Abstract:
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 including an electron conducting matrix, a separator positioned between the negative electrode and the positive electrode, an electrolyte including a salt, and a charging redox couple located within the positive electrode, wherein the electrochemical cell is characterized by the transfer of electrons from a discharge product located in the positive electrode to the electron conducting matrix by the charging redox couple during a charge cycle.

Description:
TECHNICAL FIELD 
     This invention relates to batteries and more particularly to lithium (Li) based batteries. 
     BACKGROUND 
     A typical Li-ion cell contains a negative electrode, the anode, a positive electrode, the cathode, and a separator region between the negative and positive electrodes. One or both of the electrodes contain active materials that react with lithium reversibly. In some cases the negative electrode may include lithium metal, which can be electrochemically dissolved and deposited reversibly. The separator and positive electrode contain an electrolyte that includes a lithium salt. 
     Charging a Li-ion cell generally entails a generation of electrons at the positive electrode and consumption of an equal amount of electrons at the negative electrode with the electrons 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 opposite reactions occur. 
     Li-ion cells with a Li-metal anode may have a higher specific energy (in Wh/kg) and energy density (in Wh/L) compared to batteries with conventional carbonaceous negative electrodes. This high specific energy and energy density makes incorporation of rechargeable Li-ion cells with a Li-metal anode in energy storage systems an attractive option for a wide range of applications including portable electronics and electric and hybrid-electric vehicles. 
     At the positive electrode of a conventional lithium-ion cell, a lithium-intercalating oxide is typically used. Lithium-intercalating oxides (e.g., LiCoO 2 , LiNi 0.8 Co 0.15 Al 0.05 O 2 , Li 1.1 Ni 0.3 Co 0.3 Mn 0.3 O 2 ) are typically limited to a theoretical capacity of ˜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). 
     Moreover, the low realized capacities of conventional Li-ion cells reduces the effectiveness of incorporating Li-ion cells into vehicular systems. Specifically, a goal for electric vehicles is to attain a range approaching that of present-day vehicles (&gt;300 miles). Obviously, the size of a battery could be increased to provide increased capacity. The practical size of a battery on a vehicle is limited, however, by the associated weight of the battery. Consequently, the Department of Energy (DOE) in the USABC Goals for Advanced Batteries for EVs has set a long-term goal for the maximum weight of an electric vehicle battery pack to be 200 kg (this includes the packaging). Achieving the requisite capacity given the DOE goal requires a specific energy in excess of 600 Wh/kg. 
     Various materials are known to provide a promise of higher theoretical capacity for Li-based cells. For example, a high theoretical specific capacity of 1168 mAh/g (based on the mass of the lithiated material) is shared by Li 2 S and Li 2 O 2 , which can be used as cathode materials. Other high-capacity materials include BiF 3  (303 mAh/g, lithiated) and FeF 3  (712 mAh/g, lithiated) as reported by Amatucci, G. G. and N. Pereira, “Fluoride based electrode materials for advanced energy storage devices,”  Journal of Fluorine Chemistry,  2007. 128(4): p. 243-262. Unfortunately, all of these materials react with lithium at a lower voltage compared to conventional oxide positive electrodes. Nonetheless, the theoretical specific energies are still very high (&gt;800 Wh/kg, compared to a maximum of ˜500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes). 
     One Li-based cell that has the potential of providing a driving range above 300 miles incorporates a lithium metal negative electrode and a positive electrode reacting with oxygen obtained from the environment. The weight of this type of system is reduced since the positive-electrode active material is not carried onboard the vehicle. Practical embodiments of this lithium-air battery may achieve a practical specific energy of 600 Wh/kg because the theoretical specific energy is 11,430 Wh/kg for Li metal, and 3,460 Wh/kg for Li 2 O 2 . 
     During discharge of the lithium-air cell, Li metal dissolves from the negative electrode, while at the positive electrode, lithium ions (Li +  ions) in the electrolyte react with oxygen and electrons to form a solid discharge product that ideally is lithium peroxide (Li 2 O 2 ) or lithium oxide (Li 2 O), which may coat the conductive matrix of the positive electrode and/or fill the pores of the electrode. In an electrolyte that uses a carbonate solvent the discharge products may include Li 2 CO 3 , Li alkoxides, and Li alkyl carbonates. In non-carbonate solvents such as CH 3 CN and dimethyl ether the discharge products are less likely to react with the solvent. The pure crystalline forms of Li 2 O 2  and Li 2 O are electrically insulating, so that electronic conduction through these materials will need to involve vacancies, grains, or dopants, or short conduction pathways obtained through appropriate electrode architectures. 
     Abraham and Jiang published one of the earliest papers on the “lithium-air” system. See Abraham, K. M. and Z. Jiang, “A polymer electrolyte-based rechargeable lithium/oxygen battery”;  Journal of the Electrochemical Society,  1996. 143(1): p. 1-5. Abraham and Jiang used an organic electrolyte and a positive electrode with an electrically conductive carbon matrix containing a catalyst to aid with the reduction and oxidation reactions. Previous lithium-air systems using an aqueous electrolyte have also been considered, but without protection of the Li metal anode, rapid hydrogen evolution occurs. See Zheng, J., et al., “Theoretical Energy Density of Li-Air Batteries”;  Journal of the Electrochemical Society,  2008. 155: p. A432. 
