Electrode material for lithium batteries
1. A composition consisting essentially of Nb(1-x)MxPO5,wherein M is at least one transition metal selected from among the Group 5 transition metals and the Group 6 transition metals,wherein x is a value that is greater than 0 and less than 1, andwherein M is selected from the group consisting of tantalum, vanadium, chromium, molybdenum, tungsten, or any combinations thereof.
Substitution of tantalum into the lattice of monoclinic niobium phosphate results in improved reversibility, near 0% irreversible loss, and similar excellent 20 C high rate behavior in large grain material without the formation of electronically conducting nanocomposites. Tantalum substitution into niobium pentaphosphate enables an improved stabilization of the difficult to fabricate monoclinic niobium phosphate phase. Such tantalum-substituted niobium phosphates show excellent potential for use as electrodes in lithium or lithium-ion batteries.
|Substituted lithium phosphates and solid electrolytes therefrom|
Patent #US 4,009,092 A
Current AssigneeE. I. du Pont de Nemours and Company
Sponsoring EntityE. I. du Pont de Nemours and Company
|Metal fluoride and phosphate nanocomposites as electrode materials|
Patent #US 8,518,604 B2
Current AssigneeRutgers University
Sponsoring EntityRutgers University
|BLENDED CATHODE MATERIALS|
Patent #US 20140138591A1
Current AssigneeA123 Systems Incorporated
Sponsoring EntityA123 Systems Incorporated
|COMPOSITE CATHODE ACTIVE MATERIAL, PREPARATION METHOD THEREOF, CATHODE INCLUDING THE MATERIAL, AND LITHIUM BATTERY INCLUDING THE CATHODE|
Patent #US 20160190585A1
Current AssigneeIndustry Foundation Of Chonnam National University, Samsung SDI Company Limited, Samsung Electronics Co. Ltd.
Sponsoring EntityIndustry Foundation Of Chonnam National University, Samsung SDI Company Limited, Samsung Electronics Co. Ltd.
|NEGATIVE ELECTRODE ACTIVE MATERIAL FOR POWER STORAGE DEVICE|
Patent #US 20180219221A1
Current AssigneeNippon Electric Glass Company Limited
Sponsoring EntityNippon Electric Glass Company Limited
- 1. A composition consisting essentially of Nb(1-x)MxPO5,
wherein M is at least one transition metal selected from among the Group 5 transition metals and the Group 6 transition metals, wherein x is a value that is greater than 0 and less than 1, and wherein M is selected from the group consisting of tantalum, vanadium, chromium, molybdenum, tungsten, or any combinations thereof.
- 5. A composition comprising Nb(1-x)TaxPO5 and at least one other chemical component,
wherein x is a value that is greater than 0 and less than 1.
The present application claims the benefit of U.S. Provisional Patent Application No. 62/263,993 which has a filing date of Dec. 7, 2015 and is incorporated by reference herein in its entirety.
The invention disclosed herein was made with government support under a contract from the U.S. government. The United States government has certain rights in the invention.
The present invention relates generally to electrochemical cells and a method for making same. More particularly, the present invention relates to a substituted niobium phosphate material suitable for use as an electrode of a lithium or lithium-ion battery.
The increase in demand for longer-lasting battery-powered devices such as portable electronics and electric vehicles creates a need for energy storage technologies that provide higher energy and power densities. The relatively sluggish progress of lithium-ion batteries, first commercialized by Sony in 1991, demonstrates the need for new electrode materials that meet consumer demands and expectations. In the last few decades, intercalation materials containing phosphate groups (PO4) have garnered interest due to several intrinsic advantages. The robust structure of the PO4 group provides an open 3D network allowing for long term cycling and high ionic diffusion rates. The inherent stability of the PO4 group derives from the tetrahedral coordination of the phosphorous-oxygen covalent bonds, which engender several desirable properties including resistance to thermal degradation and overcharge. The most well-known phosphate, the triphylite LiFePO4, was first introduced in 1997. This phospholivine of type LiMPO4 (M=Co, Cu, Fe, Mn, Ni) is an inexpensive, environmentally friendly, but low energy cathode material that requires various conductive additives to enable exceptionally high power. Subsequent to this pioneering work, there has been great interest in phosphate intercalation compounds for positive and negative electrodes in lithium batteries. Metal phosphates are also being investigated as model intercalation materials to further the understanding of the intrinsic reaction mechanisms and limitations to elucidate new pathways towards improved battery technology.
