Chemical bonding and intercalation processes

Solid State Ionics 40/41 North-Holland

CHEMICAL

( 1990) 3-9

BONDING

AND INTERCALATION

PROCESSES

P. HAGENMULLER Laboratoire de Chimie du Solide du CNRS. UniversitP de Bordeaux I, 351 cows de la Liberation, 33405 Talence Cedex, France

In contrast with amphoteric graphite the layer-type oxides or chalcogenides play generally the role of acceptors in intercalation reactions. Due to the more ionic character of the M-O bonds the structural evolution of the oxides may usually be explained on hand of electrostatic considerations, or in terms of cation oxide-reduction. On the contrary for the more covalent chalcogenides occupancy of energy levels in a rigid band structure model by the transferred electrons constitute mostly the determining feature, influencing strongly not only the structural changes but also the physical properties. The factors accounting for the keen tendency of the layer oxides to undergo 2D-+3D transformations are emphasized.

Graphite due to its monolayer structure and to the existence of weakly bound n-electrons has a certain number of specific properties: ability to play the role either of an acceptor or of a donor, formation of a large variety of multistage intercalation compounds respecting at long range the cluster rule of Daumas and Herold [ 1] with strong repulsions between the C-layers (see, for instance, ref. [ 21). The sandwich-type oxides and chalcogenides differ clearly from the graphite intercalation compounds not only by the structures (see fig. 1) but also by the chemical behavior: as a rule the sheets have an acceptor character, i.e. are Lewis acids. Mostly they lead to stage 1 insertion despite their increasing negative charge during the chemical or electrochemical intercalation process. In fact the ionic character of the M-O bonds within the oxide sheets and the stronger covalency of the homologous bonds in 2D-chalcogenides result in behaviors differing from the structural point of view as well as by the related physical properties. It is particularly true when the intercalated species is an alkali cation and even an 1B cation such as Cu+ or Ag+. It has been shown that in the sheet compounds, for instance by NMR Knight shift determinations, as the result of an ionization process these atoms occupy the Van der Waals gap as cations with only a very small local presence of external s-electrons [ 3 1. As a consequence we have to distinguish between the 0167-2738/90/S ( North-Holland

03.50 0 Elsevier Science Publishers )

B.V.

influence in the intercalation compounds of the A + cations and that of the liberated s-electrons which occupy the upper available levels in the energy diagram of the sheets using a rigid band model. The A+ cations play essentially a double role: on one hand they consume mechanical strain energy in pushing away the sheets from each other, on the other hand they stabilize the materials thanks to appearance of Coulomb attraction with neighboring sheets. The stronger influence of the first factor explains that during the intercalation reaction in chalcogenides the c parameter mostly grows, especially at beginning of the process. As a rule AC increases for homologous compounds with rising size of the A+ cation and decreasing value of the initial Van der Waals gap [ 41. But as a result of the strong ionicity of the M-O bonds c decreases when x increases (fig. 2). The behavior of the Li,NbOz system is intermediate due to higher covalency of the Nb-0 bonds: as strain and Coulomb energy factors compensate, c does not vary with x [26]. The fact that the charge of the inserted cations has more influence in oxides than in the more covalent chalcogenides, explains why most AXM02 compounds only exist when x>O.5, V205 and MOOR being exceptions due to strong covalency of M-O bonds [ 161. The electronegative character of fluorine accounts for the absence of sandwich-type fluorides. Such structures are on the contrary quite usual

P. Hagenmuller

feuillets

/ Chemical bonding and intercalation processes

de graphite

empilement

octa;dres-t&ra;dres

sNb

feuillet feuillets

de Ti

feuillets

de Nb

S,

Se

Fig. I. Some typical sheet structures.

LixTi S2 (01)

L&Co 02 (02)

LiMS, AC (%I

0.5

q,

Ti 52 +,zr s2 +,st%

0

i.

‘fNb% 3 /

NiPSS:Ni2,3

35

AVd.W.@J

(411j3S2

Fig. 2. In sulfides, where the M-S bonds are strongly covalent, the c-parameter increases generally by intercalation due to geometrical reasons. In oxides an opposite phenomenon is mostly detected as intercalation weakens the strong O-O repulsion between adjacent layers and compensates even the steric effect of the A+ cation owing to Coulomb attraction. A larger intersheet space reduces the dilatation observed in the sulfides as a result of the intercalation reaction.

