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Kinetics and Mechanisms of the Oxygen Electrode Reactions in Lithium-Air Batteries

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Kinetics and Mechanisms of the Oxygen Electrode Reactions in Lithium-Air Batteries 20-4 Kinetics and Mechanisms of the Oxygen Electrode Reactions in Lithium-air Batteries K. M. Abraham*, Amell Alsudairi, Sanjeev Mukerjee Department of Chemistry, Northeastern University, Boston, MA 02115 *Telephone: 781-444-8453; E-mail: [email protected] Edward J. Plichta and Mary A. Hendrickson US Army RDECOM CERDEC C2D Army Power Division, RDER-CCA Aberdeen Proving Grounds, MD 21005 Abstract: When Li salts, LiX, are dissolved in When a Li salt is dissolved in a non–aqueous organic solvents to prepare electrolytes, they form solvent to form an ion-conducting electrolyte, the Li+ + - solvates of the formula, Li (solvent)nX which cation is complexed by the solvent to form solvates + influence the kinetics of the O2 reduction reactions of the general formula Li (solvent)nX, where n is and the stability of the intermediates formed in the usually 4. The solvation of Li+ in turn modulates the discharge of Li-air batteries. In solvents with Lewis acidity of Li+ to make this normally hard Lewis Guttmann Donor number (DN) of about 25 or more, acid to acquire increasing amounts of softness. - + the initial ORR reduction product O2 is stabilized as Thus, Li gains softer Lewis acid characteristics + - Li (solvent)n-1-O2 , consistent with the HSAB theory. depending upon the basicity (DN) of the solvents In solvents with DN less than about 20, the stable with which it forms solvates. Electron donor solvents ORR reduction product is solid Li2O2. The solvent’s with High Guttmann Donor Number (DN) , i.e; DN Lewis basicity affects ORR catalysis also. Thus, an >25, are strong Lewis bases which make Li+ softer appropriate catalyst in the porous carbon cathode Lewis acids by solvation than Li+ solvated by low DN promotes ORR catalysis in low DN solvents while solvents having DN <20. Two organic solvents with the same catalyst is less effective in high DN high DN are dimethyl sulfoxide (DMSO) (DN = 29.8) solvents. In the latter ORR reactions, the solvent and dimethyl acetamide (DMAc) (DN=27). In the Li+- promotes an outer Helmholtz plane charge transfer conducting electrolytes formed in these solvents, Li+ process even in the presence of the catalyst. solvates behave as soft Lewis acids. The structural Conversely, an inner Helmholtz plane reaction that formula of the solvate formed in a LIPF6 solution in promotes ORR catalysis takes place in electrolytes DMAC is depicted in Scheme I where the solvate based on low DN solvents. exists as a solvent-separated ion pair with the anion. - Keywords: Li-air battery, non-aqueous electrolytes, Interestingly, when the anion is triflate (CF3SO3 ) the dimethyl acetamide, dimethyl sulfoxide, Donor solvate exists as a contact ion pair since both the Number, HSAB concept solvent and the triflate anion bond with Li+ (Scheme II). The non-aqueous Li-air battery composed of a Li metal anode and gaseous oxygen cathode, with a theoretical specific energy of 5200 Wh/kg, has been the subject of world-wide research and development in the last decade (1, 2). The four-electron O 2 reduction reaction (ORR) in non-aqueous electrolytes used in this battery proceeds through the intermediates LiO and Li O before converted to 2 2 2 Li O. Our studies have revealed that the kinetics 2 and mechanisms of the various chemical and electrochemical reactions in the overall reduction of O to Li O are influenced by the electron donor- 2 2 acceptor properties of the non-aqueous solvents used and the electron acceptor property of the solid Scheme I: Molecular Structure of the solvent catalyst employed in the porous carbon electrode in + - separated ion pair, Li (DMAc) PF formed, in a the Li-air battery (2-5). 4 6 solution of LiPF6 in DMAc. 347 forms an ion-pair with solvated Li+ whose life time, before decomposing according to reaction [2], depends on the electron donor property of the solvent as measured by its DN. The four electron reduction of LiO2 to Li2O can be accomplished in potentiodynamic electrochemical reductions, such as cyclic voltammetry (CV) scans, in which the potential is swept to the region where the reaction in equation [4} occurs. On the other hand, solid Li2O2 is the final discharge product in Li- air cells because in these cells, the discharge under a continuous feed of O2 gas takes place at a nearly constant potential so that the reaction mechanism is Scheme II: Molecular Structure of the contact dominated by equations [1] and [2}. Because Li2O2 + - ion pair, Li CF3SO3 (DMAc)3 in a solution of is insoluble in all of the solvents examined to date, it LiCF3SO3 in DMAc. precipitates out in the pores of the carbon cathode. In low donor number solvents such as the aliphatic ethers DME and TEGDME (DN = 16.6 and 20) Two solvents