Layered and spinel lithium titanates and processes for preparing the same

Abstract

A process for producing lithium titanate which includes the steps of synthesizing a lithium titanate hydrate intermediate via aqueous chemical processing, and thermally treating the lithium titanate hydrate intermediate to produce the lithium titanate. The lithium titanate hydrate is preferably (Li.sub.1.81H.sub.0.19)Ti.sub.2O<<2H.sub.2O. The lithium titanate is preferably Li.sub.4Ti.sub.5O.sub.12 (LTO). Synthesizing the lithium titanate hydrate intermediate may include mixing a titanium-containing compound with a lithium-containing compound in a solvent to produce a lithium-titanium precursor mixture. Preferably the titanium-containing compound includes titanium tetrachloride TiCl.sub.4. Also, a lithium titanate obtained according to the process and a lithium battery including the lithium titanate.

Claims

1. A process for producing lithium titanate (LTO) comprising the steps of: (a) providing a lithium titanate hydrate (LTH) intermediate of Formula I: (Li.sub.2-xH.sub.x)Ti.sub.2O.sub.5.nH.sub.2O, wherein 0≦x≦0.5, and wherein 0<n<4; and (b) thermally treating said LTH intermediate of formula I to produce said LTO.

2. The process according to claim 1, wherein said LTH in step (a) is obtained by mixing a titanium-containing compound with a lithium-containing compound in an aqueous solvent to produce a lithium-titanium precursor mixture.

3. The process of claim 2, wherein the step of mixing is done at a temperature below 20° C.

4. The process of claim 3, wherein the step of mixing is done at a temperature below 10° C.

5. The process of claim 2, wherein the step of mixing is done at a pH higher than 10 and Li/Ti molar ratio 6.

6. The process of claim 2, wherein said titanium-containing compound comprises a titanium tetraalkoxide or titanium tetrachloride.

7. The process of claim 6, wherein said titanium-containing compound comprises titanium tetraisopropoxide, titanium tetrabutoxide, or a combination thereof.

8. The process according to claim 6, wherein said titanium-containing compound comprises titanium tetrachloride.

9. The process of claim 2, wherein said lithium-containing compound comprises LiOH, LiNO.sub.3, LiCl, Li.sub.2CO.sub.3, Li.sub.2O, LiHCO.sub.3, Li.sub.2SO.sub.4, lithium acetate, or a combination thereof.

10. The process according to claim 9, wherein said lithium-containing compound comprises lithium hydroxide.

11. The process according to claim 2, wherein said aqueous solvent comprises a mixture of an organic solvent and water.

12. The process of claim 11, wherein said organic solvent comprises a ketone, an alcohol or a combination thereof.

13. The process of claim 12, wherein said organic solvent comprises acetone or ethanol, or a combination thereof.

14. The process according to claim 2, wherein said aqueous solvent is water or an alkaline aqueous solution.

15. The process of claim 1, further comprising isolating a precipitate and subjecting said precipitate to aging at a temperature below 100° C. to produce the LTH intermediate.

16. The process of claim 15, wherein said step of isolating a precipitate comprises centrifugation.

17. The process of claim 15, wherein said aging of the precipitate is carried out at a temperature of between 50° C. and 100° C.

18. The process of claim 17, wherein said temperature is about 80° C.

19. The process of claim 15, wherein said aging of the precipitate is carried out for up to 36 hours.

20. The process of claim 15, wherein said aging of the precipitate is carried out without stirring.

21. The process of claim 15, wherein said aging of the precipitate is carried out with stirring.

22. The process of claim 15, wherein said aging further comprises adding the precipitate in an aqueous lithium hydroxide solution.

23. The process of claim 22, further comprising a step of isolating the LTH intermediate produced.

24. The process of claim 22, wherein said aging of the precipitate is done in a lithium hydroxide solution with a concentration of 0.1 to 0.6M.

25. The process of claim 1, wherein said step of providing a LTH intermediate of formula I further comprises a step of drying the LTH intermediate obtained.

26. The process according to claim 1, wherein said LTH intermediate in step (a) is (Li.sub.1.81H.sub.0.19)Ti.sub.2O.sub.5.2H.sub.2O.

27. The process according to claim 2, wherein the step of mixing further comprises adding a dopant-containing compound.

28. The process of claim 27, wherein said dopant-containing compound comprises one or more of acetates, nitrates, chlorides, fluorides, and bromides of one or more metal elements Al, Mg, Ga, Fe, Co, Se, Y, Mn, Ni, Cr, or V.

29. The process according to claim 1, wherein said step (b) of thermally treating said LTH intermediate comprises heating the LTH intermediate at a temperature within a range of from 350° C. to 700° C.

30. The process of claim 29, wherein said temperature is within a range of from 400° C. to 600° C.

31. The process of claim 1, wherein said step of thermally treating the LTH intermediate is done for a period of time between 1 hour and 12 hours.

32. The process of claim 1, further comprising a step of grinding the LTO obtained.

33. A lithium titanate (LTO) obtained according to the process as defined in claim 1.

34. The LTO according to claim 33, wherein said LTO is Li.sub.4Ti.sub.5O.sub.12.

35. The LTO of claim 34, wherein said LTO is spinel Li.sub.4Ti.sub.5O.sub.12.

36. The LTO of claim 33, wherein said LTO has a purity equal to or above 98%.

37. The LTO according to claim 32, wherein the LTO is nanostructured with nanoparticles or nanosheets size below 100 nm.

38. The LTO according to claim 33, wherein said LTO comprises a nanoparticle structure.

39. The LTO according to claim 33, wherein said LTO comprises a nanosheet structure.

40. A lithium battery, said lithium battery comprising lithium titanate obtained according to the process as defined in claim 1.

