Metal/air flow battery

Abstract

In one embodiment, a battery system includes a negative electrode, a separator adjacent to the negative electrode, a positive electrode separated from the negative electrode by the separator, the positive electrode including an electrode inlet and an electrode outlet, an electrolyte including about 5 molar LiOH located within the positive electrode, and a first pump having a first pump inlet in fluid communication with the electrode outlet and a first pump outlet in fluid communication with the electrode inlet and controlled such that the first pump receives the electrolyte from the electrode outlet and discharges the electrolyte to the electrode inlet during both charge and discharge of the battery system.

Claims

1. A battery system, comprising: a negative electrode; a separator adjacent to the negative electrode; a positive electrode separated from the negative electrode by the separator, the positive electrode including an electrode inlet and an electrode outlet; an electrolyte including LiOH located within the positive electrode; a reservoir having a reservoir inlet configured to receive the electrolyte from the positive electrode, a first reservoir outlet, and a second reservoir outlet, the second reservoir outlet located lower than the first reservoir outlet such that an unattached solid proximate the first reservoir outlet is forced by gravity toward the second reservoir outlet; a first pump having a first pump inlet configured to take a suction on the first reservoir outlet and discharge a first effluent through a first pump outlet to the electrode inlet and controlled such that the first pump receives the electrolyte from the reservoir inlet and discharges the first effluent to the electrode inlet during both charge and discharge of the battery system; and a second pump having a second pump inlet configured to take a suction on the second reservoir outlet and a second pump outlet configured to discharge a second effluent to the electrode inlet, the system controlled such that the first effluent is not mixed with the second effluent during discharging of the battery system, and such that the first effluent is mixed with the second effluent during charging discharging of the battery system.

2. The battery system of claim 1, wherein the reservoir is located lower than the electrode outlet such that an unattached solid within the positive electrode is forced by gravity toward the reservoir inlet.

3. The battery system of claim 1, wherein the reservoir comprises: a nucleation structure configured to facilitate crystallization of super-saturated LiOH electrolyte into LiOH.H.sub.2O.

4. The battery system of claim 1, wherein the second pump is a peristaltic pump.

5. The battery system of claim 1, further comprising: a mixer component in fluid communication with the first pump, the second pump, and the electrode inlet, the mixer configured to receive and mix the first effluent from the first pump outlet and the second effluent from the second pump outlet, and to provide the mixed electrolyte to the electrode inlet.

6. The battery system of claim 5, further comprising: a first heat exchanger located between the mixer and the electrode inlet.

7. The battery system of claim 6, wherein the first heat exchanger is configured to maintain the electrolyte provided to the electrode inlet at a temperature above a solubility limit for LiOH in the mixed first effluent and second effluent.

8. The battery system of claim 6, further comprising a second heat exchanger, the second heat exchanger located within the reservoir.

9. The battery system of claim 8, wherein the second heat exchanger is controlled to heat electrolyte within the reservoir during charging.

10. The battery system of claim 9, wherein the second heat exchanger is controlled to cool electrolyte within the reservoir during discharging.

11. The battery system of claim 5, further comprising: a mixing control valve in fluid communication with the first pump, the second pump, and the mixer, and located between the mixer and the first pump outlet and between the mixer and the second pump outlet.

12. The battery system of claim 1, wherein the second pump is controlled such that during discharge of the battery system the second pump is not energized and during charge of the battery system the second pump is energized.

13. A method of operating a battery system comprising: providing a negative electrode; providing a separator adjacent to the negative electrode; providing a positive electrode separated from the negative electrode by the separator, the positive electrode including an electrode inlet and an electrode outlet; providing an electrolyte including LiOH located within the positive electrode; storing LiOHH.sub.2O within a reservoir having a reservoir inlet in fluid communication with the electrode outlet, a first reservoir outlet, and a second reservoir outlet, the second reservoir outlet located lower than the first reservoir outlet such that an unattached solid proximate the first reservoir outlet is forced by gravity toward the second reservoir outlet; operating a first pump to take a suction from the first reservoir outlet and discharge a first effluent to the electrode inlet during both charge and discharge of the battery system; controlling a second pump to take a suction from the second reservoir outlet and discharge a second effluent such that during discharge of the battery system the second pump is not energized and during charge of the battery system the second pump is energized; and mixing the first effluent with the second effluent during charge of the battery system.

14. The method of claim 13, further comprising: crystallizing super-saturated LiOH electrolyte into LiOH.H.sub.2O using a nucleation structure within the reservoir during discharge of the battery system.