In an embodiment of the present invention, a composition comprises a substituted niobium phosphate of the formula MxNb(1-x)PO5, wherein M is one or more transition metals. In an embodiment, each of the transition metals is from one of Group 5 and Group 6. In an embodiment, the one or more transition metals include tantalum. In an embodiment, the one or more transition metals are substituted for up to 20% of the niobium (i.e., x≤0.20). In an embodiment, the one or more transition metals are substituted for up to 10% of the niobium (i.e., x≤0.10). In an embodiment, the one or more transition metals are substituted for up to 5% of the niobium (i.e., x≤0.05). In an embodiment, the one or more transition metals are substituted for up to 2% of the niobium (i.e., x≤0.02). In an embodiment, the one or more transition metals are substituted for up to 1% of the niobium (i.e, x≤0.01).
In an embodiment, the substituted niobium phosphate is predominately in a monoclinic crystalline form. In an embodiment, at least 80% of the substituted niobium phosphate is in a monoclinic crystalline form. In an embodiment, at least 90% of the substituted niobium phosphate is in a monoclinic crystalline form. In an embodiment, at least 95% of the substituted niobium phosphate is in a monoclinic crystalline form. In an embodiment, the substituted niobium phosphate consists essentially of its monoclinic crystalline form. In an embodiment, the substituted niobium phosphate has a crystalline structure having the lattice parameters of a=13.1 Å (+/−0.2 Å), b=5.3 Å (+/−0.2 Å), c=13.2 Å (+/−0.2 Å) and the Beta angle=120.7° (+/−1°). In an embodiment, the substituted niobium phosphate contains is essentially free of amorphous materials.
In an embodiment, the substituted niobium phosphate is present in an electrode in a lithium battery. In an embodiment, the substituted niobium phosphate is present in an electrode in a lithium-ion battery. In an embodiment, the substituted niobium phosphate is present in a positive electrode in a lithium battery. In an embodiment, the substituted niobium phosphate is present in a negative electrode in a lithium battery.
For a more complete understanding of the present invention, reference is made to the following detailed description of exemplary embodiments considered in conjunction with the accompanying drawings, in which like structures are referred to by like numerals throughout the several views, and in which:
In an embodiment, the present invention advantageously employs the effects of crystalline-phase transitions and metal ion substitutions in monoclinic β-phase niobium phosphate (β-NbPO5) to produce a novel electrode material for lithium and lithium-ion batteries. Isolation of lower-voltage phase transitions of pure β-NbPO5 is highly effective in improving the long term cycling stability of the material. An analogous impact to cycling stability was identified through the use of effective solid solutions based on cations such as pentavalent tantalum (Ta5+). The resulting materials exhibited excellent cycling stability, and exceptionally low first cycle irreversible loss, without the need for carbonaceous nanocomposites. Surprisingly, the Ta-substituted β-NbPO5 also shows very fast rate capabilities (charge and discharge).
