Patent Application
20170047593
1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, to a low temperature molten salt battery with a corrosion-proof separator system.
2. Description of the Related Art
Flow-through batteries have been intensively studied and developed for large-scale energy storage due to their long cycle life, flexible design, and high reliability. A battery is an electrochemical device in which ions (e.g. metal-ions, hydroxyl-ions, protons, etc.) commute between the anode and cathode to realize energy storage and conversion. In a conventional battery, all the components including the anode materials, cathode materials, separator, electrolyte, and current collectors are contained within a volume-fixed compartment. Consequently, the battery's corresponding energy and capacity are fixed according to this configuration. A flow-through battery consists of current collectors (electrodes) separated by an ion-exchange membrane, while at least one of the anode or cathode materials are present in a separate storage tank. These materials are circulated through the flow-through battery in which electrochemical reactions take place to deliver and to store energy. Therefore, the battery capacity and energy are determined by (1) the type of anode and cathode active materials, and (2) the concentrations of anode and cathode active material.
A low temperature molten salt (LTMS) may be used as a cathode (catholyte) for rechargeable batteries operating at temperatures of less than 150° C., as described in the above-referenced parent application Ser. No. 13/564,015, which demonstrated high capacity solution-based catholytes and slurry cathodes.
The fundamental reason for the LATP corrosion is the strongly acidic environment arising from the hydrolysis of molten salts. For example, FeCl3.6H2O═Fe(OH)3+3Cl−+3H++3H2O. Generally, Li+-solid conductors, including LATP, Li7La3Zr2O12 (LLZO), LiLa2/3−xTiO3 (LLTO), LixPOyNz (LiPON), or Li10GeP2O12 (LGPS), cannot persevere in such acidic environments without corrosion.
In order to improve the battery stability, especially for LATP stability in acidic solutions, some strategic methods could be adopted. For example, buffer solutions can be used to modulate the pH value of the LTMS catholyte. Alternatively, polymer electrolytes could be used to modify the solid electrolyte surface to retard its corrosion/decomposition. Otherwise, inorganic compounds (e.g., TiN) could be coated onto the solid electrolyte surface to suppress corrosion. However, these solutions add complexity to the battery and their full consequences are not completely understood.
It would be advantageous if the cell structure of an alkali metal/LTMS catholyte battery could be modified to prevent the corrosion of the SE separator and improve battery performance.
Disclosed herein is a new cell structure for alkali metal low temperature molten salt (LTMS) batteries to improve their stability during cycling. The cell consists of three primary compartments which include: (1) an anode compartment containing an alkali metal, alkaline earth metal, or aluminum, (2) a cathode compartment containing a catholyte (e.g., LTMS), and (3) a buffering space. The buffering space is located between the anode and cathode compartments. A cation-permeable solid electrolyte separates the anode compartment and the buffering space, while an anion permeable membrane is interposed between the cathode compartment and the buffering space. Following at least one charge/discharge cycle, the buffering space becomes filled with a cation-anion salt solution. In one aspect, the LTMS catholyte can be refreshed by employing a flow-through mode cathode compartment. Advantageously, lithium (Li)/LTMS batteries can operate at temperatures below 100° C.
Accordingly, a method is provided for transporting ions in a battery having a corrosion-resistant ion-exchange membrane system. The method is applied to a battery with a cathode including an acidic catholyte, an anode including a metal that is chemically reactive towards water, and an ion-exchange membrane system. Some examples of anode metals include alkali metals, alkaline earth metals, and aluminum (Al). The ion-exchange membrane system includes a solid, cation-permeable, water-impermeable first membrane adjacent to the anode, prone to decomposition upon chemical reaction with an acid, an anion-permeable second membrane adjacent to the cathode, and a buffer compartment including a solution. The ion-exchange membrane system is interposed between the first membrane and the second membrane. During discharging of the battery, the solution in the buffer compartment accepts cations from the anode and anions from the cathode, forming a cation-anion salt solution in the buffer compartment.
The first membrane permits both the transport of cations from the anode to the buffer compartment, and prevents the transportation of anions from the buffering compartment to the anode. Some examples of first membrane materials include Li1+XAlXTi2−X(PO4)3 (LATP), Li7La3Zr2O12 (LLZO), LiXPOYNZ (LiPON), LixLa2/3−xTiO3 (LLTO), Li10GeP2O12 (LGPS), Na2M2TeO6, beta-alumina, Na1+xZr2SixP3−xO12, metal-organic frameworks (MOFs), (1−x)Mg(NO3)2−xAl2O3, magnesium zirconium phosphates, Al2(WO4)3, KSbO3, NaSbO3, K1−xAl1−xRxO2, and NaxAlyRzO2, where M is a transition metal, and where R may be silicon (Si), germanium (Ge), or titanium (Ti). The second membrane acts to prevent the transportation of cations from the buffer compartment to the cathode, as well as preventing the transportation of protons (Elk) from the catholyte to the buffer compartment. As a result of preventing the transfer of the protons from the catholyte to the buffer compartment, corrosion of the first membrane is inhibited.
