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Polymer Based Carbons as Potential Materials for Energy Storage in Lithium Oxygen Batteries

Mojtaba Mirzaeian and Peter J. Hall. Chemical and Process Engineering, University of Strathclyde, 75 Montrose Street, James Weir Building, Glasgow, United Kingdom

Due to their high energy density, high specific energy and high operating voltage, rechargeable lithium batteries are expected as the potential power sources for many applications such as consumer electronics and electric vehicles (EV). Commercial rechargeable lithium batteries use lithium transition metal oxides, typically LiCoO2, as cathode and graphite as anode. Their specific capacity depends on the capacities of both the cathode and anode materials. Graphite has a theoretical capacity of 372 mA h g-1. Energy storage in these batteries is limited by the cathode and does not exceed 200 mA h g-1.

The capacity of a lithium battery system can be enhanced remarkably by using a completely different approach which combines Li as anode directly with oxygen as cathode active material in a Li/oxygen cell. Oxygen accessed from environment is reduced catalytically on an air electrode surface to form either an oxide or peroxide ion. The catalytically formed anions react with lithium cations supplied by the anode and delivered by the electrolyte to form Li2O2 on the air electrode surface during discharge process.

The O2 electrode in lithium/oxygen batteries is a carbon having a porous structure in which several electrochemical and transport processes occur simultaneously. The porous structure acts as transport pores for the diffusion of electrolyte for transfer of lithium ions and diffusion of oxygen to the carbon-electrolyte interface (reaction zoon), formation and storage of Li2O2 during the discharge process and also electrochemical decomposition of Li2O2 during the charge process. It has been also discussed that the end-of-discharge of the cell is reached when the carbon pores are filled or choked by the deposition of Li2O2.

This shows that the cell performance strongly depends on the morphology and structure of the carbon. Therefore the main challenge in this issue is the development of new carbon electrode materials with controlled structure to improve the kinetics of the air cathode and enhance the capacity, energy and power densities and the stability of the energy delivered by these systems.

In this work porous carbon aerogels are prepared by polycondensation of resorcinol (R) and formaldehyde (F) catalyzed by sodium carbonate (C) followed by carbonization of the resultant aerogels at 800 C in an inert atmosphere. The porous texture of the carbons has been adjusted by the change of the molar ratio of resorcinol to catalyst (R/C) in the gel precursors in the range of 100 to 500. The porous structure of the aerogels and carbon aerogels are characterized by N2 adsorption-desorption measurements at 77 K. It is found that total pore volume and average pore diameter of the carbons increase with increase in the R/C ratio of the gel precursors. The prepared carbon aerogels are used as active materials in fabrication of composite carbon electrodes. The electrochemical performance of the electrodes has been tested in a Li/O2 cell. Through the galvanostatic charge/discharge measurements, it is found that with an increase of R/C ratio, the specific capacity of the Li/O2 cell fabricated from the carbon aerogels increases from 716 to 2077 mA h g-1. The resulting voltage profiles for the first ten charge/discharge cycles indicate that the carbon samples possess excellent stability on cycling.