Researchers Submit Patent Application, "Reducing Oxygen and Electrolyte Transport Limitations in the Lithium/Oxygen Battery through Electrode Design and Wetting Control", for Approval
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 Li2O. 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.
"FIG. 1 depicts a chart 10 showing the range achievable for a vehicle using battery packs of different specific energies versus the weight of the battery pack. In the chart 10, 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 20 which identifies the specific energy and energy density of various lithium-based chemistries. In the chart 20, 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 20, 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 50 is depicted in FIG. 3. The cell 50 includes a negative electrode 52, a positive electrode 54, a porous separator 56, and a current collector 58. The negative electrode 52 is typically metallic lithium. The positive electrode 54 includes electrode particles such as particles 60 possibly coated in a catalyst material (such as Au or Pt) and suspended in a porous, electrically conductive matrix 62. An electrolyte solution 64 containing a salt such as LiPF.sub.6 dissolved in an organic solvent such as dimethoxyethane or CH.sub.3CN permeates both the porous separator 56 and the positive electrode 54. The LiPF.sub.6 provides the electrolyte with an adequate conductivity which reduces the internal electrical resistance of the cell 50 to allow a high power.
"A portion of the positive electrode 52 is enclosed by a barrier 66. The barrier 66 in FIG. 3 is configured to allow oxygen from an external source 68 to enter the positive electrode 54 while filtering undesired components such as contaminant gases and fluids. The wetting properties of the positive electrode 54 prevent the electrolyte 64 from leaking out of the positive electrode 54. 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 68 enters the positive electrode 54 through the barrier 66 while the cell 50 discharges and oxygen exits the positive electrode 54 through the barrier 66 as the cell 50 is charged. In operation, as the cell 50 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 54 in a typical cell 50 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 cathode 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/cm.sup.2 or more.
"Materials which 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 undergoes 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 cathode surface, and electrochemical oxidation potentials of >3V (vs. Li/Li.sup.+)).
"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), morphology changes in the metal upon extended cycling that result in a large overall volume change in the cell, changes in the structure and composition of the passivating layer that forms at the surface of the metal when exposed to certain electrolytes, which may isolate some metal and/or increase the resistance of the cell over time. Many of the foregoing are significant hurdles in 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.sup.+) 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. For example, reaction between the Li.sub.2O.sub.2 and the conducting matrix 62 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 62 of the positive electrode 54. Poor contact may result from oxidation of the discharge product directly adjacent to the conducting matrix 62 during charge, leaving a gap between the solid discharge product and the matrix 52.
"Also, complete disconnection of the solid discharge product from the conducting matrix 62 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 nonaqueous electrolytes, but in each case mass-transport effects can limit the thickness of the various regions within the cell, including the cathode. Reactions among O.sub.2 and other metals besides lithium may also be carried out in various media.
"What is needed therefore is a metal/oxygen battery that provides increased oxygen and electrolyte transport within the battery."
As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventors' summary information for this patent application: "In one embodiment a battery system in one embodiment includes a negative electrode, a separator layer adjacent to the negative electrode, and a positive electrode adjacent to the separator layer, the positive electrode including a gas phase and an electrically conductive framework defining at least one wetting channel, the wetting channel configured to distribute an electrolyte within the electrically conductive framework.
"In another embodiment, a method of forming a battery system includes providing a negative electrode, providing a separator layer adjacent to the negative electrode, forming at least one wetting channel within an electrically conductive framework, the wetting channel configured to distribute an electrolyte within the electrically conductive framework, forming a positive electrode adjacent to the separator layer with the electrically conductive framework, providing an electrolyte within the positive electrode, and providing a gas phase along with the electrolyte within the positive electrode.
"In another embodiment, a positive electrode within a battery system includes an electrically conductive framework, an electrolyte, at least one wetting channel defined within the electrically conductive framework, the wetting channel configured to distribute the electrolyte within the electrically conductive framework, and a gas phase.
BRIEF DESCRIPTION OF DRAWINGS
"FIG. 1 depicts a plot showing the relationship between battery weight and vehicular range for various specific energies;
"FIG. 2 depicts a chart of the specific energy and energy density of various lithium-based cells;
"FIG. 3 depicts a prior art lithium-oxygen (Li/oxygen) cell including two electrodes, a separator, and an electrolyte;
"FIG. 4 depicts a discharge and charge curve for a typical Li/oxygen electrochemical cell;
"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 a schematic view of a lithium-oxygen (Li/oxygen) cell with two electrodes, one of which is configured to control the distribution of oxygen and electrolyte within the electrode, in a fully charged state;
"FIG. 7 depicts a schematic view of the lithium-oxygen (Li/oxygen) cell of FIG. 6 in a partially discharged state; and
"FIG. 8 depicts a schematic view of a lithium-oxygen (Li/oxygen) cell with two electrodes, one of which is configured to control the distribution of oxygen and electrolyte within the electrode, in a fully charged state using generally horizontally extending channels."
For additional information on this patent application, see: Albertus, Paul;
Keywords for this news article include: Chemistry, Chalcogens, Electrolytes, Electrochemical,
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