CA 02726435 2010-11-30 19798P0004CA01 Process for the preparation of lithium titanium spinel and its use The present invention relates to doped and undoped lithium titanate Li4Ti5O12 as well as a process for its preparation. The use of lithium titanate Li4Ti5O12, or lithium titanium spinel for short, as a substitute for graphite as anode material in rechargeable lithium-ion batteries was proposed some time ago. A current overview of anode materials in such batteries can be found e.g. in Bruce et al., Angew.Chem.Int.Ed. 2008, 47, 2930- 2946. The advantages of Li4Ti5O12 compared with graphite are in particular its better cycle stability, its better thermal rating and the higher operational reliability. Li4Ti5O12 has a relatively constant potential difference of 1.55 V compared with lithium and achieves several 1000 charge/discharge cycles with a loss of capacity of < 20%. Thus lithium titanate has a clearly more positive potential than graphite which has previously usually been used as anode in rechargeable lithium-ion batteries. However, the higher potential also results in a lower voltage difference. Together with a reduced capacity of 175 mAh/g compared with 372 mAh/g (theoretical value) of graphite, this leads to a clearly lower energy density compared with lithium-ion batteries with graphite anodes. 1 CA 02726435 2010-11-30 However, Li4Ti5O12 has a long life and is non-toxic and is therefore also not to be classified as posing a threat to the environment. Recently, LiFePO4 has been used as cathode material in lithium- ion batteries, with the result that a voltage difference of 2 V can be achieved in a combination of Li4Ti5O12 and LiFePO4. Various aspects of the preparation of lithium titanate Li4Ti5O12 are described in detail. Usually, Li4Ti5O12 is obtained by means of a solid-state reaction between a titanium compound, typically Ti02, and a lithium compound, typically Li2CO3, at high temperatures of over 750 C (US 5,545,468). This high-temperature calcining step appears to be necessary in order to obtain relatively pure, satisfactorily crystallizable Li4Ti5O12, but this brings with it the disadvantage that primary particles are obtained which are too coarse and a partial fusion of the material occurs. The product obtained in this way must therefore be ground extensively, which leads to further impurities. Typically, the high temperatures also often give rise to by- products, such as rutile or residues of anatase, which remain in the product (EP 1 722 439 Al). Sol-gel processes for the preparation of Li4Ti5O12 are also described (DE 103 19 464 Al). In these, organotitanium compounds, such as for example titanium tetraisopropoxide or titanium tetrabutoxide, are reacted in anhydrous media with for example lithium acetate or lithium ethoxide to produce Li4Ti5O12. However, the sol-gel methods require the use of titanium starting compounds that are far more expensive than TiO2 and the titanium content of which is lower than in Ti02, with the result that 2 CA 02726435 2010-11-30 preparing a lithium titanium spinel by means of the sol-gel method is usually uneconomical, in particular as the product still has to be calcined after the sol-gel reaction in order to achieve crystallinity. In addition, preparation processes by means of flame spray pyrolysis are proposed (Ernst, F.O. et al. Materials Chemistry and Physics 2007, 101(2-3) pp. 372-378) as well as so-called "hydrothermal processes" in anhydrous media (Kalbac, M. et al., Journal of Solid State Electrochemistry 2003, 8(1) pp. 2-6). Further possibilities for preparing lithium titanate, in particular by means of solid-state processes, are for example described in US 2007/0202036 Al and US 6,645,673, but they have the disadvantages already described above, that impurities such as for example rutile or residues of anatase are present, as well as further intermediate products of the solid-state reaction such as Li2TiO3 etc. Furthermore, in addition to the preparation of non-doped Li4Ti5O12r the preparation and properties of Al-, Ga- and Co-doped Li4Ti5O12 have also been described (S. Huang et al. J. Power Sources 165 (2007), pp. 408 - 412). There was therefore a need to provide an alternative preparation process for non-doped and doped lithium titanate which in particular makes the preparation of phase-pure non-doped or doped lithium titanate possible. Surprisingly, it was found that doped and non-doped lithium titanate Li4Ti5O12 can be obtained by the thermal