P. Hagenmuller / Chemical bondingand intercalationprocesses

__

C

A

__

A-0-C

C

C--,AdB

B

C-A

C

6-C

--

A

A C

B

B

A

A --)

P3

__

03

1

third

sheet

second

sheet

I

1

first

sheet

Fig. 3. Sheet gliding observed in the P3+03 transformations obtained for instance by Na+ intercalation in the NaxCr02, Na,. COO* and NaxNiOt systems (the letters P and 0 refer to the surrounding of sodium, number 3 points out the amount of sheets per unit cell).

03-01 'It P3 I

P2-02 t!

06 II

Fig. 4. Phase transformations resulting from sheet gliding processes in some anionic stackings.

for the hydroxides thanks to formation of hydrogen bonds. Delmas et al. [S-7,25 ] have shown that the structural changes observed during the electrochemical intercalation-deintercalation reactions can be explained in a rather simple way, no structural modi-

5

lication occuring within the sheets (the temperature is too low to allow breaking of the internal M-O bonds), but the sheets gliding with respect to each other. This gliding process results from the modification of the electrostatic interactions between oxygen layers facing each other in the Van der Waals gap. Two phase-transformation families related by simple sheet gliding and respecting each ABC circular permutations in the oxygen packing have been shown to result either from an intercalation reaction or from a low temperature ion exchange process. Fig. 3 and 4 illustrate these structural evolutions. Such electrostatic phenomena account clearly for the fact that in an A,MO, or A,M!$ series increasing x values favor the passage from a trigonal prismatic surrounding of A+ to a more stable octahedral one, leading to strong weakening of the A+ ion conductivity (fig. 5 ) [ 18 1. Let us mention as a significant example the structural investigations of Fouassier et al. [ 81 on the NaXCoOz series (fig. 6). In a similar way Trichet et al. [ 91 have shown that in KT& potassium has a TP surrounding due to strong covalency of the Ti-S bonds in opposition to the 0 environment in KZrS2, where weakening of the bonds within the sheets increases the formal charge of sulfur. Similar considerations explain the TP coordination of the large A+ cations (Rb, Cs) and the 0 coordination of lithium in homologous series [ 10,17,27 1. Tetrahedral surrounding occurs only for Li+ and Cu+ cations, leading to strong covalent bonding. In the delafossite-type phases the Cu+ or Ag+ ions are situated along the edges of trigonal oxygen prisms, sp hybridization leading to linear coordination (e.g. in CuAlO*). This particular structure is even observed for Pd+ or Pt+ in PdCoOz or PtCoOz probably due to dZ2 s hybridization (fig. 7 ) ~241. The influence of the outer electrons injected in the host lattice differs with the materials concerned. When they occupy discrete levels the upper intercalation limit depends not only on the available sites (as in the MOCl layer compounds) but also on the lowest oxidation state which is stable in the conditions of the reaction [ 111. The intermediate compounds are characterized by a classical hopping mechanism. As an example Nadiri [ 13 ] has shown that only two lithium or sodium atoms can be in-

P. Hagenmuller /Chemical

bonding and intercalation processes

Fig. 5. Pathways of mobile ions in 2-D-arrays with TP and 0 surroundings. Whereas in the former case the rectangular anionic windows correspond to low activation energy barriers (and hence to high mobility), in the latter the triangular facesof the intermediate tetrahedra slow down the diffusion process.

Na, COO?

---

0

M

700 "C

(A) (b)

0 icj A(a 0 (Cl

---

0 (8) M(c) 0 (A)

A (b) O(C) %!) A(c) O(A) M(b) O(C) AW

---M

(b)

O(B) ---M(c) O(A)

0'3(x=0.77) 03 (x=1) Fig. 6. The various phases of the Na,CoO, system obtained at 700°C (0’3 refers to a Jahn-Teller distorted 03 phase).

