Abstract:
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.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 61/670,461, filed on Jul. 11, 2012, the entire contents of which are herein incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to batteries and more particularly to metal/oxygen based batteries. 
       BACKGROUND 
       [0003]    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. 
         [0004]    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. 
         [0005]    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 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. 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 3  (303 mAh/g, lithiated), FeF 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 (&gt;800 Wh/kg, compared to a maximum of ˜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. 
         [0006]      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 U.S. Department of Energy has established a weight limit of 200 kg for a battery pack that is located within a vehicle. Accordingly, only a battery pack with about 600 Wh/kg or more can achieve a range of 300 miles. 
         [0007]    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 &gt;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. 
         [0008]    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 6  dissolved in an organic solvent such as dimethoxyethane or CH 3 CN permeates both the porous separator  56  and the positive electrode  54 . The LiPF 6  provides the electrolyte with an adequate conductivity which reduces the internal electrical resistance of the cell  50  to allow a high power. 
         [0009]    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 2 O 2  or Li 2 O in accordance with the following relationship: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0010]    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 2 O 2  in the cathode volume. The ability to deposit the Li 2 O 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 μm should have a capacity of 15 mAh/cm 2  or more. 
         [0011]    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 &gt;3V (vs. Li/Li + )). 
         [0012]    A number of investigations into the problems associated with Li-oxygen 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. 
         [0013]    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. 
         [0014]    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. 
         [0015]    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  62  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  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 . 
         [0016]    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. 
         [0017]    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 2  and other metals besides lithium may also be carried out in various media. 
         [0018]    What is needed therefore is a metal/oxygen battery that provides increased oxygen and electrolyte transport within the battery. 
       SUMMARY 
       [0019]    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. 
         [0020]    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. 
         [0021]    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 
         [0022]      FIG. 1  depicts a plot showing the relationship between battery weight and vehicular range for various specific energies; 
           [0023]      FIG. 2  depicts a chart of the specific energy and energy density of various lithium-based cells; 
           [0024]      FIG. 3  depicts a prior art lithium-oxygen (Li/oxygen) cell including two electrodes, a separator, and an electrolyte; 
           [0025]      FIG. 4  depicts a discharge and charge curve for a typical Li/oxygen electrochemical cell; 
           [0026]      FIG. 5  depicts a plot showing decay of the discharge capacity for a typical Li/oxygen electrochemical cell over a number of cycles; 
           [0027]      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; 
           [0028]      FIG. 7  depicts a schematic view of the lithium-oxygen (Li/oxygen) cell of  FIG. 6  in a partially discharged state; and 
           [0029]      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. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    A schematic of an electrochemical cell  100  is shown in  FIG. 6 . 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. Other metals may also be used to form the negative electrode, such as Zn, Mg, Na, Fe, Al, Ca, Si, and others. 
         [0031]    The positive electrode  104  in this embodiment includes a current collector  108  and an electrically conductive framework  110 . The electrically conductive framework  110  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 electrically conductive framework  110  defines wetting channels  112  and non-wetting channels  113 . The separator  106  prevents the negative electrode  102  from electrically connecting with the positive electrode  104 . 
         [0032]    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. 6 , 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 dimethoxyethane (DME), acetonitrile (MeCN), ethylene carbonate, or diethyl carbonate. 
         [0033]    A barrier  116  separates the positive electrode  104  from a reservoir  118 . The reservoir  118  may be any vessel suitable to hold oxygen supplied to and emitted by the positive electrode  104  or even the atmosphere. While the reservoir  118  is shown as an integral member of the electrochemical cell  100  attached to the positive electrode  104 , in one embodiment the reservoir  118  is the positive electrode  104  itself. Various embodiments of the reservoir  118  are envisioned, including rigid tanks, inflatable bladders, and the like. In  FIG. 6 , 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 . Alternatively, the retention of cell components such as volatile electrolyte may be carried out separately from the individual cells, such that the barrier  116  is not required. 
         [0034]    In the case in which the metal is Li, the electrochemical cell  100  discharges 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 . The Li +  ions travel within the wetting channels  112  and are dispersed throughout the electrically conductive framework  110 . 
         [0035]    The wetting channels  112  are configured to achieve uniform wetting of the electrically conductive framework  110  with the electrolyte  106 . In one embodiment, the surfaces of the electronically conductive framework  110  are provided with a surface treatment to provide the desired wetting. Mixtures of materials with different surface treatments, more wetting and less wetting, are included as desired to encourage the segregation of electrolyte into the wetting channels  112 . In one embodiment, all or part of the electronically conductive framework  110  is subjected to heat treatment in reductive gas. In other embodiments, fluorination, and/or silanation is used. Silanation with organosilanes (R n —Si(OR′) 4-n ) allows for a wide range of surface chemistries to be realized for this purpose. 
         [0036]    In some embodiments including a non-aqueous electrolyte  114 , non-polar surfaces are incorporated. An example of a suitable material is graphene, which is both electronically conductive and very nonpolar. 
         [0037]    While the embodiment of  FIG. 6  shows a uniform pattern of wetting channels  112 , in some embodiments the pattern and/or the surface treatment of the electronically conductive framework  110  is varied. Such variation is used in applications wherein certain portions of the electrode  104  tend to flood, and in embodiments wherein certain portions of the electrode  104  tend to exhibit excessive drying. 
         [0038]    The electronically conductive framework  110  further includes non-wetting channels  113 . In some embodiments, non-wetting channels  113  are not included. The non-wetting channels  113  serve as oxygen gas channels throughout the electrode  104 . 
         [0039]    While hydrophobic materials are beneficial for the wetting of embodiments incorporating non-aqueous electrolytes, the use of hydrophilic materials, including those achieved through surface treatments that introduce polar groups (e.g., hydroxyl groups), facilitates the creation of non-wetting channels and regions  113  for oxygen gas flow. 
         [0040]    Accordingly, oxygen is supplied from the reservoir  118  through the barrier  116  as indicated by the arrow  122 . Therefore, free electrons e flow into the positive electrode  104  through the current collector  108  as indicated by arrow  124 . 
         [0041]    The oxygen atoms and Li +  ions within the positive electrode  102  form a discharge product  130  inside the positive electrode  104  (see  FIG. 7 ). 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 electrically conductive framework  110 . 
         [0000]    
       
