Insertion reactions in electrode materials

Solid State Ionics 18 & 19 (1986) 847-851 North-Holland, Amsterdam

847

INSERTION REACTIONS IN E L E C T R O D E M A T E R I A L S

D.W. MURPHY AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974, USA

A structural comparison of host structures and lithium insertion compounds is presented in order to illustrate the factors that influence insertion reactions. Particular emphasis is placed on comparing polymorphs of the same formula such as cubic and layered TiS2 and cubic, ruffle and anatase TiO2. In general it is concluded that cation interactions play a major role in oxides but are much less important in sulfides.

below.

1. INTRODUCTION It is now widely recognized that intercalation or insertion reactions which occur topotactically can provide a mechanism for

For

a

determines the

good

posxtive battery electrode material

capacity, 2)

the

voltage, 3)

the

rate;

1) 4)

rechargeability, and 5) requires no reactivity with the electrolyte.

kinetically fast and reversible solid state electrochemical reactions.

For a combination solar ,~ell and battery 6)

In retrospect, it has been shown that this type of mechanism is

maintenance of semiconductivity and carrier mobility with changes

important in both the NiOOH/Ni(OH)2

and MnO2/Mn203

in stoichiometry. For an electrochromic there must be a color

couples in NiCd and LeClanchd cells. More recently insertion

change as a function of x, and for sensors there must be selectivity.

reactions have been used or proposed in ambient temperature secondary

lithium

batteries,

electrochromics, electrocatalysis,

translates to

Several excellent review articles on intercalation chemistry and electrode materials are available?-3

This paper is not

electrochemical sensors, energy storing solar cells, and as ohmic

intended to be an extensive review of the applications or of all of

contacts for ionic conductivity measurements. For each of the uses

the promising materials under study, but rather will address the

above the requirements for suitable electrode materials vary,

importance of structural features in controlling intercalation

necessitating a need for a fundamental understanding of the

reactions in general by comparing reactivity with Li in polymorphs

factors influencing the reactions and the nature of the products. In

of the same stoichiometry and by comparing reactivity of compounds with systematically varying structural features.

each case the reactions are charge transfer reactions usually with a change in oxidation state of a transition metal ion as in Equation 1

2. TiS2

for alkali metal reactions.

One of the most studied intercalation systems is the layered

A + MnXy - - A+Mn-XXy

(1) A.,TiS2.

The structure of the two dimensional TiS2 host is

The important properties of the reactions and the materials which

illustrated in Fig. 1. For A - L i the system forms a homogeneous

must be considered include:

single phase for 0 ~< x ~< 1.0 and the chemical diffusion coefficient

1)

The

A-MXy

phase

diagram

identifying limiting

compositions, intermediate phases and phase widths. 2) The thermodynamics of the reactions, including AG, AH and AS. 3) The chemical diffusion coefficient, D, of the interealant in the host. 4) Structural changes, both macroscopic (volume) and microscopic (distortions and defects).

of Li is 10-8 - 10-9 cm2lsec.2'4 These values make possible a high rate Li/TiS2 battery with a theoretical energy density of 480 Whr/Kg. 5-6 The volume increases by 10.7% as x increases from 0 to 1 with 80% of the increase attributable to c axis (interlayer) expansion.7 The larger alkali metal Na forms a stage two compound (Na in every other layer) at x-0.33 and two different three layer repeat structures for 0.38 < x < 0.68 and 0.79 < x < 1.s In these latter two phases the TiS2 layers have slipped with

5) Thermal and chemical stability of the compounds.

respect to each other such that Na

6) Electronic properties of the materials.

coordination at intermediate x and octahedral coordination at the

The factors important to a few applications are outlined

highest x.

0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

has trigonal prismatic

The diffusion coefficient of Na is three orders of

848

DI W, Murphy / Insertion reactions in clectrc;de materials

ions of different sizes because of the ease of interlayer separation.

--A

Recently a cubic defect spinel TiS2 has been synthesized. I°

--B

The structure is compared to the layered structure in Fig. I.

