Electrical behavior of lithium intercalated layered In-Se compounds

Mat. R e s . B u l l . , Vol. 20, p p . 287-292, 1985. P r i n t e d in t h e USA. 0025-5408/85 $3.00 + .00 C o p y r i g h t (c) 1985 Pergamon P r e s s L t d .

ELECTRICAL BEHAVIOR OF LITHIUM INTERCALATED LAYERED In-Se COMPOUNDS

C . J U L I ~ •, E.HATZIKRANIOTIS °•, A.CHEVY .°° and K.KAMBAS °° • Laboratoire de Physique des Sol des, associ~ au CNRS Universit~ P.et M.Curie 4,place Jussieu Paris France "" Solid State Physics Department Aristotle University, Thessalon ki Greece "'' Laboratoi~e de Physique des Mil eux Tr~s Condenses associ~ au CNRS Universit~ P.et M.Curie 4,place Jussieu Paris France

( R e c e i v e d December 12, 1984; R e f e r e e d )

ABSTRACT : InSe and In2Se 3 was lithium intercalated by mean of a spontaneous intercalation reaction. The electrical conductivity was studied and was found to be altered by 3-orders of magnitude with respect to non-intercalated samples. An interesting time dependent anisotropy appeared during intercalation, associed to the initial non-uniform Li distribution and the movement of Li concentration kinks.

INTRODUCTION : Much effort has been invested in the last decade into the search of new, and the development of existing, rechargeable cathode materials for secondary non-aqueous Li cells. Layered compounds form n class of materials in which significant energy density can be stored in the form of intercalated layered conductors. The main interest is focussed today on using cathode which are chargeable in volume, such as reversible intercalated compounds (I). In-Se layered compounds are interesting candidates as intercalation electrodes, and their remarkable photoconductive behavior and photomemory effects (2) permit the design of a dual device : a secondary battery, combined with a photovoltaic system.

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InSe and In2Se 3 are widely studied members of the generic class MIIIxVI with M one of the metals Ga, In, Al and X : S, Se, Te. This class includes a large number of layered compounds. In those crystals, the molecular unit bonded by first-order covalent or ionic forces extends in two dimensions. This bonding scheme is the key to the unique properties of layered compounds, which are radically different from those of more classical semiconductors as regards band structure, vibrational spectra, optical properties or mechanical behavior. InSe and In2Se 3 are lamellar compounds composed by alternating selenium-indium planes that form neutral sandwiches, with units which are covalently bonded (3). Sandwiches are stacked one over the other and held together by weak van der Waals forces. Due to this particular bonding between the sandwiches, real van der Waals gaps appear in the structure between successive layers. These gaps, empty in pure and perfect materials, can be thought of as channels in which intercalation guest atoms or molecules can diffuse, without significantly altering the host structure. Structural and electrical properties of InSe and In2Se 3 are reported in table I.(4,5,6). In a previous paper (6), we have dealt with Li intercalation into In2Se 3 by galvanostatic electro-chemical method. The chemical Li diffusivity was found to be D£ = 5.5xlO-1Ocm2s -I and D# = lO-13cm2s-lo In an identical cell, the chemical Li diffusivity of LixInSe was found D = lo-llcm2s -I at 25°C for x = I. In this paper we present the electrical properties of lithium intercalated InSe and In2Se 3,

Material

Unit sandwiches form

InSe

Se-ln-ln-Se

In2Se 3

Se-ln-Se-ln-Se

Unit cell parameters

(~)

a=4.00 c=25,32 a=4.02 c=19,12

in-plane conductivity

Energy gap Eg(eV)

0.02-0.2

I .29

3-30

I .42

TABLE i. Structural and electrical data of In~e and In2Se 3 single crystals.

EXPERIMENTAL PART : InSe was gro~n by the Bridgman-Stockbarger method from high purity elements with an excess of indium (5). Rombohedral Y-3R polytype is formed that way. The typical resistivity of the sample was 35 ohm.cm at room temperature and the free carrier concentration n = I015 cm-3. in2Se3 was grown by direct fusion of high purity elements in stoichiometric proportion (7). Hexagonal W-phase In 2 Se 3 polytype is prepared that way. Crystals were n-type and the typical resistivity of the samples was 3.8 ohm.cm and the free carrier concentration was n = 1018 cm -3.