41. An electrode comprising a LTO obtained by a process according to claim 1.

42. The electrode of claim 41, wherein said electrode in an anode.

43. A battery comprising an anode, an electrolyte and a cathode, wherein said anode is an electrode of claim 42.

44. The process according to claim 1, wherein 0.1≦x≦0.3.

45. The process of claim 1, wherein 0.15≦x≦0.25.

46. The process of claim 1; wherein 1≦n≦3.

47. The process of claim 24, wherein said lithium hydroxide solution has a concentration of 0.2 to 0.4 M.

48. The process of claim 24, wherein said lithium hydroxide solution has a concentration of 0.25 to 0.3 M.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a pseudo-binary phase diagram of the Li.sub.2O—TiO.sub.2 system. [prior art]

(2) FIG. 2 Existing conventional synthesis processes (solid-state, hydrothermal, and sol-gel processes) for Li.sub.4Ti.sub.5O.sub.12 (LTO). [prior art]

(3) FIG. 3 Comparison of an embodiment of the process of the present invention with various lithium titanate production processes.

(4) FIG. 4 Comparison of an embodiment of the process of the present invention with various lithium titanate production methods.

(5) FIG. 5 is a flowchart showing a process for producing LTH intermediate and LTO, which includes a hydrothermal treatment step.

(6) FIG. 6 is a flowchart showing a process for producing LTH intermediate and LTO in accordance with an embodiment of the present invention, which does not include a hydrothermal treatment step.

(7) FIG. 7(a) is a diagram showing a process for producing LTH intermediate and LTO in accordance with an embodiment of the present invention.

(8) FIG. 7(b) is a diagram showing a process for producing LTH intermediate and LTO in accordance with an embodiment of the present invention. In this option the cold neutralized slurry is heated prior to S/L separation and aging of the gel.

(9) FIG. 8(a) is a scanning electron microscope (SEM) image of the microstructure of a sample of LTH intermediate prepared in accordance with an embodiment of the process of the present invention.

(10) FIG. 8(b) is a scanning electron microscope (SEM) image of the microstructure of a sample of Li.sub.4Ti.sub.5O.sub.12 (LTO) produced from the LTH intermediate in accordance with an embodiment of the process of the present invention; the LTO was produced after 1.5 hours of annealing at 400° C. of the LTH intermediate.

(11) FIG. 8(c) is an x-ray diffractogram (XRD) of the sample of Li.sub.4Ti.sub.5O.sub.12 (LTO) produced from the LTH intermediate in accordance with an embodiment of the process of the present invention as in 8(b).

(12) FIG. 9(a) shows x-ray diffractograms (XRD) of a sample of LTH intermediate before and after aging at 80° C., and after annealing at 400° C. confirming the production of LTO.

(13) FIG. 9(b) is a scanning electron microscope (SEM) image of the microstructure of a sample of LTH intermediate obtained with 80° C. aging.

(14) FIG. 9(c) is a scanning electron microscope (SEM) image of the microstructure of a sample of LTO produced from the LTH intermediate after 400° C. annealing of the LTH intermediate.

(15) FIG. 10(a) is a scanning electron microscope (SEM) image of a sample of LTH intermediate showing the nanostructure of the LTH nanosheets; the LTH nanosheets are aggregated into a primary particle with a diameter of about 300 nm.

(16) FIG. 10(b) is a transmission electron microscope (TEM) image of a sample of LTH intermediate; the LTH is comprised of many small sheets each of about 60 nm in size.

(17) FIG. 11 presents SEM images of lithium titanate LTO samples after 2 hours annealing at: (a) 400° C., (b) 500° C., (c) 600° C., and (d) 700° C.; the images show the initial growth of the nanosheet structure and subsequent aggregation of the nanosheets into primary particles of about 100 nm in size.

(18) FIG. 12 presents high magnification SEM images of the lithium titanate LTO samples after 2 hours annealing at: (a) 400° C., (b) 500° C., (c) 600° C., and (d) 700° C.

(19) FIG. 13 presents XRD diffractograms of lithium titanate LTO samples after 2 hours annealing at: (a) 400° C., (b) 500° C., (c) 600° C., (d) 700° C., and (e) 800° C.; the diffractograms show the increase in crystallinity of the LTO samples with annealing temperature.

(20) FIG. 14 presents XRD diffractograms of lithium titanate hydrate LTH samples: (a) as-precipitated at <10° C. with no additional heating; (b) as-precipitated at <10° C. then heated at 30° C. for 1 hour; (c) as-precipitated at <10° C. then heated at 30° C. for 3 hours; (d) as-precipitated at <10° C. then heated at 30° C. for 6 hours; (e) after annealing at 500° C. for 2 hours sample (a); (f) after annealing at 500° C. for 2 hours sample (b); (g) after annealing at 500° C. for 2 hours sample (c); and (h) after annealing at 500° C. for 2 hours sample (d).