     An electrochemical cell  10  is depicted in  FIG. 1 . The cell  10  includes a negative electrode  12 , a positive electrode  14 , a porous separator  16 , and a current collector  18 . The negative electrode  12  is typically metallic lithium. The positive electrode  14  includes carbon particles such as particles  20  possibly coated in a catalyst material (such as Au or Pt) and suspended in a porous, electrically conductive matrix  22 . An electrolyte solution  24  containing a salt such at LiPF 6  dissolved in an organic solvent such as dimethyl ether or CH 3 CN permeates both the porous separator  16  and the positive electrode  14 . The LiPF 6  provides the electrolyte with an adequate conductivity which reduces the internal electrical resistance of the cell  10  to allow a high power. 
     The positive electrode  12  is enclosed by a barrier  26 . The barrier  26  in  FIG. 1  is formed from an aluminum mesh configured to allow oxygen from an external source  28  to enter the positive electrode  14 . The wetting properties of the positive electrode  14  prevent the electrolyte  24  from leaking out of the positive electrode  14 . Oxygen from the external source  28  enters the positive electrode  14  through the barrier  26  while the cell  10  discharges, and oxygen exits the positive electrode  14  through the barrier  26  as the cell  10  is charged. In operation, as the cell  10  discharges, oxygen and lithium ions combine to form a discharge product such as Li 2 O 2  or Li 2 O. 
     A number of investigations into the problems associated with Li-air batteries have been conducted as reported, for example, by Beattie, S., D. Manolescu, and S. Blair, “High-Capacity Lithium-Air Cathodes,”  Journal of the Electrochemical Society,  2009. 156: p. A44, Kumar, B., et al., “A Solid-State, Rechargeable, Long Cycle Life Lithium-Air Battery, ”  Journal of the Electrochemical Society,  2010. 157: p. A50, Read, J., “Characterization of the lithium/oxygen organic electrolyte battery,”  Journal of the Electrochemical Society,  2002. 149: p. A1190, Read, J., et al., “Oxygen transport properties of organic electrolytes and performance of lithium/oxygen battery,”  Journal of the Electrochemical Society,  2003. 150: p. A1351, Yang, X and Y. Xia, “The effect of oxygen pressures on the electrochemical profile of lithium/oxygen battery,”  Journal of Solid State Electrochemistry:  p. 1-6, and Ogasawara, T., et al., “Rechargeable Li 2 O 2  Electrode for Lithium Batteries,”  Journal of the American Chemical Society,  2006. 128(4): p. 1390-1393. Nonetheless, several challenges remain to be addressed for lithium-air 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, 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. 2 . In  FIG. 2 , the discharge voltage  40  (approximately 2.5 to 3 V vs. Li/Li + ) is much lower than the charge voltage  42  (approximately 4 to 4.5 V vs. Li/Li + ). 
     The equilibrium voltage  44  (or open-circuit potential) of the lithium/air 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 2 O 2  and the conducting matrix  22  may form an insulating film between the two materials. Additionally, there may be poor contact between the solid discharge products Li 2 O 2  or Li 2 O and the electronically conducting matrix  22  of the positive electrode  14 . Poor contact may result from oxidation of the discharge product directly adjacent to the conducting matrix  22  during charge, leaving a gap between the solid discharge product and the matrix  22 . 
     Another mechanism resulting in poor contact between the solid discharge product and the matrix  22  is complete disconnection of the solid discharge product from the conducting matrix  22 . Complete disconnection of the solid discharge product from the conducting matrix  22  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/air cells. By way of example,  FIG. 3  depicts the discharge capacity of a typical Li/air cell over a period of charge/discharge cycles. 
     What is needed therefore is an energy storage system that can recover disconnected and or poorly connected discharge particles electrochemically. A further need exists for a lithium based energy storage system that exhibits reduced over-potential of the cell during charging operations. 
     SUMMARY 
     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 including an electron conducting matrix, a separator positioned between the negative electrode and the positive electrode, an electrolyte including a salt, and a charging redox couple located within the positive electrode, wherein the electrochemical cell is characterized by the transfer of electrons from a discharge product located in the positive electrode to the electron conducting matrix by the charging redox couple during a charge cycle. 
     In a further embodiment, an electrochemical cell includes a negative electrode, a positive electrode spaced apart from the negative electrode and including an electron conducting matrix, a separator positioned between the negative electrode and the positive electrode, an electrolyte including a salt, and a charging redox couple located within the positive electrode, wherein the electrochemical cell is characterized by the transfer of electrons from an electrically insulating discharge product located in the positive electrode to the electron conducting matrix by the charging redox couple during a charge cycle. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  depicts a schematic view of a prior art lithium-ion cell including two electrodes and an electrolyte; 
         FIG. 2  depicts a discharge and charge curve for a typical Li/air electrochemical cell; 
         FIG. 3  depicts a plot showing decay of the discharge capacity for a typical Li/air electrochemical cell over a number of cycles; 
         FIG. 4  depicts a schematic view of a lithium-air (Li/air) cell with two electrodes and a reservoir configured to exchange oxygen with a positive electrode for a reversible reaction with lithium which includes a concentration of charging redox couples which function as electron shuttles during charging of the Li/air cell; 
         FIG. 5  depicts a schematic view of the Li/air cell of  FIG. 4  with discharge products formed on the conductive matrix of the positive electrode and some disconnected discharge product located on the bottom of the positive electrode; and 
         FIG. 6  depicts a schematic view of the Li/air cell of  FIG. 5  with gaps formed between the discharge products formed on the conductive matrix and the conductive matrix as a result of charging or discharging the Li/air cell. 
     