reaction of a 3 CA 02726435 2010-11-30 composite oxide containing Li2TiO3 and Ti02. The Ti02/Li2TiO3 ratio is in a range from 1.3 to 1.85, preferably from 1.41 - 1.7, still more preferably from 1.51 - 1.7. The stoichiometric ratio of Ti02 to Li2TiO3 in the composite oxide is in a range around the theoretical stoichiometric value of 1.5, which is due in particular to the volatility of the lithium starting compound under the chosen reaction conditions, in order to obtain a phase-pure product (cf. e.g. Dokko et. al. Elektrochimica Acta 51 (2005) 966-971, Jiang et. al. Electrochimica Acta 52 (2007), 6470 - 6475, Huang et. al. Electrochem. Comm. 6 (2004), 1093 - 97, Hao et. al., J. Alloys and Compounds (2006) doi: 10.1016/j. jallcomm. 2006.08.082. Preferably, a slight excess of the lithium compound is used, quite particularly from approx. 4-10 % compared with the theoretical value. A slight deficit of the lithium compound is less preferred, but the precise value also often depends on the reactivity of the Ti02 starting product which can vary from one manufacturer to another. In the case of the preparation of non-doped lithium titanium spinel, the composite oxide consists only of these two constituents. The term "composite oxide" means according to the invention that the constituents of the composite oxide form a completely homogeneous mixture which is achieved by a chemical and/or thermal treatment. The term "composite oxide" according to the invention is therefore not used for the purely mechanically prepared mixtures of the corresponding constituents, since 4 CA 02726435 2010-11-30 completely homogeneous mixtures cannot usually be obtained mechanically. The lithium titanate obtained according to the invention has an extremely low particle size, which leads to the current density in an anode that contains the lithium titanate material according to the invention being particularly high and wherein this anode further has a high cycle stability. The term "lithium titanate" or "lithium titanate according to the invention" here refers to both the non-doped and the doped forms. Quite particularly preferably, the lithium titanate according to the invention is phase-pure. The term "phase-pure" or "phase-pure lithium titanate" means according to the invention that no rutile phase can be detected in the end-product by means of XRD measurements within the limits of the usual measurement accuracy. In other words, the lithium titanate according to the invention is rutile-free in this preferred embodiment. In preferred developments of the invention, the lithium titanate according to the invention is doped with at least one further metal, which leads to a further increase in stability and cycle stability when the doped lithium titanate is used as anode. In particular, this is achieved by the incorporation of additional metal ions, more preferably Al, Mg, Ga, Fe, Co, Sc, Y, Mn, Ni, Cr, V or several of these ions, into the lattice structure. Aluminium is quite particularly preferred. The doped lithium titanium spinels are also rutile-free in particularly preferred embodiments. 5 CA 02726435 2011-12-16 The doping metal ions which can sit on lattice sites of either the titanium or the lithium are preferably present in a quantity of 0.05 to 3% by weight, preferably 1-3% by weight, relative to the total spinel. According to one aspect of the present invention, there is provided lithium titanate Li4Ti5O12 having a particle size d90 of 25 pm obtained by the thermal reaction of a composite oxide containing Li2TiO3 and TiO2, wherein the molar ratio of TiO2 to Li2TiO3 lies in a range from 1.3 to 1.85. According to another aspect of the present invention, there is provided a process for the preparation of a composite oxide containing x parts Li2TiO3 in cubic phase and y parts TiO2 in anatase modification, wherein x and y independently of each other stand for a number between 0.1 and 4, comprising the steps of a) providing an aqueous solution of LiOH; and b) reacting the aqueous LiOH solution by adding solid TiO2 at a temperature in the range from 100 - 250 C over a period of 10 to 30 hours. According to another aspect of the present invention, there is provided a process for the preparation of phase-pure doped or non-doped lithium titanate Li4Ti5O12r characterized in that a composite oxide containing x parts Li2TiO3 and y parts TiO2 as well as z parts of a metal oxide, wherein x and y independently of each other are a number between 0.1 