Patent Application
20230121670
Embodiments described herein generally relate to energy storage devices and methods of forming energy storage devices. More specifically, the energy storage device is a solid-state lithium thin-film battery.
Solid-state lithium thin-film batteries are utilized to enable enhanced energy storage performance, improved cycle life, enhanced safety, and high specific energies. The current approach to fabricating thin film batteries (TFBs) utilizes a series of vacuum deposition operations to deposit the cell components on a macroscopically thick substrate. Examples of these techniques include thermal evaporation, sputtering, chemical vapor deposition, pulsed laser deposition, and related approaches. TFBs are utilized for applications within smart sensors, micro-computers, biomedical health devices, tiny robots, etc.
TFBs are currently limited in overall energy density. Therefore, thin-film cathode materials with higher specific energy are needed. The specific energy of the thin-film cathodes largely determine the overall energy density of the cell. For the cathode side, many polycrystalline, inorganic cathode compounds have been developed, but have limited specific energy or poor charge reversibility. Further, polycrystalline thin film cathodes typically require high temperature annealing to achieve the preferred crystalline phase and optimal energy storage performance. High temperature annealing adds significant processing time and additional cost, while limiting material compatibility of the substrate.
Therefore, what is needed are cathode materials with higher specific capacities and reduced processing requirements.
The present inventors have developed novel cathode materials that can be employed in a variety of lithium thin-film battery applications. The novel cathode materials are based on lithium, ruthenium, cobalt, and oxides thereof, and provide energy densities that are equal to or greater than current cathode materials. Additionally, the novel cathode materials can be prepared in the absence of a thermal annealing step. By forgoing the high temperature conditions required for thermal annealing, the cathode materials disclosed herein can be assembled on a wide variety of substrates, including lower melt-temperature, flexible thermoplastic materials.
The present disclosure is generally directed towards energy storage devices and methods of forming energy storage devices. In one embodiment, an energy storage device is described. The energy storage device includes a cathode, an anode, and an electrolyte. The cathode includes lithium, ruthenium, cobalt, and oxygen. The anode is disposed adjacent to the cathode. The electrolyte is disposed between the cathode and the anode.
In another embodiment, an energy storage device is described which includes a support substrate, a platinum film disposed on a portion of the support substrate, a cathode disposed on the platinum film, an anode disposed adjacent to the cathode, and an electrolyte disposed between the cathode and the anode. The cathode includes lithium, ruthenium, cobalt, and oxygen. The anode includes lithium. In some embodiments, the cathode includes a cathode material with amounts of lithium, ruthenium, cobalt, and oxygen based on the formula Li2+xRu1−xCoxO3, where x is 0.1, 0.2, or 0.3. In some aspects, in the formula Li2+xRu1−xCoxO3, x is any one of, less than, greater than, between, or any range thereof of 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, and 0.30. In some embodiments, the cathode includes a cathode material with amounts of lithium, ruthenium, cobalt, and oxygen based on the formula (1−x)Li2RuO3+xLiCoO2+yLi2O) where y ranges from 0.05 to 0.6 and x ranges from 0.05 to 0.5. In some embodiments, in the formula (1−x)Li2RuO3+xLiCoO2+yLi2O), y is any one of, less than, greater than, between, or any range thereof of 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.40, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.50, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, and 0.60. In some embodiments, in the formula (1−x)Li2RuO3+xLiCoO2+yLi2O), x is any one of, less than, greater than, between, or any range thereof of 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.40, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, and 0.50.
In yet another embodiment, a method of forming an energy storage device is described. The method includes depositing a cathode film onto a support substrate within a process volume of a processing chamber, depositing an electrolyte over the cathode layer, and depositing an anode over the electrolyte. The deposited cathode layer includes lithium, ruthenium, cobalt, and oxygen.
In yet another embodiment, an energy storage device is described. The energy storage device includes a cathode comprising lithium, oxygen, and two or more metals. The two or more metals are selected from a group of ruthenium, cobalt, tin, iridium, and manganese. The energy storage device further includes an anode disposed adjacent to the cathode and an electrolyte disposed between the cathode and the anode.
In yet another embodiment, a method of forming an energy storage device is described. The method includes depositing a cathode film onto a support substrate within a process volume of a processing chamber, depositing an electrolyte over the cathode film, and depositing an anode over the electrolyte. The cathode film includes lithium, oxygen, and at least two of ruthenium, cobalt, tin, iridium, and manganese.
So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments.
Embodiments of the present disclosure are directed towards a high-capacity thin-film cathode for solid-state lithium thin film batteries. In some embodiments, the solid-state lithium thin film batteries are fabricated with sub-millimeter dimensions, such as on the order of 100 μm×100 μm. In some embodiments, the solid-state lithium thin film batteries are from a few micrometers to tens of micrometers thick, such as about 5 μm to about 30 μm thick, such as about 5 μm to about 20 μm thick, such as about 5 μm to about 15 μm thick. The cathode composition is synthesized as a solid solution of LiRu2O3 and LiCoO2 and contains lithium, ruthenium, cobalt, and oxygen. The thin-film cathode is fabricated on a substrate by radio-frequency magnetron sputtering techniques and the as-deposited thin-film cathode has roughly two times the discharge capacity of current thin-film cathodes, such as LiCoO2. The cathode is also shown to be fully functional and reversible in the as-deposited, un-annealed state.
The cathode composition may roughly have a nominal composition of (1−x)Li2RuO3+xLiCoO2+yLi2O. In embodiments wherein y is the same as x, the formula is shown as Li2+xRu1−xCoxO3. As described herein, y may be varied between about 0.05 to about 0.6, such as about 0.1 to about 0.4. For example, in certain embodiments, y is substantially equal to 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, etc. X may similarly be varied between about 0.05 to about 0.5, such as about 0.1 to about 0.3. For example, in certain embodiments, xis substantially equal to 0.1, 0.2, 0.3, 0.4, 0.5, etc. The lithium, ruthenium, cobalt, and oxygen containing material described herein, may be described as LRCO material and films formed from the LRCO material may be described as LRCO thin films. The atomic ratio of each of the elements within the LRCO material may vary as described herein, but include each of lithium, ruthenium, cobalt, and oxygen, in some embodiments.
In some embodiments, other materials other than the LRCO material are utilized, such that the cathode is formed from an anion redox active material. The anion redox active material includes the LRCO material as well as other materials as described herein. The anion redox active material may be an anion redox active over-lithiated transition metal oxide. The anion redox active material is a solid solution of one or a combination of lithiated ruthenium oxide (Li2RuO3) and lithiated iridium oxide (Li2IrO3) along with at least one lithium metal oxide. The lithium metal oxides include lithium oxides of iron (Fe), cobalt (Co), nickel (Ni), manganese (Mn), tin (Sn), titanium (Ti), palladium (Pd), silver (Ag), zinc (Zn), gallium (Ga), indium (in), and vanadium (V). In some embodiments the lithium metal oxides include one or a combination of iron (Fe), cobalt (Co), nickel (Ni), manganese (Mn), tin (Sn), titanium (Ti), and vanadium (V). The lithium metal oxides include one or a combination of lithium iron oxide, lithium cobalt oxide, lithium nickel oxide (LNO), lithium manganese oxide (LMO), lithium tin oxide, lithium titanium oxide (LTO), and lithium vanadium oxide. The lithium metal oxides therefore includes one or a combination of LiFeO2, LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li2MnO3, LiSnO, Li2TiO3, and LiV3O8.
