Topochemical reactions of rutile related structures with lithium

Mat. Res. Bull. Vol. 13, pp. 1395-1402, 1978. Pergamon P r e s s , Inc. Printed in the United States.

TOPOCHEMICAL REACTIONS OF RUTILE RELATED STRUCTURES WITH LITHIUM D. W. Murphy, F. J. Di Salvo, J. N. Carides and J. V. Waszczak Bell Laboratories Murray Hill, New Jersey 07974

ABSTRACT The topochemical lithiation of rutile related MO 2 with n-BuLl, and in nonaqueous lithium electrochemical-cells is reported. This series illustrates the importance of electronic conductivity and cell volume to substantial lithium incorporation. The Li MO^ compounds z are metastable and decompose at temperatures less than 250°C.

Topochemical reactions, particularly those involving lithium, have attracted interest in recent years because of their utility as cathodic reactions in high energy density secondary (r ech a rge abl e ) b a '' ~ e r i -~ . (i- 9) Topochemical reactions[ iO) involve a "host" lattice into which a "guest" is inserted with the product maintaining the basic structural features of the "host." If the structural changes accompanying a topochemical reaction are minimal (a special case termed intercalation), the overall reaction will likely be readily reversible. If the reaction is of a redox type, the reversibility of the intercalation reaction may serve as the basis of a secondary battery. Several classes of "host" structures suitable for use as cathodes in room temperature batteries utilizing lithium as the "guest'; h~ve bxeedes~d e ~ t - ~ ednden~hPlSd~s~7t~a~ .~tl~em~tald;halc°genlbdoeSd~e~ ~)ayered transition metal dichalcogenides are a premier example, wherein the structural changes between MX 2 and LiMX 2 (M=Group IVB, VB or VIB and X=S, Se) (Eqn. l) xLi + MX 2 = Lix~X 2

(1)

involve only a small separation of the MX~ layers to accommodate the lithium. The structures and properties oF these compounds have been recently reviewed. (7,8) The LixMX2. may form a single ternary phase or

1395

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D . W . MURPHY, et al.

Vol. 13, No. 12

be multiphase as a function of lithium content (x). Single phase systems (e.g. LivTiS o) appear to be readily reversible, whereas multiphase systems may be reverslble (LI VSe^) or poorly reverslbl (Li VS^).(ll) The variety of behavlor f~ound in thls seemlngly slmple cla~s ~f compounds is even more complex in other classes of materials. In particular, we have attempted to understand the factors affecting topochemical incorporation of lithium in metal oxides which might lead to improved electrodes. Although it is known that a number of metal oxides such as V~O~,MoO~, and MnO 2 u~dergo $opochemical reaction with lithium at a~b~ent ~emperatures, (2,5,12) much work remains to characterize the products of reaction with Li and systematize results. Simple model systems may afford valuable insight which could then be applied to other materials. One large class of compounds, rutile related metal dioxides, M02(12) exhibit a range of possibly important parameters for lithium incorporation such as the size of vacanies available to Li and its diffusion pathway, electronic conductivity, and various crystallographic distortions. Previous work suggested that rutiles might exhibit interesting behavior with lithium. Johnson(lh) reported a diffusion coefficient of l0 -8 cm2/sec for Li 02Ti02 at room temperature, comparable tQ . LiTiS2.(7,15) Rutile ~ 0 2 gives a larger HMF in a cell with Li (12) than predicted (see Table II) for the displacement reactions illustrated in Eqn. 2 and 3,

2 Li + 2MnO 2 = Li20 + Mn203 h Li + MnO 2 = 2Li20 + Mn

(2.63 volts)

(1.70 volts)