and 4, and 0 < z < 1 and 6 CA 02726435 2012-04-27 the metal is selected from metals of the group consisting Al, Mg, Ga, Fe, Co, Sc, Y, Mn, Ni, Cr, V or mixtures thereof, is sintered at a temperature of S 750 C. According to a further aspect of the invention, there is provided a process for the preparation of phase-pure doped or non-doped lithium titanate Li4Ti5O12, comprising the steps of: a) providing an aqueous solution of LiOH; b) reacting the aqueous LiOH solution by adding solid TiO2 and optionally a compound containing Al, Mg, Ga, Fe, Co, Sc, Y, Mn, Ni, Cr, V or mixtures thereof at a temperature in the range from 100 - 250 C over a period of 10 to 30 hours, thereby obtaining a composite oxide containing x parts Li2TiO3 and y parts TiO2 as well as z parts of a metal oxide, wherein x and y independently of each other are a number between 0.1 and 4, and 0 < z <_ 1 and the metal is selected from Al, Mg, Ga, Fe, Co, Sc, Y, Mn, Ni, Cr, V or mixtures thereof; c) grinding the composite oxide; and d) sintering the composite oxide at a temperature of 750 C. The preparation of the doped lithium titanium spinels is described in detail below. Surprisingly, it was found that the non-doped and doped lithium titanate obtainable according to the invention has a particle size d90 <_ 25 pm in an unground sample, i.e. directly after reaction and separation (see below) and no fusion phenomena are 6a CA 02726435 2012-04-27 to be observed in SEM micrographs of the product. Particularly preferably, it has a particle size d50 of < 1 pm, quite particularly preferably in the range from 0.3-0.6 pm. As already stated, a small particle size leads to a higher current density and also to a better cycle stability, with the result that the lithium titanate can also be used particularly advantageously as a constituent of an anode in rechargeable lithium-ion batteries without further mechanical grinding steps. Of course, the product obtained can also be ground even more finely, should this be necessary for a specific use. The grinding procedure is carried out with methods known per se to a person skilled in the art. Surprisingly, it was also found that the doped and non-doped lithium titanate obtained according to the invention has a relatively high BET surface area in the range from 2 - 15 m2/g. For the non-doped or doped lithium titanate obtainable according to the invention with the above-described properties, it has proved to be advantageous if the Li2TiO3 of the composite oxide is 6b CA 02726435 2010-11-30 present in cubic phase at the start of the reaction. It is also preferred if the TiO2 of the composite oxide is present, not in the rutile, but rather in the anatase, modification. The object of the present invention is further achieved by providing a process for the preparation of a composite oxide containing x parts Li2TiO3 and y parts TiO2 with 0.1 <- x, y < 4, wherein the Li2TiO3 is present in cubic phase and the TiO2 in anatase modification. The composite oxide preferably serves as starting material for the lithium titanate according to the invention. In this case, the constituents of the composite oxide are naturally present in the corresponding stoichiometric quantities, for example 2 parts Li2TiO3 and 3 parts TiO2, for the subsequent reaction to produce the lithium titanate. As already stated, the ratio of TiO2 to Li2TiO3 of the composite oxide for the subsequent reaction preferably lies in a range from 1.3 to 1.85, quite particularly preferably in the range from 1.4 - 1.7. In principle, it is possible according to the invention to set the ratio of the components of the composite oxide to each other in such a way that typically all lithium titanium spinels of the type Lil+XTi2-XO4 with 0 <- x <- 1/3 of the space group Fd3m and generally also any mixed lithium titanium oxides of the generic formula Li,,TiyO (0 <x,y <1) can be obtained in the subsequent thermal reaction (see below). If doped spinels are to be prepared, a further - preferably - metal oxide compound of the doping metal(s) is additionally present in the composite oxide. 