Lithium insertion in different TiO2 modifications

2. Experimental TiO:(A) was used as recieved (Rizde! de Hahn) after vacuum drying and TiO2(R) was prepared by heating this to 1000°C for 3 h. TiO2(B) was synthesized from K:Ti~O9 according to Marchan, B~ohan and Tournoux [ 1]. However, it proved difficult to stabilize the TiO2(B) modification wi:hout a certain content of K2TisOT, as potassium-free samples were converted to anatase by the dehydration at 500°C. In the present work a sample received from Brohan and Marchand containing only a very small amount of K2TisO7 was used. Characterization of the m~w,~.k hy X-ray p , , , n , ~ diffraction was performed with a Guinier camera. To avoid moisture from the air ~he samples were covered with adhesive tape or mylar film. This coverage preserved the samples, but the quality of the diffraction patterns was somewhat reduced. Also Li insertion tends to broaden the lines of the pattern, possibl7 due to non-equilibrated, strained crystal-
F.vsisk-Kemisk Institut, The Technical University of Denmark, DK-2800 Lyngby, Det 1987
The insertion of Li into the three titanium dioxide modifications, anatase, rutile, and TiO2(B), is studied primarily by electrochemical techniques. At 25°C at potentials above 1.4 V versus Li/LiAsF6,PCthe maximal Li uptake is 0.5 Li/Ti in anatase and TiO.,(B), while rutile does no~ ingert Li. At 120°C utilizing LiCFsSOs/PEO electrolyte an amount of 0.8, 0.5, and 0.5 Li/Ti, respectively, is inserted above 1.3 V. The different behaviour of the chemicallyidentical, but structurally non-equivalent oxides, is discuss~,d.
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liquid organic electrolyte has been obtained [4-11 ]. Up to 0.5 Li/Ti the EMF is constant and beyond this value a steadily declining curve is reported. By powder neutron diffraction on a n-BuLi treated sample Cava and co-workers [ 12,13] have found the Li atoms inserted in half of the empty octahedra, but with each Li being coordinated to five oxygen atoms only. In TiO2(R) Li insertion is curtailed. For the reaction with n-BuLi insertion of 0.03 Li/Ti has been found [ 14], while reports on electrochemical insertion scatter between 0.1 and 1 Li/Ti [4-6,9 ]. For the TiO2(B) modification insertion of 0.85 Li/ Ti by n-BuLi has been reported while discharge of electrochemical cells leads to uptake of 0.8 Li/Ti. NMR investigations suggested the Li atoms to occupy the square pyramidal site at the top and bottom of the open channel when oriented as in fig. 1 [ 1,15 ]. The same site is occupied in the isotypic Li2V3Os [161. In the present work Li insertion in these three titanium dioxides is compared. The investigations are based on chemical lithiation with n-BuLi and on electrochemical measurements on cells with either organic liqUid electrolyte operated at ambient temperature or with polymer electrol~e (120°C).
RNNJ
Fig. 1. The structures of anatase (A), TiO2(B) (B), and ruffle (R) represented by titanium oxygenoctahedra. TiO2(A) is CCP, but the packing is not dense as the crystal class is tetragonal with a c/a ratio of 2.5. The T i - O bonds along the c-axis are, however, not significandy longer than those perpendicular m the caxis leaving the empty octahedra rather large. Lithium insertion has been reported for all three oxides. TiO2(A) consumes 0.6-0.7 Li/Ti by reaction with n-butyiiithium (n-BuLl) [2,3], and electrochemically intercalation of 0.5-1 Li/Ti in cells with
Solid State lonics 28-30 (1988) 1176-1182 North-Holland, Amsterdam
L I T H I U M INSERTION IN D I F F E R E N T TiO., M O D I F I C A T I O N S
B. Z A C H A U - C H R I S T I A N S E N , K. WEST, T. J A C O B S E N and S. A T L U N G
1. Introduction
The various titanium dioxides, anatase, rutile, and TiO2(B), offer chemically identical but structurally non-equivalent h o s t lattices for insertion of lithium atoms. Anatase (TiO2(A)) and rutile (TiO2(R)) are naturally occurring minerals, whereas TiO2 (B) is a synthetic metastable polymorph prepared by soft chemistry [1 ]. When heated to 550°C T i Q ( B ) is transfGrmed "o TiO,(A), which upon further heating is converted to TiO,(R) at 1000°C. In all three oxides titanium is octahedrally coordinated to oxygen, leaving unoccupied sites available for insertion; the different structures represented by TiO6 octahedra are shown in fig. 1. The different packing density of the oxides is reflected by their densities: 3.76 g/cm 3, 3.89 g/cm 3, and 4.25 g/cm 3 for TiO2(B), TiO.~(A), and TiO2(R), respectively. The diffusion path for Li insertion is three-dimensional in TiO2(A), but unidirectional in TiO2(B) and TiO2(R). Due to the ,,',ique primitive tetragonal packing (PTP) of TiO2(R) the empty channels are very narrow; the "'cation radius" of the empty octahedral sites is 0.40 A. The radius of a Li ion varies with coordination but is normally 0.68 ~. The packing of TiO2(B) can be ~egarded as a CCP of oxygen where I of the oxygen atoms are missing. The insertion sites are therefore only 5-coordinated to oxygen. The free radius of the channels is approximately 0.58 A. Also