Patent Application
20180048042
This invention relates to promoters for batteries.
Lithium-ion (Li-ion) batteries have enabled the recent proliferation of lightweight, long-lived, portable electronic devices. Unfortunately, Li-ion batteries are too expensive and their energy density is too low to enable mass-production of electric vehicles, and there is significant interest in developing new low-cost, high-energy density battery systems. See, M. Armand and J. M. Tarascon, Nature, 2008, 451, 652, and P. G. Bruce, S. A. Freunberger, L. J. Hardwick and J.-M. Tarascon, Nat. Mater., 2012, 11, 19, each of which is incorporated by reference in its entirety. The lithium-air/lithium-oxygen (Li—O2) battery chemistry currently enjoys great scientific attention as a next-generation rechargeable battery, owing to their high theoretical gravimetric energy density (2000 Wh·kg−1). See, Y.-C. Lu, B. M. Gallant, D. G. Kwabi, J. R. Harding, R. R. Mitchell, M. S. Whittingham and Y. Shao-Horn, Energy Environ. Sci., 2013, 6, 750, which incorporated by reference in its entirety. Analysis by Gallagher et al. predicts gravimetric energy densities of ˜300 Wh·kg−1 for system-level applications in electric vehicles, a twofold increase in energy density relative to Li-ion cells. See, K. G. Gallagher, S. Goebel, T. Greszler, M. Mathias, W. Oelerich, D. Eroglu and V. Srinivasan, Energy Environ. Sci., 2014, 7, 1555, which is incorporated by reference in its entirety. However, many challenges must be resolved before practical Li—O2 devices can be produced. In particular, Li—O2 batteries suffer from high charging potentials, low round-trip efficiency, and limited cycle life, which have been attributed to the reactivity of Li—O2 discharge products and poor oxidation kinetics of Li2O2 formed upon discharge. See, M. M. Ottakam Thotiyl, S. A. Freunberger, Z. Peng, Y. Chen, Z. Liu and P. G. Bruce, Nat. Mater., 2013, 12, 1050, B. M. Gallant, R. R. Mitchell, D. G. Kwabi, J. Zhou, L. Zuin, C. V. Thompson and Y. Shao-Horn, J. Phys. Chem. C, 2012, 116, 20800, B. D. McCloskey, A. Speidel, R. Scheffler, D. C. Miller, V. Viswanathan, J. S. Hummelshøj, J. K. Nørskov and A. C. Luntz, J. Phys. Chem. Lett., 2012, 3, 997, M. M. Ottakam Thotiyl, S. A. Freunberger, Z. Peng and P. G. Bruce, J. Am. Chem. Soc., 2013, 135, 494, and Y. Shao, S. Park, J. Xiao, J.-G. Zhang, Y. Wang and J. Liu, ACS Catal., 2012, 2, 844, each of which is incorporated by reference in its entirety.
A metal-air electrochemical system can include a first electrode and a second electrode, and an electrolyte in contact with the first electrode and the second electrode, wherein the second electrode includes a promoter including a transition-metal-containing species. In certain embodiments, the first electrode can include lithium (Li). The second electrode can include oxygen.
In certain embodiments, the transition-metal-containing species can be molybdenum (Mo)-containing species. In certain embodiments, the promoter can be in form of nanoparticles.
In certain embodiments, the promoter further can include a metal selected from a group consisting of Ru, Ir, Pt, Au, Cr, and Ni. In certain embodiments, the promoter can include a transition metal oxide. In certain embodiments, the promoter can include a molybdenum oxide.
In certain embodiments, the promoter can include a lithiated molybdenum oxide. In certain embodiments, the promoter can include a Mo metal, a molybdenum oxide, a lithiated molybdenum oxide, a molybdenum sulfide, or any combination thereof. In certain embodiments, the promoter can further include carbon.
In certain embodiments, the second electrode can be pre-filled with Li2O2. In certain embodiments, Li2O2 can be formed during discharge.
In certain embodiments, the electrolyte can be non-aqueous. In certain embodiments, the electrochemical system can include a conductive support. In certain embodiments, the conductive support can include Au or Al.
In certain embodiments, the second electrode can further include a binder. In certain embodiments, the binder can be an ionomer.
In certain embodiments, the promoter can be partially dissolved in the electrolyte.
An electrode can include a Mo-containing promoter. In certain embodiments, the Mo-containing promoter is in form of nanoparticles. In certain embodiments, the Mo-containing promoter can further include a metal selected from a group consisting of Ru, Au, Cr, and Ni.
In certain embodiments, the Mo-containing promoter can include a molybdenum oxide. In certain embodiments, the Mo-containing promoter can include a lithiated molybdenum oxide. In certain embodiments, the promoter can include a Mo metal, a molybdenum oxide, a lithiated molybdenum oxide, a molybdenum sulfide, or any combination thereof. In certain embodiments, the Mo-containing promoter can further include carbon.
In certain embodiments, the electrode can be pre-filled with Li2O2. In certain embodiments, the electrode can further include a binder. In certain embodiments, the binder can be an ionomer.
In certain embodiments, the electrode can be a cathode in a Li-air battery.
A composition can include a Mo-containing material, where the composition is a promoter for an electrode in an electrochemical system. In certain embodiments, the Mo-containing material can be in form of nanoparticles. In certain embodiments, the Mo-containing material further can include a metal selected from a group consisting of Ru, Au, Cr, and Ni.
In certain embodiments, the Mo-containing material can include a molybdenum oxide. In certain embodiments, the Mo-containing material can include a lithiated molybdenum oxide. In certain embodiments, the promoter can include a Mo metal, a molybdenum oxide, a lithiated molybdenum oxide, a molybdenum sulfide, or any combination thereof. In certain embodiments, the Mo-containing material can further include carbon.
In certain embodiments, the composition can further include a binder. In certain embodiments, the binder can be an ionomer.
In certain embodiments, the electrochemical system is a Li-air battery.
A method of generating oxygen can include providing a first electrode and a second electrode, and an electrolyte in contact with the first electrode and the second electrode, wherein the second electrode includes a promoter, where the promoter includes a transition-metal-containing species, and applying an oxygen-generating voltage across the first electrode and the second electrode.
In certain embodiments, the method can further include lithiating the transition-metal-continaing species to a lithiated transition-metal-containing species, and delithiating the lithiated transition-metal-containing species to the metal-containing-containing species. In certain embodiments, the method can further include generating oxygen by repeating the lithiating the transition-metal-continaing species and the delithiating the lithiated transition-metal-containing species.
In certain embodiments, the first electrode can include Li. In certain embodiments, the second electrode can include oxygen.
In certain embodiments, the transition-metal-containing species is Mo-containing species. In certain embodiments, the promoter can be in form of nanoparticles. In certain embodiments, the promoter can further include a metal selected from a group consisting of Ru, Ir, Pt, Au, Cr, and Ni. In certain embodiments, the promoter can include a transition metal oxide. In certain embodiments, the promoter can include a molybdenum oxide. In certain embodiments, the promoter can include a lithiated molybdenum oxide. In certain embodiments, the promoter can include a Mo metal, a molybdenum oxide, a lithiated molybdenum oxide, a molybdenum sulfide, or any combination thereof In certain embodiments, the promoter can further include carbon.
In certain embodiments, the method can further include pre-filling the second electrode with Li2O2. In certain embodiments, the method can further include forming Li2O2 during discharge.
In certain embodiments, the electrolyte can be non-aqueous. In certain embodiments, the method can further include providing a conductive support. In certain embodiments, the conductive support can include Au or Al.
In certain embodiments, the method can further include providing a binder. In certain embodiments, the binder can be an ionomer.
In certain embodiments, the method can further include selecting the promoter and the electrolyte such that the promoter is partially dissolved in the electrolyte.
In certain embodiments, the method can further include lithiating the Mo-continaing species to a lithiated Mo-containing species, and delithiating the lithiated Mo-containing species to the metal-containing-containing species. In certain embodiments, the method can further include generating oxygen by repeating the lithiating the Mo-continaing species and the delithiating the lithiated Mo-containing species.
An electrochemical system can include a first electrode and a second electrode, and an electrolyte in contact with the first electrode and the second electrode, where the second electrode includes a promoter, wherein the promoter includes molybdenum (Mo), cobalt (Co), or manganese (Mn).
An electrode can include a promoter including a transition metal, where the transition metal is Mo, Co, or Mn.
A composition can include a transition metal, where the transition metal is Mo, Co, or Mn, and where the composition is a promoter for an electrode in a battery.
A method of generating oxygen can include providing a first electrode and a second electrode, and an electrolyte in contact with the first electrode and the second electrode, where the second electrode includes a promoter including a Cr-containing species, applying an oxygen-generating voltage across the first electrode and the second electrode, lithiating the Cr-continaing species to a lithiated Cr-containing species, and, delithiating the lithiated Cr-containing species to the Cr-containing species.
In certain embodiments, the method can further include generating oxygen by repeating the lithiating the Cr-continaing species and the delithiating the lithiated Cr-containing species.
In certain embodiments, the first electrode can include Li. In certain embodiments, the second electrode can include oxygen.
In certain embodiments, the promoter can be in form of nanoparticles. In certain embodiments, the promoter can further include a metal selected from a group consisting of Ru, Ir, Pt, Au, Mo, and Ni. In certain embodiments, the promoter can include a chromium metal oxide. In certain embodiments, the promoter can include a lithiated chromium oxide. In certain embodiments, the promoter can include a Cr metal, a chromium oxide, a lithiated chromium oxide, or any combination thereof In certain embodiments, the promoter can further include carbon.
In certain embodiments, the method can further include pre-filling the second electrode with Li2O2. In certain embodiments, the method can further include forming Li2O2 during discharge.
In certain embodiments, the electrolyte can be non-aqueous. In certain embodiments, the method can further include providing a conductive support. In certain embodiments, the conductive support can include Au or Al.
In certain embodiments, the method can further include comprising providing a binder. In certain embodiments, the binder can be an ionomer.
In certain embodiments, the method can further include comprising selecting the promoter and the electrolyte such that the promoter is partially dissolved in the electrolyte.
Other aspects, embodiments, and features will be apparent from the following description, the drawings, and the claims.
Lithium-oxygen batteries have been referred to as the “holy grail” of battery chemistries for its potential to provide three times the gravimetric energy density of Li-Ion batteries and as such enable similar ranges as current internal combustion engines at comparable system weights. Thus far however, the Li—O2 electrochemistry is confronted with severe instabilities of electrolyte and carbon-based cathodes which results in poor cycle life and efficiencies. More fundamentally, recharge requires large voltages for oxidation of the insulating Li2O2 deposited on discharge resulting in low round trip efficiencies.