--A

The homogeneous stoichiometry range LixTiS2 (0 ~< x ~< 1) is the

Insertion of Li is remarkably similar to the layered polytype. II

same and the EMF vs. composition curves shown in Fig. 2 arc

--B

quite similar. The volume change on Li insertion is quite close to that of the layered phase, but there is no easy expansion direction and the expansion is isotropic. The c-LixTiS2 system is closer to an ideal solid solution as evidenced by the validity of Vegard's law for the lattice parameter (Fig. 3) and a close fit of the EMF vs. composition curve to a mean field lattice gas model. The fit to this

--A

model and to Vegard's law suggests that Li occupies the same crystallographic site at all compositions.

The empty set of

B

octahedral sites is the only one of the appropriate stoichiometry.

C

Spinels ordinarily have a limiting composition AB2X4 with the A ion being tetrahedral, but A2B2X4 structures with octahedral A

A

ions have been reported for Li2V204 t2 and Li2Ti204 I~ as discussed in the next section.

No data

is yet available on diffusion

coefficients of Li in c-LixTiS2 but indications from cell cycling data suggest that Li diffusion is comparable to that in the layered polymorph. The Ti-S stoichiometry in layered TiS2 strongly influences Li diffusion with a few per cent excess Ti between the layers dramatically slowing intercalation. The c-TiS2, however, may be regarded as a layer structure with 1/4 of the interlayer sites filled by Ti with a corresponding number of intralayer vacancies. All ol

~.0

FIGURE 1 The TiS2 structure (Cdl2 type, HCP anion packing) is illustrated at the top. The structure of c-TiS2 (middle) may be derived from the layer structure by shifting the layers to the CdCI2 structure (CCP anion packing) and moving one quarter of the Ti to interlayer octahedra. The bottom illustrates the positions of Ti in one layer (e) relative to the layer above (0) and below (~).

2.6

03 0 >

2.2 SPINEL--/

t.8

1.4

magmtude faster at compositions where Na is trigonal prismatic4 which results in a limited cycling range at room temperature, but allows complete cycling at 280°C. 9 In addition to such facile

1.0 0.0

I 0.2

I 0.4

I 0.6

1 08

10

x IN L i x T i S z

polytype transformations, layer structures can also intercalate electrolyte solvent or solvent coordinated alkali metals which can cause large volume changes and loss of mechanical integrity of electrodes. Layer structures in general are able to accommodate

FIGURE 2 Data from ref. I1 for a Li/LiAsF6, propylene carbonate/c-TiS2 cell compared to a layered TiS2 cell.

D. W. Murphy / Insertion reactions in electrode materials

849

the c-TiS2 made in our laboratory to date has at least 2.5% excess

recently synthesized polymorph TiO2(B) which is a Wadsley type

Ti and as much as 7% does not appear to dramatically slow

shear

intercalation, although potential Li sites are filled by the Ti.

Li75TiO2(B)) 8 For the comparisons in this section, however, we

Although this three dimensional framework presents no size constraints on lithium insertion, we have been unable to insert Na.

phase

has

a

limiting

insertion

stoichiometry

of

will limit discussion to the close packed R, A, and spinel structures. Whereas the defect spinel TiS2 forms a homogeneous single

In addition to these two crystalline polymorphs of TiS2, an amorphous form has also been prepared. An open circuit potential

phase for c-LixTiS2 (0 < x <

of 2.55V were reported but no other cell characteristics. Chemical

phases with narrow stoichiometry widths at x - 0 . 5 and x - l . 0 .