In some embodiments where tin is substituted for cobalt, the solid solution of LiRu2O3 and LiCoO2 is replaced by a solid solution including LiRu2O3 and LiSnO2. In some embodiments where iridium is substituted for cobalt, the solid solution of LiRu2O3 and LiCoO2 is replaced by a solid solution including LiRu2O3 and Li2IrO3. Similarly, alternative transition metals such as manganese (Mn), may be substituted with ruthenium (Ru) within the composition. In some embodiments where manganese is substituted with ruthenium, the solid solution of LiRu2O3 and LiCoO2 is replaced by a solid solution including LiMn2O4 and LiCoO2 or a solid solution including LiMnO2 and LiCoO2. In some embodiments, one or a combination of LiRu2O3 and LiMn2O4 may be combined with any one or a combination of LiCoO2, LiSnO2, and Li2IrO3. Other combinations of over-lithiated transition metal oxides are contemplated, but not explicitly disclosed herein. Substitution of compounds within the composition is enabled at least in part by the ability of oxygen atoms within the compounds to participate in redox reactions. At least Li2RuO3 and Li2IrO3 have improved electrical conductivity compared to other lithium metal oxides. Therefore, at least one of the Li2RuO3 and Li2IrO3 compounds are utilized. Improved electrical conductivity improves thin film battery performance. At least one of the anion redox active materials is the LRCO material described herein.
In some embodiments, the cathode material is deposited on a rigid substrate. In further embodiments, the cathode material is deposited on a flexible substrate. As used herein, “flexible” is defined as being capable of at least one of bending, stretching, and/or compressing without causing cracks, breaks, fine cracks and the like. In some embodiments, a flexible substrate can be made of or can include a metal. Non-limiting examples of flexible metal substrates are platinum foil and aluminum foil. In some embodiments, a flexible substrate is a thermoplastic substrate. Thermoplastic substrates include amorphous thermoplastics, semi-crystalline thermoplastics, crystalline thermoplastics, and elastomeric and include, without limitation, polyimides, poly(aryletherketone) (PAEK), poly(butylene terephthalate) (PBT), poly(butyrate), poly(ether ether ketone) (PEEK), poly(etherimide) (PEI), poly(2-hydroxyethyl methacrylate) (pH EMA), poly(isocyanurate) (PIR), poly(methyl methacrylate) (PMMA), poly(oxymethylene) (POM); poly(phenylsulfone) (PPSF), poly(styrene) (PS), poly(trimethylene terephthalate) (PTT), poly(urea) (PU); poly(amide)-based thermoplastics like aliphatic poly(amides), poly(phthalamides) (PPA), and aramides (aromatic poly(amides)); poly(carbonate)-based thermoplastics; poly(ester)-based thermoplastics like poly(ethylene) naphthalate (PEN), and poly(ethylene terephthalate) (PET); poly(olefin)-based thermoplastics like poly(ethylene) (PE), poly(propylene) (PP), poly(propylene carbonate) (PPC), poly(methylpentene) (PMP), and poly(butene-1) (PB-1); poly(stannane)-based thermoplastics; poly(sulfone)-based thermoplastics; poly(vinyl)-based thermoplastics like poly(vinyl chloride) (PVC), poly(vinylidene fluoride) (PVDF), poly(vinyl fluoride) (PVF), poly(vinyl nitrate) (PVN), and poly-(4-vinylphenol) (PVP); and cellulose-based thermoplastic like cellulose ester-based thermoplastics and cellulose ether-based thermoplastics.
In some embodiments, the electrolyte comprises lithium phosphorus oxynitride (LiPON). In further embodiments, the electrolyte can comprise LiAlSiO4, lithium lanthanum titanate (LLTO), lithium phosphorous sulfuric oxynitrides (LiPSON), lithium boron oxynitride (LiBON), LiPON, and combinations thereof.
As described herein, an LRCO cathode is deposited in a thin film format. The thin film format and LRCO cathode described herein are theorized to provide almost double the charge storage capacity relative to previous cathode materials. As described herein, the LRCO thin film batteries were prepared by RF magnetron sputtering, and then integrated into thin film batteries. The LRCO thin films may be formed using an LRCO containing sputtering target. The LRCO containing sputtering target may be sputtered onto a substrate using an RF magnetron sputtering technique, such that an RF power is applied to a magnetron assembly of a process chamber and the LRCO material is sputtered from the sputtering target onto a substrate disposed within the process chamber.
Experimental methods utilized to form exemplary LRCO cathodes and thin film batteries are further described herein. The resultant LRCO cathodes and thin film batteries were then tested as described herein.
The substrate 102 may be an inorganic material, an organic material, or a combination thereof. Inorganic materials include silicon, aluminum oxide (Al2O3), quartz, and some polymers. In other embodiments, the methods described herein enable the use of an organic material for the substrate 102, such as one or more polymers. As described herein, methods used to form the cathode 106 enable the use of lower cost, flexible substrates, such as polymer substrates. As described herein, the substrate 102 is a support substrate and is generally used to support the other elements of the thin film battery 100. The other elements of the thin film battery 100 are formed on top of the substrate and the substrate may be later diced or cut to form a plurality of thin-film batteries 100.
The current collector 104 is formed on top of the substrate 102. The current collector 104 may be a thin-film current collector deposited onto the substrate 102. The current collector 104 may be formed of an electronically conductive material. In some embodiments, the current collector 104 is formed of gold, silver, platinum, aluminum, carbon based current collectors, or a combination thereof. Other electronically conductive materials are envisioned, but not listed herein for brevity. In some embodiments, the current collector 104 is a platinum metal current collector. The current collector 104 may have a collector thickness T1 of greater than about 50 nm, such as greater than about 200 nm, such as about 200 nm to about 1000 nm, such as about 200 nm to about 500 nm. In some embodiments, the collector thickness T1 is about 50 nm to about 200 nm.
The cathode 106 is formed on top of the current collector 104, such that the current collector 104 provides a low electronic resistance connection to the cathode 106. The cathode 106 as described herein is an LRCO electrode. The LRCO electrode contains each of lithium, ruthenium, cobalt, and oxygen. The general composition of the LRCO electrode layer may follow the atomic ratio provided by the formula (1−x)Li2RuO3+xLiCoO2+yLi2O). Y is varied between about 0.05 and 0.6, such as about 0.1 to about 0.4, such as about 0.2 to about 0.3. X is varied between 0.05 and 0.5, such as about 0.1 to about 0.3, such as about 0.2 to about 0.3. As described herein, x and y may be equal to 0.1, 0.2, or 0.3. As described above, the LRCO composition may be formed using a combination of Li2RuO3 and LiCoO2. The ratio of Li2RuO3 to LiCoO2 may be changed to vary the cobalt concentration within the composition. In embodiments wherein x is greater than about 0.3, the discharge capacity has generally been shown to be lower. In embodiments where x and y are estimated to be the same, the formula is Li2+xRu1−xCoxO3. The atomic ratio provided by the formula Li2+xRu1−xCoxO3 is an approximation and the composition ratio may be altered slightly to fall within the atomic ratios described herein.
In some embodiments, the cathode is formed on top of the current collection 104, but the current collector 104 is a free standing current collector. The free standing current collector is not disposed on a substrate, such as the substrate 102. In this embodiment, the current collector 104 may be a thin foil, such as an electrically conductive foil. In some embodiments, the cathode is formed on top of the current collector 104 and the current collector 104 is later removed from the substrate 102, such that the substrate 102 is removable.
As described herein, the atomic ratio of lithium to ruthenium within the cathode 106 may be about 6:1 to about 2:1, such as about 5:1 to about 2.2:1, such as about 4:1 to about 2.5:1. In some embodiments, the atomic ratio of lithium to cobalt is controlled to be about 23:1 to about 4:1, such as about 21:1 to about 5:1, such as about 15:1 to about 6:1, such as about 12:1 to about 6:1. The ratio of cobalt has been shown to directly impact the electrochemical performance of the thin film battery 100, such that cobalt concentrations of about x=0.1 to about x=0.3 have improved electrochemical performance. In some embodiments, the cobalt concentrations of about x=0.2 to about x=0.3 have improved electrochemical performance over cobalt concentrations of x=0.1. The atomic ratio of ruthenium to cobalt may be about 10:1 to about 1:1, such as about 8:1 to about 2:1, such as about 7:1 to about 2:1.
