Patent Application
20170133661
The present invention is in the field of battery technology and, more particularly, in the area of high-energy materials for use in electrodes in electrochemical cells.
Cathodes formed from active materials of the composition LiM2O4 are a common class of active materials in lithium ion batteries, where M includes a transition metal. The LiM2O4 active material can have a spinel structure. Transition metals such as manganese have been used as the transition metal in LiM2O4 active materials and, in some instances, both manganese and nickel have been used.
LiMn2O4 has been widely used in commercial batteries, and LiMn1.5Ni0.5O4 (LMNO) is an excellent candidate to replace LiMn2O4 because LMNO has about a 30% energy density improvement as compared to LiMn2O4. This energy density improvement is due to the higher voltage potential (4.7 V) and the higher capacity of the LMNO materials as compared to LiMn2O4. For the sake of comparison, a LiMn2O4 active material has a theoretical gravimetric energy density of about 492 Wh/kg and a LiMn1.5Ni0.5O4 has a theoretical gravimetric energy density of about 691 Wh/kg.
Further, LMNO materials used in lithium ion batteries have demonstrated substantial rate capability, such as a rate capability of greater than 120 mAh/g at a 20 C rate. However, the state-of-the-art LMNO that demonstrates this high rate capability still shows relatively poor cycle life (that is, the capacity retention diminishes substantially with increasing cycles). The poor cycle life limits the operation of batteries containing LMNO active materials when they are used at high voltages and/or at high temperatures.
The challenges of limited cycle life in LiM2O4 active materials can be addressed by certain embodiments of the invention described herein.
Embodiments of the invention include a method for producing an active material for use in a lithium ion battery. The method can include providing a lithium precursor, a transition metal precursor, and a process control agent. The method can further include milling the lithium precursor, the transition metal precursor, and the process control agent to form a milled powder. The method can further include heating the milled powder for a time and temperature to form the active material.
In some embodiments, the process control agent includes a carbon source. In some embodiments, the carbon source includes carbon black. In some embodiments, the carbon source includes graphite. In some embodiments, the carbon source includes an ethylene oxide polymer. In some embodiments, the carbon source includes poly(ethylene oxide).
In some embodiments, the milled powder contains unreacted process control agent. In some embodiments, the heating consumes substantially all of the unreacted process control agent.
In some embodiments, the transition metal precursor includes manganese. In some embodiments, there is an additional transition metal precursor. In some embodiments, the additional transition metal precursor includes nickel. In some embodiments, the active material includes LiMn1.5N10.5O4.
Embodiments of the invention include batteries having an electrode formed from any of the active materials disclosed above.
Embodiments of the invention include processes for making the active materials disclosed above as described herein.
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 “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 “spinel” refers to the metal oxide material with a spinel structure generally understood to be useful for electrode materials in lithium ion batteries.
The term “specific capacity” refers to the amount (e.g., total or maximum amount) of electrons or lithium ions a material is able to hold (or discharge) per unit mass and can be expressed in units of mAh/g. In certain aspects and embodiments, specific capacity can be measured in a constant current discharge (or charge) analysis, which includes discharge (or charge) at a defined rate over a defined voltage range against a defined counter electrode. For example, specific capacity can be measured upon discharge at a rate of about 0.05 C (e.g., about 7 mA/g) from 4.95 V to 3.0 V versus a Li/Li+ counter electrode. Other discharge rates and other voltage ranges also can be used, such as a rate of about 0.1 C (e.g., about 14 mA/g), or about 0.5 C (e.g., about 70 mA/g), or about 1.0 C (e.g., about 140 mA/g).
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 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 the intermediate values.
Embodiments of the present invention provide novel materials for use as active materials in cathodes of an electrochemical cell. The materials of the present invention address the challenges described above for existing cathode active materials.
Specifically, embodiments disclosed herein use a new synthesis method for producing active materials for lithium ion batteries. In some cases, the active materials are spinel active materials. In other cases, other active materials can be produced with the synthetic method disclosed herein, include oxides and phosphates.
A notable feature of the embodiments disclosed herein is the use of an additive during the synthesis process. The additive can be referred to as a process control agent, as it affects the synthesis process to produce an active material with substantially improved performance. A significant feature of certain of the process control agents disclosed herein is that there is almost no residual material from the process control agents in the final synthesized material. Because there is almost no residual material from the process control agents, no additional purification steps are required. In preferred embodiments, the process control agent is a carbon source included with the active material precursors during the active material synthesis.
As discussed in further detail below, the synthesis of the active materials involves a milling process, where precursors of the components of the active material are combined and reacted. A calcination process is used to further react the precursor materials and produce the final active material. The introduction of a process control agent during the milling and/or calcination can facilitate the production of active materials with improved performance. In particular, spinel active materials benefit from the use of a process control agent.
