TITANIUM DIOXIDE-BASED MATERIAL

Abstract: Electrochemical insertion of lithium into titanium dioxide initially in its rutile form, the insertion being performed at an elevated temperature (for example 120 DEG C), produces a material LixTiO2 with x between 0.5 and 1.0. This material is suitable as an active cathode material in a secondary cell as it can be repeatedly recycled. The material has a crystal structure, different from that of the initial rutile titania, which is believed to be hexagonal.

Description: Titanium Dioxide-Based Material This invention relates to a material based on titanium dioxide, a method of making the material, and to use of this material in an electrochemical cell. Titanium dioxide occurs in two forms: anatase and rutile. It has hitherto been found that lithium can be inserted into these two forms to very different extents. The chemical lithiation of anatase titania with n-butyl lithium results in a maximum stoichiometry of Lin.7 Tiθ2 (see M.S. Whittingham et al. , J. Electrochem. Soc. , 124, 1388 (1977)), whereas it has been reported that only small amounts of lithium, no more than 2 atomic percent, can be chemically inserted into the rutile form (see D. . Murphy et al. , Solid State Ionics, 9 & 10, 413 (1983) ). The lithiated anatase material Li x Tiθ2 has been cycled in an electrochemical cell over the range of x from 0.15 to 0.45 using a lithium perchlorate/propylene carbonate electrolyte, with an average cell voltage of about 1.7 V (see F. Bonino et al. , J. Power Sources, 6, 261 (1981)), whereas rutile titania in such a cell was found to have poor capacity and to have a significantly lower discharge voltage. According to the present invention there is provided a material of stoichiometric formula Li χ Tiθ2r where x is between 0.5 and 1.0, made by electrochemical insertion at an elevated temperature of lithium into titanium dioxide in its rutile form. The material is believed to be of hexagonal form. In a second aspect, the invention provides * a method of making a material of stoichiometric formula Li x Tiθ2, where x is between 0.5 and 1.0, the method comprising the successive steps of: (i) assembling an electrochemical cell having an anode comprising lithium as active material; a non-aqueous lithium ion conducting electrolyte; and a cathode containing titanium dioxide in its rutile form as 5 active cathode material; and (ii) discharging the cell at an elevated temperature such that the cathode material is transformed into material of formula Li x Tiθ2« 10 Preferably the elevated temperature is above 100°C, for example between 100° and 150 β C, most preferably about 120°C. Preferably the electrolyte comprises a complex of poly(ethylene oxide) and a lithium salt such as 15 L1CF3SO3 or LiClθ4, this providing a solid-state electrolyte. . In a third aspect, the invention provides a rechargeable electrochemical cell comprising an anode • 20 comprising lithium as active anode material, a non-aqueous lithium ion conducting electrolyte, and a cathode containing as the active material a material of stoichiometric formula i x Tiθ2r where x lies between 0.5 and 1.0, formed by electrochemical insertion of lithium 25 ions into titanium dioxide in its rutile form. It will be appreciated that this cell can be made by the previously defined method of making the Li x Tiθ2 material, the material being formed in situ. 30 The invention will now be further described with reference to the accompanying drawings, in which: Figure 1 shows discharge graphs for electrochemical cells 35 incorporating anatase titanium dioxide; Figure 2 shows discharge graphs for electrochemical cells incorporating rutile titanium dioxide; Figure 3 shows graphicaly the variation in capacity with discharge cycle for the cells of Figure 1 and of Figure 2; and Figures 4a to d show graphically X-ray diffraction patterns at successive stages during the first discharge of a cell incorporating rutile titanium dioxide. Experiments have been carried out using the two different forms of titanium dioxide, anatase and rutile. Composite cathodes containing titanium dioxide sample, ketjen carbon black, poly(ethylene oxide) (PEO) (MW 4000000) and LiClθ4 were prepared by doctor blade casting from the appropriate solvent slurry onto a nickel current collector. The resulting cathodes had the composition 45 vol.% titanium dioxide, 5 vol.% carbon and 50 vol.