The cathode 106 further has a cathode thickness T2. The cathode thickness T2 is large enough to cover the current collector 104 while forming a thin film cathode for the thin film battery 100. As described herein, the cathode 106 can have a cathode thickness T2 of greater than about 50 nm, such as about 50 nm to about 40,000 nm, such as about 50 nm to about 4,000 nm, such as about 50 nm to about 750 nm, such as about 100 nm to about 500 nm, such as about 100 nm to about 350 nm, such as about 250 nm or about 300 nm. The cathode composition described herein enables a thin film cathode to be formed with relative uniformity across the cathode. As described herein, the cathodes have greater uniformity when deposited as an amorphous layer using physical vapor deposition (PVD) and before being annealed.
In embodiments described herein, the cathode 106 within the thin-film battery 100 may be either an annealed or an unannealed film. When the cathode 106 is deposited using a PVD operation, no binder is utilized and the cathode 106 may be fully amorphous and non-crystalline. In embodiments herein, the crystal grain size is below 300 nm to reduce the likelihood of shorts within the thin film battery 100. The reduced crystal grain size further provides higher Coulombic efficiency and reduced capacity fading. In some embodiments, the crystal grain size is less than 250 nm, such as less than 200 nm, such as less than 150 nm, such as less than 100 nm. With increased crystal grain size, the electrolyte 108 thickness would be increased to reduce the likelihood of shorts and therefore limiting the columbic efficiency, cost, and size of the thin-film battery 100.
The cathode 106 has a surface roughness (Ra) of less than about 1000 nm, such as less than about 700 nm, such as less than about 500 nm, such as less than about 300 nm. The reduced surface roughness enables improved formation of the electrolyte thereon and reduces the potential for shorting of the thin film battery 100. The surface roughness is directly correlated to the crystal grain size and is reduced with the reduction in crystal grain size.
The electrolyte 108 is formed on top of the cathode 106 and the substrate 102, such that the electrolyte 108 entirely covers the surface of the cathode 106. The electrolyte 108 is a solid-state electrolyte and may be deposited on the substrate 102 in a similar manner to the cathode 106, such as by a solution casting or a CVD or PVD process. As described herein, the electrolyte 108 may be a solid lithium-ion conductor. The solid lithium-ion conductor is utilized to conduct lithium ions between the cathode 106 and the anode 110. The electrolyte 108 may be described herein is a Lithium phosphorus oxynitride (LiPON) material. The LiPON material has a general formula of LixPOyNz, where x−2y+3z−5 for various combinations of y and z. One exemplary atomic ratio is Li3.3PO3.9N0.17. Additional examples of potential electrolytes may be found in any one of BATES, J. B. (1992). Electrical properties of amorphous lithium electrolyte thin films. Solid State Ionics, 53-56, 647-654. https://doi.org/10.1016/0167-2738(92)90442-r, Bates, J. B., Dudney, N. J., Gruzalski, G. R., Zuhr, R. A., Choudhury, A., Luck, C. F., & Robertson, J. D. (1993). Fabrication and characterization of amorphous lithium electrolyte thin films and rechargeable thin-film batteries. Journal of Power Sources, 43(1-3), 103-110. https://doi.org/10.1016/0378-7753(93)80106-y, as well as Yu, X., Bates, J. B., Jellison, G. E., Hart, F. X. (1997). A stable thin-film lithium electrolyte: Lithium phosphorus oxynitride. Journal of The Electrochemical Society, 144(2), 524-532. https://doi.org/10.1149/1.1837443.
An electrolyte thickness T3 is a thickness of the electrolyte 108 formed on top of the cathode 106. The electrolyte thickness T3 may be the distance separating the cathode 106 and the anode 110. The electrolyte thickness may be about 0.05 μm to about 3 μm, such as about 0.5 μm to about 2 μm, such as about 1 μm to about 1.5 μm.
The anode 110 is disposed on top of the electrolyte 108. The anode 110 may be deposited using a slurry coating or a PVD process. Other deposition processes may also be utilized, such as a chemical vapor deposition (CVD) or atomic layer deposition (ALD). The anode 110 can be prefabricated, for example, a lithium metal foil. The anode 110 may be a graphite, a lithium metal, silicon alloys, titanium oxides, or other metallic materials. Other metallic materials may include germanium, indium, aluminum, tin, magnesium, zinc, silver, or gold. In some embodiments, the anode 110 includes Li4Ti5O12, TiO2, or SnO2. In embodiments described herein, the anode 110 is a lithium metal anode. In one example, the anode 110 is a lithium metal thin film. The anode 110 has an anode thickness T4. The anode thickness T4 can be about 0.05 μm to about 10 μm, such as about 1 μm to about 10 μm, such as about 1 μm to about 5 μm, such as about 1 μm to about 3 μm, such as about 2 μm.
In some embodiments, the anode 110 is replaced by a metallic thin film. The metallic thin film may be formed of a metal or a metal alloy. In some embodiments, the metallic thin film is one or a combination of nickel or copper. The metallic thin film is similarly deposited onto the electrolyte 108. When using the metallic thin film, lithium metal plates onto the metallic film during charging of the thin film battery 100 and is stripped from the metallic thin film during discharge of the thin film battery 100. In some embodiments, the electrolyte 108 comprises a lithium ion conductor and the lithium metal plates and is stripped from the metallic thin film during charging and discharging. In this embodiment, the metallic thin film serves as a current collector and the thin film battery 100 is an anode-free thin film battery.
The total thickness To of the thin film battery 100, excluding the substrate 102, is less than about 150 μm, such as less than about 100 μm, such as less than about 50 μm, such as less than about 30 μm, such as less than about 25 μm, such as less than 20 μm, such as less than 10 μm. The small total thickness T0 of the thin film battery 100 enables the thin film battery 100 to be utilized in a large amount of applications, such as in medical devices, micro-computers, tiny robots, etc. Additional layers and materials may also be used within the thin film battery 100.
In some embodiments, the substrate 102 may be omitted and the current collector 104 is utilized as a base of the thin film battery 100. In these embodiments, the current collector 104 may form the entire bottom surface of the thin film battery 100.
In yet other embodiments, the structure of the thin film battery 100 may be flipped, such that the anode 110 is disposed on a current collector on the substrate 102, an electrolyte 108 is disposed on top of the anode 110 and the substrate 102, and the cathode 106 is formed on top of the electrolyte 108. In this embodiments, the anode 110 and the cathode 106 of
In some embodiments, multiple thin film batteries 100 are stacked on top of each other, such that each individual thin film battery 100 forms a cell and two or more cells are stacked on top of each other. In these embodiments, a second current collector and/or a second cathode is disposed on top of the anode 100, a metallic thin film, and/or the electrolyte 108. A second electrolyte is formed on top of the second cathode/the second current collector, and a second anode is formed on top of the second electrolyte. A third and/or a fourth battery cell may be disposed on top of the second cell. Stacking of the thin film batteries 100 is enabled by the lack of an annealing operation, such that the thin film battery 100 is kept below about 700° C., such as below about 500° C., such as below about 300° C. Maintaining a low temperature formation process enables stacking as the electrolyte 108 material is not damaged or destroyed as it would be at elevated temperatures, such as temperatures greater than about 300° C.
