Patent Application
20200321662
The need for high performance and reliable energy storage in the modern society is well documented. Lithium batteries represent a very attractive solution to these energy needs due to their superior energy density and high performance. However, available Li-ion storage materials limit the specific energy of conventional Li-ion batteries. While lithium has one of the highest specific capacities of any anode (3861 mAh/g), typical cathode materials such as MnO2, V2O5, LiCoO2 and (CF)n have specific capacities less than 200 mAh/g.
Recently, lithium/oxygen (Li/O2) or lithium air batteries have been suggested as a means for avoiding the limitations of today's lithium ion cells. In these batteries, lithium metal anodes are used to maximize anode capacity and the cathode capacity of Li air batteries is maximized by not storing the cathode active material in the battery. Instead, ambient O2 is reduced on a catalytic air electrode to form O22−, where it reacts with Li+ ions conducted from the anode. Aqueous lithium air batteries have been found to suffer from corrosion of the Li anode by water and suffer from less than optimum capacity because of the excess water required for effective operation.
Abraham and Jiang (J. Electrochem. Soc., 143 (1), 1-5 (1996)) reported a non-aqueous Li/O2 battery with an open circuit voltage close to 3 V, an operating voltage of 2.0 to 2.8 V, good coulomb efficiency, and some re-chargeability, but with severe capacity fade, limiting the lifetime to only a few cycles. Further, in non-aqueous cells, the electrolyte has to wet the lithium oxygen reaction product in order for it to be electrolyzed during recharge. It has been found that the limited solubility of the reaction product in available organic electrolytes necessitates the use of excess amounts of electrolyte to adequately wet the extremely high surface area nanoscale discharge deposits produced in the cathode. Thus, the required excess electrolyte significantly decreases high energy density that would otherwise be available in lithium oxygen cells.
Operation of Li/02 cells depends on the diffusion of oxygen into the air cathode. As such, high oxygen solubility in the electrolyte is desired for the cell to operate under high rate discharge conditions. J. Read (J. Electrochem. Soc., 149(9) A1190-A1195 (2002)), in studying the cathodes of lithium air cells, demonstrated the dependence of cathode capacity on oxygen absorption. Oxygen absorption is a function of electrolyte Bunsen coefficient (α), electrolyte conductivity (σ), and viscosity (η). The trend of decreasing cathode lithium reaction capacity with increasing viscosity and decreasing Bunsen coefficient is apparent in Read's data. It is known that as the solvent's viscosity increases, there are decreases in lithium reaction capacity and Bunsen coefficients. Additionally, the electrolyte has an even more direct effect on overall cell capacity as the ability to dissolve reaction product is crucial. This problem has persisted in one form or another in known batteries.
Indeed, high rates of capacity fade remain a problem for non-aqueous rechargeable lithium air batteries and have represented a significant barrier to their commercialization. The high fade is attributed primarily to parasitic reactions occurring between the electrolyte and the mossy lithium powder and dendrites formed at the anode-electrolyte interface during cell recharge, as well as the passivation reactions between the electrolyte and the LiO2 radical which occurs as an intermediate step in reducing Li2O2 during recharge.
During recharge, lithium ions are conducted across the electrolyte separator with lithium being plated at the anode. The recharge process can be complicated by the formation of low density lithium dendrites and lithium powder as opposed to a dense lithium metal film. In addition to passivation reactions with the electrolyte, the mossy lithium formed during recharge can be oxidized in the presence of oxygen into mossy lithium oxide. A thick layer of lithium oxide and/or electrolyte passivation reaction product on the anode can increase the impedance of the cell and thereby lower performance. Formation of mossy lithium with cycling can also result in large amounts of lithium being disconnected within the cell and thereby being rendered ineffective. Lithium dendrites can penetrate the separator, resulting in internal short circuits within the cell. Repeated cycling causes the electrolyte to break down, in addition to reducing the oxygen passivation material coated on the anode surface. This results in the formation of a layer composed of mossy lithium, lithium-oxide and lithium-electrolyte reaction products at the metal anode's surface which drives up cell impedance and consumes the electrolyte, bringing about cell dry out.
