The patent's assignee is
News editors obtained the following quote from the background information supplied by the inventors: "Rechargeable lithium-ion batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. As discussed more fully below, a typical Li-ion cell contains a negative electrode, a positive electrode, and a separator region between the negative and positive electrodes. Both electrodes contain active materials that insert or react with lithium reversibly. In some cases the negative electrode may include lithium metal, which can be electrochemically dissolved and deposited reversibly. The separator contains an electrolyte with a lithium cation, and serves as a physical barrier between the electrodes such that none of the electrodes are electronically connected within the cell.
"Typically, during charging, there is generation of electrons at the positive electrode and consumption of an equal amount of electrons at the negative electrode, and these electrons are transferred via an external circuit. In the ideal charging of the cell, these electrons are generated at the positive electrode because there is extraction via oxidation of lithium ions from the active material of the positive electrode, and the electrons are consumed at the negative electrode because there is reduction of lithium ions into the active material of the negative electrode. During discharging, the exact opposite reactions occur.
"When high-specific-capacity negative electrodes such as a metal are used in a battery, the maximum benefit of the capacity increase over conventional systems is realized when a high-capacity positive electrode active material is also used. For example, conventional lithium-intercalating oxides (e.g., LiCoO.sub.2, LiNi.sub.0.8Co.sub.0.15Al.sub.0.05O.sub.2, Li.sub.1.1Ni.sub.0.3Co.sub.0.3Mn.sub.0.3O.sub.2) are typically limited to a theoretical capacity of .about.280 mAh/g (based on the mass of the lithiated oxide) and a practical capacity of 180 to 250 mAh/g, which is quite low compared to the specific capacity of lithium metal, 3863 mAh/g. The highest theoretical capacity achievable for a lithium-ion positive electrode is 1794 mAh/g (based on the mass of the lithiated material), for Li.sub.2O. Other high-capacity materials include BiF.sub.3 (303 mAh/g, lithiated), FeF.sub.3 (712 mAh/g, lithiated), and others. Unfortunately, all of these materials react with lithium at a lower voltage compared to conventional oxide positive electrodes, hence limiting the theoretical specific energy. Nonetheless, the theoretical specific energies are still very high (>800 Wh/kg, compared to a maximum of .about.500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes, which may enable an electric vehicle to approach a range of 300 miles or more on a single charge.
"While metal-oxygen batteries can be used in a wide range of applications, using the metal-oxygen batteries to provide power to electric and hybrid vehicles is one area of particular interest. FIG. 1 depicts a chart 2 showing the range achievable for a vehicle using battery packs of different specific energies versus the weight of the battery pack. In the chart 2, the specific energies are for an entire cell, including cell packaging weight, assuming a 50% weight increase for forming a battery pack from a particular set of cells. The
"Various lithium-based chemistries have been investigated for use in various applications including in vehicles. FIG. 2 depicts a chart 4 that identifies the specific energy and energy density of various lithium-based chemistries. In the chart 4, only the weight of the active materials, current collectors, binders, separator, and other inert material of the battery cells are included. The packaging weight, such as tabs, the cell can, etc., are not included. As is evident from the chart 4, lithium/oxygen batteries, even allowing for packaging weight, are capable of providing a specific energy >600 Wh/kg and thus have the potential to enable driving ranges of electric vehicles of more than 300 miles without recharging, at a similar cost to typical lithium ion batteries. While lithium/oxygen cells have been demonstrated in controlled laboratory environments, a number of issues remain before full commercial introduction of a lithium/oxygen cell is viable as discussed further below.
"A typical lithium/oxygen electrochemical cell 10 is depicted in FIG. 3. The cell 10 includes a negative electrode 14, a positive electrode 22, and a porous separator 18. The negative electrode 14 is typically metallic lithium. The positive electrode 22 includes electrode particles such as particles 26 possibly coated in a catalyst material (such as Au or Pt) and suspended in a porous, electrically conductive matrix 30. An electrolyte solution 34 containing a salt such as LiPF6 dissolved in an organic solvent such as dimethoxyethane or CH3CN permeates both the porous separator 18 and the positive electrode 22. The LiPF6 provides the electrolyte with an adequate ionic conductivity which reduces the internal electrical resistance of the cell 10 to enable a high power capacity.
"A portion of the positive electrode 22 is enclosed by a barrier 38. The barrier 38 in FIG. 3 is configured to allow oxygen from an external source 42 to enter the positive electrode 22 while filtering undesired components such as contaminant gases and fluids. The wetting properties of the positive electrode 22 prevent the electrolyte 34 from leaking out of the positive electrode 22. Alternatively, the removal of contaminants from an external source of oxygen, and the retention of cell components such as volatile electrolyte, may be carried out separately from the individual cells. Oxygen from the external source 42 enters the positive electrode 22 through the barrier 38 while the cell 10 discharges and oxygen exits the positive electrode 22 through the barrier 38 as the cell 10 is charged. In operation, as the cell 10 discharges, oxygen and lithium ions are believed to combine to form a discharge product Li.sub.2O.sub.2 or Li.sub.2O in accordance with the following relationship:
"The positive electrode 22 in a typical cell 10 is a lightweight, electrically conductive material which has a porosity of greater than 80% to allow the formation and deposition/storage of Li.sub.2O.sub.2 in the positive electrode volume. The ability to deposit the Li.sub.2O.sub.2 directly determines the maximum capacity of the cell. In order to realize a battery system with a specific energy of 600 Wh/kg or greater, a plate with a thickness of 100 .mu.m should have a capacity of 15 mAh/cm2 or more.
