Abstract:
An essentially carbon-free cathode for a lithium/air secondary battery and methods for making are provided. The cathode includes a hollow porous conductive metal oxide particle such as indium tin oxide, an optional functional layer, and an electrically conductive binder.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 62/272,887 filed Dec. 30, 2015, and U.S. Provisional Application No. 62/356,877, filed Jun. 30, 2016, the entire contents of which are hereby incorporated by reference. 
     
    
     FIELD 
       [0002]    The invention generally relates to cathodes for secondary batteries, and more particularly to a porous metal oxide cathode for a lithium-air battery. 
       BACKGROUND 
       [0003]    Rechargeable lithium batteries are attractive energy storage devices for portable electric and electronic devices and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. A typical lithium cell contains a negative electrode, a positive electrode, and a separator located between the negative and positive electrodes. Both 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 contains an electrolyte with a lithium cation, and serves as a physical barrier between the electrodes such that none of the electrodes are electrically 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. During discharging, opposite reactions occur. 
         [0005]    During repeated charge/discharge cycles of the battery undesirable side reactions occur. These undesirable side reactions result in the reduction of the capacity of the battery to provide and store power. 
         [0006]    A typical lithium/air battery cell includes gas-diffusion electrodes based on porous carbon materials (e.g., carbon black, graphite, graphene, carbon fibers or carbon nanotubes). The carbon materials undergo oxidation during the charge cycle, because of the harsh environment in the cell. These include formation of pure oxygen, superoxide, and peroxide ions, formation of solid lithium peroxide on the cathode surface, and electrochemical oxidation potentials greater than 3 volts (vs. Li/Li + ). The oxidation of the carbon material in combination with the presence of lithium ions leads to the formation of a surface layer of insulating lithium carbonate (Li 2 CO 3 ) on the cathode. The oxidation reaction and the surface layer cause an increasing charging resistance of the cell resulting in charge potentials greater than 4 volts (vs. Li/Li + ). This can reduce the charging efficiency and in some cases prevent the cell from being fully recharged. Additionally, the presence of decomposing carbon in a lithium/air battery can also cause the formation of peroxycarbonate ions, which can substantially degrade many electrolytes. 
       SUMMARY 
       [0007]    Recently, attempts have been made to eliminate carbon from the cathode. Materials, such as nanoporous gold and titanium carbide, have met with limited success due to high cost and oxidation respectively. What is therefore needed is an inexpensive, processable, conductive, carbon-free thin film cathode that is minimally reactive to oxygen. 
         [0008]    A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below. 
         [0009]    Embodiments of the disclosure are related to systems and methods for a cathode which is essentially free of oxidizable carbon. 
         [0010]    In one embodiment, the disclosure provides a cathode comprising: a porous conductive metal oxide particle, wherein the cathode is essentially free of oxidizable carbon. 
         [0011]    In another embodiment, the disclosure provides a method of making a cathode comprising: forming a dispersion of polystyrene beads; adding a metal oxide precursor; forming a sol-gel; depositing the sol-gel onto a support; and removing the polystyrene beads. 
         [0012]    The details of one or more features, aspects, implementations, and advantages of this disclosure are set forth in the accompanying drawings, the detailed description, and the claims below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic diagram illustrating a cathode electrode including a metal foil and conductive metal oxide particles, in accordance with some embodiments. 
           [0014]      FIG. 2  is a schematic diagram illustrating a pore-shell metal oxide particle, in accordance with some embodiments. 
           [0015]      FIG. 3  is a schematic diagram illustrating a pore-shell metal oxide particle with a functional layer, in accordance with some embodiments. 
           [0016]      FIG. 4  is a schematic diagram illustrating a battery including a battery cell, in accordance with some embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    One or more specific embodiments will be described below. Various modifications to the described embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the described embodiments. Thus, the described embodiments are not limited to the embodiments shown, but are to be accorded the widest scope consistent with the principles and features disclosed herein. 