As illustrated along the 50:50 line composition of the phase diagram in
It has been demonstrated in the prior art (Patoux, S. & Masquelier, C. Chemical and Electrochemical Insertion of Lithium Into Two Allotropic Varieties of NbPO5. Chem. Mater. 14, 2334-2341 (2002), which is incorporated by reference herein in its entirety) that monoclinic β-NbPO5 has some reversibility down to 1.85V and achieves 90 mAh·g−1 through 100 cycles and 120 mAh·g−1 for 45 cycles down to 1.7V. Potentiostatic Intermittent Titration Technique (PITT) characterization revealed three intercalation plateaus upon lithiation at x=0.2, 0.5×0.7, and x=0.8 in β-LixNb1-xP05. X-ray diffraction revealed a solid solution mechanism for the two small plateaus, and a large two-phase reaction that corresponds to β-Li0.45NbPO5 and β′-Li0.75NbPO5. The second β′-Li0.75NbPO5 phase corresponds to a P21/c space group with a=13.145(3) Å, b=5.131(1) Å, c=13.415(3) Å, =120.02(1°), and V=783.4 Å3. As β-NbPO5 is further lithiated, an irreversible amorphization occurs beyond 1V. In contrast to the monoclinic phase, the tetragonal niobium phosphate was shown to demonstrate significant losses over 50 cycles with a sustained capacity of 90 mAh·g−1 vs Li/Li+. Both phases of NbPO5 operate on the Nb5+/Nb4+ redox, located at 1.65V for the tetragonal phase and 2V for the monoclinic phase. These potentials are higher than the standard reduction potential around 1.6V. At low voltages, both phases experience an irreversible amorphization. An orthorhombic phase that is structurally similar to the monoclinic phase forms at lower temperatures, but no electrochemistry has been reported.
As disclosed herein, and in accordance with embodiments of the present invention, the electrochemical performance of β-NbPO5 is improved through adjusting a variety of synthesis techniques, isolating phase reactions, and through various ionic substitutions. In embodiments of the present invention, solid state and solution methods are used alongside metal ion substitution into the β-NbPO5 structure. In embodiments of the present invention, transition metals are selected for substitution based on their ionic radii and proximity on the periodic table as detailed in Table I. Because of similar ionic radii to the niobium ion, a stable pentavalent oxidation state, and the β-TaPO5 end member being isostructural with β-NbPO5, Ta5+ is selected as an exemplary metallic ion. Examples wherein Mo5+ is substituted in place of Ta5+ are also presented for an ionic substitution comparison.
The following experimental examples are presented to illustrate representative embodiments of the present invention and are in no way intended to limit the scope of the invention. Variations and modifications of the representative embodiments, such as may be recognized by those having ordinary skill in the art and possession of the present disclosure are within the scope of the present invention.
Synthesis of Exemplary Materials
Both solid state and solution techniques were trialed to identify an optimum synthesis process. In the solid state process, stoichiometric ratios of the precursors: Nb2O5 (Aldrich), Ta2O5 (Alfa Aesar), and (NH4)2HPO4 (Aldrich) were ground in 4 g batches in a mortar and pestle until homogeneous. Phase evolution was progressed through a series of anneals at 700° C., 900° C., and 1350° C. with heating and cooling rates detailed in Table II. Samples were removed and ground in a mortar and pestle between each annealing step to improve batch homogeneity.
For the solution process, (NH4)2HPO4 was dissolved in 5 mL of deionized water (Aldrich) and then mixed with the corresponding metal oxides. The slurry was mixed and heated in a 20 cc alumina crucible with a magnetic spin bar on a hot plate until the H2O evaporated (approximately 30 minutes). Once dry, the sample was transferred directly to the furnace to follow the same heating protocol as noted above. Samples were quenched in air from 1350° C. (or other specified temperature) to room temperature using a drop furnace where the floor of the furnace was simply lowered, thereby exposing the crucibles to ambient temperatures. Previous experiments by Patoux, referenced above, required a very rapid quench to transform the tetragonal α-NbPO5 to the β-NbPO5, but an air quench provides sufficient cooling rates for such small samples.
Nanocomposites, when utilized, were prepared using a Spex 8000 mill with hardened steel balls and milling cells containing 10% weight of conductive carbon (Super P, MMM). Samples were milled in dry air or argon for 15 or 30 minutes. Milling times were minimized to reduce the probability of iron contamination. Initial tests revealed that forming carbon nanocomposites was unnecessary as the samples performed well without conductive additives. As such, unless otherwise specified, all materials presented in the present disclosure were not fabricated as nanocomposites.