Preferred process control agents include those capable of providing sacrificial carbon during the milling and calcination process of synthesizing active materials. For example, carbon sources such as carbon black (e.g., SuperP from TIMCAL), graphite, polymers and/or oligomers containing ethylene oxide. For comparison, carbon black has a relatively larger surface area than graphite (˜20 m2/g). If measured by BET analysis (as described below, the BET of carbon black is typically about 60 m2/g and the BET of graphite is typically about 20 m2/g.
Without being bound to any theory or mechanism of action not recited in the claims, the addition of a process control agent can alter the synthesis of an active material in a few ways. In the case of spinel synthesis, which includes a mixing step and a calcination step, the addition of a process control agent can alter the synthesis. In the mixing step (and specifically when the mixing step is a milling step), the process control agent acts as an interface between milling balls (in the case of a balling milling apparatus) and the active materials to reduce the milling energy imparted to the active material precursors. During the milling process, the process control agent can form a kind of coating, layer, or buffer on the active material surface, which reduces the agglomeration of the active materials. So, the process control agent can also affect the particle size yielded by the milling process as well as the particle growth during the calcination process. Further, the process control agent can alter the surface morphology and/or surface chemistry of the particles of active material that emerge from the milling process.
In the calcination process, the particle size, surface morphology, and surface chemistry are all relevant factors that influence the reactivity of the milled mixture. Thus, the process control agent can influence the calcination process simply by altering the yield of the milling process. Alternately or additionally, the process control agent can affect the atmosphere of the calcination process as it is consumed. For example, a carbon source used as a process control agent can provide a reductive environment (as opposed to a neutral or oxidative environment). The presence of the process control during calcination may also alter the rate of particle sintering and/or particle growth. The presence of the process control during calcination may help reduce the amount of surface defects in the final particles of active material.
In preferred embodiments, the process control agent is substantially, almost completely, or completely consumed during the active material synthesis process. In the case of a carbon source process control agent, the process control agent that is unreacted during the milling step may react during the calcination step. Residual process control agent may burn off during the high temperature calcination.
Thus, the process control agents of embodiments disclosed herein not only change the synthesis process, which changes the final product physical properties and the electrochemical performance, but also do not leave substantial residuals from the process control in the final product.
In certain embodiments, the process control agent is added to the precursor mixture prior to milling in an amount of at least 0.5% by weight, at least 1.0% by weight, at least 1.5% by weight, at least 2.0% by weight, at least 2.5% by weight, at least 3.0% by weight, at least 3.5% by weight, at least 4.0% by weight, at least 4.5% by weight, at least 5.0% by weight, at least 5.5% by weight, at least 6.0% by weight, at least 6.5% by weight, at least 7.0% by weight, at least 7.5% by weight, at least 8.0% by weight, at least 8.5% by weight, at least 9.0% by weight, at least 9.5% by weight, at least 10.0% by weight, or at least 10.5% by weight of the total weight of the mixture.
In certain embodiments, powder resulting from the milling process was calcined at a temperature greater than about 650 degrees C., greater than about 660 degrees C., greater than about 670 degrees C., greater than about 680 degrees C., greater than about 690 degrees C., greater than about 700 degrees C., greater than about 710 degrees C., greater than about 720 degrees C., greater than about 730 degrees C., or greater than about 740 degrees C., greater than about 750 degrees C., greater than about 760 degrees C., greater than about 770 degrees C., greater than about 780 degrees C., greater than about 790 degrees C., greater than about 800 degrees C., greater than about 810 degrees C., greater than about 820 degrees C., greater than about 830 degrees C., or greater than about 840 degrees C., greater than about 850 degrees C., greater than about 860 degrees C., greater than about 870 degrees C., greater than about 880 degrees C., greater than about 890 degrees C., greater than about 900 degrees C., greater than about 910 degrees C., greater than about 920 degrees C., greater than about 930 degrees C., or greater than about 940 degrees C., or greater than about 950 degrees C.
In certain embodiments, powder resulting from the milling process was calcined at a temperature less than about 950 degrees C., less than about 940 degrees C., less than about 930 degrees C., less than about 920 degrees C., less than about 910 degrees C., less than about 900 degrees C., less than about 890 degrees C., less than about 880 degrees C., less than about 870 degrees C., less than about 860 degrees C., less than about 850 degrees C., less than about 840 degrees C., less than about 830 degrees C., less than about 820 degrees C., less than about 810 degrees C., less than about 800 degrees C., less than about 790 degrees C., less than about 780 degrees C., less than about 770 degrees C., less than about 760 degrees C., less than about 750 degrees C., less than about 740 degrees C., less than about 730 degrees C., less than about 720 degrees C., less than about 710 degrees C., less than about 700 degrees C., less than about 690 degrees C., less than about 680 degrees C., less than about 670 degrees C., less than about 660 degrees C., or less than about 650 degrees C.
