Abstract
Although possessing extremely high energy density, lithium-oxygen (Li-O2) battery suffered from large charge overpotential, low round-trip efficiency and poor cycling life, which limited the practical application of this smart system. Ru particles functionalized graphene aerogels (Ru-GAs) were designed and directly used as a free-standing cathode for Li-O2 battery. The Ru-GAs showed hierarchical pores, which had the pore volumes of 2.8 and 14.1 cm3 g−1 below and above critical pore diameter of 100 nm. The Ru-GA cathode in Li-O2 battery delivered a high capacity of more than 12 000 mAh g−1 and excellent cycling retention, which was mainly attributed to the three-dimensional porosity, abundant active sites with Ru particles and chemical stability arising from the character of binder-free cathode. Based on the results of in situ gas chromatography–mass spectrometry analysis, the reaction mechanism during charge process in aprotic electrolyte was proposed by the theory of three oxidation stages. By overcoming common limitations of carbon cathodes for lithium–oxygen batteries, scientists in China have produced a high-capacity battery. Lithium–oxygen batteries are very promising for powering electric vehicles and domestic devices. Carbon would be an excellent cathode material for such batteries except that it both self-decomposes and catalyses decomposition of the electrolyte. A team led by Haoshen Zhou of Nanjing University has used graphene aerogels functionalized with ruthenium nanoparticles as a freestanding cathode for a lithium–oxygen battery. The battery displayed a high specific capacity and a superior cycling retention, which the researchers attribute to the three-dimensional hierarchical pores in the cathode acting as channels for oxygen and electrolyte diffusion. The high abundance of active sites on the ruthenium nanoparticles and good chemical stability also contributed to the favourable battery properties. The graphene aerogel (GA) cathode has unique properties with a hierarchically porous structure that facilitates electrolyte permeation and oxygen diffusion, three-dimensional network structures that can enable easy electron transfer through GA, high specific surface area that offers abundant active sites for electrochemical reaction and an ultra-large pore volume that can accommodate plenty of discharge products. Ru nanoparticles supported on graphene sheets also has superior catalytic activity toward oxygen evolution reaction and can efficiently catalyze the decomposition of the discharge product Li2O2.
Highlights
The rechargeable lithium-oxygen (Li-O2) battery system is considered the most promising battery system for meeting the requirement of electric vehicles and household electric devices because of their extremely huge theoretical specific energy, environmental friendliness and low cost.[1,2,3,4] In a nonaqueous Li-O2 battery, oxygen is reduced at the cathode to form insoluble Li2O2 by combining with Li ions during discharge
The synthesis of GAs and Ru particles functionalized graphene aerogels (Ru-GAs) is illustrated in Supplementary Figure S1, and detailed information is given in the Experimental procedure section
The broad X-ray diffraction (XRD) peak at about 26.0° of resulting products indicated the poor ordering of graphene sheets along their stacking direction and reflected that frameworks of samples were composed of few-layer stacked graphene sheets (Supplementary Figure S3)
Summary
The rechargeable lithium-oxygen (Li-O2) battery system is considered the most promising battery system for meeting the requirement of electric vehicles and household electric devices because of their extremely huge theoretical specific energy, environmental friendliness and low cost.[1,2,3,4] In a nonaqueous Li-O2 battery, oxygen is reduced at the cathode to form insoluble Li2O2 by combining with Li ions during discharge. In the subsequent charge process, solid Li2O2 deposited in the cathode reversibly decomposes and releases O2 gas. An ideal cathode material for a nonaqueous Li-O2 battery should possess the following attributes: sufficient electronic conductivity, low density, stability over the operating voltage (typically 2.0–4.0 V vs Li+/Li), stability toward nucleophilic attack by O2 − and O22 − , low cost, nontoxicity and ability to be formed into a porous electrode.[5]. Carbon is widely used as a cathode material because of its excellent electronic conductivity, light weight, low cost and porosity.[6,7,8,9,10].
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