Metal nitric oxide electrochemical cells and methods of making and using thereof

Abstract

Disclosed herein are metal nitric oxides batteries as well as methods of making and using thereof.

Description

[0001]Attorney Docket No.103361-602WO1 Metal Nitric Oxide Electrochemical Cells and Methods of Making and Using Thereof CROSS-REFERENCE TO RELATED APPLICATIONS This application claims benefit of priority of U.S. Provisional Application No. 63 / 543,731, filed October 11, 2023, which is incorporated herein by reference in its entirety. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT This invention was made with government support under Grant / Contract No.2124604 awarded by the National Science Foundation. The government has certain rights in the invention. BACKGROUND Redox flow batteries (RFBs) represent one of the most promising electrochemical energy storage technologies for supplying intermittent renewable energy into the electrical grid. RFBs store energy with liquid electrolytes comprised of redox-active materials. Electrolytes on the positive side (catholyte) and negative side (anolyte) are pumped through an electrochemical cell where electron transfer reactions occur to generate or store energy. The unique configuration of RFBs allows simple scaling of energy storage capacity by increasing (1) the volume of electrolytes or (2) the concentration of the redox-active solute (Figure 1A). Despite these advantages, the sustainability issue of RFBs remains unresolved. The production of conventional RFBs relies on finite and unsustainably-sourced transition metals, such as vanadium. If this dependence continues, the cost and environmental impact of mining metal will start to restrict the adoption of RFBs. For example, the price of vanadium has increased by more than 10-fold from 2009 to 2019. Accordingly, improved RFBs as well as improved methods of making RFBs are needed. SUMMARY The strong ties between energy storage technologies and finite metal resources must be broken for a path toward sustainable energy storage to emerge. New battery technology based on abundant resources, such nitrogen oxides (NOx) can provide a more sustainable route to renewable energy storage. In contrast to the frequent disruptions of global mineral Attorney Docket No.103361-602WO1 supplies, the prices of nitrogen sources (ammonia and diammonium phosphate) have been stable at a low 0.6-1.0 $ / kg (four orders of magnitude cheaper than vanadium) over the past decade as a result of the substantial production of fertilizers. The choice of nitrogen over vanadium is advantageous from both environmental and socio-economic perspectives. Described herein are metal nitric oxides batteries that can exhibit a high energy density of a maximum specific energy (e.g., an energy density of a maximum specific energy of 488 Wh / kg, which is twenty times higher than state-of-the-art vanadium redox flow battery (20 Wh / kg) and comparable to Li-ion battery (380-460 Wh / kg)). By way of example, described herein is a high-potential (0.5 V vs. Ag / Ag+) catholyte for a redox flow battery (RFB, where the charged and discharged species are both gaseous nitrogen oxides (NOx). In some examples, these species can be liberated from the liquid electrolyte and stored in a separate gas container, allowing scale-up of storage capacity without increasing the concentration and volume of the electrolyte. The oxidation of NO in the presence of NO3- affords N2O3, and the reduction of N2O3 regenerates NO and NO3-, together affording the electrochemical reaction: NO3- + 3NO ⇄ 2N2O3 + e- with a low mass / charge ratio of 152 grams per mole of stored electron. A proof-of-concept NOx symmetric H-cell shows 200 stable cycles over 400 hours with >97% Coulombic efficiency and negligible capacity decay. DESCRIPTION OF DRAWINGS Figures 1A-1C. (1A) Factors that impact the energy storage in a redox flow battery. An increase in concentration (C) or electrolyte volume (V) will improve the energy stored, however, at the cost of cycling performance. (1B) Prices of vanadium (vanadium pentoxide) and nitrogen (ammonia and diammonium phosphate) from 2009 to 2019. The unit price was normalized based on the weight percentage of the element in each source. (1C) Illustration of an example Li-NOx battery. The gaseous NOx can be stored in a tank and supplied via a gas pump. Figure 2. Schematic of an example Li-NOx RFB cell with NOx cathode and Li anode, performing the redox reaction: 3NO(g)+NO3− 2 N2O3 + e-. N2O3 dissociates into NO2(g) and NO(g) in gas phase, both of which can stored in a connected gas tank. Figure 3. Cyclic voltammograms of 27 mM N2O3and 0.99 mM NO in 0.1 M TBAClO4 / MeCN electrolyte on a glassy carbon electrode (d = 3 mm) at scan rates of 0.1 V / s. Attorney Docket No.103361-602WO1 Figures 4A-4C. (4A) Schematic of a symmetric NOx cell. (4B) Selected charge- discharge profile (voltage vs time) for the cycling of NOx cathode: NO3−+ 3NO ⇌ 2 N2O3 + e−(5 mM NO3−in MeCN). (4C) Coulombic efficiency (%) and capacity (mAh / g) vs. cycle number for 200 cycles. Figures 5A-5B. (5A) Charge-discharge profile (voltage vs capacity) for NOx cathode with (red trace) and without (black trace) nitrate. (5B) Charge-discharge profile (voltage vs capacity) for NOxcathode with limited amount of NO (2 ml NO and 1 mL O2for each side) and excess amount of NO (NO filled in the head space and 1 mL O2). Figures 6A-6B. (6A) UV-vis profile of an N2O3 solution as a function of time. (4B) Digital picture of Schlenk UV-vis cuvette for in-situ UV-vis study. Figure 7. Digital picture of H-cell for in-situ IR study. Figures 8A-8C. (8A) Illustration of the preliminary Swagelok cell setup. (8B) Components in the electrochemical cell. (8C) Picture of the next-generation Swagelok cell. Figure 9. Plot of potential (V) vs. capacity (mAh) for the first cycle of Li-NOxcell with MeCN (4.2 M LiTFSI) as electrolyte. Figures 10A-10C. (10A, 10B) Digital picture of Li-NOx cells for testing the performance of solid-state electrolyte for Li-NOxcell. (10C) A simplified diagram of Li- NOx cell with SSE. Figure 11. Schematic of Li-NOx flow cell. Red(left): high pressure zone (up to 12 bars), Blue(right): low pressure zone (1-2 bars). Figure 12. Scheme illustrating the electrochemical reduction of N2O3, which produces NO3−and NO, which can be recharged back to NO and NO2 reversibly. Figures 13A-13D. An example Li-NOxcell with Li metal as the negative electrode and carbon felt as the positive electrode. Figure 14. Schematic showing the Li-NOx cell connected to a gas supply system that delivers NOx in a controlled flow rate of NO and NO2. Figures 15A-15F. (15A) Schematic of the battery configuration of the Li-NOxcell. (15B) Plot showing that the discharge of the Li-NOx cell exhibits a capacity of 25 mAh cm−2or 1,570 mAh gcarbon−1. (15C) Plot showing the UV-vis calculated Coulombic efficiency (%) of the Li-NOxcell across the discharge capacity ranging from 1 to 25 mA h cm–2. (15D) Plot showing the X-ray diffraction (XRD) spectra of the carbon electrode after discharge compared with the X-ray diffraction (XRD) spectra of LiNO3. The comparison suggest that the carbon electrode is passivated by LiNO3precipitate. (15E) Scanning Attorney Docket No.103361-602WO1 Electron Microscopy (SEM) images of the carbon felt electrode after full discharge. The images show the formation of a particular coating (LiNO3precipitate) on the carbon electrode. (15F) Plot showing the TPPCo-NO calculated Coulombic efficiency (%) of the Li-NOx cell across a range of gas feeds. Figures 16A-16F. (16A) Plot shows that the capacity of the Li-NOxcells increases as a function of the concentration of LiNO3. (16B) Schematic illustrating the transparent 6- mL volume H-cell used in analysis. (16C) Plot showing the UV-vis calculated Coulombic efficiency (%) of the Li-NOx cell across a range of LiNO3 concentrations. (16D) Plot of change / discharge cycles showing a reduced cell voltage from 3.85 V to 3.5 V following purging with N2, suggesting NO2 is required for discharge. (16E) Plot showing the analysis of the charge / discharge reactions of the Li-NOx cell probed under controlled gas and salt conditions. With excess NO in the headspace, the round-trip voltage profile of charge state (NO + NO2) or the discharge state (NO + LiNO3) remained consistent. (16F) Plot showing the analysis of the charge / discharge reactions of the Li-NOx cell probed with NO2-only or NO without LiNO3. The results show poor cycling with increased overpotential and poor reversibility due to the different redox mechanisms, underscoring the role of excess NO in the headspace for maintaining stable operation of the Li-NOx cell. Figures 17A-17F. (17A) Plot of the round-trip efficiency and Coulombic efficiency of a Li-NOxcell as a function of cycle number (n). (17B) Plot showing that even at a high current of 2.0 mA cm-2, the Li-NOx cell maintains a steady discharge plateau with at approximately 3.3 V. (17C) A polarization curve suggesting that the Li-NOx cell achieves a maximum power of approximately 10.5 mW cm-2. (17D, 17E) Plots comparing the performance of the example Li-NOx cell to other types of battery systems. (17F) Plot showing that the Li-NOx cell retains 99.1% of its discharge capacity after resting for 24 hours, suggesting that the cell does not exhibit self-discharge and maintains its stability of the charge species (N2O3) under resting conditions. Figure 18. An example Li-NOx closed cell. DETAILED DESCRIPTION The present invention can be understood more readily by reference to the following detailed description, examples, drawings, and claims, and their previous and following description. However, before the present articles, systems, and / or methods are disclosed and described, it is to be understood that this invention is not limited to the specific or Attorney Docket No.103361-602WO1 exemplary aspects of articles, systems, and / or methods disclosed unless otherwise specified, as such can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting. The following description of the invention is provided as an enabling teaching of the invention in its best, currently known aspect. To this end, those skilled in the relevant art will recognize and appreciate that many changes can be made to the various aspects of the invention described herein, while still obtaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be obtained by selecting some of the features of the present invention without utilizing other features. Accordingly, those of ordinary skill in the pertinent art will recognize that many modifications and adaptations to the present invention are possible and may even be desirable in certain circumstances and are a part of the present invention. Thus, the following description is again provided as illustrative of the principles of the present invention and not in limitation thereof. Definitions It is appreciated that certain features of the disclosure, which are, for clarity, described in the context of separate aspects, can also be provided in combination in a single aspect. Conversely, various features of the disclosure, which are, for brevity, described in the context of a single aspect, can also be provided separately or in any suitable subcombination. As used in the description and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. As used herein, the terms “optional” or “optionally” mean that the subsequently described event or circumstance may or may not occur and that the description includes instances where said event or circumstance occurs and instances where it does not. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting. As used in the specification and in the claims, the term “comprising” can include the aspects “consisting of” and “consisting essentially of.” Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In this specification and in the claims, which follow, reference will be made to a number of terms that shall be defined herein. Attorney Docket No.103361-602WO1 For the terms "for example" and "such as," and grammatical equivalences thereof, the phrase "and without limitation" is understood to follow unless explicitly stated otherwise. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Furthermore, when numerical ranges of varying scope are set forth herein, it is contemplated that any combination of these values inclusive of the recited values may be used. Further, ranges can be expressed herein as from “about” one particular value and / or to “about” another particular value. When such a range is expressed, another aspect includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint and independently of the other endpoint. Unless stated otherwise, the term “about” means within 5% (e.g., within 2% or 1%) of the particular value modified by the term “about.” Throughout this disclosure, various aspects of the invention can be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as individual numbers within that range, for example, 1, 2, 2.7, 3, 4, 5, 5.3, 6 and any whole and partial increments therebetween. This applies regardless of the breadth of the range. It will be understood that, although the terms "first," "second," etc., may be used herein to describe various elements, components, regions, layers, and / or sections. These elements, components, regions, layers, and / or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, or section from another element, component, region, layer, or a section. Thus, a first element, Attorney Docket No.103361-602WO1 component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of example embodiments. As used herein, the term "substantially" means that the subsequently described event or circumstance completely occurs or that the subsequently described event or circumstance generally, typically, or approximately occurs. As used herein, the term “substantially,” in, for example, the context “substantially identical” or “substantially similar” refers to a method or a system, or a component that is at least about 80%, at least about 85%, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, or about 100% by similar to the method, system, or the component it is compared to. While aspects of the present invention can be described and claimed in a particular statutory class, such as the system statutory class, this is for convenience only, and one of ordinary skill in the art will understand that each aspect of the present invention can be described and claimed in any statutory class. Unless otherwise expressly stated, it is in no way intended that any method or aspect set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not specifically state in the claims or descriptions that the steps are to be limited to a specific order, it is in no way intended that an order be inferred in any respect. This holds for any possible non-express basis for interpretation, including matters of logic with respect to arrangement of steps or operational flow, plain meaning derived from grammatical organization or punctuation, or the number or type of aspects described in the specification. The present invention may be understood more readily by reference to the following detailed description of various aspects of the invention and the examples included therein and to the Figures and their previous and following description. Electrochemical Cells Described herein are electrochemical cells, including batteries (e.g., metal nitric oxide batteries). For example, provided herein are batteries that comprise an anode comprising a metal as an active anodic ingredient; a cathode in contact with a gas comprising nitric oxide (NO); and an electrolyte disposed between the anode and the cathode. Attorney Docket No.103361-602WO1 The battery can comprise a catholyte design based on the redox interconversion of gaseous NOx. In some embodiments, the battery can comprise a catholyte comprising a nitrogen oxide of formula NxOy, wherein x is 1 or 2 and y is an integer of 1 to 4. In certain embodiments, the catholyte comprises N2O3. In certain embodiments, the battery comprises a catholyte utilizing a NO3− + 3NO ⇌ 2 N2O3+ e- redox couple. In some embodiments, the gas comprises a gas mixture comprising NO and NO2. In certain embodiments, the gas is stored in a gas tank and supplied to the cathode through a gas mass flow controller. In some embodiments, the battery comprises a flow cell. In other embodiments, the battery comprises a static cell. In some embodiments, the battery is rechargeable. Also provided herein are electrochemical cells that comprise an anode comprising lithium as an active anodic ingredient; a cathode in contact with a gas mixture comprising NO and NO2, wherein the NO and the NO2are present at a molar ratio of 1:1 to 3:1, such as a molar ratio of about 2:1; a non-aqueous electrolyte disposed between the anode and the cathode; and a separator disposed between the anode and the cathode. The negative electrode (anode) can comprise a metal selected from the group consisting of lithium, sodium, potassium, magnesium, calcium, aluminum and zinc. In certain embodiments, the anode can comprise lithium. In some embodiments, the positive electrode (cathode) comprises a porous, gas- permeable electrode. In other embodiments, the cathode can comprise a gas-impermeable electrode. In some examples, the cathode can comprise a suitable conductive material, such as a metal electrode or a carbon electrode. In some examples, the cathode can comprise a conductive material and a binder. In such cases, the cathode may be constructed by mixing the conductive material and the binder and applying the mixture to a current collector of appropriate shape. The positive electrode may contain an electrically-conductive material which is chemically stable in the potential window of use of the cell. In some cases, the conductive material is porous and has a large specific surface area to provide high output. An example of such material may include but is not limited to a carbonaceous material such as Ketjen black, acetylene black, vapor grown carbon fiber, graphene, natural graphite, artificial graphite and activated carbon. Other suitable conductive materials may be conductive Attorney Docket No.103361-602WO1 fibers, such as a metal fiber, metal powder, such as nickel and aluminum, and organic conductive materials, such as a polyphenylene derivative. In some embodiments mixtures of these materials may be employed. Other suitable conductive materials may be conductive ceramics such as titanium nitride and titanium carbide. Examples of binders known to one of ordinary skill which are chemically stable in the potential window of use of the cell may include thermoplastics and thermosetting resins. For example, suitable binders may include polyethylene, polypropylene, polytetrafluoroethylene (PTFE), Polyvinylidene fluoride (PVDF), styrene butadiene rubber, a tetrafluoroethylene hexafluoro ethylenic copolymer, a tetrafluoroethylene hexafluoropropylene copolymer (FEP), a tetrafluoroethylene perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE resin), polychlorotrifluoroethylene resin (PCTFE), a propylene- tetrafluoroethylene copolymer, an ethylene-chlorotrifluoroethylene copolymer (ECTFE) and an ethylene-acrylic acid copolymer. These binders may be used independently, or mixtures may be used. The components may be wet blended in the presence of a suitable solvent or dry blended using a mortar or other conventionally known mixing equipment. The mixture may then be applied to a charge collector by conventionally known methods. Any suitable charge collector may be employed. Examples of charge collectors may be any of carbon, stainless steel, nickel, aluminum and copper. In some cases, the collector is a porous body, such as mesh. In certain embodiments the charge collector may comprise a protective coating of an oxidation-resistant metal or alloy to protect the collector from oxidation. In some embodiments, the cathode can further comprise as oxidation reduction catalyst. Such cathodes may be constructed by mixing the redox catalyst, a conductive material, and optionally the binder and applying the mixture to a current collector of appropriate shape. The oxidation reduction catalyst may be any material which facilitates the reversible redox interconversion of NO3− + 3NO ⇌ 2 N2O3+ e-. Examples may include but are not limited to an alkali or alkali earth metal in the form of its oxide (Li2O, Na2O, K2O, MgO, CaO, SrO, BaO), hydroxide (LiOH, NaOH, KOH, Mg(OH)2, Ca(OH)2, Sr(OH)2, Ba(OH)2), carbonate (Li2CO3, Na2CO3, K2CO3, MgCO3, CaCO3, SrCO3, BaCO3), or any combination thereof. The active component can be impregnated on a high surface area oxide support such as A12O3, ZrO2, TiO2, CeO2, or any mixed oxide thereof. The reaction rate may also be increased by the addition of a precious metal such as Pt, Pd, Rh, or Attorney Docket No.103361-602WO1 any combination thereof. In other embodiments, no oxidation reduction catalyst is present in the cathode. The electrolyte composition can vary. In some embodiments, the anode comprises lithium metal, and the electrolyte comprises an electrolyte that is stable towards Li metal. In some embodiments, the electrolyte can be an electrolyte solution prepared by dissolving an electrolyte salt in an appropriate solvent. In other embodiments, the electrolyte can comprise a solid-state electrolyte. In other embodiments, the electrolyte can comprise a molten salt, such as a nitrate / nitrate molten salt (e.g., a LiNO3-KNO3 eutectic molten salt). Examples of suitable electrolytes include, but are not limited to, tetrabutylammonium salts (e.g., TBANO3, TBAClO4, etc.), nitrate salts, and lithium salts (e.g., LiPF6, LiClO4, LiAsF6, LiBF4, Li(CF3SO2)2N (LiTFSI), Li(CF3SO3), LiN(C2F5SO2)2, etc.). In some embodiments, the electrolyte comprises a nonaqueous electrolyte solution. Examples of such electrolytes include solutions comprising a salt and one or more organic solvents such as cyclic carbonates, chain carbonates, cyclic esters, cyclic ethers and chain ethers. Examples of a cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate and vinylene carbonate. Examples of a chain carbonate includes dimethyl carbonate, diethyl carbonate and methylethyl carbonate. Examples of a cyclic ester carbonate include gamma butyrolactone and gamma valerolactone. Examples of a cyclic ether include tetrahydrofuran and 2-methyltetrahydrofuran. Examples of a chain ether include dimethoxyethane and ethyleneglycol dimethyl ether. In some examples, the solvent can comprise DMSO. In some examples, the solvent can comprise DMF. In some examples, the solvent may be a nitrile system solvent such as acetonitrile. In other embodiments, the electrolyte comprises an aqueous electrolyte (e.g., a water-in-salt electrolyte). In some examples, the solvent can comprise an ionic liquid. Examples of ionic liquids include any of cations such as imidazolium cation, piperidinium cation, pyrrolidinium cation and ammonium cation and any of anions such as bis(trifluorometahnsulfonyl)imide anion, bis(fluorosulfonyl)imide anion, tetrafluoroborate anion and hexafluorophosphate anion. In one example, the solvent is an ionic liquid such as N-methyl-N-propylpiperidinium bis(trifluoromethylsulfonyl)imide (PP13TFSI). In some embodiments, the battery further comprises a separator disposed between the anode and cathode. In some examples, the separator can comprise a membrane separator. Such membrane separators can comprise a gel, a polymer, a ceramic, a Attorney Docket No.103361-602WO1 composite of a polymer and a ceramic, or a combination thereof. In another example, the separator can be a material that prevents NOx migration from the cathode to the anode (e.g., while allowing Li ions to pass). In some embodiments, the membrane separator comprises a ceramic Li-ion conducting membrane. In some embodiments, the anode, the cathode, and the electrolyte are disposed within a housing. The housing can be formed from a material that is resistant to NOx, such as stainless steel, a fluoropolymer such as polytetrafluoroethylene (PTFE), quartz, or a combination thereof. Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. EXAMPLES The invention will be described in greater detail by way of specific examples. The following examples are offered for illustrative purposes, and are not intended to limit the invention in any manner. Those of skill in the art will readily recognize a variety of non- critical parameters which can be changed or modified to yield essentially the same results. Example 1. Metal-Nitric Oxide Electrochemical Cells Batteries based on abundant resources, such as nitrogen oxides (NOx), can provide a more sustainable route to renewable energy storage. In contrast to the frequent disruptions of global mineral supplies, the prices of nitrogen sources (ammonia and diammonium phosphate) have been stable at a low 0.6-1.0 $ / kg (four orders of magnitude cheaper than vanadium) over the past decade as a result of the substantial production of fertilizers (Figure 1B, yellow and blue bars). The choice of nitrogen over vanadium is advantageous from both environmental and socio-economic perspectives. In this example, we establish the fundamental (electro)chemistry required for reversible redox of nitrogen oxides (NOx) in energy storage systems (Figure 1C). Preliminary results showed stable cycling of NOx in a symmetric cell for 200 cycles without noticeable decay. The negligible cost of NOx, high redox potential (3.6 V vs. Li / Li+), the low molecular weight-to-electron ratio of NOx (152.02 g / mol of stored electron), and the high solubility of discharge product (NO3−, up to 5 M in organic solvents) provide a compelling case for the development of NOx cathodes as high-specific-energy (up Attorney Docket No.103361-602WO1 to 488 Wh / kg) and cost-competitive catholyte in energy storage devices. In this example, we will describe Li-NOx cells (Li anode, NOx cathode) with a long lifetime, high specific energy, and specific power through the selection of electrolyte, cell component, flow rate, temperature, solution-gas-phase equilibria of gaseous NOx, etc. Specific aims include: Aim 1: Establish the electrochemistry of using gaseous NOx as catholyte: NO3−+ NO ⇆ N2O3+ e−(E1 / 2 =3.6 V vs. Li / Li+). Develop electrolytes and cycling conditions that stabilize all the reactive intermediates during the interconversion of NOx. Aim 2: Demonstrate stable cycling of Li-NOx static cells with a Li anode and a gaseous NOx cathode. Understand NOx decomposition pathways through in-situ spectroscopic study. Aim 3. Establish prototype Li-NOx flow cells with pressure regulation, gas diaphragm pump, and mass flow control. Optimize flow rate, temperature, and solution-gas equilibria of NOx. Perform technoeconomic analysis. Introduction Tradeoff relationship between energy density and cycling performance in RFBs. A major research thrust in RFBs is to develop highly soluble redox-active materials, e.g., V, Fe, Cr coordination compounds, TEMPO, quinone, cyclopropenium, pyridinium, etc.. The energy density of RFBs increases as the concentration of the redox-active materials increases. However, the performance of electrolytes often suffers at high concentrations due to high solution viscosity and accelerated cross-contamination (Figure 1A). The inherent tradeoff relationship between energy density and performance is a major obstacle. As a result, all vanadium redox flow batteries remain a rare example of commercialized RFBs, since they circumvent cross-contamination by using vanadium as both catholyte and anolyte (catholyte: V5+⇌ V4+; anolyte: V3+⇌ V2+). Similar symmetric cell architectures have been explored by developing bipolar organic molecules, though they are still far from commercialization. To address the tradeoff relationship between the energy density and the performance of RFBs, we propose to design a rechargeable half-cell reaction where the charged and discharged species are both gaseous NOx. With this approach, the redox-active gas no longer needs to remain dissolved in the electrolyte all the time. Instead, the redox-active components (NOx gas) are stored in a gas tank and supplied through a gas mass flow controller (Figure 2). This electrochemical cell allows scale-up of storage capacity without Attorney Docket No.103361-602WO1 increasing the concentration of the redox-active solute or the amount of solvents employed, which is another main cost driver in nonaqueous RFBs (Figure 2). Previous examples of using redox-active gas as energy storage media and associated challenges. There are limited examples of gaseous battery materials. The two most well-studied metal-gas batteries are Li-O2and Li-CO2. Both Li-O2and Li-CO2cells suffer from slow charging processes. The oxidation of solid Li2O2and Li2CO3back to O2or CO2 requires high overpotential. The kinetic issues of Li-O2 and Li-CO2 batteries aggravate at high rates or during deep discharge cycles, as the insolating Li2O2 and Li2CO3 can coat the current collectors completely, leading to so-called “sudden death”. Besides Li-O2and Li-CO2, flow cells with H2 gas and Br2 / FeIIIhave been demonstrated. However, the safety issues associated with H2 gas limit its applications in grid energy storage. Therefore, alternative redox-active gases with high reversibility and fast kinetics are needed for RFB application. Motivation for using gaseous NOx as energy storage media. Nitrogen oxides (NOx), including NO, NO+ (nitrosonium), NO2− (nitrite), NO3−, N2O3, NO2(nitrogen dioxide), N2O4(dinitrogen tetraoxide), etc., are abundant due to their ease of production from the oxidation of ammonia, a key industrial feedstock produced at a 140 M tons / year scale. Representing four oxidation states of nitrogen (II, III IV, V), these redox-active NOx species are connected with electrochemical reactions with potentials spanning over 0.4 to 1.85 V vs. Ag+ / Ag, making them highly attractive as energy storage materials. However, NOx has yet to be successfully employed in any batteries due to their highly reactive nature. A NO / NO+ couple has been explored in a Li-cell. However, this charge-discharge cycle exhibited very low Coulombic efficiency (ca.30%) due to the decomposition of highly reactive NO+. The decomposition of NOx is a common issue in both aqueous and nonaqueous environments. Most of the parasitic reactions of NOx have been attributed to NO2 / N2O4. Employ the equilibrium N2O3⇌ NO2+ NO to stabilize highly reactive NO2. Recognizing these hurdles, we sought to circumvent the highly reactive NO2 / N2O4 by capturing them with NO to form a stable species N2O3 in solution. In solution, the equilibrium N2O3 ⇌ NO2 + NO favors the formation of N2O3 with a dissociation constant (KD) of 8.2 × 10-5M in acetonitrile (equation 1). This low dissociation constant KDsuggests that the highly reactive NO2 / N2O4can be effectively captured by NO as relatively inert N2O3in solution. In the gas phase, however, the equilibrium N2O3⇌ NO2+ NO favors the Attorney Docket No.103361-602WO1 formation of NO and NO2 with KD of 193 kPa at 298 K (equation 2). NO regenerated in this process may re-enter the solution to assist in the stabilization and delivery of NO2to the gas phase. The combined result of equations 1 and 2 is that the presence of NO gas ensures a minimal concentration of NO2 in the organic electrolyte while allowing NO2 to leave the solution as a gas. Although the solubility of N2O3 in organic solvents is modest (ca.20 mM), a supply of NO and NO2 will ensure a sustained concentration of N2O3 during discharge. Furthermore, solvated N2O3is very stable in organic solvents. We generated an acetonitrile solution of N2O3 (0.37 mM) by mixing a 4:1 ratio of NO and O2 and monitored the absorbance of N2O3 at 641 nm (ε = 10.7 M-1cm-1). More than 95% of N2O3 was preserved during a 15-day period at room temperature. Aim 1. Demonstration of N2O3as a viable cathode material With these concepts in mind, we aim to develop nitrogen oxides (NOx) as catholyte in a recharge-able battery. During battery discharge, the dissolved dinitrogen trioxide (N2O3) is reduced to nitric oxide NO (equation 5). During charge, NO is oxidized back to N2O3 (equation 9). One equivalent of redox-inert NO3− in the electrolyte is generated / consumed to maintain charge neutrality. In contrast to conventional metal-gas batteries, both the charge and discharge products in NOx cells are gaseous; therefore, the electrode is NOT expected to be passivated at high current densities. The negligible cost of NOx, high redox potential (3.6 V vs. Li / Li+), the high solubility of NO3− (up to 5 M in organic solvents), and the low molecular weight-to-electron ratio of NOx (152.02 gram per Attorney Docket No.103361-602WO1 mole of stored electron) provide a compelling case for the development of NOx cathodes as high-energy-density and cost-competitive catholyte in nonaqueous RFBs. Preliminary Results Electrochemistry of N2O3. Electrochemistry of N2O3 in acetonitrile was evaluated with solution-phase cyclic voltammogram studies. A solution of N2O3and NO was prepared by mixing a 5:1 ratio of NO and O2in a 0.1 M TBAClO4in acetonitrile (Figure 3). Cyclic voltammogram of N2O3 solution reveals a cathodic peak at 0 V (vs. Ag+ / Ag), which is assigned to the reduction of N2O3 to NO and NO2− through the intermediacy of NO2 (equation 3). The electrochemically generated NO2− is consumed rapidly by another equivalent of N2O3 to generate NO3− and NO (equation 4). Combination of the equation 1, 3, and 4 gives the overall electrochemical reduction of N2O3 to NO and NO3− (equation 5). During the anodic scan, two distinct peaks were found at Ep= 0.8 V and 1.2 V. Notably, TBANO3itself did not show any anodic peak until 1.5 V, suggesting the current should be associated with the oxidation of NO. First, the anodic peak at 1.2 V is assigned to the oxidation of NO to NO+ (Figure 3, black trace). The anodic peak at 0.8 V is due to the oxidation of NO to NO+, followed by a rapid chemical reaction between NO+ and NO3− (reaction 7). A cyclic voltammogram of LiNO2 presented the same two anodic peaks caused by the combination of NO and NO3−, further confirming the assignment of anodic peaks at 0.8 V and 1.2 V as the oxidation of NO. The NO2 / N2O4generated from reaction 7 is expected to be captured by additional NO to afford N2O3 (reaction 8). The combination of equations 6, 7, 8 affords the overall charging process of NO and NO3- to N2O3 (equation 9). In summary, our CV study indicates that (a) the reduction of N2O3 generates NO3− and NO (equation 5) and (b) the oxidation of NO in the presence of NO3− regenerates N2O3 (equation 9). This reversible redox interconversion of NO3− + 3 NO ⇌ 2 N2O3 + e− couple presents a charge / discharge cycling that supports the feasibility of NOx for battery application. Through equations 1 and 2, NO improves the stability of NO2 / N2O4via the formation of N2O3, which serves as a redox surrogate of NO2 / N2O4. Symmetric NOx cell cycling. Encouraged by the reversible redox behavior of N2O3, we performed galvanostatic discharge−charge of a symmetric NOx cell, where NO3− + 3NO ⇌ 2 N2O3+ e- couple was used as both catholyte and anolyte in an H-cell under galvanostatic control. The voltage cutoff for the charge was determined based on CV data and set to 0.8 V (vs. Ag+ / Ag), while the discharge was set to 1.62 mAh. The symmetric H- Attorney Docket No.103361-602WO1 cell was assembled by filling both compartments of the H-cell (separated with a glass frit) with 6 mL, 5 mM solution of TBANO3, NO (1 atm), and an excess amount (5.5 equivalents with respect to NO3−) of gaseous N2O3(Figure 4A). Carbon rods were used as both working and counter electrodes, along with an Ag+ / Ag reference electrode. Upon injection of a mixture of NO2and NO into the headspace of the H-cell, the color of the solution quickly changed from colorless to dark blue within a few seconds, indicating the rapid formation of solvated N2O3. As the charged species of the electrochemical reaction is gaseous and not limited by its solution concentration, the capacity of the battery can be set to any value by controlling the discharge time, similar to metal-O2 and metal-CO2 cells. To maintain the symmetric cell configuration, we discharged the cell to 0.81 mAh, at which point all the NO3- in the one compartment was fully consumed (100% state of charge, SOC, Figure 4B). The polarity of the H-cell was then reversed to convert all the N2O3back to NO3- (0% SOC). This simple H-cell setup allows a direct evaluation of the stability of NOx during electrochemical cycling. The charge-discharge cycling was performed at 100% SoC (based on TBANO3) with a rate of 1C. After 200 charge-discharge cycles, >99 % of the initial capacity was preserved with a Coulombic efficiency > 97% (Figure 4C), suggesting the NO3− + 3NO ⇌ 2 N2O3 + e- couple has excellent stability in a symmetric cell setting. Mechanism study. Since both NO3- and NO are part of the charging reaction, the absence of either one is expected to significantly