Process For The Preparation Of Lithium Titanium Spinel And Its Use

Abstract

The present invention relates to a composite oxide with x wt.-parts Li.sub.2TiO.sub.3, preferably in its cubic modification of space group Fm-3m, t wt.-parts TiO.sub.2, z wt.-parts of Li.sub.2CO.sub.3 or LiOH, u wt.-parts of a carbon source and optionally v wt.-parts of a transition or main group metal compound and/or a sulphur containing compound, wherein x is a number between 2 and 3, y is a number between 3 and 4, z is a number between 0.001 and 1, u is a number between 0.05 and 1 and 0v<0.1 and the metal of the transition or main group metal compound is selected from Al, Mg, Ga, Fe, Co, Sc, Y, Mn, Ni, Cr, V or mixtures thereof. Further the present invention relates to the use of the composite oxide in a process for the preparation of a composition of a non-doped and doped lithium titianate Li.sub.4Ti.sub.5O.sub.12 comprising secondary agglomerates of primary particles and its use as anode material in secondary lithium-ion batteries.

Claims

1. A composite oxide with x wt.-parts Li.sub.2TiO.sub.3, y wt.-parts TiO.sub.2, z wt.-parts of Li.sub.2CO.sub.3 and/or lithium hydroxide, u wt.-parts of a carbon source and optionally v wt.-parts of a transition or main group metal compound and/or a sulphur containing compound, wherein the Li.sub.2TiO.sub.3 is present in its cubic crystal structure, wherein x is a number between 2 and 3, y is a number between 3 and 4, z is a number between 0.001 and 1, u is a number between 0.05 and 1 and 0v0.1 and the metal of the transition or main group metal compound is selected from the group consisting of Al, Mg, Ga, Fe, Co, Sc, Y, Mn, Ni, Cr, V and mixtures thereof.

2. A composite oxide according to claim 1, wherein the TiO.sub.2/Li.sub.2TiO.sub.3 weight ratio is in a range from 1.3 to 1.85.

3. A composite oxide according to claim 1, wherein the TiO.sub.2/Li.sub.2TiO.sub.3 weight ratio is in a range from 1.41-1.7.

4. A process for the preparation of a composite oxide according to claim 1, comprising the steps of: a) providing an aqueous solution of a lithium source; b) reacting the aqueous solution by adding solid TiO.sub.2 and a carbon source to form a slurry at a temperature in the range from 120-180 C.; and, c) spray-drying the slurry and collecting the composite oxide, wherein the slurry obtained in step b) is directly supplied to step c).

5. The process according to claim 4, wherein the lithium source contains lithium hydroxide.

6. The process according to claim 4, wherein the lithium source is a lithium salt of an organic acid. The process according to claim 4, wherein the lithium source is Li.sub.2SO.sub.4.

8. The process according to claim 4, wherein the carbon source is selected from elemental carbon or a carbon precursor.

9. The process according to claim 4, wherein the reaction takes place over a period of 1 to 20 hours.

10. The process according to claim 4, wherein before or during step b) a transition or main group metal compound containing Al, Mg, Ga, Fe, Co, Sc, Y, Mn, Ni, Cr, V or a sulphur containing compound or mixtures thereof is added.

11. The process according to claim 4, wherein the solids content of the slurry before spray drying is in the range of 10-25 wt. %.

Description

[0069] The invention is described in more detail below with reference to drawings and examples which are not, however, to be considered as limiting for the scope of the invention. It is shown in:

[0070] FIG. 1 a SEM micrograph of the lithium titanate composition according to the invention,

[0071] FIG. 2 a SEM micrograph of lithium titanate of prior art,

[0072] FIG. 3 charge-discharge cycles of an anode containing lithium titanate of prior art as active material,

[0073] FIG. 4 charge-discharge cycles of an anode containing lithium titanate of the invention as active material,

[0074] FIG. 5 the particle-size distribution of a lithium titanate of prior art,

[0075] FIG. 6 the particle-size distribution of a lithium titanate according to the invention,

[0076] FIG. 7a a XRD chart of the composite oxide of the invention, and

[0077] FIG. 7b another XRD chart of the composite oxide of the invention.

EXAMPLES

1. General

[0078] Determination of the particle-size distribution:

[0079] The particle-size distributions for the secondary agglomerates are determined using a light scattering method using commercially available devices. This method is known per se to a person skilled in the art, wherein reference is also made in particular to the disclosure in JP 2002-151082 and WO 02/083555. In this case, the particle-size distributions were determined by a laser diffraction measurement apparatus (Master sizer 2000 APA 5005, Malvern Instruments GmbH, Herrenberg, Del.) and the manufacturer's software (version 5.40). Measurements are performed in water with a set refractive index of 2. 200. The sample preparation and measurement took place according to the manufacturer's instructions.

