Lithium (Li) thin film battery cells are the currently preferred battery materials because they offer outstanding cycle life times and long term shelf life. One of the important advantages that Li thin film battery cells offer beyond these attributes is the robustness inherent in the solid-state design; that is, the ability to tolerate temperature extremes, mechanical shock, vibration and moderate flexture far better than conventional Li-ion or Li polymer cells. For example, cells with Li anodes plated in situ can be exposed to solder reflow temperatures of up to 250° C. for ten minutes without any degradation in performance. This remarkable robustness is particularly important for aerospace applications, wherein battery performance must meet long term power demands in critical circuits under elevated temperatures. For example, the application of thin film battery cells used in a power system externally mounted on a LEO spacecraft, the cells will likely be exposed to temperatures of about 120° C.
However, an inherent limitation of state-of-art Li thin film batteries is their sensitivity to deterioration when the cells are cycled at elevated temperatures. Cells incorporating LiCoO2 cathodes, which currently represent the most widely employed cathode for this type of battery, can be charged and discharged at 25° C. over tens of thousands of cycles and experience capacity losses of only about 0.002% per cycle. In contrast, LiCoO2 based cells that are operated at 60° C. experience a factor of ten greater capacity loss per cycle. Recent laboratory experimentation has resulted in the discovery that the capacity fade per cycle is even more severe at even higher temperatures, wherein the extant cells display marked capacity fade to 50% of initial values after only 100 cycles when these cells are operated at temperatures of 150° C.
In order to develop thin film battery cells with excellent cyclability at elevated temperature, it is imperative to understand the failure mechanisms for these devices. Wang et al. measured increases in cell resistance of LiCoO2 thin film battery cells with cycling, which was exacerbated when cycling at elevated temperatures. This resistance was attributed to strain-induced structural changes in the cathode layer that reduced Li+ ion mobility. Dudney et al. found that thin film battery cells with nano-crystalline LixMn2-yO4 cathodes experienced modest increases in resistance with cycling at room temperature, resulting in lower practical capacity due to polarization losses. When these cells were cycled at 100° C., the capacity fade was much greater, though the authors note the aging mechanisms proceeded differently than at room temperature. Again, the exact nature of the physiochemical changes in cell structure with cycling at elevated temperature was not clear, though deleterious phase transformations seem to have been indicated.
Alternative thin film cathodes were investigated to identify materials that could better tolerate microstructural and phase change transformations with cycling. Molybdenum trioxide (MoO3) is an attractive candidate from several standpoints. The thermodynamically favored orthorhombic α-MoO3 can reversibly insert via a topotactic reaction up to 1.5 Li atoms per MoO3 molecule, corresponding to a specific capacity of 279 mAh/g and a discharge cutoff voltage of 1.5V vs. Li/Li+. Assuming fully densified films, this would equate to a specific capacity of 131 μAh/(cm2-μm), as compared with 69 μAh/(cm2-μm) for LiCoO2. Its polymorph, β-MoO3 has been shown to intercalate up to 2 Li atoms per MoO3. It is known that MoO3 upon the first lithiation and subsequent delithiation undergoes significant irreversible microstructural changes such as fracture and disintegration of the grains. However, lithium reversibility in MoO3 appears to be quite insensitive to these crystallographic and morphological changes, provided the cathode material remains intact on the electrode.
The invention disclosed herein addresses the need to improve Li thin film battery performance in the area of long cycle life when the batteries are operated 10 under elevated temperature conditions. The object of the invention disclosed herein addresses the feasibility of improving Li thin film battery cell performance in this area by development of a cathode composition comprising MoO3 or Tungsten trioxide (WO3). In contrast to Li thin film battery cells containing LiCoO2 cathodes, Li thin film battery cells containing the new cathode compositions display markedly improved long cycle life without significant fade in their specific capacity when the cells are evaluated under high temperature conditions.
In a first aspect, the present invention is a method of preparing a cathode electrode suitable for use in a thin film battery that includes applying an adhesion layer on a substrate; forming a current collector layer on the adhesion layer; and forming a layer of a Group 6 oxide composition on the current collector layer. The Group 6 oxide composition for instance consists essentially of MoO3 or WO3.
