General concern about energy consumption and sustainability has dramatically increased over the last decades. With that perspective, energy storage technologies emerge as a key feature when moving from the present fossil fuel based energy production scenario to a new one based on renewable but intermittent energy sources, since they will enable to match generation and demand. Currently, lithium-ion batteries power most of the portable electronic devices and are the preferred choice for automotive applications due to their high energy density among other characteristics. However, for stationary applications in which more than energy density, cost is the main driver, room temperature sodium-ion batteries show the potential to be a good alternative. Sodium is one of the most abundant elements in the Earth’s crust and unlike lithium ores which are concentrated in remote or politically sensitive areas, sodium ones are widely distributed all over the world which make sodium based components more available and therefore cheaper .
In this work, the electrochemical characterization of a hard carbon/layered oxide sodium-ion battery is presented. Transition metal layered oxides with the general formula NaMO2 (M = one or several transition metals) are attractive cathode materials due to their ease of synthesis, good tap density and high energy density . Meanwhile, the choice of hard carbon as the anode is done on the bases of its low cost and availability. Specifically, Na0.95Ni0.32Ti0.32Mg0.16Mn0.21O2 synthesized by conventional solid state synthesis route and a commercial hard carbon, have been selected as the electrode components in this study.
Individual components are characterized in terms of morphology, chemical structure and electrochemical cycling behaviour against metallic sodium. Reversible capacities of ca. 140 mAh g-1 and 200 mAh g-1 are obtained at C/10 respectively for the cathode and the anode. A full sodium-ion cell is assembled with balanced cathode/anode capacities which delivered 180 mAh g-1 and 115 mAh g-1 for the first charge and discharge capacity, respectively with an average discharge voltage of ≈ 3 V. Cyclability tests show 80% of capacity retention and coulombic efficiencies above 99% after 100 cycles (Fig. 1) which make this combination of materials an attractive choice for the development of a sodium-ion battery.
Fig. 1 a) Voltage profile and b) cycle life of the HC/Na0.95Ni0.32Ti0.32Mg0.16Mn0.21O2 cell cycled at C/10 in 1 M NaPF6 in EC:DEC electrolyte.
 K. Kubota et al., J. Electrochem. Soc. 162(14) 2015 A2538-A2550.
 M.H. Han et al., Energy Environ. Sci. 8 2015 81-102.
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