In one aspect, the catholyte is a low temperature molten salt (LTMS). Some examples of a LTMS catholyte include FeCl3.6H2O and LiNO3, and FeCl3.6H2O and LiCl, Mn(NO3)3.6H2O, Mn(NO3)2.4H2O, MnCl2.4H2O, FeBr3.6H2O, KFe(SO4)2.12H2O, FeCl3.6H2O, Fe(NO3)3.9H2O, FeCl3.2H2O, Fe(NO3)2.6H2O, FeSO4.7H2O, CoSO4.7H2O, Co(NO3)2.6H2O, Ni(NO3)2.6H2O, Cd(NO3)2.4H2O, and Cd(NO3)2.H2O. The battery has an operating voltage range responsive to the pH value of the LTMS, which is less than 7. Advantageously, the battery (LTMS catholyte) has a liquid phase operating temperature of less than 100 degrees C.
Additional details of the above-described method and a battery having a corrosion-resistant ion-exchange membrane system are provided below.
An anion-permeable second membrane (AM) 214 is adjacent to the cathode 202. A buffer compartment 216 is interposed between the first membrane 212 and the second membrane 214, and comprises a solution 222 of material including cations 218 from the anode 206 and anions 220 from the cathode 202. Note: the cations and anions may only be present in the solution 222 in the buffer compartment after the battery 200 completes at least one charge/discharge cycle. In some aspects, the solution 222 (e.g., water or non-aqueous electrolyte) may contain no cations or anions prior to the initial charge/discharge cycle.
The first membrane 212 prevents the transportation of anions 220 from the buffer compartment 216 to the anode 206. The second membrane 214 prevents the transportation of cations 218 from the buffer compartment 216 to the cathode 202. The second membrane 214 also prevents the transportation of protons 224 from the catholyte 204 to the buffer compartment 216. As a result of preventing the transfer of the protons (also referred to as hydrons (H+)) 224 to the buffer compartment 216, corrosion of the first membrane 212 is prevented.
In one aspect as shown, an anode 206 is configured as a compartment with an electrolyte 226. However, the electrolyte is not always necessary if the anode metal 208 directly abuts the first membrane 212. Some examples of first membrane materials include Li1+XAlXTi2−X(PO4)3 (LATP), Li7La3Zr2O12 (LLZO), LiXPOYNZ (LiPON), LixLa2/3−xTiO3 (LLTO), Li10GeP2O12 (LGPS), Na2M2TeO6, beta-alumina, Na1+xZr2SixP3−xO12, metal-organic frameworks (MOFs), (1−x)Mg(NO3)2−xAl2O3, magnesium zirconium phosphates, Al2(WO4)3, KSbO3, NaSbO3, K1−xAl1−xRxO2, and NaxAlyRzO2;
where M is a transition metal; and,
where R is silicon (Si), germanium (GO, or titanium (Ti). In addition, the solid electrolyte first membrane may be a derivative of one of the above-listed materials, as these materials may be doped with elements such as aluminum (Al), magnesium (Mg), niobium (Nb), tantalum (Ta) and similar metals. Typically, the anode metal 208 is an alkali metal, alkaline earth metal, or aluminum (Al).
In one aspect as shown, the cathode 202 is configured as a cathode compartment containing a low temperature molten salt (LTMS) catholyte 204. Some examples of a LTMS catholyte 204 include FeCl3. 6H2O and LiNO3, and FeCl3.6H2O and LiCl, Mn(NO3)3.6H2O, Mn(NO3)2.4H2O, MnCl2.4H2O, FeBr3.6H2O, KFe(SO4)2.12H2O, FeCl3.6H2O, Fe(NO3)3.9H2O, FeCl3.2H2O, Fe(NO3)2.6H2O, FeSO4.7H2O, CoSO4.7H2O, Co(NO3)2.6H2O, Ni(NO3)2.6H2O, Cd(NO3)2.4H2O, and Cd(NO3)2.H2O.
With a LTMS as the catholyte, the battery 200 has a liquid phase operating temperature of less than 100 degrees C. The operating voltage range is responsive to the pH value of the LTMS, which is less than 7. The operating voltage range is a defined range in which the battery is able to be charged and discharged. During discharge, a metal anode is oxidized to metal ions and the ions in the catholyte are reduced from a high valence to a lower valance. During charge, this process is reversed. As used herein, “chemically reactive” refers to a scenario wherein a species can be transformed via chemical interaction(s) with one or more different species to form a new species that is chemically distinct from any of the original species. As used herein, “decompose” refers to a chemical transformation of species (such as those that constitute the first membrane 212) via chemical reaction with another species (such as a proton) and through which chemical composition(s) and/or physical properties are altered relative to the original such that performance (within the context of the intended function) is reduced relative to the original. Such decomposition may be catalyzed through chemical reaction with another chemical species.