In a second aspect, the present invention is a method of preparing a thin film battery cell that include applying an adhesion layer on a substrate; forming a current collector layer on the adhesion layer; applying a first shadow mask of a first defined area on the current collector layer to provide a shadow masked current collector area; forming a layer of a group 6 oxide on the shadow masked current collector area to provide a cathode electrode layer; forming a solid electrolyte film layer comprising LiaPbOcNd on the cathode electrode layer; applying a second shadow mask of a second defined area on the solid electrolyte film layer to provide a shadow masked solid electrolyte film layer; forming a metal anode layer on the shadow masked solid electrolyte film layer to complete the thin film battery cell; and sealing the thin film battery cell with a suitable sealant. The symbol a comprises a value from about 3 to about 3.3, the symbol b comprises a value of about 1, the symbol c comprises a value from about 3 to about 4, and the symbol d comprises a value from about 0.1 to about 0.3. The second defined area is coincident with or a subset of the first defined area.
In a third aspect, the present invention is a cathode electrode suitable for use in a thin film battery cell that includes a substrate; an adhesion layer applied on the substrate; a current collector layer formed on the adhesion layer; and a cathode layer comprising a group 6 metal oxide formed on the current collector layer. The resultant cathode electrode displays a specific capacity in the range from about 190 mAh/g to about 300 mAh/g or a specific capacity from about 90 μAh/(cm2-μm) to about 140 μAh/(cm2-μm).
In a fourth aspect, the present invention is a thin film battery cell that includes a substrate; an adhesion layer applied on the substrate; a current collector layer formed on the adhesion layer; a cathode layer comprising a group 6 metal oxide formed on the current collector layer; a solid electrolyte film layer composed of Li3.3PO3.8N0.22 formed on the cathode layer; a metal anode layer comprising Li deposed on the solid electrolyte layer to complete the thin film battery cell; and a sealant. The resultant thin film battery cell displays a performance attribute that includes (1) a specific capacity from about 90 μAh/(cm2-μm) to about 160 pAh/(cm2-μm) or (2) a specific capacity that does not appreciably deteriorate with cycling of the thin film battery cell at a temperature of greater than about 100° C.
The present invention makes use of the discovery of solid-state Li thin film cells using MoO3 and WO3 cathodes that have superior cycle life and specific capacity compared with state-of-art LiCoO2 based Li thin film cells. At 150° C., the MoO3 cells could be cycled at deep charge and discharge voltages over thousands of cycles with no apparent long term capacity fade, in contrast to LiCoO2 cells which experienced severe capacity fade over a few hundred cycles at this temperature. The practical specific capacity of the MoO3 cathodes, approximately 140 μAh/(cm2-μm), is about twice that of state-of-art LiCoO2 cells. The rate capability of the MoO3 cells at 150° C. is very good, with cells experiencing little polarization at rates of about 1 mA/cm2. Thin film cells containing these novel cathode compositions will be of interest for use in elevated temperature applications. The fabrication process for preparing these novel cathode compositions and their suitability in thin film battery cells are described below.
Cathode Compositions, Fabrication, and Attributes
The present invention is directed to cathode compositions of oxides of metals from group 6 of the Periodic Table, including Chromium (Cr), Molybdenum (Mo), Tungsten (W), and Seaborgium (Sg). More preferably, the cathode compositions consist essentially of Mo oxides or W oxides. Most preferably, the cathode compositions consist essentially of Mo oxides. The preferred valency of group 6 metal oxides is MOn, where M represents a metal from group 6 of the Periodic Table, 0 represents oxygen, and the value of n is in the range from about 2.7 to about 3.3, including 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, and 3.3. Preferred cathode compositions include MoO3 and WO3.
Preferred cathode compositions need not be pure group 6 metal oxides for achieving the performance characteristics of the present invention. Mixed metal oxide compositions, such as MoO3/WO3 mixtures, wherein one or more group 6 metals are present in the cathode layer are feasible. Further, mixtures of metal oxides of mixed valency, such as MO2.7/MO3.3 mixtures, may be present in the cathode layer without substantially compromising cathode electronic performance. Finally, cathode layers containing small amounts of contaminants such as non-group 6 elements or non-metal oxides, are tolerated. As elaborated below, non-group 6 metal oxide compositions may arise from small impurities being present during the sputtering process, such as that which may be associated a contaminated sputter target. Without being limited to any particular theory, the preferred cathode compositions of the present invention may contain other materials or contaminants to the extent that these materials do not interfere with the processes of Li+ ion intercalation and deintercalation occurring within individual metal oxide layers as Li+ ions move between metal oxide layers within the cathode composition when cells containing such cathode compositions are cycled at high temperatures.