% PEO-LiClθ4 ( [EO units]/[Li] = 12) with a capacity of about 1 mA h cm~2 based on a value of 335 mA h g~l , (corresponding to the insertion of one lithium into Tiθ2). Sheets of the electrolyte PEO-LiClθ4( [EO units]/[Li] = 12) were cast from acetonitrile solution onto silicone release paper. Solid state cells containing a lithium foil anode with an active area of 40 cm^ were constructed in a dry room (T = 20°C, dewpoint temperature -30 β C) and assembled using a combination of heat and pressure, each cell consisting of a nickel foil current collector, a composite cathode layer about 36 microns thick, a polymer electrolyte layer about 85 mϊcrons thick, and a lithium foil 100 microns thick. The cells were packaged and placed in an oven at 120°C for testing. Measurements of a.c. impedance of the two types of cell after 2 hours in the oven indicated that those incorporating rutile had an overall impedance several times larger than that of the anatase cells. This is consistent with the lower electronic conductivity of the rutile material. The open-circuit voltages for both anatase and rutile cells were in the region 2.4 - 2.6 V. The cells were then discharged, and cycled using a computer-controlled charge/discharge apparatus in a constant current mode between preset voltage limits, all at a constant temperature of 120°C. Initial cell cycling was carried out between the voltage limits 3.0 V and 1.2 V at the C/8 rate (i.e. at such a rate as to discharge it completely after 8 hours if it were 100% efficient) . Referring to Figure 1 there are shown graphically the 1st, 2nd, 10th and 20th discharge curves for the anatase cells. The primary discharge of anatase results in two distinct-plateaux. The first at 1.78 V continues up to the composition Lin # 6 T:L0 2? tne final composition is Li .0 τ i°2r corresponding to a theoretical energy density of about 565 W h kg~l. The shape of the second discharge curve resembles that of the first, albeit at a lower capacity, implying that the anatase structure is retained following the initial insertion of lithium ions. This has been confirmed by X-ray diffraction where the pattern obtained from an anatase cathode taken from a cell discharged to a depth Lig.7TiC>2 is essentially the same as that obtained from an uncycled cathode. The cause of the poor capacity retention in the anatase material that occurs during cycling has not been established. Referring now to Figure 2 there are shown graphically the 1st, 2nd, 10th and 20th discharge curves for the rutile cells. The initial discharge has a short step at about - o 1.75 V that can be attributed to a small quantity of anatase impurity in the rutile sample, with the majority of the discharge capacity (about 90%) occurring between 1.50 and 1.40 V. Complete discharge to a 1.2 V cutoff gives a final composition Li^nTiC^ corresponding to a theoretical energy density of about 500 W h kg-*. (The insertion of one lithium per titanium into Tiθ2 rutile in this lithium polymer electrolyte cell is in complete contrast to the very low levels of lithium successfully incorporated into rutile titanium dioxide at ambient temperature) . The second discharge shows a cell voltage that decreases continuously with capacity to a value approximately half that obtained in the initial discharge (i.e. cycling of Li x Tiθ2 where x varies between 0.5 and 1.0). The average cell voltage of 1.73 V correspnds to a theoretical energy density of about 290 W h kg-1. The subsequent discharge curves are all very similar both in terms of shape and capacity. Referring now to Figure 3, the variation in the capacity of the two types of cell with cycle number is shown graphically for the first thirty cycles. It is apparent that the lithiated rutile material has excellent reversibility. Indeed the rutile cells have been found to suffer very little decrease in capacity after as many as 250 cycles, even at a C or C/2 discharge rate. Referring now to Figure 4, X-ray diffraction patterns have been obtained for composite cathodes removed from cells made with rutile titanium dioxide discharged to different depths. These patterns were obtained with a diffractometer employing copper K-alpha radiation and incorporating pulse height discrimination and a curved graphite secondary onochromator, data collection and processing over the range for 29 from 10 to 70 degrees being computer controlled. Figure 4(a) shows the pattern from undischarged rutile