Patent Grant
7547490
This application claims priority to EP application 01810752.4 filed Jul. 31, 2001, and to International Application No. PCT/CH02/00425 filed Jul. 29, 2002 and published in English on Feb. 13, 2003, the entire contents of both are incorporated herein by reference.
The invention relates to a Lithium Titanate Spinel oxide material, more precisely a Li4Ti5O12 material, which may be used in energy storage devices and more specifically in cathodes and anodes of Lithium Ion batteries or of hybrid capacitance devices.
The invention also concerns a process which may be used for obtaining a Li4Ti5O12 spinel material.
Spinel oxides Li1+xTi2−xO4; 0≦x≦⅓ (space group Fd3 m) were described in 1971 and electrochemically characterized in the early 1990s (K. M: Colbow, J. R. Dahn and R. K Haering, J. Power Sources, 26,397 (1989) and T. Ohzuku, A. Ueda and N. Yamamoto, J. Electrochem. Soc., 142,1431 (1995)). The end members of the series, i.e. LiTi2O4 and Li4/3Ti5/3O4 (Li4Ti5O12) are metallic (super-conducting below 11 K) and semi-conducting, respectively (M. R. Harrison, P. P. Edwards and J. B. Goodenough, Phil Mag. B, 52, 679 (1985)). Both materials exhibit similar Li-insertion electrochemistry, the formal potential of Li-insertion being (1.36-1.338) V for LiTi2O4 and (1.55-1.562) V for Li4Ti5O12, respectively (S. I. Pyun, S. W. Kim and H. C. Shin, J. Power Sources, 81-82,248 (1999)).
In principle, therefore, this latter material can be coupled with a 4V electrode, as LiMn2O4 or LiCoO2, to provide a cell with an operating voltage of approximately 2.5V, which is twice that of nickel-cadmium or nickel-metal hydride cells.
Li4Ti5O12 accommodates Li with a theoretical capacity of 175 mAh/g (.based on the mass of the starting host material) according to the equation:
(Li)8a(Li1/3, Ti5/3)16dO432e+e−+Li+⇄(Li2)16c(Li1/3, Ti5/3)16dO432e (1)
where the superscripts stand for the number of equivalent sites with Wyckoff symbols for the space group Fd3m. Hence, Li+ occupies tetrahedral (8a) and octahedral (16c, 16d) sites of the lattice, and the overall insertion capacity is controlled by the number of free octahedral sites. A more detailed analysis points at two-phase equilibrium, which explains the invariance of the electrode potential on the electrode composition. The spinel host structure accommodates Li+ without significant changes of lattice constants. Consequently, these materials show excellent cycle life and the Li+ diffusion coefficient of about 2.10−8 cm2/s was reported (K. Zaghib, M. Simoneau, M. Armand and M. Gauthier, J. Power Sources, 81-82, 300 (1999)). For an entire battery, both anode and cathode active materials, the charge/discharge cycle may be simply represented by the following equation.
In previous communications the spinel Li4Ti5O12 was prepared by a solid-state reaction of stoichiometric amounts of TiO2 and Li2CO3 or LiOH; the reaction typically occurs within 12-24 hours at 800-1000° C.
A Li4Ti5O12 material with smaller particle size was prepared by high-energy ball milling of the conventional microcrystalline spinel. The product exhibited particles around 600 nm in size, but its electrochemical performance was not significantly different from that of the non-milled starting material.
The above-mentioned Li4Ti5O12 materials suffer from relatively low Li-insertion capacity at high charging rates. Therefore, there is a need for Li4Ti5O12 electrode materials with improved electrochemical performance.
The inventors unexpectedly found that Li4Ti5O12 material made of nano-sized particles which—according to the nitrogen absorption surface area measurement method of Brunauer-Emmet-Teller (BET method)—have a BET surface area of at least 10 m2/g (i.e. corresponding to particles having a theoretical size of less than 100 nm) exhibit different Li-insertion electrochemistry and show specific electrochemical performances.
In one embodiment of the invention the particles are characterized by a BET surface of between 10 and 200 m2/g.
In another embodiment the particles are characterized by a BET surface area of between 20 and 160 m2/g .
In another embodiment the particles have a BET surface between 30 and 140 m2/g.
In a preferred embodiment the particles are characterized by a BET surface area of between 70 and 110 m2/g.
For producing the above cited nano-sized particles the inventors developed new synthetic methods.
