Enhanced cycle stability of rechargeable Li-O2 batteries using immobilized redox mediator on air cathode

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Abstract

Overcoming the low round-trip energy efficiency and poor cycle stability of lithium-oxygen (Li-O2) batteries still remains a challenge. Here, we show that 2,2,6,6,-tetramethylpiperidinyl-1-oxyl (TEMPO)-immobilized air cathode can effectively reduce the charge voltage and increase the cycle stability in Li-O2 batteries. The TEMPO-immobilized air cathode is prepared using a gas diffusion layer by a simple dip coating method, in which polydopamine is used as a linker. In this method, the immobilized TEMPO on the cathode does not crossover to the anode, and the consumption of TEMPO by side reactions is minimized. As a result, the redox mediation by TEMPO is well maintained in its immobilized state. This highlights that the use of an immobilized redox mediator can be a rational strategy for expanding the practical applications of Li-O2 batteries.

Introduction

Aprotic lithium-oxygen (Li-O2) batteries have recently attracted significant interests because of their high theoretical specific energy of up to 13,000 Wh kg−1, which overcomes the limitation of intercalation electrodes [1], [2], [3], [4], [5]. However, the practical application of Li-O2 batteries requires overcoming several challenges such as the low round-trip energy efficiency and poor cycle stability [6], [7]. Since insoluble and insulating polycrystalline lithium peroxide (Li2O2) is formed on the cathode surface during the discharge process, a high overpotential is required in the charging process to reduce Li2O2, leading to the extremely low round-trip efficiency [8], [9], [10]. Carbonaceous materials with high electric conductivity, light weight, and low fabrication cost have been used for cathodes of Li-O2 batteries. However, they are prone to parasitic reactions such as the oxidative decomposition of the cathode during the charging process at high potentials [11]. The decomposed products block the surface of the carbon cathode, and this is considered as the major reason for the poor cycle stability [7]. Therefore, reduction of the overpotential during charging is essential for the application of Li-O2 batteries, because it can provide higher energy efficiency and long term cycle stability.

Heterogeneous catalysts including nanoparticles and metal oxides have been widely used to reduce the charge overpotential in Li-O2 batteries [12], [13], [14], [15]. A redox mediator (RM) is used by dissolving it in the electrolyte, as the former can act as electron carriers between the electrode surface and Li2O2 via the reversible reaction such as RM  RM+ + e [16], [17], [18]. However, dissolution of the RM in the electrolyte results in the diffusion through the separator and reduction at the Li anode. As the RM is consumed during the electrochemical process, the redox mediation is not possible anymore [19]. Hence, the strategy of dissolving the RM in the electrolytes has a certain disadvantage. Several approaches using size-exclusive separators [20], [21], negatively charged separators [22], and gel polymer electrolytes [23] have been tried to address the undesirable consumption of RM. However, no clear solutions have been arrived at as yet, primarily because of the self-discharge process, which is an inherent disadvantage in the current configurations of Li–O2 batteries.

In this study, we tried to immobilize the RM on a gas diffusion layer (GDL) cathode to prevent its diffusion to the Li metal anode. Polydopamine (PDA), a mussel-inspired functional material, was used as a linker to connect the RM to GDL, because PDA can be easily coated on various substrates [24], [25] and has a catechol group that can react with the amino group in RM via Michael addition or Schiff base formation [26], [27]. As 2,2,6,6,-tetramethylpiperidinyl-1-oxyl (TEMPO) and its derivatives have been applied as mobile RM in Li-O2 batteries owing to their appropriate redox potential, fast electrochemical kinetic, good redox reversibility, and high solubility in aprotic electrolytes [16], [28], [29], 4-amino-2,2,6,6,-tetramethylpiperidinyl-1-oxyl (4-amino-TEMPO) was used as the RM for immobilization. TEMPO-attached graphene oxide has been used as a heterocatalyst for the oxidation of alcohols to aldehydes or ketones, demonstrating that the immobilized TEMPO can still participate in redox reactions [30], [31]. Breton et al. reported that TEMPO-attached electrode exhibited electrocatalytic activity [32], [33]; however, there are no report on immobilized RM on the cathode of Li-O2 batteries. The Li-O2 battery prepared using our modified air cathode exhibited an enhanced cycle stability with reduced charge overpotential, compared with those of the conventional Li-O2 batteries without any RM or with RM dissolved in electrolyte.

Section snippets

Experimental

TEMPO-immobilized cathode was prepared using the gas diffusion layer (GDL, JNT-20A, JNTG) as a carbon cathode and substrate on which the PDA layer was coated. The GDL was cut into 3 cm × 3 cm pieces and washed with ethanol. The GDL was immersed in 20 mM precursor solution (dopamine hydrochloride, Sigma Aldrich) while maintaining the pH at 8.5 in 20 mM Tris buffer and then stirring for 30 min. The GDL treated with PDA was immersed overnight in 20 mM TEMPO-precursor solution

Results and discussion

The GDL was readily functionalized via the consecutive coating by dopamine and grafting of TEMPO onto the PDA layer, as shown in Fig. 1. When dopamine hydrochloride was added into a weak alkaline buffer solution (pH 8.5), dopamine was immediately coated on the surface by oxidative polymerization, changing the color of the solution from colorless to brownish black. The self-polymerization of dopamine produces an adhesive coating layer of PDA onto the GDL cathode. 4-Amino TEMPO was grafted onto

Conclusions

In summary, a nonaqueous Li-O2 battery with high performance was fabricated using a TEMPO-immobilized cathode that was prepared by a simple coating and grafting process. The effect of immobilized TEMPO on the cathode was verified by comparing the electrochemical performances of Li-O2 batteries with immobilized TEMPO on the cathode, TEMPO dissolved in the electrolyte, and in the absence of TEMPO. The immobilized TEMPO on the cathode suppressed the degradation of TEMPO at the Li metal anode and

Declarations of interest

None.

Acknowledgments

Funding: This work was supported by the National Research Foundation of Korea (NRF), Korea government (MSIT) (No. NRF-2018R1A5A1024127), Korea government (MEST) (NRF-2016R1E1A1A01942936)), and the Supercomputing Center/Korea Institute of Science and Technology Information with supercomputing resources, which provided technical support (KSC-2017-C2-0029).

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    1

    J.-H.B. and S.Y.L. contributed equally.

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