1), the analogous oxides form

insertion was accomplished with n-BuLi to a stoichiometry of

Removal of Li from c-Lio.sTiO2 results in an amorphous LixTiO2

Lil.o9TiS2.

which will not reinsert Li. Neutron diffraction has shown that at

These

values

are

consistent

with

the

two

crystallographic forms, but more electrochemical data is needed

x-0.5

for a more detailed comparison. Reasonable cycling of cells with

tetrahedral sites, and at x - l . 0 the Li occupies only the octahedral

amorphous MoS2 has been reported) 4

sites and the Ti remains unchanged, t9 Three points need to be

Intercalation of Li in

the compound is a normal spinel with Li occupying

1T

addressed in comparing the oxide and sulfide: 1) the narrow

polytype 15 which is stable on cycling and is the basis of a commercial cell produced by Moli Energy L t d ) 6

composition range of the oxide compared to the sulfide, 2) the

crystalline 2H-MoS2

results in a transformation

to the

tetrahedral site occupancy in c-Lio.sTiO2, and 3) the instability of c-TiO2. To a large extent these have a common origin which is much larger cation-cation interactions in the oxide. Given the c-

10.1

6.?..

TiO2 framework, the tetrahedral site is an electrostatic minimum at x - 0 . 5 whereas the octahedral site is favored at x - l . 0 .

#6.1 ';

1o.o

in

with

lattice

gas

models

either

phase

disproportionation or ordering of ions will occur if interactions

6.o

between ions are sufficiently attractive or repulsive, respectively.

9.9 .J

accordance

Further.

These interactions may be coulombic, but also include effects on

5.9

the host such as expansion or metal-metal bond formation. For c-

9.8 5.8

LixTiS2 which is homogeneous for 0 < x < 1, a fit of the E M F versus composition curve gives a repulsive interaction of 3.0kT

5-7 0

0.2

0.4

0.6

0.8

9.?'

which is presumably larger in the oxide. The defect spinel c-TiX2

X IN LixTIS 2

is expected to be much less stable for the oxide than the sulfide because of increased cation-cation repulsion since each Ti is

FIGURE 3 The cubic lattice parameter for c-Li,TiS2 and the interlayer separation in layered LixTiS2 as a function of x.

required to have six closest neighbors (through edge shared octahedra) whereas rutile and anatase have only two and four respectively.

This interaction is relatively unimportant in the

sulfide because of increased bond lengths and polarizability. An

3. Ti02

alternative covalent interpretation is based on a preference for

A comparison of different TiO2 based structures shows a

planar three coordination for oxygen versus pyramidal for sulfur. 2°

more critical relationship of structure to insertion in the oxides 17

Although the defect spinel structure is less stable for oxides, it is

than in the sulfides. Of the two well known TiO2 structures, rutile

known for MnO2 .19

(R) and anatase (A), substantial insertion occurs in LixTiO2(A)

Addition of Li to the

HCP

rutile structure requires

(0 < x < .7) whereas insertion TiO2(R) is rapid (D ~ 10-7) but

occupation of octahedra or tetrahedra that share faces with Ti

occurs only to the extent of a percent or so. The spinel framework

octahedra for which coulombic repulsion of cations would be

discussed previously for c-TiS2 is also known for the oxide, but

significant.

only for the specific compositions LiTi:O4 and Li2Ti204.

substantial amount in TiO2(R), insertion is facile in MOO2, WO2

A

Although

Li

insertion

does

not

occur

to

any

D.W. Murph.v / Insertion reactions in electrodv materials

850

R u O 2 and lrO2 .22-24 In each of these cases there appears to be an

ReO 3

increase in metal-metal bonding which can overcome unfavorable

PdF 3

coulombic interactions. The R u - R u bond distance decreases from 3.11 to 2.78,~ on insertion of Li into RuO2. The C C P structure of anatase and spinel minimize coulombic repulsion by allowing the closest approach of ions to be edge sharing.

In addition, metal

metal bond formation which also occurs in Li~TiO2(A) is an added driving force for insertion.

4. Re03 RELATED STRUCTURES The discussion above illustrates that geometrical constraints are more critical in oxides than in chalcogenides and that special attention

must

be

paid

interactions in oxides.

to

minimization

of

cation

cation

One approach to this problem is to use

non-close packed structures such as ReO3, WO3, and shear structures derived from them. ReO3 is a cubic structure comprised of [ReO6] octahedra connected exclusively by corner sharing such that all R e - O - R e bonds are linear.

