Electrochemical insertion of sodium in pitch-based carbon fibres in comparison with graphite in NaClO4-ethylene carbonate electrolyte

The electrochemical intercalation of sodium ions into pitch carbon fibres and natural graphite UF4 was studied using an electrolyte composed of ethylene carbonate as the solvent and NaClO4 as the salt. The reversible electrochemical capacity for a current equal to 7 mu A mg(-1) is negligible for graphite, a little bit higher for P100 fibres (28 mAh g(-1)) and reaches 55 mAh g(-1) for P75 fibres. The sodium uptake can be increased by grinding our materials: the reversible capacity is then around 60 and 83 mAh g(-1) for ground P100 and P75 carbon fibres, respectively. During the first reduction, a great part of the current is used for the formation of a passivating layer on the carbon surface. Analysis of this layer by transmission electron microscopy (TEM) has shown that it is composed of sodium carbonate Na2CO3 and alkylcarbonates ROCO2Na. Formation of Na2CO3 occurs at potentials in the 1-0.8 V range versus Na+/Na, and formation of sodium alkylcarbonates then follows at potentials below 0.8 V. TEM also revealed the presence of sodium chloride, attributed to NaClO4 reduction. (C) 1999 Elsevier Science Ltd. All rights reserved.