Mat. Res. Bull., Vol. 20, pp. 1347-1352, 1985. Printed in the USA. 0025-5408/85 $3.00 + .00 Copyright (c) 1985 Pergamon Press Ltd.

LITHIUM INSERTION INTO Li2Ti307

C.J. Chen and M. Greenblatt~ Department of Chemistry

Rutgers, The State University of New Jersey New Brunswick, NJ 08903

(Received August 1, 1985; Communicated by A. Wold)

ABSTRACT

Li2Ti307 with the ramsdellite-type structure undergoes lithium insertion reactions with n-BuLl. Li2+xTi~O 7 phases form with x = 0.5 and 1.0 at room temperature and at 50 C, respectively. The ESR spectrum of LI3TI307 confirms the partial reduction of Ti 4+ ions to Ti 3+. The electrical conductivity of the fully lithiated phase is several orders of magnitude higher than that of the host compound, suggesting charge hopping in the mixed valent lithlated compound.

Introduction

Recently we have reported on lithium insertion into the two polymorphs of LiFeSnO 4 with ramsdellite related structures (I). Li2Ti307 also crystallizes with the ramsdellite (y-MnO2) structure (2,3) shown in Fig. i. The structure is built up of edge-sharing distorted TIO 6 octahedra forming double chains (one octahedron by two octahedra in cross section) running parallel to the a axis, with one chain displaced i/2a along the a axis relative to the other chain. These double chains are interconnected by corner sharing in such a way that rectangular channels are formed along the a axis. In the tunnels, there are five possible sites: 4 tetrahedral (TI,T2,T3,T4) and one octahedral (Oc) (4). X-ray diffraction studies have shown (5) that the lithium ions occupy the T I and T 2 tetrahedral sites (Li I and Li 2 in Fig. I). Taking into account the distances between neighboring sites, four A cations per cell can be located in these tunnels leading to the limiting formula A4B408. Li2Ti307 (i.e. Li2.29Ti3.4308) can thus be considered as a partially occupied ramsdelllte. Since there are too few Ti 4+ ions to completely fill the octahedral sites (one out of 7 is vacant), these empty sites may be occupied by Li + ions. The single crystal x-ray diffraction structure refinements (5) indicate that the most likely formula is (Li I 72 ~ 2 28)[(LI0 57Ti3.43)08], where [] are vacancies, but the alternative formula with all the Li + in the channels, Li2.29 ~ 1.71)[(Ti3.45 ~ 0.57)08] fit the diffraction data almost equally

~Author to whom correspondence should be addressed.

1347

1348 C. J . CHEN, et al . Vol. 20, No. 11

olo

l o Li, Ti ot z= l /4 ¢

/

b ' o Li, Ti at Z=5/4

g ~. CHANNEL J Li SITE

0 0 ot z=l /4 ~ 0 o 1 z=3/4

Fig. i

Projection of the ramsdelllte structure of Li2Ti307 along [i00].

as well. Indeed the anlsotropy of ionic conductivity observed in single crystals supports the latter configuration (6). From the structural point of view, the ramsdelllte Li2Ti307 represents a framework structure with isolated one- dimensional channels. However, the long-range anisotroplc conductivity is probably due to the presence of vacant Ti sites which allow the hopping of Li ions between channels.

Since Li2Ti307 is a good ionic conductor (6,7), the titanium ions are in their highest oxidation states and there are many vacancies in the structure, this compound appeared to be a good candidate for lithium insertion study. Moreover, y-MnO 2 a commercially important cathode material in a number of llthlum-based primary cells, also has the ramsdelllte structure. However, little

information is available on the structural changes that occur upon lithium insertion into this material, because the unavailability of y-MnO 2 in good quality crystalline form. Thus a study of lithium insertion in Li2Ti307 serves as a model for lithium insertion in a ramsdellite material.

Experimental

Li2Ti307 was prepared by mixing high purity LI2CO 3 and TiO 2 in a 1:3 mole ratio and heating the sample at 750°C for one and at I050°C for three days. Chemical llthlatlon of the product was carried out by treatment with ~I.5N n-butyllithium (n-BuLl) in hexane at room temperature and at elevated (50-60°C) temperature, respectively.

