Lithium resources are unevenly distributed worldwide and might not meet the increasing global demand for energy. Rechargeable Na-ion batteries can be considered as a serious alternative due to the natural abundance and the low cost of sodium especially for electrical energy storage associated with the deployment of sustainable and renewable energy sources. In addition, aqueous electrolytes improves the safety of the batteries and are environmentally benign. Thus, there is an increasing interest in the research of new electrode materials that can be used in such systems. Transition metal phosphates with NASICON structure are good candidates due to their high ionic conductivity and the possibility of tuning the potential by selecting the transition metal. However, they present low electronic conductivity that require doping, nanostructuration or coating to improve electrochemical performances.  

Na_1+xFe_xTi_2-x(PO₄)₃/C composites, with x=0-1, could be used as anode or cathode materials for Na-ion batteries since they exhibit three different potential plateaus at 4 V, 2.2 V and 0.4 V vs. Na⁺/Na⁰ depending on the amount of inserted Na⁺. These composite materials were obtained by solid state reaction with sucrose as carbon source for conductive coating and were characterized by different experimental and theoretical methods (Figure 1). The electrochemical mechanisms were identified from operando techniques, especially for the intermediate potential plateau at 2.2 V vs. Na⁺/Na⁰ that is related to changes in the oxidation states of both Ti and Fe. For example, the reduction of Fe³⁺ into Fe²⁺ was observed at the beginning of the discharge from ⁵⁷Fe Mössbauer spectroscopy. High electrochemical performances were obtained at this potential including long cycle lifetime, high specific capacity and high power rate that are explained by the morphology of the particles, the carbon coating and the substitution of Fe for Ti (Figure 2).

Acknowledgments

Financial supports by Campus France (PHC Toubkal 008/SM/13), the Centre National de la Recherche Scientifique (CNRS, France), the Centre National de la Recherche Scientifique et Technique (CNRST, Maroc), the Institut de Recherche en Energie Solaire et en Energies Nouvelles (IRESEN, Maroc), the Swedish Research Council (contract 2012-3392) and StandUp for Energy (Sweden) are gratefully acknowledged.