In some embodiments, the thin film battery 100 includes a cathode 106 comprising an anion redox active material. The anion redox active material is a solid solution of one or a combination of lithiated ruthenium oxide (Li2RuO3) and lithiated iridium oxide (Li2IrO3) along with at least one lithium metal oxide. The lithium metal oxides include lithium oxides of iron (Fe), cobalt (Co), nickel (Ni), manganese (Mn), tin (Sn), titanium (Ti), palladium (Pd), silver (Ag), zinc (Zn), gallium (Ga), indium (In), aluminum (Al), and vanadium (V). In some embodiments the lithium metal oxides include one or a combination of iron (Fe), cobalt (Co), nickel (Ni), manganese (Mn), tin (Sn), titanium (Ti), aluminum (Al), and vanadium (V). The lithium metal oxides include one or a combination of lithium iron oxide, lithium cobalt oxide, lithium nickel oxide (LNO), lithium manganese oxide (LMO), lithium tin oxide, lithium titanium oxide (LTO), lithiated nickel-manganese oxide (NMC), lithiated nickel-cobalt-aluminum oxide (NCA), and lithium vanadium oxide. The lithium metal oxides therefore includes one or a combination of LiFeO2, LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li2MnO3, LiSnO, Li2TiO3, LiNi1−x−yMnxCoyO2, LiNi0.8Co0.15Al0.05O2, and LiV3O8.
The thin film battery 100 further includes an anode 110 disposed adjacent to the cathode 106 and an electrolyte 108 disposed between the cathode 106 and the anode 110. The anode 110, the electrolyte 108, the current collector, and the substrate 102 are similar to those previously described. The methods of forming the thin film battery 100 may be the same for the different combinations of the two or more metals. In one embodiment, the method of forming the thin film battery 100 includes depositing a cathode 106 onto a support substrate 102 within a process volume of a processing chamber, depositing an electrolyte 108 over the cathode 106, and depositing an anode 110 over the electrolyte 108. The cathode 106 includes lithium, oxygen, and at least two of ruthenium, cobalt, tin, iridium, and manganese.
The chamber body 202 includes a process volume 208 disposed therein. The process volume 208 may be isolated from the atmosphere around the chamber body 202, such that the process volume 208 is vacuum isolated. The chamber body 202 may include a plurality of openings disposed therethrough to enable other components to be inserted into the chamber body 202 and the process volume 208.
The substrate support assembly 204 is disposed within the process volume 208 and includes a support pedestal 224 and an actuator 226 coupled to the support pedestal 224. The support pedestal 224 is configured to support the substrate 250 and includes a substrate support surface 205. The support pedestal 224 may be configured with one or more heaters, one or more cooling channels, and one or more backside gas lines disposed therein (not shown). The actuator 226 is configured to move the support pedestal 224, such that the actuator 226 may move the support pedestal 224 in a vertical direction or it may rotate the substrate about an axis of the support pedestal 224. In embodiments described herein, the support pedestal 224 is configured to be raised and lowered to be moved proximate to a sputtering target 218 during substrate processing.
The sputtering assembly 206 is disposed above the substrate support assembly 204. The sputtering assembly 206 is configured to sputter one or more materials onto the substrate 150. The sputtering assembly 206 includes a sputtering target 218, a magnetron assembly 220, and a power source 222. The power source 222 is configured to supply power to the magnetron assembly 220. The power source 222 may further bias the sputtering assembly 206 by biasing the sputtering target 218. The power source 222 may be an AC or RF power source. The magnetron assembly 220 includes a plurality of magnets disposed therein and configured to move within a volume of the magnetron assembly 220. At least one side of the sputtering target 218 is exposed to the process volume 208, such that a side of the sputtering target 218 facing the substrate 250 is disposed within the process volume 208. In other embodiments, the location of the sputtering assembly 206 and the substrate support assembly 204 is reversed such that the sputtering assembly 206 is disposed below the substrate support assembly 204 and the substrate 102.
A gas source 210 is in fluid communication with the process volume 208 and is configured to supply one or more process gases into the process volume 208 through one or more openings 212 disposed within the chamber body 202. The gas source 210 may be configured to flow a single gas, a gas mixture, or multiple gases into the process volume 208. In some embodiments, the gas source 210 comprises multiple gas sources. In embodiments described herein, the gas source 210 is configured to supply one or more inert gases, such as helium, neon, or argon. The gas source 210 may also be configured to supply a process gas such as nitrogen or oxygen into the process volume 208.
An exhaust pump 214 is also in fluid communication with the process volume 208 and is configured to remove one or more process gases from the process volume 208. The exhaust pump 214 removes the process gases through one or more exhaust openings 216 disposed through the chamber body 202. The exhaust pump 214 may apply a vacuum to the process volume 208. In embodiments described herein, the gas source 210 and the exhaust pump 214 may purge the process volume 208 between process operations or when a substrate, such as the substrate 250 is moved into or out of the process volume 208.
The controller 230 is configured to control the processing of the substrate 250 within the processing chamber 200. The controller 230 as described herein, may be configured to control the actuator 226, the sputtering assembly 206, the gas source 210, and the exhaust pump 214. The controller 230 may be one or a plurality of individual controllers. The controller 230 is a general use computer that is used to control one or more components found in the processing chamber 200. The system controller 230 is generally designed to facilitate the control and automation of one or more of the processing sequences disclosed herein and typically includes a central processing unit (CPU) 232, memory 234, and support circuits (or I/O) 236. Software instructions and data can be coded and stored within the memory 234 (e.g., non- transitory computer readable medium) for instructing the CPU 232. A program (or computer instructions) readable by the processing unit within the system controller determines which tasks are performable in the processing chamber 200. For example, a non-transitory computer readable medium includes a program which when executed by the CPU 232 are configured to perform one or more of the methods described herein. Preferably, the program includes code to perform tasks relating to monitoring, execution and control of the movement, support, and/or positioning of a substrate along with the various process recipe tasks and various processing module process recipe steps being performed.
The formation of the cathode film may be performed using a suitable deposition technique. As described herein, the cathode film may be deposited either using a slurry coating or using a CVD or PVD operation. Forming the cathode film on the substrate using the PVD operation has been shown to allow more uniform deposition with smaller crystal grain sizes. When depositing the cathode film using the PVD operation, the cathode film may be an amorphous film. The PVD operation includes sputtering an LRCO material from a sputtering target, such as the sputtering target 218, onto the substrate. The sputtering target is formed from an LRCO material, such as an LRCO-1, LRCO-2, or LRCO-3 material as described herein. The LRCO material forming the sputtering target includes each of lithium, ruthenium, cobalt, and oxygen.
During deposition of the LRCO material, the processing chamber is evacuated to a base pressure of about 1·10−8 Torr to about 1·10−5 Torr, such as about 1·10−7 Torr to about 1·10−6 Torr, such as about 5.10−7 Torr. After evacuating to the base pressure, the processing chamber is brought to a working pressure. The working pressure is about 1 mTorr to about 5 Torr, such as 10 mTorr to about 1 Torr, such as about 15 mTorr. The working pressure is the pressure at which the cathode film is formed using the PVD process.
A power is applied to a magnetron assembly, such as the magnetron assembly 206 of
The substrate is disposed at a sputtering distance from a surface of a sputtering target, such as the sputtering target 218. The sputtering distance is the distance between the surface of the sputtering target which faces the substrate and a top surface of the substrate. The sputtering distance during the PVD process is about 3 cm to about 25 cm, such as about 5 cm to about 20 cm, such as about 5 cm to about 15 cm, such as about 5 cm to 10 cm. In exemplary embodiments described herein, the sputtering distance is one of 5 cm or 10 cm.
