Expanded graphitic materials prepared from micro- and nanometric precursors as anodes for sodium-ion batteries

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 discharge-charge 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 the 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 the accessibility of Na+ ions to surface oxygen-containing functional groups with the consequent increase of the surface capacity which was found to be the main contribution to the total specific capacity.Financial support from IBERDROLA SPAIN FOUNDATION (www.funfacioniberdrola.org, Projects 2015-2016) and Spanish Ministry of Economy and Competitiveness MINECO (under Projects ENE2011-28318-CO-02 and ENE2014-52189-C2-2-R) is gratefully acknowledged. A. Ramos and N. Cuesta, respectively, thank the Spanish Research Council for Scientific Research (CSIC) for a JAE-Doc contract, co-funded by the European Social Fund (ESF), and the Spanish Ministry of Economy and Competitiveness (MINECO) for a pre-doctoral grant (BES-2012-052711).Peer reviewe