A new method to prevent degradation of lithium–oxygen batteries: reduction of superoxide by viologen

Abstract

Lithium-oxygen battery development is hampered by degradation reactions initiated by superoxide, which is formed in the pathway of oxygen reduction to peroxide. This work demonstrates that the superoxide lifetime is drastically decreased upon addition of ethyl viologen, which catalyses the reduction of superoxide to peroxide.Lithium-oxygen batteries can potentially deliver 5 times more energy than lithium-ion batteries of the same weight. [1][2][3][4][5][6][7] The energy is provided by the electrochemical reaction between Li and O2, typically forming Li2O2. Since all these compounds are very light, the mass of the battery can be very small, leading to a high specific energy (i.e. energy per mass). However, several factors hamper the practical performance of current lithium-oxygen batteries. First of all, superoxide is formed as the first product of the reduction of O2: 8,9 O2 + e -+Li + → LiO2(1) It has been shown that superoxide reacts irreversibly with most known electrolytes 10 and it is also probably involved in the corrosion of carbon electrodes. 11 Intensive research has been done on finding alternative electrolytes and electrode materials that are resistant towards degradation by superoxide. A few solvents like dimethyl sulfoxide 12 , some glymes 13, 14 and pyrrolidinium or piperidinium based ionic liquids [15][16][17][18][19] have been identified as promising electrolyte choices. Gold 12 and TiC 20 electrodes have shown much better capacity performance than carbon, in terms of cycling stability and suppression of side reactions. An alternative approach to solve the superoxide problem is the introduction of catalysts to promote the reduction of superoxide to peroxide: LiO2 + e -+Li + → Li2O2(2) This would decrease the lifetime of superoxide and, with this, the long-term durability of the battery. If the superoxide lifetime were short enough, it would be conceivable that a wider range of materials could be incorporated in lithium-oxygen batteries without serious problems of degradation reactions. In addition, such catalysts would also be beneficial in order to speed up the overall 2-electron reduction of oxygen to Li2O2. In the absence of such catalyst, Li2O2 is usually formed by disproportionation of LiO2: 8, 9 2LiO2 →O2+ Li2O2 (3) However, as will be shown below, the rate of this reaction is very slow at low superoxide concentrations. Another important issue in lithium-oxygen batteries is that the discharge product, Li2O2, deposits on the surface of the electrode, blocking its electrochemical activity. This produces a very early end of discharge, with little discharge product being formed, and little capacity being delivered. An innovative solution to solve this problem is the introduction of soluble redox shuttles that can displace the formation of Li2O2 from the electrode surface to the solution, by reducing oxygen through a homogenous chemical reaction. 21,22 Soluble redox couples have also been used as mediators for the evolution of oxygen in lithium-oxygen batteries, resulting in major performance i...

Organic radicals for the enhancement of oxygen reduction reaction in Li–O2 batteries

“…Similar results were observed using EtV as soluble redox couple. 10,11 In our case the discharge voltages are higher, but the effect is still observed, thanks to the higher reduction potential of TTM compared to EtV 2+ (about 2.6 V instead of 2.4 V vs. Li 10 ). From Figures 3a-b, another notable aspect is the fact that the specific discharge capacity increases significantly when TTM radical is added.…”

“…9 To solve this problem, the introduction of solution-phase catalysts has been proposed recently to displace the formation of Li 2 O 2 from the cathode to the solution. In particular, soluble redox shuttles such as ethyl viologen (EtV) [10][11][12] or organic-electrolytedissolved iron phthalocyanine 13 14 Derivatives of polychlorotrisphenylmethyl (PTM) radicals, are molecules of low citotoxicity 16 known as inert carbon free radicals for their stability to oxygen and many other aggressive reagents. They are reported to show redox behavior in aprotic solvents and are characterized by an extremely high persistence and stability.…”

“…37,38 The cathodic potential is close to those observed for other non-radical redox couples that have been mentioned as effective catalysts for ORR. 10,11,13,14 In order to test the stability of the TTM-anion in the battery environment, bulk electrolysis of TTM-radical was carried out in the same electrolyte in presence and in absence of oxygen.…”

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

“…High donor number salts have been shown to increase the capacity fourfold and reduce the discharge overpotential by ∼30-50 mV over low donor number salts 22 . Viologens 27,28 , phthalocyanines 29 and quinones 30 have been investigated as possible soluble reduction catalysts. Although the studies of such catalysts are important, in most cases there is little or no direct evidence demonstrating that they promote formation of Li 2 O 2 in solution and not on the electrode surface because they rely on electrochemical measurements alone.…”

“…The reduction proceeds through the following general steps: 22,[27][28][29][30] . High donor number salts have been shown to increase the capacity fourfold and reduce the discharge overpotential by ∼30-50 mV over low donor number salts 22 .…”