Figure 7. The free energy of intercalationof limium into titanium disulfide.
e
TIME IN HOURS
Figure 8. Voltage changes as lithium is continuously intercalated and delntercalated into TiS2at 2 malcm2.
,
'i
POLYMER-BONDED
NON AOUEOUS SOLVENT +Li SALT
CELL REACTION:Li+TiS2-LiTiS21215WH/LB.l
Figure 0. Schematic of lithium titanium disulfide cell.
% UTILISATION
Figure 9. Electrochemical cycling in Li/TiS2cell at 2 ma/cm2(3). Volume 57, Number 8, August 1980 1 571
,TaS7
LAYER
YLAMINE
Figure 11. Structure of interleavedn-aikylamines
Figure 12. Staging behavior during intercalation (8).
Figure 10. Structure of 0-alkylamines (6).
reversible, and the efficiency of the energy storage (energy outlenergy in) is greater than 90%. The calculated energy storage is 480 W hrlkg based on the weights of the active components, and in practical devices is expected to attain over 100 W hrlkg a t power densities of 100 W hrlkg; this is about 3-4 times the energy stored in alead-acid battery but stillan order of magnitude less than that stored in an equal volume of gasoline. A number of other alkalis and hydrogen also intercalate the layered sulfides and other host structures such as graphite, FeOCI, Moos, WOa, NiPS3, and VzOs. Many of these exhibit similar properties. Thus the tunnel structure of LaNi5 can reversibly take-up 6 to 7 hydrogen atoms a t ambient temperature and pressure; this solid stores hydrogen a t a density greater than that in liquid hydrogen and is one of a number of solids being actively pursued as a hydrogen storage media. Tungsten oxide also incorporates hydrogen with a sharp change of color from pale yellow to dark blue so that it is being considered for use as a disolav . . in such devices as watches in place of liquid rrvit:ds. 'l'ltese arc knwm ar elerrrwhr~mic disvlws . nnd their efiectioen~:~dwends rritirnllv on the high diffusivity of the intercalating cat&, just as do the hydrogen storage and battery materials.
-
Other Intercalates Simple cations are not the only species that can he intercalated into layered host materials. Almost any molecule that can donate electrons or is a Lewis base can be incorporated into the layered disulfides. Thus ammonia, alkylamines, and pyridines all form compounds with tantalum disulfide. These molecules are able to diffuse between the sulfide layers, and depending on the number of carbon atoms they can either lie down or stand-up between the layers. Figure 10 shows schematically the structure of the C18 amine intercalate; the sulfur layers have become separated by over 50 A, an order of magnitude greater than the thickness of the sulfide sandwich and 572 / Journal of Chemical Education
Figure 13. Double sandwich compounds between rnetallocene and tantalum disulfide (9).
yet the sulfide layers still keep their registry relative to one another. The amine groups are adjacent to the sulfur atoms with hvdrocarhon tails in the center similar to the behavior . - ~ the - - ~ ~ - ~ - of soaps in waterloil mixtures. When the amine concentration is decreased, two different behaviors are observed. The actual concentration in a given layer can be decreased by interleaving the hydrocarbon chains (xx) as shown in Figure 11. Alternatively, layers can be completely emptied giving 2nd or lower stage materials as shown in Figure 12. The latter case is found for ammonia itself and the larger alkali metals. Some rather large organometallic molecules can he intercalated. The sandwich metallocenes such as cobaltocene intercalate into the layered sulfides by ionization to give a double-sandwich compound, [Co(C,)2]1,4TaS2, with the orientation shown in Figure 13. These ions at room temperature d~~
~~~
u
IV
V
VI
VII Vlll,
VlllZ
VII13
IB
PERlOOlC POSITION
Figure 14. The catalytic activity of transition metal sulfidesfw UK1 removal d sulfurfrom dibenrothiophene( 14. diffuse readily from site to site, but as the temperature is lowered only the cyclopentadienyl rings freely move and they rotate even a t liauid helium temneratures in contrast to the pure snlid. A ranre ofo1hc.r mewlltcenes, with different metals and rine sizcs. can nlso he intrtrculated orovided t he ionizatiun potentik is lo& enough, below about 7 k ~For . other hosts the ionization potential limit may be different, thus ferrocene (IP = x.xx eV) will intercalate FeOCl hut not TaSz. One could contemplate incorporatina a catalytically active molecule into one of these host latticesthus essentially anchoring the catalyst and increasing the selectivity of the reaction because only those molecules that could diffuse into the structure would be able to get to the active catalyst site-compare zeolites. Sulfide Catalysts So far only sulfides with layered srructurej have been de. scritml. Hnumever, not all transition meral sulfides are layered, and not all properties are necessarily determined by the crystal structure. We havr been in\,estigating aspriesof these sulfide materials as catdlvsts rl0). 'l'hev are uf ~arriculnrinterest because two of their members, M O S ~ a d WSz, when nromoted are maior comnonents of hvdrotreatina- catalvsts used in the prtroleum induztry to remove sulfur and nitrogen from crude oil. Surnrisinels. little is understond rerardina the general fundamental basii for oil and origin of t i e catalytic activity of sulfide catalysts. A simple model reaction for hydrodesulfurization is the removal of sulfur from dibenzothiophene giving a mixture of biphenyl and cyclohexyheuzene,
the product ratio depending on the hydrogenation activity of the catalyst, the activity for sulfur removal from dehenzothiophene at 400°C for a wide range of transition metal sulfides is shown in Figure 14. The ability of these metals to catalvze this reaction varies continuouslv across the periodic tablbover three orders uf magnitude. A typical wlc&wtypr plot is observed with the maximum activity near Ru and 0s.
composition, x Figure 15. Recombination coefficientof oxygen atoms on tungsten bronzes. ~ , w o ~ . o = ~ a , A ==K(II). ~i.e The stable states of the catalvsts under reaction conditions are either the bulk suliides oisurface sulfides in the raw of 0 s and lr. The most active sulfide catalyit, are thost. containing metals with maximum d-character as has been noted in other transition metal catalyzed reactions. Also for maximum activitv the heats of formation of the sulfide must take on intermezate values so that the ability to both form and reeenerate sulfur vacancies is maximized. The reactine molecule must be capable of being chemisorbed but not so stronelv that it cannot subseauentlv be desorhed and the active sit; regenerated.) ~ l e a r l ; crystalline structure per se is not imoortant for these nuresulfides, but it is important for the promotion of MoS2 by metals such as cobaltand nickel where there is close intercalation between the two metals giving activities close to the peak of the volcano. The promotion of catalytic activity by the addition of an additional metal has also been studied (11) in the case of tungsten oxide where a series of bronzes, MxW03 (M=alkali, O