As illustrated in
The cathode 100 is prepared on a substrate 101 composed of thin materials, such as thin non-metallic/non-polymer substrates, thin metal foils, and polymer materials. Thin materials are preferred because one object of the present invention is the fabrication of thin battery cells having a high specific capacity. This performance attribute is achieved by using thin substrate materials that contribute nominally to the overall weight of the battery cell. Examples of thin non-metallic/non-polymer substrates include silica, mica, silicate Fe—K compositions, silicon (Si) substrates, and Si3N4-coated Si substrates. Examples of thin metal foils include foils composed of titanium (Ti), gold (Au), and Aluminum (Al), among others. Examples of polymer materials would be any polyimide composition having high heat resistance, such as Kapton. For the purposes of preparing different cathode compositions for performance evaluation or experimental work, thin silica substrates are preferred substrates owing to the convenience, economic cost, and availability of these materials. Commercial substrates composed of thin metal foils having a material composition other than a precious metal, such as Au, are preferred, owing to the economic cost of such materials.
All film layers are preferably formed in cathode 100 by using a sputter deposition technique. Sputter deposition is performed on substrates in a planar RF magnetron sputtering chamber, evacuated to a base pressure of less than 5×10−6 Torr with a turbomolecular pumping system. Sputter deposition techniques are well known in the art, such as those disclosed in “A LOW Pt CONTENT DIRECT METHANOL FUEL CELL ANODE CATALYST: NANOPHASE PtRuNiZr” by Sekharipuram R. Narayanan, Ph.D. and Jay F. Whitacre, Ph.D., U.S. patent application Ser. No. 11/060,629, filed Feb. 17, 2005, the entire contents of which are hereby incorporated by reference. The advantage of using sputtering in the present invention is the degree of flexibility the technique affords one for forming material compositions of defined stoichiometry within the resultant deposition layers.
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The sputtered films will typically vary in color, from transparent with a slight yellow to purple tint, and are generally featureless as shown in SEM micrographs (
As deposited, the MoO3 films are amorphous. Following an annealing step in a temperature range from about 280° C. to about 500° C. for one hour, the MoO3 film crystallized as mixed phases of layered α-MoO3 and monoclinic β-MoO3 (
Li Thin Film Battery Cell Compositions, Fabrication, and Attributes
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Sputtering depositions are disfavored for forming the Li anode layer because a Li sputtering target would melt during sputtering deposition, owing to the low melting temperature of Li. Thermal evaporation is preferred method to form a Li anode layer onto the shadow masked electrolyte layer. Thermal evaporation techniques for forming a Li anode layer are well known in the art, such as that exemplified by Bates et al. (1993).
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If foil covering is selected as the protective sealant, it should be noted that the anode film layer should have the same elemental composition as the foil composition. For example, a Li foil, rather than a Na foil, should be used as a sealant for battery cell 200 having anode layer 208 composed of Li. This is due to fact that the elemental intermixing occurs between elements of the foil covering and the anode layer, wherein the resultant ions must migrate through the individual layers of the cathode composition for efficient electrical conductivity. Though the examples disclose the use of protective Li foil coverings to serve as an experimental sealant, preferred commercial embodiments of battery cell 200 would not contain a foil covering, owing to the desire to manufacture a thin film battery cell of minimum weight and enhanced specific capacity.
The first MoO3 film cell discharge shows two distinct plateaus, yielding a specific capacity of about 90 μAh/(cm2-μm) (
The rate capability of the MoO3 cathodes was very good, as shown in
Potentiostatic Intermittent Titration Technique (PITT) measurements indicated the chemical diffusion coefficient of Li in MoO3 was 7.5×10−11 cm2/s at 153° C. at 2.24V for a 10 mV step size (
A dramatic quality of the MoO3 thin film batteries is the cycle life at elevated temperatures. Whereas LiCoO2 cells fade to about 50% of their initial capacity after only 100 cycles, the MoO3 cells experience a slight capacity drop followed by recovery of the capacity, improving with increasing cycle number up to at least 5500 cycles, as shown in
Sudden catastrophic failure, as opposed to gradual capacity degradation, was found to be the chief failure of the cells. Such failure was attributed to short-circuiting of the solid electrolyte as evidenced by a sudden drop in the cell resistance by several orders of magnitude to about 10 Ω. This electrolyte failure is not unusual for thin film batteries and is typically mitigated by using a thicker electrolyte film at the expense of greater cell resistance.
