Patent Application
20150372302
The present invention is in the field of battery technology and, more particularly, in the area of improved active materials for use in electrodes in electrochemical cells.
Olivine-type LiFePO4 is one of the most promising cathode materials for large-scale lithium batteries because of its low-cost, non-toxicity, and extremely high stability. But, this cathode material has certain shortcomings, including comparatively poor conductivity and a comparatively low theoretical specific energy (about 530 Wh/kg) due to the comparatively low operating voltage of about 3.4 V.
Isostructural LiMnPO4 is another interesting candidate for a new cathode material, in part because its comparatively flat 4.0V plateau versus Li/Li+ is compatible with commercial 4V class cathodes such as layered LiCoO2 and spinel LiMn2O4. Further, the theoretical energy density is 684 Wh/kg, (derived as 171 mAh/g multiplied by 4.0 V), and this theoretical energy density is 1.2 times larger than that of LiFePO4 (derived as 170 mAh/g multiplied by 3.4 V). Further, the isostructural LiMnPO4 is compatible with the use of well-known electrolyte components, such as propylene carbonate (PC), ethylene carbonate (EC), and dimethoxyethane (DME).
However, as compared to other promising cathode materials LiMnPO4 has demonstrated much lower reversible capacities than desired. Several hypotheses have been proposed in the literature to explain such poor performances, including: (i) lower intrinsic electronic conductivity, (ii) local lattice distortion around Jahn-Teller-active Mn3+ ions, and (iii) larger mechanical strains being developed at the boundary between Li-rich (lithiated) and Li-poor (delithiated) phases. Further, it has been reported that the electrical conductivity of LiMnPO4 was measured to be lower by about five orders of magnitude than that of LiFePO4. Thus, there remains a need to improve the performance of olivine-type cathodes. Certain research has been done into doping in LiMnPO4, including U.S. Pat. No. 7,060,238; U.S. Patent Publication No. 2013/0140496; and European Patent No. 2178137. As described in further detail below, none of this research provides for improvements to olivine-type cathodes in the manner described herein.
According to some embodiments of the invention, a composition and method for making an electrode includes a material represented by Li1+xM1yMnzPO4 where 0.01≦x≦0.2, 0.01≦y≦0.1, and 0.95≦z≦1; and where M1 is a dopant. M1 can include an alkaline earth metal, such as Mg. In some embodiments, the electrode includes Li1.02Mg0.03MnPO4. According to some embodiments of the invention, a composition and method for making an electrode includes a material represented by Li1.05-xM1xMnPO4 where 0.01≦x≦0.04 and M1 comprises an alkaline earth metal.
According to some embodiments of the invention, a composition and method for making an electrode includes a material represented by Li1+xMgyMnzM1xPO4 where 0.01≦x≦0.2, 0≦y≦0.1, 0.85≦z≦1, and 0.01 w 0.2; and M1 is one or more dopants. M1 can include a transition metal. M1 can include Fe, Co, Zn, Mg, V, Ni, Nb, Cu, Cr, Ti or a combination thereof. M1 can include three different elements selected from the group consisting of Fe, Co, Zn, Mg, V, Ni, Nb, Cu, Cr, and Ti. M1 can include Fe and two elements selected from the group consisting of Mg, Co, and Zn.
According to some embodiments of the invention, a composition and method for making an electrode includes a material represented by Li1.05-xMgxMn0.9−y−zM1yM2zFe0.1PO4 where 0.0051≦x≦0.04, 0≦y≦0.05, and 0≦z≦0.05; and M1 and M2 each comprise a transition metal or alkaline earth metal. M1 and M2 can each comprise a different transition metal or alkaline earth metal.
The following definitions apply to some of the aspects described with respect to some embodiments of the invention. These definitions may likewise be expanded upon herein. Each term is further explained and exemplified throughout the description, figures, and examples. Any interpretation of the terms in this description should take into account the full description, figures, and examples presented herein.
The singular terms “a,” “an,” and “the” include the plural unless the context clearly dictates otherwise. Thus, for example, reference to an object can include multiple objects unless the context clearly dictates otherwise.