(2) (3)

indicating an alternative mechanism, likely topochemical. Besenhard "15"(~ has reported reversible i~corporation of Li into CrO 2 in an electrochemical cell to near 0.5 Li/Cr. Treatment of rutile type MO^ with n-BuLl in hexane results in substantial lithiumincorporatio~ to form Li MO~ ( x ~ 1.0) for M=Mo, Ru, Os and Ir (the compounds of Tc, RhXan~ Pt were not studied). Limiting compositions of Li 0- TiO2 (black), Li0.2MnO2 , and Li0.8CrO 2 were obtained with the last's~owing some side reaction as well as a small amount of unreacted Cr02, while V02,NbO 2 and ReO 2 gave no reaction. The stoichiometries of the substantially lithiated products and their unit cell parameters from x-ray powder diffraction data are summarized in Table I. The ideal rutile structure may be described as an hcp oxygen lattice with octc~,edrally coordinated metal ions forming edge shared infinite chains along [o01S of the tetragonal cell. The chains are crosslinked by sharing corners to form an equal number of identical vacant channels (Fig. i). It is in these channels,which contain two tetrahedral and

Vol. 13, No. 12

TOPOCHEMICAL

REACTIONS

1897

TABLE I CRYSTALLOGRAPHIC PARAMETERS FOR RUTILESaAND LITHIATED RUTILES O

o

O

a(A)

C(A)

c/a

Ti02

4.594

2.958

0.644

62.4

vo2(C)

4.552

2.846

0.625

58.97

CrO 2

4.422

2.916

0.660

57.02

Li0.8Cr02

4.84

2.88

0.595

67.5

MnO 2 NbO (c)

4.398 4.567

2.874

0.653

55.58

2.994

0.618

70.297

4.82

2 80

0.581

65.05

Lil.0Mo02 (

5.13

2 78

0.542

73.16

wo2(c)

4.88

2 78

0.570

66.20

5.16 4.491

2

75 3 106

0.533

73.22

0.692

62.64

Lil.3Ru02 (

5.043

2 784

0.552

70.80

0s02

4.497

3.182

0.7O8

64.36

Lil.50s02 (c)

5.162

2.795

0.541

74.45

IrO 2

4.499

3.155

o.7ol

63.85

Lil.51r02 (c)

4.873

3.190

0.655

75.75

MQ½V~O 2

4.64

2.86

o.616

61.66

Lil.0Mo½V~0 2

5.06

2.79

0.551

71.43

_ _2(c)

Moo 2

c)

~

c)

Lil.0W02 ( RuO 2

c)

V(A 3 )

AV/V

dec T(°C)b

0.184

0.125

210

0.106

80

0.130

210

0.157 0.186

>80

o.158

130

a.values from ref. 13. b. values determined by irreversible magnetic susceptibility changes. c. pseudotetragonal parameters.

FIG. i The rutile structure illustrating the channels available for lithium incorporation.

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D . W . MURPHY, et al.

Vol. 1S, No. 12

one octahedral site per metal ion , that lithium incorporation is likely. A number of MO^ (M=V, Nb, Mo, W, Tc and Re) have distorted structures due to covalent bonding of pairs of metal ions along [001], resulting in tetragonal or monoclinic superlattices. The values in Table I are pseudotetragonal for these cases to allow for easy comparison.

Strong metal-metal bonding results in low c/a ratios~13)e.g., f c/a is 0.692 for RuOp and 0.577 for MOO2, the latter containing a metal-metal bond. 0fi lithiation, the a axis increases for all the rutiles, while the c axis either increases or decreases, pres~mably as a result of correspondingly decreased or increased metal-metal interaction. The c/a ratio decreases slightly for most lithiated rutiles compared to the parent compound except for those of RuO^ and 0s0^ which decrease sharply indicating significant metal-metal ~onding. ZIn all cases the volume of fully lithiated rutiles increases by 10-18% over that of the pure host.

All rutiles which readily incorporate lithium are metallic conductors. High electronic conductivity could aide incorporation of lithium by screening coulombic repulsion between lithium ions. High electronic conductivity is not sufficient, as VOp fails to incorporate lithium even at temperatures above the metal-insGlator transition temperature (69°C), rather some decomposition occurs at elevated temperatures. In addition V n oMon i0~(17) which is metallic and undistorted at room temperat{~@, ~ T l ~ to incorporate lithium.