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289

Crystals could be easily cleaved in planes perpendicular to the c-axis. The samples we used were cut using razor blade as to obtain platelets of lateral dimensions of a few mm and typical thickness of 200-300 ~m. Almost damage-free surfaces were obtained during sample preparation. The electrical properties of the samples were determined by resistivity and Hall effect measurements before and after Li intercalation. The resistivity was measured using the four probe direct current van der Pauw method (8). Electrical contacts were made by ultrasonic soldering of pure In metal to the edges of the sample. Contacts appeared to have ohmic behavior for currents in the range I0 ~A to I0 mA. Lithium intercalation of InSe and In2Se 3 was performed by direct reaction with n-butyl lithium. Samples were immersed into 0.1M solution of n-butyl lithium dissolved in hexane. Experiments were carried out into a controlled atmosphere of high purity argon. The electrical properties of intercalated samples were monitored during intercalation. An Apple II microcomputer was used to control the timing and collect the data. Resistivity measurements were taken by van der Pauw method. A low current (250 ~A) was applied to the sample for a short period (less than 30 sec.) and was switched off after each measurement. Measurements were taken one every 20 min., so that the intercalation process could remain practicaly undisturbed. The ohmic behavior of the contacts was checked after each measurement and was verified in detail at the end of the experiment.

RESULTS AND DISCUSSION : A typical resistivity versus time curve is given in figure I. and figure 2. for InSe and In2Se 3 respectively. No correction to the resistivity value has been made due to the expansion of the c-axis. Table 2. gives the resistivity values at t=O and t>10 hours and the estimated c-axis expansion.

Material

InSe In2Se 3

Resistivity at t=O (ohm.cm)

Resistivity at t>10h. (ohm.cm)

35 3.8

0.08 0.01

c-axis expansion

<3% 5"/.

TABLE 2. Typical data of Li intercalated samples.

The reaction for lithium intercalation into InSe, for example, can be written in electrochemical form as : ÷xe"

x Li÷+ InSe

~ LixlnSe

(I)

If we consider that, the Li÷/Li electronic level is far above the Fermi level of the material, then the intercalated lithium remains ionized in the form Li ÷. The excess x electrons (eq.l), must find a position in the band structure of the host, and this results in the increase of the conductivity. Actually, the conductivity of Li intercalated samples is altered by approximately 3 orders of magnitude, with respect to the non-intercalated samples.

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I

1.0

J

[' '

Vol. 20, No. 3

60 5O

|

tv

L ~, lose

Se)

Li x In:

Ja0

)-

>30 5

t.

o, %

o el.2

I0 OI 0

I IO

S

I IMll

I IS boy." I

2~

~5

25

11M£

FIG.I Resistivity v.s time curve during lithium InSe intercalation at RT.

kovrm

FIG.2 Resistivity v.s time curve during lithium In2Se 3 intercalation at RT.

Conductivity varie very rapidely at short times, and further varies at a considerably lower rate. Since intercalation is a spontaneous reaction, the rate of Li transfer across the sample/electrolyte interface sets the boundary condition to the diffusion problem. Let us write Cs the interface concentration at any time, and C~ the one which would be in equilibrium with Li concentration in the electrolyte far from the interface. Mathematically the boundary condition at the sample/electrolyte interface is set as : dC - O ~ = f(C~-Cs)

at x=O

(2)

where D is the diffusion constant and f the exchange constant. Let Co be the equilibrium intercalant concentration through-out the host material at time t=O. Noting also C=C(x,t) the concentration at a point x remote from the interface at time t>O. For the shake of simplicity we assume plannar diffusion and semi-infinite geometry. The solution to such a diffusion problem is given by Crank (9) as : C -Co C -C----'-o= erfc 2 ~ t -

exp(hx+h2Dt)erfc(2---~t+h

Dt)

