Alkali metal titanates and methods for their synthesis

Abstract

A material comprising zirconium-doped lithium titanate, wherein the zirconium-doped lithium titanate contains zirconium in a concentration of 0.1-5 mol percent based on the sum of titanium and zirconium.

Description

CROSS-REFERENCE TO RELATED REGISTRATIONS

[0001] This application claims priority over U.S. Preliminary Patent Application No. 60 / 810,942, filed on June 5, 2006, entitled “Alkali Metal Titanates and Methods for Their Synthesis”, and priority over U.S. Preliminary Patent Application No. 60 / 822,675, filed on August 17, 2006, entitled “Doped Lithium Titanate Material and Methods for Its Manufacture”, both of which are incorporated herein by reference. TECHNICAL AREA OF INVENTION

[0002] The invention relates generally to alkali metal titanates and particularly to lithium titanates. More specifically, the invention relates to doped lithium titanates and to a process for producing lithium titanate materials which exhibit excellent electrochemical properties when used in lithium batteries. BACKGROUND OF THE INVENTION

[0003] Alkali metal titanates possess electrochemical properties that make them desirable as electrode materials for a variety of devices. Lithium titanate (Li₄Ti₅O₄) 12 It has proven particularly useful as an electrode material for lithium batteries and accumulators. It is a relatively inexpensive material and exhibits high-performance properties in lithium batteries. Consequently, it is expected to have significant utility as an electrode material for high-performance batteries and accumulators, such as those used in hybrid electric vehicles and other high-performance applications.

[0004] A key characteristic of high-performance batteries is their rate capacity. This is the speed at which batteries can store and release an electrical charge. This parameter is particularly important at high charging / discharging rates, such as those found in electric vehicles and other high-performance applications.

[0005] Another important parameter for the rechargeability of lithium batteries is first-cycle reversibility. This parameter measures the decrease in storage capacity when a newly manufactured lithium battery undergoes an initial cycle. Manufacturers compensate for this initial loss by building in extra capacity within the batteries. However, this approach increases the size and cost of batteries, and the industry has always sought to limit the extent of first-cycle reversibility.

[0006] Various lithium-titanium materials are commercially available for use in the production of lithium batteries. However, these materials result in lithium batteries with a first-charge cycle reversibility of approximately 80%, which represents a significant inefficiency. Furthermore, there is a need to improve the rate capacities of existing batteries to make them suitable for high-performance applications. Clearly, there is a need for improved lithium-titanate electrode materials. SUMMARY OF THE INVENTION

[0007] A doped lithium titanate material is disclosed. In particular embodiments, the dopant may comprise a transition metal, and this metal may be one or more from Group V, Zr, Nb, Mo, Mn, Fe, Cu, and Co. The dopant may be present in amounts of up to 20 atomic percent or mol percent, and in specific cases in a range of 0.1 to 5 atomic percent. In particular, the dopant may comprise Zr.

[0008] Furthermore, a process for the production of alkali metal titanates, such as doped and / or undoped lithium titanates, is disclosed. The process involves the preparation of a mixture of an alkali metal compound, such as lithium carbonate, together with a titanium compound, such as titanium dioxide or some other oxides of titanium, including suboxides. This mixture is milled by an impact mill process, such as ball milling, attritor milling, or the like, and the resulting mixture is heated for a time and at a temperature sufficient to achieve a reaction that forms the alkali metal titanates. A dopant or doping precursor can be added to the mixture before or after the milling step.

[0009] Furthermore, electrodes comprising alkali metal titanates as described above are disclosed, as well as batteries in which these electrodes comprise the anodes. BRIEF DESCRIPTION OF THE DRAWING Fig. Figure 1 is a flowchart showing a synthesis process for the preparation of alkali metal titanates; Fig. Figure 2 is a graphical representation of time versus temperature, which shows a temperature-programmed reaction schedule that can be used to produce the titanate materials; Fig. Figure 3 is another graphical representation of time versus temperature, which shows a different temperature-programmed reaction that can be used to produce the materials; Fig. Figure 4 is a diagram showing the capacity retention of a cell containing a lithium titanate anode; Fig. 5 is a diagram showing the life cycle of the cell of Fig. 4 shows; and Fig.Figure 6 is a diagram showing the capacity loss in the first charging cycle of a cell known from the prior art and a cell containing the lithium titanate material in question. DETAILED DESCRIPTION OF THE INVENTION

[0010] According to one aspect of the invention, a doped lithium titanate material is provided. Lithium titanate is generally assumed to have the formula Li₄Ti₅O₄. 12However, it is also recognized in science that the stoichiometry of this material can vary in some cases without significantly altering its fundamental nature. Such variations can result from slight oxidation or reduction of the material, minor variations in the LiTi ratio, and the presence of various dopants. Accordingly, within the context of this disclosure, all such stoichiometric and non-stoichiometric materials are encompassed by the definition of lithium titanate.

