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2017-09-03

Li–air batteries: Decouple to stabilize




a, The cathodic reactions upon discharge and charge in the absence of any mediators. Upon discharge, O2 is reduced to form O2- that binds with Li+ to produce LiO2. Subsequently, Li2O2 forms by disproportionation (represented by dotted curved arrows) or further reduction (represented by solid curved arrows) of LiO2. Upon charge, Li2O2 is oxidized to LiO2, and then O2 is released via disproportionation (dotted curved arrows) or a second oxidation (solid curved arrows) of LiO2. The electrolyte is represented by wave-like features. b, The cathodic reactions upon discharge and charge in the presence of DBBQ and TEMPO. Upon discharge, DBBQ is reduced at the cathode surface, forming LiDBBQ. Then, LiDBBQ reacts with O2 to produce Li2O2 and regenerate DBBQ. Upon charge, TEMPO is oxidized at the cathode surface, forming TEMPO+, which in turn oxidizes Li2O2 to produce O2 and regenerate TEMPO. The O2(sol) indicates the O2 dissolved in the electrolyte. The left and right boats schematically illustrate that DBBQ transfers electrons from the cathode surface to O2, and TEMPO transfers electrons from Li2O2 to the cathode surface, respectively.

The utilization of porous carbon cathodes in lithium–air batteries is hindered by their severe decomposition during battery cycling. Now, dual redox mediators are shown to decouple the complex electrochemical reactions at the cathode, avoiding cathode passivation and decomposition.
Rechargeable batteries power devices such as mobile electronics and electric vehicles, and can store electricity generated from various renewable energy sources. Lithium-ion (Li-ion) batteries are an efficient energy-storage system, but the energy density is approaching their theoretical limit. The ever-growing need for energy storage therefore necessitates the pursuit of next-generation batteries1. Lithium–air (Li–air) batteries are promising because they have a theoretical energy density that is nearly 10 times as much as that of a conventional Li-ion battery2. However, many challenges must be addressed before Li–air batteries can compete with current rechargeable batteries, the most difficult of which is the significant decomposition of the carbon cathode and electrolyte during cycling, which severely limits the battery cycle life3. Carbon modification (for example, improving crystallization) or surface treatment (for example, coating surface defects with inorganic particles) are commonly adopted to stabilize the cathode, because these approaches decrease the contact between the active site of the carbon cathode and highly active species (LiO2 and Li2O2) and hence improve resistance to decomposition4. However, these highly active species are still present in batteries and are able to degrade the carbon cathode. Writing in Nature Energy5, Peter Bruce and colleagues from the University of Oxford, UK, now propose a strategy of using dual redox mediators to stabilize the carbon cathode without any carbon modification or surface treatment.
A typical Li–air battery is composed of a lithium anode and a porous carbon cathode, separated by an aprotic electrolyte. Its chemistry can be written 2Li+ + 2e- + O2 ? Li2O2, with the forward and reverse reaction describing discharge and charge, respectively. As shown in Fig. 1a, during discharge O2 undergoes a one-electron reduction on the cathode surface to generate O2- that then binds with Li+ to form LiO2; further reduction or disproportionation of LiO2 leads to the formation of Li2O2. The reverse reactions occur during charge. As a strong nucleophile, LiO2 is unfortunately too reactive towards the carbon cathode and the aprotic electrolyte, forming undesired by-products such as Li2CO3 and Li carboxylates6. The by-products passivate the cathode, which results in severe capacity fading upon cycling and even premature cell death. Furthermore, Li2O2 is also problematic: when Li2O2 is in contact with the carbon cathode, it forms Li2CO3 (ref. 7). In addition, the high charging voltage required to oxidize the insulating Li2O2 can also induce carbon-cathode decomposition.
Recently, a concept of using dissolved redox mediators as a soluble catalyst to reduce the overvoltages has been proposed8. Overvoltages are gaps between the practical and theoretical potential, representing the energy loss in operation. It has been demonstrated that redox mediators can facilitate either the discharge or charge process. In this work, Peter Bruce and colleagues show that using a dual redox mediator can facilitate both discharge and charge processes as required, and importantly can stabilize the carbon cathode.
Specifically, the dual redox mediator consists of 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ) and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). During discharge, the DBBQ is first reduced at the cathode surface; the reduced species (DBBQ–Li, shown in Fig. 1b) then transfers the electrons to O2 that is dissolved in the electrolyte solution (O2(sol)), and the final discharge product Li2O2 is formed. Meanwhile, DBBQ–Li is oxidized to DBBQ. During charge, TEMPO is first oxidized at the cathode surface, and the oxidized species (TEMPO+) then obtains electrons from Li2O2 to produce O2. Clearly, both DBBQ and TEMPO act as an electron-transfer agent and are discharge and charge mediators, respectively, in the battery process.
The approach shown in this work bypasses the formation of the troublesome intermediate LiO2. Importantly, it decouples the electrochemical reactions at the cathode surface from the formation and decomposition of Li2O2 from solution: the electrochemistry at the cathode surface only involves the redox reactions of DBBQ and TEMPO molecules, whereas the Li2O2 formation and decomposition occurs away from the carbon surface. Therefore, the detrimental reaction between Li2O2 and the carbon surface is minimized (Fig. 1b). In addition, the use of dual mediators ensures that Li2O2 particles in solution are efficiently oxidized without having to increase the charge voltage of the carbon cathode in the conventional approach, greatly alleviating high-potential-induced carbon-cathode decomposition. Taking all these factors together, a much better cycling performance is obtained in the Li–air cell (<0.008% carbon decomposition per cycle) than in the cell without any mediators (0.12% per cycle).
Owing to its high conductivity, light weight, and rich porous structure, carbon is by far the most attractive cathode selection in Li–air batteries. This work by Bruce and colleagues overcomes one of the major barriers for the application of carbon cathodes in Li–air batteries. However, the fact that the Li2O2 growth and decomposition now occurs in the electrolyte would increase the contact between Li2O2 and the electrolyte. Hence, the issue of the electrolyte stability needs to be addressed. In addition, because of the decoupling of the electrochemical reactions at the cathode surface from the formation and decomposition of Li2O2 from the electrolyte solution, the performance of a Li–air battery is no longer limited by the charge transport of Li2O2 but rather by the O2 mass transport within the porous carbon cathode. We note that the carbon cathode in this work, which is flooded with the electrolyte, may lack efficient gas diffusion channels for O2. This would limit the advantages of the mediators, and thus future work should also focus on the design of a porous cathode that delivers rapid mass transport.