A Binder-Free Carbon-RuO2 Composite Cathode for Rechargeable Li-O2 Batteries

Abstract

A rechargeable lithium–oxygen (Li–O2) battery is considered as a promising technology for electrochemical energy storage systems, because its theoretical energy density is about ten times higher than those of state-of-the-art Li-ion batteries. The cathode (positive electrode) for Li–O2 batteries is made of carbon and polymeric binders; however, these constituents undergo parasitic decomposition reactions during battery operation, which in turn causes considerable performance degradation. Therefore, the rational design of the cathode is necessary for building robust and high-performance Li–O2 batteries. Here, a binder-free carbon nanotube (CNT) electrode surface-modified by atomic layer deposition (ALD) of dual acting RuO2 as an inhibitor–promoter is proposed for rechargeable Li–O2 batteries. The electrode architecture was designed and fabricated by taking the following features of CNT and ALD into consideration. (i) A free-standing sheet of CNTs can be prepared via a simple filtration procedure without using any binders. (ii) Given that the surface of multi-walled CNTs is very inert, the ALD process yields preferential nucleation–growth of isolated nanoparticles on defect sites rather than producing continuous films on the entire CNT surface. With its proven activity, therefore, RuO2 anchored preferentially to defect sites can play a dual role as an inhibitor (passivating structural defects on CNTs) and a promoter (facilitating electrochemical Li2O2 decomposition). (iii) The ALD process based on sequential, self-limiting reactions allows for the deposition of highly uniform RuO2 nanoparticles within the three-dimensional CNT electrode, which leads to decreased Ru loading while maintaining high catalytic performance. The binder-free RuO2/CNT cathode shows outstanding electrochemical performance as characterized by small voltage gaps (~0.9 V) as well as excellent cyclability without any signs of capacity decay over 80 cycles. The excellent electrochemical performance could be explained by the unique features of the electrode: (i) the absence of polymeric binders with high reactivity with Li2O2 and (ii) the dual function of RuO2 nanoparticles to inhibit carbon decomposition on CNT defects and to promote electrochemical Li2O2 decomposition.