During the PVD process one or more process gases may be flowed into the process volume. The process gases include one or more inert gases and one or more process gases. The one or more inert gases may be one of helium, neon, or argon. As described herein, the inert gas is argon. The inert gas is flowed at a flow rate of about 1 sccm to about 20 sccm, such as about 2 sccm to about 10 sccm, such as about 3 sccm to about 5 sccm. A second gas, such as a process gas is also supplied to the process volume during PVD processing. The second gas may be one of nitrogen (N2), oxygen (O2), or a combination thereof. In exemplary embodiments described herein, the second gas is oxygen and is flowed into the process volume at a flow rate of about 0.5 sccm to about 5 sccm, such as about 1 sccm to about 3 sccm, such as about 1 sccm.
While the cathode layer is being deposited onto the substrate, the temperature within the process volume is less than about 100° C. The reduced temperature enables the use of additional substrate materials other than aluminum oxide, silicon, or quartz substrates. In some embodiments, a polymer containing substrate may be utilized. The polymer containing substrate may be an organic or an inorganic substrate.
The above process conditions of the PVD deposition of the LRCO cathode are exemplary. The process conditions may be adjusted to compensate for varying chamber configurations, deposition rates, and substrate size.
After depositing the cathode on the substrate, the cathode may optionally be annealed during an operation 304. The optional anneal of the cathode is performed at a duration of at least about 1 hour, such as about 1 hour to about 20 hours, such as about 1 hour to about 10 hours, such as about 1 hour to about 6 hours, such as about 1 hour. The annealing temperature is about 100° C. to about 800° C., such as about 100° C. to about 700° C., such as about 400° C. to about 650° C. The length and temperature of the cathode during operation 304 is directly correlated to the crystal grain size produced within the cathode. In embodiments described herein, the crystal grain size after the optional anneal is less than 250 nm, such as less than 200 nm, such as less than 150 nm, such as less than 100 nm. In embodiments wherein there is no anneal, the cathode remains amorphous.
In embodiments both with and without the optional anneal operation 304, an electrolyte is formed over the cathode layer during an operation 306. The formation of the electrolyte may be performed using similar deposition conditions as the deposition of the cathode layer. In some embodiments, the electrolyte is also formed using a PVD process. In other embodiments, the electrolyte is deposited using a CVD or ALD process. Other processes may also be utilized to form the electrolyte. The electrolyte is a solid lithium-ion conductor. In some embodiments, the electrolyte is formed from a LiPON material. The electrolyte may be sputtered onto the substrate using a LiPON sputtering target, such that the cathode is covered by the electrolyte.
After the cathode layer has been covered by the electrolyte, an anode is formed over the electrolyte during an operation 308. The anode may be similar to the anode 110 of
In some embodiments, the substrate 102 is omitted or replaced. In embodiments in which the substrate 102 is omitted, the current collector 104 has a greater thickness and the cathode 104, the electrolyte 108, and the anode 110 are formed on the current collector without a substrate 102 disposed beneath the current collector 104.
In some embodiments, the structure of the thin film battery 100 is flipped, such that an anode current collector is disposed on top of the substrate 100. The anode 110 may then be disposed on top of an anode current collector before forming the electrolyte 108 over the anode 110. Once the electrolyte 108 is formed, the cathode 106 may be subsequently formed over the electrolyte 108. In this embodiment, the current collector 104 may be disposed on top of the cathode 106 and distal from the substrate 100. In this embodiment, the operation 308 is performed before either of operations 302, 304, or 306. Operation 308 is modified such that the anode is deposited on the substrate. After depositing the anode, the electrolyte is deposited during the operation 306. The cathode may then subsequently be deposited on the electrolyte and annealed during operations 302 and 304.
The following non-limiting examples are provided to further illustrate embodiments described herein. However, the examples are not intended to be all inclusive and are not intended to limit the scope of the embodiments described herein.
The materials and methods described above were tested experimentally as described below. Initially, a LRCO powder was synthesized. The LRCO material was prepared using solid state methods. Li2CO3 (Alfa Aesar, 99.9% purity, 10 wt. % excess), RuO2 (Alfa Aesar, 99.9% purity), and CoCO3 (Alfa Aesar, 99.9% purity) precursors were weighed according to the desired stoichiometry, mixed with anhydrous acetone (Fisher Chemical), and ground in a planetary ball mill (DECO, PBM-V-0.4L). Excess Li2CO3 was included to compensate for possible Li loss during high temperature annealing and/or during the sputtering process in later steps. The ground/mixed powders were heated at 2° C./min in a muffle furnace (air atmosphere) to a temperature of 900-1100° C., then soaked for 12 h at the final temperature before cooling. The effect of final annealing temperature on structure and electrochemical properties was investigated.
From the LRCO powder, a LRCO target is formed. The LRCO powder, as described above, has a general chemical composition of Li2.2Ru0.8Co0.2O3. A two-inch sputtering target for thin film deposition was prepared by high temperature sintering of the Li2.2Ru0.8Co0.2O3 powder. Agglomerates in the LRCO powder were first disrupted manually using a mortar and pestle. The fine powders were then mixed with a 5 wt % solution of polyethylene oxide in N,N-dimethylformamide (DMF) binder solution, and the mixture was then heated to 70° C. to remove the DMF solvent. The LRCO and a poly(ethylene oxide) (PEO) binder mixture was cold-pressed in a two-inch diameter die at 11 metric tons for 5 minutes. The pellet was then placed in a clean alumina dish and sintered in a room-air muffle furnace. The following heating profile was used to sinter the target. The temperature was increased by 5° C./minute to 300° C. The temperature was then subsequently increased by 1° C/minute to 550° C. and dwelled at 550° C. for 0.5 hours. Burn-out of the binder is theorized to occur while at 550° C. The temperature was then subsequently increased by 20° C/minute to 900° C. and dwelled at 900° C. for about 5 hours. After dwelling at 900° C., the furnace was cooled by 2° C./minute. After the sintered target fully cooled, the target was attached to a copper backing plate (OHFC) using silver-filled, vacuum grade epoxy (Dynaloy, KL-325K). The target was cured at 70° C. under vacuum before installation in the sputtering chamber.
Once the target was installed in the chamber, a plurality of thin films were deposited on substrate within the sputtering chamber. LRCO thin films were fabricated using RF magnetron sputtering in a vacuum deposition chamber. Typical, unoptimized process parameters for the RF magnetron sputtering included a base pressure of 5.10−Torr, a working pressure of 15 mTorr, a power of 70 W, a substrate to target distance of 5 to 10 cm, an argon gas flow rate of 3 sccm into the deposition chamber, and an oxygen (O2) gas flow rate of 1 sccm into the deposition chamber. Optical grade fused quartz slides (AdValue Technology, FQ-S-001, 1″ (length)×1″ (width)×0.04″ (thickness)) were used as substrates for all thin films deposited by the target. LRCO thin films on quartz slides are typically 300 nm-thick, characterized by scanning electron microscopy (SEM).
Thin films (nominally 1 μm) of lithium phosphorous oxynitride (LiPON) were directly deposited on top of the cathode layer by radio frequency (RF) magnetron sputtering of a 2-inch Li3PO4 powder target (99.95%, Kurt J. Lesker) in ultrapure N2 atmosphere. The custom-built sputtering chamber was pumped to about 1·10−7 Torr using a combination of a mechanical and diffusion pump. Key deposition parameters were a forward power of 80 W, a nitrogen gas flow rate of 5 sccm, an operating nitrogen pressure of 20 mTorr, and a target-substrate distance of 5 cm. Approximately 2-μm-thick Li metal was thermally evaporated as the anode material in a vacuum chamber with a base pressure of about 1·10−6 Torr. A quartz crystal monitor (QCM) was used to in-situ monitor Li deposition rate.