    
    
     DETAILED DESCRIPTION 
     A schematic of an electrochemical cell  100  is shown in  FIG. 4 . The electrochemical cell  100  includes a negative electrode  102  separated from a positive electrode  104  by a porous separator  106 . The negative electrode  102  may be formed from lithium metal or a lithium-insertion compound (e.g., graphite, silicon, tin, LiAl, LiMg, Li 4 Ti 5 O 12 ), although Li metal affords the highest specific energy on a cell level compared to other candidate negative electrodes. 
     The positive electrode  104  in this embodiment includes a current collector  108  and carbon particles  110 , optionally covered in a catalyst material, suspended in a porous matrix  112 . The porous matrix  112  is an electrically conductive matrix formed from a conductive material such as conductive carbon or a nickel foam, although various alternative matrix structures and materials may be used. The separator  106  prevents the negative electrode  102  from electrically connecting with the positive electrode  104 . 
     The electrochemical cell  100  includes an electrolyte solution  114  present in the positive electrode  104  and in some embodiments in the separator  106 . In the exemplary embodiment of  FIG. 4 , the electrolyte solution  114  includes a salt, LiPF 6  (lithium hexafluorophosphate), dissolved in an organic solvent mixture. The organic solvent mixture may be any desired solvent. In certain embodiments, the solvent may be dimethyl ether (DME), acetonitrile (MeCN), ethylene carbonate, or diethyl carbonate. 
     A barrier  116  separates the positive electrode  104  from a reservoir  118 . The reservoir  118  may be the atmosphere or any vessel suitable to hold oxygen and other gases supplied to and emitted by the positive electrode  104 . While the reservoir  118  is shown as an integral member of the electrochemical cell  100  attached to the positive electrode  104 , alternate embodiments could employ a hose or other conduit to place the reservoir  118  in fluid communication with positive electrode  104 . Various embodiments of the reservoir  118  are envisioned, including rigid tanks, inflatable bladders, and the like. In  FIG. 4 , the barrier  116  is a mesh which permits oxygen and other gases to flow between the positive electrode  104  and the reservoir  118  while also preventing the electrolyte  114  from leaving the positive electrode  104 . 
     The electrochemical cell  100  may discharge with lithium metal in the negative electrode  102  ionizing into a Li +  ion with a free electron e − . Li +  ions travel through the separator  106  in the direction indicated by arrow  120  toward the positive electrode  104 . Oxygen is supplied from the reservoir  118  through the barrier  116  as indicated by the arrow  122 . Free electrons e −  flow into the positive electrode  104  through the current collector  108  as indicated by arrow  124 . 
     With reference to  FIG. 5 , the oxygen atoms and Li +  ions within the positive electrode  102  form a discharge product  130  inside the positive electrode  104 , aided by the optional catalyst material on the carbon particles  110 . As seen in the following equations, during the discharge process metallic lithium is ionized, combining with oxygen and free electrons to form Li 2 O 2  or Li 2 O discharge product that may coat the surfaces of the carbon particles  110 . 
     