The conversion of nanocrystalline TiO2 (anatase) towards Li4Ti5O12 was first explored by a reaction of colloidal TiO2 with LiOH. However, this strategy was not successful, neither its variants employing Li2CO3, LiCH3COO and LiNO3 in combination with the stoichiometric amount of colloidal anatase in acidic or alkaline media at temperatures up to 250° C. (in autoclave). In all cases, the product contained Li1+xTi2−xO4 with considerable amounts of unreacted anatase.
The inventors solved the problem by developing a method comprising a step of mixing an organo-lithium compound selected from lithium alcoholates with an organo-titanium compound selected from titanic acid esters in an organic solvent and a step of hydrolyzing said mixture. Particularly preferred alkoxides as starting reagents are Li-ethoxide and Ti(IV)isopropoxide and Ti(IV) n-butoxide. Preferably, the organo-lithium compound and the organo-titanium compound are mixed in a stoichiometric molar ratio substantially equal to 4:5. For some contemplated applications, defined mixtures of anatase and lithium titanate spinel are desirable. These may be obtained by appropriate ratios differing from the above molar ratio.
After hydrolysis, isolation and drying of the precipitate, spinel products were obtained which exhibit BET surface area values of at least 5 m2/g, and generally above 10 m2/g, which correspond to much smaller particle sizes than the particles sizes of state of the art microcrystalline Li4Ti5O12 materials. However, after alkoxide hydrolysis, the slurry still contains appreciable mounts of unreacted anatase. Therefore, a preferred embodiment of the process according to the invention further comprises the steps of processing the hydrolyzed mixture with a polymer like polyethyleneglycol (PEG) up to homogeneity, and submitting the homogenized product to a heat treatment effective for removing organic material there from. This polymer is known to form complexes with lithium and oxo-titanium species, while It may also organizes the inorganic structure by supramolecular templating (L. Kavan, J. Rathousky, M. Grätzel, V. Shklover and A. Zukal, J. Phys. Chem. B, 104, 12012 (2000)). After processing the hydrolyzed mixture with PEG and removing the same by annealing, pure spinel materials could be obtained. Materials exhibiting unexpected extremely high BET surface area values of more than 80 m2/g may be obtained.
An additional object of the invention is an electrode comprising a nano-structured Li4Ti5O12 material exhibiting BET values as above. The PEG processing step and the annealing/sintering step may be conducted to obtain BET values which may range between 50 m2/g and 200 m2/g, corresponding then to values well suitable for electrodes.
A preferred object of the invention is a thin film electrode obtained by coating a conductive support with a hydrolyzed mixture produced by a process as defined above and submitting said coated support to a heat treatment. A particularly preferred thin film electrode is obtained by coating a conductive support with the homogenized product produced by processing the hydrolyzed mixture with PEG, and submitting said coated support to an annealing treatment.
Such annealing treatments may be carried out at 400-500° C., that is to say at much lower temperatures than the solid state spinel preparations of the prior art.
Thus, the invention provides electroactive ion-insertion materials based on nanostructured, tetra-Lithium Titanate spinel allowing extremely high charge and discharge rates, a high number of charge, discharge cycles, Mesoporous electrode materials thereof and processes to produce these materials, including precipitation from a solution, doping with metallic atoms, nano-templating, Microparticles agglomeration, spray-drying, ball-milling and sintering.
This invention also provides electrodes, i.e. anode or cathode, based on nanostructured lithium titanate spinel, and their manufacturing process to build these as rigid films made from nanoparticles of the electroactive material or as flexible layers made from Mesoporous microparticles of the electroactive nanostructured material. Such Microparticles may be obtained from agglomerated and sintered lithium titanate spinel precipitated particles, which are spray dried, processed with an organic binder and coated onto a conductive substrate.
It should also be noted that the invention is not limited to a specific process such as the one previously discussed. Several other processes may be used and in particular the Altair processes disclosed in U.S. patent applications 60/306,683 and 60/362,723.
Other features and advantages of the process and products according to the invention will appear to those skilled in the art from the following detailed description and related non limitative examples.