The terms “substantially” and “substantial” refer to a considerable degree or extent. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation, such as accounting for typical tolerance levels or variability of the embodiments described herein.
The term “about” refers to the range of values approximately near the given value in order to account for typical tolerance levels, measurement precision, or other variability of the embodiments described herein.
The term “transition metal” refers to a chemical element in groups 3 through 12 of the periodic table, including scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au), mercury (Hg), rutherfordium (Rf), dubnium (Db), seaborgium (Sg), bohrium (Bh), hassium (Hs), and meitnerium (Mt).
The term “lanthanide” refers to any of the fifteen metallic chemical elements with atomic numbers 57 through 71, including lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).
The term “alkali metal” refers to any of the chemical elements in group 1 of the periodic table, including lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).
The term “alkaline earth metals” refers to any of the chemical elements in group 2 of the periodic table, including beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).
A rate “C” refers to either (depending on context) the discharge current as a fraction or multiple relative to a “1 C” current value under which a battery (in a substantially fully charged state) would substantially fully discharge in one hour, or the charge current as a fraction or multiple relative to a “1 C” current value under which the battery (in a substantially fully discharged state) would substantially fully charge in one hour.
To the extent certain battery characteristics can vary with temperature, such characteristics are specified at room temperature (about 30 degrees C.), unless the context clearly dictates otherwise.
Ranges presented herein are inclusive of their endpoints. Thus, for example, the range 1 to 3 includes the values 1 and 3 as well as intermediate values.
The terms “milling” and “mixing” are used interchangeably, except in the instances where low energy mixing processes are specified. In such instances, the materials were predominantly mixed rather than milled.
Embodiments described herein provide improved electrochemical performance for certain olivine-type cathode materials. In particular, the performance of isostructural LiMnPO4 is improved via doping the material with selected transition metals. The improved LiMnPO4 material includes an excess amount of lithium in addition to the dopants. That is, the amount of Li is non-stoichiometric as compared to conventional LiMnPO4. Notably, it is the combination of excess lithium and multiple dopants that provide the improvements. The dopants are present in the Li site and on the Mn site.
According to certain embodiments, the performance of LiMnPO4 is improved by synthesizing materials according to the formula:
Li1+xM1yMnzPO4 (i)
where 0.01≦x≦0.2, 0.01≦y≦0.1, and 0.95≦z≦1. In these embodiments, excess lithium is present and some of the Li sites are doped with M1. M1 can be a transition metal, a lanthanide, an alkali metal, or an alkaline earth metal. In a preferred embodiment, M1 is an alkaline earth metal. In a still further preferred embodiment, M1 is Mg. Particularly useful compositions include those in which M1 is present such that y=0.03. Among the preferred embodiments are the following compositions:
As discussed further herein, preferred embodiments include an excess of lithium and such embodiments demonstrate significant improvement over cathodes formed from conventional LiMnPO4 active materials.
According to certain embodiments, the performance of the compositions of Formula (i) above are further improved by synthesizing materials according to the formula:
Li1+xMgyMnzM1wPO4 (ii)
where 0.01≦x≦0.2, 0≦y≦0.1, 0.85≦z≦1, and 0.01≦w≦0.2. In these embodiments, the LiMnPO4 active material includes an excess of lithium and Mg is doped in the Li site. M1 is one or more dopants in the Mn site, and M1 can be a transition metal, a lanthanide, an alkali metal, or an alkaline earth metal. In a preferred embodiment, M1 is a transition metal. In a preferred embodiment, M1 is an alkaline earth metal. In still further preferred embodiments, M1 is Fe, Co, Zn, Mg, V, Ni, Nb, Cu, Cr, Ti or a combination thereof. Notably, these double doped compounds can provide performance improvements over the single doped compounds.
The following examples describe specific aspects of some embodiments of the invention to illustrate and provide a description for those of ordinary skill in the art. The examples should not be construed as limiting the invention, as the examples merely provide specific methodology useful in understanding and practicing some embodiments of the invention.
Materials and Synthetic Methods.