Powder samples and electrodes were characterized using X-ray diffraction (XRD) with a Bruker D8 Advance diffractometer (Cu Kα, λ=1.54056 Å). Ex-situ XRD measurements were processed by disassembling the cell in an argon filled glovebox and extracting residual electrolyte from the cycled electrodes by rinsing with dimethyl carbonate (DMC). Electrodes were placed on a glass slide and sealed using Kapton film to minimize oxidation and moisture contamination. Rietveld refinements were run using TOPAS software utilizing a 5th order Chebychev polynomial background fit, sample height displacement and atomic position refinements assuming a pseudo-Voigt peak shape. No corrections were added for the conductive carbon and binder additives.
To fabricate electrodes for electrochemical evaluation, β-NbPO5 powders were cast through a doctor blade process in dry room conditions with humidity of less than 1% using the Bellcore method described in U.S. Pat. No. 5,418,091. The electrodes contained 54.7% active material (i.e., β-NbPO5), 7.8% conductive carbon (Super P, MMM), 15.6% poly(vinylidene fluoride-co-hexafluoropropylene) binder (Kynar 2801, Elf Atochem), and 21.9% dibutyl phthalate plasticizer (Aldrich) in acetone. This composition equates to a 70 weight percent active material electrode after extraction of the plasticizer. Plasticizer was extracted from the electrode tapes in 99.8% anhydrous diethyl ether (Aldrich) and punched into ½″ disks for storage and assembly in an argon-filled glove box. Cells were cycled against a pure lithium metal anode (FMC) in an Al-clad 2032 coin cell (Hohsen) with a 1M LiPF6 in ethylene carbonate:dimethyl carbonate (EC:DMC 1:1 in volume) (BASF) electrolyte and glassfiber separator (Whatman).
Galvanostatic cycling was carried out on a Maccor cycler (Maccor Corporation Model 4000 battery cycler) as specified herein. High rate efficiency was evaluated from 7.5 mA·g−1 (approximately C/20) to 2100 mA·g−1 (approximately 20 C) with voltage cutoffs ranging between 1.25V and 2.80V at 24° C. Potentiostatic intermittent titration technique (PITT) tests were run on a Macpile potentiostat (Biologic Co., France) with 10 mV steps and a 2 mA·g−1 cutoff current.
To evaluate the optimum synthesis methodology, samples were fabricated using the aforementioned solid state and solution methods along with quenching from various temperatures. After heating at 1350° C., unsubstituted samples were either immediately quenched or slowly cooled to 300° C. before quenching in air to room temperature.
Reports in the prior art indicate that a large excess of phosphorus is needed to produce a pure phase. It is also reported that P2O5 volatizes at 1070° C., which is much lower than the temperature required to form the high temperature monoclinic phase. However, as shown in the present disclosure, the methods of preparing substituted and nonsubstituted β-NbPO5 disclosed herein surprisingly show that shortening the anneal times over those in the prior art minimizes the amount of phosphate evolution, such that only a 50% stoichiometric excess of phosphorus was needed to yield greater than 99% purity of the monoclinic phase. Table III details the purity of samples containing varying amounts of excess phosphorus heated to 1350° C. and quenched at 1350° C.
To evaluate the effect of Ta5+ cation substitution on the stabilization of the β-phase, a range of quenching temperatures were tested for both unsubstituted and 2% Ta-substituted samples in a solution synthesis. The purities of the quenched samples are reported in Table IV. The addition of 2% Ta stabilized a pure β-phase at quenches starting from as low as 200° C., greatly contrasting attempts in the prior art, which required extremely rapid quenches above 1300° C.