Electrochemical systems, electrodes, and compositions including catalytic materials are described, where the catalytic material includes a transition metal. In some cases, the transitional metal can be a molybdenum (Mo). The systems can operate with improved activity, e.g., at low absolute value of the overpotential, high current density, significant efficiency, stability, or a combination of these. The catalytic materials can also be free of expensive precious metals or precious metal oxides. The systems also can operate at or higher than neutral pH, without necessarily requiring highly pure solvent sources, or any combination. The systems, electrodes, systems, and compositions are useful in applications such as energy storage, energy use, and oxygen generation.
Electrolytic devices, fuel cells, and metal-air batteries are non-limiting examples of electrochemical devices provided herein. Energy can be supplied to electrolytic devices by photovoltaic cells, wind power generators, or other energy sources.
Electrolysis refers to the use of an electric current to drive an otherwise non-spontaneous chemical reaction. For example, electrolysis involves a change in redox state of at least one species, and/or formation and/or breaking of at least one chemical bond, by the application of an electric current. Electrolysis of water generally involves splitting water into oxygen gas and hydrogen gas, or oxygen gas and another hydrogen-containing species, or hydrogen gas and another oxygen-containing species, or a combination. In some embodiments, the systems described herein are capable of catalyzing the reverse reaction. That is, a system can be used to produce energy from combining hydrogen and oxygen gases (or other fuels) to produce water.
A power source may supply DC or AC voltage in an electrochemical system. Non-limiting examples include batteries, power grids, regenerative power supplies (e.g., wind power generators, photovoltaic cells, tidal energy generators), generators, and the like. The power source can include one or more such power supplies (e.g., batteries and a photovoltaic cell). In a particular embodiment, the power supply can be one or more photovoltaic cells. In some cases, an electrochemical system may be constructed and arranged to be electrically connectable to and able to be driven by a photovoltaic cell (e.g., the photovoltaic cell may be the voltage or power source for the system). Photovoltaic cells include a photoactive material, which absorbs and converts light to electrical energy.
An electrochemical system may be combined with additional electrochemical system to form a larger device or system. This may take the form of a stack of devices or subsystems (e.g., fuel cell and/or electrolytic device and/or metal-air battery) to form a larger device or system.
Various components of a device, such as the electrodes, power source, electrolyte, separator, container, circuitry, insulating material, gate electrode, etc. can be fabricated by those of ordinary skill in the art from any of a variety of components, as well as those described in any of those patent applications described herein. Components may be molded, machined, extruded, pressed, isopressed, infiltrated, coated, in green or fired states, or formed by any other suitable technique. Those of ordinary skill in the art are readily aware of techniques for forming components of devices herein.
Generally speaking, an electrochemical system includes two electrodes (i.e., an anode and a cathode) in contact with an electrolyte. The electrodes are electrically connected to one another; the electrical connection can, depending on the intended use of the system, include a power source (when the desired electrochemical reactions require electrical energy) or an electrical load (when the desired electrochemical reactions produce electrical energy). An electrochemical system can be used for producing, storing, or converting chemical and/or electrical energy.
Further details of devices and systems, including details of electrode construction, are known in the art. In this regard, see, for example, US Patent Application Publication No. 2009/0068541, which is incorporated by reference in its entirety.
An electrochemical system can include a first electrode and a second electrode; and an electrolyte in contact with the first electrode and the second electrode; wherein the second electrode includes a promoter, where the promoter includes a transition-metal-containing species.
Promoter is defined as a chemical compound capable of being chemically lithiated by lithium oxides and proceeding through delithiation.
The transition-metal-containing-species can include transition metals including Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt, Au, or Hg. In certain embodiments, the transition-metal-containing-species can include transition metal oxides, lithiated transition metal oxides, or transition metal sulfurs. The species can be a molecule, an oxide, a carbide or a sulfide of the transition-metal. Particularly useful transition metal species can include Mo, Cr, Ru, Mn, Fe, Co, Ni, Cu, oxides thereof, lithiated oxides thereof including chemically litiated oxides, or sulfurs thereof. In certain embodiments, the transition metal-containing-species can also include rare earth metals or alkaline earth metals as well as transition metals. Rare earth metals include Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Alkaline earth metals include Be, Mg, Ca, Sr, Ba, and Ra. Transition metals include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt, Au, and Hg. Particularly useful rare earth metals can include La. Particularly useful alkaline earth metals can include Ca, Sr, and Ba. Particularly useful transition metals can include first-row transition metals, for example, Cr, Mn, Fe, Co, Ni, and Cu. Representative materials include LaCrO3, LaMnO3, LaFeO3, LaCoO3, LaNiO3, LaNi0.5Mn0.5O3, LaCu0.5Mn0.5O3, La0.5Ca0.5MnO3-δ, La0.5Ca0.5FeO3-δ, La0.75Ca0.25FeO3-67 , La0.5Ca0.5CoO3-δ, LaMnO3-δ, and Ba0.5Sr0.5Co0.8Fe0.2O3-δ.
The binder can be a polymer. For example, the polymer can be a polyolefin or a fluorinated polyolefin. In some examples, the binder can be an ionomer, such as sulfonated tetrafluoroethlyene, for example, Nafion, or an ion-exchanged Nafion such as lithim nafion. The promoter disclosed herein enables to decompose at a lower voltage and at faster kinetics, the main product of reaction (Li2O2) formed during the typical discharge of a lithium (Li)-air (or Li—O2) battery. As a result, the lithium-air battery using such a promoter is rechargeable and its columbic efficiency is improved. Also, the kinetics of the electrochemical reactions is improved, i.e. the charge of the battery can be faster. Also this promoter can promote O2 formation during charge and this for several cycles.
In Li-air (Li—O2) battery, Li and O2 combine during the discharge to form Li2O2. During the charge Li2O2 should decompose and go back to its initial state as O2 and Li. The decomposition process of Li2O2 is known to be sluggish and to happen with a high overvoltage compared to the expected thermodynamic voltage.
Therefore, it is desired (1) to increase the current associated with Li2O2 decomposition or Li-air (Li—O2) battery charging, (2) to decrease/speed up the reaction time for Li2O2 decomposition happening during the charge of a Li-air (Li—O2) battery, (3) to propose a promoter as efficient as precious metal but presenting a lower cost (e.g. Ru-containing promoter is efficient for Li2O2 decomposition but is expensive), and (4) to promote the O2 formation during charge for several cycles, instead of unwanted other species such as CO2 coming from electrolyte decomposition.
Cr-based compounds (e.g. Cr—NP, Cr2O3, LaCrO3) have been proposed as catalysts for a Li-air battery. See, K. P. C. Yao, Y.-C. Lu, C. V. Amanchukwu, D. G. Kwabi, M. Risch, J. Zhou, A. Grimaud, P. T. Hammond, F. Bardé and Y. Shao-Horn, Phys. Chem. Chem. Phys., 2014, 16, 2297, which is incorporated by reference in its entirety. A review article describes the use of 7 various families of materials to act as catalyst for aqueous or non-aqueous Li-air batteries. See, Z-L Wang, D. Xu, J-J. Xu, X-B. Zhang, Chem. Soc. Rev., 2014, 43, 7746, which is incorporated by reference in its entirety. Recently, molybdenum disulfide was proposed as a catalyst for a Li-air battery. See, Mohammad Asadi, Bijandra Kumar, Cong Liu, Patrick Phillips, Poya Yasaei,
Amirhossein Behranginia, Peter Zapol, Robert F. Klie, Larry A. Curtiss, and Amin Salehi-Khojin, ACS Nano, Articles ASAP, Publication Date (Web): Jan. 20, 2016, which is incorporated by reference in its entirety. A Mo2C/CNT composite was also proposed as a cathode for a Li—O2 battery, Won-Jin Kwak, Kah Chun Lau, Chang-Dae Shin, Khalil Amine, Larry A Curtiss, and Yang-Kook Sun, ACS Nano, 2015, 9 (4), pp 4129-4137, which is incorporated by reference in its entirety.
Disclosed herein is a Li-air battery or Li—O2 battery using molybdenum (Mo)-containing materials as “promoter” for the air cathode used in a metal-air battery. The Li-Air battery or Li—O2 battery can be non-aqueous. In certain embodiments, the Mo-containing promoter can include Mo metal particles. In certain embodiments, the Mo-containing promoter can be in form of nanoparticles, or a composite including nanoparticles. In certain embodiments, the Mo-containing promoter can include a second or a third material based on carbon, such as Mo/CNT, Mo/CNF, and Mo/graphene or another metal such as for example Mo/Ru, Mo/Au, Mo/Cr, and Mo/Ni. In certain embodiments, the Mo-containing promoter can include an oxide, for example, MoPw, with 0<w<4, such as MoO2, MoO3, etc. or a mixture of such oxides. In certain embodiments, the Mo-containing promoter can include a lithiated oxide with formulae LixMoyOz with 0<x<7 and 0<y<3 and 1<z<10. For example, the Mo-containing promoter can include Li2MoO4, Li4MoO5, Li2MoO3, LiMoO2, where y=1, Li6Mo2O7 . . . where y=2, Li4Mo3O8 . . . where y=3, or a mixture of such oxides. In certain embodiments, the Mo-containing promoter can include a mixture of any of the component described above; for example, Mo/MoOw or Mo/LixMoyOz or Mo/MoOw/LixMoyOz.
A Li-air battery or Li—O2 Battery also can include chromium (Cr)-containing materials as “promoter” for the air cathode used in a metal-air battery. The Li-Air battery or Li—O2 battery can be non-aqueous. In certain embodiments, the Cr-containing promoter can include Cr metal particles. In certain embodiments, the Cr-containing promoter can be in form of nanoparticles, or a composite including nanoparticles. In certain embodiments, the Cr-containing promoter can include a second or a third material based on carbon, such as Cr/CNT, Cr/CNF, and Cr/graphene or another metal such as for example Cr/Ru, Cr/Au, Cr/Mo, and Cr/Ni. In certain embodiments, the Cr-containing promoter can include an oxide, for example, CrOw, with 0<w, such as Cr2O3, CrO2, CrO3 etc. or a mixture of such oxides. In certain embodiments, the Cr-containing promoter can include a lithiated oxide with formulae LixCryOz with 0<x<10 and 0<y<4 and 0<z<10. For example, the Cr-containing promoter can include Cr metal particles, chromium oxides, lithiated chromium oxides, or a mixture of such oxides. The lithiated oxide can be chemically lithiated and then electrochemically delithiated in the battery. In certain embodiments, the Cr-containing promoter can include a mixture of any of the component described above; for example, Cr/CrOw or Cr/LixCryOz or Cr/CrOw/LixCryOz.