         [0018]    An embodiment of a cathode  100  is shown in  FIG. 1 . In the example of  FIG. 1  the cathode  100  may be essentially free of oxidizable carbon (e.g., carbon, carbon black, graphite) comprising a metal foil  110  having a first side and a second side, and a first cathode layer  120  comprising a porous electrically and ionically conductive material deposited on the first side of the metal foil  110 . The metal foil  110  may additionally be coated on the second side with a second cathode layer  130  comprising a porous conductive material. The porous material may be electrically and/or ionically conductive. The thicknesses and compositions of the first cathode layer  120  and second cathode layer  130  may be same or different. In a further embodiment, the composition of the first cathode layer  120  and second cathode layer  130  are the same. In an alternate embodiment, the metal foil  110  of the cathode  100  may be replaced with a temporary support material (e.g., a soluble polymer (e.g., polystyrene, polyethylene oxide, polyvinyl alcohol)) which facilitates the deposition of the first cathode layer  120  and is then subsequently removed using a solvent (e.g., water, chlorinated solvents (e.g., chloroform), aromatic solvents (e.g., toluene)). Further cathode layers may optionally be deposited. In some embodiments, the first cathode layer  120  and/or second cathode layer  130  additionally comprises a binder (e.g., polyvinylidene fluoride). In some embodiments, the first cathode layer  120  and/or second cathode layer  130  additionally comprises a functional layer (e.g., a catalyst). In some embodiments, essentially free of oxidizable carbon may be less than about 3 volume percent of oxidizable carbon, less than about 1 volume percent of oxidizable carbon, less than about 0.5 volume percent of oxidizable carbon, less than about 0.3 volume percent of oxidizable carbon, or less than about 0.1 volume percent of oxidizable carbon, based on the total volume of the cathode. 
         [0019]    In some embodiments, the porous conductive material may comprise a porous conductive metal oxide particle  200  of  FIG. 2 . The porous conducting metal oxide particle  200  comprises a pore-shell structure in which the pore  210  is hollow and the shell  220  comprises a film of a conductive metal oxide. The amount of hollow volume defines the porosity of the porous conducting metal oxide particle  200 . The porosity of the porous conducting metal oxide particle  200  is the volume percent void space based on the total volume of the particle. During battery operation the hollow pore  210  allows the formation and deposition/storage of oxidation products such as lithium peroxide (Li 2 O 2 ) formed during the discharge of the battery. The ability to deposit the oxidation product directly determines the maximum power obtainable from a battery. The shell  220  of the porous conductive metal oxide particle  200  comprises a conductive metal oxide film. 
         [0020]    Suitable metal oxides include, but are not limited to, zinc oxides, tin oxides, nickel oxides, manganese oxides, iron oxides, vanadium oxides, ruthenium oxides, rhenium oxides, iridium oxides, indium oxides, aluminum oxides, and combinations thereof. In certain embodiments, the conductive metal oxide comprises indium tin oxide, indium zinc oxide, or aluminum zinc oxide. 
         [0021]    In various embodiments, the thickness of the shell  220  may be at least about 1 nanometer, at least about 2 nanometers, at least about 3 nanometers, at least about 4 nanometers, at least about 5 nanometers, less than about 50 nanometers, less than about 40 nanometers, less than about 30 nanometers, less than about 20 nanometers, less than about 15 nanometers, less than about 10 nanometers, or less than about 7 nanometers. 
         [0022]    In various embodiments, the shape of the porous conducting metal oxide particle  200  may be an opal, inverse opal, sphere, spherical, oblate spheroid, prolate spheroid, teardrop, and combinations thereof. 
         [0023]    In some embodiments, the diameter of the porous conducting metal oxide particle  200  may be at least about 40 nanometers, at least about 50 nanometers, at least about 60 nanometers, at least about 70 nanometers, at least about 80 nanometers, less than about 400 nanometers, less than about 300 nanometers, less than about 200 nanometers, less than about 150 nanometers, or less than about 100 nanometers. 
         [0024]    In some embodiments, the porosity of the porous conducting metal oxide particle  200  may be at least about 40 volume percent, at least about 50 volume percent, at least about 60 volume percent, at least about 70 volume percent, or at least about 80 volume percent. 