[0080] The D.sub.90 value gives the value at which 90% of the particles in the measured sample have a smaller or the same particle diameter according to the method of measurement. Analogously, the D.sub.50 value and the D.sub.10 value give the value at which 50% and 10% respectively of the particles in the measured sample have a smaller or the same particle diameter according to the method of measurement.

[0081] According to a particularly preferred embodiment of the invention, the values mentioned in the present description are valid for the D.sub.10 values, D.sub.50 values, the D.sub.90 values as well as the difference between the D.sub.90 and D.sub.10 values relative to the volume proportion of the respective particles in the total volume. Accordingly, the D.sub.10, D.sub.50 and D.sub.90 values mentioned herein give the values at which 10 volume-% and 50 volume-% and 90 volume-% respectively of the particles in the measured sample have a smaller or the same particle diameter. If these values are obtained, particularly advantageous materials are provided according to the invention and negative influences of relatively coarse particles (with relatively larger volume proportion) on the processability and the electrochemical product properties are avoided. Preferably, the values mentioned in the present description are valid for the D.sub.10 values, the D.sub.50 values, the D.sub.90 values as well as the difference between the D.sub.90 and the D.sub.10 values relative to both percentage and volume percent of the particles.

[0082] The secondary particle-size distribution (of the agglomerates) of the composition according to the invention can be directly determined as follows using SEM photographs:

[0083] A small quantity of the powder composition sample is suspended in 3 ml acetone and dispersed with ultrasound for 30 seconds. Immediately thereafter, a few drops of the suspension are dropped onto a sample plate of a scanning electron microscope (SEM). The solids concentration of the suspension and the number of drops are measured so that a large single-ply layer of powder particles forms on the support in order to prevent the powder particles from obscuring one another. The drops must be added rapidly before the particles can separate by size as a result of sedimentation. After drying in air, the sample is placed in the measuring chamber of the SEM. In the present example, this is a LEO 1530 apparatus which is operated with a field emission electrode at 1.5 kV excitation voltage, an aperture of 30 m, an SE2 detector, and 3-4 mm working distance. At least 20 random sectional magnifications of the sample with a magnification factor of 20,000 are photographed. These are each printed on a DIN A4 sheet together with the inserted magnification scale. On each of the at least 20 sheets, if possible at least 10 free visible particles of the material according to the invention, wherein the boundaries of the particles of the material according to the invention are defined by the absence of fixed, direct connecting bridges. Of each of these selected particles, those with the longest and shortest axis in the projection are measured in each case with a ruler and converted to the actual particle dimensions using particle, the arithmetic shortest axis is defined the scale ratio. For each measured particle, the arithmetic mean from the and the shortest axis is defined as particle diameter. The measured particles are then divided analogously to the light-scattering method into size classes. The differential particle-size distribution relative to the number of particles is obtained by plotting the number of the associated particles in each case against the size class. The cumulative particle-size distribution from which D.sub.10, D.sub.50 and D.sub.90 can be read directly on the size axis is obtained by continually totaling the particle numbers from the small to the large particle classes.

[0084] BET measurements were carried out according to DIN-ISO 9277.

[0085] Spray drying was performed in a Nubilosa spray dryer 1.25 min diameter, 2.5 m in cylindrical height and 3.8 m in total height. The spray dryer was equipped with pneumatic nozzles type 970 form 0 S3 with an open diameter of 1.2 mm and type 15 94 0-4 3 form 0 S2 with an open diameter of 1.8 mm both of Dusen-Schlick GmbH, HutstraBe 4, D-96253 Untersiemau, Germany. Drying gas was supplied by a controlled suction fan and heated electrically before entering the spray dryer. The dried particles were separated from the gas stream by a bag filter and recovered by a pulsed jet dedusting system. Amount of drying gas, gas inlet temperature and outlet temperature were controlled by a process control system. The outlet temperature control governed the speed of the slurry feed pump. Atomization gas was supplied by the compressed air distribution of the plant and its pressure was controlled by a local pressure controller.