15. The method of claim 13, wherein: mixing the first effluent and the second effluent comprises mixing the first effluent and the second effluent with a mixing component; and the method further comprises providing the mixed first effluent and second effluent to the electrode inlet.

16. The method of claim 15, further comprising: maintaining the mixed first effluent and second effluent provided to the electrode inlet at a temperature above a solubility limit for LiOH in the mixed first effluent and second effluent.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 depicts a plot showing the relationship between battery weight and vehicular range for various specific energies;

(2) FIG. 2 depicts a chart of the specific energy and energy density of various lithium-based cells;

(3) FIG. 3 depicts a prior art lithium-oxygen (Li/oxygen) cell including two electrodes, a separator, and an electrolyte;

(4) FIG. 4 depicts discharge curves for a metal/oxygen battery showing an increasing difference between the actual capacity of a battery and the theoretical capacity of the battery as the rate of discharge increases;

(5) FIG. 5a depicts a portion of a carbon matrix including a pore opening into the matrix through a neck;

(6) FIG. 5b depicts the carbon matrix of FIG. 5a after discharge product has begun to coat portions of the neck and pore;

(7) FIG. 5c depicts the carbon matrix of FIG. 5a when the entire surface of the pore and neck have been coated, thereby precluding electrons from being available to form additional discharge products;

(8) FIG. 5d depicts the carbon matrix of FIG. 5a when the neck has been blocked by discharge products, thereby precluding Li.sup.+ and O.sub.2 from being available to form additional discharge products;

(9) FIG. 6 depicts a schematic view of a lithium-oxygen (Li/oxygen) system with a reservoir configured to store discharge products outside of the positive electrode;

(10) FIG. 7 depicts the system of FIG. 6 after discharge of the system has caused discharge products to be collected in the reservoir;

(11) FIG. 8 depicts the system of FIG. 6 after charging of the system has been initiated, and the discharge products in the reservoir are being dissolved into the electrolyte; and

(12) FIG. 9 depicts a schematic view of a lithium-oxygen (Li/oxygen) system with a reservoir configured to store discharge products outside of the positive electrode where a nucleation structure is provided in the reservoir.

DETAILED DESCRIPTION

(13) A schematic of a battery system 100 is shown in FIG. 6. In one embodiment, the battery system 100 is used to power a vehicle motor 102 through a bi-directional inverter 104. The battery system 100 includes a cell or cell stack 106 which includes a lithium negative electrode 108, a separator 110, and a positive electrode 112. The separator 110 is a dense solid electrolyte that transports metal ions but is a barrier to electrons, liquid electrolyte, and oxygen or other species.

(14) The positive electrode 112 is a porous matrix of electronically conducting material defining a continuous network of passages. The positive electrode 112 includes carbon or some other electronically conductive material that provides a continuous path for electrons, optionally catalyst materials (especially for oxygen reduction and evolution in aqueous chemistries), and optionally binder material. The electrode is porous, with electrolyte 160 in the pores and, in some embodiments, includes gas channels. The conductive material is contacted to an electronically conductive gas-diffusion layer (e.g., from carbon fiber) at the back side (opposite side from the separator 110). The gas-diffusion layer (GDL) is open to oxygen or air (from the environment or from an oxygen tank).

(15) The positive electrode 112 is in fluid connection with a reservoir 118 through a header 120. The reservoir 118 includes two outlets 122 and 124. The outlet 122 is connected to the suction side of a centrifugal pump 126 by a discharge header 128. A discharge header control valve 130 is located in the discharge header 128. In one embodiment (not shown), the discharge header control valve 130 is located in an outlet header 132 which connects the centrifugal pump 126 to a supply header 134.

(16) The outlet 124 is connected to the suction side of a peristaltic pump 140 by a charging header 142. A charging header control valve 144 is located in the charging header 142. An outlet header 146 connects the pump 140 to the supply header 134. A mixing control valve 148 is located in the outlet header 146.

(17) The supply header 134 is in fluid communication with the positive electrode 112. A mixer 150 and a heat exchanger portion 152 of a temperature control unit 154 are operably positioned within the supply header 134. Additionally, an exchanger portion 156 of a temperature control unit 158 is operably positioned within the reservoir 118.

(18) In the embodiment of FIG. 6, the reservoir 118, positive electrode 112, and headers 128, 132, 134, 142, and 146 are substantially filled with an aqueous electrolyte including about 5 molar (M) LiOH and any desired additives. The aqueous electrolyte further includes dissolved O.sub.2.