The specific surface area of the promoter is a key criterion. The specific surface area is typically measured using N2 (or other gases) adsorption tests on the material based on the Brunauer, Emmett and Teller (BET) method. From these measurements, for example, the BET value of the specific surface area is determined and expressed in m2/g. The promoter can have preferentially nanometer particle size. The promoter can preferentially present an enthalpy of reaction normal to Li2O2 which is negative. The promoters can preferentially present an ability to get partially dissolved in the electrolyte solution. The promoter can be one of the components of the positive air (or O2) electrode. In certain embodiments, the promoter can be contained in a Li2O2 pre-filled electrode. In certain embodiments, Li2O2 can be formed in situ during the discharge process. In certain embodiments, the air electrode can contain carbon. In certain embodiments, the battery described above can preferentially contain an electrolyte which favors Li2O2 as main discharge reaction products. In certain embodiments, such electrolytes can be dimethoxyethane (DME), glymes, dimethyl sulfoxide (DMSO), ionic liquid (DEME, PP13 . . . ), polymer, gel, or ceramic solid state electrolyte.
For example, in certain embodiments, Mo nanoparticles can be used as a promoter for Li2O2 decomposition in a carbon-free electrode containing Li2O2 and the promoter deposited on a conductive support (Au or Al) (see
Briefly, gold foil supported carbon-free and aluminum foil supported carbon-containing electrodes were fabricated at a fixed promoter:Li2O2 ratio of 0.667:1. Both carbon-free and carbon-containing electrodes were fabricated following methods reported previously (see, K. P. C. Yao, Y.-C. Lu, C. V. Amanchukwu, D. G. Kwabi, M. Risch, J. Zhou, A. Grimaud, P. T. Hammond, F. Bardé and Y. Shao-Horn, Phys. Chem. Chem. Phys., 2014, 16, 2297, which is incorporated by reference in its entirety) and described below. In carbon-free electrodes, ratios were set to promoter:Li2O2=0.667:1 and pressed at 5 tons onto a ½ inch gold substrate upon homogenization in isopropanol. All electrodes and electrochemical cells fabrication were performed baring atmospheric exposure in Argon filled gloveboxes (MBraun, H2O2 content below 0.1 ppm, O2 content below 1%). In addition to fabrication in water-free environment, all electrodes were dried at 70° C. in a Buchi oven under less than 30 mbar vacuum for a minimum of 12 hours. The cells consisted of a 15 mm diameter lithium foil with 150 μL 0.1 M LiClO4/DME on 2 Celgard C480 capped with a Li2O2-preloaded electrode. The 0.1 M LiClO4 in 1,2 dimethoxyethane electrolyte was acquired from BASF with measured water content below 10 ppm by Karl Fischer titration. The cell is charged at a constant potential fixed to 3.9V vs. Li/Li+ in this example.
In certain embodiments, Mo nanoparticles can be used as a promoter for Li2O2 decomposition in a carbon-containing electrode containing the promoter, Li2O2, carbon and a binder (see
Briefly, carbon-containing electrodes using Vulcan XC72 carbon (VC) as electrically conducting backbone were deposited on battery grade aluminum foil at the ratio of promoter: VC:Li2O2:LiNafion binder=0.667:1:1:1. All electrodes and electrochemical cells fabrication were performed baring atmospheric exposure in Argon filled gloveboxes (MBraun, H2O2 content below 0.1 ppm, O2 content below 1%). In addition to fabrication in water-free environment, all electrodes were dried at 70° C. in a Buchi® oven under less than 30 mbar vacuum for a minimum of 12 hours. The cells consisted of a of a 15 mm diameter lithium foil with 150 μL 0.1 M LiClO4/DME on 2 Celgard C480 capped with a Li2O2-preloaded electrode. The 0.1 M LiClO4 in 1,2 dimethoxyethane electrolyte was acquired from BASF with measured water content below 20 ppm by Karl Fischer titration. The cell is charged at a constant potential fixed to 3.9V, 3.8V or 3.7V vs. Li/Li+ in this example.
Briefly, carbon-containing electrodes using Vulcan XC72 carbon as electrically conducting backbone were deposited on battery grade celgard C480 separator at the ratio of promoter:VC:LiNafion binder=0.667:1:1:1). All electrodes and electrochemical cells fabrication were performed baring atmospheric exposure in Argon filled gloveboxes (MBraun, H2O content below 0.1 ppm, O2 content below 1%). In addition to fabrication in water-free environment, all electrodes were dried at 70° C. in a Buchi® oven under less than 30 mbar vacuum for a minimum of 12 hours. The cells consisted of a of a 15 mm diameter lithium foil with 150 μL electrolyte on 2 Celgard C480 capped with a carbon-containing electrode. The water content in electrolyte was below 20 ppm by Karl Fischer titration. The cell is charged at a constant potential fixed to 3.9V vs. Li/Li+ in this example.
In this example, Li2O2 was produced in situ in the cell, not added in the electrode. During the cell cycling, in situ DEMS was performed and gas released during charging identified and quantified. Using the promoters described here, mainly O2 gas is released and this for several consecutive cycles (
The effort of identifying the best materials has yet to probe the mechanism of enhancement and thereby obtain predictive capability. In the sections presented below, the mechanistic origin of the influence of transition metals and oxides on the Li2O2 oxidation kinetics was examined. The results suggest that these materials act as reaction promoters rather than promoters. The enthalpies of conversion of the reactant Li2O2 and transition metal (oxides) towards formation of a lithium metal oxide is strongly correlated to electrochemical activity, which offers a rule for identifying promoters of high activity.
Solid-State Activation of Li2O2 Oxidation Kinetics and Implications for Li—O2 Batteries
As one of the most theoretically promising next-generation chemistries, Li—O2 batteries are the subject of intense research to address their stability, cycling, and efficiency issues. The recharge kinetics of Li—O2 are especially sluggish, prompting the use of metal nanoparticles as reaction promoters. In this work, the underlying pathway of kinetics enhancement by transition metal and oxide particles was probed using a combination of electrochemistry, X-ray absorption spectroscopy, and thermochemical analysis in carbon-free and carbon-containing electrodes. Disclosed herein is the high activity of the group VI transition metals Mo and Cr, which are comparable to noble metal Ru and coincide with XAS measured changes in surface oxidation state matched to the formation of Li2MoO4 and Li2CrO4. A strong correlation between conversion enthalpies of Li2O2 with the promoter surface (Li2O2+MaOb±O2→LixMyOz) and electrochemical activity is found that unifies the behaviour of solid-state promoters. In the absence of soluble species on charge and the decomposition of Li2O2 proceeding through solid solution, enhancement of Li2O2 oxidation is mediated by chemical conversion of Li2O2 with slow oxidation kinetics to a lithium metal oxide. The mechanistic findings shown below provide new insights into the selection and/or employment of electrode chemistry in Li—O2 batteries.
The kinetics of Li2O2 oxidation in Li—O2 batteries have been investigated by a number of groups, who show that the charging performance is strongly impacted by the morphology of the Li2O2 produced during discharge. For thin layers of Li2O2, McCloskey et al. have computed and experimentally measured low charging overpotentials (<0.2 V by cyclic voltammetry) to posit that electrocatalysis for the oxygen evolution reaction (OER) from Li2O2 oxidation may not be necessary. See, Y.-C. Lu and Y. Shao-Horn, J. Phys. Chem. Lett., 2012, 4, 93, J. S. Hummelshøj, A. C. Luntz and J. K. Nørskov, J. Chem. Phys., 2013, 138, Y. Mo, S. P. Ong and G. Ceder, Phys. Rev. B, 2011, 84, 205446, B. D. McCloskey, R. Scheffler, A. Speidel, D. S. Bethune, R. M. Shelby and A. C. Luntz, J. Am. Chem. Soc., 2011, 133, 18038, and B. D. McCloskey, R. Scheffler, A. Speidel, G. Girishkumar and A. C. Luntz, J. Phys. Chem. C, 2012, 116, 23897, each of which is incorporated by reference in its entirety. Similarly, Lu et al. have reported evidence showing that electrocatalysis is unnecessary during the removal of the first sub- nanometer of deposited Li2O2, where electrochemical oxidation of Li2O2 can proceed from first delithiation to form lithium-deficient Li2-xO2 followed by oxygen evolution from Li2O2. See, Y.-C. Lu, B. M. Gallant, D. G. Kwabi, J. R. Harding, R. R. Mitchell, M. S. Whittingham and Y. Shao-Horn, Energy Environ. Sci., 2013, 6, 750, and Y.-C. Lu and Y. Shao-Horn, J. Phys. Chem. Lett., 2012, 4, 93, each of which is incorporated by reference in its entirety. This concept is consistent with DFT findings and recent results by Ganapathy et al. showing solid-solution lithium deficient Li2-xO2 using in operando X-ray diffraction during charge. See, S. Kang, Y. Mo, S. P. Ong and G. Ceder, Chem. Mater., 2013, 25, 3328, and S. Ganapathy, B. D. Adams, G. Stenou, M. S. Anastasaki, K. Goubitz, X.-F. Miao, L. F. Nazar and M. Wagemaker, J. Am. Chem. Soc., 2014, each of which is incorporated by reference in its entirety.