[0011] In a specific group of formulations, the lithium titanate is doped with a transition metal in an amount of up to approximately 20 atomic percent (mol percent), and some such transition metals include one or more of the metals V, Zr, Nb, Mo, Mn, Fe, Cu, and Co. In a particular case, such as the one discussed here, the dopant is Zr, which in certain cases is present in an amount of 0.1–5 atomic percent of the material.

[0012] In a series of experiments, undoped lithium titanate of conventional type and lithium titanate doped with approximately 1% zirconium according to the present teaching were incorporated into the anodes of half-cells, and the performance of these anodes was evaluated using standard procedures with respect to charge capacity as a function of different charge rates C. The data from the experimental series are summarized in Table 1 below, both for the doped and undoped lithium titanate materials. Table 1 sample Capacity at different speeds (mAh / g) Reversibility (%) C / 10 1C 3C 6C 10C 20C A:PZ-J28A Standard LTO 174 170 160 149 129 91 95 B:PZ-J28B LTO with 1% dopant Zr 174 170 166 161 157 140 95 (BA) / A Change in value 0% 0% 4% 8% 22% 54% 0%

[0013] As can be seen from the table, doped materials are suitable for cells that exhibit high charge capacity at high charge and discharge rates. These improvements are greatest at very high rates (10-20C), and as a result, cells manufactured using doped lithium titanate material offer particular advantages for use in high-speed and high-power applications, such as electric vehicles and backup power systems. Similar results to those above can be expected when other transition metals are used as dopants. Dopant concentrations generally reach up to 20 atomic percent of the material.

[0014] There is a considerable amount of prior art relating to the synthesis of lithium titanate materials, and various known processes can be used to produce the doped lithium titanate materials described above. While such prior art processes can be used, it has been further discovered within the scope of the present invention that very high-quality alkali metal titanates—both doped and undoped—can be produced by a process that includes impact milling of the starting materials to produce an intimate mixture. This mixture is then subjected to a reaction at elevated temperatures to produce the alkali metal titanates.

[0015] The Fig. Figure 1 shows a flowchart of a process that can be used to synthesize both doped and undoped lithium metal titanates. As shown in Fig.As shown in Figure 1, the lithium titanate is prepared in step 10 from a mixture of Li₂CO₃ and TiO₂ with a molar ratio of 2:5. These precursor materials are mixed with a solvent, such as isopropanol, in step 20. Other solvents, including organic solvents, aqueous liquids, and the like, may be used as long as they do not interfere with the process. The mixture is then subjected to ball milling in step 30. A typical milling process is carried out in ceramic vessels using zirconia milling media for approximately 48 hours, although milling times can typically range from 10 minutes to 240 hours. In one specific application, milling takes at least 12 hours. While step 30 represents a ball milling process, any impact milling method, such as attritor milling, vibratory milling, or the like, can be used.After milling, the precursor mixture is dried in step 40 to remove the solvent and crushed in step 50 to produce a fine powder. The mixed precursor substances are then subjected to a temperature-programmed reaction (TPR) under air or oxygen or an inert gas in an oven in step 60.

[0016] The material is typically heated to a temperature not exceeding 1000°C. In a typical process, the material is heated from room temperature to 400°C over a period of 0.5 hours; held at 400°C for 2.5 hours; heated to 800°C over a period of 3 hours; held at 800°C for 12 hours; and then cooled to room temperature, as shown in the time-temperature diagram in Fig.Figure 2 shows that in another representative process, the material is heated from room temperature to a temperature of 800°C over a period of one hour, held at 800°C for two hours, and then cooled to room temperature, as shown in Figure 2. Fig. 3 is shown.

[0017] In cases where a dopant is incorporated into the lithium titanate material, this dopant can be added to the starting material mixture before the impact milling step. In a representative synthesis process, doped lithium titanate is produced from starting materials containing Li₂Co₃ and TiO₂, along with a dopant precursor compound, which, for illustrative purposes, is a zirconium oxide dopant. The precursor can be a carbonate, acetate, chloride, alkoxide, or another dopant compound. In the case of zirconium oxide, the molar ratio Li:(Ti+Zr) is 4:5, with a Zr concentration corresponding to 0.1–5 mol% of Ti+Zr. The precursors are mixed in a suitable solvent, milled, and further processed as described above to produce the doped material.

[0018] It has been shown that titanate materials—both doped and undoped—forming titanate products after the preceding process in which precursor materials are milled together, exhibit improved properties, as demonstrated in cells containing these products. As such, the processes and materials of the present invention differ from those of the prior art, which is recognized as including the use of impact milling steps applied to the titanate material from which it was prepared.

