Abstract:
A vehicular battery system includes a vehicular battery system stack including at least one negative electrode including a form of lithium, an oxygen reservoir having a first outlet operably connected to the vehicular battery system stack, a multistage compressor having a first inlet operably connected to the vehicular battery system stack, and a second outlet operably connected to a second inlet of the oxygen reservoir, and a cooling system operably connected to the multistage compressor and configured to provide a coolant to the multistage compressor to cool a compressed fluid within the multistage compressor.

Description:
This application claims priority under 35 U.S.C. §119 to U.S. Provisional Application No. 61/767,605, filed on Feb. 21, 2013, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     This disclosure relates to batteries and more particularly to metal/oxygen based batteries. 
     BACKGROUND 
     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 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 Li 2 O. Other high-capacity materials include BiF 3  (303 mAh/g, lithiated), FeF 3  (712 mAh/g, lithiated), Zn, Al, Si, Mg, Na, Fe, Ca, and others. In addition, other negative-electrode materials, such as alloys of multiple metals and materials such as metal-hydrides, also have a high specific energy when reacted with oxygen. Many of these couples also have a very high energy density 
     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. 
       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. 
     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. 
     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 dimethyl ether 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. 
     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 2 O 2  or Li 2 O in accordance with the following relationship: 
     
       
         
           
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     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 must have a capacity of about 20 mAh/cm 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 &gt;3V (vs. Li/Li + )). 
     While there is a clear benefit to couples that include oxygen as a positive electrode and metals, alloys of metals, or other materials as a negative electrode, none of these couples has seen commercial demonstration thus far because of various challenges. 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. 
     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 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 + ) 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 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 . 
     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 2  and other metals may also be carried out in various media. 
     In systems using oxygen as a reactant, the oxygen may either be carried on board the system or obtained from the atmosphere. There are both advantages and disadvantages to operating a battery that reacts gaseous oxygen in a closed format by use of a tank or other enclosure for the oxygen. One advantage is that if the reaction chemistry is sensitive to any of the other components of air (e.g., H 2 O, CO 2 ), only pure oxygen can be added to the enclosure so that such contaminants are not present. Other advantages are that the use of an enclosure can allow for the operation at a high partial pressure of oxygen at the site of the reaction (for uncompressed atmospheric air the pressure of oxygen is only 0.21 bar), can prevent any volatile species from the leaving the system (i.e., prevent “dry out”), and other advantages. The disadvantages include the need to carry the oxygen at all times, increasing the system mass and volume, potential safety issues associated with high-pressure oxygen, and others. 
     In order to realize the advantages that come with the use of a closed system in a vehicle it is necessary to compress the oxygen so that the oxygen volume is not too large on board the vehicle. In particular, a pressure in the fully charged state of greater than 100 bar, such as 350 bar (about 5000 psi), is desirable. 
     What is therefore needed is an economic, efficient, and compact method to compress and store the oxygen produced during the charge of a battery system that consumes oxygen on discharge. 
     SUMMARY 
     In one embodiment, a vehicular battery system includes a vehicular battery system stack including at least one negative electrode including a form of lithium, an oxygen reservoir having a first outlet operably connected to the vehicular battery system stack, a multistage compressor having a first inlet operably connected to the vehicular battery system stack, and a second outlet operably connected to a second inlet of the oxygen reservoir, and a cooling system operably connected to the multistage compressor and configured to provide a coolant to the multistage compressor to cool a compressed fluid within the multistage compressor. 
     In another embodiment, a method of operating a vehicular battery system includes charging a vehicular battery system stack including at least one positive electrode including a form of lithium, transferring oxygen formed by charging the vehicular battery system stack to a multistage compressor, compressing the transferred oxygen in a first compression stage of the multistage compressor, compressing the compressed oxygen from the first compression stage in a second compression stage of the multistage compressor, providing coolant to the multistage compressor, and transferring the compressed oxygen from the second compression stage to an oxygen reservoir operably connected to the vehicular battery system stack. 
    
    
     
       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; 
         FIG. 6  depicts a schematic view of a vehicle with an adiabatic compressor operably connected to a reservoir configured to exchange oxygen with a positive electrode for a reversible reaction with lithium; 
         FIG. 7  depicts a chart showing the increase in temperature when a gas is adiabatically compressed starting from a pressure of 1 bar and a temperature of 298.15 K with constant gas properties (i.e., gamma) assumed; 
         FIG. 8  depicts a chart showing compression work for an ideal gas (diatomic and constant properties are assumed for adiabatic) as a function of pressure with the initial pressure at one bar; and 
         FIG. 9  depicts a process for how the temperature of the final compressed gas or of the gas at an intermediate stage is used by the battery control system to change the flow rate of the cooling fluid to ensure the correct final temperature is reached. 
     