Thicker deposits of Li2O2 (i.e. greater depth of discharge) have been shown to require greater overpotentials to oxidize, particularly on carbon electrodes. See, B. M. Gallant, R. R. Mitchell, D. G. Kwabi, J. Zhou, L. Zuin, C. V. Thompson and Y. Shao-Horn, J. Phys. Chem. C, 2012, 116, 20800, M. M. Ottakam Thotiyl, S. A. Freunberger, Z. Peng and P. G. Bruce, J. Am. Chem. Soc., 2013, 135, 494, Y.-C. Lu and Y. Shao-Horn, J. Phys. Chem. Lett., 2012, 4, 93, R. R. Mitchell, B. M. Gallant, C. V. Thompson and Y. Shao-Horn, Energy Environ. Sci., 2011, 4, 2952, F. Li, R. Ohnishi, Y. Yamada, J. Kubota, K. Domen, A. Yamada and H. Zhou, Chem. Commun., 2013, 49, 1175, R. Black, J.-H. Lee, B. Adams, C. A. Mims and L. F. Nazar, Angew. Chem. Int. Ed., 2013, 52, 392, and Y. Cao, S.-R. Cai, S.-C. Fan, W.-Q. Hu, M.-S. Zheng and Q.-F. Dong, Faraday Discuss., 2014, each of which is incorporated by reference in its entirety. This phenomenon is attributed to two different effects: (1) the formation of byproducts during discharge that require a greater potential to oxidize (see, B. M. Gallant, R. R. Mitchell, D. G. Kwabi, J. Zhou, L. Zuin, C. V. Thompson and Y. Shao-Horn, J. Phys. Chem. C, 2012, 116, 20800, B. D. McCloskey, A. Speidel, R. Scheffler, D. C. Miller, V. Viswanathan, J. S. Hummelshøj, J. K. Nørskov and A. C. Luntz, J. Phys. Chem. Lett., 2012, 3, 997, M. M. Ottakam Thotiyl, S. A. Freunberger, Z. Peng and P. G. Bruce, J. Am. Chem. Soc., 2013, 135, 494, B. D. McCloskey, J. M. Garcia and A. C. Luntz, J. Phys. Chem. Lett., 2014, 5, 1230, and S. A. Freunberger, Y. Chen, N. E. Drewett, L. J. Hardwick, F. Bardé and P. G. Bruce, Angew. Chem. Int. Ed., 2011, 50, 8609, each of which is incorporated by reference in its entirety) and (2) the insulating nature of Li2O2, which increases the potential needed to drive the oxidation reaction (see, V. Viswanathan, K. S. Thygesen, J. S. Hummelshøj, J. K. Nørskov, G. Girishkumar, B. D. McCloskey and A. C. Luntz, J. Chem. Phys., 2011, 135, S. P. Ong, Y. Mo and G. Ceder, Phys. Rev. B, 2012, 85, 081105, M. D. Radin, J. F. Rodriguez, F. Tian and D. J. Siegel, J. Am. Chem. Soc., 2012, 134, 1093, and P. Albertus, G. Girishkumar, B. McCloskey, R. S. Sanchez-Carrera, B. Kozinsky, J. Christensen and A. C. Luntz, J. Electrochem. Soc., 2011, 158, A343). One group of the main byproducts is carbonates such as Li2CO3, which can form from electrolyte decomposition and/or from an interaction between Li2O2 and carbon electrodes. High charging overpotentials (typically greater than 1 V) have been reported for a variety of carbon electrodes, from simple porous carbon to graphene, to carbon nanofibers'17 and nanotubes6 at moderate rates 50 to 100 mA·g−1carbon. See, M. M. Ottakam Thotiyl, S. A. Freunberger, Z. Peng and P. G. Bruce, J. Am. Chem. Soc., 2013, 135, 494, Y.-C. Lu and Y. Shao-Horn, J. Phys. Chem. Lett., 2012, 4, 93, F. Li, R. Ohnishi, Y. Yamada, J. Kubota, K. Domen, A. Yamada and H. Zhou, Chem. Commun., 2013, 49, 1175, Y. Cao, S.-R. Cai, S.-C. Fan, W.-Q. Hu, M.-S. Zheng and Q.-F. Dong, Faraday Discuss., 2014, T. Cetinkaya, S. Ozcan, M. Uysal, M. O. Guler and H. Akbulut, J. Power Sources, 2014, 267, 140, R. R. Mitchell, B. M. Gallant, C. V. Thompson and Y. Shao-Horn, Energy Environ. Sci., 2011, 4, 2952, and B. M. Gallant, R. R. Mitchell, D. G. Kwabi, J. Zhou, L. Zuin, C. V. Thompson and Y. Shao-Horn, J. Phys. Chem. C, 2012, 116, 20800, each of which is incorporated by reference in its entirety. In contrast, several groups have reported improved charging performance when carbon-free electrodes were used, such as nanoporous gold, TiC, and Ru on TiSi2. See, Z. Peng, S. A. Freunberger, Y. Chen and P. G. Bruce, Science, 2012, 337, 563, M. M. Ottakam Thotiyl, S. A. Freunberger, Z. Peng, Y. Chen, Z. Liu and P. G. Bruce, Nat. Mater., 2013, 12, 1050, and J. Xie, X. Yao, I. P. Madden, D.-E. Jiang, L.-Y. Chou, C.-K. Tsung and D. Wang, J. Am. Chem. Soc., 2014, 136, 8903, each of which is incorporated by reference in its entirety. Regarding, the insulating nature of Li2O2, Viswanathan et al. have estimated that 5-10 nm layers of insulating Li2O2 is sufficient to drive overpotentials greater than 0.6 V. See, V. Viswanathan, K. S. Thygesen, J. S. Hummelshøj, J. K. Nørskov, G. Girishkumar, B. D. McCloskey and A. C. Luntz, J. Chem. Phys., 2011, 135, each of which is incorporated by reference in its entirety.
Several reports have shown that the addition of metal nanoparticles (using either noble or transition metals) show a quantifiable reduction in charging overpotential (see, F. Li, R. Ohnishi, Y. Yamada, J. Kubota, K. Domen, A. Yamada and H. Zhou, Chem. Commun., 2013, 49, 1175., R. Black, J.-H. Lee, B. Adams, C. A. Mims and L. F. Nazar, Angew. Chem. Int. Ed., 2013, 52, 392, Z. Jian, P. Liu, F. Li, P. He, X. Guo, M. Chen and H. Zhou, Angew. Chem. Int. Ed., 2014, 53, 442, F. Li, Y. Chen, D.-M. Tang, Z. Jian, C. Liu, D. Golberg, A. Yamada and H. Zhou, Energy Environ. Sci., 2014, 7, 1648, C. Kavakli, S. Meini, G. Harzer, N. Tsiouvaras, M. Piana, A. Siebel, A. Garsuch, H. A. Gasteiger and J. Herranz, ChemCatChem, 2013, 5, 3358, K. Song, J. Jung, Y.-U. Heo, Y. C. Lee, K. Cho and Y.-M. Kang, Phys. Chem. Chem. Phys., 2013, 15, 20075, J. R. Harding, Y.-C. Lu, Y. Tsukada and Y. Shao-Horn, Phys. Chem. Chem. Phys., 2012, 14, 10540, K. P. C. Yao, Y.-C. Lu, C. V. Amanchukwu, D. G. Kwabi, M. Risch, J. Zhou, A. Grimaud, P. T. Hammond, F. Bardé and Y. Shao-Horn, Phys. Chem. Chem. Phys., 2014, 16, 2297, J. Ming, W. J. Kwak, J. B. Park, C. D. Shin, J. Lu, L. Curtiss, K. Amine and Y. K. Sun, Chemphyschem, 2014, 15, 2070, and B. G. Kim, H.-J. Kim, S. Back, K. W. Nam, Y. Jung, Y.-K. Han and J. W. Choi, Sci. Rep., 2014, 4, each of which is incorporated by reference in its entirety), and can enhance the kinetics of the Li2O2 oxidation reaction, yet the origin of this enhancement is not fully understood. No soluble species derived from solid Li2O2 have yet been identified on charge using electron paramagnetic resonance, Raman, and rotating ring-disk techniques, which would support a heterogeneous catalysis mechanism. See, R. Cao, E. D. Walter, W. Xu, E. N. Nasybulin, P. Bhattacharya, M. E. Bowden, M. H. Engelhard and J.-G.
Zhang, ChemSusChem, 2014, 7, 2436, Z. Peng, S. A. Freunberger, L. J. Hardwick, Y. Chen, V. Giordani, F. Bark P. Novak, D. Graham, J.-M. Tarascon and P. G. Bruce, Angew. Chem. Int. Ed., 2011, 50, 6351, M. J. Trahan, I. Gunasekara, S. Mukerjee, E. J. Plichta, M. A. Hendrickson and K. M. Abraham, J. Electrochem. Soc., 2014, 161, A1706, M. J. Trahan, S. Mukerjee, E. J. Plichta, M. A. Hendrickson and K. M. Abraham, J. Electrochem. Soc., 2013, 160, A259, and C. N. Satterfield, Heterogeneous catalysis in practice, McGraw-Hill New York, 1980, each of which is incorporated by reference in its entirety. McCloskey et al. attribute the measured enhancement to the catalysis of electrolyte decomposition and efficient removal of parasitic products. See, B. D. McCloskey, R. Scheffler, A. Speidel, D. S. Bethune, R. M. Shelby and A. C. Luntz, J. Am. Chem. Soc., 2011, 133, 18038, which is incorporated by reference in its entirety. In addition, Black et al. proposed that catalyst surfaces promote efficient transport of Li2-xO2 species on the electrode surfaces. See, R. Black, J.-H. Lee, B. Adams, C. A. Mims and L. F. Nazar, Angew. Chem. Int. Ed., 2013, 52, 392, which is incorporated by reference in its entirety. Moreover, experiments with soluble redox mediators such as tetrathiafulvalene, 2,2,6,6-tetramethylpiperidinyloxyl, and iodine have shown to greatly reduce the overpotential required to charge Li—O2 batteries, which suggests that the Li2O2 oxidation kinetics can be directly influenced by redox exchange with a promoter for surface charge transfer. See, G. V. Chase, S. Zecevic, T. W. Wesley, J. Uddin, K. A. Sasaki, P. G. Vincent, V. Bryantsev, M. Blanco and D. D. Addison, Soluble oxygen evolving catalysts for rechargeable metal-air batteries, USPTO, 2012/0028137, 2012, Y. Chen, S. A. Freunberger, Z. Peng, 0. Fontaine and P. G. Bruce, Nat. Chem., 2013, 5, 489, B. J. Bergner, A. Schürmann, K. Peppler, A. Garsuch and J. Janek, J. Am. Chem. Soc., 2014, 136, 15054, and H.-D. Lim, H. Song, J. Kim, H. Gwon, Y. Bae, K.-Y. Park, J. Hong, H. Kim, T. Kim, Y. H. Kim, X. Lepró, R. Ovalle-Robles, R. H. Baughman and K. Kang, Angew. Chem. Int. Ed., 2014, 53, 3926, each of which is incorporated by reference in its entirety. In summary, it is not yet understood how solid-state metal nanoparticles can alter the reaction pathways and enhance the kinetics of Li2O2 oxidation.