7 CA 02726435 2010-11-30 The process for the preparation of a composite oxide according to the invention comprises the steps of a) providing an aqueous solution of LiOH b) reacting the aqueous LiOH solution by adding solid Ti02 at a temperature in the range from 100 - 250 C. Optionally, there is the step of c) separating the product obtained by the reaction from step b) Instead of the optional separation, e.g. by means of filtration, etc., the reaction product or the suspension that contains the reaction product from step b) can e.g. also be subjected to a spray pyrolysis or other product-isolation methods known per se to a person skilled in the art. Preferably, the Ti02 is used in its anatase modification within the framework of the process according to the invention. If, in the subsequent thermal reaction of the composite oxide, a doped lithium titanium spinel is to be prepared therefrom, a corresponding metal compound, in particular a metal compound of Al, Mg, Ga, Fe, Co, Sc as well as Y, Mn, Ni, Cr, V can be added either before the addition of the TiO2 or at the same time as the addition of the Ti02. In the latter case, that is in the case of simultaneous addition, the corresponding metal oxide is preferably used. If the metal compound is already present before the addition of the Ti02 in 8 CA 02726435 2010-11-30 solution together with the LiOH, either a soluble metal compound, such as an acetate, nitrate and the like, which reacts to produce hydroxide or oxide at reaction temperature can be used or a suspension of the corresponding metal oxide. It is understood that several different metal oxides or metal compounds of the above-named metals can of course also be added, in order for example to then obtain mixed-doped lithium titanium spinels. In these cases, the composite oxide according to the invention therefore contains, in addition to the above-named two main constituents Li2TiO3 and TiO2r other appropriate metal compounds, in particular oxides of the afore-named doping metals. It is further particularly advantageous that the aqueous LiOH solution is kept at a temperature of 100 - 250 C during the reaction in step b), since this particularly encourages the reaction of the educts to produce the composite oxide according to the invention containing Li2TiO3 and TiO2. If the temperature is too low, impurities occur in the end-product. It is preferred that the reaction of the educts takes place over a period of 1-30 h, quite particularly preferably over a period of 15-25 h. Surprisingly, it was found that the composite oxide obtainable by the process according to the invention and containing Li2TiO3 and TiO2 which is separated in step c) for example by filtration is obtained in a uniform particle size in the range from 100 - 300 nm. The separated product is dried at a temperature of 70 to 120 C and for example ground with an air-jet mill, which takes place particularly easily, since surprisingly only a very small agglomeration of the obtained product particles occurs. 9 CA 02726435 2010-11-30 The object of the present invention is further achieved by the provision of a process for the preparation of doped or non-doped lithium titanate, starting from the composite oxide according to the invention, wherein the composite oxide is sintered at a temperature of <- 850 C. Quite particularly preferably, the sintering takes place at even lower temperatures of < 700 C. It was surprisingly found that, unlike all previous solid-state synthesis processes for lithium titanate, a much lower temperature and also a much shorter reaction time can be chosen and yet the disadvantages of the state of the art, in particular the occurrence of further reaction products, can be avoided and lithium titanate is obtained. When preparing doped lithium titanate, it is to be borne in mind that, in addition to the reaction according to the invention of a composite oxide already containing a doping-metal compound or a doping-metal oxide, the compound of the doping metal is also to be added after the synthesis of the (non-doped) lithium titanium spinel or also of the composite oxide in solid or liquid form (e.g. steeping) and then heated or calcined anew. In contrast, a purely mechanical mixture consisting e.g. of Li2TiO3 and TiO2 must be sintered at temperatures of more than 800-850 C, wherein different phases and products are obtained. Typically, with the process according to the invention, the duration of the sintering is 0.5 to 20 hours and is thus clearly shorter than with conventional solid-state processes or compared CA 02726435 2010-11-30 with a purely mechanical stoichiometric mixture for example of the two starting compounds Li2TiO3 and Ti02. Within the framework of the present invention, the addition of strong bases during the total synthesis of lithium titanate can be advantageously dispensed with, since the