influence the rechargeability of NO3− + 3NO ⇌ 2 N2O3+ e-. Indeed, without NO3−, the charging potential of NOx catholyte increased to ca.0.94 V, which corresponds to the oxidation of NO to NO+ (Figure 5A, red trace). Although the highly anodic potential of NO / NO+ redox couple is attractive for energy storage applications, the Coulombic efficiency of this process was found to be very low (ca.78%, not shown), suggesting instability of NO+. The presence of NO3− not only prevents the buildup of highly reactive NO+ via reaction NO++ NO3−+ 2 NO → 2 N2O3, but also reduces the charging potential down to ca.0.6 V vs. Ag+ / Ag. An excess amount of NO is also necessary for the efficient charging of NOx catholyte. H-cells assembled with- out excess NO show high charging potential (> 0.7 V, Figure 5A, blue trace). This is likely due to the liberation of NO to the gas phase during discharge, causing it to lose contact with the electrode / electrolyte interface. We also evaluated the impact of excess NO on the discharge of NOx catholyte. With a limited amount of N2O3, the discharge cycle with excess NO delivers 81% expected Attorney Docket No.103361-602WO1 capacity based on the amount of N2O3 (Figure 5B black trace). The discharge capacity did not reach 100% due to the equilibria of N2O3in the gas vs. solution phase. In the absence of excess NO, however, the discharge capacity decreased to 72% due to more facile dissociation of N2O3 to gaseous NO2 and NO. In summary, nitrate and excess NO are essential to both the charge and discharge process of NOx catholyte. Summary of preliminary results. In conclusion, our preliminary results demonstrated the first example of catholyte design based on the redox interconversion of gaseous NOx. Despite the conventional conception of the high reactivity of NOx, we show that the redox of NOx can be harnessed for energy storage applications by providing carefully constructed chemical and electrochemical sequences. The combination of NO and NO3− is crucial for preventing parasitic reactions of highly reactive NOx, such as NO+, NO2, and N2O4. The roles of NO and NO3− combination are two-fold: (1) NO3− prevents the buildup of highly reactive NO+(E1 / 2 = 0.87 V vs. Fc+ / Fc) during charge via reaction NO++ NO3− 2 NO2 and reduces the charging potential; (2) sequentially, the highly reactive NO2 is quenched by excess NO via NO2 + NO ⇌ N2O3 equilibrium providing additional protection from side reactions. The NOx cathode can be assembled at any SoC with NO3− and a mixture of NO and O2. Unlike Li-O2and Li-CO2batteries, the redox of NOx is efficient and stable without relying on noble metal catalysts. The redox-active species NO and NO2 can be readily dis-solved in the electrolyte through the equilibrium NO2 + NO ⇌ N2O3, overcoming the common problem of the low solubility of gases in the electrolyte. Summary of advantages of NOx catholyte High voltage: The demonstrated N2O3 redox couple has a high voltage of 3.6 V vs. Li / Li+, near the limit of organic solvent window, delivering high energy density. Redox kinetic: Compared to traditional gaseous electrode materials (O2and CO2), NOx does not suffer from poor redox kinetics and electrode passivation, since both the charge and discharge mate-rials in NOx catholyte is gaseous. Membrane crossover: The gaseous nature of NOx allows storage of redox active gas in a tank separated from the electrolyte, keeping the concentration of NOx low in solution, therefore slowing down crossover and parasitic reactions. The Li anode is known to form passivation layer in the presence of nitrite and nitrate salt, therefore, could prevent continuous decomposition even if crossover occurs. Attorney Docket No.103361-602WO1 Low cost: NOx has negligible cost (1 / 1000 of V metals) due to large scale production of fertilizer. Environmentally benign: NOx is produced from ammonia generated from Haber- Bosch, a potentially sustainable process if coupled with H2 produced from solar energy. Future work Our preliminary results, including the N2O3 / NO half-cell, represent just the first step of applying NOx toward energy storage. Further study in this proposal will address the rechargeability of NO3− + NO ⇆ N2O3 + e− half-reaction as a function of solvents, electrolytes, membrane, partial pressure of NO, etc., as well as designing a suitable full cell that can accommodate gaseous NOx energy storage. Enhancing the stability of N2O3via optimization of solvent and NO:N2O3ratio. Although our preliminary study has shown that N2O3 has good stability in acetonitrile with 1 M TBAClO4(t1 / 2 = 202 days), its lifetime still needs to be further increased for grid energy storage systems expected to last for years. We will study the stability of NOx in a wide range of solvents. The half-life of N2O3 will be calculated based on its decay monitored by in-situ UV-vis spectroscopy (Figure 6, 641 nm, ε = 10.7 M-1cm-1). N2O3 will be generated by mixing NO and O2and injected into a sealed UV-vis cuvette with various nonaqueous electrolytes, e.g., ethers, carbonates, DMSO, DMF, etc. The decomposition of N2O3 will be monitored at high temperatures to simulate an accelerated aging process. Our UV-vis spectrometer is equipped with a Unisoko cryostat that can heat UV-vis samples up to 100 °C. Additionally, we will study how the ratio of NO:N2O3 affects the half-life of N2O3. Previous literature shows that the parasitic reactions of NOx are mostly due to NO2. In solution, a small amount of N2O3can decompose to generate more reactive NO2via N2O3⇌ NO2+ NO. Based on Le Chatelier’s principle, a high concentration of NO could prevent this detrimental dissociation, therefore increasing the lifetime of N2O3. There remains a gap in the literature on how NOx reacts with organic solvents. We will perform detailed spectroscopic studies on these parasitic reactions. After ca.50% of N2O3has decayed, we will remove the excess amount of N2O3by vacuum and examine the decomposition product with1H NMR,13C NMR, IR, Raman, and GCMS. If the concentrations of organic byproducts are too small for1H NMR analysis, we will perform the15N NMR study with15N-labelled N2O3generated from the reaction of15NO and O2. Understanding the effects of salts, “water-in-salt” electrolytes, and molten salt electrolytes. After we identify the most stable solvents, we will next study the effect of salts Attorney Docket No.103361-602WO1 in organic electrolytes. Since our preliminary study employs non-coordinating TBA salts, the effect of Lewis acidic metal cations, e.g., Li+, Na+, K+, remains unknown. We will first examine how LiNO3, NaNO3, and KNO3influence the cycling stability of NOx in symmetric cells. We do not anticipate significant complications of NOx (electro)chemical reactions since none of the NOx species during the charge / discharge sequences is particularly Lewis basic. However, post-mortem analysis will be performed if decomposition occurs. In our preliminary study, we have shown that nitrate is generated during the cycling of N2O3to balance the charge. Therefore, a suitable electrolyte system must have a high nitrate solubility. Toward this end, the water-in-salt (WiS) and LiNO3–KNO3 eutectic molten salts are expected to achieve high energy density. Both WiS and nitrate / nitrite molten salts are highly stable toward Li metal, making them promising candidates for Li- NOx cells in Aim 2 and 3. The stability of N2O3under electrochemical conditions. High stability of N2O3 in the electrolyte is just the first criterion of a successful N2O3cell. We chose to use N2O3as the benchmark to screen electrolytes since the concentration of N2O3is the highest under the cycling condition. However, a range of other NOx is also produced during the electrochemical reactions, e.g., NO, NO2, N2O4, NO2-, and NO3-. The decomposition of any NOx redox congeners could lead to capacity fading. To understand the stability of NOx under cycling conditions, we will next study the decomposition of N2O3 in the optimized electrolyte using in-situ IR spectroscopy. The in-situ IR study will be performed with a cell similar to the published H-cell but with an in-situ IR probe attachment (ReactIR 15, Figure 7). Due to the polar nature of N-O bonds, all the charged and discharge products during the cycling of NOx cells, e.g., N2O3, NO, NO2, N2O4, NO2-, and NO3-, have characteristic IR features that allow their quantification during the cycling. This in-situ IR study will allow us to monitor the speciation of NOx during the charge / discharge cycle, filling a significant knowledge gap in the literature. Full cell demonstration and crossover behavior of NOx. Although we have demonstrated the cycling of NOx in symmetric cells, an important question remains on the crossover behavior of NOx. Crossover con-tamination of redox flow batteries is a critical issue, as it leads to irreversible fading of the capacity. To investigate the crossover behavior of NOx, we will study the performance of NOx catholyte in a full cell with a quinoxaline- Attorney Docket No.103361-602WO1 based anolyte. NOx are known to react with organic radicals; quinoxaline is chosen in this case as both the charged and dis-charge products are closed-shell com-pounds. The crossover of NOx will be monitored using in-situ IR cells illustrated in Figure 7. We will evaluate a wide range of separators, e.g., Neosepta, Fumasep FAB PK130, for their stability, conductivity, and selectivity against NOx. Separate experiments will be performed to monitor the solution concentration of NOx in both catholyte and anolyte chambers. We recognized that membrane design for nonaqueous RFB is still a challenge. Even if the crossover of NOx cannot be completely prevented, it will be informative to compare the crossover rate with aqueous vanadium flow batteries. If a membrane with satisfactory performance cannot be identified, we will pursue inorganic solid-state electrolytes, which are described in detail in Aim 2. Expected outcomes. The expected outcomes of this aim include (1) identifying an electrolyte system that improves the t1 / 2 of N2O3from 202 days to >5 years, (2) understating the major parasitic reaction(s) that limits the lifetime of NOx under cycling conditions through in-situ spectroscopic studies, (3) determining the compatibility of NOx with non-traditional electrolytes, e.g., WiS and LiNO3-KNO3molten salts. Aim 2. Demonstration of Li-NOx cycling in a static cell The next aim will be to develop a full-cell with Li anode and NOx cathode. Li metal is chosen as the anode for the following reasons: First, lithium metal is stable toward a wide range of NOx due to the formation of a stable solid electrolyte interface (SEI). In fact, nitrite and nitrate salts have been used as additives in Li-O2 and Li-S batteries to prevent the irreversible reaction of Li anode with O2and polysulfide. Molten salts of LiNO3and KNO3have been used in high-temperature Li-O2 batteries. These previous studies indicate that despite the reactive nature of alkaline metals Li, the initial formation of nitrite- and nitrate- based SEI may prevent their continuous reaction with NOx. Additionally, Li-NOx battery is expected to have a high cell voltage of 3.6 V. The solubility of LiNO3salts in organic solvents can be as high as 5 M. Assuming the maximum solubility of LiNO3 in organic electrolytes, the theoretical energy density of Li-NOx battery is ca.488 Wh / kg, comparable to Li-S batteries (450 Wh / kg) and Li-polysulfide flow battery (97 Wh / kg). We have obtained key preliminary results that demonstrate the viability of Li-NOx cell. A comparison of the preliminary Li-NOx battery with Li-ion, Li-O2, Li-CO2, and Li-S batteries is shown in the table below. Attorney Docket No.103361-602WO1 Preliminary results We have performed preliminary studies on the cycling behavior of Li-NOx using a Swagelok cell. In contrast to the high electrical resistance in H-cells, this cell configuration has a significantly shorter distance between the anode and cathode. The Swagelok cell was attached to a gas tank, where NOx was stored (Figure 8A, 8B). The Swagelok cell was assembled in Ar glovebox with a Li metal anode, a carbon paper cathode, and a glass fiber separator. Acetonitrile (MeCN) with 4.2 M LiTFSI was used as the electrolyte due to its demonstrated stability towards Li metal and NOx. The cell was sealed and taken out of the