The invention relates to Li-based batteries, and in particular relates to LiMPO4-based compositions of matter for cathodes for high-performance batteries such as Li rechargeable batteries, where M is at least one transition metal.
Li rechargeable batteries are presently the most promising candidates for large-scale energy storage applications such as for the next generation of electric and hybrid electric vehicles, electronic devices and other high performance tools, machines, etc. Among the various types of Li rechargeable batteries, olivine-structured LiFePO4 or LiMPO4 in general where M can be a transition metal or transition metals, is one of the most promising cathode materials in the electric vehicles due to its high capacity and structural stability (safety). However, its low intrinsic electronic conductivity of 10−9S/cm prevents the full use of this material to its theoretical capacity. Furthermore, the Li+ ion diffusion coefficient is very low, i.e., 10−11 to 10−10 cm2S−1.
These two intrinsic drawbacks of olivine-structured LiMPO4 are a bottleneck for its use in commercial applications. In addition, the reversible capacity loss at high current densities is another shortcoming of LiMPO4 such as LiFePO4 and LiMnPO4 etc. The poor performance at high current densities is believed to be directly associated with the polarization caused by low intrinsic electronic and ionic conductivity. There have been many attempts to modify LiFePO4, including doping different elements into LiFePO4, but these attempts have not overcome the aforementioned shortcomings to achieve a LiFePO4-based cathode with good performance at high charge and discharge current densities.
An aspect of the invention provides a method of enhancing the conductivity of lithium metal phosphate materials modified through doping with ruthenium.
Another aspect of the invention is a composition of matter that comprises a material defined by a general formula LiMPO4, where M is at least one transition metal (e.g., Fe, Mn, Ni and Co), and wherein the material has a general lattice structure. The material further includes Ru ions that substitute for at least one of Li ions and ions of the at least one transition metal M in the general lattice structure.
An example material (compositions of matter) has the general formula Li1-αxRuxMPO4 where α is constant and equal to the valence of Ru in stoichiometric form, and is not equal the valence of Ru in nonstoichiometric form, and 0<x<1/α. In an example, the parameter α is equal to the valence of Ru, which can be from 2+ to 8+ (i.e., 2≦α≦8).
Another example material is a lithium manganese phosphate material that uses manganese (Mn) as the transition metal and has the general formula LiMn1-αx/2RuxPO4 where α is constant and equal to the valence of Ru ions in the stoichiometric form, and is not equal to the valence of Ru ions in nonstoichiometric form, and 0<x<1/α. In another example, the parameter α is equal to the valence of the Ru ions, which can be from 2+ to 8+. The lithium manganese phosphate material has applications as an active cathode material in Li-ion rechargeable batteries.
Thus, another aspect of the invention is a composition of matter having a general formula of Li1-αxRuxMPO4 or LiRuxM1-αx/cPO4, where α is equal to the valence of Ru ions, M is at least one transition metal, and c is a valence of M.
Another aspect of the invention is a composition of matter comprising a lithium metal phosphate material system having a general formula of Li1-axRuxMPO4 or LiRuxM1-αx/cPO4 where α is not equal to the valence of the Ru ions, and c is the valence of M.
Another aspect of the invention is composition of matter that comprises a lithium metal phosphate material system having either a first formula Li1-αxRuxMPO4 or a second formula LiRuxM1-αx/cPO4. The parameter α is a constant and is not equal to a valence of Ru ions for the first formula and is not equal to a valence of Ru ions for the second formula, x is defined by 0<x<1, M is at least one transition metal, and c is a valence of the at least one transition metal M. In an example, the parameter α is in the range from 1 to 8 for the first formula and is in the range from c to 8 for the second formula.
Another aspect of the invention is a method of synthesizing a composition of matter based on a lithium metal phosphate material. The method includes mixing a lithium compound, a metal compound, a phosphorous compound, and a Ru compound to form a mixture. The method also includes pre-calcining and firing the mixture to form the composition of matter defined by the formula LiMPO4, where M is at least one transition metal, The composition of matter has a general lattice structure and Ru ions substitute for at least one of the Li ions and M in the general lattice structure.