(19) In operation, the cell stack 106 provides DC power to the bidirectional inverter 104 which in turn powers the motor 102. Power in the cell stack 106 is generated by O.sub.2 reduction in the positive electrode 112 according to the following equation:

(20) $2{\mathrm{Li}}^{+}+\frac{1}{2}{O}_2+H_{2}O.Math.2\mathrm{Li}\mathrm{OH}\left(\mathrm{positive}\mathrm{electrode}\right)$

(21) As the battery system 100 is discharging, the pump 126 draws a suction on the reservoir 118 through the discharge header control valve 130 which is open, and discharges the electrolyte 160 from the reservoir 118 into the supply header 134. During discharge, the charging header control valve 144 and/or the mixing control valve 148 is shut. Accordingly, the electrolyte 160 within the supply header 134 passes through the mixer 150 and the heat exchanger 152 and then into the positive electrode 112. The heat exchanger 152 maintains the electrolyte 160 that is supplied to the positive electrode 112 at a desired temperature. In one embodiment, the electrolyte 160 that is supplied to the positive electrode 112 is maintained at about 25 C.

(22) The flow of electrolyte 160 into the positive electrode 112 provides a source of dissolved O.sub.2 for use in further oxidation. Additionally, oxygen is introduced into the positive electrode 112 through the GDL. In some embodiments, a GDL located along the flow path of the electrolyte outside of the positive electrode is provided in addition to or as an alternative to the GDL in the positive electrode 112.

(23) Accordingly, LiOH is generated in the positive electrode in accordance with the equation above as lithium moves from the negative electrode 108 through the separator 110 and into the positive electrode 112. As LiOH is generated, the concentration of the LiOH in the positive electrode 112 increases. The freshly supplied electrolyte 160 flushes the electrolyte 160 with the increased concentration of LiOH out of the positive electrode 112 and into the reservoir 118. The concentration of LiOH flushed into the reservoir 118 thus increases to above 5M.

(24) The reservoir 118 is configured to encourage formation of monohydrate crystals as the concentration of LiOH in the electrolyte 160 within the reservoir 118 increases during charge. In different embodiments, one or more of gravitational separation, mechanical, evaporative, and thermal separation, or nucleation is used to encourage formation of monohydrate crystals. Thermal management is used in some embodiments to change the concentration (via evaporation) or solubility limit (via cooling) of the LiOH within the electrolyte 160, with both mechanisms resulting in precipitation of the monohydrate crystals. Filtration in some embodiments includes the use of a fine mesh that collects solid particles larger than the mesh size. The filtration system in some embodiments is self-cleaning so that the collected particles are moved to a separate storage area. In some embodiments, nucleation particles are injected into the reservoir 118. In embodiments including an ethanol/H.sub.2O solvent mixture, water is injected resulting in a lowering the solubility limit of LiOH in the electrolyte 160, thus resulting in spontaneous LiOH.H.sub.2O precipitation

(25) By way of example, in the embodiment of FIG. 6 the heat exchanger 156 may be used to cool the electrolyte 160 within the lower portion of the reservoir 118 to less than 25 C. The solubility limit of LiOH is 5.3M at 25 C. Thus, by cooling the electrolyte 160 within the reservoir 118 to a temperature below 25 C., monohydrate crystals will form as the LiOH enriched electrolyte 160 from the positive electrode 112 enters into the reservoir 118.

(26) The embodiment of FIG. 6 further incorporates a gravitational mechanism to encourage formation of monohydrate crystals within the reservoir 118. As noted above, the charging header control valve 144 and/or the mixing control valve 148 is shut during discharge. Accordingly, a dead zone is created at the lower portion of the reservoir 118 since suction is taken by the centrifugal pump 126 at a level (outlet 122) located above the level of the outlet 124. Accordingly, gravity produces a density gradient of the electrolyte 160 and monohydrate crystal mix in the reservoir 118 with solid products including LiOH.H.sub.2O, to settle to the bottom of the reservoir 118.

(27) Discharge of the system 100 thus results in the configuration of FIG. 7. In FIG. 7, monohydrate crystals 162 and electrolyte 160 are located within the lower portion of the reservoir 118 as a slurry. Because the LiOH is precipitated within the lower section of the reservoir 118, the concentration of the electrolyte 160 at the outlet 122 is maintained at about 5.3 M. Accordingly, the electrolyte 160 provided to the electrode 112 is maintained at about 5M even during discharge. This reduces the potential of crystal formation within the positive electrode 112.