glovebox. The gas storage chamber was charged with a mixture N2O3and NO through valve A. Valve B was used to introduce the NOx gas into the cell chamber. After the solution concentration of N2O3 and NO reached equilibrium, a galvanostatic charge- discharge cycle was performed (Figure 9). Preliminary cycling results showed that the Li- NOx cell has a discharge voltage of 3.4 V. Impressively, the charging of the Li-NOx occurred at 3.8 V with a very low overpotential of 390 mV. The voltage hysteresis of Li- NOx cell is much lower than other gaseous battery materials, e.g., Li-O2(> 1 V) and Li-CO2(>1.2 V), since the discharge product is not an insoluble deposit on the electrode surface. Future work Membrane-free Li-NOx cell. To further develop Li-NOx cells, we envision two possible cell configurations. First, we will attempt to develop a “membrane-free” system where the Li metal anode is NOT separated from the NOx catholyte. The membrane-less Li-NOx is a high-risk, high-reward direction. Therefore, it will be pursued first. The reported stability of Li-NOx-based SEI gives us confidence that Li might be compatible with NOx without separation. This simple setup was inspired by Li-O2, Li-CO2, and Li- polysulfide flow batteries. Notably, the cycling data in our preliminary Li-NOx study (Figure 9) was obtained from a membrane-less Li-NOx cell, where the Li metal is in contact with the NOx in the solution. The cell showed the expected charge and discharge profile with a small voltage hysteresis. We believe the stability of Li toward NOx can be attributed to (1) the formation of SEI on Li surface and (2) the low concentration of N2O3 (10-20 mM) Attorney Docket No.103361-602WO1 in the electrolyte, which slows down parasitic reactions. Unlike traditional redox flow batteries, the redox-active components, e.g., NO, NO2, and N2O3, do not need to remain dissolved in the electrolyte; instead, they are stored in the gas storage tank (Figure 8). Examination of cell materials Although we have confirmed that a membrane-less Li-NOx cell exhibits the expected charge and discharge potential, several important issues remain. First, the charge did not stop at the expected capacity. This is perhaps due to the parasitic reaction between 304 stainless steel and N2O3, which produces NO2- or NO3- salts that sustain the charging current beyond the theoretical 100% Coulombic efficiency. Some corrosion of the Swagelok cell was observed after the cycling, further supporting this hypothesis. Unfortunately, the chemical compatibility of N2O3 with common metal materials is not very well documented. To avoid the parasitic reaction of NO2, NO, and N2O3, the next-generation Li-NOx cells (Figure 8C) will prepare with 316 stainless steel, Teflon, or quartz, which are expected to be chemically resistant to NOx. The transparent quartz cell body will allow us to monitor the cell during cycling, e.g., color of the N2O3 electrolyte, morphology of the Li surface, etc (Figure 8C). We will also explore the use of different current collectors, including 316 stainless steel, graphite foil, Ti foil, or Cu foil. A short cycling test (100-200 mAh gcarbon-1) will be performed with the membrane-free cell configuration demon-strated in the preliminary results. This preliminary cycling serves as a quick test of the stability of the cell materials. A stable cell design should show a proper potential hysteresis around 0.4 V and a galvanostatic charge-discharge curve with an endpoint less than 100% limit of the discharge curve. Identification of electrolyte / separator system compatible with the anode. After identifying the most stable cell materials, we will examine the long-term compatibility of anode, cathode, electrolyte, and separator. Li metal is compatible with nitrite and nitrate, but its stability toward N2O3and NO is not fully understood. To determine if Li metal can form a stable interface with NOx, we will soak Li metal in a solution containing LiNO2, LiNO3, and N2O3. The surface of Li will be monitored with XRD, XPS, Raman, and EDX to understand the chemical composition of SEI (potentially a mixture of LixOyand LiNO2, and LiNO3). The amount of Li metal consumed during the formation of SEI will be quantified by quantitative7Li NMR of the electrolyte. We do not anticipate significant compatibility issues between Li metal and nonaqueous electrolytes themselves. However, the mixture of NOx with organic electrolytes might lead to different reactivity. Toward this end, we will also monitor the chemical composition of the NOx solution with solution IR to confirm the Attorney Docket No.103361-602WO1 stability of NOx. If the in situ solution IR study is not sensitive enough, we will perform a15N NMR study with15N labeled15N2O3as well as an ion-chromatography study to quantify the amount of nitrite and nitrate salt generated during the formation of SEI. Once a suitable electrolyte stable toward both Li and NOx is identified, we will perform deep galvanostatic charge-discharge cycles (>2000 mAh g carbon-1) with the membrane-less Swagelok cell demonstrated in the preliminary results. The deep charge- discharge cycles will ensure the buildup of enough parasitic side products for chemical analysis. The Coulombic efficiency and overpotential of Li-NOx cycling as a function of time will be used to evaluate the viability of the membrane-free configuration. We will also monitor the side products during electrochemical reactions in the system (possibly LixOy at the anode surface) to clarify the side reaction of Li with intermediate NOx. The concentration of nitrite and nitrate ions in the electrolyte will be quantified using ion chromatography. This will further support the viability of the Li-NOx battery as a promising energy storage system. Alternative approach: Li-NOx cells with solid-state electrolytes. Solid-state electrolytes (SSE) are promising alternatives to traditional liquid electrolytes due to their intrinsic safety, stability, and ability to prevent crossover. A large number of SSE materials, including inorganic, organic polymers, and composites, have been examined in all-solid- state Li-ion batteries. For example, β-Al2O3, LIPON, antiperovskites, and Garnet exhibit high electrochemical-stability windows (>4.0 V) and high Li-ion conductivities close to 1 mS cm-1even at room temperature. If the compatibility of Li and NOx becomes a challenge, we will use Li-selective SSE to prevent the crossover NOx to the Li anode. These crossover issues in K-Air and K-S batteries are similar to the NOx crossover issues in Li-NOx cells. Therefore, we are confident that a suitable Li-based SSE will enable the stable cycling of Li-NOx cells. In addition, the dendritic growth of Li metal anode can be mitigated by adopting the SSE further stabilizing the operation of Li-NOx cell. Another inspiration for our cell configuration is the use of solid-state Li conductors in Li-flow cells. In this case, Li / Li+ anode in organic solvent was used in tandem with an otherwise incompatible aqueous I- / I3- catholyte. Inspired by this cell design, we will build a similar cell with a NOx chamber and Li chamber. A digital picture of the cell is shown in Figure 10A and 10B. The advantage of this cell configuration is that the electrolyte (including both solvent and salt) in each chamber are completely separated by the Li- selective SSE, significantly minimizing the crossover reactivity of NOx with Li metal Attorney Docket No.103361-602WO1 (Figure 10C). This cell has several built-in ports for in-situ IR analysis (Figures 10A-10C). A wide range of inorganic Li-selective SSE, e.g., LATP, β-Al2O3, LIPON, antiperovskites, and Garnet, will be tested. Since the compatibility of these solid-state electrolytes with Li metal has been established before, their compatibility with NOx is the limiting factor. We believe that inorganic SSE has a higher chance of success than organic polymer-based electrolytes due to the intrinsic resistance of inorganic materials to the high oxidative ability of NOx. Expected outcomes. Demonstration of Li-NOx cell > 80% capacity retention after 500 cycles in static mode. The experiments outlined above are designed to demonstrate the usage of NOx anode in a full cell by pairing it with a Li anode. If a stable electrolyte with high LiNO3 solubility can be identified, Li-NOx cells can achieve a maximum specific energy of 488 Wh / kg, similar to Li-S batteries and Li-polysulfide flow batteries. We will first investigate if a membrane-free Li-NOx cell can be accomplished. If compatibility of Li metal with NOx is a persistent issue, Li-selective solid-state electrolytes will be used to compartmentalize the Li and NOx. We will systematically optimize each cell component, including cell materials, cell design, current collector, electrolyte, and separator in a static mode. Aim 3. Demonstration of Li-NOx cycling in a flow cell In Aim 3, the most promising combination of cell materials, cell configuration, electrolyte, and separator will be employed to enable the operation of Li-NOx flow cells. The difference between the NOx cell and traditional RFBs is that the transfer of redox- active components to the electrochemical cell does not require the pumping of electrolytes. Instead, redox-active gas will be circulated between the storage tank and the electrochemical cell in a closed-loop fluid control system (Figure 11). Both the pressure (force exerted in all direction) and flow (direction movement) of the NOx gas has to be precisely controlled to ensure optimal performance of Li-NOx cell. To reduce the footprint of the whole energy storage device, the NO2 and NO gas will be stored in a high-pressure zone (highlighted in red, up to 12 bars). The electrochemical cell will be operated at a low- pressure zone (highlighted in blue, 1-2 bar) to reduce the crossover of NOx to Li anode chamber and parasitic reaction with electrolyte. The flow rate of the gas from the high-pressure zone to the low-pressure zone will be regulated with a gas mass flow controller to ensure that the upstream pressure from the storage tank does not influence the chemical condition inside the electrochemical cell. Upon Attorney Docket No.103361-602WO1 electrochemical reaction, the gas from the electrochemical cell will be pumped back to the high-pressure zone with a diaphragm compressor. A max pressure of 8-12 bar can be achieved with commercial mini diaphragm pumps. The NOx storage tank will be connected to NO and O2 tanks to ensure precise mixing of NOx at various ratios. Both the undivided cell in Figure 8 and the divided cell in Figure 10 have gas inlets and outlets that can be used to connect to a NOx storage tank. With a flow cell in hand, we will first compare the discharge capacity of Li-NOx cells under static mode and flow mode to evaluate the effective usage of the capacity in the gas tank. The performance of Li-NOx cell will be monitored as a function of electrolyte volume, gas flow rate, the ratio of N2O3 and NO, and the overall gas pressure. These factors are expected to influence the solubility as well as the diffusion of NOx, which will influence cell voltage and cell capacity. Temperature. After the use of flow mode is demonstrated, we will investigate the effect of temperature. The cycling temperature is expected to have a significant impact on performance through the influence on (1) the Li-ion conductivity of the membrane / solid- state electrolyte (high conductivity at high T, low conduc-tivity at low T), (2) the solubility of the N2O3 and NO (low solubility at high T, high solubility at low T), (3) charge transfer kinetics (fast at high T, slow at low T), and (4) rates of parasitic reactions (faster at high T, slower at low T). Since a high cell temperature has both positive and negative effects on cell performance, the systematic temperature-dependent study will help us identify the optimal operating temperature. Gas flow rate and pressure. The effect of gas flow rate and pressure on the electrochemical performance of Li-NOx cell. In a conventional redox flow battery, the liquid flow rate can influence the voltage profile of charge / discharge. Similarly, the circulation rate of the redox active NOx gas from the storage chamber to the electrochemical cell is expected to have a significant impact. The flow rate of NOx can be tuned by the gas mass flow controller. The