[0086] Calcination was performed 1500-3 of HTM Reetz GmbH, in a rotary kiln type LK 900-200-1500-3 of HTM Reetz GmbH, Kopenicker Str. 325, D-12555 Berlin, Germany. Its heated rotary tube was 150 mm in diameter and 2.5 m in length. It provided a preheating zone, three heated separately controlled temperature zones, and a cooling zone. The inclination of the tube could be adjusted and its variably controlled. The product was supplied by a controlled screw feeder. Product supply, the kiln itself and product outlet could be blanketed by nitrogen. The amount of calcined product could be continuously monitored by a balance. Besides the calcination in a rotary kiln, the calcination has also been performed in state-of-the-art batch furnaces.

2. General description of the process according to the invention for the preparation of the composite oxide according to the invention and of a lithium titanate.

2.1. Composite Oxide

[0087] The compounds used for the process according to the invention 15 for the preparation of a composite oxide are, as starting products, initially LiOH.H.sub.2O and TiO.sub.2 in anatase form. A carbon source and optionally a transition or main group metal compound and/or a sulphur containing compound as defined in the foregoing of the corresponding dopant are added. The water content varies in the case of commercially available LiOH.H.sub.2O (from Merck) from batch to batch and was determined prior to the synthesis.

[0088] LiOH.H.sub.2O was initially dissolved in distilled water at temperatures between 15 and 50 C. Once the lithium hydroxide has completely dissolved, a corresponding quantity (depending on the desired end-product) of solid TiO.sub.2 in anatase modification (available from Sachtleben) and graphit(from GK Graphit Kropfhl AG) was added under constant stirring. After homogeneous distribution of the anatase, the suspension was placed in an autoclave, wherein the reaction then took place under continuous stirring at a temperature of 100 C. to 250 C., typically at 120 to 180 C. for a period of approx. 18 hours.

[0089] Parr autoclaves (Parr 4843 pressure reactor) with double stirrer and a steel heating coil were used as autoclaves.

[0090] After the end of the reaction, the composite oxide was subjected to spray drying.

[0091] The drying of the suspension/slurry was carried out at gas entry temperatures in the spray-drying apparatus of 120-500 C., usually between 200-350 C., in the present case at 210 C. The exit temperatures are in the range of 70-120 C., in the present case at 110 C. The separation of the solid product from the gas can be done with any commercially available gas-solid separation system, e.g. a cyclone, an electrostatic precipitator or a filter, preferably with a bag filter with a pulsed jet dedusting system.

2.2 Lithium Titanate

[0092] The composite oxide according to the invention was then calcined.

[0093] 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 Li.sub.2TiO.sub.3 and 3 parts TiO.sub.2 are typically implemented at temperatures of >800-850 C. and reaction times of more than 15 hours.

[0094] 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 750 C., phase-pure lithium titanate compositions even formed from the foregoing composite oxide after only 3 hours.

[0095] Only minor particle growth during the synthesis of the phases pure lithium titanate composition compared with the starting material of the corresponding composite oxide was recorded. However, the particle size increased markedly as the calcining temperature increased.

[0096] In contrary to the invention prior art, the calcined product has not been milled. The agglomerated, or partly agglomerated product was separated from possible coarse agglomerates.

[0097] For the separation step, either a standard sieving technique of a classification technique can be used.

2.3. Examples

Example 1

[0098] LiOH.H.sub.2O was dissolved in distilled water at temperatures between 15 and 50 C. The lithium hydroxide solution has been filled in an autoclave and TiO.sub.2 in anatase modification has been added under stirring. The ratio of Li/Ti ratio was in between 4/5 to 6/7. Additionally, graphite was added under constant stirring.

[0099] After homogeneous distribution of the all components, the suspension was heated to a temperature of 160 C. for approximately 12 hours.

[0100] After the end of the reaction, the composite oxide was subjected to spray drying.

[0101] The drying of the suspension/slurry was carried out at gas entry temperatures in the spray-drying apparatus between 120-250 C. The exit temperatures were in the range of 100-120 C.

[0102] After spray drying, the product was then calcined at a temperature between 760 C. and 780 C. for 2 hours.

[0103] The BET of the obtained product was 5 m2/g and the D50 was at 10.37 m.

Example 2

[0104] Water-free LiOH was dissolved in distilled water at temperatures between 40 and 50 C. in an autoclave. 3% of the lithium hydroxide has been substituted with lithium actetate. AEROXIDE P 25 from Evonik, a TiO2 compound comprising anatase and rutile has been added under stirring as well as 0.5% of aluminum hydroxide. Lithium acetate and aluminum hydroxide were added in ratios according to the invention as carbon source and dopant. The total Li/Ti ratio was in between 4/5 to 6/7. The reaction was performed at 120 C. for approximately 18 hours.