P. Hagenmuller /Chemical bondingand intercalationprocesses

7

vide 20 electrons for a maximum population of 24 in the discrete molecular levels. The intercalation is thus limited to two divalent or four monovalent cations, as shown by Schollhorn [ 15 1. On the contrary in LiW309F with HTB-related structure the intercalation does not result from an upper limitation of electronic levels but from the available lattice sites: the maximum of two extra Li+ ions observed for reversible intercalation corresponds to occupation of two new positions out of four. Any lithium excess leads to Li+-Li+ repulsions and non reversible amorphization. The electrons provided occupy apparently a conduction band (characterized by Pauli paramagnetism) [ 261. Rouxel and Liang [ 41, [ 12 ] have shown that for the chalcogenides electronic occupation of such a conduction band accounts largely as well for the reactivity of the MX1 compounds as for the structure obtained and the corresponding physical properties, in particular the electrical properties. The existence of a large tz,-type conduction band explains for instance the easy intercalation up to x= 1 of lithium in o-type T&, the obtained Li,Ti$ compounds having a metallic behavior. Easy intercalation of lithium in the P-variety of Ta!$ (with a trigonal prismatic coordination of tantalum) appears to result from the progressive filling of a low lying dZ2-band, which compensates for a stronger repulsion between S-layers belonging to the same sheets. The material as expected is metallic, except when x= 1 leads to full band occupancy (fig. 8). Another significant example is

Fig. 7. In delafossite-type layer oxides, the Cu+, Ag’. Pd+ and Pt+ have linear coordination due to sp or 4~s hybridization.

tercalated in the cage structure of Fel(SOa)9 or NaTi* ( P04)3, where four empty sites are available but in which iron can only be reduced to oxidation 2 + and titanium to 3 + . The presence of alkali cations gives rise to unit cell shrinking, whereas the reduced oxidation state of Fe or Ti favors dilatation, but a careful examination of the precise variation of the cell parameters can bring evidence of the type of site occupied by the alkali cations in the host matrix. In the Chevrel phases built up by Mo6 octahedra enclosed in chalcogen cubes only a part of the six available sites are occupied, as the MO, clusters pro-

___

BE i C

a r b

n(E)

Zr 52 Li interc. without str. change: wide qapl/2 cond.+ met.

Fig. 8. Rigid band structure of O-Z&,

Ta 52-0 met -met.

- .--’

I

w

*n(E) Ta Sp-P

poor met.+poor met. (eventl&ond for x=1 as for isotr. MO SrP)

O-TaS2 and P-TaS2 (with a stabilized d.2 band due to TP surrounding of tantalum).

P. Hagenmuller /Chemical

bonding and intercalation processes

Li, MOS2-P ‘t

--a

\

b

A

w

Li-

c

B

S(p)

Unstab.

explained

addit. d-electrons electron.

by high energy despite

strong

entropy

Fig. 9. Band filling of Li,MoS, due to intercalation of lithium into P-MoS2 and resulting in unstability of the material.

intercalation of lithium in MO& which has the structure of P-TaS2 but an already filled up h2-band: intercalation of lithium becomes difficult in comparison with TaS2 due to occupancy by the extra

electrons of an antibonding high energy band. MO& which is a semiconductor becomes metallic (fig. 9). An obvious consequence of the bonding difference on the intercalation processes in oxides and chalcogenides is the easy adoption of a 3-D-character by the oxides, which contrasts with the tendency of the intercalants to order in the Van der Waals gap of the sulfides (as a consequence of strong cation-cation repulsion) [ 141. This feature is clearly illustrated by the tendency of the defect NaCl-type lithium intercalated layer oxides to lead to spinels making advantage of the existence of a similar cfc oxygen packing [ 19,201. Stabilization of Li+ in tetrahedral sites is illustrated by lithiation of rutile-type P_Mn02 which does not lead to a NiAs-type phase with the same hc oxygen packing but after sheet gliding to a spine1 phase of formulation close to LiMn204, as shown by Goodenough et al. [ 23 1. A negative aspect of this tendency to 2D+3D transformation is easy migration of the transition element into the intersheet space of the layer structure oxides, which indeed shrinks the c parameter and makes more difficult further A+ intercalation (fig. 10) [ 2 1,221. A similar migration occurs in sulfides only for very high overvoltages.