Under Ar-atmosphere, 1.4 g (0.2 mol) of lithium metal (Aldrich) was dissolved in 110 ml of absolute ethanol and mixed with 71 g (0.25 mol) of titanium (IV) isopropoxide (pract Fluka) or, alternatively, with 85 g (0.25 mol) of titanium (IV) n-butoxide (pract. Fluka). Still another alternative consisted in using lithium ethoxide or lithium methoxide powders from Aldrich.s 50 mL of the solution of Li+Ti alkoxides was hydrolyzed in 300 mL of water, and the produced slurry was concentrated on rotary evaporator (40° C. 20 mbar) to a concentration of 10-20 wt %. Polyethylene glycol (molecular weight 20 000, Merck) was added in the proportion of 50-100% of the weight of Li4Ti5O12, and the mixture was stirred overnight. The resulting viscous liquid was deposited on a sheet of conducting glass (F-doped SnO2, TEC 8 from Libbey-Owens-Ford, 8 Ω/square) using a doctor-blading technique (L. Kavan, M. Grätzel. J. Rathousky and A. Zukal, J. Electrochem. Soc., 143, 394 (1996)) and finally annealed at 500° C. for 30 min. Sometimes, the slurry was homogenized using a titanium ultrasonic horn (Bioblock Scientific; 80 W, 30×2 s pulses) before deposition. The mass of active electrode material was typically 0.1-0.3 mg/cm2; the projected electrode area 1 cm2 and the layer thickness about 2-6 μm. For comparison, an analogous electrode was prepared from commercial Li4Ti5O12 (LT-2 from Titan Kogyo Japan). The material had a BET surface of 2.9 m2/g (manufacturer's specification; 3.1 m2/g by own measurement). The LT-2 powder was dispersed by mortaring with acetylacetone, and the paste for doctor-blading was prepared by addition of hydroxypropylcelulose and Triton X-100 as described elsewhere (L. Kavan, M. Grätzel. J. Rathousky and A. Zukal, J. Electrochem. Soc., 143, 394 (1996))
The BET surface areas of the prepared materials were determined from nitrogen adsorption isotherms at 77 K (ASAP 2010, Micromeritics). The film thickness was measured with an Alpha-step profilometer (Tencor Instruments). Powder X-ray diffractometry (XRD) was studied on a Siemens D-5000 difractometer using CuKα radiation. The samples for BET and XRD were obtained by mechanical scraping of the film from a glass support.
The BET surface areas of the as-prepared materials were 105 m2/g (synthesis employing Ti(IV) isopropoxide) and 153-196 m2/g(synthesis employing Ti(IV) butoxide), respectively. If the slurry was autoclaved at 150° C. for 10 hours, the surface areas decreased to 53 m2/g (isopropoxide-synthesis) or 119 m2/g (butoxide-synthesis), which is due to hydrothermal particle growth by Ostwald ripening.
Fig I/(A) shows the X-ray diffractogram of a material resulting from the butoxide-synthesis (surface area 183 m2/g). All peaks can be indexed as Li4Ti5O12. The crystal size (dc) can be estimated from the X-ray line width (w) (Scherrer formula):
dc=0.9λ/w cos θ (2)
(λ is the X-ray wavelength (0.1540562 nm) and θ is the diffraction angle). Eq. (2) gives dc about 4-5 nm. This value roughly matches the particle size (dp) estimated from BET area (S=183 m2/g). Assuming spherical particles, the value of dp can be approximated as:
dp=6/Sρ (3)
which gives dp≈9 nm for S=183 m2/g and ρ=3.5 g/cm3. Analogous evaluation routine for the ex-propoxide material produces dc≈15 nm and dp≈19 nm. FIG. 1/(B) shows the XRD plot for a material prepared as that in FIG. 1/(A) except that the addition of polyethylene glycol was omitted. In this case, anatase is clearly distinguished at 2θ≈25 deg. FIG. 1/(C) displays the XRD plot for a material prepared as that in FIG. 1/(A), but the particles were grown hydrothermally (the product's surface area was 119 m2/g, dc≈dp≈14 nm). The lattice constant of the hydrothermally grown material (FIG. 1/(C)) equals 0.8366 nm, which is in good agreement with the lattice constant of Li4Ti5O12 made by the conventional high-temperature synthesis: 0.8367, 0.8365 and 0.8358. However, the lattice constant of the nanocrystalline materials (without hydrothermal growth, cf. FIG. 1/(A)) is significantly smaller. The actual values fluctuate between 0.8297 nm to 0.8340 nm for various samples, both from the butoxide-and isopropoxide-synthesis.