The invention relates generally to Li-based batteries, and in particular relates to Li rechargeable batteries. Aspects of the invention are directed to the use of ruthenium-doped olivine-structured LiMPO4 fine powders as active cathode materials for Li-ion and Li rechargeable batteries, where M is at least one transition metal such as Fe, Mn, Ni and Co. Aspects of the invention include methods for greatly enhancing the electrochemical performance of the cathode material at high current densities to achieve high-performance Li rechargeable batteries.
An aspect of the invention includes lithium metal phosphate materials for forming Li-battery cathodes. In an example, the lithium iron phosphate materials have a general lattice structure with an olivine structure (i.e., comprises a modified olivine-structured LiFePO4) formed via doping with trace amounts of ruthenium (Ru). In another example, the lithium metal phosphate material is in the form of a lithium manganese phosphate material that has a general lattice structure with an olivine structure (i.e., comprises a modified olivine-structured LiMnPO4) formed via doping with trace amounts of ruthenium. In both examples, the general lattice structures remain unchanged even thought Ru ions occupy lattice sites in the structure via doping.
An example composition of matter disclosed herein comprises a lithium metal phosphate material defined by a general formula LiMPO4, where M is at least one transition metal (e.g., Fe, Mn, Ni and Co), and wherein the material has a general lattice structure. The material further includes Ru ions that substitute for at least one of Li ions and ions of the at least one transition metal M in the general lattice structure.
Another example composition of matter comprises a lithium metal phosphate material system having either a first formula Li1-αxRuxMPO4 or a second formula LiRuxM1-αx/cPO4. The parameter α is a constant and is not equal to a valence of Ru ions for the first formula and is not equal to a valence of Ru ions for the second formula, x is defined by 0<x<1, M is at least one transition metal, and c is a valence of the at least one transition metal M. In an example, the parameter α is in the range from 1 to 8 for the first formula and is in the range from c to 8 for the second formula.
Thus, in an example, the lithium metal phosphate material can be in stoichiometric form (i.e, represented by a first formula) when α is equal to the valance of Ru ions or a nonstoichiometric form (i.e., represented by a second formula) when α is not equal to the valance of Ru ions, namely:
Li1-αxRuxMPO4 and LiM1-αx/cRuxPO4 (0<x<1/α)
where α is constant and is greater than 0, and c is the valence of M, where M is at least one transition metal. In an example, the parameter α is an integer and is equal to the valence of Ru, which can be from 2+ to 8+ (i.e., a can range from 2 to 8). In other examples, the parameter α is not equal to the Ru valence. The Ru-doped lithium metal phosphate material has a general lattice structure that is charge-neutral. In an example of Li1-αxRuxFePO4, charge neutrality is maintained via the relationship:
0=(1Li
Due to the presence of Ru with a high oxidation state, vacancies form in the general lattice structure. The amount (number) of defects in the general lattice structure depends on both the oxidation state and the amount of Ru used. In the nonstoichiometric form, electronic defects are present.
In an example, the Ru ions are substituted for Li ions in the general lattice structure via Ru doping, with the general lattice structure having same number of lattice positions and remaining unchanged relative to undoped lithium iron phosphate general lattice structure.
In another example, the Ru ions are substituted for ions of the at least one transition metal in the lattice structure via Ru doping, with the general lattice structure having same number of lattice positions and remaining unchanged relative to the undoped lithium iron phosphate general lattice structure.
When used as a cathode in a Li battery, the lithium metal phosphate material provides enhanced electrochemical performance at high charge and discharge current density without substantially compromising the battery capacity.
An example method of synthesizing the lithium metal phosphate material includes mixing compounds or elemental Ru with other raw materials by mixing, ball milling, co-precipitation, mechanical activation, mechanical alloying, sol-gel or other mixing methods, wherein the mixing methods generally form a homogeneous mixture. Here, the term “mixing” is generally used to include any one of the above mixing methods or combinations thereof.
The method also includes calcining and firing the mixture for a sufficient time and temperature in a furnace or furnaces having an inner or reductive atmosphere or vacuum protection to achieve homogeneous reaction of the mixture.
Precursors with compounds of Li, M, P, and Ru sources are initially mixed to achieve a certain degree of homogeneity. In an example, the precursors is mixed into acetone, ethanol or distilled water and then ball milled at different energies for 0.5 to 200 hours. In producing the particular lithium iron phosphate material, the mixed powder can be pre-calcined directly or the mixed powder can be further compacted into pellets that are then pre-calcined at temperatures in the range from 200 to 600° C. for a time duration from 30 min to 1440 min or longer.