Disclosed herein is the enhancement of Li2O2 oxidation kinetics with transition metal nanoparticles, such as Co, Mo, Cr and Ru, using electrodes preloaded with commercial crystalline Li2O2 in both carbon-free and carbon-containing electrodes developed recently (see, J. R. Harding, Y.-C. Lu, Y. Tsukada and Y. Shao-Horn, Phys. Chem. Chem. Phys., 2012, 14, 10540, and K. P. C. Yao, Y.-C. Lu, C. V. Amanchukwu, D. G. Kwabi, M. Risch, J. Zhou, A. Grimaud, P. T. Hammond, F. Bardé and Y. Shao-Horn, Phys. Chem. Chem. Phys., 2014, 16, 2297, each of which is incorporated by reference in its entirety). Using Li2O2-loaded electrodes minimizes the interference of catalyst-dependent parasitic discharge products as well as crystallinity and morphology variations in electrochemically formed Li2O2 on the Li2O2 oxidation kinetics. See, B. M. Gallant, R. R. Mitchell, D. G. Kwabi, J. Zhou, L. Zuin, C. V. Thompson and Y. Shao-Horn, J. Phys. Chem. C, 2012, 116, 20800, S. A. Freunberger, Y. Chen, N. E. Drewett, L. J. Hardwick, F. Bardé and P. G. Bruce, Angew. Chem. Int. Ed., 2011, 50, 8609, and B. G. Kim, H.-J. Kim, S. Back, K. W. Nam, Y. Jung, Y.-K. Han and J. W. Choi, Sci. Rep., 2014, 4, each of which is incorporated by reference in its entirety. As the surfaces of these nanoparticles are likely oxidized, the activation of Li2O2 oxidation kinetics was also compared using corresponding metal oxides including MoO3, Cr2O3, RuO2, Co3O4, and α-MnO2. Ex situ X-ray absorption spectroscopy (XAS) and inductively coupled plasma atomic emission spectra (ICP-AES) of electrodes before and after charging are used to provide insights into processes potentially responsible for the activation of Li2O2 kinetics. Correlating the enhanced Li2O2 oxidation kinetics with the enthalpy of conversion Li2O2+MaOb±O2→LixMyOz allows us to propose a unifying descriptor and a pathway for the solid-state activation of Li2O2 electro-oxidation activity across transition metal nanoparticles and oxides. In light of the proposed mechanism, the added nanoparticles are referred as “promoters” throughout the text.
I. Increased Li2O2 Oxidation Kinetics With Nonprecious Transition Metal Nanoparticles
Carbon-containing and carbon-free Li2O2-loaded electrodes promoted by bulk transition metals nanoparticles Mo, Cr, Ru, Co, and Mn were examined, which revealed high activities of group VI Mo and Cr nanoparticles. Note that aluminum foil was used as support for carbon-free Mo electrodes due to embrittlement of the Au support in presence of Mo.
The current profile versus time for the same five representative metal nanoparticle promoters are further analysed in
Metal oxides including MoO3, Cr2O3, RuO2, Co3O4 and α-MnO2, were investigated in carbon-containing electrodes (
To examine the intrinsic activities across all the promoters studied, area-specific activities (normalized to the BET surface area of the promoter) in carbon-containing electrodes are shown in
II. Ex Situ XAS of Preloaded Li2O2 Electrodes During Electrochemical Oxidation
There were considerable changes in the oxidation state of Cr and Mo particles during charging using XAS data. The chemical changes in charged carbon-free Cr:Li2O2 electrodes at 3.8 VLi using XANES spectra of the Cr K edge were probed, as shown in
Comparing the Mo L edge spectra of MoO2 and MoO3 and a Mo foil, a significant fraction of Mo on the surface of Mo powder can be assigned to metallic Mo in addition to some with oxidation states of Mo4+ and Mo6+ (
The spontaneous chemical reaction of Mo with Li2O2 was confirmed by the presence of Li2MoO4 using XAS (
III. Promoter Dissolution During Li2O2 Oxidation and Implication on the Li2O2 Oxidation Kinetics
Table 1 summarizes the results of probing the presence of soluble metal species in the electrolyte post-charging. The molar amount of soluble metal in the electrolyte generally increases with greater activation of Li2O2 oxidation and XAS-resolved oxidation state changes in the promoter: Mo>Cr>Co≈Co3O4>α-MnO2. It is conceivable that dissolved promoter- derived complexes in the electrolyte are acting as redox mediators to the electrochemical oxidation of Li2O2. However, the measured concentrations of dissolved species are one order of magnitude lower compared to the typical concentrations of more than 10 mM of redox mediators used in the literature. See, Y. Chen, S. A. Freunberger, Z. Peng, O. Fontaine and P. G. Bruce, Nat. Chem., 2013, 5, 489, and B. J. Bergner, A. Schürmann, K. Peppler, A. Garsuch and J. Janek, J. Am. Chem. Soc., 2014, 136, 15054, each of which is incorporated by reference in its entirety.
To examine the influence of these soluble species on the observed enhancement of Li2O2 oxidation with Cr, Mo and Ru, a promoted high activity electrode (Mo, Cr, and Ru) was allowed to fully charge at 3.9 VLi in 0.1 M LiClO4/DME electrolyte (see EXAMPLES), likely resulting in dissolved transition metal species in the electrolyte. Immediately afterwards, a carbon electrode (VC:Li2O2=1:1, without promoter) was substituted into the cell (reusing the exact previous electrolyte layer containing the dissolved metal species) and similarly charged at 3.9 VLi. The absence of electrochemical activation in all three VC:Li2O2 electrodes in
IV. Influence of Water On The Li2O2 Oxidation Kinetics
Meini et al. demonstrate that impurities such as water (produced from electrolyte degradation in operando) can enhance the electrode activation. See, S. Meini, S. Solchenbach, M. Piana and H. A. Gasteiger, J. Electrochem. Soc., 2014, 161, A1306, which is incorporated by reference in its entirety.
Similarly to observations made for the leached metal species, the absence of electrochemical activation in all three VC:Li2O2 electrodes in
V. Unified Mechanism of Solid-State Activation of Li2O2 Oxidation
Further insights into the enhanced Li2O2 kinetics are gained from examining the enthalpies for conversion reactions: Li2O2+MaOb±O2→LixMyOz, where MaOb is the surface composition of the promoter. Values of computed enthalpies for a number of representative Li2O2 reactions with transition metal (oxides) towards formation of lithiated metal oxides are tabulated in Table 2.
Phys. Rev. B, 2011, 84, 045115, which is incorporated by reference in its entirety).
½ O2 + Li2MnO3
⅓O2 + ⅔Li3MnO4
O2 + 2LiMn2O4
⅙ O2 + Li2MnO3
1/9O2 + ⅔Li3MnO4
2Li3MnO4
⅓O2 + 2LiCoO2
Li2Cr2O7
2LiCr3O8
½O2 + 2LiCrO2
Li2CrO4
⅙O2 + ⅔Li3CrO4
Li2Cr2O7
Li2MoO4
½Li4MoO5
⅓Li6Mo2O7
Li2MoO3
2LiMoO2
½Li4Mo3O8
Li4Mo5O17
Li2MoO4 + ½O2
Li2RuO3
1/7O2 + 2/7Li7RuO6
2LiRuO2
½O2 + Li2RuO3
3/7O2 + 2/7Li7RuO6
Based on the L edge XAS results of pristine Cr, Mo, and Co particles, their surfaces were identified as Cr2O3, Mo/MoOx, and Co3O4, respectively. It is assumed that the surface of Mn particles was covered by Mn3O4 as reported by American Elements and that of Ru by Ru/RuO2 based on previous studies. See, K. S. Kim and N. Winograd, J. Catal., 1974, 35, 66, which is incorporated by reference in its entirety. In the case of the metal oxides, the surfaces of MoO3, Cr2O3, Co3O4, α-MnO2 and RuO2 are comparable to the bulk. Additionally, the reaction intermediates of Cr and Mo are Li2CrO4 and Li2MoO4, respectively, as revealed from XAS measurements. Increasing enthalpy for chemical reaction between Li2O2 and the promoter was correlated with increasing specific Li2O2 oxidation currents in both carbon free and carbon-containing electrodes, as shown in
i˜P.R˜e
−ΔH/KT.eα.n.e.η
i˜e
−Δh+α.N.Eη
/KT
log*)˜−ΔH+α.n.e.ηapplied
In the particular case of Mn, activity is limited by the delithation step, which would not be possible at the 3.9 VLi applied potential here.
Phys. Rev. B, 2004, 70, 235121, P. Lanz, C. Villevieille and P. Novak, Electrochim. Acta, 2013, 109, 426, and S. Sarkar, P. Mahale and S. Mitra, J. Electrochem. Soc., 2014, 161, A934, each of which is incorporated by reference in its entirety. On the other hand, the delithiation reaction will likely result in a metal oxide deposit but not necessarily the regeneration of the original promoter. The proposed pathway can be used to explain the surface behavior during Li2O2 oxidation of the reported TiC and Ti4O7 promoters. See, M. M. Ottakam Thotiyl, S. A. Freunberger, Z. Peng, Y. Chen, Z. Liu and P. G. Bruce, Nat. Mater., 2013, 12, 1050, and D. Kundu, R. Black, E. J. Berg and L. F. Nazar, Energy Environ. Sci., 2015, each of which is incorporated by reference in its entirety. X-ray photoelectron spectra (XPS) after first discharge on TiC and Ti4O7 in Li—O2 batteries reveal the growth of peaks at ˜458.5 and ˜464 eV, indicative of Ti4+2p3/2 and Ti4+2p1/2 in Li2TiO3. See, H. Deng, P. Nie, H. Luo, Y. Zhang, J. Wang and X. Zhang, J. Mater. Chem. A, 2014, 2, 18256, which is incorporated by reference in its entirety. The thermodynamically spontaneous reactions between Li2O2 and TiC and Ti4O7 in presence of oxygen such as Li2O2+TiC+3/2O2→Li2TiO3+CO2 (ΔHcalc=−1459 kJ/mol), Li2O2+TiC+O2→Li2TiO3+CO (ΔHcalc=−1071 kJ/mol) and 4Li2O2+Ti4O7→4Li2TiO3+3/2O2 (ΔHcalc=−753 kJ/mol) have high enthalpies. See, A. Jain, G. Hautier, S. P. Ong, C. J. Moore, C. C. Fischer, K. A. Persson and G. Ceder, Phys. Rev. B, 2011, 84, 045115, which is incorporated by reference in its entirety. Regarding delithiation of the intermediate, Li2TiO3 is stable against delithiation above 4.7 V which would explain the relatively low surface-area-normalized activity of Ti4O7 (˜4V at ˜8.4.10−3 μA·cm−2BET) electrodes loaded with crystalline Li2O2 and the persistence of the Ti4+ XPS peak during cycling beyond the first discharge.