       
                 
         
             
             
         
      
     
     As discharge continues, some of the discharge product  130  may flake off or in some other way become dislodged from the carbon particles  110  as depicted by the disconnected discharge product  132 . 
     When desired, the electrochemical cell  100  may be charged from the discharged state. Electrochemical cell  100  may be charged by introducing an external electric current which oxidizes the Li 2 O and Li 2 O 2  discharge products into lithium and oxygen. The internal current drives lithium ions toward the negative electrode  102  where the Li +  ions are reduced to metallic lithium, and generates oxygen which diffuses through the barrier  116 . The charging process reverses the chemical reactions of the discharge process, as shown in the following equations. 
     
       
                 
         
             
             
         
      
     
     The discharge products  130  in the form of Li 2 O and Li 2 O 2  donate electrons according to the foregoing equations which are transported to the current collector  108  by the electrically conductive matrix  112 . This reaction may occur most rapidly with the discharge products  130  immediately adjacent to the particles  110  resulting in a gap  134  as depicted in  FIG. 6 . In some instances, the gap  134  may electrically isolate the discharge products  130  from the electrically conductive matrix  112 . In other instances, the gap  134  enables portions of the discharge product  130  to flake off, resulting in an increase in the disconnected discharge product  132 . 
     A gap  134  may also form as a result of charging a cell. By way of example, the Li 2 O 2  adjacent to the electronically conducting matrix may react first due to the low electronic conductivity of Li 2 O 2 , thereby liberating O 2 , Li+, and electrons and leaving a gap between the conducting matrix and the remaining Li 2 O 2 . 
     Regardless of the mechanism by which a disconnected discharge product  132  or poorly connected discharge product  130  is formed, reduction of the disconnected discharge products  132  and the poorly connected discharge products  130  in the electrochemical cell  100  is enabled by the electrolyte solution  114 . Specifically, the electrolyte solution  114  includes a charging redox couple which scavenges electrons from the discharge products  132  and the discharge products  130  and transports the electrons to the electrically conductive matrix  112  whereat the charging redox couple is oxidized as shown in the following equations:
 
Li 2 O 2 +2R→O 2 +2Li + +2R −  (discharge products)
 
Li 2 O+2R→1/2O 2 +2Li + +2R (discharge products)
 
2R − →2R+2e −  (conductive matrix)
 