The room temperature llthlation reactions were carried out in a hellum-filled dry box. For the high temperature llthlatlon the reactants (the Li2Ti307 powder and n-BuLi/hexane) were contained in a pyrex glass tube of ~7" length which was vacuum-sealed after the reactants were frozen in a dewar filled with liquid nitrogen. The glass tube was heated for several days in an oil bath (50-60°C) and occasionally shaken to facilitate mixing of the reagents. The reaction was terminated when the glass tube was opened in the dry box. The dark blue to black powder obtained by filtration of the solution was vacuum-drled for several hours. The amount of lithium inserted was determined by acid babe titration of the excess n-BuLl and by plasma emission spectroscopy of the llthlated products. Delithiation reactions were carried out on the lithium inserted compounds with ~O.IN iodine in acetonltrile (12/CH3CN). The amount of 12 reacted was determined by titration of the excess with standared thiosulfate solution.

Li2Ti307, its lithlum-lnserted and dellthlated phases were identified by powder x-ray diffraction using Ni filtered Cu radiation and silicon powder as an internal standard.

Vol. 20, No. i i LITHIUM INSERTION 1349

The qualitative electrical conductivities of thin cylindrical pellet samples were measured inside the dry box using a 2-probe method. The pellets were made by compacting the fine powder samples by stainless steel plungers in a teflon cylinder. The electron spin resonance (ESR) spectrum of the powder samples contained in sealed quartz tubes were recorded at liquid-nitrogen temperature on a Varian E-12 (X-Band) spectrometer calibrated with a Hewlett-Packard Model 5245L frequency counter and a Mn(ll) standard.

Results and Discussions

The color of the host solid, Li2Ti307 prepared by solid state reaction was dependent upon impurities present in the starting materials and on the container used. The color of pure Li2Ti307 is white; faint yellow color was observed when samples were prepared in platinum crucibles. The pellet forms of all samples are very hard. The powder x-ray diffraction patterns of Li2Ti307 products with differences in color are identical, and agree with previously reported data (3).

On lithium insertion at ambient and elevated temperatures, the original color of the host, Li2Ti307 turned blue in a few minutes and changed to an increasingly darker blue with time.

The maximum amount of the lithium ions inserted into the crystal structure of the ramsdellite, Li2Ti307 is x = 1.0 at high temperatures (50°-60°C). For the room temperature lithiation, x = 0.5. The stoichiometry of the high-temperature product, Li3Ti307 scaled to the ramsdellite formula is Li3.43Ti3.4308; thus the limits of the formula A4B408 predicted for ramsdellite with maximum possible occupation of sites is not quite reached. Powder x-ray diffraction patterns for both Li2.5Ti307 and Li3Ti307 are similar to that of the host material as shown in Table I. Data is given only for the high temperature product because for Li2.5Ti307 the shift in the x-ray diffraction pattern compared to the host is insignificant. The unit cell dimensions (c.f. Table i) were determined by least-squares fit to the observed data. The lattice dimensions increase along the b and c axes and decrease along the a axis. The unit cell volume of Li3Ti307 increased by only 0.67% because of the compensating changes in cell dimensions. The variation of the cell parameters is consistent with the structural properties (c.f. Fig. I) in which the b and c directions correspond to the in-plane dimensions of the tunnel which must increase to accommodate the guest Li + ions. In contrast, the slight shrinkage of the a axis may be due to increased polarization of oxygens by the Li + ions along the tunnels where the guest ions are presumed to be located. All three cell dimensions were found to increase upon lithium insertion in LiFeSnO 4 (high temperature polymorph), another ramsdellite-type compound (i). However, more significant increases of the b and c axes relative to that of the a axis were noted.

The original color of the host material could be restored by delithiation of the lithium-inserted compounds. The color change, lithium analysis and the x-diffractlon patterns (Table i) of the delithiated phases indicate that most of the inserted lithium ions may be extracted from the ramsdellite structure. The similarity of the lattice parameters and the x-ray diffraction line intensities of the host, lithiated and delithiated compounds suggests that these redox reactions are topotactlc and reversible.

Table I

~O

O1

Compound

lattice parameters

X-ray Diffraction Data of LixLi2Ti307

Ll2Ti307(x ffi 0)

I

Li3Ti307(x =

1.0 ±

0.1) 2

Li2+xTi307(x

~ 0)

3

a 2

.94

0(1

) 2

.92

3(1

) 2

.93

4(1

) b

5.0

10

(2)

5.0

54

(2)

5.0

22

(2)

e 9.537(3)

9.573(4)

9.522(5)

V 140.45

141.43

140.30

h k

i do

bg

dca I

0 2

0 4.77

4.77

i i

0 4.44

4.44

i 2

0 3.458

3.454

i 3

0 2.685

2.684

1 0

1 2.535

2.536

0 2

1 2.503

2.503

1 1

1 2.450

2.450

1 2

1 2.239

2.239

1 4

0 2.152

2.153

1 3

1 1.982

1.982

2 3

0 1.968

1.968

0 4

1 1.854

1.852

2 2

i 1.770

1.770

2

4

0 1

.72

6

1.7

27

3 2

0 1.575

1.576

1 5

i 1.517

1.518

i.