Example lithium compounds include Li2CO3, Li2O and LiOH. Example iron compounds include FeO, FeC2O4, FeC2O4.2H2O, Fe2O3 and Fe(AC)2. Example phosphorous compounds include NH4H2PO4, (NH4)2HPO4, (NH4)3PO4, P2O5, H3PO4. Example of manganese compounds include Mn(CH3COO)2, MnO, etc. Example ruthenium compounds include RuO2 and RuCl3.
After pre-calcining, the mixed powder can be fired with a fire temperature ranging from 500 to 1000° C. for a time duration from about 30 minutes to about 6000 minutes or longer. Alternatively, the calcined powder or calcined pellets is/are ball milled, or ground or treated by other means to achieve a fine powder having a substantially homogenous chemical and size distribution before firing at the aforementioned temperature and time duration ranges. In an example, the calcining and the firing of the mixed powder can be performed with intermittent grinding. In an example, the firing temperature can be maintained substantially constant over the firing time duration.
In an example, an amount of carbon can be mixed into the precursor during the regrinding process to enhance the conductivity and electrochemical properties of the final lithium iron phosphate material. The properties and quality of the final lithium iron phosphate material may be altered by changing one or more of the process parameters, such as the firing temperature, the firing duration, the heating and cooling rates, etc. As such, the various synthesis parameters can be controlled in manners understood by one skilled in the art in order to achieve desirable properties and performance of the lithium iron phosphate material.
An example embodiment of the lithium iron phosphate material has a Ru valence of 4 (i.e., α=4, the valence of Ru4+ used here), so that the formula for the lithium iron phosphate material is Li1-4xRuxFePO4. Based on this formula, the content of Ru can be varied from x=0.0001 mole to a maximum 0.2500 mole.
To synthesis the example lithium iron phosphate material, stoichiometric amounts of Li2CO3, FeC2O4.2H2O, NH4H2PO4 and RuO2 are mixed according to corresponding formula in an acetone liquid, ethanol, or distilled water and then ball-milled for a few hours to form a homogeneous mixture using a low-energy horizontal mill.
After ball milling, a firing step is carried out, which in an example is a two-step solid-state reaction carried out at 350° C. and 600° C., respectively, in a furnace with flowing argon. Intermittent mixing and grinding are performed to ensure chemical homogeneity in the resultant active powder.
After the firing step, the active powder is well mixed with 15% carbon black powder and 5% polyvinylidene fluoride (PVDF) to form a slurry. The slurry is cast on to an aluminium foil to form an electrode. In the electrochemical test, metal Li is used as an anode as well as the reference electrode.
The Li1-4xRuxFePO4 cathode material also shows excellent cycability at high charge/discharge current density. As shown in
Another example material is LiMnPO4, where Ru is doped in the transition metal lattice (i.e. Mn lattice position) forming LiMn1-2xRuxPO4. Based on this formula, the content of Ru can be varied from x=0.0001 mole to a maximum of 0.5000 mole.
To synthesize the example lithium manganese phosphate material, stoichiometric amounts of CH3COOLi.2H2O, Mn(CH3COO)2.4H2O, NH4H2PO4 and RuO2 are mixed according to corresponding formula in an acetone liquid, ethanol, or distilled water and then ball-milled for a few hours to form a homogeneous mixture.
After ball milling, a firing step is carried out, which in an example is a two-step solid state reaction carried out at 350° C. and 600° C., respectively, in a furnace with flowing argon. Intermittent mixing and grinding are performed to ensure chemical homogeneity in the resultant active powder.
After the firing step, the active powder is well mixed with 20% carbon black powder and 10% polyvinylidene fluoride (PVDF) to form a slurry. The slurry is cast on to an aluminium foil to form an electrode. In the electrochemical test, metal Li is used as an anode as well as the reference electrode.
This Application claims priority from U.S. Provisional Patent Application Serial No. 61/335,688,entitled “Modified LiFePO4 cathode materials with ultrafast charge rate and ultrahigh power density for high performance Li Batteries,” filed on Jan. 11, 2010.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/SG2011/000010 | 1/10/2011 | WO | 00 | 6/24/2012 |
Number | Date | Country | |
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61335688 | Jan 2010 | US |