         [0025]      FIG. 3  depicts another embodiment of a porous conducting metal oxide particle  300 . The porous conducting metal oxide particle  300  comprises a pore-shell structure in which the pore  310  is hollow and the shell  320  comprises a film of a conductive metal oxide. The pore  310  is the same as described for the pore  210  above. The shell  320  is the same as described for shell  220  above. The porous conducting metal particle  300  additionally comprises a functional layer  315  on an internal face of the shell  320 . The functional layer  315  may comprise one or more functional materials as a mixture, a plurality of sub-layers, and combinations thereof. The materials of the functional layer may impart various functionality to the porous conducting metal oxide particle  300  to promote or inhibit various physical, chemical, or electrochemical processes in the electrode (e.g., wettability, non-wettability, catalysis). In certain embodiments, the functional layer includes a catalyst (e.g., α-manganese oxide, manganese, cobalt, ruthenium, platinum, silver, and combinations thereof). In certain embodiments, the functional layer includes a material that imparts non-wettability (hydrophobicity) (e.g., alkyl silanes). In certain embodiments, the functional layer includes a material that imparts wettability (hydrophilicity). In various embodiments, the thickness of the functional layer  315  may be at least about 0.5 nanometers, at least about 1.0 nanometers, at least about 1.5 nanometers, at least about 2.0 nanometers, at least about 3.0 nanometers, at least about 4.0 nanometers, at least about 5.0 nanometers, less than about 40 nanometers, less than about 30 nanometers, less than about 20 nanometers, less than about 15 nanometers, less than about 10 nanometers, or less than about 7.0 nanometers. 
         [0026]    In some embodiments, the cathode  100  may comprise a plurality of different porous conducting metal oxide particles  200  and/or  300 . The particles may comprise the same or different metal oxides, the same or different functional layers, the same or different shapes, the same or different diameters, and combinations thereof. In certain embodiments, the cathode  430  comprises a first porous conductive metal oxide particle  300  having a first functional layer  315  and a second porous conductive metal oxide particle  300  having a second functional layer  315 . In one embodiment the cathode  100  comprises a first porous conducting metal oxide particle  300  having a non-wetting functional layer  315  and a second porous conducting metal oxide particle  300  having a catalyst functional layer  315 . 
         [0027]    In embodiments, the cathode  100  may be fabricated by a sol-gel process comprising forming a dispersion of polystyrene beads; combining the dispersion with a metal oxide (e.g., zinc oxide) precursor; forming a sol-gel by gelation; and depositing the sol-gel on a perforated metal foil or removable support. The suspension can then be dried to create a lattice of hexagonally closest packed polystyrene spheres having a metal oxide film coating. The polystyrene and removable support (if present) are then removed by dissolution or thermal decomposition. A binder, if desired, may be added to the metal oxide particles  200  before or after the dissolution of the polystyrene spheres. Alternatively, the polystyrene spheres can be sedimented prior to coating with a metal oxide precursor. In embodiments, metal oxides particles may be formed by the technique described by Zhang et al.,  Energy Environ. Sci.  2014, 7, 1402-1408, which teaches a self-induced assembly of polystyrene spheres on a substrate, infiltration with metal oxide precursors, followed by removal of the polystyrene spheres and crystallization of the metal oxide by annealing, the contents of which are hereby incorporated by reference in their entirety. In other embodiments, the metal oxide particles  300  comprise a functional layer  315 . The functional layer  315  can be deposited on the polystyrene beads prior to treatment with the metal oxide precursor. 
         [0028]    In various embodiments, the proportions of binder to porous conductive metal oxide particles  200 ,  300  can be varied to achieve the desired porosity of the cathode  100 . In some embodiments, the binder is present in the amount of about 5 to about 70 volume percent and the porous conductive metal oxide particles  200 ,  300  are present in the amount of about 30 to about 95 percent by volume based on the total volume of the cathode  100 . 
         [0029]    An embodiment of a battery  400  is shown in  FIG. 4 . The battery  400  includes a battery cell  402 , an anode current collector  405 , an anode  410 , a separator  420 , a cathode  430 , and a cathode current collector  435 . In various examples, the anode  410  comprises an oxidizable metal (e.g., lithium). In various examples, suitable materials for the separator  420  may include porous polymers, ceramics, and two dimensional sheet structures such as graphene, boron nitride, and dichalcogenides. In various examples the cathode  430  may comprise the cathode  100  of  FIG. 1 . 
         [0030]    In some examples the cathode  430 , separator  420 , and anode  410  comprise an ionically conductive electrolyte that contains a salt, such as lithium hexafluorophosphate (LiPF 6 ) that provides the electrolyte with an adequate conductivity which reduces the internal electrical resistance of the battery cell. In embodiments, the cathode  430 , can contain a lithium ion conducting ceramic (e.g., garnet). 
         [0031]    The embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling with the spirit and scope of this disclosure. 
         [0032]    It is believed that embodiments described herein and many of their attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components without departing from the disclosed subject matter or without sacrificing all of its material advantages. The form described is merely explanatory, and it is the intention of the following claims to encompass and include such changes.