(21) FIG. 15 presents XRD diffractograms of lithium titanate hydrate LTH samples prepared under the following conditions: (a) as-precipitated at <10° C. with no additional heating; (b) as-precipitated at <10° C. then heated at 50° C. for 1 hour; (c) as-precipitated at <10° C. then heated at 50° C. for 3 hours; and (d) as-precipitated at <10° C. then heated at 50° C. for 6 hours;

(22) FIG. 16 presents XRD diffractograms of LTH samples prepared by neutralization at <10° C. for 2 hours (a) without heating, (b) with heating at 30° C., (c) with heating at 50° C. for 6 hours; then aging each of the samples at 80° C. for 36 hours, and drying at 80° C.

(23) FIG. 17 shows (a) a XRD diffractogram and (b) a SEM image of the spinel LTO sample obtained after annealing at 400° C. for 2 hours.

(24) FIG. 18 presents XRD diffractograms of samples produced by annealing at 700° C. for 2 hours of LTH intermediates that were synthesized in different pH environments: (a) pH 0.5, (b) pH 8.7, (c) pH 11.7, and (d) pH 12.

(25) FIG. 19A presents XRD diffractograms of intermediate LTH with different Li/Ti ratio: (a) 4, (b) 6, and (c) 8, prepared at 50° C., and (d)-(f) are diffractograms showing LTO obtained after 500° C. annealing for 1.5 hours of (a)-(c) intermediates.

(26) FIG. 19B presents XRD diffractograms of intermediate LTH with different Li/Ti ratio: (a) 0.8, (b) 2, and (c) 6, prepared by cold neutralization (T<10° C.) and aging of the precipitate at 100° C. for 24 hours. (d)-(f) are diffractograms showing LTO obtained after 700° C. annealing for 1.5 hours of (a)-(c) intermediates.

(27) FIG. 20 presents (a) galvanostatic charge/discharge voltage profiles and (b) corresponding differential capacitance of lithium coin cells comprising LTOs of the invention, at C/24 in the potential range from 2.5 V to 1.0 V or 1.2 V vs. Li/Li.sup.+.

(28) FIG. 21 shows transmission electron microscope (TEM) images of the LTO after annealing at (a) 400° C., (b) 500° C., and (c) 600° C. of a LTH obtained from aging with stirring in presence of LiOH; and annealing at (d) 400° C., (e) 500° C., and (f) 600° C. of a LTH obtained from aging without stirring or LiOH addition.

(29) FIG. 22 shows the XRD diffractograms of the corresponding LTO materials shown in FIG. 21.

(30) FIG. 23 shows galvanostatic charge/discharge voltage profiles of lithium coin cells comprising LTOs of the invention prepared according to Example 9, at C/24 in the potential range from 2.5 V to 1.2 V vs. Li/Li.sup.+.

DETAILED DESCRIPTION OF THE INVENTION

(31) According to one embodiment of the present invention, there is provided a process for producing lithium titanate (LTO) which includes the steps of: (a) providing a lithium titanate hydrate (LTH) intermediate of formula I: (Li.sub.2-xH.sub.x)Ti.sub.2O.sub.5.nH.sub.2O, wherein 0≦x≦0.5, wherein 0.1≦x≦0.3, or wherein 0.15≦x≦0.25; wherein 0<n<4, wherein 1≦n≦3, or n=2; and (b) thermally treating the LTH intermediate of formula I to produce the lithium titanate (LTO).

(32) In accordance to another embodiment of the invention, the step of providing a LTH comprises synthesizing the LTH via aqueous chemical processing.

(33) Aqueous Chemical Synthesis of the LTH Intermediate

(34) In accordance to a particular embodiment, the LTH intermediate of step a) is synthesized via aqueous chemical processing.

(35) Accordingly, synthesizing the LTH intermediate may include mixing a titanium-containing compound with a lithium-containing compound in a solvent to produce a lithium-titanium precursor mixture. For example, the ratio Li/Ti of the titanium-containing compound and lithium-containing compound is above 4, preferably between 4 and 8, and more preferably of about 6. To tailor the electrochemical properties of the final lithium titanate product, dopant-containing compound may be mixed along with the lithium-containing compound and titanium-containing compound into the solvent. According to embodiments of the invention, synthesizing the lithium titanate hydrate intermediate occurs at a temperature range from 0° C. to 100° C.

(36) The lithium-containing compound may be selected, for example, from the group consisting of LiOH, LiNO.sub.3, LiCl, Li.sub.2CO.sub.3, Li.sub.2O, LiHCO.sub.3, Li.sub.2SO.sub.4 and lithium acetate. When LiOH is used in the process of the present invention, it serves both as source of lithium (Li) ions and as alkaline reagent providing an alkaline pH environment for the synthesis process. Non-alkaline or low-alkaline lithium-containing compounds may be used with a “foreign” base, for example NH.sub.4OH, to provide a desired alkaline pH environment.

(37) The titanium-containing compound according to embodiments of the invention includes, for example, titanium tetrachloride (TiCl.sub.4). TiCl.sub.4 is commonly used as raw material for industrial production of pigment, and it is hence cost-effective. TiCl.sub.4 is organic-free and hence is also environmentally friendly. Nevertheless, the titanium-containing compound may include a titanium tetraalkoxide such as titanium tetraisopropoxide and titanium tetrabutoxide for example.

(38) For a doped lithium titanate Li.sub.4Ti.sub.5O.sub.12 final product, different dopant-containing compounds may be mixed along with the lithium- and titanium-containing compounds. Soluble compounds, including acetate, nitrate, chloride and the like for example, comprising one or more of the following metal elements Al, Mg, Ga, Fe, Co, Se, Y, Mn, Ni, Cr, V, or following anions Cl.sup.−, Br.sup.− or F.sup.−, may be used as the doping-containing compounds. Other dopants may be used according to other embodiments of the invention.