For the cathode annealing process, 300nm-thick films of as-deposited LRCO (chemical composition of Li2.2Ru0.8Co0.2O3) were prepared, then moved to tube furnace for annealing under O2 atmosphere. As described herein, the LRCO has a molar ratio of x=0.2 when using the molecular formula Li2+xRu1−xCoxO3. However, the cobalt concentration within the material may be varied. When x=0.1, the LRCO is said to be LRCO-1. When x=0.2, the LRCO is said to be LRCO-2. When x=0.3, the LRCO is said to be LRCO-3. Unless otherwise specified herein, x=2 and the LRCO material has a composition similar to LRCO-2. The furnace temperature was first ramped up to set values at a ramping rate of 2° C./min, then held for the desired time, and finally allowed to naturally cool down to room temperature.
To determine the structure of LRCO powders and thin films, X-ray powder diffraction (XRD) was conducted using a Rigaku Synergy-S diffraction system with Cu Kα microfocus X-ray source. XRD powder patterns were refined via MDI Jade 9 software. Elemental compositions (Li, Ru, and Co) of LRCO were characterized using inductively coupled plasma-mass spectrometry (ICP-MS) (Perkin Elmer NexION 2000) to determine exact stoichiometry of LRCO powder. For post-mortem analysis, the cell was carefully disassembled in an Ar-filled glove box. All samples were dried under vacuum, put into a hermetically sealed plastic bottle, and then transferred to various analysis systems. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were performed on a ZEISS Crossbeam 340 FIB-SEM system using an accelerating voltage of 7.5 kV. For analysis of the samples' cross-section, sample substrates were fractured by hand.
For liquid coin cells, a slurry was prepared by adding a binder solution containing 10 wt. % polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) solvent to the LRCO powder mixture containing 80 wt. % active material and 10 wt. % Super P conductive carbon. The slurry was cast on aluminum foil current collectors and dried under vacuum at 100° C. overnight before cell assembly. Metallic Li foil (1.9 mm×0.75 mm, 99.9%, Alfa Aesar) was scraped, rolled, and cut into a disc with a diameter of 1.43 cm and put on a microporous glass fiber separator of slightly larger diameter which acted as a spacer to define the electrode separation. The electrolyte used was 100 μl of 1.0 M LiPF6 in 1:1:1 EC:EMC:DMC (Gotion). The coin cells were cycled from 2.0 V to 4.5 V using a battery cycler (BT-2043, Arbin Instruments). The first two cycles were set at a 0.02 C rate as formation period. Cell temperature was controlled at 25° C. The electrode area was defined by the geometric area of the cathode side. Potentiostatic electrochemical impedance (PEIS) measurements were executed with a 10 mV AC amplitude at room temperature over a frequency range of 3 MHz to 0.1 Hz.
Thin-film batteries were cycled inside an Ar-filled glovebox from 2.0 V to various charge potentials using a battery cycler (BT-2043, Arbin Instruments). The electrode area was defined by the geometric area of the cathode side. Potentiostatic electrochemical impedance (PEIS) measurements were executed with a 10 mV AC amplitude at room temperature over a frequency range of 3 MHz to 0.1 Hz.
To determine the optimal annealing temperature in LRCO powder synthesis, the effect of the annealing temperatures (900° C., 1000° C. and 1100° C.) on the microstructure of Li2.2Ru0.8Co0.2O3 was investigated.
As the annealing temperature increases, new diffraction peaks were not observed and all observed reflections were indexed to the LRO PDF card, suggesting phase purity. At low Co content (x=0.2), it is difficult to observe the diffraction peaks from LCO phases. However, the sample at higher calcination temperature (1100° C.) tends to be less crystalline due to weaker peak intensity and characteristic peaks becoming obscure.
To correlate the LRCO structure obtained at the three annealing temperatures to electrochemical performance, lithium-ion battery cathodes were prepared by incorporating the LRCO powders into a typical electrode slurry and casting the slurry onto Al current collectors. The LRCO cathodes were incorporated into coin cells with Li metal anodes and standard liquid electrolyte. The battery cells were cycled galvanostatically at approximately 0.1 C (current density of 30 mA g−1) from 2.0 V to 4.5 V vs. Li+/Li. The cathode slurries were not optimized for long-term cycling studies. Thus, the electrochemical characterization provides a comparative study to understand the effect of annealing temperature on electrochemical properties (e.g. initial charge capacities, lithiation potentials, and charge reversibility). The corresponding voltage profiles and integrated capacities are provided in
When cycling the LRCO-2 half coin cells after calcinating the cathode at a temperature of 900° C., the first discharge capacity was 255 mAh g−1, the initial Coulombic efficiency was 70.5%, and the capacity retention at the 70th cycle was 53.6%. When cycling the LRCO-2 half coin cells after calcinating the cathode at a temperature of 1000° C., the first discharge capacity was 238 mAh g−1, the initial Coulombic efficiency was 94.3%, and the capacity retention at the 70th cycle was 73.3%. When cycling the LRCO-2 half coin cells after calcinating the cathode at a temperature of 1100° C., the first discharge capacity was 211 mAh g−1, the initial Coulombic efficiency was 83.9%, and the capacity retention at the 70th cycle was 70.3%.
At a calcination temperature of 900° C., the cycling stability was observed in
To determine the optimal Co substitution in LRCO, a series of compounds were prepared with x=0.1, 0.2, and 0.3 in Li2+xRu1−xCoxO3. The compositions were varied through the ratio of CoCO3 included in the solid-state synthesis of the LRCO powder. The compositions of the synthesized powders were confirmed using ICP-MS.
Similar microstructural and electrochemical characterization was conducted to determine the optimal Co substitutions amounts in LRCO. Microstructural characterization of Li2+xRu1−xCoO3 series powders (x=0.1, 0.2 and 0.3) were characterized via XRD (shown in
As illustrated in
As shown in
To assess the effect of cobalt substitution on electrochemical performance, LRCO-1 and LRCO-3 were incorporated into lithium-ion half cells and cycled galvanostatically using the previous cycling protocols.
As seen in
The synthesis of LRCO-2 (Li2.2Ru0.8Co0.2O3) cathode powder was conducted based on desired calcination temperature and cobalt content, followed by preparation of a sputtering target. The LRCO-2 composition was consolidated into a 2″ diameter sputtering target (LRCO sputtering target) by high temperature sintering. Thin film cathodes were prepared via RF-sputtering of the LRCO sputtering target. The resulting cathodes were dense lithium insertion compounds (e.g. lithium metal oxides) that are fabricated in a “thin” supported film format and function as the positive electrode in a solid-state electrochemical cell. These electrochemical cells include a support substrate, a thin film cathode, a solid lithium-ion conductor (i.e. a solid electrolyte), and an anode (such as lithium metal).
To further investigate morphology differences of LRCO thin films deposited from the LRCO-2 target at a target-sample distance of 5 cm as a function of annealing temperature, top-down and cross-sectional SEM images were compared in
At annealing temperatures of 300 and 400° C. (
Thin-film XRD patterns were presented in
SEM of annealed LRCO-2 thin films (450° C. for 3 h) are shown in
The LRCO thin films as discussed herein are fully functional, reversible cathodes and have substantial capacity in the as-deposited, unannealed state. The electrochemical characterization and energy storage performance tests discussed herein are therefore focused primarily on amorphous thin films of LRCO. There is practical motivation for utilizing as-deposited/unannealed thin films as well. Utilizing as-deposited thin films eliminates the annealing steps and significantly broadens substrate compatibility while reducing production costs. For electrochemical characterization described herein, a 250 nm-thick thin film cathode was deposited by sputtering the LRCO-2 target onto a platinum current collector supported on a quartz substrate. One micrometer of LiPON and nominally 2 micrometers of Li were sequentially deposited on top of the LRCO film. The thin film cell was cycled between 2.0 V and 4.5 V at a rate of 20 μA/cm2 and the cathode area was used to compute current density. The cycling between 2.0 V and 4.5 V at a rate of 20 μA/cm2 is described herein as a first charging pattern. A stable capacity of 105 μAh/cm2-μm could be reversibly accessed over 20 charge/discharge cycles. This is a substantial improvement over LiCoO2 cathodes, which provide a capacity of 67.5 μAh/cm2-μm. Further, LCO films are thermally annealed at temperatures exceeding several hundred degrees Celsius to fully crystallize the LCO microstructure and obtain optimal energy storage performance.