with low DN useful in rechargeable Li crystalline Li2O2 is formed even in a CV scan since + - batteries are dimethoxy ethane (DME) (DN=20) and the life time of Li (solvent)n--O2 is very low in these tetraethyleneglycol dimethylether, (TEGDME solvents and it decomposes quickly to Li2O2 as (DN=16.5). In Li+-conducting electrolytes formed in depicted in equation [2]. these solvents, the Li+ solvates behave as hard Lewis acids. Our electrochemical data and NMR and Raman spectral results have revealed that soft Lewis acids - 2- - The oxygen reduction products O2 , O2 and O2 are such as the tetrabutyl ammonium (TBA) cation of characterized by varying soft to hard Lewis base TBA salts (e.g., TBAPF6) or 1-ethyl-3- - properties with O2 functioning as a soft Lewis base methylimidazolium imide (EMITFSI) cation of the 2- 2- and O2 and O behaving as hard Lewis bases. room-temperature ionic liquid (RTIL) 1-ethyl-3- Consequently, O2 reduction reactions in non- methylimidazolium bis(triflouromethanesulfonyl) aqueous solvents are governed by the Hard-Soft imide (EMITFSI), stabilize the one-electron reduction - Acid-Base (HSAB) theory which state that hard product O2 in both high and low DN solvents Lewis acids want to combine with hard Lewis bases (Scheme III) (2, 5). In these solutions further whereas soft Lewis acids want to combine with soft reduction of superoxide is very difficult which Lewis bases to form stable reaction products (2) supports the validity of the HASAB concept. We have found that ORR can be catalyzed by metal The overall conversion of O2 to Li2O in the phthalocycanine such as cobalt and iron electrochemical reduction of O2 in a non-aqueous phthalocycanines (CoPC and FePC) appropriately solvent proceeds through the intermediates LiO2 and dispersed in the porous carbon cathode (5, 7 ). The Li2O2 as shown in equations [1] – [4]. catalysis proceed via either a homogeneous + - + - (Outer Helmholtz Plane, OHP) or a heterogeneous O + Li (solvent) + e Li (solvent) --O [1] 2 n n 2 (Inner Helmholtz Plane, IHP) reduction pathway + - depending on the Donor Number of the organic 2Li (solvent) --O n 2 solvent and the d electron configuration of the metal Li O (s) + O + n solvent [2] 2 2 2 phthalocyanine MPC catalyst. These are depicted in + - - Figures 1 and 2. In these and later figures CoPC600 Li (solvent) --O + e n 2 represents the active catalyst formed by heating the Li O (s) + n solvent [3] o 2 2 carbon-CoPC mixture at 600 C (5, 7). In the case of 7 + - CoPC, the Co(II) oxidation state with a 3d electronic Li O (s) + 2 Li (solvent) + 2 e 2 2 n configuration is the desired catalysts specie. When FePC is used, it is the Fe(I) oxidation state with the 2Li O(s) + nsolvent [4] 7 2 3d electronic configuration that is desired for catalysis. A heterogeneous Inner Helmholtz Plane Note that the reactions in equations [1]–[4] involve + reaction catalyzed by the metal in the MPC is solvated Li reactants. Similarly, the soluble initial - favored in the low DN solvent one-electron O2 reduction product superoxide, O2 , 348 tetraethyleneglycoldimethyl ether, (TEGDME The higher discharge voltage of catalyzed cells is (DN=16.5) in which catalytic ORR is observed. In due to the catalyst lowering the charge transfer these Li-air cells, discharge occurs at a higher resistance of ORR, manifested as a lower iR drop in discharge voltage than in uncatalyzed cells (see the cell discharge voltage. The catalyst doubled the figure 3) and the charge takes place at lower cycle life of Li-O2 battery cells cycled with the use of voltages (figure 4b). TEGDME-LiSO3CF3 electrolyte. These catalysts are less active in high DN solvent-based electrolytes such DMSO/LiSO3CF3 and DMAc/ LiSO3CF3 (5, 6). Figure 2: Electrochemical double layer at the interface of a CoPC600 catalyzed carbon - + + Scheme III: Stabilized superoxide (O2 ) in TBA electrode and a Li -DMSO or Li -DMAc electrolyte. (upper in acetonitrile) and EMIImide (lower in Highlighted section (A) depicts outer sphere one- EMITFSI) salt solutions electron transfer. Highlighted section (B) shows the one-electron product superoxide chemisorbed to the Cobalt catalyst promoting further electrochemical - reduction of O2 . Figure 1: A schematic representation of the electrochemical double layer at the interface of a + Figure 3: Discharge voltages of a Li-air cell with CoPC600 catalyzed carbon electrode in a Li - (upper) and without (lower) CoPC600 catalyst. (CH3CN)n electrolyte. Highlighted section A shows the outer Helmholtz plane one electron charge transfer as it occurs on an uncatalyzed surface. Highlighted section B shows inner Helmholtz plane charge transfer process facilitated by the presence of a catalyzed surface. 349 Acknowledgment: Financial support from the US Army CERDEC contract No: GTS-S-15-015 is gratefully acknowledged References 1. Abraham, K. M.; Jiang, Z., Journal of the Electrochemical Society 1996, 143 (1), 1-5.
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