     Once the charging redox couple has been oxidized, it is available to transport additional electrons from additional discharge products  132  and discharge products  130 . Nonetheless, to provide optimal performance of the charging redox couple, the selected charging redox couple may exhibit a high solubility in the electrolyte solution  114  to ensure that a sufficient concentration of the charging redox couple is present in the electrolyte solution  114  to function as a rapid redox shuttle between the discharge product  132 , the discharge products  130 , and the electrically conductive matrix  112 . When provided as an additive in the electrolyte solution  114 , the charging redox couple is typically selected such that the charging redox couple does not react with the electrolyte, binder, separator, negative electrode, or current collectors. In one embodiment, the charging redox couple is a minor constituent of the electrolyte so that it does not adversely affect the transport properties of the electrolyte. 
     Performance of the electrochemical cell  100  is further optimized by proper selection of the equilibrium voltage of the charging redox couple. For example, the equilibrium voltages for Li 2 O 2  and Li 2 O are, respectively, 2.96 and 2.91 V. Thus, selecting an equilibrium voltage for the charging redox couple that is slightly above 2.96 V, such as between 3 and 3.1 V, limits the over-potential during cell charge. 
     Exemplary classes of compounds that could be used as a charging redox couple in the electrochemical cell  100  include, but are not limited to, metallocenes (e.g., ferrocene), halogens (e.g., I-/I3-), and aromatic molecules (e.g., tetramethylphenylenediamine). Some specific materials within the foregoing classes which are suitable for use in a Li/air cell with an equilibrium voltage between 2.9 and 4.5 V include Ferrocene which has an equilibrium voltage between 3.05 to 3.38 V, n-Butylferrocene which has an equilibrium voltage between 3.18 to 3.5 V, N,N-Dimethylaminomethylferrocene which has an equilibrium voltage between 3.13 to 3.68 V, 1,1-Dimethylferrocene which has an equilibrium voltage between 3.06 to 3.34 V, 1,2,4-Triazole, sodium salt (NaTAZ) which has an equilibrium voltage of 3.1 V, and Lithium squarate which has an equilibrium voltage of about 3.1 V. 
     For a given embodiment, the charging redox couple may be selected to provide high reversibility approaching 100% coulombic efficiency. A highly reversible charging redox couple is desirable to allow the charging redox couple to be cycled many times during a single cell charging step. A charging redox couple that exhibits fast kinetics (i.e., its exchange current density is high) is also desirable. Fast kinetics results in a small difference between the charging redox couple&#39;s charge and discharge voltage, resulting in more efficient charging. 
     As described above, the charging redox couple activity is confined to the positive electrode. Therefore, in contrast to overvoltage redox couples, used to provide overvoltage protection, which require high mobility to travel between the positive electrode and the negative electrode, a high mobility is not necessary for a charging redox couple. For example, while movement on the order of 10 s of μm is needed in providing overvoltage protection, the charging redox couples in the electrolyte solution  114  may travel about 1 μm or less. 
     If desired, a charging redox couple with high mobility may be used to function as a rapid redox shuttle between the discharge product  132 , the discharge products  130 , and the electrically conductive matrix  112 . Because the high mobility charging redox couple, if unconstrained, may also be reduced at the negative electrode, transport of the oxidized species to the negative electrode may be blocked by applying a protective layer on the negative electrode. The charging redox couple is thus confined to the positive electrode and the separator. One material that may be used as a protective layer is Li 1.3 Ti 1.7 Al 0.3 (PO 4 ) 3 , a lithium-ion conducting glass-ceramic material commercially available from Ohara Corporation of Rancho Santa Margarita, Calif. 
     By incorporation of an optimally selected charging redox couple, the over-potential of the electrochemical cell  100  during charging is lowered. By way of example, for an exemplary electrochemical cell  100  which has discharge products  130  and disconnected discharge products  134  of Li 2 O 2  or Li 2 O, the equilibrium voltage of the discharge products  130  and disconnected discharge products  134  is about 2.9 to 3 V. By selecting a charging redox couple (R/R−), wherein species R− is the reduced form of species R) with an equilibrium voltage of 3.1 V, all of the charging redox couple will be in a reduced form (species R−) during discharge, when the cell voltage is below the equilibrium voltage of the discharge product. 
     During charge of the exemplary electrochemical cell  100 , as the potential of the positive electrode with respect to Li/Li+ climbs above 3.1 V, the reduced species R− will be oxidized at the surface of the conducting matrix  112  to form species R. Species R can then react with the discharge product Li 2 O 2  or Li 2 O (chemically or via a corrosion reaction) to form species R−, Li+, and O 2 , because the discharge product  130  and disconnected discharge product  134  have an equilibrium voltage lower than that of the charging redox couple. The freshly formed species R− can subsequently yield its charge to the conducting matrix  112 , while the liberated Li+ can migrate toward the negative electrode  102 , where it is plated as Li metal. 
     Accordingly, even poorly connected discharge product  130  or disconnected discharge product  134  can be consumed electrochemically during charge at a voltage only slightly above that of the charging redox couple. Assuming a discharge voltage of 2.8 V, reducing the charge voltage from ˜4 V to ˜3.2 V could yield an improvement in energy efficiency from 70% to more than 87%. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. Only the preferred embodiments have been presented and all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.