Attempts to use active (non-lithium metal) anodes to eliminate dendritic lithium plating have not been successful because of the similarities in the structure of the anode and cathode. In such lithium air “ion” batteries, both the anode and cathode contain carbon or another electronic conductor as a medium for providing electronic continuity. Carbon black in the cathode provides electronic continuity and reaction sites for lithium oxide formation. To form an active anode, graphitic carbon is included in the anode for intercalation of lithium and carbon black is included for electronic continuity. Unfortunately, the use of graphite and carbon black in the anode can also provide reaction sites for lithium oxide formation. At a reaction potential of approximately 3 volts relative to the low voltage of lithium intercalation into graphite, oxygen reactions would dominate in the anode as well as in the cathode. Applying existing lithium ion battery construction techniques to lithium oxygen cells would allow oxygen to diffuse throughout all elements of the cell structure. With lithium/oxygen reactions occurring in both the anode and cathode, creation of a voltage potential differential between the two is difficult. An equal oxidation reaction potential would exist within the two electrodes, resulting in no voltage.
As a solution to the problem of dendritic lithium plating and uncontrolled oxygen diffusion, known aqueous and non-aqueous lithium air batteries have included a barrier electrolyte separator, typically a ceramic material, to protect the lithium anode and provide a hard surface onto which lithium can be plated during recharge. However, formation of a reliable, cost effective barrier has been difficult. A lithium air cell employing a protective solid state lithium ion conductive barrier as a separator to protect lithium in a lithium air cell is described in U.S. Pat. No. 7,691,536 of Johnson. Thin film barriers have limited effectiveness in withstanding the mechanical stress associated with stripping and plating lithium at the anode or the swelling and contraction of the cathode during cycling. Moreover, thick lithium ion conductive ceramic plates, while offering excellent protective barrier properties, are extremely difficult to fabricate, add significant mass to the cell, and are rather expensive to make.
Thick lithium ion conductive ceramic plates have also been employed, particularly in lithium water cells. Having thicknesses in the range of 150 um, these plates offer excellent protective barrier properties, however, they are difficult to fabricate and expensive. In addition, these ceramic plates add significant mass to the cell, resulting in a reduction in specific energy storage capability. This reduction can be sufficient to negate the otherwise high energy density performance available using lithium-air technology.
As it relates to the cathode, the dramatic decrease in cell capacity as the discharge rate is increased is attributed to the accumulation of reaction product in the cathode. At high discharge rate, oxygen entering the cathode at its surface does not have an opportunity to diffuse or otherwise transition to reaction sites deeper within the cathode. The discharge reactions occur at the cathode surface, resulting in the formation of a reaction product crust that seals the surface of the cathode and prevents additional oxygen from entering. Starved of oxygen, the discharge process cannot be sustained.
Another significant challenge with lithium air cells has been electrolyte stability within the cathode. The primary discharge product in lithium oxygen cells is Li2O2. During recharge, the resulting lithium oxygen radical, LiO2, an intermediate product which occurs while electrolyzing Li2O2, aggressively attacks and decomposes the electrolyte within the cathode, causing it to lose its effectiveness.
High temperature molten salts have been suggested as an alternative to organic electrolytes in non-aqueous lithium-air cells. U.S. Pat. No. 4,803,134 of Sammells describes a high lithium-oxygen secondary cell in which a ceramic oxygen ion conductor is employed. The cell includes a lithium-containing negative electrode in contact with a lithium ion conducting molten salt electrolyte, LiF—LiCl—Li2O, separated from the positive electrode by the oxygen ion conducting solid electrolyte. The ion conductivity limitations of available solid oxide electrolytes require that such a cell be operated in the 700° C. range or higher in order to have reasonable charge/discharge cycle rates. The geometry of the cell is such that the discharge reaction product accumulates within the molten salt between the anode and the solid oxide electrolyte. The required space is an additional source of impedance within the cell.
Molten nitrates also offer a viable solution and the physical properties of molten nitrate electrolytes are summarized in Table 1 (taken from Lithium Batteries Using Molten Nitrate Electrolytes by Melvin H. Miles; (1999)).
The electrochemical oxidation of the molten LiNO3 occurs at about 1.1 V vs. Ag+/Ag or 4.5 V vs. Li+/Li. The electrochemical reduction of LiNO3 occurs at about −0.9V vs. Ag+/Ag, and thus these two reactions define a 2.0V electrochemical stability region for molten LiNO3 at 300° C. and are defined as follows:
LiNO3→Li++NO2+½O2+e− (Equation 1)
LiNO3+2e−→LiNO2+O−− (Equation 2)
The work with molten nitrates was not performed with lithium air cells in mind; however, the effective operating voltage window for the electrolyte is suitable for such an application. As indicated by the reaction potential line in Scheme 1, applying a recharge voltage of 4.5V referenced to the lithium anode can cause lithium nitrate to decompose to lithium nitrite, releasing oxygen. On the other hand, lithium can reduce LiNO3 to Li2O and LiNO2. This reaction occurs when the LiNO3 voltage drops below 2.5V relative to lithium. As long as there is dissolved oxygen in the electrolyte, the reaction kinetics will favor the lithium oxygen reactions over LiNO3 reduction. Oxide ions are readily converted to peroxide (O22−) and aggressive superoxide (O2−) ions in NaNO3 and KNOB melts (M. H. Miles et al., J. Electrochem. Soc., 127, 1761 (1980)).