Stoichiometric ratios of LiH2PO4, MnCO3, Fe(C2O4)2H2O, Li2CO3, Carbon KJ600, and TiO2 were combined in reaction vessels along with 19 g of 5 mm diameter chrome steel ball bearings. Li2CO3 was added to provide excess Li compared stoichiometric LiMPO4. A milling solvent such as hexane is optional. The wells were sealed, removed from glove box, and then milled with high energy milling. The milled precursors were heated to the 530 degrees C. at a rate of 5 degrees per minute for 3 hours under flowing nitrogen gas. Then, the vessels were cooled to room temperature at a rate of 5 degrees per minute. 5 wt % carbon was added based on final product mass, and the active material mass used to calculate specific capacity includes the carbon amount.
Electrode Formulation.
Cathodes based on the activated phosphate material were prepared using a formulation composition of 93:5:2 (active material:binder:conductive additive) according to the following formulation method. 94.3 mg PVDF (Sigma Aldrich) was dissolved in 12.5 mL NMP (Sigma Aldrich) overnight. 37.7 mg of conductive additive was added to the solution and allowed to stir for several hours. 40 mg of the activated phosphate material was then added to 1 mL of this solution and stirred overnight. Films were cast by dropping about 50 mL of slurry onto stainless steel current collectors and drying at 150 degrees C. for about 1 hour. Dried films were allowed to cool, and were then pressed at 1 ton/cm2. Electrodes were further dried at 150 degrees C. under vacuum for 12 hours before being brought into a glove box for battery assembly.
Electrochemical Characterization.
All batteries were assembled in a high purity argon filled glove box (M-Braun, O2 and humidity contents <0.1 ppm), unless otherwise specified. Cells were made using lithium as an anode, Celgard 2400 separator, and 90 mL of 1M LiPF6 in 1:2 EC: EMC electrolyte. Electrodes and cells were electrochemically characterized at 30 degrees C. with a constant current C/10 charge rate followed by constant voltage step till the current reaches C/100 and discharge rate C/10 between 4.3 or 4.5 V and 2.0 V for the first two cycles. Starting from cycle 4, both charge and discharge rate is 1 C with slow rate C/10 on every 25th cycle between 4.5 and 2 V.
Li1.05-xM1xMnPO4 (iii)
where for the control material, x=0, and for the doped materials, x=0.01 or 0.03. The identity of the dopant appears on the top x-axis and the concentration of the dopant appears on the bottom axis. For
Li1.05−xMgxMn0.9−y−zM1yM2zFe0.1PO4 (iv)
where 0.005≦x≦0.04, 0≦y≦0.05, and 0≦z≦0.05. In this embodiment, the samples of LMP active material can be Fe doping on the Mn site, and the further dopants M1 and M2 are chosen among Mg, Co, or Zn. The identity and the amount of one dopant appear on the top x-axis. The results of the electrochemical testing of following materials are show in
In
Similarly,
Still further,
The improvements generated by the compounds of the present invention are significant and unexpected in view of the prior art. For example, European Patent No. 2178137 discloses Mg doping of the Li site of a lithium phosphate material, but not in the presence of excess lithium. The ratio of Li plus the Mg dopant to P and to the other metals present in the lithium phosphate is fixed at 1. The compounds of European Patent No. 2178137 would not yield the performance improvements disclosed herein.
The same can be said for U.S. Pat. No. 7,060,238 and U.S. Patent Publication No. 2013/0140496 in that the ratio of Li to P is fixed at 1. Thus, these references do not disclose an excess of Li and in particular do not disclose the double doping of the Li site and the Mn site.
According to the embodiments disclosed herein, a combination of Mg doping on Li site and excess lithium (that is, (Li+Mg)/M1>1) shows electrochemical performance improvement.
While the invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention as defined by the appended claims. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, method, or process to the objective, spirit and scope of the invention. All such modifications are intended to be within the scope of the claims appended hereto. In particular, while the methods disclosed herein have been described with reference to particular operations performed in a particular order, it will be understood that these operations may be combined, sub-divided, or re-ordered to form an equivalent method without departing from the teachings of the invention. Accordingly, unless specifically indicated herein, the order and grouping of the operations are not limitations of the invention.
