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 a nd natural graphite UF4 was studied using an electrolyte composed of ethyle ne carbonate as the solvent and NaClO4 as the salt. The reversible electroc hemical capacity for a current equal to 7 mu A mg(-1) is negligible for gra phite, a little bit higher for P100 fibres (28 mAh g(-1)) and reaches 55 mA h 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 g round P100 and P75 carbon fibres, respectively. During the first reduction, a great part of the current is used for the formation of a passivating lay er on the carbon surface. Analysis of this layer by transmission electron m icroscopy (TEM) has shown that it is composed of sodium carbonate Na2CO3 an d alkylcarbonates ROCO2Na. Formation of Na2CO3 occurs at potentials in the 1-0.8 V range versus Na+/Na, and formation of sodium alkylcarbonates then f ollows at potentials below 0.8 V. TEM also revealed the presence of sodium chloride, attributed to NaClO4 reduction. (C) 1999 Elsevier Science Ltd. Al l rights reserved.