(39) The term “solvent” means the medium in which the compounds are dissolved. The solvent may comprise, for example, a mixture of organic solvent and water, or water. Examples of organic solvents include alkyl ketone and lower alkyl alcohols, such as acetone or ethanol. According to embodiments of the invention, the solvent is water; it may be distilled water or deionized water to avoid introducing impurities.

(40) According to one embodiment of the invention, the lithium-containing compound (along with any dopant-containing compounds) is added and mixed into the solvent. Then the titanium-containing compound is added slowly (for example, using a drop-wise technique) and mixed (for example, via stirring) into the solvent to produce the lithium-titanium precursor mixture that is homogeneous, although other non-homogenous mixtures may be employed. The mixing of the compounds is according to embodiments of the invention done at a temperature below 10° C. to favour the nucleation of the LTH intermediate; such mixing is also termed “cold neutralisation” (FIG. 6). In a preferred embodiment, the mixing is carried out in an alkaline pH, preferably a pH of 9 or higher, more preferably at a pH of 10 or higher. Cold neutralization may be followed by heating at room temperature to 100° C. (FIG. 7) to promote crystallization before the as-neutralized precipitate (gel) is separated and subjected to aging at temperatures T<100° C. (FIGS. 6 and 7) in order to produce the LTH intermediate according to the present invention.

(41) In accordance to one embodiment, the neutralized precipitate is allowed to stand in a step called aging at a temperature ranging from room temperature (RT) to 100° C. for several hours (up to 36 hours) without stirring.

(42) In accordance to another embodiment, the neutralized precipitate is allowed to stand in a step called aging at a temperature ranging from room temperature (RT) to 100° C. for several hours (up to 36 hours) with stirring. For example, stirring may be carried out at a constant rate ranging from 200 rpm to 1000 rpm, for example about 500 rpm.

(43) In accordance to both of the above embodiments, aging further comprises the addition of the recovered precipitate in an aqueous lithium hydroxide solution with or without stirring, preferably, the concentration of the aqueous lithium hydroxide solution is in the range of 0.1 to 0.6 M, more preferably of 0.2 to 0.4 M, or most preferably 0.25 to 0.3 M.

(44) Via the aqueous reaction process of the invention, (as exemplified by FIGS. 6 and 7) the synthesis of the LTH intermediate may be advantageously accomplished. According to an embodiment, the LTH intermediate produced responds to the formula: (Li.sub.2-xH.sub.x)Ti.sub.2O.sub.5.nH.sub.2O, wherein 0≦x≦0.5, or wherein 0.1≦x≦0.3, or wherein 0.15≦x≦0.25. In one embodiment, the LTH intermediate prepared is (Li.sub.1.81H.sub.0.19)Ti.sub.2O.sub.5.2H.sub.2O.

(45) According to another embodiment, 0.2M TiCl.sub.4 aqueous solution is mixed with LiOH having a concentration in the range of 0.2 to 1.5 M at 10-100° C. under constant agitation for 0.5 to 6 hours.

(46) The LTH intermediate may be layered (Li.sub.1.81H.sub.0.19)Ti.sub.2O.sub.5.2H.sub.2O with a base-centered (C) orthorhombic crystal structure with lattice constant a.sub.0=1.662 Å, b.sub.0=3.797 Å, c.sub.0=3.007 Å.

(47) To recover the LTH intermediate from the processed mixture, for example to recover the as-neutralized precipitate (FIG. 6) from the synthesis liquor or after aging in a LiOH solution (FIG. 6, on the right), phase separation techniques, such as filtration, washing, centrifuging, may be applied according to embodiments of the invention. The precipitate may then be dried at a temperature in the range of 30° C.-100° C.

(48) According to embodiments of the invention, the process of the invention allows for the production of the LTH intermediate while bypassing the formation of undesirable intermediate phases, such as Li.sub.2TiO.sub.3 or amorphous/anatase/rutile TiO.sub.2 that can complicate the downstream production of pure nanosized lithium titanate LTO.

(49) Thermal Treatment of LTH Intermediate for the Production of Lithium Titanate (LTO)

(50) One advantage of converting lithium titanate hydrate (LTH) intermediate phase into lithium titanate (LTO) is its higher theoretical Li/Ti ratio, which is 0.905, than 0.8 of Li.sub.4Ti.sub.5O.sub.12. The over-stoichiometric ratio with uniform distribution in atomic scale can compensate for Li loss during annealing, and provide high homogeneity and purity in the final LTO product.

(51) In accordance to a particular embodiment of the invention, after recovering the LTH intermediate, the precipitate of LTH intermediate may be annealed by heating to 350-700° C. for 1-12 hours to produce the lithium titanate, for example Li.sub.4Ti.sub.5O.sub.12 (LTO) and more specifically spinel Li.sub.4Ti.sub.5O.sub.12. Further grinding (mechanical grinding or high-energy ball-milling) to reduce particle size is normally not required unless it is necessary for a particular application.

(52) A low annealing temperature in the range of 400-600° C. and a 1- to 6-hour annealing time may be employed according to embodiments of the invention to avoid hard agglomeration or further grain growth during annealing.