TiC>2. Figure 4(b) shows the pattern from a composite cathode discharged to the composition Lin.25 T:Lθ 2' where the rutile structure is clearly intact. (The peaks appearing at 2θ values of 19.0° and 23.5° are associated with the crystalline polymer). At 50% depth of discharge, Figure 4 (c) r that is at a composition of Lin.5Tiθ2r there is a drastic reduction in the intensity of the strongest rutile reflection at 27.5° with the predominant phase being an unidentified lithium titanium oxide characterised by the d-spacings given in Table 1. (The composite nature of the cathode samples limits the quality of the X-ray diffraction data and so only major reflections are given) . At a composition Figure 4 (d) , the majority phase has a X-ray diffraction pattern similar to that of a hexagonal form of LiTiC>2 first reported by Hewston and Chamberland (see J.Phys. Chem. Solids, Vol. 48, No. 2, 97 (1987)), while the peaks associated with the .unidentified phase seen at composition Lin.5Tiθ2 have disappeared. The intercalation of lithium into TiC>2 rutile appears to occur via an intermediate phase of approximate composition in.5Tiθ2 that on further lithiation leads to the formation of a hexagonal form of Li iθ2« Presumably the activation barrier to the formation of the intermediate phase precludes its formation at ambient temperature. Table 1 X-ray Diffraction Data for Lin.51102 Intensity Interplanar spacing/nm 95 .344 100 .249 65 .222 80 .174 A significant decrease in cell resistance, from about 5 ohms to about 1 ohm, is observed during the first discharge of the Tiθ2 rutile cells in the region up to a composition about Lin.sTiθ2, while during subsequent cycling the cell resistance is essentially independent of the state of discharge. This is consistent with the initial resistance drop being associated with a structural transformation of the Tiθ2 rutile such as that described above resulting in a material of higher conductivity. A similar decrease in cell resistance is not observed during the first discharge of the anatase Tiθ2 cells. It is thus apparent that one mole of lithium can be electrochemically intercalated into both anatase and rutile poly orphs of Tiθ2 in a lithium polymer electrolyte cell .operating at 120°C. On cycling the anatase form demonstrates poor capacity retention. During the initial discharge the rutile material undergoes a structural transformation resulting in the formation of a hexagonal form of LiTiθ2» This hexagonal LiTiθ2 prepared in situ from rutile exhibits excellent reversibility over the composition range Li x Tiθ2, where x is between 0.5 and 1.0, and good rate performance. The average cell voltage is about 1.73 V and corresponds to a theoretical energy density of about 290 W h kg~l.

Claims: Claims 1. A material of stoichiometric formula Li x Tiθ2, characterised in that x is between 0.5 and 1.0, and that the material is made by electrochemical insertion at an elevated temperature of lithium into titanium dioxide in its rutile form. 2. A material as claimed in Claim 1 wherein the elevated temperature is between 100°C and 150°C. 3. A method of making a material of stoichiometric . formula i x Tiθ2, characterisd in that x is between 0.5 and 1.0, and that the method comprises the successive steps of: (i) assembling an electrochemical cell having an anode comprising lithium as active material; a non-aqueous lithium- ion conducting electrolyte; and a cathode containing titanium dioxide in its rutile form as active cathode material; and (ii) discharging the cell at an elevated temperature such that the cathode material is transformed into material of formula Li x Tiθ2. 4. A method as claimed in Claim 3 wherein the elevated temperature is between 100°C and 150°C. 5. A method as claimed in Claim 4 wherein the elevated temperature is about 120°C. 6. A method as claimed in any one of Claims 3„to 5 wherein the electrolyte is a solid state electrolyte. 7. A rechargeable electrochemical cell comprising an anode comprising lithium as active anode material, a non-aqueous lithium ion conducting electrolyte, and a cathode containing as the active material a material as claimed in Claim 1 or Claim 2.

Inventors: MACKLIN WILLIAM JAMES, NEAT ROBIN JOHN, MACKLIN, WILLIAM, JAMES, NEAT, ROBIN, JOHN
Applicants: ATOMIC ENERGY AUTHORITY UK, UNITED KINGDOM ATOMIC ENERGY AUTHORITY
Application year: 1992
Main classification: H01M10
IPC codes: H01M4, H01M10