Carbon black cathodes for lithium oxygen batteries: Influence of porosity and heteroatom-doping
Introduction
Lithium oxygen battery is one of the most promising energy storage candidates for meeting the future demands of the electric vehicles (EVs) or hybrid electric vehicles (HEVs) [1], [2]. However, the challenges for this battery system, such as the rate capability, cycle life, power performance, etc. should be overcome before lithium oxygen batteries can be used in practical applications [3]. It was reported that during discharge process of the battery, the product, Li2O2, deposits on the surface of the electrodes and eventually blocks the path ways for electrolyte and oxygen transportation, terminating the discharge process. Therefore, the electrodes are directly determining the battery performance.
To date, carbon materials are still the most studied cathode materials for lithium oxygen batteries, and efforts have been made to increase the oxygen solubility and diffusion, to decrease the accumulation of reaction products, and to create effective three-phase electrochemical interface of these materials [4], [5], [6], [7], [8]. For example, one-dimensional carbon materials (CNTs, CNFs) have been reported to exhibit good performance because they could form an interconnected porous electrode with high void volume [9], [10]. Graphene nanosheets, a two-dimensional material showed significantly improvement for the battery performance due to its unique morphology and structures [11], [12], [13]. It was also reported that heteroatom-doping to carbon nanotubes and graphene nanosheets further increased the battery performance because of the active sites introduced into the pristine samples [14], [15], [16]. Due to abundance and low cost, carbon black has also been extensively studied as a cathode in lithium oxygen batteries. For example, several carbon black powders have been studied by Xiao et al. [4] and the results suggested that the pore volume and the pore size affect the battery performance. Hall et al. [8] also suggested that electrode made of carbon aerogel with appropriate pore volume and pore diameter delivered high discharge capacity. The findings indicated that the limited discharge capacity was associated to pore clogging as claimed by others [17]. However, recently Luntz et al. [18], [19] reported that the electrical passivation caused by the formation of discharge product layer on the electrode was the limiting factor by using the electrochemical experiments and modeling. They found that even a very thin layer (4–5 nm) of the insulated film of Li2O2 was sufficient to terminate the discharge reaction due to the increased electrical resistance at the electrode/electrolyte interface, then preventing further O2 reduction. This would imply that the specific capacity should be related to the effective carbon surface area, accessible to the electrolyte and oxygen. In the present paper, various carbons all derived from the same starting commercial pristine carbon black (N330) were obtained by threating this carbon black in different treating atmospheres and for different times. The resulting carbons have been used as cathode in lithium oxygen batteries, revealing that the discharge capacity is proportional to the specific surface area of mesopores in these carbon electrodes. For the first time, the influence of various parameters resulting from the heat-treatment of the same starting carbon black, such as the content of disorganized carbon and heteroatom-doping effects, is studied in detail as well on the performance of the lithium oxygen battery.
Section snippets
Sample preparation
Commercial N330 furnace carbon black (from Sid Richardson Carbon Corporation) was used as the starting material. It was heat-treated under NH3 or CO2 (with or without H2) atmospheres. The percentage of mass that was lost during the heat-treatment, W, was calculated as follows:
Results and discussion
The morphology and the distribution of particle sizes of some samples resulting from the heat-treatment of N330 in NH3 are shown in Fig. 1. The distribution of particle sizes of the carbon black was obtained from the analysis of SEM micrographs based on about 300 particles. It can be seen that the shape of the particles is kept almost the same, even after a loss of 75% of the pristine N330 carbon mass. However, the particle size is decreasing after the heat treatment. Several large particles
Conclusions
In summary, commercial carbon black (N330) was treated under various atmospheres (NH3, CO2 and CO2/H2). The mesopore surface areas increased as the treating time increased while the micropore surface area only increased until the mass loss reached 35% and then decreased. It is suggested that the surface area of mesopores plays an important role for the discharge capacity of lithium oxygen batteries due to the passivation effect of discharge product film on the carbon surface. Nitrogen and
Acknowledgements
This research was supported by Natural Sciences and Engineering Research Council of Canada, Canada Research Chair Program, Canada Foundation for Innovation, Ontario Early Researcher Award and the University of Western Ontario.
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