(53) As compared to conventional solid-state synthesis processes, a much lower temperature and reaction time may be chosen due to the formation of the LTH intermediate which allows the transformation to the spinel LTO to be triggered at a temperature in the range of 350-450° C., for example.

(54) The proposed advanced aqueous-based process of the invention to synthesize lithium titanate is scalable as well as potentially more environmentally-friendly and cost-effective than current methods. It offers the possibility to use inexpensive inorganic raw materials and to carry out the synthesis at a relatively low temperature (lower than boiling point of water) and hence reduce the energy demand of the process. The process of the invention also allows for reproducible control of the synthesis conditions to avoid undesirable by-products for optimum performance of the LTO product as electrode in LIBs.

(55) FIGS. 2 to 7 provide a means of comparing various existing LTO production processes (synthetic, conventional solid-state, traditional wet chemical) to one embodiment of the process of the present invention (advanced aqueous chemical).

(56) The invention is also directed to a lithium titanate obtained according to the process defined above. The lithium titanate obtained is, according to embodiments of the invention, Li.sub.4Ti.sub.5O.sub.12 (LTO), and more specifically spinel Li.sub.4Ti.sub.5O.sub.12. Advantageously, the process may yield LTO of relatively high purity and of finer nanostructure which implies that the product is phase-pure, i.e. any undesired phase (e.g. rutile or Li.sub.2TiO.sub.3) that may be present in the end-product is negligible. It is possible, according to embodiments of the invention, to achieve nanostructured spinel LTO of particle size below 100 nm, and for example in the range of from 5 to 80 nm, from 50 to 80 nm, or from 5 to 50 nm. Of course, a high-purity nanostructured LTO-containing electrode is of benefit for use as an electrode in LIBs.

EXAMPLES

(57) The following non-limiting examples illustrate the invention. These examples and the invention will be better understood with reference to the accompanying figures.

Example 1

(58) A sample of LTH intermediate prepared in accordance with an embodiment of the process of the present invention was examined using scanning electron microscopy. FIG. 8(a) shows the scanning electron microscope (SEM) image of the microstructure of the obtained LTH intermediate.

(59) After annealing the LTH intermediate sample for 1.5 hours at 400° C., the product was analysed using x-ray diffraction (XRD) and scanning electron microscopy (SEM). The x-ray diffraction identified the product as Li.sub.4Ti.sub.5O.sub.12 (LTO) [see FIG. 8(c)]. FIG. 8(b) is a scanning electron microscope (SEM) image of the microstructure of the sample of Li.sub.4Ti.sub.5O.sub.12 (LTO).

Example 2

(60) An LTH intermediate and LTO final product were produced according to the procedure illustrated in FIG. 6 (left side: aging of precipitate without stirring or LiOH addition).

(61) a) Synthesis of LTH Intermediate:

(62) A volume of 20 mL of a 2 M TiCl.sub.4 aqueous solution was added dropwise to 180 mL of an ice-cold 1.33 M lithium hydroxide stirred aqueous solution (Li/Ti molar ratio=6). The temperature was maintained below 10° C. during addition. The terminal pH of the reaction was within the range of 11.5-12, and the mixture was stirred for an additional 2 hours. The as-neutralized precipitate was collected by centrifugation, and washed 3 times with deionized water. The recovered product was transferred to a closed vessel for aging at 80° C. without stirring during 36 hours. The LTH intermediate was then further dried in an oven at 80° C.

(63) b) Annealing of LTH to Produce LTO:

(64) The LTH intermediate obtained in step (a) was subjected to annealing for 2 hours at 400° C. to produce the desired LTO.

(65) Three samples from the above process were then compared: LTH intermediate without aging, LTH intermediate with 80° C. aging, LTO after 400° C. annealing of the LTH intermediate.

(66) From x-ray diffraction, the sample without aging is mainly amorphous (FIG. 9(a), lower diffractogram). However, after aging at 80° C., highly crystalline lithium titanate hydrate (LTH) was prepared with a delicate nanostructure (see FIG. 9(a), second diffractogram). The top diffractogram of FIG. 9(a) also shows the characteristic LTO pattern obtained after annealing according to step (b) above.

(67) From scanning electron microscopy, the prepared lithium titanate hydrate (LTH) is seen to have “nanosheet” (nanostructured) morphology. FIG. 9(b) is a scanning electron microscope (SEM) image of the microstructure of the sample of LTH intermediate obtained with 80° C. aging. FIG. 9(c) is a scanning electron microscope (SEM) image of the microstructure of the sample of LTO after 400° C. annealing of the LTH intermediate.

Example 3

(68) FIG. 10(a) is a SEM image of a sample of the LTH intermediate produced according to step (a) of Example 2, showing the nanostructure of the LTH nanosheet material; the LTH is aggregated into a primary particle with a diameter of about 300 nm.

(69) FIG. 10(b) is a transmission electron microscope (TEM) image of a sample of LTH intermediate; the LTH is comprised of many small sheets each of about 60 nm in size.

Example 4

(70) Lithium titanate hydrate (LTH) samples produced according to step (a) of Example 2 were annealed for 2 hours at: (a) 400° C., (b) 500° C., (c) 600° C., and (d) 700° C. The microstructure of the LTO samples was then analysed via SEM and XRD.