Optionally as shown, the cathode 202 is a flow-through cathode. The cathode compartment has an input flow port 227, and an output flow port 228. A reservoir 230 containing LTMS catholyte is connected to the input 227 and output 228 flow ports. As another option, a pump 232 may be connected between the cathode compartment 202 and the reservoir 230 to supply a flow of LTMS catholyte from the reservoir in response to the LTMS catholyte 204 in the cathode compartment becoming discharged below a minimum threshold voltage, or the LTMS catholyte in the cathode compartment becoming charged above a maximum threshold voltage.
In the example shown, the anode metal 208 is lithium (Li), the catholyte (LTMS) 204 is FeCl3.6H2O and LiCl, the cations 218 are Li+, the anions 220 are Cl−, and the protons 224 are H+.
As noted, the battery cell structure is not limited to only the LTMS catholyte battery shown, but may also be adopted for use in other batteries containing water-sensitive/reactive anodes and water-based cathodes/catholytes separated by an acid-sensitive solid ionic electrolyte membrane 212. The cell structure includes at least three compartments. A first compartment 202 contains the LTMS catholyte 204 as the battery cathode. A second compartment 206 contains a water-reactive metal anode 208 (alkali metal in the example shown). The third compartment (ion-exchange membrane system) 210, which is located between the cathode and anode compartments, functions as a buffering space. An alkali-ion solid electrolyte membrane 212 separates the anode compartment 206 and the buffer compartment 216, while an anion permeable membrane 214 separates the cathode compartment 202 from the buffer compartment 216. The battery 200 can be operated at temperatures of 100° C. or lower, since the liquid phase operating temperature of the LTMS catholyte is 100° C. or lower. In general, the LTMS catholyte is acidic due to the reaction of metal-ion hydrolysis. In fact, all Li+-solid conductors, such as LATP, Li7La3Zr2O12 (LLZO), LixLa2/3−xTiO3 (LLTO), LixPOyNz(LiPON) and Li10GeP2O12 (LGPS), are readily corroded in the acidic solutions.
The acidity of the LTMS catholyte is dependent upon the types of metal ions as well as their activity in the liquid. Aside from decomposing the solid ionic electrolyte first membrane 212, the acidic degree (or pH) of the LTMS catholyte 204 impacts the potentials of redox couples as well as its own stability. For example, if a LTMS catholyte contains Fe-ions, its hydrolysis reaction can be expressed as the following:
Fe3++3H2O═Fe(OH)3+3H+; (1)
In terms of the reaction, the corresponding pH value of the solution can be derived in theory from the following:
whereby aFe
In order to evaluate the pH value due to Fe3+ under different conditions, four types of solutions were prepared. Sample (a) was 1M Fe(NO3)3 dissolved in water, Sample (b) was 1M FeCl3 dissolved in water, Sample (c) was an LTMS catholyte consisting of FeCl3.6H2O and LiNO3 (mol %:mol %=1:1), and Sample (d) was an LTMS catholyte consisting of FeCl3.6H2O and LiCl (mol %:mol %=1:1). At a temperature of 20.4° C., their corresponding pH values were 0.4, 1.0, −1.8, and −1.3, respectively. According to Eq. (2), the pH value of the 1M Fe3+ solution should be 1.15. The experimental pH values for 1M Fe(NO3)3 and 1M FeCl3 solutions were in agreement with the theoretical values. The difference between the experimental and theoretical data might arise due to the effect of anions (Cl− and NO3−) in the solutions on the activities of Fe. Iron nitrate [Fe(NO3)3] solution was more acidic than iron chloride [FeCl3] solution.
In LTMS catholytes, Fe ions react with crystal water and show a very strong acidity. The addition of lithium salts, for example LiCl and LiNO3, significantly affect properties such as the pH values. For instance, NO3− made the liquid mixture more acidic than Cl−, which was very similar to their behaviors in the water solutions. Another consideration is the melting point of the molten-salt system. In general, it was observed that FeCl3.6H2O+LiNO3 was not completely liquefied at room temperature and had a high viscosity while, in contrast, the FeCl3.6H2O+LiCl system had the appearance of a solution.
The pH value of the molten-salt catholyte determines the operating voltage range for the lithium/LTMS battery since water decomposition must be avoided in the system. According to the Nernst equation, the potential dependence of pH is
E=E
o−0.059 pH (3)
Therefore, the water decomposition voltages are 3.15-4.38 V and 3.12-4.35 V versus Li/Li+ for FeCl3.6H2O+LiNO3 and FeCl3.6H2O+LiCl systems, respectively. Noteworthy is the fact that commercial solid electrolytes (e.g. Li1+x+yAlxTi2−xSiyP3−yO12) are not stable in acidic solutions. Therefore, the novel cell structure disclosed herein was developed for the LTMS catholyte battery to avoid corrosion of the solid electrolyte by acidic species.
