DRAMATIC EFFECT OF OXIDATION ON LITHIUM INSERTION IN CARBONS MADE FROM EPOXY-RESINS

Carbons made by pyrolyzing epoxy novolak resins at 1000 degrees C are made up predominantly of single graphene sheets, having a lateral dime nsion of about 25 Angstrom, which are arranged somewhat like a ''house of cards.'' This structure implies significant micro- or nanoporosity . Such carbons can reversibly react with large amounts of lithium in a mechanism thought to involve the adsorption of lithium on the interna l surfaces of nanopores. We have studied the effect of controlled oxid ation of these samples, which changes the pore structure, on the subse quent electrochemical insertion of lithium in these materials. Using B runauer, Emmett, and Teller surface area measurements, methylene blue adsorption tests, powder x-ray diffraction, and small angle x-ray scat tering (SAX), we are able to correlate the changes in pore structure t o the electrochemical behavior of these samples. Initially, the pores are small (of the order of 15 Angstrom), as are their openings, and th e electrolyte cannot penetrate the pores, so excellent behavior is obs erved. Reversible specific capacities for Li as large as 570 mAh/g hav e been observed, with little irreversible capacity. As the samples are oxidized, the pores do not grow significantly in volume, as measured by SAX, but the size of their openings apparently does, to the point w here the electrolyte can penetrate the pores, leading to irreversible electrolyte decomposition reactions during the first electrochemical r eaction of lithium with the carbon, and hence large irreversible capac ity. Burnoffs as small as 5% are enough to transform the samples from excellent to poor. In addition, chemisorbed oxygen (from the oxidation treatment) appears to react with lithium, leading to increases in bot h irreversible and reversible capacities. However, the reversible capa city due to the chemisorbed oxygen shows large hysteresis. A pictorial model is proposed that is consistent with the results.