pressure of in the Li-NOx cell (low-pressure zone) will be regulated the pumping rates of the diaphragm pump. We will control the NOx flow rate and pressure to maintain the solution concentration of N2O3at 10-20 mM. This concentration is chosen to (1) maintain a high enough discharge voltage, (2) reduce the crossover of NOx to Li anode chamber, (3) reduce parasitic reaction with electrolyte as the rate of the decomposition reaction, which has a first-order dependence on the concentration of the dissolved N2O3. Attorney Docket No.103361-602WO1 Next, we will study the effect of NO:N2O3 ratio. As our preliminary results showed, NO itself does not participate in the energy storage process, but it prevents the generation of highly reactive NO2and N2O4through N2O3⇌ NO2+ NO equilibrium. We will keep the concentration of N2O3in the tank constant and adjust the pressure of NO to examine its influence on cell voltage, rate capability, and stability. This study could also be coupled with in-situ IR measurements to minor how the concentration of N2O3 and NO changes as a function of gas flow rate and pressure. Electrolyte volume and conductivity. Next, we will study the ion conductivity of the electrolyte at different state-of-charge (SoC). As the discharge continues, the solution concentration of LiNO3will slowly increase to its saturation value (close to 5 M). As the SoC increases from 0% to 100%, we expect the electrolyte conductivity to increase and then decrease due to the viscosity. We will adjust the electrolyte volume accordingly to compensate for the increased viscosity. The plots of power density vs. current density will allow us to determine the optimal SoC range and peak power range. These studies relevant to real-life cycling conditions will allow us to perform a comprehensive techno-economic analysis of the installation cost and operational cost of LiNOx batteries. We will consider energy storage efficiency (round trip efficiency, pumping loss) as well as the cost of separators, solvent, NOx, electrodes, tanks, etc. Post-mortem analysis. As the concentration of all redox active components increases, new parasitic reactions might be introduced. Post-mortem analysis, e.g., IR,1H NMR, PXRD, SEM, GCMS, and ion-chromatography will be performed to understand the possible mechanism for electrode decomposition under deep discharge conditions. The stability of the Li metal anode will be studied by the XRD, SEM, and XPS analysis to understand the stability of the SEI during deep cycling. Expected outcomes. A comprehensive understanding of how gas flow rate, gas pressure, electrolyte volume, temperature, and state-of-charge influence the cycling performance of Li-NOx cells. The optimized performance of Li-NOx flow cells will also allow us to perform a techno-economic comparison to other technologies used for grid energy storage. Attorney Docket No.103361-602WO1 Example 2. Rechargeable Li-NOxBattery with a 3.85 V Cell Voltage. Summary In this example, we report a rechargeable Li-NOxbattery that use a lithium metal anode and gaseous nitrogen oxides (NOx) as the cathode. This battery discharges via oxidation of lithium metal and reduction of NOxgas, promising a high theoretical specific energy of 1496 Wh kgLiNO3–1. We found that the redox of 2:1 mixture of NO and NO2gas is highly reversible, achieving stable cycles over 300 hours with average 89.1% energy efficiency at a high current density of 0.5 mA cm−2. The cell delivers a discharge volage of 3.85 V vs. Li. Electrochemical and spectroscopic studies suggest that (a) the reduction of NO / NO2 mixture produces NO3−and (b) the oxidation of NO3−in the presence of NO regenerates NO2. The combination of these two redox processes affords a rechargeable NOx full cell reaction: LiNO3 + NO ⇌ 2 NO2 + Li. With a full Li-NOx cell, we demonstrated a high specific energy of 640 Wh / kgcellor 150 Wh / Lcell, rivaling the state-of-the-art lithium- ion batteries (380-460 Wh / kg). Introduction Li-ion batteries (LIBs) are at the forefront of energy storage technology, powering a wide range of applications from electric vehicles to wearable devices. However, the sustainability issue of LIBs poses a critical challenge due to their reliance on finite metal resources such as lithium, cobalt, or nickel. This dependence not only impedes its adoption due to rising costs but also raises environmental impacts associated with metal mining. One promising avenue for next-generation energy storage is the development of a battery system based on redox-active gases. These gases, under effective utilization, can offer a high energy density, minimal voltage hysteresis, and excellent reversibility, providing a sustainable alternative to LIBs. Over the past decades, rechargeable gas batteries–such as metal-O2, metal-CO2, metal-SO2, metal-Cl2, metal-SO2Cl2, etc. have been explored. Despite significant progress, one of the key challenges is the sluggish recharging processes, where insolating discharge products–such as Li2O2, Li2CO3, Li2SO4, LiCl, and NaCl–accumulate and increase the kinetic barriers when oxidized back to corresponding gas. The kinetic issues of metal-gas batteries are exacerbated when the cells are deeply cycled under high currents, requiring electrode engineering such as novel metal catalyst, redox mediator, or nanostructured carbon electrode and thus limiting the practical viability of these systems. Additionally, many of the gas batteries operate at relatively low voltage (2.4-2.8 V vs. Li / Li+for Li-O2and Li-CO2), which further restricts their efficiency Attorney Docket No.103361-602WO1 compared to commercial LIBs (> 3.6 V). Therefore, identifying novel electrochemistry is important in achieving a next-generation energy storage system with sustainable electrode materials. Gaseous nitrogen oxides (NOx), such as NO and NO2, can undergo reversible redox processes with potentials more than 3.8 V vs. Li+ / Li, making them promising energy storage materials. NOxcan be synthesized cost-effectively from ammonia, a key industrial chemical produced at a 140 M tons / year scale. While NOx has been previously explored as cathode materials in aqueous batteries, the cell voltage is limited by the stability window of aqueous environment. To circumvent the limitation of water-based system, it is crucial to explore their application in an aprotic environment. However, studies addressing the NOx electrode in nonaqueous batteries have been very limited; most proof-of-concept cells exhibit either low Coulombic efficiency (ca.30%) or high voltage polarization (> 2.0 V). This irreversibility of NOxredox in nonaqueous environments is due to the highly reactive nature of NOx. Numerous electrochemical and chemical side reactions of NOx with organic solvents make it difficult to take advantage of their high specific energy for energy storage. Additionally, a range of (electro)chemical mechanisms have been proposed for the redox of NOx in aqueous and non-aqueous environments, including NO3− / NH3, NO / NO+, NO2 / NO+O2−, and N2O3 / NO+NO3−, further complicating the design of metal-NOx batteries. In this Example, we report reversible cycling of a Li-NOxbattery in nonaqueous environment. The battery uses Li metal as anode and a 2:1 mixture of NO and NO2 gases as cathode, separated by a ceramic solid-state electrolyte. The battery delivers a high discharge voltage of 3.85 V at a capacity of 25 mAh cm−2or 1,570 mA h gcarbon-1. The cell is stable over 300 hours of cycling with a small voltage polarization of 0.2 V at a high current density of 0.5 mA cm−2, achieving a high energy efficiency of 87% (ratio of energy output over energy input per cycle) and a specific energy of 640 Wh / kgcellor 175 Wh / Lcell, rivaling the state-of-the-art lithium-ion batteries (380-460 Wh / kg). Results and Discussion The fundamental principle of Li-NOxbattery is based on the electrochemical reaction of Li metal with NO2, affording an expected cell voltage of 3.8 V: Li++ e − NO + NO3NO + LiNO= 3.8 V3 Attorney Docket No.103361-602WO1 The key challenge of this design is that NO2 is a highly reactive gas that can cause rapid degradation of the electrolyte. To circumvent this issue, we use a mixture of NO and NO2instead of pure NO2. In non-aqueous environment, NO2reacts with NO to generate N2O3with a high affinity of KD = 193 kPa (298 K). N2O3 is a close-shell species and highly stable in organic solvents. Electrochemical reduction of N2O3produces NO3−and NO, which can be recharged back to NO and NO2reversibly (Figure 12). The resulting net electrochemical reaction remains the same, but now the reactive NO2 gas is stabilized as N2O3: For simplicity purpose, the mixture of NO, NO2, and N2O3 is referred to as NOx. With the electrochemical reactions of NOx in mind, we assembled a Li-NOx cell with Li metal as the negative electrode and carbon felt as the positive electrode (Figures 13A-13D). Since NOx can react with Li metal, we separate the anode and cathode with a commercial ceramic lithium-ion superion conductor (LISICON, Li1.5Al0.5Ge1.5(PO₄)3). In addition to the LISICON separator, 1 M LiTFSI in diglyme was used as electrolyte for both cathode and anode. A customized Li-NOxcell was fabricated and connected to a gas supply system that delivers NOx in a controlled flow rate of NO and NO2 (Figure 14). The NO / NO2 gas mixture is generated in-situ by providing the NO and O2in 6:1 v / v ratio, so that the headspace of cathode chamber is filled with NO2(0.333 atm) and NO (0.667 atm). This ratio of NO2 and NO ensures that all NO2 is converted to N2O3 in solution. A scheme of the battery configuration is provided in Figure 15A. The Li-NOxbattery has an open circuit potential of ca.4.0 V and was first discharged at current density of 0.5 mA cm–2with cut-off volatge of 3.0 V. The discharge exhibits a capacity of 25 mAh cm−2or 1,570 mAh gcarbon−1(Figure 15B). The high gravimetric capacity per carbon is due to dissolution of the discharge product LiNO3in the electrolyte, which prevents the passivation of the carbon electrode. A single discharge plateau at 3.85 V was observed, corresponding to the electrochemical reduction of N2O3 to NO3−and NO. To confirm the production of LiNO3, we performed an ex-situ Griess test. The addition of Griess reagent (sulfanilamide and N-(1-naphthyl)ethylenediamine dihydrochloride in HCl) to the electrolyte from the fully discharged cell does not lead to any Attorney Docket No.103361-602WO1 observable color change, suggesting the absence of NO2−ions in the electrolyte. However, if the electrolyte is treated with Griess reagent along with VCl3(capable of reducing NO2−to NO3−), a color change from colorless to pink is observed. UV-vis analysis of the resulting solution suggests the formation of LiNO3 in 3.67 M concentration, corresponding to 99.7% Coulombic efficiency. This result is further supported by ex-situ UV-vis analysis as a function of discharge capacity, which shows consistent behavior across the discharge capacity ranging of 1 to 25 mA h cm–2with average Coulombic efficiency of 99.9% (Figure 15C). The discharge of the cell ends around 25 mAh cmcarbon−2, which corresponds to the formation of LiNO3 to 3.66 M concentration, near its saturation point in diglyme (ca.4 M). We hypothesize the end of the discharge is caused by passivation of carbon electrode by LiNO3precipitate. Indeed, X-ray diffraction (XRD) study of the carbon electrode after discharge shows a diffraction pattern consistent with LiNO3(Figure 15D). Scanning Electron Microscopy (SEM) images of the carbon felt electrode after full discharge shows the formation of particular coating on the carbon electrode (Figure 15E). Energy- Dispersive X-ray Spectroscopy (EDX) analysis further indicates the precipitate contains nitrogen and oxygen, consistent with the formation of LiNO3. The discharge reaction is also expected to produce a gaseous discharge product NO. To quantify the formation NO during discharge, we performed the discharge under conditions of limited gas feed. The cell was sealed with a rubber septum, and the headspace was filled with 4:1 volumetric ratio of NO and O2 gas, respectively, assuming all NO and O2are converted to the N2O3. The observed discharge capacity under various mixture of NO and O2 conditions reached ≈90% of the theoretical capacity. The less than 100% capacity is due to the dissociation of N2O3to