Whereas the lattice constant, a, of a Li1+xTi2−xO4 spinel is known to decrease with x (ref.) (M. R. Harrison, P. P. Edwards and J. B. Goodenough, Phil Mag. B, 52, 679 (1985)) according to the relation:
α=0.8405−0.0143x (4)
this reasoning cannot account for the observed decrease of a for the nanocrystalline material according to the invention. The latter conclusion is supported by two arguments: (i) the lattice constant attains its “normal” value after hydrothermal growth and (ii) the nanocrystalline material according to the invention shows the electrochemistry of a Li-rich spinel, vide infra. It may be noticed that a 5 nm-sized particle of Li4Ti5O12 contains about 200 unit cells only, and such a small particle exhibits marked lattice-shrinking.
Electrochemical measurements were carried out in a one-compartment cell using an Autolab Pgstat-20 controlled by GPES-4 software. The reference and auxiliary electrodes were from Li metal, hence potentials are referred to the Li/Li+ (1 M) reference electrode. LiN(CF3SO2)2 (Fluorad HQ 115 from 3M) was dried at 130° C./1 mPa. Ethylene carbonate (EC) and 1,2-dimethoxyethane (DME) were dried over the 4A molecular sieve (Union Carbide). The electrolyte solution, 1 M LiN(CF3SO2)2+EC/DME (1/1 by mass) contained 10-15 ppm H2O as determined by Karl Fischer titration (Metrohm 684 coulometer). All operations were carried out in a glove box.
Cyclic voltammogram of the ex-butoxide material (A) evidences the Li-insertion into Li4Ti5O12 spinel (
Details of the Discovery of the Particle Size Requirements for High Rate Charging of tetra-lithium titanate (Spinel):
A unique set of various size samples of tetra-Lithium Titanate was collected for study by the inventors. This unique set of samples ranged in surface area from approximately 1 m2/gm to approximately 200 m2/gm. The charging characteristics of tetra-Lithium Titanate (spinel) was determined as a function of Surface Area of the is active electrode powder. As a result of this study the charging performance of tetra-Lithium Titanate as a function of the surface area of the active electrode grade powder was discovered.
The unique and comprehensive set of samples was collected from 3-sources:
Preparation of electrodes used in measuring the charging characteristics of particulate tetra-Lithium Titanate samples acquired from commercial sources (item 1. above) and produced using the Altair NanoMaterials process (Item 2. above) proceeded as follows.
The Li4Ti5O12 powder was dispersed in aqueous medium to form a viscous paste. The powder (1.0 g) was ground for at least 20 min in an agate or porcelain mortar under slow addition of 4×0.2 mL of 10% aqueous solution of acetylacetone. The mixture was diluted with 5 ml H2O and mixed with 2 mL of 4% aqueous solution of hydroxypropylcellulose (MW 100,000) and 2 mL of 10% aqueous solution of Triton-X100 (Fluka). The resulting viscous liquid was stirred overnight before use. When necessary, the mixture was further homogenized using a titanium ultrasonic horn (Bioblock Scientific; 80 W, 30×2 s pulses). The obtained paste was deposited on a sheet of conducting glass (F-doped SnO2, 8 Ω/square) using a doctor-blading technique. The sheet of conducting glass had dimensions: 3×5×0.3 cm3. A Scotch-tape at both edges of the support (0.5 cm) defined the film's thickness and left part of the support uncovered for electrical contact. The film was finally calcined for 30 min in air at 500° C. After cooling down to room temperature, the sheet was cut into ten electrodes 1.5×1 cm2 in size; the geometric area of the TiO2 film was 1×1 cm2. The as-deposited films were controlled by optical microscope, by a simple scratch test (surface scratched by a piece of glass to check the sintering of particles) and by an alpha-step profilometer. The latter method provided information about layer thickness and surface corrugation. The film's mass was determined after scraping the TiO2 layer from the SnO2(F) support by a piece of glass sheet. The layer thickness was about 1-5 μm. No signs of electrode “aging” (cracking, delaminating) were found, even after many repeated tests of the same electrode. The mass of active electrode material was typically 0.1-0.4 mg/cm2; the projected area was 1 cm2.
It is to be noted that when Li4Ti5O12 was produced as a slurry, this slurry was mixed with hydroxypropylcellulose and Triton-X in the same proportions as with the powder samples. The electrodes were fabricated and tested by the same methods as mentioned above.