To explain the working mechanisms of the novel cell structure, a Li/FeCl3.6H2O LTMS battery is used as an example, as shown in
In the Li/FeCl3.6H2O LTMS battery example, the anode includes a lithium metal with/without organic electrolyte. During discharge, metallic lithium is oxidized to Li+-ions and electrons. Li+-ions pass through the solid electrolyte (SE) first membrane 212 and into the buffer compartment 216. The electrons move to the cathode through the external circuit (load) and reduce Fe3+-ions to Fe2+-ions. At the same time, Cl−-ions move through the anion-exchange membrane (AM) 214 from the cathode compartment 202 into the buffer compartment 216 in order to neutralize charges. Due to the repulsion caused by the anion-exchange membrane 214, protons 224 are confined to the cathode compartment 202 and cannot contact with the lithium ionic conductor first membrane 212. Therefore, corrosion/decomposition of the solid electrolyte first membrane 212 by the action of protons is eliminated. As a result, the LTMS catholyte battery demonstrates stable behavior.
Step 302 provides a battery comprising a cathode including an acidic catholyte, an anode including a metal that is chemically reactive towards water, and an ion-exchange membrane system. Some examples of anode metals include alkali metals, alkaline earth metals, and aluminum (Al). The ion-exchange membrane system comprises a solid, cation-permeable, water-impermeable first membrane adjacent to the anode, prone to decomposition upon chemical reaction with an acid, an anion-permeable second membrane adjacent to the cathode, and a buffer compartment including a solution. The ion-exchange membrane system is interposed between the first membrane and the second membrane. Some examples of first membrane materials include Li1+XAlXTi2−X(PO4)3 (LATP), Li7La3Zr2O12 (LLZO), LiXPOYNZ (LiPON), LixLa2/3−xTiO3 (LLTO), Li10GeP2O12 (LGPS), Na2M2TeO6, beta-alumina, Na1+xZr2SixP3−xO12, metal-organic frameworks (MOFs), (1−x)Mg(NO3)2−xAl2O3, magnesium zirconium phosphates, Al2(WO4)3, KSbO3, NaSbO3, K1−xAl1−xRxO2, and NaxAlyRzO2;
where M is a transition metal; and,
where R is Si, Ge, or Ti.
Step 304 discharges the battery. In response to discharging the battery, in Step 306 the solution in the buffer compartment accepts cations from the anode and anions from the cathode. Step 308 forms a cation-anion salt solution in the buffer compartment. In one aspect prior to charging and discharging the battery, Step 302 initially provides a solution in the buffer compartment free of cations and anions.
In Step 307a the first membrane prevents the transportation of anions from the buffer compartment to the anode. In Step 307b the second membrane prevents the transportation of cations from the buffer compartment to the cathode. In Step 307c the second membrane prevents the transportation of protons from the catholyte to the buffer compartment. In response to preventing the transfer of protons to the buffer compartment, Step 307d prevents the corrosion of the first membrane.
In one aspect, the catholyte provided in Step 302 is a LTMS catholyte, such as FeCl3.6H2O and LiNO3, and FeCl3.6H2O and LiCl, Mn(NO3)3.6H2O, Mn(NO3)2.4H2O, MnCl2.4H2O, FeBr3.6H2O, KFe(SO4)2.12H2O, FeCl3.6H2O, Fe(NO3)3.9H2O, FeCl3.2H2O, Fe(NO3)2.6H2O, FeSO4.7H2O, CoSO4.7H2O, Co(NO3)2.6H2O, Ni(NO3)2.6H2O, Cd(NO3)2.4H2O, and Cd(NO3)2.H2O. The LTMS catholyte has a liquid phase operating temperature of less than 100 degrees C., and has an operating voltage range responsive to the pH value of the LTMS, which is less than 7. The pH remains at a value of less than 7 regardless of whether the battery is charged or discharged, as protons don't participate in the electrochemical reactions.
A battery with a corrosion resistant ion-exchange membrane system has been provided. Examples of catholyte and anode materials have been presented to illustrate the invention. However, the invention is not limited to merely these examples. Other variations and embodiments of the invention will occur to those skilled in the art.
The application is a Continuation-in-Part of a pending application entitled, BATTERY WITH LOW TEMPERATURE MOLTEN SALT (LTMS) CATHODE, invented by Yuhao Lu et at, Ser. No. 13/564,015, filed on Aug. 1, 2012, Attorney Docket No. SLA3165, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13564015 | Aug 2012 | US |
| Child | 15339769 | US |