This leaves a cuboctahedral

cavity as the vacant space for insertion. WO3 has a slight triclinic distortion of the ReO3 structure. Three distinct phases LixReO3 occur at x < 0.3, x - -

1 . 0 a n d 1.8 < x < 2.0. L i , W O 3 forms a

FIGURE 4 The relationship between the ReO3 and PdF3 structures. The structures can be interconverted without bond breaking by bending the M-X bonds. The bottom illustrates the transformation of the cuboctahedral cavity in ReO3 to two octahedral sites in PdF3.

phase similar to the low x ReO3 phase with a wider limit 0 < x < .7. Structures of each of these phases have been determined by neutron diffraction. Both Li0.2ReO32s and Lio.35WO326 have ki in

structures

the center of the cuboctahedral cavity in which four coplanar

crystallographic shears add oxygens to the square faces of the

oxygens have been displaced toward the lithium, resull~ing in a

cuboctahedral cavities with the number of capped faces depending

doubling of the unit cell and a 4% reduction in cell volume. Both

on the type of shear.

LiReO3 and Li2ReO3 have structures in which substantial bending

V6013, VO2(B), TiO2 (B), and FeV308 all cavities are equivalent,

of the

PdF3

whereas for others such as Nb2Os, WsNb18069, and W 5V2sO7

PdF3

more than one type of cavity occurs.

linear R e - O - R e

frameworkfl 7 The

bonds has occurred to give a

relationship

between

the

ReO3

and

preventing contraction

around

the

lithium.

These

For some of these structures such as V205,

A neutron structure of

structures is illustrated in Fig. 4. This framework is H C P and Li

Li2FeV30 8 showed that the cavity structure was maintained on

occupies all the octahedral sites within the framework in Li2ReO3

insertion

and half the octahedral sites in an ordered way in kiReO3. The

pyramidal sites as shown in Fig. 5. 29 The member of this class

kinetics are rapid in the low x phase, but the contraction to close

most extensively studied electrochemically is

packed structures in the high x phases reduces kinetics to such a

in a ki cell is dependent on stoichiometry, but is on the order of I

and

found that

Li occupies five coordinate square

V6OI3. The capacity

degree that equilibrium titration curves could not be obtained.

ki/V. 3° Chemical diffusion coefficients of 3.5 x 10-~cm2/sec and

Substitution of N a for Li in L i x N a y W O 3 (0 < x < 0.5, x + y <

4.5 x 10-9cm2/sec in two crystallographic directions at 120°C in

0.93)

ki017V60 13 have

has

the

effect of stabilizing the

cubic

structure

and

polymer

electrolytes.

L i / V 6 0 I~

room temperature have been measured. 28

A b r a h a m 31, found that carbon was necessary for high rate ambient

cells

have reported

using

Chemical diffusion coefficients for Li as high as 10 -7 cm2/sec at

ReO3 contain edge shared octahedra which add rigidity to their

North

measured

Hooper

A number of crystallographic shear structures related to

and

been

enhancing diffusion of Li because the N a increases the cell volume.

with

polymer

high efficiency cycling of electrolytes

at

120-140"C.

temperature performance and that the cell was easily d a m a g e d by over discharge.

D. W. Murphy / Insertion reactions in electrode materials

/'l"'" /'" "

t.961

","

"'"

4.

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28.

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" "\,

-i 4.96/ ,;'

,/'"

FIGURE 5 Location of Li in Li2FeV3Os. 5. C01~CLU,510N The systems described here and others under investigation in other laboratories, structure

and

demonstrate the important effects of

electronic

properties

on

insertion

reactions.

Although this article has discussed only insertion of Li, these are the same considerations that must be applied to other ions.

A CKNO WLEDGEMENTS 1 am indebted to many colleagues for collaborative work in this area, especially S. Sinha for the work on c-TiS2 and R. J. Cava for neutron diffraction.

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851

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3.

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