NO and NO2gas under limited NO concentration, leading to the loss of NO2 gas to the headspace. Nevertheless, we quantified the released NO gas by trapping it by cobalt (II) propyhyrin (TPPCo) to form TPPCo nitrosyl complex (TPPCo-NO). UV-vis and NMR measurement of the resulting solution of TPPCo-NO confirmed that the average Coulombic efficiency was 84.8% (Figure 15F). The charging process exhibits a steady voltage plateau at 4.0 V and eventually rise to the cut off of 4.75 V. The capacity of the cell increases as a function of the concentration of LiNO3 (Figure 16A). The voltage of charging plateau suggests that Li metal is plated to on the anode surface and the NO / NO3−mixture is oxidized back to N2O3, which at high concentration dissociates into NO and NO2. The formation of N2O3was confirmed by Attorney Docket No.103361-602WO1 reappearance of blue color using a transparent 6-mL volume H-cell (Figure 16B). UV-vis spectrometry analysis of the electrolyte after charging shows N2O3is produced in 97.8% yield (average 95.2% Coulombic efficiency) using its characteristic absorption feature at 656 nm (Figure 16C). SEM and XRD analyses of the carbon electrode show complete removal of the LiNO3precipitation. The gaseous nature of both charge and discharge product (NO2and NO, respectively) was confirmed by purging the battery headspace with a stream of N2. When the cell was discharged to 3.0 V then purged with a stream of N2 to remove the dissolved gas, the cell loose most of its reversible capacity, suggesting the NO is necessary for charging. Similarly, when the cell was charged to 4.5 V then purged with a stream of nitrogen to remove the dissolved gas, the following discharge cycle shows a reduced cell voltage from 3.85 V to 3.5 V, suggesting NO2is required for discharge (Figure 16D). The charge / discharge reactions of the Li-NOxcell were systematically probed under controlled gas and salt conditions. Under providing excess amount of NO in headspace, the round-trip voltage profile of charge state (NO + NO2) or the discharge state (NO + LiNO3) remained consistent. Combining the spectroscopic evidence with voltage profiles of Li- NOxcell, a reversible redox mechanism of Li + 2NO2⇌ LiNO3+ NO was confirmed (Figure 16E). A controlled cell assembled under NO2-only or NO without LiNO3 shows a poor cycling with increased overpotential and poor reversibility due to the different redox mechanisms, underscoring the pivotal role of excess NO in headspace for maintaining stable operation of Li-NOx batteries (Figure 16F). The Li-NOx battery demonstrated excellent cyclability, operating for over 300 hours at a reversible capacity of 1 mAh cm−2at a current density of 0.5 mA cm−2. In light of the high operation potential and small overpotential of Li-NOxbattery, the cell maintains average 89.1% of energy efficiency with 99.9% Coulombic efficiency, which achieves much higher energy efficiency than Li-O2 (ca.60-80%) and Li-CO2 batteries (ca.55-75%) (Figure 17A). A fast- and high-capacity (current density of 1.0 mA cm–2and cutoff capacity of 2.0 mA h cm–2) also deliver stable 20 cycles with ca.78.5% energy efficiency, although the parasitic dendritic growth of Li metal anode and mechanical failure of ceramic electrolyte pose a challenges for long-time operation. Importantly, the rate capability of the Li-NOx cell is exceptional comparing to state- of-the-art Li-O2 and Li-CO2 batteries, a key issue in current metal-gas batteries. Even at a high current of 2.0 mA cm-2, the cell maintain a steady discharge plateau with a ca.3.3 V Attorney Docket No.103361-602WO1 (Figure 17B). A polarization curve suggests that the Li-NOx cell achieves a maximum power of 10.5 mW cm-2(Figure 17C). The high rate capability and small voltage hysteresis of Li-NOxis attributed to the fact that both the charge and discharged products are partially soluble in the electrolyte, preventing the passivation of the carbon electrode. The energy preservation of Li-NOxbatteries was further demonstrated by resting the cell after full charging. The reversible capacity of discharge capacity after 24 hours resting reserves 99.1% capacity, supporting no self-discharge and maintain its stability of the charge species (N2O3) under resting condition (Figure 17F). In conclusion, we demonstrated reversible cycling of Li-NOxbattery in non-aqueous environment. The highly reactive charged product of NOx is stabilized using a mixture of NO and NO2. Excess amount of NO traps the highly reactive NO2 as N2O3, preventing the degradation of the electrolyte. The cell represents the highest cell voltage observed in Li- gas battery, achieving a 3.85 V stable discharge voltage, a specific energy of 640 Wh / kgcell, and a high energy density of 150 Wh L–1. Such charge / discharge cycling is sustainable for more than 300 hours without significant decay. Since the charge-discharge mechanism of the battery involved solvated species, LiNO3, this energy storage capacity will increase proportionally to the volume of the electrolyte, similar to flow batteries. Comparing to the state of the art, a energy density of or 175 Wh / Lcell is realistic, calculated based on the solubility of LiNO3in the electrolyte. We note that the Li-NOx cell can operate without the anode electrolyte towards compact configuration; however, parasitic dendritic growth poses a significant challenge to stable operation. Looking forward, the development of advanced ceramic separators holds promise for achieving a more compact and stable Li-NOx cell design in the future Materials and Methods Preparation of LAGP (Li1.5Al0.5Ge1.5(PO₄)3) ceramic separator. LAGP powder (particle size 300-500 nm) was purchased from MSE Supplies®. The ceramic powder was pressed into the pellets using a φ25.4 mm pellet pressing die (MSE Supplies®) and hydraulic press at ca.175 MPa for half-an-hour. The pellet was then sintered in air at 120 °C for two hours, followed by sintering at 850 °C for six hours, with ramping rate of 3 C min–1. The sintered pellet was wet-polished to a thickness of ca.1.0 mm by using 400- and 4000-grit sandpaper. Li-NOxcell fabrication. The battery was fabricated in an Ar-filled glovebox. The electrolyte was prepared by dissolving the 1.0 M of Lithium Attorney Docket No.103361-602WO1 bis(trifluoromethane)sulfonimide (LiTFSI, anhydrous, 99.99%, Sigma Aldrich) in the diethyl glycol dimethyl ether (99.5%, anhydrous, Sigma Aldrich). AvCarb soft graphite battery felt (FuelCellStore®, USA) was pre-heated at 500 °C for four-hours in air and then punched to φ10 mm circular disc. Lithium metal disc (MSE Supplies®) was punched to φ10 mm circular disc and polished before use. A lab-built Li-NOxcell setup is detailed in Figures 13A-13D. The valve remained closed until the cell was connected to the gas supply system. A closed cell (Figure 18) was assembled by using rubber septa. All electrochemical measurement was recorded using a battery testing system (LAND CT2001A, China) or a potentiostat (Bio-logic SAS, SP-150, France). Nitrate (NO3–) Detection. Nitrate concentrations in the electrolyte was determined using the modified Griess test. A color reagent was prepared by dissolving sulfanilamide (0.50 mmol, 86 mg) and N-(1-naphthyl)ethylenediamine dihydrochloride (0.25 mmol, 65 mg) in 5.00 mL of 37% hydrochloric acid, then diluting to 100 mL with distilled water. Vanadium (III) chloride (0.30 mmol, 47 mg) was dissolved to a 20.0-mL above solution. The reagent was used within one day due to the sensitivity of vanadium (III) to light and air. A discharged Li-NOx cell was dissembled, and a 10.0 µL aliquot of the electrolyte was dried at 150 °C overnight. The residue was dissolved in 20.0 mL of distilled water. The solution was diluted 10,000-200,000 times to adjust the concentration. A 3.00 mL sample was mixed with an equal volume of color reagent and heated at 85 °C for 15 minutes. The nitrate concentration was analyzed by UV-vis spectroscopy (Instrument info) at λ = 540 nm at 20 °C. The calibration plot for NO3–concentration was established using various concentrations of LiNO3 in distilled water. LiNO3Detection. A discharged Li-NOxcell was disassembled, and the carbon felt was fully dried at 150°C overnight. The carbon felt was ground using mortar and pestle and analyzed by using X-ray diffractometer (Bruker D8 Powder X-ray Diffractometer; Cu Kα as X-ray source, λ = 1.5406 Å) and scanning electron microscopy (Apero I, Thermo Scientific Co., Ltd, USA) equipped with energy-dispersive X-ray spectroscopy detector. Nitric oxide (NO) Detection. A color reagent was prepared in a glovebox. In a rubber-sealed vial, 5,10,15,20−tetraphenyl−21H,23H−porphine cobalt(II) (TPPCo) was dissolved in 15.0 mL of tetrahydrofuran. The TPPCo concentration was controlled from 3.00 mM to 5.00 mM based on the experimental conditions. The vial was brought out of the glovebox. The headspace gas in the charged cell was extracted using a 20.0 mL syringe Attorney Docket No.103361-602WO1 while purging with 20.0 mL of N2 gas. The extracted gas was injected directly into the color reagent solution to allow NO to react with TPPCo forming TPPCo-nitrosyl (TPPCo- NO). TPPCo-NO species is stable under air exposure. The solution was diluted, and the concentration was analyzed by UV-vis spectroscopy at 20°C. The peak was deconvoluted by using the UV-vis spectra of TPPCo (0.1 mM; λ = 532 nm) and TPPCo-NO (0.1 mM; λ = 542 nm) solution as standard. Dinitrogen Trioxide (N2O3) Detection. A rubber-sealed, lab-built UV-vis cuvette was vacuum-cleaned and filled with nitrogen gas. NO gas was purged into the cuvette to prevent the dissociation of N2O3 to NO and NO2 gas. 3.00 mL of catholyte from the discharged six-mL H-type cell was extracted while connected to the NO gas and transferred to the cuvette. The N2O3 concentration was analyzed by UV-vis spectroscopy at λ = 656 nm at 20 °C. To prepare the standard N2O3 solution, the rubber-sealed vial was charged with 3.00 mL of electrolyte. The vial was brought out of the glovebox. The headspace was purged with excess amount of NO gas (20 mL), followed by injecting a certain volume of O2(from 0.50 to 5.00 mL), and then an additional 10.0 mL of NO gas to convert O2 gas to N2O3. Each solution was transferred to the NO-filled cuvette and characterized using UV-vis spectroscopy at λ = 656 nm. The compositions and methods of the appended claims are not limited in scope by the specific compositions and methods described herein, which are intended as illustrations of a few aspects of the claims. Any compositions and methods that are functionally equivalent are intended to fall within the scope of the claims. Various modifications of the compositions and methods in addition to those shown and described herein are intended to fall within the scope of the appended claims. Further, while only certain representative compounds, components, compositions, and method steps disclosed herein are specifically described, other combinations of the compounds, components, compositions, and method steps also are intended to fall within the scope of the appended claims, even if not specifically recited. Thus, a combination of steps, elements, components, or constituents may be explicitly mentioned herein or less, however, other combinations of steps, elements, components, and constituents are included, even though not explicitly stated. The term “comprising” and variations thereof as used herein is used synonymously with the term “including” and variations thereof and are open, non-limiting terms. Although Attorney Docket No.103361-602WO1 the terms “comprising” and “including” have been used herein to describe various embodiments, the terms “consisting essentially of” and “consisting of” can be used in place of “comprising” and “including” to provide for more specific embodiments of the invention and are also disclosed. Other than where noted, all numbers expressing geometries, dimensions, and so forth used in the specification and claims are to be understood at the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, to be construed in light of the number of significant digits and ordinary rounding approaches. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of skill in the art to which the disclosed invention belongs. Publications cited herein and the materials for which they are cited are specifically incorporated by reference.

