Elsevier

Energy Storage Materials

Volume 9, October 2017, Pages 134-139
Energy Storage Materials

Dense graphene monolith oxygen cathodes for ultrahigh volumetric energy densities

https://doi.org/10.1016/j.ensm.2017.06.003Get rights and content

Abstract

A convenient strategy is developed to prepare template-assisted, high density, porous graphene monolith (THPGM) cathodes with high densities for compact Li-O2 batteries. Graphene oxide is used as the primary building block to construct condensed carbon electrodes by self-assembly followed by capillary drying. SiO2 nanoparticles are incorporated onto the dense graphene monolith to function as sacrificial pore former. The bimodal pores of diameters ranging 1–6 and ∼ 40 nm created in the close-grained graphene monolith facilitate ion transport and oxygen diffusion, while providing sufficient space to accommodate the discharge products. The oxygen cathodes made from THPGM possess the advantageous features of high volumetric densities, a fully-developed porous structure and a robust architecture, resulting in unprecedented volumetric energy densities and excellent cyclic stability for Li-O2 batteries.

Graphical abstract

The dense yet porous graphene-based material is fabricated via three-dimensional densification of graphene hydrogel and using a sacrificial pore former (SiO2) on the dense graphene monolith. The bimodal pores with diameters ranging 1–6 nm and 40 nm created in the graphene-based monolith facilitate ion transport and oxygen diffusion, while providing sufficient space to accommodate the discharge products. Benefiting from a developed pore structure and robust microstructure, it delivers an unprecedented volumetric energy density with excellent long-term stability for Li-O2 batteries.

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Introduction

The rapid depletion of traditional fossil fuels has led to an increasing demand for cost-effective electrochemical energy storage (EES) systems with consistently high power outputs, excellent safety and long life. Conventional power sources, like lithium ion batteries (LIBs) as one of the most popular storage devices, have achieved great success in powering portable electronics [1], [2]. However, they cannot satisfy the requirement of a higher energy density in the fields of next-generation electric vehicles and large-scale electrical grids. Lithium-oxygen (Li-O2) batteries have gained increasing attention as a novel EES device due to their ultrahigh specific energies [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], but are not free of weaknesses. Apart from the common problems of a long recharge time and a low energy conversion efficiency [18], they also suffer from the large volumes required to obtain a sufficient energy supply and life span. Judging from the essential requirements of future EES devices, the two key factors including the gravimetric and volumetric energy densities must be simultaneously taken into account [19]. The gravimetric energy density, which mainly depends on the intrinsic nature of the electrode materials [20], [21], [22], [23], indicates how much energy can be stored in a unit weight of the assembled cell, whereas the volumetric energy density measures how much energy can be harvested from a unit volume [24]. To improve the volumetric energy density, both the performance of the electrode materials and the configuration of the cell components must be considered [25], [26]. Unfortunately, however, the vast majority of published reports have focused mainly on the gravimetric energy density of Li-O2 batteries and the concept of volumetric energy density has been barely mentioned. It is reasonable to believe that it can make Li-O2 battery system more competing if we can improve its volumetric performance while keeping its superior gravimetric energy density over other conventional energy storage devices.

Carbon materials are the main cathodes for Li-O2 batteries because of their many advantages, such as excellent electrical conductivities and porous structures [27], [28]. The porous structure and the pore volume of carbon affect the oxygen diffusion and electrolyte penetration significantly [29], while they are also closely associated with the volumetric energy densities of electrodes [30]. A well-developed porous structure in carbon often leads to a low density ranging 0.3–0.6 g cm-3, giving rise to a poor volumetric energy density of a packed battery [26]. In contrast, a higher volumetric density requires a lower pore volume, which in turn does not favour oxygen diffusion and the growth of discharge products. Striking a balance between the high volumetric density and the appropriate porous structure is extremely important in designing the cathode for Li-O2 batteries.

Here, we report a graphene-based porous carbon material with a well-developed porous structure with a pore volume of 0.51 cm3 g-1 and an exceptionally high density of 1.04 g cm-3 for Li-O2 batteries. Graphene emerging as an ideal building block for all types of sp2 carbon was used to build highly porous microstructures by self-assembly [31], [32], [33]. A close-grained structure was synthesized by conjugating the graphene sheets into a 3D hydrogel using a hydrothermal assembly where silica was added as a sacrificial template. A capillary evaporation-induced drying (CEID) technique was employed to densify the carbon network [34], where the silica template was removed after annealing while creating mesopores transporting ions. Compared to many previous studies that ignored the volumetric performance of packed batteries, we report a significantly increased density of the cathode while retaining efficient ion transport for Li-O2 batteries. The graphene-based oxygen cathode delivers a volumetric capacity as high as 384.8 mA h cm−3 (based on the cathode sheet), equivalent to about three times of that of conventional Ketjen black. Furthermore, the battery presents a maximum volumetric energy density of 1109 W h L-1 with extraordinary cyclic stability up to 157 cycles at a current density of 0.1 A g-1.

Section snippets

Synthesis of surface-modified SiO2 nanospheres

With reference to Stöber method, the specific volume 3 ml of concentrated ammonium hydroxide (NH4OH) was added into 60 ml ethanol and 1 ml H2O. After adding 2 ml tetraethylorthosilicate (TEOS) and stirring for another 4 h at 50 °C, the white precipitate can be obtained through filtration. The SiO2 samples were then added into the mixture solution consisting of 200 ml isopropanol and 200 μL γ-aminopropyltriethoxysilane (KH-550) and refluxed at 80 °C under continuous stirring. The solution was washed with

Results and discussion

To optimize the relationship between materials density and pore structure, graphene-based electrodes with different densities and porosities were synthesized using interfacial gelation techniques with different drying methods to tune the materials texture. Fig. 1 is a schematic of the synthesis process of a template-assisted, high density, porous graphene monolith (THPGM). The SiO2 with a mean particle diameter of ~ 40 nm was first prepared by the Stöber method (Fig. S1) [36]. The surface of SiO2

Conclusions

In summary, we have developed a high density, graphene-based porous structure which was used as the cathode for Li-O2 batteries. The bimodal pores with diameters ranging 1–6 nm and 40 nm created in the graphene-based monolith THPGM facilitated ion transport and oxygen diffusion, and provided sufficient space to accommodate the discharge products. The robust microarchitecture ensured stable reactions during the ORR/OER processes. It had a very high density of 1.04 g cm-3, giving rise to exceptional

Acknowledgements

The authors appreciate the financial support from the National Science Fund for Distinguished Young Scholars, China (No. 51525204), National Natural Science Foundation of China (Nos. U1401243, 21506212, and 51302146) and National Basic Research Program of China (2014CB932400), and Shenzhen Basic Research Project (Nos. ZDSYS20140509172959981 and JCYJ20150529164918734), as well as from the Research Grants Council (GRF Project: 613612 and 16212814) and the Innovation and Technology Commission (

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