In 2015, Vincent Giordani of Liox Power, Inc. reported high temperature molten salt system using nitrates. Nitrate and halide salts have the stability needed for the lithium oxygen environments, high ion conductivity and the ability to dissolve lithium oxygen and lithium carbonate reaction products. The challenge faced with these systems is primarily associated with disposition of reaction products. Similar to the non-aqueous, organic electrolyte cells, accumulation of discharge reaction product within the cell tends to interfere with migration of reactants to reaction sites and thereby limit cell performance.
A need remains for a lithium air cell which overcomes problems associated with those of the prior art.
A rechargeable lithium air battery according to an embodiment of the disclosure contains a lithium based anode containing a solid lithium ion conductive electrolyte forming a first chamber that encloses lithium metal, an oxygen electrode, a solid oxygen ion conductive electrolyte forming a second chamber, and a molten electrolyte contained in the second chamber and coupled between the oxygen ion conductive electrolyte and the lithium ion conductive electrolyte, and the molten salt electrolyte has no contact with air.
The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
In the drawings:
This disclosure generally relates to energy storage, and more particularly to a lithium air electrochemical cell. For the purposes of this disclosure, the terms lithium air cell, lithium air battery, lithium air electrochemical engine, rechargeable lithium air battery, and lithium oxygen battery are used interchangeably.
Certain terminology is used in the following description for convenience only and is not limiting. The words “proximal,” “distal,” “upward,” “downward,” “bottom” and “top” designate directions in the drawings to which reference is made. The words “inwardly” and “outwardly” refer to directions toward and away from, respectively, a geometric center of the device, and designated parts thereof, in accordance with the present invention. Unless specifically set forth herein, the terms “a,” “an” and “the” are not limited to one element, but instead should be read as meaning “at least one.” The terminology includes the words noted above, derivatives thereof and words of similar import.
It will also be understood that terms such as “first,” “second,” and the like are provided only for purposes of clarity. The elements or components identified by these terms, and the operations thereof, may easily be switched.
Aspects of the disclosure relate to a lithium air battery which exhibits a high rate of cell charge/discharge with limited capacity fade, high energy density, high power density and the ability to operate on oxygen from ambient air. As such it removes significant barriers that have prevented the commercialization of lithium air cells. For example, the mossy lithium powder and dendrites at the anode-electrolyte interface formed during cell recharge are eliminated by using molten lithium supplied as a flow reactant to the anode side of a stable solid state ceramic electrolyte. A flow system for removing reaction product from the cathode is also described.
The reactions of lithium with oxygen are as follows:
2Li+O2→Li2O2 Eo=3.10 V
4Li+O2→2Li2O Eo=2.91V
To avoid the problems associated with past approaches to lithium air cells, aspects of the disclosure include a lithium air cell that operates at elevated temperature, in the wide range of about 250° C. to 650° C., more preferably about 250° to 400° C. or about 400° C. to 650° C., depending on the specific electrolyte contained in the battery. Specifically, as described in further detail below, the lower operating temperature range is preferred when the molten electrolyte contains siloxanes and the higher operating temperature range is preferred when the electrolyte contains only inorganic molten salts. Operation at elevated temperature enables faster kinetics for higher power density, thus eliminating a major problem associated with lithium air technology. Further, operation at elevated temperature also allows the use of high temperature organic electrolytes and inorganic, molten salt electrolyte solutions that have high electrochemical stability, thus avoiding another of the major problems that has plagued the conventional approach to lithium air cells. Selected inorganic molten salts have good solubility of lithium/oxygen reaction products, thus allowing better control of cell kinetics.
The rechargeable lithium air battery according to aspects of the disclosure contains a lithium based anode comprising a lithium ion conductive electrolyte forming a first chamber that encloses lithium metal, an oxygen electrode, a solid oxygen ion conductive electrolyte forming a second chamber, and a molten electrolyte contained in the second chamber and coupled between the oxygen ion conductive electrolyte and the lithium ion conductive electrolyte, in which the molten electrolyte has no contact with air. Each of these components will be described in more detail below.