In summary, mechanistic insights into the kinetics of Li2O2 oxidation has been presented by coupling electrochemical Li2O2 oxidation trends of metal and oxide promoters with spectroscopic measurements and the reactivity energetics between Li2O2 and the promoter. The measured activities of Cr, Mo and Ru particles are an order of magnitude greater than those of Co and Mn as well as those of corresponding oxides. Upon Li2O2 oxidation, XAS measurements show that Cr and Mo particles become highly oxidized to M6+ in CrO42− and MoO42− environments such as Li2CrO4 and Li2MoO4, respectively, which is accompanied with soluble Cr and Mo-based species in the electrolyte. However, those soluble species as well as other potential impurities such as water generated in operando are not the main source for the order of magnitude enhancement in electrode activity in presence of Mo, Cr, and Ru for example. A strong correlation between increasing specific Li2O2 oxidation currents in both carbon free and carbon-containing electrodes and increasing enthalpy for chemical reaction between Li2O2 and the promoter were found. This result proposes a universal mechanism for promoting Li2O2 oxidation kinetics via solid-state activation, which involves thermochemical conversion of the promoter surface and Li2O2 towards a lithium metal oxide, which can subsequently undergo electrochemical delithiation. The influence of such solid-state activation of Li2O2 oxidation for the voltage and faradaic efficiency of rechargeable Li-air batteries require further studies.
The Li—O2 system holds promise in revolutionizing gravimetric energy density in the battery energy storage field. A variety of transition metal based nanoparticles are candidate promoters in lowering recharge potentials and boosting its round trip efficiency. Chemical lithiation followed by electrochemical delithiation provides kinetic enhancement measured in presence of promoters such as Mo, Cr, and Ru. The present work focuses on the process efficiency during charging of Li—O2 batteries in presence of Mo, Cr, and Ru metal promoters using differential electrochemical mass spectrometry (DEMS). Oxygen consumption during discharge abides by the 2 e−/O2 desired for formation of Li2O2 for three cycles of all three promoters. On potentiostatic charging at 3.9 VLi, in agreement with current state of the art, all three promoters display sub-stoichiometric oxygen regeneration albeit with negligible CO2, CO, and H2O generation. Mo, with the highest activity enabled by its large conversion enthalpy with Li2O2 operates farthest from ideal at 4.82 e−/O2, while Cr, and Ru with comparable conversion enthalpies and electrochemical Li2O2 oxidation activities operate with ˜3.0 e−/O2. This study reinforces that low cost transition metals such as Cr are excellent substitutes for the noble metal Ru used extensively in promoting the charging of Li—O2 batteries.
The Li-Ion battery system has taken center-stage in high-energy and high-power applications; it is currently the chemistry of choice for powering portable electronics and the upcoming electric vehicles. However, their typical gravimetric energy density of ca. 100 Wh·kg−1 falls short of the US electric vehicle (EV) target 350 Wh·kg−1. See, P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845, and USCAR, Energy Storage System Goals, Accessed Jan. 1, 2016, 2016, each of which is incorporated by reference in its entirety. Several next generation chemistries generally based on conversion of oxygen or sulfur with lithium or sodium are in various stage of development. See, P. G. Bruce, S. A. Freunberger, L. J. Hardwick and J.-M. Tarascon, Nat. Mater., 2012, 11, 19, which is incorporated by reference in its entirety. Li—O2 batteries have captured vigorous scientific interest owing to their promise of providing double to triple the energy density of state-of-the-art Li-Ion batteries. See, K. G. Gallagher, S. Goebel, T. Greszler, M. Mathias, W. Oelerich, D. Eroglu and V. Srinivasan, Energy Environ. Sci., 2014, 7, 1555, and Y.-C. Lu, B. M. Gallant, D. G. Kwabi, J. R. Harding, R. R. Mitchell, M. S. Whittingham and Y. Shao-Horn, Energy Environ. Sci., 2013, 6, 750, each of which is incorporated by reference in its entirety.
However, their viability is hindered by several cell-level factors. Severe degradation of the solvents is observed for most aprotic electrolytes including alkyl carbonates used in Li-Ion cells, ethereal solvents and organosulfurs. See, S. A. Freunberger, Y. Chen, Z. Peng, J. M. Griffin, L. J. Hardwick, F. Bardé, P. Novák and P. G. Bruce, J. Am. Chem. Soc., 2011, 133, 8040, B. D. Adams, R. Black, Z. Williams, R. Fernandes, M. Cuisinier, E. J. Berg, P. Novak, G. K. Murphy and L. F. Nazar, Adv. Energy Mater., 2015, 5, S. A. Freunberger, Y. Chen, N. E. Drewett, L. J. Hardwick, F. Bardé and P. G. Bruce, Angew. Chem. Int. Ed., 2011, 50, 8609, and D. G. Kwabi, T. P. Batcho, C. V. Amanchukwu, N. Ortiz-Vitoriano, P. Hammond, C. V. Thompson and Y. Shao-Horn, J. Phys. Chem. Lett., 2014, 5, 2850, each of which is incorporated by reference in its entirety. The electrolyte degradation couples with the formation of parasitic discharge products and the poor electronic conductivity of the main discharge product Li2O2 to cause high recharge overpotentials, low round trip efficiencies and limited cycle life. See, B. D. McCloskey, A. Speidel, R. Scheffler, D. C. Miller, V. Viswanathan, J. S. Hummelshøj, J. K. Nørskov and A. C. Luntz, J. Phys. Chem. Lett., 2012, 3, 997, B. D. McCloskey, A. Valery, A. C.
Luntz, S. R. Gowda, G. M. Wallraff, J. M. Garcia, T. Mori and L. E. Krupp, J. Phys. Chem. Lett., 2013, 4, 2989, 0. Gerbig, R. Merkle and J. Maier, 2013, 25, 3129, and S. P. Ong, Y. Mo and G. Ceder, Phys. Rev. B, 2012, 85, 081105, each of which is incorporated by reference in its entirety. To address the coupled issues of high overpotentials and poor round trip efficiencies, reaction promoters consisting of metal (oxides) nanoparticles are commonly employed. See, K. P. C. Yao, Y.-C. Lu, C. V. Amanchukwu, D. G. Kwabi, M. Risch, J. Zhou, A. Grimaud, P. T. Hammond, F. Barde and Y. Shao-Horn, Phy. Chem. Chem. Phys., 2014, 16, 2297, K. P. C. Yao, M. Risch, S. Y. Sayed, Y.-L. Lee, J. R. Harding, A. Grimaud, N. Pour, Z. Xu, J. Zhou, A. Mansour, F. Barde and Y. Shao-Horn, Energy Environ. Sci., 2015, 8, 2417, F. Li, R. Ohnishi, Y. Yamada, J. Kubota, K. Domen, A. Yamada and H. Zhou, Chem. Commun., 2013, 49, 1175, R. Black, J.-H. Lee, B. Adams, C. A. Mims and L. F. Nazar, Angew. Chem. Int. Ed., 2013, 52, 392, and Z. Jian, P. Liu, F. Li, P. He, X. Guo, M. Chen and H. Zhou, Angew. Chem. Int. Ed., 2014, 53, 442, each of which is incorporated by reference in its entirety. Recent systematic probing of electrochemical and thermochemical trends aided by ex-situ X-ray absorption spectroscopy revealed the chemical conversion of the promoter with the discharge product Li2O2 to form lithiated metal oxides. See, K. P. C. Yao, Y.-C. Lu, C. V. Amanchukwu, D. G. Kwabi, M. Risch,
J. Zhou, A. Grimaud, P. T. Hammond, F. Barde and Y. Shao-Horn, Phy. Chem. Chem. Phys., 2014, 16, 2297, K. P. C. Yao, M. Risch, S. Y. Sayed, Y.-L. Lee, J. R. Harding, A. Grimaud, N. Pour, Z. Xu, J. Zhou, A. Mansour, F. Bardé and Y. Shao-Horn, Energy Environ. Sci., 2015, 8, 2417, and D. Kundu, R. Black, B. Adams, K. Harrison, K. Zavadil and L. F. Nazar, J. Phys. Chem. Lett., 2015, 6, 2252, each of which is incorporated by reference in its entirety. The latter delithiation of the lithiated metal oxide intermediates is revealed to be the source of the observed enhanced kinetics of Li2O2 oxidation. See, K. P. C. Yao, M. Risch, S. Y. Sayed, Y.-L. Lee, J. R. Harding, A. Grimaud, N. Pour, Z. Xu, J. Zhou, A. Mansour, F. Barde and Y. Shao-Horn, Energy Environ. Sci., 2015, 8, 2417, which is incorporated by reference in its entirety. A mechanism which differs significantly from traditional oxygen evolution (OER) catalysis in which the catalyst lowers the barrier of the rate limiting step through tuned binding of oxygenated intermediates on the surfaces. See, I. C. Man, H.-Y. Su, F. Calle-Vallejo, H. A. Hansen, J. I. Martinez, N. G. Inoglu, J. Kitchin, T. F. Jaramillo, J. K. Nørskov and J. Rossmeisl, ChemCatChem, 2011, 3, 1159, and J. Suntivich, K. J. May, H. A. Gasteiger, J. B. Goodenough and Y. Shao-Horn, Science, 2011, 334, 1383, each of which is incorporated by reference in its entirety. In light of this finding, it becomes imperative to investigate the process efficacy of the OER from Li2O2 oxidation necessary to regenerate a Li—O2 cell for the following discharge.
McCloskey et al. employed differential electrochemical mass spectrometry (DEMS) to investigate the OER during the charge reaction of Li—O2 batteries using either polycarbonate:dimethoxyethane (PC:DME) or 1, 2 dimethoxyethane (DME) as electrolyte solvent. See, B. D. McCloskey, R. Scheffler, A. Speidel, D. S. Bethune, R. M. Shelby and A. C. Luntz, J. Am. Chem. Soc., 2011, 133, 18038, which is incorporated by reference in its entirety. Their work concluded that metal nanoparticles in Li—O2 cells only affected the removal of soluble parasitic products in PC-based electrolytes evolving CO2 on charge, while no effect is observed in DME-based electrolytes where the desired Li2O2 product is being oxidized to evolve O2. Later work by the same author comparing the Li—O2 and Na—O2 systems further suggests that in the absence of carbonate side products, recharge of the alkali-air cell should be efficient without needing promoter nanoparticles. See, B. D. McCloskey, J. M. Garcia and A. C. Luntz, J. Phys. Chem. Lett., 2014, 5, 1230, which is incorporated by reference in its entirety. These conclusions do not agree with the clear charging trends observed for Li2O2 decomposition using carbon-free electrodes preloaded Li2O2 where little to no carbonates are expected. See, K. P. C. Yao, M.