The voltage profiles and cycle capacities of the as-deposited LRCO-2 are provided in
The Coulombic efficiency, sometimes referred to as reversibility, of the cells can be further improved by charging the LRCO to lower potentials at lower current densities.
As shown in
The ability to obtain significant capacity in as-deposited LRCO allows for the formation of a thin film battery without thermal annealing. Thermal annealing adds significant processing cost and time to the preparation of LCO thin film cathodes. Typically, LCO films are annealed at temperatures in excess of 500° C. The thermal annealing requires that the substrate is also thermally stable. Typical substrates are inorganic materials, such as aluminum oxide (Al2O3) or quartz. In this initial demonstration of LRCO thin films, quartz substrates were used, however, the methods disclosed herein also include thin film preparation in the absence of annealing on flexible, lower melt-temperature substrates such as a polymer substrates.
In some embodiments, thermal annealing is used to crystallize the as-deposited LRCO, which is expected to enhance the specific energy, Li+ diffusivity and (de)lithiation potentials. In some embodiments, LRCO thin films were annealed at 450° C. and 650° C. for 3 hours. The annealed LRCO thin films are reversible and demonstrate stable charge capacities as shown in
As illustrated in
Lithium ruthenium cobalt oxide (LRCO) cathode materials with a chemical formula of (1−x)Li2RuO3+xLiCoO2+yLi2O where Y is between about 0.05 and 0.6 and X is between about 0.05 and 0.5 were used to prepare energy-dense cathode thin films at low temperatures. The LRCO thin films were prepared by RF magnetron sputtering and were shown to be smooth, uniform, and electrochemically active without further thermal annealing, enabling the successful fabrication and operation of thin film batteries on flexible thermoplastic substrates. Compared to lithium cobalt oxide (LCO) films with a specific capacity of 39 μAh cm−2 μm−1, LRCO provide a high discharge capacity of 110 μAh cm−2 μm−1 at a 0.3 C rate and greater than 92% capacity retention over 150 cycles. This discharge capacity exceeds even the optimized, near-theoretical capacity of annealed, polycrystalline LCO (67 μAh cm−2 μm−1).
LRCO was prepared by conventional solid-state synthesis. Li2CO3 (Alfa Aesar, 99.9% purity, 35 wt. % excess), RuO2 (Alfa Aesar, 99.9% purity), and CoCO3 (Alfa Aesar, 99.9% purity) precursors were weighed according to the desired stoichiometry ((1−x)Li2RuO3−xLiCoO2−yLi2O), mixed with anhydrous acetone (Alfa Aesar), and ground in a planetary ball mill (DECO, PBM-V-0.4L). Excess Li2CO3 was included to compensate for Li loss during high temperature annealing and/or subsequent sputtering. The powder mixture was heated at a rate of 2° C./min in a muffle furnace (air atmosphere) to a temperature of 1000° C., held for 12 hours, then cooled under ambient conditions.
A two-inch sputtering target for thin film deposition was prepared by high temperature sintering of the synthesized LRCO powder. LRCO powder agglomerates were first ground using a mortar and pestle. The fine powders were then mixed with a 5 wt. % solution of polyethylene oxide (PEO) in N,N-dimethylformamide (DMF) binder solution, and the mixture was then heated to 70° C. to remove the DMF solvent. The LRCO and PEO binder mixture was cold pressed in a two-inch (5.08 cm) diameter die at 48 MPa for 5 mins. The pellet was then removed from the die, placed in a clean alumina dish, and sintered at 900° C. for 5 hours in a muffle furnace. After the sintered target fully cooled, it was attached to a copper backing plate (OHFC) using silver-filled, vacuum grade epoxy (Dynaloy, KL-325K). The target was cured at 70° C. under vacuum before installation in the sputtering chamber.
LRCO and LCO thin films were fabricated via RF magnetron sputtering in a custom-built vacuum deposition chamber. LCO sputtering target (99.9% purity) with a size of (2.00″ (5.08 cm) diameter×0.125″ (0.3175 cm) thick) was purchased from Kurt J. Lesker. Process parameters for the RF magnetron sputtering are provided in Table 1. Optical grade fused quartz slides (AdValue Technology, FQ-S-001, 2.54×2.54×0.1 cm) were used as substrates for all thin-film battery assembly. Before thin film cathode deposition, 100 nm-thick Pt, as cathode current collector, was deposited via direct current (DC) sputtering (Denton Vacuum DESK-II DC Sputtering System). Thin film cathodes with an area of 0.6 cm2 were typically deposited to 300 nm thickness, confirmed by scanning electron microscopy (SEM).
Thin films (1 μm and 5 cm2) of lithium phosphorous oxynitride (LiPON) were directly deposited on top of the cathode layer by radio frequency (RF) magnetron sputtering of a 2-inch (5.08 cm) Li3PO4 powder target (99.95%, Kurt J. Lesker) in ultrapure N2 atmosphere. The custom-built sputtering chamber was pumped to ˜5×10−7 Torr via a mechanical diffusion pump before deposition. Key deposition parameters were a forward power of 90 W, a nitrogen gas flow rate of 5 sccm, an operating pressure of 20 mTorr, and a target-substrate distance of 5 cm. 2-μm-thick Li metal, as anode and current collector, was thermally evaporated in custom vacuum chamber with a base pressure of ˜6×10−7 Torr. Quartz crystal monitor (QCM) was used to in-situ monitor Li deposition rate. A schematic illustration of the thin film battery fabrication process is shown in
Inductively coupled plasma-mass spectrometry (ICP-MS) was performed using Perkin Elmer NexION 200 to identify the chemical composition of the LRCO target. To determine the structure of LRCO powders and thin films, powder X-ray diffraction (XRD) and thin-film XRD were conducted using a Rigaku Synergy-S diffraction system and a Bruker D8 Advance system with Cu Kα microfocus X-ray source, separately. XRD patterns were refined via MDI Jade 9 software. X-ray photoelectron spectroscopy (XPS) was performed using a Krato Axis Ultra DLD XPS system with a monochromatized Al Kα source at 15 kV and 10 mA. Survey scans were conducted from 1200 to 5 eV with a 1 eV step and 160 eV pass energy. For calibration, the aliphatic C 1s peak was assigned to 284.6 eV. Ten high-resolution scans were employed for O 1s, Ru 3d, and Co 2p with a pass energy of 20 eV. Detailed peak deconvolution was analysed via CasaXPS software. For post-mortem analysis, the cell was carefully disassembled in an Ar-filled glove box. All samples were put into a hermetically sealed plastic bottle, and then transferred to various analysis systems. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were performed on a ZEISS Crossbeam 340 FIB-SEM system using an accelerating voltage of 7.5 kV. For analysis of the samples' cross-section, sample substrates were fractured by hand.
Thin-film batteries were cycled inside an Ar-filled glovebox under various charge potentials using a battery cycler (BT-2043, Arbin Instruments). The electrode area was defined by the geometric area of the cathode side. Cyclic voltammetry was conducted from 2.0 V to 3.9 V reversibly with a scanning rate of 0.1 mV/s. Rate performance tests (5 cycles as an increase period) were employed at 3, 10, 15, 27, 33, back to 3 μA/cm2, 2.0-3.9 V (LRCO) and 3.0-4.2V (LCO) vs. Li/Li+ and at 25° C.