The data in Table 1 indicate the importance of unit cell volume to easy lithiation. Of the 3d MOp, only CrOp readily incorporates a significant quantity of lithium, ~nd even it-does not form a pure compound free of side reaction products. When treated with n-BuLi at 50°C, M n 0 p f o r m s a red-metallic looking bronze after qO.5 hour followed by a deep-blue bronze shortly thereafter, each product containing only ~0.01 Li/Mn. After two days the composition LiMnO 2 is obtained, but the structure corresponds to that previously descrlbed for LiMn0_(18) • 2 prepared at hlgh temperature, which has a rock salt structure. The initial rutile MnO~ volume is 55.6~3, whereas the cubic LiMnOo has a volume (for the same number of formula units) of 73.9A 3 , or a 33% volume increase. It appears to be impossible to have such large volume changes while maintaining a constant structure. The compounds LiVO 2 and LiCrOo have also been prepared at high temperature and have a layer type structure(19) with volumes of 7 7 . 3 ~ 3 a n d 78.5A 3 respectively which would require 30-40% increases in volume over the rutile, oAll the Li MO~ rutile type compounds formed have volumes at least 70A3 for X x near or~beyond one, and expanslons of 10-15% over the parent rutile. For the Mo V. Or series, lithium incorporation occurs readily at room temperatur y "~toYgive Li~ ~Mo V~ O~ for y > 0.5. The volume of • I.U I-~" Mo y V~ O~ Increases linearl~ wi{h y. Volume considerations are not ~-y

Vol. 13, No. 12

TOPOCHEMICAL REACTIONS

1399

sufficient to ensure lithium incorporation as demonstrated by the lack of lithium incorporation in the non-metallic rutiles TiO~ and Nb02, even though their cell volumes are 62.4 and 70.3~ 3 respectively. No correlation was found to indicate any importance associated with the various structural distortions other than the effect on electronic conductivity. Lithiated rutiles were readily delithiated by treatment with iodine or simply with water to return the parent rutiles, confirming the close structural relationship between the MO 2 and LixMO 2. Lithiation occurs electrochemically when the rutile is used as the cathode in cells of the type Li/Li+/M02, wherein the cathodic reaction is given by Eqn. 3. xLi + + MO 2 + xe = LixMO 2

(3)

Discharge and charge curves for selected rutiles are shown in Fig. 2 and the voltages are summarized in TaDle II, along with calculated EMF's for displacement reactions such as represented by Eqn. 4. hLi + M02 = 2Li20 + M

(4)

TABLE II RELEVANT THERMODYNAMIC VALUES

MO 2

Ti02

-AG~ (Kjoule/mole) a

889

Product b (theor. v)

EMF of Li/LixMO2(v)

Ti (0.6) Ti203 (1.12)

VO 2

663

V (1.18) V203(1.97)

MnO 2

466

Mn (i.70) Mn203 (2.63)

2.7_2.0 (11)

MoO 2

536

Mo (1.51)

i. 7-1.1

WO 2

537

W (1.51)

i.i-0.5

NbO2

744

Nb (0.97)

RuO 2 IrO 2

251 172

Ru (2.25) Ir (2.46)

2.1 2.1

0s02

207

0s (2.37)

1.5

a - Values from J. G. Gibson and J. L. Sudworth, "Specific Energies of Galvanic Reactions", Chapman and Hall LTD., London (1973). b - Li20 is the other product of these displacement reactions.

1400

D . W . MURPHY, et al.

0.5Li '

3.0

Vol. 13, No. 12

1.5Li

1.0Li ~'I

5/

/

2.5

...........