Risch, S. Y. Sayed, Y.-L. Lee, J. R. Harding, A. Grimaud, N. Pour, Z. Xu, J. Zhou, A. Mansour, F. Bardé and Y. Shao-Horn, Energy Environ. Sci., 2015, 8, 2417, which is incorporated by reference in its entirety. In the work of Kundu et al. exploring the effect of Mo2C on charge, a charging plateau below 3.6 VLi (strong enhancement effect) and online electrochemical mass spectrometry (OEMS) measurement of mostly O2 with only trace CO2 are observed. See, D. Kundu, R. Black, B. Adams, K. Harrison, K. Zavadil and L. F. Nazar, J. Phys. Chem. Lett., 2015, 6, 2252, which is incorporated by reference in its entirety. The authors report by X-ray photoelectron spectroscopy the conversion of the promoter surface to LixMoO3 per the proposed mechanism by Yao et al. (Energy Environ. Sci., 2015). Furthermore, comparison of the oxidation kinetics of Li2O2 in Li—O2 and NaO2 in Na—O2 in this case (B. D. McCloskey, J. M.
Garcia and A. C. Luntz, J. Phys. Chem. Lett., 2014, 5, 1230, which is incorporated by reference in its entirety) disregards the anticipated slower kinetics of a two-electron transfer vs. a one-electron transfer reaction as well as the possible differences in charge transport from one to the other.
In the present work the process efficiency of Li—O2 cells was investigated in presence of the Mo, Cr, and Ru which are the most active Li2O2 oxidation promoters (described above) using DEMS. Characteristic similarities between Cr and noble metal Ru and their difference from Mo are revealed. Those similarities and differences as found explicable by values of conversion enthalpies of the promoter with Li2O2 towards lithiated metal oxides upon charging. First the discharge process in presence of Mo, Cr, and Ru in carbon supported electrodes was investigated during galvanostatic discharge at 200 mA·g−1Carbon=300 mA·g−1Promoter. The desired discharge reaction in Li-—O2 batteries is the conversion of lithium with oxygen in the gas phase to form a lithium oxide (LiO2, Li2O2, and/or Li2O). Since the first publication by Kumar et al. (B. Kumar, J. Kumar, R. Leese, J. P. Fellner, S. J. Rodrigues and K. M. Abraham, J. Electrochem. Soc., 2010, 157, A50, which is incorporated by reference in its entirety), the Li—O2 electrochemical system in absence of parasitic decomposition of the electrolyte or carbon cathode is been reported to discharge through formation of Li2O2 as the final discharge product (2Li++2e−+O2⇄Li2O2). See, S. A. Freunberger, Y. Chen, N. E. Drewett, L. J. Hardwick, F. Bardé and P. G. Bruce, Angew. Chem. Int. Ed., 2011, 50, 8609, and Y.-C. Lu, D. G. Kwabi, K. P. C. Yao, J. R. Harding, J. Zhou, L. Zuin and Y. Shao-Horn, Energy Environ. Sci., 2011, 4, 2999, each of which is incorporated by reference in its entirety) The stoichiometry of this reaction dictates the consumption of one oxygen molecule per two electrons passed (2e−/O2).
The most significant enhancement effect of the promoter nanoparticles is observed on the Li2O2 oxidation reaction during cell charging. The previous probing by X-ray absorption spectroscopy of the chemical processes occurring at 3.9 VLi in presence of metal nanoparticles revealed the chemical conversion of the promoter with Li2O2 towards formation of a lithiated metal oxide LixMyOz (Li2O2+MaOb±O2→LixMyOz). Therefore, the potential effect of this pathway was investigated on the regeneration of O2 (Li2O2⇄2Li++2e−+O2) and compare the actual process efficiencies across the high activity promoters Mo, Cr, and Ru identified previously.
Scheffler, D. C. Miller, V. Viswanathan, J. S. Hummelshøj, J. K. Nørskov and A. C. Luntz, J. Phys. Chem. Lett., 2012, 3, 997, each of which is incorporated by reference in its entirety. The decomposition of these carbonates on subsequent charging explains the greater amount of CO2 compared to preloaded electrodes where discharge is bypassed for the purpose understanding the Li2O2 oxidation reaction with minimal interference from parasitic products.
Nonetheless, rates of oxygen evolution upon charging is sub-stoichiometric compared to the current observed considering the 2e−/O2 reaction (Li2O2 2Li++2e−+O2) both in Li2O2-preloaded electrodes (
2Li++MOx+½(y−x)O2) of the chemically lithiated metal oxide which contributes to the externally measured activity of electrodes cannot be expected to result in 2e−/O2, hence likely to cause greater stoichiometric deviations. Prior studies utilizing DEMS or OEMS for gas quantification generally report sub-stoichiometric O2 regeneration from Li2O2 oxidation in Li—O2 cells. See, B. D. McCloskey, J. M. Garcia and A. C. Luntz, J. Phys. Chem. Lett., 2014, 5, 1230, S. Meini, S. Solchenbach, M. Piana and H. A. Gasteiger, J. Electrochem. Soc., 2014, 161, A1306, B. D. McCloskey, D. S. Bethune, R. M. Shelby, T. Mori, R. Scheffler, A. Speidel, M. Sherwood and A. C. Luntz, J. Phys. Chem. Lett., 2012, 3, 3043, and S. Meini, N. Tsiouvaras, K. U. Schwenke, M. Piana, H. Beyer, L. Lange and H. A. Gasteiger, Phys. Chem. Chem. Phys., 2013, 15, 11478, each of which is incorporated by reference in its entirety. McCloskey et al. report values of 2.59 e−/O2 in recharging O2-electrodes with the LiTFSI/monoglyme (DME) electrolyte. See, B. D. McCloskey, R. Scheffler, A. Speidel, D. S. Bethune, R. M. Shelby and A. C. Luntz, J. Am. Chem. Soc., 2011, 133, 18038, and B. D. McCloskey, D. S. Bethune, R. M. Shelby, T. Mori, R. Scheffler, A. Speidel, M. Sherwood and A. C. Luntz, J. Phys. Chem. Lett., 2012, 3, 3043, each of which is incorporated by reference in its entirety. Gasteiger et al. used OEMS to report values of 2.6 e−/O2 and 2-2.4 e−/O2 in preloaded electrodes with LiTFSI/diglyme electrolyte and carbon-only electrode. See, S. Meini, N. Tsiouvaras, K. U. Schwenke, M. Piana, H. Beyer, L. Lange and H. A. Gasteiger, Phys. Chem. Chem. Phys., 2013, 15, 11478, and S. Meini, S. Solchenbach, M. Piana and H. A. Gasteiger, J. Electrochem. Soc., 2014, 161, A1306, each of which is incorporated by reference in its entirety. The potentiostatic DEMS investigation at 4.4 VLi (chosen to enable reasonable rate of oxygen evolution in VC-only electrodes) of VC:Li2O2:LiNafio=1:1:1 electrodes yielded 2.89 e−/O2 with a relatively greater amount of CO2 evolved (
The consumption and regeneration of O2 during cycling of Mo, Cr, and Ru-promoted O2-electrodes was investigated (
As discussed above, charging of Li—O2 cells generally does not follow the desired 2e−/O2 decomposition of the Li2O2 formed on discharge. Upon charging in
In conclusion, metal nanoparticle promoters offer an avenue for reduction of the large overpotential pervasive during Li—O2 cells recharge and thereby increase recharge efficiency and lower parasitic oxidation of the organic electrolyte. Here the process efficiency of promising promoter nanoparticles Mo, Cr, and Ru are shown. The following four major findings are highlighted: (i) Li2O2 with 2 e−/O2 is the major discharge product independent of the presence of Mo, Cr, or noble metal Ru. The discharge pathway (2Li++O2 Li2O2) is unaffected by the promoter nanoparticle as revealed through comparable discharge voltage of ˜2.6 VLi at 200 mA·g−1Carbon=300 mA·g−1Promoter for all three promoters studied. (ii) Oxidation of the Li2O2 discharge product results in sub-stoichiometric regeneration of O2 in agreement with literature reports. In particular, Mo electrodes depart strongly from 2 e−/O2 with significant fluctuations likely as a result of the greater thermodynamic driving force (−939 kJ·mol−1 for Li2O2+Mo+O2
Li2MoO4) for conversion of Mo with Li2O2 towards Li2MoO4. In contrast Cr and Ru with medium and similar conversion enthalpies (approximately −440 kJ·mol−1 for Li2O2+½Cr2O3+½O2
Li2CrO and Li2O2+Ru+½O2
Li2RuO3), display values around 3 e−/O2 prior to fluctuations observed beyond full recharge. Remarkably, the correlation between conversion enthalpy and promoter electrochemical activity is further reflected in the similarity between Cr and Ru in terms of both current and oxygen evolution rates at 3.9 VLi in Li2O2 preloaded as well as O2-electrodes. Low cost Cr nanoparticle promoted electrodes would be an excellent substitute for higher cost noble metal Ru electrodes extensively used in Li—O2 batteries. (iii) Only minor amount of CO2, CO and H2O are measured during cycling charging at 3.9 VLi, which emphasize the utility of promoter nanoparticles to enable charging voltage below 4.0 VLi, for electrolyte stability.
Electrode Preparation
The electrochemical oxidation kinetics of Li2O2 were studied using promoters including metal nanoparticles of Mo (US Research Nanomaterial Inc., Purity=99.9%, SSABET=4 m2·g−1), Cr (US Research Nanomaterial Inc., 99.9%, 26 m2·g−1), Co (US Research Nanomaterial Inc., 99.8%, 21 m2·g−1), Ru (Sigma Aldrich, ≧98%, 23 m2·g−1), Mn (American Elements, Mn3O4 shell, 99.9%, 24 m2·g−1) and metal oxide particles of MoO3 (Sigma Aldrich, 99.98%, 1.8 m2·g−1) Cr2O3 (Sigma Aldrich, 99%, 20 m2·g−1), Co3O4 (Sigma Aldrich, 99.5%, 36 m2·g−1), RuO2 (Sigma Aldrich, 99.9%, 16.2 m2·g−1) nanoparticles and α-MnO2 nanowires (Synthesized, SSABET=85 m2·g−1, X-ray diffraction pattern provided in
Carbon and binder free gold-supported electrodes, having a fixed promoter:Li2O2 mass ratio of 0.667:1, were prepared using the following method reported previously. See, K. P. C. Yao, Y.-C. Lu, C. V. Amanchukwu, D. G. Kwabi, M. Risch, J. Zhou, A. Grimaud, P. T. Hammond, F. Bardé and Y. Shao-Horn, Phys. Chem. Chem. Phys., 2014, 16, 2297, which is incorporated by reference in its entirety. Due to embrittlement of the gold foil in presence of Mo, Mo-promoted electrodes were deposited on battery grade aluminum foil. Masses of 10 mg promoter and 15 mg of ball-milled Li2O2 (Alfa Aesar, ≧90%, ˜345 nm after ball-milling) were mixed in 1 mL anhydrous 2-propanol (IPA, Sigma Aldrich, 99.5%) and horn-sonicated at 50% pulses of 30 W for 30 minutes. After sonication, 40 μL of the slurry is dropcasted onto ½ inch diameter gold foil, resulting in a material loading of ˜0.8 mg·cm−2. Upon evaporation of the IPA, the gold disk was enclosed between two dried aluminum sheets and sealed in an argon-filled heat-seal bag. The sealed bag was removed from the glovebox and pressed at 5 tons under a hydraulic press to secure the promoter:Li2O2 mixture onto the gold foil.