For structural and morphological characterization, LRCO thin films were first prepared on flat, model Si wafer substrates. The Si wafers were coated with a 300 nm-thick thermal oxide layer to prevent potential Li interdiffusion into Si during sputtering.
Returning now to
The LRCO target was prepared based on Li-rich solid solution of ((1−x)Li2RuO3−xLiCoO2−yLi2O), where x is content for Co and y is the ratio for excess Li in the target. The chemical composition of the LRCO target of 0.79Li2RuO3-0.18LiCoO2-0.66Li2O was confirmed by ICP-MS (see Table 2). For liquid coin cells, a slurry was prepared by adding a binder solution containing 10 wt. % PVDF in N-methyl-2-pyrrolidone (NMP) solvent to the LRCO powder mixture containing 80 wt. % active material and 10 wt. % Super P carbon. The slurry was cast on an Al foil current collector and dried under vacuum at 100° C. overnight before cell assembly. Metallic Li foil (1.9 mm×0.75 mm, 99.9%, Alfa Aesar) was scraped, rolled and cut into a disc with a diameter of 1.43 cm and put on a microporous separator of slightly larger diameter (glass fiber) which acted as a spacer to define the electrode separation. Electrolyte used was 100 μl of 1.0 M LiPF6 in 1:1:1 EC:EMC:DMC (Gotion). The coin cells were cycled from 2.0 to 4.5 V using a battery cycler (BT-2043, Arbin Instruments). The first two cycles were set at a 0.02 C rate as the formation period. Cell temperature was controlled at 25° C. Excess Li was added to the target to compensate for Li loss during high temperature calcination of the sputtering target and long-distance sputtering deposition. Increasing the substrate-target distance is expected to reduce the lithium to transition metal ratio, since Li is the lightest metal element, resulting in a lower sputter yield in the final film composition. A Ru:Co atomic ratio of 4:1 was confirmed by EDS (provided in
To investigate the crystallinity and microstructure of the LRCO thin films, XRD was conducted on the smooth, featureless films (deposited at 10 cm distance). X-ray diffractograms of the precursor LRCO powders for the sputtering target, LRCO thin film, and substrate (thermal oxide on Si wafer) are provided in
LRCO TFBs were successfully fabricated by sequential deposition of films of Pt, LRCO, LiPON, and Li metal onto a quartz substrate, as shown by the 3D schematic in
For each charge potential, the cells exhibit sloping voltage profiles (
While all cells demonstrate excellent performance over the first 100 cycles, extended cycling reveals important distinctions in failure modes of LRCO cathodes charged to 3.9 and 4.0V (
To further understand the contribution of both Co and Ru redox in the cycling of cathode, additional electrochemical characterization of as-deposited LRCO was conducted. For comparison, unannealed LiCoO2 (LCO) thin films were also prepared and characterized. The cyclic voltammograms (CV) of both LRCO and LCO at a scanning rate of 0.1 mV/s is provided in
In addition to the CV, TFBs with 300 nm-thick as-deposited LCO films were also cycled; the results are reported in
The comparison graph in
By possessing a high specific capacity in the as-deposited, nanocrystalline morphology, LRCO extends the range of compatible substrates for thin film batteries. Beyond typical rigid, inorganic substrates such as silicon, quartz, and alumina, low cost and flexible thermoplastic substrates can be used as substrates. This has been demonstrated using polyethylene terephthalate (PET) and polyimide (Kapton®) films as substrates.
As another attractive option, Kapton® films can be employed over a much wider temperature range (−250 to 400° C.). Kapton® films were also employed as substrate layers to further demonstrate the outstanding flexibility and mechanical stability of as-deposited LRCO TFB. For an additional comparison of the effect of bending on cell operation, Kapton®-based TFB remained in a flat conformation for the first 60 cycles, and then bent for the subsequent cycles (
The capacity retention vs. capacity diagram in
The development of a high specific capacity, long cycle life cathode that does not require high temperature post-deposition processing enables multi-cell vertical stack battery configurations. LiPON ionic conductivity is known to substantially degrade above about 350° C. If cathode post-deposition annealing is required, this high temperature processing prevents the sequential deposition of multiple cells into high voltage serial batteries. By employing the unannealed LRCO cathode in the cell design, multiple cells may be sequentially deposited on a single substrate to fabricate multi-cell TFB. Such a design can dramatically reduce the substrate-to-active material mass ratio resulting in overall higher battery specific energy.
LRCO thin-film cathodes were successfully fabricated and demonstrated in thin film batteries. Increasing the distance between sputtering target and substrate results in deposition of smooth, isotropic films, important for the uniformity of ion transport. XRD suggests the LRCO films are nanocrystalline due to the presence of characteristic LRCO reflections. Oxidation states of 3+ and 4+ for cobalt and ruthenium, respectively, were confirmed by XPS. After fabrication of LRCO TFBs on fused quartz substrates, the highest charge potential was also tested for optimal cycle life. Starting from 3.8 V, the specific capacity and capacity retention increase as charge cut-off potential increases, reaching a maximum of 111.7 μAh/cm2-μm and 94.4% over 100 cycles with a current density of 10 μA/cm2 at charge potential of 4 V vs. Li/Li+. To avoid structural instability of LRCO at higher potentials, a charge potential of 3.9 V vs. Li/Li+ provides a stable compromise between specific capacity and reversibility. Given the low temperature preparation of the as-deposited LRCO, completed thin film batteries were also fabricated directly on flexible plastic substrates without further ex situ annealing. With PET substrates, outstanding specific capacity and capacity retention as high as 101.5 μAh/cm2-μm and 97.5%, respectively, was achieved over 120 cycles. The excellent performance of as-deposited LRCO, substrate versatility, and potential for unannealed multi-cell battery configurations may enable the utilization of TFBs in future flexible applications.
As discussed herein, the LRCO thin film cathodes are shown to have improved performance when integrated into a solid-state thin film battery. As-deposited amorphous LRCO films have been shown to provide good charge capacity and electrochemical reversibility. Unannealed LRCO has been shown to provide discharge capacities exceeding 110 μAh/cm2-μm, which is almost twice the specific energy of LCO thin-film cathodes. Unannealed LRCO films also provide the ability to develop thin film batteries on flexible/polymeric substrates. Improvements in the LRCO energy storage properties are achievable through crystallization. However, annealed films have been shown to exhibit rapid crystalline grain growth and rough particulate film morphologies.
By forgoing the high-temperature annealing associated with conventional lithium battery cathode production, a wide variety of substrate materials can be employed, including substrates made from flexible thermoplastic materials. Lithium batteries that include the flexible-substrate cathodes can be used in conventional battery applications. Lithium batteries that include the flexible-substrate cathodes can also be used in newer applications, such as wearable, flexible consumer electronics, where device flexibility is required. The novel, flexible cathode materials disclosed herein will enable the development of flexible electronic devices that require flexibility in the battery component.
The energy density of existing flexible batteries remains low as there are limitations regarding the thickness of the materials and total electrolyte loading. The advent of a high performance flexible thin film battery will accelerate the development of next-generation fully flexible electronic systems in combination with existing flexible components such as display, memory, interactive user interfaces and LED. The flexible batteries disclosed herein exhibit high electrochemical performance and excellent mechanical deformability, by virtue of being provided on flexible, low-melt temperature substrates.
It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.
While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
This application claims priority to the U.S. Provisional Patent Application Ser. No. 63/255,814, filed Oct. 14, 2021, which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 63255814 | Oct 2021 | US |
