No assignee for this patent application has been made.
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.
"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 dimethyl ether 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 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 must have a capacity of about 20 mAh/cm.sup.2.
"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 undergo an oxidation process during charging of the cell, due at least in part to the harsh environment in the cell (pure oxygen, superoxide and peroxide ions, 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 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 acceptable 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.sup.+) is much lower than the charge voltage 72 (approximately 4 to 4.5 V vs. Li/Li.sup.+). 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.
"Another mechanism resulting in poor contact between the solid discharge product and the matrix 62 is complete disconnection of the solid discharge product from the conducting matrix 62. 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 may also be carried out in various media.
"While the above described challenges are not insignificant, the most common failure mode for cells with Li metal anodes is that of dendrite growth and increase in electrode surface area. Needle-like dendrites can grow through the separator during charging of the cell, resulting in an internal short. 'Soft shorts' that burn out rapidly result in a temporary self discharge of the cell, while 'strong shorts' consisting of a higher, more stable contact area can lead to complete discharge of the cell, cell failure, and possibly thermal runaway.
"While dendrites grow through the separator during charge, shorts can sometimes develop during discharge depending on the external pressure placed on the cell and/or internal volume changes that occur in both the negative and positive electrodes. Because Li metal is highly electronically conductive, surfaces tend to roughen as the metal is plated and stripped. Peaks in the surface grow as dendrites during charge. While the surface is smoothed during discharge, some roughness typically remains at the end of discharge and, depending on the depth of discharge, the overall roughness can be amplified from one cycle to the next.
"Because the metal is essentially at the same electrochemical potential throughout, potential and, to a lesser extent, concentration gradients in the electrolyte phase drive the change in morphology. Previous Li dendrite growth modeling work has shown that the moving front of a dendrite tends to accelerate during cell charge due to the higher current density localized at the dendrite tip relative to its base. Application of thermodynamic models has shown that dendrite initiation (i.e., initial roughening of an almost perfectly smooth surface) can be suppressed by applying mechanical stress and selecting solid electrolytes with shear moduli on the order of 10 GPa at room temperature. The same models indicate that surface tension at metal-fluid interfaces is insufficient to suppress dendrite initiation.
"Related to dendrite initiation and growth is development of the Li morphology, which tends to increase the electrode surface area with cycling and consumes solvent to generate fresh passivation layers. Formation of high-surface-area mossy Li tends to occur during low-rate deposition from a liquid electrolyte, especially if the salt concentration is high. The high surface area combined with high reactivity of Li and flammability of the organic solvent makes for a very reactive and dangerous cell.
"Because of the enormous challenge involved in stabilizing the Li surface chemically and mechanically through the use of electrolyte additives, such that passivation remains in effect over hundreds to thousands of cycles, the preferred treatment for rechargeable Li-based cells is the use of a solid-electrolyte membrane that is mechanically robust and chemically stable against both electrodes. Such a barrier removes several simultaneous constraints that the liquid electrolyte otherwise must satisfy, but the requirements for its properties are nonetheless multifaceted and challenging to obtain in a single material. The barrier must be chemically stable with respect to some or all of the following: the liquid electrolyte in the positive electrode, electronic conductors and catalysts in the positive electrode, the metallic Li negative electrode, reactive species such as oxygen molecules and reaction intermediates, and (in aqueous cells) water.
"Solid electrolytes must also have sufficient Li+ conductivity over the operating temperature range of the cell, negligible electronic conductivity, and high elastic modulus to prevent Li dendrite initiation.
"One approach which uses a solid barrier involves the use of a poorly conducting amorphous material known as LiPON, which has been used successfully in thin film lithium-metal batteries. However, because of LiPON's low lithium conductivity, it is difficult to make cells with thick, high capacity electrodes and still maintain a desired rate of discharge.
"Another approach involves the use of a block copolymer that includes lithium-conducting channels in a matrix of inactive polymer that has a high shear modulus, perhaps high enough to prevent lithium dendrite formation. This approach has several drawbacks: 1) the composite conductivity is too low at room temperature because the intrinsic conductivity of the conducting phase is low, and the high-shear-modulus phase does not conduct lithium ions, thus diluting the composite conductivity further; 2) polymers generally absorb liquids and therefore are not an effective barrier between lithium metal and liquid electrolytes in the positive electrode or separator. Hence, lithium-metal cells with such polymer electrolytes are typically used without any liquid electrolyte in the positive electrode, and instead the positive electrode must contain polymer electrolyte in order to provide a conducting network for lithium ions. All such cells must be operated at high temperature (70.degree. C. or higher) in order to achieve desired performance (energy density and power density).
"A related approach that has been proposed recently is to embed grains of lithium-conducting ceramic or glass inside a conducting polymer (or block copolymer as described above). The candidate ceramics or glasses tend to have at least an order of magnitude higher ionic conductivity than the polymer matrix. Moreover, they provide some additional mechanical stiffness to the composite, which may be sufficient to prevent lithium dendrite initiation. Proposers of this approach hypothesize that lithium will conduct more rapidly through the ceramic or polymer grains, thereby raising the conductivity of the composite to an acceptable level while simultaneously improving its mechanical properties. However, this approach still does not prevent transport of liquids from the positive electrode to the negative electrode, and is therefore unlikely to provide, on its own, a solution for high-energy-density, high-power rechargeable lithium-metal batteries.
"What is needed therefore is an inexpensive, robust, lightweight protection against dendrites formed in a cell. It would be beneficial if a solution to dendrite formation was a relatively thin (
As a supplement to the background information on this patent application, NewsRx 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, a separator positioned between the negative electrode and the positive electrode, and a first lithium ion conducting and ionically insulating composite solid electrolyte layer positioned between the negative electrode and the positive electrode.
"In another embodiment, a method of forming an electrochemical cell includes positioning a separator between a negative electrode including a form of lithium and a positive electrode, and positioning a first lithium ion conducting and ionically insulating composite solid electrolyte layer between the negative electrode and the positive electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
"The above-described features and advantages, as well as others, should become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying figures in which:
"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; and
"FIG. 6 depicts a schematic view of a lithium-oxygen (Li/oxygen) cell with two electrodes and a reservoir configured to exchange oxygen with a positive electrode for a reversible reaction with lithium which includes a solid electrolyte composite."
For additional information on this patent application, see:
Keywords for this news article include: Cell Surface Extensions, Chalcogens, Chemistry, Dendrites, Electrochemical, Electrolytes, Inorganic Chemicals, Neurons, Patents.
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