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The electrochemical performance as potential anodes for lithium-ion batteries of graphitized biogas-derived carbon nanofibers (BCNFs) is investigated by galvanostatic cycling versus Li/Li + at different electrical current densities. These graphitic nanomaterials have been prepared by high temperature treatment of carbon nanofibers produced in the catalytic decomposition of biogas. At low current density, they deliver specific capacities comparable to that of oil-derived micrometric graphite, the capacity retention values being mostly in the range 70-80 % and cycling efficiency  100 %. A clear tendency of the anode capacity to increase alongside the BCNFs crystal thickness was observed. Besides the degree of graphitic tri-dimensional structural order, the presence of loops between the adjacent edges planes on the graphene layers, the mesopore volume and the active surface area of the graphitized BCNFs were found to influence on battery reversible capacity, capacity retention along cycling and irreversible capacity. Furthermore, provided that the development of the crystalline structure is comparable, the graphitized BCNFs studied show better electrochemical rate performance than micrometric graphite. Therefore, this result can be associated with the nanometric particle size as well as the larger surface area of the BCNFs which, respectively, reduces the diffusion time of the lithium ions for the intercalation/de-intercalation processes, i.e. faster charge-discharge rate, and increases the contact area at the anode active material/electrolyte interface which may improve the Li + ions access, i.e. charge transfer reaction.

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A series of expanded graphitic materials are prepared from two different precursors: micrometric synthetic graphite and graphitized carbon nanofibers, and tested as anodes for sodium-ion batteries. The materials preparation involves the oxidation of the precursors followed by partial thermal reduction. Overall, the expanded synthetic graphite materials show better electrochemical performance as anode than the expanded graphite nanofibers, providing higher specific capacity, leading to lower capacity losses in the first charge-discharge cycle and exhibiting outstanding cycling stability. Specific capacities of ~150 mA h g -1 at 37 mA g -1 and ~110 mA h g -1 at 100 mA g -1 are attained, and up to 50 % of initial capacity at 19 mA g -1 is kept at 372 mA g -1. Unexpectedly, higher capacity losses are measured for the nanostructured electrodes by progressively increasing the current density. These differences are attributed to the lower surface area and porosity of expanded synthetic graphite materials which favors the formation of thinner and more stable SEI, thus reducing the electrode resistance and enhancing Na + ions accessibility to surface oxygen groups with the consequent increase of the surface capacity which was found to be the main contribution to the total specific capacity.