Electrochemical Measurements of Electrodes:
Electrochemical measurements were carried out in a one-compartment cell using an Autolab Pgstat-20 (Ecochemie) controlled by GPES-4 software. The reference and auxiliary electrodes were made of Li metal, hence, potentials are referred to the Li/Li+ (1M) reference electrode. LiN(CF3SO2)2 was dried at 130° C./1 mPa. Ethylene carbonate (EC) and 1,2-dimethoxyethane (DME) were dried over the 4A molecular sieve. The electrolyte solution, 1 M LiN(CF3SO2)2+EC/DME (1/1 by mass) contained 10-15 ppm H2O as determined by Karl Fischer titration. All operations were carried out in a glove box.
A total of 25 electrodes were prepared and tested according to the method of preparation and the experimental setup given above. XRD analysis of these samples showed that they are formed of pure Li4Ti5O12 with less than 1% free TiO2 in the rutile or anatase phase.
For each example, galvanostatic chronopotentiometry curves at different charging and discharging rates were measured.
The results obtained with all the samples are summarized in Table 1, the raw data table. Separate measurements confirm that up to a charging rate of 2 C (such that complete charging would be completed in ½ h; a charging rate of 1 C corresponds to full charge in 1 h), all samples exhibit the same maximum charge quantity. This number is considered as the full capacity of the given sample. Table 1 shows the specific surface area as well as the charging capacity (mC, milli Coulombs) measured for each example. Please note that as the actual mass of tetra-Lithium Titanate (spinel) varied on each electrode, so did the actual charge/discharge currents at the given charging rates.
Table 1, the raw data table, was processed to produce Table 2, which normalizes the data set to 100% charge capacity at 2 C_rate. Table 2 expresses observed capacities as a % of the capacity of sample at 2 C. A number smaller than 100% corresponds to a loss of capacity at higher charging rate.
The results of Table 2, the processed data table, are plotted on a logarithmic scale in
Some general trends are apparent. Clearly, as the charging rate is increased, the charging capacity decreases for the particles with small surface area, while the charging capacity of the particles with large surface area is substantially maintained. As the production of the data set Table 1 is a substantial art, the data is necessarily “noisy”. Despite the “noise” of the measurement process some clear performance plateaus are visible. These performance plateaus provide sufficient guidance for a manufacturer to design and produce tetra-Lithium Titanate that conforms to “guaranteed” performance requirements.
These data show that particulate tetra-Lithium Titanate qualifies for use in anode or cathode service in energy storage devices based on Li+ion electron pair insertion/desertion cycles that function at a 50 C_rate. Using a pass/fail test based on equal to or greater that 90% working charge capacity passes and less than 90% fails, the test data may be screened to produce two subsets. This screening process is shown graphically in
These data show that particulate tetra-Lithium Titanate qualifies for use in anode or cathode service in energy storage devices based on Li+ ion electron pair insertion/desertion cycles that function at a 100 C_rate. Using a pass/fail test based on equal to or greater that 90% working charge capacity passes and less than 90% fails, the test data may be screened to produce two subsets. This screening process is shown graphically in
These data show that particulate tetra-Lithium Titanate qualifies for use in anode or cathode service in energy storage devices based on Li+ ion electron pair insertion/desertion cycles that function at a 150 C_rate. Using a pass/fail test based on equal to or greater that 80% working charge capacity passes and less than 80% fails, the test data may be screened to produce two subsets. This screening process is shown graphically in
These data show that particulate tetra-Lithium Titanate qualifies for use in anode or cathode service in energy storage devices based on Li+ ion electron pair insertion/desertion cycles that function at a 200 C_rate. Using a pass/fail test based on equal to or greater that 80% working charge capacity passes and less than 80% fails, the test data may be screened to produce two subsets. This screening process is shown graphically in
These data show that particulate tetra-Lithium Titanate qualifies for use in anode or cathode service in energy storage devices based on Li+ ion electron pair insertion/desertion cycles that function at a 250 C_rate. Using a pass/fail test based on equal to or greater that 80% working charge capacity passes and less than 80% fails, the test data may be screened to produce two subsets. This screening process is shown graphically in
One of the preferred BET-SA of particulate tetra-Lithium Titanate has been discovered by the inventors to be at least 70 m2/g but not more than 110 m2/g.
The above examples show that the charging capacity corresponding to a given rate of charge increases with increasing surface area, reaches a plateau, then decreases as the surface area increases further.
Thereby (Examples 1a through 5a), the excellent electrochemical performance of nanocrystalline Li4Ti5O12 is clearly demonstrated. Furthermore, it is to be noted that several different processes may be used to manufacture the particles according to invention.
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| 01810752 | Jul 2001 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
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| PCT/CH02/00425 | 7/29/2002 | WO | 00 | 1/29/2004 |
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