The embodiment of the disclosure shown in
The solid lithium ion conductive electrolyte is preferably a ceramic material which is stable in contact with lithium metal and forms a chamber or enclosure for containing the lithium metal. Together with the lithium metal, the solid lithium ion conductive electrolyte forms the anode for the battery.
Preferred materials for the solid lithium ion conductive electrolyte include lithium ion conducting glasses such as lithium beta alumina, lithium phosphate glass, lithium lanthanum zirconium oxide (LLZO), alumina doped LLZO (Al2O3:Li7La3Zr2O12), lithium silicon phosphate (Li7SiPO8), lithium aluminum germanium phosphate (LAGP), and lithium aluminum titanium phosphate (LATP). The most preferred material is lithium silicon phosphate.
In a preferred embodiment, the anode chamber which is formed from the solid lithium ion conductive electrolyte is maintained at relatively uniform temperature.
The solid oxygen ion conductive electrolyte forms a chamber for the molten salt electrolyte. Preferred materials for the solid oxygen ion conductive electrolyte include ceramics such as, but not limited to, scandium-stabilized zirconia (SSZ) and yttria-stabilized zirconia (YSZ), stabilized by either 3 mol % Y2O3 (3YSZ) or 8 mol % Y2O3 (8YSZ).
Although illustrated in
The air cathode or oxygen electrode is porous so that oxygen can flow through the pores to and from reaction sites where it is oxidized or reduced as the cell is discharged or charged respectively. During discharge, oxygen enters the cell by flowing to oxidation sites where it is oxidized into oxygen ions and electrons. The electrons are conducted through load 40 to anode electrode terminal 28. The oxygen ions are conducted through solid electrolyte 6 into molten electrolyte 18. The opposite occurs during charge. Oxygen ions are conducted from the molten electrolyte through solid electrolyte 6 to reaction sites in the cathode where it is reduced to oxygen and released to external air.
The cathode may be constructed of an electrically conductive sintered metal oxide, such as lanthanum strontium iron oxide, lanthanum strontium iron cobalt oxide (LSCF), praseodymium strontium iron oxide (PSF), barium strontium cobalt iron oxide (BSCF), lanthanum strontium copper oxide (LSC), and lanthanum strontium manganese oxide (LSM). The preferred cathode material is LSM. It is also within the scope of the disclosure for the cathode to include silver or other suitable electron conductive materials.
The molten electrolyte is preferably an inorganic molten salt eutectic.
Alternate example molten electrolytes include lithium metaborate, lithium orthoborate, lithium tetraborate, LiPON in bulk form, lithium fluoride doped lithium metaborate, silicon doped lithium tetraborate, lithium metaborate doped lithium carbonate (LiBO2—Li2CO3), lithium orthoborate doped lithium carbonate (Li3BO3—Li2CO3), lithium carbonate doped lithium orthoborate (Li2CO3—Li3BO3), silicon dioxide doped Li3BO3—Li2CO3 (SiO2—Li3BO3—Li2CO3), and lithium fluoride doped Li3BO3—Li2CO3 (LiF—Li3BO3—Li2CO3). Other example electrolytes include molten inorganic salts, for example, alkali nitrates such as lithium and sodium nitrate, alkali chlorides and bromides such as lithium, potassium and sodium chlorides and bromides, alkali carbonates such as sodium and lithium carbonates, as well as eutectic mixtures such as sodium nitrate-potassium nitrate (NaNO3—KNO3) and lithium chloride-potassium chloride (LiCl—KCl) eutectic for operating in the 400 to 650° C. temperature range and silane and siloxane-based compounds including, for example, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, and dodecamethylhexatetrasiloxane with or without polyethylene oxide groups for operating in the 250 to 400° C. temperature range. Particularly preferred materials include doped Li9.3C3BO12.5 (LCBO), such as LCBFO (LCBO doped with fluorine), LCBSO (LCBO doped with sulfur), LBCSiO (LCBO doped with silicon), LBCSiFO (LBCSiO doped with fluorine) and LBCGeO (LCBO doped with germanium), as well as LBCSO (LBCO doped with sulfur) for operating in the 400 to 650° C. temperature range.
The non-aqueous electrolyte is chosen for stability in contact with lithium. Thus, a breach in the lithium conductive enclosure will not result in rapid reactions, particularly because oxygen ingress into the cell will be controlled.