The host solid

2.

The fully llthlated phase

3.

The delithlated phase

I/I 0

dnhA

d

ea

l

I00

4

.46

4

.47

8

3

.47

3

.46

44

2.6

95

2

.69

8

3 2.524

2.530

24

2.494

2.495

28

2.447

2.446

33

2.2

36

2

.23

7

6 --

--

16

1.9

83

1

.98

2

7 --

--

4 1.852

1.8

52

25

1.776

1.775

22

1.7

38

1

.73

9

2 1.589

1.589

23

1.526

1.527

I/I

0

100

18

28 7

28

12

31

36

m

3

12

15

6

16

dn

hg

4.7

8

4.45

3.459

2.684

2.534

2.499

2. 448

2.236

1.9

82

1.9

68

m

1.7

70

1

.72

8

1.5

77

1.522

dea|

4.7

6

4.4

4

3.4

55

2

.68

3

2.5

33

2

.49

8

2.4

48

2

.23

6

1.980

1.969

1.771

i. 7

28

1.5

79

1

.52

2

I/I

O

9 i0

0

12

27 2

39

27

24

15

12

33

2O

<1

I0

C3

~o

O

Vol. 20, No. 11 LITHIUM INSERTION 1351

Qualitative electrical resistivity data measured on pressed powder pellets at room temperature in the dry box show that the low electrical conductivity (p>106~-cm) of the insulating host material Li~Ti307 is enhanced by several orders of magnitude upon lithium insertion (0~lOO~-cm for LI3TI307). The expected oxidation states of the cations in the fully llthiated compound are Li3+Ti24+Ti3+07 . The Ti 3+ and Ti 4+ cations are located in equivalent octahedral sites of the ramsdellite structure (see Fig. i), hence these mixed valent cations may be responsible for the observed increase in electrical conductivity.

IOOG

Fig. 2

ESR spectrum of Li3Ti307 at 77K

The ESR spectrum of the lithlated compound LiqTiqO 7 confirms the presence of Ti 3+ ions which result by reduction of Ti 4+ upon lithium insertion. Fig. 2 shows a very broad (AH>IO0 Gauss), symmetrical ESR band at 77K. No dlscernable spectra could be obtained at room temperature. The g-factor, is 1.967 and falls within the range characteristic of d I ions in a distorted octahedral site (8). Sites which the Ti 3+ (d I) ions are expected to occupy in the ramsdelllte structure (c.f. Fig. I). The g value and the shape of the ESR band are similar to those reported for TinO2n_ I (n>5) mixed valent titanium oxide phases (9) in which the line width (AH~50 Gauss) was attributed to magnetic dipole-dipole interactions. The absence of resolved hyperflne

structure expected for the two titanium isotopes 47Ti (7.75% abundant, I = 5/2) and 49Ti (5.5% abundant, I = 7/2) with nuclear nagnetic moment suggests that the unpaired spin is shared by several Ti nuclei (i0).

Acknowledgement

The authors thank Dr. P.K. Bharadwaj for his assistance in obtaining the ESR spectrum. This work was supported in part by the Office of Naval Research.

References

i. M. Greenblatt, E. Wang, H. Eckert, N. Kimura, R.H. Herber, and J.V. Waszczak, Inorganic Chem. 24, 1661 (1985).

2. A.M. Bystrom, Acta Chem. Scad. 3, 163 (1949).

3. M. Lundberg and S. Anderson, Acta Chem. Scand. 18, 817 (1964).

4. J. Choisnet, M. Hervieu B. Raveau, and P. Tarte, J. Solid State Chem. 40, 344 (1981).

1352 C . J . CHEN, et al . Vol. 20, No. 11

5. B. Morosin and J.C. Mikkelsen, Jr., Acta Cryst. Sect. B 35, 798 (1979).

6. J.B. Boyee and J.C. bllkkelsen, Jr., Solid State Commu. 31, 741 (1979).

7. J.B. Boyce and B.A. Huberman, Phys. Rep. 51, 190 (1979).

8. J.E. Wertz and J.R. Bolton, "Election Spin Resonance: Elementary Theory and Practical Applications", McGraw-Hill, Inc. 1972, p. 316-320.

9. J.F. Houllhan and L.N. Mulay, Mat. Res. Bull. ~, 737 (1971).

i0. P.K. Bhartia and R.L. Mieher, J. Solid State Chem, 12, 295 (1975)