Significance Statement
Energy storage system has been developed to supply electric energy in a stable and efficient way. Sodium sulfur battery as one of energy banks for energy storage system offers high theoretical energy density, high energy efficiency, and long operational lifetime with no self-discharge. They also offer similar advantages in terms of non-toxicity, low cost and ready availability.
In a recent study by Kim et al. (2016) and published in Journal of Power Sources introduced an innovative method of preparing a surface modified carbon felt treated with inorganic nanoparticles for a current collecting matrix of the cathode.
NaS battery consists of sodium for an anode, sulfur for a cathode and β”-alumina solid electrolyte (BASE) for a separator. Carbon felt is known to be a good candidate for commercial NaS battery as a current collector in the cathode because of a high corrosion resistance to corrosive sodium sulfides.
The internal resistance of battery increases due to sulfur precipitation hindering the migration of Na+ into the anode section during charge and degrades the charge recovery characteristics due to decrease of electric conduction during charge (Sudworth and Tilley, The Sodium Sulfur Battery, 1985). Various attempts to suppress formation of sulfur-insulating layer has been reported by simply avoiding unnecessarily fast electron donating-accepting reactions during charge.
Alumina impregnating method is one of the methods used to suppress formation of sulfur insulating layer. However, it has disadvantages such as reduced space for sulfur due to high content of alumina particles, poor adhesion of alumina particles to carbon felt. The method currently adopted is a needle-punching process: a glass fiber mat is placed on a carbon felt and needle-punched to insert a part of the glass fibers into the carbon felt. This process leads to unavoidable mechanical damages and inhomogeneous distribution of glass fibers along thickness direction of the carbon felt.
This has led to Kim et al. (2016) treatment of carbon felt with inorganic nanoparticles on carbon-felt takes a negligible space for sulfur and coating process can be applied to any shape of carbon-felts because the coating is achieved by a sol-gel process using an inorganic-organic hybrid sol.
Kim et al. (2016), prepared an inorganic-organic hybrid sol for surface modification of carbon felt by sol-gel process which provides nanoscale alumina/silica particles upon calcination. The prepared carbon felt was observed by scanning electron microscopy (SEM) and field emission SEM with Energy Dispersive X-ray (EDX). Thickness of coating layer was observed by Scanning Transmission Electron Microscopy (STEM) with EDX. Cell components including stainless steel current collectors, O-rings and alumina gasket were cleaned by sonication in acetone and later dried in order to eliminate impurities and moisture. Sodium and sulfur-impregnated carbon-felt were inserted into anode and cathode compartment, respectively, in an argon-filled glove box. The cell was discharged up to depth-of-discharge (DoD) of 70% and charged up to 2.5 V for 3 cycles at current densities of 60 and 100 mAcm-2 in order to observe change in charge capacity at various current densities.
Results from STEM-EDX images showed a cross-section of fiber revealing a coating layer of about 6 nm in thickness where aluminum, silicon and oxygen are mapped homogeneously supporting the fact that carbon-felt can be modified with an insulating layer by simple sol-gel process.
Results demonstrating the possibility of surface modified carbon-felt showed cell with coated carbon-felt having a better performance than barely carbon felt in terms of charge capacity and voltage drops as current density increases. The degradation of charge capacity and drastic increase in charge voltage with increased current densities at barely carbon-felt was due to increase of rapid formation of sulfur layer at electrolyte surface while results of coated carbon-felt cell showed sulfur deposition hardly occurs at surface of electrolyte.
Further results show that discharged capacity decreased from 334 mAhg-1 at first cycle to 330 mAhg-1 at 70th cycle by 1.2% and columbic efficiency remaining steady throughout number of cycles. This means that effective resistance of the cell is not increased through the cycles.
Kim et al. (2016) fabrication of innovative electronically-conducting cathode matrix showed a columbic efficiency as measures to be more than 99.9% and discharge capacity retained higher than 98% of the discharge of the first to 70th cycle at constant current densities of 100 mAcm-2 in discharge and 80 mAcm-2 in charge.
Journal Reference
Seong In Kim1,2, Won Il Park1, Keeyoung Jung3, Chang-Sam Kim1. An Innovative Electronically-Conducting Matrix of the Cathode for Sodium Sulfur Battery. Journal of Power Sources, 2016, Volume 320, pp 37-42.
Show Affiliations- Center for Energy Convergence Research, Korea Institute of Science and Technology, Seoul 02792, South Korea
- Division of Materials Science and Engineering, Hanyang University, Seoul 04763, South Korea
- Energy Storage Materials Research Group, Research Institute of Industrial Science and Technology (RIST), Pohang 37673, South Korea
Go To Journal of Power Sources
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