LiOH which is used in the first synthesis step when preparing the composite oxide according to the invention acts as a base or "activator". Thus, a total synthesis of doped or non-doped lithium titanate can be provided without using strong and also corrosive bases, such as NaOH or KOH, such as are indispensable in most of the above-named wet-chemical or hydrothermal processes of the state of the art. Moreover, this advantageously results in sodium or potassium impurities being avoided in the end-product. As already stated above, it was surprisingly found that the necessary temperatures in the calcining step which leads to the phase-pure lithium titanate Li4Ti5O12 according to the invention are extremely low compared with the state of the art. Compared with temperatures of more than 800-850 C of the state of the art, according to the invention temperatures of only < 750 C, preferably < 700 C are necessary. For example, a clean product was already obtained after 15 hours' reaction time at a temperature of only 700 C (see below). A further advantage of the process according to the invention compared with the usual solid-state synthesis routes for the preparation of lithium titanium spinels is further that a calcining with neither LiOH=H2O nor Li2CO3 need be carried out. Both compounds usually used are highly reactive and corrosive at 11 CA 02726435 2010-11-30 the high temperatures used of more than 850 C and thus strongly attack the walls of the reactors in which the calcining takes place. With the Li2TiO3 used according to the invention, no reaction with the materials of the reactors takes place. Preferably, the doped or non-doped lithium titanate according to the invention is used as anode material in rechargeable lithium- ion batteries. Thus, the present invention also relates to a rechargeable lithium-ion battery comprising an anode and cathode as well as an electrolyte, wherein the anode contains lithium titanate Li4Ti5O12 according to the invention. The anode according to the invention has a capacity retention of at least 90%, quite particularly preferably of at least 95% at a rate of 20 C and a specific charge/discharge capacity of >160 Ah/kg. The invention is described in more detail below with reference to drawings and embodiment examples which are not, however, to be considered limiting. There are shown in: Figure 1 an X-ray diffractogram of the lithium titanate according to the invention Figure 2 an SEM micrograph of a composite oxide Li2TiO3/TiO2 according to the invention 12 CA 02726435 2010-11-30 Figs. 3a - 3c SEM micrographs of the phase-pure lithium titanate according to the invention which was obtained at different calcining temperatures Figure 4 an SEM micrograph of lithium titanate according to the invention which was calcined at a temperature of 8500C Figure 5 the particle-size distribution of a lithium titanate according to the invention Figure 6 a graph of the cycle stability of the lithium titanate according to the invention as anode material Figure 7 charge/discharge curves of the lithium titanate according to the invention as anode material. 13 CA 02726435 2010-11-30 Embodiment examples: 1. General description of the process according to the invention for the preparation of the composite oxide according to the invention The compounds used for the process according to the invention for the preparation of a composite oxide containing x Li2TiO3 /y TiO2 (x and y have the meanings defined above) are, as starting products, initially LiOH=H2O and TiO2 in anatase form. Optionally, oxides of the corresponding doping metals are added. The water content varies in the case of commercially available LiOH=H2O (from Merck) from batch to batch and was determined prior to the synthesis. LiOH=H2O is initially dissolved in distilled water and heated to a temperature of 50 to 60 C. Once the lithium hydroxide has completely dissolved, a corresponding quantity (depending on the desired end-product) of solid TiO2 in anatase modification (available from Sachtleben) is added to the 50 to 60 C hot solution under constant stirring. After homogeneous distribution of the anatase, the suspension is placed in an autoclave, wherein the reaction then took place under continuous stirring at a temperature of 100 C to 250 C, typically at 150 to 200 C for a period of approx. 18 hours. Parr autoclaves (Parr 4843 pressure reactor) with double stirrer and a steel heating coil were used as autoclaves. 