    
    
     DETAILED DESCRIPTION 
     For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the disclosure is thereby intended. It is further understood that this disclosure includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the disclosure as would normally occur to one skilled in the art to which this disclosure pertains. 
     A schematic of vehicle  100  is shown in  FIG. 6 . The vehicle  100  includes a vehicular battery system stack  102  and an oxygen reservoir  104 . A pressure regulator  106  governs provision of oxygen to the vehicular battery system stack  102  during discharge while a multi-stage oxygen compressor  108  is used to return oxygen to the oxygen reservoir  104  during charging operations. 
     The vehicular battery system stack  102  includes one or more negative electrodes (not shown) separated from one or more positive electrodes (not shown) by one or more porous separators (not shown). The negative electrode (not shown) 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 other materials that can react reversibly and electrochemically. 
     The positive electrode (not shown) in one embodiment includes a current collector (not shown) and electrode particles (not shown), optionally covered in a catalyst material, suspended in a porous matrix (not shown). The porous matrix (not shown) 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 (not shown) prevents the negative electrode (not shown) from electrically connecting with the positive electrode (not shown). 
     The vehicular battery system stack  102  includes an electrolyte solution (not shown) present in the positive electrode (not shown) and in some embodiments in the separator (not shown). In some embodiments, the electrolyte solution 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. 
     In the case in which the metal is Li, the vehicular battery system stack  102  discharges with lithium metal in the negative electrode ionizing into a Li +  ion with a free electron e − . Li +  ions travel through the separator toward the positive electrode. Oxygen is supplied from the oxygen storage tank  104  through the pressure regulator. Free electrons e −  flow into the positive electrode (not shown). 
     The oxygen atoms and Li +  ions within the positive electrode form a discharge product inside the positive electrode, aided by the optional catalyst material on the electrode particles. 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. 
     
       
         