(71) SEM images of the samples [FIG. 11] show that after two-hour annealing at 400° C., the nanosheet structure is maintained while some modest growth is observed. The structure shown in FIG. 11(a) is in the form of 10 nm thick nanosheets with ˜102 m.sup.2/g specific surface area. However, after two-hours annealing at 500° C., the nanosheet structure is seen to have collapsed, and with an increase in the annealing temperature, the morphology is seen to have changed into particles. Annealing at 600° C. for two hours resulted in nanoparticle LTO material with a specific surface area of about 28 m.sup.2/g. After two-hour annealing at 700° C., the particle size of the LTO has grown to 100 nm. FIG. 12 presents high magnification SEM images of the lithium titanate LTO samples after 2 hours annealing at: (a) 400° C., (b) 500° C., (c) 600° C., and (d) 700° C.

(72) From the XRD analysis, it can be seen that crystallinity of the LTO samples increased with annealing temperature. FIG. 13 presents XRD diffractograms of lithium titanate LTO samples after 2 hours annealing at: (a) 400° C., (b) 500° C., (c) 600° C., (d) 700° C., and (e) 800° C.; the diffractograms show the increase in crystallinity of the LTO samples with annealing temperature.

Example 5

(73) FIG. 15 presents XRD diffractograms of lithium titanate hydrate LTH samples produced under various conditions: (a) as-precipitated at a temperature below 10° C. with no additional heating, (b) as-precipitated at a temperature below 10° C. then heated at 50° C. for 1 hour, (c) as-precipitated at a temperature below 10° C. then heated at 50° C. for 3 hours, and (d) as-precipitated at a temperature below 10° C. then heated at 50° C. for 6 hours.

(74) FIG. 16 presents XRD diffractograms of LTH samples prepared by neutralization at a temperature maintained below 10° C. for 2 hours: (a) without heating, (b) with heating at 30° C., (c) with heating at 50° C. for 6 hours; then aging each of the samples at 80° C. for 36 hours, and drying at 80° C. for 12 hours. The XRD analysis shows an increasing trend in the crystallinity of the LTH samples.

Example 6

(75) According to another embodiment, the lithium titanate hydrate (LTH) intermediate can also be prepared by mixing 1.3M LiOH and 1.33M TiCl.sub.4 solutions as illustrated in step 1 of FIG. 7(a), with Li/Ti molar ratio equal to 6. This aqueous chemical processing involves neutralization and in situ aging at 50° C. for 2 hours. The LTH intermediate is then annealed at 400° C. for 2 hours to obtain a phase-pure spinel LTO sample. FIG. 17 shows (a) an XRD diffractogram and (b) a SEM image of the spinel LTO sample obtained after annealing at 400° C. for 2 hours.

(76) According to the present invention, it was found that the surface area of layered LTO may be larger than 80 m.sup.2/g, for example in the range of 90-110 m.sup.2/g, which corresponds to a particle size in the range of 15-40 nm. Surprisingly, the surface area of spinel LTO which is obtained from layered LTO after annealing is in the range of 20-30 m.sup.2/g without any post treatment such as grinding, corresponding to the isotropically growing nanosheet structures. According to FIG. 17(b), there is some agglomeration among primary particles of spinel LTO nanosheets; further grinding may be carried out to obtain finer LTO for specific applications.

Example 7

(77) Specific conditions, namely reaction pH, compound ratios, and aging temperature/time for the formation of the LTH intermediate were studied. The results obtained are illustrated in FIGS. 18 and 19A,B.

(78) FIG. 18 shows XRD diffractograms of samples produced from LTH intermediates that were prepared in different pH environments during “cold neutralization” (refer to FIG. 6) of (a) pH 0.5; (b) pH 8.7; (c) pH 11.7; and (d) pH 12, followed by aging at 80° and then annealed at 700° C. In an acidic environment, the sample produced contained TiO.sub.2 and the anatase phase. In a neutral or slightly basic environment (pH 7-9), the sample produced contained a mixture of rutile and anatase. When the pH value was increased to higher than 10, the sample produced after annealing contained the desired LTO phase with little to no impurities.

(79) FIG. 19A (a),(b),(c) shows XRD diffractograms of LTH intermediate samples prepared as in example 6 (neutralization and aging at 50° C. for 2 hours) with varying initial concentrations of reactants, LiOH and TiCl.sub.4, expressed as Li/Ti molar ratio. In one trial, when the ratio of LiOH and TiCl.sub.4, Li/Ti, was 4, the sample prepared contained the intermediate phase LTH. In another trial, when Li/Ti is 6 or 8, the sample contained the non-desirable α-Li.sub.2TiO.sub.3 phase. After annealing at 500° C. for 2 hours, the sample with a Li/Ti ratio of 8 (FIG. 19A(f)) yielded β-Li.sub.2TiO.sub.3 instead of the desired LTO while the samples with Li/Ti ratio of 4 and 6 (FIG. 19A (d),(e)) yielded LTO but with variable types of contaminants, like beta-Li.sub.2TiO.sub.3, Anatase or Rutile.

(80) FIG. 19B (a),(b),(c) shows XRD diffractograms of intermediate LTH samples prepared with varying initial concentrations of reactants, LiOH and TiCl.sub.4, expressed as Li/Ti molar ratio. The preparation involved cold neutralization at T<10° C. followed by aging of the precipitate at 100° C. for 24 hours. In one trial, when the ratio of LiOH and TiCl.sub.4, Li/Ti, was 6, the sample prepared contained the intermediate phase LTH (FIG. 19B(c)). In another trial, when Li/Ti is 2 or 0.8, the sample contained the protonated titanate H.sub.2Ti.sub.2O.sub.5 with poor crystallinity. After annealing at 700° C. for 1.5 hours, the sample with Li/Ti=0.8 was converted into pure rutile TiO.sub.2 instead the desired LTO; Li/Ti=2 yielded a mixture of TiO.sub.2 and lithium titanate (LTO). However the LTH prepared with Li/Ti=6 yielded only lithium titanate (LTO) after annealing.