The X-ray patterns for Ta substitutions ranging from 0% to 100% prepared at 1350° C. and subsequently quenched at 1350° C. are shown in
The TaxNb1-xPO5 compositions were slowly cycled against a lithium metal anode in 1M LiPF6 (EC:DMC 1:1 in volume) at 7.5 mA·g−1 (C/12−C/15) to obtain the voltage profiles of the 1st and 75th cycles shown in
The rate capabilities of TaxNb1-xPO5 cells with x=0, 0.02, 0.05, 0.10, and 0.20 were evaluated up to rates of 2100 mA·g−1 (19 C-26 C) and then subjected to continued cycling at 22.5 mA·g−1 (C/5-C/4) using the same voltage range of 2.8V to 1.75V.
The voltage profile of the unsubstituted NbPO5 sample suggests a possible deleterious reaction occurring at voltages less than 1.8V. To test such theory, various discharge cutoff voltages were tested for the unsubstituted sample and their voltage profiles are overlaid in
The improved cycling efficiency obtained from changing the cutoff voltage to 1.85V is compared to the cycling stabilities of various Ta substitutions in
To further investigate the effects of Ta substitution, the isostructural material β-TaPO5 was synthesized and cycled versus a lithium metal anode. β-TaPO5, represents the end member of the Ta-substitution series of β-TaPO5, and its electrochemical properties apparently have not previously been investigated. Because TaPO5 operates on the Ta5+/Ta4+ reduction/oxidation around 1.25V, much lower than Nb5+/Nb4+, the cell'"'"'s discharge cutoff voltage was lowered to 1V, 0.75V, and 0.075 V (see
Discussion of Experimental Results
At least two electrochemical results disclosed herein appear to be previously unknown in the art, and may be considered to be surprising. First, the data shows a distinct improvement in an already proficient cycling stability and a decrease in polarization with cycle number when the lower voltage region of the unsubstituted β-NbPO5 less than 1.8V is avoided. Second, an analogous improvement of cycling and a decrease of polarization with cycle number is demonstrated by increasing Ta5+ substitution for Nb5+. The voltage profiles of the latter with increasing substitutions of Ta also systematically decrease the apparent presence of the aforementioned low voltage reaction. The connection between the presence of the lower voltage reaction and the presence of Ta is quite clear, but to confirm the lower voltage reaction mechanism was indeed evolving with Ta substitution, high-resolution electrochemical techniques were utilized.
Potentiostatic Intermittent Titration Technique (PITT) experiments were run to obtain insight on the reaction mechanisms occurring for the unsubstituted samples as a function of low voltage cutoff and Ta substitution. Lower voltage cutoffs of 1.85V, 1.60V, and 1.25V were tested for both the unsubstituted and substituted samples and are shown in
Interestingly, the specific capacity represented by the multiphase delithiation reaction is in far excess of the specific capacity associated with the lower voltage lithiation reaction avoided by the 1.85V PITT cutoff. In all cases, the latter part of the delithiation seems to progress in a single-phase or near single-phase reaction.
To confirm that the multiphasic reaction depletion was a result of Ta substitution, a similar PITT study was executed with 5% and 10% Mo substitutions into NbPO5 using the aforementioned synthesis techniques with MoO3 (Aldrich) to achieve 100% purity (50% stoichiometric excess of phosphorus). As seen in
To understand the changes in the lithiation reaction preceding the multiphase reaction region, several ex-situ X-ray diffraction studies were completed. Cells were discharged to similar voltage cutoffs and Li contents to compare the discharge mechanisms.
For the 1.90V cutoff, the ex-situ XRD results summarized in Table VII reveal similar lattice volumes of about 773 Å3 for all samples. In contrast, the lower 1.75V discharge reveals a distinct and systematic increase in lattice volume as a function of Ta substitution. The unsubstituted sample shows a slight increase in volume to 778 Å3 whereas the substituted samples of 5% and 10% Ta show a significant increase in volume to 795 Å3 and 797 Å3, respectively. The volume expansion to 788 Å3 for the highly substituted 20% sample reverts the trend slightly, while still showing a significantly greater volume than the unsubstituted sample. This observation may be linked to a lower degree of lithiation recorded for this sample (Table VII). In general, the increase in volume for all the lithiated Ta substituted samples is manifested as a systematic increase in the b and c lattice parameters while maintaining relatively invariant a lattice parameter and β angle.