[0019] Table 2 below summarizes some physical parameters that were measured for known lithium titanate materials, referred to in the table as “known LTO”, and for materials that were produced in the manner described above and referred to in the table as “T / J LTO”. Table 2 parameter well-known LTO T / J LTO Particle size (µm) <1 ~ 5 Surface area (m²) 2 / G) 60 4 Ionic conductivity (S / cm) 1.14 × 10 -5 1.35 × 10 -4 Reversibility 80 % 95 %

[0020] As can be seen from the table, the material produced according to the present method has a larger particle size than the known material. Likewise, the surface area of ​​the materials of the present invention is correspondingly smaller, which means that the materials of the present invention may be more stable or safer in an electrochemical environment than known materials. The ionic conductivity of the material of the present invention is approximately an order of magnitude higher than that of known materials. Most significantly, the first-charge cycle reversibility of cells containing material of the present invention is approximately 95%, while the reversibility of known materials is only 80%.

[0021] Cells containing lithium titanate anodes manufactured according to the aforementioned method and those containing conventional lithium iron phosphate cathodes were provided and examined. Fig. Figure 4 shows the charging potential (rate capability) of cells manufactured using the lithium titanate materials in question. As can be seen, the cell from Fig. 4. Excellent charging potential with 98% capacity retention at a 20C discharge rate, and 91% capacity retention at a 50C rate. Cells of this type offer excellent utility in demanding high-performance applications.

[0022] Fig. Figure 5 shows the life cycle of a cell of the in Fig. 4 of the type shown and represents the discharge capacity retention as a function of charge / discharge cycles performed at 3C / -3C.

[0023] As can be seen, this cell retains over 90% of its capacity after 6000 cycles.

[0024] Further tests were carried out to measure the capacity loss during the initial charging cycle of the aforementioned batteries, and the results are presented in Fig.Figure 6 shows that these batteries exhibited a capacity loss of approximately 5% for the first charge cycle, compared to typical losses of approximately 20% for similar cells manufactured using commercially available lithium titanate anode materials. Furthermore, the anode materials of the present invention delivered approximately 160-175 mAh / g over repeated cycles and retained a capacity of at least 100 mAh / g during 20C charge cycles. The performance of the materials of the present invention far exceeds that of known anode materials, both those based on titanate and those based on graphite.

[0025] As can be seen, the present invention provides high-quality lithium titanate materials and methods for their synthesis. The materials of the present invention have properties that allow the production of lithium batteries that are stable, efficient, and capable of reliably delivering very high current levels. These properties, together with the low costs achieved through the use of the disclosed methods, make this technology particularly advantageous for the production of high-performance battery systems, such as those used in electric vehicles, large power tools, power backup systems, and the like.

[0026] Although the invention has been described with reference to certain lithium titanate materials, it is understood that it can also be used for the production of other alkali metal titanates. It is also understood that the present invention is broadly applicable to doped and undoped materials, although specific lithium titanate materials doped with transition metals have been described, and that in these cases, where doped materials are used, dopant substances other than transition metals can be employed.

[0027] In light of the teaching presented here, further modifications and variations are obvious to the person skilled in the art. Accordingly, the foregoing is to be understood as an exemplary description and not as a limitation of the applicability of the invention. It is the claims, including all equivalents, that define the scope of the invention.

Claims

[1] A material comprising zirconium-doped lithium titanate, wherein the zirconium-doped lithium titanate contains zirconium in a concentration of 0.1-5 mol percent based on the sum of titanium and zirconium. [2] A process for producing a zirconium-doped alkali metal titanate, the process comprising the steps of: Provision of an alkali metal compound; Provision of a titanium compound with an initial amount of titanium; Provision of a zirconium compound with a second amount of zirconium, wherein the concentration of zirconium is 0.1-5 mol percent based on the sum of titanium and zirconium; Preparation of a mixture of the alkali metal compound, the zirconium compound and the titanium compound by impact milling of the alkali metal compound, the zirconium compound and the titanium compound; and Heating the mixture for a time and at a temperature sufficient to transform the mixture into the zirconium-doped alkali metal titanate. [3] Method according to claim 2, wherein the alkali metal compound comprises Li2CO3, the titanium compound comprises a titanium oxide, and the alkali metal titanate comprises lithium titanate doped with zirconium in a range of 0.1-5 atomic percent at the combined atomic percent of titanium in the first amount and zirconium in the second amount. [4] Method according to claim 2, wherein the step of creating a mixture comprises mixing approximately 2 mol of Li2CO3 and approximately 5 mol of TiO2. [5] Method according to claim 2, wherein the impact grinding method comprises ball grinding or attritor grinding. [6] Method according to claim 2, wherein the step of heating the mixture comprises heating the mixture to a temperature of not more than 1000°C. [7] Method according to claim 6, wherein the step of heating the mixture comprises holding the mixture at a temperature of 800°C. [8] The method of claim 2, wherein the reaction mixture is held at the temperature for at least one minute. [9] The method of claim 2, wherein the reaction mixture is held at the temperature for at least 2 hours. [10] Method according to claim 2, wherein the heating is carried out under an atmosphere which is an inert atmosphere or an oxidizing atmosphere. [11] An electrode comprising a zirconium-doped lithium titanate, wherein the zirconium-doped lithium titanate contains zirconium in a concentration of 0.1-5 mol percent based on the sum of titanium and zirconium. [12] A battery comprising the electrode of claim 11.

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