Towards a Stable Organic Electrolyte for the Lithium Oxygen Battery

Abstract

mass of both reactants and a favorable 3.0 V potential. [3][4][5][6] The theoretical specifi c energy of 3505 Wh kg -1 , and energy density of 3435 Wh L -1 (based on Li 2 O 2 ), is very attractive compared to contemporary Li-ion couples that are calculated to have theoretical values near 400 Wh kg -1 and 1400 Wh L -1 . Practical values for Li-air batteries are estimated near 1000 Wh kg -1 , [ 7 ] which compare favorably to current Li-ion commercial cells (210 Wh kg -1 ), but realizing this specifi c energy over continuous operation requires surmounting many challenges in the underlying chemistry.The most prominent of these is creating an electrolyte for the non-aqueous cell which is inert to nucleophilic attack on discharge and charge of the battery, is stable to metallic lithium and solvates Li salts. Although the search for more stable systems has resulted in many investigations of different solvent/ salt combinations, it is generally agreed that there is presently no electrolyte that fi ts these requirements. Dimethylacetamide (DMA) [ 8 ] and dimethylformamide (DMF) [ 9 ] have recently been shown to be quasi-stable in combination with the LiTFSI salt. Nonetheless, both solvents react to form Li-X salts on cycling (X = formate, acetate and carbonate). [ 10 ] These decomposition products, particularly Li 2 CO 3, precipitate on the cathode where they increase impedance and create high cell polarization on charge owing to their high oxidation potentials. [11][12][13] Similar problems are created by the carbon support typically used for the gas diffusion membrane cathode which has been shown to react with the peroxide discharge product and produce Li 2 CO 3 interfacial impedance layers. [ 10 ] Two promising solutions to this dilemma have been presented by Peng et al., [ 14 ] who employed dimethyl sulfoxide (DMSO) as a solvent in combination with a nanoporous gold foil as a gas diffusion membrane, or Thotiyl et al., [ 15 ] utilizing TiC as a stable cathode material. Decomposition of DMSO leads to soluble products such as dimethyl sulfone and lithium sulfate, [16][17][18] which do not passivate the cathode surface to the same extent as do the carbonates. The high reactivity of DMSO with the lithium metal anode, and the eventual precipitation of the decomposition products renders this a fi rst-step solution. The higher rate capability of the noncarbonaceous cathode materials is also highly benefi cial because it favors formation of quasi-amorphous Li 2 O 2 thin-fi lms, which exhibit lower charging potentials. [ 19,20 ] TiC exhibits limited cycling behavior at fast rates even with tetraglyme (TEGDME).A new lithium-ether-derived chelate ionic liquid is synthesized to serve as an electrolyte for the Li-O 2 battery that is stable to metallic lithium, and whose ethereal framework is much more inherently stable to superoxide-initiated hydrogen abstraction than the simple glyme, dimethoxyethane (DME). Reactions of chemically generated superoxide with this electrolyte show that virtually no decomposition products such as lith...

Why Do Lithium–Oxygen Batteries Fail: Parasitic Chemical Reactions and Their Synergistic Effect

“…[7,9] Thec ategorization based on the various decomposition pathways as summarized above is new.T he knowledge presented here is not limited to the reactive oxygen species,but also applicable to other radicals or anions that might exist in the system, such as redox mediators,d issolution of metal catalysts or electrolyte decomposition intermediates.T of acilitate the understanding of the systems,wefurther list different discussions in Table 1, where the mechanisms and chemical structures are correlated for easy reading.These efforts provide us with new insight into the role of oxygen species on the decomposition of the electrolytes.I ti sc onceivable that ethers can be stabilized by the substitution of the Honthe carbon backbone with inert groups such as -CH 3 .T his hypothesis has been recently verified by experimental efforts by Nazar et al [20] Similarly,the methylation of b positions on PYR cation may improve the stability of the related ionic liquid cation. Nevertheless,t he synthesis and purification of modified electrolytes may incur high cost, which can be an issue for practical applications.I na ddition, the introduction of bulky substitution groups may decrease the oxygen diffusivity, leading to high overpotentials.…”

“…[41] Their reactivity toward auto-oxidation, [20] nevertheless,p resents significant problems ( Figure 1). For example,t he a-H in ethers has been shown reactive toward superoxide radicals.…”

“…See Section 2.5 for discussions of the SEI formation and functionality. [31,35] --Ether/polyether -O ,P [20,42,44,51] -O [43] DMSO B, [32] N [30] B, [32] N, [30] P [46] R [19] Ionic liquid -B [47,48] R [47] Amide -N [39,40] R [55] Notes:N:nucleophilic attack;B:acid-base reaction;O:auto-oxidation; P: proton-mediated process;R:r eduction by Li.…”

“…[15][16][17] No stable electrolytes have been identified, although DME (dimethoxyethane), TEGDME (tetraethylene glyco dimethyl ether) and DMSO (dimethtylsulfoxide) have been popularly used. [18][19][20] Without as table solid-electrolyte interface (SEI) layer, Li as an anode material faces critical problems. [21] But replacing it with other Li-containing materials will greatly reduce the achievable capacities,u ndermining the potentials held by Li-O 2 batteries.…”

Erratum: Promoting solution phase discharge in Li–O2 batteries containing weakly solvating electrolyte solutions

“…The slightly higher yields indicate that the relatively high surface area of the Li 2 O 2 film that grows on the electrode in the absence of DBBQ leads to more decomposition of the electrolyte solution than is the case for the large particles in solution. It has also been suggested that LiO 2 is responsible for solvent decomposition on discharge 26,41,42 and, as discussed below, our analysis points to a mechanism that avoids this reactive intermediate. Attempts to charge the cells after discharge proved fruitless (see Supplementary Fig.…”