"Materials that provide the needed porosity include carbon black, graphite, carbon fibers, carbon nanotubes, and other non-carbon materials. There is evidence that each of these carbon structures undergo an oxidation process during charging of the cell, due at least in part to the harsh environment in the cell (possibly pure oxygen, superoxide and peroxide ions and/or species, formation of solid lithium peroxide on the positive electrode surface, and electrochemical oxidation potentials of >3V (vs. Li/Li+)).
"A number of investigations into the problems associated with Li-oxygen batteries have been conducted as reported, for example, by Beattie, S.,
"While some issues have been investigated, several challenges remain to be addressed for lithium-oxygen batteries. These challenges include limiting dendrite formation at the lithium metal surface, protecting the lithium metal (and possibly other materials) from moisture and other potentially harmful components of air (if the oxygen is obtained from the air), designing a system that achieves favorable specific energy and specific power levels, reducing the hysteresis between the charge and discharge voltages (which limits the round-trip energy efficiency), and improving the number of cycles over which the system can be cycled reversibly.
"The limit of round trip efficiency occurs due to an apparent voltage hysteresis as depicted in FIG. 4. In FIG. 4, the discharge voltage 70 (approximately 2.5 to 3 V vs. Li/Li+) is much lower than the charge voltage 72 (approximately 4 to 4.5 V vs. Li/Li+). The equilibrium voltage 74 (or open-circuit potential) of the lithium/oxygen system is approximately 3 V. Hence, the voltage hysteresis is not only large, but also very asymmetric.
"The large over-potential during charge may be due to a number of causes. With reference to FIG. 3, the reaction between the Li.sub.2O.sub.2 and the conducting matrix 30 may form an insulating film between the two materials. Additionally, there may be poor contact between the solid discharge products Li.sub.2O.sub.2 or Li.sub.2O and the electronically conducting matrix 30 of the positive electrode 22. Poor contact may result from oxidation of the discharge product directly adjacent to the conducting matrix 30 during charge, leaving a gap between the solid discharge product and the matrix 30.
"In some cases, poor contact between the discharge product and the conducting matrix 30 leads to a complete disconnection of the solid discharge product. Complete disconnection of the solid discharge product from the conducting matrix 30 may result from fracturing, flaking, or movement of solid discharge product particles due to mechanical stresses that are generated during charge/discharge of the cell. Complete disconnection may contribute to the capacity decay observed for most lithium/oxygen cells. By way of example, FIG. 5 depicts the discharge capacity of a typical Li/oxygen cell over a period of charge/discharge cycles.
"Other physical processes which cause voltage drops within an electrochemical cell, and thereby lower energy efficiency and power output, include mass-transfer limitations at high current densities. The transport properties of aqueous electrolytes are typically better than non-aqueous electrolytes, but in each case mass-transport effects can limit the thickness of the various regions within the cell, including the positive electrode. Reactions among O.sub.2 and other metals besides lithium may also be carried out in various media.
"One problem that reduces the available capacity of lithium-air systems occurs when only a fraction of the positive electrode is utilized before Li+ ions and oxygen cease to combine with each other. By way of example, FIG. 6 depicts the cell 10 after discharge occurs. In the cell 10, carbon particles 28 are fully plated with a discharge product 32, with the remaining carbon particles 26 remaining unplated. The discharge product 32 is typically Li.sub.2O.sub.2. The arrangement of the plated carbon particles 28 proximate to the barrier 38 prevents oxygen from the external source 42 from being transported into the regions of the positive electrode 22 surrounding the unplated carbon particles 26. FIG. 7 depicts another example where the discharge product 32 covers carbon particles 29 that are proximate to the negative electrode 14. The arrangement of plated carbon particles 29 prevents lithium from the negative electrode from penetrating the positive electrode 22.
"The uneven plating of the cell 10 in FIG. 6 is caused, in part, by an uneven distribution of oxygen in the positive electrode 22. Oxygen is introduced into the positive electrode 22 through the barrier 38, and then diffuses through the electrolyte 34 towards the porous separator 18. The highest concentration of oxygen is near the barrier 38, reducing to a lower concentration at a boundary 46 between the positive electrode 22 and the porous separator 18. Moreover, electrons are supplied to the positive electrode 22 at a location proximate to the barrier 38. Oxygen, electrons, and Li.sup.+ ions, which are available from the electrolyte 34, react with each other rapidly. The rapid reaction quickly plates carbon particles 28 near barrier 38 with non-conductive discharge product 32. Oxygen diffusion into the positive electrode 22 through barrier 38 is impeded by the plated particles 28, and this can prevent the cell 10 from fully discharging.