Example 8

a) Preparation of Electrochemical Cells

(81) To test the electrochemical performance of the nanostructured LTO produced in Examples 4(a) annealed at 400° C., and 4(c) annealed at 600° C., coin cells in which a metallic lithium foil was used as the counter electrode were prepared. The employed organic electrolyte was a mixture of 1M LiPF.sub.6 with ethylene carbonate (EC), diethyl carbonate (DEC) in a ratio 3:7 (volume ratio) with 2 wt % by total weight of vinylene carbonate (VC) as an additive. The composite electrodes were made of the active material, synthesized nanostructured LTO, (89 wt %), conductive carbon black (6 wt %) and polyvinylidene fluoride (PVDF) binder (5 wt %) homogeneously dispersed in N-methyl pyrrolidinone (NMP), coated on Al-foil. The loading of prepared electrodes was 4.73±0.04 mg/cm.sup.2 for the 400° C. annealed LTO and 4.12±0.02 mg/cm.sup.2 for the 600° C. annealed LTO. Then, these electrodes were dried at 110° C. in vacuum for 12 hours. The coin cells were assembled in an argon-filled glove box (M. Braun Co., [O.sub.2]<1 ppm, [H.sub.2O]<1 ppm). Each of the two nanostructured LTO materials exhibited unique electrochemical performance properties as will be shown below.

b) Initial Charge/Discharge at C/24

(82) The electrochemical performance of the cells, including rate capability and charge/discharge capacity, was then evaluated at room temperature. A constant current protocol (C/24) was used for formation cycles in the potential range from 2.5 V to the cutoff voltage of 1.0 V and 1.2 V vs. Li/Li.sup.+, respectively. The galvanostatic discharge/charge voltage and corresponding differential capacitance are shown in FIG. 20. Table 2 summarizes the initial charge/discharge capacity data for the two LTO materials (Ex, 4(a) from 400° C. annealing, and Ex. 4(c) from 600° C. annealing).

(83) TABLE-US-00002 TABLE 2 Initial charge/discharge capacity in C/24 formation cycle Cut-off voltage Capacity (mAh/g) Efficiency LTO vs. Li/Li.sup.+ (V) Charge Discharge (%) Ex. 4(a) 1 170.3 228.3 74.6 (400° C.) 1.2 187.2 199.6 93.8 Ex. 4(c) 1 162.4 187.3 86.7 (600° C.) 1.2 158.0 176.9 89.3

(84) Thus the LTO nanosheets of Example 4(a) yielded a curved voltage profile typical of nanosized intercalation compounds with first discharge capacity equal to 228 mAh/g for 1.0 V cutoff at a C/24 rate. The LTO nanoparticles of Example 4(c) exhibited a voltage plateau at 1.55 V with first discharge capacity equal to 187.3 mAh/g, i.e. well above the theoretical value of 175 mAh/g.

c) Discharge Rate Performances

(85) The discharge rate performances were also tested for the LTO containing cells, in which the charge rate was kept constant at 0.25 C. The discharge rate was varied from 1 C (0.74 mA/cm.sup.2) to 15 C over the operating voltage range from 2.5 to 1.0 or 1.2 V vs. Li/Li+. Table 3 summarizes the initial capacity data at different discharge rates.

(86) TABLE-US-00003 TABLE 3 Charge/discharge capacity at varying discharge rates Cut-off voltage Charge/discharge capacity (mAh/g) LTO vs. Li/Li.sup.+ (V) 0.25Ch/1D 0.25Ch/4D 0.25Ch/10D 0.25Ch/15D Ex. 4(a) 1 157.5/151.3 143.0/133.7 110.7/88.1  77.8/17.6 1.2 155.4/153.5 151.4/146.6 137.5/131.9 132.3/115.0 Ex. 4(c) 1 159.6/158.5 154.6/151.3 133.0/110.1 108.9/5.4  1.2 156.6/155.2 152.5/150.0 134.9/127.7 126.3/68.4 

(87) It can be noted that both materials exhibited higher rate capability when the cutoff voltage was 1.2 V rather than 1.0 V. In other words the extra capacity afforded by the lower cutoff voltage (1.0 V) was lost upon increasing the discharge rate. This loss in capacity was more pronounced with the LTO nanosheet material (400° C.). At cutoff voltage 1.2 V both LTO nanomaterials exhibited essentially the same rate capability up to 10 C rate. Thus at 1 C rate the discharge capacity was 153.5 vs. 155.2 mAh/g, while at a 10 C rate the corresponding values for the LTO nanosheets and nanoparticles were 131.9 vs. 127.7 mAh/g respectively. However, at higher rate (15 C) the nanosheet LTO material performed better than the nanoparticle LTO material (115.0 vs. 68.4 mAh/g), which might be reflecting the shorter diffusion path characterizing nanosheets.

d) Cell Cyclability

(88) The cyclability of the two nanostructured LTO materials was evaluated by 0.25 C charge and 1 C discharge over 130 cycles. The 600° C. annealed LTO material (nanoparticles) retained 97% capability after 130 cycles (at about 150 mAh/g), while the 400° C. annealed LTO nanosheets showed a 15% capacity loss, which might reflect their higher degree of crystallinity. Both materials, however, exhibited nearly 100% columbic efficiencies after a few cycles. In addition, the LTO nanosheet material's reversible capacity stabilized at about 135 mAh/g after 120 cycles.