The lithium based anode is comprised of lithium contained in a sealed ceramic enclosure or chamber formed by the solid lithium ion conductive electrolyte. The anode comprises metallic lithium in a molten state; lithium has a melting point of about 180° C. Lithium metal is stable in direct contact with the molten salt electrolyte because there is no oxygen gas or air inside the molten salt enclosure. The benefit of the molten lithium anode within the ion conductive enclosure is that it limits undesirable dendrite growth and short circuits in the cell. The solid lithium electrolyte enclosure maintains lithium in a contiguous state so that all of the molten lithium remains in electrical contact with the anode terminal. During discharge, lithium is oxidized into lithium ions and electrons at the solid electrolyte interface. The electrons are conducted through load 40 to cathode electrode terminal 30. The lithium ions are conducted through solid electrolyte 16 into molten electrolyte 18 with oxygen ions being simultaneously conducted through the oxygen ion conductive enclosure. The opposite occurs during charge. Lithium ions are conducted from the molten salt through molten electrolyte 18 and reduced to lithium metal within reservoir 20 as electrons are coupled to terminal 28 from positive electrode 12.
Oxygen ions are conducted into the molten salt through the wall of the molten salt enclosure, which is oxygen ion conductive, and the molten salt does not contact air. There is no direct contact between the molten salt and the air, so that there is no evaporation of the molten salt from the cell. Oxygen is iodized into ions at the outer surface of the containment chamber and conducted through the solid containment wall into the molten salt.
An exemplary design is a 1875 kWh battery designed for maximum power output at a discharge rate of 1C, i.e., the battery totally discharges in 1 hour. Lithium has a specific energy of 11,580 Wh/kg. For a 1.875 kWh cell, 162 g of lithium is needed. Lithium has a discharge current capacity of 3.86 Ah/g so that the Amp-hour capacity of the cell would be 625 Ah, (162 g*3.86 Ah/g/1 hr).
Because of its operating temperature, the primary reaction product of the cell is Li2O. The atomic mass of lithium is 6.9 g/mole. For the 4Li+O2>2Li2O discharge reaction product, 0.5 mole of oxygen is required for per mole of lithium. Assuming 162 g (23.48 moles) lithium, 11.74 moles (187.82 g) of oxygen are required to balance the reaction. If the mass of the oxygen is included, the net energy density is 5,385 Wh/kg for lithium oxide (Li2O) as the reaction product.
The amount of air flow required to sustain a 1C discharge rate can be determined from the required oxygen flow. Air is 23% oxygen by mass so that the total amount of air needed for the reaction is 816.6 g (187.82 g O2/(0.23 g O2/g Air). For the 1C discharge, the air mass flow rate is 816.6 g/hr or 0.23 g/sec, and using the density of air of 0.00123 g/cm3 yields a volumetric flow rate of 187 cm3/sec.
Referring to
The conductivity of the molten salt electrolyte 18 at 600° C. is 4 S/cm as shown in
Using scandium stabilized zirconia [(Zr2)0.9(Sc2O3)0.1, SSZ] as the oxygen ion conducting electrolyte 6, the surface area at a radius of 2.09 cm is 288 cm2 (2π*2.09 cm*22 cm) and the maximum power current density at this radius is 2.17 A/cm2 (625 Ah/1 h/288 cm2). From
For this example, the total voltage drop relative to open circuit voltage during a high rate, 1 hour full discharge will be 1.65 volts.
The energy density can be approximated by considering the mass of the components needed to construct the cell.
The biggest single material impacting volumetric energy density is lithium at 300 cm3. A volume of 437 cm3 has be allocated within electrolyte reservoirs 6 and 8 to accommodate the 200 cm3 of molten salt plus 173 cm3 of lithium-oxygen reaction product at full discharge. The assessment allocated 200 grams for balance of plant components that may be shared with other cells within an overall battery system including an air blower and conduits, thermal insulation, recuperative heat exchange, electrodes and terminal interconnects.
Based on this example analysis, the approximate volumetric energy density is 3.1 kWh/l, (1,875 Wh/604 cm3) and the fully discharged specific energy is 1.29 kWh/kg. It should be noted that reducing the molten salt allocation to 300 grams would result in a fully discharged specific energy of 1.6 kWh/kg.
It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.
This application claims priority to co-pending U.S. Provisional Application No. 62/829,108, filed Apr. 4, 2019, the disclosure of which is herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62829108 | Apr 2019 | US |
