| dc.description.abstract | This thesis describes the structural and lithium-insertion properties of pyrolytic carbons derived from peanut shells. Peanut shells were treated with different weight ratios of a proprietary porogenic agent and carbonized between 600 and 900°C. The work covers three areas: (1) optimization of the porogen-to-peanut shell weight ratio (P) and the pyrolysis temperature, (2) comparison of the lithium-insertion properties of carbons obtained from untreated and porogen-treated peanut shells, and (3) charge-discharge studies with pre-lithiated carbons.
Porogen treatment was implemented in order to alter the pore structure and effect a manifold increase in the surface area of the carbonaceous product. Both the untreated and porogen-treated shells yielded carbons with poor crystallinity, but the pore diameter of the latter was twice as large and the surface area was 66 times greater than the untreated carbon. Both types of products were primarily non-parallel single sheets of carbons, as determined by the values of their R factors. While porogen can increase the number of uncorrelated graphene fragments, leading to more lithium accommodation sites, the pyrolysis temperature can induce breakage of the links between adjacent sheets and encourage their parallel alignment. The products obtained with P = 5 at 500°C gave a first-cycle lithium insertion capacity of 4765 mAh/g, which is the highest value reported for any lithium-insertion material so far. At a pyrolysis temperature of 600°C, the P = 5 product gave the optimal insertion and deinsertion capacities, their values in the first cycle being 3504 and 1650 mAh/g, respectively. The deinsertion capacity of this sample in the tenth cycle was very high at 1504 mAh/g. However, the irreversible capacities of these carbons, especially in the first cycle, were too large to be practical.
The large irreversible capacities were reflected in the cyclic voltammograms of the carbons, where the absence of a significant anodic peak indicated that only part of the inserted lithium could be retrieved. In the case of the P = 0 carbon, lithium insertion was observed below 0.7 V vs. Li+/Li, while in the P = 5 carbon, the insertion process commenced from about 1.3 V. Moreover, the decrease in the insertion current with cycle number was lower in the case of the porogen-treated carbon than with the untreated carbon, suggesting the former had better capacity retention. No distinguishable current peaks were seen in the cyclic voltammograms, indicating lack of any long-range ordering, which precludes staging behavior during the insertion and deinsertion processes. The P = 5 carbon also exhibited higher exchange current densities, which would imply that the kinetics of the insertion reaction was faster than when the carbon was untreated. Electrochemical impedance studies showed that the resistance due to the formation of surface film increased when the carbon was charged. However, the slight increase in resistance suggests that the products of the surface reduction are either soluble in the electrolyte or are loosely held to the surface.
Charge-discharge studies with the porogen-treated carbon, pre-charged and discharged prior to use in coin cells, indicated that the first-cycle reversible capacity was the greatest when the charge-discharge rate was 0.4 C. At this rate, the carbon maintained capacities of about 325 mAh/g for 20 cycles, and then stabilized at around 380 mAh/g for over 70 cycles. Signature curves of the carbon showed that the deliverable capacities at charge-discharge rates of 0.2, 0.4, 0.8 C were 900, 700 and 500 mAh/g, respectively. Even at the 1.6 C rate, more than 300 mAh/g could be tapped from the carbon after 130 cycles. | en_US |