"In FIG. 7, the presence of Li.sup.+ ions results in the discharge product 32 quickly plating the particles 29. Once plated, the particles 29 impede additional lithium from the negative electrode 14 from entering the positive electrode 22 to react with oxygen and plate the remaining particles in the conductive matrix 30.
"Another important challenge for Li-oxygen batteries, and metal/air batteries more generally, is that when the discharge product has a fixed composition (i.e., does not make use of the alloy or intercalation reaction mechanisms) and is completely insoluble or nearly insoluble in the electrolyte, a non-uniform current distribution results in a non-uniform product distribution that can lead to pore clogging and thereby low capacity, energy, and power, and possibly introduce safety problems.
"What is needed therefore is a battery that permits oxygen and lithium to combine more uniformly throughout the positive electrode. What is further needed is a battery where the distribution of electrical current in the positive electrode is more balanced than prior art devices."
As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventors' summary information for this patent application: "In accordance with one embodiment, an electrochemical cell includes a negative electrode including a form of lithium, a positive electrode spaced apart from the negative electrode and configured to use a form of oxygen as a reagent, a separator positioned between the negative electrode and the thick positive electrode, and an electrolyte including a salt concentration of less than 1 molar filling or nearly filling the positive electrode.
"In accordance with another embodiment, a method of forming an electrochemical cell with an improved product distribution includes forming a negative electrode including a form of lithium, forming a thick positive electrode configured to use a form of oxygen as a reagent, forming a separator such that when assembled, the separator is positioned between the negative electrode and the thick positive electrode, and inserting an electrolyte including a salt concentration of less than 1 molar in the positive electrode.
"In a further embodiment, a method for producing a uniform deposition of a reaction product in a metal/air cell having composition and potential that do not change significantly with the degree of discharge in the cell includes at least one of (a) controlling of the electrolyte ionic impedance, (b) adjusting the oxygen concentration and pressure, and the overall gas flow rate, © forming a porosity gradient in an electrode structure, (d) forming an electrical conductivity gradient in an electrode, (e) adjusting an ionic conductivity of an electrolyte, and (f) controlling an electric current level during a charge and discharge cycle of the cell.
BRIEF DESCRIPTION OF DRAWINGS
"FIG. 1 depicts a prior art plot showing the relationship between battery weight and vehicular range for various specific energies;
"FIG. 2 depicts a prior art chart of the specific energy and energy density of various lithium-based cells;
"FIG. 3 depicts a prior art flooded lithium-oxygen cell including two electrodes and an electrolyte with a 1 molar concentration of salt in a charged state;
"FIG. 4 depicts a prior art discharge and charge curve for a typical Li/oxygen electrochemical cell; and
"FIG. 5 depicts a plot showing decay of the discharge capacity for a typical Li/oxygen electrochemical cell over a number of cycles;
"FIG. 6 depicts the prior art lithium-oxygen cell of FIG. 3 in a discharged state when the discharge reaction occurs primarily at the positive electrode/current collector boundary;
"FIG. 7 depicts the prior art lithium-oxygen cell of FIG. 3 in a discharged state when the discharge reaction occurs primarily at the separator/positive electrode boundary;
"FIG. 8 depicts a schematic view of a metal-oxygen cell with two electrodes and a separator, with the positive electrode being flooded and containing an electrolyte having a concentration of a salt of less than one molar, to increase the uniformity of distribution and removal of a discharge product during operation of the cell;
"FIG. 9 depicts a relationship between a molar concentration of salt in an electrolyte with the ionic conductivity of the electrolyte;
"FIG. 10 depicts the metal-oxygen cell of FIG. 3 after being discharged where the positive electrode is more uniformly plated with discharge product; and
"FIG. 11 depicts a process of forming an electrochemical cell including an electrolyte having a salt concentration of less than one molar;
"FIG. 12 depicts a schematic view of a metal-oxygen cell with two electrodes and a flooded positive electrode having a conductive matrix with a porosity gradient;
"FIG. 13 depicts a schematic view of a metal-oxygen cell with two electrodes and a mixed phase positive electrode having a conductive matrix with a porosity gradient;
"FIG. 14 depicts a block diagram of a battery pack including a plurality of cells and a battery management system;
"FIG. 15 depicts a schematic view of a metal-oxygen cell with a flooded positive electrode and a pump that adjusts a pressure of oxygen in a positive electrode of the cell; and a diffuser that provides an inert gas to the positive electrode; and
"FIG. 16 depicts a schematic view of a metal-oxygen cell with a mixed phase positive electrode and a pump that adjusts a pressure of oxygen in a positive electrode of the cell; and a diffuser that provides an inert gas to the positive electrode."
For additional information on this patent application, see: Albertus, Paul;
Keywords for this news article include: Chemistry, Chalcogens, Electrolytes, Electrochemical,
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