(28) When the battery system is to be charged, it is not necessary to change the direction of flow. The solid LiOH.H.sub.2O is in equilibrium with dissolved LiOH.H.sub.2O. Therefore, simply reversing the direction of the current in the cell 106 will result in consumption of LiOH.H.sub.2O as long as the flow is maintained. Accordingly, charging of the system 106 includes opening the charging header control valve 144 along with the mixing valve 148. The pump 126 and the pump 140 then take suction on the reservoir 118 as depicted in FIG. 8.

(29) The pump 126 pumps predominantly electrolyte 160 which, after discharge, may be slightly higher than 5M. The pump 140 pumps a mixture of predominantly electrolyte 160 and perhaps some monohydrate crystals 162. The electrolyte 160 pumped by the pump 140 will be at about the solubility limit for LiOH. The mixer 150 mixes the effluent from the two pumps 126/140. Accordingly, at least some of the monohydrate crystals 162 dissolve through the mixer 150. Additionally, the heat exchangers 156 and 152 may be used to increase the temperature of the electrolyte 160, thereby allowing for additional dissolution of the monohydrate crystals 162.

(30) Within the positive electrode 112, Li.sup.+ is removed during charge, thereby reducing the concentration of LiOH within the electrolyte 160. The electrolyte 160 with the lower concentration of LiOH is then moved into the reservoir 118. The lower concentration of LiOH in the electrolyte 160 entering the reservoir 118 allows for further dissolution of the monohydrate crystals 162 (via thermodynamic equilibrium) into the electrolyte 160 until either charging is completed or all monohydrate crystals 162 are removed.

(31) The above described processes are modified for various embodiments. In one embodiment, the discharge header control valve 130 is closed, and only effluent from the pump 140 is provided to the supply header 134. In some embodiments, the heat exchanger 156 is used to heat the slurry of electrolyte 160 and monohydrate crystals 162 prior to pumping, thereby increasing the molarity of the electrolyte in the lower portion of the reservoir 118 above 5.3M.

(32) Additionally, while the embodiment of FIG. 6 depicts a single cell 106, other embodiments include multiple cells 106 (a cell stack). Thus, a single reservoir services multiple cells 106. In yet another embodiment, multiple reservoirs 118 are provided.

(33) FIG. 9 depicts yet another embodiment of a battery system 200 incorporating features of the disclosure. The battery system 200 is used to power a vehicle motor 202 through a bi-directional inverter 204. The battery system 200 includes a cell or cell stack 206 which includes a lithium negative electrode 208, a separator 210, and a positive electrode 212 which may be constructed in the same manner as the like components in the embodiment of FIG. 6.

(34) The positive electrode 212 is in fluid connection with a reservoir 218 through a header 220. The reservoir 218 includes a single outlet 222. The outlet 222 is connected to the suction side of a centrifugal pump 226 by a discharge header 228. A discharge header control valve 230 is located in the discharge header 228. In one embodiment (not shown), the discharge header control valve 230 is located in an outlet header 232 which connects the centrifugal pump 226 to a supply header 234.

(35) The supply header 234 is in fluid communication with the positive electrode 212. A mixer 250 and a heat exchanger portion 252 of a temperature control unit 254 are operably positioned within the supply header 234.

(36) In the embodiment of FIG. 9, the reservoir 218, positive electrode 212, and headers 228, 232, and 234 are substantially filled with an aqueous electrolyte 160 including about 5 molar (M) LiOH and any desired additives. The aqueous electrolyte further includes dissolved O.sub.2.

(37) The battery system 200 further includes a nucleation structure 256. The nucleation structure 256 includes channels/piping/tubing with a bellmouth or other similar construction to facilitate crystallization of super-saturated LiOH electrolyte 260 into LiOH.H.sub.2O. The monohydrate crystals form over some fins, bellmouth or other similar construction.

(38) Operation of the system 200 is similar to the operation of the system 100. During discharge, crystal formation is avoided in the headers 228, 232, and 234 and the positive electrode 212 because of the flow of the electrolyte. The nucleation structure 256, however, provides a relatively stationary flow area. Accordingly, the monohydrate crystals preferentially form on the fins, bellmouth or other similar construction of the nucleation feature 256. During charging, 5M LiOH is circulated in the positive electrode 212 with Li.sup.+ being removed from the solution to the Li-anode 208. As this progresses, LiOH solution concentration falls below 5M, allowing monohydrate crystals to dissolve (via thermodynamic equilibrium) into the electrolyte 260 until either charging is completed or all monohydrate is removed. Further charging causes a drop in LiOH concentration.