[0105] After the end of the reaction, the composite oxide was subjected to spray drying.

[0106] The drying of the suspension was carried out at gas entry temperatures in the spray-drying apparatus of 250-350 C. The exit temperatures were in the range of 110-120 C. Afterwards, the composite oxide was sintered at a temperature of 730 C. for 3 hours and a classification technique with an AFG 100 jet mill, equipped with a static coarse sifter and a cyclone preseparator to a pure sifter device has been used for separation from coarse particles.

[0107] The BET of the final product was 10 m2/g and the D.sub.50 was at 17.5 m.

Example 3

[0108] LiOH was dissolved in distilled water at temperatures between 10 and 50 C. 3% of saccharose has been added under stirring, as well as TiO.sub.2 in an agglomerated rutile form. The suspension was filled in an autoclave and heated under constant stirring to 180 C. for approximately 3 hours.

[0109] Before spray drying, lithium sulphate has been added as dopant. The total Li/Ti ratio was in between 4/5 to 6/7. The suspension was then spray dried at an entry temperature of 350-450 C. and an outlet temperature of 120 C.

[0110] The composite oxide was then sintered at 750 C. under nitrogen for 3 h. The material has been sieved.

[0111] The BET of the final product was 12 m.sup.2/g and the 0 50 was at 5.5 m.

Example 4

[0112] SEM micrographs

[0113] FIG. 1 shows a SEM micrograph of the agglomerates of a lithium titanate composition according to the invention compared to prior art lithium titanate in FIG. 2 (WO2009/146904). The particles, i.e. the agglomerates in FIG. 1 are distinctly separated from one another, smaller and more uniform in size as compared to the prior art product.

Particle Size Distribution

[0114] FIG. 6 shows measurements of the particle-size distribution of a lithium titanate composition according to the invention and which shows a markedly monomodal product. Here, the D.sub.50 value of the secondary agglomerates is 10.22 m the D.sub.90 value is 19, 41 m. The tiny fraction to the left of the peak with sizes <1 m consists of primary particles. The product of prior art (WO2009/146904) shows a bimodal particle size distribution.

XRD-Charts of the Composition Oxides of the Present Invention

[0115] XRD charts were recorded of the composite oxide which are 10 shown in FIGS. 7a and 7b. The XRD chart of FIG. 7a shows traces of anatase (x), rutile (.diamond-solid.), Li.sub.2TiO.sub.3 (*) and a very small amount of Li.sub.2CO.sub.3 (+). FIG. 7b shows another XRD chart of a composite oxide obtained, showing only reflections assigned to anatase (x), small traces of rutile (.diamond-solid.) and Li.sub.2TiO.sub.3 (*).

Electrochemical Properties

[0116] FIGS. 3 and 4 show a graph of the cycle stability of a nondoped lithium titanate composition according to the invention in FIG. 4 (the material was calcined at 750 C. for 15 hours) as anode of a half cell compared with metal lithium and of prior art material in formulation consisted (Li.sub.4Ti.sub.5O.sub.12), obtainable FIG. 3 of 90% according (WO2009/146904). The electrode formulation consisted of 90% by weight lithium titanate (Li.sub.4Ti.sub.5O.sub.12), obtainable according to the process according to the invention, 5% Super P and 5% PVdF. The active-mass content of the electrode was 10 mg/cm2, which is in the typical range of commercial lithium ion batteries.

[0117] The specific charge-discharge capacity which is achieved at low rates of roughly 165 to 170 Ah/kg in FIG. 4 is close to the theoretical value and even slightly better than approximately 160-165 Ah/kg for a an anode containing lithium titanate Li.sub.4Ti5O.sub.12 which was obtained to WO2009/146904.

[0118] The capacity and the cycle stability of an anode containing the Li.sub.4Ti.sub.5O.sub.12 composition according to the invention as an active material in a typical half cell compared with metal lithium are remarkably good at Crate with an average decline (fading) of the order of 0.03%/cycle.

[0119] All cycles of the test cells were operated in the range from 1.0 V-2.5 Vat 20 C.

[0120] It is shown that the lithium titanate according to the present invention has a higher capacity at C/10 as the material of the prior art. Furthermore, it is proved that the preparation process for the lithium titanate of the present invention using the composite oxide of the present invention is more economical as processes of the prior art because no filtering step is needed and no waste water is formed.