Cathode Thickness as an Important Design Consideration
An important design attribute of the cathode material compositions for both cathode performance in particular and battery cell performance in general is the role that cathode film layer thickness has upon battery cell integrity. The MoO3 layers that form the cathode of the present invention will dilate (swell) during battery cell discharge, owing to the movement of Li+ ions into the MoO3 layers. Should the cathode layer 205 formed inside battery cell 200 have a thickness that is not sufficiently small to accommodate the dilation of the MoO3 layers, then the MoO3 layers will expand and crack the solid electrolyte layer 206 that lies above the cathode layer 205. Consequently, the integrity of the cell will be preserved if a thin cathode layer 205 is used in battery cell 200. The preferred thickness of cathode layer 205 will of course depend upon the particular application of battery cell 200; however, a dimensional thickness of less than about 1 micron is preferred for the cathode layer.
All solid-state Li thin film battery cells were fabricated on glass slides or Si3N4 coated Si substrates. The deposition of all the films (except the anode layer) was carried out in a planar RF magnetron sputtering chamber, evacuated to a base pressure of less than 5×10−6 Torr with a turbomolecular pumping system. The first layer consisted of a Ti adhesion layer and Pt current collector that was patterned through a shadow mask defining a 1.69 cm2 square pad. Using the same shadow mask, the LiCoO2 or MoO3 layer was sputtered onto the cathode current collector, and then annealed in room air. The LiCoO2 films were sputtered from a cold-pressed and sintered LiCoO2 target as discussed by Neudecker et al. (2000), and annealed to 700° C. for one hour in air. The MoO3 films were sputtered from a MoO3 target (K. J. Lesker) and annealed for one hour in air. Next, the solid electrolyte film of Li3.3PO3.8N0.22 (LiPON) was deposited onto the cathode layer by sputtering a Li3PO4 target in N2, following Yu et al. (1997). Finally, a Li metal anode layer was thermally evaporated onto the electrolyte through a second shadow mask defining an area of 0.7 cm2 in the center of the cathode pad to complete the cell. In order to protect the cells during elevated temperature testing, the Li film was covered with Li foil cut to match the size of the Li pad, and then the entire cell was covered with Kapton tape. The deposition parameters for each layer for the MoO3 based cells are shown in Table 1.
Since the intent was to develop thin film batteries with a high tolerance to abusive conditions, deep charge and discharge cutoff voltages were employed, using moderately high current densities at a temperature well in excess of the targeted value of 120° C. To this end, the MoO3 cells' charge cutoff voltage was 5V, the discharge cutoff voltage was 1V, and the cycling temperature was 150° C., at a (dis)charge current density of 0.7 mA/cm2. A 60 second current taper step was employed on the charging. For the LiCoO2 cells, the same conditions for cycling were employed with the exception that the charge cutoff voltage was 4.25V and the discharge cutoff voltage was 3V.
Film material was characterized using a Siemens D500 diffractometer run in the theta −2 theta geometry, with a Cu anode at an accelerating voltage of 40 kV and a tube current of 20 mA. Surface morphology was studied using a Hitachi field-emission scanning electron microscope (SEM).
The electrochemical characterization of the films was performed using a Princeton Applied Research 273A potentiostat, driven by Corrware Software (Scribner Associates). Cyclic voltammetry measurements were performed with sweep rates between 0.05-5 mV/s. The chemical diffusion coefficient was measured using potentiostatic intermittent titration technique (PITT) using a 10 mV step size. Cycling experiments were carried out using an Arbin battery cycler. All cells were charged and discharged in an Ar filled glove box. For elevated temperature testing, the cells were placed on a hot plate in the glove box with the temperature monitored using a thermocouple.
The results of these experiments are presented in
All printed publications, patents, and patent applications cited in this disclosure are hereby incorporated by reference herein in their entireties.
The foregoing description and drawings merely explain and illustrate the invention and the invention is not limited thereto. Those of the skill in the art who have the disclosure before them will be able to make modifications and variations therein without departing from the scope of the present invention.
This application claims benefit of priority from U.S. Provisional Application Ser. No. 60/590,726, filed Jul. 23, 2004, which is hereby incorporated by reference.
The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. § 202) in which the Contractor has elected to retain title.
Number | Date | Country | |
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60590726 | Jul 2004 | US |