Carbon-containing electrodes, with Vulcan XC72 carbon as electrically conducting backbone, were deposited on battery grade aluminum foil at a mass ratio of promoter:VC:Li2O2:LiNafion binder=0.667:1:1:1 using a #50 Mayer rod. See, J. R. Harding, Y.-C. Lu, Y. Tsukada and Y. Shao-Horn, Phys. Chem. Chem. Phys., 2012, 14, 10540, and K. P. C. Yao, Y.-C. Lu, C. V. Amanchukwu, D. G. Kwabi, M. Risch, J. Zhou, A. Grimaud, P. T. Hammond, F. Barde and Y. Shao-Horn, Phys. Chem. Chem. Phys., 2014, 16, 2297, each of which is incorporated by reference in its entirety. Prior to ink casting, 75 mg of Vulcan XC72, 50 mg of promoter, 75 mg of Li2O2, and 75 mg equivalent of IPA-dispersed lithium-substituted Nafion (LiNafion, Dupont) were horn-sonicated in IPA at 50% pulses of 30 W for 30 minutes. All electrodes were dried at 70° C. in the Buchi® vacuum oven for a minimum of 12 hours and transferred into the glove box without ambient exposure. The fabrication of electrochemical cells was performed without atmospheric exposure in an Argon-filled glovebox (Mbraun, H2O<0.1 ppm, O2<0.1%).
The oxidation kinetics of Li2O2 was studied in electrochemical cells consisting of an 18 mm diameter lithium foil (Chemetall Germany), 150 μL of 0.1 M LiClO4 in 1,2 dimethoxyethane (0.1 M LiClO4/DME, BASF, H2O<20 ppm by Karl Fischer titration), two pieces of Celgard C480, and an Li2O2-preloaded electrode. These cells were tested potentiostatically using a VMP3 potentiostat (BioLogic Inc.).
Ex situ X-ray absorption spectroscopy was performed at the SGM beamline of the Canadian Light Source at first-row transition metal L edges in vacuum. Molybdenum L edges were recorded in vacuum at the SXRMB beamline of the Canadian Light Source and in a helium atmosphere at the 9-BM-B beamline station at the Advanced Photon Source. Chromium K-edges were collected in a helium atmosphere at beamline X11A of the National Synchrotron Light Source. All spectra were acquired in the surface sensitive electron yield mode at room temperature. The spectra were processed as reported previously. See, K. P. C. Yao, Y.-C. Lu, C. V. Amanchukwu, D. G. Kwabi, M. Risch, J. Zhou, A. Grimaud, P. T. Hammond, F. Bardé and Y. Shao-Horn, Phys. Chem. Chem. Phys., 2014, 16, 2297, and M. Risch, A. Grimaud, K. J. May, K. A. Stoerzinger, T. J. Chen, A. N. Mansour and Y. Shao-Horn, J. Phys. Chem. C, 2013, 117, 8628, each of which is incorporated by reference in its entirety. Energy axes are calibrated to appropriate metal references. The promoter metal (Mo, Cr, Co, Mn) L-edges were collected for the nanoparticle powder, a pristine electrode, a partially charged electrode, and fully charged electrode. Mo L edge spectra of MoO2 (Alfa-Aesar, 99%), MoO3 (Sigma Aldrich, 99.98%), Li2MoO4 (Alfa Aesar, 99.92%), Mo foil (Sigma Aldrich, 99.9%) and Cr K edge K2CrO4 (Alfa Aesar, 99%) were collected and used as references.
Inductively coupled plasma atomic emission spectra (ICP-AES) were collected from the electrolyte after electrochemical oxidation of Li2O2 in presence of Mo, Cr, Co, Co3O4, and α-MnO2. As any dissolution of transition-metal-containing species could plate on the lithium anode, “2-compartment” cell was utilized reported by Gasteiger et al., which consists of lithium foil||Celgard C480 with 50 μL 0.1 M LiClO4/DMEH||Ohara solid electrolyte||Celgard C480 with 100 μL 0.1 M LiClO4/DME||Carbon-free Li2O2-loaded electrode. See, R. Bernhard, S. Meini and H. A. Gasteiger, J. Electrochem. Soc., 2014, 161, A497, which is incorporated by reference in its entirety. The C480 separator in contact with the Li2O2 electrode was collected post charging, and was immersed in DME (BASF, H2O<20 ppm by Karl Fischer titration), which was combined with DME that was used to rinse the surface of the solid electrolyte for a total of 3 mL DME.
The resulting DME solution was then centrifuged at 7000 rpm for 10 minutes to remove solid particulates, which was pipetted subsequently out into a new vial and evaporated slowly at 40° C. on a hot plate. 0.5 mL of 37 wt % HCl was added to the dried vial to dissolve any solid precipitates, which was then evaporated slowly on a hot plate. Finally, the vial was rinsed with 10 mL of 2 wt % nitric acid (Sigma Aldrich, TraceSelect®) to create the ICP sample. ICP standards at 0, 1, 2, and 5 ppm were also generated for Mo (RICCA CHEMICAL COMPANY® 1000 ppm in 3% HNO3 with trace HF), Cr, Co, and Mn from standard solutions (Fluka TraceCERT®, 1000 ppm in 2% HNO3). ICP-AES data were collected using a Horiba ACTIVA-S spectrometer.
E lectrode Preparation for DEMS Experiments
The most active metal nanoparticles discovered above, namely Mo (US Research Nanomaterial Inc., purity=99.9%, SSABET=4 m2·g−1), Cr (US Research Nanomaterial Inc., 99.9%, 26 m2·g−1), Ru (Sigma Aldrich, ≧98%, 23 m2·g−1) were selected for further study using DEMS. Vulcan XC72 (VC, Premetek, ˜100 m2·g−1) carbon-supported electrodes containing these three promoter nanoparticles where fabricated in an argon-filled glovebox (MBraun, water content <0.1 ppm, O2 content <1%). Fabrication tools consisting of a #50 mayer rod, battery grade aluminum foil (Targray Inc.), and Celgard C480 cell separator sheet (Celgard Inc.) were dried at 70° C. prior to use. Nanoparticles powders of VC, Mo, Cr, and Ru were dried at 100° C. under a 30 mbar vacuum in a Buchi® B585 oven. Transfer of the dried nanoparticles occurred with isolation from ambient air within the Buchi® vacuum tube.
Oxygen electrodes of VC:(Mo, Cr, Ru):LiNafion=1:0.667:1 (mass ratios) were obtained by ink-casting on a sheet of Celgard C480. A mixture of 75 mg of Vulcan XC72, 50 mg of promoter, and 75 mg equivalent of IPA-dispersed lithium-substituted Nafion (LiNafion, Dupont) was homogenized in IPA by horn-sonication at 50% pulses of 30 W for 30 minutes. Similarly, Li2O2-preloaded electrodes of VC:(Mo, Cr, Ru):Li2O2:LiNafion=1:0.667:1:1 (mass ratios) were obtained by ink-casting on a sheet of aluminum. A mixture of 75 mg of Vulcan XC72, 50 mg of promoter, 75 mg of Li2O2 (Alfa Aesar, ≧90%, ˜345 nm after ball-milling), and 75 mg equivalent of IPA-dispersed LiNafion was homogenized in IPA by horn-sonication at 50% pulses of 30 W for 30 minutes.
Within the anaerobic environment of the glovebox, half-inch diameter discs were punched and secured in the vacuum tube of the Buchi® oven tube and dried at 70° C. for a minimum of twelve hours before cell assembly.
Electrochemical cells made of either O2 electrodes or Li2O2-preloaded electrodes were fabricated in an argon glovebox (MBraun, water content <0.1 ppm, O2 content <0.1 ppm) and subjected to DEMS measurement. All cells consisted of 150 μm lithium foil (RockWood Lithium Inc.), 0.1 M lithium bis(trifluoromethane)sulfonimide (LiTFSI) in diglyme (20 ppm nominal after drying on molecular sieves) and an O2 or Li2O2-preloaded electrode. Cells consisting of lithium foil||2 Celgard C480 separators with 150 μL of 0.1 M LiTFSI in Diglyme||0.5 inch electrode were assembled in a custom cell with an internal volume of ca. 2.9 mL. An in-house DEMS based on a design reported by McCloskey et al. and Jonathon et al. 25,26 was utilized to monitor oxygen consumption during discharge and gas evolution on charge. See, B. D. McCloskey, D. S. Bethune, R. M. Shelby, G. Girishkumar and A. C. Luntz, J. Phys. Chem. Lett., 2011, 2, 1161, J. R. Harding, C. V. Amanchukwu, P. T. Hammond and Y. Shao-Horn, J. Phys. Chem. C, 2015, 119, 6947, and J. R. Harding, in Chemical Engineering, Massachusetts Institute of Technology, hdl.handle.net/1721.1/98707, 2015, each of which is incorporated by reference in its entirety. Oxygen consumption during galvanostatic discharge at 200 mA·g−1Carbon=300 mA·g−1Promoter of O2 electrodes was quantified via pressure drop monitoring at two second intervals. O2, CO2, and H2O evolution during potentiostatic charge of both O2 and Li2O2-preloaded electrodes was quantified at 15-minute intervals using a mass spectrometer coupled with pressure monitoring. Linear interpolation is used to match electrochemical and DEMS measurement in the all figures presented herein. Details of DEMS and cell technical construction are available online. See, J. R. Harding, in Chemical Engineering, Massachusetts Institute of Technology, hdl.handle.net/1721.1/98707, 2015, which is incorporated by reference in its entirety.
Other embodiments are within the scope of the following claims.
This application claims the benefit of prior U.S. Provisional Application No. 62/121,036 filed on Feb. 26, 2015, which is incorporated by reference in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2016/019951 | 2/26/2016 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62121036 | Feb 2015 | US |