                 
         
             
             
         
       
     
         [0042]    In accordance with the foregoing embodiment, the amount and distribution of non-aqueous electrolyte and oxygen gas in the cathode is carefully controlled such that transport limitations are minimized. The cell  100  thus provides increased power density, increased energy density, a higher round-trip energy efficiency at a given power or current density. The cell  100  also exhibits increased ability to provide electrolyte throughout the electrode  104  even as Li 2 O 2  is deposited on the electrically conductive framework  110 . 
         [0043]    In general, the cell  100  is optimally configured such that in a fully charged state, the electrically conductive framework  110  occupies about 10% by volume of the electrode  104 . The electrolyte  106  occupies about 25% by volume of the electrode  104 . The gas phase of the oxygen occupies about 65% by volume of the electrode  104 . This configuration provides uniform wetting of electrolyte  106  throughout the electrode  104  and uniform distribution of gas volume fraction within the electrode  104 . 
         [0044]    Upon fully discharging the cell  100 , the electrically conductive framework  110  occupies about 10% by volume of the electrode  104 . The electrolyte  106  occupies about 25% by volume of the electrode  104 . The Li 2 O 2    130  occupies about 55% by volume of the electrode  104 . The gas phase of the oxygen occupies about 10% by volume of the electrode  104 . 
         [0045]    The cell  100  thus provides optimization of the volume fractions and distribution of components by engineering the wetting of the electrolyte  106  on the surfaces of the electrically conductive framework  110 . The configuration of the cell  100  ensures good access of the oxygen gas phase throughout the cathode by ensuring a pore structure and product structure that includes gas channels or an otherwise open pore structure. 
         [0046]    In addition to or as an alternative to the above described use of wetting materials, gas transport through the electrode  104  in some embodiments is accomplished using reduced tortuosity of aligned electrode structures. In the embodiment of  FIG. 6 , the non-wetting channels  113  are non-tortuous while the wetting channels  112  are tortuous. In one embodiment, aligned carbon nanotubes are used. In one embodiment, long fibers are used to encourage porosity and reduce overall tortuosity while smaller electrode particles with higher surface area are incorporated to provide gas transport without sacrificing active surface area. 
         [0047]    Moreover, while the non-wetting channels  113  and the wetting channels  112  are depicted as generally vertical, the actual orientation of the channels will vary depending upon the particular embodiment. Accordingly,  FIG. 8  depicts an electrochemical cell  200  including a negative electrode  202  separated from a positive electrode  204  by a porous separator  206 . The positive electrode  204  in this embodiment includes an electrically conductive framework  210 . The electrically conductive framework  210  defines wetting channels  212  and non-wetting channels  213 . 
         [0048]    The electrochemical cell  200  includes an electrolyte solution  214  present in the positive electrode  204  and in some embodiments in the separator  206 . A barrier  216  separates the positive electrode  204  from a reservoir  218 . 
         [0049]    The electrochemical cell  200  is thus substantially the same as the electrochemical cell  100 . One difference is that the wetting channels  212  and non-wetting channels  213  extend generally horizontally. In other embodiments, a mixture of horizontally and vertically extending channels are used. In other embodiments, randomly oriented channels are used or intermixed with horizontally or vertically extending channels. 
         [0050]    In some embodiments, low boiling solvents or high temperatures are used during electrode formation to induce a “mudcracking” effect of channels throughout the electrode. Gas transport is thus improved by the intentional introduction of defects in the electrode structure. 
         [0051]    In addition to the above described configurations, some embodiments include gas-driven convection to provide both electrolyte and gas mixing. The oxygen gas, which in some embodiments includes inactive components from air, is used to mix the electrolyte and gas volumes within the cathode to provide a desired uniform distribution of oxygen gas and electrolyte. 
         [0052]    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.