Electrolyte-Directed Reactions of the Oxygen Electrode in Lithium-Air Batteries

Abstract

Non-aqueous electrolytes play a prominent role in the redox reactions of the oxygen electrode in the non-aqueous Li-air battery. In all electrolytes the initial O 2 reduction reaction (ORR) product is superoxide, O 2 − , whose stability is determined by the Lewis acidity of the ion-pairing cations present in the medium as governed by the Hard Soft Acid Base (HSAB) theory. Our results suggest that depending on the basicity of the solvent as measured by its Donor Number (DN), the superoxide will be stabilized to varying lengths of time before transforming to O 2 2− via a chemical or an electrochemical reaction. Organic solvents decrease the Lewis acidity of Li + through the formation of solvates, Li + (solvent) n . As a result, high DN solvents such as dimethyl sulfoxide (DMSO), which decrease Li + acidity more than low DN solvents, provide longer life-time for the soft base O 2 − by forming the ion pair, Li + (DMSO) n -O 2 − . In low DN solvents such as 1, 2-dimethoxy ethane (DME) and tetraethyleneglycol dimethyl ether (TEGDME), the LiO 2 rapidly decomposes to the peroxide, Li 2 O 2 . As the ionic bond strength between the O 2 reduction product and the conducting salt cation becomes stronger, its rechargeability becomes poorer. Solvent electron donor property affects ORR catalysis also. There is intense world-wide research and development of a rechargeable lithium battery based on the Li/O 2 chemical couple, popularly known as the Li-air battery, encouraged by its very high theoretical specific energy (5200 Wh/kg) and low cost of the materials from which it can be fabricated.1,2 The theoretical specific energy of the Li-air battery is surpassed only by a battery based on the Li/F 2 couple.3 However, F 2, because of its high reactivity, is an impractical electrode material to construct batteries for powering consumer products such as cellphones, laptop computers, tablets, digital cameras, and electric vehicles. This leaves Li-air as the candidate of choice for an ultra-high energy density, low cost (air is free) rechargeable lithium battery. The Li-air battery is environmentally friendly as nontoxic electrolytes and other materials, besides Li and O 2, needed for fabricating it can be formulated. It is a safe battery despite having a Li metal anode because the cathode active material is not stored inside the battery cell.The Li-air battery is one of many metal-air batteries that could conceivably be fabricated (Table I).Among the various metal-O 2 couples, the Li-air is the most attractive battery since the cell discharge reaction involving metallic Li and oxygen to yield Li 2 O, according to the reaction 4Li + O 2 → 2Li 2 O, has an open circuit voltage of 2.91 V and a theoretical specific energy of 5200 Wh/kg (Table I). In practice, oxygen does not have to be stored in the battery which makes the theoretical specific energy excluding oxygen to be 11140 Wh/kg when the battery is initially fabricated. The battery weight will increase and its specific energy will decrease as it is discharged due to the accumulation ...

1,2‐Dimethoxyethane Degradation Thermodynamics in Li−O2 Redox Environments

“…Step 2: The O 2 − radical anion can attack the slightly acidic hydrogen atoms on the DME solvent molecule …”

Effect of Solvents on the Behavior of Lithium and Superoxide Ions in Lithium–Oxygen Battery Electrolytes

“…It is cleart hat in DMSO the association process should be inhibited relative to that in ACN because the barrier in DMSO is 2.5 times higher than that in ACN. [6,8,9] Thus, the interaction of even as ingle pair of ions may be accompanied by bond rupture and formation.I no ur simulation, Li is equidistant from both Oa toms of superoxide in the CIP,a nd the LiÀOb ond lengths are 2.15 and 2.06 in DMSO and ACN, respectively. However,D ME does not follow this trend.…”

“…[6] It was observed that high donor-number (DN) solvents increase the lifetimeo ft he superoxide anion, O 2 À ,a nd the rever-sibility of the oxygen reduction reaction. [8] Strong solvation of the Li + ions slows down association with O 2 À and increases the thermodynamic stability of solvent-separated ion pairs (SSIP). In thef ramework of Pearson's hard-soft acid-base (HSAB) theory,t his effect can be explained by increased stability of the solvation shell around the Li + ions in high DN solvents.…”

Lithium salts for advanced lithium batteries: Li–metal, Li–O2, and Li–S

“…LiO 2 as well as the discharge capacity of Li-O 2 cells [74][75][76]. Degradation of LiTFSI and LiTf both result in the formation of LiF and -CF 3 157,159,[164][165][166].…”