Claims

Attorney Docket No.103361-602WO1 WHAT IS CLAIMED IS:

1. A metal nitric oxide battery, the battery comprising: an anode comprising a metal as an active anodic ingredient; a cathode in contact with a gas comprising nitric oxide (NO); and an electrolyte disposed between the anode and the cathode.

2. The battery of claim 1, wherein the gas comprises a gas mixture comprising NO and NO2.

3. The battery of any one of claims 1-2, wherein the gas is stored in a gas tank and supplied to the cathode through a gas mass flow controller.

4. The battery of any one of claims 1-3, wherein the battery comprises a catholyte design based on the redox interconversion of gaseous NOx.

5. The battery of any one of claims 1-4, wherein the battery comprises a catholyte comprising a nitrogen oxide of formula NxOy, wherein x is 1 or 2 and y is an integer of 1 to 4.

6. The battery of claim 5, wherein the catholyte comprises N2O3.

7. The battery of any one of claims 1-6, wherein the battery comprises a catholyte utilizing a NO3− + 3NO ⇌ 2 N2O3+ e- redox couple.

8. The battery of any one of claims 1-7, wherein the cathode comprises a porous, gas- permeable electrode.

9. The battery of any one of claims 1-7, wherein the cathode comprises a gas- impermeable electrode.

10. The battery of any one of claims 1-9, wherein the cathode comprises carbon.Attorney Docket No.103361-602WO1 11. The battery of any one of claims 1-10, wherein the battery further comprises a separator disposed between the anode and cathode.

12. The battery of claim 11, wherein the separator comprises membrane separator.

13. The battery of claim 12, wherein the membrane separator comprises a gel, a polymer, a ceramic, a composite of a polymer and a ceramic, or a combination thereof.

14. The battery of any one of claims 1-13, wherein the electrolyte comprises a solid- state electrolyte.

15. The battery of any one of claims 1-13, wherein the electrolyte comprises a nonaqueous electrolyte solution.

16. The battery of any one of claims 1-15, wherein the electrolyte comprises a tetrabutylammonium salt.

17. The battery of any one of claims 1-16, wherein the electrolyte comprises a water-in- salt electrolyte.

18. The battery of any one of claims 1-16, wherein the electrolyte comprises a molten salt, such as a nitrate / nitrate molten salt (e.g., a LiNO3-KNO3eutectic molten salt).

19. The battery of any one of claims 1-18, wherein the battery comprises a flow cell.

20. The battery of any one of claims 1-18, wherein the battery comprises a static cell.

21. The battery of any one of claims 1-20, wherein the battery is rechargeable 22. The battery of any one of claims 1-21, wherein the anode, the cathode, and the electrolyte are disposed within a housing.Attorney Docket No.103361-602WO1 23. The battery of claim 22, wherein the housing is formed from a material that is resistant to NOx.

24. The battery of any one of claims 22-23, wherein the housing is formed from stainless steel, a fluoropolymer such as polytetrafluoroethylene (PTFE), quartz, or a combination thereof.

25. The battery of any one of claims 1-24, wherein the anode comprises lithium.

26. An electrochemical cell, the cell comprising: an anode comprising lithium as an active anodic ingredient; a cathode in contact with a gas mixture comprising NO and NO2; and an electrolyte disposed between the anode and the cathode; wherein the battery comprises a catholyte utilizing a NO3− + 3NO ⇌ 2 N2O3 + e- redox couple.

27. The electrochemical cell of claim 26, wherein the gas mixture is stored in a gas tank and supplied to the cathode through a gas mass flow controller.

28. An electrochemical cell, the cell comprising: an anode comprising lithium as an active anodic ingredient; a cathode in contact with a gas mixture comprising NO and NO2, wherein the NO and the NO2are present at a molar ratio of 1:1 to 3:1, such as a molar ratio of about 2:1; a non-aqueous electrolyte disposed between the anode and the cathode; and a separator disposed between the anode and the cathode.

29. The electrochemical cell of claim 28, wherein the separator prevents NOx migration from the cathode to the anode.

30. The electrochemical cell of any one of claims 28-29, wherein the separator comprises membrane separator.Attorney Docket No.103361-602WO1 31. The electrochemical cell of any one of claims 28-30, wherein the membrane separator comprises a gel, a polymer, a ceramic, a composite of a polymer and a ceramic, or a combination thereof.

32. The electrochemical cell of any one of claims 28-31, wherein the membrane separator comprises a ceramic Li-ion conducting membrane. .

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