0

0

0000 0000

0

00

0000

0000

00

0

NaTi 02 (03)

NaxTiOn

*

*

(hT$

-TO,_X_Y I (T&

Ti

0 Na

0.-/4x<

1

o +02

Oooo

(K,_yTiy)(Ti+yNaJO,

a

reversible

-0

000

irreversible x-=0.7

*

*

reversible

(2D+3D)

Fig. 10. Migration of titanium in the Van der Waals gap of 03 type NaxTi02 during strong deintercalation of NaTi02.

P. Hagenmuller /Chemical

bonding and intercalation processes

References [ 1 ] N. Daumas and A. Herold, C.R. Acad. Sci., 268 ( 1971) 373. [2] M.S. Dresselhaus, Phys. Today 37 (1984) 60. [3] B.G. Silbemagel, Solid State Commun. 17 (1975) 361. (41 J. Rouxel, Chem. Scripta 28 (1988) 33. [8] C. Delmas, J.J. Braconnier, C. Fouassier and P. Hagenmuller, Mat. Res. Bull. 3/4 (1981) 165. [6] J. Rouxel, M. Danot and J. Pichon, Bull. Sot. Chim. Fr. 3390 (1971). [ 7 ] J. Rouxel, in: Intercalated layered materials, ed. F.A. Levy, Vol. 6 (Reidel, Dordrecht, 1979) p. 2 10. [ 8 ] C. Fouassier, C. Delmas and P. Hagenmuller, Mat. Res. Bull. 10 (1975) 443. [9] A. Le Blanc, M. Danot, L. Trichet and J. Rouxel, Mat. Res. Bull. 9 (1974) 191. [‘O J. Rouxel, J. Solid State Chem. 17 ( 1976) 223. [I’ G.A. Fatseas, P. Palvadeau and J.P. Venien, J. Solid State Chem. 51 (1984) 17. [‘2 W.Y. Liang, Physics and chemistry of electrons and ions in condensed matter, NATO ASI 130 (Reidel, Dordrecht, 1984). [‘3 ‘I A. Nadiri, PhD (University of Bordeaux I, 1985).

[ 141 A.H. Thompson, Phys. Rev. Letters 40 ( 1978) 15 11. [ 151 R. Schiillhom, Angew. Chem. Eng. Ed. 19 (1980) 983. [ 161 C. Fouassier, G. Matejka, J.M. Reau and P. Hagenmuller, J. Solid State Chem. 6 (1973) 532. [ 17 ] C. Delmas, C. Fouassier and P. Hagenmuller, Mat. Res. Bull. 11 (1976) 1483. [ 18 ] A. Maazaz, C. Delmas, C. Fouassier, J.M. Reau and P. Hagenmuller, Mat. Res. Bull. 14 (1979) 329. [ 191 M.G. Thomas, WI. David, J.B. Goodenough and P. Groves, Mat. Res. Bull. 20 ( 1985 ) 1137. [ 201 L.A. De Picciotto and M.M. Thackeray, Mat. Res. Bull. 20 (1985) 187; Solid State Ionics 18/19 (1986) 773. [ 2 1 ] A. Maazaz, C. Delmas and P. Hagenmuller, J. Incl. Phenom. 1 (1983) 45. [ 221 A. Maazaz and C. Delmas, C.R. Acad. Sci. 295 ( 1982) 759. [23] WI. Bruce, M.M. Thackeray, P.G. Bruce and J.B. Goodenough, Mater. Res. Bull. 19 (1984) 99. [24] J.P. Doumerc, A. Ammar, A. Wichainchai, M. Pouchard and P. Hagenmuller, J. Phys. Chem. Solids 48 (1987) 37. [25] S. Kikkawa, S. Miyazaki and M. Koizumi, J. Solid State Chem. 62 (1986) 35. [26] Chang, Soon-Ho, PhD (University of Bordeaux I, 1989). [27] C. Delmas, C. Fouassier and P. Hagenmuller, Physica 99B (1980) 81.