(39) In some embodiments, an agitator is provided in the reservoir (118 or 218) to assist in dissolution of the monohydrate crystals during charge operations.

(40) In some embodiments, operation of the various components is controlled by a battery management system which includes a memory and a processor. The processor executes program instructions which are stored in the memory in order to control the various components. Accordingly, the processor can control the temperature of the electrolyte within the system, as well as the pump speed. Thus, during lower power operation, electrolyte is fed into the positive electrode at a slower rate.

(41) Additionally, various sensors may be provided throughout the system to assist in control of the system. The sensors include temperature sensors positioned to sense the temperature at the upper and lower portion of the reservoir, at the mixer, after the heat exchanger in the supply header, and at the discharge from the positive electrode. Pressure sensors and flow sensor are also provided in various embodiments.

(42) The processor in some embodiments is further configured to provide control of the position of the various control/mixing valves. Accordingly, the processor controls the amount of monohydrate crystals which may be pumped into the supply header so as to avoid clogging the pores of the positive electrode.

(43) By storing solid product (e.g., monohydrate crystals) in a reservoir, passivation of the cathode surface, which limits capacity and current density, is avoided. Additionally, mechanical stress on the cathode structure, caused by precipitation on the surface and in the pores of the cathode, is also reduced.

(44) By locating the reservoir outside of the current path, current gradients, which tend to be formed in the direction of gravity, or gradients of liquid porosity are avoided. Such gradients could result in non-uniform Li stripping (during discharge) or plating (during charge), hot spots, poor utilization of active electrode area, non-uniform aging, or other deleterious operation of the cell.

(45) The above described embodiments also prevent reaction of solid discharge products with cell components such as the cathode and the solid-electrolyte protection layer on top of the Li anode. Accordingly, any irreversible reaction of LiOH with CO.sub.2 to form Li.sub.2CO.sub.3 is avoided. Similarly, in aprotic embodiments, irreversible reactions of Li.sub.2O.sub.2 with carbon cathode materials to form Li.sub.2CO.sub.3 is avoided.

(46) The above described embodiments also separate the energy storage capacity and the power capability of the system. Hence, a wide variety of power/energy ratios can be easily implemented, simply by increasing or decreasing the size of the reservoir tank (as well as the Li anode metal thickness) relative to the total active area of the cell stack.

(47) Additionally, by controlling the flow through the cathode, a desired level of convective mass transfer is achieved that may increase the limiting current of the system, thereby enabling higher current densities and smaller (cheaper) cell stacks.

(48) Finally, because the discharge product is not stored within the positive electrode, the cells can be made thinner and more economical. A thinner cell design will have lower impedance (hence, higher current density is enabled).

(49) While the foregoing embodiments depicted aqueous cells, the system can be modified to extend to aprotic systems with some peroxide solubility. For example, certain solvents or additives can provide some solubility of Li.sub.2O.sub.2. The Li.sub.2O.sub.2 will precipitate as a solid once its solubility limit is surpassed. Hence, the above described modes of operation in which LiOH.H.sub.2O is the discharge product for aqueous systems are substantially identical to aprotic systems with peroxide solubility.

(50) By way of example, in embodiments incorporating aprotic Li/O.sub.2 cells, where Li.sub.2O.sub.2 is the discharge product, the electrolyte may contain a solvent that includes NH groups such that NH . . . O bonding provides solubility of the peroxide dianion (O.sub.2.sup.2). The electrolyte in some embodiments further includes an additive such as an anion receptor that recognizes the peroxide dianion (O.sub.2.sup.2), thereby providing solubility to the Li.sub.2O.sub.2 discharge product. Examples of anion receptors include a class of boron-based anion receptors as reported by Xie et al., New electrolytes using Li.sub.2O or Li.sub.2O.sub.2 oxides and tris(pentafluorophenyl) borane as boron based anion receptor for lithium batteries, Electrochem Comm., 2008, p. 1195, as well as a class of cryptands that make use of NH . . . O bonds to stabilize the O.sub.2.sup.2 as reported by Lopez et al., Reversible Reduction of Oxygen to Peroxide Facilitated by Molecular Recognition, Science 335, 450 (2012).

(51) Additionally, while the above described embodiments incorporated pumps to assist electrolyte flow, wherein the flow was always in the same direction, some embodiments are bi-directional flow embodiments, with the flow of electrolyte reversed for charging operations.

(52) 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.