In certain embodiments, the calcination is performed for a time greater than about 0.1 hour, greater than about 0.5 hour, greater than about 1.0 hour, greater than about 1.5 hours, greater than about 2.0 hours, greater than about 2.5 hours, greater than about 3.0 hours, greater than about 3.5 hours, greater than about 4.0 hours, greater than about 4.5 hours, greater than about 5.0 hours, greater than about 5.5 hours, greater than about 6.0 hours, greater than about 6.5 hours, greater than about 7.0 hours, greater than about 7.5 hours, greater than about 8.0 hours, greater than about 8.5 hours, greater than about 9.0 hours, greater than about 9.5 hours, greater than about 10.0 hours, greater than about 10.5 hours, greater than about 11.0 hours, greater than about 11.5 hours, greater than about 12.0 hours, or greater than about 12.5 hours.
In certain embodiments, the calcination is performed for a time less than about 12.5 hours, less than about 12.0 hours, less than about 11.5 hours, less than about 11.0 hours, less than about 10.5 hours, less than about 10 hours, less than about 9.5 hours, less than about 9.0 hours, less than about 8.5 hours, less than about 8.0 hours, less than about 7.5 hours, less than about 7.0 hours, less than about 6.5 hours, less than about 6.0 hours, less than about 5.5 hours, less than about 5.0 hours, less than about 4.5 hours, less than about 4.0 hours, less than about 3.5 hours, less than about 3.0 hours, less than about 2.5 hours, less than about 2.0 hours, less than about 1.5 hours, less than about 1.0 hour, or less than about 0.5 hour.
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.
Synthesis. Spinel materials were synthesized by a milling and calcination process. Typically, stoichiometric amount of precursors for the lithium, manganese, and nickel content of the final material were milled. Suitable precursors include, but are not limited to, Li2CO3, Mn2O3, and Ni(OH)2. Other precursors capable to donating lithium and the desired transition metals may be used. In embodiments including a process control agent, the process control agent was added to the precursor mixture prior to milling in an amount from about 1% by weight to about 10% by weight of the total weight of the mixture. The powder resulting from the milling process was calcined at a temperature range from about 700 degrees Celsius to about 900 degrees Celsius. Calcination times varied from about 0.5 hours to about 12 hours under optional air flow (about 20 L/min)
Battery Cell Formation. Battery cells were formed in a high purity argon filled glove box (M-Braun, O2 and humidity content <0.1 ppm). The electrodes were prepared by following method. For the cathode, a spinel was mixed with poly(vinylidene fluoride) (Sigma Aldrich), carbon black (Super P Li, TIMCAL), using 1-methyl-2-pyrrolidinone (Sigma Aldrich) as solvent, and the resulting slurry was deposited on an stainless steel current collector and dried to form a composite cathode film. For the anode, a thin Li foil is cut into required size and used as anode. Each battery cell including the composite cathode film, a polypropylene separator, and lithium foil anode was assembled in a CR2032 coin cell (Hohsen). A conventional electrolyte mixed with an additive was used.
Testing. The battery cell was sealed and cycled between 3 V to 4.9 V at room temperature for an initial 4 cycles at a rate of 0.1 C rate. Later cycles were at elevated temperature (about 50 degrees Celsius) at a rate of 1 C.
The LMNO active materials depicted in
The micrographs in
The differences between the particles of LMNO active material synthesized with and without a process control agent can be quantified using Brunauer-Emmett-Teller (BET) surface area analysis. BET analysis provides specific surface area evaluation of materials by gas adsorption measured as a function of relative pressure.
The measured BET surface area for the LMNO synthesized with a process control agent was 7.5 m2/g, while the BET surface area for the LMNO synthesized without a process control agent was 0.8 m2/g. The addition of the process control agent during synthesis yielded an almost ten-fold increase in surface area for the LMNO active material. Since there is almost no residue left from the process control agent in the final materials and the particle size change is relatively small, this substantial increase in surface area is unexpected based on prior known use of process control agents.
Without being bound to a particular theory or mechanism of action not recited in the claims, it is hypothesized that the increased surface area and/or the more regular shape of the active material particles will contribute to substantial improvements in the electrochemical performance of batteries including cathodes formed from active materials synthesized using a process control agent. The effect of the process control agent on the performance of batteries including cathodes formed from active materials synthesized using a process control agent is demonstrated in the following figures.
Further,
Improvements have been demonstrated for process control agents including both graphite and poly(ethylene oxide). Improvement on both capacity and capacity retention are shown for graphite in
The embodiments disclosed herein provide improvements in the electrochemical performance of active materials, and in particular spinel active materials. The improvement in capacity retention for these materials is significant, since diminished capacity retention is one of the biggest challenges to commercializing high voltage spinel active materials.
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.