Example 9

(89) An LTH intermediate and LTO final product were also produced according to the following procedure procedure illustrated in FIG. 6 (right hand side: aging of precipitate in LiOH addition subjected to stirring).

(90) a) Synthesis of LTH Intermediate:

(91) i. Neutralization:

(92) A volume of 20 mL of a 2 M TiCl.sub.4 aqueous solution was added drop wise to 180 mL of an ice-cold 1.33 M lithium hydroxide stirred (1000 rpm) aqueous solution (Li/Ti molar ratio=6). The temperature was maintained below 10° C. during addition. The terminal pH of the reaction was within the range of 11.5-12, and the mixture was stirred at 1000 rpm for 2 hours. The as-neutralized precipitate was separated by centrifugation at 6000 rpm, and washed 3 times with deionized water.

(93) ii. Aging:

(94) The washed precipitate (about 5-10 g) was added to 50 mL of a pre-prepared 0.25M LiOH solution and the mixture was stirred (500 rpm) at 80° C. for 16 hours. The precipitate was centrifuged and washed as in step (i). The LTH intermediate obtained was then further dried in an oven at 80° C. for 12 hours.

(95) b) Annealing of LTH to Produce LTO:

(96) The LTH intermediate obtained in step (a) is then subjected to annealing for 2 hours at a temperature of (i) 400° C., (ii) 500° C., and (iii) 600° C. The material obtained was then subjected to mild regrinding.

(97) The morphology of the LTO obtained was compared with the LTO of Examples 4(a) to (c). For example, FIG. 21 shows transmission electron microscope (TEM) images of the LTO obtained by the above procedure and annealing at (a) 400° C., (b) 500° C., and (c) 600° C., while images (d), (e), and (f) were obtained with LTO resulting from the procedures of Example 4 at the same temperatures. The images show that the LTO annealing product of LTH obtained through an aging process involving agitation and LiOH 0.25M maintained a nanosheet structure at all three annealing temperatures. As a comparison, the nanosheet structure of the LTO annealing product of an LTH prepared as in Example 2 (aging step without agitation and LiOH) gradually collapsed and transformed into particulate material as the annealing temperature increased.

(98) FIG. 22 compares XRD analysis of the LTO product from this example (FIG. 22a-c) with the product of Example 4 (a-c) (FIG. 22d-f). These demonstrate that the purity of the left hand LTOs is over 98% while the diffractograms on the right depict traces of TiO.sub.2 in the product obtained through an aging process not involving agitation and LiOH.

(99) Table 4 below further illustrates the physical characteristics of LTOs obtained by annealing at 400° C., 500° C. and 600° C. of an LTH obtained through an aging process involving LiOH (0.25M) and agitation, or an LTH obtained through an aging process involving neither LiOH nor agitation. These results show that a small grain size and high surface area especially for the LTO materials obtained through the annealing of the LTH obtained via an aging process involving agitation and addition of LiOH.

(100) TABLE-US-00004 TABLE 4 Physical characteristics of LTO products [LiOH] Agitation Annealing Temp Grain Size Specific area (M) (rpm) (° C.) (nm) (m.sup.2/g) 0.25 500 400 7.2 86.4 500 10.8 90.6 600 15.1 66.5 0 0 400 9.4 102 500 16.4 53.7 600 31.8 28.0

(101) The LTO product of the second line of Table 4 was also characterized using the Rietveld Refinement XRD method that confirmed the spinel Li.sub.4Ti.sub.5O.sub.12 structure at 98.7% (by weight) purity. The rest was Ti—O compounds, e.g. TiO.sub.2. The electrochemically inactive β-Li.sub.2TiO.sub.3 could not be detected.

Example 10

(102) The LTO materials prepared in Example 9 (annealed at 500° C.—FIG. 21(b) and FIG. 22(b)) were used in the preparation of coin cells according to Example 8(a), and the cells thus obtained were tested for their electrochemical response in terms of initial charge/discharge capacities. The obtained results are shown in FIG. 23 and Table 5. The LTO nanosheet material of Example 9 (annealed at 500° C.—FIGS. 21(b) and 22(b)) yielded a first discharge capacity equal to 198 mAh/g for 1.2 V cutoff at a C/24 rate that was the same with the equivalent value for the Ex. 4(a)-400° C. material (199.6) but higher than that for Ex. 4(c)-600° C. material (176.9). At the same time the voltage profile (FIG. 23) is less curved than that of Ex. 4(a)-400° C. approaching that of Ex. 4(c)-600° C. implying that overall has the features to deliver stable cycling at high rate performance.

(103) TABLE-US-00005 TABLE 5 Initial charge/discharge capacity in C/24 formation cycle Cut-off voltage Capacity (mAh/g) Efficiency LTO vs. Li/Li.sup.+ (V) Charge Discharge (%) Ex. 9 1.2 152 198 76.9 (500° C.)

(104) Numerous modifications could be made to any of the embodiments described above without departing from the scope of the present invention. Any references, patents or scientific literature documents referred to in this application are incorporated herein by reference in their entirety for all purposes.

REFERENCES

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