           
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     The vehicular battery system stack  102  does not use air as an external source for oxygen. External sources, such as the atmosphere, include undesired gases and contaminants. Thus, while the oxygen that reacts electrochemically with the metal in a metal/oxygen battery may come from the air, the presence of CO 2  and H 2 O in air make it an unsuitable source for some of the media in which the metal/oxygen reactions are carried out and for some of the products that form. For example, in the reaction of Li with oxygen in which Li 2 O 2  is formed, H 2 O and CO 2  can react with the Li 2 O 2  to form LiOH and/or Li 2 CO 3 , which can deleteriously affect the performance and rechargeability of the battery. As another example, in a basic medium CO 2  can react and form carbonates that precipitate out of solution and cause electrode clogging. 
     In  FIG. 6 , all of the components are stored on board the vehicle  100 . In the embodiment of  FIG. 6 , the oxygen storage reservoir  104  is separated from the vehicular battery system stack  102  where the reactions take place, but in other embodiments the oxygen storage is more closely integrated with the stack (for example, incorporated within the cells). In the embodiment of  FIG. 6 , the oxygen storage is done in a tank or other enclosure that is spatially separated from the stack or cells where the reactions are carried out such that a minimal amount of high-pressure housing is required for the vehicle  100 . 
     During discharge (in which oxygen is consumed), the pressure of the oxygen gas is reduced by passing it through the pressure regulator  106  such that the pressure of the oxygen that reaches the stack is close to ambient (i.e., less than about 5 bar). During discharge the compressor  108  does not operate. During charge the compressor  108  is operated to compress the oxygen that is being generated within the stack or cells where the reactions are taking place. 
     The compressor  108  in various embodiments is of a different type. In one embodiment which is suitable and mature for a vehicle application in which it is desired to pressurize a gas to more than 100 bar in a unit with a compact size is a multi-stage rotary compressor. When embodied as a multi-stage rotary compressor, each compression step is nearly adiabatic because it involves the rapid action of a piston to compress the gas. Commercial units of the appropriate size are widely available at a reasonable cost; they are used for a variety of applications that require air compression. 
     Because each stage of the compressor is nearly adiabatic, in addition to an increase in the pressure there is also an increase in the temperature, as explained with reference to  FIG. 7 .  FIG. 7  shows the temperature at the end of a single adiabatic compression step starting at a pressure of 1 bar and a temperature of 298.15 K assuming constant gas properties. The figure shows that it is impractical to use a single compression step to achieve a pressure of, for example, 350 bar, because the output temperature would be far too high to inject into a tank of standard materials, which in turn is integrated in a vehicle that may have heat-sensitive components. In addition, the final pressure shown in  FIG. 7  is for the temperature at the end of the compression step; thus, after cooling, the pressure will fall. It is important for the temperature of the compressed gas released into the tank to be within a certain range so that it is compatible with the tank material, which in different embodiments is a metal such as aluminum or a polymer, depending on the type of tank. 
     In order to prevent the temperature from rising too high it is necessary to cool the gas at the end of each adiabatic compression step. This is accomplished using the radiator  110  shown in  FIG. 6 . The radiator  110  in some embodiments is the same radiator that is used to cool the vehicular battery system stack; in such embodiments the heat exchange loop also extends into the other components of the battery system such as the vehicular battery system stack  102  and battery system oxygen storage  104 . Typically, fluid is passed through the oxygen compressor  108 , removing heat from the oxygen gas after each compression step and bringing the temperature towards that of the radiator fluid. The fluid is passed through the radiator  110  where heat is exchanged with the atmosphere. The compressor is also insulated to prevent the exposure of other parts of the battery system or the vehicle  100  to high temperatures. 
     The cooling of the oxygen after each compression step allows the system to operate closer to the isothermal compression work line shown in  FIG. 8 . In particular,  FIG. 8  shows the difference in the work required for a single-stage adiabatic compression (assuming a diatomic gas and constant properties) compared to the compression work required for isothermal compression. As the figure shows, significantly more work is required for adiabatic compression than isothermal compression. For a multi-stage adiabatic compression process with cooling between stages the amount of work required is between the pure isothermal and single-stage adiabatic lines. Thus, the amount of work required for the compression can be lowered compared to adiabatic compression by using multiple compression stages with cooling of the gas at the end of each compression. 
     The magnitude of the compression energy compared to the reaction energy also depends on the negative electrode material with which oxygen is reacting. For example, if the oxygen is reacting with Li to form Li 2 O 2  on discharge, the reaction energy is 159 Wh/mole O 2 . Thus, if the charging process takes place with 85% efficiency, about 24 Wh/mole O 2  would be required for cooling for the reaction, suggesting that the amount of cooling required for the compression should be smaller than that required for cooling the stack or cells. 
     In the embodiment of  FIG. 6 , all processes associated with the operation of the battery system are controlled by a battery control system  112 . The battery control system  112  controls the flow rate of the fluid that is passed through the radiator  110  and the oxygen compressor  108  and possibly other components on the vehicle  100 . The battery control system  112  includes a memory (not shown) in which program instructions are stored and a processor (not shown) which executes the program instructions to control the temperature of the oxygen which is compressed into the storage system  104 . The processor is operatively connected to temperature sensors (not shown) in the vehicular battery system stack  102 , the oxygen storage  104 , the radiator  110 , and at various stages in the compressor  108  in order to more precisely control the system. In some embodiments, more or fewer temperature sensors are included. A schematic that shows how the temperatures are used by the battery control system  112  is shown in  FIG. 9 . 
     In  FIG. 9 , the processor obtains a signal indicative of the temperature at the output of the compressor  108  and controls the flow rate of fluid based upon the obtained temperature. In some embodiments, the temperature of one or more intermediate stages of the compressor  108  is obtained, and cooling flow throw the particular stages is modified based upon the temperature. In some embodiments, the temperature of the cooling fluid is obtained, and used to determine or control the flow rate of the cooling fluid. 
     The vehicular battery system stack  102  thus makes use of oxygen (which may be pure or contain additional components) stored within a battery cell or external to a cell in a tank or other volume. The oxygen reacts electrochemically with the metal (which may include Li, Zn, Mg, Na, Fe, Al, Ca, Si, and others) to produce energy on discharge, and on charge the metal is regenerated and oxygen gas (and perhaps other species, such as H 2 O) are evolved. 
     Beneficially, the battery system in the vehicle  100  is thus a completely closed system and species present in ambient air (e.g., H 2 O, CO 2 , and others) that may be detrimental to the cell operation are excluded. The battery system provides electrochemical compression of oxygen on charge, and the use of compressed oxygen on discharge, to reduce energy losses associated with mechanical oxygen compression (which is typically carried out adiabatically, including in a multi-stage adiabatic process) and to reduce the cost and complexity of a mechanical compressor. The components of the battery system are configured to handle the pressure of the compressed oxygen, including flow fields, bipolar plates, electrodes, separators, and high-pressure oxygen lines. 
     The battery system in some embodiments includes high-pressure seals, an electrode, gas-diffusion layer, and flow field design that provide sufficient mechanical support to prevent pressure-induced fracture or bending (including with pressure cycling) that would be deleterious to cell performance and life, and a separator that is impervious to oxygen (even at high pressures, including up to 350 bar or above). The minimum pressure in some embodiments is chosen to eliminate delamination of cell components from one another. The minimum pressure in some embodiments is chosen to reduce mass transfer limitations and thereby increase the limiting current. 
     The above described system provides a number of advantages. For example, the use of a multi-stage compressor results in a vehicle with a battery system that is smaller and more economical, and with a higher efficiency, than other compression strategies. 
     Additionally, a higher oxygen pressure in the tank can be achieved if the compressor is properly cooled than if there is not a good cooling solution. In addition the compression can be carried out more efficiently if the oxygen can be adequately cooled between each stage. 
     Moreover, the vehicle can be charged using only a wall outlet if a compressor is integrated into the vehicle system itself rather than stored externally from the vehicle. 
     Integration of the compressor on the vehicle allows for a completely closed gas handling system. If a compressor is stored separately from the vehicle a connection between the external compressor and the gas handling system on the vehicle may introduce contamination. 
     While the disclosure 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 disclosure are desired to be protected.