(3)

where h=f/D and erfc are the usual abbreviation for the complement of the error function defined as : err (z) = ~----/o:x P (-x2) dx

(4a)

erfc(z) = I - erf(z)

(4b)

In figure 3. we have plotted the concentration distribution (eq.3) for different values of the dimensionless parameter h~D't. It can be easily seen, that the rate at which the total amount of Li per unit cross-sectional area changes, mainly depends on the diffusion constant D of Li ions from the interface into the bulk material, while the value of the parameter h actually includes the concentration at the interface. It should also be noted that at long times the concentration profiles decay toward an homogeneous distribution.

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20, No.

I n - S e COMPOUNDS

3

291

If l is the lateral dimension, the condition for homogeneous Li distribution is set by the unequallity : 12/4Dt << I

(5)

1 o0~

"

i

=

I

w'1

ooo Li.lnSe Ll.ln2Se ~

cl~ *°° ". °°°.o. oJ ,8lo .6

14

,

\

4

Z" ,,

L

O|

I.Z

~.i Z'J I/(4Oll'~

.4

%%•-o.,

0

.,

I 4

TIME

*°°°%°°

I e

,,

I 12 hours

I 16

20

FIG.4

FIG.3 Concentration distribution for the case of spontaneous intercalation (eq.3). The different curves correspond to various values of h ~ (h=f/D).

Time dependence of the van der Pauw resistance ratio RI/R2 as intercalation proceeds. The induced "anisotropy" is due to the non-uniform Li distribution.

An interesting feature, however, is the time variation of the "resistance" ratio RI/R2. Figure 4. shows this behavior for InSe and In2Se 3. According to van der Pauw formalism (I0), for an isotropic sample the ratio RI/R2 depends only on the geometry of the four contacts. The time induced "anisotropy" is therefore related to the intercalated lithium. According to the diffusion model, the condition (5) is not met for the time scale of our experiment (t=25 hours). This indicates that the Li distribution is non-uniform. Concentration gradient acts as the driving force for the diffusing ionic species. As Fick's first law predicts : dC JLi.= - O d-'x

(6)

The transport of ions in mixed electronic ~nd ionic conductors proceeds through the simoultaneous movement of electrons, so that the two current densities are connected through the requirement of local electrical neutrality. For a monovalent ion as Li, this condition is set as : JLi += Je"

(7)

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This means that the Li concentration profiles through out the material correspond to local variation of the Fermi level. The appearence time-depended "anisotropy" should therefore be associated to the time evolution of lithium concentration profiles (II). Diffusing cations act as mobile donors giving rise to such "anisotropic" behavior. ACKHOWLEDGEHENTS :

The authors would l i k e to thank Prof. M.I~4LKANSKI for helpful discussions and also Mr. G.HOUGET for his technical collaboration. REFERENCES : I . M.S. Whittingham, Progr.Solid State Chem., I_.22, 41, (1978). 2. A. Segura, J.P. Guesdon, J.M. Besson and A. Chevy, J.Appl.Phys., 5.4, 876, (1983). 3. A. Likforman and M. Guittard, CR Acad.Sci.(Paris), 4. A. Likforman, Thesis, Universit~ R.Oescartes,

279C, 33, (1974).

Paris, (1977).

5. A. Chevy, Thesis, Universit~ P.et H.Curie, Paris, (1981). 6. M. Balkanski, K. Kambas, C. Julien, J. Hammerberg and D. Schleich, Solid State lonics, 5, 3S7, (1981). 7. K. Kambas and J. Spyridelis, Mat.Res.Bull., S. L. van der Pauw, Philips Res.Rept.,

1.3, 653, (1978).

L3, I, (1958).

9. J. Crank, The Mathematics of Diffusion, Oxford Univ.Press, I0. L. van der Pauw, Philips Res.Rept., II. E. Hatzikraniotis, (to be published).

(1967).

I_66, 187, (1961).

C. Julien and M. Balkanski,

Solid State lonics,