Looking specifically at the unsubstituted samples and the 10% Ta sample presents an ideal comparison with similarly intercalated Li values of 0.694 and 0.667 at 1.90V and 0.814 and 0.827 at 1.75V, respectively. In addition, these two samples demonstrate a distinct contrast in electrochemical stability and phase progression upon delithiation via PITT. At 1.90V, the unsubstituted lattice volume of 773.2 Å3 is in close agreement with the 10% Ta substitution lattice volume of 775.3 Å3. At the lower voltage of 1.75V, a significant contrast in lattice volume of 775.3 Å3 for the unsubstituted β-NbPO5 exists with that of 797.1 Å3 for the 10% Ta. To recall, this lower voltage region is where the lattice volume difference is observed, and also contains the electrochemically deleterious region for the unsubstituted sample as demonstrated by the change in discharge cutoff potential (see
In order to further demonstrate the disparity between the delithiation mechanisms in the unsubstituted and 10% Ta samples of contrasting electrochemical performance, additional ex-situ XRD experiments were performed. Both samples were tested at the beginning of delithiation at 1.75V to contrast the different electrochemical behaviors shown by PITT of the strong two-phase behavior for the unsubstituted sample versus single-phase like reaction for the 10% Ta-substituted sample. X-ray patterns and the corresponding voltage profile are show in
In summary, much improved cycling for β-NbPO5 is manifested when the lower lithiation voltage is raised, suggesting a failure mode associated with a phase transition induced by higher degrees of lithiation approaching x=1. Lower voltage cycling is shown to be much improved by 5 to 20% Ta substitution within β-NbPO5. This improved cycling seems to be associated with a distinct change in phase progression. Although unsubstituted and substituted β-NbPO5 initially have similar lattice parameters, upon lithiation the Ta-substituted β-NbPO5 has a significantly larger monoclinic unit cell. Upon delithiation, the slope of the voltage profile, high resolution PITT, and ex-situ XRD reveals that the delithiation reaction progresses as a two-phase reaction for the unsubstituted β-NbPO5, while Ta-substituted β-NbPO5 evolves via a single-phase de/insertion reaction. While the voltage is not necessarily optimal for a high energy battery, Ta-substituted β-NbPO5 represents a model of lithium insertion phosphates with near 0% irreversible loss, excellent cycling stability, and excellent 20 C rates in large grains without the formation of a conductive nanocomposite.
Summarizing the foregoing discussion, in embodiments of the present invention, substitution of Ta into the lattice of β-NbPO5 results in improved reversibility, near 0% irreversible loss and similar excellent 20 C high rate behavior in large grain material without the formation of electronically conducting nanocomposites. Further, Ta substitution into NbPO5 enables an improved stabilization of the difficult to fabricate β-NbPO5 phase, which is isostructural with the β-TaPO5 end member. Much improved cycling stability can be extracted from β-NbPO5 if a lower voltage transformation is eliminated. We find that this can be accomplished by raising the cutoff voltage or substituting at least 5% Ta into NbPO5. In addition, avoiding the lower voltage region below 1.8V decreases the polarization and evolution of polarization while improving the reversibility of the cell. A similar effect is seen with a 10% substitution while still cycling down to 1.75V. Through the use of a comprehensive series of PITT and XRD analyses, this improvement was correlated to a distinct change in the two-phase delithiation pathway for the β-NbPO5 versus that of the single-phase reaction for β-TaxNb1-xPO5.
It will be understood that the embodiments of the present invention described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention, selected embodiments of which are also encompassed by the attached list of claims.