14 CA 02726435 2010-11-30 After the end of the reaction, the composite oxide x Li2TiO3/y Ti02 is filtered off and an SEM micrograph produced. In the case of the composite oxide x Li2TiO3 / y Ti02 with a Ti02/LiTiO3 ratio of 1.68 (Figure 2) it was found that, during the hydrothermal reaction, no particle growth compared with the starting material anatase occurred and also no agglomeration of the free primary particles with a particle size in the range from 100 - 300 nm took place. After washing the filter cake, this was dried at 80 C and then ground. An air-jet mill for example is used for the grinding. The composite oxide x Li2TiO3/ y Ti02 according to the invention was then calcined. It was found that the composite oxide according to the invention was extremely reactive in the subsequent conversion to lithium titanate through the preceding synthesis. The reaction temperatures of conventional processes for the preparation of lithium titanate starting from a purely physical mixture e.g. of 2 parts Li2TiO3 and 3 parts Ti02 are typically implemented at temperatures of > 800-850 C and reaction times of more than 15 hours. It was further found that even at low temperatures, for example at 650 C, phase-pure products (i.e. lithium titanate) form after only 15 hours' reaction time. At a temperature of for example CA 02726435 2010-11-30 750 C, phase-pure lithium titanate even formed from the foregoing composite oxide after only 3 hours. No particle growth during the synthesis of the phase-pure lithium titanate compared with the starting material of the corresponding composite oxide was recorded. However, the particle size increased markedly as the calcining temperature increased: Figures 3a - 3c show the effect of the calcining temperature on the particle size of the lithium titanate. The sintering temperatures were 700 C for Figure 3a, 750 C for Figure 3b and 800 C for Figure 3c. As can be seen from Figures 3a - 3c, the higher the calcining temperature, the larger the particles and the more difficult was then the grinding of the obtained material. Figure 1 shows the X-ray diffractogram of a sample of non-doped lithium titanate obtained according to the invention which was calcined at 700 C for 15 hours and shows only reflexes which can be ascribed to pure Li4Ti5O12. In particular, this sample does not show reflexes that have to be ascribed to Ti02 in the rutile modification. Figure 4 shows an SEM micrograph of a non-doped lithium titanate calcined at 850 C, but the particles are clearly larger than those that were obtained at low temperatures (see Figures 3a-3c), with the result that the particles are strongly caked with each other and a later grinding procedure is made clearly more difficult. 16 CA 02726435 2010-11-30 Figure 5 shows measurements of the particle-size distribution of lithium titanate according to the invention which was obtained at 700 C over 15 hours starting from a composite oxide 2 Li2TiO3 /3 Ti02 according to the invention and which shows a very finely dispersed product. The d50 value is 0.36 pm. The coarse fraction with sizes > 1 pm consists only of agglomerates and not of primary particles. Figure 6 shows a graph of the cycle stability of non-doped lithium titanate according to the invention (the material was calcined at 750 C for 15 hours) as anode of a half cell compared with metal lithium. The electrode formulation consisted of 85% by weight lithium titanate (Li4Ti5012), obtainable according to the process according to the invention, 10% Super P and 5% Kynar. The active-mass content of the electrode was 2.2 mg/cm2. The specific charge-discharge capacity which is achieved at low rates of roughly 165 to 170 Ah/kg is close to the theoretical value as against a value of approx. 130 Ah/kg for a lithium titanate Li4Ti5O12 which was obtained in a conventional solid- state reaction from Ti02 and Li2CO3 at high temperature. The capacity and the cycle stability of the Li4Ti5O12 according to the invention in a typical half cell compared with metal lithium are remarkably good at the C rate with an average decline ("fading") of the order of 0.03%/cycle. Figure 7 shows the charge (Fig. 7a)/discharge curves (7b) of the lithium titanate according to the invention (see below Fig. 6). As can be seen in Figure 7, an anode according to the invention shows a capacity retention during discharge of 96% even at a C 17 CA 02726435 2010-11-30 rate of 20. All cycles of the test cells were operated in the range from 1.0 V - 2.0 V at 20 C. 18