(n 2.0

,.-7-------'7

1.5

-;:-'j

1

RuO2

1.0 0.5 0

0

I

I

I

i

i

I

I

|

1

2

3

4

5

6

7

8

I

I

I

i

|

I

9 10 11 12 13 TIME (HOURS)

I

i

14 15 16

I

I

I

I

17 18

19 20 21

2

I

I Li

1.5, I

7

"2 I

,

,

I

,

2

,

:5

',

4

,'

O.5Li

,

7

,

8

,

9

ILi

i

3.0

,

5 6 T I M E (HOURS)

I.SLi

i

I

MO 0 2

2.5

II

I0

1

2.0 o~

~

1.5 1.0

0.5

I --

: 1

I 2

L _

S

I 4

I 5

] 6

I 7

~ 8

I 9 TIME,

L 10

1J1

2L 1

I 13

I 14

I 15

I 16

I 17

J 18

I 19

(HOURS)

Fig. 2 (top):

9.7 mg Ru02, 0.15 ma

(middle):

7.8 mg. W02, 0.15 ma

(bottom):

8 mg MOO2, 0.15 ma

1M LiC10 h in propylene carbonate was electrolyte. indicate-cycle number.

Numbers on curves

20

Vol. 13, No. 12

TOPOCHEMICAL REACTIONS

1401

In each case the lithiated rutiles are found to be thermodynamically unstable with respect to such reactions, but are observed to be kinetically stable at room temperature (CAUTION!! Scraping a vial containing Lil.5IrO 2 with a spatula in a helium atmosphere resulted in vigorous decomposition). Decomposition temperstures for the lithiated compounds determined by irreversible magnetic susceptibility changes and by changes in the x-ray diffraction patterns are given in Table I. The voltages of Ru02, 0sO 2 and IrO 2 cells are relatively constant as a function of state of charge suggestive of a two phase system. For Ru02,x-ray data have confirmed that nominal compositions LixRuO 2 are approriate ratios of the limiting compositions RuO 2 and Lil. BRUO 2. For MoO 2 and WO 2 distinct breaks occur in the EMF as a function of state of charge indicating more complex behavior. The capacity of all the Li/Li+/M02 cells is gradually diminished with cycling. This may reflect a slow disproportionation of LixM02 to thermodynamically more favored products. Although WO_ could not be lithiated with n-BuLl, the cell 2 Li/LiC10h, PC/WO 2 afforded the electrochemical preparation of LiWO 2 at 1.1 to 0.5 volts, confirming that the free energy of reaction with lithium is too low to occur with n-BuLi. All other rutiles for which litbiation failed with n-BuLi also gave no lithiation in cells. The EMF for lithiation of W0_ is low enough to consider cells using LiWOp as anode. Preliminary date from cells of the type LiWO2/LiCIG4, PC/RuO 2 has been obtained. This cell delivers 1.5-1.0 volts. Low potential lithium intercalation reactions offer the possibility of circumventing the poor plating efficiency of pure lithium. Further irreversible reduction of RuO2,0sO 2 and IrO 2 occurs nesr one volt vs. Li/Li + indicating the onset of a major structural change or a displacement reaction. Excess n-BuLl also gives overreduction complicating the determination of the exact stoichiometry of the limiting compositions. For these rutiles lithiation clearly exceeds one Li/M and, since only one octahedral site is available per M, we assume that Li occupies at least some tetrahedral sites in these compounds. The variety of properties exhibited by the rutile related M02 have enabled us to demonstrate the importance of cell volume and electronic conductivity to reversible topochemical incorporation of lithium. Electronic conductivity is believed to reduce coulomb repulsion between lithium ions, and the volume correlates with the size of the available lithium sites and/or the lithium ~on diffusion path. We believe that the general guidelines determined here will be applicable to other classes of compounds to varying degrees. Much work remains even within the rutiles to c~arify questions of structure, thermodynamics, and the kinetics and anisotropy of lithium diffusion.

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D . W . MURPHY, et al.

Vol. 13, No. 12

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2.

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7.

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ll.

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